Measurements of Phononic and Electronic Heat Transfer Coefficients at T < 0.1 K for Magnetic Microcalorimetry

Magnetic Micro-Calorimetry (MMC) has been developed for high-precision ²⁴¹Pu beta spectroscopy in the search for a keV-scale sterile neutrino. In this approach, a gold absorber containing the ²⁴¹Pu source is thermally coupled to an MMC device, which consists of a paramagnetic sensor and a Superconducting QUantum Interference Device (SQUID). The Q value of each particle decay is fully converted into thermal energy, which is detected via the correlated magnetic flux of doped ions in the paramagnetic film. The temperature of the detector system is then cooled to the base temperature through various thermal pathways to the detector holder. To achieve high statistical precision in the experiment, minimizing the detector cooldown time is crucial. We measured the heat transfer coefficients of two major pathways: electronic heat transfer via gold wires between the paramagnetic film and the detector holder, and phononic heat transfer via the paramagnetic film–silicon substrate–copper holder surface boundaries, in the context of optimizing heat flow to reduce the detector cooldown time. This was accomplished by measuring the detector cooldown times at various temperatures between 20 mK and 100 mK, both with and without gold wirebonds for comparison.