As reported in the last issue, Stanford Synchrotron Radiation Lab (SSRL) is upgrading its storage ring to a 3rd generation light source, SPEAR3, capable of running at design values of 3 GeV and 500 mA stored beam. The SPEAR3 installation progress continues on track with more than 1,000,000 lb of accelerator and beamline hardware moved out of the storage ring. This completes the first phase of the installation with two other phases (preparation of facilities and the installation of components) to be completed by the end of October 2003. In close collaboration with the SSRL beamline physicists and engineers, James Liu, Hesham Khater, and Alyssa Prinz of the Radiation Physics Department have prepared the shielding design for the SPEAR3 ring that is currently under review by the Stanford Linear Accelerator Center (SLAC) Radiation Safety Committee.
A significant restriction in design of shielding for the SPEAR3 ring was that the existing bulk shielding structure of SPEAR2 (61-cm-thick concrete lateral walls, a 30-cm-thick concrete roof, and 61- or 91-cm-thick concrete ratchet walls) could not be changed. Instead, extensive local shielding will be used to augment the existing bulk shielding of the SPEAR ring at locations identified as potential beam loss points. Both analytical and Monte Carlo codes were used in design of shielding for the project. The shielding will be implemented according to the two phases of operations: 1.5-W/100-mA for October 2003 operation and 5-W/500-mA in October 2004. Radiation Physics Department staff are currently working with SPEAR3 designers on a detailed commission plan for this fall; at the same time work on a generic design for shielding of the synchrotron beamline is continuing.
The Sub-Picosecond Photon Source (SPPS) project at SLAC started generating ultra-short X-ray beams on Monday, May 19. The SPPS is a collaboration involving SSRL/SLAC, Argonne National Laboratory, Brookhaven National Laboratory, Deutsches Elektronen Synchrotron (DESY), Lund University, University of California, University of Copenhagen, University of Michigan, and Uppsala University. The most unique feature of the SPPS is its combination of brightness and subpicosecond pulse length generated by the 28-GeV SLAC linac electron beam going through an undulator (on loan from the Argonne Advanced Photon Source) and monochromated by a multilayer monochromator.
The electron bunch is compressed in three stages in the 2-mile long linear accelerator where the final compression in the Final Focus Test Beamline (FFTB) compresses the 28.4-GeV, 3.2-nC-electrons per pulse (10 Hz) to 80 femtoseconds full-width half-maximum (FWHM). The peak brightness of SPPS will exceed that of any existing hard x-ray source by several orders of magnitude (more than 4E7 x-rays per pulse of 1.5 angstroms). While the peak power emitted by the undulator is 8.85 GW, the average power is only 0.025 W with a critical energy of 200 keV. The combination of brightness and subpicosecond pulse length will allow collection of diffraction images of the atomic positions of scattering materials as they undergo changes in their atomic arrangements following an impulse from an ultrafast optical laser beam.
The generated x-rays produced in the 2.5-m-long undulator will be deflected on a 1-cm-thick mirror (multilayer monochromator) by 8.5 degrees and will then pass through a 6-in. opening in the FFTB shielding wall and through three collimators before reaching an experimental hutch more than 50 m away from the x-ray deflection point in the FFTB.
The FLUKA Monte-Carlo particle transport code was used for design of the SPPS hutch shielding where the shielding was dominated by Bremsstrahlung radiation produced by the interaction of the electron beam halo with the FFTB beam line components. Radiation safety aspects of the SPPS projects were presented at the 2003 American Nuclear Society meeting in San Diego and will be available also as a SLAC publication (SLAC-PUB-10010).
The last phase of the E158 Experiment in the End Station A, "A Precision Measurement of the Weak Mixing Angle in Møller Scattering," also resumed in June. This experiment is using a 48-GeV, 550-kW electron beam on a 0.173-radiation-length-thick liquid hydrogen target. This is the third and final phase of the E158 Experiment. Previous radiation surveys have shown that the gamma and neutron levels outside the shielding meet the design goal. Radiation safety aspects of this experiment will be presented at the Health Physics Society (HPS) annual meeting in San Diego by Stan Mao. See the abstract later in this newsletter.