Influence of secondary neutrons on alpha-particle induced reaction cross section measurement below the Coulomb barrier

TL;DR

This study addresses anomalously high cross sections for $^{nat}$Pt$(\alpha$,x)$^{195m}$Pt measured below the Coulomb barrier by attributing them to secondary neutrons produced in the stacked Pt foils. The authors couple PHITS-based neutron-field simulations (using INC and NDL options) with renormalized $^{194,195,196}$Pt$(n,x)^{195m}$Pt cross sections from JENDL-5/A, fitted via least-squares to EXFOR data, to estimate the neutron-induced yield $Y$ and the per-foil cross section contribution $\sigma_n=Y/n$. They find that secondary neutrons can largely account for the observed subbarrier production, while secondary light charged particles contribute negligibly; neutrons above $20$ MeV have only a minor impact ($\lesssim$1%). The results emphasize the importance of accounting for secondary-neutron backgrounds in low-energy charged-particle activation studies and suggest practical corrections (e.g., end-of-stack foils) and potential benchmarking with neutron dosimetry for robust cross-section measurements.

Abstract

The influence of the secondary neutrons on measurements of alpha-particle activation cross sections below the Coulomb barrier was studied for the $^\mathrm{nat}$Pt($α$,x)$^\mathrm{195m}$Pt reaction. We characterized the secondary neutron field by using the particle transport simulation code PHITS, and estimated the $^\mathrm{nat}$Pt(n,x)$^\mathrm{195m}$Pt yields by using the characterized neutron spectra and the $^\mathrm{nat}$Pt(n,x)$^{195m}$Pt cross sections in the JENDL-5/A library. We confirmed that the unexpectedly high $^\mathrm{nat}$Pt($α$,x)$^\mathrm{195m}$Pt cross sections below the Coulomb barrier measured by us are explained well by the $^\mathrm{nat}$Pt(n,x)$^\mathrm{195m}$Pt reaction induced by the secondary neutrons. This indicates that the secondary neutron effect is sometimes not negligible even in low energy charged-particle activation cross section measurements. We also studied the influence of the secondary light charged particles by the same approach, and confirmed that their influence is negligible.

Figures (7)

• Figure 1: Comparison of double differential thick target neutron yields for Bi irradiated by 30 MeV alpha-particles measured at RIKEN Sugihara2020 with those predicted by PHITS INC and NDL calculations. The error bars of the experimental yields are small and not visible.
• Figure 2: $^{194}$Pt(n,$\gamma$)$^{195m}$Pt , $^{195}$Pt(n,n')$^{195m}$Pt and $^{196}$Pt(n,2n)$^{195m}$Pt cross sections in the JENDL-5/A library Iwamoto2023 multiplied by the isotopic abundances of the target nuclides (left: original, right: renormalized) and the experimental cross sections Luo2010Fan1985Zhao1984Hankla1972.
• Figure 3: Neutron energy spectra calculated with the INC option in the 1st (Upstream), 5th (Center) and 9th (Downstream) platinum foils from the upstream side under the 30 MeV alpha-particle irradiation (left), and in the 1st (Upstream), 10th (Center) and 19th (Downstream) platinum foils from the upstream side under the 50 MeV alpha-particle irradiation (right). Each ten neutron energy groups introduced in Section \ref{['sec:nfield']} collapse to one bin for better visual clarity.
• Figure 4: Comparison of neutron energy spectra with and without GEM in the 5th platinum foil from the upstream side under the 30 MeV alpha-particle irradiation calculated with the INC option. Each ten neutron energy groups introduced in Section \ref{['sec:nfield']} collapse to one bin for better visual clarity.
• Figure 5: $^\mathrm{nat}$Pt($\alpha$,x)$^{195m}$Pt cross sections under 30 MeV (left) and 50 MeV (right) alpha-particle irradiation. The data points shown at -2, -4 and -6 MeV on the right panel are for the 17th, 18th, and 19th platinum foils, which are not exposed to alpha-particles.