NASA's first ground-based Galactic Cosmic Ray Simulator: Enabling a new era in space radiobiology research

Abstract

With exciting new NASA plans for a sustainable return to the moon, astronauts will once again leave Earth's protective magnetosphere only to endure higher levels of radiation from galactic cosmic radiation (GCR) and the possibility of a large solar particle event (SPE). Gateway, lunar landers, and surface habitats will be designed to protect crew against SPEs with vehicle optimization, storm shelter concepts, and/or active dosimetry; however, the ever penetrating GCR will continue to pose the most significant health risks especially as lunar missions increase in duration and as NASA sets its aspirations on Mars. The primary risks of concern include carcinogenesis, central nervous system (CNS) effects resulting in potential in-mission cognitive or behavioral impairment and/or late neurological disorders, degenerative tissue effects including circulatory and heart disease, as well as potential immune system decrements impacting multiple aspects of crew health. Characterization and mitigation of these risks requires a significant reduction in the large biological uncertainties of chronic (low-dose rate) heavy-ion exposures and the validation of countermeasures in a relevant space environment. Historically, most research on understanding space radiation-induced health risks has been performed using acute exposures of monoenergetic single-ion beams. However, the space radiation environment consists of a wide variety of ion species over a broad energy range. Using the fast beam switching and controls systems technology recently developed at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory, a new era in radiobiological research is possible. NASA has developed the "GCR Simulator" to generate a spectrum of ion beams that approximates the primary and secondary GCR field experienced at human organ locations within a deep-space vehicle. The majority of the dose is delivered from protons (approximately 65%-75%) and helium ions (approximately 10%-20%) with heavier ions (Z ≥ 3) contributing the remainder. The GCR simulator exposes state-of-the art cellular and animal model systems to 33 sequential beams including 4 proton energies plus degrader, 4 helium energies plus degrader, and the 5 heavy ions of C, O, Si, Ti, and Fe. A polyethylene degrader system is used with the 100 MeV/n H and He beams to provide a nearly continuous distribution of low-energy particles. A 500 mGy exposure, delivering doses from each of the 33 beams, requires approximately 75 minutes. To more closely simulate the low-dose rates found in space, sequential field exposures can be divided into daily fractions over 2 to 6 weeks, with individual beam fractions as low as 0.1 to 0.2 mGy. In the large beam configuration (60 × 60 cm2), 54 special housing cages can accommodate 2 to 3 mice each for an approximately 75 min duration or 15 individually housed rats. On June 15, 2018, the NSRL made a significant achievement by completing the first operational run using the new GCR simulator. This paper discusses NASA's innovative technology solution for a ground-based GCR simulator at the NSRL to accelerate our understanding and mitigation of health risks faced by astronauts. Ultimately, the GCR simulator will require validation across multiple radiogenic risks, endpoints, doses, and dose rates.

Fig 1. Three key areas that must be developed together to ultimately provide the GCR simulator at NSRL.

Development focused on establishing irradiation requirements and balancing facility capabilities and limitations, including constraints imposed by animal and cellular model systems. GCR, galactic cosmic radiation; NSRL, NASA Space Radiation Laboratory.

Fig 2. Relative contribution to fluence (squares), dose (diamonds), and dose equivalent (circles) of different elements in the free-space GCR environment during solar minimum conditions (June 1976) as described by the Badhwar–O'Neill 2010 GCR model [14] (Adapted from Durante and Cucinotta [3]).

Plot data available in S1 Data. GCR, galactic cosmic radiation.

Fig 3

GCR particle spectra at solar minimum conditions (June 1976) denoted by solid lines and solar maximum conditions (June 2001) denoted by dashed lines in (A) free space and (B) behind 20 g/cm² of aluminum to female BFOs as described by the Badhwar–O’Neill 2010 GCR model [14], HZETRN transport code [13,16,17], and human phantoms [15,18,19]. Plot data available in S1 Data. BFO, blood-forming organ; GCR, galactic cosmic radiation; HZETRN, High Charge and Energy Transport.

Fig 4. Vehicle shielding is combined with shielding afforded by a crew member’s body surrounding critical organs to determine the primary and secondary radiation environment at points within the crew member.

(A) Human phantoms are used to calculate the body’s self-shielding of critical organs. (B) Shield thickness provided by the vehicle are depicted as green intersecting rays in a crew exploration vehicle (similar to Orion).

Fig 5. Three basic strategies for beam selection.

(A) Beam selection is representative of the external, free-space GCR spectrum and is approximated by discrete ion and energy beams delivered onto a shielding and tissue equivalent material placed within the beam line, in front of the biological target. (B) Beam selection is representative of the shielded tissue spectrum found in space (e.g., average tissue flux behind vehicle shielding) and is approximated by discrete ion and energy beams delivered directly onto the biological target. (C) Beam selection is representative of energies less than free space with thinner amounts of vehicle shielding and variable thicknesses of tissue equivalent materials to represent the differences in body self-shielding between the physical sizes of species. GCR, galactic cosmic radiation.

Fig 6. Reference field spectra in the female BFOs behind 20 g/cm² of aluminum shielding during solar minimum conditions.

(A) Neutron, hydrogen, and helium energy spectra. (B) The corresponding differential LET spectra with and without contributions from hydrogen and helium. Based on calculations from Slaba and colleagues, 2016 [8]. Plot data available in S1 Data. BFO, blood forming organ; LET, lineal energy transfer.

Fig 7. Illustration of beam selection strategy for GCR simulator.

The total LET spectrum (light blue) and the HZE spectrum (dark blue) are shown separately. The green bars are representative of the number of single-ion beam experiments performed at NSRL as a function of LET (scaled for plot clarity). The black line is representative of ICRP-60 quality factor weighting [11] to estimate biological damage (scaled for plot clarity). Plot data available in S1 Data. GCR, galactic cosmic radiation; HZE, high charge and high energy ions; ICRP, International Commission on Radiological Protection; LET, linear energy transfer; NSRL, NASA Space Radiation Laboratory.

Fig 8. Representation of the reference field using discrete monoenergetic beams.

The hydrogen and helium energy spectra are considered directly (A), whereas HZE ions are represented within the LET spectrum (B). Solid blue lines are the reference spectra from Fig 6. The bin widths for 1 GeV/n protons and helium particles are at lower fluences and not shown on the figure; however, these data are included in supplementary data file. All plot data available in S1 Data. HZE, high charge and high energy ions; LET, linear energy transfer.

Fig 9. Mouse and rat voxel models used to evaluate dose distributions in tissues from exposure to GCR simulation.

Digimouse (A) has been scaled by a factor of 3.15 to obtain and estimate of a rat’s body self-shielding, referred to here as “digirat” (B). Transport of full GCR simulation field provides homogeneous dose distribution within voxel mouse model (A) and scaled rat model (B). GCR, galactic cosmic radiation.

Fig 10. Cumulative dose as a function of LET comparing simulated environments within phantoms to the reference field and GCR simulation beam exposure.

Plot data available in S1 Data. GCR, galactic cosmic radiation; LET, linear energy transfer.

Fig 11. Facility layout of NSRL at BNL.

(A) Tools to reliably control system hardware settings, from ion production by the LIS through booster injection, acceleration, extraction, and delivery to the NSRL target room were developed to sequentially deliver the GCR simulator ion-energy beam combinations. (B) Position of imaging chamber behind target (top, left-hand side), cut-off chamber (top, right-hand side) near beam entrance to target room, and photo of large-area degrader (binary filter) system (bottom) in NSRL beam line to maintain control and uniformity of 60 × 60 cm² beam. BNL, Brookhaven National Laboratory; EBIS, Electron Beam Ion Source; GCR, galactic cosmic radiation; LINAC, Linear Accelerator; LIS, laser ion source; NSRL, NASA Space Radiation Laboratory.

Fig 12

Housing array for mouse (A) and rat (B) irradiations in the 60 × 60 cm² beam. Exposure boxes, made of approximately 2-mm thick polyethylene, stack together and are held in an array using a fabricated frame strucure. (C) Ventilation lids for air circulation are provided. The nonventilated sides of the lids are painted red to serve as a quick visual cue that the lids are in the correct orientation for air flow.

Fig 13

Modified incubator for use in beamline (A) with a holder that can accommodate up to 15 T75 flasks in a 3 × 5 array or 44 T25 flasks (B).

Fig 14. Computer screen shot measuring GCR simulator doses per particle for the 20.8 mGy cycle.

GCR, galactic cosmic radiation.