Radiation Monitor and Alarm
protection against radiological risk
Date: 27 Oct 1998
Place: Dept. of Nuclear Engineering, Universitad Politčcnica de Catalonya
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.
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
Finally, inter-lot variability was not investigated, and further investigations are
warranted to assess the importance of this source of uncertainty.