The CR39 Coincidence Counting Technique for Enhanced

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• The CR-39 Coincidence Counting Technique for
  Enhanced
• Signal-to-Background in a Large Range of
  Charged-Particle
• and Neutron Measurements at OMEGA and the NIF
• D. T. Casey, J. A. Frenje, J. R. Rygg, F. H.
  Séguin, C. K. Li, S. C. McDuffee and R. D.
  Petrasso
• Plasma Science and Fusion Center, Massachusetts
  Institute of Technology,
• Cambridge, Massachusetts, 02139
• V. Yu. Glebov, D. D. Meyerhofer, S. Roberts and
  T. C. Sangster
• Laboratory for Laser Energetics, University of
  Rochester,
• Rochester, New York, 14623

American Physical Society 49th Annual Meeting of
the Division of Plasma Physics Orlando,
Florida November 12th - 16th, 2007
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Abstract
CR-39 detectors have been used extensively in
several types of nuclear instruments for
diagnosing Inertial Confinement Fusion (ICF)
plasmas produced at the OMEGA laser facility. A
Coincidence Counting Technique (CCT) is now being
used to identify signal particle tracks in CR-39
when the signal-to-background ratio (S/B) is very
low. Two orders of magnitude improvement in the
S/B can be achieved in some applications. In this
poster, we describe the effect of the CCT when
applied to a large range of charged-particle and
neutron data obtained from measurements performed
at OMEGA and for proposed work at the NIF. This
work was supported in part by the U.S. Department
of Energy (Grant No. DE-FG03-03SF22691), LLE
(subcontract Grant No. 412160-001G), LLNL
(subcontract Grant No. B504974).
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Motivation for developing the CCT
The Coincidence Counting Technique (CCT) was
developed to significantly improve the
signal-to-background (S/B) ratio for several low
S/B applications, such as measuring products from
secondary and tertiary reactions.
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The CCT will be applied to several low S/B
applications that use CR-39 detectors

• Secondary proton (Sec-p) measurements (cryogenic
  D2 implosions at OMEGA)
• D  D ? n 3He (0.8 MeV)
• 3He (0.8 MeV) D ? a p (12.5 17.4 MeV)
• Knock-on proton (KO-p) and knock-on deuteron
  (KO-d) measurements for several types of
  implosions at OMEGA and the NIF
• n H ? n' p (lt14 MeV)
• n D ? n' d (lt12.5 MeV)
• Down-scattered neutron measurements (at OMEGA)
  and tertiary neutron measurements (at the NIF)
• Down-scattered n D ? n' D ' (lt12.5 MeV)   
• n T ? n' T ' (lt10.6 MeV)  
• Tertiary D ' T ? a n ' ' (12.0 30.1
  MeV)  
• T ' D ? a n ' ' (9.2 28.2 MeV)

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• KO-p, KO-d, Sec-p and n signals are orders of
  magnitude
• smaller than the primary neutron yield (Yn)
  resulting in low S/B

n'
CH
DT
KO-p
n'
n
KO-d
S rR x Yn B Yn
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The CCT is a valuable tool for several low S/B
spectrometry applications that use CR-39
CPS magnet

• Charged Particle Spectrometer (CPS) measurements
  of secondary protons, KO-ps and KO-ds.
• Wedge-range-filter (WRF) spectrometer
  measurements of KO-ps and Sec-ps.
• Magnetic Recoil Spectrometer (MRS) measurements
  of down-scattered and tertiary neutrons.

Al-wedge filter
MRS magnet
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1. In CPS measurements, charged particles are
momentum analyzed in a magnetic field and
detected by CR-39 detectors
Target
50 keV
200 keV
2cm
Neutron background region
600 keV
30 MeV
1 MeV
3 MeV
10 MeV
cm
KO-d data
Counts / Pixel
CR-39 detector
D. G. Hicks, PhD. Thesis, MIT (1999).
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2. The WRF measures the proton energy from the
recorded CR-39 track diameter and local filter
thickness
Incident protons
Al-wedge filter
t
Al-wedge filter
CR-39
Proton tracks
F. H. Seguin et al., Rev. Sci Instrum 74, 975
(2003).
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3. In the MRS, charged recoil particles are
momentum analyzed in a magnetic field and
detected by CR-39
Magnet
CH-foil or CD-foil
Target
Entrance
Exit
10 cm
215 cm
CR-39
6-28 MeV (p) 3-14 MeV (d)
J. A. Frenje et al., GO6.00009
J. A. Frenje et al., Rev. Sci Instrum 72, 854
(2001).
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The principle of the Coincidence Counting
Technique (CCT)
Surface after etching
Interface before etching
CR-39
Thin CR-39
Front-piece
Back-piece
Incident Neutrons
Incident protons / deuterons
Coincidence tracks can be found by overlaying
scanned images and correlating tracks that are in
the same approximate location on each
surface. Careful alignment of the two images is
crucial.
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The correlation radius (Rc) is the maximum
distance a particle can laterally straggle and
still be considered a coincidence
Interface before etching
Surface after etching
Incident proton / deuteron
Correlation Radius (Rc)
CR-39
CR-39
Front-piece
Back-piece
Most particles
Calculated lateral-straggling distribution of
deuterons going through 200 mm of CR-39
Dy mm
Rc
No particles
Dx mm
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The CCT background (BCCT) level is strongly
dependent on track density and correlation radius
(Rc)
The CCT background is
Where nF frontside track density nB backside
track density S Signal B Background A
Detection area
(1)
The track densities are
nF nB (SB)/A
So the background becomes
Eq. (1)
Data
106
105
BCCT
104
And therefore
103
102
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The CCT will improve S/B if the background track
density is less than the inverse search area 1/(p
Rc2)
The S/B with CCT is
Where S Signal B Background A Detection
area
Demand S/BCCT gt S/B
Assume
S/B will be improved if
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Thin and thick CR-39 response must be well
understood to optimally implement the CCT
Interface before etching
Surface after etching
E0
Incident proton / deuteron
Efront
Eback
CR-39
Thin CR-39 Front-piece
Back-piece
100 CR-39 detection efficiency region
Eback
Efront
Track Diameter
E0
Charged-particle Energy
McDuffee has recently undergone a systematic
study of thin and thick CR-39 response to protons
using an accelerator (to be published, RSI 2007)
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A S/B improvement of 15 times was demonstrated
when CCT was applied to KO-d data obtained by the
CPS at OMEGA
Standard Analysis
CCT Analysis
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The random coincidence background level in the
KO-d data is observed by misaligning pieces
during the CCT analysis
Analysis when pieces are aligned
Analysis when pieces are misaligned 200mm
Dy mm
Counts / Pixel
Rc
Rc
Dx xfront - xback mm
2000 random background tracks
Dx mm
6000 signal tracks 2000 background tracks
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The KO-d spectrum obtained from the CCT
analysis agrees with the standard analysis
S/B3
Shot 43498 (Y1n 1.5 x 1013) ltrRgtfuel 11 mg/cm2
S/B0.2
rR inferred from the CCT analyzed KO-d
spectrum is consistent with the results published
by Li et al. (2001)
Li et al., Phys. Plasmas 8, 4902 (2001).
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A S/B improvement of 25 times was demonstrated
when CCT was applied to KO-p data obtained by the
CPS at OMEGA
Front piece was contaminated during manufacture
creating an extraordinarily high
intrinsic background level
y cm
Counts / Pixel
CCT improved S/B 25 times
x cm
Counts / Pixel
y cm
x cm
Problem has been corrected by the manufacturer.
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B must be reduced at least 750 times to
achieve S/B gt 10 for down-scattered neutron
measurements using the MRS
Fn 7.7x10-7 cm-2 from TART modeling
Then for rR 0.1 g/cm2
Bn/B required 750 times
J. A. Frenje et al., GO6.00009
D. T. Casey et al., APS 2005
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Polyethylene shielding around the detector
housing will reduce the background 50 times
An additional background reduction of 15 times
is required to reach S/B 10
Neutronics Analysis from TART - D. T. Casey et
al., APS 2005
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The CCT will dramatically improve the S/B for
down-scattered neutron measurements with the MRS
For a rR 0.1 g/cm2
102
Shielding CCT
S/B gt 10
101
100
S/B
Shielding
10-1
10-2
No shielding
10-3
1015
1012
1013
1014
Primary Neutron Yield (Yn)
eMRS J. A Frenje et al., MRS System Design
Review 4/27/2006 (for 6-10MeV ?E3 MeV)
Includes CR-39 intrinsic background
characterized by F. H. Seguin et al., Rev. Sci
Instrum 74, 975 (2003).
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Conclusions and future work

• Conclusions
• The CCT can dramatically improve the S/B for low
  S/B applications that use CR-39
• The CCT has been demonstrated for proton and
  deuteron measurements at OMEGA
• Future work
• Develop fiducial based alignment of front and
  backside pieces to aid implementation in low
  signal applications
• Implement CCT with OMEGA MRS measurements

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Number of neutron induced coincidences 1 Neutron
background (Bn)
100 false
Neutron background region
Dy mm
CCT applied to KO-D data
y cm
Counts / Pixel
Dy mm
x cm
6000 CCT counts
Dx xfront - xback mm
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Introduction
Description of low S/B Applications
CCT Principle
CCT for Charged Particle Spectrometry
CCT for Neutron Spectrometry