Elsevier

Nuclear Physics A

Volume 1010, June 2021, 122192
Nuclear Physics A

Gamma-ray spectrum from Cd induced by fast neutrons in indoor experiments

https://doi.org/10.1016/j.nuclphysa.2021.122192Get rights and content

Abstract

Spectrum of gamma-rays from nuclear reactions with fast neutrons on cadmium measured by the time-of-flight method is analyzed. Impact of neutrons re-scattered on the experimental facilities and surroundings down to thermal energies is considered for indoor experiments. New unfolding procedure is proposed and used for determination from experimental amplitude spectrum of the gamma spectrum induced by fast neutrons only. The flux of the thermal neutrons is estimated by MCNP code. Spectrum of gamma ray from Cd induced by fast neutrons is corrected and its systematical uncertainties are estimated. The re-calculated gamma-spectrum is smaller than previous one and in the energy range ∼ 3.5÷6.5 MeV the differences between these spectra can reach ∼ 30%. Two wide peaks are observed as before in exponentially dropping yield for gamma-ray energies near neutron separation energy (∼ 4.5÷10 MeV) but with lesser intensities.

Introduction

The accurate values of the cross sections of gamma-ray production induced by neutron interactions are required for reactor technology development, such as radiation damage of reactor pressure vessel and internal construction elements undergoing neutrons irradiation, shielding calculations for fusion reactors etc. [1], [2]. Investigations of nuclear reactions with fast neutrons of the energies near 14 MeV are important for these tasks because of producing the great number of different particles and gamma-rays which can irradiate reactor elements. In some cases there are disagreements between nuclear data obtained by different authors [3] as well as between experimental results and theoretical calculations. In particular, they were found in the experiments for the isotopes of cadmium [4], which are commonly used in the reactor industry. This disagreement can be conditioned by the physical effects dismissed in the theoretical codes. For example, the absence of low-energy (pygmy) resonance in the photon strength functions [5], [6], [7], [8], [9], a simplification of collective enhancement of the nuclear level density [10]. It can be also resulted from simplification of the measurement procedure. In this contribution, the effects resulting from indoor measurements are discussed. The analysis is given for the experiments performed at Nuclear Physics Department of Taras Shevchenko National University of Kyiv (TSNU) [4], [11], [12], [13].

Section snippets

Outlines of the measurements and calculations

The gamma-ray spectra were measured for the following samples: natFe, natBi, natCd, natNi and natSn [4], [11], [12], [13]. Pulse neutron generator (PNG-200) was used to generate fast neutrons via H(d3,n)4He reaction on Ti-T target. Deuterons were accelerated up to 130 keV energies by low-voltage accelerator with klystron bunching of the beam. Pulse generation frequency was equal to 7.25 MHz, average neutron intensity I=107 s−1.

The PNG-200 room dimensions are 9.9 × 6.1 × 4.4 m and for the

MCNP simulation of neutron fluxes

The input geometry for MCNP simulations was presented in Fig. 1. The concrete walls, floor, roof and oil filled bucket for high voltage supply were also taken at simulations, but not shown in the Fig. 1(right panel) because of the differences in the size scales.

The neutron fluxes FΔ(En) per oneof source DT-neutron in the energy bin ΔEn at the energy En were estimated for three places in experimental hall: near the sample location (1), behind the shielding (2) and near the wall (3) (see Fig. 1

Gamma spectrum determination with extraction of re-scattered neutrons

As it was indicated in Section 2 for the TOF experiments, the amplitude spectrum Aγ(ε,En) of gamma-rays from (n,xγ) reactions at fixed neutron energy En is measured as a function of the energy deposited ε in the detector volume. The inclusive spectrum σγ(Eγ,En) of gamma-rays is determined by unfolding the Fredholm integral equation (1), which can be written in simplified designations asAf(ε,En)Aγ(ε,En)=F(En)αn(En)0UQΔε(ε,Eγ)σ(Eγ,En)dEγ, where U=Sn+En is the maximum excitation energy of

Results and conclusions

Impact on gamma-ray spectrum of fast neutron moderation upon the experimental facility and surroundings down to the thermal energies is analyzed for the indoor experiment with natCd sample. New unfolding procedure is proposed and used for determination of gamma-ray spectrum induced by fast neutrons from experimental amplitude spectrum. The flux of the thermal neutrons was estimated by MCNP code. The gamma-spectrum in the gamma-energy range ∼4.5÷10 MeV in reactions with fast neutrons on cadmium

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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