History of the GeDet group

The group was formed to study germanium detectors for the GERDA experiment. The first job was to verify that germanium detectors can work while submerged in liquid nitrogen or argon which was a novel idea for the GERDA experiment. Industrially, germanium detectors are always operated in vacuum.

The first test facility to operate a detector in a cryoliquid was called "Milchkanne" (milk churn), as can be seen in Figure 1.

It was not constructed for large segmented detectors but for a small unsegmented germanium n-type detector named ''Mikesch''. Mikesch is mounted on a stick holding all cables and electronics, see Figure 1, on which it is dunked into the cryogenic liquid inside the Milchkanne. The detector was protected by a teflon wrap to avoid mechanical damage. Measurements were only possible for periods of hours, but this test facility made the first operation of an n-type detector submerged in liquid nitrogen and liquid argon possible. With this test stand it was actually shown that the operation of germanium detectors is possible in liquid argon as well as in liquid nitrogen. The energy resolution stays the same in both liquids; spectra are shown in Figure 2.

The operation of Milchkanne was a bit rustic. So the next step was Gerdalinchen I.

K1 consisted of a conventional vacuum cryostat for one detector which was cooled via a cooling finger submerged in a liquid nitrogen dewar. The setup was originally built by Canberra France and later modified several time by the MPI group. The electronics and the preamplifier units were housed in two copper cylinders, "ears", attached to the side of the aluminum cryostat. There is one preamplifier per segment plus one for the core.

The facility was used to study the detector response to photons from a variety of radioactive sources like Co-60, Eu-152, Th-228. Low energy photons from Eu-152 were collimated for surface scans. K1 was used to characterise the 18-fold segmented true-coaxial detectors originally foreseen for GERDA and to study their potential to reduce photon-induced background. It was shown that segmentation itself provides a powerful tool to reduce background by one order of magnitude. It was also shown that the detectors are unique to investigate the general properties of germanium detectors. Segmentation was used in a novel way to measure the position of the crystal axes and a first measurement of the dependence of the lengths of the pulses on the temperature was made. The latter brought a surprise as the mobility of electrons in germanium does not follow the predicted T^3/2 behaviour, but can be described by a Boltzmann-like ansatz.