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Re: I-125/I-129 Calibration Sources
Hello Radsafers:
The emission data I use for examining the photon output of the two iodines
in question (taken from the RadDecay program):
I-129 for Calibration and Performance Verification Procedures
Emission Mean #/Decay Energy
G1 0.0801 39.5 keV
X1 0.3814 29.7 keV
X2 0.1968 29.4 keV
X3 0.1052 33.6 keV
X4 0.0221 34.5 keV
0.7856 (~79%)
I-125 for Calibration and Performance Verification Procedures
Emission Mean #/Decay Energy
G1 0.0666 35.4 keV
X1 0.7615 27.4 keV
X2 0.3906 27.2 keV
X3 0.2056 30.9 keV
X4 0.0426 31.8 keV
1.4669 (~147%)
Regarding detection:
If I'm correct, most thin-crystal PMT detection systems have ~2 - 5
microsecond recovery (or resolving) times. That seems sort of long if you
look into Knoll, but if you combine scintillator decay times (230 ns) with
the dead times of the PMT and rest of the instrumentation the figure is
probably ball-park (Paul Frame - are you lurking about). I've gotten the 2
- 5 microsecond figures from both Ludlum and Victoreen.
With I-125 there is a statistical chance of more than one photon being
emitted per decay, however, what are the chances of those being detected
separately? Wouldn't that be a function of the dead time of the detection
system, and/or the time differential between each photon (in that
particular decay) being emitted? And isn't there a only fifty percent
(max) chance of the two photons being emitted in the "capture" geometry (2
Pi ) of the detector?
I am operating under the assumption (looking at decay schemes for I-131 -
don't have a detailed one for I-125) that the time difference between
emissions from the same decay is on the order of nano or pico seconds. All
the detector would register would be a higher energy (sum peak) photon
rather than two separate events (which could complicate the calculation of
efficiencies and activities). So isn't it reasonable to assume that for
I-125/I-129 the "calibration factor" would be on the order of 1.0/0.7856 =
1.273 (and not 2 as suggested in a previous post).
In reality I haven't used I-129 as a calibration or a check source. But
the least expensive supplier of NIST traceable I-125 sources no longer
produces them (Test-R out of IL), so using I-129 might become an economical
necessity rather than an option.
Any comments or criticism regarding the thoughts provided above are welcomed!
Regards,
-Erick Lindstrom
Erick Lindstrom
Radiation Safety Officer
Montana State University
309 Montana Hall
Bozeman, MT 59717-2440
Phone: (406) 994-2108
Fax: (406) 994-4792
avrel@gemini.oscs.montana.edu
"Strange as it may seem, no amount of learning can cure stupidity, and
formal education positively fortifies it." - Stephen Vizinczey