University of MilanoDepartment of Physics and INFN
HIGH DYNAMIC RANGE LOW-NOISE PREAMPLIFICATION OF NUCLEAR
SIGNALS
A. Pullia, F. Zocca, C. Boiano, R. Bassini, S. Riboldi, D. Maiocchi
Department Conference “Highlights in Physics 2005” October 14, 2005
40 cm
Bea
m
AGATA detector AGATA detector arrayarray
AGATA: an Advanced GAmma-ray Tracking Array
Proposed for high resolution γ-ray spectroscopy with exotic beams
Employing highly segmented HPGe detectors, newly developed pulse-shape analysis and tracking methods
HPGe segmented HPGe segmented detectordetectorγ (1 MeV)
p K
(50 MeV)
Background of energetic particles
Segments
Core
10 cm
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 segmen
t
FR
FCSecond stage
Anti-alias AD
C
The new nuclear experiments with exotic beams pose challenging requirements to the
front-end electronics
Besides having a LOW NOISE, an extremely HIGH
DYNAMIC RANGE is required !
ADC overflow voltage level
New mixed reset technique:
continuous + pulsed
Saturated output without
pulsed-reset
Ideal non-saturated output without pulsed-reset
Preamplifier output with continuous-reset
(50s decay time constant)
Output with pulsed-reset
An ADC overflow condition would saturate the system
for a long while
A pulsed-reset mechanism could permit a fast recovery of the output quiescent value, so minimizing the system dead
time
1st stage 2nd stage
Charge loop
Passive P/Z Amplification
From detector
Cold part of
preamplifier
Warm part of preamplifier
Schmitt trigger comparator
Discharge current From
ADC OVR(optional)
Output
Capacitance to be
discharged to de-
saturate 2nd stageDe-De-
saturation saturation circuitrycircuitry
/Output
1
-1
3rd stage
Implemented mixed reset technique: a time-variant
charge preamplifierCircuit architecture: fast de-saturation of the 2nd stage
Noise is not at risk as no new path is connected to the input node !
Signal acquired at 1st stage output… …and at preamplifier output
1st stage output voltage swing
The realized pulsed-reset technique does not act on the 1st stage and so can’t “protect” it against saturation
The architecture of the 1st stage has been studied to provide a large output voltage swing ( 10 V) and so to a prevent a risk of an overflow condition
Triple AGATA segment preamplifier on alumina substrate
(Mod. “PB-B1 MI” – Milano)
Top view
Bottom view
PZ trimmers
Mechanical dimensions:57x56x5 mm
MDR26 connectors
Segment preamplifiers
Segment preamplifiersCore
preamplifier
0 2 4 6 8 10 120
1
2
3
4
11
109
87
65
43
2
1A
mpl
itude
[V]
Time [µs]
Action of pulsed-reset device
Curve (1)-(10) = from 5 to 50 MeV Curve (11) = 100
MeV
IC
C
qC
I
dT
dE FT
dET / dT = 7.8 MeV/μs
Event energy = 100 MeV : Reset time 13μs !
In a first approximation, a directly proportional relationship exists between the pulsed-reset time T and the event energy ET
I = reset current
= 55 mV/MeV (1st stage conversion gain)
C = 2nd stage capacitance (to be discharged)CF = feedback capacitance
Ψ = 2.92 eV/pair (for HPGe)
Es: CF=1pF, C=4.7nF, I=2mA
Passive P/Z stage: pole
21 RRCP superposition theorem :
1) large signal:
2) tail of previous events:
3) reset current:
Pt
eHtv )(01
Pt
ehtv )(02
P
t
eRRItv 1)( 2103
2121)( RRIeRRIHhtV Pt
PZ
sum of the three contributions: expression of the reset transient
Tt ,0for
by equating to zero at t=T, we derive the relationship between
the total signal amplitude and the reset time :
121
PT
eRRIhH
Detailed analysis of the reset transient
“Reset time-energy” relationship
...TC2
IT
C
IEEE 2
PCST
If we convert the voltage amplitudes H and h in the equivalent energies Es and Ec (by using the conversion gain ), we obtain the
relationship
1eRR
IEEE P
T
21CST
ES = energy of the large signal
EC = equivalent energy of the tail of previous signals
We can expand the exponential term with no loss of accuracy since T<<τP :
large signal energy Es estimated from the reset time T and the tail
contribution Ec
ET = equivalent total energy subjected to reset
T = reset time
C2
PS E...T
C2
IT
C
IE
Energy estimate of a large individual event from the measurement of the
reset time
CP
S ETC
IT
C
IE ...
22
O2112
21S EVVkTbTbE
Contribution of the tail of previous
events
ES = energy of the individual large event
T = reset time
V1 , V2 = pre- and post-transient baselines
b1 , b2 , k1 , E0 = fitting parameters
Tests of the large-signal measurement technique performed with a prototype of
the circuit and a bulky HPGe detector
(Padova, July 2004)
reset device
A spectroscopy-grade pulser injects a large pulse at the preamplifier input
A 60Co source provides a background of lower events which destroys the
large signal resolution if no correction is made
Measurement of large pulses from reset time
Rate of 60Co background
events
Resolution @ 10 MeV in Ge (FWHM)
1 kHz 0.26 %
2 kHz 0.32 %
4 kHz 0.30 %
8 kHz 0.37 %
16 kHz 0.57 %
32 kHz 0.56 %
Rate of 60Co events = 32 kHz
Measurement performed at Padova with HPGe detector (courtesy of D. Bazzacco and R. Isocrate)Measurement performed at Padova with HPGe detector (courtesy of D. Bazzacco and R. Isocrate)
**F. Zocca, ”F. Zocca, ”A new low-noise preamplifier for A new low-noise preamplifier for -ray sensors with smart device for large signal management-ray sensors with smart device for large signal management ”, Laurea ”, Laurea Degree Thesis, University of Milano, October 2004 (in Italian). Degree Thesis, University of Milano, October 2004 (in Italian). See http://topserver.mi.infn.it/mies/labelet_iii/download_file/capitolo6.docSee http://topserver.mi.infn.it/mies/labelet_iii/download_file/capitolo6.doc
OS EVVkTbTbE 2112
21
** EESS = equivalent energy release = equivalent energy release
T = reset timeT = reset time
bb11, b, b22, k, k11, E, E00 = fitting parameters = fitting parameters
VV11, V, V22 = pre- and post-pulse baselines = pre- and post-pulse baselines
Energy range in Energy range in normal mode normal mode ~ 2MeV 2MeV
1408 keV1408 keV
2.02 keV fwhm2.02 keV fwhm
Extending the energy range by reconstruction of the large signals from
reset time
Extended range
+ pulser+ pulser
122 keV122 keV
344 keV344 keV
Future developments
Tests of the pulsed-reset device with a triple AGATA preamplifier coupled to an AGATA HPGe segmented detector
Tests of the large-signal measurement technique when applied to measure the energy of real highly energetic events (photons or energetic particles in the 10-50 MeV range)
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