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Bayesian Analysis of Time-intervals for Radiation Monitoring
Peng Luoa
T. A. DeVola and J. L. Sharpb
a. Environmental Engineering and Earth Sciencesb. Applied Economics and Statistics
2011 SRC-HPS Technical Seminar
Time-Interval ?
Timet2t1 t3 t4 t5
…Background
2011 SRC-HPS Technical Seminar
Time-Interval ?
Time
…Presence of
a source
t2’t1’ t3’ t4’ t5’ t6’ t7’ t8’ t9’
2011 SRC-HPS Technical Seminar
Bayesian Statistics Bayes’ theorem• x – observation, counts or
time-interval; r – count rate
)()|( rPrxPP(r|x)
P(r): gives the prior prob. of observing r before data is collected,
P(x|r): gives the prob. to obtain the observation x given r
P(r|x): summarizes the knowledge of r given the prior and data
PriorLikelihoodPosterior
Bayesian statistics give direct probability statements about the parameter based on prior information and actual data.
Subjective
2011 SRC-HPS Technical Seminar
Conjugate Prior: Gamma distribution Poisson Distribution
( )( )
!
krtrt
P k ek
Poisson Distribution
( ) rtf t re
Time-Interval Distribution
1( , )( )
rGamma r e
Gamma Distribution
Gamma Distribution
2011 SRC-HPS Technical Seminar
Example 1:Background D
ata
0
1
2
3
4
5
6
0 5 10 15 20
coun
ts/o
bser
vatio
n
number of observation
rbkg.=2.0 cpsDetectionLimit= 4.3
Gam
ma
Pri
or Posterior
2011 SRC-HPS Technical Seminar
Example 2: A Source is Present
2011 SRC-HPS Technical Seminar
Data Acquisition & Analysis
Time-interval data acquisition: experiments (~105 pulses) and simulation (~106 pulses).
Three methods are used to analyze the data: classical (1.65σ), Bayesian with counts (cnt), and Bayesian with time-intervals (ti). Fixed count time is 1 second.
Compared the three methods in terms of average run length (time to detect the source) and detection probability (1- ) where is false negative (FN) rate.
2011 SRC-HPS Technical Seminar
Bayesian Analysis Methodology
Ifi=1
Posterior (i) Likelihood Prior
Prior(1)
If r > r0
Prior(i+1)=Posterior(i)
i=i+1
Yes
Yes
No
No
Obs.(i)
……
1
2
3
Obs
erva
tion
s
2011 SRC-HPS Technical Seminar
Average Run Length
Average Run Length: the average time needed to make a decision whether an alarm is issued.
Time-interval can make a quick decision.
(s)
0
5
10
15
20
25
30
35
402.9 104
3 104
0 2 4 6 8 10
1.65 Bayesian (cnt)Bayesian (ti)
aver
age
ru
n le
ngth
(s)
mean count rate (cps)
0
1
2
3
4
5
6
7
3 4 5 6 7 8 9 10
1.65 Bayesian (cnt)Bayesian (ti)
ave
rag
e r
un
len
gth
(s)
mean count rate (cps)
ab
Experimental result
Mean count rate (cps)
Ave
rage
run
leng
th
(s)
0
5
10
15
20
25
900092009400
0 2 4 6 8 10
1.65Bayesian (cnt)Bayesian (ti)
aver
age
ru
n le
ngth
(s)
mean count rate (cps)
a
0
1
2
3
4
5
6
3 4 5 6 7 8 9 10
1.65 Bayesian (cnt)Bayesian (ti)
ave
rage
ru
n le
ngth
mean count rate (cps)
b
Simulated result
Mean count rate (cps)A
vera
ge r
un le
ngth
(s
)
2011 SRC-HPS Technical Seminar
Detection Probability
Detection probability is defined as 1-
5s bkg.+20s source +5s bkg.
104 trials 95% detection limit (DL)
Bayesian methods give lower false positive (FP)rates
Also lower detection probabilities at low levels
5 s
20 s
5 s
0
0.2
0.4
0.6
0.8
1
1 2 3 4 5 6
1.65 Bayesian (cnt)Bayesian (ti)de
tect
ion
prob
abi
lity
mean count rate (cps)
Det
ectio
n pr
obab
ility
Mean count rate (cps)
FP ()
FN ()
2011 SRC-HPS Technical Seminar
Detection Probability
5 s20 s
5 s
0.5
0.6
0.7
0.8
0.9
1
1.5 2 2.5 3 3.5 4 4.5 5 5.5
1.65 Bayesian (cnt)Bayesian (ti)de
tect
ion
prob
abi
lity
mean count rate (cps)
Det
ectio
n pr
obab
ility
Mean count rate (cps)
95 % 60% DL
ARL – 60% DL
0
5
10
15
20
200
0 2 4 6 8 10
1.65 Bayesian (cnt)Bayesian (ti)
aver
age
ru
n le
ngth
mean count rate (cps)
Ave
rage
run
le
ngth
Mean count rate (cps)
2011 SRC-HPS Technical Seminar
Source Time Effect
2s, 5s, 20s and 50s source time
Bayesian analysis has the ability to reduce both false positive and false negative rates.
a b
c d
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
1.65 Bayesian (cnt)Bayesian (ti)
dete
ctio
n pr
oba
bilit
ymean count rate (cps)
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
1.65 Bayesian (cnt)Bayesian (ti)de
tect
ion
prob
abi
lity
mean count rate (cps)
0
0.2
0.4
0.6
0.8
1
1 2 3 4 5 6
1.65 Bayesian (cnt)Bayesian (ti)
dete
ctio
n pr
oba
bilit
y
mean count rate (cps)
0
0.2
0.4
0.6
0.8
1
1 2 3 4 5 6
1.65 Bayesian (cnt)Bayesian (ti)de
tect
ion
prob
abi
lity
mean count rate (cps)
2s 5s
20s 50s
2011 SRC-HPS Technical Seminar
If source time < count time
0.5 s source 1 s count time
Time-interval may result in a higher detection probability when the source time is shorter than the fixed count time.
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
1.65 Bayesian (cnt)Bayesian (ti)
dete
ctio
n pr
oba
bilit
y
mean count rate (cps)
Det
ectio
n pr
obab
ility
Mean count rate (cps)
2011 SRC-HPS Technical Seminar
Effect of Change Point
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20
1.65 Bayesian (cnt)Bayesian (ti)
dete
ctio
n pr
oba
bilit
y
change point (s)
0 - 20s5 s
5 s Change point --- a point at which the radiation level changes to an elevated level.
Change point determines the amount of background data that are included in Bayesian inferences.
The detection of radioactive sources may be delayed.
2011 SRC-HPS Technical Seminar
Enhanced Reset
ifp(r>r0)
discriminator
1st limit
no
start over
yes new data point(s)
incorporated
2nd limit start over
2011 SRC-HPS Technical Seminar
Moving Prior
t
Dat
a
-202468
101214
0 5 10 15 20
coun
ts/o
bser
vatio
n
number of observation
2.0cpsrbkg.=2.0 cps
Critical Level = 4.3
Prior
The moving prior relies on the latest information to calculate the posterior probability by updating the prior probability with each new data point.
2011 SRC-HPS Technical Seminar
Result of Modified Methods 10 (pulses)/20 (pulses)
for ‘Enhanced Reset’. 10 (pulses) is set for
‘moving prior’. Both modified Bayesian
methods result in a higher detection probability than the typical Bayesian analyses.
The performances of two modified methods are independent of the change point.
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20
1.65 Bayesian (ti)Enhanced resetmoving prior
dete
ctio
n pr
oba
bilit
y
change point (s)
Time-Interval Data
2011 SRC-HPS Technical Seminar
Summary ARL indicates that Bayesian(ti) could more rapidly detect the source
than other two methods.
When source time < count time, Bayesian(ti) results in a higher detection probability than Bayesian(cnt), which are both less than the frequentist method.
For no source present, both Bayesian(ti) and Bayesian(cnt) result in lower false positive rates than the frequentist method.
When source time > count time, Bayesian analysis results in a lower detection probability relative to the frequentist method, which increases with source time.
Modified methods can improve the performance of Bayesian analysis by reducing the effect of the background.
2011 SRC-HPS Technical Seminar
Acknowledgement
• Funded by DOE Environmental Management Science Program.
(Contract number: DE-FG02-07ER64411)
• Dr. Fjeld and Dr. Powell (EE&ES)