Speaker
Description
LaBr$_3$(Ce)-based detectors, which provide excellent time response together with high gamma detection efficiency and good energy resolution [1, 2], are optimal devices for a wide range of applications. Their scintillation properties are strongly influenced by their size and shape. Consequently, we have designed novel geometries of LaBr$_3$(Ce) crystals aimed at enhancing the light collection, and thus their scintillation characteristics. Moreover the new geometries make it possible to construct rings of detectors surrounding implantation points (detectors or catcher foils) with higher packing factor than cylindrical geometries, increasing the γ-ray detection efficiency.
In this work we evaluate the overall performance of two new geometries of LaBr$_3$(Ce) crystals with special attention to the time response. The crystals were coupled to Hamamatsu R9779 photomultiplier tubes (PMTs). The signals were processed with NIM analog electronics including ORTEC Constant Fraction Discriminator (CFD) 935 and Time to Amplitude Converter (TAC). Energy resolution and efficiency were measured using standard calibration sources and timing measurements were performed at $^{60}$Co and $^{22}$Na γ-ray energies against a fast BaF$_2$ reference detector. The time resolution was optimized by the choice of the PMT bias voltage and the fine-tuning of the CFD parameters.
In addition to energy resolution and γ-ray detection efficiency, we report the individual FWHM time resolutions for the two geometries, which are the best results reported to date for LaBr$_3$(Ce) crystals of these sizes. This achievement has been possible thanks to the new geometry designs in combination with the use of the ORTEC 935 CFD at very short delays. Monte Carlo simulations using the Geant4 toolkit have been performed to gain a better understanding of the effect of the new geometries on the light transport, and thus on the performance of the crystals. We report on the results from the measurements and the simulations and on the best crystal choice for fast-timing applications.
Bibliography
[1] Saint Gobain, Brillance 380 Brochure
[2] V. Vedia, H. Mach, L. M. Fraile, J. Udías, S. Lalkovski, Enhanced time response of 1-inch LaBr3 (Ce) crystals by leading edge and constant fraction techniques, NIM A 795 (2015)
[3] V. Vedia, M. Carmona-Gallardo, L.M. Fraile, H. Mach, J.M. Udías, Performance evaluation of novel LaBr3(Ce) scintillator geometries for fast-timing applications, NIM A in press, http://dx.doi.org/10.1016/j.nima.2017.03.030
