Introduction
Recently, the establishment of the Shanghai laser electron gamma source (SLEGS) beamline at the Shanghai Synchrotron Radiation Facility has provided a promising platform for studying MeV-level photonuclear reactions in China [1]. The laser Compton scattering gamma source offers advantages over traditional γ-ray sources, including quasi-monoenergetic, high brightness, and high polarization. Photonuclear reactions, which are characterized by simple reaction mechanisms and clean final-state products, serve as effective probes for nuclear structures and aid in the measurement of key reaction rates in nuclear astrophysics [2-8]. However, the most important photonuclear reaction experiments suffer from the drawbacks of small reaction cross sections and low-energy reaction products.
Active target time projection chambers (AT-TPCs) play a crucial role in low-energy nuclear physics and are considered novel and powerful detector tools [9-11]. They have been widely used in various fields, such as the ACTAR TPC of the Grand Accélérateur National d’Ions Lourds for studying shell evolution [12]; O-TPC of the University of Warsaw for studying cluster structure [13], nuclear astrophysics [14], and exotic decay [15]; MAIKo of Kyoto University for studying shell evolution and cluster structure [16, 17]; CAT-TPC of Peking University for studying cluster structures in unstable nuclei[18]; MATE of the Institute of Modern Physics, Chinese Academy of Sciences for studying heavy-ion fusion reactions at stellar energies[19, 20]; MTPC of the Institute of High Energy Physics, Chinese Academy of Sciences for measuring cross sections of neutron-induced light-charged particle emission and fission reactions [21, 22]; and TexAT of Texas A&M University for studying shell evolution and nuclear astrophysics [23]. AT-TPCs are designed to simultaneously use different gases as targets and detectors, providing nearly 4π solid angle coverage and a low-energy detection threshold. Furthermore, because the target itself serves as the detector, a thicker target does not compromise the energy resolution and detection efficiency, which is particularly beneficial for photonuclear reactions and low-intensity beam experiments.
AT-TPCs are particularly useful for nuclear cluster studies. The study of cluster structures in light nuclei represents a prominent research frontier in nuclear physics [24-31]. A well-known example is the Hoyle state of 12C, which was first posited by F. Hoyle in 1953 to explain nucleosynthesis in stars [32]. Although various studies have indicated the presence of significant cluster components in the ground and low-lying excited states of light α-conjugate nuclei [13, 33, 34], the exact properties and configurations of α clusters remain elusive. Questions remain regarding the distinction between α cluster states and free α particles and whether α clusters adopt a Bose-Einstein condensate state [35-37] or specific geometric configurations [38-40]. Thus, the properties and structures of clusters in α-conjugated nuclei, such as 12C, 16O, and 20Ne, and non α-conjugated nuclei, such as 6Li and 9Be, including the Hoyle state in 12C and analogous Hoyle-like cluster states in other α-conjugated nuclei [37, 41, 42], is an important open question.
Considering these advantages, an AT-TPC, namely, the Fudan multipurpose active target time projection chamber (fMeta-TPC), was built. The fMeta-TPC was designed and constructed with a special focus on the study of photonuclear reactions, particularly the properties of α clusters in the excited states of light nuclei.
The remainder of this article is organized as follows: In Sect. 2 we describe the design of the fMeta-TPC. The features of the readout board are presented in Sect. 2.2, and the design and simulation of the electronic field uniformity of the field cage are presented in Sect. 2.3. An overview of the electronic system and its basic performance characteristics is provided in Sect. 2.4. Offline performance tests of the detector covering the electron drift velocity, electronic field homogeneity, angular resolution, and energy resolution of the TPC are detailed in Sect. 3. Finally, a summary is provided in Sect. 4.
Design of fMeta-TPC
The fMeta-TPC was developed at Fudan University, Shanghai, China. The four main components of the fMeta-TPC are the gas chamber, anode pad plane, field cage, and electronic system. A schematic view is shown in Fig. 1.
Gas chamber
As shown in Fig. 1, fMeta-TPC is housed in a cubic stainless steel chamber with a volume of 600 mm(L)×450 mm(W)×475 mm(H). This chamber is equipped with five removable flanges, located at the bottom and along the four sides, to allow easy mounting and testing of the detectors. On the beam injection side, the flange has a circular entrance window, with a diameter of 15 mm, and four quartz windows. A 0.3 mm thick Kapton foil capable of withstanding a pressure differential of 1 atm is used to maintain isolation between the gas volume and the external environment. Owing to the low stopping power of the gas, charged particles with several MeV energy can easily escape from the field cage. In view of future experimental plans regarding rare isotope beams, the remaining flanges were designed to support auxiliary detectors. For example, ΔE-E telescopes composed of a double sided silicon strip detector and a 50 mm thick CsI(Tl) detector were designed; however, these have not been described in this paper.
Resistive Micromegas
As shown in Fig. 1, the anode pad plane coupled with the field cage is housed by the top flange, and its voltages are supplied through safe high voltage connectors located on the top flange. In this study, the resistive Micromegas detector was used for electron amplification and collection. A photograph and schematic view of the pad plane are shown in Fig. 2. The Micromegas detector was fabricated at the University of Science and Technology of China using the thermal bonding technique [43]. The mesh of the Micromegas has 325 lines per inch with 23 μm diameter wires and a 49% opening rate. The avalanche gap between the mesh and the surface of the printed circuit board (PCB) is approximately 100 μm. A 5 mm thick aluminum plate was glued and screwed to the back of the PCB to increase its mechanical rigidity.
The anode pad plane has a sensitive area of 144 mm×288 mm, consisting of 2048 readout channels. Owing to the characteristics of photodisintegration reactions, the sensitive area is segmented into 32×64 rectangular pixels, which increase in size from the inner to the outer region, as shown in Fig. 2. The pixel size in the x-direction (short side) increases from 2 mm to 6 mm, whereas it remains constant at 4.5 mm in the z-direction (long side). The smaller pixels in the inner region are designed to improve the angular resolution of the detector for short-range particles. The larger pixels in the outer region cause the anode pad plane to cover a larger sensitive area with a limited number of electronic channels. All pixels are read out through high-density (0.5 mm pitch) Hirose FX10A-140P/14-SV1(71) connectors containing two rows of 70 pins each. Further details on the angular resolution tests are provided in Sect. 3.3.
Field cage
The vertical uniform electric field within the sensitive volume of the TPC is formed by the field cage with a volume of 330 mm(L)×180 mm(W) × 180 mm(H). Two versions of the field cage were developed for different experimental purposes: a double-wire-plane field cage and a PCB field cage.
The double-wire-plane field cage was designed specifically for radioactive ion beam experiments, which are known to produce high-energy reaction products. To ensure accurate measurements, designing a field cage that is transparent to both the beam ions and the reaction products while maintaining a uniform electric field is critical. Therefore, the double-wire-plane structure was selected as the most effective approach. This design features five wire planes surrounded by gold-plated tungsten wires soldered together, as shown in Fig. 3. The four-sided wire planes are composed of 70 wires, each with a diameter of 50 μm and a vertical spacing of 5.08 mm. The cathode plane is composed of 30 μm wires with a distance of SI3.05mm between each wire. The horizontal distance from the inner to the outer wire plane is 3 mm for all the wire planes. This design renders the double-wire-plane field cage nearly 99% optically transparent.
The PCB field cage was specifically designed for the photonuclear reaction experiment in SLEGS. Except for the beam entrance plane, which remained the double-wire plane, the other side planes of the PCB field cage were constructed using etched copper plate lines with a diameter of SI1mm. In this design, a 3 mm thick copper-clad PCB with dimensions of 180 mm×330 mm serves as the cathode. The wires in the side plane are connected in series with 10 MΩ (0.1% error) surface mount resistors for both field cages to homogeneously degrade the voltage from the micromesh (GND) to the cathode (-HV). Different drift and avalanche field strengths can be set by adjusting the voltages at the cathode and anode.
To verify the uniformity of the electric field in both the field cages, a finite element calculation was performed using the neBEM code [44], which was integrated into the Garfield++ code [45]. The simulation showed that the field cage maintained the distortion of the electric field within 3% for the double-wire-plane field cage and within 1% for the PCB field cage in the sensitive volume, as shown in Fig. 4.
Readout electronics system
In view of possible future experiments, the electronic systems of the fMeta-TPC should meet requirements such as high integration, compactness, short dead time, low power consumption, and high dynamic range. The fMeta-TPC was designed for 2048 readout channels and future upgrades for higher spatial resolution, which places specific requirements on the high integration and compactness of the readout electronics. High dynamic ranges of the detected energy are also required. For example, in reactions such as 16O(γ, α) 12C, the ionization energy loss ratio between 12C and α can be several hundred times, requiring a wide dynamic range for charge measurement in the electronics.
The key parameters of the electronic system are listed in Table 1. It has a wide dynamic range, from 2 fC to 3 pC, and a short dead time of 25 μs (much less than the dead time of GET electronics) for high event rates [46].
| Parameter | Value |
|---|---|
| Channels | 2048 |
| Dead time (μs) | 25 |
| Sampling rate (MSPS) | 40 |
| Quantization accuracy (bit) | 12 |
| Sampling window width (μs) | 25.6 |
| Dynamic range | 2 fC to 3 pC |
As shown in Fig. 5, the readout system is divided into two components: front- and back-end modules. The front-end module, located at the top of the chamber (as shown in Fig. 1), interfaces with the detector through two Flexible Printed Circuit (FPC) boards.
The front-end module consists of eight groups, each containing the pre-amplifier module (PAM), analog-to-digital module (ADM), and power clock management module (PCMM). Within each group, there are four PAM modules responsible for charge integration, four ADM modules responsible for waveform digitization, and one PCMM module responsible for supplying power to the front-end modules.
Upon receiving a signal from the detector, the PAM initiates amplification. A single PAM board contains 64 channels and consumes approximately 1 W of power. After amplification, the signal is transmitted to the ADM via the FPC board. The ADM then performs the simultaneous digitization of 64 channels of analog signals at a sampling rate of 40 MHz and a quantization accuracy of 12 bits. Each ADM board consumes approximately 8 W of power. Finally, the PCMM provides power and clock distribution to the group of four PAMs and ADMs.
The backend modules contains two parts: a trigger clock module (TCM) and a data concentrator module (DCM). The TCM generates a global trigger and synchronous clock by accepting hit information and distributing the trigger and synchronous clock to the DCM. The DCM is responsible for data acquisition, digital filtering, buffering, and data upload to the server via FPGA. It also distributes the synchronous clock, instructions, and triggers to the front-end electronics. In fMeta-TPC, the back-end modules have two DCMs and one TCM. Each DCM board is connected to 16 ADM boards using fiber optics.
Performance test
This section describes the several methods used to characterize the detector performance. The gain variation of the Micromegas detector was tested using a 55Fe 5.9 keV X-ray source. A UV laser that can ionize gas molecules and form a straight line in the gas was used to study the drift velocity, field homogeneity, and intrinsic angular and positional resolutions of the detector. An 241Am alpha source was used to determine the energy resolution of the detector. In this study, tests were mainly performed in Ar+ CH4 (9:1, P10 gas), Ar+ iC4H10 (93:7), and He+ CO2 (96:4) gas mixtures at different pressures.
Drift velocity
One of the key advantages of a TPC is its ability to record the 3D trajectories of charged particles. The determination of 3D information is based on the product of the electron drift velocity and drift time. Therefore, the accurate measurement of the electron drift velocity in a gas is crucial for particle trajectory reconstruction. In this study, the velocity was determined using a 266 nm laser with a power of 20 mW and a frequency of 7.5 kHz. The laser entered the detector through quartz windows mounted on the front flange of the chamber, as shown in Fig. 1. Prior to the test, the orientation of the laser was determined based on the location of the incident point on the quartz window and the end point of the track in the field cage. Owing to limitations imposed by the maximum processing frequency of the ADM chip and the maximum transmission capacity of the gigabit network. Data acquisition was triggered by an external 100 Hz trigger, and the acquisition window was set to 25 μs. Considering that the pixel size remained constant at 4.5 mm in the z-direction, calculating the relative differences in the drift height of each pad after establishing the laser direction was possible. Therefore, one of the fired pads was chosen as the reference pad, and the drift velocity was determined by calculating the disparity in the drift height and time with respect to this reference pad.
Figure 6 shows the measured drift velocity in different gases compared with the theoretical calculations of Magboltz [44] and other experimental results [19, 47-49]. The measured results in the 600-mbar P10 gas, as shown in Fig. 6 (left), were in good agreement with other experimental results. However, a slight deviation from the theoretical results was observed when E/P<0.23 V/cm/torr. The results shown in Fig. 6 (right) represent the electron drift velocities in 600 mbar Ar+ iC4H10 (93:7) and 500 mbar He+ CO2 (96:4) mixed gases. Both measurement results agreed with the theoretical calculations within an uncertainty of 5%. This indicated that our test method was applicable to the measurement of the electron drift velocity.
Drift field homogeneity
Ensuring homogeneity of the electric field is important for achieving accurate 3D trajectory reconstruction. Ideally, the electric field should be perpendicular to the micromesh, have no horizontal components, and maintain uniform scalar magnitude in the vertical orientation. However, practical considerations introduce several factors that affect the uniformity of the electric field, including wire deformation induced by voltage, feedback from positive ions, and edge effects of the electric field.
To evaluate the homogeneity of the electric field within the sensitive volume, the chamber was filled with 600 mbar of Ar+ iC4H10 (93:7) mixed gases. The cathode voltage was set to -1800 V, resulting in a drift electric field strength of 100 V/cm, while the anode avalanche voltage was set to +350 V. Considering that distortions are more pronounced at the edges, the uniformity of the electric field at the edge of the sensitive area better reflects the quality of the field cage design and fabrication. Therefore, two trajectories at the edges are provided as examples. In Fig. 7, the upper two panels show the measured trajectories of the laser beam along the z-axis at the edge of the sensitive area, with the red lines representing two-dimensional weighted linear fits.
The two lower panels of Fig. 7 present the distributions of the residual distances between the two-dimensional linear fitting curves and the measured positions on the anode pad plane. The residual distance di can be computed as follows:
Angular and spatial resolution
Because UV laser light is free of straggling effects, it can be used to test the intrinsic angular and spatial resolutions of the detector. In this study, θ is defined as the angle between the projection of the track on the readout plane and the z axis. The detector was filled with Ar+ iC4H10 (93:7) gas mixture at 400 mbar. During the measurement, the drift field strength was set to 44 V/cm and the avalanche voltage was set to +345 V. Fig. 8 (top) shows the measured angular resolution of the laser with an incident angle of 6.05. An angular resolution of 0.17 (σθ) was achieved in this incident direction. The incident angle of the laser beam was determined by three-dimensional weighted linear fitting, with the weighted coefficient being the ratio of the deposited charge to the pixel area.
Notably, both angular and spatial resolutions depend on the angle of incidence and the pad structure. Therefore, measuring the intrinsic angular resolutions at different incident angles was necessary, particularly for readout pixels of different sizes, as shown in Fig. 2. Figure 8 (below) shows the measured intrinsic angular resolution with the laser light incident through two quartz windows at different angles of incidence. A remarkable angular resolution of 0.06 (σθ) was achieved at an angle of incidence of 5.6, whereas the worst measured angular resolution remained within 0.30 (σθ) when the laser mainly traversed the pixel with a size of 6 mm × 4.5 mm. For the detector with a sensitive length of 288 mm, the corresponding position resolutions (FWHM) can be derived from 288 ×2.355 σθ. Thus, the optimal position resolution was 0.7 mm and the worst resolution was within 3.4 mm.
Gain uniformity
Calibration of the gain map plays a critical role in energy reconstruction of charged particles. In addition, the uniformity of the detector is primarily influenced by two factors. The primary factor is the uniformity of the avalanche gap, while the secondary factor is the uniformity of the electronic gain.
First, the gain variations in the electronics were tested by applying a common pulse to the PAM board. In each PAM module, all 64 channels were connected to the Sub-Miniature-A connector via 1 pF capacitors with 5% accuracy for gain calibration. The resulting waveform acquired from the 64 channels within the PAM module is visually represented in the top panel of Fig. 9. By adjusting the pulse amplitudes to record the waveform amplitudes for each channel at different collected charge levels, the resulting normalized gain variation distribution for each channel is shown in the bottom panel of Fig. 9. The results showed that the gain variation across the 2048 channels of the electronic system was within 4% (σ/μ).
An 55Fe X-ray source with an activity of 1.43×105 becquerels and a diameter of 10 mm was used to evaluate the gain uniformity of the Micromegas detector. The source was positioned at the center of the bottom flange, at a distance of 20 cm from the micromesh. This ensured that every pixel was illuminated. For the gain uniformity test, the chamber was filled with P10 gas at a pressure of 600 mbar. The anode voltage was set to +465 V, and the field ratio of the avalanche field to the drift field was set to 300, ensuring that more than 95% of the primary electrons could be collected in the avalanche gap at this field ratio.
To validate the results obtained, the gain map was adjusted to correct the 55Fe spectrum measured at the anode. A comparison of the spectrum before and after the correction is shown in Fig. 11. The spectrum before correction was derived assuming a uniform pixel gain. After applying the gain map, the width (σ) of the 55Fe spectrum decreased from 1.03 keV to 0.92 keV, indicating that the gain map obtained by multiple linear regression had some reliability.
Energy resolution
The energy resolution of the detector was further investigated using an 241Am alpha source located 39 mm from the sensitive region. The detector was filled with 600 mbar P10 gas, and the voltages applied to the anode and cathode were set to +405 V and -2430 V, respectively. The total charge deposited by the alpha particles was obtained on an event-by-event basis by summing all the individual charges collected on each pad. To minimize the effect of the dead zone between the alpha source and the sensitive area on the energy resolution, the alpha source had to be collimated by restricting the emission angle of alpha to within ±5 during data analysis. The resulting energy spectrum for the alpha source is shown in Fig. 12. Fitting the spectrum to a single Gaussian distribution yielded an energy resolution of 6.85% for the alpha particles with a deposited energy of 3.0 MeV.
Summary
A new 2048-channel prototype active target detector system, fMeta-TPC, was designed and constructed for low-energy nuclear experiments. In this study, a resistive Micromegas with an avalanche gap of 100 μm was used for signal readout. As verified by an 55Fe X-ray source, the gain uniformity of the detector was approximately 10% (RMS/mean), and the contribution of the electronic gain fluctuations was within 4% (σ/μ). The energy resolution obtained from the total charge collected on the pad plane was deduced to be 6.85% for 3.0 MeV alpha particles.
Considering the characteristics of photodisintegration reactions, the readout plate was divided into 2048 rectangular pixels of unequal sizes. It was tested with laser light at different injection angles to evaluate the effect of pad size on angular resolution. The results showed that the readout board can achieve a remarkable angular resolution of 0.06 (σθ); the worst angular resolution measured was within 0.30 (σθ). In addition, the electron drift velocity and homogeneity of the drift field by the laser light were also tested. The results showed that the homogeneity of the drift field was satisfactorily maintained in the sensitive volume. The measured electron drift velocity was in good agreement with other experimental results and theoretical calculations.
For future low-pressure experiments, the plan is to upgrade the detection system by using a Micromegas detector with a double micromesh gaseous structure [50, 51] or with a larger avalanche gap. In addition, the first commissioning run of the 7Li(γ,t)4He ground-state cross section was performed at SLEGS, and the data are currently being processed.
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. Nucl. Instrum. Meth. A 976,Yu-Gang Ma, De-Qing Fang and Xi-Guang Cao are the editorial board members for Nuclear Science and Techniques and was not involved in the editorial review, or the decision to publish this article. All authors declare that there are no competing interests.
