Pergamon Radiorim Measuremenrs, Vol. 24, No. 2. pp. I29- 138, I995 13504487(94)ooo94-8 Copyright c 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1350-4487/95 $9.50 + .oo MEASUREMENTS OF THE SECONDARY PARTICLE ENERGY SPECTRA IN THE SPACE SHUTTLE GAUTAM D. BADHWAR, JAGDISH U. PATEL, FRANCIS A. CUCINOTTA* and JOHN W. WILSON* NASA Johnson Space Center, Houston, TX 77058-3696, U.S.A.; and *NASA Langley Research Center, Hampton, VA 23681-0001, U.S.A. zyxwvutsrqponmlkjihgfedcbaZYXWVUTS (Received 6 September 1994) Abstract-Measurements of the energy spectra of secondary particles produced by galactic cosmic rays and trapped protons due to the nuclear interactions of these particles with the Shuttle shielding provide a powerful tool for validating radiation transport codes. A code validated in this way can be used to better estimate the dose and dose equivalent to body organs, measurements that cannot be made directly. The principal cause of single event upsets in electronic devices in the region of the South Atlantic Anomaly is secondary particles, and even in the region of galactic cosmic radiation a significant fraction is produced by secondary particles. In this paper, we describe the first direct measurements of the energy spectra of secondary protons, deuterons, tritons, 3He and 4He produced by galactic cosmic rays inside the Space Shuttle using a charged particle spectrometer. A comparison of these spectra with radiation transport code HZETRN showed reasonably good agreement for secondary protons. However, the code seriously underestimated the flux of all other light ions. The code has been modified to include pick-up and knock-on processes. The modified code leads to good agreement for deuterons and ‘He but not for other light ions. This revised code leads to about 10% higher dose equivalent than the original code under moderate shielding, if we assume that higher charge ion fluxes are correctly predicted by the model. INTRODUCTION The radiation environment in space is quite complex and cannot be duplicated using ground-based acceler- ators. The galactic cosmic radiation (GCR) contains nuclei from hydrogen to uranium with energies of a few MeV to hundreds of TeV. The free space radi- ation environment is modified by the Earth’s mag- netic field and its intensity is reduced by the Earth’s shadow. This modified energy spectrum of galactic particles impinges on the Space Shuttle and is further modified by nuclear interactions with the spacecraft material (mostly aluminum). It is this modified spec- trum that is needed for estimating radiation risk to crew members and electronic devices, such as the Shuttle General Purpose Computers (GPCs). There are a large number of studies dealing with the charge and energy spectra of ions in free space or at balloon altitudes. Secondary particles produced by GCR par- ticles in the atmosphere have been studied, because this background has to be subtracted from balloon measurements to obtain the spectra above the Earth’s atmosphere (Papini et al., 1993). Studies of interactions of GCR particles with nu- clear emulsion targets (Powell ef al., 1959) have demonstrated the prolific production of deuterium and tritium. Low energy deuterium and ‘He, in particular, have received considerable attention be- cause of their fundamental importance in cosmic ray propagation models. However, since it has not been experimentally possible to separate the primary par- ticles from atmospheric secondary particles, the fluxes of secondary ions have been calculated. However, the radiation transport codes necessary for these calcu- lations need independent verification. This can be accomplished using data from accelerators and space flights. The Space Shuttle offers a unique platform for the study of these secondary particles, particularly for light ions (defined as protons, deuterium, tritium, 3 He and 4He) because of reasonably high production cross-sections and the short duration of these flights. In the absence of a solar particle event, a spec- trometer will see both the trapped particles from the South Atlantic Anomaly (SAA) and charged particles from GCR. For spacecraft observations in low earth orbit, the SAA particles are easily separated from the GCR particles because: (1) there are only about 7 orbital passes lasting about 20 min each through the SAA of a total of about 18 passes a day lasting about 90min each; and (2) the SAA is geographically confined to a relatively small area of the South Atlantic. The GCR particles are observed over the full latitude and longitude range of the Shuttle orbit. Thus, for an instrument with time-resolved infor- mation, the SAA and GCR data can be separated. Even more relevant, if ions are observed in the region outside the SAA with energies below the minimum 129