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Measurements of neutron production in shielding materials for radiation protection in space

Categories:
  • Division 6
  • 6.4 Ionenbeschleuniger und Referenzstrahlungsfelder
  • News from the annual report
  • Fundamentals of Metrology
20.12.2019

The harmful effects of space radiation on an astronaut’s health represent a major limiting factor for long-duration human space missions beyond low Earth orbit. National space agencies have recognized the risks related to exposure to space radiation and are developing complex model-based risk mitigation strategies. The space radiation risk models are based on the interplay of basic physical and radiobiological modelling. In both fields, there are still significant gaps of knowledge which need to be addressed by collecting high-quality experimental data, developing the corresponding theoretical models, and finally validating and improving the simulation codes employed in radiation risk assessment.

One significant gap is the lack of experimental data of neutron and light ion production in thick shielding by galactic cosmic rays. Due to their high mean free path length and high biological effectiveness, neutrons represent a significant threat to the astronauts’ health. Neutrons are easily produced through all phases of a nuclear fragmentation process. Their abundance coupled with their low interaction rate makes them one of the major issues in the design of passive shielding. The issue of neutron production in thick shields is particularly important in design studies of planetary habitats. Current studies of major particle transport codes, used in the space radiation community, suggest that the neutron and light ion production in thick shields still causes significant discrepancies among the codes. The reason is the limited amount of double-differential cross-sectional data describing the scattering at large angles, and uncertainties in the nuclear physics models describing the breakup and de-excitation of the projectile and target.

Recently, the Department 6.4 Neutron radiation of PTB has joined a research project of the Biophysics Department of the GSI Helmholtzzentrum für Schwerionenforschung Darmstadt and the Trento Institute for Fundamental Physics and Applications (TIFPA-INFN). The aim of the joint project is to develop a novel experimental method for characterizing the secondary radiation field behind thick shielding. The exposure to galactic cosmic rays is approximated by a high-energy Fe-56 ion beam (1 GeV per nucleon) produced by the heavy-ion synchrotron SIS18 at GSI, and the shielding is simulated by a massive aluminum target. The particles of the secondary radiation field, produced via nuclear reactions, are emitted in all directions. The quality and composition of the secondary radiation field behind the shielding will be evaluated -- under specific angles relative to the incident beam axis -- with a setup of four complementary detector systems: Bonner sphere spectrometer NEMUS of PTB, Time-of-Flight method using scintillator telescopes of GSI, neutron ball dosimeters of GSI, and tissue-equivalent proportional counter of TIFPA-INFN. The combination of the results from the individual detectors will made it possible to determine the energy spectra and angular distributions of the high-energy secondary neutrons, protons and light ions, and also to draw conclusions about the expected biologically relevant effects of the secondary mixed field. The results obtained with this multi-detector approach will be used for benchmarking of major particle transport codes.

The joint project was selected in the program AO-2017-IBER (Announcement of Opportunity for Investigations into Biological Effects of Radiation using the GSI accelerator facility) of the European Space Agency (ESA) and was granted beam time at GSI. The first test of the method took place in April 2019 and showed promising results. The full-scale measurement campaign is planned for March 2020. Firstly, standard shielding materials like aluminum and polyethylene will be used for the proof-of-principle of the method. Further, novel high-performance shielding materials can be examined with this method.

 

Fig.: Experimental setup at the high-energy irradiation facility Cave A of GSI during the test beam time in April 2019. The high-energy ion beam comes from the right side and hits the aluminum target (silvery cylinder block) on the right side of the photograph, marked by a blue arrow. The particles of the secondary radiation field are emitted in all directions. The red arrow marks the Bonner sphere (white sphere) of the PTB neutron spectrometer NEMUS, located at the angle of 15° with respect to the beam axis. The green arrows mark the GSI balls neutron dosimeters (black spheres) at angles of 15° and 40° with respect to the beam axis. The plane of the beam is 2 m above ground.

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