Extension of the dE/dx-E Technique Using Silicon Detectors to below 1 MeV/nucleon

The "2012 Solar and Space Physics Decadal Survey" and the "2014 Heliophysics Roadmap" acknowledged the importance of better understanding how ions and electrons are accelerated to high energies in the solar corona, the interplanetary medium, and the local galactic medium. Over the past two decades, missions such as ACE and Wind have shown that the existence of a seed population of suprathermal particles plays a central role in the efficient acceleration of high-energy particles. Thus, it is important to understand the mechanisms that produce and maintain the suprathermal particle distribution and accelerate a fraction of it to much higher energies. We propose the development of a sensor that would make possible instruments that can span the ion energy range from <0.5 MeV/nuc to several MeV/nuc with high efficiency and thereby address these objectives. Stacks of silicon solid-state detectors have been widely used since early in the space age for making measurements of energetic particles. By combining the energy loss, Delta-E, from a detector that the particle penetrates with the residual energy, E', in following detectors, the total energy, the nuclear charge, and, in some cases, the mass of the particle can be derived. The lower limit to which this technique can be applied is set by the thickness of the first detector, which the particle must penetrate. Until very recently, silicon detectors with good thickness uniformity have only been available in thicknesses down ~30 microns, which sets a lower energy limit of several MeV/nucleon. Over a number of years, we have developed a new technique for fabricating thin silicon detectors that leads to devices that can be significantly thinner, much more uniform in thickness, and mechanically more robust than earlier detectors. Detectors of this new type are being flown in thicknesses of 12 microns and 25 microns in the EPI-Hi instrument on the Parker Solar Probe mission. We have worked with a commercial detector manufacturer to transition the fabrication process to an industrial setting, thus making this type of detector available to the broader scientific community. The detector fabrication process is thought to be suitable for making devices that are thinner than the 12-micron detectors used in EPI-Hi. We propose the fabrication and testing of prototype detectors with thicknesses down to ~5 microns, which would lower the threshold for the Delta-E vs E' technique using silicon detectors to ~0.4 MeV/nucleon. To optimize the performance of such devices, we will also investigate the use of thinner dead layers, which can be made by low-energy ion implantation, and the use of a novel scheme for coupling the detector to a charge sensitive amplifier that should allow significant improvements in the signal-to-noise ratio in these high-capacitance devices. In addition, we will perform basic environmental tests to establish the suitability of this type of detector for use in spaceflight instruments.

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