U.S. patent number RE35,908 [Application Number 08/662,229] was granted by the patent office on 1998-09-29 for neutron individual dose meter neutron dose rate meter, neutron detector and its method of manufacture.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Shigeru Izumi, Akihasa Kaihara, Hiroshi Kitaguchi.
United States Patent |
RE35,908 |
Kitaguchi , et al. |
September 29, 1998 |
Neutron individual dose meter neutron dose rate meter, neutron
detector and its method of manufacture
Abstract
A neutron individual dose meter and a neutron dose rate meter,
both capable of implementing the effective dose equivalent
response. The neutron individual dose meter is capable of being
accomplished by providing a composite layer made up of a converter
such as boron, and a proton radiator, on the surface of a
semiconductor neutron detection element. The neutron dose rate
meter is capable of being accomplished through such a structure as
to surround a neutron detector with a neutron moderator and a
thermal neutron absorber which has openings. Thus, a neutron
individual dose meter and a neutron dose rate meter, both capable
of implementing the effective dose equivalent response and
measurement at lower operating voltage have been provided. Further,
these meters are capable of being implemented by utilizing a single
semiconductor detection element, respectively.
Inventors: |
Kitaguchi; Hiroshi (Naka-machi,
JP), Izumi; Shigeru (Tokyo, JP), Kaihara;
Akihasa (Hitachi, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
|
Family
ID: |
14526825 |
Appl.
No.: |
08/662,229 |
Filed: |
June 14, 1996 |
PCT
Filed: |
April 26, 1991 |
PCT No.: |
PCT/JP91/00574 |
371
Date: |
December 27, 1991 |
102(e)
Date: |
December 27, 1991 |
PCT
Pub. No.: |
WO91/17462 |
PCT
Pub. Date: |
November 04, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
778876 |
Apr 26, 1991 |
05321269 |
Jun 14, 1994 |
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Foreign Application Priority Data
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Apr 24, 1990 [JP] |
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2-110092 |
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Current U.S.
Class: |
250/370.05;
250/252.1; 250/390.03 |
Current CPC
Class: |
G01T
3/08 (20130101); G01T 1/026 (20130101) |
Current International
Class: |
G01T
1/02 (20060101); G01T 3/00 (20060101); G01T
3/08 (20060101); G01T 003/08 (); H01L 031/115 ();
H01L 031/00 () |
Field of
Search: |
;250/370.05,390.03,252.1 |
References Cited
[Referenced By]
U.S. Patent Documents
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4074136 |
February 1978 |
Heinzelmann et al. |
4489315 |
December 1984 |
Falk et al. |
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Foreign Patent Documents
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1386485 |
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Dec 1964 |
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FR |
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55-95868 |
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Jul 1980 |
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JP |
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55-95886 |
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Jul 1980 |
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JP |
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58-167988 |
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Oct 1983 |
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JP |
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64-39778 |
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Feb 1989 |
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JP |
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1-164071 |
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Jun 1989 |
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JP |
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1-253971 |
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Oct 1989 |
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JP |
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Other References
Dale E. Hankins, "Phantoms for Calibrating Albedo Neutron
Dosimeters." Health Physics, vol. 39, No. 3 pp. 580-584, Pergamon
Press Ltd. Sep. 1980..
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Primary Examiner: Hannaher; Constantine
Attorney, Agent or Firm: Antonelli, Terry, Stout &
Kraus, LLP
Claims
What is claimed is:
1. A neutron individual dose meter comprising: a neutron detector
comprising a plurality of grains of a first material which
generates charged particles through a nuclear reaction with thermal
neutrons, and a second material different from the first material,
both provided in a mixture state bonded to the surface of a
semiconductor detection element; a processing circuitry for
processing signals obtained from said neutron detector; and a power
source for supplying power to said neutron detector and the
processing circuitry.
2. The neutron individual dose meter claimed in claim 1, wherein
said first material is characterized by being boron.
3. A neutron individual dose meter claimed in claim 1, wherein said
second material is a hydride compound, and said first material is
bonded by calcination on the surface of the semiconductor
element.
4. A neutron individual dose meter comprising: a neutron detector
comprising a grain-formed material which generates charged
particles through a nuclear reaction with thermal neutrons and
another material which generates no charged particles through
interactions with fast neutrons, both formed on the surface of a
semiconductor detection element; a processing circuitry for
processing signals from said neutron detector; and a power source
for supplying power to said neutron detector and processing
circuitry.
5. The neutron individual dose meter claimed in claim 4 wherein,
said grain-formed material characterized by being boron.
6. A meter according to claim 4, wherein said another material has
a thermal expansion coefficient approximately equal to that of said
semiconductor detection element or an elasticity which absorbs a
thermal expansion of said grain-formed material, both said
grain-formed material and said another material are bonded by
calcination to a surface of said semiconductor detection
element.
7. A neutron dose rate meter comprising: a neutron detector having
a plurality of grains of a first material which generates charged
particles in a semiconductor radiation detection element through a
nuclear reaction with thermal neutrons and a second material
different from the first material, both provided in a mixture state
in the surface of the semiconductor detector element; a thermal
neutron absorber disposed in the direction of incidence of said
neutrons with respect to said neutron detector; a processing
circuitry for processing signals from said neutron detector; and a
power source for supplying power to said neutron detector and
processing circuitry.
8. A neutron dose rate meter comprising: a neutron detector having
a plurality of grains of a first material which generates charged
particles in a semiconductor radiation detection element through a
nuclear reaction with thermal neutrons and a second material
different from the first material, both provided in a mixture state
on the semiconductor radiation detection element; a thermal neutron
absorber disposed so as to surround said neutron detector; a
processing circuitry for processing signals from said neutron
detector; and a power source for supplying power to said neutron
detector and processing circuitry.
9. The neutron dose rate meter of claim 8 wherein, said thermal
neutron absorber is characterized by having openings.
10. A neutron dose rate meter claimed in claim 8, further
comprising a neutron moderator, said neutron moderator and said
thermal neutron absorber both being disposed so as to surround said
neutron detector.
11. A method of fabricating a neutron detector characterized by
comprising steps of: bonding by calcination grain-formed boron
which generates charged particles through a nuclear reaction with
thermal neutrons on the surface of a semiconductor detection
element; and infiltrating a hydride compound into the interstices
of the bonded grain-formed boron to be retained firmly therein.
12. A neutron individual dose meter, comprising:
a neutron detector having a layer made up of grain-formed material
which generates charged particles through a nuclear reaction with
thermal neutrons and another material which generates charged
particles through an interaction with fast neutrons, said another
material being infiltrated into interstices in the grains of said
grain-formed material, the layer being formed on a surface of a
semiconductor detection element;
a processing circuitry for processing signals from said neutron
detector; and
a power source for supplying power to said neutron detector and
processing circuitry.
13. A neutron individual dose meter claimed in claim 12, wherein
said grain-formed material is boron.
14. A neutron individual dose meter claimed in claim 12, wherein
said another material is a hydride compound, and said grain-formed
material is bonded by calcination on the surface of the
semiconductor detection element.
15. A neutron individual dose meter claimed in claim 12, wherein a
neutron sensitivity of said dose meter is adjusted to a desired
dose equivalent response by varying an amount of said grain-formed
material and said another material in said layer.
16. A neutron individual dose meter claimed in claim 12, wherein a
grain thickness is varied so as to improve a detection sensitivity
to thermal neutron.
17. A neutron individual dose meter, comprising:
a neutron detector having a layer made up of a first material which
generates charged particles through a nuclear reaction with thermal
neutrons and a second material which generates charged particles
through an interaction with fast neutrons, said first material
being provided in a substantially plate form having openings
therein, said second material being provided in said openings, the
layer being formed on a surface of a semiconductor detection
element;
a processing circuitry for processing signals from said neutron
detector; and
a power source for supplying power to said neutron detector and
processing circuitry.
18. A neutron individual dose meter claimed in claim 17, wherein a
neutron sensitivity of said dose meter is adjusted to a desired
dose equivalent response by varying areas of the openings of said
second material.
19. A neutron dose rate meter, comprising:
a neutron detector having a layer made up of a grain-formed
material which generates charged particles through a nuclear
reaction with thermal neutrons and another material which generates
charged particles through an interaction with fast neutrons, said
another material being infiltrated into interstices in the grains
of said grain-formed material, said layer formed on a surface of a
semiconductor detection element;
a neutron moderator provided either on or above said neutron
detector in the direction of incidence of said neutrons;
a processing circuitry for processing signals from said neutron
detector; and
a power source for supplying power to said neutron detector and
processing circuitry.
20. A neutron dose rate meter, comprising:
a neutron detector having a layer made up of a grain-formed
material which generates charged particles through a nuclear
reaction with thermal neutrons and another material which generates
charged particles through an interaction with fast neutrons, said
another material being infiltrated into interstices in the grains
of said grain-formed material, said layer being formed on a surface
of a semiconductor detection element;
a neutron moderator disposed so as to surround said neutron
detector;
a processing circuitry for processing signals from said neutron
detector; and
a power source for supplying power to said neutron detector and
processing circuitry.
21. A neutron dose rate meter as claimed in claim 20, wherein said
grain-formed material is boron, and said another material is a
hydride compound.
22. A neutron dose rate meter, comprising:
a neutron detector having a plurality of grains of a first material
which generates charged particles in a semiconductor detection
element through a nuclear reaction with thermal neutrons and a
second material different from said first material, both provided
in a mixture state on said semiconductor detection element;
a human body simulating phantom firmly attached to said neutron
detector;
a processing circuitry for processing signals from said neutron
detector; and
a power source for supplying power to said neutron detector and
said processing circuitry.
23. A neutron dose rate meter as claimed in claim 22, wherein said
phantom is constituted of acrylic having a surface area of 40
cm.times.40 cm and a height of 15 cm, or a vessel of the same size
containing water, and said neutron detector being fixed firmly on a
surface of said phantom.
24. A neutron detector comprising a layer made up of a grain-formed
material which generates charged particles through a nuclear
reaction with thermal neutrons and another material which generates
charged particles through an interaction with fast neutrons, said
another material being infiltrated into interstices in the grains
of said grain-formed material, the layer being formed on a surface
of a semiconductor detection element.
25. A neutron detector comprising a layer made up of a first
material which generates charged particles through a nuclear
reaction with thermal neutrons and a second material which
generates charged particles through an interaction with fast
neutrons, said first material being provided in a substantially
plate form having openings therein, said second material being
provided in said openings, the layer being formed on a surface of a
semiconductor detection element.
26. A neutron individual dose meter, comprising:
a neutron detector having a semiconductor detection element made up
of a single wafer of semiconductor, electrodes formed on obverse
and reverse sides of said wafer, at least one electrode on a p-n
junction side of said electrodes being formed so as to cover a
portion of a depletion region therein, said at least one of said
electrodes having a reduced area so that an amount of charged
particles entering said semiconductor element is increased, a power
source for applying voltage to said electrodes;
a processing circuitry for processing signals from said neutron
detector; and
a power source for supplying power to said neutron detector and
said processing circuitry.
27. A neutron detector, comprising:
a single wafer of semiconductor;
electrodes provided on obverse and reverse surfaces of said
wafer;
a semiconductor detection element, in which at least one of said
electrodes on a p-n junction side of said electrodes is formed so
as to cover a portion of a depletion region therein, said at least
one of said electrodes having a reduced area so that an amount of
charged particles entering said semiconductor element is increased;
and
a power source for applying voltage to said electrodes.
28. A neutron individual dose meter, comprising:
a neutron detector having a semiconductor detection element made up
of a single wafer of semiconductor, electrodes formed on obverse
and reverse sides of said wafer, a p-n junction formed inside the
obverse side of said semiconductor element, at least one of said
electrodes being connected to said p-n junction with a small area
compared with that of said p-n junction and being formed so as to
cover a portion of a depletion region therein, a power source for
applying voltage to said electrodes;
a processing circuitry for processing signals from said neutron
detector; and
a power source for supplying power to said neutron detector and
processing circuitry.
29. A neutron detector, comprising:
a single wafer of semiconductor;
electrodes provided on obverse and reverse surfaces of said
wafer;
a semiconductor detection element, in which a p-n junction is
formed inside the obverse side of said semiconductor element, at
least one of said electrodes being connected to said p-n junction
with a small area compared with that of said p-n junction and being
formed so as to cover a portion of a depletion region therein;
and
a power source for applying voltage to said electrodes. .Iadd.
30. A neutron detecting apparatus comprising one semiconductor
detection element having different materials for producing charged
particles in accordance with incident neutrons and for detecting
the charged particles produced by the different materials, wherein
the neutron detecting apparatus has at least one sensitivity value
for neutrons having an energy over 1 MeV which is at least about 20
times greater than at least one sensitivity value for neutrons
having an energy under 10 KeV. .Iaddend..Iadd.31. A neutron
detecting apparatus as claimed in claim 30, wherein the different
materials includes a plurality of a first material for producing
the charged particles in accordance with the incident neutrons
having the energy under 10 KeV, and a second material for producing
the charged particles in accordance with the incident neutrons
having the energy over 1 MeV. .Iaddend..Iadd.32. A neutron
detecting apparatus as claimed in claim 31, wherein the first
material and the second material are provided in a mixture state
and are bonded to the surface of the one semiconductor detection
element. .Iaddend..Iadd.33. A neutron detecting apparatus as
claimed in claim 31, wherein the first material is boron.
.Iaddend..Iadd.34. A neutron detecting apparatus as claimed in
claim 31, wherein the second material is a hydride compound.
.Iaddend.
Description
FIELD OF THE TECHNOLOGY
The present invention relates to a portable neutron dose meter for
use by individuals who are engaged in radiation handling in
radiation handling facilities such as nuclear power plants,
reprocessing facilities and the like, a neutron exposure dose rate
meter for monitoring neutrons within the compounds of neutron
handling facilities, and a neutron detector which serves as their
detection element, and more specifically it relates to such that
employs a semiconductor detector as its neutron detection
element.
BACKGROUND TECHNOLOGY
As prior art neutron individual dose meters using semiconductor
detection elements, there are such that have been described in
Radiation Protection Dosimetry, Vol. 27, No. 3, pp145-156 (1989)
(hereinafter referred to as the prior art 1) and in U.S. Pat. No.
322787 as the prior art 2). Because the (hereinafter referred to
semiconductor detection elements cannot detect neutrons directly,
they manage to detect indirectly the neutrons by means of detecting
charged particles which are generated by the interaction of the
neutrons with other substances. For this purpose, the structure of
the neutron detector in the prior art referred to in the former has
a boron layer on the surface of the semiconductor detection element
for detecting low energy thermal neutrons, and a polyethylene layer
formed on the boron layer for detecting high energy fast neutrons.
Further, in front of them, neutron moderator material is disposed
to moderate neutron energy. The neutron detector in the prior art
referred to in the latter case has a boron layer formed on the
surface of a semiconductor detection element likewise the former
example, the periphery of which layer is further surrounded by
neutron moderator material in order to detect fast neutrons.
On the other hand, many of the prior art neutron exposure dose rate
meters for monitoring neutrons in radiation handling facilities
employ BF.sub.3 counters or .sup.3 He counters as described in the
Japanese Patent Publication 63-235646 (1988) (hereinafter referred
to as the prior art 3). It is disclosed, further, in the same
publication that, in order to cut thermal neutrons and detect only
fast neutrons, said counter is surrounded by neutron moderator, the
surface of which is further surrounded by thermal neutron absorber,
still further the surface of which is covered by neutron moderator,
respectively.
Each country concerned is required to provide for various radiation
detectors which satisfy energy response characteristics so that the
effective dose equivalent evaluation conforming to the
recommendation of the International Committee on Radiological
Protection (ICRP) may be performed. Also in Japan, in accordance
with the ICRP recommendation, the domestic radiation hazard
prevention laws and regulations have been revised in April, 1989.
Generally, radiation damages (dose quantities) differ depending on
materials, even when they are exposed to radiation of the same
energy. The so-called effective dose equivalent refers to a dose
value for evaluation which precisely reflects an exposure quantity
to neutrons of a human body. In order to implement this effective
dose equivalent, it is necessary to survey and evaluate respective
quantities of dose of each energy in a human body over a wider
spectrum range of energy existing in radiation handling facilities.
The range of energy includes a region for thermal neutrons having
energy in the thermal neutron region below 0.5 eV, to fast neutrons
ranging above 0.5 eV up to 10 MeV. Here, a sensitivity curve with
respect to each specific energy is called a response. Since the
difference in the effective dose equivalent responses in terms of
sensitivities between the thermal neutron region and the MeV region
is as large as more than 50 times, it is extremely difficult to
implement this required response. The required response will be
called as a dose equivalent response hereinunder. In order to
satisfy the dose equivalent response, it is important (1) to make
the shapes of sensitivity curves to coincide with each other or
minimize the difference in the sensitivities, and (2) to increase
sensitivity to each energy.
First, the above case (1) will be considered from a viewpoint of
implementing a neutron individual dose meter.
In the prior art 1, in order to render the sensitivity curves to
coincide with each other, in addition to the above-mentioned
neutron detector, another neutron detector which has only a
polyethylene layer on a semiconductor detector is provided
therewith, then the responses of the two neutron detectors are
added. However, as shown in FIG. 11 in page 155, the response
performance has not been satisfied in an energy range from 10 K eV
up to 1 MeV. In addition, there is a problem that the device tends
to become large-sized because it utilizes two neutron detectors,
and its processing circuitry is more complicated.
Next, we will consider the case (2). The prior arts 1 and 2 have a
structure such that a layer of boron 10(.sup.10 B) is formed on the
surface of a semiconductor detection element, and charged particles
(.alpha. rays) generated in the layer when neutrons enter therein
are detected by the semiconductor detection element. Such materials
which generate charged particles when thermal neutrons enter will
be called a converter hereinafter. In the neutron detector
according to the prior art 1, the thickness of a boron 10 layer is
formed to 1 micron m thickness by plasma doping techniques. Boron,
however, has drawbacks that its melting point is as high as 2300
degree C., and its processibility is extremely poor. Further, the
thickness of a boron film cannot be increased more than 1 micron m
in order to prevent peel-off of the film due to temperature
changes, because thermal expansion coefficients between boron and
silicon which constitutes a semiconductor detection element differ
as large as 3.5 times. The number of .alpha. rays generated in
boron being increased in proportion to its film thickness, there
has been a problem in implementing a neutron detector which has a
sufficient detection sensitivity. While in the prior art 1 there
has been described diffusion injection means for injecting boron,
it has not been successful in increasing boron concentrations in
diffused layers, thus failing to attain a neutron detector having a
sufficient detection sensitivity. Boron enters into a nuclear
reaction with a high probability when it encounters thermal
neutrons, generating a lot of .alpha. rays, but it does not respond
so vigorously to fast neutrons (high-energy neutrons over several
ev). Therefore, it has a structure such that a polyethylene layer
is provided, as has been described in the prior art 1, so as to
enable detection of charged particles (protons) to be generated
when fast neutrons enter therein by the semiconductor detection
element. Such substance which generates protons when encountered
with incident fast neutrons is called a proton radiator. It,
however, has had a problem that it could not improve sensitivity to
neutrons in an energy range from 10 KeV to 1 MeV.
Now, let's consider the neutron dose rate meter. According to the
prior art 3, the dose equivalent response has not been satisfied.
Further, because of a higher operating voltage at the time of
neutron detection, which results in a more complicated measuring
circuits, the prior art 3 has had a problem that it somewhat lacked
reliability.
Further, the prior arts 1 and 2 have not been contemplated to be
applied to neutron dose rate meters, hence there has been devoid of
any such consideration.
A fist object of the present invention is to provide for a portable
neutron individual dose meter which attains the dose equivalent
response.
A second object of the present invention is to provide for a
portable neutron individual dose meter which attains the dose
equivalent response by using a single semiconductor detection
element.
A third object of the present invention is to provide for a
portable neutron individual dose meter which has a high sensitivity
to thermal neutrons.
A fourth object of the present invention is to provide for a
portable neutron individual dose meter which has a high sensitivity
to fast neutrons.
A fifth object of the present invention is to provide for a neutron
exposure dose rate meter which implements the dose equivalent
response.
DISCLOSURE OF THE INVENTION
In order to accomplish the first and second objects of the present
invention, there have been provided: a neutron detector having a
layer comprising a first material which emits charged particles
through a nuclear reaction with incident thermal neutrons and a
second material which emits charged particles through interactions
with incident fast neutrons, the same layer being bonded on the
surface of a semiconductor detection element; a processing circuit
for processing signals obtained from the neutron detector, and a
power source for supplying power to the neutron detector and the
processing circuit.
In order to accomplish the third object of the present invention,
there have been provided: a neutron detector having a grain-formed
material which emits charged particles through a nuclear reaction
with incident thermal neutrons and another material which does not
emit charged particles through interactions with incident fast
neutrons, each being formed on the surface of a semiconductor
detection element; a processing circuit for processing signals
obtained from the neutron detector; and a power source for
supplying power to the neutron detector and the processing
circuit.
In order to accomplish the third or fourth object of the present
invention, there are provided: a single semiconductor wafer; a pair
of electrodes formed on the obverse and reverse surfaces of the
semiconductor wafer; a power source for supplying power to the
electrodes; a neutron detector having a semiconductor detection
element wherein the electrode through which the charged particles
enter is formed so as to cover a portion of a depletion region
thereof; a processing circuit for processing signals obtained from
the neutron detector; and a power source for supplying power to the
neutron detector and the processing circuit.
In order to accomplish the fifth object of the present invention,
there are provided: a neutron detector having a layer comprising a
first material which emits charged particles through a nuclear
reaction with incident thermal neutrons and a second material which
emits charged particles through interactions with incident fast
neutrons, the same layer being formed on the surface of a
semiconductor detection element; at least one of a neutron
moderator and a thermal neutron absorber which are installed in the
direction of the incident neutrons with respect to, or in such a
manner as to surround, the neutron detector; a processing circuit
for processing signals obtained from the neutron detector; and a
power source for supplying power to the neutron detector and the
processing circuit.
In order to accomplish the fifth object of the present invention,
there have been provided: a neutron detector having a layer
comprising a first material which emits charged particles through a
nuclear reaction with incident thermal neutrons and a second
materials which emits charged particles through interactions with
incident fast neutrons, the layer being formed on the surface of a
semiconductor detection element; a phantom which simulates a human
body on which to attach the neutron detector; a processing circuit
for processing signals obtained from the neutron detector; and a
power source for supplying power to the neutron detector and the
processing circuit.
In order to accomplish the fifth object of the present invention,
there are provided: a neutron detector having a material bonded on
its surface, which emits charged particles through interactions
with incident neutrons; a neutron moderator and a thermal neutron
absorber disposed such as to surround the neutron detector; a
processing circuit for processing signals obtained from the neutron
detector; and a power source for supplying power to the neutron
detector and the processing circuit.
The dose equivalent response Sa(E) is capable of being expressed by
equation 1 as follows.
where Da(E) (see FIG. 2 (b)) represents a neutron sensitivity
response, indicating a sensitivity response to neutron energy E by
the neutron detector itself, fa(e) (see FIG. 2(d)) represents a
phantom sensitivity response, indicating a response to the neutron
detector of incident neutrons entering a human body or human body
simulator, and Ia(E) (see FIG. 2(c)) indicates an incident neutron
spectrum. Thereby, the effective dose equivalent response S(E) in
accordance with the ICRP recommendation is a preferred response
which corresponds to a case when an incident neutron spectrum Ia(E)
in a unit spectrum I(E). Thereby, it is expressed by equation 2 as
follows.
Thereby, with respect to a neutron dosimeter to be used carried by
an individual, it is necessary for the neutron sensitivity response
Da (E) to indicate a preferred response D(E), because the human
body itself indicates a preferred phantom response D(E) of
necessity. On the other hand, with respect to the neutron dose rate
meter, it is necessary for the effective dose equivalent response
S(E) to be satisfied at least by the product of responses of the
neutron detector and a human body simulating phantom. Namely, if
the neutron detector satisfies the effective dose equivalent
response D(E) by itself, the phantom may be constituted simply to
simulate a human body. On the other hand, if the neutron detector
cannot satisfy the effective dose equivalent response by itself,
the phantom may be constituted, though a total sensitivity may
drop, to complement the insufficiency so as to satisfy the
effective dose equivalent response S(E) as a whole. Means and its
functions to solve the problems based on the above concepts will be
described in the following.
First, the individual neutron exposure dosimeter will be explained.
With respect to the individual neutron exposure dosimeter, it is
desired for the response of the neutron detector to be such as
shown in FIG. 2(b). The neutron sensitivity response D(E) according
to the present invention involves roughly two curves, this is based
on the knowledge that; a first curve, D.sub.1 (E) depends on a
converter which emits low energy neutrons consisting mainly of
thermal neutrons and .alpha. rays; and a second curve D(2)(E)
depends on a proton radiator which emits protons through
interactions with high energy neutrons consisting mainly of fast
neutrons. Thereby, it is not necessary to provide the neutron
moderator disposed in front of the neutron detector as described in
the prior art 1. If, however, the converter and the proton radiator
are provided in separate layers, for example, in the sequence of a
converter directly on a semiconductor detection element, then a
proton radiator thereupon, protons generated in the proton radiator
cannot reach the semiconductor detection element being prevented by
the converter. Hence, according to the present invention, the
converter and proton radiator are provided in the same composite
layer each coexisting therein, bonded to the surface of a
semiconductor detection element. As a result, the neutron
sensitivity response D(E) is capable of being satisfied by
adjusting respective amounts of the converter and proton radiator,
and a total thickness of the layer. The response of a semiconductor
neutron detection element having a converter and a proton radiator
varies greatly according to its structure how the converter and
proton radiator are built. Namely, when a large quantity of
converter is provided on the surface of a detection element, an
increased sensitivity to thermal neutron components will be
attained, while, on the contrary, an increased sensitivity to fast
neutron components will be attained if a large quantity of proton
radiator is provided therein. Thereby, by adding a power source and
a processing circuit to such neutron detector, a neutron individual
exposure dosimeter is capable of being implemented. Since it is
only required for a preferred converter to react with thermal
neutrons to generate charged particles, boron, lithium or the like
may be utilized. Further, as a preferred proton radiator, there are
hydride compounds such as paraffin, polyethylene, and other organic
compounds or resins composed of them. Since boron and hydride
compound are typically employed for the converter and the proton
radiator, respectively, related description hereinunder will be
made with these typical two materials as examples.
Next, means for increasing sensitivity to thermal neutrons will be
explained. An increased sensitivity to thermal neutrons is capable
of being attained by increasing the thickness of a converter, i.e.,
the thickness of a boron layer. Boron, however, has such drawbacks
that it has a melting point as high as approximately 2300.degree.
C., an extremely poor processibility, and that the difference in
thermal expansion coefficients between a boron film and silicon
constituting a semiconductor detection element is as large as 3.5
times, thus failing to provide a boron film having a thickness more
than 1 .mu.m. In an embodiment according to the present invention,
an effective thickness of a film is enabled to be increased by
providing boron in a grain form or powder mixed with other
materials. These other materials are preferably such that which has
a thermal expansion coefficient not too much different from that of
silicon, or that which has elasticity so as to absorb the thermal
expansion of boron. Further, in case boron is prepared in grain,
and a boron layer is formed by calcination of grain boron, its
processibility will be much simplified. Further, as the boron layer
is composed of grain boron, a peel-off problem of the layer due to
the difference in the thermal expansion coefficients with silicon
caused by temperature changes is solved. Thus, a sufficiently
thicker boron layer than by the prior art is capable of being
formed, providing for an increased sensitivity to thermal neutrons.
When the other material of the above is a hydride compound to be
utilized as the proton radiator, by arbitrarily setting its grain
size (average grain diameter) of boron, it is possible to control
the quantity of the hydride compound to be bonded to the surface of
silicon. The sensitivity to fast neutrons being changed
accordingly, the response characteristics of the detector are
capable of being adjusted with respect to a total neutron energy,
which is also preferable in implementing the effective dose
equivalent response. Further, they being elastic, the hydride
compounds are preferable in view of mitigating the peel-off
problem. Further, in such a structure where the interstices of the
abovementioned grain converters are infiltrated or plugged with
proton radiators, a grain size of the grain converters is likely to
control a total response. For example, when the grain size becomes
smaller, sensitivity to thermal neutron components is increased. In
case boron(.sup.10 B) is used for converters, .alpha. rays of 1.47
MeV will be produced through a nuclear reaction with thermal
neutrons. Likewise, proton radiators will produce proton beams
through interactions with fast neutrons. When these charged
particles (.alpha. rays, proton beams) enter the detection element,
pairs of charges of electrons and holes are generated therein
through the charged particles' ionization actions. Since a range
for .alpha. rays is less than 10 .mu.m, a thickness more than that
for the converters will make no difference in sensitivity
responses. On the other hand, a range for proton rays emitted from
the proton radiators is approximately 1 mm at an energy level of 10
MeV. Thereby it is typical with the detection elements having such
structures as above that a thin layer of converters of several 10
micron m thickness is formed on the surface of the element, and
proton radiators are infiltrated into interstices between converter
grains, and proton radiators are further laminated thereupon in a
thickness of 1-2 mm.
Because neutron dosimeters for personal use have to be portable,
such a structure surrounded in a neutron absorber of a large
dimension as in the neutron dose rate meter will not be
advantageous in practical applications. When a personal dosimeter
is used fixed to the chest of a human body, the human body itself
serves as a neutron moderator. That is, fast neutrons are scattered
and moderated in the human body, and the scattered components which
are typically caleed albedo neutrons are effected to enter the
dosimeter. Through adjustment of the grain size of boron and the
thickness of proton radiators under the above conditions, a neutron
dosimeter for personal use having the dose equivalent response is
capable of being implemented.
Next, preferred means for increasing sensitivity at least to one of
the thermal neutrons and fast neutrons will be described in the
following. According to the prior arts, charged particles generated
in the converters and proton radiators are blocked to enter a
semiconductor detection element by its electrode portion and oxide
film on the surface, as a result dropping its sensitivity. Thereby,
in the prior art 2, electrodes are disposed collectively on the
reverse surface of the converters and proton radiators. This type
of devices, however, are not easy to manufacture. According to the
present invention, an electrode disposed at the side of an incident
window for charged particles is formed such as to cover only a
portion of a depletion region therein. By such arrangement, an area
of the electrode has been reduced, thereby blockage of incident
charged particles by the electrode has been minimized. As a result,
the number of charged particles to enter the semiconductor
detection element has increased, thereby substantially improving
detection sensitivity.
In the last, a preferred neutron exposure dose rate meter will be
described below. As hereinbefore referred to, with respect to its
construction, if a neutron detector itself is able to satisfy the
effective dose equivalent response D(E), it is necessary simply to
construct a human body simulating phantom. If, on the contrary, the
neutron detector itself is not able to satisfy the effective dose
equivalent response D(E), though a total sensitivity may drop, a
phantom may be constituted such as to complement the insufficiency
to satisfy the effective dose equivalent response S(E) as a
whole.
From the view point of the former, it is possible to provide for a
neutron dose rate meter which satisfies the effective dose
equivalent response S(E), simply by attaching a neutron exposure
dosimeter for personal use on the phantom which simulates a human
body. As such a phantom, there is water, acrylic, or the like.
Further, according to the present invention, neutron moderators or
thermal neutron absorbers are substituted for the phantom. For
instance, the semiconductor detection element may be surrounded by
neutron moderators or thermal neutron absorbers. Then, by
increasing the thickness of the neutron moderator, it is possible
to adjust a rate of sensitivity, i.e., decreasing the sensitivity
to thermal neutrons and increasing that to fast neutrons. Further,
the thickness of thermal neutron absorbers or the width of openings
are adjusted. A thermal neutron absorber with openings is fully
transparent to fast neutrons permitting then to transmit
therethrough. Sensitivity to thermal neutrons is capable of being
adjusted by controlling in opening ratio. Through such arrangement,
sensitivity to each energy source is capable of being adjusted,
hence providing for a phantom which simulates human body.
From a view point of the latter, any neutron detectors consisting
of semiconductor detection elements without utilizing silicon may
be also employed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a neutron individual exposure dosimeter according to a
first embodiment of the present invention.
FIG. 2 shows an effective dose equivalent response, the response of
a neutron detector, and the response of a phantom, relative to
incident energy spectra.
FIG. 3 illustrates how an individual neutron dosimeter according to
the present invention is attached to the body.
FIG. 4 shows a schematic diagram of a semiconductor neutron
detector and a block diagram of a measurement circuit for the
neutron individual dosimeter.
FIG. 5 shows a relationship between the number of nuclear reactions
N(I) and the thickness of converters.
FIG. 6 shows the result of comparison of responses between the
neutron individual dosimeter according to the present invention and
the effective dose equivalent.
FIG. 7 illustrates a state at calibration of the personal dosimeter
of the present invention.
FIG. 8 shows a personal neutron dosimeter of a second embodiment of
the invention.
FIG. 9 shows a personal neutron dosimeter of a third embodiment of
the invention.
FIG. 10 shows a personal neutron dosimeter of a fourth embodiment
of the invention.
FIG. 11 shows a neutron dose rate meter of a first embodiment of
the present invention.
FIG. 12 is a schematic diagram illustrating a neutron detector
portion of a first embodiment of the invention.
FIG. 13 shows a structure of a neutron detector and a block diagram
of a radiation measurement circuit 60.
FIG. 14 shows energy responses of the neutron dosimeters according
to the present invention, when the thickness of neutron moderators
was varied.
FIG. 15 shows the result of comparison of the responses between the
neutron dosimeters of the present invention and the effective dose
equivalent.
FIG. 16 shows a neutron dose rate meter of a second embodiment of
the present invention.
FIG. 17 shows directivity in a perpendicular direction for the
neutron dose rate meter of the second embodiment of the
invention.
FIG. 18 shows a neutron dose rate meter of another embodiment of
the invention.
FIG. 19 is a cross-sectional view of the embodiment of FIG. 18.
FIG. 20 shows a neutron dose rate meter of still another embodiment
of the invention.
FIG. 21 shows a neutron dose rate meter of furthermore embodiment
of the invention wherein a thermal neutron absorber is included
inside a neutron moderator.
FIG. 22 shows a neutron dose rate meter of still further embodiment
of the invention wherein a thermal neutron absorber is disposed in
a space inside a neutron moderator.
FIG. 23 illustrates an exemplary distribution of wave heights of
signals measured according to the present invention.
PREFERRED EMBODIMENTS FOR IMPLEMENTING THE INVENTION
Referring to the embodiments and drawings attached, detailed
descriptions of the present invention will be made in the
following. FIG. 1 shows a personal semiconductor neutron dosimeter
of a preferred embodiment of the present invention. A semiconductor
neutron detection element 1; a radiation measurement circuit
(hybrid circuits) 33 including a preamplifier, a discriminator, a
reverse bias application circuit and the like; an arithmetic
display unit 34; and a power source 35 are contained in a portable
carrying case 32. This portable dosimeter is attached to the chest
portion of a worker with a clip 36. FIG. 3 illustrates how this
semiconductor neutron dosimeter 32 is attached to a human body
31.
FIG. 4 shows the structure of a semiconductor detection element,
and a block diagram of the measurement circuit for a personal
neutron dosimeter. The structure of a semiconductor detection
element 1 will be described by using n-type silicon as an example.
At first, p-n junction (p-layer) 8 is formed in the surface of
n-type silicon 1 through impurity (boron) diffusion. The surface of
the junction 8 is then insulated and protected by a silicon oxide
film (SiO.sub.2). A pointed junction electrode 10 for loading out
signals is provided as shown in the figure, and an earth electrode
11 provided in ohmic contact on the surface opposite the junction
is led out. When a reverse bias voltage is applied between the
electrodes 10 and 11, a depletion region 7 is effected to expand
below the junction 8. On the surface of the semiconductor detection
element 1 having the oxide film 9 interposed therebetween, a mass
of grain (powder) shaped boron 5 is bonded tightly by calcination.
The interstices between the grains are infiltrated by proton
radiators (hydride compounds) and the outer surface thereof is
further covered by the same 6. As preferable hydride compounds to
be used, there are paraffin, epoxy, polyethylene or the like. The
whole detection element is hermetically sealed in a can, though not
shown, and lead wires from respective electrodes at p-layer 8 and
n-layer 4 are drawn outside the can. The electrode 10 in contact
with the p layer 8 is formed into a point in order to prevent
attenuation of .alpha. rays and protons. The oxide film 9 which
serves as an incident window for .alpha. rays and the like is
readily formed into a thickness of approximately 100 .ANG.. By
providing layers of the abovementioned converters and proton
radiators in the incident window, attenuation of charged particles
in the electrode 10 and oxide film 9 is capable of being minimized.
Further, it sometimes occurs that neutron moderator (not shown) is
required to be used outside or inside the sealing can. Incident
thermal neutrons(Nth) incoming from outside into a dosimeter of the
present invention enter into a nuclear reaction in thermal neutron
converter 5 to generate .alpha. rays. Likewise, fast neutrons (Nf)
generate repulsion protons through scattering actions with proton
radiators. These .alpha. rays and proton rays produce electric
charges in the depletion region 7 in the detection element 1. Such
electric charges produced in accordance with the incident thermal
neutrons (Nth) and fast neutrons (Nf), after being guided through
ac coupling capacitance 13, are amplified in a preamplifier 14 and
a linear amplifier 15, respectively. Amplified signals undergo wave
height discrimination in a discriminator 16, and their pulses are
measured in a counter circuit 17. Based on the values counted in
the counter circuit 17, quantities of dose are calculated and
displayed in an arithmetic/display unit 18.
The principle of neutron detection according to the present
invention is roughly classified into two categories as follows. One
is that low energy thermal neutrons enter into a nuclear reaction
in boron 5 as represented by equation 3, hence emitting .alpha.
rays therefrom.
where .alpha. rays have energy of 1.47 MeV, and produce charges of
pairs of electrons e and holes h in the depletion region 7 in the
detection element 1, which charges cause to vary the detector
current. The other one is that incident fast neutrons interact
(scatter) with proton radiators 6, generating repulsion protons,
which in turn produce charges of pairs of electrons e and holes h
in the depletion region 7, which cause to vary the detector
current. The preferred embodiment of the invention described
herewith is capable of providing an improved detection sensitivity
and a wider range of energy detection through improving
probabilities of occurrence of respective interactions according to
the above two categories, and efficiencies of collecting introduced
charged particles (.alpha. rays, protons). The details of their
operations will be described below.
The number N (l) of nuclear reactions taking place in boron 5 which
serves as a converter is expressed as follows.
where, .phi.:the number of incident neutrons (n/cm.sup.2.S)
n: atomic density of converter (1/cm.sup.3)
.sigma.: cross-section of nuclear reaction (barn)
l: thickness of converter (cm)
As is obvious from the equation, the number N(l) of nuclear
reactions is directly proportional to the thickness 1 of the
converter, which numeric relationship is shown in FIG. 5. On the
other hand, the flight range in boron and silicon of .alpha. rays
generated in the converters is up to 7 .mu.m. Thereby, in such a
structure wherein the thickness of boron converters is more than 7
.mu.m, or when a flight range more than 7 .mu.m is required in
silicon, .alpha. rays will soon be attenuated, thus contributing
none to the improvement in the sensitivity of the detector. Namely,
the contribution of .alpha. rays saturates at 7 .mu.m or more of
the thickness of converters.
On the other hand, anoccurrence probability .sigma.p of protons to
be generated by fast neutrons by scattering in the proton radiator
6 will be expressed by the following equation, wherein an incident
energy of neutrons is given by En(MeV). ##EQU1## Namely, the
occurrence probability .sigma.p changes in proportion to the power
of -1/2 of incident neutron energy En, which occurrence
probability, however, does not depend on the scattering angle of
protons. On the other hand, the energy of the protons hereinabove
generated depends on scattering angles, thereby, an energy
distribution for protons generated by monochrome neutrons involves
a continuous distribution from 0 eV energy up to incident energy
En. The range of the protons is approximately 1 mm at 10 MeV, but
low energy protons are rapidly attenuated while passing through
boron.
The preferred embodiment of the invention shown in FIG. 4, has been
contemplated taking into account the abovementioned considerations.
Thereby, a layer thickness 1c composed of grain boron 5 easy to
process having an average in size of 10 .mu.m is set at 30 .mu.m
with a porosity rate of 75%, which corresponds to a layer thickness
of 7 .mu.m consisting of 100% boron, so that a calcinated layer
thickness as thick as possible to maximize the number N(l) of
nuclear reactions without exceeding the range of .alpha. rays may
be obtained. The interstices in grain boron 5 are infiltrated with
proton radiator 6, and an entire outer surface is coated further
with a layer of the proton radiator 6 to a thickness (1 p=) of 2
mm. By means of such detector construction, most of the low energy
components of protons generated in the proton radiators 6 are
permitted, without passing through boron, thereby without
attenuation, to get into the detection element 1.
Further, .alpha. rays generated in the grain shaped boron 5 have
energy of 1.47 MeV, thereby they are scarcely attenuated while
passing through several .mu.m of proton radiators composed of
hydride compounds having a small atomic number. Further, on the
side of the semiconductor detection element 1, the oxide film 9 and
p-layer 8 being insensitive to charged particles, and only causing
attenuation to .alpha. rays and the like, their thickness is
controlled within 0.3 .mu.m in order to prevent drop of
sensitivity.
As hereinabove described, the present embodiment of the invention,
has advantages as follows simply as a personal neutron dosimeter.
Because of an increased effective thickness of boron enabled by
being provided in grain form, detection sensitivity to thermal
neutron is capable of being improved approximately by 10 times
greater than the prior arts. Further, the shield effect of boron on
fast neutrons, in particular, on the fast neutrons in lower energy
side is capable of being reduced, thereby, detection sensitivity
for fast neutrons is capable of being improved as a whole. Still
further, with respect to the manufacturing techniques, there are no
specific problems, and calcination (evaporation solidification) of
grain boron is readily performed. Further, permeable coating using
permeable hydride compounds is effective for preventing reduction
in a boron layer by peel-off due to temperature changes. There may
be conceivable some other methods other than the calcination
method. For example, a composite layer comprising boron and proton
radiators may be fixed to a semiconductor element with a plastic
fixing film. Lastly, by means of a pointed electrode provided in
the side of the charged particle converter such as boron and the
like, a portion of the electrode which blocks the passage of
charged particles into the semiconductor detection element is
capable of being eliminated. In addition, the oxide film 9 is
capable of being made thinner. Thereby, transmissivity of charged
particles into the semiconductor detection element is capable of
being substantially improved, thus increasing the detection
sensitivity over a wider range of neutron energy.
Next, a personal neutron dosimeter of another embodiment of the
invention capable of evaluation of the effective dose equivalent
conforming to the recommendation of the ICRP will be described.
According to the today's radiation hazard prevention laws and
regulations, it is required to have a neutron detector which has
energy response characteristics capable of performing the effective
dose equivalent evaluation conforming to the ICRP recommendations.
The hereinabove embodiment detector also answers to such needs.
Namely, with reference to FIG. 4, the detection sensitivity to fast
neutrons is capable of being adjusted by changing the coating layer
thickness 1p of the proton radiator 6, the particle size of the
grain boron 5, or a ratio of quantities between the proton radiator
and the grain boron. A solid line curve in FIG. 6 shows a
relationship between the neutron energy and the effective dose
equivalent (which indicates degrees of influence upon human body)
according to the ICRP recommendation. Circles o in the figure
indicate characteristics of the embodiments of the present
invention, while x marks indicate those of the prior art detectors.
Data available in FIG. 6 are limited in the range of thermal
neutron, and from several 100 KeV up to 15 MeV. Data between
several 100 KeV and thermal neutrons are interpolated by Monte
Carlo simulation. This is because that data in such intermediate
region cannot be gathered experimentally by the present
state-of-the-art technologies.
As a personal neutron dosimeter is utilized being attached to the
chest of human body as shown in FIG. 3, calibration tests for the
personal neutron dosimeter must be performed taking into account
neutrons scattered from within the body. Data in FIG. 6 is obtained
by using a dosimeter fixed not on the chest of human body but at
the center of a phantom simulating human body, made of acrylic or
water, of a size of 40.times.40.times.15 cm. FIG. 7 shows the
behavior of a neutron in a phantom simulating a human body. A part
of fast neutrons interacts with proton radiators in the dosimeter,
but most of them reaches a phantom 30 and are scattered therein.
Some of the scattered beams enter a neutron detection element 1. It
is natural that the energy of neutrons which enter the dosimeter
includes, with the incident neutrons being the maximum, up to
thermal neutrons. It is possible readily to obtain the response of
the effective dose equivalent by adjusting the kinds of converters
in the dosimeter, its grain size, layer thickness, and also the
kinds of proton radiators and its layer thickness.
Energy response characteristics of the personal dosimeter of the
hereinabove described preferred embodiment according to the present
invention show a good agreement even in a higher energy region in
comparison with the prior art examples. Further as they agree well
with the response of the effective dose equivalent, it will be
obviously appreciated that a practical neutron dosimeter for
personal use is capable of being provided.
Further, with respect to the embodiment of FIG. 4, charged
particles are generated through such a structure that grains of
boron are calcinated, and their interstices and a surface thereon
are plugged with proton radiators. Such structure, however, may be
substituted for with a compound or mixture of boron and hydride
compounds.
Another neutron detector of a second embodiment of the present
invention will be described below with reference to FIG. 8. This
embodiment has such a structure that a boron plate 50 having
openings is provided in the surface of a semiconductor detection
element 1, and the openings are plugged with proton radiators 51.
According to this embodiment, it is possible to adjust the response
of the detector by adjusting the area of openings (in case the
opening is a circle, its diameter). A method of manufacturing boron
plates having arbitrary shapes and openings may be implemented,
first by forming a boron layer in front of a metal mask using
sputtering techniques or the like, then removing the metal mask.
The boron plate prepared as above is attached firmly on the surface
of a semiconductor detection element 1. Attachment may be
accomplished simply by bonding using adhesives or with mechanical
fixture at peripheral portions.
Substantially the same advantages as in the first embodiment are
capable of being attained according to this embodiment of the
invention.
Further, a third embodiment of the invention is shown in FIG. 9.
This embodiment consists of a semiconductor detection element 1 and
a proton radiator 53 formed thereon, thus providing a detector
which is sensitive only to fast neutrons. The proton radiator 53 is
capable of being provided simply by applying paraffin or the like
as in the case of FIG. 4. According to this embodiment of the
invention, it is capable of providing a personal neutron dosimeter
which has an especially high detection sensitivity to fast
neutrons.
A fourth embodiment of the invention is shown in FIG. 10. In this
embodiment, a layer of grains of boron 55 is formed on the surface
of a semiconductor detection element 1, an outer surface of which
is coated by substance devoid of hydrogen such as aluminum
evaporation film 56 for protection. This detector prepared as above
is sensitive only to thermal neutrons, and a peel-off problem due
to temperature changes is minimized through the employment of
grains of boron with a maximized thickness permitted for the layer,
thus substantially having improved detection sensitivity to thermal
neutrons. According to this embodiment of the invention, it is
possible to provide for a personal neutron dosimeter having an
especially high detection sensitivity to thermal neutrons.
Boron is utilized as material for effecting nuclear reactions with
thermal neutrons in each of the above embodiments of the invention.
This, however, may be replaced by lithium effecting a nuclear
reaction of .sup.6 Li(n,.alpha.).sup.3 H, or by uranium. In this
case, however, since energy of .alpha. rays emitted therefrom
differs from that of boron, it is necessary to determine the
thickness of converters appropriately according to their flight
ranges. Further, in the hereinabove description, as an exemplary
sensitivity enhancing substance, there have been utilized proton
radiators such as hydride compounds or the like, which, however,
may be any other substance only if it generates heavy charged
particles through interactions with fast neutrons. Further, silicon
semiconductors utilized for semiconductor detection elements in the
above description may be replaced by chemical compound
semiconductors using cadmium telluride, mercury iodide or the
like.
Next, we will describe neutron dose rate meters. FIG. 11 shows a
semiconductor neutron dose rate meter of an embodiment of the
invention. According to the present embodiment, a human body
simulating phantom is composed of a spherical neutron moderator 2
and a thermal neutron absorber 3 which have openings, the latter
disposed over the outer shell. Inside the phantom, the above
semiconductor neutron detector 40 is installed. Neutron detection
signals are transmitted outside to a radiation measurement circuit
60 via a signal cable. This type of system using the spherical
neutron moderator 2 is capable of providing a preferable dose rate
meter which retains a maximized nondirectivity. FIG. 12 illustrates
a schematic diagram of a neutron detection portion according to the
present invention. In this embodiment, a semiconductor detector as
shown in FIG. 4 is installed at the center of a neutron moderator 2
made of paraffin or polyethylene, and a thermal neutron absorber 3
made of cadmium plate or the like having openings. In FIG. 12, a
metal case (can) for hermetically sealing in a semiconductor
detection element 1 and a signal cable for taking out signals are
omitted. Incident thermal neutrons(Nth) entering the dose rate
meter from outside are partially absorbed in the thermal neutron
absorber 3 and neutron moderator 2, and a portion of which reaches
the detection element 1. Here, they enter into a nuclear reaction
with thermal neutron converters 5, producing .alpha. rays. Fast
neutrons (Nf) without being absorbed in the thermal neutron
absorber 3 reaches the neutron moderator 2, where some of them are
decelerated to thermal neutrons, but some other reaches a proton
radiator 2. In the proton radiator, repulsion protons are produced
through scattering action with fast neutrons. Charges of these
.alpha. rays and proton beams produced as above are collected in a
depletion region 7 in the semiconductor detection element 1.
FIG. 13 shows a neutron detector structure and a block diagram of a
radiation measuring circuit 60. For the structure of the neutron
detector is the same as that of FIG. 4, explanation will be
omitted. The radiation measuring circuit 60 is basically the same
as that in FIG. 4, but has two types of discriminators 16 and
counter circuits 17, respectively. Suffix a subsystem represents an
energy dependent total counter, while suffix b sub-system indicates
a circuit for discriminating fast neutrons having energies greater
than a prescribed value to be described later.
FIG. 14 shows examples of measurements of energy responses by the
neutron dose rate meter of the invention. This figure shows the
results of measurements obtained by varying the thickness of the
neutron moderator as a parameter. Main specifications of the
structure of this neutron dose rate meter is as follows.
______________________________________ grain size of boron 20 .mu.m
thickness of proton radiator 2 mm opening ratio for thermal 70%
(opening coefficient) neutron absorber (0.5 mm thick cadmium)
______________________________________
As above, it is possible to change the response of the dose
equivalent through changing the thickness of the neutron
moderator.
FIG. 15 shows the response of the neutron dose rate meter according
to the present invention in comparison with the response of the
effective dose equivalent. The thickness of the neutron moderator
was 80 mm (the same data as for the moderator thickness of 80 mm in
FIG. 14). The data was interpolated likewise that in FIG. 6. As
obviously seen from the results of the comparison, the response of
the neutron dose rate meter of the invention.Iadd., as shown by the
circles o substantially lying the solid line curve, .Iaddend.agrees
well with the dose equivalent response at a high precision within
.+-.30% over an energy range from the thermal neutrons up to 15
MeV. Concurrently, some examples of the response of a prior art
dose rate meter are shown. The prior art dose rate meter.Iadd., as
shown by x marks connected by the one-dot chain line curve,
.Iaddend.has shown that its sensitivity falls exceeding several
MeV.
As hereinabove described, it is possible to implement the effective
dose equivalent response through adjustment of the thickness of the
neutron radiation moderator. Further, although the thickness of the
neutron radiation moderator is adjusted in the above description,
the same effect may be attained by adjusting the thickness of the
thermal neutron absorber or its opening ratio. The neutron
radiation dose rate meter according to the present invention is
capable of being applied to an area monitor, environmental monitor,
surveillance monitor and so on to be employed for radiation
handling facilities such as nuclear power generation plants or the
like, thus providing a novel and practical measuring device.
Still another neutron radiation dose rate meter of a second
embodiment of the invention is shown in FIG. 16. In order to retain
nondirectivity, a neutron radiation moderator of a nearly spherical
body, more specifically, a right circular cylinder body having the
same length for the diameter and the height, is utilized. A
semiconductor neutron detector 40 is placed in the center of the
neutron radiation moderator 2, over the outer surface of which
thermal neutron absorbers 3 having openings are attached. Neutron
detection signals are transmitted, via a signal cable, to a
measurement circuitry 60 and an indicator unit 20 which includes a
data processing unit. A neutron radiation moderator 2 having an
outer form of a sphere is expensive to manufacture. However, a
substantial cost reduction in production cost will be attained by
forming the neutron moderator 2 into a right circular cylinder
substantially resembling a sphere. There arises a possibility to
give rise to directivity by rendering the neutron radiation
moderator 2 anything but a sphere form. FIG. 17 shows the result of
the investigation on the directivity obtained in a perpendicular
direction and other incident angles with respect to this
embodiment. As is obvious from this result, the directivity is
capable of being ignored within an allowance of .+-.10%. This is
because of the effect of a repeated scattering of the neutrons
within the moderator so as to permit the directivity to be lessened
substantially.
According to the above verification, another modified embodiment of
the present invention is capable of being contemplated as follows.
FIG. 18 illustrates a neutron moderator 2 of the invention shaped
into a square cube, every corner of which is cut out. FIG. 19 shows
a raised plan view of FIG. 18. A neutron detector 40 is disposed in
the center in each of these embodiments described herewith. FIG. 20
is still another modification of the neutron moderator 2 of the
invention formed into a right circular cylinder the corner of which
to be cut away likewise. Each of these modified embodiments is
capable of fully retaining the performance and advantages of the
present invention. In FIGS. 18, 19 and 20, illustrations of a
thermal neutron absorber 3 are omitted.
FIG. 21 shows a thermal neutron absorber 3 disposed in another
modified version of the invention. In FIG. 21, the thermal neutron
absorber 3 having openings is installed inside a neutron moderator
2. FIG. 22 shows another thermal neutron absorber 3 disposed in
still another modified version of the invention, wherein a thermal
neutron absorber 3 having openings is disposed in an interstice
between a neutron moderator 2 and a neutron detector 40. When the
thermal neutron absorber 3 is disposed deeper inside the neutron
moderator 2, the quantity of the thermal neutron absorber 3 is
capable of being reduced.
FIG. 23 shows an example of the wave height distribution (spectrum)
of signals measured according to the present invention. .alpha.
region between the wave height values A and B represents a wave
height region for .alpha. rays generated by the nuclear reactions
with thermal neutrons. The region exceeding wave height value B
represents a wave height region for protons generated through
interactions with fast neutrons having several MeV. A region below
wave height B indicates wave height values for gamma rays. By
dividing discrimination levels into A, B, or into multilevels
beyond B, it is capable of readily performing concurrent
discriminations of respective energy components for fast neutrons
from each output signal (see the block diagram of the measuring
circuitry in FIG. 13). Of course, energy of neutrons, calibration
coefficients for wave height values and sensitivity coefficients
for effecting neutron detection in respective wave height regions
should be obtained in advance, and their data conversion processing
into absolute values are necessary. Because there are great needs
for simultaneous discrimination measurement of fast neutrons, in
particular, in such facilities where high energy accelerators are
utilized, applicability of the present invention will be extremely
wide and extensive.
Further, with resect to the thermal neutron absorber, boron or
lithium is capable of being substituted for cadmium. In the above
embodiments, although the detection sensitivity to thermal neutron
components is adjusted by changing the opening ratio in the neutron
absorber, it is, however, possible to effect the sensitivity
adjustment by changing the thickness of the thermal neutron
absorber.
Still further, although the semiconductor detection elements are
utilized as detection elements in the above embodiments as in the
case of the personal dosimeter, chemical compound semiconductors
such as cadmium telluride or mercury iodide may be utilized
instead.
A neutron dose rate meter is capable of being provided which is
capable of implementing the effective dose equivalent response,
through installing a neutron detector capable of implementing the
effective dose response on a certain volume of water, acrylic or
the like serving as a phantom simulating a human body, which has
been described in the personal dosimeter using neutron moderators
and thermal neutron absorbers.
In the hereinabove case, the neutron detectors are limited to such
that are capable of attaining the effective dose equivalent
response. The dose equivalent response, however, is capable of
being changed by varying the thickness of the neutron moderators or
thermal neutron absorbers as well. Hence, although a neutron
detector cannot realize the effective dose equivalent response by
itself, it is still possible to implement the effective dose
equivalent response as a whole of a neutron dose rate meter by
changing the thickness or the like of the neutron moderator or
thermal neutron absorber which surround the same neutron detector.
In this case, from a view point of providing a neutron dose rate
meter capable of implementing the effective dose equivalent
response, the neutron detector may be a BF.sub.3 counter or .sup.3
He counter.
Lastly, in the hereinabove descriptions, the neutron moderators and
thermal neutron absorbers are disposed in enclosure In such a case,
however, when a neutron dose rate meter is disposed along a wall,
because an incident direction of neutron radiations is limited, the
neutron moderator or thermal neutron absorber may be installed only
in such a specific incident direction.
INDUSTRIAL APPLICABILITY
As hereinabove described, according to the present invention, it is
capable of providing a personal neutron dosimeter which implements
the dose equivalent response.
It is further capable of providing a personal neutron dosimeter
utilizing a single wafer of semiconductor detection element which
is capable of implementing the dose equivalent response.
Still further, it is capable of providing a personal neutron
dosimeter which has an especially high sensitivity to thermal
neutron radiations.
Furthermore, it is capable of providing a personal neutron
dosimeter which has an especially high sensitivity to fast neutron
radiations.
Lastly, it is capable of providing a neutron dose rate meter which
is capable of implementing the dose equivalent response.
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