Scintillator performance investigation for MSL/RAD

The Mars Science Laboratory (MSL) mission is a NASA Mars rover mission planned to be launched in 2011. One of the instruments onboard MSL is the Radiation Assessment Detector (RAD), a comprehensive particle measurement sensor. The instrument is divided into a RAD Sensor Head (RSH) and a RAD Electronics Box (REB). The sensor head consists of a silicon detector telescope stack, placed in front of a cesium-iodide scintillator calorimeter, followed by an additional tissue-equivalent plastic scintillator, designed to measure the Martian neutral particle field. An Anti-Coincidence (AC) scintillator surrounds both scintillators. To save mass and power, photo multiplier tubes are avoided in the instrument and all scintillators are read out using multiple, independent photo diodes. The unavoidable low signal-to-noise ratios of the photo diode readout of the plastic scintillators pose a challenge for calibration and verification of the instrument’s working principles. The scope of this work is to obtain information about the physical details of the scintillator readout and its implications and limits for RAD’s performance. To this aim, an optical Monte-Carlo model of each scintillator is set up. After demonstrating that it adequately explains the major effects seen in RAD’s scintillators, further analyses are performed based on this model. It is used for quantitative exploration of noise limits and onboard energy reconstruction methods for the scintillation detectors. Further, it is integrated with a variant of a preexisting particle physics model of the sensor head. In addition, the electronic readout is modeled, too, to finally provide a sequential model explaining all major effects in RAD. This model, supplied with suitable parameters from measurements, is then used to forecast the instrument’s behaviour in Martian conditions, especially the behaviour of the AC. To complete the picture, initial results on neutral particle modeling are analyzed and compared to measurements to check the analysis path for neutral particle measurements on Mars. As a byproduct, the analysis yields insights into possible ways of processing data from MSL/RAD and further a set of data analysis tools. Parts of those tools are already in use for investigations regarding other, future missions.


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