Double Differential Neutron Yields Produced by Proton, Helium, and Iron Interactions in Thick Aluminum Targets

Recent calculations of galactic cosmic ray (GCR) transport in enclosed, shielded space environments indicate that a minimum dose equivalent is achieved with aluminum shielding thicknesses near 20 g/cm² [grams per centimeter squared]. Increases in the absorbed dose and dose equivalent with shielding thicknesses above 20 g/cm² are believed to be caused by the production of light ions and neutrons in the thick shielding. However, uncertainties surround these calculations due to limited cross section and yield data for high-energy projectiles incident on thick targets. Thick-target neutron yields are particularly valuable measurements since they are produced over a wide range of energies by primary and secondary particles and include neutrons modified by transport through a material. Thus, a database of thick-target neutron yield measurements will help validate transport code calculations and quantify uncertainties between experimental and simulated data.

In March 2016, secondary neutron yields from GCR-like projectiles impinging upon thick targets were measured at Brookhaven National Laboratory’s NASA Space Radiation Laboratory. 400 and 800 AMeV [megaelectron volt per nucleon] iron and proton, and 400 AMeV helium projectiles were set incident upon 20, 40, and 60 g/cm²-thick aluminum targets, and a second 60 g/cm² aluminum target was centered downstream to study backscattered neutrons at a later date. Upstream target neutron yields were measured with liquid scintillators at 10° [degrees], 30°, 45°, 60°, 80°, and 135° off the beam axis using the time-of-flight technique. Measurements were converted to double differential thick-target yields and compared with PHITS and MCNP transport model calculations.

Comparisons with PHITS and MCNP revealed inconsistencies at low to intermediate energies, in addition to overestimations of the experimental yields at the 10° high-energy peak. Wide-angle yields at the shoulder energies were fairly well modeled for most systems, and yields at 135° were underestimated for the 400 AMeV projectile beams. Overall, both codes would benefit from improvements in their neutron production models, particularly below the peak or shoulder energies. This systematic study on secondary neutrons produced by thick-target interactions will be incorporated by NASA into a rigorous uncertainty quantification procedure, which will ultimately help determine optimal shielding thicknesses for future space applications.