Download Measurement of large pulses from reset time

University of Milano
Department 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
AGATA:
an
Advanced GAmma-ray Tracking Array
AGATA detector array
 40 cm
 Proposed for high resolution γ-ray spectroscopy with exotic beams
 Employing highly segmented HPGe detectors, newly developed
pulse-shape analysis and tracking methods
The new nuclear experiments with exotic beams pose
challenging requirements to the front-end electronics
HPGe segmented detector
charge preamplifier
RF
Core
Background of
energetic
particles
From
detector
segment
CF
Second
stage
Antialias ADC
Segments
 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
Besides having a LOW NOISE,
an extremely HIGH DYNAMIC
RANGE is required !
New mixed reset
technique:
continuous + pulsed
Ideal non-saturated
output without
pulsed-reset
Saturated
output without
pulsed-reset
ADC overflow voltage level
Preamplifier output with
continuous-reset (50s
decay time constant)
An ADC overflow condition
would saturate the system
for a long while
Output with
pulsed-reset
A pulsed-reset mechanism could
permit a fast recovery of the
output quiescent value, so
minimizing the system dead time
Implemented mixed reset technique:
a time-variant charge preamplifier
Circuit architecture: fast de-saturation of the 2nd stage
Cold part of
preamplifier
Warm part of
preamplifier
2nd stage
1st stage
From
detector
Charge loop
Passive P/Z
Amplification
Discharge
current
Schmitt trigger
comparator
3rd stage
1
Output
-1
/Output
Capacitance to
be discharged
to de-saturate
2nd stage
De-saturation
circuitry
From
ADC OVR
(optional)
Noise is not at risk as no new path is connected to the input node !
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
Signal acquired at 1st stage output…
…and at preamplifier output
Triple AGATA segment preamplifier on
alumina substrate (Mod. “PB-B1 MI” – Milano)
MDR26 connectors
Top
view
PZ
trimmers
Bottom
view
Segment
preamplifiers
Mechanical dimensions:
57x56x5 mm
Core
preamplifier
Segment
preamplifiers
Action of pulsed-reset device
In a first approximation, a directly
proportional relationship exists
between the pulsed-reset time T
and the event energy ET
Curve (1)-(10) = from 5 to 50 MeV
Curve (11) = 100 MeV
4
11
dET
I
 CF


I
dT
C q C
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
Amplitude [V]
3
2
3
4
5
6
7
8
9
10
2
1
1
0
0
2
4
6
8
Time [µs]
dET / dT = 7.8 MeV/μs
Event energy = 100 MeV : Reset time  13μs !
10
12
Detailed analysis of the reset transient
Passive P/Z stage: pole  P  C  R1  R2 
superposition theorem :
1) large signal: v01 ( t )  H e
 t
P
2) tail of previous events: v02 ( t )  h e
 t
P
3) reset current:
 t


v03 ( t )   I R1  R2  1  e  P 


sum of the three contributions:
expression of the reset transient
VPZ ( t )  h  H  I R1  R2  e
for t  0, T 
 t
P
 I R1  R2
by equating to zero at t=T, we derive
the relationship between the total
signal amplitude and the reset time :
T

H  h  I R1  R2  e  P  1 


“Reset time-energy” relationship
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
 T P

ET  E S  EC   R1  R2   e
 1



I
T = reset time
ET = equivalent total energy subjected to reset
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 :
ET  E S  EC 
ES 
I
I
T
T 2  ...
C
2  C P
I
I
T
T 2  ...  EC
C
2  C P
large signal energy Es estimated from the
reset time T and the tail contribution Ec
Energy estimate of a large individual event
from the measurement of the reset time
ES 
I
I
T
T 2  ...  EC
C
2 C  P
Contribution of the tail
of previous events
E S  b1T  b2T 2  k1 V1  V2   EO
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)
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
reset device
Measurement of large pulses from reset time
ES  b1T  b2T  k1 V1  V2   EO
2
*
ES = equivalent energy release
T = reset time
b1, b2, k1, E0 = fitting parameters
Rate of 60Co events = 32 kHz
V1, V2 = pre- and post-pulse baselines
Rate of 60Co
Resolution @ 10 MeV
background
in Ge (FWHM)
events
1 kHz
0.26 %
2 kHz
0.32 %
4 kHz
0.30 %
8 kHz
0.37 %
16 kHz
0.57 %
32 kHz
0.56 %
Measurement performed at Padova with HPGe detector (courtesy of D. Bazzacco and R. Isocrate)
Zocca, ”A new low-noise preamplifier for g-ray sensors with smart device for large signal management”, Laurea
Degree Thesis, University of Milano, October 2004 (in Italian). See http://topserver.mi.infn.it/mies/labelet_iii/download_file/capitolo6.doc
*F.
Extending the energy range by reconstruction of
the large signals from reset time
122 keV
344 keV
+ pulser
1408 keV
2.02 keV fwhm
Extended range
Energy range in
normal mode ~ 2MeV
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)