Accelerator-based measurements relevant for shielding design in space

Abstract

Experimental work on light charged ion production from thick target shielding began this past May at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL). This paper presents the measured secondary light charged ion and neutron yields produced by 0.4- and 2.5-GeV protons and 0.4- and 1.0-AGeV iron ions striking a 30 g/cm2 aluminum target. Neutron and light charged ion (protons, deuterons, and tritons) measurements were taken with liquid scintillators and sodium iodide (NaI) detectors positioned at seven locations between 10 and 135 degrees off the beam axis to best cover the angular distributions of secondary particles, as determined by MCNPX simulations. In the liquid scintillators, neutron-gamma separation was achieved with pulse shape discrimination, and particle species were identified and isolated by analyzing the total charge deposited in the detector versus particle time of flight (TOF). After isolation, the TOF technique was utilized to produce energy spectra for protons, deuterons, tritons, and neutrons at various locations. Additionally, the stopping powers of light charged ions were compared in NaI detector pairs to generate energy spectra for protons, deuterons, and tritons. Preliminary results demonstrated promising agreement with MCNPX Monte Carlo transport code simulations for protons, deuterons, tritons, and neutrons, despite the lack of a full background characterization and optimization of detector settings. Results are expected to improve over the next three years with an increase in beam time, inclusion of specific liquid scintillator detection efficiencies, and an investigation of the physics model parameters in MCNPX. Future experiments will include the use of both forward and back targets composed of high-density polyethylene or aluminum with thicknesses of 20, 40, and 60 g/cm2. Furthermore, proton, helium, carbon, silicon, and iron projectiles will be utilized at energies of 0.4, 0.75, 1.5, and 2.5 AGeV. Ultimately, these measurements will be incorporated in the uncertainty analysis for the engineering codes that NASA uses to develop optimal shielding thicknesses for spacecraft and space habitat design.