The Personnel Protection System for a Synchrotron Radiation Accelerator Facility: Radiation Safety Perspective

The Personnel Protection System (PPS) at the Stanford Synchrotron Radiation Laboratory is summarized and reviewed from the radiation safety point of view. The PPS, which is designed to protect people from radiation exposure to beam operation, consists of the Access Control System (ACS) and the Beam Containment System (BCS). The ACS prevents people from being exposed to the very high radiation level inside the shielding housing (also called a PPS area). The ACS for a PPS area consists of the shielding housing and a standard entry module at every entrance. The BCS prevents people from being exposed to the radiation outside a PPS area due to normal and abnormal beam losses. The BCS consists of the shielding (shielding housing and metal shielding in local areas), beam stoppers, active current limiting devices, and an active radiation monitor system. The system elements for the ACS and BCS and the associated interlock network are described. The policies and practices in setting up the PPS are compared with some requirements in the U.S. Department of Energy draft Order of Safety of Accelerator Facilities. Works Supported by Department of Energy Contract DE-AC03-76SFOO5 15. INTRODUCTION The Stanford Synchrotron Radiation Laboratory (SSRL) is located within the site of the Stanford Linear Accelerator Center (SLAC). Although they are operated independently, SSRL conforms to the same requirements as SLAC with regard to environment, safety, and health issues. Figure 1 shows that SSRL complex consists of an Injector and a storage ring called SPEAR (Stanford Positron Electron Asymmetric Ring). The Injector is comprised of a 2.5 MeV RF (radio frequency) gun, a linear accelerator capable of accelerating the electrons up to 150 MeV, and a 3 GeV Booster synchrotron. There are three beam transport lines. The Linac-to-Diagnostic room (LTD) line transports the beam from the Linac room to the Faraday cup in the Diagnostic room for beam diagnosis. The Linac-to-Booster (LTB) line directs the beam from the Linac room to the Booster for further acceleration through a 760 keV maximum RF cavity. The Booster-to-SPEAR (BTS) line transports the beam from the Booster to SPEAR. Ban-ier D and gate E are used to separate the Booster from SPEAR. SPEAR is also capable of accelerating its stored beam (100 mA, 4.9 x lo11 e-) through its two RF cavities. Note that the beam parameters (energy and intensity) shown in Figure 1 are design limits, not physical limits. Figure 1 also shows the nine main synchrotron radiation beamlines: the synchrotron radiation in beamline numbers 1, 2, 8, and 3 are *generated from bending magnets, the synchrotron radiation in beamline numbers 7, 10,6, and 4 are from wigglers, and the synchrotron radiation in beamline number 5 is from an undulator. The possible radiation protection problems for the workers and the general public from the SSRL operation are shown in Figure 2. The problem for the general public outside SLAC is mainly the neutron-photon skyshine radiation to the site boundary, which are monitored continuously with six active peripheral monitoring stations at SLAC. The x-ray radiation hazard is mainly from the klystrons, which provide RF power to the Linac cavities for electron acceleration. However, the x-ray has been reduced to an acceptable level with local lead shielding, which was placed around the collector as part of the klystron strncture. Minor problems for the workers include the induced activity, ozone in the insertion device beamlines, radioactive gases, and muons, and some has been described in a comprehensive report (1). This paper will only discuss the Personnel Protection System (PPS), which is designed for the major radiation protection problems for the workers. The major problems for the workers/users at SSRL are the very high neutron, photon, and electron radiation levels inside the shielding housing (called a PPS area), the neutron-photon radiation outside the PPS areas, and the synchrotron radiation and the gas bremsstrahlung problems in the beamlines. Recognizing the radiation protection problems between accelerator and nuclear facilities are different, the US Department of Energy (DOE) is preparing an Order of the Safety of Accelerator Facilities (SAF) (2) and the associated implementation guidance document (3). Some of the radiation protection policies and practices used in setting up the PPS at SSRL are compared with the requirements in the draft Order of SAF. PERSONNEL PROTECTION SYSTEM (PPS) FOR THE WORKERS The PPS is defined in this paper to be a system protecting people from exposure to beam radiation (this is the prime purpose) and electrical hazards. Figure 3 shows that the PPS’s two major systems and the associated interlock network: the Access Control System (ACS) which keeps people from being inside a PPS area where beam may be running, and the Beam Containment System (BCS) which not only limits the beam power, but prevents beam from escaping from its prescribed channel. There are four major PPS area classifications at SSRL: the Linac and Diagnostic rooms (called the Linac here), the Booster, SPEAR, and the synchrotron radiation experimental hutches. The beam can be in one or more PPS areas, while the remaining PPS