Gamma-Ray Spectroscopy With LaBr$_3$: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