APD Response Time Measurements for Future TOF-E Systems

In space physics, the ability to detect ions is crucial to understanding plasma distributions in the solar wind. This usually typically requires the determination of the particle's mass, charge, and total energy. Current ion detection schemes are implemented in three sequential parts; an electrostatic analyzer for energy per charge (E/Q) measurements, a time-of-flight (TOF) for mass per charge (M/Q) measurements, and a solid-state detector (SSD) for total energy (E) measurements. Recent work has suggested the use of avalanche photodiode detectors (APD) for a simultaneous TOF and total energy (TOF-E) measurement system, which would replace traditional SSDs, simplify design, and reduce costs. Although TOF based ion spectrometry typically requires timing resolution of <1ns, the timing profile for ion detection by APDs is not well understood. In this study we examine the timing profile of 3 different APDs for ion measurements over a suprathermal energy range of 50-300 keV. The three APDs differ by their doping type (N or P) and their detector thickness (30 μm or 150 μm). We find that APD P30, which is P doped and 30μm thick, provides the fastest rise times of the three APDs. Furthermore, these rise times are species independent and less than 1 ns. Our study shows that APDs are capable of sub-nanosecond response times for low energy ions and thus supports the future use of APDs in replacing SSDs in some TOF-E systems.