Comprehensive Radiological Safety Program at SLAC for LCLS Shielding

1. LCLS Radiological Considerations Sayed H. Rokni, SLACApril 24, 2002 • Radiological safety program at SLAC (S. H. Rokni) • Shielding methodology for LCLS (W. R. Nelson) • Radiation safety systems for the LCLS (S. Mao) Sayed Rokni, SLAC

2. Outline of the Presentation • Radiological safety program at SLAC • ALARA/dose limitation • Design criteria • Radiation safety systems • Passive/active components (BCS) • Access control systems (PPS, HPS) • Administrative • Overview of the systems specific to LCLS • Existing facility (FFTB) • New components Sayed Rokni, SLAC

3. Radiation Safety at SLAC • Ensure ALARA • Provide adequate shield for facilities, beam lines and experimental areas • Control access to areas where beam could be present • Incorporate dose reduction, contamination reduction, and waste minimization features in planning stage • Prevent any person from receiving more radiation exposure than limits Sayed Rokni, SLAC

4. Radiological Safety at SLAC-Cont. • Engineering and administrative systems to implement the program • SLAC Radiological Control Manual • Radiation Safety Systems, Technical Basis Document • Guidelines for Operation • Implemented by Radiation Physics, Operational Health Physics, Accelerator and SSRL Safety Offices, Controls Departments Sayed Rokni, SLAC

5. Engineering Controls for Radiation Safety • Passive • Bulk Shielding, local shielding: attenuate radiation • Protection collimators: limiting apertures prevent beams from striking shielding • Active Sensors • Beam Containment System (BCS): Sensors that turn beams off when parameters exceed pre-set limits • Ion chambers • Torroids Sayed Rokni, SLAC

6. Engineering Controls-Cont. • Access Control System • Prevents access to areas where potential for beam operations exist • Personnel Protection System (PPS) • Hutch Protection System (HPS) Sayed Rokni, SLAC

7. Design of Engineering Controls • Radiation Physics Department staff have been involved in design of shielding, BCS and PPS specifications for the LCLS from the start: Design Study Report, Environmental Assessment, Conceptual Design Report • Staffing: • K. R. Kase W. R. Nelson • H. Khater A. Prinz • J. C. Liu S. H. Rokni • S. Mao H. Vincke Sayed Rokni, SLAC

8. Radiological Safety at SLAC- Operations • Radiation physics has oversight for beam operations in conjunction with: Accelerator and SSRL Safety Offices • Administrative controls: • Conduct of Operations, Configuration Control • Guidelines for operations • Beam Authorization Sheets (BAS) • Radiation Safety Work Control Forms (RSWC) • Accelerator and SSRL safety procedures Sayed Rokni, SLAC

9. Training Instrumentation Radiological Postings Dosimetry Radiological Surveys & Monitoring Radiological Hygiene Radioactive Material Inventory Control Radioactive Waste Management Radiological Environmental Protection Emergency Preparedness Radiological Safety at SLAC-Cont OHP administers the following services: Sayed Rokni, SLAC

10. ES&H Division and Radiation Safety Management Sayed Rokni, SLAC

11. Shielding Design Criteria • Normal mode of operation • The integrated dose equivalent outside the the FFTB tunnel must not exceed 1 rem/yr • The integrated dose equivalent to personnel working inside and around the experimental hutch shielding barriers must not exceed 0.1 rem • Maximum Credible Incident • The dose equivalent-rate is limited to less than 25 rem/h, and integrated dose equivalent of less than 3 rem Sayed Rokni, SLAC

12. Final Focus Test Beam • Designed for: • 2.5 kW • 50 GeV • e- or e+ beams • FFTB Tunnel • 4’ thick concrete side walls • 3’ thick concrete roof • FFTB Dump • 9’ thick iron, concrete walls • 60’ of iron to shield muons Sayed Rokni, SLAC

13. LCLS Radiation Safety Systems • Additional components for the LCLS • Injector shielding at sector 20 • lateral shielding walls of the optical front end • front, back and lateral shielding walls for the electron dump • experimental hutches shielding • Front End and hutch beam stoppers • entrance maze • HPS • Initial design completed • Optimize the design Sayed Rokni, SLAC

14. Sayed Rokni, SLAC

15. LCLS Radiological Considerations Walter R. Nelson, SLACApril 24, 2002 • Radiological Safety Program at SLAC (S. H. Rokni) • Shielding Methodology for LCLS (W. R. Nelson) • Radiation Safety Systems for the LCLS (S. Mao) Sayed Rokni, SLAC

16. Shielding Methodology – Outline of Talk • Radiation fields around an electron accelerator • Electromagnetic cascades (soft showers) • Photoproduction of neutrons and muons • LCLS geometry “sources of radiation” • Shielding methodology (computer codes) • Example results using the EGS4, MUON89 and MuCarlo codes will be presented • Relevant shielding results will be presented in the talk that follows (by Stan Mao) Sayed Rokni, SLAC

17. Electron-photon interactions and EM showers • Understanding the electromagnetic cascade shower is key to understanding radiation fields at electron accelerators • High-energy electrons and positrons produce lots of x-rays • X-rays in turn produce more electrons and positrons (inpairs) • This continues back in forth – but it eventually stops when all of the energy is used up • Process is called an electromagnetic cascade “shower” • First observed in photographs like the one shown in the… … next slide Sayed Rokni, SLAC

18. Cloud chamber photo of a shower (circa 1950) (Note: only charged particles are visible) • High-energy electron enters from top • Strikes Pb sheet (not visible in photo) • Many more charged particles produced • They bend because of magnetic field • Charged particles are both e+e- (pairs) • e+e- get swept out by the magnetic field ------------------------------------------ Take note of the two other Pb sheets ------------------------------------------ • Shower regenerated by invisible photons • Without the magnetic field, the shower would be very forward-directed Sayed Rokni, SLAC

19. Monte Carlo simulation showing initiation of a shower 100 MeV electron impinges from the left onto a 1-cm slab of Pb. Particles are produced and are identified by their color. Sayed Rokni, SLAC

20. Ten 1-GeV electrons strike a 15-cm Cu target All particles are shownOnly electrons and positrons shown Sayed Rokni, SLAC

21. …Real example of the power of a full EM shower A Cu beam stopper (PPS/BCS device) that was destroyed by the beam Melt-down at shower maximum e- Sayed Rokni, SLAC

22. …Real example of the power of a full EM shower (cont.) A Cu beam stopper (PPS/BCS device) that was destroyed by the beam Note: THREE shower maxima Sayed Rokni, SLAC

23. Photoproduction of other particles (radiation) • Although most of the energy in an EM shower goes into ordinary “energy deposition” (i.e., heat), other particles are produced • About 0.2% goes into hadron production --- i.e., neutrons • Half into low-energy neutrons (giant resonance excitation) • Half into high-energy neutrons (hadronic cascade) • And muons are pair produced at a rate that is about 1/40,000th that of electron-positron pairs • Although these numbers may seem small, these other radiations are more difficult to attenuate than photons Sayed Rokni, SLAC

24. General schematic of radiation and shielding • Electron beam enters from the left and produces a shower • Photons from shower itself must be shielded • Neutrons produced by photons must be shielded • Muons produced by photons must be shielded (forward direction) Sayed Rokni, SLAC

25. A closer look at how radiation gets shielded • Low-energy neutrons attenuate rather easily • They are followed by the photons • When the shield gets “thicker” the high-energy neutron component starts to dominate • Ultimately we may be stuck with muons, which attenuate very slowly • This is what happens at 0° • At 90° things look similar…except we don’t have muons • For the LCLS we have to consider both the forward and side directions Sayed Rokni, SLAC

26. Components along the LCLS (Sources of Radiation) • An Injector (at Sector 20) feeds into the main SLAC Linac • 15 GeV electron beam with 2 kW of power at the east end • 7 x 109 electrons/pulse at 120 Hz (130 nA of beam current) • The electrons pass through an Undulator that generates a coherent beam of x-rays • A beam line allows x-rays to go to experimental Hutches • Another beam line sends the 15 GeV electrons to a Dump • Collimators and stoppers are required for PPS purposes • All must be considered in the radiation safety analysis Sayed Rokni, SLAC

27. Sources of radiation (cont.) • Undulator • Collimators • Stoppers • Beam dump Electrons will hit all of these components… … and so will the photons The result: radiation !!! Sayed Rokni, SLAC

28. Sources of radiation (cont.) • We have sources resulting from the 15 GeV electron beam • Beam halo • Gas bremsstrahlung • Mis-steering • Beam-diagnostic devices • The electron dump • We also have x-ray sources created by the undulator • Coherent (FEL) radiation • Synchrotron radiation Sayed Rokni, SLAC

29. Shielding Methods • The Radiation Physics Department has successfully shielded the SLAC Two-Mile Accelerator, and its beam lines, for about 40 years • A good starting reference is: Chapter 26: “Shielding and Radiation” in The Stanford Two Mile Accelerator, R. B. Neal, Editor (W. A. Benjamin, Inc., New York, 1968). • Many problems can be solved by using existing computer codes, which we categorize into three groups • Monte Carlo: EGS4, FLUKA, andMuCarlo • Analytic: MUON89, STAC8andPHOTON • Semi-empirical (scaling): SHIELD11 • The four codes highlighted above in “blue” were developed within the RP department Sayed Rokni, SLAC

30. Monte Carlo codes: EGS4, FLUKA, and MuCarlo • EGS4 is a program that specializes in the production and transport of electrons, positrons and photons through matter. It is the most thoroughly benchmarked shower code and is generally considered the standard by which other EM codes are judged. • FLUKA is a general program for producing and transporting more than 30 different particles, including neutrons, muons, and for solving the EM shower problem. • MuCarlo takes muons that are produced by electron beams striking beam dumps and transports them through very complex geometries (including magnet fields). Sayed Rokni, SLAC

31. Analytic codes: MUON89, STAC8 and PHOTON • MUON89 determines the fluence of muons through thick shields after they have been produced in an EM shower initiated by a beam of electrons striking a dump. • PHOTON is an x-ray shielding code that was specifically designed to determine radiation levels associated with beam lines at synchrotron light facilities. • STAC8 was developed from PHOTON and contains many improvements, such as undulator radiation, build-up factors, angular-dependent coherent and incoherent scattering, self-shielding by scatterers (including inclined scatterers), etc. Sayed Rokni, SLAC

32. Semi-empirical codes: SHIELD11 • SHIELD11 is the “workhorse” code of the department and is based on the scaling of experimental data taken during the early days of SLAC. The components and methods of the code have been used with great success in shielding design not only at SLAC, but also at KEK and Jefferson Lab. Sayed Rokni, SLAC

33. An example calculation using EGS4 • EGS4 was used to determine where the electron beam produces showers and how much energy gets deposited in stoppers Sayed Rokni, SLAC

34. An example calculation (cont.) • Energy absorbed all along the undulator • Specific peaks at: • Collimator C1a • Collimator C2 • Collimator C3 • Stopper ST3 • Energy fraction absorbed in ST3 is about 6x10-4, which corresponds to 12 mW (for a 2 kW electron beam) Sayed Rokni, SLAC

35. Another example calculation using MUON89 • We used MUON89 for the forward dump shielding analysis • Note the isodose (rate) lines below the ground level Sayed Rokni, SLAC

36. Summary • The shielding of muons from the primary electron beam striking the dump is well understood • The energy deposition along the LCLS itself – from the Undulator down through the Hutches – has been quantified • Using this information, an extensive shielding analysis has been done by the Radiation Physics Department and the results will be discussed in the next talk (by Stan Mao) Sayed Rokni, SLAC

37. LCLS Radiological Considerations Stan Mao, SLACApril 24, 2002 • Radiological Safety Program at SLAC (S. H. Rokni) • Shielding Methodology for LCLS (W. R. Nelson) • Radiation Safety Systems for the LCLS (S. Mao) Sayed Rokni, SLAC

38. Radiation Safety Systems for the LCLS • Injector at LINAC Sector 20 • Electrondump line • Beam containment system • Shielding • Front End Enclosure • Experimental Hutch • Front End Enclosure stoppers and Hutch stoppers Sayed Rokni, SLAC

39. Injector at LINAC Sector 20 Sayed Rokni, SLAC

40. Injector at LINAC Sector 20 Sayed Rokni, SLAC

41. Electron dump line Sayed Rokni, SLAC

42. Beam containment system (BCS) Sayed Rokni, SLAC

43. Dump shielding 1 Sayed Rokni, SLAC

44. Dump shielding 2 Sayed Rokni, SLAC

45. Front End Enclosure Sayed Rokni, SLAC

46. Front End Enclosure - continued Sayed Rokni, SLAC

47. Experimental Hatch Sayed Rokni, SLAC

48. Front End Enclosure stoppers and Hutch stoppers Sayed Rokni, SLAC

49. LCLS Radiological Considerations- Summary • Radiation safety issues for the LCLS are similar to issues normally encountered at high energy electron linacs and synchrotron radiation facilities. • Staff from Radiation Physics Department are involved in the design of shielding and specifications for active radiation safety systems for the LCLS. • SLAC has a well established radiological safety program to oversee the safe operation of the LCLS. Sayed Rokni, SLAC