Low-energy Nuclear Physics Experiments Research Articles

Recent progress in the construction of KoBRA for low-energy nuclear physics experiments

A multi-purpose experimental instrument, called as KoBRA (Korea Broad acceptance Recoil spectrometer and Apparatus), has been constructed for low-energy nuclear physics experiments at RAON (Rare Isotope Accelerator complex for ON-line experiments), and is being prepared in order to produce rare isotope beams at an energy of about 20 MeV/nucleon during the first beam commissioning phase. A test was performed to measure the positions of 4He ions at the dispersive and achromatic foci of KoBRA using an 241Am α-source placed at the production target position, so as to examine the momentum dispersion and ion beam transport. We report on the results of the test and the detailed design of KoBRA including ion optics, together with the status of detectors for particle identification of rare isotopes.

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.

Development of Micro-Pattern Gaseous Detectors for Nuclear Reaction Studies

One of the frontiers of today’s nuclear physics research is the synthesis of Super Heavy Elements (SHE). Fusion-fission dynamics, namely the competition between quasi fission and fusion is one of the key challenges to optimize the SHE. To have an insight into the dynamics, one requires the study of fission fragment mass and angular distribution near barrier energies for heavy-ion induced fission reactions. Recent successful installation of linear accelerators in India offers a unique opportunity to study the dynamics of nuclear reactions and formation process of SHE. For the effective utilization of these current, as well as upcoming facilities, development of novel detectors to study reaction dynamics, formation process of SHE with heavier projectiles and higher beam energies is needed. Gaseous detectors have undergone a rapid improvement in terms of spatial, temporal and energy resolution, rate capability, radiation hardness, ion feedback etc., ushering in a new genre of micro-structured devices based on semi-conductor technology, commonly known as Micro-Pattern Gaseous Detectors (MPGDs). Although many of the MPGD structures were primarily developed for high-rate tracking of charged particles in high energy physics experiments, stability of operation, simplicity of construction and relatively low cost make these detectors suitable for other applications, such as low-energy nuclear physics experiments. The present activities encompass a detailed evaluation of the operational conditions of Micromesh-Multi Wire and THGEM-Multi Wire hybrid detector operated in low-pressure isobutane gas with a view to optimizing their use in the detection of charged particles and fission fragments.

Conduction-Cooled Superconducting Magnet for Active-Target TPC Experiments at RAON

A compact superconducting magnet was constructed for low-energy nuclear physics experiments at the RAON heavy-ion accelerator facility. Conduction-cooled Helmholtz coils with a radius of 40 cm generate a uniform magnetic field of up to 1.51 Tat a current of 71 A. The magnet has two crossing beam windows that allows the beam to pass either parallel (solenoid mode) or perpendicular (dipole mode) to the field direction. The magnet operates at a ramping rate of 0.01 A/s without quenching and generates a highly uniform magnetic induction within 0.1% of the central value of 1.51 T over the volume of 10 × 10 × 10 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> corresponding to the size of an active target time projection chamber. Herein, we report our first excitation test and field mapping results.

Beam-induced space-charge effects in time projection chambers in low-energy nuclear physics experiments

Tracking capabilities in Time Projection Chambers (TPCs) are strongly dictated by the homogeneity of the drift field. Ion back-flow in various gas detectors, mainly induced by the secondary ionization processes during amplification, has long been known as a source of drift field distortion. Here, we report on beam-induced space-charge effects from the primary ionization process in the drift region in low-energy nuclear physics experiment with Active Target Time Projection Chamber (AT-TPC). A qualitative explanation of the observed effects is provided using detailed electron transport simulations. As ion mobility is a crucial factor in the space-charge effects, the need for a careful optimization of gas properties is highlighted. The impact of track distortion on tracking algorithm performance is also discussed.

Automatic trajectory recognition in Active Target Time Projection Chambers data by means of hierarchical clustering

The automatic reconstruction of three-dimensional particle tracks from Active Target Time Projection Chambers data can be a challenging task, especially in the presence of noise. In this article, we propose a non-parametric algorithm that is based on the idea of clustering point triplets instead of the original points. We define an appropriate distance measure on point triplets and then apply a single-link hierarchical clustering on the triplets. Compared to parametric approaches like RANSAC or the Hough transform, the new algorithm has the advantage of potentially finding trajectories even of shapes that are not known beforehand. This feature is particularly important in low-energy nuclear physics experiments with Active Targets operating inside a magnetic field. The algorithm has been validated using data from experiments performed with the Active Target Time Projection Chamber developed at the National Superconducting Cyclotron Laboratory (NSCL). The results demonstrate the capability of the algorithm to identify and isolate particle tracks that describe non-analytical trajectories. For curved tracks, the vertex detection recall was 86% and the precision 94%. For straight tracks, the vertex detection recall was 96% and the precision 98%. In the case of a test set containing only straight linear tracks, the algorithm performed better than an iterative Hough transform.

NPTool: a simulation and analysis framework for low-energy nuclear physics experiments

The Nuclear Physics Tool (NPTool) is an open source data analysis and Monte Carlo simulation framework that has been developed for low-energy nuclear physics experiments with an emphasis on radioactive beam experiments. The NPTool offers a unified framework for designing, preparing and analyzing complex experiments employing multiple detectors, each of which may comprise some hundreds of channels. The framework has been successfully used for the analysis and simulation of experiments at facilities including GANIL, RIKEN, ALTO and TRIUMF, using both stable and radioactive beams. This paper details the NPTool philosophy together with an overview of the workflow. The framework has been benchmarked through the comparison of simulated and experimental data for a variety of detectors used in charged particle and gamma-ray spectroscopy.

High voltage studies of inverted-geometry ceramic insulators for a 350 kV DC polarized electron gun

Jefferson Lab is constructing a 350 kV direct current high voltage photoemission gun employing a compact inverted-geometry insulator. This photogun will produce polarized electron beams at an injector test facility intended for low energy nuclear physics experiments, and to assist the development of new technology for the Continuous Electron Beam Accelerator Facility. A photogun operating at 350kV bias voltage reduces the complexity of the injector design, by eliminating the need for a graded-beta radio frequency �capture� section employed to boost lower voltage beams to relativistic speed. However, reliable photogun operation at 350 kV necessitates solving serious high voltage problems related to breakdown and field emission. This study focuses on developing effective methods to avoid breakdown at the interface between the insulator and the commercial high voltage cable that connects the photogun to the high voltage power supply. Three types of inverted insulators were tested, in combination with two electrode configurations. Our results indicate that tailoring the conductivity of the insulator material, and/or adding a cathode triple-junction screening electrode, effectively serves to increase the hold-off voltage from 300kV to more than 375kV. Electrostatic field maps suggest these configurations serve to produce a more uniform potential gradient across the insulator.

Fabrication of calcium-gold and calcium-lead targets for ion beam experiments at TRIUMF ISAC-II

Multi-layer thin targets are vital in low-energy nuclear physics experiments employing the use of accelerated beams. In this paper, we describe the fabrication of encapsulated calcium targets produced by evaporation on gold and lead backings for radioactive beam experiments at TRIUMF’s ISAC-II facility.

A New approach for cross section measurements at low energies using a monolithic silicon telescope

We propose a new experimental technique for the study of low-energy nuclear reactions using a monolithic silicon telescope, which has a thin silicon layer with a thickness of 1 μ m. A feasibility study of this technique for the investigation of the 8 Li( α ,n) 11 B reaction at low energies has been performed using the GEANT4 simulation code and a calibrated alpha source. Our results indicate the possibility of a novel technique for low-energy nuclear physics experiments relevant to astrophysics.

Three-Compton telescope: theory, simulations, and performance

The advent of highly segmented gamma-ray detectors with good energy resolution has made a new class of gamma-ray detectors possible. These instruments record the positions and energies of each individual gamma-ray interaction with high precision. Analysis of the individual interactions can provide energy and directional information, even for events with only partial energy deposition. Advantages over traditional gamma-ray detectors include enhanced efficiency, background rejection, gamma-ray imaging, and sensitivity to polarization. Consider those gamma-rays that interact three or more times in the detector. The energy of the gamma-ray that initiated one of these events is uniquely determined by measuring the energies of the first two interactions and the scatter angle of the second interaction. The precision of this measurement is limited by the energy and position resolution of the detector, but also from Doppler broadening that results from gamma-ray scattering off-bound electrons in the detector. It is also essential to correctly sequence the first three interactions. The importance of Doppler broadening is greater in higher Z-materials, thus silicon becomes a good choice for the detector material. We discuss performance and simulations of the multiple Compton telescope. Possible applications include an advanced Compton telescope (ACT) for astrophysics, a medical multiple-gamma detector high-energy imaging survey instrument, and a gamma-ray tracking detector for future low-energy nuclear physics experiments.

A thin-film scintillator detector for light ions in low-energy nuclear physics

A thin-film plastic scintillator detector has been developed for use in time-of-flight measurements of fast light ions in low-energy nuclear physics experiments. The design of the detector is described, and its response to protons, deuterons and alpha particles is discussed with respect to light output, time resolution, and detection efficiency.

An on-line computer system in use with low-energy nuclear physics experiments

A description is given of a computing system used on-line with experiments performed at the 4 MeV Van de Graaff and the tandem Van de Graaff accelerators at Argonne National Laboratory. The requirements for the system are listed and the hardware and software obtained to fulfill these requirements are described. Some typical applications of the system are given.

Experience with an On-Line Computing System in Low-Energy Nuclear Physics Experiments

During the past three years, an online computing system has been designed, installed and used extensively in connection with low-energy nuclear physics experiments. The system is used by experimenters and both the 4.5 MeV Van de Graaff and the 12 MeV Tandem accelerators at Argonne National Laboratory. At the heart of the system are two inter-connected small computers (ASI 210 and 2100). These machines have fast operation times and both have 8192 words of core memory. Peripheral devices include punched card and paper tape equipment, typewriters, line-printers, two magnetic tape units, and a display oscilloscope with a light pen. ASI 210 is interfaced to a 4096-channel pulse-height analyzer and to 4 ADC units in the Tandem experimental area. The ASI 2100 has long-line links to the 4 MeV Van de Graaff and to the laboratory's CDC 3600 central processor. Experiments at both accelerators can interrupt the 3600 and use its high computing power to great advantage. The system is currently being augmented by the addition of the large external core memory (98,304 words) addressable by both small computers. The chief function of this memory is to act as a large two-parameter pulse-height analyzer and as additional storage for program. A special on-line data-handling program has been developed which greatly facilitates the acquisition and manipulation of experimental data. A brief description of the entire system will be given together with a description of the various ways in which it has been used.