Gordon,
Duke Energy uses the following beta correction factors for the RO-20:
* 4 for bare instrument.
* 5 for single bagged instrument.
* 6 for double bagged instrument.
* 7 for a single bagged instrument measuring a noble gas atmosphere.
These factors were developed using slab sources of Tc-99, Tl-204 and U-238 at
contact. The sources were wider than the instrument beta window. Note that
geometry is important. We measured significantly lower correction factors
using on axis point sources. The beta correction factor for a bare instrument
changes only slightly with changes in beta half value layer (energy) because
the window thickness approximately equals the 7 mg/cm^2 depth at which skin
dose is defined.
Inspection of the detector geometry explains the measured bare instrument
correction factor of 4. The anode blocks the rear half of the detector chamber
making it insensitive to all but high energy beta radiation. This accounts for
a correction factor of 2. Partial shielding of the space around the side walls
of the chamber for radiation entering off axis brings the bare instrument
correction factor to 4.
We do not perform a beta calibration on each instrument since once the
instrument's overall gamma response is accurately calibrated, the correction
for beta is fixed since the beta correction factor depends only on the geometry
of the chamber, the thickness of the anode and the thickness of the window. We
do confirm the beta response of individual instruments.
Bob Sorber
704-382-7259
_____________________________________________
From: [email protected] [mailto:[email protected]] On Behalf Of
[email protected]
Sent: Wednesday, January 04, 2012 10:44 AM
To: [email protected]
Subject: Powernet: Beta Correction Factor for Handheld Ion Chamber Instruments
Palo Verde requests the following information:
What beta source is used to establish beta correction factors for handheld ion
chambers (RO-20, RO-2/2A, RSO-50E) at your facility?
Is a generic beta correction factor is assigned (such as 4 or 5) or is the
specific value established during calibration assigned (such as 3.8)?
Thanks
Gordon Nelson
Radiological Engineering
Palo Verde Nuclear Generating Station
Phone (623) 393-5200
Cell (623) 238-0542
Pager (877) 466-7534
[email protected]
