A solid ionization chamber operated at 20 mK

We have built a germanium ''solid ionization chamber'' with aluminium evaporated electrodes. We obtained very low leakage current complete charge collection at 23 mK. However, we found that the spectra evolved as a function of time. To investigate this effect, the detector has been irradiated with the 43, 87, 105, and 662 keV photons of Eu-155 and Cs-137 and we analyzed this time evolution taking into account the different penetration depths of the photons. We also studied the influence of the ''history'' of the detector (the previous polarization voltages). We found that the electric field due to the built-in space charge (presumably induced by carriers trapping) plays an important role, and that the time evolution cannot be explained solely by the trapping of the carriers generated by each photon.