ISSN 10628738, Bulletin of the Russian Academy of Sciences. Physics, 2014, Vol. 78, No. 5, pp. 350–354. © Allerton Press, Inc., 2014. Original Russian Text © S.V. Zuyev, E.S. Konobeevski, M.V. Mordovskoy, 2014, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2014, Vol. 78, No. 5, pp. 532–536.
A Data Acquisition System Based on Digital Signal Processors for a Setup with the Detecting of Coincident Events S. V. Zuyev, E. S. Konobeevski, and M. V. Mordovskoy Institute for Nuclear Research, Russian Academy of Sciences, Moscow, 117312 Russia email:
[email protected] Abstract—A setup is described for detecting several charged and neutral particles in coincidence and deter mining their energy and time parameters. The CAEN DT5742 (DT5720) desktop digitizer based on a digital signal processor is chosen as a central element of the data acquisition system. Using measurements of the parameters of secondary particles in the d + 2H → 2He + 2n reaction as an example, it is shown that this con figuration allows all necessary information to be collected using a minimal set of electronic units, while the processing of digitized signals offers broad opportunities for interpreting the obtained data. DOI: 10.3103/S1062873814050232
INTRODUCTION Investigations of NN interaction in reactions with fewnucleon systems were performed recently at the Institute for Nuclear Research, Russian Academy of Sciences. A basic condition of these experiments is the detection of a charged particle and a neutron in coin cidence. In measuring when several particles are detected, the need arises to obtain a large amount of data on the amplitude and time parameters of signals obtained from different detectors. This requires a con siderable amount of analog and digital equipment, leading to significant expenditures of time (including accelerator time) for equipment adjustment and debugging, and to large difficulties in transferring the acquisition system to other setups. In recent years, various companies have manufac tured digital waveform analyzers or digitizers in the form of digital pulse processors (DPP). As was noted in [1], digitizers enable us to have a compact mobile system for data acquisition, to arrange it near the detecting apparatus, and to operate with minimal dead time. In our work, this acquisition system employed DT5742 [2] and DT5720 [3] digitizers (CAEN, Italy). Multichannel units allow us to receive signals immediately from preamplifiers or photomultipliers of detectors, to digitize simultaneously signals from all detectors, and to transform them to a packet of reduced data for transfer to a main computer. Digitiz ers replace several analog and digital modules: shapers, amplifiers, peak detectors, amplitudetodig ital converters (ADCs), chargetodigital converters, discriminators, and so on. The buffer memory avail able in these devices allows considerable reductions in dead time. Signal processing is feasible in the online mode using algorithms executed in reprogrammed control microprocessors.
The operating principle of a digital signal processor is the same as that of a digital oscilloscope: an input signal is continuously digitized by a parallel analogto digital converter. Upon the arrival of a triggering sig nal, a certain number of events (corresponding to a predetermined time range of acquisition) is stored in the buffer memory. Differences include the use of (i) an ADC looped datastorage system, from which the buffer memory is filled for the transfer of individ ual events; (ii) a programmable mode of online pro cessing; and (iii) relatively cheap single channel of recording. The multichannel design of these units in combination with the inwardly reconfigurable arrangement of coincidences is especially important for experiments with coincident events. To verify and debug the procedure for making these measurements and determining the basic parameters of the detector setup, test investigations of the d + 2H → 2He + 2n reaction were performed with the recording of both charged 2He particle (two protons) and neu tron. To obtain the required information on this reac tion, we needed ⎯to obtain for a charged particle data on the energy loss in the ΔE and E detectors, to select events according to the particle type using a ΔE–E diagram, to separate the events of the twoproton passage through the ΔE–E telescope, and to determine the energy released in the telescope (the 2He energy); ⎯to obtain for a neutron data on its time of flight from the target to the neutron detector (with coinci dence of the neutron signal and the twoproton signal in the ΔE–E telescope). Our kinematic simulation results showed that to reconstruct the full kinematics and collect data on the energy of the quasibound state of two neutrons with good accuracy, the following system parameters are necessary:
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Fig. 2. Example of signals recorded by DT5742. Curves 1, 2, and 3 denote the signals from the ΔE, E, and neutron detectors, respectively. Dashed line 4 shows the shape of a signal from a gamma quantum; time intervals 5 and 6 are typically selected for analysis via pulse shape.
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Fig. 1. General layout of the setup: (1) scattering chamber; (2) CD2 target; (3) neutron detector; (4) ΔE detector; (5) E detector; (6) preamplifiers of Si detectors; (8) main computer; the other units of the data acquisition system scheme: using DT5742: (7) DT5742; using 5720: (9) fast amplifier, (10) spectrometric amplifier, (11) time shaper, (12) delay line, (13) timetoamplitude converter (TAC), and (14) DT5720.
⎯a relative energy resolution for the charged parti cle no worse than 0.01 (for separating sorts of particles using ΔE–E diagrams), ⎯a relative neutron energy resolution no worse than 0.02 (corresponding, e.g., to a time resolution of 0.5 ns with a timeofflight base of 1.5–2 m), ⎯the feasibility of neutron–gamma discrimina tion of events in the neutron detector with a discrimi nation figure of merit (FOM) no worse than 1. Figure 1 shows a schematic of the experimental setup. The 15MeV deutron beam of the accelerator at the Skobel’tsyn Institute of Nuclear Physics, Moscow State University, was used. The 10–20 nA beam was incident on a target of deuterated polyethylene 2 mg cm–2 thick. Two protons were recorded by a ΔE–E telescope arranged at an angle of 30°. A fully depleted surfacebarrier silicon detector ~25 μm thick (ORTEC, United States) was used as ΔE detector, while an ORTEC silicon detector 100 μm thick was employed as E detector. The dis tance between the detectors was ~2 cm. Neutrons were detected by an EJ301 liquid hydrogencontaining scintillator (an analog of the NE213 unit) at an angle of 34° corresponding to the kinematics of the twopar ticle d + 2H → 2He + 2n reaction. Signals from the sil icon detectors were supplied to ORTEC H242A hybrid preamplifiers with two output signals, timing and amplitude. The bias voltage was supplied by a CAEN SP5600 unit. The energy of charged particles was determined from their energy losses in the ΔE–E
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telescope; the neutron energy⎯from the time of flight with a base of ~2 m. The DT5742 digitizer has the following character istics: number of inputs, 16 + 1; maximum sampling rate, 5 × 109 samples per second (timebase step, 0.2 ns; 1024 channels); amplitude resolution, 12 bits; buffer memory, 128 events. The similar parameters for DT5720 were four input channels, 250 × 106 samples per second (step, 4 ns; 16000 channels), 12bit resolu tion, and 1250event memory. Figure 1 presents two versions of the data acquisi tion scheme. The first acquisition system was assem bled on the basis of DPP DT5742 (containing appara tus units 6, 7, and 8 only). The signals were supplied to the DPP inputs directly from the photomultiplier of the neutron detector, and from the time outputs of preamplifiers H242A of the silicon detectors. Record ing of the suppliedsignal oscillograms was triggered by the operation of a TR0 internal discriminator upon the arrival of a signal from the ΔE or E detector. The accessible time range of recording was ~200 ns (1024 channels with a step of 0.2 ns). Digitized signals were written into the buffer memory; after it was filled, they were transferred to the main computer. Figure 2 shows an example of recorded oscillograms. When using DPP DT5720, four oscillograms were collected and processed. These were oscillograms of the signals from the timetoamplitude converter (TAC) (whose signal is proportional to the time differ ence between the signals of the ΔE and neutron detec tors) and the signals from the amplitude channels of all three detectors. Data processing in both versions of our experiment (DT5742 and DT5720) was performed only in the off line mode. It included determining the pulse ampli
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QL − QS , QL where QS is the sum of digitized amplitudes in a short time interval, while QL is the sum of digitized ampli tudes over a long time interval encompassing the entire slow component of the fall. The time intervals for determining this function (denoted by 5 for QS and by 6 for QL) are indicated in Fig. 2. Events corresponding to the neutrons or gamma quanta recorded in the neu tron detector can be selected by analyzing a two dimensional pulse amplitude–PSD diagram. Two individual domains form in it that determine a parti cle’s type. The silicon detector signals from the time output of the H242A preamplifiers with rise times of less than 5 ns and falling edge times of less than 100 ns corre spond to the DT5742 time range and allow good time resolution to be achieved. In this case, the dependence of the amplitude of these signals on the input energy is nonlinear in the range of our interest. To achieve better energy resolution, another version of the setup with DPP DT5720 was assembled and measuring was per formed. The DT5720 time interval was considerably longer: 16000 × 4 ns, permitting digitization of the sig nals from the preamplifier spectrometric outputs. In order to shape these signals into the standard quasi Gaussian form (with rise time on the order of 1 μs) and to achieve the required time resolution, additional units were used: a TAC for determining the difference between time of arrival of a charged particle and a neu tron with an error of no more than 0.5 ns, fastsignal amplifiers and time discriminators for forming the sig nals needed for the TAC, amplifiers in the slow chan nels of the Si detectors, and power supplies for the additional equipment. Figure 3a displays a twodimensional diagram of the difference tE – tΔE obtained in processing coinci dence events in two detectors (ΔE and E) for the scheme using DT5742. Oscillogram recording was triggered by the signal of the E detector. The time markers (in the channels of the ADC, the scale factor of a channel was 0.2 ns) were obtained from the signal oscillograms via constant fraction discriminator pro cessing. The overwhelming majority of recorded events resided in domain 1 and corresponded to the minimum (negative) time shift between the signals from the E and ΔE detectors, i.e., to the real passage of a single charged particle through the telescope. Ran dom coincidences can be estimated from vertical domain 2, where a spot with a positive time shift cor responds to the coinciding of signals in the E detector with signals in the ΔE detector from particles of the next accelerator beam pulse (time between pulses, 100 ns). Diagram analysis (see Fig. 3a) thus allows us to visualize the entire timing picture of particle recording in the E and ΔE detectors, and to separate PSD =
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tudes and areas, obtaining the times of signals arising in the detectors (constant fraction discrimination, pulse shape approximation, and so on), digital analysis of pulse shapes for discrimination between events for different particle types, selecting coincident events, and obtaining the final energy and time spectra. Events induced by neutrons and gamma quanta can be discriminated for the neutron channel in the acqui sition system based on DPP DT5742, due to the dif ference between the pulse shape for a neutron and a gamma quantum recorded in the liquid scintillator. The neutron signal has a slower pulse fall. Dashed line 4 (Fig. 2) near neutron signal 3 shows the pulse shape for a gamma quantum. By selecting different
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N, events Fig. 4. Twodimensional ΔE–E diagram using the (a) DT5720 and (b) DT5742 digitizers. The amplitudes of signals from amplitudetodigital converters can be seen: those of the ΔE detector are on the ordinate axis; those of the E detector are on the abscissa axis. Loci 1–4 corre spond to protons, deuterons,3He, and 4He, respectively. Kinematic domain 5 corresponding to twoproton events is marked with solid lines.
the events corresponding to real events. Figure 3b pre sents the corresponding spectrum of time differences between events in the E and ΔE detectors. After separating events coincident in time in the E and ΔE detectors, we must separate events with the recording of two protons. To accomplish this, the two dimensional diagrams of the dependence of ampli tudes of ΔE detector signals on the E detector ampli tudes (Fig. 4) must be analyzed. The loci corresponding to protons, deuterons, 3He, and 4He can be seen in Fig. 4a. The reverse course of the proton and deuteron loci is associated with incom plete absorption of the proton and deuteron energies in the E detector 100 μm thick. Results from our reac tion kinematic simulation with allowance for the pas sage of particles through the detectors and other absorbing layers showed that twoproton events must occur above the deuteron locus and below the 3He locus. This corresponds to domain 5 between solid lines in Fig. 4a. The diagram corresponding to the use of DT5742 and fast outputs of the preamplifiers (Fig. 4b) displays worse energy resolution than that of DT5720, compli cating the separation of twoproton events. We emphasize that these diagrams correspond to coinci dent events in the detectors of the charged and neutral particles.
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Fig. 5. (a) Spectrum of total energy ΔE + E for selected events of recording two protons (DT5720); the dashed anddotted line gives the simulation results. (b) Timeof flight spectrum of neutrons for selected events 2He – n. The arrow indicates model calculations.
The silicon detectors were calibrated both with α sources and from bend points of the proton and deu teron loci for the detector 100 μm thick. With allow ance for energy calibration, absolute values of energy losses in the ΔE and E detectors and the total energy released in the telescope were obtained. In Fig. 5a, the total energy spectrum is compared to the results from simulating the energy losses of two protons from the
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d + 2H → 2He + 2n reaction under the test experi ment’s conditions for experimental points from domain 5, marked by the curves in Fig. 4a. Good agreement can be seen between the experimental and the simulated spectra. Figure 5b shows the timeofflight spectrum of neutrons selected in coincidence with the twoproton signal in the ΔE–E telescope. The arrow indicates the simulated mean timeofflight of neutrons in the d + 2 H → 2He + 2n reaction under conditions of the test experiment: Ed = 15 MeV, Θ 2 He = 30°, and Θn = 34°.
various means of data processing and the advantage of simplicity for the data acquisition apparatus. Our setup based on DPP DT5742 on condition of a good amplitude resolution for fast time signals is at an advantage of minimum amount of additional equipment. The use of DPP DT5720 ensures a good amplitude resolution of detectors; however, it demands that additional units be included in the data acquisition system. The further steps in modernization of this setup will be directed both to employing the preamplifiers with spectrometric fast outputs and to using the synchronous operation of the two processors (5742 and 5720).
CONCLUSIONS Our test experiment confirmed the methodical fea sibility of performing the planned experiments and determining the required energy and time characteris tics of detected charged and neutral particles using the described data acquisition system. It was shown that the use of digital signal processors in setups with the registration of several coincident particles allows the collection of complete information for the analysis of reaction kinematics, offering the opportunity to use
REFERENCES 1. CAEN. Digital Pulse Processing in Nuclear Physics. White Paper WP2081. http://www.caen.it 2. CAEN. Digital Electronic Instrumentation. DT5742 digitizer. http://www.caen.it 3. CAEN. Digital Electronic Instrumentation. DT5720 digitizer. http://www.caen.it
Translated by M. Samokhina
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