Assessment of the radiation tolerance of LaBr3:Ce scintillators to solar proton events

Abstract

Radiation effects caused by solar proton events will be a common problem for many types of sensors on missions to the inner solar system because of the long cruise phases coupled with the inverse square scaling of solar particle events. In support of the BepiColombo and Solar Orbiter missions we have undertaken a comprehensive series of tests to assess the effects on a wide range of sensors. Sensors based on scintillators have been proposed for both missions. In this paper, we report on a series of controlled irradiations on a set of five LaBr3:Ce scintillators. The crystals are 1in. right circular cylinders of LaBr3, packaged in aluminium housings and viewed through BK7 optical windows. Four crystals were exposed to simulated solar proton events over the energy range 60–200MeV having a spectral shape approximating that of the August 1972 solar particle event. Each crystal was exposed to a different total fluence. One crystal was exposed to an integral fluence of 109protonscm−2, a second to 1010protonscm−2, the third to 1011protonscm−2 and the fourth to 1012protonscm−2. The latter corresponds to an absorbed dose in silicon of 1Mrad or in SI units, 10kGy. The fifth crystal served as a reference. The crystals were characterized both before and after the irradiations in terms of energy resolution, light output and background count rate. The key conclusions of the study are that LaBr3 is radiation tolerant showing no measurable degradation effects when exposed to experimentally simulated solar proton flare spectra with fluences up to 1012protonscm−2 (∼1Mrad or 10kGy equivalent in silicon) and integrated above a 60MeV energy threshold. LaBr3 behaves as a generic intermediate mass material showing similar activation yields as CsI(Tl) and Ge above 1010protonscm−2 (∼10krad (100Gy) equivalent in silicon) and significantly less than Ge below this fluence.