Gamma-ray spectroscopy with LaBr3:Ce scintillator readout by a silicon drift detector

In this work we 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 behaviour, 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 far to exploit the best noise performances achievable with a SDD obtained at shaping times in the order of 1 μs. 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 the work and are compared with the performances achieved with other photodetectors, coupled to the same scintillator crystal. The SDD has an active area of 30 mm². Antireflective coatings have been implemented. Good energy resolutions were measured also at room temperature, thanks to the low leakage current of the detector: a resolution of 2.7% was measured at the ¹³⁷Cs 661.7 KeV line and a resolution of 6.1% at the ⁵⁷Co 122 KeV line. A discussion of the noise performances of the SDD is carried out in the last part of the paper.