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AGATA preamplifier performance on large signals from a 241Am+Be
source
F. Zocca, A. Pullia, D. Bazzacco, G. Pascovici
AGATA Week - LNL (PD), Italy, 12-15 November 2007
Outline
Recalls :
Fast reset device of AGATA preamplifiers
TOT technique for the estimate of large saturated signals
Test measurements with the AGATA capsule at LNL in last July :
Test of the TOT technique on large pulser signals from 3 to 50 MeV
Calibration procedure and effects of high count rates
Spectra acquired from a 241Am+Be source in “reset” mode in the energy range from 3 to 10 MeV
HPGe segmented detectorHPGe segmented detectorγ (≈ 1-10MeV)
p± K±(≈ 10-100 MeV)
Background of energetic particles
Core
Needed wide-dynamic-range front-end electronics
Exotic nuclei are to be disentangled in a hostile environment of high background radioactivity: (Bremsstrahlung, neutrons, charged particles…)
Segments
≈ 10 cm
HPGe segmented detectorHPGe segmented detectorγ (≈ 1-10MeV)
p± K±(≈ 10-100 MeV)
Background of energetic particles
Segments
Core
≈ 10 cm
Needed wide-dynamic-range front-end electronics
Besides having low noise and large bandwidth, an extremely WIDE DYNAMIC RANGE is
also required !
Individual highly energetic events or bursts of piled-up events could easily cause ADC SATURATION and introduce
a significant SYSTEM DEAD TIME
charge loop
charge preamplifier
From detector
FR
FCSecond stage
Anti-alias ADC
Exotic nuclei are to be disentangled in a hostile environment of high background radioactivity: (Bremsstrahlung, neutrons, charged particles…)
Mixed reset technique: continuous + pulsed
ADC overflow voltage level
Saturated output without pulsed-resetIdeal non-saturated
output without pulsed-reset
Preamplifier output with continuous-reset (50µs decay time constant)
Output with pulsed-reset
A pulsed-reset mechanism allows a fast recovery of the output
quiescent value, so minimizing the system dead time
An ADC overflow condition would saturate the system
for a long while
Fast-reset device of AGATA preamplifiers
PACAGA5A PACAGA5A (GANIL)(GANIL)
PBPB--B1B1-- MI MI (MILANO)(MILANO)
AGATA_AGATA_corecore--pulser pulser (KOELN)(KOELN)
Fast-reset device of AGATA preamplifiers
PACAGA5A PACAGA5A (GANIL)(GANIL)
PBPB--B1B1-- MI MI (MILANO)(MILANO)
AGATA_AGATA_corecore--pulser pulser (KOELN)(KOELN)
Fast-reset device of AGATA preamplifiers
PACAGA5A PACAGA5A (GANIL)(GANIL)
PBPB--B1B1-- MI MI (MILANO)(MILANO)
AGATA_AGATA_corecore--pulser pulser (KOELN)(KOELN)
Time-Over-Threshold (TOT) techniquesecond-order time-energy
relation offset term
( ) OEVVkTbTbE +−−+= 2112
21
E = energy of the large signal
T = reset time
Time-Over-Threshold (TOT) techniquesecond-order time-energy
relation offset term
( ) OEVVkTbTbE +−−+= 2112
21
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
Time-Over-Threshold (TOT) techniquesecond-order time-energy
relation offset term
( ) OEVVkTbTbE +−−+= 2112
21
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
Experimental setup : AGATA capsule, core preamplifier + built-in pulser
Encapsulated AGATA HPGe crystal at LNL
1.8 Ω
HV
Pulser signal
Warm partCold part
Core preamplifier + pulser Core preamplifier + pulser
47 Ω
Cold part Warm part
Segment preamplifier Segment preamplifier
HPGe crystal (36+1 segments) HPGe crystal (36+1 segments) 9 cm
built-in pulser
Core preamplifier
Calibration procedure (1)
( ) OETbTbVVkE ++=−+ 221211
Parameters calculated by a fitting procedure
Parabolic fitting curve
Calibration pulser signals are completely disentangled from
the background
Calibration procedure (2)
Pulser energies within the 10MeV
ADC range
Main issue : energy calibration of pulser lines beyond ADC range (from 10 MeV on)
as no γ-rays of known energies from a portable calibration source are available at these higher energies
TOT technique applied to over-threshold pulser signals (1)
Pulser signal @ 5.97 MeV60Co background rate = 1.3 kHz 60Co background rate = 14.5 kHz
TOT technique applied to over-threshold pulser signals (2)
60Co background rate = 1.3 kHz 60Co background rate = 14.5 kHz
Resolution @ 5.97 MeV = 10.5 keV (0.18 %)
Resolution @ 5.97 MeV= 15.2 keV (0.25 %)
TOT technique applied to over-threshold pulser signals (3)
Background event rate = 800 Hz
0.039 %19.16 keVE6 = 49.434 MeV
0.043 %14.33 keVE5 = 33.369 MeV
0.067 %12.55 keVE4 = 18.797 MeV
0.11 %11.28 keVE3 = 10.656 MeV
0.19 %11.42 keVE2 = 5.9720 MeV
0.35 %11.68 keVE1 = 3.3501 MeV
Resolution (fwhm) Pulser line energy
Less than 0.4% over the full range
Event rate Resolution (fwhm)
1.3 kHz 10.50 keV 0.18 %
2.3 kHz 11.79 keV 0.20 %
4.2 kHz 12.57 keV 0.21 %
8.2 kHz 13.23 keV 0.22 %
14.5 kHz 15.18 keV 0.25 % 0.21 %22.56 keV14.5 kHz
0.17 %17.87 keV8.2 kHz
0.13 %14.02 keV4.2 kHz
0.12 %12.97 keV2.4 kHz
0.11 %12.07 keV1.2 kHz
Resolution (fwhm)Event rate
Pulser energy = 10.65 MeV
0.21 %~ 40 keV14.2 kHz
0.16 %~ 30 keV8.2 kHz
0.10 %18.64 keV4.2 kHz
0.083 %15.56 keV2.3 kHz
0.069 %12.94 keV1.3 kHz
Resolution (fwhm)Event rate
Pulser energy = 18.8 MeVPulser energy = 5.97 MeV
TOT technique applied to over-threshold pulser signals (3)
Background event rate = 800 Hz
0.039 %19.16 keVE6 = 49.434 MeV
0.043 %14.33 keVE5 = 33.369 MeV
0.067 %12.55 keVE4 = 18.797 MeV
0.11 %11.28 keVE3 = 10.656 MeV
0.19 %11.42 keVE2 = 5.9720 MeV
0.35 %11.68 keVE1 = 3.3501 MeV
Resolution (fwhm) Pulser line energy
Less than 0.4% over the full range
Pulser energy = 10.65 MeV Pulser energy = 18.8 MeVEvent rate Resolution (fwhm)
1.3 kHz 10.50 keV 0.18 %
2.3 kHz 11.79 keV 0.20 %
4.2 kHz 12.57 keV 0.21 %
8.2 kHz 13.23 keV 0.22 %
14.5 kHz 15.18 keV 0.25 %
Pulser energy = 5.97 MeV
0.21 %22.56 keV14.5 kHz
0.17 %17.87 keV8.2 kHz
0.13 %14.02 keV4.2 kHz
0.12 %12.97 keV2.4 kHz
0.11 %12.07 keV1.2 kHz
Resolution (fwhm)Event rate
0.21 %~ 40 keV14.2 kHz
0.16 %~ 30 keV8.2 kHz
0.10 %18.64 keV4.2 kHz
0.083 %15.56 keV2.3 kHz
0.069 %12.94 keV1.3 kHz
Resolution (fwhm)Event rate
Obtained resolutions less than 0.25% for all the tested count rates
Peak shift at increasing count rates
1.3 kHz count rate 14.5 kHz count rate
Time (µs) Time (µs)
The baseline shifts downwards owing to the AC-coupling between the preamplifier and the core electrode
Peak shift at increasing count rates
1.3 kHz count rate 14.5 kHz count rate
Time (µs) Time (µs)
The baseline shifts downwards owing to the AC-coupling between the preamplifier and the core electrode
Estimate of the energy peak shift according to Campbell’s theorem : λ = event rate
< E > = mean event energy
T = reset time
TEEshift ><=∆ λ
Experimental setup: 241Am+Be source
AGATA capsule at LNL
241Am+Be source with Ni target
Fast neutrons thermalized in paraffin and captured by natural metallic nickel
γ-photons produced in the 4 to 9 MeV range
Resolution (fwhm) in “pulse-height” mode Energy
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 %
241Am+Be spectrum in pulse-height mode
Resolution (fwhm) in “pulse-height” mode Energy
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 %
241Am+Be spectrum in pulse-height mode
Resolution (fwhm) in “pulse-height” mode Energy
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 %
241Am+Be spectrum in pulse-height mode
241Am+Be spectrum
“reset” mode (by TOT technique)
“pulse-height” mode (by ADC)
Energy Resolution (fwhm)in pulse-height mode
Resolution (fwhm)in reset mode
4.440 MeV (12C) 104 keV 2.34 % 104 keV 2.34 %
~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 (Fe) 11 keV 0.14 %
7.6456 MeV (Fe) 11 keV 0.14 %
8.9984 MeV (Ni) 15 keV 0.17 % 18.9 keV 0.21 %
18.8 keV(29.4 keV
for the double-peak)
0.25 %(0.38 % for
the double-peak)
Comparison on the double-peak Fe line (7.6312-7.6456 MeV)
“reset” mode“pulse-height” modeFWHM = 18.8 keV ( 0.25 % )FWHM = 11 keV ( 0.14 % )
Comparison on the 8.99 MeV Ni line
“reset” mode“pulse-height” modeFWHM = 19 keV ( 0.21 % )FWHM = 15 keV ( 0.17 % )
At high energies the performance in reset mode approaches the performance in pulse-height mode
The ideal acquisition chain: “dual-channel” core preamplifier
1st channel
2nd channel
~ 5 MeV
~ 20 MeV
Reset threshold ~ 10 MeV
The ideal acquisition chain: “dual-channel” core preamplifier
1st channel
2nd channel
~ 5 MeV
~ 20 MeV
Reset threshold ~ 10 MeV
Pulse-height mode (ADC ~ 5 MeV)
Pulse-height mode (ADC ~ 20 MeV)
Reset mode (from ~ 20 MeV on)
Result :
The ideal acquisition chain: “dual-channel” core preamplifier
1st channel
2nd channel
~ 5 MeV
~ 20 MeV
Reset threshold ~ 10 MeV
Pulse-height mode (ADC ~ 5 MeV)
Pulse-height mode (ADC ~ 20 MeV)
Reset mode (from ~ 20 MeV on)
Result :
A prototype of the dual core board has already been realized and will be tested in these days here in Legnaro
with the AGATA capsule.
ConclusionsThe potentiality of the TOT technique for γ-ray spectroscopy has been proved. The obtained resolution in “reset mode” was of < 0.4% in all the tested range from 3 MeV to 50 MeV. A remarkable resolution of 0.21% was obtained on the Ni spectrum line at the energy of 8.998 MeV.
The purpose of the TOT technique is not that of replacing the standard pulse-height analysis : reset-mode spectroscopy is to be applied BEYOND the range of the ADC in order to extend the energy measurement range.
Future tests and developments are foreseen to address the discussed issues regarding the calibration procedure at high energies and the energy peak shift at increasing count rates.
Acknowledgements
to B. Million, A. Bracco and Milano nuclear-physics group for strongly supporting this work and for valuable suggestions and hints