LET Augmentation from Moderator Block Dosimetry Agitation
In this work we demonstrate an approach to simulate the space radiation environment in a laboratory setting. For simplicity, we focus on the IVA radiation spectrum measured on different spacecrafts. Our goal is to numerically develop a target moderator block that can be easily constructed from materials with multiple layers of varying geometry to generate specific nuclear reactions and spallation products.
The moderator block is designed so that the final field closely simulates the IVA linear energy transfer (LET) spectrum measured on previous spaceflights. The LET quantifies how much energy is lost in a material and is typically given in units of kilo electron volts per micron (keV/μm) for quantification of radiobiological damage. This target moderator block can, for example, be placed in front of a 1 giga electron volt per nucleon (GeV/n) iron (56Fe) single-particle beam with no modifications to the beamline infrastructure. As the iron beam passes through the moderator block, nuclear spallation processes can create modest amounts of the desired fragments resulting in a complex mixed field of particle nuclei with dif- ferent atomic numbers Z in the range 0 < Z ≤ 26 and LETs up to approximately 200 keV/μm. Modifications to the internal geometry and chemical composition of the materials in the target moderator block allow for a shaping of the simulated IVA LET to specific spectra. The concept is shown in the figure below.
Our approach thus leverages available beamline technologies to provide an enhancement to current ground-based analogs of the space radiation environment by reproducing the measured IVA LET spectrum.
Figure 1. Left: Concept of the moderator block geometry. A primary beam of 56Fe (iron) is selectively degraded with a carefully designed moderator block to produce a desired distribution of energies and ions simulating the intravehicular space radiation environment. Top right panel: Integrated LET per day (black dotted line), the results of our model (blue solid line), and the beam line measurements prototype moderator block that replicates the numerically determined geometry (red solid line). The LET of five single-ion exposures are shown to highlight the lack in breadth of energies in current radiobiological studies. Bottom right panel: Relative abundance of intravehicular ions in the exiting field created by the moderator block plotted against the predicted as a function of Z. Both distributions have been normalized to the most prolific ion, hydrogen (Z = 1).