LaBr3 gamma?ray spectrometers for space applications

Abstract

LaBr3 has been developed into large volume scintillator detectors within an ESA and TU Delft programme during this thesis work. The programme, which aimed at the space applications of LaBr3, also led to extensive experiments within a collaborative framework which included representatives for all the development aspects, i.e., from crystal growers to scintillator materials researchers to final users. The research presented in this thesis is the result from my in-depth involvement in that programme. The thesis’s first achievement was the assessment of the radiation tolerance of LaBr3 in Chapter 2. In fact, BepiColombo’s payload must withstand proton fluence as high as 6 × 1010 protons/cm2 to guarantee successful scientific observations. It was found that LaBr3 exceeds that requirement without substantial deterioration of its performance. Such an ability, together with the relative low mass and power resources demand that is typical for scintillation detectors, has made LaBr3 the choice for the BepiColombo onboard gamma-ray spectrometer. LaBr3 detectors is a new technology, and experimental studies were necessary to support instrument design and optimization for space applications. The deterioration of performance with increased detector size has been closely monitored in Chapter 4. The limits of LaBr3 application were also studied and, with the use of synchrotron radiation, it was found that LaBr3 is a non-ideal scintillator for photon detection in the X-ray domain, below 100 keV. The same technique proved to be an effective tool for non-proportionality studies and extensively applied by TU Delft colleagues. Scintillation readout of LaBr3 pushes the PMT to its operational limits. Experimental campaigns were performed to study the response to high energy gamma-rays as reported in Chapter 5. This gave a unique opportunity to observe the correlation between gamma-ray energy and energy resolution up to 15 MeV. In addition, it offered the benchmark to verify in flight energy calibration capability. As demonstrated in Chapter 5 of this thesis, results are satisfactory, which makes LaBr3 applicable for the BepiColombo mission. However the results also suggest to initiate new programmes to develop alternative scintillation readout techniques as with silicon Photomultipliers and Silicon Drift Detectors. The major disadvantage of LaBr3 is a lack of sensitivity because of the intrinsic activity generated by 138La decays, in particular at 1.4 MeV, which is relevant for the detection of 40K. Full characterization of this activity in Chapter 6 led to the first experimental determination of the low-energy end of a second-order-unique-forbidden ? continuum. A deviation with the standard theoretical models on nuclear decay was found of which an explanation is not at hand yet. Development of alternative materials able to challenge LaBr3 for energy resolution and efficiency but with much reduced intrinsic activity is presently ongoing. A selection of recent results achieved with CeBr3 spectrometers were presented in Chapter 7, showing that CeBr3 detection sensitivity at 1.4 MeV is about 8 times higher compared to LaBr3.