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Date: 27 Oct
1998
Place: Dept. of Nuclear Engineering, Universitad Politčcnica de
Catalonya
Materials
One randomly
selected RadAl-1/4, S/N 1014, equipped with an RS-232 cable and connected
to a laptop computer equipped with program
Radal.ht.
One Am241 source, reported to be 12 µCi ± 10%
One
Cs137 source, certified to be 7.11 µCi on 30 Sep
1986, computed to be 5.39 µCi on present date.
Experimental
The instrument
was turned on and calibrated on the local background, which resulted to
be:
BGD: 2428 ± 35 (average of two readings) expressed as counts over
10 minutes (Standard Count Rate or scr)
The Am source was placed at a
distance of 83 cm (est. ± 1 cm) along the vertical to the reference point
marked on the instrument. Four readings gave:
12 µCi
Am241: 2939 ± 27 scr
The Cs source was placed at
a distance of 83 cm (est. ± 1) as above. Five readings gave:
5.39 µCi
Cs137: 3145 ± 25 scr
Results and
Calculations
Am241 sensitivity, 12 µCi source: 511 ± 44 scr @ 83
cm = 352 ± 30 scr @ 1 m.
Sensitivity is 29.3 ± 5.4 scr/µCi @ 1 m
(including calibration source uncertainty).
Cs137 sensitivity, 5.39 µCi source: 717 ± 43 src @
83 cm = 494 ± 30 @ 1m.
Sensitivity is 91.6 ± 5.6 src/µCi @ 1 m
The
alarm threshold can be computed based on knowledge of the alarm algorithm.
The alarm algorithm triggers the alarm when the measured signal is 6
standard deviations over background. In this case,
Threshold = 296 scr
over BGD
Am241 alarm threshold (50% level): 296/29.3 = 10.1
µCi @ 1 m, 2.51 µCi @ 50 cm, 0.63 @ 25 cm
Cs137 alarm threshold (50% level): 296/91.6 = 3.23
µCi @ 1 m, 0.80 µCi @ 50 cm, 0.20 µCi @ 25 cm
An error of 20% should be
placed on the computed thresholds, in consideration of source and
statistical errors as well as possible Geiger-Müller tube inter-lot
variability.
Conclusions and
Comments
The background at
location was very high compared to that found at most installation
(typically 1300 to 3000 scr). Hence these results are conservative, and
better sensitivity should be expected in locations with lower background,
as well as by use of lead shielding.
The RadAl-1/4 instrument clearly
complies with the FEMA requirements of being able to detect 1 µCi
Cs137 across a 1 m distance, if used in pairs placed at 1
m distance.
The claim of higher sensitivity to 60 keV vs. 661 keV is
supported when converting to dose rate rather than source strength. In
first approximation, taking into account the 661 keV to 60 keV gamma ray
energy ratio, and a 36% gamma ray conversion efficiency for
Am241 vs. 85% for Cs137, the ratio of signals at equal dose rate
becomes (29.3/91.6)*(85/36)*(661/60) = 8.36
In other words, 1 µGi/h
(for example) due to Am241 will produce a signal 8.4 times higher than
the same 1 µGi/h due to Cs137. This behavior makes the RadAl-1 optimal for the
detection of Special Nuclear Materials and other alpha emitters, which are
the most dangerous of all radioactive materials.
Finally, inter-lot
variability was not investigated, and further investigations are warranted
to assess the importance of this source of
uncertainty.
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