Beta-spectrum shape measurements with a 4π semiconductor spectrometer

Abstract

A 4π semiconductor spectrometer employing diffused window lithium-drifted silicon detectors has been constructed. It was tested by varying the parameters of the system, so as to eliminate possible sources of distortion. It was further tested by measuring the shapes of two simple β-spectra ( 143Pr and 185W) and two inner group β-spectra ( 198Au and 86Rb) in coincidence with the subsequently emitted γ-rays. The γ-ray pulse, detected by a NaI well counter that envelops the beta spectrometer, supplied the gate to the 400-channel pulse-height analyser. In all cases, the results of these measurements are in good agreement with those previously reported. The beta spectrum of the 170Tm decay to the first excited level in 170Yb was measured in coincidence with the 84 keV gamma ray. The experiment was performed under the same measurement conditions used for the 198Au and 86Rb experiments. The spectrum exhibits a definite non-statistical energy dependence, falling below any straight line Fermi-Kurie plot fit at low energies. The shape factor was fitted by a polynomial of the form 1+ aW+ b/ W+ cW 2. A good fit was obtained for a = 1.11, b = −0.85, c = − 0.24. The end-point of this transition was determined to be 879±7 keV.