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3.22 Measurement of Neutron Spallation Cross Sections

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3.22 Measurement of Neutron Spallation Cross Sections JAERI-Conf 96-008 3.22 Measurement of Neutron Spallation Cross Sections E.Kim,T.Nakamura,A.Konno(CYRIC,Tohoku Univ.),M.Imamura,N.Nakao T.Shibata(INS,Univ.of Tokyo),Y.Uwamino,N.Nakanishi(Institute of Physical and Chemical Research),Su.Tanaka,H.Nakashiman, Sh.Tanaka,(JAERI) Neutron spallation cross sections of l2C(n,2n)"C and 2O9Bi(n,xn)2IOxBi were measured in the quasi-monoencrgetic p-Li neutron fileds in the energy range of 20McV to 210MeV which have been established at four AVF cyclotron facilities of 1)INS of Univ. of Tokyo, 2)TIARA of JAERI, 3)CYRIC of Tohoku Univ., and 4)RIKEN. Our experimental data were compared with other experimental data and the ENDF/B-VI high energy file data. 1. Introduction At present, a demand for neutron reaction data is world-wide increasing from the viewpoints of intense neutron source of material study, induced radioactivity and shielding design of high energy accelerators. Nevertheless, neutron reaction data in the energy range above 20MeV are very poor and no evaluated data file exists at present mainly due to very limited number of facilities having quasi-monoenergetic neutron fields. In this study, we measured the neutron spallation cross sections of I2C, 27A1,59Co and 209Bi which has been and will be used for high energy neutron spectrometry, by using quasi-monoenergetic p-Li reutrons in the energy range of 20MeV to 210MeV. 2.Experiment The experiments were performed at four cyclotron facilities of 1 institute for Nuclear Study ( INS ),University of Tokyo for 20 to 40 McV, 2)Cyclotron and Radioisotopc Center (CYRIC),Tohoku University for 20 to 40 MeV, 3)Takasaki Research Establishment, Japan Atomic Energy Reserch Institute (TIARA) for 40 to 90MeV and 4)Institute of Physical and Chemical Research (RIKEN) for 80 to 210MeV. The 7Li-targets of 2 to 10mm thickness were bombared by proton beams of 20 to 210MeV energies which were extracted from these cyclotrons. The neutrons produced in the forward direction from the target were transported through the collimater for sample irradiation and the proton beams passed through the target were swept out by the magnet to the beam dump at CYRIC,TIARA and RIKEN. The cross section measurements between 20 and 40MeV were performed at INS, because the neutron flucnce of the CYRIC neutron field was too low for sample irradiation. The samples were placed only 10cm away from the Li-target in the forward direction at INS,and the neutron spectra were measured at CYRIC where the same target configuration was prepared, because the INS neutron field has not enough space for neutron spectrometry with the TOF method. In the CYRIC, TIARA and RIKEN experiments, the neutron spectra were measured with the TOF method using a organic liquid scintillator. The absolute neutron flucnce of the monoenergy peak was determined with the PRT (Proton Recoil counter Telescope) at TIARA, and with the Li activation method to detect the 7Be activity from the 7Li(p,n)7Be reaction at CYRIC and RIKEN. Figs.l and 2 show the neutron spectra for 43,58,68 and 88MeV proton incidence at TIARA and for 90, 100, 110,and 120 MeV proton incidence at RIKEN, respectively . 236- JAIiRI-Conf 96-008 1 104 6 10' 43McV(Ep) 9OMeV(Ep) 58MeV(Ep) 5 103 . 10OMeV(Ep) « 8 10' u (iSMeV(Ep) 1 lOMeV(Ep) a. !2OMcV(Ep) 88MeV(Ep) 1 \ 4 10- 2 *: "\r\i I 3 1O3 * 410' 210J e 3 2 10' 4J I 1 10' 0 10° 0 10° 140 20 40 60 80 100 40 60 80 100 120 Neutron Energy(MeV) Neutron Energy(MeV) Fig I. Neutron spectra of 43,58,68 and Fig 2. Neutron spectra of 90,100,110, 88MeV p-Li reactions at TIARA and 120MeV p-Li reactions at RIKEN The irradiation samples are l2C,27Al,29Cu, 59Co and 209Bi . Tables 1, 2 and 3 show the physical data of irradiation samples in INS,TIARA and RIKEN experiments. Table 1. Physical data of irradiation samples at INS Sample Diameter Thickness Weight Purity I2C 20mm 2.2mm 1.57g 98.81% 27A1 30mm 2.0mm 3.8Og 100.00% :wBi 15mm 0.6mm 0.65g 99.99% Table 2. Physical data of irradiation samples at TIARA Sample Diameter Thickness Weight Purity 12C 33mm 1 mm 1.83g 98.81% :7AI 29mm 2mm 3.64g 100.00% s9Co 33mm 2mm 20. Ig 99.9% :wBi 29mm 2mm 6.90g 99.99% Table 3. Physical data of irradiation samples at RIKEN Sample Diameter Thickness Weight Purity 1:C 80mm 10.0mm 104g 98.81% :7AI 80mm 10.0mm I49g 100.00% wBi 80mm 10.8mm 530g 99.99% The samples were irradiated 10cm, 400cm, and 837cm behind the Li target at INS,TIARA and RIKEN, respectively. Irradiation time consisted of short irradiation time (1 - 237- JAKKI-Conf 96-008 to 2 hours under 120McV, 30min above 120McV) and long irradiation time (about 20 hours) by considering the half life of produced nuclei. During sample irradiation, proton beam currents were monitcrcd with the digital current integrator and sealer. The gamma rays emitted from irradiated samples were measured with a high purity Gc detector by coupling with the 4096 multi-channel analyzer. The carbon samples were put into an aluminum case to absorb the positron energy from "C nuclei produced by the ':C(n,2n)"C reaction, since the annihilation gamma rays of 51 lkcV were measured with the Gc detector. The peak efficency of the Gc detector was obtained from the mixed standard source and the self absorption of samples was calculated with the PEAK code (I) and the EGS4 codc(2). 3. Analysis The reaction rates of identified radioisotopes were obtained by analyzing gamma-ray spectra after corrected for the peak efficiency, sum-coincidence and self-absorption effects, also for the beam current fluctuation during sample irradiation. The reaction rate corrected for the beam current fluctuation becomes R= . ___n N.£.y.e-A'l\..(l_e-XT,,,).£ |Q..c-A(n-i)At| i (1) where is decay constant, C is counts of gamma-ray peak area , N is atomic number of the target, ~ is peak efficiency, * is branching ratio of gamma rays, Tni is counting time , Tc is cooling time and Q, is beam current for irradiation time interval At. As the neutron energy spectra used in this activation cross section measurement have a monocncrgctic peak and the lower energy continuum as seen Figs. 1 and 2, the reaction rate is divided into two components as follows, |-r,,,,,, |-nllux R=N o( n)- cK R)CIF.+N o( n)- <\i n)dn Mil -'Emin (2) where N is the number of target nucleus, -<E) the spallation cross section, (E) the neutron tluencc, Elh the threshold energy, Emin the minimum energy of the neutron peak and Enux the maximum energy of the neutron peak. The first term corresponds to the contribution from the lower energy neutrons, and the second term from monocnergctic peak neutrons. R is written as /"*--min R= o(E)- (})(E)dE+a(EPeak)- cI>(Ep,aK) (3) where -<Epi.,lk) is the average cross section at the monoencrgetic peak Epeali ,and <I>(Epeak)= (4) is the peak neutron tluence which is given by PRT or Li activation method. 238- JAERI-Conf 96-008 Finally, from Eq. (3) the cross section at Epeak can be estimated by Cmin ( o(E).t<EME #(Epeak) (5) If the threshold energy Elh is higher than Emin, the second term of the numerator of Eq.(5) is zero, otherwise, this term can be estimated by successive subtraction method using the neutron flux (E) having lower peak energy. The -<E) values in lower energy region were estimated from the evaluated data file, ENDF/B-VI[3], experimental data compiled by McLane et al.[4] and our experimental data obtained in lower energy region. The errors of cross section data were obtained from the error propagation law by combining the error of activation rate (2~ 10%), peak neutron fluence (5~ 15%), contribution from the reference cross section used to estimate the low energy neutron activation (4—48%). 4.Results and Discussions At present,we obtained the cross section values of 209Bi(n,4n)206Bi to 209Bi(n,10n) 200Bi, and l2C(n,2n)"C reactions. As examples, Figs.3,4,5 and 6 give the obtained cross section data. The cross section data of 2o9Bi(n,xn)2IOxBi reactions were compared with the ENDF/B-VI high energy file data calculated with the ALICE code[5]. Our experimental results are the first experimental data and arc generally in good agreement with them except for 2O9Bi(n,9n)2OIBi. A big discrepancy, about a factor of 4, between our experiment and ENDF/B-VI may come from the uncertainty of the decay scheme of 2OIBi,where we assumed the 100% branching ratio of 628keV gamma rays from the first excited state to the ground state. If this ratio is 25%, then our data well agree with the ENDF/B-VI. Our cross section data of l2C(n,2n) "C shown in Fig. 6 are lower in the peak region around 40MeV than the ENDF/VI high energy file data, but much higher than the ENDF/VI data above 60MeV. Our results below 40MeV show good agreement with the Brill's data[6]. Our first experimental data above 40MeV show almost a constant value of 20mb, which reveals that the ENDF/VI data of l2C(n,2n)"C reaction may be inaccurate. References [llT.Nakamura and T.Suzuki, Monte Carlo Calculation of Peak Efficiencies of Ge(Li) and Pure Gc Detector to Voluminal Sources and Comparison with Environmental Radioactivity Measure ment.
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