Characteristics of High Reliability TEPC for Long Term Space Missions

The Tissue Equivalent Proportional Counters (TEPC) have become a major part of the dosimetry system used on the International Space Station (ISS). TEPCs provide near real-time measurements of absorbed dose and dose equivalent in different parts of the ISS. The current TEPC model used at ISS uses two spherical tissue equivalent proportional counters with their charge sensitive preamplifiers encased in an aluminum vacuum chambers filled with propane gas at low pressure. Both detectors operate at low pressure that simulates a site size of 2 μm in tissue. This site diameter is used because of extensive experience with similar detectors used in industrial applications for mixed field, neutron and gamma, dosimetry. One concern limiting proposals to use TEPCs for dosimetry on manned missions beyond low earth orbit has been the potential for vacuum leaks which result in gradual degradation of proportional counter performance. The potential for leakage can be eliminated by filling the detector with counter gas at atmospheric pressure. This results in a simulated site size of approximately 68 μm for a 3.8 cm detector or 32 μm for a 1.8 cm detector. Many of the secondary protons produced by neutrons have ranges of as little as 10 μm so TEPCs simulating sites larger than 2 μm may underestimate the dose equivalent in some situations. The ranges of nearly all of the charged particles in space are hundreds of micrometers or more, so dose equivalent can be evaluated using much larger simulated site sizes. Monte Carlo calculations were used to evaluate the behavior of two TEPCs, 3.8 cm and 1.8 cm diameter. The source particles used were proton, helium, and iron ions. Two different simulations were run for each particle; 1000 MeV/n and the Badhwar-O’Neill flux model distribution. The results show that both detectors operating at atmospheric pressure can estimate dose equivalent in space; the results are essentially identical to those produced by a 2 μm site in the space radiation environment