7. Shielding design for the COBRA R&D set-upset-up
7.3. Shielding against environmental background
To shield the CdZnTe detectors operated in the R&D set-up at LNGS from environmental radiation these are surrounded by layers of copper and lead.
Depending on the configuration up to 16 CdZnTe detectors of approximately (10×10×10)mm3 size are assembled in a supporting structure made of laser cut POM sheets. Up to four of these layers can be installed above each other in-side a copper structure usually referred to as the NEST (compare [Mü07]). The NEST furthermore is designed to be equipped with five calibration tubes made of PTFE above, below and between the four detector layers. The NEST has a di-mension of(10×10×10)cm3and its front was in the old design closed by two 5 cm thick copper bricks leaving a small horizontal gap in between to run cables to the preamplifiers. The structure holding the detector layers is surrounded by 5 cm thick copper bricks so that in total outer dimensions of(20×20×20)cm3 are reached. In the old design the copper was surrounded by at least 15 cm of lead of unknown quality to shield the detectors from γ radiation from the envi-ronment. The cables were run through a special lead brick with a V-shaped gap to prevent any lines of sight to the detectors (compare Figure 7.4a).
The old design of the cable feedthroughs in the copper and lead layers aimed at the usage of thin flat cables made of Kapton® for the signal read out as well as the supply with HV. Due to discharges the usage of Kapton® for the HV side was not suitable and had to be changed to single cables. Since usual cables require larger bending radii than the ones necessary for cables made from foil, the design of the feedthroughs in copper and lead had to be modified (compare Figure 7.4).
Furthermore, it was decided to surround the copper shielding layer, which is very radiopure, with lead bricks of an intrinsic activity of 210Pb of less than 3 Bq/kg to reduce the intensity of bremsstrahlung of high energetic β particles emitted in the decay of the210Pb daughter nuclide210Bi.
The cables used for the supply of the high voltage are glued to the bottom POM plates of each layer. To reduce the bending radii of the cables two feedthroughs
(a) Feedthrough and NEST before upgrade.
(b) Feedthrough and NEST after upgrade.
Figure 7.4.: Cross section through feedthrough and NEST before and after upgrade.
Only the bottom POM plate of each detector layer is drawn, since the HV supplying cables are mounted to it. The holes for the calibration tubes are visible. The vertical positions of the new feedthroughs are centred to the lower edges of two layers each.
Drawings not to scale.
were foreseen so that the cables of two layers each run through one of these. Since the Kapton® cables used for the signal read out are much more flexible than the HV cables the vertical position of the feedthroughs is centred to the distance be-tween the lower edges of the upper and lower detector layer pair. The mechanical drawings for the different new and modifications to existing parts were done by the author. The machining of the copper blocks as well as the outer straight lead feedthrough was done by the mechanical workshop at LNGS to reduce the expo-sure of the copper parts to cosmic radiation and limit the activation to a minimum level. The lead feedthrough formed like a wave was machined according to the authors drawings from lead with less than 3 Bq/kg 210Pb by Plombum, Poland, who provided the other high purity lead bricks as well.
Due to the additional layer of high purity lead the total thickness of the new lead shielding is 20 cm. To reduce lines of sight through the lead shielding, after a stock taking of all available parts the arrangement of the complete lead shielding was planned with CAD by the author. These drawings were prepared so that
Figure 7.5.: View into the opened new COBRA set-up with the EMI shielding consisting of galvanised steel sheets. The radon shielding made of Tropac III foil has been replaced recently by a housing made of polycarbonate. In the back the additionally installed per-amplifier cooling system is visible. The cable feed trough in the EMI shielding can be seen at the left edge of picture.
they are readable for COBRA shifters installing new detectors in the R&D set-up without requiring knowledge of the used CAD software.
To prevent the diffusion of airborne222Rn into the NEST and reduce the back-ground induced by it and its daughter nuclides a system for flushing with filtered gaseous nitrogen was designed in [Mü07] and improved in [Ale09, Sch11]. To increase the efficiency of the flushing it was planned to surround the lead and copper shielding with a special foil impermeable for diffusing radon gas. Based on the results of Mamedov et al. [M ˇCSŠ11] Tropac III foil was chosen. The gas-tight enclosure for the lead shielding was constructed by J. Tebrügge [Teb11] by using aluminium sheets and the aforementioned foil so that it possible to keep the set-up under a slight overpressure.