radiation detection systems
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
Health, Safety and Environmentwww.hse.ubc.ca
Radiation Detection Systems
8. 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
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
Sample 124
800 cpm
Eff=39%
Distintegrations Per Minute = Counts Per Minute / % Efficiency
Sample 123
1000 cpm
Eff=50%
Activity / CalibrationA ~ 2.22 MBq
N = Activity x (Efficiency x Geometry Factor) • A ~ 2.22x106
dps• Efficiency ~ 50 %• GF ~ 0.5
Amp
Counter N
N =
Activity / CalibrationIf you detect 555,000 cps, is the If you detect 555,000 cps, is the activity of the source 2.22 MBq?activity of the source 2.22 MBq?
Consider other contributing Consider other contributing factors :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?
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
ID21
1 4 9
Inverse-Square Law
0 1 2 3
If I α 1 (D)2
What is the intensity at twice the distance?
I1 = (D2)2
I2 (D1)2
Let D2 = 2D1 I2 = I1/(D1)2 / (2D1)2
I2 = I1 (D1)2 / (D2)2OR
I2 = I1 / 4
Distance ExampleAt 10 cm you measure the field intensity to be 160 μSv/ h.
What is the field intensity at 1 m?I1 = D1 =
I2 = D2 =
ShieldingMaterial 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 containerPlexiglas
• 125 I 1 mm Lead sheet
• 99m Tc 12 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!
END DAY 1