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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.