Gamma-Ray Spectroscopy With LaBr :Ce Scintillator Readout by a Silicon Drift Detector

In this paper, the authors propose a gamma-ray spectrometer based on a LaBr₃ :Ce scintillator coupled to a silicon drift detector (SDD). The SDD is a photodetector characterized by a very low noise thanks to the low value of output capacitance independent from the active area. With respect to a PMT, the SDD offers a higher quantum efficiency which reduces the spread associated to the statistic of photoelectrons generation. Also with respect to an APD, the SDD offers a lower photoelectrons statistic contribution, which, in the APD, is worsened by the excess noise factor with respect to pure Poisson statistics. Moreover, the SDD has a stable behavior, less sensitive to temperature and bias drift. In the past years, good energy resolutions were measured using a SDD coupled to a CsI:Tl crystal. However, the long shaping time, to be used with this scintillator to prevent ballistic deficit, was too far to exploit the best noise performances achievable with a SDD obtained at shaping times in the order of 1 mus. On the contrary, this optimum shaping time is fully compatible with the short decay time of the LaBr₃ :Ce crystal (about 25 ns). The results of the experimental characterization of the LaBr₃ :Ce-SDD gamma-ray spectrometer are presented in this work and are compared with the performances achieved with the same crystal coupled to a PMT and to a CsI(Tl) crystal coupled to the same SDD. The SDD has an active area of 30 mm². Antireflective coatings have been implemented. Good energy resolutions were measured at room temperature, thanks to the low leakage current of the detector: 2.7% at the¹³⁷ Cs 661.7 KeV line and 6.1% at the ⁵⁷Co 122 KeV line. A resolution of 5.7% at 122 KeV line was measured at 0 degC