The 41 Ar(n,γ) 42 Ar reaction R. N. Sahoo 1 , M. Paul 1, ∗ , U. Köster 2 , R. Scott 3 , M. Tessler 4 , A. Zylstra 5 , M. L. Avila 3 , C. Dickerson 3 , H. Jayatissa 3 , M.S. Kohen 1 , J. McLain 3 , R.C. Pardo 3 , K. E. Rehm 3 , I. Tolstukhin 3 , R. Vondrasek 3 , T. Bailey 6 , L. Callahan 6 , A. Clark 6 , P. Collon 6 , Y. Kashiv 6 , and A. Nelson 6 1 The Hebrew University of Jerusalem, Jerusalem, Israel 91904 2 Institut Laue-Langevin, Grenoble, France 3 Argonne National Laboratory, Argonne, IL 60439, USA 4 Soreq Nuclear Research Center, Yavne, Israel 5 Lawrence Livermore National Laboratory, Livermore, CA, USA 6 University of Notre Dame, Notre Dame, IN 46556, USA Abstract. The cross-section of the thermal neutron capture 41 Ar(n,γ) 42 Ar(t 1/2 =32.9 y) reaction was measured by irradiating a 40 Ar sample at the high-flux reactor of Institut Laue-Langevin (ILL) Grenoble, France. The sig- nature of the two-neutron capture has been observed by measuring the growth curve and identifying the 1524.6 keV γ-lines of the shorter-lived 42 K(12.4 h) β − daughter of 42 Ar. Our preliminary value of the 41 Ar(n,γ) 42 Ar thermal cross section is 240(80) mb at 25.3 meV. For the first time, direct counting of 42 Ar was performed using the ultra-high sensitivity technique of noble gas accelerator mass spectrometry (NOGAMS) at Argonne National Laboratory, USA. 1 Introduction Neutron capture reactions and their cross section are es- sential for basic and applied nuclear physics. It was recog- nized by Cameron [1] and Burbidge, Burbidge, Fowler and Hoyle [2] that they play a crucial role in stellar production of heavy elements. The quest for experimental determina- tion of neutron capture cross sections has been intensely pursued for the study of the slow ( s) process [3]. How- ever, no experimental pathway exists to determine neutron capture rates on nuclei far from stability [4, 5] which are relevant to the rapid (r) process [6]. Various techniques have been proposed for providing indirect measurements of neutron-capture cross sections far from stability [7, 8]. Obtaining reliable data on neutron capture cross section for unstable isotopes remains a challenge and an essential task in contemporary research [9, 10]. Production of 42 Ar and its properties are not exten- sively studied. In the 1950’s and 1960’s, the half-life of 42 Ar was measured as 32.9±1.1 y and the cross section of the 41 Ar(n,γ) 42 Ar reaction at thermal energy was deter- mined as 0.5(1) b [11, 12]. 42 Ar(32.9 y) is thus known to undergo 100% β − decay to shorter-lived 42 K(12.36 h), itself further β − decaying to stable 42 Ca (Fig. 1). In an ex- periment approved at the National Ignition Facility (NIF) of Lawrence Livermore National Laboratory [13], we are considering 42 Ar as a candidate for the experimental ob- servation of a rapid two-neutron capture reaction on 40 Ar. The extreme high-density plasma and high-density neu- tron environment of a laser-induced Inertial Confinement ∗ e-mail: paul@vms.huji.ac.il Figure 1. Simplified decay scheme of 42 Ar and 42 K. Fusion shot at NIF is the closest terrestrial analog of stellar explosive nucleosynthesis. The experiment will consist of a high-power laser shot on a DT filled capsule seeded with 40 Ar atoms, where 42 Ar could be produced by the two- neutron 40 Ar(n,γ) 41 Ar(n,γ) 42 Ar reaction within ≈ 100 ps. The objectives of the present preparatory study, per- formed before the approved experiment at NIF, were twofold: (i) production of 42 Ar in a long irradiation of 40 Ar in a high flux of thermal neutrons and a new measurement of the 41 Ar(n,γ) 42 Ar reaction cross, and (ii) first demon- stration of direct detection of 42 Ar at ultra-high sensitivity, as required for the NIF experiment. 2 40 Ar sample preparation and irradiation A 0.768 cc high-purity quartz ampoule was filled with 99.992 % enriched 40 Ar gas [14] at 314(1) Torr and shipped to the high-flux reactor of Institut Laue-Langevin EPJ Web of Conferences , 01037 (2023) 284 ND2022 https://doi.org/10.1051/epjconf/202328401037 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http ://creativecommons.org/licenses/by/4.0/). s