Radiation Safety Considerations for the Parasitic Final Focus Test Beam at SLAC*

A low intensity electron beam parasitic to the operation of the Stanford Linear Collider (SLC) has been transported through the Final Focus Test Beam (FFTB) facility making secondary test beams available for users. Photons generated in collimation of the SLC electron and positron beams in the linac pass through a splitter magnet that deflects the primary beams away from the linac axis into the SLC beam lines. These photons are converted to electrons and positrons in a secondary production target located down beam on the hnac axis. The secondary electrons are then transported through the FFTB beam line onto experimental detectors. The average power of the parasitic beam is very low, thus, it presents no hazards. However, various accident scenarios involving failure of the splitter magnet and the active protection devices could send much more powerful SLC beams (up to 90 kilo-watts) into this zero-degree secondary beam line. For the accident case, the average power in the transmitted beam was calculated using the Monte Carlo programs EGS4 and TURTLE. Results from analysis of the radiation protection systems that assure safety during the parasitic operation are presented. Paper presented at the Thirtieth Midyear Topical Meeting of the Health Physics Society, San Jose, CA (January 5-8,1997) *This work was supported by the Department of Energy contract DE-AC03@76SF00515. Introduction The FFTB components are installed in a shielded enclosure in the straight-ahead channel at the end of SLAC linac (Balakin et al. 1994, see Fig. 1). This tunnel is comprised of two sections; the first 107 m of the FFTB is in the beam-switch yard (a two-level structure that is shielded on the roof by more than 12 m of concrete and earth). The remaining 88 m of the beam line is in a concrete structure that extends beyond the beam-switch yard into an area known as the research yard. The thickness of the roof and side walls of this section of the FFTB tunnel are 1.2 m and 1 m, respectively. While the full FFTB (which is a 47 GeV electron beam that has an average power of 1 kW) can be parked on beam stops in the first section of the tunnel in the beam-switch yard, resulting in negligible increase in radiation levels outside the tunnel, the shielding in the research yard limits the amount of the beam loss in this section of the tunnel to less than 0.1% of the beam (1 watt) only before the design limits are exceeded. In addition to the shielding, other components of the radiation protection systems for the primary FFTB beam line are the Beam Containment System (BCS) and the Personnel Protection System (PPS). The BCS is designed to ensure that beam parameters (current and energy) do not exceed the preset values, and that the beam is delivered to the main dump with minimal loss. The PPS controls entry to the tunnel, ensuring that personnel are excluded from the tunnel during the FFTB operation. A detailed discussion of the radiation safety features of the primary FFTB is given in Rokni et al., 1996. Theprimary FFTB operates in a dedicated mode in which the average beam power in the linac is 1 kW. However, in the parasitic mode, SLC beams with an average power of 45 kW each, are present in the linac. During the operation of SLC 5 to 10% of the primary electron and positron beams are scraped off at collimators located in the last three sectors of the linac. A splitter dc magnet, dipole 50B1, directs the positron and electron beams into the SLC lines. Bremsstrahlung photons generated in these collimators travel straight ahead, pass through the splitter magnet, and strike a secondary production target located down beam on the linac axis (Fig. 2). The secondary electrons can then be transported to various beam lines. The maximum electron intensity in the FFTB is calculated to reach up to 3000 electrons per pulse at 15 GeV. However, for checking response of most of the experimental detectors a beam intensity of around 1 electron per pulse is sufficient. This technique, established at End Station A previously (CavelliSforza et al. 1993), was used to generate electron beams in the FFTB facility as well. The main safety concern for the parasitic operation of the FFTB is the accident case in which the splitter magnet trips and the full SLC beams are transported through the FFTB causing large radiation levels outside the FFTB tunnel in the research yard. Accident Beam Safety Analysis In order to analyze the worst accident case it was assumed that the splitter magnet and all the active protection devices (see next section on BCS) fail and both SLC beams impinge on the secondary production target. The Monte Carlo shower program EGS4 (Nelson et al. 1985), and ray-tracing program TURTLE (Carey 1978) were used to estimate the average power in the transmitted beam for such a scenario. In TURTLE, the moveable apelltures in the FFTB were assumed to be open for maximum transmission to the I experimental detector location. The beam spot size on the secondary target was taken to be 1 mm in radius. The angular and energy acceptance of the FFTB given by TURTLE