Radiation Measurements 41 (2006) 1216 – 1226 www.elsevier.com/locate/radmeas EVA space suit proton and electron threshold energy measurements by XCT and range shifting M.F. Moyers a , ∗ , P.B. Saganti b, c , G.A. Nelson a a Department of Radiation Medicine, Loma Linda University, 11234 Anderson St., Loma Linda, California 92354, USA b Space Radiation Health Project, NASA—Johnson Space Center, 2101 NASA Road 1, Houston, Texas 77058, USA c Department of Physics and NASA—Center for Applied Radiation Research, Prairie View A & M University, Prairie View, Texas 77446, USA Received 9 September 2005; accepted 12 April 2006 Abstract Construction of the International Space Station (ISS) will require more than 1000h of extravehicular activity (EVA). Outside of the ISS during EVA, astronauts and cosmonauts are likely to be exposed to a large fluence of electrons and protons. Development of radiation protection guidelines and mitigation of risks requires the determination of the minimum energy of electrons and protons that penetrate the astronaut EVA suits at various locations. Measurements of the water equivalent thickness of both United States (US) and Russian EVA suits were obtained by performing X-ray computed tomography (XCT) scans. Selected regions of interest of the suits were further evaluated using a “differential range shift” technique. This technique involved measuring thickness ionization curves for 6 MeV electron and 155 MeV proton beams with ionization chambers using a constant source-to-detector distance. The thicknesses were obtained by stacking polystyrene slabs immediately upstream of the detector. The thicknesses of the 50% ionizations relative to the maximum ionizations were determined. The detectors were then placed within the suit and the stack thickness adjusted until the 50% ionization was re-established. The difference in thickness between the 50% thicknesses was then used with standard range tables to determine the threshold energy for penetration. This paper provides a detailed description of the experimental arrangement and the obtained results. © 2006 Elsevier Ltd. All rights reserved. Keywords: Space suit; Proton; Electron; CT; Polystyrene 1. Introduction Construction of the International Space Station (ISS) is still anticipated to involve several hundreds of hours of extravehicu- lar activity (EVA). During an EVA, both astronauts and cosmo- nauts are likely to be exposed to a large fluence of electrons and protons. Figs. 1 and 2 contain typical energy spectra of elec- trons and protons anticipated at the orbit of the ISS. When the current generation EVA suits were designed, the suit designers had not envisioned the multitude of extensive-duration EVAs required for ISS construction and radiation shielding was not ∗ Corresponding author. Tel.: +1 909 370 1103. E-mail address: MFMoyers@adelphia.net (M.F. Moyers). 1350-4487/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2006.04.033 of paramount importance. Threshold radiation measurements and radiation protection guidelines were therefore required before extensive construction of the ISS could commence. These guidelines were based on calculational models that required validation. Obtaining direct measurements of the minimum energy of electrons and protons that can penetrate the suits at various locations was one of the validation steps. Additionally, these measurements may influence the designs of future space suits. Transverse XCT scans provided maps of the water equiv- alent thickness of both US and Russian EVA spacesuits. The minimum energy of electrons and protons that can penetrate these thicknesses were interpolated from standard range-energy tables. Specific regions of interest of the suits were then evalu- ated directly with electron and proton beams using a differential range shift technique.