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A Mobile System for Detecting A Mobile System for Detecting Gamma-Radiation SourcesGamma-Radiation Sources
Part I.Part I.
Physical and Statistical BackgroundPhysical and Statistical Background
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Ludmil Tsankov Dept of Nuclear Engineering, University of Sofia, Dept of Nuclear Engineering, University of Sofia,
BulgariaBulgaria [email protected]
Mityo MitevDept of Electronic Technique, Technical Dept of Electronic Technique, Technical
University of Sofia, BulgariaUniversity of Sofia, Bulgaria [email protected]
Tchavdar Lenev
Institute Institute forfor Nuclear Research and Nuclear Nuclear Research and Nuclear Energy, BAS, Sofia, BulgariaEnergy, BAS, Sofia, Bulgaria
lenevlenev@@inrneinrne.bas..bas.bgbg
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IntroductionIntroduction
Two general methods are used by the national nuclear safety services to recover out-of-control ('orphan') gamma-radiation sources:
o passive ('trapping‘) methods - to set up large high sensitive stationary detectors at the borders, at the entrances of the scrap recycling facilities and at other potentially suspicious sites;
o active ('hunting') methods - to use mobile equipment able to discover signals from the sources during a survey made either by car (CGS) or by aircraft (AGS).
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PurposePurpose
To perform a theoretical analysis of a CGS system in order to express the minimum detectable activity as a function of its basic parameters
To develop algorithms for real time data processing in order to come near to the theoretical limits
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Statement of the problem Detector D has effective sensitive area S and moves across
a point source s with a constant activity A.
Assumptions:
• the effective area of the detector S does not depend on the source-detector disposition;
• the detector moves uniformly with respect to the source at a velocity V;
• the absorption of the gamma-rays emitted from the source in the air is negligible.
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Integral countingIntegral counting
- radiation flux density
- flux
Signal-Noise Ratio
The background is estimated from preceding measurements and extrapolated: 2
00 ,B
)(4)(
222 tVd
SAt
d
Vtarctg
d
Vtarctgt
dV
AStt 12
021 4),(
212
2012021
21
)()(),(
),(
ttttBtt
ttR
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Optimal Optimal systemsystem
R() has always a maximum
V=10m/s
A=3.7.107Bq (1mCi), S=0.0058m (3"x3"), d=10m,
B0=800cps,
{B0}=1.63cps
(tBG=300s)Vd /4.1max
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Nearly-optimal Nearly-optimal systemsystem
Optimal system
Nearly-optimal system
for all 0.10
1.00
10.00
100.00
1000.00
0 5 10 15 20 25
V [m/s]
tau
[s]
3)( R
max)( R
R() depends on V:
s1smV /20
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Adaptive algorithms for data analysis
Starts e e d B S
e s tim a te
R ece ive d a ta
(T-Tes t)
20,, BB Bn
),( 21 ttN
0
?
0 BS
u p d a te B S 2
0021 ,,),( BnttN B
N o
Yes
Yes
N o
A la rmu p d a te S S
2021 ,,),( SS SnttN
re s e t S S
5?
Sn
N ew B ackg rou nds ave B S
1?
Sn
BSSS
Yes
N o
1. Two samples are created:• Background (BS)• Signal (SS)2. New data are first regarded as a signal3. While the process is stationary, BS is extended
1?
00 ||t
BSt
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Spectral registration of gamma-quanta
NaI(Tl) detectors have a good energy resolution which can be used to improve sensitivity.
Statistical hypothesis is changed:
now we have to compare not two numbers but
two probabilities distributions:
)()( 2 testtestT 2?
1
2)(1
r
k kk
kBkS
SB BS
SnBn
nn
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Possible errors in decisions based on statistical tests
Type I (false alarm) Type II (to bypass a source)
Both types of errors have to be minimised simultaneously
Relationship between Type I and Type II errors depends on the choice of the significancy level (SNR threshold value)
Repeated test at yields a higher sensitivity
than a single test at at the cost of 1s reaction
delay .
)3(10 3 R610
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Conclusion
Spectrometric registration mode is more complex, but yields a higher sensitivity
The problem for the optimal setting of the alarm threshold level is consistently deduced from the theory of statistical hypotheses testing
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Acknowledgements
This work is supported by the Bulgarian Nuclear This work is supported by the Bulgarian Nuclear Safety Authority under contract NoSafety Authority under contract No 286-02.286-02.
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To be continued …
A Mobile System for Detecting Gamma-Radiation Sources:
PART II. Design and First Experiments
(At the poster session)