We have recently completed the second part of resurrecting an old coaxial HPGe detector disarmed for over 20 years in the lab’s stockroom. The first part was to find out all info about the detector, a difficult task considering we had no tech sheets or accompanying documents. Thankfully, the kind people of Areva/Canberra tracked down a copy of the original (but very limited) specs sheet of our detector based on the serial number and model number. We proceeded to fully characterize our old buddy with point-like radiation sources, estimating its linearity, absolute efficiency and misc. characteristics (inc. DC voltage, FWHM vs energy, LN2 cooling times, etc).
Our GEROS (Germanium for Environmental Radioactivity Studies, an acronym meaning literally “Old man” in greek) detector has moved to phase two of characterization some time ago. Vaso Angelopoulou has taken over the task to try and measure the actual dimensions of the crystal (a coaxial-type by manufacturing) based on the absorption of γ-rays, in fashion similar to tomography. Briefly, a weak γ-emitting source was placed behind a thick lead shield with an open narrow window, availing the bombardment of the detector with known-energy gamma rays. The source was moved across and along a grid of specific (X,Y) coordinates in front of the detector. Depending on the source position and due to the coaxial geometry, different numbers of photons were recorded by the detector. Reconstructing the 3D image, we managed to have a detailed picture of the inside of the detector (See picture below).
We are now able to simulate the detector using MCNP and Geant4 based on the crystal dimensions consolidated experimentally. Also, a full CT-scan will take place some time soon to get a full 3D image of the inside, obtaining information on additional technical data, such as supporting screws, preamp position etc.