Radiation Protection Service
University of Glasgow
THE EXTERNAL
HAZARD
Radiation Protection Service
University of Glasgow
External Hazard
‘That (biological) hazard arising from the immersion
of a body in a radiation field’
Source types exhibiting an external hazard
• Sealed sources
• Unsealed sources
• Electrical equipment generating em radiation
• Natural sources
Radiation Protection Service
University of Glasgow
Current Annual Occupational Limits
Skin (incl hands/feet) – 500 mSv public 50 mSv
Eye – 150 mSv public 15 mSv
Abdomen of female – 13 mSv in any 3 mnth period
These are equivalent dose limits
Equivalent dose (HT) = Absorbed dose (DT) X WR
Adequate Shielding Level = 7.5 μSvh-1
(unclassified radiation workers)
Radiation Protection Service
University of Glasgow
Estimating the External Hazard - Calculation
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Estimate of biological damage – i.e. Absorbed dose
Type of isotope - α/β/γ
Radiation generators – e.g. X-ray
Geometry of source – point sources are isotropic
Activity of source
Distance from the source
Exposure time
Include natural radiation? e.g aircraft crew
Radiation Protection Service
University of Glasgow
Alpha emitters
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Not generally considered to be an external hazard
Penetrate less than 4 cm in air
Generally considered an Internal Hazard
Bremstrahlung a problem?
Radiation Protection Service
University of Glasgow
Beta emitters
• Dose depends on number of beta particles per unit area
• Independent of beta energy
• The range of 14C (low energy) betas in tissue @ 1 mm
• The range of 32P (high energy) betas in tissue @ 1 cm
• The dose rate Db in mSv/hr produced by a point source of
beta activity M MBq at distance 0.1m is given by
• Db = 1000 M mSv/hr at 0.1m
• This translates to 1 beta particle cm-2 s-1 ~ 1 mSv/hr
Radiation Protection Service
University of Glasgow
EXAMPLE:
ESTIMATE THE RADIATION DOSE RECEIVED AT THE
EYES OF A WORKER USING A SMALL UNSHIELDED 32P
SOURCE OF ACTIVITY 5 MBq FOR A PERIOD OF 15
MINUTES. ASSUME EYE-SOURCE DISTANCE IS 0.3M
Dβ = 1000 M μSv/hr at 0.1m
DOSE RATE = 5000 mSv/hr at 0.1m
DOSE RATE = 5000 mSv/hr at 0.3m (inverse square law)
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DOSE RECEIVED IN 0.25 HR = 5000 x 0.25 = 139 mSv
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Radiation Protection Service
University of Glasgow
Gamma emitters – e.g. Cr51, Co60
The radiation dose delivered by a flux of gamma radiation
depends upon the number of photons incident on unit area
and it also depends upon photon energy, ie the dose
increases with photon energy.
A point source of gamma radiation of activity M MBq with a
total gamma photon energy per disintegration Eg (Mev)
produces a dose rate Dg = MEg mSv/hr at 1.0m (>0.1MeV)
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To find the dose rate at other distances we apply the inverse
square law
Radiation Protection Service
University of Glasgow
EXAMPLE
Find the gamma dose rate at a distance of 0.5m
from a 60Co source of activity 50 MBq. Each
disintegration of 60Co results in the emission of
two gamma ray photons of energy 1.17 and 1.33
MeV respectively
It follows that E = 1.17 + 1.33 = 2.5 MeV
Dose rate at 1m = 50 x 2.5 = 17.9 mSv/hr
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Dose rate at 0.5m = 17.9 x 4 = 71 mSv/hr
Radiation Protection Service
University of Glasgow
X-ray generators
Radiation Protection Service
University of Glasgow
X-ray tube
• X-rays produced by the bremstrahlung effect
• Dose rate is dependent on:
• Target material (atomic number)
• Applied tube voltage (kV)
• Tube current (mA)
• distance from the source (mm)
• Filtration?
Rule of thumb formula: (with 1 mm Be filter)
D = 670 ZVI
d2
mGy/s
Radiation Protection Service
University of Glasgow
Example
Calculate dose rate at 50 mm from an X-ray tube
Using a copper target and tube voltage 50 kV at
10 mA
Z = 29 for copper
D = 670 x 29 x 50 x 10 / 2500
D = 4 Gy/s
i.e. finger dose limit of 500 mSv reached in 125 ms
Radiation Protection Service
University of Glasgow
In context:
Dose rates @ 1 m for various (unshielded) sources
37 MBq (1 mCi) C14 – 0 mSv/h
37 MBq P32 – 0.5 mSv/h
400 GBq Cs137 – 34 mSv/h
X-ray source @ 75 kV and 10 mA – 3.5 Sv/h
(2 mm Al filter)
Radiation Protection Service
University of Glasgow
ESTIMATION OF DOSE RATE BY MONITORING
SIGN ON SIDE OF MONITOR GIVES RESPONSE TO 10mSv hr-1
THUS BY USING MINI MONITOR CPS CAN BE APPROXIMATELY
CONVERTED TO DOSE RATE ie,
IF
20 CPS = 10mSv hr-1
15 CPS = 7.5 mSv hr-1
THE ADEQUATE SHIELDING LEVEL
Radiation Protection Service
University of Glasgow
Minimising the External Hazard
Radiation Protection Service
University of Glasgow
ALARP PRINCIPAL
USE LEAST ACTIVITY 
Radiation Protection Service
University of Glasgow
LEAST ACTIVITY
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Use the least activity required to get good results
Incorporation
Counting efficiency / error / noise level etc
Standard deviation varies as 1/√n
i.e. doubling activity only improves statistical error by
√2
• 95% confidence level (2σ level) ~ 1000 counts above b/g
Radiation Protection Service
University of Glasgow
ALARP PRINCIPAL
USE LEAST ACTIVITY 
USE LEAST TIME 
Radiation Protection Service
University of Glasgow
LEAST TIME
• Conduct a dummy experiment to pinpoint
deficiencies in the technique before using
radioactivity
• All apparatus required should be ready
prior to starting experiment
• Remember dose = dose rate x time
Radiation Protection Service
University of Glasgow
Example:
A classified radiation worker is permitted to receive up
to 20 mSv per year ~ 400 mSv per week. How many hours
of each week can he spend in an area having an average
dose rate of 100 µSv/hr ?
Dose = Dose Rate x Time
 400 µSv = 100 µSv x T
 T = 4h
Radiation Protection Service
University of Glasgow
ALARP PRINCIPAL
USE LEAST ACTIVITY 
USE LEAST TIME 
USE DISTANCE PROTECTION 
Radiation Protection Service
University of Glasgow
DISTANCE PROTECTION
• If we double the distance from a point source of
radiation, the number of particles/unit area in a
given time is reduced by a factor of 4. This is an
example of the inverse square law ie…
• if the dose rate produced by a small radioactive
source is 1µSv/hr at 1 m then the dose rate at
hand distance (0.1 m) = 100 µSv/hr
• finger distance (0.01m) = 104 µSv/hr = 10 mSv/hr
Radiation Protection Service
University of Glasgow
Demonstration
Radiation Protection Service
University of Glasgow
ALARP PRINCIPAL
USE LEAST ACTIVITY 
USE LEAST TIME 
USE DISTANCE PROTECTION 
USE SHIELDING 
Radiation Protection Service
University of Glasgow
SHIELDING
• It is always best and most economical to shield the
source not the personnel.
• Placing a shield round the immediate vicinity of the
source requires a small amount of shielding material
and also obviates the possibility of irradiation of hands
etc
• Use the correct shielding material for the isotope
concerned
Radiation Protection Service
University of Glasgow
• For alpha emitters – thin sheet of paper or plastic
• For beta emitters – plastic / perspex thickness dependent on energy
• For gamma emitters – lead shielding or leaded glass
• For high activity sources bremsstrahlung may be an issue
Radiation Protection Service
University of Glasgow
Demonstration
Radiation Protection Service
University of Glasgow
Remember the adage:
Time!
Distance!
Shielding!