Topic 10  Shielding
External Radiation Protection



Basic Principles
 Minimize
exposure time
 Maximize
distance from the source
 Shield
the radiation source
Distance Effects on Dose Rate
 Radiation
Field Geometry Considerations
 Point
Source
 Line Source
 Plane
Source
Calculating Fluence and Flux From a Point
Source
 F = Fluence g/cm^{2}
 f = Flux g/cm^{2}s
 D = total number of photons emitted
 S_{0} = source strength (photons/s)
 r = distance from source
Point Source Calculations  Photons
 Dose
rate dictated by inverse square law
 I_{1} =
radiation intensity at distance R_{1} from the source
 I_{2} =
radiation intensity at distance R_{2} from the source
Example
 ^{60}Co
 2 photons
(1.17, 1.31 MeV)
 Given
3700 MBq source
 What is
the exposure rate a 1 m?
 What is
the exposure rate at 7.2 m?
Point Source Calculations  Photons
 Dose
equivalent rate
 G = specific gamma ray constant
 A
= source activity
 d
= distance
 w_{r} =
radiation weighting factor
 V
= velocity of approach to source
 t
= total time of exposure
Example
 10
Ci ^{60}Co source failed to retract into its shield
 Operator
walks to source at 1 m/s
 Stops
1 m away, looks for 15 s and leaves at a rate of 2 m/s
 What
is the dose commitment?
Line Source (Cember)
 Pipe
carrying contaminated waste
 C_{l} is
linear concentration of activity (e.g., Ci/cm or MBq/m)
 Source
strength isG
Line Source (Cember)
 Dose
rate at point p from small amount dl is
Line Source (Cember)
 Dose
rate at point p from total length of pipe is
Example
 ^{24}Na
in a cooling water line passes through a pipe in an access room 6 m wide
 Door
to the room is in the center of the 6m wide wall, 3 m from the pipe
 Activity
concentration is 100 MBq/m
 What
is the dose equivalent rate in the doorway at point D_{1} & D_{2}
Example Layout
Line Source Exposure Rate  Photons
 S
= source strength in photons per s per unit line length (cm)
 h
= perpendicular distance from point of interest P to the horizontal plane
of the source>
 f = flux at point of interest in photons per cm2 per
s
 r
= distance from source to point of interest, P
 q = angle in radians
 Z_{2} 
Z _{1} = line length of source in cm
Ratio of Dose Rates as a Function of Distance Midpoint from a Line Source
Calculating Flux From Complex Geometries
 Point
Kernel method
 Source
broken into many small kernels
 Contribution
from each kernel evaluated for a common point
 Contributions
are summed
Plane Source
Plane Source Activity
Plane Source Activity, Std Units
Ratio of Dose Rates as a
Function of Distance from a Plane Source
Example
 50
MBq of ^{24}NaCl solution spilled over a circular area of 50cm
 Find
the dose equivalent rate at
 30
cm
 1
m
Volume Source
 Uniformly
distributed activity
 Estimate
based on “effective surface activity”
 Allow
for selfabsorption in slab
 Slab
 Thickness
t
 Contains
C _{v }MBq m^{3} uniformly distributed activity
 Linear
absorption coefficient of slab is µ
 Observed
intensity at surface due to activity in layer dx at depth x is:
 D(C_{a})
= C_{v}dx·e^{µx}
