~ Pergamon Radiation Measurements, Vol. 26, No. 2, pp. 147-158, 1996 Publishedby ElsevierScience Ltd Printed in Great Britain 1350-4487(95)00290-1 J 350-4487/96$15.00 + 0.00 MEASUREMENTS OF THE LINEAR ENERGY TRANSFER SPECTRA ON THE MIR ORBITAL STATION AND COMPARISON WITH RADIATION TRANSPORT MODELS G. D. BADHWAR,* A. KONRADI,* W. ATWELL,* M. J. GOLIGHTLY,* F. A. CUCINOTTA,f J. W. WILSON,t V. M. PETROV,f I. V. TCHERNYKH,:~ V. A. SHURSHAKOV:~ and A. P. LOBAKOV§ *Space Radiation Analysis Group, NASA Johnson Space Center, Houston, TX 77058-3696, U.S.A.; tHigh Energy Physics Branch, NASA Langley Research Center, Hampton, VA 23681-0001, U.S.A.; :~Space Radiation Department, Institute of Biomedical Problems, Ministry of Health, Moscow, Russia 123007; and §NPO Energia, Kaliningrad, Moscow Region, Russia (Received 7 March 1995; in final form 20 June 1995) Abstract--A tissue equivalent proportional counter designed to measure the linear energy transfer spectra (LET) in the range 0.2-1250 keV//~m was flown in the Kvant module on the Mir orbital station during September 1994. The spacecraft was in a 51.65 ° inclination, elliptical (390 x 402 km) orbit. This is nearly the lower limit of its flight altitude. The total absorbed dose rate measured was 411.3 + 4.41 t~Gy/daywith an average quality factor of 2.44. The galactic cosmic radiation (GCR) dose rate was 133.6#Gy/day with a quality factor of 3.35. The trapped radiation belt dose rate was 277.7/~Gy/day with an average quality factor of 1.94. The peak rate through the South Atlantic Anomaly was ~ 12/~Gy/minand nearly constant from one pass to another. A detailed comparison of the measured LET spectra has been made with radiation transport models. The GCR results are in good agreement with model calculations; however, this is not the case for radiation belt particles and again points to the need for improving the AP8 omni-directional trapped proton models. INTRODUCTION The radiation burden of crew members in low earth orbital flights, depends strongly on the spacecraft inclination, altitude, the time the mission occurs during the solar cycle, and the spacecraft shielding distribution. During solar quiet times the two main sources of radiation are the trapped protons and electrons from the South Atlantic Anomaly (SAA) region and the galactic cosmic ray (GCR) nuclei. Realistic estimates of the radiation-induced risk to crew members depend upon the type, amount, and energy distribution of radiation to which various critical body organs are exposed. Errors in risk esti- mates result from uncertainties in the radiobiological effectiveness of various radiation components. The biological damage per unit absorbed dose depends on the type of radiation. Currently, these differences are expressed in terms of the energy loss per unit length of track in the material or the linear energy transfer (LET). The equivalent biological response, expressed as dose equivalent (H), is defined as H = QD, (I) where H is the dose equivalent, Q isthe quality factor and D is the absorbed dose. The quality factor (Q) is related to the L E T of the radiation and has been defined in I C R P Publications 26 and 60 (ICRP, 1977, 1991). It is possible to deter- mine the effective quality factor, Q, experimentally as O = (1/D)SQ (L)D (L)dL, (2) where D(L)dL is the absorbed dose in the LET interval L to L + dL at the point of interest. Q(L) is the quality factor as a function of LET, and D is the absorbed dose at the point of interest and is given by O (L) = k SF(L)L dL, (3) where F(L) is the differential LET spectrum and k is the proportionality factor to express the absorbed dose in appropriate units. During the last thirty years a considerable effort has been made to measure the absorbed dose at various locations in various spacecraft and on the skin of crew members (Richmond et al., 1968; Atweil, 1990). These measurements have been made using passive thermoluminescent detectors (TLD). TLD detectors rapidly lose detection efficiency with increas- ing LET above ~ 7 keV//t m (Frank and Benton, 1979) and thus miss the contribution to dose from high LET particles. There have been very limited attempts to measure dose rates. Measurements of the LET spectra have been made using plastic nuclear track detectors (PNTD), such as CR-39, Lexan, poly- carbonates, and cellulose nitrate (Benton, 1983). The 147