Introduction
Measurement of the lifetimes of nuclear excited states is a fundamental aspect of nuclear structure studies. These lifetimes are directly related to the reduced transition probabilities, which provide essential information about the underlying nuclear structure, including the degree of overlap between the initial and final nuclear wave functions. Precise lifetime data are crucial for testing nuclear models and understanding the collective and single-particle excitations across various regions of the nuclear chart. Over the past two decades, the development of the fast scintillator LaBr3(Ce) has opened the possibility of performing fast timing measurements on the lifetimes of nuclear states ranging from nanoseconds to picoseconds [1]. Several dedicated fast-timing arrays have been developed since then, including FATIMA [2], KHALA [3], and VESPA [4]. When coupled with a Ge array, these arrays are used in triple Ge-LaBr3(Ce)-LaBr3(Ce) coincidence measurements, allowing a wide range of excited-state lifetimes in neutron-rich nuclei to be studied. This method leverages the superior energy resolution of Ge detectors to isolate specific transitions of interest, whereas the coincident γ-γ events recorded by the LaBr3(Ce) array were used to perform precise fast-timing analyses. More recently, extensive lifetime measurement studies have been performed using combined setups such as EXILL & FATIMA [5], ν-ball & FATIMA [6] and GAMMASPHERE & FATIMA [7] using thermal and fast neutron-induced fission, as well as 252Cf spontaneous fission source. However, achieving high selectivity and effective background suppression is a major challenge in such studies. In any fission process, hundreds of different fission fragments are produced, with each fragment typically emitting approximately seven γ-rays. The accurate association of these γ-rays with their corresponding fission products with an intense background is experimentally demanding. To obtain clean data, it is essential to enhance selectivity and minimize background contributions.
To overcome the limitations associated with using a Ge array alone, various fission-tagging devices have been developed to enhance the selectivity. One notable example is the integration of a liquid scintillator-based active fission target into the FIPPS spectrometer at the ILL [8]. This active target enables the identification of fission fragments and their temporal correlation with the subsequent γ decay, allowing efficient suppression of the uncorrelated γ-ray background originating from β-decay. To further improve the selectivity and suppress the background originating from nonspecific fission products, mass-identified fission fragment FIFI spectrometers with a resolution of approximately six mass units have been employed in isomer decay spectroscopy studies [9].
To enhance the selectivity and suppress the background, a dedicated hybrid setup HALIMA (Hybrid Array for Lifetime MeAsurement, HALIMA) was developed at the Institute of Modern Physics, Chinese Academy of Sciences, China. HALIMA is specifically designed to measure the sub-nanosecond lifetimes of neutron-rich nuclei produced via fission. The array consists of eight Compton-suppressed HPGe and twenty LaBr3(Ce) detectors with CsI(Tl)-based anti-Compton shields for γ-ray detection. In addition, two auxiliary detector arrays, 36 solar cell detectors, and 36 fast plastic scintillators were integrated into the HALIMA setup to detect fission fragments and β particles. With fragment tagging via solar cells and β-particle tagging via plastic scintillators, HALIMA enables the simultaneous application of both FF-Ge-LaBr3(Ce)-LaBr3(Ce) and β-Ge-LaBr3(Ce)-LaBr3(Ce) fast timing techniques. A key component of the HALIMA setup is the solar cell array, which registers the implantation of fission fragments and correlates them spatially and temporally with their subsequent γ decay, thereby effectively serving as a fission tag. As demonstrated later, the application of mass-identified fission tagging reduces the background from other fragments by more than a factor of four. In addition to fission tagging, the use of CsI(Tl)-shielded LaBr3(Ce) detectors in Compton-suppression mode enhances the peak-to-total ratio by a factor of 1.5. To further optimize the detection, the LaBr3(Ce) array in HALIMA was designed to have twice the efficiency of the Ge array, thereby maximizing the overall Ge-LaBr3(Ce)-LaBr3(Ce) coincidence efficiency.
In this paper, a comprehensive description of the HALIMA detection system is presented, including its detector components, readout electronics, and fully digital data acquisition system. The results of the first experiment using a 252Cf spontaneous fission source are discussed to illustrate the performance of the setup. Section 2 details the detector hardware and data acquisition system. Section 3 describes the data analysis techniques employed for fast-timing measurements across different time ranges. A summary and outlook are provided in Sect. 4.
Instruments
An overview of the HALIMA detection system is shown in Fig. 1, which illustrates the arrangement of the HPGe and LaBr3(Ce) detectors, along with the data acquisition setup. The following sections describe the configuration and integration of each component in detail.
HPGe detectors
The HALIMA setup includes eight n-type coaxial HPGe detectors, each equipped with BGO shielding, mounted on the central ring perpendicular to the fission axis. The crystals of the HPGe detectors are 71 mm in diameter and 72 mm long. The distance between the entrance window of the HPGe detectors and the center of the array is 17.5 cm. Each detector offers a relative efficiency of 70% compared to a standard 3 inches × 3 inches NaI(Tl) detector, with a typical energy resolution of 2.5 keV (FWHM) at 1332 keV. The full-energy peak (FEP) efficiency at 1 MeV was measured as 1.05% using standard 152Eu, 133Ba, and 60Co radioactive sources. The resulting absolute efficiency curves are presented in Fig. 2, along with a fit using the empirical formula described in Ref. [10].
Each HPGe detector in the HALIMA setup is surrounded by a BGO anti-Compton shield composed of eight optically isolated BGO crystals, each coupled to a photomultiplier tube (PMT). For each BGO anti-Compton shield, the PMTs are daisy-chained to obtain a single summed output signal. These BGO Compton suppressors significantly reduce the background, providing a suppression of approximately 77% in the 400-800 keV energy range when tested with a 60Co source.
LaBr3(Ce) detectors
To facilitate the lifetime measurements, 20 individual LaBr3(Ce) detectors were incorporated into the HALIMA setup, all configured identically. Each detector consists of a cylindrical LaBr3(Ce) crystal with a diameter of 2 inches and length of 3 inches, coupled to a Hamamatsu R13089 photomultiplier tube. As shown in Fig. 1, two rings consisting of eight LaBr3(Ce) detectors each have been mounted on both sides of the central ring of HPGe detectors, positioned at angles of 56° and 124° relative to the fission axis. The distance from the detector face to the center of the array is 16.5 cm. Additionally, a third ring composed of four LaBr3(Ce) detectors is mounted on the topmost layer of the array, with each detector located 22 cm from the center and oriented at an angle of 29° with respect to the fission axis. This configuration provides a solid-angle coverage of approximately 13% for an isotropic source located at the center of the array.
Owing to the compact size of LaBr3(Ce) crystals, Compton scattering is significant, resulting in a pronounced Compton continuum that reduces the peak-to-total ratio, particularly for low energy γ rays [11]. To reduce Compton continuums, Régis [12] and Gierlik [13] et al. developed a BGO-based Compton suppressor for a LaBr3(Ce) detector and achieved significant improvements in the peak-to-total ratio [12] and peak-to-Compton ratios [13]. In the present work, a more cost-effective, compact, and novel CsI(Tl)-based anti-Compton shield was developed and installed in the HALIMA setup. Figure 3 shows a technical drawing of a single LaBr3(Ce) detector housed within a CsI(Tl) anti-Compton shield supported by an aluminum shell. Each LaBr3(Ce) detector is enclosed by a CsI(Tl) anti-Compton shield composed of four separated CsI(Tl) crystals. The scintillation signals from these crystals are read out using twenty-eight 6.4 mm × 6.4 mm silicon photomultipliers (SiPMs), with seven SiPMs coupled to each crystal. These signals were summed and read out as a single channel. A comparative study using a calibrated 152Eu source (Fig. 4) demonstrates that the intense Compton background is significantly reduced when the CsI(Tl) anti-Compton shield is applied. A more detailed characterization and performance analysis will be presented in a forthcoming publication.
To optimize the time resolution of LaBr3(Ce) detectors in γ-γ coincidence mode, the PMTs of the LaBr3(Ce) detectors were operated at a bias voltage of approximately –1070 V. At this voltage, the 662 keV γ-rays from a 137Cs source produced a uniform output signal amplitude of 200 mV across all detectors. Because the inherent nonlinearity in the energy response of LaBr3(Ce) detectors is largely attributed to the voltage sensitivity of PMTs, a second-order polynomial function was employed for energy calibration. Standard γ-ray sources, 133Ba and 152Eu, were used for this calibration, covering an energy range of 81 keV to 1408 keV. The energy resolution of the LaBr3(Ce) detectors was found to be 2.67% at the 1332 keV line of a 60Co source. The absolute full-energy peak (FEP) efficiency at 1 MeV for the entire LaBr3(Ce) array was measured to be 2.74%. The corresponding absolute efficiency curves along with the fitting results are presented in Fig. 2.
The two primary timing characteristics of the LaBr3(Ce) detector array are the time resolution and time walk. The time resolution was determined from the summed time difference spectra of all pairwise combinations among the 20 LaBr3(Ce) detectors. To achieve this, the individual time spectra were aligned by applying appropriate time shifts to each detector relative to a chosen reference detector, after which the summed time difference spectrum was obtained using 60Co source. The time information was extracted using the TSINC digital algorithm [14] and Constant Fraction Discrimination (CFD). Figure 5 displays the resulting time difference distribution for 20 LaBr3(Ce) detectors with 1173 keV–1332 keV γ-rays of 60Co source. The time resolution is 348.9(2) ps in FWHM, which is comparable to the resolution of FATIMA [2] despite the larger crystal volume of the LaBr3(Ce) detectors used in our setup.
To extract lifetimes shorter than the intrinsic time resolution of LaBr3(Ce) detectors to a few picoseconds (ps), it is essential to determine the Prompt Response Difference (PRD) as a function of energy. PRD calibration is used to describe the time walk characteristics using the coincident γ-ray cascade emitted by 152Eu source over the energy range of 200–1400 keV. The PRD was measured using Leading-Edge Discrimination (LED) in conjunction with the TSINC interpolation algorithm [14]. The resulting PRD curve is presented in the top panel of Fig. 6 and was fitted using the following equation:
FFs detectors: Solar cells
The detection of fission fragments (FFs) in an intense background dominated by light charged particles remains a major challenge in fission experiments. Solar cells, originally developed for photovoltaic energy conversion, were first introduced by Siegert in 1979 for the detection of FFs at the Institut Laue-Langevin (ILL) facility [16]. Subsequently, Ajitanand et al. demonstrated their radiation hardness during a 252Cf experiment, thereby confirming their suitability for use in high-background environments [17]. Owing to its low cost, flexible geometry and insensitivity to alpha particle, solar cells have been effectively employed in fission detection arrays, such as SAPHIR [18, 19] and DEATH-STAR [20, 21].
In this study, the rise time and pulse height characteristics of three commercially available solar cells were evaluated. Among them, TOPCON-type solar cells were selected as FF detectors because of their high signal amplitude (400–600 mV) and relatively fast rise time (200–800 ns). These solar cells have a thickness of ~ 130 μm and a very small depletion depth (~ 1 μm), making them largely insensitive to light charged particles, such as alpha particles, because of their charge collection mechanism, specifically the funneling mechanism [22]. As illustrated in Fig. 7, the solar cells array, consisting of thirty six 11 mm × 11 mm solar cells, mounted on a printed circuit board (PCB) positioned at the center of HALIMA setup. The raw pulse-height spectrum of the FFs from a 252Cf source detected by a solar cell is shown in Fig. 8. To assess the mass resolution of the TOPCON solar cells, the 2E method [23] which is based on the conservation laws of mass and momentum, was applied using an open 252Cf source. Schmitt calibration [24] was used in this analysis, with the primary equation expressed as:
When solar cells are used as FF detectors in fission experiments, it is essential to evaluate their radiation damage tolerance. For this purpose, a comparative radiation damage test was performed using a 20 mm × 20 mm silicon detector and a solar cell of the same size. Both detectors were placed 8 cm from a 252Cf source with an activity of 100 μCi (3.7 MBq). The variation in the pulse height as a function of the incident flux is shown in Fig. 10. For an integrated flux of approximately 109 particles/cm2, the FFs detected by the solar cell exhibited only a 10%–15% reduction in pulse height, indicating a minor degradation. In contrast, the silicon detector exhibited a complete signal loss, demonstrating its significantly lower radiation tolerance under similar conditions.
β detectors: Fast plastic scintillators
An auxiliary detector array comprising 36 fast plastic scintillators (11 mm × 11 mm) was integrated into the HALIMA setup for β-particle tagging. Each scintillator, fabricated from EJ-200 plastic scintillator material and 3 mm in thickness, is optically coupled to a Hamamatsu S14160 Multi-Pixel Photon Counter (MPPC) with a sensitive area of 6.4 mm × 6.4 mm. These scintillators and MPPCs were mounted on a PCB, positioned ~ 5 mm downstream from the center of the HALIMA setup. The configuration of β ancillary array is illustrated in Fig. 11. The time resolution of each β detector was measured as 942 ps.
Electronics and DAQ system
The electronics and data acquisition (DAQ) system of HALIMA is based on the Pixie-16 system developed by XIA LLC, USA. Energy signals from the HPGe detectors, BGO and CsI(Tl) anti-Compton shields, and solar cells are digitized by 16 channel, 100 MHz, 14-bit modules, while the energy signals from LaBr3(Ce) detectors and β detectors are digitized by 16 channel, 500 MHz, 14-bit modules, wherein the rise time can be accurately measured. The data were recorded using a general-purpose digital data acquisition system (GDDAQ) [27, 28] and operated in a triggerless mode, which means that all live events are recorded without predefined conditions. This triggerless architecture provides significant flexibility in offline data analysis and event reconstruction [29].
To optimize the fast-timing performance of LaBr3(Ce) detectors, particularly the time resolution and time walk, the waveforms of LaBr3(Ce) detectors are recorded on an event-by-event basis for offline analysis. The waveform recording length was set to 0.2 μs, providing sufficient temporal detail for precise timing studies. The high-voltage supply system is segmented into two parts: two CAEN N1470 modules are used to power the HPGe detectors, while three ISEG EHS modules are dedicated to supplying the LaBr3(Ce) detectors, CsI(Tl) anti-Compton shields, and BGO detectors.
Commissioning with a 252Cf source
A commissioning measurement was performed using a semi-closed 252Cf source with an activity of 100 μCi. The source was encapsulated with a 50 μg/cm2 Au sputter and mounted on a 0.127 mm thick platinum-clad nickel backing. This configuration ensures that, in each spontaneous fission (SF) event, one fission fragment (FF) recoils into the vacuum chamber, while the complementary FF is absorbed in the backing. Figure 12 illustrates the technical layout of the HALIMA setup in the commissioning phase. The 252Cf source was positioned at the bottom of the vacuum chamber and surrounded by a 40 mm-thick tungsten shield with a cone angle of 18°. This shielding configuration effectively blocks the prompt γ rays emitted from the source during fission. Following fission, the recoiling FF traverses the 8 cm vacuum gap and is detected by the centrally located solar cell array. In this configuration, the solar cells function as implantation detectors, enabling event-by-event correlation between the FFs and their associated γ rays. Because the FFs of different masses possess varying velocities, their time-of-flight (TOF) over the 8 cm path introduces a measurable spread, which can be utilized for isomer selection. By gating the signals from the solar cells, isomer-specific events were effectively selected, and the overwhelming background was significantly suppressed. Approximately 4400 h of data acquisition was completed during this commissioning run, yielding approximately ~ 1.3×1011 fission events.
In the present work, two complementary approaches were employed to measure the lifetimes of the excited states of interest produced by fission, as illustrated in Fig. 13. The first method, as shown in Fig. 13(a), utilizes the FF-Ge-LaBr3(Ce)-LaBr3(Ce) coincidence technique. In this configuration, a fission event is initially detected by the solar cell array, followed by observation of an isomeric decay involving three or more γ rays. The excited state of interest, with the lifetime denoted by τ is fed and decayed via γ2 and γ3, which are detected in coincidence by two LaBr3(Ce) detectors. The preceding transition γ1 is recorded using an HPGe detector. The second approach, as illustrated in Fig. 13(b), involves a β-Ge-LaBr3(Ce)-LaBr3(Ce) coincidence technique based on the fast plastic scintillator array. In this case, γ rays are emitted following β decay of fission fragments, with the triple γ-ray detection configuration remaining similar to that of the first method.
Prior to the coincidence analysis, the time alignment of all detectors was performed relative to a reference LaBr3(Ce) detector to ensure precise timing. Based on this preliminary data analysis, the FFs selectivity was validated by comparing γ-ray spectra gated with and without FF detection. This comparison clearly demonstrates the advantage of FF gating in terms of significant background suppression and improved peak-to-background ratio. The lifetimes of the three excited states ranging from a few picoseconds to several hundred nanoseconds were determined by implementing distinct coincidence and lifetime extraction methods. The extracted values showed good agreement with the literature data, confirming the capability of the HALIMA setup to perform precise lifetime measurements over a broad temporal range.
Time alignment
Owing to variations in electronics, cable lengths, and the use of digitizer cards with different sampling frequencies and bit resolutions, each detector type within the HALIMA setup exhibited a unique timing offset. These offsets lead to misalignments in the time distributions of the detectors when referenced to the reference LaBr3(Ce) detector. To correct this, the timing information, including the initial timestamp and the timing information provided by the CFD filter algorithm, is extracted, allowing for the implementation of time alignment for each detector on a run-by-run basis. For each detector, the relative time difference with respect to the reference LaBr3(Ce) detector was determined by performing a Gaussian fit of the time distribution. The centroid of each distribution was then shifted to 0 ns to achieve synchronization. Figure 14 shows the time difference as a function of the detector ID matrices before and after time alignment. Each unit on the horizontal axis represents individual detector IDs with the HPGe detectors, solar cells, LaBr3(Ce) detectors, and fast plastic scintillators represented by IDs 0-7, 8-43, 44-55 and 60-67, 56-59 and 68-99, respectively. As illustrated in Fig. 14(a), the centroids of the time distributions are scattered owing to inherent offsets. After applying the alignment procedure, all the time distributions were successfully centered at approximately 0 ns, as shown in Fig. 14(b), indicating a uniform timing response across all detectors in the array.
Validation of FFs Selectivity
The γ-ray spectrum originating from the spontaneous fission of 252Cf is notably complex owing to the presence of emission lines from hundreds of neutron-rich fission fragments. To enhance the selectivity of γ-ray events and suppress the background, the HALIMA setup utilizes a solar cell array to detect fission fragments (FFs), which are then used as triggers for implementing FF-γ coincidence measurements. To evaluate the improvement in selectivity, HPGe-HPGe coincidence spectra were generated under different gating conditions and compared, as shown in Fig. 15. Figure 15(a) shows the HPGe coincidence spectrum of Ge-Ge double events, which were generated using a Ge gate on the 314 keV transition. The γ rays originating from both the isomeric decay and β decay are shown in the spectrum. In contrast, Fig. 15(b) shows the coincidence spectrum obtained by applying an additional gate on the solar cells, thereby selecting events correlated with fission fragments. Under these conditions, the isomeric state (19/2-, 10 ns) in 137Xe was selectively tagged, and its subsequent γ-ray transitions were prominently visible. As a result, unwanted γ rays from β-decay processes are effectively suppressed, yielding a significantly cleaner spectrum dominated by the 400 keV and 1220 keV transitions.
Beyond its role in enhancing isomer selectivity, gating on FFs offers a significant advantage in suppressing the Compton background, which is particularly critical in fast-timing measurements using LaBr3(Ce)-LaBr3(Ce) coincidences. In such measurements, the background beneath the full energy peaks can introduce considerable uncertainties, thereby diminishing the precision of lifetime extraction. Furthermore, scattered γ rays originating primarily from neutron-induced inelastic scattering and Compton scattering of prompt γ-rays within the surrounding components of the HALIMA setup substantially contribute to the background in the LaBr3(Ce) spectra. By utilizing the FF selectivity, this intense background is reduced, thereby improving the peak-to-background ratio. To quantify the performance of this selectivity, a comparative analysis was conducted for the nucleus 134Te and LaBr3(Ce)-LaBr3(Ce) coincidence spectra were obtained under different gating conditions. In Fig. 16, the LaBr3(Ce) coincidence spectrum was obtained by using a double gate on both 115 and 297 keV, respectively, for Ge and LaBr3(Ce). The peak-to-background ratio obtained for the energy width corresponding to the full width at half maximum of 1279 keV was measured to be 1.6, as shown in the inset of Fig. 16. Upon applying an additional gate on the solar cell signals (gating on FFs), a noticeable reduction in the background, particularly below 600 keV, was observed. Under this condition, the P/B ratio for the 1279 keV peak increased substantially to 4.3. The third case is the LaBr3(Ce) coincidence spectrum employing an additional gate on a specific mass of 134Te. Compared to the Ge-LaBr3(Ce)-LaBr3(Ce) coincidence events, the background in the LaBr3(Ce) spectrum were further suppressed, resulting in a more than fourfold improvement in the P/B ratio.
It should be emphasized that the 252Cf source employed in the present work is a semi-closed source, disabling the employment of the 2E method for determining the masses of FFs detected by solar cells. Therefore, an alternative approach known as the M-V method [30] was exploited, where M and V are the mass and velocity of the FFs, respectively. This method is based on the conservation laws of mass and momentum. Combing the Eq. 2 and the following formulas:
Preliminary results
This subsection presents the lifetime measurements of the three nuclear excited states, ranging from a few picoseconds to several hundred nanoseconds. Specifically, the lifetimes of the two isomers in 134Te and 138Ba were measured using FFs-Ge-Ge/FFs-LaBr3(Ce)-LaBr3(Ce) and β-Ge-LaBr3(Ce)-LaBr3(Ce) coincidence techniques. Additionally, the lifetime of an excited state in 132Te was extracted using the Generalized Centroid Difference (GCD) method based on FFs-Ge-LaBr3(Ce)-LaBr3(Ce) coincidences. The measured lifetimes were in good agreement with previously reported values, validating the reliability of the experimental techniques and analysis methods employed in the present work.
In the case of 134Te, the 6+ state was identified as a relatively long-lived isomer [32]. As illustrated in Fig. 17, the FFs-Ge-Ge and FFs-LaBr3(Ce)-LaBr3(Ce) coincidence techniques were employed to measure its lifetime. The time difference spectra of both HPGe and LaBr3(Ce) were obtained using a Ge/LaBr3(Ce) gate for both 115 and 297 keV transitions. The coincidence time windows between two γ rays were set to ± 200 ns for Ge-Ge and ± 4 ns for LaBr3(Ce)-LaBr3(Ce), respectively. Using the slope method, the extracted lifetimes were determined to be 164.04(95) ns from the FFs-Ge-Ge data and 164.7(7) ns from the FFs-LaBr3(Ce)-LaBr3(Ce) data. Both values are in excellent agreement with the previously reported value of 164.1(9) ns [32].
The primary objective of the present work was to measure the sub-nanosecond lifetimes of the nuclear excited states. In the case of 138Ba, the 4+ state was identified as a short-lived isomer [33] and its lifetime was determined using the β-Ge-LaBr3(Ce)-LaBr3(Ce) coincidence technique. The coincidence time window for detecting the feeder (547 keV) and decay (463 keV) γ-ray transitions with LaBr3(Ce) detectors was set to ± 4 ns, whereas the third γ-ray detected by HPGe was gated on the 1436 keV transition. The coincidence time windows between β and triple γ rays were determined based on the correlated prompt time difference distribution. The γ-γ coincidence matrix from LaBr3(Ce) is shown in the top panel of Fig. 18. This matrix clearly reveals 463 keV and 547 keV transition, confirming the correlated decay pathway of the 4+ state in 138Ba. The time difference spectrum was obtained using an LaBr3(Ce) gate at 463 keV and another LaBr3(Ce) gate at 547 keV, as illustrated in the bottom panel of Fig. 18. A convolution function consisting of Gaussian and exponential distributions was used to fit the time distribution. The extracted half-life of the 4+ state in 138Ba was found to be 2.02(9) ns, which is in good agreement with the previously reported value of 2.08(6) ns [33].
The Generalized Centroid Difference (GCD) method provides a more suitable approach for excited states with lifetimes shorter than the intrinsic time resolution of LaBr3(Ce) detectors. The lifetime of the 
Summary
A novel hybrid detection system, named HALIMA, was developed and installed at the Institute of Modern Physics to facilitate the measurement of the sub-nanosecond lifetimes of neutron-rich nuclei produced via fission. This system is composed of eight BGO-shielded HPGe detectors and 20 fast LaBr3(Ce) detectors, each shielded with CsI(Tl), offering high-resolution γ-ray energy and timing capabilities. To enhance event selectivity, two specialized ancillary detector arrays were incorporated: a solar cell detector array for detecting fission fragments (FFs) and a fast plastic scintillator array for β-particle detection. These additions enable advanced coincidence techniques, such as FFs/β-Ge-LaBr3(Ce)-LaBr3(Ce), significantly improving the spectral quality and peak-to-background, thereby allowing precise lifetime measurements. In particular, the implementation of FF gating proved critical in isolating γ-ray cascades of interest by effectively suppressing background contributions from unrelated fission products and β decays. This additional selectivity not only reduced the intense background but also improved the peak-to-background ratio by more than a factor of four, thereby increasing the accuracy of the lifetime measurements. A comprehensive overview of HALIMAs components has been provided, detailing the energy and timing performance of the detectors, the data acquisition (DAQ) system, and the associated electronics architecture.
A commissioning experiment using a 252Cf source was conducted to validate the performance of the HALIMA setup. By applying combined gating to FFs and fast plastic scintillators, the lifetimes of three excited nuclear states in 134Te, 138Ba, and 132Te were measured, covering a range from a few picoseconds to several hundred nanoseconds. The results were in good agreement with the literature values, demonstrating the capability and precision of the HALIMA setup. Furthermore, several excited states with previously unmeasured lifetimes produced by 252Cf fission were identified for the first time, thereby opening new avenues for nuclear structure studies of neutron-rich nuclei.
Looking ahead, the HALIMA system will be used not only to measure the lifetimes of excited states following isomeric or β decay, but also for the lifetimes of prompt γ rays directly emitted by the FFs of 252Cf, and the source will be placed at the center of the HALIMA array without shielding. To enhance the mass resolution of fission fragments, new ancillary detectors based on Silicon Carbide (SiC) are being developed as next-generation fission detectors. With its advanced detection capabilities, HALIMA will significantly contribute to the systematic study of neutron-rich nuclei produced via neutron-induced fission, extending our understanding of nuclear structures in regions far from stability.
γ-γ fast timing with high-performance LaBr3(Ce) scintillators
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