AGATA - Nuclear Physics - University of Liverpool

Position Sensitive Semiconductor
Detectors for Use in Nuclear Structure
Helen Boston
On behalf of the
AGATA
Community

Introduction
• Challenges ahead for Nuclear Physics experiments
• AGATA – The Advanced GAmma Tracking Array
• Pulse Shape Analysis (PSA)
• Gamma Ray Tracking (GRT)
• Expected results
• Time line
• Future experiments

Experimental Conditions and Challenges
FAIR
SPIRAL2
SPES
REX-ISOLDE
EURISOL
ECOS
• Low intensity
• High backgrounds
• Large Doppler broadening
• High counting rates
• High g-ray multiplicities
Need instrumentation
High efficiency
High sensitivity
High throughput
Ancillary detectors

Future Arrays
• Next generation g-ray spectrometer based on gammaray
tracking
• First “real” 4 germanium array no Compton
suppression shields
• Versatile spectrometer with very high efficiency and
excellent spectrum quality for radioactive and high
intensity stable beams

The Concept
Without
Compton
suppression
shields
Compton
continuum.
=> Large
peak to
total ratio
With BGO
shielding
With highly
segmented
detectors
E
'
g
1
( E
g
E
/ mc
g
2
)(1 cos
)
Less solid
angle
coverage
=> Big drop in
efficiency
Path of g-ray reconstructed to form
full energy event
=> Compton continuum reduced
=> Excellent efficiency ~50% @1MeV
=> Greatly improved angular
resolution (~1 0 ) to reduce Doppler
effects
Gamma-ray tracking arrays

The AGATA Concept
• Spherical array of 180 asymmetric HPGe detectors
• Each detector has a 36-fold segmented outer
Contact ( FWHM ~ 2keV @ 1.3MeV)
• The crystal ;
• 90mm long
• 40mm Maximum
diameter (10 o taper)
P +
• Completed array - HPGe covers
full 4 solid angle
back
+4000V
n +
front

AGATA
(Design and characteristics)
4 g-array for Nuclear Physics Experiments at European accelerators
providing radioactive and stable beams
Main features of AGATA
Efficiency: 43% (M g =1) 28% (M g =30)
today’s arrays ~10% (gain ~4) 5% (gain ~1000)
Peak/Total: 58% (M g =1) 49% (M g =30)
today ~55% 40%
Angular Resolution: ~1º
FWHM (1 MeV, v/c=50%) ~ 6 keV !!!
today
~40 keV
Rates: 3 MHz (M g =1) 300 kHz (M g =30)
today 1 MHz 20 kHz
• 180 large volume 36-fold segmented Ge crystals in 60 triple-clusters , 12
pentagonals
• Shell of Ge with inner radius of 23.5cm will consist of 230kg of Germanium
• Solid angle coverage of 80%
• Digital electronics and sophisticated Pulse Shape Analysis algorithms allow
operation of Ge detectors in position sensitive mode g-ray tracking

Position Sensitive Detectors
Highly segmented detectors
• Each event x, y, z, t, E
• x, y, z determined with PSA (IC + RC)
/ mc
• Experimental pulse shapes are compared to theoretical
basis
- Impossible to scan 180 detectors
- Simulation can be adapted as neutron damage
occurs
• Tracking: Compton scatter formula relates scatter
angle to energy deposited
- Allows reconstruction of FEE
- Increases P/T
- Optimum use of HPGe coverage
E
'
g
1
( E
g
E
g
2
)(1 cos
)

Ingredients of g-Tracking
g
1
Highly segmented
HPGe detectors
Identified
interaction
points
(x,y,z,E,t) i
4
Reconstruction of tracks
e.g. by evaluation of
permutations
of interaction points
·
· ·
·
Pulse Shape Analysis
to decompose
recorded waves
2 3
Digital electronics
to record and
process segment
signals
reconstructed g-rays

Pulse Shape Analysis (PSA)
• Location of the interaction position within detector
gathered by parameterisation of the information from
the pulse shape
• Real charge deposited and transient charges observed
100%
T90
T50
50%
DE
T30
0%

Magnitude (keV)
Magnitude (keV)
Magnitude (keV)
X Y position
Image charge asymmetry varies as a function of
lateral interaction position
- Calibration of asymmetry response
Pixilation 5x5x20mm becomes 1mm 3
Asymmetry
Area
Area
left
left
Area
Area
right
right
700
AC03
700
AC04
700
AC05
700
AC06
700
AC07
600
600
600
600
600
500
500
500
500
500
400
400
400
400
400
300
300
300
300
300
200
200
200
200
200
100
100
100
100
100
0
0 1000 2000 3000
Time (ns)
0
0 1000 2000 3000
Time (ns)
0
0 1000 2000 3000
Time (ns)
0
0 1000 2000 3000
Time (ns)
0
0 1000 2000 3000
Time (ns)
h
e

Azimuthal Detector Sensitivity
r = 24mm
z = 7.3mm
= 124.1 o
F1
A1
E1
0 o
B1
D1
C1

21
Azimuthal Detector Sensitivity
r = 24mm
z = 7.3mm
= 133.7 o
F1
A1
E1
0 o
B1
D1
C1

Azimuthal Detector Sensitivity
r = 24mm
z = 7.3mm
= 143.2 o
F1
A1
E1
0 o
B1
D1
C1

Azimuthal Detector Sensitivity
r = 24mm
z = 7.3mm
= 152.8 o
F1
A1
E1
0 o
B1
D1
C1

Azimuthal Detector Sensitivity
r = 24mm
z = 7.3mm
= 162.3 o
F1
A1
E1
0 o
B1
D1
C1

Azimuthal Detector Sensitivity
r = 24mm
z = 7.3mm
= 171.9 o
F1
A1
E1
0 o
B1
D1
C1

Depth Position
• Depth of
position
gained
from side
scan
• Asymmetry
• D6 and D1
no
neighbour
segments
so area
divided by
gamma ray
energy

Pulse Shape Analysis : Current Status
Results from the analysis of an in-beam test with the first triple module,
e.g. Doppler correction of gamma-rays using PSA results
d( 48 Ti,p) 49 Ti, v/c ~6.5%
REACTION CHANNEL: (d,p)
Best result
γ detector,
seg. mult. 1
Full
dataset
Simulation
FWHM FWHM FWHM
1382 keV
Psa
Seg
Det
Detector 32 keV 35 keV
Segment 11.1 keV 12 keV
PSA 4.8 keV 5.3 keV 5.0 keV
Results obtained with Grid Search PSA algorithm (R.Venturelli et al.)
Many different methods are under development

Gamma Ray Tracking (GRT)
• Basic assumptions w.r.t. Energy and Klein Nishina
• 1 st interaction deposits most energy
• Scatter will be forward focused

Expected Results
• Simulation of
rotational band
structure
emitted by a
recoiling
nucleus with
very high
velocity
• Gain in
efficiency and
resolution
=50%
Agata: FWHM(1.3 MeV)~6 keV
Euroball: FWHM(1.3 MeV)~60 keV
From W Lopez-Martens

Characterisation and Scanning
Comparison of real and calculated pulse shapes. Validate codes.
Coincidence scan for
3D position determination
Three symmetric capsules scanned
in Liverpool
374
keV
662
keV
288
keV
Commissioning of further scanning
systems
at Orsay and GSI
Scan of an asymmetric capsule C001
in Liverpool - now

Liverpool Scanning Table
• Calibration of the detector response as a function of interaction position.
• Scanning asymmetric detector with a collimated beam of mono-energetic photons.

Intensity Profile – 662keV
Intensity profile – 662keV
• Max count = 1400 in front
• Background = 100 cps
• ~500 keV CFD threshold
• 60s per position
• 920MBq Cs-137 source
• 1mm steps
• 1mm diameter collimator
z

Intensity Profile – 662keV
• Detector
rotated and
scanned
along the
depth of the
detectors

Coincidence Front Face Scan
374keV in AGATA and 288keV in the BGO detectors

Coincidence Scan Positions
Line 5
Line 13
Line 1
Line 2
Line 6
Line 3
Line 4
Azimuth 3
F
A
E
Azimuth 4
B
D
Line 18
Line 7
Line 8
Line 9
Azimuth 2
Line 17
Azimuth 1
Line 16
C
Line 15
Line 12
Line 11 Line 14
Line 10
• √r, grid adopted
• 1200 positions
0 o

Electric Field Simulations : MGS
I
Geometry
II
Potential
Elec field
III
Drift
velocities
IV
Weightin
g fields
• Electric field simulations have
been performed and detailsed
comparisons have been made
with experimental pulse shape
data.
AGATA symmetric crystal
simulation
Gamma-ray tracking arrays

AGATA Triple – Detector Module
First prototype summer 2005
3 encapsulated asymmetric Ge crystals in one cryostat
111 preamplifiers with cold FET
~230 vacuum feedthroughs
LN 2
dewar, 3 litre, cooling power ~8 watts

The AGATA timescale
3 different
asymmetric
hexagonal shapes are
used
Triple cluster
modular units in a
single cryostat
The AGATA
demonstrator: 5 triple
clusters, 540
segments. Scheduled
for completion 2010
Completed array (6480
segments) with support
structure
2 of
completed
array

AGATA Experimental Program
Efficiency (%)
2009 LNL
6TC
2010 GANIL/SPIRAL
≥ 8TC
2012 GSI / FRS
~15TC (1)
AGATA D. + PRISMA
AGATA radius
23.5 cm
Eff (%)
vs. distance
AGATA + VAMOS
AGATA D. ≥8TC
EXOGAM 8 seg. Clovers
Total Eff. > 10%
Setup works also as
Compton Polarimeter
AGATA @ FRS
50
45
40
35
30
25
20
15
Solid Angle (%)
Efficiency M = 1
Efficiency M = 10
Efficiency M = 20
Efficiency M = 30
= 0 = 0.5
6%
AGATA D.
LNL
13.5 cm
10
5
0
1 2

Commissioning Preliminary Plan
- Phase 0: commissioning with radioactive sources starting
when detectors and electronics are available
(even partially).
- Phase 1: easy test with tandem beams with no ancillary
detectors. Radiative capture or fusion-evaporation
reactions with light targets in inverse kinematics.
- Phase 2: test with a “simple” ancillary detector with
limited number of parameters (DANTE).
Coulomb excitation reactions with medium mass
beams (A

• March 2009 at Legnaro
• 1 asymmetric triple AGATA detector
• 30 Si@70MeV + 12 C (216mg/cm 2 )
• Two detector
positions :
close (5.5 cm)
for 54h
far (23.5 cm)
for 20h
• 14TB data
• >1M events in
1823keV peak
Latest In-beam Test
40
K
1823 keV
40
K
2333 keV

The AGATA Demonstrator
Objective of the final R&D phase 2003-2010
1 symmetric triple-cluster
5 asymmetric triple-clusters
36-fold segmented crystals
540 segments
555 digital-channels
Eff. 3 – 8 % @ M g = 1
Eff. 2 – 4 % @ M g = 30
Full EDAQ
with on line PSA and g-ray tracking
In beam Commissioning
Technical proposal for full array
Cost ~ 6 M € Capital

AGATA demonstrator at Legnaro
(2009-10)
5 triple clusters coupled to PRISMA
Schematics of the mounting frame
holding (up to) 15 clusters
Peak efficiency
3 – 8 % @ M g = 1
2 – 4 % @ M g = 30

AGATA Demonstrator at Legnaro
Principal physics opportunities :
High-spin spectroscopy of
moderately neutron-rich nuclei
produced in deep-inelastic reactions
Good experience from CLARA + Prisma
Heavy-ion beams from PIAVE + ALPI
with suitable intensities and energies

AGATA Demonstrator at GANIL
Main physics opportunities:
(~2010/11)
• Spectroscopy of heavy elements towards SHE
• Gamma-ray spectroscopy of neutron-rich nuclei
populated in Deep Inelastic Reaction (with the GANIL
specific aspects)
• Gamma-ray spectroscopy with reactions at intermediate
energies (up to 50 A.MeV)
• Classical high-spin physics and exotic shapes
Range of beams, fragmentation, SPIRAL, direct beam line

AGATA Post-Demonstrator Array at GSI ~ 2012
Main physics opportunities:
Gamma-ray spectroscopy with reactions
at relativistic energies (> 50 A.MeV) - Coulomb excitation, few nucleon
removal etc
with slowed-down beams (10-20 A.MeV) - direct reactions, inelastic
scattering

Summary
• Challenges ahead for nuclear spectroscopy
• New Arrays such as AGATA use highly segmented detectors to
gain position sensitivity and allow gamma ray tracking
• AGATA at Legnaro
• Shell evolution in neutron-rich nuclei and high-spin states
in island of inversion etc
• AGATA at GANIL
• High-spin spectroscopy of neutron-rich nuclei populated in
deep inelastic reactions, studies of proton-rich nuclei and
proton-neutron pairing correlations
• AGATA at GSI
• Proton drip-line and N=Z physics, neutron-rich nuclei,
shell melting and studies along the r-process path

Acknowledgements
A.J. Boston a , R.J. Cooper a , J.R. Cresswell a , M.R.
Dimmock a , I. Lazarus b , S. Moon a L. Nelson a ,
P.J. Nolan a , J. Simpson b , C. Unsworth a .
a
Oliver Lodge Laboratory, The University of
Liverpool, UK
b
STFC Daresbury Laboratory, Daresbury,
Warrington , UK
On behalf of the AGATA Collaboration

HPGe Specifications
• Hexaconical Ge crystals:
- 90 mm long.
- 80 mm max diameter.
- 36 segments, 1 centre contact
- Al encapsulation, 0.6 mm spacing,
• Symmetric detectors
– 3 delivered
• Asymmetric detectors
– 19 ordered (9 accepted, 4 in test, 2
not accepted, 4 to be delivered)
• Preamplifiers available
– Core (Cologne);
– Segment (Ganil & Milano)
• Test cryostats for characterisation
– 5 delivered
• Triple cryostats
– 5 ordered
– 1 complete, 2 being assembled, 2
ordered
0.8 mm thickness.
-37 vacuum feed-throughs.

AGATA : Advanced GAmma
Tracking Array
CAARI 2008
Helen Boston
On behalf of the AGATA Community

Digital Electronics
o Digital sampling of preamplifier response allows analysis of pulse shape
o CCLRC Daresbury designed GRT4 cards currently in use
The GRT4 Cards
4-channel VME cards
14 bit, 80MHz FADCs
2 XILINX Spartan II FPGAs per input provide:
• Buffering
• Internal CFD
• Event timestamp
• Energy calculation via Moving Window
Deconvolution
Count rate limited by data readout through VME
bus (40Mb/s/crate)

Efficiency (%)
AGATA-15 at the GSI-FRS
Forward Quadrant with 45 crystals in 15 triple-clusters
50
45
40
35
Solid Angle (%)
Efficiency M = 1
Efficiency M = 10
Efficiency M = 20
Efficiency M = 30
30
25
20
15
10
5
0
= 0 = 50 %
v/c 1 = 0 v/c
2= 0.5