No abstract

We investigate energy measurements in Timepix hybrid detectors in hole collection mode above pixel input energy depositions of about 2.2 MeV or 600,000 electron/hole pairs in Silicon. Measurement of the detector response well above this regime is important for our application measuring the heavy ion exposure of NASA astronauts. We find that high input charges produce a double pulse structure in the Timepix front end which creates a taxonomy of different clusters which are observed at accelerators, but not in typical operation on orbit. We investigate the LET response of the Timepix to different heavy ion species and find that for a given polar angle the so called 'volcano effect' can be compensated with a simple power law model. We also characterize the pixel response of the Timepix using mono-energetic stopping protons and derive a per pixel calibration method for correction of the pixel response up to about 8 MeV. We conclude that with an appropriate calibration procedure the Timepix can correctly measure the LET values in excess of 500 keV μm-1 in Silicon for tracks with a polar angle exceeding 60̂, corresponding to the entirety of the cosmic ray spectrum up to Nickel.

We demonstrate techniques to improve the accuracy of the energy calibration of Timepix pixel detectors, used for the measurement of energetic particles. The typical signal from such particles spreads among many pixels due to charge sharing effects. As a consequence, the deposited energy in each pixel cannot be reconstructed unless the detector is calibrated, limiting the usability of such signals for calibration. To avoid this shortcoming, we calibrate using low energy X-rays. However, charge sharing effects still occur, resulting in part of the energy being deposited in adjacent pixels and possibly lost. This systematic error in the calibration process results in an error of about 5% in the energy measurements of calibrated devices. We use FLUKA simulations to assess the magnitude of charge sharing effects, allowing a corrected energy calibration to be performed on several Timepix pixel detectors and resulting in substantial improvement in energy deposition measurements. Next, we address shortcomings in calibration associated with the huge range (from kiloelectron-volts to megaelectron-volts) of energy deposited per pixel which result in a nonlinear energy response over the full range. We introduce a new method to characterize the non-linear response of the Timepix detectors at high input energies. We demonstrate improvement using a broad range of particle types and energies, showing that the new method reduces the energy measurement errors, in some cases by more than 90%.