LaBr3 gamma?ray spectrometers for space applications

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.Radiation Science & TechnologyApplied Science

Experiments and theory of ¹³⁸La radioactive decay

RST/Luminescence MaterialsRST/Fundamental Aspects of Materials and Energ

Energy resolution and related charge carrier mobility in LaBr3:Ce scintillators

The scintillation response of LaBr3:Ce scintillation crystals was studied as function of temperature and Ce concentration with synchrotron X-rays between 9?keV and 100?keV. The results were analyzed using the theory of carrier transport in wide band gap semiconductors to gain new insights into charge carrier generation, diffusion, and capture mechanisms. Their influence on the efficiency of energy transfer and conversion from X-ray or ?-ray photon to optical photons and therefore on the energy resolution of lanthanum halide scintillators was studied. From this, we will propose that scattering of carriers by both the lattice phonons and by ionized impurities are key processes determining the temperature dependence of carrier mobility and ultimately the scintillation efficiency and energy resolution. When assuming about 100?ppm ionized impurity concentration in 0.2% Ce3+ doped LaBr3, mobilities are such that we can reproduce the observed temperature dependence of the energy resolution, and in particular, the minimum in resolution near room temperature is reproduced.RST/Radiation, Science and TechnologyApplied Science

Localization of cosmic gamma-ray bursts in interplanetary space with MGNS/BepiColombo and HEND/Mars Odyssey experiments

This paper describes the methods and results for the localization by triangulation of cosmic gamma-ray bursts (GRBs) independently observed by two space experiments: the Mercury Gamma-ray and Neutron Spectrometer (MGNS) and the High Energy Neutron Detector (HEND). MGNS is onboard the MPO/BepiColombo mission and on a stage of cruise to Mercury whereas HEND is onboard Mars Odyssey mission and in orbit around Mars. An analysis is performed of the accuracy of localization of the GRBs jointly observed by the two instruments at interplanetary distances by comparing their light curves. Notable achievements and scientific opportunities are described also in light of the recent inclusion of MGNS within the program of interplanetary network for gamma-ray burst localization (IPN).Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.RST/Luminescence Material

BepiColombo Science Investigations During Cruise and Flybys at the Earth, Venus and Mercury

The dual spacecraft mission BepiColombo is the first joint mission between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) to explore the planet Mercury. BepiColombo was launched from Kourou (French Guiana) on October 20th, 2018, in its packed configuration including two spacecraft, a transfer module, and a sunshield. BepiColombo cruise trajectory is a long journey into the inner heliosphere, and it includes one flyby of the Earth (in April 2020), two of Venus (in October 2020 and August 2021), and six of Mercury (starting from 2021), before orbit insertion in December 2025. A big part of the mission instruments will be fully operational during the mission cruise phase, allowing unprecedented investigation of the different environments that will encounter during the 7-years long cruise. The present paper reviews all the planetary flybys and some interesting cruise configurations. Additional scientific research that will emerge in the coming years is also discussed, including the instruments that can contribute.RST/Luminescence Material