Highly Miniaturized and Sensitive Thermal Neutron Detector for Space Applications
Z. Vykydal 1*, M. Holik 1, V. Kraus 1, S. Pospisil 1, J. Solc 1, D. Turecek 1
1 Institute of Experimental and Applied Physics, Czech Technical University in Prague
Devices from the Medipix family prove to be an excellent tool for measurement and characterization of complex radiation fields including neutrons. The use of a neutron detector in planetary remote sensing is an essential tool in the search for hydrogenous materials and specifically the presence of water which is the essential ingredient in the search for extraterrestrial life. In addition, high sensitivity neutron measurements used in combination with X-ray and gamma-ray measurements, improves the analysis of the atomic composition of regolith, which in turn, is used to interpret surface geology and ultimately planetary evolution.
High spatial resolution (matrix of 256 x 256 pixels of 55 x 55 µm2 pitch) and sensitivity of the Medipix detector allows visualizing directly the energy loss and charge collection processes in the sensor material (300 µm thick silicon in this case). The different radiation types have different characteristic shapes and it is possible to use this information for very effective background suppression. Since silicon itself is insensitive to thermal neutrons a 6Li in the form of 6LiF powder was used to convert thermal neutrons into alpha particles via the reaction 6Li + n → α + 3H. Several approaches of 6LiF conversion layer production have been tested and compared regarding detection efficiency. Measured detection efficiency is above 1% in all cases.
In order to meet ESA communication standards for space equipment we have developed compact, low power and lightweight FPGA based readout system communicating via SpaceWire interface (see Fig. 1). Dimensions of the whole device including Medipix chipboard is 160 x 75 x 15 mm3 and its total weight is 70 g. Power consumption of the device is 1.4 W during measurement and 0.75 W when the detector is switched off. Readout speed is 7 fps with single Medipix device which is sufficient for the target application. Furthermore, the whole detection system is very mass and power efficient in comparison with the gas proportional detectors which are standardly used in space applications for thermal neutron detection.
Fig. 1. FPGA based SpaceWire readout system with attached Medipix chip.