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MUHC Survey Meter Protocol

MUHC Survey Meter Protocol. Dr. Robert Corns Department of Radiation Oncology The Sir Mortimer B. Davis Jewish General Hospital. Radiation Safety. Availability of calibrated gamma survey meters legally required Room and source surveys License requirement Patient survey

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MUHC Survey Meter Protocol

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  1. MUHC Survey Meter Protocol Dr. Robert Corns Department of Radiation Oncology The Sir Mortimer B. Davis Jewish General Hospital

  2. Radiation Safety • Availability of calibrated gamma survey meters legally required • Room and source surveys • License requirement • Patient survey • Contamination control • Emergencies

  3. Do It Yourself? • Why bother developing a protocol? • External services available • $200 + shipping and handling • 10+ meters used in Medical Physics and Radiation Oncology • > $2000 per year

  4. Regulatory Document R117(It’s the law) • Describes minimum requirements for survey meter calibrations • Pre-calibration checks • Known calibration sources (NIST) • Requirements on measurement setup • Records CNSC site at www.nuclearsafety.gc.ca

  5. Calibration Goals • Annual calibration • Calibrate scales up to 10 mSv/hr (1 R/hr) • Typically 3 or 4 for scales per meter • Check scales beyond 10 mSv/hr • Scales ± 20% of true value • Measure at 25% and 75% of each scale’s maximum

  6. d02 e–µd X = kA d2 But How to Do It? • Relate source activity to exposure rate in air • Combination of activities and distances to produce all desired exposure rates • Problem: Does not include scatter effects

  7. X = Xprimary + Xscatter X ≈ Xscatter The Shadow Knows Shadow Block Technique

  8. Nothing is Ever Simple

  9. 1 Cs-137 Yield 0 0 200 400 600 Energy (keV) 1 Ir-192 Yield 0 800 0 200 400 600 Energy (keV) A Source by Any Other Name… • Cs-137 • 3 mCi & 30 mCi • Simple spectrum • Ir-192 • 4 – 10 Ci • Complex spectrum • Well chamber • NIST traceable

  10. 1.2 Response 1.0 0.8 10 1000 100 Energy (keV) Energy Crisis? • Meters can have an energy dependence • Potential differences using Cs-137 and Ir-192 • Cs-137 used for low and intermediate scales • Possible to cross calibrate • Ir-192 at a large distance against the 30 mCi Cs-137 at a short distance • Reasonable agreement • Some meters are used to monitor Ir-192

  11. 425 149.9 Setup Distances

  12. Optical Bench d dob

  13. Ir-192 Jig

  14. d??? Chamber Source X Marks the Spot! X • Where is d measured from? • A 2 cm chamber 40 cm from a source has a 10% gradient in the exposure rate • Most meters have their centers marked

  15. Meters

  16. Safety First! • Cs-137 needles are handled manually • Upto 8 mR/hr at 1 m • 1/2 hour exposure time per session • < 0.04 mSv body dose per session • Ir-192 source handled remotely • Upto 2 to 4 R/hr at 1 m • Meters read remotely • Camera or computer interface

  17. Cs-137 Pig

  18. MicroSelectron

  19. Summary I • Annual calibration with a goal of ± 20% • 2 Cs-137 needle sources and an Ir-192 source used to calibrate • Exposure rate changed via source choice and distance • Shadow block method removes scatter

  20. Summary II • Measured exposure adjusted to agree with calculated exposure • Nist traceable sources • Radiation Safety • Records and certificates

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