Impact of embedded $$^{163}$$Ho on the performance of the transition-edge sensor microcalorimeters of the HOLMES experiment

We present a detailed investigation of the performance of transition-edge sensor (TES) microcalorimeters with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mn>163</mml:mn></mml:mmultiscripts> </mml:math> Ho atoms embedded by ion implantation, as part of the HOLMES experiment aimed at neutrino mass determination. The inclusion of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mn>163</mml:mn></mml:mmultiscripts> </mml:math> Ho atoms introduces an excess heat capacity due to a pronounced Schottky anomaly, which can affect the detector's energy resolution, signal height, and response time. We fabricated TES arrays with varying levels of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mn>163</mml:mn></mml:mmultiscripts> </mml:math> Ho activity and characterized their performance in terms of energy resolution, decay time constants, and heat capacity. The intrinsic energy resolution was found to degrade with increasing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mn>163</mml:mn></mml:mmultiscripts> </mml:math> Ho activity, consistent with the expected scaling of heat capacity. From the analysis, we determined the specific heat capacity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mn>163</mml:mn></mml:mmultiscripts> </mml:math> Ho to be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo> <mml:mn>2.9</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.4</mml:mn> <mml:mrow><mml:mo>(</mml:mo> <mml:mi>stat</mml:mi> <mml:mo>)</mml:mo></mml:mrow> <mml:mo>±</mml:mo> <mml:mn>0.7</mml:mn> <mml:mrow><mml:mo>(</mml:mo> <mml:mi>sys</mml:mi> <mml:mo>)</mml:mo></mml:mrow> <mml:mo>)</mml:mo></mml:mrow> </mml:math> J/K/mol at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo> <mml:mn>94</mml:mn> <mml:mo>±</mml:mo> <mml:mn>1</mml:mn> <mml:mo>)</mml:mo></mml:mrow> </mml:math> mK, close to the literature values for metallic holmium. No additional long decay time constants correlated with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mn>163</mml:mn></mml:mmultiscripts> </mml:math> Ho activity were observed, indicating that the excess heat capacity does not introduce weakly coupled thermodynamic systems. These results suggest that our present TES microcalorimeters can tolerate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mn>163</mml:mn></mml:mmultiscripts> </mml:math> Ho activities up to approximately 5 Bq, with only about a factor of three degradation in performance compared to detectors without <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mn>163</mml:mn></mml:mmultiscripts> </mml:math> Ho. For higher activities, reducing the TES transition temperature is necessary to maintain or improve the energy resolution. These findings provide critical insights for optimizing TES microcalorimeters for future neutrino mass experiments and other applications requiring embedded radioactive sources. The study also highlights the robustness of TES technology in handling limited amounts of implanted radionuclides while maintaining high-resolution performance.