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2,4%¿67
DIFFERENTIAL ION CHAMBER
Filed Jan. 20, 1944
2` Sheets-Sheet 2
{oQ.wP¿izfegasl}
E.F15.
5»0§ 0
.370
20
Patented pr. 20, 1948
nel) sTATas
2,440,167
A
DIFFERENTIAL 10N C ~1
y“,
James W. Broxon and William P. Jesse, Chicago,
Ill., assignors to the United States of America
as represented by the United States Atomic En
ergy Commission
Application January 20, 1944, Serial No. 518,972
20 Claims. (Cl. Z50-83.6)
l
.
2
addition, serve together with the cylindrical elec
trodes C1, Cz, and Ca, to define the volumes of
Our invention relates to a device for measur
ing radiation intensity, particularly that of slow
regions or sub-chambers Ci-Cz and Cz-Ca, re
,
spectively. The purpose oi’ the guard rings may
An object of our invention is to provide an
be better understood by considering the iield
electrical measuring device for measuring the
conditions, for example, involving insulators 20
effects of slow neutron radiation separately from
and il. The upper insulator 2li is subjected to
those `of other penetrating radiations, such as
an _intense electric ñeld due to the potential
gamma rays and fast neutrons.
diiîerence maintained between electrode C1 and
A more speciiic object of our invention is to
provide a device in the form of a differential ion 10 the guard system which includes plate 9. Some
surface 1eakage is likely to occur even on very
chamber which is selectively operable to meas
good insulators. Also, insulators subjected to
ure either the combined effects or the separate
neutrons.
effects of slow neutrons and other penetrating
radiations such as gamma rays and fast neutrons.
Other objects and advantages of our inven
tion will become more apparent from the fol
strong electric fields become polarized, that is,
the positive and negative charges thereon are
15 separated, but not suñlclently to be pulled out of
. their original atoms or molecules.
Either sur
face charges or virtual charges due to polariza
tion of the insulator would añect the potential
of the collecting electrode Cz unless prevented
lowing description and drawings, in which:
Fig. 1 is a cross-sectional view of a differ
ential ionization chamber >wherein the electrical
circuit is illustrated schematically; Fig. 2 is a 20 from doing so by `a constant-potential conductor
such as the guard ring 8. Since surface leak
graph showing galvanometerA deiiections due to
age in particular is likelyto' be quite erratic,
neutron-induced. currents plotted against the
shielding of this nature is important. Refer
voltage applied to one of the electrodes of the
ence is now made to the lower insulator li lo-_
device shown in Fig. 1 and showing a plurality
of “saturation” curves for different radiation in 25 cated between electrode C2 and end plate lll of
the guard system. Since the potential diñerence
tensities.
I
'.
’
Referring more particularly to Fig. l, Cl, Cz,
between these conductors is never allowed to be
and O3 denote separate cylindrical electrodes
come very great, it is possible to prevent appre
which, in conjunction with a guard system to be
ciable polarization of lower insulator ll. How
described hereinafter, denne two adjacent sub 30 ever, electrode C3 is maintained at a potential
dilîering greatly from that oi’ the guard system.
chambers or regions, Ci-Cz and Cz-Ca respec
tively. These regions are of substantially equal
Consequently, guard ring 23 is desired to insure
volume, for example, about 800 c. c. each.V The
that lower insulator Il will not become polar
outermost region, Cz-Cs, has interiorly on its
ized under the influence of the strong electric
walls a coating of material which emits alpha 35 ñeld between electrode C3 and the guard sysì
particles when bombarded by slow neutrons, such
~ tem.
Such polarization of insulator ll would
añect the potential of C2 and hence change the
bide ls very suitable since it can be readily ap
reading obtained with current indicating instru
plied as a coating. A certain potential rela
ment 3. Since' the inner edges of these guard
tive to the guard or instrument case is applied 40 rings partially deñne the eñ’ectlve volumes of the
to electrode C1 by a battery l and a potential
sub-chambers or regions, then by making the
of opposite sign is applied to electrode C3 by'a ' end plates 8 and lll vertically adjustable, say by
battery 2, whereas the collecting electrode Cz.
the introduction of gaskets of diiîerent thickness
assumes a potential. not d_iiîering greatly from
between the end plates 9 or [email protected] and the shoulders
that of the guard or instrument case by virtue 45 of outer casing S,- or by merely replacing these
as one containing boron or lithium. Boron car
of a connection throughs:I current-indicating in
strument 3, such as an electrometer or a galva
nometer, which is connected between electrode
Cz, and a metallic 'outer casing 4. Casing ß
may be grounded if desired. A plurality of cylin 50
rings with ringsvof different sizes,'these volumes
are made adjustable.
'
comprising the guard system are supported on
end plates 9 and I0 (for example, as shown) and
serve to minimize errors due to insulation leak
Electrode C3 is supported by a plurality of
insulated supports and spacing assemblies such
as l2 each provided with insulation washers l2a.
An insulated lead-in le employing insulation
washers lia is provided for electrode C3. Elec
trode C1 is supported by insulating washers 20
from end plate 9 much as C; is supported from
age and electrostatic induction effects and, in
end "plate lq, `-Electrode Ca-is supported on end
drical metallic guard rings- 5, -6, l, 8, and 23
i
2,440,167
3
plate I0 by insulation washers li which elec
trically insulate this electrode from said end
plate. Washers Il, I2a, Ma, and 20 are prefer
ably made of quartz, amber or other excellent
insulating material. However, the insulation for
C; and C3 need not be so extremely high as the
insulation for C2, and consequently washers Iza,
Ma, and 20 may be made of lower quality of ma
terial although they must be capable of with
4
lisions with the gas. Hence. for each gas therel
is an optimum pressure depending on the ‘elec
` trode spacing and the coating material that will
just allow the alpha particles to travel the region
between the electrodes. Such optimum pressure,
in a given case, can be readily determined by test
or calculation.
The operation of the device is as follows:
Ii’ a positive potential (say about 360 volts) is
standing several hundreds or even more than a 10 aplied to'electrode C1 and negative potential (say
thousand volts.
Small electrical leaks over the s
washers |2a, Ha, and 2t are not important since
they do not register on the indicating instru
about 1035 "volts) is applied to electrode Cs, it
will be apparent that if the device is subjected to
slow neutron radiations and other vpenetrating
ment 3. The end plates i6 and 9 are secured in
vacuum or pressure .tight relatlonship'to the 15 radiations such as gamma rays and fast neutrons.
all occurring simultaneously, the other penetrat
outer casing 4’ by means of a plurality of set
ing radiations (gamma rays, fast neutrons, etc.)
screws such as Il and I8 which are symmetrically
will either directly or indirectly ionize the gaseous
spaced around and pass through rings i5 and I6,
medium
in intercommunicating regions Ci--Cz
respectively. The rings l5 and i5 are threaded
into the ends of the cylindrical case 4, and either 20 and Cri-Ca. Gamma rays bombard the gas in
cluded in the electric field and produce beta rays
these rings or the screws il and I 8 are used to
in an amount proportional to the volume of such
exert pressure upon end plates 9 and i0, forcing
gas. Negative ions developed in region Ca-Cs
tongues on these against gaskets of material such
move towards electrode Cz, whereas negative ions
as rubber, tin, lead, or other yielding material
located in grooves as shown near the ends of the 25 developed in region Ci-Cz will move away from
electrode C2. Inasmuch as the _volumes of these
cylindrical case 4. The several insulating wash
regions, as well as the pressures, are substantially
ers supporting electrodes C1 and
C2 and support- '
ing the lead-in for electrode Ca, together with ‘ equal, the net munber of negative ions collected
by C2 is zero ii’ conditions are ideal, and as ex
associated systems of thin gaskets, metal wash
ers, and nuts, as shown,. serve to complete the so plained hereinbelow, is close enough to zero _for
practical purposes, under ordinary conditions of
vacuum seal. As will be understood by those
operation of the device. A similar situation exists
-relative to the positive ions in the two regions.
Hence, so far as the eifects of `gamma rays or
in end plate i0 (or in end. plate t' or case 4),
through which the chamber may be evacuated 35 fast neutrons are concerned, there will be no
reading on the galvanometer, electrometer, or
and ñlled with various gases.
other indicating-instrument 3. In 'other words,
The lower cover plate 2l _serves to keep out
dust. It is fastened by one or more bolts 24 which '_ ' ion currents due to gamma rays and fast neutrons
are edectively cancelled, apart from discrepancies ~
pass through apertures in the plate 2| and into
familiar with the art, suitable means may also '
be provided such as a needle valve (not shown)
due to divergence of radiation beams and absorp
tion and production of subsidiary radiations- by
dust and _is fastened by .one or more bolts (not " y' thechamberwalls, electrodes, etc. These uncan
shown) similar to bolt 24. vCover-plate.21',rto- ’ celled eß’ects are commonly small. However, _the
slow neutron radiations, upon _bombarding the
gether with `a tube 26,> continues- the' guard sys
tern for connecting _electrode Cà. andthe _lead-in 45 interior boronäcarbide'coating of 'region Ca-_Ca
will cause emission of -alpha'particles from such
21 to the current indicating device 3.. _ ‘
‘
boron-containing coating whichv will ionize the
The entire chamber is-preferably filled with an
corresponding threaded bores 25 in-.plate 9i.> Sim
ilarly,- the upper. cover plateg22 serves to keep out
gas in region Cz-Ca but not in region C1-_--C2
because the thickness of C2 is too great for the
alpha particles to penetrate; and there are no4
inert gas such as helium or argon or similar gas,
at or near atmospheric or even athigher pres
sures, with which .gas it is easy to -attain satura-v
tion currents at 'relativelyA high radiation inten»
locally-produced alphal particles to cause ioniza
tion in region Ci-Cz, since Ci-Cz is not coated
sities,` using ordinary _applied voltages. Helium
interiorly. Hence, an excess 'of ions will be de~
veloped in region Cz-Cs'on acount oi.' the slow
. appreciable ionization is: produced by the neutron 55 neutrons, and the excess negative ions will be
collected by electrode C2. Current will thus flow
induced alpha particles: saturation currents with
through the indicating device 3, this current
a particular electrode conñguration and. radia- 1
" being practically proportional> to the radiation
tion intensity can be readily obtained with rela
intensity of the slow neutrons which is sometimes
tively small applied potential diiîerences; these
expressed by 1wl (neutron »densityxvelocitm ~in
gases are not chemically active and willnot cause
vneutrons per square cm. per‘sec.) V‘provided the
deterioration Aof` the insulation or other exposed ' »impressed pûtential différence-S
are .sumcient to
parts; 'in the event of leakage from the container, '
practical saturation currents. the escaping gas is not harmful; and helium does v’ v provide
If instead or
__
.
and argon are especially suitable lbecause -they
have the following advantageous'characteristics:
measuring theeilects of slow-neu- '
not become undesirably radioactive when bom
barded by neutrons, although argon may become 65 trons, as-described above, it is desired to measure
>the combined eiîects of slow neutrons and of other ’
somewhat disturbingly radioactive ii’ subjected
penetrating radiations, such as gamma rays and
to'very intense neutron radiation for long in
tervals of time. lSaturation is more diill'cult to ' ¿fast neutrons, electrodes C11-'and Ca-»are brought
obtain the higher the pressure `because recom--_- '_ ¿tov potentials of the same sign relative tothe guard
system. This can be, done readily. by reversing
bination of the ions is more dimcult to reduce at
ï switch iti.> Now the negative ions in 1‘egion'Ch---higher pressures. However, at low pressures the
C2 will i'low towards the electrode C: as well as
range of the alpha particles is increased and the
alpha particles may go clear across to another y ‘~ those in region Cz-Ca. Hence, the combined
electrode Aand waste- some of their energy that ' » current will be that produced by the slow neutrons
in region C12-Ca, having the interior boron car
otherwise could have been used in» ionizing> C01“ 7 Vbide
coating, plus that produced ’_ by. the other
g.,
2,440,167
5
penetrating radiations such as gamma rays and
fast neutrons in both the regions Ci-Ca and
Cz-Cs. If it is desired to measure the effects.
of penetrating radiations due to gamma and fast
6
bination, a pair of sub-chambers having wall
portions constituting electrodes, only one of said
sub-chambers enclosing a material which emits
alpha particles when bombarded by slow neu
trons, the other of said sub-chambers being de
void ot said material, means for applying elec
trical potentials to each of said electrodes, said
electrodes including collecting electrode means
for collecting ions formed in both sub-chambers,
neutrons only, the previous reading due to slow
neutrons alone is subtracted from the latter read
ing and will give the combined ion currents in
both regions, Ci-Cz and C2---Ca. Since these
regions are of equal volume, half this reading will
denote the ion current in one of the regions due 10 and means for measuring the ñow of the collected
ons.
only to gamma rays and fast neutrons. In this
-2. Apparatus as recited in claim 1 in which
manner the relative intensities of slow neutron
said sub-chambers are of substantially equal vol
and other penetrating radiations occurring, as
ume and include electrostatic guard means which
well as the absolute value of either, can be readily
partially define said volumes.
determined upon appropriate calibration of the
3. Apparatus for measuring the radiation in
apparatus.
tensity of slow neutrons separately from that of
Fig. 2 shows a graph of galvanometer deiiec
other coexistent penetrating radiations such as
tions versus voltage applied to electrode C: (the
gamma rays or fast neutrons comprising, in com
currents being due to slow neutrons) and shows
a plurality of “saturation” curves representing 20 bination, three electrodes which define a pair of
sub-chambers, the interior walls of only one of ‘
diiîerent radiation intensities. In obtaining these
said sub-chambers being coated with a material
curves, the potential applied to C1 was made equal
which emits alpha particles when bombarded by
and opposite to that applied to C3 for voltages
slow neutrons, the other of said sub-chambers
less than 315 volts, but the potential applied to
C1 was then kept constant while that applied 25 being devoid of said material, means for applying '
electrical potentials to two of said electrodes, the
to C3 was increased to higher values. This was
third electrode serving as a collecting electrode
due to the fact that such potentials were more
for collecting ions from both sub-chambers, and
than adequate to provide saturation in the inner
means for measuring the ion ñow to said collect
region, i. e. in the region free from boron. It
will be noted that the voltage applied to electrode 30 ing electrode. '
_
4. Apparatus for measuring the radiation in
C3 should be one which is sufficiently highrso as
tensity of slow neutrons separately from that of
to effect practical saturationof the chamber and
thus to allow operation in the region of the ap
other penetrating radiations such as gamma rays
proximately flat part of the “saturation” curve.
or fast neutrons comprising, in combination,
That is to say, ii’ practical saturation is obtained, . three coaxial cylindrical electrodes which denne
a pair of sub-chambers of substantially equal vol
recombination of the ions is substantially elim
inated and the ions are collected substantially
ume, the interior walls of only one of said sub
as fast as they are formed. By operating with
chambers being -coated with a material which
an electrode supply voltage corresponding to a
emits alpha particles when bombarded by slow
region well within the nearly fiat or “saturation"
neutrons, the other of said sub-chambers being
part of the "saturation” curve, ñuctuations in the
devoid of said material, means for applying elec
supply voltage will have negligible eiîect on the » trical potentials to two of said electrodes, the
currents collected and the currents will be sub
third and intermediate electrode serving as a col
stantially directly proportioned to the slow neu
lecting electrode for collecting ions from both sub
45
tron radiation intensities when the potentials of
chambers, and means for measuring the ion iiow
Ci and C: are of opposite sign relative'to the
through said collecting electrode.
`
guard. Each of the curves represents a different
5. Apparatus as recited in claim 1 together
radiation intensity. It will be noted that if the
with means for selectively applying potentials of
applied voltage is about 150 volts or more, prac
opposite sign or of the same sign to said first
50
tical saturation values are obtained with the ra
mentioned two electrodes for measuring slow neu
diation intensity employed.
tron intensity either alone or taken together with
The diiïerential ion chamber following the
the intensity of other penetrating radiations, re
teachings of our invention is particularly advan
spectively,
tageous in substantially eliminating the errors
6. Apparatus for measuring the intensity of
otherwise caused by radioactivity of the chamber 55 slow neutrons exclusive of that of other penetrat
parts or the contained gas after they are sub
ing radiations such as gamma rays or fast neu
jected for long periods to intense radiations which
may induce radioactivity of the chamber or its
contents. Since radiations due to such induced
trons comprising, in combination, three elec
trodes which deñne two sub-chambers,- only one
of the sub-chambers having its interior walls
radioactivity would produce substantially equal 60 coated with a material which emits alpha par
and opposite effects in the regions C1-Cz and
ticles when bombarded by slow neutrons, means
Cz-Cs, they would substantially cancel each
for applying electrical potentials of opposite sign
other and the meter reading would not be ma
terially affected thereby, when the instrument is
to two of said electrodes, the third electrode being
an intermediate collecting electrode for collecting
being employed for the measurement of slow neu~ 65
ions from both of said sub-chambers, and means
tron radiation as differentiated from other pene
for measuring the flow of ions to said third col
trating radiations.
It will be readily apparent that modifications
may be made without departing from the spirit
and scope of the following claims.
Y
We claim:
1. Apparatus for measuring the radiation in
tensity of slow neutrons separately from that of
other coexistent penetrating radiations such as
gamma rays or fast neutrons comprising, in com
lecting electrode.
7. Apparatus as recited in claim 1 in which said
70 sub-chambers are filled with an inert gas at a
pressure of the order of atmospheric pressure.
8. Apparatus as recited in claim 1 in which said
sub-chambers are ñlled with argon.
9. Apparatus as recited in claim l in which said
75 sub-chambers are filled with helium.
7
2,440,167
10. Apparatus recited in claim 1 in which said
material contains boron.
l1. Apparatus recited in claim 1 in which said
material contains lithium.
l2. Apparatus recited in claim 1 in which said
18. The method oi' measuring radiation inten.
sity of slow neutrons separately from that of other
coexistent penetrating radiations, such as gamma
rays or fast neutrons, that comprises the steps
of developing ion currents resulting from gamma.
material comprises boron carbide.
and fast neutron radiations, directing said ion
13. Apparatus recited in claim l in which said
currents in opposition to nullify said ion currents,
sub-chambers contain an ionizaole gas at an ap
developing an ion current resulting solely from
propriate pressure so that practical saturation
slow neutron radiationI collecting said last men
currents are readily attained while e?cient use 10 tioned current, and directing it through a current
is made of the ability of the alpha particles to
indicating device.
produce ions.
19. The method of determining the percentage
14;. Apparatus recited in claim l in which said
of gamma and other penetrating radiation in
potentials are sumciently high to insure practical
tensity compared to slow neutron radiation in
saturation currents in both said sub-chambers
tensity that comprises the steps of applying po
and said potentials are within a region corre
tentials of opposite sign to a pair of electrodes in
Y- sponding to the substantially fiat portion of the
a gaseous atmosphere,V one of said electrodes being
saturation curve oí’ current flow plotted versus l
coated with a material that emits alpha particles
potential applied to one of the ilrst mentioned
electrodes, -
when bombarded by slow neutrons, measuring ion
20 current ñow between said electrodes and a third
15. Apparatus as recited in claim ‘i in which
electrode, then applying potentials of the same
-electrostatic guard rings are provided closely ad
sign to said pair of electrodes, measuring the ion
jacent to the edges oi' the cylindrical electrodes
curent :Elow between said pair oi’ electrodes and
for defining the volumes of the chambers and for
said third electrode, and then comparing said two
reducing insulation leakage and electrostatic iri-v 25 values of measured ion current now.
duction eñects.
20. The method of determining the effects ci
16. The method of determining the percentage
slow neutron radiation intensity exclusive of the
of gamma and other penetrating radiation in
eiïects of gaa and fast neutron radiation in
tensity compared to slow neutron radiation in
tensity that comprises, applying potentials of cp
tensity that comprises applying potentials of op 30 posite sign to a pair of electrodes in a gaseous
posite sign to a pair of electrodes contained in
atmosphere, one of said electrodes being coated
an ionizable atmosphere, only one of said elec
with a material that emits alpha particles when
trodes being coated with a material that emits
bombarded by slow neutrons, and measuring the
alpha particles when bombarded by slow neutrons,
ion current iiow between said pair of electrodes’
measuring the net ion now, then applying poten 35 and a collecting electrode.
tials oí' the same sign to said pair of electrodes
and measuring the total ion dow, then compar- ’
JALEES W. BROXON.
ing the two values oi ion ñow.
WULIAM P. JESSE.
17. The method of measuring radiation inten
sity of slow neutrons separately from that oi.’ other 40
REFERENCES
coexistent penetrating radiations, such as gamma
The following references are of record in the
rays or fast neutrons, that comprises the steps of
ñle of this patent:
developing a plurality of ion currents resulting
UNITED STATES PATENTS
from gamma and fast neutron radiations, nullify
ing said ion currents, bombarding with slow neu 45 Number
Name
Date
trons a material that emits alpha particles when '
so bombarded in an ionizable atmosphere having
a voltage gradient, thereby causing anion current
flow, and measuring said last mentioned ion cur
rent ñow.
50
2,220,509
2,288,718
2,349,753
Brons _____________ __ Nov. 5, 1940
Kallmann _________ __ July 7, 1942
Pontecorvo _______ __ May 23, 1944
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