Abstract
We measured the spectrum of energies deposited by γ-radiation, emanating from radioactive materials in the laboratory that houses our mK cryostat, and by cosmic ray muons. This allows us to quantify the heat input that adversely affects the lowest temperature accessible in sub-mK experiments. We use our nuclear stage, stage plate and experimental cell as a prototype “model” system, and calculate the power deposited due to low energy (below 2.65 MeV) background radiation γ quanta (~20 pW). This is significantly less than the power (~120 pW) deposited in the nuclear and experimental stages by muons. Installation of a 5 cm thick lead wall around the cryostat reduced the energy due to the flux of γ quanta by a factor of ~10 to ~2 pW, and the number of γ quanta by a factor of ~20. The lower energy, “soft” cosmic ray component was also affected by introducing the same thickness of lead, reducing the overall count of cosmic ray derived particles by ~15% and the heat leak to ~100 pW.
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