Gammaray tracking detectors: Physics opportunities and status of GRETINA

1
Gamma-ray tracking detectors Physics
opportunities and status of GRETINA

• I-Yang Lee (??? )
• Lawrence Berkeley National Laboratory
• The 10th International conference on
  Nucleus-Nucleus Collision
• August 16-21, 2009
• Beijing, China

2
Outline

• Frontier of nuclear structure studies
• Experimental challenges
• Principles of gamma-ray tracking
• Status of GRETINA
• Summary

3
Frontier of nuclear studies

• There are 300 stable nuclei (black).
• There are 2300 particle bound nuclei (out to
  the driplines).
• We have limited access with stable beam-target
  combinations (blue).

4
Physics questions

• How do extreme proton-to-neutron asymmetries
  affect nuclear properties, such as shell
  structure and collectivity?
• What are the properties of nuclei at the limits
  of mass and charge?
• What are the properties of nuclei at the limits
  of angular momentum?
• What is the origin of the elements in the cosmos?
• What are the nuclear reactions that drive stars
  and stellar explosions?

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Needed tools

• Radioactive beam accelerators produce nuclei
  far from stability
• NSCL MSU
• HRIBF ORNL
• GANIL
• GSI
• ISAC TRIUMF
• RIKEN
• HIRFL-CSR Lanzhou
• Sensitive detector systems study rare nuclei
• Gamma-ray detectors
• Particle spectrograph
• Neutron detectors
• Magnetic spectrograph

6
Challenges and requirements for g-ray spectroscopy

• High efficiency
• High position resolution, Doppler correction
• High P/T
• High counting rate

• Low beam intensity/
• Cross section
• Large recoil velocity
• Fragmentation
• Inverse reaction
• High background rate
• Beam decay
• Beam impurity

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Principle and advantages of g-ray tracking

• Efficiency (50 ?)
• Proper summing of scattered gamma rays, no
  solid angle lost to suppressors
• Peak-to-background (60) Reject Compton events
• Position resolution (1-2 mm) Position of 1st
  interaction
• Polarization
• Angular distribution of the 1st scattering
• Counting rate (50 kHz) Many segments

3D position sensitive Ge detector shell
Resolve position and energy of interaction points
Determine scattering sequence
8
Evolution of g-ray detector technology
The calculated resolving power is a measure of
the ability to observe faint emissions from rare
and exotic nuclear states.
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New shell structure
One nucleon transfer reactions radioactive
beams low intensity high background GRETA
advantage efficiency peak-to-background
How does structure evolve at extremely large
neutron number?
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New shell structure
Nucleon knock out reactions radioactive
beams high velocity high background GRETA
advantage Doppler correction efficiency peak-to
-background
Wave functions from knock out reactions
GRETA Simulation
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Shape changes
Deep inelastic reactions many reaction
channels low cross sections high background
two-body final state GRETA advantage Doppler
correction efficiency
What drives changes in collectivity?
Gammasphere CHICO 1n transfer
GRETA SuperCHICO 6n transfer
Critical point description
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Structure of heaviest nuclei
Fusion reactions low cross sections high
fission background GRETA advantage efficiency
high counting rate

• What are the heaviest magic numbers?
• How do the fission barriers change
• with spin?

GS FMA
GRETA BGS
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Extreme spin and deformation
Can hyperdeformed nuclei exist?
Fusion reactions low cross sections high g-ray
multiplicity GRETA advantage efficiency peak
-to-background
Woods Saxon Potential
Single Particle Levels (MeV)
?
ND SD HD
Quadrupole Deformation
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GRETINA
Collaboration Institutions LBNL, ANL, MSU,
ORNL, Wash. U.
Cover ¼ of 4p solid angle Seven 4-crystal
detector modules All the required software

• Start Construction June 2005
• Start of Operation February 2011
• Engineering and commissioning
• runs at LBNL 2011
• Operation at 2012 -2013
• NSCL MSU
• HRIBF ORNL
• ATLAS ANL

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Technical achievements
We have demonstrated all the technology

• Advances in Ge detector production
• Segmentation size 2 cm ? 30-40 segment/crystal
• Closely pack irregular tapered hexagon shape
• Fast electronics
• ADC with 10 nsec sampling rate, 14 bit
  resolution
• Efficient algorithms
• Signal analysis position resolution better
  than 2 mm
• Tracking multiple gamma rays
• Computing power
• Cluster with 200 computer to process 20,000 ?/sec

G. J. Schmid et al., Nucl. Instrum. Methods Phys.
Res. A430, 69 (1999). K. Vetter et al., Nucl.
Instrum. Methods Phys. Res. A452, 105 (2000). K.
Vetter et al., Nucl. Instrum. Methods Phys. Res.
A452, 223 (2000). M. Descovich et al., Nucl.
Instrum. Methods Phys. Res. B241, 931 (2005). M.
Descovich et al., Nucl. Instrum. Methods Phys.
Res. A545, 199 (2005). M. Descovich et al., Nucl.
Instrum. Methods Phys. Res. A553, 535 (2005).
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GRETINA Detector module
8 cm
20 º
9 cm
36 segments per Ge crystal
4 crystals per module

• Two types of crystal, one cryostat simple
  geometry
• Warm FET for the segments easy to replace,
  higher availability

30 modules for 4p
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Detector modules
Three received
A-type
36 segments/crystal 4 crystal/ module 148
signal channels /module
B-type
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Electronics
All modules produced
Trigger Timing Control module (ANL) Fast
trigger (lt300 nsec) Trigger conditions
Multiplicity Sum energy Hit pattern
Digitizer module (LBNL) 14bit, 100
MHz Energy Leading edge time Constant fraction
time Pulse shape
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Electronics installation
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Computing
37 segments per detector
Data from GRETINA Detectors
Segment events
Crystal Event Builder
Crystal events
Signal Decomposition
Interaction points
1-28 crystals
Data from Auxiliary Detectors
Global Event Builder
Global Events
70 nodes 2 cpu / node 4 core / cpu
Tracking
Goal Processing 20,000 Gamma rays /sec
Analysis Archiving
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Tracking principle

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

source
Eg Ee1 Ee2 Ee3
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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Signal Decomposition
Adaptive Grid Search algorithm Start on a
course grid, to roughly localize the
interactions, then refine the grid close by.
Computing time T N N number of interactions
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Excellent fit to data

• Measured signals with multiple gamma ray hits
  (red), fitted with a linear combination of basis
  signals (blue) , using Grid search followed by
  least-square fitting.
• The analysis gives (x, y, z, E) of the
  interaction points.

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Pencil beam results
Using the latest decomposition program and
basis, we have achieved lt2 mm (RMS) resolution
in PIII
Pencil beam of 662 keV, 1 mm diameter
Position resolution ?x 1.5 mm ?y 1.7 mm
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In-beam test at MSU

• 36Ar at 86 MeV/u, v/c 0.4
• Be target, 0.54 mm(100mg/cm2) thick
• PIII in coincidence with S800

• Demonstrated coincidence measurement using
  time-stamp technique
• Obtained effective position resolution of 2.2 mm

Gamma - 28Si coincidence
2
E g (keV)
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Summary

• Gamma-ray tracking detectors provide a number of
  unique properties
• High efficiency
• Good position resolution
• Imaging
• They will have major impacts on nuclear physics.
• GRETINA is under construction, and is schedule to
  complete in February 2011.