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
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!
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?
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. Distance
If I α 1 (D) 2 What is the intensity at twice the distance? I 1 = (D 2 ) 2 I 2 (D 1 ) 2 Let D 2 = 2D 1 I 2 = I 1 /(D 1 ) 2 / (2D 1 ) 2 I 2 = I 1 (D 1 ) 2 / (D 2 ) 2 OR I 2 = I 1 / 4
Distance Example At 10 cm you measure the field intensity to be 160 μSv/ h. What is the field intensity at 1 m? I 1 = D 1 = I 2 = D 2 =
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
Recommended Shielding 32 P 12 mm Plexiglas 14 C Glass container Plexiglas 125 I1 mm Lead sheet 99m Tc12 mm Lead
Contamination Control Wipe Test Survey Meter Combination Purpose is to ensure that all work and non-work surfaces do not pose a risk to health
Wipe tests 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!