Introduction
Xenon is an exceptional medium for particle detection due to its high density, large atomic mass, and excellent scintillation properties. The dual-phase xenon time projection chamber leverages the superior properties of xenon and is extensively utilized in dark matter [1-6] searches, neutrino detection [7-11], and related experiments. It is primarily based on the precise reconstruction of scintillation signals (S1) and ionization signals (S2) generated by particles that deposit energy in liquid xenon (LXe). Scintillation photons, detected by photomultiplier tubes (PMTs), generate a pulse signal referred to as S1. The ionization electrons, under the influence of an extraction electric field, drift into the gaseous xenon phase and emit secondary scintillation light through the electroluminescence process, and are then recorded as S2. The spatial coordinates of an event were reconstructed from the patterns of S1 and S2, with photoelectron counts proportional to the energy magnitude of the signal. The geometric variation and inhomogeneous distribution of the electric field and light collection efficiency influence the detector and lead to a significant position dependence of the signal intensities of S1 and S2, which not only reduces the precision of the energy of events and three-dimensional position reconstruction but also weakens the ability to distinguish between nuclear and electronic recoil events [12]. Therefore, it is essential to use a calibration source that can be uniformly distributed in LXe and yield monoenergetic signals to calibrate the detector response.
Owing to its uniform mixing properties with xenon, the 37Ar gaseous source has emerged as an ideal calibration source. The radioactive isotope 37Ar, with a half-life of 35.01 days, can decay to 37Cl and neutrinos [13] via the electron capture process. During this process, the atomic nucleus captures an electron from the K, L, or M shell. The resulting vacancies were filled by outer electrons, accompanied by the emission of X-rays or Auger electrons. The total energy deposition of these processes corresponds to the binding energyies of each shell: 2.82 keV (K-shell), 0.27 keV (L-shell), and 0.01 keV (M-shell), with decay branch ratios of 90.2%, 8.7%, and 1.1%, respectively [14-17]. The energy depositions of the K and L shells were close to the energy threshold of the LXe dark matter detectors, making 37Ar an ideal calibration source. Furthermore, 37Ar can be removed using a cryogenic distillation tower similar to that of 85Kr [18], thereby improving its potential application in detector calibration.
The production of 37Ar has long been a subject of interest owing to its potential applications in various fields, including low-background detection and fundamental nuclear research. In the atmosphere, the primary source of 37Ar is the reaction of fast neutrons produced by cosmic rays, 40Ar(n,4n)37Ar [19]. Although 40Ar constitutes up to 99.60% of natural argon, the cross-sectional effects result in a low yield of 37Ar, accompanied by the production of numerous other radioactive isotopes, particularly long-lived 39Ar, which is highly undesirable. Another method for producing 37Ar involves irradiating 40Ca in calcium oxide (CaO) with fast neutrons [20]. This approach has been commonly used in the past owing to its high yield [21]. However, to facilitate the extraction of 37Ar from CaO, the target material must be prepared in powdered form. Additionally, 37Ar gas was subsequently distilled at high temperatures in a sealed container. This high-temperature distillation process imposes stringent requirements on the technology and equipment involved. Moreover, powdered CaO may be carried along with gas into the xenon detector, causing contamination. Impurities such as radon, which is co-distilled with 37Ar, can also interfere with low-background experiments. Thermal neutron irradiation of 36Ar is an effective technique for preparing radioactive isotopes 37Ar. Although the reaction cross-section for 36Ar(n,γ)37Ar is lower than that for 40Ca(n,α)37Ar, the preparation of the target material is simpler, and the range of products is more limited. This method is particularly suitable for high-sensitivity, low-background experiments such as those used for dark matter detection.
We performed a detailed simulation based on Geant4 to identify the various nuclei expected to be produced after irradiation. In particular, considering the complexity of the energy distribution of the reactor neutron source, we need to avoid producing by-products such as 39Ar which would produce a low-energy electronic recoil background in large-scale LXe detectors and would be difficult to remove. This is because 37Ar gas can be distributed in gaseous xenon at room temperature. We adopted the GXe TPC to measure 37Ar radioactivity.
The remainder of this paper is organized as follows. Section 2 describes in detail the preparation of 37Ar, including the simulation and feasibility assessment. Section 3 shows the measurement results of the activity of 37Ar obtained through the operation and analysis of the gaseous xenon detector.
Preparation of 37Ar calibration source
Experimental Setup and Principles
The target isotope 37Ar was produced by irradiating high-purity (99.935%) 36Ar with thermal neutrons. This process involved sealing 36Ar in a precisely specified quartz ampoule with a diameter of 1 cm, length of 4 cm, and wall thickness of 1 mm. The relative pressure of the package was negative. 37Ar is produced by the neutrons captured by 36Ar. The reactor neutron source [22] generated a thermal neutron flux of 1.5×1013 n/(cm2·s) with an irradiation duration of 2.17 hours. Additionally, because of the intrinsic properties of the neutron source, an accompanying epithermal neutron flux of 6.25×1011 n/cm2/s was present. The uncertainty in the neutron flux measurements was estimated at 5%. Sealing of the quartz ampoule was a critical step in the experiment. A melt-seal technique was used in this process, as illustrated in Fig. 1, we used liquid nitrogen on the bottom side of the quartz ampoule to create a low-temperature environment for enrichment of 36Ar. The other side was sealed with a high-temperature hydrogen torch. This method ensures the airtightness and structural integrity of the seal. Figure 2 shows the quartz ampule in its pre- and post-neutron irradiation states. The transformation of the ampule to a dark purple color was hypothesized to be the result of microscopic structural and chemical alterations induced by irradiation. Neutron irradiation catalyses the formation of color centers within the silicon dioxide matrix. These color centers introduce new energy levels within the electron bandgap, which leads to photothermal absorption. The superposition of various absorption bands results in the creation of absorption maxima, which in turn impart a tinting effect on the vitreous material [23, 24].
Following irradiation, the quartz ampoule was placed within a pressure-transfer apparatus, as indicated by the red arrow in Fig. 3. The apparatus is shown in Fig. 3 was used for the precise recovery of all gases generated after irradiation. The process begins with the evacuation of the apparatus to achieve vacuum, thereby eliminating any extraneous atmospheric influences. The release of the trapped gas was achieved by applying pressure to the ampoule placed in the vacuum chamber via a pressure transfer apparatus with a maximum capacity of 100 N. The gas then diffuses and homogenizes within the system, allowing for controlled and quantified extraction of the gas according to experimental requirements, ensuring both the accuracy and integrity of the sample.
Based on the simulation results (Sect. 2.2), the yields and activities of nuclides such as 37Ar and 39Ar can be determined. Furthermore, the “burn-up” effect [25], which refers to the potential reaction of newly formed nuclides with neutrons to produce other particles, was evaluated. The calculations indicated that the "burn-up" effect was negligible under our experimental conditions.
Thermal Neutron Irradiation Simulation
39Ar is devastating for dark-matter search experiments. Consequently, the mitigation of background signals is essential. To precisely identify the nuclides generated during the production of 37Ar and exclude those with extended half-lives that are difficult to eliminate once they are introduced into the detector, we performed a detailed simulation experiment. The purpose of this simulation was to emulate the actual irradiation conditions and to evaluate the probability of producing other potential nuclides. To achieve this, we established the following parameters for the simulation.
Based on the neutron flux in the reactor, a simulation was performed to ensure that the thermal neutron proportion was maintained at 24/25, and the remaining fraction consisted of epithermal neutrons. All neutrons were introduced randomly from the side to simulate the natural variability of the neutron incidence. To enhance the yield of isotopes other than 37Ar, particularly to amplify reactions with low probabilities during our simulation, we increased the proportion of isotopes other than 36Ar, which serves as the target nucleus for the production of 37Ar. When statistically analyzing the results, we adjusted the proportions to reflect the actual yields and effectively scaled the amplified ratios back. Table 1 presents the composition and mass fractions of all gases before the actual irradiation. This approach allows for a more accurate assessment of the production of nuclides during the irradiation process, ensuring that the sensitivity of the detector to dark matter signals is not compromised by the presence of long-lived background isotopes.
| Isotopes | Mass fractions (%) |
|---|---|
| 36Ar | 99.935 |
| 38Ar | 0.049 |
| 40Ar | 0.004 |
| CH4 | 0.002 |
| CO/N2 | 0.002 |
| O2 | 0.003 |
| CO2 | 0.004 |
| H2O | 0.001 |
Our simulation, as indicated by the data presented in Table 2, provided the cross-sections of the thermal neutron irradiation reaction and the half-lives of the selected argon isotopes [25]. This table lists the cross-sections associated with the
| Isotopes | 36Ar | 37Ar | 38Ar | 39Ar | 40Ar | 41Ar | 42Ar |
|---|---|---|---|---|---|---|---|
| σ (barn) | 5.2 ± 0.5 | 2040 ± 340 | 0.8 ± 0.2 | 600 ± 300 | 0.66 ± 0.01 | 0.5 ± 0.1 | - |
| τ1/2 | stable | 35.01 d | stable | 269 yr | stable | 1.83 h | 33 yr |
| Target nuclide | Generated nuclide | Yield (s-1) | Decay mode | Half-life (τ1/2) | Decay product |
|---|---|---|---|---|---|
| 28Si | 29Si | - | stable | - | - |
| 33S (stable) | (3.95 ± 0.20)×105 | - | - | - | |
| 36Ar | 36Cl | (1.65 ± 0.09)×105 | β-/β+ | 3.01×105 yr | 36S (stable)/36Ar (stable) |
| 37Ar | (4.43 ± 0.22)×108 | ϵ | 35.01 d | 37Cl (stable) | |
| 35S | - | β- | 87.35 d | 35Cl (stable) | |
| 38Ar | 38Cl | - | β- | 37.24 min | 38Ar (stable) |
| 39Ar | (3.08 ± 0.15) × 104 | β- | 268 yr | 39K (stable) | |
| 37S | - | β- | 5.505 min | 37Cl (stable) | |
| 40Ar | 40Cl | - | β- | 1.35 min | 40Ar (stable) |
| 41Ar | (2.02 ± 0.10) × 103 | β- | 109.61 min | 41K (stable) |
During the simulation, specific attention was directed towards two nuclides: 29Si and 41Ar. Although 29Si exhibits a comparatively elevated yield, it is derived from the neutron irradiation of 28Si present in quartz and is not expected to enter the gas source. In contrast, 41Ar, despite its certain yield, has a half-life of merely 109.61 min, indicating that it decays rapidly. Furthermore, the presence of 39Ar, if mixed uniformly with xenon within the detector, poses a challenge for removal, thus significantly increasing the background level of the detector. The simulation results substantiate our rationale for proceeding with subsequent experimental endeavors.
The Gaseous Xenon Time Projection Chamber
Before injecting the 37Ar calibration source into the ton-level detectors, it was injected into a GXe TPC to validate its performance. The detector was operated using gaseous xenon at room temperature. Gas detectors represent a crucial subset of instruments utilized in particle and nuclear physics experiments. Time projection chambers that are made with gas as the medium have many applications in nuclear reactions and particle detection. Xenon is chosen as the detection medium due to its pivotal role in dual-phase time projection chambers (LXe TPCs) used in dark matter and neutrino experiments such as PandaX-4T, XENONnT, LZ and so on. The GXe TPCs have several notable advantages. First, GXe TPC avoids the operational complexities associated with cryogenic and slow control systems. Second, GXe TPCs feature a lower detection threshold and reduced background compared to LXe TPCs because the background is dominated by gamma rays and cosmic muons. Additionally, argon and xenon, as members of the same group in the periodic table, exist in the gaseous phase at room temperature, enabling a uniform distribution within the detector. This uniformity is advantageous for measuring the activity of calibration sources and for facilitating the verification of activity estimations. Although gaseous xenon emits fewer photons than liquid xenon, leading to reduced efficiency in detecting S1-S2 paired events, S2-only analysis can estimate the decay rate with high detection efficiency.
A schematic diagram of the GXe TPC used in this measurement is shown in the top panel of Fig. 4. This TPC served as a prototype detector for the RELICS experiment [11]. The TPC was mounted inside a double-walled cryostat to provide thermal insulation and structural support. It was equipped with 14 Hamamatsu R8520-406 PMTs, which were compactly placed on the top and bottom of the TPC and optimized for high VUV photon detection efficiency. These PMTs operate at a working voltage of -800 V. Each array comprised seven PMTs in a regular hexagonal pattern positioned above and below the drift region. The TPC walls are made of Teflon, which has excellent VUV reflectivity and enhanced light collection efficiency. This arrangement provides relatively high light-collection efficiency and improves the spatial resolution of the detected events.
The bottom panel of Fig. 4 shows the operational principle of the GXe TPC for detecting decay 37Ar. 37Ar decay produces scintillation and ionization electrons in the GXe. The scintillation photons were detected directly by the PMTs as the S1 signal. The ionization electrons drift under an electric field toward the proportional luminescence region, where they emit secondary scintillation light (S2). The top and bottom arrays of photomultiplier tubes (PMTs) capture S1 and S2 signals, enabling precise event reconstruction, including its energy and three-dimensional positions.
The detector system integrates various subsystems, including cryogenic, gas purification, data acquisition, and recycling equipment subsystems. The TPC operates at a pressure of approximately 170 kPa, with gaseous xenon continuously circulated through a hot-getter system for purification. The purification process removes electronegative impurities, such as oxygen and water, which may absorb scintillation light and ionization electrons, reducing the detection and identification efficiency of 37Ar decays. The electron drift region of the TPC is defined by a set of electrodes, including the anode, gate, cathode, and five shaping rings, which establish a uniform electric field for electron drift and convert electrons to proportional scintillation photons. The anode was maintained at a voltage of +1200 V to amplify the S2 signals, whereas the gate, cathode, and screen were set to -1800 V, -2400 V, and -800 V, respectively. This voltage configuration ensures stable operation, minimizes the risk of electrical breakdown, and provides the available conditions for the readout of single-electron S2 signals. This measurement was based on the GXe TPC operation mode to evaluate the radioactivity of the source.
Measurement of 37Ar radioactivity within the GXe TPC
Injection of the 37Ar source
The 37Ar source was stored in a Stainless Steel container with a volume of 500 mL. A dedicated pipeline was developed to allow controlled introduction of a fixed portion of the 37Ar source into the gaseous xenon detector system. A simplified diagram illustrating the injection and gas recycling routes is shown in Fig. 5. This dosing system is designed to allow seamless calibration source injection during detector operation while minimizing the impact on xenon gas purity. The activity of the injected source was calculated based on the volumetric relationships between the pipeline (including the cryostat containing the GXe TPC), storage container, and drift region of the TPC, assuming a uniform distribution of 37Ar. Detailed information regarding the volumes within the injection system is provided in Table 4.
| Component | Volume |
|---|---|
| 50 cm long, VCR-1/2 pipeline | 63.3 mL |
| Source bottle | 500 mL |
| TPC drift region | 181 mL |
| Total system | 28 L |
The 37Ar source is introduced through multiple injections. The circulation pipe enclosed by valves V1, V2, and V3 was defined as the dilution volume for the source injection. Each injection was performed using several steps. First, the dilution volume was pumped into a vacuum. Then, 37Ar was introduced to the dilution volume by opening V1. Consequently, 11% of the total source was introduced into the dilution volume and injected into the circulation. The source was then uniformly distributed in the system with a total volume of
Data acquisition and signal processing
To achieve a high detection efficiency of low-energy signals from the source 37Ar, all waveforms from the PMTs were digitized using CAEN V1725 digitizers, which employ a dynamic acquisition window (DPP-DAW) firmware for self-triggering readout. The digitized raw data were stored on a server, and subsequent event reconstruction and analysis were performed on dedicated analysis servers. Data acquisition was carried out over an 8-hour period both before and after the injection of the 37Ar source, allowing background subtraction. A software package was developed to process the data acquired from each PMT and to group them into peaks. A peak is defined as a waveform that features two or more PMT signals within ~300ns. Scintillation and ionization signals from interactions with energy depositions in the GXe TPC, including decays in 37Ar, produce peaks in the data.
The area of a peak is proportional to the number of photons detected by the PMTs and is expressed in units of photon-electron (PE), as calibrated by single-photon counting with an LED. S1 peaks, induced by scintillation photons produced by direct excitation of the Xe atom or by recombination of electron and ion pairs from ionization, have a narrow distribution in time with a typical spread below ~200ns. S2 peaks, induced by the electroluminescence of the electrons drifting in GXe at a strong electric field (notably between the Gate and Anode electrodes), have a wider distribution in time with a typical spread above ~200ns. The time spread of a peak is characterized by the leading time, defined as the time interval between the 0% and 50% percentiles of the waveform area. The relative peak area distribution on the PMT arrays depends on the light collection efficiency of each PMT, and is used to reconstruct the position of an interaction. For the S2 peaks induced by interactions in the drift region, the horizontal distribution was reconstructed from the area distribution pattern on the top PMT array. S2 peaks can also be produced above the anode or below the cathode, because the detector is operated in the GXe mode. The area fraction of the top (AFT), which is the ratio of the area recorded by the top PMTs to the total area, is distinguishable for the S2 peaks produced in the drift region and below the cathode or above the anode.
The distribution of the peaks in the area and leading-time space is shown in Fig. 6. The peaks collected before and after injection of the 37Ar source are shown in the top and bottom panels of Fig. 6, respectively. The pulses with a leading time above the dashed red line and an area greater than 100 PE are attributed to beta or gamma interactions within the drift region of the GXe TPC. pulses with an area of ~ 20 PE and a leading time of ~ 700 ns characterize S2 produced by single electrons drifting between the gate and anode. pulses with an area below 500 PE and a leading time below the dashed red line correspond to S1s.
Some additional populations appear after the injection of the 37Ar source: signals with an area of approximately 2000 PE correspond to S2s from the K-shell 37Ar electron capture events in the drift region. Pulses with an area of approximately 200 PE correspond to S2s from the L-shell 37Ar electron-capture events in the drift region. Pulses with an area below 10 PE and leading time below the dashed red line correspond to S1s from the K-shell 37Ar electron capture events. The identification is based on the known energy spectrum of 37Ar, in which the K-shell and L-shell electron capture lines are at approximately 2.8 keV and 0.27 keV, respectively. Given the W-value of gaseous xenon (approximately 22.0 eV) and a single-electron gain of approximately 20 PE, the expected S2 area for K-shell events is approximately 2000 PE, and the L-shell contribution is approximately one-tenth that, which matches well with the observed populations.
In this study, we focus on the signals corresponding to 37Ar K-shell decay events that occur within the region between the drift regions. The events detected outside this region were classified as background events. To suppress these background events, it is necessary to know properties such as the light collection efficiency distribution and electron transport processes, which have not been thoroughly simulated, and the photon detection efficiency of PMTs remains insufficiently understood. These factors introduce constraints in the accurate analysis of signals. Consequently, a data-driven analysis approach was used to reduce the background and estimate the activity of the source 37Ar. This method compensates for the lack of comprehensive detector simulations and allows evaluation of 37Ar source activity.
The analysis focuses on the S2 signals, represented by the regions above the dashed red lines in Fig. 6. Accurately determining the activity of 37Ar requires meticulous data selection to minimize the impact of background noise. As shown in Fig. 7, three different types of background noise are identified and removed.
First, events occurring between the anode and gate exhibited a positive correlation between the S2 area of these background signals and their leading time. These events were located in the lower region of the distribution shown in the top panel of Fig. 7, indicating a relationship between the event timing and background signal intensity. Second, when the photomultiplier is set to -800 V with a positive anode voltage, the ionized electrons generated by high-energy events can drift toward the anode under the influence of the electric field between the anode and top PMT array. This drift results in peaks with a larger proportion of top PMTs. Similarly, events that occur between the cathode and screen tend to produce a signal with a smaller area fraction of the top. Furthermore, some pulses exhibited reduced light collection efficiency in specific regions, which appear on the left side of the distribution in the top panel of Fig. 7. To correct for this bias, a crystal-ball model was employed to describe this phenomenon and fit the signal count.
These background events are effectively removed by selecting a waveform based on the area fraction of the top (AFT) and the leading time. The distribution of the AFT for events at a fixed area in the drift region is described by a skew-Gaussian distribution to determine the acceptance of the cut. The cut boundary corresponding to the selection efficiencies of 2.5% and 97.5%, respectively, was determined to be (0.627, 0.703). Events occurring between the anode and gate have a similar area fraction to the top ith signal. They are characterized by shorter leading times compared to events occurring in the drift region, as the drift lengths for these ionization electrons are shorter. Peaks with leading times shorter than approximately 1030 ns were excluded from this measurement, resulting in a selection efficiency of
37Ar K-shell activity estimate
The magnitude distribution of the area was obtained after selecting the peaks. The selected S2 spectrum from the 37Ar K-shell decay was analyzed using Gaussian and Crystal Ball distributions to determine the event rates, as shown in Fig. 8. The crystal band distribution was selected because it provides a more accurate representation of the spectrum, accounting for the effects of low photon detection efficiencies in certain regions of the projection chamber. The Crystal Ball function combines a Gaussian core with a power-law tail, offering flexibility to model the asymmetric features observed in the spectrum. Mathematically, it is expressed as
A fit using the Crystal Ball distribution yielded an observed activity of approximately 14.96 Bq. Considering that K-shell decays constitute 90.2% of all 37Ar decays, and factoring in the selection efficiency of 94.0% achieved through the area fraction of top (AFT) and leading time cuts, the total activity within the drift region is estimated at 17.646 ± 0.025 (stat.) ± 0.007 (sys.) Bq. This activity level is well suited for calibrating liquid xenon dark matter detectors, such as PandaX-4T and XENONnT.
Summary
In this study, we successfully synthesized the radioactive isotope 37Ar using a reactor-derived thermal neutron source. With a half-life of 35.01 days, 37Ar is particularly valuable for calibrating LXe TPCs in low-energy regions. The isotopes were produced by irradiating high-purity 36Ar with thermal neutrons in a quartz ampoule. Geant4 simulations were used to predict the types and activities of the products, ensuring the minimal production of long-lived isotopes such as 39Ar.
The prepared 37Ar source was injected into a GXe TPC for the preliminary measurements. Upon injection, a notable increase in the peak counts around 2000 PE was recorded, confirming the successful synthesis and deployment of the source. A data-driven analysis approach was applied to reduce the background noise and focus on the S2 signals of 37Ar K-shell decay. The activity of the 37Ar K-shell decay was measured to be approximately 14.96 Bq. The conversion of 36Ar to 37Ar via neutron activation is a critical factor for determining the expected activity levels. An inaccurate estimation of the initial content of 36Ar can lead to errors in calculating the decay rates and activities of 37Ar. This highlights the importance of precisely controlling the argon content during the preparation phase. To mitigate this issue, a thorough review of the gas sealing process, particularly the impact of the temperature distribution during fusion sealing, could identify procedural errors that could contribute to underestimation.
In conclusion, this study successfully prepared and measured the activity of 37Ar, demonstrating its feasibility as a calibration source for low-energy dark-matter searches in LXe TPCs. These findings establish a solid foundation for future applications in detector calibration and dark-matter research.
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