Improved accuracy in the determination of the thermal cross section of ${}^{14}{\rm N}({\rm n},{\rm p}){}^{14}{\rm C}$ for neutron lifetime measurement

Abstract

In a neutron lifetime measurement at the Japan Proton Accelerator Complex, the neutron lifetime is calculated from the neutron decay rate and the incident neutron flux. The flux is obtained by counting the protons emitted from the neutron absorption reaction of ${}^{3}{\rm He}$ gas, which is diluted in a mixture of working gas in a detector. Hence, it is crucial to determine the amount of ${}^{3}{\rm He}$ in the mixture. In order to improve the accuracy of the number density of the ${}^{3}{\rm He}$ nuclei, we have suggested using the ${}^{14}{\rm N}({\rm n},{\rm p}){}^{14}{\rm C}$ reaction as a reference because this reaction involves similar kinetic energy to the $^3$He(n,p)$^3$H reaction and a smaller reaction cross section to introduce reasonable large partial pressure. The uncertainty of the recommended value of the cross section, however, is not satisfied with our requirement. In this paper we report the most accurate experimental value of the cross section of the $^{14}$N(n,p)$^{14}$C reaction at a neutron velocity of 2200 m s$^{-1}$, measured relative to the $^3$He(n,p)$^3$H reaction. The result was 1.868 $\pm$ 0.003 (stat.) $\pm$ 0.006 (sys.) b. Additionally, the cross section of the $^{17}$O(n,$\alpha$)$^{14}$C reaction at the neutron velocity is also redetermined as 249 $\pm$ 6 mb.

Neutron lifetime measurement with pulsed cold neutrons

“…6, where the details are described in Ref. [42,43] 1 . Here, the volume ratio of the vacuum chamber for the TPC to the small container was determined as (1.497 ± 0.028) × 10 4 by measuring the pressure change when He gas was released from the container to the vacuum chamber.…”

Improvement of systematic uncertainties for the neutron lifetime experiment at J-PARC

“…where 𝑆3 He and 𝑆14 N are the number of measured events of 3 He(𝑛, 𝑝) 3 H reactions and 14 N(𝑛, 𝑝) 14 C reactions respectively, 𝜎3 He and 𝜎14 N are their capture cross-sections, and 𝜌 N 2 is the number density of nitrogen gas. We have performed the measurement of 𝜎14 N with high accuracy (1.868 ± 0.007 barn) [4]. The results of the two methods were consistent, and the uncertainty of 𝜌 workgas by the N 2 method achieved to 0.4% as shown in Table 1.…”

“…A new neutron lifetime experiment is in progress at J-PARC MLF BL05. In this method, the neutron flux and decay electrons are measured simultaneously by a time projection chamber (TPC) filled with a working gas mixture of 4 He of 85 kPa and CO 2 of 15 kPa, and 3 He gas to evaluate the neutron flux. The systematic uncertainties are different from the previous beam method because we measure the decay electrons, whereas the previous beam experiments measured decay protons.…”

“…A free neutron decays into a proton, an electron, and an antineutrino in a lifetime of about 880 s. The neutron lifetime dominates the uncertainty on 4 He abundance in the Big Bang Nucleosynthesis, and it also determines 𝑉 𝑢𝑑 term in the Cabibbo-Kobayashi-Maskawa quark mixing matrix. Although the neutron lifetime is very important in modern physics, there is a 4.6𝜎 (9.5 s) discrepancy between the results of the two typical methods: the beam method (888.0 ± 2.0 s) and the storage method (878.4 ± 0.5 s) [1,2].…”

Empirical Neutron Cross Section Formulae for (n, p) and (n, d) Reactions of He-3 Target

“…During calculation of the lifetime of neutron [18], protons emitted from the (n, p) reaction of the target He-3 were studied. Also, cross section of this reaction at a range of incident neutron energy 0.15 -150 keV was measured [19].…”