Froehner, F.H.

Kaeppeler, F.

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Froehner, F. H.

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June 1970Institut für Ang ewandte KernphysikAbsolute Neutron Flux DeterminationF. Käppele r , F. H. FröhnerKFK 1226IAEA-CJ\T-26j'9KERNFORSCHUNGS ZENTRUM KARLSRUHEJuly 1970 KFK 1226IAEA-CN-26/9*Institut für Angewandte Kernphysik*Absolute Neutron Flux DeterminationF. Käppeler and F.H. FröhnerPaper presented at the Second International Conference onNuclear Data for Reactors, Helsinki, June 15 - 19, 1970.Gesellschaft für Kernforschung mbH., KarlsruheABSTRACTA proton recoil counter with a solid radiator and asolid statedetector for absolute measurements of neutron beam fluxes above~250 keV is described. The geometry is such that only protonsemerging within a narrow cone around the forward direction aredetected. Thus only relatively high-energy protons reach the de-tector and background correction is facilitated. A specially de-velopped Monte Carlo program is used to normalize the spectra.The counter is fast enough (,v18 nsec time resolution) for manytime of flight applications.ZUSAHMENFASSUNGFür die Absolutmessung von Neutronenflüssen oberhalb <~ 250 keVwird ein Protonenrückstoßdetektor mit einer festen Streuprobeuridelnem Halbleiteriähler besChrütben •. Die Geom.etrle wurde sogewählt, daß nur solche Rückstoßprotonen den Detektor treffen,die unter kleinen Winkeln gegen die Einfallsrichtung der Neutronenaus der Streuprobe austreten. Da so nur Protonen mit relativhoher Energie nachgewiesen werden, kann leichter gegen niederener-getischen Untergrund diskriminiert werden. Für die Normalisierungder Spektren wurde ein eigenes Monte Carlo Programm entwickelt.Mit ungefähr 18 nsec Zeitauflösung ist der Detektor für vieleFlugzeitexperimente geeignet.CN-26/9ABSOLUTE NEUTRON FLUX DETERMINATIONF. KÄPPELER AND F.H. FRÖHNERINSTITUT FüR ANGEWANDTE KERNPHYSIKKERNFORSCHUNGS ZENTRUM KARLSRUHEF.R. GERMANY1. INTRODUCTIONFor neutronenerg-ies higher than abouf 100 keV anypartialIlElutr-öh 6rosssectiOhmEiäsurement er gOodaccura.cj"häs torefer tothe hydrogen (n,p) scattering cross section. This is the only standardcross section with an accuracy better than 3 %in this energy region,weIl established by experimental as weIl as by theoretical work [1] •Therefore many efforts were made to design proton recoil countersfor neutron flux measurements. In general one can distinguish betweencounters with solid radiator and counters filled with gas containinghydrogen [2,3J • The use of a solid radiator (for example a thin layerof stearic acid) has the advantages that the detector geometry i5 weIldefined even in a divergent neutron flux and that it is possible todetermine the background by covering the radiator. The disadvantage ofsuch a system is its emaIl efficiency: Since the solid radiator has tobe so thin that practically all protons can escape from it theefficiency is about three orders of magnitude lower than for a gas-filled counter.The recoil protons from a solid radiator can be detected in aproportional counter or with a solid-state detector. The proportionalcounter has a good efficiency - the protons are detected in 2\[-geometry - but it is sensitive to y-rays. The y-flash of an accelerat-or causes a background which masks the lower part of the step-likerecoil proton spectrum. Because the proportional counter is too slow- 2 -for time-of-flight-discrimination of the y-ray background, one always hasto extrapolate the spectrum to zero pulse height in order to find thefull count rate. This extrapolated portion of the pulse-heightdistribution and thus the uncertainty of the number of recoil protonsgets larger with decreasing energy. Additional uncertainties arisefrom the amount of protons absorbed in the radiator. These difficult-ies remain when the proportional counter is replaced by asolid statedetector located close to the surface of the radiator as in Ref.[4J •The uncertainty due to extrapolation of the recoil proton spectr-um can be avoided with asolid state detector positioned at a certaindistance from the radiator. Then only forward-peaked protons aredetected. The minimum proton energy Ep min = En cos2 ema x is definedby the maximum scattering angle ema x between the radiator and thedetector. Thus, it is not at all necessary to extrapolate the spectrato zero pulse height. Furthermore, all recoil protons impinging onthe solid state detector have enough energy to be detected. No recoilprotons are lost by absorption in the radiator or in the detectorwindow. The penalty for the improvement in background discriminationis a further reduction of the efficiency. Nevertheless, this solutionis valuable for neutron energies below about 1 MeV, where other recoildetector systems fail because of background problems.2. EXPERIMENTAL ARRANGEMENT2.1 The Proton Recoil CounterIn its first form, this detector was.suggefited by E. Pfletschin-~er. The prine-iple was to measure the entirespectrum of recoilprotons. When the spectrum is integrated for the calculation of thedetector efficiency, it is not necessary to make corrections for anextrapolated part of the spectrum. Fig. 1a shows the counter. Thematerials used were bronze for the walls and brassfor the lid andthe flanges. During operation, the counter is evacuated to about 0.1Torr. It was designed as light-weight as possible to keep the amountof scattered neutrons small. A coolant baffle filled with liquidnitrogen prevents oil vapors from contaminating the counter. Storageand handling of the counter is always done in Argon atmosphere.The solid state detector and the radiator are mounted on the lid.Therefore deformations due to the air pressure outside the countercannot change the solid angle between them.2.2 The Solid State DetectorAn ion-implanted silicon detector was used as the proton detect-or ~J . Its surface was metallized with 20 pg/cm2 gold and the t~ick­ness of the insensitive region, measured by a collimated beam ofalpha particles impinging under different angles, was about 20 pg/cm2- 3 -silicon under experimental conditions. The detector had an energyresolution of about 2 %and a sensitive area, defined by a mask withan aperture}of 1.76 cm2• During measurements the detector can becovered with a 0.1 mm thick bronze sheet during every second cycle ofan automatie sampIe changer. The distance between the aperture of thedetector and the bronze sheet was 1.0 cm. The background spectrummeasured with the detector covered can be stored in the memory of anon~line computer under exactly the same conditions as the recoilproton spectrum.2.3 RadiatorThe radiators were made by the CBNM/Euratom in Geel [6J • Stearicacid (C18H3602) was evaporated at a constant temperature of 240 0 C on-to a 1 mm thick sheet of stainless steel. The polish of the backingis better than 1 pm, which was measured with an optical method.Evaporation times from one to severßl hours were used, depending onthe layer thickness. Stearic acid was chosen because cf the largehydrogen content of about 12 %, the high purity of this materialand its resistance against air moisture.The diameter of the layer was 4 cm and the thickness variedbetween 60 pg/cm2 and 180 pg/cm2, corresponding to 0.6 and 2.2 mg ofstearic acid. The total mass was determined by weighing the sampIeswith a vacuum balance before and after evaporation. The uncertaintyis estimated to be + 10 Mg and is caused by the error of the balance.After storage in air as weIL as in Argon over long time periods(several months) no change in the weight of the sampIes greater than+ 10-pgwas- -f'ounds-A second method to determine the hydrogen content of the radia-tors is a quantitative chemical analysis which is supposed to beaccurate to about 1 %. Such an analysis is being prepared.2.4 Neutron SourceNeutrons were produced with the Karlsruhe 3 MV pulsed Van deGraaff accelerator via the 7Li(p,n)7Be reaction. The pulse width was1 n~ec and the reuetition rate 106 ~. The neutron ener~y was- ---- ---- ---- -- -L - -- - -- -- - _. ;:sec ~Vdetermined with a Li-glass detector at a distance of 1.80 m and witha time resolution < 2 n~ec. Normally the 7Li-targets were 30 to 70 keVthick. Ta-backings for the 7Li-targets and a shielding of 2 cm Pbreduced the y-flash of the accelerator.2.5 ElectronicsFig. 1b shows a block diagram of the electronics. The pulses froma charge-sensitive preamplifier located elose to the silicon detector- 4 -are shaped and amplified by a spectroscopy amplifier. The width ofthe output pulses was 250 nsec. Time determination is achieved witha zero-crossing trigger, the time resolution depending on the pulseheight, e.g. 18 nsec at 500 keV. The time-of-flight-spectrum producedin a first analog-to-digital converter (ADC1 in Fig. 1b) contained256 channels. By the computer program they were compressed to 64channels in such a way, that the important part of the spectrum withthe recoil protons is preserved with the original resolution. Thetwo resulting 64 x 64 channel matrixes (for covered and uncoveredsilicon detector) contain pulse height spectra versus time of flight.For data transmission from the buffer memory to the computer itis necessary that there are two coincident signals, one from everyADC.3. MONTE CARLO PROGRAMFor the calculation of the efficiency and far the simulation cfpulse-height spectra of the proton recoil counter a Monte Carloprogram was written. Similar to that of Bame et al.L?] , the neutronsfrom a point source hit the (circular) radiator sheet, producingrecoil protons. The laboratory scattering angles are sampled from theappropriate distributions, then the energy and the direction of theknock-on proton can be determined for a given neutron energy anddirection. This is done for different places and depths in the radia-tor. To save computer time, angle sampling for each interaction isrestricted to a cone containing the solid state detector. The geometryfactors of the efficiency M, determined with about 105 hits of thesQ;1idsta-tedetecto-ragreed w~thi-n the statistical uncertaint~eswiththe values of Bame et al. [7J • Energy losses in the radiator and inthe entrance window of the solid state detector are calculated fromknown energy-range and dE/dx relationships [8,9,10,1fJ •4. RESULTS AND DISCUSSIONFirst experiments without any radiator material confirmed thatthere was no difference in the spectra measured with the detector un-covered or covered by the 0.1 mm thick bronze sheet. Fig. 2a shows aspectrum taken with the radiator in place. Also shown is the back-g~ound. The spectrum is due to neutr~ns with energies of (550 + 20)keV from a pulsed 3 MV Van de Graaff accelerator. It demonsträtesthat measurements can be done with good discrimination against back-ground. In Fig. 2b the experimental results at three neutron energiesare plot ted together with the theoretical curves. The agreement issatisfactory. Absolute fission cross section measurements with thisdetector are in progress.- 5 -ACKNOWLEDGEMENTThe authors wishes to thank E. Pfletschinger for the initiation of,and Prof. Beckurts for his support and permanent interest in,thiswork. We appreciate the careful preparation of the radiator foils byH.L. Eschbach and E.W. Kruidhof as weIl as the good work of the vande Graaff operators.- 6 -REFERENCES[1J GAMMEL, J.L., Fast Neutron Physics - 11 (J.B. Marion andJ.L. Fowler, Eds.) Intersei., N.Y. (1963) 2185.[2J FERGUSON, A.T.G., Fast Neutron Physics - I (J.B. Marion andJ.L. Fowler, Eds.) Intersei., N.Y. (1963) 179.[3J[4J[5J[6JJOHNSON, C.H., Fast Neutron Physics - I (J.B. Marion andJ.L. Fowler, Eds.) Intersei., N.Y. (1963) 24%WHITE, P.H., J. Nucl. Energy 12 (1965) 325.MEYER, 0., IEEE Trans. Nuc. Sei. NS-15 3(1968) 232.ESCHBACH, H.L., priv. comm.[7J BAME, S.J., Jr., EUGENE HADDAD, PERRY, J .E., Jr., SlUTH, R.K.,and BLAIR SWARTZ, Rev. Sei. Instr. 21 (1960) 911~[8J WHALING, wARD, Handbuch der Physik 34 (1958) 193.~J BICHSEL, HANS, Phys. Rev. ~ (1958) 1089.[10J HIRSCHFELDER, J.O., and MAGEE, J.L., Phys. Rev. 73 (1948) 207.~1J WILLIAMSON, C~, and BOUJOT, J.P., Cß~-2189 (1962 Saclay).- 7 -FIGURE CAPTIONSFig. 1aFig. 1bFig. 2aFig. 2bSchematic view of the proton recoil counterBlock diagram of the electronicsSpectrum with covered and uncovered solid state detectorExperimental and calculated spectra at three energiesPULSEHEIGHTTRIGGER256 CHANNELTOF SPECTRAV'-TIMEDETERMTRIGGERivLINEARGATEr,.STRETCHER64 CHANNELPULSE HEIGHTSPECTRARUBBER SEALBRONZE $HEET(O.1mm)--SOLID STATEDETECTORWITH MA$KO.1mm BRONZERADIATOR,0.4mmBRONZENEUTRON iWINDOW( 03 mm B~ONZE )1J I nI .MOlOR WITHGEARVACUUMPUMP-~FIG.la FIG.lboo SPECTRUM• BACKGROUND--+--BACKGROUND .SUBTRAC TEDI30 keV90keV50 keV870 keV670 keV180 119cmr180 !JQc"m""-MAX. NEUTRON ENERGY 350 keVRADIATOR THICKNESS 80 l!fJ..cmNEUTRON ENERGYSPREAD• MEASURED+ CALCULATEDt+•~..+•~i-++" .+ +• •~."t-.J'\+' • +~. .+. ~.If. •er .,.•+ '-+..+ •+ -Je'!f- ~ ...+ .~~e!- +.+. ++ ...'" '+~~++~+, , ,~, '100 500 1100PROTON ENERGY (keV)+s.+• .f-e!--l'+~--,-"t- , , , , Iil+,. fe:'i ~.~ +.. .If-+ +01-eIe ...1 ~-+..... .01- 4e+ ++ ~ I++ ~t I ,I ,++'"oFIG.2b0::UJa...~z::>~0::I-iB0::<{..JUJZZ<(i5~z::>8ooCHANNEL NUMBER570 keV180:@.cm00oo 000 0000NEUTRON 40 keVENERGY SF'READMAX.NEUmONENERGYRADIATORTHICKNESSjI\l-tI 1I 1o 1 11 II \0I 1, I+ 1o I \1 II I1 II II 1o f 1, It tI Io f \I \Ollt :I II II ,I II I+ \! \0I II I0.. ;L Ioii •••• / \i·t • ••• •• •1 :f.... \/ e •• -+ti- .'0f .~, 0,f I.t+ \.0-J:"*" " .0,+' "(t .Q+. 0+'+ \+'+++~oot>._-~-++ I 3'0 4'0I 2015001000..JUJZZ<{J:U 5000::UJJa... 0•~z 0::> •0u •• •• Iil•• 0000 .to 000++ + +0 10FIG.2a

Absolute Neutron Flux Determination

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Absolute Neutron Flux Determination

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