Overview of gamma ray tracking

1
Overview of gamma ray tracking

• I-Yang Lee
• Lawrence Berkeley National Laboratory
• Second Joint Meeting of the Nuclear Physics
  Divisions of the APS and The Physical Society of
  Japan
• September 1822, 2005 Maui, Hawaii

2
Outline

• Principle of gamma-ray tracking
• Advantages of gamma-ray tracking
• Technical developments
• Status of GRETINA
• Summary

3
Compton Tracking Principlesource location is
known

• Assume full energy is deposited
• 2) Start tracking from the source

Eg Ee1 Ee2 Ee3
source
Eg
Eg
For N! possible permutations, check each
interaction point for Compton scattering
conditions
N5
Select the sequence with the minimum ?2 lt ?2
max ? correct scattering sequence ? rejects
partial energy event ? reject gamma rays with
wrong direction
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Compton Tracking Principlesource location is
unknown Compton camera
1) Assume full energy is deposited 2) Start
tracking from the first interaction
Cone with half-angle of ?1
Tracking results ?correct sequence ?reject
partial energy events ?confine gamma-ray
direction in a cone
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Advantages of g-ray tracking

• For a 4p array
• Efficiency proper summing of scattered
• gamma rays
• Peak-to-background reject Compton events
• Doppler correction - Position of 1st interaction
• Polarization angular distribution of the 1st
• scattering
• Counting rate - segmentation

• For Compton Imaging
• Efficiency no collimation
• Spatial distribution 3D (near field)
• Directional distribution 2D (far field)
• Sensitivity reduced background

6
Examples of detector arraycurrently operational
RISING
MINIBALL
SeGA
KD4 Dinca
KD3 Ideguchi
KD9 Fukuchi
EXOGAM
GRAPE
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Examples of detector arrayunder construction
TIGRESS
AGATA Demo
KD6 Garrett
GRETINA
KD7 Fallon
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Examples of Compton imaging detector
EJ2 Gono
KD10 Motomura
KD11 Hirakuri
ANL
LLNL
KD8 Vetter
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Examples of Compton imaging detector

• Naval Research Laboratory Ge
• University of Michigan
• Z. He CZT
• N. Clinthorne Si NaI
• Kyoto University TPC
• University of Tokyo CdTe NeXT Misson

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Physics opportunities with a 4p array (e.g.
GRETA)

• ? Angular resolution (0.2º vs. 8º)
• N-rich exotic beams
• Coulomb excitation
• Fragmentation-beam spectroscopy
• Halos
• Evolution of shell structure
• Transfer reactions
• ? Count rate per crystal (100 kHz vs. 10 kHz)
• More efficient use of available beam intensity
• ? Linear polarization
• ? Background rejection by direction

• ? Resolving power 107 vs. 104
• Cross sections down to 1 nb
• Most exotic nuclei
• Heavy elements (e.g. 253,254No)
• Drip-line physics
• High level densities (e.g. chaos)
• ? Efficiency (high energy) (23 vs. 0.5 at
  E?15 MeV)
• Shape of GDR
• Studies of hypernuclei
• ? Efficiency (slow beams) (50 vs. 8 at E? 1.3
  MeV)
• Fusion evaporation reactions
• ? Efficiency (fast beams) (50 vs. 0.5 at E?
  1.3 MeV)
• Fast-beam spectroscopy with low rates -gt RIA

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Advantages of g-ray Tracking In particular for
Radioactive Beams

• High position resolution
• High efficiency
• High peak to background
• High counting rate
• Background rejection

• Large recoil velocity
• Fragmentation and
• Inverse reactions
• Low beam intensity
• High background rate
• Beam decay
• Beam impurity

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GRETINA Design

• 7 modules with 4 crystals each cover 1p solid
  angle (cover 4p will take 30 modules).
• Modules can be placed at 31.7º (5 positions),
  58.3º (4), and 90º (8).
• On-line processing gives gamma-ray energy and
  position.

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Doppler Broadening
V?DV
Doppler shift
Moving nucleus
D?N
?
D?D
g-ray detector

• Broadening of detected gamma ray energy due to
• ? Spread in speed DV
• ? Distribution in the direction of velocity D?N
• ? Detector opening angle D?D
• ?Need accurate determination of V and ?.
• ? Position sensitive g-ray detector and particle
  detector

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Performance examplen-rich nuclei from
fragmentation reactions
Simulation SeGA
Simulation GRETINA
30Mg (pn) ? 30Na (100 MeV/u) v/c0.43 charge
exchange reaction Gamma-gamma coincidence NSCL
data SeGA (E. Rodriguez-Vieitez et al.)
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Performance exampleHigh spin state from fusion
reactions
v/c0.04
64Ni ( 48Ca, 4n) 108Cd, Gammasphere
Simulation GS, e 0.09
3-fold, I10-4
Simulation GRETA, e 0.25
Simulation GS
4-fold, I10-5
3-fold, I10-3
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Technical Challenges

• Advances in detector segmentation
• Fast electronics development
• Efficient algorithms
• Signal analysis
• Tracking
• Image processing
• Computing power for data processing

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GRETINA Three-crystal prototype
Received June 4, 2004

• Tapered hexagon shape
• Highly segmented 6 ? 6 36
• Close packing of 3 crystals
• 111 channels of signal

• Tests performed
• Mechanical dimension
• Temperature and LN holding time
• Energy resolution
• Singles and coincidence scan
• In-beam measurements

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Detector Module Design

• Design Choices
• 4 crystals per cryostat
• Warm FETs

• Reasons
• Simpler geometry 2 types of crystal, one
  cryostat
• Easier FET replacement 1 day vs. 9 days
• Higher availability near 100 vs. 85
• Lower price 400k cost difference, with one
  more crystal

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Signal digitizer

• Variable gain control
• Digitization at 100MHz, 12 bits
• Flexible trigger- internal, external, validation
• On board data processing
• Leading Edge Time
• Constant Fraction Time
• Energy from Trapezoidal Shaping
• P/Z correction
• Raw data sample of charge signals
• VME (readout/control)

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Data Acquisition System

• Signal digitizer
• Network switch
• Workstations

• Trigger/timing system
• Processing farm
• Data storage

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Achievements of GRETINA signal analysis

• Demonstrated that signals from segmented detector
  provide a position sensitivity of about 1 mm( 1
  interaction/segment).
• Developed algorithms to decompose multi-hit
  multi-segment events
• Calculated signals which accurately reproduce
  real signals
• K. Vetter et al., Nucl. Instrum. Methods Phys.
  Res. A452, 105 (2000).
• K. Vetter et al., Nucl. Instrum. Methods Phys.
  Res. A452, 223 (2000).
• Improved algorithms and program could achieve a
  speed of processing 20,000 g/sec with 75
  computers
• D. Radford 2004
• Obtained 2 mm position resolution from in-beam
  experiments using prototype II (1 seg. events)
  and prototype III detectors (1 and 2 seg.
  events).
• M. Descovisch, P. Fallon, D. Radford 2005

KD2 Radford
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In-beam test of prototype II

• Experiment
• LBNL 88 cyclotron (July 03)
• Prototype II detector
• 82Se 12C _at_ 385 MeV
• 90Zr nuclei (b 8.9)
• 2055 keV (10?8) in 90Zr
• Detector at 4 cm and 90
• Three 8-channels LBNL signal
• Digitizer modules

• Analysis
• Event building
• Calibration cross talk
• Signal decomposition
• Doppler correction

beam
?
target
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In-beam test ResultsSum all segments in layers 3
and 4, except segment E
Doppler Corrected using first hit position
determined by signal decomposition
FWHM14.9 keV ?x 2.0 mm (rms)
FWHM28.3 keV
Corrected using center of segment only
No correction
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In-beam test ResultsGRETINA prototype II
?t0, time alignment between measured and
calculated signal, is the major contributor to
the position resolution M. Descovisch et. al.,
NIMA
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Tracking multiple g-rays

• First step cluster creation

• Any two points with
• lt ?p are grouped
• into the same cluster

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Tracking algorithm
INPUT Position and Energy of Interactions From
Signal Decomposition

• Split Clusters
• Use 3-D position of interactions
• Determine principle axes of
• cluster moment
• Split cluster perpendicular to
• the axes

Cluster Identification Based on Angular Separation
Tracking Clusters Using Compton and
Pair-production Formulae
Principle axis
Split
Good
Add
Split
Bad
Split-Add
Add
OUTPUT Gamma Rays Reconstructed Energy,
Interaction Points, and Scattering Sequence
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Tracking results
Mg 25, a 10?
Eg 1.33 MeV, Dr 1 mm
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Computing Requirements

• Assumptions
• Movable among labs, expandable to more detectors
• Performance Requirements
• Process 20,000 gamma/sec, store 10 Mbytes/sec
• Position resolution 2 mm (RMS) average for Eint
  gt 300 keV
• Data Processing Components
• Readout, event building, signal decomposition,
  tracking
• Services
• Controls, configuration management, online
  monitoring, data archiving

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Neutron Damage Effects

• Pulse Shape have been calculated for different l
• Energy and position resolution have been
  extracted
• Degradation in E P resolution depends on hole
  path
• Energy is corrected for interaction position

• Neutron damage has more effect on energy than on
  position resolution
• The detectors needs to be annealed before the
  position resolution will be affected.

(l c )DE 30 cm (l c )Dr 17 cm
10 keV 1 mm
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GRETINA Schedule (fiscal year)
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Remaining Technical Challenges

• Detector
• Production reliability
• Long term stability
• Cost
• Signal analysis algorithm
• Handle more complicated events
• Improve speed
• Tracking algorithm
• Handle high multiplicity events
• Compton image reconstruction algorithm
• Computing power

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Acknowledgement
Kai Vetter Greg Schmid Austin Kuhn Martina
Descovich Rod Clark Marie-Agnes Deleplanque Mario
Cromaz Paul Fallon Augusto Macchiavelli John
Pavan Frank Stephens David Ward
D. Bazzacco Th. Kroell T. Teranishi N. Aoi
and GRETINA Advisory Committee
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Summary

• Gamma ray tracking provides new capabilities in
  nuclear science and applications
• Considerable advances have been made in all
  technical areas
• A number of working systems are providing
  promising results
• Many new ideas are being pursuit which will
  results in further improvements