Re: ICRU Report 37, Radiological Health Handbook

[kmq@cosmail1.ctd.ornl.gov (Kim McMahan)]

At 10:43 AM 10/29/96 -0600, steven t hand wrote:
>The covering on the film for our monitoring has
>a thickness of 35 mg/cm sq plastic ( I am assuming for the moment that
>it is the A-150) and the skin thickness is 7 mg/cm sq.  I am looking 
>for the energy of the beta particle under which monitoring would not
>be useful, feasible, practical based on this information.  We currently
>do not monitor for H-3, C-14, and S-35, however the data on page 114 of
>the ICRU document lists a .150 MeV beta as having an average range of
>0.03 mg/cm sq (rounded up).  I am confused by this information.  Does the
>factor of 8x thickness for the plastic have to do with this ?  Is there
>also a limit to the thickness of the plastic around the film to keep 
>light from entering?  


I don't have a copy of ICRU 37 handy, but I do have the HP/RHH you refer to.

First, from the chart on p. 184, the range of a (monoenergetic) 150 keV
electron is 0.03 grams/cm2 (not mg/cm2). Is this part of the problem or just
a typo in the e-mail?

Second, since beta decay results in a spectrum of energies, even though your
dosimeter can TECHNICALLY detect beta emitters with energies above 150 keV,
PRACTICALLY the nuclide will have to have an endpoint energy much higher
than that. Very simply, if only 2% of the decays exceed 150 keV, then the
calibration factor would be 50 at best (assuming 100% detection of those
making it through the filter - quite a stretch). Your LLD for this
hypothetical nuclide would be  h u g e. 

Third, as for what endpoint energy your dosimeter might reliably see, I'm
afraid all our experience is with TLD, which will be comparing apples and
oranges. Even a thin TLD has a sensitive volume that is much thicker than
your film emulsion, so our correction factors will have a non-trivial
component due to depth dose effects. But, if it helps, for our nominally 7
mg/cm2-filtered element and Pm-147 (Emax ~ 225keV), the correction factor is
about 50.

Most dosimetry vendors calibrate to a particular nuclide(s), and you should
find out what that is. Some will only give you a choice of one from a list;
some use an algorithm to get a best guess of the energy and apply a
correction based on that. Depending on your particular radiation fields,
you'll have to choose which is best for you.

I can't comment on the thickness of filter needed to block out light from
film. Maybe others can supply this information.

Kim McMahan, CHP
Office of Radiation Protection				    Sola fide ...
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e-mail: mcmahankl@ornl.gov