| Research Abstract |
The noises current originated by thermal generation of electron-hole pairs in semiconductor detectors is basically governed by the Boltzmann factor of exp(-E_<th>/kT), where E_<th> denotes the characteristic energy, k Boltmann constant, and T the absolute temperature. Silicon semiconductor detectors can be operated at room temperature, because the number of density of electron-hole pairs, n_<e-h>, in silicon is on he order of 10^<10> cm^<-3> at T=25℃ owing to the high value of E_<th>(1.1 eV). On the contrary, n<e-h> in germanium is as high as 10^<13> cm^<-3> at T=25℃ due to the low value of E_<th>(0.7 eV), making the cryogenic condition compulsory to the operation of germanium semiconductor detectors. However, n_<e-h> in germanium at moderate cryogenic condition (T=-65℃) is already as low as that in silicon at room temperature. With an electrode as small as the pixel size assumed in the present work (100 μm × 100 μm), furthermore, the noise current should be reduced less than 100 nA pe
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r pixel, since it is proportional to the volume of the semiconductor each pixel electrode covers. The number of density of electron-hole pairs generated by ionizing particles is, on the other hand, independent from the detection volume, increases with the energy of ionization particles, and localized within a few hundreds μm. This argument suggests that, when pixilated in a few hundreds μm, the germanium semiconductor can be operated near room temperature. The present study has confirmed that the noise current is already reduced to 10 mA at room temperature in a tiny germanium detector with a volume of 1 mm × 1mm, and is further reduced by two orders of magnitude at T=-73℃. This implies that the noise current will be reduced below 30 nA with a germanium crystal pixilated into a proposed standard size of 0.2 mm × 0.2 mm × 0.3 mm, and supports the operation possibility of a pixilated germanium detector at near room temperature. However, there was a dark current observed with an applied voltage higher than 10 V, which is presumably a surface leakage current. The latest version of pixilated germanium detectors fabricated has, therefore, a crystal volume of 5 mm × 5 mm, a pixel size of 0.75 mm × 0.75 mm, and a pixel number 2 × 2, together with a guard-ring electrode that is to remove the dark current. Less
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