Development of beta-ray full absorption detector for measurement of decay energy of unstable nuclei far off stability.

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We have developed a new detector, namely, a sum-detector, for beta-ray decay energy determination of nuclei far off stability. It consists of two identical HPGe detectors, which are located in face to face. It can detect electrons efficiently, which back-scatter with each detector and measure the energy sum of two detectors. In conclusion, this idea is much effective for beta-ray measurements. Simulation in high energy gave a perspective for detector design. If putting much larger crystals in close geometry without Be windows is much powerful for rare isotopes like new isotopes.We proceeded this project as follows.1. We ordered special Ge detectors (LEPS by ORTEC) which have 36mm^<phi>xl3mm^t crystals and thin Be window (50 mum^t) . The distance between the Be window and the crystal surface is 7 mm.2. Response functions for monoenergetic electrons/positrons were measured with the Ge detectors. Using a beta-ray spectrometer in Kyoto University Reactor Research Institute (KURRI) , quasi-monoenergetic electrons/positrons were produced with radioactivities (^<90>Sr/Y and ^<38>Cl for electrons and ^<68>Ge/Ga and ^<34m>Cl for positrons) to 4 MeV Pair production by high energy photons produced by 30MeV electron linear accelerator (KUR-LINAC) was utilized up to 9 MeV.The obtained response functions were evaluated as three components, namely, full energy absorption, scattering and bremsstrahlung, respectively. The response functions of the sum-detector were determined with 625 keV conversion electrons of ^<137>Cs experimentally and simulated using a code (EGS4) in higher energy region.3. The radioactivities of ^<32>P ^<42>K and ^<38>Cl (Q_<beta> : 1.7-4.9MeV) were produced with thermal neutron irradiation in KUR and measured with the sum-detector. Two Ge detector were put as close as possible. The distance between crystals was 17 mm. In comparison with a beta-ray singles spectrum, typical efficiency was 20% in ^<42>K.These results are consistent with the simulation.