Gas bremsstrahlung considerations in the shielding design of the Advanced Photon Source synchrotron radiation beam lines

Abstract

The Advanced Photon Source (APS) currently under construction at Argonne National Laboratory will be one of the world's brightest synchrotron radiation (SR) facilities. The storage ring, capable of storing currents up to 300 mA at 7.0 GeV, or 200 mA at 7.5 GeV, will produce very intense and energetic synchrotron radiation (Ec = 24 keV for bending magnets, and Ec = 37.4 keV for wigglers, where Ec is the critical energy). The synchrotron radiation beam lines, consisting of experimental enclosures and transport lines, will have to be shielded against synchrotron radiation and gas bremsstrahlung scattered from beam line components. For insertion devices placed in the straight sections (length = 15 m), the gas bremsstrahlung produced by the interaction of the primary stored beam with residual gas molecules or ions in the storage ring vacuum chamber dominates the SR beam line shielding. Gas bremsstrahlung in the forward direction will be stopped by a tungsten beam stop, 18cm thick, located at the back of the experimental enclosure and placed in the median plane of the storage ring. The forward-directed gas bremsstrahlung is characterized, and the effectiveness of the tungsten beam stop is assessed. The Monte Carlo code FLUKA was used to determine the dose equivalent rates from gas bremsstrahlung in a cylindrical tissue phantom with and without the tungsten beam stop. Simulations were performed using an air target of length 15 m at a pressure of 1 atm and 110atm (1 atm = 101.325 kPa = 760 torr) both with and without suppressing positron multiple scattering (M.S.) and Bhabha and Möller scattering in the air target. At a given pressure the dose equivalent rates are higher without positron multiple scattering in the air target than with multiple scattering. For simulations at Ps = 1 atm, the minimization of Bhabha/Möller scattering is important for areas with scoring radii less than 0.4 cm. The maximum dose equivalent rate (for E0 = 7 Gev, I = 300 mA, and P = 0.133 μPa or 10−9 torr) in the phantom (at a depth of 29.5 cm) for a scoring area of 0.0013 cm2 (radius = 0.02 cm) is 2.1 Sv/h, whereas for a scoring area of 0.5 cm2 (radius = 0.4 cm) it is 0.57 Sv/h. Scoring photon fluence and converting it to dose equivalent using Rogers' fluence-to-dose equivalent conversion factors results in an overestimation of dose equivalent. The maximum dose equivalent rate behind the tungsten beam stop, at a depth of 2.5 cm in the tissue, is 0.1 + 0.04 μSv/h for a scoring radius of 1 cm. The corresponding dose equivalent rates without the stop are 32 mSv/h (depth = 2.5 cm) and 0.17 Sv/h (depth = 29.5 cm). A simple expression has been derived for the upper limit on the dose equivalent that an individual can receive due to loss of vacuum in the straight section.