Re: X-Ray Shielding Requirements

[tdc@ehssun.lbl.gov (Ted M. de Castro)]

The consequence of the 100 mREM limit may not be as great as you suspect.

I think it might depend upon when the initial shielding calculations were 
done.

Restricting my comments (for now) to low voltage x-rays in the medical 
diagnostic range say less than 150 kVp:

I have NEVER specified a shielding to just meet the legal limit!  I think 
500 mREM is far too much to give the general public when these low energy 
x-rays are so easily and cheaply attenuated.  In the old days of following 
the NCRP tables I would almost always double the work load given and the 
usual techniques of lumping all the kVps together at the highest were 
grossly conservative and almost always gave at least 1 TVL more shielding 
than minimum required.  Now that I use a spreadsheet I wrote that can 
accurately combine multiple techniques at their actually kVps and properly 
calculate the combination - the conservative factors are gone and I 
purposely add a TVL and make my clients aware of this.

So - I guess this is all to say that your comments about the 
conservativeness of NCRP methods may indeed by so and a more accurate 
analysis may indicate that you already meet the newer levels.

The other consideration is that if the existing shielding was for 
conditions and equipment say 20 years ago - then techniques have gone down 
and again you may be already in compliance.

Your alternative to a more precise calculation would also be to post 
environmental TLDs and log all usage and establish your actually exposure 
conditions.  Again I suspect you may already be in compliance.

Now for higher energy orthovoltage and up (250 kVp and up).

Here is likely to be real problems.  Here shielding is not so cheap and easy 
and therefore it is usually precisely calculated and the "excesses" of 
conservativeness not applied quite so liberally.

Also here techniques have not gone down.

Your best bet here is environmental TLDs and or surveys and then act 
accordingly.

I have done this many times for industrial radiography facilties and - 
you'll be sorry to hear - it has almost always resulted in more shielding.  
This is just one you will have to "bite the bullet" on.

That's my 2 bits worth!