backscatter -Reply

[ABELQUIE@ORAU.GOV]

Will-

In the context of surface activity measurements, beta backscatter from
different types of surface materials was considered and evaluated in
NUREG-1507.  Specifically, we deposited approximately 4,000 dpm of a
liquid SrY-90 source on a very thin sheet of Mylar, and placed this
activity between the detector and various surface materials (wood, SS,
concrete, drywall, floor tile, etc.) 

We normalized the detector response for each material to the
instrument's response to this activity in air.  The backscatter factor
(using a gas proportional detector w/ 3.8 mg/cm^2 window) varied from
1.14 on wood to 1.37 on SS.  We also performed a similar experiment
with Tc-99, and the backscatter factor varied from 1.02 on wood to 1.28
on SS.  The backscatter factor varied with the detector used - depending
on how responsive the detector was to the lower energy backscattered
betas. 

It is interesting to note what backscatter factors the calibration source
manufacturers are using - e.g. The Source in Santa Fe uses 25% for
Tc-99 and 40% for SrY-90.  I do not know where their data came from -
referenced or self-derived? Any answers out there?

Anyway, our purpose for looking at the backscatter factor was to allow
for more realistic assessments of surface activity in the field (during
decommissioning).  How often do we use the detector efficiency
determined in the lab on a clean, high-backscatter source to make
measurements in the field on a variety of surfaces and under numerous
surface conditions (dirty, damp, rusty, etc)?    For those interested, the
approach recommended in the MARSSIM allows for surface type and
condition corrections to be made - it is based on guidance from
ISO-7503.  

Regards,
Eric Abelquist
abelquie@orau.gov 


>>> D1.OUT."WMCCABE@tnrcc.state.tx.us" 01/31/98 02:43pm >>>
Can anybody provide references or info on calculating the amount of or
probability of backscatter of different types of radiation from various
materials?

Please respond to me directly or to RADSAFE if you feel it is of general
interest.  Thanks.

- Will McCabe
wmccabe@tnrcc.state.tx.us