MUHC Survey Meter Protocol

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
• gt 2000 per year

4
Regulatory Document R117(Its 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 scales maximum

6
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
The Shadow Knows
Shadow Block Technique
8
Nothing is Ever Simple
9
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
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
Setup Distances
12
Optical Bench
d
dob
13
Ir-192 Jig
14
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
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Safety First!

• Cs-137 needles are handled manually
• Upto 8 mR/hr at 1 m
• 1/2 hour exposure time per session
• lt 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
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MicroSelectron
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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