The total amount of beam time purchased by NASA for research with ions heavier than protons is currently 100 hours per year. This includes the time not only for the physics experiments referred to above, but also for the biological irradiations and the dosimetry for biological experiments, which may take as much time as the biological irradiations themselves. This amount of beam time compares with about 400 hours per year previously available for similar research studies at the now-closed Berkeley BEVALAC. From the predicted cross sections for the secondary particles and the maximum count rates of the most sensitive detectors to detect these particles in the apparatus, it is possible to estimate typical time periods necessary to accumulate a sufficient number of counts at a specified beam rate, so that the random error in total counts is minimized. A reasonable estimate for measurements of secondary particle spectra is about 1 hour of beam on target for each data point, i.e., one incident particle type at one energy level for one target composition at one thickness. Each shield material would need to be tested with at least three particle types, not including protons. Each particle type would need to be accelerated to about five different levels of energy, and five shielding thicknesses should be tested at each energy. Including the time needed for setting up experiments and testing equipment (which may equal the time needed to accumulate data), the task group estimated that about 100 hours are needed for each shielding material examined with one particle type for data collected along the primary beam axis, with the beam time increasing geometrically with scattering angle. (A semipermanent or dedicated facility could drastically reduce set up times, since equipment may be left in place between experiments.) If collection of off-axis data is also considered, a conservative estimate of the time needed for obtaining data on particle types, numbers, and energies is about 300 hours for each particle for each shielding material, or about 900 hours for three materials. This amount of time would increase by a factor of 2 or more if data were collected at off-axis scattering angles and could correspond to about 1 or 2 equivalent chronological years of dedicated research at most DOE accelerators.
The composition of the GCR particle spectra dictates a focus on iron as the highest-atomic-number (Z) particle of critical interest. Studies with additional ions are required, however, to benchmark the theoretical cross section code against the Z range of interest. Because the uncertainty in the calculated cross sections is reflected in the uncertainty in the level of required shielding, reduction in the uncertainty of these data values will have major cost reduction implications.
Needed in addition to validation of the cross section calculation codes is validation of the transport code itself. Experimental measurements of particles emanating from a thick laminated shield need to be compared with theoretical calculations to benchmark the transport codes and reduce the uncertainty in the calculation of the amount of shielding needed. The laminate shields should be chosen carefully to reflect the full scope of materials that might be used; SiC, 6LiH, Al, regolith and other hydrogen-containing materials that show promise.
As indicated above, radiation has not been considered a serious hazard in low-Earth-orbit missions of relatively short duration at low inclinations, given that the doses within spacecraft have been well within NCRP guidelines.2 For the mission to Mars, however, loss of the shielding benefits of Earth's magnetic field and the longer time periods over which dose is accumulated require that the shielding design be reevaluated. The concept of “just add a little more aluminum” is not a satisfactory solution.
Two options can be considered for shielding in space: active or passive shielding. The passive approach of bulk shielding was chosen for the NASA low-Earth-orbit missions and resulted in a reliable system at reasonable cost. An active system would require the use of very large magnetic field strengths to deflect charged particles away from the spacecraft. Minimizing energy requirements would entail the use of superconducting systems and would involve the associated complexity of such systems. Since cosmic radiation is essentially isotropic, a fully encompassing magnetic shield would be desirable but from a practical point of view would be very difficult to achieve. Thus, the construction of a satisfactory active shield is questionable. NASA has chosen to not pursue design of an active shielding system primarily because of doubts about its reliability and this report focuses on the requirements for a passive shielding system.