Poenitz, W.P.

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Poenitz, W. P.

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Januar 1968Institut für Angewandte KernphysikKFK 744A "Greyli Neutron Detector for the Intermediate Energy Regionw. P. PönitzM. B. H.NUCLEAR INSTRUMENTS AND METHODS 58 (968) 39-44; © NORTH-HOLLAND PUBLISHING co.A "GREY" NEUTRON DETECTOR FOR THE INTERMEDIATE ENERGY REGIONW. P. PÖNITZ*Institut für Angewandte Kernphysik, Kernj'orschungszentrum KarlsruheReceived 13 August 1967A neutron detector for the accurate measurement of relativeneutron flux intensities in the keV energy region is described. Thedetector consists, in its simplest arrangement, of a paraffin pileand a NaI(Tl) detector to detect the 2.2 MeV capture y-rays. Theefficiency curve is relatively flat in the considered energy range.The efficiency curve depends only slightly on a change in theparameters used for its ca1culation. The application ofthe detectorin neutron cross section experiments is described.that all neutrons are slowed down and captured in themoderator (no leakage).b. There should be no y-ray absorption in the sphere.With these assumptions and neglecting the capturein components of the moderator other than hydrogen,the 2.2-MeV y-ray flux integrated over the surface ofthe sphere is independent of the primary neutronenergy of the source and equal to the neutron sourcestrength. This means that the efficiency of the detectorwill be expected to be a constant. Due to this importantbasic principle of the detector, calculated efficiencycurves taking into account the y-ray absorption in thesphere are very reliable as long as no neutron leakageoccurs.* Now at Argonne Nat. Lab., Argonne, Illinois, U.S.A.1. IntroductionThere exist already several fast neutron detectorsbased upon the slowing down of fast neutrons andmeasuring of the thermalized neutrons. The best-known detector of this type is the "long counter" asdescribed by Hanson and McKibben1) and byMcTaggart2). Some other detectors of this kind are the"paraffin pile"3), the "graphite sphere,,3), the "boronpile"4) and the "Iarge liquid scintillator"3). The firsttwo detectors are heterogeneous systems, as is the longcounter, and they are therefore more sensitive to aneutron field variation than a homogeneous system.The latter two detectors are homogeneous and havelarge efficiencies. However, these facilities are relativelyexpensive. Moreover, in the case of the large liquidscintillator the background problem plays an impor-tant role.In the present paper an improved form of the "greyneutron detector", [Pönitz and Wattecamps5)J will bedescribed and its properties will be discussed. It will beshown that this detector allows accurate neutron beamintensity measurements in the energy rang(from zeroto a few MeV. This is possible even though the detectoris a very simple facility consisting of a paraffin (orwater) sphere (or pile) and a"'NaI(Tl)"detector.- "Water orParaffin Sphere•Neutron Sourcey Ray- DetectorMultiplier SignalNa! (Tl) trystal2. Tbe principle of tbe detectorWe first consider an isotropically emitting neutronsource in the center of a water or paraffin sphere. ANaI(Tl) detector is located at the edge of the sphere asshown in fig. I. The source neutrons are slowed down inthe moderator. Apart of the thermalized neutrons iscaptured in the hydrogen, and another part leaks out ofthe sphere. An increase in the primary neutron energyresuIts in a stronger leakage of neutrons. The neutron;apture events are the sources of the 2.2-MeV y-raysJme of which are detected by the NaI(Tl) detector.In order to understand the basic principle of thetector, we make two additional assumptions:'. The radius of the sphere should be large enough39dFFig. 1. Principle of the detector.40 w. P. PÖNITZ(2)3. Calculation of the efficiency curve of the detectorWeintend to measure only the photopeak of the2.2-MeV capture y-ray which simplifies considerablythe calculation ofthe efficiency ofthe detector.The y-ray intensity going through a surface elementdF of the sphere and originating in the volume elementdV (fig. 1) isf:o Ia(E)·<f>Cr;E,Eo)dE·. {(e-l'l)j(4nZZ)}cosljtdcos8dcprzdr. (1)<p(r;E,Eo) is the neutron flux in the sphere at the radiusrand energy E, depending on the primary neutronsource energy Eo and the radius R of the sphere.I a( E) is the macroscopic capture cross section ofhydrogen in the moderator. f1 is the total attenuationcoefficient for the 2.2-MeV capture y-rays in themoderator. The quantities 1, ljt, 8, cp and rare shown infig. 1. The energy integral ofthe first two terms in eq. (1)gives the radius dependent capture rate which weassume to be proportional to the thermal neutron flux.Thus we get for the efficiency 11 of the:detector11 ~ f~=o f:~l Z-ze-I'lcosljtdx'rz<pth(r)dr,where x = cos8, 1= [RZ+ rZ- 2RrxJ4- and cos ljt =R - rx)/1. R is the radius of the sphere.Because of the principle of the detector, we canexpect that the particular choise of <pth(r) should nothave a strong influence on the calculated efficiencycurve as lang as no leakage occurs. We verify this usingthree different flux functions:A. The thermal neutron flux was calculated with agetheory [ref. 6), p. l64J assuming an infinite mediumgiving the result<pth(r) ~ r -1 e -<fLZ{e -rfL[l + erf(tn-t -Tt/L)J--erfL[l-erf(tn-4- +T4-jL)J}. (3)B. The thermal neutron flux was calculated with a twoneutron group theory [ref. 6), p. l64J for an infinitemedium giving<pth(r) ~ (LZ-T)-l r -1{exp( -rjL)-exp( -r/T4-)}, (4)where L is the diffusion length and T the Fermi age.c. The thermal neutron flux was calculated using agetheory [ref. 6), p. l65J assuming a finite sphere with aradius R yielding00<pth(r) ~ Im{l+(mnLjR)Z}-l.m. [exp {-(mnT4-jR)z}Jr- 1 sin(mnr/R). (5)Fig. 2 shows the efliciency curves calculated witheq. (2) and the different neutron flux functions as givenin eqs. (3)-(5). The curves are normalized at 30 keV.0.1L=2.755cm I\A~1:=1.0cm2.u=0.047cm-1R=25.0cm hl-n \8 \/ \c\/ \\\\eV keV MeV \t53 1221 195.3 1.56 3QO 100.0 300.0 1.0 2.5 7.01.061.061.041102~ .-"oM~ 1.00~F0.960.960.941.0 10.0 cm 2 100.0E1:Fig. 2. The efficiency of the detector vs energy calculated for different thermal neutron fiux functions.A "OREY" NEUTRON DETECTOR1.08r--------------r-~-----_.__~__,_,_----__,______,l=2.755cmtSt=5cmu=0.0474cm1.06J.-------':~~=~-+--+----~_H~r_~--+___lt 1.04+---------l----\-----+-fII'-----+--+_--10.94t----------+------+--+---r----t----t+-lQ92 f-_+--__.::..:v ..,..L__+--ik::..V~_+:_~:-:L-_:':_----l~.:..--.L-~~....... E1.53 12.21 195.3 1.56 30.0 10110 3000 1.0 7.0'-----------+--------+:----~-----:c:-~----1I-"tm W m ~ ~Fig. 3. The efficiency of the detector vs energy. The variable parameter is the radius of the water sphere.41The result confirms the expected feature of this detector.The three curves are in agreement be10w a few hundredkeV primary neutron energy where the neutron leakageis very small for a water sphere with R = 25.0 cm.4. Influence of the parametersThe calculated efficiency curves depend on the para-meters R, /l, Land Lh where 'r = Lh+'r1.44 eV. 'r1.44eV isthe age of the neutrons at the energy of the indiumE"t1.06t 1.04~ 1.02..><oM:E- 1.00@F0.980.960.940.92R=25cmll."t=5.0cm2l =2.75cm/./'\,lJ=0.03~ \ \-,lJ=0.05 \ \\ \eV keV MeV \ \1.53 12.21 195.3 1.56 30.0 100.0 300.0 1.0 2.5 7.0I ,-OJ 1.0 10.0 100.0Fig. 4. The efficiency of the detector vs energy. The variable parameter is the y-ray attenuation coefficient.42 W. P. PÖNITZ1.06~ 1.028!=! 1.00F0.980.960.940.92R=25.0cm It.-C=5.0cm 2I~=0.047cm-1v~/L=100~~ ~ ~L=2.50cm=2.75c = .75cm=2.50~~ - =3.00cml".N keV MeV \\1.53 1221 195.3 1.56 30.0 100.0 300.0 1.0 2.5 7.0 l, , 1-E-CQ1 1.0 10.0 100.0Fig. 5. The efficiency of the detector vs energy. The variable parameter is the diffusion length.resonance and Li']; is the age from the indium resonanceto thermal energy, For the following considerations weuse '];1.44 eV of water as given by Goldstein et al. 7). Thedifference between the slowing down age and the fluxage is much to small to be important in the energyregion of interest.In fig. 3. the efficiency curves are shown for differentradii R of the sphere. The increase of the efficiency forhigher neutron energy is due to the smaller 'l'-rayattenuation for capture 'l'-rays which originate on alarger mean radius than for primary thermal neutrons.The efficiency decrease at high energies is due to neutronleakage.The influence of the parameter J.l is shown in fig. 4.From the basic principle of the detector, it follows thatfor J.l-'>O the efficiency curve is exactly flat in the regiontoS,-----~---,-~-----__,_---R=25cmL=2.75cm~=0.0474cm-10.9S+-------+-------t----------\-r------I0.94t--------f--------f-------\-i--10.92 ~-~-..:ev:..---~L...-_+-...;:.::...:..___~ _ _.L_+_--:+-:--'-'-_-~::+:_'--.. E1.53 12.21 195.3 1.56 30.0 100.0 300D 1.0 7.0L-------~------__+---~:__-~~ -C0.1 1.0 10.0 cm2 100.0Fig. 6. The efficiency of the detector vs energy. The variable parameter is the neutron age LI. from 1.44 eV to thermal energies.A "GREY" NEUTRON DETECTOR 43where no leakage occurs. Therefore, the caIculation of'1 for fl = 0 gives the leakage as determined by the4Jth(r) chosen for the calculation and the sphereradius R. Values of fl are available from theoreticalcalculations8) or they can be measured using radio-active 'l'-sources. There is an additional effect whichshould be considered. The energy loss of the 'l'-rays dueto Compton scattering can be smaHer than the resolu-tion ofthe NaI(TI)-detector. A decrease of 10% for thetheoretical value of fl was calculated and this was foundin agreement with our experimental determination of fl.In general, the uncertainty offl should be less than 10%.Figs. 5 and 6 show the efficiency curves fot differentvalues of Land LI. respectively. As one can see, theinfluence of these parameters is small in the energyregion where the neutron leakage is small. Vlllues of Lfor water and paraffin have been measured very oftenand they are uncertain by only 1 or 2% [ref. 6), p. 368and p.370]. A measured value9) ofLI. = 1.0±0.5 cm2 isin relatively good agreement with a theoretical pre-diction of 0.8 cm2[ref. 6), p. 351].5. The application of the detectorA possible arrangement for the use of the greyneutron detector in a neutron cross section experimentis shown in fig. 7. There should be a weH collimatedmonoenergetic neutron beam which strikes the sampIeor the detector and enters into the grey neutron detectorthrough a channel. The grey neutron detector should beshielded by boron-paraffin to avoid the detection ofbackground neutrons and the NaI(Tl) detector shouldbe shielded by lead to reduce the background 'l'-radi-ation. However, the lead shielding should be positionedin such a way that the NaI(Tl) crystal can look at thewhole paraffin sphere.Comparing figs. 1 and 7 one can see that the realisticdetector in fig. 7 violates one assumption used for thecalculation of the efficiency curve. This is the isotropiedistribution of source neutrons. First, there is theeffect of the increase with increasing energy of theaverage path length for the first coHision. The efficieneyof the deteetor ean be corrected by a factorwhere R is the radius of the sphere, I/I; the averagepath length and I/I;3o is the average path length forneutrons at the normalization point. Eq. (6}is valid forthe arrangement shown in fig. 7 if one replaees the realneutron field by its center. The influenee of the hole inthe moderator (the channel in fig. 7) was investigatedby placing an additional NaI(TI) deteetor at 0° to theneutron beam direetion and observing the countingratios in the iwo deteetors as a funetion of energy. Inthis experiment the moderator was a 60 x 60 x 60-cmparaffin pile with a 10 x 10 x 30-cm neutron entrancechannel. No effeet was found within the 2% eountingstatistics. Thus, the error resulting from this effect in•NeutronSourceSample andDetectorsystemto detect theReaction RateNeutron BeamEntrance ChannelCollimatedNeutron BeamParaffin V \Spherep:~~~ I -=---Boron Paraffin ShieldFig. 7. Principle experimental setup for neutron cross section measurements using a "grey" neutron detector.44 W. P. PÖNITZthe NaI(TI) detector at 90° should be Iess than 2%.The use of a Iarger number of NaI(TI)-detectorswould increase the efliciency of the detector; however,it would not change the signal to room y-radiationbackground ratio.The energy range in which the grey neutron detectoris applicable is given by the variation and certainty ofthe efliciency curve with the energy. It should beapplicable from zero to a few hundred keV or a fewMeV depending on the size of the detector.It should be possible to measure with the arrange-ment shown in fig. 7 many kinds of cross sections wherethe "reaction method" [a separate determination oftheneutron flux and the reaction rate, definition in ref. 10)Jis applicable. This concerns mainly fission and capturecross sections.The capture cross sections of goIdll ,12) and ofuranium13) have been measured in the 25-500 keVenergy range with an accuracy of about 5% using thegrey neutron detector. A detailed description of theseexperiments will be given eIsewhere12, 13).The author wishes to acknowledge the help of Prof.Beckurts for interest in these investigations and Dr.H. O. Menlove and E. Wattecamps for interestingdiscussions.References1) A. O. Hanson and J. L. McKibben, Phys. Rev. 72 (1947) 673.2) M. H. McTaggart, AWRE-Report NR/AL/59 (1958).3) W. D. Allen, in Fast Neutron Physics 1 (Interscience, NewYork, 1960) p. 361 and references cited therein.4) D. W. Co1vin and M. G. Sowerby, EANDC-33 "U" (1963)44.5) W. P. Pönitz and E. Wattecamps, ibid. 102.6) K. H. Beckurts and K. Wirtz, Neutron Physics (Springer,Berlin, 1964).7) H. Goldstein et al., ORNL-2639-34.8) G. W. Grodsttin, NBS Circular 583 (1956).9) L. M. Barkowet al., J. Nucl. Energy 4 (1957) 94.10) W. P. Pönitz, IAEA Conf. Paris 1966, Nuclear Data JorReactors 1 (1967) 277.11) W. P. Pönitz, IAEA Panel, Brussels (1967).12) W. P. Pönitz et al., to be published (1967).13) H. O. Menlove and W. P. Pönitz, to be published (1967).

A \u27Grey\u27 neutron detector for the intermediate energy region

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A \u27Grey\u27 neutron detector for the intermediate energy region

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