Nuclear data and modelling for monitoring
compliance with the Comprehensive Test
Ban Treaty
R Britton, A Davies
UK Radionuclide Laboratory
GBL15
[email protected], [email protected]
Overview
Introduction
CTBTO
GBL15
Example Measurements
Key Radionuclide signatures
Compton Suppression
Cosmic Suppression
Monte-Carlo Modelling
Detector Characterisations
Design Studies
Summary
“The CTBT bans all nuclear explosions on
Earth whether for military or peaceful purposes”
Verification Technologies
The International Monitoring System
RN68 Tristan da Cunha
RN67 St Helena
RN69 Antarctica
RN66 BIOT
GBL15
AWE hosts the UK Radionuclide laboratory (GBL15)
in support of the CTBT
Many years’ experience of carrying out radionuclide
measurements and diagnosis
1952 - 1991 – supporting UK’s nuclear test
programme (AGT/UGTs)
Samples were sent from distant nuclear test sites
for analysis of fission and activation products, and
residual device materials
2004 – certified by CTBTO as GBL15 and part of
International Monitoring System
Ultra low-background detectors for routine analysis
2 x BE5030, 2 x BE6530, 1 x 93% eff. extended
p-type
Research and non-CTBT detectors
3 x Compton suppression, 1 x cosmic veto, 1 x
Gamma-Gamma coincidence system, 10 x HPGe
Radiological and Nuclear Forensics
To further develop capabilities from pre and post-detonation scenarios
What is it?
Where is it from?
Who is involved?
Where has it been?
Laboratory instrumentation
Alpha spectrometry
Alpha Analyst (x 24 PIPS)
Portable units (x 3 dual)
Alpha/Beta
iMatics (x 3)
iSolo (x 3)
Liquid scintillation
Quantulus 1220
Radon mobile laboratory
SAUNA Radioxenon Laboratory
Key Radionuclide Signatures
Three types of release in a nuclear
explosion – fuel products, fission products
and activation products
Detection probabilities must consider:
The amount of RA material released
The half-life of the radioisotopes
The type & branching ratios of the decay
radiation
Key nuclides include Pu, Cs, Ba, La, Np,
I, Zr, Mo, Tc, Ru, Ce, Am
Research systems
Cosmic veto
Although lead shielding reduces the gamma-background,
it increases cosmic-ray interactions and the cosmicbackground.
Reduction of this component is achievable by:
1.Installation of the gamma-spectrometer in an underground
laboratory
2.Increasing the overburden above the detector
3.Using a cosmic veto device such as plastic scintillation
plates surrounding the lead shielding in anti-coincidence with
the primary detector
10000
1000
Counts
A gamma-spectrum shows peaks at
595.9, 691.0 and 834.0 keV
attributable to prompt neutron
capture reactions. The spectra was
collected over 63 days.
100
10
1
0
5
10
15
20
25
Energy (MeV)
A cosmic radiation spectrum measured using a BC408 plastic detector.
Davies AV, Burnett JL (2012) JRNC, Vol 292-3, pp 1007
Cosmic veto
Version 3.0 – Hardware gating using two Lynx MCAs
Canberra extended p-type HPGe detector
within low-background graded shield
Surrounded by five 55 cm x 55 cm Bicron
BC408 plastic scintillation plates.
Combined control using Canberra Lynx MCAs
with pre-amplifiers.
Hardware gating for real-time processing..
Gate
Inside an extended range p-type detector
HPGe
ICR
Scintillation plates
Background reduction
7-day background spectra reduced by
a mean of 75.2%.
Reduction of Ge neutron capture
peaks by 70.5 – 77.4%.
Davies AV, Burnett JL (2013) JRNC, Vol 298-2, pp 987
MDA improvement
During 5-day count, MDA for 85 CTBT radionuclides improved by 20.0 – 64.8% with mean of 45.6%.
Ba-140 MDA of 19.2 mBq (2 days) and 11.3 mBq (7 days). This compares to 35.9 mBq and 21.7 mBq.
Ba-140 MDA of 24 mBq at 1.5 days.
MDAs for Zr-95, Mo-99, Ce-141 and Nd-147 improved by 52.7%, 37.3%, 38.5% and 52.1% after 7
days.
MDAs for I-131, Cs-134 and Cs-137
improved by 42.0%, 46.6% and 49.5%.
MDA improvements for the 85 CTBT radionuclides
after 5 days. Some key indicators of nuclear weapons
tests (black diamonds) and the Fukushima incident
(grey squares) are labelled.
Davies AV, Burnett JL (2013) JRNC, Vol 298-2, pp 987
Upgrade of RN67 St Helena
What is Compton suppression?
Typically consists of a high-resolution Ge
detector coupled with an anti-Compton NaI(Tl)
shield.
The shield allows the veto of Compton
scattered events, where the full energy of an
incident gamma-photon is not completely
absorbed by the Ge detector.
The NaI(Tl) shield measures the escaping
scattered photon, allowing simultaneous (Ge +
NaI) Compton background events to be rejected.
This produces significant suppression of the
Compton background and a large increase in the
measured peak to background ratio.
Configuration
Hardware gating for real-time processing
Utilise Canberra Lynx MCA with no dedicated
coincidence electronics
Easily configurable and cost effective solution
Additional time-stamping functionality
Gate
HPGe
ICR
NaI(Tl)
Improved sensitivity
Produces a step-change in detection
sensitivity
Used by AWE for measurement of complex
mixtures of radionuclides
Unsuppressed sample
Suppressed sample
The improved sensitivity offers potential
advantages for samples containing multiple
interferences, including high yield fission
products and their daughters such as Ba-140
(La-140) and Te-132 (I-132)
CTBT samples
Using SRID 52200905290611 (suppression on/off)
Compton suppression reduced background by 28 %.
Suppression factors of 0.1 – 38.1 increasing with energy.
Improved MDA values typically by 40 %.
Problem – True coincidence summing
Cascade summing occurs when a
single nuclear disintegration emits two
or more photons and one is observed in
the Ge detector.
The NaI detector may measure the
second photon escaping from the Ge
crystal and reject the initial photon by
Compton suppression.
For any radionuclide subject to
summing, this shall produce a loss of
measured counts and apparent
reduction in detection efficiency.
Application for non-summing radionuclides.
Of the 85 CTBT radionuclides, 52 are
subject to true coincidence summing
and are measured with reduced
efficiency using the Compton
suppression detector.
Fukushima incident
Detection limits for Ba-140
reduced from 37 mBq to 25 mBq
134Cs
609 KeV gamma in cascade peak
areas reduced ~ 4
5000
4500
4000
3500
100000
3000
Counts
131I
No cascade gamma’s peak areas
remain the same.
Baseline reduced ~1.6
2500
2000
1500
1000
10000
500
0
600
601
602
603
604
605
Energy K
1000
Counts
18200
16200
14200
100
10200
8200
6200
4200
10
500
450
400
2200
350
300
200
360
Counts
Counts
12200
361
362
363
364
365
250
200
150
Energy KeV
100
1
50
0
365
365.5
366
366.5
367
367.5
Energy KeV
368
368.5
369
369.5
370
0
500
1000
1500
Energy KeV
2000
2500
PTE sample
91Y
Peak uncertainty
unsuppressed
Typical Compton Reduction
Suppression factor of 20-40
Suppressed
11.5%
3.4%
1000000.0
Counts
100000.0
10000.0
1000.0
100.0
670.0
770.0
870.0
970.0
1070.0
1170.0
Energy (keV)
1270.0
1370.0
1470.0
1570.0
Monte-Carlo Simulations
Monte-Carlo simulations rely upon repeated random sampling to provide
numerical results
GEANT4/MCNPX are Monte-Carlo toolkits that simulate the passage of particles
through matter
Why?
Better understanding of detectors
Allows us to test unusual configurations & focus any future detector designs
on our specific requirements
Simulations are free, building test detectors is both expensive and time
consuming
How?
Create simulations using Monte Carlo package
Test and validate these against existing detectors
Use these models to develop new detector configurations
All these are alrgely reliant on the nuclear data libraries upon which they are built!
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Example Simulations
We can test scenarios that are inaccessible in the laboratory
X-ray reduction improved by a factor of 10
Compton Scattering of the source contributes 10x more events to the spectra than
x-rays
Cosmic muons increase the background rate as the amount of shielding increases
– however the overall effect is to reduce the background
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Britton R, Burnett JL, Davies AV, Regan PH (2013) JRNC, Vol 298-3, pp 1491
Simulating Compton Suppression
To design a new system, Monte-Carlo models must be validated against existing
systems
GEANT4 models were created for NaI(Tl) detectors
Achieved agreement between data and simulation of greater than 91% from 30 –
3000 keV
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Britton R, Burnett JL, Davies AV, Regan PH (2012) JRNC, Vol 295-1, pp 573
Simulating Compton Suppression
Moved onto HPGe crystals
Used more advanced features of GEANT4
This was optimised and validated against
experimental data, achieving agreement
>97%
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Britton R, Burnett JL, Davies AV, Regan PH (2012) JRNC, Vol 295-3, pp 2035
Simulating Compton Suppression
Built GEANT4 simulations of our Compton Suppression
system
Results show excellent agreement for a Broad-energy HPGE
(BEGe) detector in conjunction with a NaI(Tl) guard detector
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Britton R, Burnett JL, Davies AV, Regan PH (2014) JRNC, Vol 300-3, pp 1253
Coincidence system corrections
Cascade Summing corrections often have to be applied to radionuclide abundance
calculations to account for ‘summing out’ of ‘summing in’ of each peak
GEANT4 models must also reproduce this effect to be valid
It is also crucial to be able to model this when simulating a full Compton Suppression system
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Britton R, Burnett JL, Davies AV, Regan PH (2015) JRNC, Vol 299-1, pp 447
Coincidence system corrections
Coincidence systems suffer from multiple
problems, including time-walk of low-energy
events and accidental coincidences
By collecting all data in ‘list-mode’, it can be
reprocessed to extract far more coincidence
information than is typically available
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Routines have been developed to correct for
these effects
Delay gate width reduced by 18.4% with
timewalk correction
Effective removal of accidental coincidences
resulted in ~100% of 'accidental' signals being
removed
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Britton R, Burnett JL, Davies AV, Regan PH (2015) NIMA, Vol 769, pp 20
Coincidence system corrections
Coincidence systems suffer from multiple
problems, including time-walk of low-energy
events and accidental coincidences
By collecting all data in ‘list-mode’, it can be
reprocessed to extract far more coincidence
information than is typically available
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Routines have been developed to correct for
these effects
Delay gate width reduced by 18.4% with
timewalk correction
Effective removal of accidental coincidences
resulted in ~100% of 'accidental' signals being
removed
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Britton R, Burnett JL, Davies AV, Regan PH (2015) NIMA, Vol 769, pp 20
Improving the
Current System
Validated GEANT4 models were utilised
to design a new system
Based upon old design, primary and
secondary detectors were optimised
Guard material changed to BGO
Additional ‘base’ guard detector to veto
high energy gamma radiation that
penetrates the system
Resulted in substantial improvement in
sensitivity for a range of nuclides over
current system (factor of 10)
Primary detector - Britton R, Burnett J, Davies A,
Regan P (2014) J Env Rad, Vol 134, pp 1
Secondary detector - Britton R, Burnett J, Davies A,
Regan P (2014) NIMA, Vol 762, pp 42
Summary
Thank you