Measurements
To determine the effect of the NBR algorithm, measurements with different
sources were carried out. As sources 60Co with an activity of 174 kBq, 137Cs with
an activity of 40 kBq, and 133Ba with an activity of 70 kBq were used. The count
rate of the source in the detector was varied by changing the distance of the
source to the detector. The natural background was increased by a Thorium
mantle which was identified as natural by the NBR system and which was directly
attached to the detector.
The NBR signals are ratios of count rates in different energy regions. The
following figures compare the NBR signals and the NBR alarms at normal
background and with elevated background. There are two NBR signals and
alarms, one for the Cs energy region and one for the Co energy region. The
background level for normal background was 2700 cps and with the Thorium
mantle it was 6900 cps. As can be seen in the figures the additional artificial count
rate needed to trigger the NBR alarm is only slightly increased. If the natural
background increases by the same amount the alarm threshold of a detection
system not equipped with the NBR algorithm had to be set at a high value thereby
missing small sources. The other possibility would be to use a lower threshold
which would lead to a permanent alarm with the higher background.
Conclusion
The technique of natural background rejection allows to enhance the detection
limit for artificial radiation for mobile gamma search systems significantly.
Therefore, the usage of NBR functionality for OSI gamma survey can enhance the
performance of radiation survey, especially for carborne survey.
Introduction
The CTBTO verification system comprises an On-Site Inspection (OSI) to verify
the suspicion of a banned nuclear test. An OSI comprises different methods of
verification, one of them being the radiological survey of the inspection area.
The radiological survey is done by airborne survey, carborne survey, and
environmental sampling, thereby narrowing the inspected area with each step.
Our institute operates a measurement car with highly sensitive neutron and
gamma detection systems. We investigated search strategies with the gamma
detectors of this system. Of course large area surveys should be done by gross
gamma counting. One problem with gamma survey are variations in background
which may lead to incorrect identification of relevant spots or, even worse, to a
miss of relevant spots due to high background. Therefore, our gamma detectors
are equipped with special software, called NBR (natural background rejection).
Detectors
Neutron detection
is performed by two rows of
large-area neutron slab
counters with 3He detectors
(reaction:
n + 3He → p + 3H + 765 keV).
Both sides are evaluated
separately, therefore it is
possible to differentiate
between left and right.
The typical measurement
interval for the neutron measurement is 2 s.
Gamma detection
is performed by two NBR
probes (Natural Background
Rejection). It is determined if
the radiation is natural,
artificial or anormal. Anomaly
is signaled if an outsized part
of high energy gamma
radiation, an exceptional high 40K or 228Th part of the
ambient radiation or neutron
radiation is present.
Reduction of false alarms
The total dose rate of the background may vary significantly without source
whereas the artificial component of the background remains low. In the following
example the background varied up to a factor of 3 due to different environments.
Natural background rejection (NBR)
To determine if an increasing count rate is caused by a varying background or an
artificial radiation source the shape of the energy spectra is classified. Therefore
the count rate in different energy regions of the spectrum is compared. The overall
shape of pure natural background always is nearly constant. If the shape
significantly differs from natural background a NBR alarm is issued. The NBR
method allows to reject count rate increases which are only due to an increase in
background. Therefore, it significantly enhances the detection limit for artificial
radiation and reduces false alarms.
T2-P12
Nuclear Security Policy and
Detection Techniques
Neutron slab counters left
Neutron slab counters right
Gamma detectionNBR detector left
Battery (130 Ah)
Voltage distribution
Neutron analyzer
Ruggedized computer
Stowing box /
accessories
Hand-held Germanium detector
Power supply
unit
Gamma detection NBR detector right
Touch screen monitor
14 kgTotal weight
52 x 28 x 20 cm3Suitcase dimensions
Typ. 20000 cps/µSvh-1Sensitivity
20 % artificial dose rateDetection limit NBR
Approx. 2 nSv/hDetection limit gross counting
Approx. 100 keVEnergy threshold
5 literVolume
plastic scintillatorDetector type
FHT 1376 (Thermo)Type
1
2 3,4
Finding and Identifying Radioactive Material by Carborne Search for OSI Deployment
T. Köble, W. Berky, H. Friedrich, M. Risse, W. Rosenstock, O. Schumann
Fraunhofer INT, Euskirchen, Germany, contact: [email protected]
11:10 11:15 11:20 11:25 11:30 11:35 11:40 11:45
0
20
40
60
80
100
3
4
2
γ-d
ose r
ate
[n
Sv/h
]
time [hh:mm]
total dose rate
artificial lower energy
artificial higher energy
1
0,12 % (for a single Slab Counter with 252Cf source in a distance of one meter from the detector surface).
Efficiency
Polyethylene (minimum 1,7 cm)Moderator
6 tubes per counter6 counters per side of car => in total 36 tubes / side
Tube number
33 cmTube length
2,54 cmTube diameter
3He (4,2 ⋅ 105 Pa)Gas
JCC 71SS (Canberra)Type
2
Total ambient dose rate [nSv/h]
50 nSv/h70 nSv/h
NBR signal of artificial sources with normal and elevated natural background:
NBR-Cs no alarm NBR-Cs alarm NBR-Co no alarm NBR-Co alarm
The map shows the trace of
the measurement car. The
diagram shows the
corresponding total and
artificial dose rates. Different
paving stones can change
background significantly.
The measurement car
The main use of the measurement car is the
search and detection of concealed nuclear
material and radioactive sources from roads,
pathways or other vehicle accessible areas. The
measurement system in the car consists of
sensitive neutron detectors and two scintillation
gamma detectors. The position of the car is
recorded by GPS synchronized with the measured
data. The complete system is mounted
in a square steel rod assembly so that it fits in all standard station wagons easily.
Between the racks for the neutron detectors we have fastened a voltage converter
(with back-up battery), the electronics and a electrical cooled HPGe detector. The
figure shows the measurement car completely equipped. The NBR detectors are
located in the black boxes in the rear windows. The small suit-case on the left side
contains an additional Identifinder (HM5).
2000 3000 4000 50004
6
8
10
NB
R
Total count rate [cps]
Co
Normal background Elevated background
6000 8000 10000 120004
6
8
10
NB
R
Total count rate [cps]
2000 3000 4000 50004
6
8
10
12
14
NB
R
Total count rate [cps]
Cs
6000 8000 10000 120004
6
8
10
12
14
NB
R
Total count rate [cps]
2000 3000 4000 5000
5
10
15
20
NB
R
Total count rate [cps]
Ba
6000 8000 10000 12000
5
10
15
20
NB
R
Total count rate [cps]
