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The cross-section of the thermal neutron capture⁴¹Ar(n,γ)⁴²Ar(t_1/2=32.9 y) reaction was measured by irradiating a⁴⁰Ar sample at the high-flux reactor of Institut Laue-Langevin (ILL) Grenoble, France. The signature of the two-neutron capture has been observed by measuring the growth curve and identifying the 1524.6 keV γ-lines of the shorter-lived⁴²K(12.4 h) β⁻daughter of⁴²Ar. Our preliminary value of the⁴¹Ar(n,γ)⁴²Ar thermal cross section is 240(80) mb at 25.3 meV. For the first time, direct counting of⁴²Ar was performed using the ultra-high sensitivity technique of noble gas accelerator mass spectrometry (NOGAMS) at Argonne National Laboratory, USA.

 

The thermodynamical conditions and the neutron density produced in a laser-induced implosion of a deuterium-tritium (DT) filled capsule at the National Ignition Facility (NIF) are the closest laboratory analog of stellar conditions. We plan to investigate neutron-induced reactions on 40 Ar, namely the 40 Ar( n , 2 n ) 39 Ar( t 1/2 =268 y), the 40 Ar( n , γ) 41 Ar(110 min) and the potential rapid two-neutron capture reaction 40 Ar(2 n , γ) 42 Ar(33 y) in an Ar-loaded DT capsule. The chemical inertness of noble gas Ar enables reliable collection of the reaction products. 

Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, λ = g A / g V , through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter a with a precision of δ a / a = 10 −3 and the Fierz interference term b to δ b = 3 × 10 −3 in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio λ with a precision of δλ/λ = 0.03% that will allow an evaluation of V ud and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects.