U.S. patent application number 13/504125 was filed with the patent office on 2013-01-17 for detector, method for manufacturing a detector and imaging apparatus.
This patent application is currently assigned to Finphys Oy. The applicant listed for this patent is Risto Orava, Tom Schulman. Invention is credited to Risto Orava, Tom Schulman.
Application Number | 20130015363 13/504125 |
Document ID | / |
Family ID | 42136513 |
Filed Date | 2013-01-17 |
United States Patent
Application |
20130015363 |
Kind Code |
A1 |
Orava; Risto ; et
al. |
January 17, 2013 |
DETECTOR, METHOD FOR MANUFACTURING A DETECTOR AND IMAGING
APPARATUS
Abstract
A detector (100) for detecting neutrons comprises a neutron
reactive material (102) adapted to interact with neutrons to be
detected and release ionizing radiation reaction products in
relation to said interactions with neutrons. The detector also
comprises a first semiconductor element (101) being coupled with
said neutron reactive material (102) and adapted to interact with
said ionizing radiation reaction products and provide electrical
charges proportional to the energy of said ionizing radiation
reaction products. In addition electrodes are arranged in
connection with said first semiconductor element (101) for
providing charge collecting areas (106) for collecting the
electrical charges and to provide electrically readable signal
proportional to said collected electrical charges. The thickness of
the first semiconductor element (101) is adapted to be electrically
and/or physically so thin that it is essentially/practically
transparent for incident photons, such as background gamma
photons.
Inventors: |
Orava; Risto; (Helsinki,
FI) ; Schulman; Tom; (Porvoo, FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Orava; Risto
Schulman; Tom |
Helsinki
Porvoo |
|
FI
FI |
|
|
Assignee: |
Finphys Oy
Vantaa
FI
|
Family ID: |
42136513 |
Appl. No.: |
13/504125 |
Filed: |
October 26, 2010 |
PCT Filed: |
October 26, 2010 |
PCT NO: |
PCT/EP10/66193 |
371 Date: |
October 4, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61254828 |
Oct 26, 2009 |
|
|
|
Current U.S.
Class: |
250/390.11 |
Current CPC
Class: |
G01T 3/08 20130101; H01L
31/115 20130101; H01L 31/1185 20130101 |
Class at
Publication: |
250/390.11 |
International
Class: |
G01T 3/06 20060101
G01T003/06; G01T 3/08 20060101 G01T003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 5, 2010 |
GB |
1003663.0 |
Mar 8, 2010 |
US |
12719284 |
Claims
1. A detector for detecting neutrons, wherein the detector
comprises: a neutron reactive material for interacting with
neutrons incident thereon to be detected to release ionizing
radiation reaction products responsive to interactions with the
incident neutrons, a first semiconductor element arranged to
interact with the ionizing radiation reaction products for
providing electrical charges in proportion to the energy of said
ionizing radiation reaction products, wherein the first
semiconductor element is configured with electrodes for providing
charge collection areas for collecting the electrical charges and
to provide electrically readable signals proportional to said
collected electrical charges, and wherein the first semiconductor
element is configured to inhibit interaction with the incident
photons.
2-22. (canceled)
Description
TECHNICAL FIELD
[0001] The invention relates especially to a detector, method for
manufacturing said detector, and imaging apparatus. In particular,
but not exclusively, the invention relates to a neutron detector
and neutron imaging apparatus.
BACKGROUND
[0002] Different kinds of detectors are known for detecting,
tracking, and/or identifying ionizing radiation and high-energy
particles, such as particles produced by nuclear decay, cosmic
radiation, or reactions in a particle accelerator. Some examples of
ionizing radiation types and particles producing ionizing radiation
via collisions with other particles are: Alpha particles (helium
nuclei), beta particles (electrons), neutrons, gamma rays (high
frequency electromagnetic waves, X-rays, are generally identical to
gamma rays except for their place of origin), and charged hadrons,
as an example. Neutrons are not themselves ionizing but their
collisions with nuclei lead to the ejection of other charged
particles that do cause ionization.
[0003] There are dedicated detectors for different type of
radiation and particles. To detect radiation, the interaction
process with matter is utilized where the interacting medium
converts the invisible radiation to detectable signals. If the
radiation consists of charged particles, such as alphas, electrons
or positrons, the electromagnetic interaction create charges which
can be collected and detected. It can also initiate further
processes, which can give rise to registrable signals in the
detector medium. The radiation or particle (such as neutrons) has
to interact with matter and transfer its energy to charged
particles (e.g. electrons). For example, the electrically neutral
gamma radiation interacts with matter with electromagnetic
processes and transfer part or all its energy to charge carriers.
For the registration of thermal neutrons, neutron capture is needed
that results e.g. in a charged particle (such as an alpha
particle).
[0004] All detectors use the fact that the radiation interacts with
matter, mostly via ionization. The detector converts deposited
energy of the ionizing radiation to registered signals, usually
electric signals. The interaction with the radiation takes place in
an interacting medium and creates charges that are collected and
detected. A very typical detector nowadays is a semiconductor
detector that uses a semiconductor (usually silicon or germanium)
to detect traversing charged particles or the absorption of
photons. In the semiconductor detectors radiation is measured by
means of the number of charge carriers set free in the detector or
more particularly the detector material, which is arranged between
two electrodes. The number of the free electrons and the holes
(electron-hole pairs) produced by the ionizing radiation is
proportional to the energy transmitted by the radiation to the
semiconductor. As a result, a number of electrons are transferred
from the valence band to the conduction band, and an equal number
of holes are created in the valence band. Under the influence of an
electric field, the electrons and the holes travel to the
electrodes, where they result in a pulse that can be measured in an
outer circuit. The holes travel into the opposite direction than
the electrons and both can be measured. As the amount of energy
required to create an electron-hole pair is known, and is
independent of the energy of the incident radiation, measuring the
number of electron-hole pairs allows the energy of the incident
radiation to be measured.
[0005] The semiconductor detectors are based on a wafer, which is a
thin slice of semiconducting material, such as a silicon crystal,
upon which e.g. microcircuits are constructed by doping (for
example, diffusion or ion implantation), chemical etching, and
deposition of various materials. Most silicon particle detectors
work, in principle, by diode structure on silicon, which are then
reverse biased. A diode is a component that restricts the
directional flow of charge carriers. Essentially, a diode allows an
electric current to flow in one direction, but blocks it in the
opposite direction. As charged particles pass through these diode
structures, they cause small ionization currents which can be
detected and measured. Arranging thousands of these detectors
around a collision point in a particle accelerator can give an
accurate picture of what paths particles take.
[0006] An example of a silicon detector for detecting
high-intensity radiation or particles is illustrated by WO
2009/071587, where the detector comprises a silicon wafer having an
entrance opening etched through a low-resistivity volume of
silicon, a sensitive volume of high-resistivity silicon for
converting the radiation particles into detectable charges, and a
passivation layer between the low and high-resistivity silicon
layers. The detector further comprises electrodes built in the form
of vertical channels for collecting the charges, wherein the
channels are etched into the sensitive volume, and read-out
electronics for generating signals from the collected charges. The
detector is constructed to take in the radiation or particles to be
detected directly through the passivation layer and in that the
thickness of the sensitive layer having been selected as a function
of the mean free path of the particles to be detected.
[0007] The detector of WO 2009/071587 is manufactured by using a
semiconductor-on-insulator (SOI) wafer, which comprises two outmost
layers of n-type silicon and an intermediate layer of silicon
dioxide. The manufacturing method is mainly characterized by the
steps of selecting the thickness of the silicon layer to be the
sensitive layer at the front surface as a function of the mean free
path of the particles to be detected, growing or depositing an
insulation layer on both surfaces of the wafer by leaving open a
window, etching holes into the layer to constitute the sensitive
layer to reach the silicon oxide layer, doping the holes to create
electrodes, depositing and patterning a metal layer at the front
surface of the wafer and routing the metal layer to read-out
electronic, and forming a window in the back surface of the wafer
to reach the silicon oxide layer.
[0008] The detector of WO 2009/071587 can be used e.g. for
detecting high-intensity radiation particles by having radiation or
particles entering through the entrance window into the detector,
ionizing the neutral atoms within the sensitive volume of
high-resistivity silicon, applying a voltage between electrodes
etched into the sensitive volume, and detecting the signals caused
as a result of the contact with the electrodes by means of read-out
electronics. The detector can also have a polyethylene moderator at
the entrance window for detection of neutrons.
[0009] Also some other neutron detectors are known from prior art,
such as a detector of WO 2007/030156 A2, where semiconductor-based
elements as an electrical signal generation media are utilized for
the detection of neutrons. Such elements can be synthesized and
used in the form of, for example, semiconductor dots, wires or
pillars on or in a semiconductor substrate embedded with matrixes
of high cross-section neutron converter materials that can emit
charged particles as reaction products upon interaction with
neutrons. These charged particles in turn can generate
electron-hole pairs and thus detectable electrical current and
voltage in the semiconductor elements.
[0010] Especially WO 2007/030156 A2 discloses an apparatus for
detecting neutrons, comprising: a substrate capable of producing
electron-hole pairs upon interaction with one or more
reaction-produced particles; a plurality of embedded converter
materials extending into said substrate from only a single
predetermined surface of said substrate, wherein said embedded
converter materials are configured to release said
reaction-produced particles upon interaction with one or more
received neutrons to be detected, and wherein said embedded
converter materials are adapted to have at least one dimension that
is less than about a mean free path of said one or more
reaction-produced particles to efficiently result in creating said
electron-hole pairs; and at least one pair of non-embedded
electrodes coupled to predetermined surfaces of said substrate,
wherein each electrode of said at least one pair of electrodes
comprises a substantially linear arrangement, and wherein signals
from resulting electron-hole pairs as received from a predetermined
said at least one pair of electrodes are indicative of said
received neutrons. The pillars are individually coupled to signal
collection electronics so as to indicate the direction of said
received neutrons.
[0011] In addition WO 2004/040332 discloses a neutron detector,
which utilizes a semiconductor wafer with a matrix of spaced
cavities filled with one or more types of neutron reactive material
such as .sup.10B or .sup.6LiF for releasing radiation reaction
products in relation to the interactions with neutrons. The
cavities are etched into both the front and back surfaces of the
device such that the cavities from one side surround the cavities
from the other side. The cavities may be etched via holes or etched
slots or trenches. In another embodiment, the cavities are
different-sized and the smaller cavities extend into the wafer from
the lower surfaces of the larger cavities. In a third embodiment,
multiple layers of different neutron-responsive material are formed
on one or more sides of the wafer. The new devices operate at room
temperature, are compact, rugged, and reliable in design.
[0012] There are however some problems related to the known
solutions, namely since most of the neutron sources or reactions
are accompanied by a gamma or X-ray background and because the
neutral gamma or X-ray radiation interacts with the semiconductor
matter of the detectors, the gamma or X-ray background will disturb
the accurate measuring, which is an undesired effect especially in
connection with neutron imaging apparatuses.
SUMMARY
[0013] An object of the invention is to alleviate the drawbacks
related to the known detectors. Especially an aim of the invention
is to provide a detector, which is sensitive for detecting neutrons
but at the same time "transparent" for the background gammas and/or
X-rays. In addition a goal of the invention is to provide a
detector with fast charge collection and with excellent radiation
hardness.
[0014] An embodiment of the invention relates to a detector
according to claim 1, another embodiment to a neutron detecting
device according to claim 17, a further embodiment to an
arrangement according to claim 18, a yet further embodiment to a
neutron imaging apparatus according to claim 21 and a still yet
further embodiment to a method of manufacturing the detector
according to claim 22.
[0015] According to an embodiment of the invention the detector
comprises a neutron reactive material functioning as a neutron
sensitive converter adapted to interact with neutrons to be
detected and release ionizing radiation reaction products or recoil
nucleus in relation to said interactions with neutrons, such as
.sup.7Li, .sup.3H, .sup.155Gd, .sup.158Gd, .sup.114Cd, proton,
alpha particle, triton particles, fission fragments, electrons of
internal conversion and/or gamma photons depending of the neutron
reactive material used in the detector.
[0016] In addition the detector comprises first semiconductor
element being coupled with said neutron reactive material and
adapted to interact with said ionizing radiation reaction products
and provide electrical charges (electron-hole pairs) proportional
to the energy of said ionizing radiation reaction products. The
first semiconductor element is typically or even advantageously
silicon wafer, but also other semiconducting material can be used,
such as e.g. gallium arsenide (GaAs) or cadmium telluride
(CdTe).
[0017] The detector also comprises electrodes, which are arranged
in connection with said first semiconductor element for providing
charge collecting areas and for collecting the electrical charges
generated by the ionizing radiation reaction products upon
interacting with said first semiconductor. The detector also
comprises read-out electronics electrically connected with said
electrodes to provide an electrically readable signal proportional
to said collected electrical charges.
[0018] According to an embodiment the thickness of the first
semiconductor (depletion layer) is adapted to be physically so thin
that it does not, or very little, interact, with incident photons,
i.e. it is essentially and practically transparent for incident
photons, such as background gamma photons. According to an
illustrative embodiment said thickness of said first semiconductor
element is about 10 .mu.m. According to a typical embodiment of the
invention the thickness of said first semiconductor element is
between 10-30 .mu.m.
[0019] The thinness of the first semiconductor element can be
achieved e.g. either by physically removing the semiconductor
material (mechanically back thinning) or by appropriately doping
the semiconductor so as to create only a thin active layer or i.e.
electronically by arranging the electrodes to collect charges
within a certain depth only (in the back side).
[0020] The ultra thin detector may offer clear advantages over the
known detectors, because when the thickness of the first
semiconducting layer is at the range of 10-30 .mu.m, the incoming
photons, such as background gammas or X-rays do not, or very
little, substantially interact with the semiconducting layer. For
example, when the thickness of the semiconducting layer is about 10
.mu.m, much less than 0.1% of background gammas will interact with
it, which is clearly negligible. Thus a thin layer of e.g. silicon
or equivalently a thin charge collection region within a silicon
detector represents negligible conversion probability for incoming
photons. For soft X-rays the conversion probability is highest, but
still remains below fractions of a percent for an effective
Si-detector thickness of 10 micrometers. The ultra thin detector
(especially ultra thin first semiconductor and converter material)
enables e.g. imaging, because of the transparency for gamma and
X-ray photons. In addition, when the detector is ultra thin the
charge carriers produced can be effectively caught by charge
collecting areas, such as electrodes.
[0021] The neutron reactive material forms advantageously a neutron
sensitive converter. According to an embodiment also the thickness
of the neutron sensitive converter may be adapted to be physically
so thin that it does not, or very little, interact with incident
photons, i.e. it is essentially and practically transparent for
incident photons, such as background gamma photons. According to an
illustrative embodiment the thickness of the neutron sensitive
converter is 10-30 .mu.m at maximum. The thinness of the neutron
sensitive converter can be achieved by the manufacturing method,
wherein the neutron reactive material is arranged on and/or inside
the first semiconductor element by applying a surface deposition
method, such as laser ablation, atomic layer deposition (ALD),
photolithography or sputtering technique.
[0022] According to an embodiment the neutron reactive material may
be introduced on the surface of the semiconductor element as a
neutron sensitive converter layer. However, according to another
embodiment also other forms can be applied. For example, the first
semiconductor element may be provided additionally with pores, like
pillars, channels, grooves and/or other cavities, which are then
filled with the neutron reactive material. According to an
embodiment the neutron reactive material may also be ion-implanted
inside the structure of said first semiconductor and possibly, even
advantageously, in the surface layer in the proximity to the charge
collecting areas so that the release ionizing radiation reaction
products can effectively reach the first semiconductor and that the
generated electron-hole pairs can be effectively caught by said
charge collecting areas.
[0023] According to an embodiment the neutron reactive material may
be arranged also between the first semiconductor element and the
read-out electronics coupled with said first semiconductor element.
In addition the neutron reactive material may be applied also on
the surface of said first semiconductor element and/or on the
surface of said read-out electronics. In addition according to an
embodiment the neutron reactive material may be adapted to form a
neutron sensitive converter, which has at least one surface the
shape of which is complex or rugged, such as sawtooth-like.
Furthermore according to an embodiment neutron reactive material
may be arranged as clusters on and/or in the surface of the first
semiconductor element, between the read-out electronics and the
first semiconductor element, and/or on the surface of the first
semiconductor element. This can be achieved for example by the
laser ablation illustrated elsewhere in this document.
[0024] The above embodiments, where the neutron reactive material
is applied in different places and has complex or rugged shapes in
the detector maximize the surface area of the neutron reactive
material in the detector so that more neutrons will interact with
the neutron reactive material. This offers clear advantages, such
as increasing
[0025] the efficiency for converting incident neutrons to reaction
products. In addition neutrons may also interact with the neutron
reactive material near the read-out electronics, the first
semiconductor element and especially the charge collecting areas
(electrodes) which makes the detector very effective for detecting
neutrons. In addition the distances for the generated reaction
products from the origin to the semiconductor and electrodes can be
effectively minimized which further improve the effectiveness of
the detector.
[0026] According to an embodiment of the invention the detector may
also comprise in addition a second semiconductor element, which is
typically much thicker than the first semiconductor element coupled
with the neutron reactive material. According to an example the
second semiconductor element is several hundred times thicker than
the first one, typically, or even advantageously, several
millimeters, and according to an example of the order of 5 mm. The
second semiconductor element may be, even advantageously, so thick
that it is sensitive for the gamma photons generated by the
neutrons when interacting with the neutron reactive material. In
addition the second semiconductor element is adapted to provide
electrical charges (electron-hole pairs) proportional to the energy
of said gamma photons. According to an embodiment the second
semiconductor element may be used e.g. to determine the kinematic
of the detected neutrons, such as e.g. a path of the gamma photon
generated by the neutron in the neutron reactive material or
reaction place of the neutron in the neutron reactive material, as
well as also energy of the incident neutron. Thus, when the
kinematic (momentum or energy and direction) of the gamma photon
and the energy of the reaction product is determined, the source or
origin of said incident neutron can be identified.
[0027] According to an embodiment the detector comprises or is
coupled with additional coincidence means for providing a time
window during which the gamma photons are detected by the second
semiconductor element. The starting point of the time window may be
triggered by the interaction of the neutron with the neutron
reactive material generating said gamma photon, or practically by
the electrical signal generated by the electrodes of the first
semiconductor element due to detecting generated electron-hole pair
as discussed elsewhere in this document. This ensures that the
gamma photon, for example, detected by the second semiconducting
element is produced by the neutron interacting with the neutron
reactive material thus excluding for example undesired background
gamma or X-ray photons. Also energy discrimination can be applied
to exclude undesired background gamma or X-ray photons the energy
of which clearly differs from that of the gamma photons generated
by the detected neutrons in the neutron converting material.
[0028] It should be noted that the first and/or second
semiconductor elements illustrated in the above embodiment can be
electrically divided into plurality of areas or pixels, whereupon
the accurate location of the neutrons hit the detector or at least
the reaction products generated by the neutrons can be determined.
The dividing can be achieved e.g. by plurality of electrodes
applied in and/or on the semiconducting material so that the
electrical charges generated in the semiconductor element is
adapted to be collected by the nearest electrode. Thus the location
of the generated electrical charge is determined based on the
location of the electrode collecting said electrical charge.
[0029] The read-out electronics may be implemented e.g. by an ASIC
or similar chip, which may be flip-chip bonded with the electrodes
of the semiconductor element e.g. via bump bond elements. The
read-out electronics are, possibly advantageously, adapted to
detect the charges collected by the electrodes and generate
electric signals proportional to the collected charges either
sensitive for the location or not. According to an embodiment the
read-out electronics may be implemented only for detecting counts
(whereupon the electrodes may be short-circuited, because the
location information is not needed), but according to another
embodiment also for determining dose or even for providing
information for neutron imaging, especially when the location
information is also provided.
[0030] Possible nuclei for the neutron converter materials are for
example:
TABLE-US-00001 .sup.10B(n,.alpha.) .sup.10B + n .fwdarw. .sup.7Li +
.alpha. 2.792 MeV (6%) .sup.7Li* + .alpha. + .gamma.(0.48 MeV)
2.310 MeV (94%) ELi + E.alpha. = Q = 2.31 m.sub.Liv.sub.Li =
m.sub..alpha.v.sub..alpha. {square root over (2m.sub.LiE.sub.Li)} =
{square root over (2m.sub.aE.sub.a)} => E.sub.Li = 0.84 MeV,
E.sub..alpha. = 1.47 MeV (94%) E.sub.Li = 1.01 MeV, E.sub..alpha. =
1.78 MeV (6%)
[0031] The neutron capture cross section: .sigma.=3842 b (0.0253
eV). The natural boron has abundance of .sup.10B 19.8%.
TABLE-US-00002 .sup.6Li(n,.alpha.) .sup.6Li + n .fwdarw. .sup.3H +
.alpha. 4.78 MeV E.sub.3H = 2.73 MeV, E.sub..alpha. = 2.05 MeV
[0032] The neutron capture cross section: .sigma.=942 b (0.0253
eV). The natural lithium has abundance of .sup.6Li 7.40%.
TABLE-US-00003 .sup.3He(n,p) .sup.3He + n .fwdarw. .sup.3H + p
0.764 MeV F.sub.3H = 0.191 MeV, Ep = 0.573 MeV
[0033] The neutron capture cross section: .sigma.=5320 b (0.0253
eV) It is commercially available, but expensive material.
.sup.155Gd
[0034] .sup.155Gd+n.fwdarw..sup.155Gd+.gamma.(0.09,0.20,0.30
keV)+conversion electrons
[0035] The neutron capture cross section: =60791b (0.0253 eV)
.sup.157Gd
[0036] .sup.157Gd+n.fwdarw..sup.158Gd+.gamma.(0.08,0.18,0.28
keV)+conversion electrons
[0037] The neutron capture cross section: =255011 b (0.0253 eV).
Natural gadolinium has abundance of 15.70% of .sup.157Gd, it emits
gamma photons. In 39% of captures conversion electrons with energy
mainly of 72 keV are emitted (electrons with higher energies are
also emitted). The conversion efficiency can reach up to 30%.
.sup.113Cd
[0038] .sup.113Cd+n.fwdarw..sup.114Cd+.gamma.(558 keV)+conversion
electrons
[0039] The neutron capture cross section: =20743 b (0.0253 eV).
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] Next the invention will be described in greater detail with
reference to non-limiting illustrative embodiments in accordance
with the accompanying drawings, in which:
[0041] FIG. 1 illustrates a planar semiconductor detector of
neutrons with a neutron converter deposited on the surface
according to an embodiment of the invention,
[0042] FIGS. 2A-C illustrate charts for ranges of alphas and/or
tritons in LiF of different effective density, where alpha
particles and tritons are products of neutron capture on
.sup.6Li.
[0043] FIGS. 3A-B illustrate dependencies of neutron detection
efficiencies as functions of the neutron converter thickness for
.sup.6LiF and .sup.10B converters. (Both types of the converters
show an optimal thickness at which the detection efficiency is the
highest. It is about 5% for both types of converters.)
[0044] FIGS. 4A-B illustrate schematics of converter side (a) and
detector side (b) irradiation showing the numbers of neutrons
captured in the neutron converter,
[0045] FIG. 5 illustrates detection efficiency as a function of the
LiF converter thickness for the front and backside irradiation,
[0046] FIG. 6A illustrates a method for manufacturing a neutron
detector with neutron reactive material according to an embodiment
of the invention,
[0047] FIG. 6B illustrates another method for manufacturing a
neutron detector with neutron reactive material according to an
embodiment of the invention,
[0048] FIGS. 7A-D illustrate an example of an electrically thin
structure manufactured by the method described in connection with
FIGS. 6A-B according to an embodiment of the invention,
[0049] FIG. 8A illustrates an example of a physically thin planar
semiconductor detector of neutrons with a neutron converter
deposited on the surface according to an embodiment of the
invention,
[0050] FIGS. 8B-C illustrates examples of a thin semiconductor
neutron detector with a neutron converter deposited on the surface
according to an embodiment of the invention,
[0051] FIG. 9 illustrates a semiconductor detector, where the
converter has more complex or rugged shape both in outer surface
and the surface coupled with the detector sensitive volume
according to an embodiment of the invention,
[0052] FIG. 10 illustrates a detector with neutron reactive
material on its surface according to an embodiment of the
invention,
[0053] FIG. 11 illustrates a semiconductor detector with pores
according to an embodiment of the invention,
[0054] FIG. 12 illustrates pixelization of the detector in order to
detect both the neutron collision and its location on the detector
according to an embodiment of the invention,
[0055] FIG. 13 illustrates a semiconductor detector with a readout
chip according to an embodiment of the invention,
[0056] FIG. 14 illustrates a semiconductor detector with an
additional second semiconducting element according to an embodiment
of the invention,
[0057] FIG. 15 illustrates a neutron detector with electrodes
comprising neutron reactive material according to an embodiment of
the invention,
[0058] FIG. 16 illustrates a device for detecting neutrons
according to an embodiment of the invention, and
[0059] FIG. 17 illustrates an arrangement utilizing neutron
detectors of the invention according to an embodiment of the
invention.
DETAILED DESCRIPTION
I Detector Structure
Neutron Converter
[0060] The semiconductor detectors (e.g. illustrated in FIGS. 1 and
10-16) are typically adapted for the thermal neutron detection and
imaging, and are supplemented with a material (neutron reactive
material) which "converts" neutrons into reaction products. The
reaction products, may be advantageously, transfer its energy to
charge carriers, which can be electrically detected directly in the
semiconductor detector. Silicon is very commonly used in the
detectors but there are besides silicon also other types of
semiconductor materials which can be used, such as silicon carbide,
germanium, gallium arsenide (GaAs), gallium phosphide, gallium
nitride, indium phosphide, cadmium telluride (CdTe), cadmium
zinctelluride (CdZnTe), mercuric iodide, lead iodide, and composite
materials based on boron nitride (BN) or lithium fluoride (LiF). An
advantage of such materials may be that the neutron converting
material can be presented directly in their volume. For example
silicon walls of even 10 .mu.m thick or less can detect heavy
charged particles which are products of neutron capture e.g. on
.sup.6Li or.sup.10B.
[0061] The semiconductor neutron (imaging) detectors according to
one or more embodiments of the invention can have high spatial
resolution, high dynamic range and can suppress gamma and electron
background efficiently. Both can be achieved while having high
detection efficiency for thermal neutrons.
[0062] The semiconductor neutron detectors can be divided into
groups determined by how the converter, i.e. the neutron reactive
material, is implemented in the detector: [0063] 1.degree. The
first type is a planar neutron semiconductor detector, such as is
depicted in FIGS. 1 and 10. It may be e.g. a simple planar diode
where the PN junction is parallel to the detector surface. The
neutron converter may be deposited on the detector surface.
Fabrication of such detector to is simple, but its neutron
detection efficiency is limited. The neutron detection efficiency
is defined as a ratio of detected and incident neutrons. The reason
for the limited detection efficiency of the planar detectors is
that all the particles created in the converter by the neutron
capture cannot reach the detector sensitive volume, as can be seen
from FIG. 1, for example. [0064] 2.degree. The second type are so
called 3D detectors, such as is depicted in FIGS. 6 and 11-13. The
abbreviation "3D" stands for 3D structures created inside, but also
on the surface the detector, where the shape, such as surface of
the neutron converter is complex or rugged. The current
semiconductor technologies allow fabrication of advanced surface
structures in the semiconductor. Such structures can be filled by a
neutron reactive material. The 3D structures increase the surface
area between the neutron converter and the detector material and
thereby also the surface area of the neutron converter. Thus, they
increase the probability that incident neutrons will be converted
and detected. This also increases the probability that particles
created in the converter by neutron rapture will be detected in the
sensitive volume of the detector. It should be noted that at least
part of the 3D-structures (such as pores or pillars inside the
semiconducting material) may also be dedicated for electrodes,
which increases the probability that charges (such as electron-hole
pairs) created in the semiconducting element will be detected.
Converter Materials:
[0065] Most of the semiconductor detectors are not able to detect
neutrons directly. A material which "converts" neutrons into
particles detectable by the semiconductor is necessary. Such
material is called the "neutron converter" or neutron reactive
material. The converter materials which produce e.g. heavy charged
particles may be considered to have two significant advantages. The
first advantage is that the heavy charged particles detected in the
detector sensitive volume deposit a large amount of energy and
therefore create a high signal, which allows an easy discrimination
of the background other than neutrons. It is an important feature
because most of the neutron sources are accompanied by a gamma
background. The second advantage applies mainly to neutron imaging
detectors. The relatively short range of heavy charged particles in
the semiconductor material allows a design of neutron imagers with
a higher spatial resolution, since the ranges of the heavy charged
particles are short.
[0066] One parameter to be noticed when selecting the material is a
range of neutron capture products in the matter and the range of
conversion electrons and gammas versus a pixel size of the imaging
detector. Moreover it should be noticed that the eventually
detected electron can be generated by Compton scattering or photo
effect at a different place than where the neutron was captured.
This will deteriorate the imaging detector spatial resolution as
well. These are reasons why the selection of the neutron converter
material is important.
[0067] The used neutron reactive material may be same or different
in different places of the detector and comprises according to an
embodiment at least one predetermined converter material
comprising: .sup.10B, .sup.6Li, .sup.3He, .sup.155Gd, .sup.157Gd,
.sup.113Cd, cadmium telluride (CdTe), cadmium zinc telluride
(CdZnTe), or composite materials based on boron nitride (BN) or
lithium fluoride (LiF, which is essentially transparent for
incident photons, such as gammas). Typically the neutron reactive
material is selected so that it's Z-number is as high as
possible.
[0068] FIG. 1 illustrates a planar semiconductor detector 100 of
neutrons with a neutron converter 101 deposited on the surface of
the semiconducting material 102 according to an embodiment of the
invention. The detector 100 is based on a planar diode detector,
where the thermal neutrons 103 are captured in the 6Li (which is in
form of LIF compound) converter 101 and secondary particles 104 are
produced. These particles 104 are subsequently detected by the
semiconducting material 102 of the detector 100.
[0069] However, the planar converter has its limitations. The
probability of neutron capture in the converter is increasing with
the increasing thickness of the converter layer. On the other hand,
with the growing converter thickness the chance that the neutron
capture products from the most distant converter levels will reach
the detector sensitive volume also decreases. For a particular
converter type an optimal converter thickness has to be found.
Unfortunately, this limited effective thickness also limits the
overall neutron detection efficiency (the detector sensitivity).
Important parameters which determine the design of the neutron
converter are ranges of neutron capture products in matter.
[0070] FIG. 2A illustrates a chart for ranges of tritons and alphas
in LiF of different effective density, where alpha particles and
tritons are products of neutron capture on .sup.6Li, as known.
According to an embodiment the LiF converter may be in form of
powder and therefore it can be pressed and have virtually an
arbitrary effective density almost up to density of LIF crystal
which is 2.64 g/cm.sup.3. LiF may be enriched by .sup.6Li to
89%.
[0071] The range of tritons in silicon crystal is 44.1 .mu.m and
the range of alpha particles is 8.6 .mu.m. FIG. 2B contains results
of a similar simulation but for amorphous boron powder illustrating
ranges of alpha particles and lithiums which are products of the
neutron capture in .sup.10B, as known from prior art. It is clear
that ranges of heavy charged particles are shorter than in the case
of LiF. Ranges of products of neutron capture on .sup.10B are in
Si:R.sub.Li=3 .mu.m/2.7 .mu.m, R.sub..alpha.=5.4 .mu.m/5.2
.mu.m.
[0072] The curves in FIG. 2C are heavy charged particle ranges as
functions of B.sub.4C density, as known from prior art. B.sub.4C is
an example of a boron compound usable as a neutron converter. It is
possible to calculate such dependencies for any boron or lithium
compound. However, a common property of all of them generally will
be significantly shortened range of neutron capture products for
.sup.10B in comparison with .sup.6Li. This somehow predetermines
the applicability of converters based either on .sup.6Li or
.sup.10B. The advantage of .sup.10B over .sup.6Li in the higher
thermal neutron capture cross section is reduced by the shorter
ranges of the capture products. This effect will be even more clear
if the heavy charged particles will have to pass through a thicker
layer of metallic contacts or a thicker insensitive layer in the
semiconductor detector. The results of heavy charged particles
ranges may also be applicable on other types of detectors which use
the same thermal neutron converters.
Neutron Detection Efficiency
[0073] FIGS. 3A-B illustrate dependencies of neutron detection
efficiencies as functions of the neutron converter thickness for
.sup.6LiF and .sup.10B converters, where both types of the
converters show an optimal thickness at which the detection
efficiency is the highest, which is about 5% for both types of
converters, as known from prior art. It can be seen from FIGS. 3A-B
that the effect of the lower neutron capture cross section of
.sup.6Li can be in comparison to .sup.10B well compensated by
longer ranges of secondary particles. The probability that neutrons
will be captured in the .sup.10B converter is higher, but on the
other hand the shorter ranges of neutron capture products from
.sup.10B prevent them reaching the detector sensitive volume and
create a sufficient signal. Secondary particles born in the
converter layer most distant from the silicon surface should be
still capable reaching the sensitive detector volume and leave a
detectable amount of energy there. Thus, the converter thickness
should be limited to a value of the longest particle range in the
converter material. A lower converter thickness increases the
chance that heavy charged particles will reach the sensitive
volume, but it reduces the probability that neutrons will interact
inside the converter. The overall maximum detection efficiency of
.about.5% is similar for both types of converters. However,
.sup.16B can offer a better spatial resolution when applied on a
neutron imaging device which has a pixel size comparable or lower
than ranges of flight of the neutron capture products. A way to
overcome the limited detection efficiency is to introduce more
complex geometrical structures of the surface between the neutron
converter and the detector sensitive volume. According to an
embodiment the surface of the neutron converter may be e.g. rugged
or other way complex so that its surface area will be
increased.
[0074] Neutrons follow the exponential attenuation law when passing
through the material. FIGS. 4A-B illustrate schematics of the
converter side (a) and the detector side (b) irradiation showing
the numbers of neutrons captured in the neutron converter. When
neutrons 10a enter from the converter 102 side A) more neutrons are
captured and converted away from the detector surface (i.e. in the
part of the neutron reactive material 102 locating most far from
the semiconducting element 101). When neutrons 10b enter from the
detector 101 side B) more neutrons are captured and absorbed in the
neutron reactive material 102 close to the semiconducting element
surface 101 where the probability that the conversion alpha
particles escape the converter 102 and penetrate into the
semiconducting element 101 is higher.
[0075] The heavy charged particles created close to the outer
surface must fly through a thicker layer of the converter to reach
the sensitive volume. Therefore, a chance that such particles will
be detected in the sensitive volume is lower. Apparently, this
effect becomes even more significant for thick converters (i.e.
with a thickness comparable or higher than ranges of charged
particles in the matter of converter).
[0076] Since the semiconductor materials are in most cases
transparent for neutrons it is possible to irradiate the whole
detector structure from the backside. Neutrons pass through the
semiconductor first and are then captured in the converter. Indeed,
a higher number of neutrons are captured closer to the boundary
between the semiconductor and the converter. The probability that
products of the neutron capture will reach the sensitive volume is
higher and the overall detection efficiency is higher. Moreover,
the converter thickness does not have to be optimized and can be
even thicker than the range of the heavy charged particles.
[0077] FIG. 5 illustrates the difference in detection efficiency as
a function of the LiF converter thickness for the front and
backside irradiation, wherein the LiF is enriched in .sup.6Li to
89%, as known from prior art. In both cases the detection
efficiency is increasing up to a layer thickness of about 7
mg/cm.sup.2. It is a surface density which is equal to the maximal
range of tritons in LiF. The curve exhibited a maximum of 4.48% at
this converter thickness in the case of the front irradiation. From
this thickness is the detection efficiency decreasing for
irradiation of the front side, but remains constant at 4.90% for
the back side irradiation. If the detector is irradiated from the
back side the converter is active only to the depth which is equal
to the longest range of neutron capture products. Deeper converter
layers do not contribute to the neutron detection at all and the
detection efficiency stays constant with the increasing converter
thickness.
[0078] However, the effect of the back side irradiation is not
significant for thin converter layers, which can also be seen from
FIG. 5. The advantage of the back side irradiation is that it is
not necessary to control the converter thickness during the
deposition precisely and that the reaction product will more
effectively reach the semiconducting material where they produce
detectable electrical signals, such as electron-hole pairs. In
principle it is enough to deposit a layer thicker than the range if
the heavy charged particles from the neutron capture reaction. The
detection efficiency is the maximal achievable with this
geometrical configuration.
[0079] The neutron detection efficiency depends also as on a
function of a pore size and shape, such as whether the shape of
pore is square or cylindrical, but also the density of the neutron
reactive material. Square (or cylindrical) pores can be relatively
easily fabricated and allow a good filling ratio of the detector
with a neutron converter.
[0080] The detection efficiency typically increases with increasing
converter density, such as especially in the case of the LiF filled
structure. This is due to the increasing macroscopic cross section
for neutron capture. .SIGMA.=.sigma.n, where is microscopic neutron
capture cross section and n is number of converter nuclei per unit
of volume. The range of heavy charged particles remains sufficient
to escape the pore even with the increasing density. The situation
is opposite in the case of .sup.10B. The highest detection
efficiency can be reached with a lower density of the converter.
More important is here the effect of the heavy charged particle
range extension with the decreasing density. The macroscopic cross
section E remains sufficiently large with the decreasing density,
i.e. the number of converter nuclei per volume unit.
[0081] The highest reached detection efficiencies are lower than in
the case of square pores. The cylindrical pores do not fill up the
volume of the detector as much as the square pores. There is more
silicon in between pores and thus this volume is insensitive to
neutrons. The ratio of the pore top surface and the surface of
surrounding silicon is higher for square pores and therefore the
overall efficiency is higher. The cylindrical pores, however,
should not be abandoned hence the bigger volume of silicon around
pores may allow also better charge collection efficiency.
[0082] A possible way according to an embodiment of the present
invention to provide more efficiency is to introduce a complex or
rugged, such as e.g. a sawtooth like surface between the neutron
converter and the detector sensitive volume (as disclosed in FIGS.
7A-D, for example). The detector may contain an array of inverted
pyramidal dips created e.g. by anisotropic etching of silicon with
KOH (Potassium Hydroxide).
[0083] According to an embodiment the surface between the neutron
converter and the detector may be doubled. Contrary to the planar
detector case the spectrum now contains events above 2.73 MeV,
because both particles (alpha and triton) can be detected
simultaneously if the reaction takes place in the region close to
the sawtooth tip. Once again the detector can be irradiated from
the back side.
[0084] According to an embodiment the converter material comprises
at least one of the following: .sup.10B, .sup.6Li, .sup.3He,
.sup.155Gd, .sup.157Gd, .sup.113Cd or cadmium telluride (CdTe) or
composite materials based on boron nitride (BN) or lithium fluoride
(LiF), or CdZnTe. According to an embodiment it is desirable that
the Z-number of the converter material is as high as possible so
that the neutrons would interact efficiently with the converter
material producing detectable radiation, such as for example gamma
rays, which can be detected by the detector material.
[0085] According to an, possibly advantageous, embodiment of the
present invention the neutron reactive material is coupled on
and/or inside the first semiconductor element or detector in a new
way, namely by applying a laser ablation. FIG. 6A illustrates a
method for manufacturing a neutron detector with neutron reactive
material according to an, possibly advantageous, embodiment of the
invention using the laser ablation, where high-power laser pulses
are used to evaporate matter from a target surface.
[0086] The laser ablation based surface deposition can be divided
into four stages: [0087] 1) Laser ablation of thee target material
109 and creation of plasma [0088] 2) Dynamics of the plasma [0089]
3) Deposition of the ablated material on the substrate 101 [0090]
4) Nucleation and growth of the film 102 on the substrate surface
101.
[0091] The manufacturing of the neutron detectors by applying the
laser ablation in depositing the conversion layer may offer
numerous advantages. Basically any material can be used for surface
deposition. In addition the low process temperature allows
deposition of heat sensitive materials. The laser ablation surface
deposition heats also the substrate (semiconducting layer 101) only
locally and retains the material properties of the target. Moreover
the surface morphology (smoothness or roughness) can be controlled,
as well as also the crystallinity of the surface can be controlled
from amorphous to microcrystalline. Furthermore the adhesion is
superior compared to other PVD (physical vapour deposition)
processes. In addition the laser ablation method is applicable also
to mass production process, so it suits very well for deposition of
the conversion layer overall.
[0092] It should be noted that the overall process for
manufacturing the neutron detector may be implemented for example
applying e.g. lithographic methods, which may comprise e.g. the
following steps: [0093] spinning of photoresist [0094] baking of
photoresist e.g. in an oven [0095] patterning of photoresist with a
mask aligner [0096] deposition of neutron converter (such as thin
film or other shape discussed e.g. in this document) for example by
sputtering, atomic layer deposition or laser ablation [0097] lift
off (removal of photoresist together with converter from detector
contact pads)
[0098] Or alternatively in other order, such as: [0099] deposition
of neutron converter (such as thin film or other shape discussed
e.g. in this document) for example by sputtering, atomic layer
deposition or laser ablation [0100] spinning of photoresist [0101]
baking of photoresist in an oven [0102] patterning of photoresist
with a mask aligner [0103] etching of converter from detector
contact pads [0104] removal of photoresist
[0105] According to another embodiment the neutron reactive
material may also be coupled on and/or inside the first
semiconductor element by applying another surface deposition
method, such as atomic layer deposition (ALD), photolithography or
sputtering technique.
[0106] FIG. 6B illustrates another method for manufacturing a
neutron detector with neutron reactive material according to an
embodiment of the invention, where an oxide layer 114 is arranged
(e.g. by ALD) on the surface of a SOI-wafer 113, such as when
growing a SOI-wafer. However, according to an embodiment of the
invention the neutron reactive material 102a is, possibly
advantageously, applied on the surface of the insulator 114 and in
addition the semiconducting layer, such as Si-layer, is then
arranged on the top of the first neutron reactive material layer
102a. In addition, according to an embodiment of the invention
additional neutron reactive material layer 102b can be arranged on
the surface of the semiconducting layer 101 in order to further
enhance the neutron conversion efficiency of the detector. The
multiple layer structure (102a, 102b) can be implemented also in
other neutron detector depicted in this document even though not
separately mentioned.
[0107] The surface deposition methods depicted above (and
especially laser ablation method) may have the advantage that the
extremely thin detector structures can be made. For example the
converter material layer 102 as well as also the first
semiconductor detector material layer 101 is, possibly
advantageously, about 10 .mu.m, or, possibly more advantageously,
10-30 .mu.m, as is illustrated by FIG. 8A. One or more embodiments
of the invention may offer clear advantages with extremely thin
semiconducting detector layer, because the extremely thin
semiconducting detector layer is in practice, although not
necessarily or absolutely, transparent to undesirable background
gamma and X-ray photons, whereupon the undesirable background
photons do not cause any undesirable effects. For example when the
thickness of the semiconducting layer 101 is about 10 .mu.m, much
less than 0.1% of background gammas will interact with it. However,
it also enables the charges to be produced by the reaction products
in the first semiconductor to reach the electrodes.
[0108] FIGS. 7A-D illustrates an example of a semiconductor
detector for neutrons according to an, possibly advantageous,
embodiment of the invention, where the first semiconducting element
101 is electrically thin (101a, typically, possibly most
advantageously, 10-30 .mu.m), and which still enables the charges
to be produced by the reaction products in the first semiconductor
to reach the electrodes 112. The detector of FIGS. 7A-D comprises a
neutron converter 102, possibly advantageously, deposited on the
"back" surface of the first semiconductor 101, so the same side of
the first semiconductor 101 than where the electrodes 112 are
applied and the same side where the read-out chip will be placed
(when it is used).
[0109] The detectors of FIGS. 7A-D can be manufactured e.g. by the
method illustrated in FIG. 6B, where the SOI wafer 113 has an
optional neutron converter layer 102a to increase the probability
of neutron conversion. An applied voltage between the n+ (or p+) 3D
pixel electrodes 112 and the p (or n) type silicon 101 creates a
depletion layer extending down to the conversion layer 102a and
sideways to the regions between the 3D electrodes 112. Grooves 313
increase the surface area of the neutron conversion layer 102b for
higher neutron absorption probability. The dimensions of the
grooves 313 and pixel plateaus are preferably chosen so as to
produce the largest possible surface area of the conversion layer
102b for a desired thickness of the active region 101. The
thickness of the neutron conversion layers 102a and 102b is
typically 5 .mu.m. The thickness of the active region, i.e. the
first semiconducting element 101 is typically 10-30 .mu.m. The
wafer substrate 113 can be of conventional thickness (as in the
drawing) or physically thinned, as described elsewhere in this
document.
[0110] The wafer substrate 113 can alternatively be a high
resistivity Si wafer without the conversion layer 102a. If a high
resistivity Si wafer is used the thickness of the active region of
the first semiconducting element can be made small by tuning the
depletion voltage appropriately or by doping a p well (n well if
the substrate is p type) around the electrodes 112. The 3D
electrodes 112 can alternatively be planar processed 2D electrodes,
such as depicted in connection with FIGS. 8B-C.
[0111] According to an embodiment the pixel electrodes can either
be shortened together (e.g. by a sputtered metal layer on top of
the pixel electrodes) for single channel readout as is depicted in
FIG. 7C or connected to a multi channel readout circuit with bump
or wire bonding or similar means as illustrated e.g. in connection
with FIG. 13. If the pixels are shortened and the detector is used
as a single channel device two detectors can be sandwiched
face-to-face for double efficiency, as illustrated in FIGS. 7C and
7D.
[0112] It should be noted that the detector may comprise an
additional detector element 115 (essentially similar to the lower
one), where the detector elements are arranged face-to-face to each
other, and possibly advantageously, so that the neutron converters
102 of the detector elements are faced to each other. Now the
read-out means, such as read-out electronics or even conductive
wires, can be applied between the detector elements. The embodiment
having two detector elements further enhances neutron conversion
efficiency.
[0113] According to an embodiment the pixel pitch of the detector
structure illustrated in FIGS. 7A-D is typically 50 .mu.m or even
smaller. The 3D electrodes, possibly advantageously, extend the
depletion layer below the grooves 313. The grooves may be
manufactured e.g. by etching. The structure of the detector
illustrated in FIGS. 7A-D enables optimum efficiency maintaining
thin depletion layer.
[0114] According to an embodiment of the invention the converter
material to be coupled with the detector material may be planar,
such as depicted in FIG. 8A. However, according to another
embodiment of the invention, such as depicted in FIGS. 7A-D, 8B-C
and 9, at least one of the surfaces of 102a, 102b the converter
material has more complex shape, such as 3D, a sawtooth-like,
convoluted or rugged surface in order to maximize the effective
surface area of the neutron reactive converter to convert neutron.
Such geometries allow a larger volume and/or surface area of the
neutron converter while keeping a high probability of the secondary
particle detection.
[0115] FIGS. 8B-C illustrates examples (side and perspective views)
of a thin semiconductor neutron detector with a neutron converter
102 deposited on the surface of the first semiconducting element
101 according to a, possibly advantageous, embodiment of the
invention. Also the electrodes 112 and depletion areas 116 can be
seen in FIGS. 8B-C, as well as the 3D structure, which increases
the surface area of the neutron converter 102 made of neutron
reactive material. According to an embodiment the pixel size is
100-300 .mu.m. The structure may be e.g. bump-bonded for position
sensitive detection, but also short-circuited if the purpose of
using is e.g. only count detection. Grooves can be manufactured
e.g. by dicing or etching. It should be noted that the similar
structure may also be applied in other detectors depicted in this
document in connection with other figures.
[0116] According to an embodiment the converter layer most distant
from the detector surface may have a complex shape, such as a
convoluted, e.g. sawtooth-like, surface. Also the surface coupled
with the detector material may have a complex shape, such as the
sawtooth-like shape. In addition according to an embodiment of the
invention also both surfaces may have a complex shape, like the
shape of sawtooth, such as illustrated by FIG. 9. This kind of
converter may efficiently convert neutrons even though they do not
enter into the detector (converter material) perpendicularly. Again
it should be noted that the neutrons to be detected may be arranged
to incident either from the front (first through the converter
material 102) or back side (first through the semiconducting
material 101) of the detector.
[0117] FIG. 10 illustrates a detector according to a, possibly
advantageous, embodiment of the invention, where the neutron
reactive material 102 is applied, such as ion-implanted on the
surface and/or inside the structure of said first semiconductor
element 101. When the neutron reactive material 102 is applied
inside the structure, it is still, possibly advantageously,
arranged in the surface layer 101a in the proximity to the charge
collecting areas (not shown in FIG. 10) so that the released
ionizing radiation reaction products can effectively reach the
first semiconductor 101 and that the generated electron-hole pairs
can be effectively caught by said charge collecting areas.
According to an embodiment the neutron reactive material 102 is,
possibly advantageously, arranged as clusters on and/or in the
surface of the first semiconductor element 101 (as in FIG. 10).
According to an embodiment of the invention the neutron reactive
material 102 can be arranged between the read-out electronics and
the first semiconductor element, and/or on the surface of the first
semiconductor element. This can be achieved for example by the
laser ablation as illustrated elsewhere in this document.
[0118] FIG. 11 illustrates a semiconductor detector with pores 105,
such as pillars or other cavities, according to a, possibly
advantageous, embodiment of the invention. The pores 105 may be
filled with the neutron reactive material 102 to convert neutrons
for detectable reaction products, such as to gamma photons or other
products described e.g. in this document. The neutron reactive
filling material is, possibly advantageously, the same as used for
neutron converter 102 in other parts of the detector. The cavities
may be in perpendicularly in relation to the converter layer
coupled with the detector material, but also in some other angle so
that the neutrons will interact with the filling material even
though they will enter into the detector in other angle than
essentially perpendicular. The detector structure having pores or
other cavities may also in additionally have more complex shapes
for the surfaces as well as comprise also neutron reactive material
as clusters on and/or inside the detector structure, possibly
advantageously the first semiconductor structure, such as disclosed
above in connection with FIGS. 9 and 10, for example (even though
it is not shown in FIG. 11 for clarity reasons).
[0119] The pores may be manufactured by the known technologies
applicable for fabrication of 3D structures (pores) in
semiconductor materials. The technologies of pore fabrication are
for example reactive ion etching and electrochemical etching. In
both cases, the etching may be preceded by a photolithographic step
which prepares a mask for the etching. The mask protects areas of
surface against the etching and opens top of patterns to be etched.
Type of the mask depends on the used technology. It can be a metal
for DRIE or SiO.sub.2 layer for the electrochemical etching.
[0120] In Deep Reactive Ion Etching (DRIE) is a highly anisotropic
etch process used in microsystem technology. It is used to create
deep and high aspect ratio holes and trenches in silicon and other
materials. Structures with aspect ratios 20:1 and more can be
produced. DRIE etch rates are 5-10 .mu.m/minute.
[0121] Another illustrative method for pore creation is the
electrochemical etching (EE), which is a low cost alternative to
deep reactive ion etching (DRIE). It allows fabrication of
structures such as walls, tubes, pillars and pores. In
electrochemical etching the applied electric field may be
concentrated e.g. on the inverted pyramid tips (for example when
the shape is like sawtooth).
[0122] FIG. 12 illustrates a pixelization 106 of the detector 100
in order to detect both the neutron collision and its location on
the detector according to a, possibly advantageous, embodiment of
the invention. The pixelization may be implemented e.g. by dividing
at least the portion of the semiconducting element 101 electrically
into plurality of areas, hereinafter pixels 106. The electrical
dividing can be achieved e.g. by using plurality of electrodes,
whereupon the electrical charges generated in the semiconductor
element are collected by the nearest electrode. Thus also the
location of the generated electrical charge in the semiconducting
element 102 can be determined based on the location of the
electrode collecting said electrical charge.
[0123] The detector with pixelization, as illustrated in FIG. 12,
can be used e.g. for neutron imaging according to an embodiment of
the invention, where the detector sensitive volume is arranged so
that it can both detect the neutron (or the radiation reaction
products produced by the collision of the neutron with the neutron
reactive material), but also its location on the detector. For
example by utilizing the high integration of contemporary
electronic parts for the design of an imaging detector can improve
parameters of current radiation imaging systems.
[0124] However, in order to read the collisions and location the
detector of FIG. 12 is, possibly advantageously, provided in
addition with a readout chip 107, as is illustrated in FIG. 13. The
detector may be for example flip-chipped or bump-bonded to the
readout chip 107, such as e.g. CMOS (Complementary
Metal-Oxide-Semiconductor) or the like. The bump-bonding may be
implemented via bump-balls 110. According to an embodiment each
pixel may, possibly advantageously, have its own readout with
preamplifier, discriminator and 15-bit counter, for example. The
readout chip may be manufactured for example according to 1 .mu.m
SACMOS (Self-Aligned Contact Metal-Oxide-Semiconductor)
technology.
[0125] According to another embodiment of the invention the neutron
imaging device may also be manufactured using 6-metal 0.25 .mu.m
CMOS technology, where the pixel size may be e.g. 55.times.55
.mu.m.sup.2 and the pixel array even 256.times.256 pixels, for
example. The sensitive area may be about 2 cm.sup.2. According to
an embodiment the readout electronics may offer a possibility to
use two discriminators to set an energy window for choosing the
measured energy of radiation. Each cell may contain e.g. a 13-bit
counter and an 8-bit configuration register which allows masking,
test-enabling and 3-bit individual threshold adjust for each
discriminator, for example. Using the serial or parallel interface,
the readout of the whole matrix containing measured data (clock 100
MHz) may take 9 ms or 266 .mu.s, respectively. The fast readout is
predestinating this detector also for applications where a fast
frame acquisition is needed. Overall the detector illustrated in
FIGS. 12 and 13 may provide huge advantages, because they provide a
high spatial resolution, high dynamic range and low noise.
[0126] The signal created by the heavy charged particles is
typically high enough to set the discriminator threshold in each
pixel far above the noise and a possible background. Counts of
events in each pixel obey a Poisson distribution with a standard
deviation determined only by the number of neutrons reacting in the
converter. Therefore, the signal to noise ratio can be improved to
an arbitrary level only by an exposition time extension. In the
case of thermal neutrons the threshold is high and thus the
background is neglectable or negligble. The signal to noise ratio
is then given only by where n is a number of counts per pixel.
[0127] FIG. 14 illustrates a semiconductor detector with an
additional second semiconducting element 108 according to a,
possibly advantageous, embodiment of the invention, which may be
applied e.g. to neutron spectroscopy or imaging and to detect the
kinematic of the reaction of the neutron with the detector. The
detector of FIG. 14 comprises neutron converter 102 and ultra thin
semiconducting element 101 similarly as discussed earlier in this
document, but in additionally the detector comprises also second
additional semiconducting element 108. The second semiconducting
element 108 is typically much thicker (108a) that the one 101
coupled with the neutron converter material 102 and comprises,
possibly advantageously, cadmium telluride (CdTe) The thickness of
the thicker second semiconducting element 108 may be according to
an embodiment of the invention even 5 mm or more.
[0128] In some neutron conversion reactions gamma rays or X-rays
are created. These gammas should not be detected by the first
semiconducting element 101 but should escape or penetrate the first
semiconducting element 101 and be detected by a separate detector,
such as the second semiconducting element 108.
[0129] A possible advantage of the neutron detector according to
FIG. 14 is that for example the gamma rays or X-rays (originated
from the collision of neutron to be detected with the neutron
reactive material) passing the ultra thin semiconducting element
101 may be detected by the thicker second semiconducting element
108 because the probability for the interaction of the gamma rays
or other reaction products with the semiconducting material
increases when the thickness of the semiconducting material
increases. Thus the kinematic of the detected neutrons, such as
e.g. a path of the gamma photons generated by the neutron in the
neutron reactive material or reaction place of the neutron in the
neutron reactive material, as well as also energy of the incident
neutron can be detected. When the kinematic (momentum or energy and
direction) of the gamma photon and the energy of the reaction
product is determined, the source or origin of said incident
neutron can be identified.
[0130] In addition the using of the detector of FIG. 14 enables
e.g. combined neutron and X-ray spectroscopy or imaging and use of
plurality of neutron and X-ray sources, because by the detector of
FIG. 14 the origin or source of the gamma or X-ray photon can be
determined. I.e. when the gamma or X-ray photon is detected by the
second semiconducting element 108 it can be determined whether it
was produced by the interaction of the incident neutron with the
neutron reactive material or whether it was originated from the
gamma or X-ray source outside the detector.
[0131] The second semiconducting element 108 also, possibly
advantageously, comprises own pixelization 106, and it should be
provided by own readout chip (not shown), which is arranged,
possibly advantageously, in electrical connection with the
pixelization 106 of the second semiconducting element.
[0132] FIG. 15 illustrates a neutron detector with electrodes 112,
where the electrodes are, possibly advantageously, e.g. etched into
the detector structure for example in the form of cylinder.
According to an embodiment of the invention the electrodes 112 may
comprise neutron reactive material 112 inside the electrode
structure 112. In other words the outer layer of the electrode
structure 112 forms an electrode and the inner portion is filled
with the neutron reactive material 102. This still increases the
surface area of the neutron reactive material and thereby also the
probability that the incident neutrons will be converted into the
reaction products. In addition when the reaction products fly
through the electrode wall, they will, possibly advantageously,
produce electrical charges (such as electron-hole pairs) in the
vicinity of the electrode, whereupon the produced charges can be
effectively caught by the electrode. Thus the detector illustrated
in FIG. 15 is very efficient both for converting the neutrons and
collecting the charges.
[0133] FIG. 16 illustrates a device 200 for detecting neutrons
according to a, possibly advantageous, embodiment of the invention.
The device 200, possibly advantageously, comprises a detector
module 201 and interface module 210, the detector module having
neutron convertor 102, such as depicted elsewhere in this
application, as well as at least one semiconducting element 101,
108. The semiconducting element 101, 108 is, possibly
advantageously, electrically coupled with the readout electronics
107, such as e.g. ASIC chip. In addition a programmable logic 202
may be adapted to provide functions of the detector module 201,
such as signal processing, timing and control operations, as well
as also to provide interface and data communication between the
detector module 201 and the interface module 210. The interface
module 210, possibly advantageously, comprises own programmable
logic 211.
[0134] In addition the interface module 210, possibly
advantageously, comprises EEPROM memory means 212, as well as also
other memory means 213 for storing data, user interface means 214
for controlling the operation of the device 200, display means 215
for displaying information, such as total counts and/or dose
related to counted neutrons or reactions, and data communication
means 216, such as wireless communication means, which may be
implemented e.g. by Bluetooth or WLAN, for example. The data
communication means 216 may also have serial communication bus,
such as USB. In addition the interface module 210, possibly
advantageously, comprises a microcontroller 217 for controlling the
operations and the data communications between the portions 211-216
of the interface module.
[0135] The neutron detectors in accordance with embodiments of the
invention have many applications. Due to their compact size, low
cost, high detection efficiency, low power consumption as well as
direct real time conversion of neutron signal they can be used for
example to real time monitoring. One possible arrangement 300
utilizing the neutron detectors in accordance with embodiments of
the invention is illustrated in FIG. 17, where the neutron
detectors form, possibly advantageously, a measuring network for
communicating measuring information for example from a measurement
point e.g. via a base stations or other nodes to a central control
point. Due to plurality of the measuring points, which are
measuring neutrons in real time, a possible neutron migration can
be detected and forecast composed for example in a rescue
viewpoint.
[0136] The arrangement 300 may, possibly advantageously, comprise a
plurality of sensor nodes 301 each of them utilizing at least one
neutron detector in accordance with embodiments of the invention.
The sensor nodes 301 are, possibly advantageously, powered e.g. by
batteries, solar cell or other way known by the skilled person, and
the data communication 302 of the sensor nodes 301 is, possibly
advantageously, implemented in a wireless way, such as utilizing
WLAN (802.1 b, g or 6LowPan) or other wireless technology known by
the skilled person. Therefore the sensor nodes 301 can be located
e.g. geographically in very difficult places. However, it should be
noted that sensor nodes 301 can also be mains powered and/or the
data communication 302 of the sensor nodes 301 can also be
implemented by a wire.
[0137] The sensor nodes 301 are, possibly advantageously, in a data
communication with a backbone node 303, such as mains powered
backbone WLAN MESH nodes utilizing WLAN (802.1 b, g or 6LowPan) or
other data communication technology known by the skilled person. In
addition the backbone nodes 303 may be in data communication 304
with each other, as well as e.g. via base stations with operators
305 for example in 3G or GPRS network, Internet or the like.
According to an embodiment also users, databases and application
servers 306 may gather measuring data e.g. via LAN, and mobile
users 307 e.g. via mobile network, such as 3G or GPRS. In addition
according to an embodiment also e.g. administrators may be in data
communication with the measuring nodes or even with the detectors
in the nodes (such as controlling the operation of them) via data
communication network illustrated in FIG. 17.
[0138] In addition the measuring nodes with the detectors may be
arranged for example in vehicles, such as airplanes and especially
Unmanned Aerial Vehicle 308, the operation of which can be
programmed beforehand but also the operation of which can be
controlled via the data communication network illustrated in FIG.
17.
[0139] The neutron detectors in accordance with embodiments of the
invention have also other application areas in addition to safety
and monitoring of background radiation, such as security
(protection against nuclear terrorism) and imaging, as well as
non-destructive tests (neutron imaging for industrial applications,
complementary to X-rays). In addition the detectors may be used for
health purposes, such as personal dosimetry for e.g. personnel
exposure at nuclear power plants and soldiers on a field.
[0140] One or more embodiments of the present invention may offer
clear advantages, such as low cost, high detection efficiency,
direct real time conversion of neutron signal, compact size, low
power consumption, well suitability for high volume production,
good discrimination power against background X-rays and/or
.gamma.-rays, and suitability for neutron imaging. In addition the
neutron spectroscopy is also possible according to embodiments of
the invention as depicted above in this document. The invention
also offers flexible modular architectures, based on a variety of
detector substrates and readout ASICs, for example.
[0141] As a conclusion the converter materials of the invention,
possibly advantageously, have high Z, such as CdTe or CdZnTe
converters have a high Z and hence they are well suitable for
example for converting neutrons for example into detectable gamma
rays. For example the natural Cd contains also .sup.113Cd which has
a high cross section for thermal neutron capture. Products of this
reaction are gamma photons and conversion electrons. When a neutron
is captured for example by a Cd nucleus, a 558 keV photon is
emitted and about 3% of photons are converted to electrons of the
same energy by the internal conversion mechanism.
[0142] The detection efficiency can be increased by introduction of
3D and/or more complex structures into the semiconductor detector
and converter material, even if the semiconductor element and/or
converting material is ultra thin. The detection efficiency can be
increased from less than 5% in the case of the planar devices to
more than 30% in the case of the 3D detectors
[0143] The invention has been explained above with reference to the
aforementioned embodiments, and several advantages of the invention
have been demonstrated. It is clear that the invention is not only
restricted to these embodiments, but comprises all possible
embodiments within the spirit and scope of the inventive thought
and the following patent claims. For example the presented
detectors are being developed especially for neutron detecting,
counting and imaging, but can find usage also in other scientific
and technical applications.
[0144] In addition, even though electrodes for collecting the
charges are not described in further details, they may be arranged
according to an embodiment of the invention as planar on the
surface of the detector (i.e. perpendicularly to the neutron flux).
However, also an embodiment where the electrodes are arranged in
other way, such as essentially parallel with the neutron flux (e.g.
as disclosed in the publication of WO 2009/071587), can be used for
collecting the charges produced.
[0145] Furthermore it should be noted that read-out means connected
to the electrodes may be implemented e.g. by an electrically
conductive wire and the separate read-out electronics may be
arranged elsewhere than on the surface of the detector or in direct
contact with the electrodes. According to an embodiment the
read-out electronics may be connected to the electrodes via wires,
and/or a conductive means, such as wire or metal plate, may be used
to short-circuit the electrodes of the detector, for example when
only the counts are detected and the location information is not
needed.
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