Recent Neutron Activation Cross Section Measurements

Journal of Nuclear Science and Technology

ISSN: 0022-3131 (Print) 1881-1248 (Online) Journal homepage: http://www.tandfonline.com/loi/tnst20

Recent Neutron Activation Cross Section Measurements J.M.Arjan Plompen, Donald L. Smith, Peter Reimer, Syed M. Qaim, Valentina Semkova, Ferenc Cserpák, Vlad Avrigeanu & Sándor Sudár To cite this article: J.M.Arjan Plompen, Donald L. Smith, Peter Reimer, Syed M. Qaim, Valentina Semkova, Ferenc Cserpák, Vlad Avrigeanu & Sándor Sudár (2002) Recent Neutron Activation Cross Section Measurements, Journal of Nuclear Science and Technology, 39:sup2, 192-197 To link to this article: http://dx.doi.org/10.1080/00223131.2002.10875073

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Journal of NUCLEAR SCIENCE and TECHNOLOGY, Supplement 2, p. 192-197 (August 2002)

Recent Neutron Activation Cross Section Measurements Arjan I.M. PLOMPEN h , DonaIdL. SMITH2, Peter REIMER 1,3, Syed M. QAIM3 , Valentina SEMKOVA 1,4, Ferenc CSERPAK1,5, Vlad AVRIGEANU 6 , Sandor SUDAR 5 1 European

Commission, Joint Research Center, Institute for Reference Materials and Measurements, 2440 Geel, Belgium 2 Argonne National Laboratory, Technology Development Division, Argonne, Illinois 60439, USA 3 Forschungszentrum Jillich, Institut /iir Nuklearchemie, D-52425 Jillich, Germany 4 Institute for Nuclear Research and Nuclear Energy, 1784 Sofia, Bulgaria 5 University of Debrecen, Institute for Experimental Physics, H-4001 Debrecen, Hungary 6 Institute for Physics and Nuclear Engineering Horia Hulubei, 76900 Bucharest, Romania

Recently cross sections have been measured with the activation technique for selected (n,xn), (n,xp) and (n,xa) reactions on isotopically enriched and natural samples of V, Cr, Co, Ni, Cu, Zn, Se, Zr, Mo, Tc, I, and Pb. Here results are presented for the measured cross sections of the 92Mo(n,p)92mNb, 95Mo(n,p)95mNb, natMo(n,xp)94Nb, 98 Mo(n,p )98m Nb, 96 Mo(n,np )95m Nb, 96 Mo(n,p )96 Nb, 97 Mo(n,p )97m Nb, 97 Mo(n,p )97m+ 9 Nb, 97 Mo(n,np Nb, 98Mo(n,np)97mNb, 98 Mo(n,np)97m+ 9 Nb, 92Mo(n,2n)91mMo, 94 Mo(n,2n)93mMo, 100 Mo(n,2n)99 Mo, 92 MoCn,a )89mZr, 100 Mo(n,a )97 Zr, 208 Pb(n,p)208 TI, 206 Pb(n,a )203 Hg, 206Pb(n,3n) 204m Pb, 204 Pb(n,n f 04m pb, 204 Pb(n,2n)203 Pb, 204 Pb(n,2n) 203m Pb, 204Pb(n,3n)202m1 Pb, 1291(n,2n)128 I, 58Ni(n,np)57 Co, reactions and for the isomeric ratio of the 59Co(n,2n)58m,gco and 58Ni(n,p)58m,gNi reactions. Cross section measurements focussed on the energy range from 16 to 20.5 MeV with few exceptions. Half-lives of the reactions studied vary from 6.3 s to 20300 years. Isotopically enriched samples and high purity natural samples were used to separate reactions leading to the same activity. The measurement results are compared with work by other authors, current evaluated data files and new model calculations. The ongoing and planned work is indicated.

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KEYWORDS: activation; nuclear reactions; measured cross sections; isomeric ratio; statistical model; preequilibrium emission; evaluation; gas production; cobalt; iodine-129; molybdenum; nickel; lead

I.

Introdu.ction

Measurement of cross sections with the activation technique is an efficient method to survey the status of our knowledge with regard to (n,xn), (n,xp) and (n,xa) reactions and, of course, on reactions leading to activation products. The new and recent measurements of this collaboration emphasize the energy range from 15 to 20 MeV and isotopes of materials that are important for nuclear reactors. 1-5) Often the measurements provide unique results because data in this energy range are scarce, and because of the use of enriched samples and the radioactive isotope 1291. The measured data are of relevance to applied issues such as hydrogen and helium gas production, the number of displacements per atom, neutron multiplication, and the accumulation of activation products (decay heat) in Accelerator Driven Systems (ADS) and fusion reactors. Although the energy range of interest for ADS extends up to a few GeV, neutrons with energies below 20 MeV playa considerable role with regard to hydrogen and helium gas production and the number of displacements per atom. 6) It may further be noted that (n,xp), (n,xa) and (n,xn) reactions above 15 MeV are often poorly studied, and that the scatter of the data around 15 MeV is often large. 1,4) The latter implies that data of interest to fusion reactors in the 14 MeV range may be obtained by interpolation of data at higher (and lower) energies since these establish the full excitation curve of a particular reaction channel.

* Corresponding author, Tel. +32-14-571381, Fax. +32- 14-571 376, E-mail: [email protected] 192

From our work, the need for benchmarking evaluated data against measured cross sections clearly emerges. 1,3) Furthermore, since in many cases of interest cross sections are not easily measured, calculations based on optimized physics modeling are the only resort. Measured data are needed to benchmark such model calculations.4 ) In this contribution we present measured cross sections for neutron-induced reactions on isotopes of molybdenum and lead, on 129 1, and finally for the 59Co(n,2n)58m+gCo and 58Ni(n,p)58m+gCo reactions and the isomer ratio of the latter two. The present results are compared with excitation functions found in evaluated cross section libraries and, for molybdenum and lead, new model calculations. Model calculations were performed with the codes STAPRE7 ) for the reactions on lead and with a modified version of STAPRE-H95 8 ) for the reactions on molybdenum. The aim was to explore the possibility of consistent model descriptions using all available experimental information relevant to the reactions studied under this collaboration. Also, the predictive power of the models for addressing nearby unmeasured reaction channels and isotopes is being assessed. Requirements for further measurements based on sensitivity analyses of model parameters and the status of the data uncertainties3 ,9) will be formulated elsewhere. Recent measurements and model calculations for vanadium and technetium were presented by Reimer et at. 3-5) Data analysis is ongoing for meas urements on Cu, Zn, Sr, Y, Zr, Ag and Au isotopes. Further model calculations are planned for earlier measurements of Cr isotopes and for the reactions on Ni and Co presented here.

193

II.

~easuremoents

The measurement procedure here closely follows that reported by Fessler et at. I)


1.

Irradiations and Normalization For the results presented here, all samples were irradiated at the GeeI 7 MV Van de Graaff accelerator. Neutrons from 16 to 20.5 MeV were obtained by use of the T(d,n) reaction with deuteron energies of 1, 2,3 and 4 MeV, and a titanium-tritide target of 2 mg/cm 2. Occasionally, neutrons with energies between 0.7 and 4 MeV were used, produced by the T(p,n) reaction. Four different setups were used, depending on the half-life of the activity. For the 204Pb(n,2n? 03m Pb reaction, where the activity has a half-life of 6.3 s, a new pneumatic transport system was used with automated control over the sample position and data-acquisition. The sample is repeatedly transported between irradiation and counting positions until sufficient statistics is accumulated. Transport takes place over 3 m and lasts 3 s. In each cycle the sample is irradiated for 3 half-lives while its activity is counted for 6 half-lives. The second irradiation setup uses a pneumatic transport system that is hand controlled. It was used for samples with half-lives between 30 s and a few minutes, and is fully described by Fessler et at. I) The third irradiation setup is a light weight holder that allows samples to be placed from 1 to 5 cm from the neutronproducing target with angles varying from 0 to 135 degrees with respect to the incident deuteron (proton) beam. It is used for activities with half-lives of at least 3 minutes and up to a few months. The setup is such that the influence of multiple scattering is less than 2% for all reactions involved in this work. The fourth irradiation setup concerned only the study of the natMo(n,xp )94 Nb reaction. Since the half-life of the activity (20300 years) is exceptionally long, a large sample mass of about 1.4 kg of high purity natural Mo was arranged around the target. Two concentric rings and one disk were placed at three nominal angles. This allows for three data points at three different energies, albeit with relatively poor energy resolution. The closest distance to the neutron source was 2.5 cm; the thickness of the samples was 1.4 cm. The irradiation was done for 100 h with 4 MeV incident deuterons at 10 f-L A of beam. In all cases, the beam intensity variation was measured and corrected for by the use of a BF3 proportional counter or a Bonner Sphere, the count rates of which were stored in multi-channel scaling mode. Neutron attenuation, neutron scattering, the sample geometry, and the neutron source angular distribution were accounted for by use of the MCNP4C code. IO) A stack of monitor foils with different thresholds was used to determine the fluences due to low-energy parasitic neutrons. 5 ) Depending on the threshold of the reaction studied, use was made of the 11 5In(n,n'), 58Ni(n,p), 27 AI(n,p), 27 AI(n,a), 93Nb(n,2n) (and/or 59Co(n,2n)) standard cross sections. Normalization of the data was done relative to the 27 AI(n,a?4Na ENDFIB-VI standard cross section.

SUPPLEMENT 2, AUGUST 2002

2.

Activity Determination Activities of all irradiated samples were determined by ,ray counting. The pertinent decay data were taken from the NUDAT database. I I ) HPGe detectors were used that were calibrated with point sources obtained from PTB and DAMRI and with 94Nb from IRMM. Corrections were made for summing coincidence effects, self attenuation, and sample size. A special case was the determination of the activity of 94Nb. For the lowest two neutron incident energies this was done in the HADES laboratory at 240 m below ground level. I 2, 13) This resulted in much lower uncertainties than those associated with conventional low-background detector arrangements. This is witnessed by the difference in uncertainties for the lower two and the highest energy results for this reaction.

III.

~odel

Calculations

Calculations were done for the reactions on molybdenum and lead to investigate if a consistent description can be obtained by use of physical models. The molybdenum calculations made use of the modified STAPRE-H95 8, 14, 15) code for determining the statistical and pre-equilibrium components to the cross sections. The choice for the level density model, and the importance of preequilibrium surface effects in the frame of the geometry dependent hybrid model, were addressed in these proceedings by Avrigeanu et al. 16 , 17) The calculations for the lead reactions were done with the STAPRE code?) using the exciton model for the preequilibrium contributions. Following comparison of the measured total cross section data for 204Pb, it appeared that the Bersillon-Cindro I8 ) and Koning l9 ) optical model potentials were the most suitable for the neutron channel. For protons the OMP of Perey20) was used, and for alphas that of McFadden-Satchler. 21 ) For the level densities, discrete levels were used up to a maximum excitation energy. Above this energy the backshifted Fermi-gas model was used. The level density parameters were obtained from the RIPL compilation,22) from Plyaskin et at.,23) or by smooth interpolation. The backshift was determined from a fit to the cumulative number of discrete levels. The calculations here use an effective moment of inertia that is 50% of the rigid rotor moment of inertia. In addition to the calculations presented here, model calculations were done for the lead reactions in which the rigid rotor moment of inertia was adopted for the Fermi-gas model approach and in which a variable moment of inertia was adopted following Junghans et at. 24) Further work in this direction is ongoing.

rv.

Results

Figure 1, Fig. 2 and Fig. 3 present the measured cross sections, the new model calculations and a comparison with the EAF-99 25 ) and JENDL-3.226) evaluated data files for the reactions on molybdenum isotopes. Figure 4 presents the measured cross sections for reactions on lead, cobalt, nickel and 1291, as well as the isomer ratio measurements for the 59Co(n,2n)58m,gco and 58Ni(n,p)58m,greactions.

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Fig. 4 Comparison of our measured crosss sections (0) with existing data (taken from EXFOR 27 ), evaluations, and STAPRE calculations 7) for some reactions on Pb Isotopes, the 129 l(n,2n)128 I and 58 Ni(n,np)57 Co reactions. Also shown are the isomer ratio measurements of the 59Co(n,2n)58m,gco and 58Ni(n,p)58m,gco reactions.

JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY


197

The results for the 129 I(n,2n)1 28 I reaction compare well with those by Murata et at. and Nakano et at. around 14 MeV. The data by Kuhry et al. appear to be high. The EAF-99 evaluation is too high by about 40-50%. JENDL-3.2 is high as well, although by about 25-50%. Our measurements for the 58Ni(n,np)57Co reaction confirm the results of two recent measurements by Pavlik et at. and Iwasaki et al.. The good agreement with these authors supports the use of this reaction as a secondary standard for the higher energy range. The ENDF/B- VI evaluation is recommended for this reaction. The JENDL-3.2 evaluation is too low at high energies (about 10%). The isomer ratio measurements for the 59Co(n,2n)58m,gco reaction agree with earlier measurements by Bormann et at. and Ghorai et at. and confirm an essentially energy independent value of about 0.7. The isomer ratio measurements for the 58Ni(n,p )58m,gco reaction show a constant value of about 0.55 above 14 MeV and a constant value of about 0.29 below 4 MeV. In both energy ranges these results disagree with certain earlier measurements. Our results for 58Ni have an impact on the discussion 2 &-30) concerning the influence of the level scheme and decay of 58CO. New model calculations for the entire energy range and both reactions are underway. It may further be noted that careful cross section measurements were done for the 59Co(n,2n)58m+ gCo and the 58Ni(n,p)58m+gCo reactions. Results closely agree with the ENDF/B- VI (standard) evaluations.

v.

Conclusions

Measured cross sections are presented for reactions on isotopes of molybdenum and lead, the 129I(n,2n)1281 and 58Ni(n,np)57Co reactions. Measured isomer crosssection ratios are given for the 59Co(n,2n)58m,gco and 58Ni(n,p)58m,gco reactions. A comparison with the EAF-99 and JENDL-3.2 evaluated data was made and areas of improvement emerged in particular for EAF-99 and to a lesser extent for JENDL-3.2. Full model calculations were performed for the molybdenum and lead reactions that often show a fair agreement with the data and can be used in several cases to improve evaluated data libraries. References I) A Fessler, A.J.M. Plompen, D.L. Smith, J.w. Meadows, and Y. Ikeda, Nuc!. Sci. Eng. 134, 171 (2000). 2) A Fessler, E. Wattecamps, D.L. Smith, and S.M. Qaim, Phys. Rev. C 58, 996 (1998). 3) AJ.M. Plompen, P. Reimer, S.M. Qaim, A Fessler, and D.L. Smith, "Neutron-Activation Cross Sections: Measurements and Model Sensitivity", Proceedings of the Physor 2000 Conference, Pittsburgh, PA, USA, May 7-12 (2000).

SUPPLEMENT 2, AUGUST 2002

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