Uniformity of Proton Beams Available at the NASA Synchrocyclotron (SREL)

Abstract

The national Aeronautics and Space Administration (NASA) has in operation a Space Radiation Effects Laboratory (SREL) that is suitably designed for radiation biology research. A 600 MeV proton synchrocyclotron, capable of simulating the space radiation environment, is the facility's major machine. Space radiation simulation is achieved by degrading either of the two primary extracted proton beams having measured energies of 595 MeV and 325 MeV. The primary extracted beams are degraded to lower energies by using copper absorbers located in the magnet hall of the facility (Fig. 1). Proton beams having energies from 595 MeV to approximately 30 MeV can be obtained at the target position. Both small- and large-area beams are available over a wide energy range. The final beam configuration at the target position, however, is dependent upon the optimization of some 25 magnets located in the proton beam transport system (PBTS). Because of this, an extensive proton beam characterization study was initiated and recently completed. This paper summarizes the characteristics of approximately 30 beam configurations and includes such parameters as proton beam energy, energy spread, intensity, intensity profile, repeatability, and area. The results reported here are primarily related to large-area beams, which are of interest to experimenters studying the radiation effects of protons on animals. Uniformity of several large-area beams is discussed in terms of dose rate (rads/min.) in muscle. Experimental Methods Monitoring Technics Foil activation is considered one of the best technics for determining high-energy proton beam intensities. The 12C (p,pn) 11C reaction is widely used since the cross section is known to within ±5 per cent over an energy range of 50 MeV to the GeV region (1). The induced 11C atoms decay to 11B by positron emission with a half-life of 20.5 minutes and a maximum energy of 0.968 MeV. For the determination of absolute 11C activity induced in a polystyrene target, a gamma counting system was chosen for this experiment. The emitted positrons were stopped in an aluminum capsule surrounding the polystyrene disk. Gamma rays produced by positron annihilation were then counted with a NaI(T1) crystal. Horizontal and vertical profiles of the small-area, high-intensity beams were obtained by scanning the beams with a silicon diode (2, 3). Figure 2 is the horizontal profile of the small-area 325 MeV proton beam. Calibration of Detector System Several precision gamma standard sources with accuracies of ± 1–2 per cent were used for calibration of the 3 × 3-inch NaI(Tl) crystal detector system. Nominal source-to-detector distances of 0, 5, and 10 cm were chosen depending on the source strength of each disk. Crystal efficiency was determined for each sample size and each source-to-detector distance.