Improving the Accessibility and Efficiency of Proton Irradiations for ⁴He/³He Thermochronology

Abstract

Synthesizing uniform and high concentrations of 3He within minerals via high‐energy proton irradiation is paramount for 4He/3He thermochronology and helium diffusion kinetic studies. Proton irradiations of geological material have hitherto exclusively been routinely conducted at the Francis H. Burr Proton Therapy Center (FHB); we thus explored alternative irradiation protocols at two European‐based facilities with the intention to improve the accessibility and efficiency in obtaining 4He/3He data. We conducted a single irradiation at the Paul Scherrer Institute (PSI) using an approach most similar to that used at FHB (wide, high‐energy beam), and four irradiations at the Helmholtz Zentrum Berlin (HZB) using a newly developed in‐vacuum irradiation protocol in a narrow, lower‐energy but high‐intensity beam. Internal shards of Durango apatite were irradiated in all experiments; 4He/3He release spectra and bulk 3He concentrations of PSI and HZB‐irradiated Durango shards were compared to those from FHB to assess the quality of each experiment in terms of the quantity and uniformity of synthesized 3He. While 3He was uniformly synthesized in PSI‐irradiated Durango shards, the bulk 3He concentration was below the required threshold due to limitations on the maximum allotted proton flux. Over the course of four irradiation experiments at HZB, the protocol evolved to ensure that uniform and high concentrations of 3He can be consistently induced. Furthermore, we demonstrate how HZB irradiations can be replicated using computer simulations, permitting the use of simulations to inform future modifications of the irradiation protocol in order to optimize the uniformity of the 3He distribution across all irradiated samples.