Gamma-ray Tracking Research Articles

Design of GRETA Cooling Systems

The Gamma-Ray Energy Tracking Array (GRETA) is a full 4π gamma-ray tracking detector capable of reconstructing the energy and three-dimensional position of gamma-ray interactions within a compact sphere of high-purity germanium crystals. The GRETA Detector Array Sphere will have the capacity to accommodate a total of 30 Germanium Quad Detector Modules (QDM). The 30 QDMs are to be cooled and maintained below 100 K using liquid nitrogen (LN) at all times while the array is in normal operation, and will require regular filling of a LN Dewar on each module. The Dewar is designed to allow the Quad Module to be operated in any orientation with a LN holding time of no less than 12 hours when the detector module is fully powered. An automated LN cooling and refilling system is required to supply LN to the 30 QDMs and ensure them maintained below 100 K. Each of the GRETA QDMs houses a total of 148 pre-amplifier units within the module, and with the high power consumption of each pre-amplifier, active cooling of the pre-amplifier compartment is required. Additionally, each Quad Module will have 4 digitizer modules attached to it, which generate heat and require cooling as well. A closed-loop liquid (Glycol) cooling system will provide the required temperature stability and dissipate power generated heat for electronics. This paper presents design of the GRETA LN cooling system for detectors and the closed-loop liquid cooling system for electronics including technical requirements, design schemes, key components, operation modes, and so on.

Agata characterisation and pulse shape analysis

The AGATA and GRETA spectrometers are large arrays of highly segmented HPGe detectors that use the technique of gamma ray tracking to reconstruct the scattering path of gamma rays interacting within the active material. A basic requirement is a precise reconstruction of the individual interaction locations within the detectors. This is possible through the use of pulse shape analysis which has to be conducted in real time due to the high data rates generated by the spectrometer. The methodologies that have been evaluated to perform this for AGATA are discussed along with the approaches used to calculate the pulse shape databases required by these algorithms. Finally, the performance and limitations of the existing approaches are reviewed.

The AGATA Spectrometer

The Advanced GAmma Tracking Array (AGATA) is a European project to develop and operate a high-resolution gamma-ray spectrometer. AGATA is based on the technique of gamma-ray energy tracking in electrically segmented high-purity germanium crystals. The tracking technique requires the accurate determination of the energy, time and position of every interaction as a gamma ray deposits its energy within the detector volume. AGATA can measure gamma rays from 10’s of keV to 10 MeV with excellent efficiency and position resolution and has a very high count rate capability. The realisation of AGATA and gamma-ray tracking is a result of many technical advances. AGATA has operated in a series of successful scientific campaigns at Legnaro National Laboratory in Italy, GSI in Germany and GANIL in France. AGATA is now in its next phase of development as it evolves to the full 4π instrument. It is presently starting its next campaign at Legnaro.

Experimental study of the S38 excited level scheme

Information on the $^{38}\mathrm{S}$ level scheme was expanded through experimental work utilizing a fusion-evaporation reaction and in-beam $\ensuremath{\gamma}$-ray spectroscopy. Prompt $\ensuremath{\gamma}$-ray transitions were detected by the Gamma-Ray Energy Tracking Array (GRETINA) and recoiling $^{38}\mathrm{S}$ residues were selected by the Fragment Mass Analyzer (FMA). Tools based on machine-learning techniques were developed and deployed for the first time in order to enhance the unique selection of $^{38}\mathrm{S}$ residues and identify any associated $\ensuremath{\gamma}$-ray transitions. The new level information, including the extension of the even-spin yrast sequence through ${J}^{\ensuremath{\pi}}={8}^{(+)}$, was interpreted in terms of a basic single-particle picture as well shell-model calculations which incorporated the empirically derived FSU interaction. A comparison between the properties of the yrast states in the even-$Z, N=22$ isotones from $Z=14$ to 20, and for $^{36}\mathrm{Si}--^{38}\mathrm{S}$ in particular, was also presented with an emphasis on the role and influence of the neutron $1{p}_{3/2}$ orbital on the structure in the region.

Experimental evaluation of the performance of a novel planar segmented HPGe detector for use in gamma emission tomography

The use of segmented HPGe detectors for gamma-ray tracking applications is well established. The spectroscopic capabilities of these detectors make them most suitable for such applications. For similar reasons, the use of such detectors in the tomographic measurement of irradiated nuclear fuel has also been envisioned. Especially, these detectors can facilitate faster fuel examination with excellent energy resolution. We have proposed and designed a novel planar segmented HPGe detector for use in gamma emission tomography. The design of the detector segmentation and the mode of operation is unique and offers six simultaneous detection channels for tomographic measurements. This work reports the first experimental evaluation of the performance of the detector. Important characteristics of the detector have been obtained, such as energy resolution of the segments in 1-fold (one segment) and 2-fold (two segments) modes, throughput curves, crosstalk energy corrections, and mislocalisation rate. Collimated source tests have been performed and the results have been compared with the MCNP simulations results. The obtained results are as expected and in good agreement with the simulation results, and it is estimated that using this detector can speed up the data collection by 3.2 times in comparison to an unsegmented detector of the same overall dimensions, in a tomographic application. Further improvements are foreseeable if scaling up to a larger detector with greater segmentation.

Gamma-ray track reconstruction using graph neural networks

Since the advent of the new generation of germanium detector arrays for low-energy nuclear physics experiments utilizing gamma-ray tracking, the challenges associated with track-reconstruction methods have been extensively studied. In the present work an approach based on recent developments in machine learning was used to address the problem. Here, a graph neural network was constructed and trained on data simulated in Geant4 in order to attempt track reconstruction of gamma rays below 1 MeV in a spherical shell geometry of pure germanium. Using a thick-shell geometry, and simulated data without energy- and position uncertainties the network achieved a reconstruction rate above 80% for complete tracks, and a combined peak-to-total value of 85% for energy spectra with four discrete peaks. For data with added noise, i.e. finite resolution in interaction-point position and energy, the corresponding peak-to-total ratio dropped to 74%. The track reconstruction was stable across multiplicities 1–10 but showed an increased error frequency in the energy range between 50 keV and 250 keV. To specifically study the complication of gamma tracks lost by out-scattering from the detector volume, a thin-shell (9 cm thickness) geometry was used together with a modified version of the GNN framework. By letting the GNN code identify and discriminate the out-scatter events, an improvement of the P/T value from 66% to 75% was found for the packed, noisy data. For the sake of comparison the new GNN model with existing gamma-ray tracking methods, a separate instance of the network was trained on slightly higher energies (up to 1.5 MeV) and multiplicities (up to 15) to evaluate 1.332 MeV photon cascade data in terms of P/T and photo-peak efficiency. The results for this GNN data set, with P/T values at 85% for single tracks and 74% for multiplicity 15, show clear promise when compared to the existing tracking methods.

First experimental measurements with the Segmented Inverted-coaxial GerMAnium (SIGMA) detector

The SIGMA project aims to develop a p-type segmented inverted coaxial point contact germanium detector for gamma-ray tracking and imaging. The performance of the first prototype of this type of detector is reported. The measurements performed verified the size and shape of the crystal and determined the location of the complex active volumes of the detector segments. The latter determines the charge collection times that are related to the position of interaction. Good agreement is found between the results and the specified crystal design. The energy resolution of the point contact has been measured to be 0.84 keV and 2.21 keV at 122 keV and 1332 keV, respectively.

Full-volume characterization of an AGATA segmented HPGe gamma-ray detector using a $$^{152}Eu$$ source

Scanning tables use collimated gamma-ray sources to perform full volume characterization of position sensitive detectors. One of such tables is hosted at IPHC Strasbourg. It was designed and built within the AGATA collaboration. It exploits the pulse shape comparison scanning (PSCS) technique to build databases of pulses used to characterize the response of high purity germanium (HPGe) detectors and perform R&D on such crystals. Ultimately, measured databases could be used by the pulse shape analysis (PSA) algorithms employed in AGATA experiments. The table can perform full volume scans of large volume detectors in short times, with a good spatial resolution and at different energies. Lately, the table was upgraded with a new \(^{152}Eu\) source, which emits gamma rays in cascades of different energies. A scan with such source is performed for the first time. It allows to build different energy databases in one single scan. The present work aims at testing the performances of the PSCS technique with a multi-energetic source and verifying some assumptions of the Shockley–Ramo theorem which are at the base of the PSA algorithms used for gamma-ray tracking arrays.

Simple and Scalable Streaming: The GRETA Data Pipeline

The Gamma Ray Energy Tracking Array (GRETA) is a state of the art gamma-ray spectrometer being built at Lawrence Berkeley National Laboratory to be first sited at the Facility for Rare Isotope Beams (FRIB) at Michigan State University. A key design requirement for the spectrometer is to perform gamma-ray tracking in near real time. To meet this requirement we have used an inline, streaming approach to signal processing in the GRETA data acquisition system, using a GPU-equipped computing cluster. The data stream will reach 480 thousand events per second at an aggregate data rate of 4 gigabytes per second at full design capacity. We have been able to simplify the architecture of the streaming system greatly by interfacing the FPGA-based detector electronics with the computing cluster using standard network technology. A set of highperformance software components to implement queuing, flow control, event processing and event building have been developed, all in a streaming environment which matches detector performance. Prototypes of all high-performance components have been completed and meet design specifications.

Optimization and Estimation of Parameters for a Compton Camera Consisting of the DSSD Scatterer and the GAGG Absorber with the Monte Carlo Simulation

With the advent of new rare-earth-based scintillators, a variety of Compton cameras with novel features in detector material and structure have been developed for the application to either molecular imaging or gamma-ray observation with space telescopes. In this study, we investigated the performance of a Compton camera consisting of a DSSD as a scatterer and an array-type GAGG scintillator as an absorber. We mainly focused on how to optimize the performance of the Compton camera in imaging resolution as well as in efficiency and thereby determined the optimal geometrical configuration with the Monte Carlo simulation using a GEANT4 toolkit. The energy discrimination levels were set in the simulation so that the general experimental environments could be taken into account. We studied different configurations with a figure of merit accounted for efficiency and resolution as a function of the gamma-ray energy, the pixel size of the absorber, the source-to-scatterer distance, the scatterer-to-absorber distance and the scatterer-to-absorber angle in this study. The figure of merit reached its maximum when the source-to-scatterer distance was 3 cm, the scatterer-to-absorber distance was 5 cm and the scatterer-to-absorber angle, that is, the absorber with 256 pixels was located at an angle of 15° with respect to the scatterer for 356-keV gamma rays used in this study. We also investigated how the event rates for Compton scattering obtained by counting the scattering events were contributed to the reconstructed image and caused unwanted imaging artifacts for the optimal configuration. Finally, calculations were performed by using the gamma-ray tracking method with inclusion of two-fold events occurring within the absorber and showed increase in imaging efficiency by 25%.

Intruder dominance in the 02+ state of Mg32 studied with a novel technique for in-flight decays

The development of advanced γ-ray tracking arrays allows for a sensitive new technique to investigate elusive states of exotic nuclei with fast rare-isotope beams. By taking advantage of the excellent energy and position resolution of the Gamma-Ray Energy Tracking In-beam Nuclear Array, we developed a novel technique to identify in-flight isomeric decays of the 0⁺₂ state in 32Mg populated in a two-proton removal reaction. We confirm the 0⁺₂→2+1γ-ray transition of ³²Mg and constrain the 0⁺₂ decay lifetime, suggesting a large collectivity. The small partial cross section populating the 0⁺₂ state in this reaction provides experimental evidence for the reduced occupancy of the normal configuration of the 0⁺₂ state, indicating the intruder dominance of this state.

Formula omitted]-ray tracking with AGATA: A new perspective for spectroscopy at radioactive ion beam facilities

The Advanced GAmma Tracking Array (AGATA) is a next generation high-resolution γ-ray spectrometer for nuclear structure studies based on the principle of γ-ray tracking. It is built from high-fold segmented germanium detectors which will operate in position-sensitive mode by employing digital electronics and pulse-shape decomposition algorithms. The unique combination of highest detection efficiency and position sensitivity allows spectroscopic studies with instable ion beams of lowest intensity. The first implementation of the array consisted of five AGATA modules; it was operated at INFN Legnaro. A larger array of AGATA modules was used at GSI for experiments with unstable ion beams at relativistic energies. At the moment the spectrometer is hosted by GANIL. In the near future AGATA will be employed at the leading infrastructures for nuclear structure studies in Europe. The presentation will illustrate the potential of the novel gamma-ray tracking method by physics cases from the different exploitation sites. Perspectives and opportunities for γ-ray spectroscopy at future radioactive ion beam facilities are presented and discussed.

Universat-SOCRAT multi-satellite project to study TLEs and TGFs

We present concept of a new multi-satellite Universat-SOCRAT project aimed to study transient phenomena in the upper atmosphere such as transient luminous events (TLEs) and terrestrial gamma-ray flashes (TGFs). It is a new space project of Lomonosov Moscow State University based on the use of a few satellites in the near-Earth orbit for real-time monitoring of radiation environment, natural (asteroids, meteoroids) and artificial (space debris) potentially dangerous objects, electromagnetic transients including cosmic gamma-ray bursts, terrestrial gamma-ray flashes, and optical and ultraviolet bursts in the Earth’s atmosphere.Study of TLEs and TGFs remains an important and demanding task despite of a multitude of recently acquired data for these phenomena. This might be explained by the absence of comprehensive theoretical understanding of physical nature of high-energy processes in the Earth’s atmosphere. Multi-wavelength synchronous observations with moderate accuracy of localization of TGF and TLE events are necessary to gain an insight of physics governing these high-energy processes in the Earth’s atmosphere. In the article, we present results of TLE observations in space experiments of Moscow State University and discuss advanced instruments for optical observations of TLEs, as well as gamma-ray burst monitor and tracking gamma spectrometer for TGFs observations.

A compact scintillator based position sensitive detector system for gamma ray tracking applications

Abstract A Cerium doped Lutetium Yttrium Orthosilicate (LYSO) crystal based precision gamma detection system has been developed using a position sensitive photomultiplier tube, with minimal pulse processing requirements. The assembly’s detailed characterization has been reported in this work. This kind of a setup with a precise spatial resolution can be very useful for scanning high purity germanium detectors that use pulse shape analysis algorithms for signal processing in gamma ray spectroscopy experiments. The highly pure LYSO crystal used here is custom-made and the overall setup including the multi anode readout photomultiplier tube is compact. The system has been tested for energy, timing and position information regarding gamma photons interacting within the crystal using a coincidence mechanism. Position sensitivity has been found to be in the anticipated range of a few millimeters. This kind of a detector can be used in various fields, vis-a-vis, medical imaging, radioactive waste management, nuclear proliferation control, and various applications that deal with precisely identifying and locating radioactivity.

Compton camera based on Timepix3 technology

The Compton camera concept is based on the reconstruction of recorded Compton scattering events of incoming gamma rays. The scattering of primary gamma ray occurs in the first detector (called scattering detector—usually thin) recording the position and energy of the recoiled electron. The scattered gamma quantum continues towards the second detector (called absorption detector - usually thick) where it is absorbed. The second detector records the energy and the position of this scattered gamma. Using the Compton scattering equation it is possible to determine the scattering angle, and estimate possible directions of the original gamma ray as a surface of a cone. When the Compton camera records the number of such events, the location and the shape of the gamma source can be reconstructed. Timepix3, a hybrid single photon counting imaging pixel detector, is a perfect device for creation of a compact Compton camera. Timepix3 is an event based readout chip (every hit pixel is immediately sent to a readout) and can record the time-of-arrival (ToA) and energy of an incident gamma simultaneously in each pixel. The chip offers high energy resolution (1 keV at 60 keV, 7 keV at 356 keV), as well as time resolution (1.6 ns). The Timepix3 readout chip can be combined with different sensor materials (Si, CdTe, CZT). In this contribution, we present a very compact detector system for imaging with gamma-rays using the Compton camera principle. The system consists of at least two layers of hybrid pixel detectors Timepix3 with the sensors being optimized for gamma-ray tracking. The front detector layer (scattering) is made of silicon of 1 mm thickness, while the last layer (absorbing) is equipped with thick CdTe or CZT sensors up to 2 mm in thickness. The total absorption of the whole detector can be very high if several CdTe or CZT layers are used. The maximal number of layers is not limited, but the practical evaluation was performed with 2 layers. Thanks to Timepix3 simultaneous measurement of ToA and energy, it is possible to precisely detect coincidence events in the detector layers. Based on the energy and position of these events, it is possible to estimate the possible direction of the original gamma. The angular resolution of the presented Compton camera depends on the detected energy, and it is in the order of 1 degree.

On the self-calibration capabilities of gamma -ray energy tracking arrays

.Gamma-ray energy tracking arrays constitute the technological frontier of high-resolution gamma -ray spectroscopy revolutionizing modern nuclear physics investigations. Their principle of operation lies on the precise reconstruction of the three-dimensional gamma -ray interaction positions within the detector volume. The most common method to obtain these interaction points in real time is to compare the experimental signals against a reliable library of signals (signal basis) that maps the detector response as a function of the gamma -ray interaction position. Obtaining a high-fidelity signal basis, however, remains a big technological challenge, which hinders the optimal operation of these state-of-the-art arrays. In this article, we propose a pioneering and notably simple method for generating experimentally a reliable signal basis. The proposed method enables the gamma -ray tracking devices to perform a self-calibration of their position sensitive response in situ, opening up the way for reaching their optimum performance for the first time.

The (Li6,Li*6[3.56MeV]) reaction at 100 MeV/u as a probe of Gamow-Teller transition strengths in the inelastic scattering channel

Background: Inelastic neutrino-nucleus scattering is important for understanding core-collapse supernovae and the detection of emitted neutrinos from such events in earth-based detectors. Direct measurement of the cross sections is difficult and has only been performed on a few nuclei. It is, therefore, important to develop indirect techniques from which the inelastic neutrino-nucleus scattering cross sections can be determined. Purpose: This paper presents a development of the (Li-6,Li-6*[T = 1, T-z = 0, 0(+), 3.56 MeV]) reaction at 100 MeV/u as a probe for isolating the isovector spin-transfer response in the inelastic channel (Delta S = 1, Delta T = 1, Delta T-z = 0) from which the Gamow-Teller transition strengths from nuclei of relevance for inelastic neutrino-nucleus scattering cross sections can be extracted. Method: By measuring the Li-6 ejectile in a magnetic spectrometer and selecting events in which the 3.56 MeV gamma ray from the decay of the Li-6*[3.56 MeV] state is detected, the isovector spin-transfer selectivity is obtained. High-purity germanium clover detectors served to detect the gamma rays. Doppler reconstruction was used to determine the gamma energy in the rest frame of Li-6. From the Li-6 and 3.56 MeV gamma-momentum vectors the excitation energy of the residual nucleus was determined. Results: In the study of the C-12(Li-6,Li-6*[3.56 MeV]) reaction, the isovector spin-transfer excitation-energy spectrum in the inelastic channel was successfully measured. The strong Gamow-Teller state in C-12 at 15.1 MeV was observed. Comparisons with the analog C-12(Li-6,He-6) reaction validate the method of extracting the Gamow-Teller strength. In measurements of the Mg-24, Nb-93(Li-6,Li-6*[3.56 MeV]) reactions, the 3.56 MeV gamma peak could not be isolated from the strong background in the gamma spectrum from the decay of the isoscalar excitations. It is argued that by using a gamma-ray tracking array instead of a clover array, it is feasible to extend the mass range over which the (Li-6,Li-6*) reaction can be used for extracting the isovector spin-transfer response up to mass numbers of similar to 25 and perhaps higher. Conclusions: It is demonstrated that the (Li-6,Li-6*[3.56 MeV]) reaction probe can be used to isolate the inelastic isovector spin-transfer response in nuclei. Application to nuclei with mass numbers of about 25 or more, however, will require a more efficient gamma-ray array with a better tracking capability.

Lifetime Measurements and Triple Coexisting Band Structure in ^{43}S.

Lifetime measurements of excited states in the neutron-rich nucleus ^{43}S were performed by applying the recoil-distance method on fast rare-isotope beams in conjunction with the Gamma-Ray Energy Tracking In-beam Nuclear Array. The new data based on γγ coincidences and lifetime measurements resolve a doublet of (3/2^{-}) and (5/2^{-}) states at low excitation energies. Results were compared to the π(sd)-ν(pf) shell model and antisymmetrized molecular dynamics calculations. The consistency with the theoretical calculations identifies a possible appearance of three coexisting bands near the ground state of ^{43}S: the K^{π}=1/2^{-} band built on a prolate-deformed ground state, a band built on an isomer with a 1f_{7/2}^{-1} character, and a suggested excited band built on a newly discovered doublet state. The latter further confirms the collapse of the N=28 shell closure in the neutron-rich region.

Performance of a segmented HPGe detector at KRISS

A 24 segmented HPGe coaxial detector was set up with a digitized data acquisition system (DAQ). The DAQ was composed of a digitizer (5 × 107sampling/s), a Field-Programmable Gate Array (FPGA), and a real time operating system. The Full Width Half Maximum (FWHM), rise time, signal characteristics, and spectra of a 137Cs source were evaluated. The data were processed using an in-house developed gamma-ray tracking system.

In-beam γ -ray spectroscopy of the neutron-rich platinum isotope Pt200 toward the N=126 shell gap

The neutron-rich nucleus \nucleus{200}{Pt} is investigated via in-beam \gamma-ray spectroscopy in order to study the shape evolution in the neutron-rich platinum isotopes towards the N = 126 shell closure. The two-neutron transfer reaction \nucleus{198}{Pt}(\nucleus{82}{Se}, \nucleus{80}{Se})\nucleus{200}{Pt} is used to populate excited states of \nucleus{200}{Pt}. The Advanced Gamma Ray Tracking Array (AGATA) demonstrator coupled with the PRISMA spectrometer detects \gamma rays coincident with the \nucleus{80}{Se} recoils, the binary partner of \nucleus{200}{Pt}. The binary partner method is applied to extract the \gamma-ray transitions and build the level scheme of \nucleus{200}{Pt}. The level at 1884\,keV reported by Yates et. al [Phys. Rev. C 37, 1889] was confirmed to be at 1882.1\,keV and assigned as the (6^+_1) state. An additional \gamma ray was found and it presumably de-excites the (8^+_1) state. The results are compared with state-of-the-art beyond mean-field calculations, performed for the even-even \nucleus{190-204}{Pt} isotopes, revealing that \nucleus{200}{Pt} marks the transition from the \gamma-unstable behaviour of lighter Pt nuclei towards a more spherical one when approaching the N=126 shell closure.