Re: Worse case vs. normal operations

[P van Rooyen <rooyen@aib-vincotte.nl>]

ICRP 33 gives a calculation method for shielding of useful beam and
scattered radiation.

For primary radiation: 

beta = P * (d ^ 2) / (W * U * T)

with 

beta =  maximum allowed transmission  [ mGy / (mA min) at 1 m] 
P = acceptable weekly exposure [mGy]
W = workload  [mA minutes per week]
U = correction factor for direction; 
(U equals fraction of W during which the beam is directed in the particular
direction)
T = occupancy grade (permanent T=1)
d = distance from evaluated point to focus [m]

For scattered radiation: 

beta = 100* P * (D^2) * (d^2) / (W * S * T)

P, W and T are the same as above 
D = distance from evaluated point to scattered surface [m]
d = distance from focus to scattered surface [m]
S = according to table and figures in ICRP 33 [% per 100 cm^2 ]
for small surfaces S = 0,1 % can be used

You will need the figures and tables from ICRP 33 to find the S values. 
After you calculated the beta values you can determine the needed shielding
thickness with use of figures given for different kV's  

As you noticed the philosophy is: 
first decide on a accepted weekly exposure (based on the allowed limit per
year),  
secondly you correct for the workload per week and for the occupancy.

I am not sure about the US regulations but I checked some European codes.
All these codes follow a system similar like the one I described above.

It is always better to stay on the safe side and add a margin. 
For fixed installations where permanent shielding is possible, in contrary
to mobile situations, a little more shielding gives a permanent effect. I
see this as an advantage: long term dose reduction is easily achieved by a
one time investment.  But personally I think that calculating for 24 hours
while the system is only used for several hours per day  is over
dimensioning your shielding, and not in accordance with my interpretation
of the ICRP philosophies.  

Compared to our neighbor countries, we have a very strict environmental
policy in our country. (As long as it is reasonable I agree totally with
strict protection of people and nature.) 
For people outside the company fences we have to calculate with 100 (yes
hundred) microSievert per year.
But, even these regulations, allow for occupancy corrections. 
The situation that the same individuals are staying at our fences for 24
hours day, 7 days a week, 52 weeks a year is rare!


Paul van Rooijen

AIB-Vinçotte Nederland BV
Postbus 6869
4802 HW Breda
The Netherlands

tel	++31 (0) 76 579 11 54
fax	++31 (0) 76 587 47 60 
e-mail	rooyen@aib-vincotte.nl




At 15:31 9-11-00 -0600, you wrote:
>This question concerns an industrial x-ray installation in the state of
>California.  When demonstrating compliance with requirements for doses to
>non-radiation workers, must you show worse-case conditions (an individual
>plants themselves next to the unit 24-hours a day and the unit operates
>24-hours a day), or may you use normal conditions, e.g., an individual is in
>the area several feet away from the unit for 8-10 hours per day and the unit
>operates 6 hours a day? 
>
>Susan McElrath
>smcelrath@atl.mediaone.net <mailto:smcelrath@mindspring.com> 


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