Все науки. №10, 2024. Международный научный журнал. Ибратжон Хатамович Алиев

1 presents MARS electronic system. It is based on desktop and VME modules from CAEN SpA:

      – TwoA1422 charge sensitive preamplifiers. They present a sensitivity of 90 mV/MeV (for Si detectors) and support a detector capacitance up to 200 pF (F2 type). The A1422 are implemented into alloy boxes, 8 channels each, and feature BNC connectors for the input (detector), SHV for high voltage supply, LEMO for the energy output and the input test (TEST IN), and a cable with a D – type 9 pin male connector for the power supply [17].

      – The DT5423 power distributor provides 4 standard D-type 9 pin fe male connectors to supply up to 4 A1422 preamplifiers with ± 12 V [18].

      An alternative VME form factor module [19] could be employed and inte grated in the same VME crate described below.

      – Three V6519P HV power supply modules provide the high voltage (up to 500 V and 3 mA of voltage and current, respectively) to the de tectors. Each one houses 6 HV power supply channels that are controlled by software. The channels share a common floating return which allows on-detector grounding, reducing the noise level. HV outputs are delivered by SHV connectors [20].– The V1725S digitizer consists of 16 (0—15) input channels capable of recording waveforms (directly from the preamplifiers A1422 [17]) along with performing advanced algorithms for online DPP [21]. Using DPP f irmware, we are able to acquire the integrated charge and carry out PSD (Pulse Shape Discrimination) and PHA (Pulse Height Analysis), as well as read out waveforms with automatic pulse identification. It features 14 bit resolution, 250 MS/s sampling rate, and 2 V input range. This implies a digital precision of 0.12 mV and a digital resolution of 0.006%, with a processing time of 65 µs. Communication with the board can be done through the VME bus and Optical Link. It is well suited for mid fast pulses such as the ones coming from silicon detectors coupled to the charge sensitive preamplifiers A1422 [17].

      – The V1718 bridge is a VMEtoUSB2.0 bridge. It acts as a VME master module and can be operated from a USB port of a standard PC [22].

      – The VME8008Bisan8slotVME cratesuitable for 6U x 160 mmboards, which serves as a support and power supply for the electronic modules V6519P, V1725S, and V1718 [23].

      2.3. MARS acquisition system

      The V1725S digitizer runs the DPP-PSD/PHA firmware. The digitizer im plements a digital replacement of discriminator, pulse shaping, and gate gener ator. These functions are performed inside the FPGA (onboard) without using external cables, delay lines and/or other boards. The V1725S digitizer itself con sists of a multi-channel data acquisition system that replaces traditional analog boards and constitutes a single compact tool for nuclear reactions spectroscopy. A PC with an Intel i7 (1.8 GHz, 8 CPUs) and 16 GB RAM handles the data acquisition by means of CoMPASS (CAEN Multi-PArameter Spectroscopy Software) [24]. CoMPASS represents the GUI, manages parameters, builds plots and saves the energy and time spectra. Within CoMPASS, we can set discriminator parameters such as: threshold, discrimination type, and trigger holdoff. In addition, we can set parameters related to PHA: trapezoid rise time, flat top, pole-zero; parameters related to PSD: pre-gate, short gate and long gate; as well as the gain and the number of channels of the spectra. It also allows to discard pile-up and saturation events besides applying energy, time, and PSD filters.

      (a) PSD firmware. The input signal (black) within the long and short gates (red and green squared pulses). At the bottom, we have the constant fraction discriminator (CFD) signal (blue). (b) PHA firmware. The input signal (black with long tail), the fast trigger (left red), and the trapezoid filter (blue) with the peaking signal (green) just below the trapezoid. Figure 2: CoMPASS screenshots showing different electronic signals for (a) PSD firmware, and (b) PHA firmware.

      Moreover, the V1725S digitizer has a logic unit that allows for measuring coincidences. Through CoMPASS, we are able to acquire events in logic AND between pairs of consecutive data acquisition chains (ch0 & ch1 || ch2 & ch3 || and so on). For this, a coincidence (time) window must be enabled and defined. Once the first detector is triggered, any signal triggering the second detector, within the specified time window, will validate an event (in coincidence). Anti coincidences can also be performed using external logic modules and using the generated gate as an input to the digitizer trigger input.

      2.3.1. PSD & PHA firmware

      For nuclear reactions spectroscopy, we are particularly interested in two f irmware packages dedicated to DPP: PSD and PHA. Both are available in MARS. The PSD firmware allows for integrating incoming pulses and producing an energy spectrum. The selection of events can be done by means of a leading edge discriminator (LED) or a constant fraction discriminator (CFD). It features a double gate integration of the input in order to calculate the PSD factor, given by:

      where Qlong and Qshort are the integrated charges in the long and short gates (Figure 2a). It represents the ratio between the signal tail and the total inte gral. PSD is suitable for processing pulses with fast decay time, such as those produced by electrons, gammas, and neutrons in scintillation detectors [24]. The PHA firmware allows the digitizer to obtain an energy spectrum by applying a trapezoidal filter to the input pulse. It transforms the typical expo nential decay pulse, generated by a detector (and transmitted through a pream plifier), into a trapezoidal signal whose height is proportional to the input pulse amplitude. The height of the trapezoid is measured at the peaking time (see Figure 2b). This firmware is ideal for processing pulses with long decay times, such as those produced by alphas and heavy ions in semiconductor detectors [24]. This is the case for charged fragments resulting from nuclear reactions. Therefore, we will report on the PHA firmware from now on.

      Figure 3: Scheme of the setup used for MARS characterization.

      3. MARS characterization

      As a proof of concept and validation, MARS has been firstly tested in a laboratory environment. Figure 3 schematize the first setup used to characterize the MARS electronic system. The test pulses were generated by the 419 ORTEC precision pulse generator [25] or the digital detector emulator DT4800 [26]. Both modules allow for emulating electronic pulses that simulate the detection of a charged particle in a silicon detector. The former works at a fixed frequency of 70 ± 10 Hz [25], while the latter can be set to different values. The DT4800 allows for emulating pulses with time and amplitude follow ing Poisson distributions. It allows for loading spectra from different sources, simulating their emission rate and pulse height distribution. It also allows for modifying the rise and decay times of the pulses. It is worth mentioning that pulses were initially delivered through the com monTESTIN (input) of A1422 preamplifiers. In practice, the electronic signals, coming from detectors, must be transmitted through individual DET IN elec tronic chains. However, unlike DET IN, TEST IN has a 50 Ω termination in series with a 1 pF capacitance. Further measurements were performed using a triple alpha source placed in front of detectors, which were then connected to individual A1422 DET IN. According to Figure 3, MARS system is divided into 8 individual electronic chains (1- 8) related to each A1422 (13323 and 13324) preamplifier and 16 data acquisition (DAQ) chains (0- 15) related to the V1725S digitizer. These numbers coincide with the numbers printed, respectively, in the front panel of A1422 preamplifiers and V1725S digitizer.

      3.1. A1422 preamplifiers response

      3.1.1. Pulse height response: the IN/OUT relation

      Following Figure 3 scheme, using the 419 ORTEC precision pulse generator [25], pulses of different amplitudes have been transmitted, via both A1422 TEST

      Table 1: Amplification factor (F), of each electronic (A1422) chain and the average value (F) obtained for each module.