GAS BREMSSTRAHLUNG CONSIDERATIONS IN THE SHIELDING DESIGN OF THE ADVANCED PHOTON SOURCE SYNCHROTRON-RADIATION BEAM LINES

The Advanced Photon Source (APS) currently under construction at Argon ne National Laboratory will be one of the world's brightest synchrotro n radiation (SR) facilities. The storage ring, capable of storing curr ents up to 300 mA at 7.0 GeV, or 200 mA at 7.5 GeV, will produce very intense and energetic synchrotron radiation (E(c) = 24 keV for bending magnets, and E(c) = 37.4 keV for wigglers, where E(c) is the critical energy). The synchrotron radiation beam lines, consisting of experime ntal 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 (len gth = 15 m), the gas bremsstrahlung produced by the interaction of the primary stored beam with residual gas molecules or ions in the storag e ring vacuum chamber dominates the SR beam line shielding. Gas bremss trahlung in the forward direction will be stopped by a tungsten beam s top, 18 cm thick, located at the back of the experimental enclosure an d placed in the median plane of the storage ring. The forward-directed gas bremsstrahlung is characterized, and the effectiveness of the tun gsten beam stop is assessed. The Monte Carlo code FLUKA was used to de termine the dose equivalent rates from gas bremsstrahlung in a cylindr ical tissue phantom with and without the tungsten beam stop. Simulatio ns were performed using an air target of length 15 m at a pressure of 1 atm and 1/10 atm (1 atm = 101.325 kPa = 760 torr) both with and with out suppressing positron multiple scattering (M.S.) and Bhabha and Mai ler scattering in the air target. At a given pressure the dose equival ent rates are higher without positron multiple scattering in the air t arget than with multiple scattering. For simulations at P-s = 1 atm, t he minimization of Bhabha/Moller scattering is important for areas wit h scoring radii less than 0.4 cm. The maximum dose equivalent rate (fo r E(0) = 7 Gev, I = 300 mA, and P = 0.133 mu Pa or 10(-9) torr) in the phantom (at a depth of 29.5 cm) for a scoring area of 0.0013 cm(2) (r adius = 0.02 cm) is 2.1 Sv/h, whereas for a scoring area of 0.5 cm(2) (radius = 0.4 cm) it is 0.57 Sv/h. Scoring photon fluence and converti ng it to dose equivalent using Rogers' fluence-to-dose equivalent conv ersion factors results in an overestimation of dose equivalent. The ma ximum dose equivalent rate behind the tungsten beam stop, at a depth o f 2.5 cm in the tissue, is 0.1 + 0.04 mu Sv/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 expre ssion has been derived for the upper limit on the dose equivalent that an individual can receive due to loss of vacuum in the straight secti on.