CROSS-SECTION MEASUREMENTS of (N,Xn) THRESHOLD REACTIONS in Au, Bi, I, In, and Ta

O. SVOBODA et al., ND2010 Proceedings –Cross-section Measurements of (n,xn) Threshold Reactions in Au, Bi, I, In and Ta

CROSS-SECTION MEASUREMENTS OF (n,xn) THRESHOLD REACTIONS IN Au, Bi, I, In, AND Ta

O. SVOBODA et al., ND2010 Proceedings –Cross-section Measurements of (n,xn) Threshold Reactions in Au, Bi, I, In and Ta

O. SVOBODA1,2*, J. VRZALOVÁ1,2,A. KRÁSA1, A. KUGLER1, M. MAJERLE1,V. WAGNER1

1Nuclear Physics Institute of the Academy of Sciences of the Czech Republic PRI,Řež near Prague, 250 68,Czech Republic

2Faculty of Nuclear Sciences and Physical Engineering, CTU in Prague, Břehová 7, 115 19 Prague, Czech Republic

*Corresponding author. E-mail :

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Accepted for Publication

O. SVOBODA et al., ND2010 Proceedings –Cross-section Measurements of (n,xn) Threshold Reactions in Au, Bi, I, In and Ta

We measured neutron cross-sections of various threshold reactions using different quasi-monoenergetic sources in the range 17 – 94 MeV. Our motivation comesfrom the “Energy plus Transmutation project”, in which Al, Au, Bi, In, Ta, Co, Y foils are used to measure the flux of high energy neutrons produced in spallation reactions. Unfortunately, no experimental cross-section data for observed threshold (n,xn) reactions are available at neutron energies above 20 MeV.

We performed eleven measurements of the (n,xn) cross-sections using the neutron source atNPI ASCR cyclotron in Řež and at TSL cyclotron in Uppsala (EFNUDAT program). Quasi-monoenergetic neutrons from p+Li reaction irradiated above mentioned foils. Irradiated foils were measured on HPGe detectors and spectroscopic corrections were applied to obtained cross-section data.

KEYWORDS : Energy plus Transmutation, (n,xn) reaction, quasi-monoenergetic neutron source

O. SVOBODA et al., ND2010 Proceedings –Cross-section Measurements of (n,xn) Threshold Reactions in Au, Bi, I, In and Ta

1. MOTIVATION AND CURRENT (n,xn) CROSS-SECTION STATUS

We are members of the international project called “Energy plus Transmutation”[1], in which spallation reactions, transport of high energy neutrons and transmutations are being studied. For the measurements of neutron flux activation detectors in the form of thin foils from Al, Au, Bi, Co, In, Ta, and Y are being used. Wide range of (n,xn) threshold reactions induced by high energy neutrons in above mentioned foils are observed, number of emitted neutrons (x) is up to 10.

Almost no experimental cross-section data exist for x higher than four and neutron energies over ~40 MeV in the EXFOR library[2]. Only Bi was measured up to (n,12n) and neutron energy of 150 MeV,but in one experiment only [3].

Evaluated libraries offer (n,xn) cross-sections for higher x and energies up to hundreds of MeV. Deterministic code TALYScan calculate (n,xn) cross-sections up to neutron energy 200 MeV[4].Both evaluated libraries and TALYS 1.0 describe well the shape of the cross-section, but they differ in absolute value.

1. EXPERIMENT BACKGROUND

In 2007, anexperiment on (n,xn) cross-section measurements was successfully proposed by us to EFNUDAT [5]. Three irradiations were performed in the summer 2008 in The Svedberg Laboratory (TSL), Uppsala, Sweden. These were supported with another four irradiations performed during 2008-09 in the Nuclear Physics Institute (NPI) in Řež, Czech Republic. After successful comparison of measured cross-sections with the EXFOR library, further irradiations in TSLwere proposed to the EFNUDAT and performed in February 2010 in order to fill in the gaps between already measured cross-section values. Other irradiations are planed to be done in NPI in the autumn 2010.

2.1Neutron sources

For (n,xn) cross-section measurements by the means of activation analysis intensive quasi-monoenergetic neutron sources with well known spectrumwereused. Neutron sourceswere based on the 7Li(p,n)7Be reaction. High energy protons from the cyclotron were directed to a thin, lithium target, the neutron flux density was in the range from105 cm-2s-1 (TSL) up to 108cm-2s-1 (NPI). Approximately a half of the intensity was in the peak with FWHM = 1 MeV (corresponds to the ground state and first excited state at 0.43 MeV in 7Be) and half of intensity was in the continuum in lower energies (corresponds to higher excited states, multiple-particle emission etc.), see(Fig. 1 and Fig. 2).Proton energy loss in the target amounted to 2-6 MeV depending on the incident beam energy and target thickness.

In TSL, proton beams with energies 25, 50, 62, 70, 80, 92 and 97 MeV were used. Rest of the proton beam was behind the Li target deflected by a magnet and guided onto a graphite beam dump (more details in [6]). The neutron beam was formed by100 cm long iron collimator with a 12.2 cm hole. Samples were placed 373 cm from the Li target, irradiation time was 8 hours.

In the NPI, proton beams with energies 20, 25, 32.5, and 37 MeV were used. Behind the Li target, graphite plug was used to stop the rest of proton beam (more detail on source description in [7]). Samples were placed 11-16 cm from the Li target, irradiation time was 15-20 hours. Neutron spectra are considered to be the same as in the work of Y. Uwamino [8].

Fig. 1.Quasi-monoenergetic neutron spectrum from 7Li(p,n)7Be at the TSL.

Fig. 2.Quasi-monoenergetic neutron spectrum from 7Li(p,n)7Be at the NPI (overtaken from [8]).

Uncertainty of the neutron spectrum determination is 10% for both sources.Uncertainty of the beam intensity determination is 5% at NPI, respectively 10% at TSL.

2.2Gamma measurements and evaluation

Measured materials (Al, Au, Bi, Co, In, Ta, Y)were in the form of foils with dimension 1.5x1.5 cm2 up to 3x3 cm2, thickness of the foils was in the range 50 m up to 1 mm. Iodine was in the form of KIO3 powder, which was pressed to compact pill and sealed to polyethylene coating. Dimensions and thickness of the samples were modified from irradiation to irradiation in order to get enough activated nuclei.

Activated foils were as soon as possible measured on the HPGe detectors (transport times were from 2 minutes in TSL up to 15 minutes in NPI). Multiple measurements of each foil were done, close geometries were necessary due to low activities of the foils (distance between detector-sample was 33 mm in NPI, respectively 40 mm in TSL).HPGe detectors were calibrated with the set of laboratory etalons, uncertainty of the efficiency calibration is under 1%.

Gamma spectra were analysed in the DEIMOS32 code[9]. Minimal statistical uncertainty from the Gauss-fit of the gamma peaks is 1%, but lover than 10% for most of the peaks. To calculate the amount of observed isotopes, all available gamma lines were used and it was done a weighted average from their yields.Half-lives of the isotopes and gamma-line intensities were taken from theLund/LBNL Nuclear Data Search [10].

Following set of spectroscopic corrections was used to correct the raw data: correction on decay during cooling, corr. on decay during irradiation, self absorption corr., non-point like emitter corr., real coincidence corr., detector efficiency and dead time corr., gamma line intensity corr. Correction on the course of the irradiation was not used due to high stability of the irradiation.

2.3Background subtraction

Contribution of neutrons coming from low energy tail (background) to the total production of observed isotope was computed for all isotopes. Deterministic code TALYS 1.0 was used to calculate the (n,xn) cross-sections, basic setting of the code was used. Calculated cross-sections were folded with the neutron spectra and were computed shapeless ratios between production in the peak and total production. Yield of studiedisotope was multiplied by this calculated ratio and the cross-section wasevaluated from this multiplied value.

Fig. 3. Cross-sections of various reactions compared with the neutron spectrum from 32 MeV proton beam on Li target (NPI). In the parenthesis are ratios of peak production to total.

This procedure subtracts the contribution of the background neutrons in the beam, but inserts another possible uncertainty to the measured cross-section data. The procedure is insensitive to the absolute value of the TALYS cross-section, but sensitive to the cross-section shape. Comparisons of the TALYS calculations with the EXFOR data and with measured cross-sections show good shape agreement and thus a proper function of the background subtraction procedure.

1. RESULTS

When using two different neutron sources, HPGe detectors, and various foil geometries, a special attention was paid to comparisons among the results, data in EXFOR and TALYS calculations. Some examples of comparisons with the data found in EXFOR and with TALYS 1.0 calculationsarein the following Fig. 4 – Fig. 6. Numerical values of measured cross-sectionsarein the Table 2.Total uncertainty is a square root of the second powers of Gauss-fit, beam intensity determination and neutron spectrum uncertainties. Evaluation of the TSL experiments performed in the February 2010 was not yet completed (62, 70, 80 and 92 MeV). Their results will be published later together with the rest of the data from previous experiments.

Fig. 4.Cross-sections of 197Au(n,2n)196Au reaction from EXFOR database, TALYS calculation, NPI and TSL experiments.

Fig. 5. Cross-sections of 197Au(n,4n)194Au reaction from TALYS calculation, NPI and TSL experiments.

Fig. 6.Cross-sections of 197Au(n,5n)193Au reaction from TALYS calculation, NPI and TSL experiments.

Table 1Energy uncertainties for different neutron energies.

Table 2Cross-section values for various reactions.

Table 2 - continuation

1. CONCLUSION

Cross-sections of threshold reactions in Al, Au, Bi, I, In, and Ta were studied in the energy region from 17 to 94 MeV by the means of activation analysis and gamma spectroscopy. Quasi-monoenergetic neutron sources at TSL Uppsala and at NPI Řežwere used. Comparison of the results with the cross-sections published in EXFOR is in good agreement, so it can be assumed that also the rest of measured cross-sections is reliably determined.

Cross-sections from the last irradiation in TSL Uppsala, performed in February 2010, were not yet finally evaluated and will be published later. Presented cross-sections are final at this moment, but we cannot exclude some cosmetic changes and improvement of the values with the connection to the results of our new experiments.

ACKNOWLEDGMENTS

We would like to thank to the staff of the TSL Uppsala (especially to Alexander Prokofiev and Torbjörn Hartman) and to the staff of the cyclotron in Řež for great support and excellent beams. Our special thanks belong to Marek Fikrle for the help with the set up of I samples. We would like also to thank to Pavel Bém, Eva Šimečková and Milan Honusek for the possibility to joint their irradiations.

This work was supported by the EFNUDAT program, grant number CTU0808214and by the F4E program of the Nuclear Reaction Department (NPI), F4E-2008-GRT-014.

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