Gamma-ray Tracking Research Articles

Response of AGATA segmented HPGe detectors to gamma rays up to 15.1 MeV

The response of AGATA segmented HPGe detectors to gamma rays in the energy range 2–15MeV was measured. The 15.1MeV gamma rays were produced using the reaction d(11B,nγ)12C at Ebeam=19.1MeV, while gamma rays between 2 and 9MeV were produced using an Am–Be–Fe radioactive source. The energy resolution and linearity were studied and the energy-to-pulse-height conversion resulted to be linear within 0.05%.Experimental interaction multiplicity distributions are discussed and compared with the results of Geant4 simulations. It is shown that the application of gamma-ray tracking allows a suppression of background radiation caused by n-capture in Ge nuclei. Finally the Doppler correction for the 15.1MeV gamma line, performed using the position information extracted with Pulse-shape analysis is discussed.

Neural network consistent empirical physical formula construction for neutron–gamma discrimination in gamma ray tracking

Gamma ray tracking is an efficient detection technique in studying exotic nuclei which lies far from beta stability line. To achieve very powerful and extraordinary resolution ability, new detectors based on gamma ray tracking are currently being developed. To reach this achievement, the neutron–gamma discrimination in these detectors is also an important task. In this paper, by suitable layered feedforward neural networks (LFNNs), we have constructed novel and consistent empirical physical formulas (EPFs) for some highly nonlinear detector counts measured in neutron–gamma discrimination. The detector counts data used in the discrimination was actually borrowed from our previous paper. The counts used here had been originally measured versus the following parameters: energy deposited in the first interaction points, difference in the incoming direction of initial gamma rays, and finally figure of merit values of the clusters determined by tracking. The LFNN–EPFs are of explicit mathematical functional form. Therefore, by various suitable operations of mathematical analysis, these LFNN–EPFs can be used to derivate further physical functions which might be potentially relevant to neutron–gamma discrimination performance of gamma ray tracking.

Consistent empirical physical formula construction for recoil energy distribution in HPGe detectors by using artificial neural networks

The gamma-ray tracking technique is one of the highly efficient detection method in experimental nuclear structure physics. On the basis of this method, two gamma-ray tracking arrays, AGATA in Europe and GRETA in the USA, are currently being developed. The interactions of neutrons in these detectors lead to an unwanted background in the gamma-ray spectra. Thus, the interaction points of neutrons in these detectors have to be determined in the gamma-ray tracking process in order to improve photo-peak efficiencies and peak-to-total ratios of the gamma-ray peaks. Therefore, the recoil energy distributions of germanium nuclei due to inelastic scatterings of 1-5 MeV neutrons were obtained both experimentally and using artificial neural networks. Also, for highly nonlinear detector response for recoiling germanium nuclei, we have constructed consistent empirical physical formulas (EPFs) by appropriate layered feed-forward neural networks (LFNNs). These LFNN-EPFs can be used to derive further physical functions which could be relevant to determination of neutron interactions in gamma-ray tracking process.

Circular polarimetry with gamma-ray tracking detectors

Measurement of circular polarization of gamma-rays with position sensitive solid state detectors is proposed. It is based on the transfer of the photon spin to the recoiled electron in the process of Compton scattering and subsequent detection of the electron spin polarization. Bremsstrahlung polarization correlations, in particular the left–right asymmetry of the photon emission from transversally polarized electrons, are used for this purpose. Compton scattering and electron bremsstrahlung events as well as the scattered and the bremsstrahlung photon absorptions take place inside the active volume of the position sensitive detector. These events are identified and their complete kinematics is reconstructed by means of gamma-ray tracking. No spin polarized targets and no inactive materials are needed for this technique. The proposed method is naturally integrated into the concept of a Compton telescope, allowing for the first time to build an imaging gamma-ray circular polarimeter. It should work in the energy region of several 100 keV up to several 10 MeV.

A novel HPGe detector for gamma-ray tracking and imaging

A novel, large-volume High Purity Germanium (HPGe) detector that will provide gamma-ray detection with both sub-mm position resolution and high efficiency is under development. This design is based on a coaxial HPGe geometry and employs a point contact along with segmentation of the outer electrode. Calculations indicate that this device will be capable of achieving values of position sensitivity which are approximately a factor of four or five greater than conventional gamma-ray tracking detectors.

Characterisation of a Broad Energy Germanium (BEGe) detector

Characterisation of Germanium detectors used for gamma-ray tracking or medical imaging is one of the current goals in the Nuclear physics community. Good knowledge of detector response to different gamma radiations is needed for this purpose. In order to develop this task, Pulse Shape Analysis (PSA) techniques have been developed for different detector geometries or setups. In this work, we present the results of the application of PSA for a Canberra Broad Energy Germanium (BEGe) detector. This detector was scanned across its front and bottom face using a fully digital data acquisition system; allowing to record detector charge pulse shapes from well defined positions with collimated sources of 241Am, 22Na and 137Cs. With the study of the data acquired, characteristics of the inner detector geometry like crystal limits or positions of contact and isolate can be found, as well as the direction of the axes for the Germanium crystal.

Gamma-ray tracking detectors: physics opportunities and status of GRETINA

Gamma-ray tracking detectors using highly segmented germanium detectors will give higher efficiency, better peak-to-total ratio and much higher position resolution than current generation of detectors. Particularly, the capability of reconstructing the position of the interaction with millimeters resolution is needed to fully exploit the physics opportunities provided by the next generation radioactive beam facilities. This paper presents the basic concepts of tracking, examples of physics opportunities, and the status of the project GRETA/GRETINA.

A Time-Over-Threshold Technique for Wide Dynamic Range Gamma-Ray Spectroscopy With the AGATA Detector

Wide-range energy measurements have been carried out with an encapsulated 36-fold segmented germanium detector of AGATA (Advanced GAmma-ray Tracking Array). An unprecedented equivalent energy range of 5 keV to 180 MeV, or ~90 dB, has been obtained using a single acquisition chain, while maintaining a high energy resolution in the whole spectrum. For energies larger than 3 MeV the pulse-height is obtained with a Time- over-Threshold (ToT) technique. The measured resolution in all the tested range from 3 to 50 MeV is of <0.4%. The ToT technique has been demonstrated using a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup> Am + Be source with Ni target generating high-energy gamma photons by neutron capture reactions. A remarkable resolution of 0.21% has been obtained on the Ni spectrum line at the energy of 8.998 MeV.

Portable gamma-ray tracking system for nuclear material safety

Using a CsI (Tl)-coupled PIN-type photodiode of 1 cm×1 cm and compact data acquisition system, a portable gamma monitoring system with lower cost has been fabricated and tested for gamma detection. After evaluating spectroscopy performance for gamma emission radioisotopes of Co-57 and Cs-137, we integrated the DAS with low-noise preamplifier, shaping amplifier, discriminator and multi-control unit (MCU) to acquire cumulative gamma detection data. The data are continuously sent to an observer, through a wireless telecommunication module of 400 MHz band, to monitor whether any gamma emitting material is out of sight or any accident takes place. A comparative study of gamma spectroscopy between our detector and a commercialized detector has been performed.

Performance of an AGATA asymmetric detector

High-resolution gamma-ray detectors based on high-purity germanium crystals (HPGe) are one of the key workhorses of experimental nuclear science. The technical development of such detector technology has been dramatic in recent years. Large volume, high-granularity, electrically segmented HPGe detectors have been realised and a methodology to improve position sensitivity using pulse-shape analysis coupled with the novel technique of gamma-ray tracking has been developed. Collaborations have been established in Europe (Advanced GAmma Tracking Array (AGATA)) [J. Simpson, Acta Phys. Pol. B 36 (2005) 1383] and the USA (GRETA/GRETINA) [C.W. Beausang, Nucl. Instr. and Meth. B 204 (2003)] to build gamma-ray tracking spectrometers. This paper discusses the performance of the first AGATA asymmetric detector that has been tested at the University of Liverpool. The use of a fully digital data acquisition system has allowed detector charge pulse shapes from a selection of well-defined photon interaction positions to be analysed, yielding important information on the position sensitivity of the detector.

Performance tests of large area position-sensitive planar germanium detectors with conventional and amorphous contacts

Large area position-sensitive planar germanium wafers are increasingly being used for a variety of gamma-ray imaging and tracking tasks. Position sensitivity can be achieved through measuring charge collected on orthogonal strip electrodes, and by digital pulse shape analysis. However, the development of this detector technology has been slow, and criteria to measure improved performance have not been well established. We have studied 93 and 88 mm square segmented detectors of 20 mm thickness made with conventional boron and lithium electrodes, and two 100 mm circular segmented detectors of 14 mm thickness with amorphous germanium contacts. We have compared these detectors with a planar detector of 15 mm thickness. Conventional energy resolution tests of individual strips are insufficient to fully categorize the performance of the position sensitive detectors. We propose some basic tests which can rapidly quantify any segmented detector characteristics, and show potential inadequacies which become important when imaging or tracking is attempted.

Global Trigger and Readout System for the AGATA Experiment

AGATA is a 4-pi array of high purity Ge detectors for in-beam gamma-ray spectroscopy based on the novel concepts of pulse shape analysis (PSA) and gamma-ray tracking. Tracking and PSA require the concurrent digitization-at a sampling rate of 100 Msamples/s-of preamplifier signals of the 36-fold segmented Ge crystals composing the array. Locally digitized data are optically transferred to remote pre-processing nodes for pulse energy computation. The design of the front-end readout and level-1 (L1) trigger in AGATA follows a synchronous pipeline model: the detector data are stored in pipeline buffers at the global AGATA frequency, waiting the global L1 decision. A global timing system provides a reference clock and time tag to the digitizers and the pre-processing units by means of a tree of optically connected timing units. Pre-processing nodes are integrated in advanced TCA-based carrier cards with full mesh connectivity in the backplane and read-out through pci-express based optical links. The front-end data readout and its integration in the global trigger and synchronization system will be described.

Gamma-ray tracking: Characterisation of the AGATA symmetric prototype detectors

Each major technical advance in gamma-ray detection devices has resulted in significant new insights into the structure of atomic nuclei. The next major step in gamma-ray spectroscopy involves achieving the goal of a 4pi ball of Germanium detectors by using the technique of gamma-ray energy tracking in electrically segmented Germanium crystals. The resulting spectrometer will have an unparalleled level of detection power for nuclear electromagnetic radiation. Collaborations have been established in Europe (AGATA) [J. Simpson, Acta Phys. Pol. B 36 (2005) 1383. [1]] and the USA (GRETA/GRETINA) to build gamma-ray tracking spectrometers. This paper discusses the performance of the AGATA (Advanced Gamma Tracking Array) symmetric prototype detectors that have been tested at the University of Liverpool. The use of a fully digital data acquisition system has allowed detector charge pulse shapes from a selection of well defined photon interaction positions to be analysed, yielding important information on the position sensitivity of the detector.

The AGATA project

Each major technical advance in gamma-ray detection devices has resulted in significant new insights into the structure of atomic nuclei. These advances have culminated in the construction of 4π arrays of escape-suppressed spectrometers that comprise a Ge detector and scintillation detector suppression shield. The next major step in gamma-ray spectroscopy involves achieving the ultimate goal of a 4π ball of just Ge detectors by using the technique of gamma-ray energy tracking in electrically segmented Ge crystals.The resulting spectrometer will have an unparalleled level of detection power for nuclear electromagnetic radiation. Its sensitivity for selecting the weakest signals from exotic nuclear events will be enhanced by a factor of up to 1000 relative to its predecessors. It will have an unprecedented angular resolution making it ideally suited for high-energy resolution even at recoil velocities up to 50% of the velocity of light. Therefore, it is ideally suited to be used in conjunction with the new generation of radioactive beam accelerators or existing stable beam facilities.A European collaboration has been established to construct a 4π tracking spectrometer called AGATA (Advanced Gamma Tracking Array). This collaboration is currently performing the research and development necessary to finalise the technology for gamma-ray tracking and hence fully specify the full 4π spectrometer. The status of this first phase of the AGATA project will be reported.

GRETINA status and recent progress: The effect of neutron damage on energy and position resolution of the GRETINA detector

GRETINA is a high precision gamma-ray detector for nuclear spectroscopy experiments, which is currently being developed. It consists of a number of highly segmented coaxial germanium detectors arranged in a close-packed configuration. GRETINA uses digital signal processing electronics and pulse shape analysis to determine the energy and position of each individual interaction resulting from the gamma-ray absorption. Gamma-ray tracking is then used to reconstruct the gamma-ray scattering sequence. This results in high efficiency, peak-to-total ratio and position resolution. Particularly, the capability of reconstructing the position of the interaction with resolution at the level of a few millimeters is a fundamental requirement and is important to understand and quantify limiting factors of position resolution. This paper reports on the effects of neutron damage on pulse shape and on its impact on position resolution. Simulation results show that even a relatively heavily damaged detector maintains high position resolution. Only for neutron flux >6×109 neutron/cm2, the position resolution becomes worse than 1mm, which is the achievable limit.

The AGATA Project

For decades, the study of the gamma-ray decay from quantal states of the atomic nucleus has played a pivotal role in discovering and elucidating a wide range of phenomena. Each major technical advance in gamma-ray detection devices has resulted in significant new insights into the structure of nuclei. To date, these advances have culminated in the construction of 4π arrays of escape-suppressed spectrometers. The global consensus of opinion is that the next major step in γ-ray spectroscopy involves abandoning the concept of a physical suppression shield and achieving the ultimate goal of a 4π Ge ball using the technique of γ-ray energy tracking in electrically segmented Ge crystals. The resulting spectrometer will have an unparalleled level of detection power for nuclear electromagnetic radiation. Its sensitivity for selecting the weakest signals from exotic nuclear events will be enhanced by a factor of up to 1000 relative to its predecessors. It will have an unprecedented angular resolution making it ideally suited for high-energy resolution even at recoil velocities up to 50% of the velocity of light. Therefore, it is ideally suited to be used in conjunction with the new generation of radioactive beam accelerators or existing stable beam facilities. In Europe, a collaboration has been established to construct a 4π tracking spectrometer called AGATA (advanced gamma tracking array). This collaboration has signed a Memorandum of Understanding for the first phase of the project to perform the research and development necessary to finalize the technology for gamma-ray tracking and hence fully specify the full 4π spectrometer. The status of this first phase of the AGATA project will be reported.

Ambiguity in gamma-ray tracking of ‘two-interaction’ events

Tracking of gamma-rays in germanium detectors can allow the full reconstruction of interactions, a feature that is useful in many applications. Scrutiny of the kinematics and geometry of gamma-rays which are Compton scattered only once prior to full absorption reveals that there are special cases where even perfect spatial and energy resolution cannot resolve the true interaction sequence and consequently the gamma-ray tracks cannot be unambiguously reconstructed. The photon energy range where this ambiguity exists is from 255 keV to about 700 keV. This is an energy region of importance for nuclear structure research and a domain where two-point interactions are probable.

GRETINA: A gamma ray energy tracking array

A gamma-ray energy tracking array (GRETA) is a new concept for the detection of gamma radiation. In such an array, the individual interactions of all the gamma rays are identified by their energies and positions. Then, using tracking algorithms based on the properties of gamma ray interactions, the scattering sequences are reconstructed. GRETA will give high peak efficiency, peak-to-background ratio, and position resolution. Recent research and development efforts have demonstrated that the construction of a gamma ray tracking array is feasible, and a plan for constructing a US array GRETINA is in place.

The advanced gamma ray tracking array AGATA

New accelerator facilities for radioactive-ion beams and high-intensity stable beams will start operation in a few years. Although these beams will provide interesting opportunities for exploring unknown territories of the nuclear landscape, the experimental conditions will be very challenging and, indeed, the nuclear structure community has realized that a new generation of powerful arrays for gamma-ray spectroscopy has to be built in order to cope with them. As a result of years of experience with Compton suppressed germanium arrays and of intensive R&D work targeted to extend their limits, it is now clear that the next 4<FONT FACE=Symbol>p g</FONT>-ray spectrometers will be built fully from germanium detectors and will be based on the technique of gamma-ray tracking. The "Advanced GAmma Tracking Array" (AGATA), proposed in Europe, will be an instrument of major importance for nuclear structure studies at the very limits of nuclear stability. It will be built out of 120/180 highly segmented Ge crystals operated in position sensitive mode by means of digital data techniques and pulse shape analysis of the segment signals. AGATA will be able to measure gamma radiation in a large energy range (from ~ 10 keV to ~ 10 MeV), with the largest possible photopeak effi ciency (25 % at Mgamma = 30) and with a good spectral response. In particular, its very good Doppler correction and background rejection capabilities will allow to perform "standard" gamma-ray spectroscopy experiments using fragmentation beams with sources moving at velocities up to beta ~ 0.5.

The GRT4 VME pulse processing card for segmented germanium detectors

A four channel VME card with 14 bit, 80 MHz digitizers and powerful on-board processing has been designed, built and used in tests of digital pulse processing techniques for gamma-ray tracking. This paper explains the background (rationale for the project), describes the VME card (known as the GRT4) and presents a 64 channel GRT4 digitizing system which was used to instrument two segmented Germanium detectors during in-beam tests. Results obtained using the GRT4 card are presented as well as some applications.