GeDet Test Facilities
The GeDet group operates germanium detectors in vacuum and submerged in cryogenic liquids. A summary can be found here [pdf] [ps].
For some more information and individual pictures, please read on.
K1 consists of a conventional vacuum cryostat for one detector which is cooled via a cooling finger submerged in liquid nitrogen, see Fig.1. It was built by Canberra France and modified at the MPI. The electronics and the preamplifier units are housed in two copper cylinders attached to the side of the aluminum cryostat, see Fig.2. There is one preamplifier per segment plus one for the core.
K1 is used to study the detector response to photons from radioactive sources made of Co-60, Eu-152, Th-228 and other isotopes. Low energy photons from Eu-152 can be collimated facilitating surface scans. K1 is also used to study the detector response to neutrons, for example from an AmBe neutron source.
Figure 1: K1 (.eps) (.jpg)
Figure 2: K1, one ''ear'' open (.eps) (.jpg)
''Galatea'' is a new vacuum test stand currently being commissioned at the MPI. The main feature is that there is no material between source and detector. This allows the usage of alpha and beta sources as well as of a laser beam to study surface effects.
The setup is optimized to hold one (18-fold) segmented n-type germanium detector.
The detector is cooled via a cold finger submerged in an internal liquid nitrogen tank located inside the vacuum tank. The detector is shielded against IR radiation (emitted by the warm vacuum tank) by a copper hat with slits to insert sources. The hat can be turned with respect to the detector. One slit is on the side of the hat, the other on top. The sources are placed inside two collimators pointing through these slits. A system of three stages allows a complete scan of the mantle of a detector, see Fig.1.
Galatea is a powerful high precision tool to investigate bulk and surface effects in germanium detectors.
Figure 1: Galatea (.eps) (.jpg)
One of the tasks of the GeDet group was to verify that germanium detectors can work while submerged in liquid nitrogen or argon. Three test facilities of increasing complexity, Milchkanne, Gerdalinchen I and Gerdalinchen II were developed for this task.
''Milchkanne'' is a small and very simple test facility. Its diameter is about 20cm and the height is about 40cm, see Fig.1.
It is now mainly of historical importance. 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 Fig.2, 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 are only possible for periods of hours, but this test facility facilitated the first operation of an n-type detector submerged in liquid nitrogen and liquid argon. 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 Fig.3.
Figure 1: Milchkanne (.eps) (.jpg)
Figure 2: Detector Mikesch (.eps) (.jpg)
Figure 3: Eu-152 energy spectrum (.eps) (.jpg)
''Gerdalinchen I'' was the first test stand built to facilitate long term measurements with large detectors submerged in cryogenic liquid. It has a diameter of about 40cm and a height of about 70cm and holds around 60l of cryogenic liquid. One or two detectors in their holders are hung from the lid, see Fig.1. The cables are fed through the lid and electronics is mounted on top.
This test facility provided the experience necessary to build a follow-up test facility that ensures safe cooling and warming procedures and it provided data on the importance of shielding against infrared radiation, IR. Measurements with and without shielding against IR demonstrated the sensitivity of the detectors. The leakage current increased by 3 orders of magnitude without shielding against IR from the lid.
Gerdalinchen I is now used for all kinds of tests in liquid nitrogen or argon. One example are durability tests of technical components. The refilling procedure in the lab is shown in Fig.2.
Figure 1: Gerdalinchen I drawing (.eps) (.jpg)
Figure 1: Gerdalinchen I refilling (.jpg)
''Gerdalinchen II'' is the follow-up test stand of ''Gerdalinchen I''. It reproduces the conditions of the GERDA experiment to a large extent. It can hold up to three detectors in their holders in a string configuration as envisioned for the GERDA experiment. The detectors are inserted from the top; the dewar is filled with cryogenic liquid after the device is closed an flushed with nitrogen. The detectors can be operated in the cryogenice liquid as long as is wanted, bascially over years.
This test facility was used to verify that large segmented n-type germanium detectors can be operated in cryogenic liquids over long periods of time. The data was also used for several analyses revealing important properties of germanium detectors. Figures 1 and 2 show schematic drawings; Fig. 3 shows the open test facility with a detector mounted waiting for insertion.
Figure 1: Gerdalinchen II schematic (.eps) (.jpg)
Figure 2: Gerdalinchen II, schematic, cut through (.eps) (.jpg)
Figure 3: Gerdalinchen 2 with a detector mounted and waiting for insertion (.eps) (.gif)