Re: x-ray shielding

[Lester.Slaback@nist.gov]

>I have a 150 kvp device which is used for ion implantation.  The device
>emits x-rays in bursts during the warm up to the operating potential.

$$ Note that this is a positive ion machine and that the xrays are
produced by electron backstreaming.  Hence the shielding need only be
strategically placed.  Second, the spectrum will be much softer than the
150 kV max accelerating potential.  So it will be easy to achieve large
reduction factors.

>Film badge placement to check on exposure found one area that requires
>shielding on the device.

>In order to protect the other individuals in the room ( a large bay area
>housing other equipment for experimental research) I am considering using
>the 100 mrem/yr limit to determine the thickness of lead to place around
>the device - instead of the 100 mrem/wk for the occupational worker.

Assuming one is using a realistic occupancy and distance I think a dose
design criteria in excess of 10-50 mrem would be a poor design.  On the
other hand is you are starting with the fencepost/100% occupancy
presumption then it probably is not a problem.  The folks using such
devices are not usually classed as 'radiation workers' because there
usually is not sufficient external leakage to justify this.  Hence they do
not have the relevant radiation training. Hence one usually is concerned 
with a public dose limit, and this should be reflected in the shield
design criteria.

Also, if the machine usage factor is included (and is a low number) make
certain you do not exceed an accessible dose RATE limit, e.g., 2 mrem/h if
you are a NRC licensee.

A pet peeve: Just because one's job is in the proximity of a source of
ionizing radiation that person does not need to be automatically
classified as a 'radiation worker' and be allowed to be exposed above 100
mrem. And just because you class the workers as 'rad workers' there is no
reason not to try to protect them as if they were the general public.

-- 
the above are the personal musing of the author,
and do not represent any past, current, or future
position of NIST, the U.S. Government, or anyone else
who might think that they are in a position of authority.
NBSR Health Physics
NIST
Gaithersburg, MD 20899
301 975-5810
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Lester.Slaback@nist.gov
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