Average Neutron Capture Cross Sections of. 151 Eu and 153Eu from 3 to 100 kev

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1 journal of NUCLEAR SCIENCE and TECHNOLOGY, 16[10], pp. 711~719 (October 1979). 711 Average Neutron Capture Cross Sections of 151 Eu and 153Eu from 3 to 100 kev Motoharu MIZUMOTO, Akira ASAMI, Yutaka NAKAJIMA, uuki KAWARASAKI, Toyojiro FUKETA Y and Hidekuni TAKEKOSHIt Tokai Research Establishment, Japan Atomic Energy Research Institute* Received May 23, 1979 Revised August 16, 1979 Neutron capture cross sections of europium isotopes 151Eu and 153Eu were measured in the neutron energy range of 3~100 kev. Experiments were carried out with the timeof-flight facility at the 52 m station of the JAERI Electron Linear Accelerator. Prompt capture g-rays were detected by a large liquid scintillation detector and the neutron flux shape was determined with a 6Li glass scintillation detector. The average capture cross sections were examined in terms of energy independent strength functions for 151Eu and 153Eu. KEYWORDS: neutron capture cross sections, europium 151, europium 153, kev range 3.0~100, strength functions, comparative evaluations I. INTRODUCTION The accurate capture cross sections of the europium isotopes are needed for the design of fast power reactors. The percent of the contribution of 153Eu isotope to the neutron absorption in the equilibrium core of the reactor is estimated to be about 2%(1). Furthermore, the possible use of europium as control rod materials is also considered because they have relatively high neutron capture cross sections in the kev region without having the problem of production of 4He as in the case of 10B control rods(2). Average capture cross sections with an accuracy of 10% for the natural europium in the energy range of 100 ev~500 kev have been requested in WRENDA 76/77(3) for fast reactor calculation with priority 1. Capture cross sections of 153Eu isotope with an accuracy of 30% have been required above 40 kev (priority 2) while the 5% for both isotopes from 1 kev to 1 MeV (priority 1). Previous measurements for these isotopes have been made in tens to hundreds kev region by several authors(4)~(7). Substantial discrepancies between recent measurements still exist. Preliminary results of present experiments have been reported(8). Average capture cross section in the continuum region may be described in terms of the energy independent strength functions and a single average radiative strength. These experimental values and atomic mass dependence are usuful in the development and application of the optical model calculation. The europium isotopes are located near a peak in the mass dependence of the s-wave strength function. The level spacings for both isotopes are very small : 0.7 and 1.3 ev for 151Eu and 153Eu, respectively(9). * Tokai-mura, Ibaraki-ken Present address : Keage t Laboratory of Nuclear Science Institute for Chemical Research, Kyoto University, Yoshida, Sakyo-ku, Kyoto

2 712 J. Nucl. Sci. Technol., In Chap. V, the average capture cross sections were analyzed by the least squares fitting program with the energy independent s-wave strength function S0. II. EXPERIMENT Measurements were carried out using the neutron time-of-flight method. Neutrons were produced by a pulsed electron beam of 100 MeV which was stopped in a laminated water-cooled Ta target. The neutrons were moderated by a 5 cm thick boron loaded paraffin. The direct g-flash and high energy neutrons which cause the overload of the detector and the time dependent backgrounds, respectively, were shielded by the 5 cm thick x 20 cm (in the beam direction) long Pb shield ring surrounding the Ta target. The linac was run at 150 pps repetition rate and 0.1 /ms burst width. The detailed description of the linac and data acquisition system has been given elsewhere(10). The capture g-ray detector was a large liquid scintillator tank of 3,500 l. Its schematic drawing is shown in Fig. 1. It has a 10 cm diameter Al inner tube through which the collimated neutron beam (4 cm diameter) passes and in which a sample changer is placed. This inner tube is covered with a 2 cm thick natural granular boron liner. The liquid scintillator consists of purified xylene containing 4 g/l p-terphenyl and 0.09 g/l a-npo. The trimethylborate was added to the scintillator in order to reduce the background due to the 2.22 MeV neutron capture g-ray of hydrogen. The scintillator was viewed by four EMI-9545B photomultipliers whose gains were individually adjusted. The time-of-flight spectra were taken within a pulse height window corresponding to equivalent g-ray energy between 3.5 and 12 MeV. The energy calibration of the discriminator was performed by placing a 60Co source at the sample position inside the detector and observing the position of the 2.5 MeV sum peak. The discriminated pulses were then sent to the data acquisition room through about 100 m of doubly shielded coaxial Fig. 1 Schematic drawing of 3,500l liquid scintillator tank 12

3 Vol. 16, No. 10 (Oct. 1979) 713 balloon cable. Time-of-flight data were taken on an on-line computer (16 kw-20 bits/word) with 8192 time channels and channel width of 100 ns. A block diagram of the experimental arrangement is shown in Fig. 2. Fig. 2 Block diagram of liquid scintillator electronics and automatic sample changer system The detector was located at 52 m from the neutron producing target. The neutron flight path of Al tube with the 50 mm mylar film windows;was evacuated during the experiment. Beam filters were inserted in the neutron beam : 200 mg/cm2 B4C to absorb low energy neutrons and 0.8 cm of Pb to reduce the g-flash further in addition to the Pb ring at the neutron producing target. Time dependent background was determined by inserting, in the beam, notch filters consisting of Al and Na. The capture time-of-flight spectra were measured in alternate cycles with sample (or Pb scatterer) and notch filters in and out of the neutron beam(11). The sample changers for the sample and notch filters were controlled by the same on-line computer in the above. The time interval of cycle was determined by the accumulated Time independent backgrounds are already subtracted. The lower curve shows the spectrum obtained with notch filters. Fig. 3 Raw data time-of-flight spectra observed with 151Eu sample 13

4 714 J. Nucl. Sci. Technol., monitor counts from an additional 5 cm diameter 6Li glass detector at 10 m from the neutron target. Thus the influence of time dependent variations on the relative counting rate was eliminated. Figure 3 shows raw data time-of-flight spectra observed with the 151Eu sample. The lower curve on the figure shows the spectrum obtained with notch filters The neutron flux as a function of energy was measured using a 11.1 cm diameter x cm thick 6Li glass scintillation detector at 56 m. The neutron flux timeof-flight spectra as well as the capture data were measured in alternate cycles. The energy calibration was carried out using energies of sharp resonances present in the filter isotopes kev of Na(12). : kev of Al and Measurements were made with the enriched 151Eu and 153Tu isotope sample of thicknesses and atoms/b, respectively. Detailed description of the samples is shown in Table 1. The samples were oxide powder contained in Al case with the 50 mm mylar windows, shown in Fig. 4. The diameter of the samples was 6 cm. A 2 mm thick Pb scatterer was used to determine the shape of the background in the capture time-of-flight spectra. Table 1 Isotopic compositions and thicknesses of samples Fig. 4 Al sample case with 50 pm mylar windows III. ANALYSIS In the neutron capture cross section measurements with the large liquid scintillato the detector counts are expressed by r, Cg=peYg+Bg, where Cg is the total number of detector counts corrected for dead time losses, p the number of neutrons incident upon the sample, Yg the capture yield, e the detector efficiency and Bg the detector background. Each step of the analysis to determine these quantities is briefly discussed below. 1. Background Determination The background Bg associated with the capture g-ray detector consists of two com - ponents ; time independent background and time dependent one. Time independent background is attributed to cosmic rays and long lived radioactivities such as 40K present in the concrete wall of the detector station. This component is determined from the constant counts at a long flight time. The time dependent background is due to the prompt g-rays following the capture of scattered neutrons in the detector or in a ny material surrounding the detector. The spectrum of the time dependent background was calculated by the following relationship : 14

5 Vol. 16, No. 10 (Oct. 1979) 715 where Cg and Cn are the capture and flux counts, Bg and Bn the respective backgrounds and a prime designates the corresponding quantities measured with notch filters in the neutron beam. The shape of the background B'g was obtained from measurements in which the capture sample was replaced by a Pb scatterer of approximately equivalent thickness. The normalization of B'g was made from capture detector counts by inserting notch filters of Al and Na which have black resonances at 88.5, 34.7 and 2.85 kev. The backgrounds of the flux detector Bn and B'n are relatively small compared to the capture detector, and fitted to a suitable exponential function of neutron energy. The only unknown values Bg in the whole region was then determined by smoothly connecting the calculated points by above mentioned relationship. To improve the statistical accuracy of the background measurements, the spectra obtained with the Pb scatterer were averaged over appropriate time channels. 2. Relative Neutron Flux Neutron flux p measured by the 6Li glass detector was corrected for dead time losses (which range from about 0.3% at 3 kev to about 1% at 80 kev) and backgrounds were subtracted. The relative efficiency of the detector was estimated using the evaluated 6Li(n, a) cross sections(13). The corrections for neutron multiple scattering and selfshielding in the detector were calculated with the Monte Carlo method(14). The effects of scattering process due to resonances of detector constituents such as 27Al, 28Si, 160, Li were also taken into account in the calculation. The neutron flux 7 data measured at 56 m were converted to the equivalent value of the flight path to the capture tank, 52 m. The correction for the attenuation of neutrons by air between the capture and the flux detectors was also made. The neutron flux spectrum is shown in Fig. 5. The structure in the neutron beam are caused by the Al. The peak at 56 kev is caused by an enhancement of detector efficiency due to resonance scattering from 140Ce contained in the 6Li-glass detector and 28Si in the photomultiplier window, which were also corrected. Our flux shape is roughly approximated with a power law (p(e)de oce-0.71). 3. Normalization The structure in the beam is due to Al and resonance scattering from 140Ce contained in the 6Li-glass detector, and 28Si in the photomultiplier window. Fig. 5 Neutron flux measured at 56 m Since the absolute efficiencies of the capture g-ray detector and of the neutron flux detector were not known, the saturated resonance technique was used by assuming the capture efficiency not to vary with neutron energy. In this experiment, difficulty was encountered in determining the background of the neutron flux at the saturated resonances, so only the relative cross sections were obtained. The cross section measurement of a natural europium sample was made(16) separately, in which transmission was also measured in order to determine the background at the saturated resonances. The 15

6 716 J. Nucl. Sci. Technol., saturated capture probabilities at the 7.42 ev 151Eu and the 8.84 ev 153Eu resonances in the natural europium were calculated by the Monte Carlo method(16) using the values of resonance parameters given in the BNL-325(9). The normalization for separated isotopes was carried out by the following relationships : where sn.g.(nat), sn.g(151) and sn.g(153) are absolute capture cross sections of natural europium, 151Eu and 153Eu respectively, a1 and a2 isotopic abundances to be and respectively, s'n.g,(151) and sn.g,(153) the relative cross sections obtained in this experiment, and e(151) and e(153) capture efficiencies for two elements. The difference of two efficiencies is due to the variation of the discriminator bias and neutron binding energy. The ratio e(151)/e(153) was estimated from capture counts between saturated resonances of 151Eu and 153Eu samples. The normalization was made in the energy range of 8~11 kev where the statistical uncertainty of the cross section obtained for the natural sample was +-1.5%. 4. Self-shielding and Multiple Scattering Correction The resonance self-shielding and multiple scattering correction due to finite sample thicknesses were estimated by a similar way as a statistical technique developed by Dresner(17) and coded by Macklin(18). In this calculation, the reduced neutron widths distributed over a Porter Thomas distribution and a strict 21+1 dependence of level density were assumed. The average parameters were taken from BA/L-325(9). The energy dependence of the combined correction factors for self-shielding and multiple scattering are less than 0.5% for our samples including the effect of oxygen in oxide samples. 5. Uncertainty The statistical uncertainty ranged 0.4~2% after the data were averaged over convenient energy interval. The major sources of uncertainty in this experiment were the determination of the background (2.5~7% and 4~11% for the 151Eu and 153Eu, respectively) and the normalization (4%). V. RESULTS AND DISCUSSION The average cross sections of 3~400 kev neutron energy range are shown in Fig. 6(a) and (b) as histograms. Numerical values are listed in Table 2. Several previous data available for comparison are also shown in the figures. The present data on "'Eu (see Fig. 6(a)) are in good agreement with those of Moxon at al.(6) (who measured with a Moxon-Rae detector) over the whole energy range, but are 20% higher above 30 kev than those of Hockenbury at al.(5) (who measured with a large liquid scintillator). The data of Konks et al.(4) (who measured with a Pb slowing down spectrometer) are systematically below our results by about 20%. Figure 6(b) shows a comparison of measurements of the Eu cross section. The data from Refs. (4)~(6) are in reasonable agreement with 153 our data below 30 kev within quoted uncertainties, while above 30 kev discrepancies become somewhat larger. The disagreement between our data and those of Kononov at al.(7) are large in both magnitude and shape for 151Eu and 153Eu. The evaluated data of ENDF/B-IV(19) for 151Eu and of JENDL-1(20) for 153Eu seem to be influenced by data of Konks at al.(4) 16

7 Vol. 16, No. 10 (Oct. 1979) 717 Fig. 6 Neutron capture cross section of 151Tu and Eu in energy range of 2.5~100 kev

8 718 J. Nucl. Sci. Technol., The average capture cross section can be fitted with the energy independent strength functions S0, S1, S2 and a single average radiation strength Gg/D. It is, however, difficult to determine the p-wave and d-wave strength functions by fitting our data since the s-wave contribution to the capture cross section is dominant due to the large neutron penetrability in our measured energy region. The only s-wave strength function was adjusted to fit the data. The values of S, and S2 were assumed to be 1.0x10-4 and not varied. The values of Gg/D, which were taken from the BNL-325 to be and for 151Eu and 153Eu, respectively, were also not varied. Our fitted values of 104 S00= for 151Eu and for 153Eu are larger than those of Rahn et al.(21) They obtained the s-wave strength function from resonance parameters below 100 ev giving the values of 104,S0= for 151Eu and 104,S0= for 151Eu. The discrepancy between our data and calculated values above higher energy region (>20 key) becomes larger (about 20% for 151Eu and about 30% for 153Eu at 100 kev). This indicates that increasing S1 would improve the fit. ACKNOWLEDGMENTS Authors wish to thank Dr. G. Rogosa and the U.S. Department of Energy for help in procuring the separated isotope samples. They also thank Mr. T. Shoji for technical assistance and Mr. M. Ohkubo for providing the computer code MCRTOF. The contribution of the operation crew of linac is gratefully acknowledged. REFERENCES (1) KIKUCHI, Y., et al.: Fission product fast reactor constant system of JNDC, JAERI-1248, (1976). (2) For a review concerning this possibility, see PASTO, A.E. : Europium oxide as a potential LMFBR control material, ORNL-TM4226, (1973). (3) LESSLER, R.M. (Ed.) : WRENDA 76/77, World request list for nuclear data, INDC (SEC)- 55/URSF, (1976). KONKS, V.A., POPOV, Yu P., FENIN, Yu I.: Soy. J. Nucl. Phys., 7, 310 (1968). HOCKENBURY, R.W., KNOX, H.R., KAUSHAL, N.N. : Proc. Conf. on Nuclear Cross Sections and Technology, Washington, D.C. 1975, (Ed. by SCHRACK, R.A., BOWMAN, C.D.), NBS Special Publ. 425, p.905 (1975). (s) MOXON, M.C., ENDACOTT, D.A. J., JOLLY, J.E. : Ann. Nucl. Energy, 3, 399 (1976). 18

9 Vol. 16, No. 10 (Oct. 1979) 719 (7) KONONOV, (1977). V.N., et al.: Proc. 4th All Union Conf. Neutron Physics, Kiev, Apr. 1977, p. (8) ASAMI, A., et al.: Topical Conf. Tech. of Neutron Capture Cross Section Measurements at NEANDC Meeting, Oak Ridge, Apr (9) Neutron cross sections (Compiled by MUGHABGHA B, S.F., GARBER, D.1.), BNL-325 (NTIS, springfield, Va., 1973), (3rd ed)., Vol. 1, Parameters, (1973). (10) TAKEKOSHI, H. (Ed.) : Design, construction and operation of JAERI-linac, JAERI-1238, (in Japanese), (1974). (11) SHOJI, T., MIZUMOTO, M., KAW ARASAK I, Y.: A sample changer system automatic measurement of neutron capture cross sections, JAERI-M 6010, (in Japanese) (12) OLSEN, D.K., et al.: Nucl. Sci. Eng., 66, 141 (1978)., (1975). (13) IGARASI, S., et al.: Japanese evaluated nuclear data library, version-1, JENDL-1, JAERI.1262, (1979). (14) MIZUMO-TO, M., ASAMI, A.: A Monte Carlo Code (ELIS) for calculating the efficiency of 'Liglass scintillator, To be published. (15) ASAMI, A., et al.: To be published. (16) OHKUBO, M.: MCRTOF-A Monte Carlo program for multiple scattering of neutrons in resonance energy region, JAERI-M 7988, (1978). 417) DRESNER, L.: Nucl. Instrum. Methods, 16, 176 (1962). as) MACKLIN, R.L.: ibid., 26, 213 (1964). (19) TAKAHASHI, H.: Evaluation of the neutron and gamma-ray production cross section of "'Eu and "'Eu, ENDF-213, (1974). 120 IIJIMA, S., et al.: J. Nucl. Sci. Technol., 14[3], 161 (1977). (21) RAHN, F., et al.: Phys. Rev., C6, 251 (1972). 19