Primary Barrier

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    Primary barrier is the barrier in which the primary beam of the treatment machine falls (Linac, co-60 units) on it. The thickness of the primary is greater than the other barriers of room. Shielding materials for primary barrier as well as other barriers are same like concrete, lead, and steel etc.
Primary barrier thickness determination:

    The required attenuation of the barrier B may be found according to a desired dose constraint that is derived from an occupational or public dose limit.  Reference (1) uses the following expression to determine the attenuation required by the barrier.

Where,


P – is the allowed dose per week (Sv/week)outside the barrier.


D – is the distance from the isocentre to the outside of the barrier, in meters.


SAD – Source to Axis Distance in meters


W – is the workload, in Gy/week at 1meter.


U – is the use factor or fraction of time that the beam is likely to be incident on the barrier.


T – is the occupancy factor or the fraction of time that the area outside the barrier is likely to be occupied (mentioned in the table 1)

    The thickness of concrete required from attenuation graphs, or by the use of TVLs.  The number of TVLs required to produce this attenuation is determined from:

The width of the primary will be calculated as follows,

    The primary barrier width is made equal to the maximum to the maximum field size at the barrier plus 1foot (0.305m) on either side to prevent radiation from leaking through the secondary barrier that abuts the primary.  Most of the linear accelerator has 40cm x 40cm as maximum field size at one meter from the target.

When the collimator is rotated to 45 degree, the above dimension becomes equal to its diagonal (56.6cm) then the horizontal barrier width (W) required is given by

W = 0.556d+ (0.305 x 2)


Where, d is the distance from the source to the barrier.


Questions:

1. Calculate the primary barrier thickness of Co-60 unit. One TVL (density 2350 kg/m3)is 21.8 cm. permissible dose (p) = 0.12mSv/week, distance (d) = 3meter, use factor (U) = 0.25, occupancy factor (T) = 1 and workload of the machine is 384 x 103 mGy/week.

a) 1033mm


b) 103mm


c) 1000mm


d) 950mm



Answer:

a) 1033mm



References:

1. Safety Report Series No. 47, IAEA, Vienna:2006

2. The Textbook of Radiological Physics, by Prof. K. Thayalan.

3. Atomic Energy Regulatory Board lecture notes.

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