Direct detection of the ≈ 8.4eV internal conversion energy of ²²⁹ᵐTh embedded in a superconducting nanowire

Abstract

We report on a direct measurement of the ≈8.4 eV nuclear excitation energy of the isomeric first-excited state ²²⁹ᵐTh via the internal conversion (IC) decay channel. Thermalized and mass-filtered recoiling ²²⁹ᵐTh ions from ²³³U α decay are delivered to the surface of a superconducting nanowire sensor and become embedded. The ion is neutralized, triggering the IC decay, and the energy released by the IC decay is detected with high quantum efficiency by the nanowire sensor. Energy resolution is enabled by the current dependence of the internal quantum efficiency of the nanowire sensor. The techniques presented here are complementary to light-based detection schemes. The IC decay channel is about eight orders of magnitude faster than the photoemission channel, thus the ability to detect IC decays with high efficiency with superconducting nanowire sensors is likely to be a valuable tool for future ²²⁹ᵐTh experiments.