Determination of high Qβ-values by beta-gamma-coincidence measurements with a plastic scintillator telescope

A βγ-coincidence system has been built to measure high-energy β-spectra of very short-lived isotopes. The β-detector telescope of this system consists of a large E-scintillation counter, which works in coincidence with a thin ΔE-detector. In addition, the E-scintillator is surrounded by a scintillator ring working in anticoincidence mode with the E-detector. All photomultipliers are cooled by Peltier elements in order to reduce and to stabilize their temperature. This telescope has been tested with monoenergetic electrons in the energy range between 500 keV and 10 500 keV at the conversion electron spectrometer BILL of the ILL. In order to demonstrate the performance of the coincidence system, the endpoint energies of β-spectra emitted in the decay of 92Kr, 139I and 139Xe have been measured at the mass separator Lohengrin. The total experimental errors of the Qβ-values derived from these measurements are smaller than 30 keV. (orig.)