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SHIELDING for a HIGH ENERGY DIGITAL RADIOGRAPHY BAY

SHIELDING for a HIGH ENERGY DIGITAL RADIOGRAPHY BAY. P.Berry, P. Heintz, S.K.Velarde Los Alamos National Laboratory University of New Mexico Medical School Three Rivers Technical Conference 2010 Rogers, MN. Unclassified per T. N. Claytor, ADC 3/15/10. WE’RE GOING HIGH ENERGY.

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SHIELDING for a HIGH ENERGY DIGITAL RADIOGRAPHY BAY

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  1. SHIELDING for a HIGH ENERGY DIGITAL RADIOGRAPHY BAY P.Berry, P. Heintz, S.K.Velarde Los Alamos National Laboratory University of New Mexico Medical School Three Rivers Technical Conference 2010 Rogers, MN Unclassified per T. N. Claytor, ADC 3/15/10

  2. WE’RE GOING HIGH ENERGY

  3. OBJECTIVES • Identify key personnel needed to build a radiography facility • Know the elements and requirements in the building • Know the basic elements in performing shielding calculations • Follow the project after the planning phase through completion

  4. PLANNING COMMITTEE • Management Representative (budget) • Radiographers Levels II and III • Radiological Physicist • Architect • Construction Representative • Equipment Vendor

  5. KEY to SUCCESS COMMUNICATION And perhaps cooperation and consensus

  6. Some Issues to be Addressed • What accelerator to purchase? • What objects will be radiographed? • What modalities will be used (DR ,CT)? • Any other activities in the facility? • Types of doors (sliding or swinging)? • Handling and storing of radioactive material? • Future use of area around the facility?

  7. Some Issues to be Addressed • Density of the concrete has to be specified • Mechanical penetrations have to be planned in advance • Plan for HVAC • If any changes are made to the original plan NOTIFY the physicist IMMEDIATELY • Ensue proper concrete pouring techniques

  8. D-primary D-leakage door

  9. Description of the Environment Outside of the Bay • Parking Lot to the left of the bay. • Grassy area in the direction of the primary beam • New office building will be constructed 100 ft in the direction of the primary beam. • Radiography bay will be inside of a larger building. • 8 foot wide hallway separating the bay and office space.

  10. PRODUCTION OF RADIATION TYPES IN A LINAC electrons photonsBREMSSTRAHLUNG Radioactive nucleus positron electron Pair-production Radioactive nucleus Photo-neutrons production Neutron capture ACCELERATOR HEAD > 10 MeV

  11. Photoneutron Production • Interaction of photon beam with heavy metal accelerator components results in neutron production. • For all practical purposes neutron production threshold is 8-10 MeV. • Neutrons are emitted isotropically and scattered many times • Photoneutrons are produced mainly from the target, and collimators– exiting through the collimator

  12. Threshold for Photoneutron Production

  13. Definitions • Control Area: limited access area with the occupational exposure controlled by the radiation supervisor • Uncontrolled area: any space not meeting the definition of a controlled area (general public) • Shielding design goals (P): effective dose values recommended by NCRP-2004 for controlled and uncontrolled areas • Primary Barrier: a wall, ceiling, or floor designed to attenuate the useful beam to the required degree • Primary radiation: radiation emitted directly from the source that is intended to be used for industrial imaging

  14. Definitions • Occupancy factor (T): the fraction of time that the maximally exposed person is present in an area while the beam is on. (always 1.0 for radiation workers) • Workload (W): the average dose of radiation produced by a source over a specified time (most often a week) at a specific location. If there is no dependable information, assume 8 hours per day , 5 days a week for a total of 40 hours per week. If this is to be exceeded, NCRP 51 page 42 gives the guidelines for this problem.

  15. Definitions • Secondary Barrier: A wall, ceiling, floor designed to attenuate leakage and scattered radiation to the required goal. • Secondary radiation: All radiation produced by scattering off of objects, or leakage through the protective source housing –all radiation in the bay except for the primary beam • Leakage radiation: All radiation , except the useful beam, coming from within the accelerator head and other beam-line components. • Use factor (U): the fraction of a primary beam workload that is directed toward a primary barrier

  16. Shielding Design Goals • Controlled Area0.1 mGy/wk • Uncontrolled Area0.02 mGy/wk

  17. Shielding Materials

  18. Assumptions for the Problem • 15 MV LINAC • Dose Rate 20,000 cGy/min@ 1 meter (200 Gy/min) • Leakage .001% • Workload: 4.8 x 105 Gy/week • Beam width at the inner wall 8.6 feet in diameter • Room size: 55 feet x 30 feet x 25 feet. • All calculations are 0.30 meters from the outer wall.

  19. D-leakage D-primary door

  20. Sample Problem (primary barrier) • Equations:B = Pd2/WUTn = log (1/B) S = TVL(1) –(n-1) TVL(2)where “n” is the number of tenth value layers, S is the total wall thickness and TVL(1) and TVL(2) are the tenth values

  21. Primary Barrier • Distance = 14.3 meters Leakage = 1P = 20X10-6 (Sv/wk)U =1, T = 1/40 (this is a grassy area and rarely occupied) • TVL(1) = 44 cm; TVL(2) = 41 cmWall thickness is 8.8 feet

  22. Secondary Barrier or Ceiling • B = P d2/ ((Leakage) WUT) n = log (1/B) S = TVL(1)- (n-1) TVL(2)

  23. Secondary Barrier • Distance = 6.38 meters Leakage = 0.001% • P = 20X10-6 (Sv/wk)U =1 T = 1/40 • TVL(1) = 36 cm; TVL(2) = 33 cmWall thickness is 2.5 feet

  24. DOOR • Reduce the energy and quantity of the neutrons reaching outside of the door. • Hydrogenous material reduces the neutron dose equivalent to an acceptable level. • Neutrons are absorbed by (n, gamma) reactions in the door. • The gamma rays (neutron-capture gamma rays are released in the range of a few MeV. • Lead in the door is required to attenuate the beam to an acceptable level.

  25. l DIRECTED SHIELDED DOOR • Typical Sliding Door with the entire door encased in steel. X-ray beam concrete gap polystyrene lead

  26. DOOR AT THE END OF THE MAZE INDIRECT • The neutron contribution as well as the photon contribution is calculated. • The main issue is the scattered radiation down the maze. • The door construction is still the same, lead- poly- lead with the entire door encased in steel. This door is a swinging door and opened and closed pneumatically. • EXCELLENT REFERENCE: McGinley, P, Shielding Techniques for Radiation Oncology Facilities,2d ed.

  27. SLIDING DOOR

  28. AFTER ALL OF THE PLANNING IS COMPLETED

  29. ITEMS DURING and POST CONSTRUCTION • Ensure that the proper shielding report is being used. • Test the concrete for the proper density. • Ensure that all radiation protection equipment-warning lights, door interlocks, emergency off buttons-have been ordered and installed. • Ensure that all registrations have been completed. • Ensure that the final radiation survey was conducted by an authorized expert.

  30. HELP HOW DO YOU DEFINE A DEAD PANEL?

  31. COMING ATTRACTIONS • My panel is dead!!!!!!!!!What does this mean physically?Please send answers to Phil Berrypberry@lanl.gov • This will be for a project: Failure Modes of Digital DetectorsWill be to determine the dose for “My panel is dead!!!!!!!!”

  32. Take Home Points • Did you identify key personnel needed to build a radiography facility • Did you consider all of the main elements and requirements in the building • Be sure you understand the basic elements in performing shielding calculations • Follow the project after the planning phase through completion

  33. THANK YOU FOR YOUR ATTENTION

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