Is it possible that you are using a lead-impregnated plexiglas shield?
We have a couple on hand in our lab for 125I work, as well as "normal"
plexiglas for 32P. You can't use the lead ones for 32P work--they will
generate Brehmstrallung radiation, as you seem to be seeing. The lead
ones are slightly more yellow than the non-lead guys (they look more
"aged", presumably due to the lead). The regular ones damp down 32P
effectively (though if you're looking at high levels, not completely).
On 2 Nov 1995, NICHOLAS THEODORAKIS wrote:
> In article <rjordan-2510951356100001 at linus.radonc.washington.edu>,
> Rob Jordan <rjordan at tezcat.com> wrote:
> >Can anyone email me some companies/catalog/etc. phone numbers that have
> >good selections of beta shielding devices. I'm looking primarily for
> >boxes, containers, shields, etc. of various sizes, but every <insert
> >common lab supply catalog here> catalog that I've looked in has a page or
> >two at best. I mean something like Acme Radiation Saftely Supply would be
> >Rob rjordan at u.washington.edu>> Well, I don't have a recommendation (many lab supply vendors sell them),
> but I do have an observation that I hope will generate some discussion.
> We have some plastic beta shields from Owl Scientific around the lab;
> when we were doing some rad. surveys with a monitor that has probes for
> both beta- and gamma rays, we noticed that 32P kept behind the shield
> generated a lot of bremstrallung (spelling?) gamma radiation when it hit
> the plastic (which did block the betas, BTW). I almost think it would
> safer not to use the shield. Has anybody else noticed this with their
> beta-shields? Any comments?
> Nick Theodorakis
>ntheo at welchlink.welch.jhu.edu> Johns Hopkins Medical School, Baltimore, MD