Towards a measurement of α-decay lifetime variation at high pressure: The case of $ ^228$Th

We suggest that a change in the lifetime of the α decay of $ ^228$Th ($T_{1/2}$ = 1.9127(5) yr) may be detected at static pressures as high as 1 Mbar, a regime nowadays achievable in the laboratory by means of diamond anvil cells. The effect is due to the perturbation of the nuclear Coulomb potential and decay Q-value by the electron environment, and is enhanced, in this case, by the high compressibility of Th. The Thomas-Fermi model, within the frame of the Feynman-Metropolis-Teller treatment of the equation of state, has been used for calculations. It is found that at a compression factor of 2 on the volume of the Wigner-Seitz cell, the fractional variation in the lifetime of $ ^228$Th is about -2 x $10^{-4}$ (conservative estimate). Detailed experimental procedures to measure this effect by compressing $ ^228$Th metal in a diamond anvil cell are presented, where diagnostics is based on activity-vs-time recording on suitable lines of the decay γ-spectrum, e.g. the 2.614-MeV peak stemming from the disintegration of the decay product $ ^208$Tl, at secular equilibrium.

本研究提出，在高达1兆巴（Mbar）的静压力环境中，可探测到²²⁸钍（²²⁸Th）的α衰变（α decay）寿命变化——该衰变的半衰期T₁/₂=1.9127(5)年。当前实验室可通过金刚石对顶砧（diamond anvil cells）实现该压力区间。该效应源于电子环境对原子核库仑势（Coulomb potential）与衰变Q值（decay Q-value）的扰动，而钍（Th）的高压缩性进一步放大了这一效应。本研究采用托马斯-费米模型（Thomas-Fermi model），结合费曼-梅特罗波利斯-特勒（Feynman-Metropolis-Teller）对物态方程（equation of state）的处理框架开展计算。研究发现，当维格纳-塞茨胞（Wigner-Seitz cell）体积的压缩因子为2时，²²⁸钍衰变寿命的相对变化量约为-2×10⁻⁴（保守估算值）。本文还详述了通过金刚石对顶砧压缩²²⁸钍金属以测量该效应的实验方案：实验诊断基于衰变γ谱（γ-spectrum）特定谱线的活度随时间的记录，例如久期平衡（secular equilibrium）下由衰变产物²⁰⁸铊（²⁰⁸Tl）衰变产生的2.614 MeV峰。