Introduction
Advances in techniques for providing radioactive isotope (RI) beams and the construction of high-intensity heavy-ion accelerator facilities [1-5] have opened vast opportunities for experimentally studying and understanding nuclei far from the beta-stability line [6-8], as well as nuclear reactions relevant to astrophysics. In experiments involving reaction targets, the selection an appropriate material, phase (solid, liquid, or gas), and thickness for target is crucial to ensure adequate luminosity and achieve the intended physical goals. The rates of RI beams are usually several orders of magnitude lower than those of stable beams for nuclear spectroscopy, wherein charged particles are measured. Hence, employing sufficiently thick reaction targets while attaining moderate energy resolution is crucial. However, such requirements are usually difficult to fulfill with ordinary solid or liquid targets because the uncertainties in reaction vertices and energy straggling impair the resolution.
An active-target time projection chamber (TPC) [9, 10] with a typical reaction gas thickness of 10 – 100 mg/cm2 provides the high luminosity necessary for nuclear spectroscopy using RI beams, with little or moderate loss of resolution. Because the reaction gas in an active-target TPC also serves as a detector medium, the reaction vertices of the reaction-induced charged particles can be determined by tracking the TPC. Because of these advantages, active-target TPC scores [11-19] have been established for experiments [20-23] with RI beams. Recently, a prototype active-target TPC with 1024 channel readouts, named the multi-purpose active-target time projection chamber for nuclear astrophysical and exotic beam experiments (MATE-1000) [24], was developed at the Institute of Modern Physics of the Chinese Academy of Sciences (IMP-CAS). This innovative system was applied to measure the 12C+12C fusion reaction at stellar energy levels [24] and energies above the Coulomb barrier [25]. Building on earlier success, a larger version of MATE with approximately 4000 channel readouts, hereafter denoted as MATE-4000, was constructed. The sensitive volume of the MATE-4000 was 300(L)×300(W)×200(H) 3. To detect the reaction-induced charged particles that penetrate the MATE active volume, a charged particle detector array, called the MATE particle detector array (MATE-PA), consisting of 20 single-sided strip silicon detectors, was designed and constructed. The MATE-4000 software (Fig.1(a)) has been configured using a 14N beam incident on 4He and employed in experiments to measure the inelastic alpha scattering of 11C and scrutinize the possible Z=6 magic number [26] on the proton-rich side.
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In this paper, we report on the construction and performance tests of the MATE-PA. Section 2 describes the design concept, geometry, and readout electronics of the array. To assess the performance of the detector array, an experiment was conducted using 14N+4He-induced reactions. Monte Carlo simulations were performed to estimate the geometrical acceptance of the MATE-PA. The details of the experiment, along with the experimental and simulation results are provided in Sect. 3. Prospects for experiments using MATE-4000 in combination with MATE-PA are also presented. Conclusions and prospects are presented in Sect. 4.
Charged Particle Detector Array for MATE
In this section, the design concept and specifications of the MATE-PA are presented. The array is designed to achieve the largest possible angular coverage within MATE-4000. Detailed descriptions of the MATE-4000 and its performance have been provided in a previous paper.
Detector array
The particle-detector array consists of 20 single-sided strip silicon detectors produced by Hamamatsu Photonics (Serial nos. S10938-6734 and S10938-6735), mounted in a single-layer 2× 3 matrix on both sidewalls and double-layer 2× 2 matrix on the front wall of the target chamber. Each silicon detector had eight strips with widths of 11.275 mm, strip gaps of 0.1 mm, and sensitive areas of 91 mm × 91 mm; the size of one silicon detector was 100 mm × 100 mm. The side silicon detectors had nominal thicknesses of 600 μm, while the front and rear forward silicon detectors had thicknesses of 150 μm and 600 μm, respectively. The upper (lower) rows of the right- and left-side silicon detectors were labeled as RUi (RDi) and LUi (LDi), respectively, where the subscript i indicates 1, 2, or 3. The forward detectors consisted of two layers of Si detectors, labeled FFj and FSj, where the subscript j indicates 1, 2, 3, or 4, as shown in Fig.1(b). For simplicity and to reduce the risk of electric discharge, we mounted the silicon detectors close to the flanges. The distance from the RUi (RDi) and LUi (LDi) Si detectors to the field cage was set as 13.35 cm (14.00 cm), while that from FFj (FSj) to the field cage was set as 17.80 cm (19.15 cm). The silicon detector array covered approximately 10.6% of the total (4π) solid angle in the laboratory frame, on the assumption that the reaction point was at the center of the TPC.
Strip silicon detectors with moderately large strip widths were chosen instead of large-area single-channel or microstrip silicon detectors is to achieve sufficient energy resolution while maintaining a manageable number of readout channels. The current strip width is sufficient because the silicon detector array will be used together with the TPC, which can provide track information, and the maximum rate of particles hitting the silicon detectors is expected to be of the order of a few hundred particles per second.
To protect the Si detectors from possible electric discharges, an electrostatic shield layer (Fig.1(a)) was placed between the MATE field cage and each side of the Si array. The shields were fabricated by soldering Cu-Be wires with diameters of 100 μm at a constant distance of 1 cm on the copper-clad surface of a glass epoxy frame. Electrical grounding was achieved by connecting the shields to a vacuum chamber. Because no electric discharge was observed during the commissioning experiment, the distances were optimized for future experiments to improve geometrical acceptance. For instance, the geometrical acceptance can be increased by approximately two times by reducing the distance between the silicon detectors and field cage from the current distance of approximately 15 cm to 5 cm.
Electronics and data acquisition system
The output signals of the silicon detectors were fed into 16-channel integrated charge-sensitive preamplifiers, designed by the China Institute of Atomic Energy. The preamplifier, named the smart preamplifier (SPA) [27], was designed as a compact and low-cost preamplifier with a fast response and low power consumption. Its smaller volume allows it to be placed closer to the detectors, leading to a reduction in the environmental noise and the achievement of a performance similar to that of other commercial preamplifier modules. Owing to the limited space in the MATE chamber, the SPA modules were mounted outside the flanges.
Every 16 readout signals from the SPA module were fed to a Mesytec MSCF-16 module before being fed to an analog-to-digital converter (ADC; Mesytec MADC-32 module). The Mesytec MSCF-16 module included 16-channel shaping and timing filter amplifiers, along with constant fraction discriminators. The timing signals from the MSCF-16 modules were sent to a time-to-digital converter (TDC; CAEN V1190A module). A typical electron circuit diagram is presented in Fig. 2. The VME and general electronics for TPC (GET-based data acquisition systems were employed for the data readout from the silicon and TPC detectors, respectively [28, 29]. Because the GET system acquires at a much slower rate (approximately 100 Hz for MATE), only reaction events were selected to trigger GET acquisition. In the current experiment, OR signals from the MSCF-16 modules and GET system were fed to a gate generator (ORTEC GG8020) to provide gate signals for the VME and GET systems. Therefore, both DAQ devices started simultaneously and operated at the same rate.
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Experiment
To evaluate the performance of the Si detector array, we performed an experiment at the RIBLL [30, 31] and at the heavy ion research facility in Lanzhou (HIRFL) [32, 33], operated by the IMP-CAS. A 14N primary beam with an energy of 117.6 MeV provided by a sector focusing cyclotron (SFC) was transported to the RIBLL. To reduce the energy of 14N and stop it in the TPC chamber, Al plates with thicknesses of 39.2 and 60.6 μm were placed at the momentum-dispersive focal plane C1 and final achromatic focal plane T2, respectively. The resulting 14N beam, with a total energy of about 36 MeV and an intensity of 105 pps, was injected into the gas chamber of the TPC, which was filled with a gas mixture of 95% 4He and 5% CO2 at 500 mbar. The gas mixture served as both the target and detection medium. Charged particles generated by reactions between 14N and the target nuclei were tracked and detected by the TPC. Some of the charged particles that penetrated the TPC were detected by the Si detectors. The data was primarily obtained using a Si ⋃ TPC trigger. To prevent the beam from triggering the TPC, the TPC sensitive area was divided into two regions: low- and high-gain regions. The low-gain region had a width of ± 1.7 cm around the beam axis for the detection of beam particles, and high-gain regions were located on both sides for the detection of reacted particles and TPC trigger generation.
For simplicity, the outputs of every four neighboring channels of the Si detectors on the left and right sides were summed into one channel, thereby reducing the number of electronic readout channels without significantly affecting the energy resolution. All the Si detectors were energy calibrated using a triple-nuclide alpha source consisting of 239Pu (5.147 MeV), 241Am (5.486 MeV), and 244Cm (5.805 MeV), prior to the experiment. The typical energy resolution (σ) was approximately 0.8% at an alpha particle energy of 5.5 MeV.
Particle identification
Forward region
Reaction-induced charged particles emitted in the forward direction were detected using FF and FS silicon detectors. Figure 3(a) shows the typical particle identification plot obtained from the measured energy deposited in the first (ΔEFF) and second layers (ΔEFS) of the forward silicon detectors. Three different loci corresponding to protons, deuterons, and α particles were clearly observed. We eliminated overflow events and events caused by accidental triggers by considering the tracking information from the TPC.
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To better understand the data, simulations using the MATEROOT program [34] were performed by assuming 4He(14N, p), 4He(14N, d), and 4He(14N,α) reactions and considering the full geometry of the detector setup, incident 14N beam energy, and 4He+CO2 gas mixture in the TPC chamber. MATEROOT is a FairRoot [35]-based data analysis and simulation platform built, on CERN’s open-source data analysis software ROOT [36] and the GEANT4 simulation framework [37], developed specifically for experiments with MATE. The simulation results (green) are compared with the experimental data (blue) in Fig. 3(a). Deviations between the experimental and simulation data were observed, and they were attributed to the insufficient energy calibration and inappropriate extrapolation along the energy-deposit axes. To eliminate these deviations, we considered the turnaround points of proton loci, which correspond to protons that penetrate the second-layer silicon, in addition to the triple-alpha-source data. The turnaround point was determined by searching for the crossing between the two curves obtained by fitting the loci of the protons that penetrated and stopped in the second-layer silicon. Four data points, which included the turnaround point and three data points from the triple-alpha source data, were used to recalibrate the energy-deposit axes. The resulting experimental loci after recalibration and simulation data are shown in Fig. 3(b). The energy resolution of the Si detectors was deduced to be approximately 1% (σ) for an energy loss of approximately 10 MeV caused by the α particles. The results demonstrated sufficient particle identification for light-charged particles. To further discriminate protons from the energetic deuterons that punch through both silicon layers, the second-layer silicon must be replaced with a thicker detector such as CsI(Tl) or gadolinium aluminium gallium garnet (GAGG).
Side region
Under the current experimental conditions, most of the light-charged particles that penetrated the TPC and headed towards the side silicon detectors stopped in the single-layer silicon detectors. Figure 4 shows a typical scattering event recorded by a detector. The z axis in Fig. 4(a) is defined as the beam axis, whereas the y axis is defined perpendicular to the beam direction. z=0 cm represents the upstream side of the field cage or entrance of the TPC region sensitive to the incident beam. The rectangles at the top and two sides of the figure represent the silicon detector strips. The same event in the x (drift direction)-z plane is shown in Fig. 4(b). The recoil α particles and residual 14N particles were observed simultaneously; the α particles penetrated the TPC sensitive area and hit the highlighted group of strips (yellow), whereas the 14N particles stopped in the sensitive area. The charge and track information of the incident 14N beam particles were not recorded for this reaction because of the space-charge effect in the low-gain beam region.
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For these events, particle identification (PID) can be performed using the charge (ΔETPC) and track length (denoted as L) obtained via the TPC and energy deposited in the side silicon detectors (ΔESi). The ΔETPC values were calibrated using the energy-loss curves simulated using the GEANT4 program. Typical ΔETPC–L and ΔETPC–ΔESi+TPC scatter plots are shown in Fig. 5. For a clearer identification of the light and heavy particles, the ΔETPC axes of the PID plots are shown on a logarithmic scale.
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Raw data plotted without the response of the front silicon detector are shown in Figs. 5(a) and (b), where the two main components can be identified as particles with atomic number Z > 2 and light particles with Z ≤ 2. In Fig. 5(a), the expected light-charged particles with relatively high energy penetrated the TPC sensitive area from approximately 10 to 28 cm depending on the emitted angles, and some of them were detected by the side silicon detector, as shown in Fig. 5(b).
The background can be mainly attributed to the electronic noise of the TPC and possible electrons induced by the beam particles within the beam region around the center of the TPC-sensitive area, along the beam direction. The electronic noise of the TPC can be eliminated by limiting the number of hits on the anode pad (e.g. 10≤Nhits≤200) of the TPC. The electrons induced by the beam particles can be filtered out by applying geometrical constraints based on the kinematics of the two-body reaction, that is, by positioning the first and last points of the trajectories within the high-gain region and selecting appropriate track angles and directions for the two particles. A typical “good" event is shown in Fig. 4. A clear PID plot was obtained after applying the aforementioned conditions, as shown in Fig. 5(c) and (d). The backgrounds, especially the low ΔETPC components, were removed, resulting in a clear separation between 14N and the α particles. For the α particles that stopped in the TPC, as shown in Fig. 5(c), energies as low as a few hundred keV were detected, consistent with the MATE-1000 results [24]. Owing to the MATE-PA, the detection energy range was extended to approximately 15 MeV for α particles, as shown in Fig. 5(d). In this experiment, protons and deuterons were barely detected owing to the insufficient gain of the TPC. The clump below the 14N band, at approximately L = 20 cm in Fig. 5 (a) and (c), is due to a malfunctioning channel in the GET electronics, which quenched the collected charges in a region close to the beam.
The results demonstrated the feasibility of identifying and separating the scattered light particles and residual heavy particles using the MATE-PA, incorporating the tracking information obtained with the TPC. Owing to the tracking information, the angles of the particles were also extracted as shown in Fig. 6, which can be useful for further analyses to refine particle identification.
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For low-energy charged particles that stop in the TPC, one must rely on the range information for particle identification. Another good alternative is to insert the TPC into a solenoid or dipole magnet and identify the particles based on their curved paths in a magnetic field [14].
Geometrical acceptance
For experiments that require measurements of absolute differential cross-sections, determining the solid angle or geometrical acceptance of the detectors is crucial. To estimate the geometrical acceptance, simulations using MATEROOT, considering the full geometrical setup, incident beam energy, gas medium, energy deposit of the beam, and reaction-induced charged particles in the detectors, were performed. For thick-target experiments, where the reaction energy changes drastically as the beam particles travel along the TPC, simultaneously iterating the geometrical acceptance simulation by considering the determined differential cross-section is essential. However, for simplicity, we assumed isotropic alpha scattering in this study.
For reactions within a very small range of excitation energies, the geometric acceptance can be represented as a function of the polar angle in a center-of-mass system. The simulated geometrical acceptance of the prototype MATE-PA for 14N+4He elastic scattering is shown in Fig. 7(a). Geometrical acceptance (ϵ) is defined as the ratio of silicon responses (total number of responses of 0 or 1 per reaction event) to the number of reaction events generated within a subtended polar angle (θ-Δθ and θ+Δθ), where θ is the polar angle, and 2Δθ is the bin size. Only events in which the TPC that responded to the recoil α particles (L > 5 cm) were considered. We assumed a uniform distribution of reaction vertices within the active volume of the TPC, and the reaction region was limited to the domain z = [0, 10] cm. For a more intuitive understanding, the geometrical acceptance and solid angle (in steradian) are shown as functions of the polar angle in the laboratory system in Fig. 7(b) by the left and right axes, respectively. The angular dependence of the geometrical acceptance primarily arises from the layout of the silicon detectors.
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To evaluate the dependence of the geometrical acceptance on the reaction vertex position, we divided the TPC sensitive area into ten parts along the z-axis. For each part of the sensitive area, we obtained an angle-integrated geometric acceptance. The simulated result for the 14N+4He elastic scattering is shown in Fig. 7(c), where the shaded region represents the TPC sensitive region.
The average total geometric acceptance was estimated to be approximately 6.0% for the 4He(14N,α)14N reaction. The energy of the 14N beam particles decreased quickly, and no reaction occurred at the end of the sensitive area. This led to a reduction in the overall geometrical acceptance. However, the actual geometrical acceptance was expected to be higher. Based on an analysis of the experimental data, some of the reaction events occurring at the so-called “dead region" between the gas entrance window and field cage were also recorded. Therefore, in the present simulation, the reaction vertex position was expanded from the TPC sensitive area to the entire gas chamber. The results show that the reactions occurring near the front of the TPC sensitive area contribute to a sizable geometrical acceptance. Hence, the MATE and MATE-PA can be used to measure the reactions occurring both within and outside the TPC sensitive area. Moderate resolution can still be achieved with careful track reconstruction and proper track extrapolation. This aspect is useful for thick target experiments that measure the excitation function using a low-energy beam.
For comparison, a simulation was also performed for the elastic alpha scattering of a 12C beam with an energy of 75 MeV/nucleon, incident on a TPC filled with a gas mixture of 95% 4He and 5% CO2 at a gas pressure of 500 mbar. The results shown in Figs. 8(a)–(c) are similar to those shown in Fig. 7. The average total geometrical acceptance was estimated to be about 12.8% from Fig. 8(c).
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The addition of the MATE-PA increased the dynamic range of the kinetic energy of the charged particles, improved particle identification, and opened up broad opportunities for experiments measuring nucleon-transfer reactions, elastic and inelastic scattering, charged-particle decay, and evaporation.
Conclusion and future prospect
A prototype particle detector array consisting of 20 single-sided strip silicon detectors was constructed for use in MATE-4000 to achieve large-angle and wide-energy-range measurements. The detector system was commissioned at the HIRFL-RIBLL, using a 36-MeV, 105-pps 14N beam incident on MATE-4000, which was filled with a gas mixture of 95% 4He and 5% CO2 (500 mbar). The test results demonstrate good separation of light-charged particles by the forward double-layer silicon detectors of the MATE-PA. The energy resolution of the Si detectors was deduced to be approximately 1% (σ) for an energy loss of approximately 10 MeV caused by α particles. Combined with the MATE-PA, MATE-4000 can offer significant opportunities for experiments in nuclear astrophysics and RI beam physics in future.
Recently, the readout cables of the MATE-PA have been replaced with Kapton ribbon cables to improve the flexibility and work efficiency. The use of ribbon cables also helps reduce environmental noise. For readout electronics, we used SPA and MSCF modules, incorporating a VME-based ADC for data acquisition. In future, we plan to migrate the GET system, used for the readout of TPC signals. An upgrade of MATE-PA with additional detectors, such as CsI(Tl) or GAGG, is also under consideration to accommodate a wide variety of experiments.
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