OFFICIAL: Sensitive
Missing Source Dose Assessment
Background
A Cs-137 ceramic source capsule ~20GBq is missing from an industrial gauge after transport of the gauge
from s 22 - Irrelevant
in Western Australia to a depot in Perth. The source is a ceramic source
~ 6mm x 8mm. The source may be located along the route where it presents a danger to members of the
public who may approach it. A search for the source wil be conducted using vehicle mounted detector
systems.
Version Changes
Version 1.2
• Added measurement integration calculations for mobile detector system
Simple gamma dose rate calculation:
Decay chain:
Cs-137 → Ba-137m → Ba137(stable)
Specific Gamma-ray dose constant (Γ) for Cs-137 = 2.393 x 10-10 mSvh-1 MBq-11
Specific Gamma-ray dose constant (Γ) for Ba-137m = 8.216 x 10-5 mSvh-1 MBq-1
by ARPANSA under FOI
𝐴𝐴
Gamma dose rate = Γ ×
𝑑𝑑2
Assuming no gamma shielding provided by obstacles or source capsule:
20,000
Gamma dose rate = 8.216x10−5 × 12 = 1.643
At 1m from the source: Dose rate = 1.643 mSvh-1
At 10m from the source: Dose rate = 0.0164 mSvh-1
Released
1 (2020) Peplow, D. Specific Gamma-Ray Dose Constants with Current Emission Data, Health Phys. 2020 Apr;118(4):402-416
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For a drive by detection of the source at 15m assuming no shielding and obstacles:
At 15m from the source: Dose rate = 0.0073 mSvh-1 , 7.3µSvh-1
This is within detection limits of vehicle mounted sodium iodide detection systems. For a source within 15m
of the vehicle, assuming no obstacles, detection wil depend on the detector system time-resolution.
Testing should be conducted to determine an appropriate vehicle speed.
Small amounts of soil or roadway between the source and detection system may attenuate the dose rate
significantly. From Rad Pro Calculator2:
At 15m with 30cm concrete (~soil density): 0.26µSvh-1
At 15m with 50cm concrete (~soil density): 0.013µSvh-1
Vehicles should travel on both sides of the road to avoid attenuation as much as possible. Mounting
location of detectors should be considered. Low-mounted detectors may encounter more attenuation from
a source behind smal ground undulation.
Point source detection from a mobile detector system
The detector integration time can limit the speed at which a vehicle mounted system can travel and still
detect a point source. The lowest integrated detector measurement occurs when the integration time
begins or ends as the vehicle is horizontally adjacent to the source. This splits the highest source detection
periods over two measurements. Distance d from the vehicle to the source varies as the vehicle passes the
source (Figure 1):
𝑑𝑑 = √(𝑥𝑥2 + 𝑦𝑦2)
Where:
d = distance from the detector in the vehicle to the point source
y = shortest distance from the point source to the detector line of travel
x = distance from the detector to the horizontally adjacent point
by ARPANSA under FOI
d
y (15m)
x
Figure 1 – vehicle mounted detector system measurement of a point source 15m from vehicle travel path
Released
2 Rad Pro Calculator does not have soil as a shielding option. Concrete was used as an approximate value and to account for
potential road attenuation. Concrete is ~2.5 gcm-3 for concrete vs 1.6 gcm-3 for soil. Values are indicative of potential
attenuation effects of terrain undulation.
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For a vehicle travelling at a constant speed of 70kmh-1 (approximately 20ms-1) with an integration time of
1s detecting a 20GBq source located 15m from the road, the measured dose rate is:
1 20 1.643𝑚𝑚𝑚𝑚𝑚𝑚ℎ−1
𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑑𝑑 𝑑𝑑𝑑𝑑𝑚𝑚𝑚𝑚 𝑚𝑚𝑚𝑚𝑟𝑟𝑚𝑚 = 20�
2 𝑑𝑑𝑥𝑥
0 �(225 + 𝑥𝑥2)
Evaluating this gives a measured dose rate of 5µSvh-1.
A three second integration time would give:
1 60 1.643𝑚𝑚𝑚𝑚𝑚𝑚ℎ−1
𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑑𝑑 𝑑𝑑𝑑𝑑𝑚𝑚𝑚𝑚 𝑚𝑚𝑚𝑚𝑟𝑟𝑚𝑚 = 60�
2 𝑑𝑑𝑥𝑥
0 �(225 + 𝑥𝑥2)
Evaluating this gives a measured dose rate of 2.4µSvh-1.
Travelling at 110kmh-1 would give ~30ms-1. With a 1s integration time:
1 30 1.643𝑚𝑚𝑚𝑚𝑚𝑚ℎ−1
𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑑𝑑 𝑑𝑑𝑑𝑑𝑚𝑚𝑚𝑚 𝑚𝑚𝑚𝑚𝑟𝑟𝑚𝑚 = 30�
2 𝑑𝑑𝑥𝑥
0 �(225 + 𝑥𝑥2)
Or 4µSvh-1.
Table 1 – comparison of vehicle mounted detector measurements assuming an ideal detector
Speed
Detector measurement (µSvh-1)
Source 15m from road, 1s
Source 15m from road, 3s
Source 15m from road, ~30cm
integration
integration
concrete attenuation
~35kmh-1
6.3
4
1
~70kmh-1
5
2.4
0.8
~110kmh-1
4
1.7
0.6
Speed
Detector measurement (µSvh-1)
by ARPANSA under FOI
Source 30m from road, 1s
Source 30m from road, 3s
Source 30m from road, ~30cm
integration
integration
concrete attenuation
~35kmh-1
1.8
1.4
0.28
~70kmh-1
1.6
1.0
0.25
~110kmh-1
1.4
0.76
0.23
Speed
Detector measurement (µSvh-1)
Source 50 m from road, 1s
Source 50m from road, 3s
Source 50m from road, ~30cm
integration
integration
concrete attenuation
~35kmh-1
0.65
0.59
0.1
Released
~70kmh-1
0.63
0.48
0.1
~110kmh-1
0.59
0.39
0.09
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An unobstructed source of this type should be detectable by the vehicle mounted detector systems even at
higher speeds. Faster measurement integration speeds significantly improve the speed the vehicle is able to
travel while maintaining good detection probability. Other factors such as attenuation by terrain, a desire
for more measurement locations or the potential of missing measurements may make a lower speed
preferable.
The above calculated detector measurement values assume an ideal detector that is correctly calibrated
and receives sufficient counts to return a valid measurement. Field testing of ARPANSA and ANSTO vehicle
mounted detectors indicate that the detected counts are sufficient to detect sources of this strength at 15
and 30m for a vehicle travelling 70kmh-1.
Varskin v1.1 assessment:
Scenario – source picked up and held for 60s by a member of the public:
Cylinder geometry 6mm x 8mm, averaged over 1cm2 of skin, dose at depth of 70µm, exposure time 60s, no
air gap. End wall is assumed to be stainless steel of 1mm thickness, 7500kgm-3 density.
A Photon dose of 870mSv to the most exposed 1cm2 of skin is predicted for this scenario (Figure 2) This
would be sufficient to cause prompt skin reactions and injuries including transient erythema, epilation,
ulceration and other effects. Electron dose is attenuated by source encapsulation (represented by a cover).
by ARPANSA under FOI
Released
Figure 2 – Varskin Assessment Scenario 1
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Scenario 2– source placed in a pocket and kept for 1h by a member of the public:
Cylinder geometry 6mm x 8mm, averaged over 1cm2 of skin, dose at depth of 70µm, exposure time 3600s,
no air gap. End wall is assumed to be stainless steel of 1mm thickness. An additional cover of 0.03mm
representing cloth clothing (density assumed 20kgm-3) is added.
A Photon dose of 5.140Sv to the most exposed 1cm2 of skin is predicted for this scenario (Figure 3 and 4).
This would be sufficient to cause prompt skin reactions and injuries including transient erythema, epilation,
ulceration and other effects.
by ARPANSA under FOI
Figure 3 – Varskin Assessment Scenario 2
Released
Figure 4 – Varskin Detailed Results Scenario 2
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Varskin calculation of Hp(10) dose rate at 1m:
Cylinder source and covers remain in place. Air gap of 1000mm is used. Dose depth is calculated at 10mm.
Varskin predicts 1.46 mSvh-1 for this scenario (Figure 5). Table 2 compares different calculation methods.
Figure 5 – Varskin Assessment 1m Hp(10)Dose Rate
Other calculation methods
by ARPANSA under FOI
The Radiation Protection Handbook gives a value of 1.07 x10-3 mSvh-1 MBq-1 at 30cm for Cs-137/Ba-137. For
a 19GBq point source at 1m this gives a value of 1.926 mSvh-1
Rad Pro Calculator gives 0.85 mSvh-1 MBq-1 at 30cm for Cs-137/Ba-137. For a 19GBq point source at 1m this
gives 1.53 mSvh-1.
Table 2 – comparison of dose rates at 1m
Method
Dose rate (mSvh-1)
Varskin 1.1
1.46
Radiation Protection Handbook
1.926
Released
Rad Pro Calculator
1.53
Specific Gamma-ray dose constant
1.643
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