Crystal growth and detector fabrication technologies have reached such a state of maturity that high-quality large-volume CeBr3 scintillators can now be produced with dimensions of 2″×2″ and well above. We present a study of CeBr3 samples of various dimensions and show that they have a number of advantages over equivalently sized LaBr3:5%Ce for gamma-ray spectroscopy applications requiring high detection sensitivity. At the present time, the achieved energy resolution of CeBr3 is about 4% FWHM at 662 keV, i.e. 25% worse than that of LaBr3:5%Ce. However, thanks to the drastically reduced intrinsic activity, CeBr3 gamma-ray detection sensitivity is about 1 order of magnitude better than that of LaBr3:5%Ce at energies of 1461 keV and 2614.5 keV, which are relevant for the detection of 40K and 208Tl (232Th), respectively. In this communication, we report on several aspects of CeBr3 gamma-ray spectrometers, such as scintillation characteristics, non-proportionality of the response, gamma-ray detection performances up to 3 MeV and radiation tolerance.

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Scintillation and detection characteristics of high-sensitivity CeBr3 gamma-ray spectrometers

The properties of large volume cylindrical 3.5″×8″ (89 mm×203 mm) LaBr 3 :Ce scintillation detectors coupled to the Hamamatsu R10233-100SEL photo-multiplier tube were investigated. These crystals are among the largest ones ever produced and still need to be fully characterized to determine how these detectors can be utilized and in which applications. We tested the detectors using monochromatic γ–ray sources and in-beam reactions producing γ rays up to 22.6 MeV; we acquired PMT signal pulses and calculated detector energy resolution and response linearity as a function of γ-ray energy. Two different voltage dividers were coupled to the Hamamatsu R10233-100SEL PMT: the Hamamatsu E1198-26, based on straightforward resistive network design, and the “LABRVD”, specifically designed for our large volume LaBr 3 :Ce scintillation detectors, which also includes active semiconductor devices. Because of the extremely high light yield of LaBr 3 :Ce crystals we observed that, depending on the choice of PMT, voltage divider and applied voltage, some significant deviation from the ideally proportional response of the detector and some pulse shape deformation appear. In addition, crystal non-homogeneities and PMT gain drifts affect the (measured) energy resolution especially in case of high-energy γ rays. We also measured the time resolution of detectors with different sizes (from 1″×1″ up to 3.5″×8″), correlating the results with both the intrinsic properties of PMTs and GEANT simulations of the scintillation light collection process. The detector absolute full energy efficiency was measured and simulated up to γ-rays of 30 MeV

Characterization of large volume 3.5″×8″ LaBr3:Ce detectors

High resolution γ-ray spectroscopy measurements at MHz counting rates were carried out at nuclear accelerators, combining a LaBr 3(Ce) detector with dedicated hardware and software solutions based on digitization and off-line analysis. Spectra were measured at counting rates up to 4 MHz, with little or no degradation of the energy resolution, adopting a pile up rejection algorithm. The reported results represent a step forward towards the final goal of high resolution γ-ray spectroscopy measurements on a burning plasma device.

High Resolution Gamma Ray Spectroscopy at MHz Counting Rates With LaBr $_{3}$ Scintillators for Fusion Plasma Applications

The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) , composed of two small satellites, is a new mission to monitor the Gamma-Ray Bursts (GRBs) coincident with Gravitational Wave (GW) events with a FOV of 100% all-sky. Each GECAM satellite detects and localizes GRBs using 25 compact and novel Gamma-Ray Detectors (GRDs) in 6 keV–5 MeV. Each GRD module is comprised of LaBr 3 :Ce scintillator , SiPM array and preamplifier . A large dynamic range of GRD is achieved by the high gain and low gain channels of the preamplifier. The energy response of GRD prototype was evaluated using radioactive sources in the range of 5.9–1332.5 keV. A energy resolution of 5.3% at 662 keV was determined from the 137Cs pulse height spectra, which meets the GECAM requirement ( 8% at 662 keV). Energy to channel conversion was evaluated and a nonlinearity correction was performed to reduce the residuals ( 1.5%). Also, a Geant4-based simulated in-flight background and a measured GRD LaBr3:Ce intrinsic activity were used to evaluate the capability of in-flight calibration. These results demonstrate the design of GRD.

Energy response of GECAM gamma-ray detector based on LaBr3:Ce and SiPM array

CeBr 3 gamma-ray spectrometers are preferable to LaBr 3 :Ce ones for low-count-rate experiments because of their low intrinsic activity and consequently increased detection sensitivity. A drawback of CeBr 3 is a nonoptimum energy resolution, i.e. 4% at 662 keV. Here we demonstrate that aliovalent co-doping techniques are effective to enhance the CeBr 3 energy resolution to at least 3%. Such an enhancement is achieved because of a more proportional energy response of the scintillation.

Co-doping of CeBr3 scintillator detectors for energy resolution enhancement

Lanthanum bromide scintillation detectors produce very high light outputs (∼60,000 ph/MeV) within a very short decay time (typically ∼20 ns) which means that high instantaneous currents can be generated in the photocathode and dynode chain of the photomultiplier tube (PMT) used for the scintillation readout. The net result is that signal saturation can occur long before the recommended PMT biasing conditions can be reached. In search of an optimized light readout system for LaBr3, we have tested and compared two different PMT configurations for detection of gamma-rays up to 15 MeV. This range was chosen as being appropriate for gamma-ray remote sensing and medium energy nuclear physics applications. The experiments were conducted at two facilities: the Laboratori Nazionali del Sud (LNS) in Catania, Italy [1] and the High Intensity Gamma-ray Source (HIγS) at Triangle University Nuclear Laboratory, in Durham, North Carolina, USA [2] . The PMT configurations we have tested are (1) a standard dynode chain operated under-biased; (2) a 4-stage reduced chain operated at nominal inter-dynode bias. The results are that shortening the number of active stages, as in configuration (2), has advantages in preserving energy resolution and avoiding PMT saturation over a large energy range. However, the use of an under-biased PMT, configuration (1), can still be considered a satisfactory solution, at least in the case of PMTs manufactured by Photonis. The results of this study will be used in support of the Mercury Gamma-ray and Neutron Spectrometer (MGNS) on board of BepiColombo, the joint ESA/JAXA mission to Mercury, scheduled for launch in 2014.

N. Brown

Papers

High energy gamma-ray spectroscopy with LaBr3 scintillation detectors