US2521656A - Ionization chamber - Google Patents

Description

Sept. 5, 1950 E. G. SEGRE ETAL 2,521,656

IONIZATION CHAMBER Filed Dec. 13, 1945 4 Sheets-Sheet 1 WITNESSES. 1N VEN TORS- Emilio G. Segre 7 W W BY Owen Chamberlain Wm Was-w Sept. 5, 1950 E. a. SEGRE ErAL 2,521,656

IONIZATION CHAMBER Filed Dec. 15, 1945 4 Sheets-Sheet 2 WITNESSES. IN V EN TORS Emilio ,G. Segre BY Owen Chamberlain Sept. 5, 1950 E. G. SEGRE ETAL 2,

IONIZATION CHAMBER Filed Dec. 13, 1945 4 Sheets-Sheet 3 W] T NES SE5 INVENTORS Emilio G. Seg

g M M BY0W8I1 Chamb lain Sept. 5, 1950 E. G. SEGRE EI'AL 2,521,656

IONIZATION CHAMBER Filed Dec. 13, 1945 4 Sheets-Sheet 4 INVENTORS Enaili'o G. Se re YOwen Cham erlac'n Patented Sept. 5, 1950 I UNITE-D "STATES PA Y dd 82,521,655?" U q gidnizmiomnmee l Emilio G. Segre, Samara ,N. Mex.) and own;

1 Chamberlain, Philadelphia; Pa.', as'signorsto T the United States of America asrepresented by a "theUnited ,States Atomic -Energy commission 2 Ap lication e m a 13,1 e45, s1'-ia1.Ng. 634,354 4"Clai m s.; (pres o-e as) l V i This invention, relates to anjimproved ioniza tion chamber.

Aprim'ary object of the invention resides in the provision of an improved ionization f chamber adapted to increase the counting rate over-prior art chambers, therebyresulting in increased, efilciency of the device.

In certain nuclear physics experiments in which radioactive materials are to be measured, it is desirable to'increase the counting speed of an ion chamber used to detect-neutrons from a' sample by increasing the effects produced by the slow neutrons directedinto} the chamber from an exterior source withoutbhanging'the source of said neutrons. Itis knownt'o effectively increase the flux of thermal energy neutrons as measured b chamber of metals snch'as' 'aluminumfalloys which have low neutron absorption propertiesin place of metals of higher neutron absorption properties such as brass. While the use o-f an hydrogenous material such as polystyrene or paraflin for the purpose of scattering and reflecting 'o'r diffusing neutrons is known per se, such material having been commonly employed as a material in which ion chambers are enclosed or housed for the above stated purpose, the'present invention employs such material in a novel manner, resu 1t-- 7 ing in a substantial i' crease in the counting-rate over known prior art devices of thi to provide an improved ion chamber wherein hydrogenous materialperforms the dual function of insulatingly supporting the various metal portions of the chamber while substantially filling the interior portion of the chamber to provide improved scattering, diffusing, and slowing of'fast neutrons to thermal energies,"th'e:percent-' age of fast neutrons slowed to thermal energies to cause ionization in the present device being substantially larger than percentages heretofore obtained with prior art devices. l

a chamber of known type by constructing the general type; A further object of the present invention is A still further object is to provide an ion cham ber of improved design which has particular utility in obtaining quantitative measurementof the fissionability of radioactive samples, the chamber being subjected to slowed neutrons from a remote source of known strength or flux.

Further Objects and advantages of. the inven tion will become apparent to persons skilled in the art upon examination of the drawings a'nd the description, the inventive aspects of which are defined by the appended claims. I

In the drawings, in which like parts are desigj hated by the same reference numere'vlsz'- 0f the def ened in'serts1 23 'engaged in plate It in the ma Figure l taken atf angularit'y toth'e' se x Figure 1S a rotated duartensectional v l -l-,-of-Figuresl'and2. P 1 Figure 4: is an enlarged fragmentary 'view of a; marginal. portion of the; cathode assembly 4'2, 3 Figure l, for the purpose of illustrating censured; new details of mean .for'thedetachable'e gagement of samples within the device Figured illustrates the manner in ices-are associated with a spacedneu- "eing awesome; asamplej of known material placed in one" chamber and a sample material placedin the other. H a Referring to Figure ;1, the device includes" an inherently rigid metallic baseplate In of fcircu lar configuration having suitably attachedto'the left face, as viewed, a'fianged annular sleeved-fl} interiorly threaded at 12 for mounting, prefer? ably in the position shown, to an 'ex'ternally threaded supporting device, 'not shown. Base plate mane sleeve ll, as well as other' nietallic; housing and supporting elements later de'scribe with the exception of sample supporting'pl'atef are preferably of metal having low neutreh ab? sorption properties, for examplealuminu'm or aluminum alley, or other suitable metali Plate 10 also provides support for a cap-like closed end cylindricalhousing l5 having atthe mounted end thereof anintegral annular'flange "H- pro vided with an annular groove in whichis disposed a resilient'gasket l8. Gasket I8 is maintained in pressure engagement between plate I0 and flange I1" byfmeans or a plurality ofcircumferentially disposedclamp screws 20 extending throughsuitable bores in flange I! to threaded-1y 'engag'ejhardner'shownp" w 1 'Dispo'sedwithin housing 1 5, in spaced relation to the side and closed end ane thereto; and'in contiguous" relation to the interior face of plate It, is ahollow cylindrical element 25 ofhydr'og'- enous material "having good 1 dielectric" proper-' ties, such'f'as =1ucite',".a methyl'meth'acrylat'e polymer. Element 25 is rigidly. maintained by clamping: engagement with cylinder 25,- also 'of" hydrogeiious rnaterial, cylinder 26 being clamped? to. plate H} bye plurality of circumferentially' disposed clamp screws 28. Afmetallic' cylinder 30, providing a guard ring; adapted to be grounded, includes a relatively thick portion adjacent plate In and :a portion of reduced thickness extending outwardly therefrom, cylinder 38 being interposed intermediate element 25 and cylinder 26. The assembly comprising element 25, cylinder 38, and cylinder 26 is -maintained clampedfibymeans-of plural;screw-s 312;; A fluid t;- 34. ex nd thro l t L o. he. n: terior of the device, which is otherwise sealed to the atmosphere.

channel 38 to the volume between housing l and element 25, channel 31, Figure'2, leading therefrom to the volume defined intermediate element and cylinder 28 to insure.gascommuni;

cation therebetween.

A pair of electrode assemblis. 4 1] ..and;4.2:.." r mounted respectively on the free endsof inner cylinder 26 and outer cylindrical element 'zi in the manner hereinafter described. As best shqwn l.

in Figure 2, a circular plate 4|, providing an anode.. its support as iollows; An ann ular metallic flange. 4,4;i's provided with a. suitable numberof integral lugs, 4,5 for: recessed engageintent; with complementary, apertures provided in thee'nd wall of cylinder 28, each such aperture ha ing an. a ne ntr lly; hr ad in i rta" n' helqedf h vdr een flisn l ndw a te i l.- n suit'able; clamp screw 4.8 extends .Jthro lug to threadedly engag'efinsert 4.1.and-Clamp qangam toitheface of cylinder 26.

Hangs 4. is. furt q=D Z d wi h. m ltiple sockets. '5.8..embedded the..m ateri al of cylinder 25.; at nonts dntermediate lugs 45, sockets 58,104 ing adapted to .detachabl'y, receive. snap-in, plugs 5 whiehfare secured to plate 4l.f0r th e=remov rees; a$ssmblv 2; s ud s an lar su pplt n fiallssfii to hi slemp a irculerrc ppes lat i ple u s. asses ed irc m eren iallya' esn cttofianee 5:, tassbowainemarsed detail K F ure, t fix r vides. su portfor a at he t; c mpedqth retosbyz brackets l; o providei s orms a plezmaterialz as; w ich may, be electrQD ated to. sheet..-fi6:- hroughe ut he en a ar a t e A admium plate 5;, s i s c a l mount dsto per .-.-plate 6.3; to provide a shield for slow neutrons, laterldescribed; Asshown. n i 1, the annular fiange' 511 supported inthe assemblyby means of;a marginally ribbed portion 58;;imbedded; in theendrwall 01' element 25 and securelyrmaintained therein by means of a plurality of clampscrews 58 which hreadedly, en age. associated imbedded inserts 5. L Similar toinsertl lz'ltdescribed above. Acylin drsical cadmium sleeve 'ldmay be removably-dise posed within housing 55. to provide additional; isisinse o Wfi H S s. d s ed. el

Suitable electrical conduits. extend from' the electrode asembliesAlland 42 to the exterior of Conduit 34 leads through.

4 the device, cathode assembly 42 being connected by lead 68 through tube 36 and through a suitable glass seal "H. Anode assembly 40 is connected by lead 12, through suitable conduits 13 and 14, and glass seal 15. I

Referring to Figure 5, a pair of the improved .ion chambers, above described, designated and .84; are disposed within opposite} end portions of a block of hydrogenous material 83 enclosed within a lead shield 84. Chambers 80 and 8| are mounted on slidably mounted plugs 85 and 88,

' -also-of hydrogenous material to permit insertion and: removal; of, the chambers within apertured endPQrtions of} block 83 to provide complete en- 'ClQSllrB:OfgthflhflfllbGlS by the block material. A

sourcesofz fast.neutrons 88 may be removably positioned midway between chambers 88 and 8| by rod 90, slidably associated within a vertical channel-.-9:'l;;provided in block 83. Source 88 is suitably attached to the lower end of rod 90. Lead blocks Maud 95.x! .Qvi e pr tective.s eldinsareundthe mouth of channel' l. Dispositionoflthepohrponents; of; the, device arasueh thation chambers 8.!,;and;8l aresubjected to the-same magnitudeof thermalenergy neutron flux from source 88..

Qpe 1 'a.tion. of j the; device as}, follows; .With housing l5 andcathode plate 63, removed from bothg'ehambers. the laminatedfanodendisc'4iis snapped into place-, c aldmium cylinder ill-inserted within housing 15 ai ter 'whichthe samplegca rry ing cathode disc 83 1s mounted as above,,with cad mium disc It; attached and housing l5; is ree p a ed. amolam t ria nq n fissi a e s. eles si a es m e r wi h. a to measured placed in the other chamber. w th mi ble-known e acu i an a n aratus en s t d ossnflW-M: t es i e s; pumped e appr x m t one millime r o msrs r n s ntemal y. Wash i h: e se I a a knownmanner, after which-argon isintroduced to a. pressure of; one and one-half atmospheres. Conduit 3.4 is then suitably sealed-and the-.assoe ci d; pparatus d s onnect With".- a; w hambers! tabl mo nted th n- 1 h hy 0a use-mat ria ass ow in. F ure n leads. 63;; and T2 of eacl chamber connected to indie vidual; counties-c1 quits-of;atype-knownwin the r thepparat si ready;fonon ration...

Since. theaters e; evice is primarilyuintended for s nor he the ount g; rate-0f. fiss on: d e 7 slow neutrons, and since the obseryed fission rate is dependent to. some degree. upon .the', presence or ifast neutrons, the; percentage. of, fission due to fasthneutrons may be accurately measured and subtractedifrom;the.totalfissioncount.in:,the. fo1

lowing, mannenv Itxis known. that ;.cadmium-. is opaque. toilslowrmeutrons. hence: byssubstantially enclosingmhe effective chamber area by cadmium shielding the. fission count duetdufastheutrons may be observed. This count. isaobtaineot with cylindrical shield. 10;: circular. disc :3.1Tdi; the anode assembly containingucadmiumz disc. 55;'= in place .as. shown .in'..Figurel 21-and2 above. described.

Whileethisshielding does. not present abomplete. barrierlto. the .transmissionof. slow neutrons due to. some leakageuofeneutronsi marginally. of disc. l Qawhich: maybeldiffusedbythe.hydrogenous material. within the. counter, it .has been. empiri: callyz determinedgth'at ;this leakage. factor-is small in; respect to, the slowlneutrons .stoppedhby :th'e cadmium; By interchange ofxsamplesubetween the chambers, errors ;-due,- to smallrdifierenoes bee tween the chambers. fllieeliminated; Pursuant to btain n hse mmt without-thecadmiuml shield, cylinder intend-the disc loareremoved andvthe sample material anode containing disc 55 replaced with the Duralumin anode and the count obtained which is due to fission caused by both fast and thermal energy neutrons. The count obtained by fast neutrons above is then subtracted from the count obtained by fast and slow neutrons to obtain a count due to the slow neutrons only. The independent recording devices mentioned may be electronic devices properly biased or otherwise adjusted to record counts above a predetermined threshold value.

Neutrons radiating from source 88 are largely slowed to thermal energies prior to entering the chambers 86 and 3 I While a large percentage of fast neutrons are slowed by the block material prior to entering the chambers, the presence of a substantial amount of hydrogenous material within the chambers presented by elements 25 and 26 provides additional slowing means for neutrons entering the chambers at above thermal energies especially with the cadmium elements removed. Since the metallic components of the device other than plate 63 are of aluminum alloy or similar metal, neutron loss through absorption by the components of the device is relatively small. Neutrons impinging upon the sample 69 of each chamber cause fission which in turn causes ionization of the gas within the device. The number of ionization pulses appearing on the anode is indicative of the fission rate of the sample and since one such rate is known, the other is computed on a comparative basis.

We claim:

1. An ionization chamber having in combination, a metallic housing sealed from the atmosphere, an hydrogenous material disposed within said housing, said material filling a major portion of the volume therein, an ionizable gas maintained within said housing to fill the volume intermediate said hydrogenous material and said housing, a pair of electrodes mutually spaced within said housing and insulatingly supported from said housing by said hydrogenous material, means preventing the transmission of slow neutrons from a point exteriorly of said chamber to said hydrogenous material comprising protective elements opaque to the transmission of slow neutrons substantially covering said electrodes and the interior of said housing, a substantial portion of said electrodes being exposed to said ionizable gas, and means maintaining said electrodes at a difference of potential.

2. An ionization chamber comprising a hollow cylinder having one closed end and one open end, said cylinder being of low neutron absorbing material, a hollow cylinder of high neutron absorbing material adapted for removable disposition within said first cylinder, a closure element for the open end of said first cylinder, means for clamping said closure element in fluid sealing engagement with said first cylinder to seal the cylinder interior to the surrounding atmosphere, a solid cylinder of hydrogenous material concentrically disposed within the first cylinder and supported at one end on said closure element, means associated with the unsupported end of said solid cylinder for the removable attachment of a planiform electrode, a metallic guard ring circumferentially enclosing said solid cylinder, a hollow hydrogenous cylinder circumferentially enclosing said solid cylinder and said guard ring, means for clamping said hollow hydrogenous cylinder and said guard ring to said solid cylinder, said hollow hydrogenous cylinder axially overlapping said solid cylinder at the unsupported end thereof, means associated with the overlapping end of said hollow hydrogenous cylinder for the detachable engagement of a second planiform electrode in parallel spaced relation to said first electrode, said detachable means permitting interchange betweenelectrodes of low neutron absorbing properties and electrodes of high neutron absorbing properties, electrical conduit means associated with said electrodes and extending to the exterior of the chamber to permit application of a difierence of potential therebetween, and means for maintaining an ionizable gas in the volume defined between said first mentioned cylinder and the assembly comprising the hydrogenous cylinders, the guard ring, and the electrodes, said guard ring being electrically connected to said.

closure element to permit grounding thereof.

3. An ionization chamber having in combination, a metallic housing sealed from'the atmosphere, supporting elements of hydrogenousmaterial disposed within said housing, said supporting elements filling a major portion of the volume therein, an inert gas maintained within said housing to fill the volume intermediate said supporting elements and said housing, a pair of electrodes in mutually spaced parallel relation within said housing and insulatingly supported from said housing by said supporting elements in a plane perpendicular to them, a substantial portion of said electrodes being exposed to said inert gas, a barrier element substantially opaque to the transmission of slow neutrons disposed within said housing between said housing and said hydrogenous material and surrounding said electrodes whereby slow neutrons radiating from at least one point exteriorly of said chamber are prevented from reaching said electrodes, and means maintaining said electrodes at a difierence of potential.

4. An ionization chamber having in combination, a metallic housing sealed from the atmosphere, supporting elements of hydrogenous material disposed within said housing, said supporting elements filling a major portion of the volume therein, an inert gas maintained withinsaid housing to fill the volume intermediate said supporting elements and said housing, a pair of electrodes mutually spaced in parallel relation with-' in said housing and insulatingly supported from said housing by said supporting elementsin a plane perpendicular to them, each of said electrodes including a removable metallic portion of substantially the same size as the electrode,effec tive as a barrier against the transmission of slow neutrons through said electrodes into the space between said electrodes, a substantial portion of said electrodes being exposed to said inert gas, and means maintaining said electrodes at a difference of potential.

EMILIO G. SEGRE. OWEN CHAMBERLAIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS v Date 1

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