U.S. patent application number 12/439117 was filed with the patent office on 2011-01-27 for ionizing radiation detector.
This patent application is currently assigned to SAINT-GOBAIN CRISTAUX ET DETECTEURS. Invention is credited to Jeremy Flamanc, Guillaume Gautier.
Application Number | 20110017914 12/439117 |
Document ID | / |
Family ID | 40674761 |
Filed Date | 2011-01-27 |
United States Patent
Application |
20110017914 |
Kind Code |
A1 |
Flamanc; Jeremy ; et
al. |
January 27, 2011 |
Ionizing Radiation Detector
Abstract
The invention concerns an ionizing radiation detector comprising
a housing containing: an avalanche photodiode in contact, through
its photosensitive face, with the scintillator material via optical
coupling, a preamplifier of the electrical signal from the
avalanche photodiode. This detector is compact, portable, and very
robust. It detects X-rays or gamma rays with excellent resolution,
which can be less than 3% at 662 keV.
Inventors: |
Flamanc; Jeremy;
(Fontanebleau, FR) ; Gautier; Guillaume; (Mennecy,
FR) |
Correspondence
Address: |
LARSON NEWMAN & ABEL, LLP
5914 WEST COURTYARD DRIVE, SUITE 200
AUSTIN
TX
78730
US
|
Assignee: |
SAINT-GOBAIN CRISTAUX ET
DETECTEURS
Courbevoie
FR
|
Family ID: |
40674761 |
Appl. No.: |
12/439117 |
Filed: |
November 28, 2008 |
PCT Filed: |
November 28, 2008 |
PCT NO: |
PCT/FR2008/052157 |
371 Date: |
May 18, 2010 |
Current U.S.
Class: |
250/362 ;
250/361R |
Current CPC
Class: |
G01T 1/2018 20130101;
G01T 1/202 20130101 |
Class at
Publication: |
250/362 ;
250/361.R |
International
Class: |
G01T 1/20 20060101
G01T001/20 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 4, 2007 |
FR |
0759555 |
Feb 13, 2008 |
FR |
0850895 |
Claims
1. Ionizing radiation detector comprising a housing containing: a
scintillator material, an avalanche photodiode in contact, through
its photosensitive face, with the scintillator material via optical
coupling, a preamplifier of the electrical signal from the
avalanche photodiode.
2. Detector according to the previous claim, characterized in that
the scintillator material is covered with a light reflector.
3. Detector according to one of the preceding claims, characterized
in that the scintillator material is a rare-earth halide single
crystal.
4. Detector according to the preceding claim, characterized in that
the single crystal is a cerium doped lanthanum bromide.
5. Detector according to one of the preceding claims, characterized
in that the volume of the housing is less than 300 cm.sup.3.
6. Detector according to the preceding claim, characterized in that
the volume of the housing is less than 60 cm.sup.3.
7. Detector according to one of the preceding claims, characterized
in that the scintillator material has a volume ranging from 1000 to
10000 mm.sup.3.
8. Detector according to one of the preceding claims, characterized
in that the housing comprises a metal.
9. Detector according to one of the preceding claims, characterized
in that the thickness of the optical coupling is less than 1
mm.
10. Detector according to one of the preceding claims,
characterized in that the optical coupling is an epoxy
adhesive.
11. Detector according to one of the preceding claims,
characterized in that the refractive index of the optical coupling
is between that of the scintillator material and that of the
avalanche photodiode.
12. Detector according to one of the preceding claims,
characterized in that the thermal expansion coefficient of the
optical coupling is between that of the scintillator material and
that of the avalanche photodiode.
13. Detector according to one of the preceding claims,
characterized in that the scintillator material does not surround
the photodiode but is completely contained in the half space
delimited by the plane passing through the photosensitive face of
the photodiode and not containing said photodiode.
14. Detector according to one of the preceding claims,
characterized in that the contact face area of the scintillator
material is less than 1.5, and preferably less than 1.2 times, and
even more preferably less than 1 times the contact face area of the
avalanche photodiode.
15. Detector according to the preceding claim, characterized in
that the contact face area of the scintillator material is from 0.8
to 1 times the contact face area of the avalanche photodiode.
16. Detector according to one of the preceding claims,
characterized in that the contact face of the scintillator material
and the contact face of the photodiode have the same form.
17. Detector according to one of the preceding claims,
characterized in that the contact face of the scintillator material
is inscribed in the contact face of the avalanche photodiode.
18. Detector according to one of the preceding claims,
characterized in that every point of the edge of the contact face
of the scintillator material is inside the contact face of the
photodiode and at a distance of between 0.1 and 3 mm, and
preferably between 0.2 and 0.7 mm, from the edge of the
photodiode.
19. Detector according to one of the preceding claims,
characterized in that the avalanche photodiode works at a voltage
of less than 450 volts.
20. Detector according to one of the preceding claims,
characterized in that there is an electromagnetic shielding element
between the avalanche photodiode and the preamplifier.
21. Detector according to one of the preceding claims,
characterized in that the two closest points, one of the
preamplifier and the other of the scintillator material, are less
than 2 cm apart.
22. Detector according to one of the preceding claims,
characterized in that its resolution at 662 keV is less than 3.5%
and even less than 3%.
23. Detector according to the preceding claim, characterized in
that its resolution at 662 keV is less than 2.9%.
24. Detector according to one of the preceding claims,
characterized in that the detection threshold measured with an
americium 241 source is less than 15 keV and preferably less than
12 keV.
25. Detector according to the preceding claim, characterized in
that the detection threshold measured with an americium 241 source
is less than 11 keV.
26. Detector according to one of the preceding claims,
characterized in that the preamplifier gain is between 0.1V/pC and
10 V/pC.
27. Detector according to one of the preceding claims,
characterized in that it does not comprise a cooling system.
28. Detector according to one of the preceding claims,
characterized in that it comprises a temperature stabilization
system, the housing is placed in a casing, a Peltier module being
placed between said housing and said casing.
29. Method of detecting X-rays or gamma rays using the detector of
one of the preceding claims.
30. Method according to the preceding claim, without collimation of
the radiation.
Description
[0001] The invention concerns the field of detecting ionizing
radiation, in particular X-rays or gamma rays, using a crystal
scintillator.
[0002] Ionizing radiation (which includes ionizing particles such
as in particular protons, neutrons, electrons, alpha particles, and
X-rays or gamma rays) are usually detected using single-crystal
scintillators that convert the incident radiation into light, which
is then converted into an electrical signal using a
photomultiplier. In the case of X-rays or gamma rays the
scintillators used can be, in particular, made of doped single
crystals of Nal, Csl or lanthanum halide. Lanthanum halide based
crystals have been the subject of recent work such as that
published under U.S. Pat. No. 7,067,815, U.S. Pat. No. 7,067,816,
US2005/188914, US2006/104880, and US2007/241284. These crystals are
promising in terms of luminous intensity and resolution, but
require special precautions as a result of their hygroscopic
character.
[0003] These detection systems are used, inter alia, in the field
of medical imaging scanners, airport security scanners, and oil
exploration. The conversion of the light emitted by the crystal
into an electrical signal is generally effected by a
photomultiplier. Photomultipliers are relatively restrictive due to
their high-voltage power supply and are bulky and fragile as a
result of the glass bulb they contain.
[0004] There is a need in particular for a system for detecting
X-rays or gamma rays that is portable and mobile. Indeed, it is
desired that certain people, specifically those assigned to
security, should be easily able to detect this type of radiation by
their mere presence. This need exists particularly in security and
safety activities concerning ionizing radiation. This involves, for
example, detecting and identifying illicit radioactive sources.
These systems must detect X-rays or gamma rays and, preferably, be
capable of identifying the nature of the radioisotope.
[0005] It is therefore useful to develop compact systems for
detecting ionizing radiation, and more particularly X-rays or gamma
rays. A system for detecting X-rays or gamma rays is elaborated in
more detail below, it being understood that the principle of this
system can be adapted to detect other types of ionizing radiation
as soon as the scintillator and the input window are adapted to
said targeted types of radiation. Such a compact detector must be
as small as possible while preserving good detection properties,
particularly as far as signal resolution and energy linearity (that
is, the proportionality between the energy of the X-ray and gamma
photons and the detector's response) are concerned. In particular,
the photomultiplier usually used to convert the scintillator light
into an electrical signal occupies a fairly large volume, of around
180 cm.sup.3, and it is desirable to be able to reduce this volume.
In addition, photomultipliers work at high voltage and are
sensitive to external magnetic fields, such as that of the Earth
for example. Photodiodes are able to detect the light, but they
generally produce noise that impairs resolution and the threshold
of minimum detectable energy (typically 60 keV). The performance
and the resolution obtained can be improved by cooling the
photodiode. There are several types of photodiode: PN, PIN,
avalanche photodiodes (in linear mode or Geiger mode), silicon
drift detectors, etc.
[0006] The energy resolution of an ionizing radiation detector in
fact determines its capacity to separate very close radiation
energies. It is usually determined for a given detector, at a given
energy, as the half-peak width of the peak considered over an
energy spectrum obtained from this detector, normalized with
respect to the peak centroid energy (see in particular G. F Knoll,
"Radiation Detection and Measurement", John Wiley and Sons, Inc.,
2nd edition, p 114). The percentage resolution is the half-peak
width of the photoelectric peak divided by the energy of this peak
and multiplied by 100. In the following text, for all measurements
carried out, the resolution is determined at 662 keV, the energy of
the principal gamma emission of Cs 137.
[0007] The article by R. Scafe et al., Nuclear Instruments and
Methods in Physics Research A 571 (2007) 355-357 teaches detection
of the light emitted by a LaBr.sub.3:Ce crystal by an avalanche
photodiode. The crystal had a diameter of 12 mm and the photodiode
an area of 5 mm.times.5 mm. The crystal was provided by
Saint-Gobain and was encapsulated in an aluminum housing with a 5
mm thick glass window. The photodiode was outside this housing and
received the light emitted by the crystal through the glass window.
A fortiori the preamplifier was also outside the housing. The gamma
radiation was received through a 0.5 mm thick aluminum window. The
observed resolution was 7.3% for incident photons of 662 keV
energy.
[0008] The article by K. S. Shah et al., IEEE Transactions on
Nuclear Science, Vol. 51, No. 5, October 2004 compares the
detection of light emitted by an LaBr.sub.3 crystal doped with 0.5%
Ce by an avalanche photodiode on the one hand and by a
photomultiplier on the other. The avalanche photodiode was cooled
to 250 K (or -23.degree. C.). This article concludes that at
ambient temperature photomultiplier detection would be chosen to
obtain the highest resolution.
[0009] The article by C. P. Allier et al., Nuclear Instruments and
Methods in Physics Research A 485 (2002) 547-550 reports on the
detection of light emitted by a LaCl.sub.3:Ce crystal by an
avalanche photodiode working at a voltage of 1500-1700 volts. The
photodiode was coupled to the crystal by means of a low viscosity
silicon grease. Taking account of the hygroscopic character of the
crystal used and this being an experimental system in a university
environment, the assembly was necessarily carried out entirely in a
glove box in an inert atmosphere, said glove box containing the
radioactive source, the crystal and the photodiode. The reported
resolution was 3.65% at 662 keV.
[0010] The article by C. P. Allier et al., 2000 IEEE Nuclear
Science Symposium Conference Record & Medical Imaging
Conference (2000: Lyon, France) teaches the detection of light
emitted by a LaCl.sub.3:Ce crystal by an avalanche photodiode
working at a voltage of 1500-1700 volts. The photodiode was coupled
to the crystal by means of an oil. Taking account of the
hygroscopic character of the crystal used and this being an
experimental system in a university environment, the assembly was
necessarily carried out entirely in a glove box in an inert
atmosphere, said glove box containing the radioactive source, the
crystal and the photodiode. The reported resolution was 3.65% at
662 keV. Taking account of the small volume of the crystal (63
mm.sup.2), obtaining a spectrum for the radiation source requires a
long duration of acquisition or a highly active source. A pinhole
(small hole) allows the nonuniformity in the sensitivity of the
photodiode to be factored out.
[0011] EP1435666 teaches a detector, in a housing, a scintillator
whose light is focused onto an avalanche photodiode by means of a
lens. It was observed that the distance of the photodiode from the
scintillator, due to the multitude of materials (glass lens, air
around the lens) between the scintillator and the photodiode, in
fact leads to mediocre results in terms of resolution.
[0012] US 2005/0127300 teaches a scintillator made of
polycrystalline ceramic (necessarily nontransparent) which
surrounds a photodiode. Light losses are inevitable at the
scintillator placed below the photodiode. This light loss
necessarily causes poor resolution. Such an assembly is used, above
all, to detect the presence of radiation without being able to
identify it, as the recording of a source spectrum is
impossible.
[0013] The possibility has now been found of producing a detection
system at ambient temperature (without the need for a stabilization
system or a cooling system) comprising an avalanche photodiode and
leading to excellent resolution, which may be less than 3.5%, or
even less than 3%, or even less than 2.9% at 662 keV. It must be
understood that the more the resolution of a detection system is
improved, the more difficult it is to improve it further. Thus with
a lanthanum bromide based crystal, passing, for example, from a
resolution of 3.0 to 2.9% is a very significant step forward.
[0014] The detector according to the invention in particular allows
X-ray or gamma ray detection with excellent resolutions and with a
very low detection threshold. This detection threshold can,
specifically, be less than 15 keV, even less than 12 keV, or even
less than 11 keV, measured with an americium 241 source. The
detection threshold is given by the abscissa of the valley between
the noise at low energy (to the left of the valley) and the source
signal (generally americium 241). A detector which has a good
detection threshold with one source will have a good detection
threshold with another source.
[0015] The detection system is based on the compactness of its
various components, all arranged in a sealed housing, with a
minimum of material and of distance between its different elements.
Indeed it has been observed that rigorous application of this
principle leads to results that are noteworthy and surprising in
terms of resolution and detection threshold. The invention thus
concerns a sealed housing for the detection of ionizing radiations
and in particular for the detection of X-rays and gamma rays,
comprising a scintillator material (in particular a rare earth
halide crystal when X-rays or gamma rays are targeted), an
avalanche photodiode coupled to the scintillator material by
optical coupling, and a preamplifier for amplifying the electrical
signal from said photodiode. It appears that the compactness of
this system, by reducing the distance between its various
components, is one of the elements allowing the noise that usually
impairs the resolution and detection threshold to be reduced to a
minimum. Reducing the distance reduces noise. Encapsulating the
components in the metal housing and adding a metal plate between
the photodiode and the preamplifier also allows noise to be reduced
by providing electromagnetic shielding. With a crystal made of
LaBr.sub.3:Ce (cerium doped lanthanum bromide) provided with a
light guide, the detection threshold is around 40 keV in a
nonencapsulated system without shielding. It is reduced by 20 keV
thanks to the encapsulation. These measurements are found in Table
1, in rows 2 and 10. The importance of the quality of encapsulation
and of the shielding will therefore be understood. According to the
invention, in an optimized system, the detection threshold can even
be less than 10 keV (row 14 of Table 1).
[0016] The housing according to the invention is small in size, it
being possible for its external volume to be less than 1000
cm.sup.3 and even less than 500 cm.sup.3, 300 cm.sup.3, 100
cm.sup.3 or 60 cm.sup.3. The inventors have even already produced a
housing with a volume as low as 50.4 cm.sup.3 and including a
parallelepipedal scintillator with the dimensions
9.times.9.times.20 mm.sup.3. A smaller crystal would allow the
volume of the housing to be reduced even more, but said crystal
would stop less radiation and the sensitivity of the detector would
be lower.
[0017] The scintillator material can have a volume between 1 and
50000 mm.sup.3. When the rays to be detected are low energy, low
scintillator volumes can suffice. For higher energies, larger
volumes are preferable. Specifically for the detection of X-rays or
gamma rays, the scintillator preferably has a volume larger than
1000 mm.sup.3 and even larger than 1300 mm.sup.3. Its volume is
generally less than 10000 mm.sup.3 and even less than 5000
mm.sup.3. Thus the housing according to the invention can even be
smaller in size than a simple photomultiplier. The size of the
scintillator located in the direction of incident radiations is
chosen in order to absorb the maximum of radiation. For example,
the previously mentioned scintillator has a thickness of 20 mm,
which allows 100% of photons with an energy of 122 keV emitted by a
Co.sup.57 source to be absorbed, and around 53% of photons with an
energy of 662 keV emitted by a Cs.sup.137 source. The higher the
proportion of absorbed radiation, the less active the source must
be in order to be identified and the shorter the acquisition of the
spectrum.
[0018] The detector according to the invention is light. The mass
of the detector previously mentioned, containing the
9.times.9.times.20 mm.sup.3 scintillator as well as the electronics
with electrical connections, is around 60 grams. Thus the detector
according to the invention can weigh less than 100 grams. Finally
the detector can be portable (it fits in the hand, in a pocket,
etc.), resistant to impacts and vibrations, resistant to bad
weather, and resistant to extreme temperatures (-20 to +50.degree.
C.).
[0019] The housing is a container, preferably comprising a metal,
and it must allow the types of radiation to be detected to pass
through to the scintillator material. It must also be opaque to
visible light and preferably provide shielding from electromagnetic
waves of all kinds (mobile phone, radio waves, television waves
etc.) that are capable of interfering with electronic circuits. The
housing can therefore be at least partly, or totally, made of a
metal that allows the radiation to be detected to pass, such as
aluminum (it should be noted that the term "aluminum" also covers
aluminum alloys compatible with the application, that is alloys
permeable to the intreated radiation and especially to X-rays and
gamma rays). Notably, one face of the housing can serve more
particularly for receiving radiation. Thus the face of the housing
acting as a window may be a little thinner than the other walls of
the housing. The housing can also be made of a plastic (a polymer
material such as, for example, PE, PP, PS) and be covered with a
thin layer or foil of metal such as aluminum. For example, the
housing may be a parallelepiped made completely of aluminum (or
aluminum alloy) and have one face thinner than the others. In the
case of detecting X-rays or gamma rays, by way of example, this
face may be made of aluminum of 0.5 mm thickness, the other walls
possibly being made for example of aluminum of 1 mm thickness. For
the detection of ionizing particles, an aluminum window must be
much thinner (of the aluminum "foil" type).
[0020] In the case of detecting X-rays or gamma rays the housing
contains the scintillator material comprising a rare earth halide.
This is generally of the single-crystal type and comprises a rare
earth halide, essentially a chloride, bromide, iodide or fluoride,
generally of formula A.sub.nLn.sub.pX.sub.3p+n in which Ln
represents one or more rare earths, X represents one or more
halogen atoms chosen from F, Cl, Br or I, and A represents one or
more alkali metals such as K, Li, Na, Rb or Cs, n and p
representing values such that: [0021] n, which can be zero, is less
than or equal to 3p [0022] p is greater than or equal to 1.
[0023] The rare earths (in the form of halides) concerned are those
in column 3 of the Periodic Table, including Sc, Y, La, and the
lanthanides from Ce to Lu. More particularly concerned are the
halides of Y, La, Gd and Lu, especially doped with Ce or Pr (the
term "dopant" here refers to a rare earth that is generally a minor
component in molar terms, replacing one or more rare earths that
are generally major components in molar terms, the minor and major
components being included under the abbreviation Ln).
[0024] More particularly concerned are especially materials of
formula A.sub.nLn.sub.p-xLn'.sub.xX.sub.(3p+n) in which A, X, n and
p have the previously given meanings, Ln being chosen from Y, La,
Gd and Lu or a mixture of these elements, Ln' being a dopant such
as Ce or Pr, and x is greater than or equal to 0.01p and less than
p, and ranges more generally from 0.01 p to 0.9p. Especially of
interest within the context of the invention are materials
combining the following characteristics: [0025] A chosen from Li,
Na and Cs, [0026] Ln chosen from Y, La, Gd, Lu or a mixture of
these rare earths, Ln being more particularly La, [0027] Ln' being
Ce, [0028] X chosen from F, Cl, Br, I or a mixture of several of
these halogens, especially a mixture of Cl and Br, or a mixture of
Br and I.
[0029] A scintillator material particularly suited to the detection
of X-rays or gamma rays is a single crystal comprising LaX.sub.3
doped with cerium (Ce), where X represents Br, Cl or I, with halide
mixtures, especially chloride/bromide mixtures, being possible.
When speaking of a cerium-doped rare earth halide, a person skilled
in the art will immediately know that the cerium is in halide form,
that is to say that the rare earth halide contains a cerium halide.
The following single crystals, especially, are particularly suited:
[0030] LaBr.sub.3 doped with 1 to 30 mol % of CeBr.sub.3; [0031]
LaCl.sub.3 doped with 1 to 30 mol % of CeCl.sub.3; [0032]
yLaBr.sub.3+(1-y)CeBr.sub.3 with y.gtoreq.0.
[0033] The scintillator material can, in particular, be cylindrical
or parallelepipedal and be larger along one axis. This axis is then
perpendicular to the plane of the photodiode. The scintillator
material is placed in the housing in immediate proximity to that
wall of the housing acting as window. A sheet of a shock-absorbing
material may be placed between the crystal and the wall of the
housing.
[0034] The scintillator material is generally covered with a light
reflector. This reflector preferably covers all the sides of the
scintillator material, apart from the area through which the light
emitted by the scintillator must pass to reach the photodiode. The
light reflector may be made of PTFE (polytetrafluoroethylene). It
can therefore be a strip of PTFE with which the scintillator
material is surrounded. Before being covered with the light
reflector, the external faces of the scintillator material are
preferably roughened (or frosted) with an abrasive material such as
abrasive paper (especially 400 grit). The roughness thus given to
the surface increases the light flux received by the
photodetector.
[0035] The housing contains an avalanche photodiode. This
photodiode is in contact with the scintillator material via an
optical coupling material. This might be a silicon grease
(polysiloxane) or any other transparent, nonadhesive material, but
this optical coupling is preferably an epoxy adhesive. Once the
epoxy adhesive has hardened, the photodiode and the crystal are
joined together. The optical coupling has a refractive index
between that of the scintillator material and that of the avalanche
photodiode. When the optical coupling is solid (in the case of an
epoxy adhesive) it is preferably chosen such that its thermal
expansion coefficient is between that of the avalanche photodiode
and that of the scintillator material. It can also be relatively
flexible. In this way the coupling better absorbs the differences
in thermal expansion between the two materials that it links. This
reduces the risks of fracturing the crystal in the case of
excessive heating.
[0036] The thickness of the optical coupling is preferably less
than 2 mm and even less than 1 mm, and more preferably still less
than 0.6 mm. For the person skilled in the art the term "optical
coupling" excludes a vacuum and gases such as air. An optical
coupling is necessarily liquid (which includes "greasy") or
solid.
[0037] The avalanche photodiode is generally a flat component with
two main faces. It is one of these main faces that is in contact
with a plane face of the scintillator material via the optical
coupling. The face of the scintillator material in contact with the
photodiode (contact face of the scintillator material) is
preferably inscribed in that face of the photodiode with which it
is in contact (contact face of the photodiode). Consideration is
taken here of the light-sensitive surface of the photodiode (or the
photosensitive face). In fact the photodiode generally comprises a
light-sensitive area surrounded by a dead area (insensitive to
light) corresponding to the encapsulation material of the
photodiode. It should therefore be borne in mind that the
expression "contact face of the photodiode" or "contact face area
of the photodiode" refers to the sensitive area of the
photodiode.
[0038] The scintillator material is placed completely opposite the
sensitive face of the photodiode. This means that the scintillator
material does not surround the photodiode but is completely
contained in the half space delimited by the plane passing through
the sensitive face of the photodiode and facing (or opposite) said
sensitive face. Said half space containing the scintillator
material does not therefore contain the photodiode.
[0039] The contact face of the scintillator material and the
contact face of the photodiode preferably have the same shape (both
are square for example), the contact face of the scintillator
material preferably being a little smaller (in a homothetic manner)
than the contact face of the photodiode. Preferably, each point on
the edge of the contact face of the scintillator material is inside
the (sensitive) contact face of the photodiode, and at a distance
of between 0.1 and 3 mm, preferably of 0.2 to 0.7 mm, from the edge
of the contact face of the photodiode.
[0040] The contact face area of the scintillator material is
preferably less than 1.5 times, and even less than 1.2 times or
even less than 1 times the contact face area of the avalanche
photodiode. Saying that the contact face area of the scintillator
material is less than 1.5 times the contact face area of the
photodiode means that if the contact face area of the photodiode is
S, the contact face area of the scintillator material is less than
1.5 multiplied by S. The contact face area of the scintillator
material can therefore be greater than the contact face area of the
photodiode. However, the contact face area of the scintillator
material preferably ranges from 0.8 to 1 times the contact face
area of the avalanche photodiode. By way of example, a photodiode
with a sensitive surface area of 0.2 cm.sup.2 is compared with a
photodiode of 1 cm.sup.2 in rows 2 and 12 of Table 1. It can be
seen that for the 10.times.10.times.10 mm.sup.3 crystal provided
with a glass window performance is very different, going from a
resolution of more than 8.5% with the smallest photodiode to almost
3.0% with the largest.
[0041] The invention thus also concerns the detection system
comprising the scintillator material and the avalanche photodiode,
preferably respecting the contact face data just given, even for a
noncompact system. A compact system is nonetheless preferred.
[0042] The avalanche photodiode preferably works at a voltage of
less than 1050 volts, and more preferably still at a voltage of
less than 450 volts.
[0043] The photodiode generally has two electrical connectors.
These two connectors are preferably soldered directly to a charge
amplifier called a preamplifier, the components of which are
located on a printed circuit that is also incorporated within the
housing according to the invention. The preamplifier might, in
particular, have the following characteristics: power supply +/-12
V or +24/0 V, gain of 0.1 V/pC to 10 V/pC. An electromagnetic
shielding element such as a copper or brass plate, electrically
connected to the housing and to ground, is preferably placed
between the avalanche photodiode and the preamplifier. The
photodiode connectors, insulated by polymer sheaths or by
insulating adhesive, pass through the shielding element via
orifices. Note that the electromagnetic shielding is an obstacle to
electromagnetic waves of all kinds (mobile phone, radio waves,
television waves etc.) that are capable of interfering with
electronic circuits. The metal housing (which can be of aluminum)
contributes to this shielding.
[0044] The photodetector used within the context of the present
invention is an avalanche photodiode, this term covering a simple
individual photodiode, but also an array of avalanche photodiodes,
that is a collection of individual avalanche photodiodes grouped on
one face of the crystal and the signals of which are summed. This
collection therefore includes a dead area (insensitive to light)
around the array, but also in general between the individual
photodiodes. The characteristics given above for the relation
between the contact areas between the avalanche photodiode and the
scintillator material remain valid and it is the cumulative area of
the light-sensitive contact surfaces of the linked avalanche
photodiodes that is taken into consideration. However, in the case
of an array of avalanche photodiodes, the surface of the contact
face of the scintillator material may be larger than the cumulative
(light-sensitive) contact surface of the avalanche photodiodes, due
to the existence of dead areas between the individual photodiodes
in the array. The silicon PM can be cited as an example of a
photodetector consisting of an array of avalanche photodiodes in
Geiger mode.
[0045] According to the invention it is not necessary to collimate
the incident radiation. Collimation, for example by surrounding the
source with a pierced absorbing material, provides a fine beam of
radiation which, as it irradiates only a particular point of the
detector, allows inhomogeneities in the crystal or in the
photodiode to be factored out. It turns out that the quality of the
combination of the scintillator, its reflector, the optical
coupling and the photodiode allows excellent performance to be
obtained without using a collimation.
[0046] The housing according to the invention can have an axis
passing through the centre of gravity of the scintillator material
and the centre of gravity of the radiation entrance window. Moving
along this axis, starting with the entrance window, the light
reflector, the scintillator material, the optical coupling, the
photodiode, the shielding and the preamplifier are successively
encountered. The photodiode, the shielding (where this is a plate)
and the preamplifier printed circuit, in particular, can be
perpendicular to this axis.
[0047] The entrance window can also be lateral, that is parallel to
an axis of the housing. If the housing is a parallelepiped, five of
its faces (the four lateral ones and the front face) can constitute
the entrance window.
[0048] The compactness of the detector according to the invention
is characterized especially by the absence of a light guide between
the crystal and the avalanche photodiode (apart from the thin layer
of optical coupling, in particular of the epoxy adhesive type,
which can have a thickness of less than 0.6 mm, along with a
possible protection layer for the surface of the avalanche
photodiode), a short distance between the preamplifier and the
scintillator material, such that the two closest points, one of the
preamplifier and the other of the scintillator material, can
generally be less than 2 cm apart.
[0049] According to the prior art, when they must operate in an
environment at ambient temperature, the photodiodes are often
cooled below 0.degree. C. The performance of the detector according
to the invention is such that a temperature stabilization system
(cooling or heating) is not necessary. Cooling systems such as a
Peltier module, a heat sink, a fan, or the circulation of a coolant
increase the size requirement, the mass or the electrical
consumption. However, in the case in which the detector may not be
used at constant temperature, the addition of a thermal insulation
system or even an active temperature stabilization system is a
possible option as the gain of an avalanche photodiode varies with
temperature. A possible design of a detector equipped with a
temperature stabilization system is the following: [0050] the
(first) thermally conductive housing containing, as previously, the
scintillator material, the avalanche photodiode and the
preamplifier, [0051] thermal insulation around this housing (for
example: expanded polystyrene, or a vacuum, or air), [0052] an
outer thermally conductive casing (or second housing) containing
the first housing covered by the thermal insulation, [0053] a
Peltier module (operating on the principle of a thermoelectric
effect) in thermal contact via one of its faces with the first
housing, its other face being in thermal contact with the outer
casing, [0054] a heat sink fixed to the outer casing, [0055] a fan
mounted on top of the heat sink.
[0056] For the case in which this detector must contend with hot
ambient temperatures (such as higher than 25.degree. C.), the cold
side of the Peltier module is in thermal contact with the inner
housing, while the hot side of the Peltier module is in thermal
contact with the outer casing. For the case in which this detector
must contend with cold ambient temperatures (such as lower than
20.degree. C.), the hot side of the Peltier module is in thermal
contact with the inner housing, while the cold side of the Peltier
module is in thermal contact with the outer casing.
[0057] The expression "thermal contact" used with regard to the
Peltier module means that it is directly touching the housing (or
casing) with which it is in thermal contact or alternatively that
an intermediate part (generally a metal part) that conducts heat
well is placed between said Peltier module and said housing (or
said casing).
[0058] The addition of a temperature probe such as a PT1000
thermistor (marketed by the company Correge) enables the
effectiveness of the stabilization provided to be checked and
allows automatic control of the system so as to regulate the
temperature inside the housing (first housing) when the outside
ambient temperature (outside the outer casing) varies.
[0059] The outer casing (or second housing) is made of a material
that conducts heat well and is permeable to the intreated radiation
and especially to X-rays and gamma rays. It may therefore be made
at least partly, or totally, of a metal allowing through the
radiation to be detected, such as made of aluminum (it should be
noted that the term "aluminum" also covers aluminum alloys that are
compatible with the application, i.e. are permeable to the
intreated radiation and especially to X-rays and gamma rays). In
particular, one face of the casing may more particularly function
as a window for receiving radiation. Thus, the face serving as a
window of the casing may be a little thinner than the other walls
of said casing. The window face of the outer casing is opposite the
"window" face of the (first) inner housing. The casing may, for
example, be a parallelepiped completely made of aluminum (or
aluminum alloy) and include one face (window face) thinner than the
others. For the case of detection of X-rays or gamma rays, by way
of example, this face may be of 0.5 mm thick aluminum, the other
walls possibly being, for example, of 1 mm thick aluminum.
[0060] The inventors have, for example, developed a detector (FIG.
5) of outer size 7.2.times.7.0.times.5.2=262 cm.sup.3 and, in an
outside environment at 50.degree. C., a stabilized temperature of
around 20.degree. C. in the inner housing (first housing) has been
obtained. The Peltier module was the Supercool PE-127-08-15 brand.
The size of the outer casing, especially the thickness of the
insulator, depends on the outside temperature to be compensated
for.
[0061] Thus the invention also relates to a detector equipped with
a temperature stabilization system. In particular, as explained
above, the housing may be placed in a casing, a Peltier module
being placed between said housing and said casing.
[0062] Although the detector according to the invention exhibits
good performance between -20.degree. and +50.degree. C. without a
temperature stabilization system, it is preferable to provide such
a system in the case of significant fluctuation in the ambient
temperature or in the case of the temperature being above
25.degree. C. or below -20.degree. C. on a permanent basis.
[0063] The invention also concerns a method of detecting ionizing
radiation such as X-rays or gamma rays using the detector according
to the invention.
[0064] FIG. 1 shows a compact detector according to the invention.
It has an axis AA'.
The reference numbers have the following meanings: 1: ground wire
soldered into the hole in the closure plate 2: aluminum housing 3:
preamplifier 4: brass plate (shielding) 5: avalanche photodiode
S-8664-1010 6: optical coupling 7: PTFE covering (light reflector)
8: single-crystal scintillator, of which the face on the photodiode
side has been polished and the other faces have been roughened 9:
0.5 mm thick aluminum entrance window 10: signal connector 11: low
voltage connector 12: high voltage connector 13: insulator The
other figures outside the housing are the dimensions in mm.
[0065] FIG. 2 illustrates the linearity, that is the
proportionality between the energy of the incident X-ray or
gamma-ray photons and the response of the detection system of the
compact detector according to an example of the invention.
[0066] FIG. 3 shows the spectrum of an Am 241 source measured using
a compact detection system according to the invention. The
detection threshold is indicated by the letter S. It is excellent
as it is around 10 keV.
[0067] FIG. 4 shows the spectrum of a Cs 137 source (energy of
incident photons of 662 keV) measured by a compact system according
to the invention.
[0068] FIG. 5 shows a detector according to the invention equipped
with a temperature stabilization system. An outer casing 20 made of
aluminum contains the housing 21 which is also made of aluminum,
said housing containing the scintillator material and the
preamplifier as for FIG. 1. A Peltier module 22 is in thermal
contact with the housing 21 on the side of its cold side and in
thermal contact with the outer casing 20 on the side of its hot
side. The thermal contact with the outer casing is made via a
spacer made of copper that conducts heat well. A heat sink
(radiator) 24 is fixed on the outside of the casing and a fan 25
helps remove the heat. The duct 26 contains the electric power
wires for the fan 25. The electric wires have not been shown inside
the casing in the interest of clarity. Overall, the device is
electrically connected to the outside via the connectors 27. The
largest dimension of the outer casing was here 72 mm. Thermal
insulation is placed between the outer casing and the housing
containing the photodiode and the scintillator material. A
thermistor (28) is placed inside the inner housing in order to
control the temperature. Wires (not shown) coming from this
thermistor as connected to a connector 27.
EXAMPLES
[0069] Single crystals of the following were used as scintillator
material:
TABLE-US-00001 CsI doped with 0.8 mol % of TI denoted by CsI NaI
doped with some TI denoted by NaI LaCl.sub.3 doped with 10 mol % of
CeCl.sub.3 denoted by LaCl LaBr.sub.3 doped with 5 mol % of
CeBr.sub.3 denoted by LaBr
These could have the following forms:
TABLE-US-00002 cylindrical: Diameter height 25.4 mm 25.4 mm denoted
by 25 .times. 25 12.8 mm 12.8 mm denoted by 13 .times. 13 6 mm 6 mm
denoted by 6 .times. 6
TABLE-US-00003 parallelepipedal: Contact face height 10 mm .times.
10 mm 10 mm denoted by 10 .times. 10 .times. 10 9 mm .times. 9 mm
20 mm denoted by 9 .times. 9 .times. 20
[0070] The energy of the incident rays was 662 keV. The gamma
radiation was not collimated. The ambient temperature was
23.degree. C..+-.2.degree. C. The avalanche photodiode (denoted by
APD in Table 1) was a Hamamatsu S8664-1010. The photodiode PIN was
a Hamamatsu S3590-08. The preamplifier was a Hamamatsu H4083.
[0071] The results are gathered in Table 1. The result in the last
row illustrates the excellence of a compact system, that is one
consisting of a housing that integrates the detector material, the
avalanche photodiode and a preamplifier at a short distance from
one another.
TABLE-US-00004 TABLE 1 Single-crystal Photodiode Detection
scintillator Light area threshold Resolution Type Form guide
Shielding Compactness Photodiode (cm.sup.2) (keV) at 662 keV LaBr
13 .times. 13 5 mm NO NO APD 1 20 4.0 glass LaBr 10 .times. 10
.times. 10 1 mm NO NO APD 1 19 3.0 glass LaBr 6 .times. 6 3 mm NO
NO APD 1 20 3.1 glass LaBr 13 .times. 13 5 mm NO NO PIN 1 270 13.8
glass LaBr 12 .times. 6 Glass NO NO PIN 1 120 12.6 LaBr 6 .times. 6
3 mm NO NO PIN 1 100 11.3 glass CsI 6 .times. 6 3 mm NO NO APD 1 30
5.6 glass NaI 10 .times. 10 .times. 10 1 mm NO NO APD 1 23 6.4
glass LaCl 13 .times. 13 5 mm NO NO PIN 1 Not glass calculable LaBr
10 .times. 10 .times. 10 1 mm NO NO APD 1 41 3.2 glass LaBr 6
.times. 6 3 mm YES NO APD 0.2 8.4 glass LaBr 10 .times. 10 .times.
10 1 mm YES NO APD 0.2 8.7 glass LaBr 25 .times. 25 5 mm NO NO APD
1 66 5.0 glass LaBr 9 .times. 9 .times. 20 <0.5 mm YES YES APD 1
10 2.8 epoxy adhesive
[0072] FIG. 2 illustrates the good linearity of the system, that is
the proportionality between the energy of the incident X-ray or
gamma-ray photons (plotted on the x-axis) and the response of the
detection system (photoelectric peak value, plotted on the y-axis).
It is evaluated by measuring the photoelectric peak value for gamma
rays incident at 1332 keV (Co 60), 1173 keV (Co 60), 662 keV (Cs
137), 122 keV (Co 57) and 60 keV (Am 241). Note the absence of a
lack of nonlinearity.
[0073] FIG. 3 shows the spectrum of an Am 241 source measured using
the compact detection system according to the invention. It can be
seen that the detection threshold is 10 keV, which is an extremely
good performance. This is the value on the x-axis of the minimum on
the left of the curve, between the noise to the left of this
minimum and the source signal. The energy of the incident photons
is 60 keV. The dimensions of the crystal are 9.times.9.times.20
mm.
[0074] FIG. 4 shows the spectrum of a Cs 137 source (incident
photon energy of 662 keV). The resolution (half-peak width divided
by the energy) is 2.8%.
* * * * *