A program has been undertaken to simultaneously measure integral and differential cross sections in order to establish the degree of consistency between integral and differentially derived spectra. An assessment is then made concerning cross section limitations in deriving high energy neutron spectra by the foil-activation spectral-unfolding technique. (SDF

Dudey, N.D.

Greenwood, L.R.

Heinrich, R.R.

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0A COMPARISON OF DIFFERENTIAL AND INTEGRALCROSS SECTION MEASUREMENTSByL. R. Greenwood, R. R. Heinrich § N. D. DudeyFor Presentation at1st ASTM-Euratom Symposium on Reactor DosimetryPetten, NetherlandsSeptember 22-25, 1975-HOllCl-IT..* icpDri wii prepved at an account of work•poMOTcd by the limited State* Goveti:meni. Neitherthe United States not the IMied SUM* EnajyRetvvcb *n& Develojiineit AdmiidttnUoa, not *ny orthtki employtcs, not any of theii contiactort.tubanitnctor*, 01 ihdr employe*, nwket anymvtaaty, txpitm or impliei, or amunet any kpJbbiiiiy or rcipDMibiyty fot the accuncy, compkt«ie»or utefvlncai of any Wattmthn, •pptniuc, piodud ordikdowd, oi represents that iu uie would notprivately O H K I richti. MASTERilofCWVUSEBM • »DISTRIBUTION OF THIS DOCUMENT !S UNLIMlTED/iARGONNE NATIONAL LABORATORY, ARGONNE, ILLINOISoperated under contract W-31-109-Eng-38 for theU S. ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION- 2 -I. Introduction - -Neutron spectra of interest to reactor dosimetry are now routinelydetermined using spectral unfolding techniques, such as the SAND-II computercode. Integral saturated activities, determined by foil irradiations, areused to unfold the neutron spectra assuming that the appropriate nuclearcross sections are known. The accuracy of the unfolded spectra are thusdirectly related to the accuracy of tabulated cross sections, such asENDF/B-IV.In order to evaluate the accuracy of the nuclear cross sections, wehave undertaken a program to simultaneously measure integral and differentialcross sections. The method is to directly measure the differential neutronspectrum and compare this measured spectrum to the spectrum unfolded fromintegral multiple-foil reaction-rate measurements. Differences between theintegral and differentially derived spectrum are due to (1) uncertaintiesIn the reaction rate measurements, (2) the spectral unfolding algorithm,or (3) the differential cross section data. If errors in the first twofactors are kept small and if this procedure were followed with severaldifferent neutron spectra, then the correct cross sections could, in prin-ciple, be unfolded. However, our primary goal is to establish the degreeof consistency between integral and differentially derived spectra in orderto determine the accuracy of integrally unfolded neutron spectra.II. Experimental ProcedureSuitable neutron spectra can ba generated by stopping a deuteron beamin a thick 9Be foil. The gBe(d,n) reaction then produces a continuum ofneutrons in a bell-shaped curve, peaking at a neutron energy that is about40% of the incident deuteron energy.A. Differential MeasurementsThe differential neutron spectra were measured using the time-of-flight technique with the pulsed deuteron beam from the Argonne tandemaccelerator. Neutrons were detected using a NE213 (5 cm x 5 cm) liquidscintillator. Pulse shape discrimination was used to reduce the randomgamma background. Neutron spectra were measured at incident deuteronenergies of 14 and 16 MeV at angles of 0°-25°, with a flight path of 2.0and 3.7 meters. Data of sufficient accuracy could be obtained in about 30minutes of beam time..The flight times were converted to neutron energies, and absoluteneutron yields were calculated using detector efficiencies computed with amo rite carlo code [1], The total number of deuterons incident on the targetwas determined by charge integration.The resultant spectra are shown in Figs. 1 and 2. The data agreequite well with measurements made by Weaver [2], as shown in Fig. 3, althoughour data include more angles and extend to lower neutron energies (0.3 MeV).Neutron spectra were actually measured as low as 200 keV. However, the- 3 -2 h10*5TO2l10'\_- ..—1117 v \\ • ••* •0L10°15°25°1I\x# N!19Be(d,n)EdS\16 MeV\ VIi——^ \7 V,\ \ . -\\ •\ -8̂ —5 - —8 12. MeV16 20Fig. 1. 9Be(d,n) Time-of-Flight Spectra are Shovm at 16 MeV for 5 Angles- 4 -2 -5 -1 at !085 -2 -10'——.--••^• • 1M«i|1VBL0°10°20°1\N•I\19Be(dE d s ( 41,n)MeV: \1——116 204 8 12En,MeVFig. 2. 9Be(d,n) Time-of-Flight Spectra are Shown et 14 MeV for 4 Angles10to10IAaia*o-Jio810'Ed = 14 MeV 9 Be(d,n)ANL3.5-+ + + UCRI20"OO O ANL23.8°A A A UCRL16 20Fig. 3. Our Spectra at 14 MeV are Compared with Those of Weaver- 6 -calculated neutron efficiencies are not reliable in this region, and measure-ments of the efficiency in this region are now in progress.B. Integral Activation MeasurementsFoil packets consisting of Al, Au, Ni, Ti, 2 3 5U, and 2 3 8U wereirradiated at 0° at Ed = 14 and 16 MeV. The foils were about 1 cm indiameter and were located about 4 cm from the center of the 9Be target.In order to determine any variations in the spectra due to scattering orabsorption effects, Au and Al foils were placed before and after each ofthe other materials. The ratios between the Al(n,tn), Au(n,y), and Au(n,'2n)yields are highly sensitive to any spectral changes. A careful analysisof these results showed no deviation from the expected geometric effects(1/R2), indicating that absorption and scattering effects were not signif-icant .The gamma yields from the irradiated foils were determined byGe(Li) counting. All foils were measured at least twice in order toconfirm the extrapolation to the end of irradiation time. Correctionswere then made for decay during the irradiations, which lasted from 5-14hours. The integrated beam current was recorded at 10 min intervals, andthe decay correction was computed by summing the corrections for each shortinterval.The resultant values of the saturated activities a? were extractedfor ten reactions. The natural Ti foil gives three contributions for the(n,p) reaction to l|6Sc, l*7Sc, and 1*8Sc; however, corrections must be madefor the (n,np) and (n,d) contributions to the activation products. Forexample, 46Ti(n,p), l|7T(n,np), and ^Tifn.d) all lead to 1|6Sc, although thelatter two reactions only contribute at higher energies. This problem washandled by using the properly weighted activation cross sections in SAND-II.The fission-foil values were averaged using 95Zr, 103Ru, andHowever, fission yields are not known for our range of neutronenergies (E ̂ 5-6 MeV) and had to be interpolated from the ENDF/B-IV valuesat 500 keV and 14 MeV. This resulted in fairly consistent saturatedactivities for the three fission products; however, the results are notas accurate as those for other reactions. In the future we intend to usea fission chamber of the NBS type in order to accurately determine thetotal fissions. Fission yields will then be deduced for our range of neutronenergies.III. Comparison of Differential and Integral MeasurementsSince the foils subtended a finite range of angles (±7°), they saw aneutron spectrum given by an integral over the measured time-of-flightspectra. The uncertainty in the geometry is also worsened by the acceler-ator beam definition system. Any system of slits will produce a beam" spot" "of finite size on the target and changes in the accelerator tuning condi-tions may cause this spot to wander on the target. An analysis of oursystem showed that this could have caused an error in our average irradia-tion angle of as much as 1°. Any drift in the beam spot on the target pro-duces a much larger shift in the average angle of irradiation since thefoils are no longer concentric about 0°. This effect cannot be eliminated 'since the target-to-foil geometry must be close in order to produce a highneutron flux (e.g., reducing the angle by a factor of 2 would have requiredextending the irradiation time by a factor of 4). However, we intend toreduce this effect in the future by redesigning the accelerator slit system.For most of the reactions, the geometric effect is very small (<2%)and a properly averaged (by solid angle) neutron spectrum can be used. Inour case, the 5° spoctrum is quite close to the average. To be quite precise,each foil saw a slightly different average neutron spectrum since they wereat slightly different distances; however, this effect is small, and simple |corrections can be made to correct all integral values to the same irradia-tion geometry. ;iThe case of the 58Ni(n,2n) reaction is particularly striking and \demonstrates that caution must be used in evaluating errors due to the !geometry used in the foil irradiations. In this case, the high threshold jenergy makes the average activation cross section extremely sensitive to ismall changes in angle since the neutron flux is dropping rapidly at high :energies and changes slope about 12-16 MeV (see Figs. 1 and 2). At Ej = 14 ;MeV, a change in the average irradiation angle of only ±1° would produce a •change in the calculated saturated activity of ±15%. For all of the other .;reactions studied, this effect is less than ±3%. Hence, we have excluded '•.the 58Ni(n,2n) reaction from our comparisons and suggest that it be studied ;at higher incident deuteron energies. :The corrected activation results were finally compared with the |time-of-flight data. The ratio of saturated activities (cr<j>) are shown in 'Table I, where the numerator is the integrated foil data and the denominatoris the averaged time-of-flight flux times the ENDF/B-IV activation cross Jsection, integrated over energy. This ratio should be equal to one for jtotal consistency between the integral and differential results, assuming )that the nuclear cross.sections are correct. Since the spectra were not ',measured below 0.8 MeV, the 197Au(n,y) calculation is somewhat sensitive jto the assumption that one makes below this energy. We assumed that the \flux was zero below E n = 0.8 MeV in Table I. Other assumptions are dis- jcussed later (see Figs. 4 and 5). I\As shown in Table I, the ratio of measured to calculated saturated §activities has a standard deviation of +14% at Erf = 16 MeV and ±28% at |E d = 14 MeV. It is interesting to note that the differences appear to be jsimilar at both incident energies, that is, the ^Tifa.p) and **7Au(n,2n) iratios are high and the fission products are low. These results, at present, |provide only tentative information regarding the accuracy of the nuclear ;jcross sections. Additional neutron spectra need to be examined and each of j>the measurements should be repeated for verification. The interpolated |fission yields may be causing the apparent discrepancy for the fission |reactions but this possibility is currently being investigated. finally,we point out that the 197Au(n,2n) reaction is also very sensitive to thehigh, energy tail of the neutron spectra, although the sensitivity is muchless than for the 58Ni(n,2n) case.- A'S- 8 -TABLE I. Comparison of Saturated Activities forIntegral (Foil) and Differential (TOF)MeasurementsKatio =Reaction197Au(n,2n)396Au197Au(n,v)198Au27Al(n,a)2I*Na58Ni(n,p)58CoTi -> "eScbTi •»• *l7ScbTi • *»8Scb«5u(n,f)Cf.td. Devfa(ENDF/B-IV)$(TOF)dE16 MeV1.141.10a0.960.901.050.851.230.880.89a. (%) 13.51411000010028MeV.36.09a.96.75.98.79.53.75.79a.2aBelow 0.8 MeV, 4>(E) is assumed to be zero.(n,np) and (n,d) contributions included exceptfor ^9Ti case.CAverage of 9SZr, 103Ru, and llt0Ba, usingaverage fission yields from ENDF/B-IV.- 9 -1010T l 0Viiui9O_J!08I0749Be(d,n)-l6MeV9 (5°)-T.0.F.— SAND H (ENDF/B-IE8\12 !6 20En(MeV)Fig. 4. Iterated SAND-II Spectrum at 16 MeV is Comparedwith the Average Time-of-Flight Spectrum- 10 -1010COIo>Q- JIUI08I07i T~ r9Be(d,n)-l4MeV6 (5°)-T.0.F.— SAND E (ENDF/B-Fig. 5.t\\\\8 12 16 20En(MeV)Iterated SAND-II Spectrum at 14 MeV is Comparedwith the Average Time-of-Flight Spectrum- 11 -In order to examine the consistency between integral and differ-ential spectrum measurement procedures, we used the measured activities tounfold the neutron spectra. These are compared to the time-of-flight spectrain Figs. 4 and 5. The banded regions at low energy reflect the uncertaintyin the starting spectra (TOF) since our measurements did not extend below0.8 MeV. Only the 197Au(n,f) calculations are very sensitive in this regionand the data suggest that the flux is fairly low in this low energy region.The SAND-II multiple-foil unfolded spectrum is considerably more consistentwith the measured reaction rates but this is expected because the spectrumis derived in a manner to force consistency. For this reason, comparisonsas in Table I provide the most insight into differential and integral crosssection consistency. •The results presented are certainly not adequate to snake definitivestatements regarding cross section inconsistencies. Our purpose, however, isto point out the approach and the type of work being carried out to assesscross section limitations in deriving high energy neutron spectra by thefoil-activation spectral-unfolding technique. Our work is continuing to (1)improve upon the accuracy of the time-of-flight spectral measurements, (2)derive energy dependent fission yields for the fission reactions, (3) obtainactivation rate data for more reactions in order to exjand the number ofcross sections which can be studied, and (4) improve the quality of acceler-ator-produced neutron spectra.REFERENCES{1] POENITZ, W.P., "The Black Neutron Detector," ANL-7915 (1972).[2J WEAVER, K.A., "Spectra of Neutrons From Deuteron Bombardment of LightNuclei," UCRL-51310 (Addendum) (1973).Figure CciptionsFig. 1 9Be(d,n) Time-of-Flight Spectra are Shown at 16 MeV for 5 AnglesFig. 2 9Be(d,n) Time-of-Flight Spectra are Shown at 14 MeV for 4 AnglesFig. 3 Our Spectra at 14 MeV are Compared with Those of WeaverFig. 4 Iterated SAND-IT Spectrum at 16 MeV is Compared with the AverageTime-of-Flight SpectrumFig. 5 Iterated SANB-r. Speci-naa ac 14 MeV is Compared with the AverageTime-of-Flight f̂ e? trun

Comparison of differential and integral cross section measurements

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Comparison of differential and integral cross section measurements

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