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: theo.koeble@int.fraunhofer.de

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]