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# Topic 10 - Shielding  ### Basic Principles

• Minimize exposure time
• Maximize distance from the source

### Distance Effects on Dose Rate

• Point Source
• Line Source
• Plane Source

### Calculating Fluence and Flux From a Point Source  • F = Fluence g/cm2
• f  = Flux g/cm2s
• D = total number of photons emitted
• S0 = source strength (photons/s)
• r  = distance from source

### Point Source Calculations - Photons • Dose rate dictated by inverse square law
• I1 = radiation intensity at distance R1 from the source
• I2 = radiation intensity at distance R2 from the source

### Example

• 60Co
• 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
• wr = radiation weighting factor
• V = velocity of approach to source
• t = total time of exposure ### Example

• 10 Ci 60Co 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
• Cl 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

• 24Na 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 D1 & D2

##  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 Z2 - 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 ## ### Example

• 50 MBq of 24NaCl 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 self-absorption 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(Ca) = Cvdx·e-µx Welcome Contact Getting Started Site Map Project 1 2 3 4 5 6 7 8 9 10 11 College of Engineering OSU Extended Campus Local 541-737-9204 Fax 541-737-2734 4943 The Valley Library Corvallis, OR 97331-4504