Operating Performance OF He3-Cooled Bolometers

Abstract

The operation of semiconductor bolometer detectors at the lowest possible temperatures may be expected to produce major improvements in their ultimate performance. Ignoring photon noise and current noise in the detector, the ultimate thermodynamic limitation on the noise equivalent power (NEP) is the sum of the Johnson and temperature fluctuation noise powers [1] given by $$ NEP \simeq 4T{{\left( {k{{G}_{e}}} \right)}^{{ \frac{1}{2} }}}$$ (1) The response time constant τ r depends on the ratio of the heat capacity C of the thermosensitive conductor element and the thermal conductance G e between it and the temperature reservoir. Assuming C ∝ T 3, we see that for a fixed time constant the thermodynamic NEP will change approximately with the 5/2 power of the temperature. Cooling to a He3 temperature of 0.38 K should therefore allow improvement in the ultimate NEP by a factor of 30 compared to operation at the usual pumped He4 bath temperature of 1.5 K.