Heavy ion charge and velocity resolution with a Medipix-based active Space Radiation Dosimeter

In preparation for the development of an active Space Radiation Dosimeter based on the Medipix2 pixel ASIC technology developed at CERN in Geneva, Switzerland, exposures were made at the HIMAC facility in Japan with a TimePix-based version with a Si detector layer to explore the potential for discrimination between tracks with differing charges and energies, but with very similar dE/dx values. Data were taken in February 2009 at 15 degree increments for a number of different beams including 600 and 800 MeV/A Si, 180 MeV/A Ne and 100 MeV/A O. Data were also obtained for 400 MeV/A Si and 500 MeV/A Fe along with 290 and 180 MeV/A N. The TimePix chips have been calibrated to achieve the maximum resolution. One of the principal objectives of these data runs was to explore the resolution of TimePix-based Si detectors to discriminate between various ions with different energies and charges, but with similar dE/dx values in Si. The ongoing analysis is intended to determine the charge and energy resolution capability of the TimePix-based Si detectors. Additional runs were made in September 2009 and preliminary analyses of the data from those runs are included in the results described in this paper. There are more runs planed for January 2010, and preliminary results from those runs should be available for presentation at the conference. The data show an ability to distinguish slow from fast particles as well as a capability to make reasonable estimates of the charge of a traversing particle. The limit of the resolution for such measurements will require additional data, but the clear ability of the technology to measure the LET (Linear Energy Transfer) of traversing charged particles has been shown. The present version of the TimePix ASIC does show evidence of saturation at the highest LET levels, and this information can be used to guide the development of the next generation of the technology.