beta factors for portable survey instruments

[John Lobdell <lobdell@fly.HiWAAY.net>]

This is in response to the question concerning beta factors for portable
survey instruments.

TVA determines beta correction factors for ion chamber survey instruments
using a 10 centimeter by 10 centimeter Co-60 slab source.  This isotope was
chosen since it is the most abundant isotope at our nuclear power plants.
The source was calibrated in mrad/hr at one centimeter from the source with
an extrapolation chamber.  We wanted to supply a "generic" factor for all
instruments so the technicians in the field could remember one number and
apply it universally for all instruments.  Each instrument is positioned 1
centimeter above the center of the slab and measurements are made with the
window open and window closed.  The known dose rate from the source divided
by the difference between the open and closed window readings provides the
correction factor.  If the factor determined in this fashion is within an
established numerical window, a generic correction factor of 5 is supplied
for the instrument.  The window was established by exposing approximately
ten of each type of instrument to the source to determine the spread.  This
beta correction factor is determined each time an instrument is calibrated
since it is possible, but unlikely, for an instrument to "fail" this test.
When it "fails", there is a problem with the ion chamber in the instrument.  

We also have additional 10 by 10 centimeter sources.  The isotopes are:
depleted uranium, Tl-204, and Sr-90.  The correction factors obtained with
these sources are not significantly different than the factor obtained with
Co-60.  The correction factor will change significantly with geometry - size
of the source and distance from the source to the instrument.   

The beta correction factors described above are good only for the geometry
described above - 10 by 10 centimeter slab at a distance of 1 centimeter.  A
question arose concerning the beta correction factor for a "hot particle".
One of these particles was "captured" at Browns Ferry.  The extrapolation
chamber was used to make dose rate measurements from this particle.
Corrections were made for geometry and a depth of 7 mg/cm squared for skin.
Instruments were then exposed to the source, and a correction factor of 230
was determined.  This factor is the ratio of average dose rate over 1 square
centimeter at a depth of 7 mg/cm squared to the dose rate measured by an ion
chamber survey instrument.        

If I can provide additional information, please contact me.

John Lobdell, Ph.D., C.H.P.
lobdell@HiWAAY.net

This information is only my opinion.