Health, Safety and Environment
www.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
Distintegrations Per Minute =
Counts
Per Minute /Sample
%
Sample
Efficiency
123
124
1000 cpm
800 cpm
Eff=50%
Eff=39%
A ~ 2.22 MBq
Activity / Calibration
Amp
Counter N
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:
1
I 
2
D
The source is assumed to be small
compared to the distance.
Inverse-Square Law
4
1
0
1
9
2
3
If
Iα 1
(D)2
What is the intensity at twice the distance?
I1 = (D2)2
I2 (D1)2
Let D2 = 2D1
OR
I2 = I1 (D1)2 / (D2)2
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
•
14
C Glass container
Plexiglas
•
125
I 1 mm Lead sheet
•
99m
Tc 12 mm Lead
12 mm Plexiglas
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
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