New technologies for High-Purity
Germanium detectors read-out
Francesca Zocca
Now with: Consejo Superior de Investigaciones Cientifica (CSIC)
Instituto de Estructura de la Materia (IEM) – MADRID
CERN - ISOLDE collaboration
Work developed while was with:
University of Milano, Dep. of Physics
Istituto Nazionale di Fisica Nucleare (INFN) - MILANO
ISOLDE seminar - October 20th 2010
Outline
AGATA project
Technical advance in front-end read-out
New read-out techniques
Experimental test with AGATA crystals
Development of integrated (CMOS) preamplifiers
for HPGe detectors
Acknowledgements
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
AGATA: the Advanced GAmma-ray Tracking Array
4π tracking spectrometer of highly-segmented HPGe detectors
for high resolution γ-ray spectroscopy with next-generation
high-intensity radioactive ion beams
Future 4π AGATA final detector array
180 hexagonal detectors
(grouped in 60 triple clusters)
+ 12 pentagonal detectors
Beam
≈ 50 cm
AGATA demonstrator (LNL- Padova)
5 triple-cluster modules
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Highly-segmented HPGe detectors
• operated at cryogenic temperature (liquid nitrogen used as cooling medium @ 77 K)
• energy of detected gamma-rays: from a few keV to a few tens of MeV
• excellent intrinsic energy resolution: 0.1% - 0.2% in the typical 1 MeV region
• segmentation of the outer electrode => position-sensitivity: < 5 mm 3D resolution
Gamma-ray tracking in highly-segmented HPGe detectors based on the
Compton-scattering formula (AGATA & GRETA)
emitting radionuclide
Determination of the photon emission
direction within an opening angle smaller
than 1° => Doppler effect correction ~ 9 cm (HPGe)
Improved energy resolution (better than 0.5%) also for emitting
nuclei with recoil velocities of up to 50% of the velocity of light
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Front-end read-out of AGATA crystals
HPGe crystal (36+1 segments) Core preamplifier + pulser
9 cm
Cold part Warm part
HV
9cm-long 36-fold
segmented HPGe crystal
37 hybrid charge 47 Ω
preamplifiers
1.8 Ω
Pulser
Segment preamplifier signal
Core preamplifier
Cold part Warm part
Built-in
pulser
Triple segment preamplifiers
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Front-end electronics specifications
low noise (high-resolution gamma spectroscopy : 0.1-0.2 % @ 1MeV)
wide bandwidth: rise time of ~ 20 ns (pulse shape analysis)
excellent stability of the preamplifier response
LARGE DYNAMIC RANGE:
- at least ~104 : from a few keV to 10-20 MeV
- up to ~30 MeV depending on the physics experiment (i.e. giant resonances)
- minimization of the dead time in a much larger dynamic range up to 100-200 MeV
γ (≈ 1-10M
eV )
In next-generation nuclear physics experiments
with high-intensity radioactive beams exotic
nuclei are to be disentangled in a hostile p± K±
environment of high background radioactivity: (≈ 10-100
MeV)
bremsstrahlung, neutrons, highly-energetic
charged particles @ high count rates
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
The issue of ADC saturation
charge preamplifier Custom-made hybrid charge
RF preamplifiers provide a large
dynamic range (180 MeV of
From
detector
CF
Second
equivalent energy) and avoid
Anti-
stage alias ADC saturation in most cases
charge loop
But the ADC has a limited
input voltage range !
In order to perform the gamma-ray tracking, the preamplifier signals must be
directly digitized: shaping and filtering techniques for pulse-height analysis
(energy) & pulse-shape analysis (position) are applied on the digitized signals
Individual highly energetic events or bursts of piled-up
events can easily yield ADC SATURATION and
introduce a significant SYSTEM DEAD TIME
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Mixed reset technique: continuous + pulsed
Saturated
output without
Ideal non-saturated pulsed-reset
output without
pulsed-reset
ADC overflow voltage level
Preamplifier output with
continuous-reset (50µs Output with
decay time constant) pulsed-reset
A pulsed-reset mechanism
An ADC overflow condition
allows a fast recovery of the
would saturate the system for
output quiescent value, so
a long while
minimizing the system dead time
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
AGATA preamplifiers structure
Cold part of Warm part of
preamplifier preamplifier
1st stage 2nd stage 3rd stage
1 Output
-1 /Output
From Charge loop Passive P/Z Amplification
detector
Capacitance to
be discharged
to de-saturate
2nd stage
Discharge
De-saturation current
Schmitt trigger From
comparator
circuitry ADC OVR
(optional)
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Time-Over-Threshold (TOT) technique
second-order time-energy
relation
E = b1T + b2T 2 − k1 (V1 − V2 ) + EO
contribution of the tail
due to previous events
E = energy of the large signal
T = reset time
V1 , V2 = pre-pulse and post-pulse baselines
b1 , b2 , k1 , E0 = fitting parameters
Within ADC range standard “pulse-height mode” spectroscopy
Beyond ADC range new “reset mode” spectroscopy
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Test of the “reset” mode spectroscopy
AGATA 36-fold
segmented crystal
241Am+Be source
with Ni target
Fast neutrons
thermalized in paraffin
and captured by natural
metallic nickel
γ-photons produced in
the 7.5 to 9 MeV range
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
241Am+Be spectrum in pulse-height mode
Resolution (fwhm) in
Energy
“pulse-height” mode
1.1732 MeV (60Co) 2.99 keV 0.25 %
1.3325 MeV (60Co) 3.24 keV 0.24 %
2.2233 MeV (H) 4.51 keV 0.20 %
4.440 MeV (12C) 104 keV 2.34 %
7.6312 MeV (Fe) 11 keV 0.14 %
7.6456 MeV (Fe) 11 keV 0.14 %
8.9984 MeV (Ni) 15 keV 0.17 %
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
241Am+Be spectrum in “reset” mode
See: F.Zocca, A.Pullia, D.Bazzacco, G.Pascovici, “A Time-over-Threshold technique for wide dynamic range gamma-
ray spectroscopy with the AGATA detector”, IEEE Trans. Nucl. Sci., vol. 56, no. 4, pp. 2384-2391, Aug. 2009
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
241Am+Be spectrum
“reset” mode
Resolution (fwhm) Resolution (fwhm)
Energy
in pulse-height mode in reset mode
4.440 MeV
104 keV 2.34 % 104 keV 2.34 %
(12C)
~5.6 MeV 10.5 keV 0.14 % 18.8 keV 0.34 %
~6.1 MeV 15.1 keV 0.17 % 17.1 keV 0.28 %
7.6312 MeV 18.8 keV 0.25 %
11 keV 0.14 %
(Fe) (29.4 keV (0.38 % for
for the the
7.6456 MeV double- double-
11 keV 0.14 %
(Fe) peak) peak)
8.9984 MeV
15 keV 0.17 % 19 keV 0.21 %
(Ni)
“pulse-height” mode
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Comparison on the double-peak Fe line
(7.6312-7.6456 MeV)
“pulse-height” mode “reset” mode
FWHM = 11 keV ( 0.14 % ) FWHM = 19 keV ( 0.25 % )
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Comparison on the 8.99 MeV Ni line
“pulse-height” mode “reset” mode
FWHM = 15 keV ( 0.17 % ) FWHM = 19 keV ( 0.21 % )
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Dual-channel core preamplifier
~ 5 MeV Reset threshold ~ 10 MeV
1st channel
~ 20 MeV
2nd channel
Result :
Pulse-height mode Pulse-height mode Reset mode
(ADC ~ 5 MeV) (ADC ~ 20 MeV) (from ~ 20 MeV on)
A dual-channel version of the core preamplifier is currently under
test at GSI with a triple cluster of AGATA
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
From ADC saturation to preamp saturation…
Hybrid DISCRETE preamplifiers
• high flexibility in the design
• use of high voltage power supply (ex: +/- 12 V)
CMOS integrated solutions
the high segmentation of the read-out electrodes yields a higher
and higher count of read-out channels
• small dimensions & low power dissipation
• radio-purity and full functionality at cryogenic temperature
Intrinsically low available voltage swing of scaled CMOS technologies (3-5V)
A decrease of the preamplifier sensitivity (energy-to-voltage gain) would
compromise the signal-to-noise ratio and the spectroscopic performances
Saturation of a CMOS preamplifier for HPGe detectors is
expected for input energies > 5-10 MeV
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
A cryogenic JFET-CMOS preamplifier
Developed in the framework of the GERDA experiment (GERmanium Detector Array)
for the search of 76Ge neutrino-less double beta decay
Preamplifier operated while directly immersed
in the cryogenic liquid (LN / LAr @ 77 / 87 K) First prototype realized in 5V 0.8 µm
CMOS technology (old but reliable)
T IN V CC
Cdet ~ 60 pF
IN
Bias2
Bias1
T out
OUT
-V EE
0.17 % 0.15 % VD
100 µm
test
23 mm
det
out
VEE
VCC 47 mm
See: A.Pullia, F.Zocca, S.Riboldi, D.Budjáš, A.D’Andragora,
C.Cattadori, “Cryogenic performance of a low-noise JFET-
CMOS preamplifier for HPGe Detectors”, IEEE Trans. Nucl.
Sci., vol. 57, no. 2, pp. 737-742, Feb. 2010.
Speaker: Francesca Zocca CMOS ET Workshop, Banff, February 18th, 2009
0.35 um CMOS solution under test
Fast active reset of the charge
sensing stage
charge-sensing stage
saturation voltage
RESET THRESHOLD
See: A.Pullia, F.Zocca, “Extending the dynamic range of a charge-preamplifier far
beyond its saturation limit: a 0.35um CMOS preamplifier for germanium detectors”,
2009 IEEE Nuclear Science Symposium Conference Record, Oct.25-31, Orlando,
Florida, USA
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
Acknowledgements…
to Prof. Alberto Pullia
Milano nuclear electronics group leader & preamplifier
group leader in the AGATA collaboration
And to…
Dr. George Pascovici
(IKP, University of Cologne, Germany)
Dr. Dino Bazzacco
(INFN-Istituto Nazionale di Fisica Nucleare, Padova)
Nuclear Physics group in University of Milano
(Prof. Angela Bracco, Dr. Benedicte Million, Prof. Franco Camera…)
the collegues of LNL- Laboratori Nazionali di Legnaro, Padova
Speaker: Francesca Zocca ISOLDE seminar – October 20th 2010
