Radiat Environ Biophys (2007) 46:107–111 DOI 10.1007/s00411-006-0088-6 123 PROCEEDINGS OF THE 4TH IWSRR Shielding of relativistic protons A. Bertucci · M. Durante · G. Gialanella · G. Grossi · L. Manti · M. Pugliese · P. Scampoli · D. Mancusi · L. Sihver · A. Rusek Received: 10 July 2006 / Accepted: 19 December 2006 / Published online: 26 January 2007  Springer-Verlag 2007 Abstract Protons are the most abundant element in the galactic cosmic radiation, and the energy spectrum peaks around 1 GeV. Shielding of relativistic protons is therefore a key problem in the radiation protection strategy of crewmembers involved in long-term mis- sions in deep space. Hydrogen ions were accelerated up to 1 GeV at the NASA Space Radiation Labora- tory, Brookhaven National Laboratory, New York. The proton beam was also shielded with thick (about 20 g/cm 2 ) blocks of lucite (PMMA) or aluminium (Al). We found that the dose rate was increased 40–60% by the shielding and decreased as a function of the dis- tance along the axis. Simulations using the General– Purpose Particle and Heavy-Ion Transport code Sys- tem (PHITS) show that the dose increase is mostly caused by secondary protons emitted by the target. The modiWed radiation Weld after the shield has been characterized for its biological eVectiveness by measur- ing chromosomal aberrations in human peripheral blood lymphocytes exposed just behind the shield block, or to the direct beam, in the dose range 0.5– 3 Gy. Notwithstanding the increased dose per incident proton, the fraction of aberrant cells at the same dose in the sample position was not signiWcantly modiWed by the shield. The PHITS code simulations show that, albeit secondary protons are slower than incident nuclei, the LET spectrum is still contained in the low- LET range (<10 keV/m), which explains the approxi- mately unitary value measured for the relative biologi- cal eVectiveness. Introduction Shielding is generally considered the only available countermeasure to reduce the exposure to galactic cos- mic radiation. However, risk mitigation produced by shielding depends on the biological response in the mixed Weld generated by the interactions of the pri- mary radiation with the shield [1]. Only a few measure- ments of damage in biological samples exposed to accelerated charged particles behind the shields are available for testing current models used for shielding design of spacecrafts [2]. Chromosomal aberrations are among the diVerent biological endpoints relevant for the assessment of the quality factor of space radiation, because cytogenetic rearrangements are considered to be correlated to genomic instability and late stochastic eVects, particularly cancer [3]. We have measured in the past years the induction of chromosomal aberra- tions in human peripheral blood lymphocytes exposed to accelerated iron ions (0.1–5 GeV/n) shielded with blocks in diVerent materials and of diVerent thickness [4–6]. Our results demonstrated that shielding produces M. Durante (&) · G. Gialanella · G. Grossi · L. Manti · M. Pugliese · P. Scampoli Department of Physics and INFN, University Federico II, Monte S. Angelo, Via Cintia, 80126 Napoli, Italy e-mail: durante@na.infn.it A. Bertucci Department of Biology, University Federico II, Monte S. Angelo, Via Cintia, 80126 Napoli, Italy D. Mancusi · L. Sihver Chalmers University of Technology, Applied Physics, Nuclear Engineering, 412-96 Gothenburg, Sweden A. Rusek NASA Space Radiation Laboratory, Brookhaven National Laboratory, Upton, NY 11973, USA