From: jgd@dixie.com (John De Armond)
Subject: Re: Brehmstrehhlung X-rays
Date: Mon, 25 Jan 93 19:14:59 GMT
Organization: Dixie Communications Public Access.  The Mouth of the South.

bbaker@cscns.com (Brian Baker) writes:


>Ok, I chalk this post up to too many people in the biotech business
>not knowing enuff chemistry.... (sarcastic joke, no offense meant!)

>I just graduated with a BS in Biochemistry, got my first job in 
>Denver doing RFLP for paternity analysis and trying to perfect a
>non-isotopic method of doing genomic Southern blots.

>Anyway, a couple of the guys in the lab are doing dot blots in attempts to
>detect CMV virus and do HLA typing.  They started with 5 millicuries of
>32-p (now they're up to 10 or 15).  They're working behind 1/2" plexiglass,
>which is sufficient to 
>block all betas coming from the source.  However, one guy took the geiger
>counter in front of the screen and noticed a few thousand counts.  I 
>told him it was somthing along the lines of X-rays coming from the plexi-
>glass.  

>The way I understand this is:

>	1)  The X-rays are caused by betas being deflected by lead
>	    in the plexiglass, which causes a rapid change in velocity
>	    and a subsequent X-ray

>	2)  These X-rays are of low energy (compared to X-rays you get
>	    at the hospital) and the extinction coefficient is only
>	    a few feet or so

>	3)  5 millicuries, which is a large amount of 32-p, will not
>	    generate enuff X-rays from plexiglass to be concerned about,
>	    certainly .5 mCi behind 1/2" plexiglass is safe

>Oh yeah...this same person claims X-rays to be 'ionizing radiation'
>Now I  had a pretty good Nuclear/Radiochemistry class and I certainly 
>don't remember X-rays being classified as ionizing radiation.  This person
>also claims X-rays and Gamma rays to be "almost" the same.

Brian, this is not a flame so please don't take it as such.  If anyone
in the nuclear industry had run a radio chemlab like that, everyone involved
would be thrown in jail.  That is some of the sloppiest radiation 
safety I've heard about in years.  

Your assumptions above are essentially correct.  Bremsstrahlung radiation
(literally "braking radiation") is generated when a high energy particle
is stopped rapidly.  The energy and thus the dose per absorbed event,
is dependent on both the beta energy and the Z of the absorber.  High
Z materials generate more radiation and thus should not be used around
beta.  To be getting any significant Bremsstrahlung from plexiglass
(normally only very low Z elements), it must be lead-loaded plexiglass.
This is exactly the WRONG shield to use in a high energy beta environment.
A LOW Z absorber should be the shield.  If you need to see the experiment
(obviously) and the beta emitter is near enough to high Z materials
such as metal lab equipment, the proper shield is something low Z such
as normal plexiglass backed by lead-loaded plexiglass.  The normal
plexiglass absorbs the betas with minimal Bremsstrahlung and the lead
loaded plexiglass behind it absorbs any Bremsstrahlung from surrounding
materials.

As far as the degree of hazard, you should not have to be
guessing, having a BRH person speculating or anything else like
that.   That hazard should have been quantified by your
radiation safety officer. You are supposed to have on hand
dosimetery and survey equipment suitable for the isotopes
involved.  You should be wearing personal dosimetry with a beta
screen.  You should also have available to you a low energy ion
chamber survey meter calibrated in mR/hr.  A geiger counter will
not do.  The reason is the GM counter equally counts an event
for  each radiation particle regardless of the energy of the
particle.  Since the dose delivered is proportional to energy,
you have no indication of the dose, only events per unit time 
That many GM counters also have a scale "calibrated" in mR/hr is 
unfortunate.  That calibration only holds at one energy point. Typically 
these things are calibrated for a degraded gamma spectrum seen in 
nuclear facilities and around aged fallout which tends to  center around 
650 kev.  No good at all for your purposes since the Bremsstrahlung
and/or fluorsence X-rays from the lead are probably  in the <100
kev range. 

You should have been trained in basic radiological health which, for 
nuclear power radiation workers, involves a 2-3 day course.  You
should also have a resident radiation safety officer (RSO) knowledgable in
these matters and who assures your experimental techniques are 
radiologically safe.

My educated *guess* is that the dose you are receiving is minimal, given the 
conditions you described.  However, please don't rely on opinion. Inquire of 
your RSO of you have one and insist on there being one if you don't.  

John
(a health-physicist in another life)
-- 
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