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Патент USA US2556769

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June 12, 1951
J. L. MCKIBBEN
2,556,768
NEUTRON DETECTOR
Filed Oct. 16, 1945
2 Sheets-Sheet l
íaurce
57
`
»
I
June 12,» 1951
J. L. MCKIBBEN
2,556,768
NEUTRON DETECTOR
Filed Oct. 16, 1.945
FIELI
â/z 2902556 5 .~
2 Sheets-Sheet 2
Patented June 12, 1951
UNITED STATES ÈÀTENT OFFICE
2,55 6,7 68
NEUTRON DETECTOR
Joseph L. Mclíibben, Madison, Wis., assignor to
the United States of America as represented by
the United States Atomic Energy `Commission
1
Application October 16, 1945, Serial No. 622,634
15 Claims. (Cl. Z50-83.6)
The present invention relates to an improved
neutron detector.
In connection with various physical investiga
tions and procedures which involve the use of
neutrons, it is necessary to employ some type
of neutron detection means in order to determine
and measure the neutron density. The neutron
detection means heretofore available in the art
operate reasonably satisfactorily when the neu
trons to be measured are of substantially equal
energies, and in instances when the neutron ener
gies do not vary over a very wide range. How
ever, neutron sources rarely emit neutrons of
equal energies. The neutron beam from a neu
tronic reactor, for example, may include neutrons
whose energies vary from thermal energy to en
ergies of several million times that amount, and
2
Fig. 3 is a diagrammatic view illustrating the
device of Fig. 1 in use;
Fig. 4 is a sectional view, similar to Fig. 1, of a
modified form of the apparatus;
Fig. 5 is an end elevation of the device shown
in Fig. 4;
‘
Figs. 6 and 7 are sectional views, also similar
to Fig. 1, illustrating two other possible forms of
the invention; and
-
Fig. 8 is a longitudinal, central sectional view
of a further modiñed form of the invention.
Each of the particular neutron detection
means illustrated in the several figures of the
drawings includes an elongated neutron respon
sive unit which is responsive to thermal energy
neutrons only, and a reflector-shield or sleeve
structure of neutron slowing material which is
in order that such a beam may be used effec
disposed peripherally about such unit and which
tively, the neutron detecting means must be
is of such design that it effectively reduces the
capable of yielding substantially straight line re 20 energy of all neutrons entering the device to
sponse to neutrons of the varying energy pres
ent in such beam. The apparatus heretofore
thermal, i. e., detectable, energies.
In use the
devices are aimed in the direction of the source
available has been incapable of doing this. When
of the neutrons to be detected with the longitu
lthe apparatus was capable of detecting thermal
dinal axis of device pointing toward the neutron
neutrons, it was not sufficiently sensitive to sat 25 source. Conveniently, the neutron responsive
isfactorily detect high energy neutrons, and vice
unit comprises an ionization chamber, which,
versa, and this deñciency seriously handicapped
except for the fact that it has an axial length
research in the ñeld.
many times its width, may follow conventional
The present invention has for a principal ob
designs of such chambers. The particular cham
ject the provision of a neutron detection and 30 ber illustrated at Il in Figures 1 through '7 of
measuring means having a response characteris
the drawings comprises a cylindrical tube I3 of
tic which is substantially independent of the
brass, or other metal, which serves as the outer
neutron energy or neutron intensity. In other
electrode of the device, and which is provided
words, it is an object of the invention to provide
with suitable insulating electrode supports i5 at
a neutron detection and measuring means which 35 either end thereof. These serve to support the
has a substantially uniform sensitivity to neu
inner or collecting electrode Il, which may con
trons so that a substantially flat response is ob
veniently comprise a small diameter wire or rod,
tained to neutrons between energies of a few kilo
as illustrated. The inner surface of the tube
volts and a few million volts. This object is oc
I3 may be coated with a material which is neu
ccmplished by combining a neutron responsive 40 tron responsive such as boron, or the 235 isotope
unit which is inherently responsive only to neu
of uranium, U235, or other material having a
trons of a particular' limited energy range,- with
large fission or absorption cross-section to ther
a neutron reflector or shield structure which is
mal neutrons, and which by neutron capture
elfective to extend very greatly the response
produces instantaneously an ionizing event in
range of he neutron responsive unit. The design
of the constituent parts of the combination is
carefully correlated in a novel manner, as will
be made apparent in the accompanying drawings
and the following description.
order that the device shall be neutron responsive.
When a coating of this character is used, the ion
chamber H should be ñlled with a readily ion
izable gas, such as argon, which may be at at
mospheric pressure, and the chamber should,
in the drawings:
50 of course, be sealed. As an alternative construc
l is a longitudinal sectional view through
tion, the inner coating may be omitted, and the
a neutron detection means constructed in ac
cordance with the invention;
2 is a sectional view on the line 2-2 of
Fig. I;
tube may be filled with a gas, such as boron tri
fluoride, BF3, which is neutron responsive. Elec
trical connections are provided, as illustrated at
55 i9 andV 2|, in order that the outer and inner elecs
2,556,768
3
trodes may be connected to a suitable source of
potential as is well known in the art.
The reflector or shield of neutron slowing ma
terial, which constitutes the other principal eie«
ment of the combination, comprises, in the einm
bodiment illustrated in Fig. l, a hollow, cylin
drical, sleeve member 23 which peripherally sur
rounds the chamber Il except at one end there
of, which end is left exposed as shown. The
sleeve or shield 23 is positioned coaxially with
the chamber Il, and it should be constructed or"
a neutron slowing material having a low neutron
absorption cross-section. The most uniform re
sponse characteristics appear to be obtained by
the use of a hydrogenous slowing material, such
as parafhn, and this material is particularly rec
ommended. However, other neutron slowing ma~
substantially all neutrons entering the device to
thermal energy. The thickness of the shield
should be suñicient to minimize leakage of neu-trons during the neutron slowing operation to a
very low value. Considering a specific example,
in a device utilizing a hollow cylindrical, paraffin
shield disposed about and in contact with anA
elongated, thermal neutron responsive, cylin~
drically shaped, ionization chamber of ordinary
dimensions (i. e., a chamber having a diameter
of the order of about one half to one inch), it
has been found that a shield having a length of
the order of about 8 inches and a wall thickness
of the order of from about 3 to 4 inches will effect
thermalization of neutrons having energy levels
from a few kilovolts up to about 2 to 3 million
electron volts, with only very small probability
of the higher energy level neutrons passing
terials, such as graphitic carbon, beryllium, water
through the shield and out of the device before
and heavy water, may be employed with good re
reaching detectable thermal energies, and with
sults. A shield construction, utilizing heavy
very low leakage. The dimensions for shields
water as the neutron slowing material, is shown
of other suitable neutron slowing materials can
in Fig. '7. This construction has the disadvan
be readily determined on the basis of the com
tages that a tank must be provided for the heavy
parative neutron slowing ability of such other
water, as illustrated at 25, and that precautions
must be taken to obviate any loss of this rela~ 25 materials.
rÍ‘he modiñcation of the invention illustrated
tively expensive material.
The extent to which the neutron responsive
in Fig. 8 includes additional structural features
adapted to improve the directional quality of
ionization chamber Il projects from the shield
the device, by minimizing the effect of neutrons
23 is not particularly critical. In some instances,
it appears advisable that a length of the cham~ '» approaching from a direction other than along
ber about equal to the width or diameter thereof
the longitudinal axis of the device, and to irn
should extend beyond the main body of the shield,
prove the fundamental accuracy of the device
by providing means for preventing particle radia
but good results are obtained with a somewhat
greater length of the chamber extending. It is
tion other than neutron radiation from activiat
also possible, and in some instances desirable, ~
ing the device. The Fig. 8 apparatus includes a
to operate the device with nothing more than
the end of the ionization chamber exposed. Also,
an ionization chamber 49 disposed within a hol
it is possible to obtain improved detection char
acteristics under certain other instances by cov
ering the exposed portion of the chamber l l, with
a relatively thin shield of neutron slowing ma«
terial, such as is indicated by the dotted out
neutron responsive detection unit, specincally
low cylindrical, sleeve-like neutron reflector or
shield 5o of suitable neutron slowing material,
similar to the reflecting shields of the previously
described embodiments. In addition, the device
includes a cup-shaped shield E8 of neutron ab~
sorbing material such as boron carbide, BiC,
which surrounds the ionization chamber 59 and
thickness than the width or diameter of the 45 the shield 5d, as illustrated. The cup-shaped
chamber Il, and it is accurate to say that this
neutron absorbing shield 65 improves the direc
end of the chamber should be substantially free
tional quality of the device by elfecting absorp
tion of such neutrons as may approach the device
of neutron slowing material, as compared with
from a direction other than along the longitudi-i
the shielded portion of the chamber.
The particular shape of the shield of neutron ,
nal axis thereof.
line 21 in Fig. l. For most conditions, this ad
ditional end shield should not be of greater
slowing material is not especially critical.
For
example, the shield may take the form of a frus~
trum of a hollow cone as illustrated at 29 in Fig.
4 and in Fig. '7, or it may be spherical as illuse
trated at 3| in Fig. 6. The cylindrical form illus- ,
trated in Figs. l, 2 and 8 is the simplest to con»
struct and is in general, the most satisfactory
form. The thickness and especially the length
of the shield or sleeve of neutron slowing ma
terial are, however, most important.
In order that the device shall operate with
maximum efficiency, the neutron slowing shield
should be of sufficient length that substantially
all neutrons of the highest energies which the
device may be required to detect shall be slowed
to detectable energies before such neutrons pass
entirely through the device. Stated somewhat
diiferently, the length of the shield should be
such that the probability of a fast neutron pass
ing entirely through the device without being
slowed to detectable energy is very small.
For
The neutron absorbing efficiency of the shield
60 can be increased substantially by the provision
of an outer enclosing layer of neutron slowing
material. In the illustrated structure, the shield
6U is disposed within a hollow cylindrical, cup
shaped body of parañin 5I which provides a layer
of neutron slowing material about 11/2 to 2 inches
thick about the neutron absorbing shield 6U.
Desirably the entire shield structure comprising
the inner, neutron slowing and reflecting shield
50, the neutron absorbing shield 60, and the outer
shield 6I, are enclosed within a supporting shell
or casing 58 of sheet steel or other conducting
material.
The casing 58 is open at the forward end of the
apparatus and is provided with an inwardly ex~
tending metallic sleeve 59 integrally attached
thereto which serves to slidably support the ioni
zation chamber unit ¿i9 for movement into and
out of the shield structure. For safety reasons,
the casing 58 is desirably maintained at ground
potential.
the particular detecting means contemplated in
The ionization chamber 49 includes a cylin
the illustrated structures, this requires that the
drical outer sleeve 55, and a hollow cylindrical in
length of the neutron slowing and reflecting
shield shall be sufficient to reduce the energy 0f 75 sulator 5l which is disposed concentrically with
2,556,268
6
in, and',` which tightly engagesîthe outer-sleeve 5.5..
invention is connected to a suitable source of
The insulator: 5i serves as a. spacer and a; sup
high. potential to. establish the necessary poten
tial gradientbetweenthe ionizationchamber elec
port> for a cylindrical shellf orviilm'-V 513. oflthe 23.5
isotope> of' uranium, U235, or other‘neutron` re
sponsive material capable of effectingY an ionizing
event when subjected to neutron- irradiation.
The shell' 53 is utilizedV as the highA voltage
electrode oi the chamber, and electrical connec~
tion thereto is provided by theinsulated lead-in
2|, asl illustrated. TheV inner electrode for the
ionization chamber is provided by a rod 52 of
conductingv material whichl extends coaxially oi
the shell 53'-, and is held in- spaced relation there-`
to by means of suitable insulating supports 55:
and 5'! which havek thel further functionA oi seal
ing the ends of the chambercasing 55;. During
trodes, and it' is desirably connected. toa linear
amplifier and recorder in order that the neutron
induced ionizations may be. counted and a record
thereof obtained. A circuit of“ this- character is
shown in Fig. 3, wherein there is illustrated a
neutron detection device similar‘to that of Figs.
1 and. 2, a potential. source 33, a linear ampli
ner. and recorder 35., and a neutron source 31.
The amplifier and recorder 35 may be oaf any
conventional design. In use, the detector is
pointed axially at the source` of neutrons with
` the end which isV unshielded, or substantially
free. of- neutron slowing material, nearestl the
use, the ionization chamber may contain a
readily ionizable gas such- as argon, as in the
source'~ The neutrons emerging from the source
nuclear reactions which are induced byl slowv or '
exactly the same except that the additional radi
thermal energy neutrons. In such instances, in
ation screen B2, disposed in front ci the input
end ofthe detection means 4S, and the neutron
slowing shield 50 serve to intercept the unwanted
radiation and to prevent thatv radiation from af
fecting the responseV of' the apparatus.
3l, as indicated by the dotted lines in Figure 3,
may include- thermal energy and high energy
previously described structures. Electrical con
nection to»thecentralA4 electrode 52 is pro-vided by 20 neutrons. These enter the exposed end of the
neutron responsive chamber and the end of the
a suitableconnection F9. It is, ofcourse, possible
shield. The thermal energy neutrons will' be
to utilize an ionization chamber having ordinary
detected immediately by the ionization events
conductingl electrodes and containing; a neutron
produced in the neutron responsive chamber.
responsive gas such as boronV trifluoride, BF3, in
The low energy, or slow neutrons, after moving
place of‘ a device such as is described above.
a relatively short distance through the neutron
The relative dimension of the severalV parts
slowing material of the shield', will be slowed to
should be such that the ionization chamber 49
thermal energies and detected, and. the higher
can be readilypushed into place within the shield
energy neutrons will move progressively farther
structure while at the> same time the inner re
flecting‘shield 50j should snugly engage the outer 30 into the shield mass before reaching detectable
energizes. In effect, the device operates by slot casingV 55' of the ionizationv chamber.
ing down substantially all ci the neutrons therm
Itis contemplated that> a device of this charn
mal. energy level. Whe-n apparatus of the» type
ac_ter` may kbe employed for the detection- and
illustrated in Fig. 3 is employed, the operation is
measurement ofI neutron radiation resulting fromY
order to secure accurate measurements of the
reaction process, it isV necessary to prevent the
thermal neutrons which induce the reaction,_from
activating the detection and measuring ap
paratus To accomplishl this desired selective
measurement, the apparatus illustrated' in Fig; 8j
~ The neutron detector of the invention has sub
stantiallyl straight line response characteristics
includes a shieldv disposed over` the end of theV
ionization chamberA ¿i9 and over the end of- the
neutron
for neutrons
energies,
having
and, extremely
in operation,
wide
it has proven
c
neutron slowing and reflecting shield â?‘w-hich is
disposed about that chamber. This shield may
conveniently comprise a thin circularv plate 62; of.
cadmium, positioned as illustrated; The shield
plate 62 will act to ñlter out thereaction produc
ine` slow or thermal; energy- neutrons incident;
a most useful device. The» features ci the» in
vention, which are believed to be new, are eX
thereon, andA this will limit the response of the
detection and; measuring mea-ns to the neutronsl
resulting from the nuclear reaction.
symmetrically
spaced",
open-ended',
cylin
pressly pointed out in- the accompanying claims.
What is claimed is:
le. In', appara-tus of the class described, neutron
detection» means which is inherently» capable of
detecting neutrons oia particular limited energy
range, and means for increasing the operable
range of said detection means, comprising a body
of neutron slowing material disposed aboutv said
drically shaped cavities E3 of about the same
neutron detection means andy having an open
diameter as the» ionization chamber t9' are pro
ing transmit-ting neutrons possessing a detect
able energy to the neutron detection means.
2. In apparatus` of the class described, neutron
detection means which is inherently capable of
detecting neutrons of thermal energy only, and
a sleeve-like shield of neutron slowing material
disposed about said detection means and having
an opening to transmit thermal neutrons to the
detection means, the dimensions of said shield
being such that said shield' is effective to slow
neutrons of the highest energy level which it is
desired to detect' to thermal energy level,` and
the dimensions of said detection means being so
vided in the end oi the neutron slowing> and re
fleeting shield 5?» which faces the source of the
neutrons being detected or measured by the ap
paratus. The cavi-tiesl 53 extend intov the shield
5U fromV behind' the cadmium shield plate 62 for'
gamma radiation to a substantialY depth, as indi
cated in the drawing». The cavitiesA 63 serve to
broaden the range oi response of' the device to
neutrons of various energies.
If desired, other neutron responsive detection
means may be used instead of an ionization
chamber. For example, a cylinder' or other mem
ber oi gold, silver or other> material> which -be
comes radioactive when subjected to neutrorr.
irradiation may be inserted in lieu of’ the ioniza
tion chamber 4S. This member-'may later be re
moved and the induced radioactivity' measured'
shield- that relative
correlated
efficient detection
to the dimensions
of 'neutronscislowed
to thermal level by said shield will be accom
plished.
3. In apparatus of the class described,A neutron
detection means which comprises an elongatedV
In» use the neutron detector apparatus of the 75 chamber and which is inherentlyY capable of de
by conventional» methods.
2,556,768
7
8
gated chamber and which is inherently capable
tecting neutrons of a particular limited energy
range, and means for extending the operable
of detecting neutrons of a particular limited en
ergy range, and a sleeve-like shield body of neu
range of said detection means comprising a
tron slowing material which peripherally sur
sleeve-like body of neutron slowing material dis
posed about said elongated detection means and 5 rounds said chamber, at least one end of said
having an opening that is transparent to neutrons
chamber being substantially free of neutron slow
possessing detectable energies.
ing material, and said shield body being pro
4. In apparatus of the class described, neu
tron detection means which comprises an elon
vided with a plurality of cavities at the end there
of which is adjacent the end of said chamber
which is free of neutron slowing material.
10. In apparatus of the class described, neu
tron detection means which comprises an elon
gated chamber and which is inherently capable
of detecting neutrons of thermal energy level
only, and a sleeve-like shield of neutron slowing
material disposed about said detection means,
said shield having an opening that is transparent
to neutrons of thermal energy, the dimensions
of said shield being such that said shield is
eiîective to slow neutrons of the highest energy
level which it is desired to detect to thermal
energy level, and the dimensions oi said detec
gated chamber and which is inherently capable
of detecting neutrons of thermal energy level
only, and a sleeve-like shield body of paraffin
disposed about said elongated chamber, at least
one end of said chamber being substantially
free of neutron slowing material, and- said shield
body being provided with a plurality of cavities
tion means being so correlated relative to the di 20 at the end thereof which is adjacent the end of
mensions of said shield that eiñcient detection of
said chamber which is free of neutron slowing
neutrons slowed to thermal energy level by said
material.
shield will be accomplished.
5. In apparatus of the class described, neu
11. In apparatus of the class described, neu
tron detection means which is inherently capa
tron detection means which comprises an elon 25 ble of detecting neutrons of a particular lim
gated ionization chamber and which is inherently
ited energy range, means for extending the oper
capable of detecting neutrons of thermal energy
able range of said detection mean comprising a
only, and means for extending the operable range
body of neutron slowing material disposed about
of said detection means comprising a sleeve-like
said neutron detection means, and a generally
body of neutron slowing material disposed about 30 cup-shaped shield of neutron absorbing mate
said ionization chamber, said body having an
rial disposed about said detection means and
opening that is transparent to neutrons of ther
said body of neutron slowing material for absorb
mal energy, the dimensions of said body of neu
ing neutrons which approach said detection
tron slowing material being such that the prob
means 'from a direction other than toward the
ability of a neutron of the highest energy level .
which it is desired to detect passing therethrough
Without being slowed to thermal level is extremely
small.
6. In apparatus of the class described, neutron
detection means which comprises an elongated
chamber which has a length at least several times
its width and which is inherently capable of
detecting neutrons of a particular limited energy
range, and means for extending the operable
range of said detection means comprising a
open end of said neutron absorbing shield and
the body of neutron slowing material having an
opening facing the open end of the shield that
is transparent to neutrons of measurable energy.
12. In apparatus of the class described, neutron
detection means which comprises an elongated
chamber and which is inherently capable of de
tecting neutrons of thermal energy only, means
for extending the operable range of said detection
means, comprising a sleeve-like body of neutron
slowing material which peripherally surrounds
sleeve-like shield body of neutron slowing ma
said chamber, except at one end thereof, which
terial which peripherally surrounds said cham
end is substantially free of neutron slowing ma
ber except at one end thereof.
terial, and a cup-shaped shield of neutron absorb
'7. In apparatus of the class described, neutron
ing material disposed about said chamber and
detection means which comprises an elongated 50 said body of neutron slowing material for absorb
chamber, having a length at least several times
ing neutrons which approach said chamber from
its cross-sectional Width, and which is inherently
a direction other than toward the end thereof
capable of detecting neutrons of thermal energy
which is free of neutron slowing material.
only, and means for extending the operable range
13. In apparatus of the class described, neutron
of said detection means, comprising a sleeve-like 55 detection means which comprises an elongated
shield body of hydrogenous material with pe
chamber and which is inherently capable of de
ripherally surrounds said chamber except at
tecting neutrons of a particular limited energy
one end thereof.
range, means for extending the operable range
8. In apparatus of the class described, neutron
of said detection means, comprising a sleeve-like
detection means which comprises an elongated 60 body of neutron slowing material which peripher
chamber and which is inherently capable of de
tecting neutrons of thermal energy only, and a
sleeve-like shield body of paraiñn disposed about
ally surrounds said chamber, and a generally
cup-shaped shield of neutron absorbing material
disposed about said chamber and said body of
neutron slowing material for absorbing neutrons
said detection means, said body having an open
ing that is transparent to thermal neutrons, said 65 which approach said detection means from a
shield body being eiïective to slow neutrons of
direction other than toward the open end of said
the highest energy level which it is desired to
neutron absorbing shield, at least one end of
detect to thermal energy level, and the dimen
said chamber being substantially free of neutron
sions of said detection means being so corre
slowing material, and said body of neutron slow
lated relative to the dimensions of said shield 70 ing material being provided with a plurality of
that eñicient detection of neutrons slowed to ther
inwardly extending cavities at the end thereof
mal energy level by said shield body will be ac
which is adjacent to the open end of said cup
complished.
9. In apparatus of the class described, neu
tron detection means which comprises an elon
shaped shield.
14. The method of detecting neutrons of vari
75 ous energies by the use of a neutron detection
2,556,768
9
means which is responsive only to neutrons of
a particular limited energy level which comprises
10
ing lengths to the detection means, whereby only
the neutrons having thermal energy are detected
adjusting the energy of substantially all of the
neutrons to be detected to said responsive level,
through the shortest path and neutrons of greater
thermal level the energy of substantially all of
the neutrons to be detected by transmitting the
neutrons to the detection means through slowing
material providing a plurality of paths of vary
FOREIGN PATENTS
Country
energies are detected through the longer paths.
by transmitting the neutrons to the detection 5
JOSEPH L. MCKIBBEN.
means by a plurality of paths, each path con
taining a means to adjust the energy of the
REFERENCES CITED
neutrons transmitted therethrough, and each
The
following
references are of record in the
path adjusting the energy of the neutrons passing
file of this patent:
therethrough by a diiîerent amount than the 1o
UNITED STATES PATENTS
other paths.
15. The method of detecting neutrons of vary
Number
Name
Date
ing energies by the use of a neutron detection
2,206,634
Fermi et al ________ __ July 2, 1940
means which is responsive only to neutrons of
2,345,119
Hare ____________ __ Mar. 28, 1944
thermal energy which comprises reducing to
2,408,230
Shoupp __________ __ Sept. 24, 1946
Number
465,045
Date
Great Britain _____ __ Apr. 26, 1937
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