Radiation Detection Systems Laboratory Radiation Surveillance

Direct Survey Meters Geiger-Mueller Scintillation Counter Measure surfaces directly Main use for contamination control

Radiation Survey Meters Maintenance Per use: Battery power Check source Check background Calibration: Yearly After maintenance or repairs

Geiger-Meuller Tube

Low Energy Gamma Scintillator (LEGS)

Survey Instrument Comparison Geiger-Muller Detection through window Detects rays (photons) Detects a few particles Shields allow differentiation between particles & photons Designed to measure activity Can be less sensitive to low counts Scintillation Counter Much more sensitive than Geiger-Muller Widespread detection

Indirect Survey Methods Liquid Scintillation Counter Gamma Counter Wipe of surfaces Detect contamination on wipes

No internal radioactive Gamma Counter No internal radioactive standard. May generate small, negative numbers when counting low activity samples: ie wipe tests. Wipe test criterion of 100 cpm above bkgnd still applies!

Scintillation Counter Distintegrations Per Minute = Counts Per Minute / % Efficiency

Scintillation Counter Distintegrations Per Minute = Counts Per Minute / % Efficiency Sample 123 1000 cpm Eff=50% Sample 124 800 cpm Eff=39%

Activity / Calibration A ~ 2.22 MBq Activity / Calibration Detector Counter N Amp N = Activity x (Efficiency x Geometry Factor) A ~ 2.22x106 dps Efficiency ~ 50 % GF ~ 0.5 N =

Activity / Calibration If you detect 555,000 cps, is the activity of the source 2.22 MBq? Consider other contributing factors :

Radiation Sources in the Workplace 9. Radiation Protection Principles

Radiation Transfer of energy, in the form of waves or particles, from one point in space to another point in space.

Time Distance Shielding Contamination Control

Time Minimize the time spent in a radiation field. Example: You are working in front of a fume hood where the field is 18 Sv/h. What is the dose you would receive after 90 minutes? after 10 minutes?

Distance Inverse Square Law The radiation intensity, I, is proportional to one over the distance squared: The source is assumed to be small compared to the distance.

Inverse-Square Law 9 4 1 1 2 3

What is the intensity at twice the distance? If I α 1 (D)2 What is the intensity at twice the distance? I1 = (D2)2 I2 (D1)2 I2 = I1 (D1)2 / (D2)2 OR Let D2 = 2D1 I2 = I1/(D1)2 / (2D1)2 I2 = I1 / 4

Distance Example At 10 cm you measure the field intensity to be 160 μSv/ h. What is the field intensity at 1 m? I1 = D1 = I2 = D2 =

Shielding Material placed between yourself and the source will reduce your exposure to radiation. The amount of reduction will depend upon the material and the radiation. Material density and thickness Radiation type: α, β, γ, or x-ray Radiation energy

Half-value Layer 20 Sv/hr

Half-value Layer Sv/hr

Half-value Layer Sv/hr

Half-value Layer Sv/hr

Recommended Shielding 32 P 12 mm Plexiglas 14 C Glass container Plexiglas 125 I 1 mm Lead sheet 99m Tc 12 mm Lead

Contamination Control Purpose is to ensure that all work and non-work surfaces do not pose a risk to health Survey Meter Wipe Test Combination

Wipe tests Be suspect of zeroes! Use filter paper/tissue etc. Wet with appropriate solvent. Standard surface area to cover is 100 cm2 for each wipe. Place in vial with scintillation cocktail, count. Always include a background. Action level for contamination is 100 cpm above bkgnd. Spurious counts may be due to static, or fluorescence not from radioactive source. Be suspect of zeroes!

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