Download A Fast-stretcher for an easy acquisition of the fast component of

A Fast-stretcher for an easy acquisition of the fast
component of BaF2 detectors signals.
C. Boiano1, R. Bassini1, A. Pullia2, F. Camera2, G. Benzoni1, A. Bracco2, S. Brambilla1, B. Million1, O. Wieland1
Abstract— A gate-free fast-stretcher circuit has been
developed for an easy acquisition of the fast and slow
components of the signals produced by BaF2 scintillators, in
order to discriminate the types of incident particles. In
experiments where many detectors are used, these
measurements are normally performed providing the signal
to two QDC’s (Charge to Digital Converter) and using logical
signals of different widths as gates. One gating signal is short
to integrate the fast part of the signal only; the other is much
longer to integrate the whole signal. Such a measurement
technique has many inconveniences. One of the most critical
is that the analog signals fed into the QDC must be delayed
through long and cumbersome transmission lines to
synchronize them with the QDC gate signals, which degrades
the quality of the fast component. We have developed a new
circuit structure that addresses these issues. The circuit uses
no delay line. It simply builds two slow Gaussian signals: the
first, called "Fast", is obtained after stretching the fast signal
component, and is proportional to its amplitude; the second,
called "Slow", is proportional to the energy of the entire
signal. These outputs are easily acquired with a standard
peak ADC with no gate-timing problem. The test has shown
an excellent Fast-Slow separation even with small input
signals of only a few millivolts.
I. INTRODUCTION
We have developed a gate-free fast-stretcher circuit able
to capture the peak of the fast component of the signals
produced by BaF2 scintillators.
As well known, after a particle interaction the light emitted
by these scintillators forms a pulse composed of a narrow
peak and a slow exponential decay. The relative weight of
these signal components depends on the nature of the
incident particles. The fast component lasts a few
nanoseconds while the slow tail falls out in some hundred
nanoseconds. In many nuclear-physics experiments one
has to determine (1) the energy of the fast signal
component, and (2) the total energy of both components in
order to discriminate the types of incident particles [3]. To
do so, measurements are normally performed using a QDC
(Charge to Digital Converter) with a short gate to integrate
the fast part of the signal, and another QDC with a longer
gate to integrate the whole signal and so provide the total
event energy. This measurement technique has many
inconveniences when these detectors are used with other
detector arrays [4] producing signals much delayed in
1
C. Boiano, R. Bassini, G. Benzoni, S. Brambilla, B. Million, O.
Wieland are with INFN sez. Milano via G. Celoria, 16 Milano 20133
ITALY (telephone: +39-02-50317282, e-mail: [email protected]).
2
A. Pullia, F. Camera, A. Bracco are with Milano University and INFN
sez. Milano via G. Celoria, 16 Milano 20133 ITALY
time. In this case the use of this integration technique is not
ideal since the BaF2 signals must be delayed considerably
and the use of the delay lines degrades the quality of the
fast component of the analog signals as shown in Fig.1.
Fig.1 Typical BaF2 signals before and after 150ns trasmission line
Moreover when many BaF2 detectors are used in the same
experiment it is difficult to produce a short common gate
aligned in such a way to capture the fast components of all
channels, unless such a common gate signal is made
relatively large with a subsequent loss in the ability to
discriminate the fast signal against the slow one. A good
alternative to this pulse integration technique is to produce
two Gaussian signals with heights proportional to the
amplitudes of the fast and slow components, respectively.
To follow this approach a fast stretcher module has been
developed and constructed.
II. FUNCTIONAL DESCRIPTION
The developed circuit permits to provide two slow
Gaussian signals: the first, called "Fast", is proportional to
the amplitude of the rapid component only of the signal;
the second, called "Slow", is proportional to the energy of
the entire signal. The working principle of the circuit
section that processes the fast component is quite simple. It
consists of a fast peak stretcher able to capture the peak of
the BaF2 signal in a time budget of as little as a few
nanoseconds. The circuit comprises a C-R passive
differentiator with a time constant of a few nanoseconds.
The derivative of the signal is transformed into a current
stretching circuit is realized with the Trasconductance
Operational Amplifier OPA660 from Burr Brown [1],
schematized as an ideal transistor. Similarly to transistors,
this device has only three terminals, a base with high
impedance, an emitter with low impedance and a collector
with a current output proportional to the voltage between
the base and emitter terminals. Note however that the OTA
mirrors the current flowing upward through its emitter into
a current flowing downward through its collector and
viceversa. The emitter of this ideal transistor has been
connected to a C-R circuit. A rapid variation of the input
base signal produces a variation of the Collector-Emitter
current until the capacitance C2 connected to the emitter
gets completely charged across resistor R2, with time
constant C2*R2. This current is the imperfect derivative,
with time constant C2*R2, of the input signal. With the
input signal depicted in Fig. 1 it will be negative in the first
part of the transient and positive right after the peak. To
get an estimate of the height of the fast signal component
we only need to integrate the negative part of the OTA
output current, flowing downward through its collector.
This is done using bipolar transistor Q1 as a current
rectifier. The collector of the OTA is connected to the
emitter of Q1, which is an NPN BJT, and opens the way
only to currents exiting its emitter, i.e. downward through
the OTA collector or upward through the OTA emitter. In
this case the same current flow provided by the OTA is
integrated upon the capacity C3. When instead the OTA
output current is opposite the current flow is blocked at the
emitter of Q1 and finds an open path through the Schottky
diode D1. When D1 goes into conduction the OTA
collector voltage rises to only ~0.4V. In this way a reduced
voltage swing appears on the emitter of Q1, which helps
reduce charge injection onto capacitance C3 through the
using a fast OTA (Operational Trasconductance
Amplifier). The negative part only of such a current is then
passed along to an integrator. To block the positive current
a device with unidirectional conduction is used, namely we
used a high-frequency transistor. A simple quasi-Gaussian
shaper amplifier with a time constant of 500ns is finally
used to transform the fast current signal into a wide voltage
pulse.
The part of the circuit that provides the "Slow" signal is
even simpler. In fact it basically consists of a quasiGaussian shaper amplifier with a time constant of 1.5µs
and with adjustable gain.
III. CIRCUIT DESCRIPTION
Fig.2 shows a simplified schematic diagram of the faststretcher circuit. The input signal is fed into two buffer
circuits implemented with a transistor in common-base
configuration. The buffered outputs are connected to the
fast stretching section and to the front panel of the module,
making a replica of the input signal available for timing
measurements. The input signal is also fed into the part of
the circuit that produces the "Slow" signal, through a highimpedance path. The most critical part of the circuit is that
used to capture the fast signal peak. The requirements of
this circuit are very stringent. In fact it has to track signals
with a risetime of a few nanoseconds, it must be very
accurate, linear and it must work in the wide dynamic
range from 10mV to 2V.
The basic idea of our stretching circuit is to use a
peculiarity of the BaF2 signals. The fast component of
these signals is always higher than the slow component
even if this latter has a greater energy. So it is always
possible to capture the amplitude of the rapid peak. The
VB2
OTA
OPA660
D1
C1
Input
Q1
Shaper amplifier
500ns τ
Buffer
R1
Buffer
R3
C3
VB1
C2
R2
Buffered Output
VCC
Shaper amplifier
1.5us τ
Slow Output
Fig. 2 Simplified schematic diagram of the fast-stretcher circuit
Fast Output
transistor parasitic capacitances. Such a charge injection
could otherwise significantly degrade the circuit ability to
capture the smallest fast signals. To obtain a faster
response, Q1 is maintained on in sleep conditions by
means of a very little bias current, of the order of some ten
microamperes. The peak value obtained on the capacitance
C3 decays with a time constant C3*R3 of the order of
some microseconds.
The signal is then fed into a CR-RC² network and
subsequently amplified in such a way to obtain a quasiGaussian signal with a time constant of 500ns, easily
acquired with a standard peak ADC for pulse-height
spectroscopy. Our fast-stretcher module also provides a
signal called "Slow" which is proportional to the total
energy of the input signal. The "Slow" signal is obtained
by integrating the input signal through an R-C network
with a time constant of 1.5µs. Subsequently the signal is
amplified and fed into a Sallen-Key type active filter.
The so-obtained quasi-Gaussian output is then passed
along to an adjustable-gain amplifier, set through DIP
switches accessible from the frontal panel.
rt=20.3
Fig. 4 Fast-stretcher NIM module.
V. EXPERIMENTAL RESULTS
Fig.5 shows the fast versus slow matrix as obtained
acquiring the internal radioactivity of a BaF2 scintillator, in
which the value of the slow component is in the horizontal
axis and that of the fast component in the vertical axis. The
two groups of events correspond to gamma and alfa
radiations, which indeed are well separated using this
technique.
ns
Fig. 3 Typical waveforms of the fast-stretcher circuit.
IV. CHARACTERISTICS OF THE MODULE
The circuit has been implemented in a single-unit NIM
module with 8 stand-alone channels Fig.4.
The eight Lemo-connector inputs are housed upon the
front panel together with the buffered outputs.
An auxiliary input used for the Slow section is also placed
on the front panel. The octal DIPswitch used for the
adjustment of the gain of the "Slow" signals is also placed
on the front panel. Moreover the sensitivity of the fast
section is adjustable through two internal jumpers allowing
the module to work with four different input ranges. The
"Fast" and "Slow" outputs are provided through two
standard 8-pin dual-row connectors placed upon the back
panel.
Fig. 5 Matrix built with “Fast (vertical)” versus “Slow (horizontal)” signal
component.
VI. REFERENCES
[1] "OPA 660" data sheet, Apr 1995, Burr Brown semiconductors.
http://www.philips.com
[2] "Large barium fluoride detectors" K.Wisshak and F.Kaeppeler NIM
227 (1984) 91-96
[3] "A note on the application of baf2 scintillators to γ-ray and charged
particle detection" E. Dafni NIM a254 (1987) 54-60
[4] A.Maj and other Nuclear Physics A571 (1994) 185
[5] “Giant Resonances” P.F.Bortignon, A.Bracco, R. A. Broglia,
Contemporary Concepts in Physics Vol.