LaX3 scintillator research

Future planetary missions such as BepiColombo and Solar Orbiter are extremely resource limited in both mass and power. Due to the vicinity of the spacecraft to the Sun, the onboard instrumentation will face harsh environments as far as radiation levels and thermal loads are concerned. Only radiation-hard detectors that need little or no cooling will be able to successfully operate after long cruise times and during the expected mission lifetimes. Until the present time the energy resolution of scintillators was considered too poor to satisfy the geochemistry resolution requirements for remote-sensing gamma-ray spectrometers – meaning, their ability to uniquely separate the elemental gamma-ray lines emanating from the surface regolith. Lanthanum halide (LaX₃:Ce) scintillators have revolutionized spectroscopic systems because of their excellent energy resolutions and proportionality of response when compared with traditional scintillating materials. LaBr₃:Ce5% has a fast light output decay (16 ns) and an emission spectrum with a peak at 380 nm. The light yield is typically 60 000 photons per MeV or 165% relative to a standard NaI(Tl) scintillator. The material density is 5.07 g cm^-3. FWHM energy resolutions of 3% at 662 keV (¹³⁷Cs) and 2.0% at 1332 keV (⁶⁰Co) have been reported for smaller volume crystals (5.3 cm³ and 12.5 cm³).

The Future Missions Preparation Office in collaboration with Saint Gobain Crystals and Detectors, the Technical University of Delft, and Cosine Research BV, began an ambitious research and development programme to advance crystal growth technology to produce large volume LaX₃ detectors. The stated goal of this programme was to produce large LaBr₃ volume scintillation crystals to be used as part of the Mercury Gamma-ray and Neutron Spectrometer (MGNS) on BepiColombo. These crystals have been successfully produced.

Specifically, the gamma-ray detector has to be:

• 3 × 3 inch right circular cylinder crystals with no cracks or imperfections
• have an energy resolution at 662 keV of ≤ 3% FWHM
• radiation hard to a total fluence of 1 Mrad equivalent into Si

Because of requirement 2, most of the work concentrated on LaBr₃.

At the start of the programme, there were four overriding questions:

• could crystals be grown to the correct size without cracking?
• would the energy resolution degrade with increasing volume?
• would radiation degrade the scintillation mechanism resulting in worse energy resolution?
• after a major solar particle event would the material activate so badly as to be unusable?

Last Update: 15 March 2012