Beta rule of thumb

[Keith Welch <welch@CEBAF.GOV>]

Mike, 

This is scary.  These two rules of thumb agree perfectly.  I'm familiar
with the use of the conversions you're talking about.  Swipes that read 100
mrem/hr are kind of hard to check on the frisker.  But the conversion I was
using was going directly from activity level to dose rate - not to
"apparent" dose rate on a meter.  Here's how I see that they agree -
suprisingly well.

The short answer is, apply the open window correction for an RO-2....(you
mentioned the factor of five difference)
 
The long answer is.....

My ROT:
A surface with 2E^6 dpm has gamma dose rate ~ 1 mrem/hr, beta, 100 mrem/hr

Now, if I MEASURE this surface with an RO-2, I get:

Closed window =~ 1 mrem/hr
Open window =~ 21 mrem/hr (assuming a beta correction factor of 5)
Open minus closed = 20mrem/hr X 5 = 100 mrem/hr
So the corrected reading agrees with the rule of thumb for 'true' dose rate.


Now, the uncorrected open window reading is 2E^6 dpm = 21 mrem/hr .....

or, 'about' 100,000 dpm = 1 mrem/hr instrument reading - your ROT.

I'm assuming you apply this correction to your beta dose rates taken with
RO-2 and similar instruments when determining "true" surface beta dose
rate.  The factor of 4 - 5 appears to be relatively standard from my
experience.  Does anybody use conversion factors terribly different from
this???

--------------------------
Date: Fri, 03 Oct 97 12:17:53 PST
From: "MIKE RUSSELL" <russelmj@songs.sce.com>
Subject: Re[2]: Beta rule of thumb

  Keith,

  Here's my two cents.

  I've always interpreted these "Rules of Thumb" not for converting
activity to
  dose rate but rather converting measurements made by friskers to
measurements
  made by ion chamber survey instruments.

  This is useful in comparing contamination survey measurements made by an
  survey meter when the count rate is too high to be handled by a frisker.

  Our empirically derived conversion is 10,000 cpm on a frisker (pancake GM
  tube) equals 1 mR/hr on an RO-2 (open window).

  This value (100,000 dpm/mR/hr) is about 5 times higher than the value
  you quoted below.

Keith Welch
Thomas Jefferson National Accelerator Facility
Newport News VA
welch@cebaf.gov
Ph: (757)269-7212
FAX:(757)269-5048