Low Energy Heavy Ion Research Articles

Design and commissioning of a two-harmonic pre-buncher for extended bunch spacing at LEAF

The Low Energy Heavy Ion Accelerator Facility (LEAF) is a low-energy, high-intensity heavy-ion linear accelerator facility at Institute of Modern Physics (IMP) for multidiscipline research. The beam bunch repetition rate coincides with the LINAC frequency of 81.25 MHz, resulting in a 12.3 ns period. However, in the context of certain nuclear physics experiments, a longer bunch spacing is deemed essential for the precise detection of experimental products. To address this need, a strategy to extend the bunch spacing to 98 ns has been proposed by integrating a pre-buncher operating at a fundamental frequency of 10.156 MHz before the radio-frequency quadrupole (RFQ) in conjunction with a fast chopper in the medium energy beam transport (MEBT) system. The pre-buncher is a pivotal component in enabling a greater beam current at the target, surpassing the capability of employing a chopper alone. To enhance the bunching efficiency, the pre-buncher operates with two frequencies: the fundamental frequency of 10.156 MHz and its first sub-harmonic of 20.3125 MHz, each of which is produced by a coaxial resonator. This paper describes the design, manufacture, off-line testing, and beam commissioning of the pre-buncher.

Electromagnetic fields in low-energy heavy-ion collisions with baryon stopping

Electromagnetic fields in low-energy heavy-ion collisions with baryon stopping

Medically relevant radioisotope production through low energy heavy ion reactions

Medically relevant radioisotope production through low energy heavy ion reactions

Development of a low-pressure Multi-Mesh THGEM detector for fission experiments at FRIB

A small-area imaging detector prototype for position measurement of fission fragments produced in low energy heavy-ion reactions is presented in this study. The detector readout is equipped with a 2-dimensional, position-sensitive gaseous avalanche readout based on the novel Multi-Mesh Thick-GEM (MM-THGEM), and is coupled to a delay-line board for particle localization. The prototype has an effective area of 10 × 10 cm2 and is operated in isobutane at low pressure (7–10 Torr). We present and discuss a series of systematic evaluation tests performed by irradiating the detector with α-particles and fission fragments emitted by small-rate sources (241Am, 249Cf and 252Cf). Position and time resolutions of about 0.42 mm and 1 ns were achieved, respectively. This work serves as a benchmark for the development of a large-scale array of detectors for experiments with fission and fission-like reaction products at the Facility for Rare Isotope Beams (FRIB).

A high-intensity, low-energy heavy ion source for a neutron target proof-of-principle experiment at LANSCE

The capability to directly study neutron capture reactions on radionuclides with half-lives on the of order minutes would allow key cross section measurements in nuclear astrophysics and energy applications. However, such experiments with stationary targets are currently impossible because of signal detection and target sample fielding issues. To overcome these challenges, a neutron target facility is being developed to permit neutron capture experiments on unstable isotopes in inverse kinematics at the Los Alamos Neutron Science Center (LANSCE). This next-generation facility will consist of a heavily moderated, high-intensity spallation neutron target coupled with a radioactive ion beam storage ring. A proof-of-principle experiment is underway to demonstrate this neutron target concept at LANSCE in the near term with stable ions and without a storage ring. Here, mA-level beams of 10-50 keV heavy ions exhibiting large resonant neutron capture cross sections will be generated by a new ion source, transported through a large-volume neutron moderator surrounding an adjacent spallation target driven by the LANSCE accelerator, and collected downstream of the moderator for subsequent decay-counting measurements. The neutron density within the moderator will be obtained from these decay yields and compared with Monte Carlo N-Particle (MCNP) simulation results. The science application, operational requirements, and performance objectives for this heavy ion source will be presented.

Diamond ∆E-E telescope with an ultra-thin ∆E-diamond detector

The limits of mechanical polishing and RIE/ICP etching process were investigated to reduce the thickness of detector-grade ultra-thin below 10 μm, preserving the high quality of starting single crystal diamond (SCD) material. Free standing electronic grade SCD films with sizes up 4.5 × 4.5 mm2 and thickness down to 5 μm with overall thickness uniformity of ±0.35 μm were fabricated. The ultra-thin films were flat demonstrating the low internal stress.Diamond ∆E − E telescope was made from 5 μm thick ΔE-detector and 500 μm thick E-detector. The telescope was installed at the MARS spectrometer (Cyclotron Institute, TAMU) and tested with 241Am and 228Th alpha sources and 20Ne ion-beam, demonstrating the good spectral resolution around 100 keV. Simulations with the LISE++ program indicate that the telescope will be suitable to measure low energy heavy ion beams at energies as low as 2–3 MeV/nu for Au and U heavy ions, and even lower energies for lighter ions. The diamond telescope will be superior to common Si radiation telescopes having a low life-time in intense heavy ion-beams.

A prototype scintillator real-time beam monitor for ultra-high dose rate radiotherapy.

FLASH Radiotherapy (RT) is an emergent cancer RT modality where an entire therapeutic dose is delivered at more than 1000 times higher dose rate than conventional RT. For clinical trials to be conducted safely, a precise and fast beam monitor that can generate out-of-tolerance beam interrupts is required. This paper describes the overall concept and provides results from a prototype ultra-fast, scintillator-based beam monitor for both proton and electron beam FLASH applications. A FLASH Beam Scintillator Monitor (FBSM) is being developed that employs a novel proprietary scintillator material. The FBSM has capabilities that conventional RT detector technologies are unable to simultaneously provide: (1) large area coverage; (2) a low mass profile; (3) a linear response over a broad dynamic range; (4) radiation hardness; (5) real-time analysis to provide an IEC-compliant fast beam-interrupt signal based on true two-dimensional beam imaging, radiation dosimetry and excellent spatial resolution. The FBSM uses a proprietary low mass, less than 0.5mm water equivalent, non-hygroscopic, radiation tolerant scintillator material (designated HM: hybrid material) that is viewed by high frame rate CMOS cameras. Folded optics using mirrors enable a thin monitor profile of ∼10cm. A field programmable gate array (FPGA) data acquisition system generates real-time analysis on a time scale appropriate to the FLASH RT beam modality: 100-1000Hz for pulsed electrons and 10-20kHz for quasi-continuous scanning proton pencil beams. An ion beam monitor served as the initial development platform for this work and was tested in low energy heavy-ion beams (86Kr+26 and protons). A prototype FBSM was fabricated and then tested in various radiation beams that included FLASH level dose per pulse electron beams, and a hospital RT clinic with electron beams. Results presented in this report include image quality, response linearity, radiation hardness, spatial resolution, and real-time data processing. The HM scintillator was found to be highly radiation damage resistant. It exhibited a small 0.025%/kGy signal decrease from a 216 kGy cumulative dose resulting from continuous exposure for 15 min at a FLASH compatible dose rate of 237Gy/s. Measurements of the signal amplitude versus beam fluence demonstrate linear response of the FBSM at FLASH compatible dose rates of >40Gy/s. Comparison with commercial Gafchromic film indicates that the FBSM produces a high resolution 2D beam image and can reproduce a nearly identical beam profile, including primary beam tails. The spatial resolution was measured at 35-40µm. Tests of the firmware beta version show successful operation at 20000Hz frame rate or 50 µs/frame, where the real-time analysis of the beam parameters is achieved in less than 1 µs. The FBSM is designed to provide real-time beam profile monitoring over a large active area without significantly degrading the beam quality. A prototype device has been staged in particle beams at currents of single particles up to FLASH level dose rates, using both continuous ion beams and pulsed electron beams. Using a novel scintillator, beam profiling has been demonstrated for currents extending from single particles to 10 nA currents. Radiation damage is minimal and even under FLASH conditions would require ≥50 kGy of accumulated exposure in a single spot to result in a 1% decrease in signal output. Beam imaging is comparable to radiochromic films, and provides immediate images without hours of processing. Real-time data processing, taking less than 50 µs (combined data transfer and analysis times), has been implemented in firmware for 20kHz frame rates for continuous proton beams.

Tuning magnetocrystalline anisotropy by Au ion induced defects in NiO thin films

Intercalations and doping effectively induce room-temperature ferromagnetism in antiferromagnetic NiO thin films. This report investigates the defect-controlled ferromagnetic properties of Au ion implanted highly crystalline NiO thin films at different ion fluences. Low energy heavy ion irradiation leads to preferential sputtering of Ni and O atoms that create various kinds of vacancies and interstitial defect states within the matrix. These defect states are responsible for bandgap narrowing and changing the crystallite size and show the defect luminescence in the NiO thin films. It has been observed that the intrinsic Ni and O defects with the non-stoichiometric ratio of these atoms during crystal growth, resulting in uncompensated spins, induce the ferromagnetic properties in the pristine sample and are further modulated with ion fluences. The presence of defects gives rise to energy variations due to the modification of the magnetic environment of Ni ions in the lattice and tune the blocking temperature and magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant (K) reaches its maximum at 70 erg/cm3 with ion fluence and decreases at the highest fluence because of the suppression of exchange interaction by a large number of defect centers. The modulation of anisotropy in NiO thin films is intricately linked to crystallite size, bandgap narrowing, blocking temperature, and the magnetization mechanism, which is a direct result of defect concentrations in the material.

Heavy ion induced e+e− pair production: Theoretical and preliminary experimental efforts

The production of real e+e− pairs is an important quantum electrodynamics phenomenon, that could have significant implications on low energy heavy ion dynamics. Here, we employ an established Schwingerinspired theory to study this effect in nuclear disintegrations. We extend the current framework with a Monte Carlo approach to include non-perturbative contributions. Simultaneously, we supplement our study with a preliminary experimental procedure, via the use of the Global Spectrometer for Positron and Electron Characterization developed at the Texas Petawatt Laser facility of the University of Texas at Austin. Theory and experiment agree within one order of magnitude, which signals promise for further investigations.

On the missing single collision peak in low energy heavy ion scattering

We present experimental and simulation data on the oblique angle scattering of heavy Sn ions at 14keV energy from a Mo surface. The simulations are performed with the binary collision approximation codes TRIM, TRIDYN, TRI3DYN, SDTrimSP, and IMSIL. Additional simulations were performed in the molecular dynamics framework with LAMMPS. Our key finding is the absence of an expected peak in the experimental energy spectrum of backscattered Sn ions associated with the pure single collision regime. In sharp contrast to this, however, all simulation codes we applied do show a prominent single collision signature both in the energy spectrum and in the angular scatter pattern. We discuss the possible origin of this important discrepancy and show in the process, that widely used binary collision approximation codes may contain hidden parameters important to know and to understand.

Measurement and Simulation of Ultra-Low-Energy Ion-Solid Interaction Dynamics.

Ion implantation is a key capability for the semiconductor industry. As devices shrink, novel materials enter the manufacturing line, and quantum technologies transition to being more mainstream. Traditional implantation methods fall short in terms of energy, ion species, and positional precision. Here, we demonstrate 1 keV focused ion beam Au implantation into Si and validate the results via atom probe tomography. We show the Au implant depth at 1 keV is 0.8 nm and that identical results for low-energy ion implants can be achieved by either lowering the column voltage or decelerating ions using bias while maintaining a sub-micron beam focus. We compare our experimental results to static calculations using SRIM and dynamic calculations using binary collision approximation codes TRIDYN and IMSIL. A large discrepancy between the static and dynamic simulation is found, which is due to lattice enrichment with high-stopping-power Au and surface sputtering. Additionally, we demonstrate how model details are particularly important to the simulation of these low-energy heavy-ion implantations. Finally, we discuss how our results pave a way towards much lower implantation energies while maintaining high spatial resolution.

Neural network-based nuclear charge Z identification from ionization chamber waveforms of low-energy heavy ions

This study investigates Z identification by analyzing the low-energy heavy ion waveforms in a high-resolution low-pressure ionization chamber through a multilayer perceptron (MLP) neural network. Five nuclides - 127I, 128,129,134Xe, and 133Cs - were measured and their waveforms normalized to minimize interference from mass and energy. The MLP model, trained on these normalized waveforms, achieved a skill score of 53.2%. Our analysis of the amplitude distribution of waveform sampling points revealed that the MLP identified Z by considering the amplitude of the sampling points where statistical characteristics are learned during training. Furthermore, the skill score increased with the number of statistical characteristics learned. Therefore, three methods were developed to enhance the skill score: mean waveform training dataset generation, training dataset mean waveform augmentation, and training dataset waveform downsampling, which increased the skill score to 75.2%, 70.5%, and 71.5%, respectively.

Measuring neutron capture cross sections of radioactive nuclei

Measuring neutron capture cross sections of radioactive nuclei is a crucial step towards a better understanding of the origin of the elements heavier than iron. For decades, the precise measurement of direct neutron capture cross sections in the “stellar” energy range (eV up to a few MeV) was limited to stable and longer-lived nuclei that could be provided as physical samples and then irradiated with neutrons. New experimental methods are now being developed to extend these direct measurements towards shorter-lived radioactive nuclei (t_{1/2}< 1 y). One project in this direction is a low-energy heavy-ion storage ring coupled to the ISAC facility at TRIUMF, Canada’s accelerator laboratory in Vancouver BC, which has a compact neutron source in the ring matrix. Such a pioneering facility could be built within the next 10 years and store a wide range of radioactive ions provided directly from the existing ISOL facility, allowing for the first time to carry out direct neutron capture measurements on short-lived isotopes in inverse kinematics.

Probing neutron-star matter in the lab: Similarities and differences between binary mergers and heavy-ion collisions

Binary neutron-star mergers and heavy-ion collisions are related through the properties of the hot and dense nuclear matter formed during these extreme events. In particular, low-energy heavy-ion collisions offer exciting prospects to recreate such {extreme} conditions in the laboratory. However, it remains unexplored to what degree those collisions can actually reproduce hot and dense matter formed in binary neutron star mergers. As a way to understand similarities and differences between these systems, we {discuss their geometry and }perform a direct numerical comparison of the thermodynamic conditions probed in both collisions. To enable a direct comparison, we employ a finite-temperature equation of state able to describe the entire high-energy phase diagram of Quantum Chromodynamics. Putting side by side the evolution of both systems, we find that laboratory heavy-ion collisions at the energy range of $E_{\mathrm{lab}}=0.4 - 0.6\ A$ MeV probe (thermodynamic) states of matter that are very similar to those created in binary neutron-star mergers. These results can inform future low-energy heavy-ion collisions probing this regime.

Bremsstrahlung photons from a hadronizing quark-gluon plasma

Assuming that quark and antiquark numbers are separately conserved during hadronization, we calculate Bremsstrahlung photon from a hadronizing quark-gluon plasma. The quark and antiquark numbers are obtained from the hadron numbers in the statistical model and the transition amplitudes for the hadronization from the constraint that all quarks and antiquarks must be consumed in the hadronization. Then Bremsstrahlung photon from the hadronization is obtained in the soft photon approximation, and we find that its contribution to the direct photon increases in low-energy heavy-ion collisions and in peripheral collisions where the lifetime of a quark-gluon plasma (QGP) is relatively short.

Theoretical uncertainty quantification for heavy-ion fusion

Many aspects of physics that spans from the origin of the elements to nuclear technologies rely on a robust description of nuclear reactions. One therefore needs predictive modelling of nuclear quantum many-body dynamics in which the uncertainties are well-understood. The authors use the time-dependent Hartree-Fock approach, and apply uncertainty quantification to the nuclear dynamics, considering a collection of low-energy heavy-ion fusion reactions. They find that the uncertainty in the theoretical results mainly arises from structure properties, such as the skin of neutron-rich nuclei, which are particularly ill-constrained. This work highlights the importance of including reaction data in the construction and constraint of new theoretical models that can treat dynamics and structure on the same footing.

First Experiments with CRYRING@ESR

The low-energy heavy ion storage ring CRYRING was transported from Stockholm to Darmstadt, modernized and reconfigured, and recommissioned as CRYRING@ESR. The machine is now in operation with all installations in service and is available as a user facility for experiments proposed through the SPARC collaboration. During the 2020–2022 period, we brought a number of experimental installations into service and used them to measure first data: the ultra-cold electron cooler for merged-beam electron–ion collisions, the gas jet target for atomic collisions, a next-generation microcalorimeter-based X-ray spectroscopy setup, and others. Ions can be injected either in low charge states from a local ion source through a 300 keV/u RFQ linac, or in high charge states from the GSI accelerator chain through ESR. This allows for very broad access to ions across the entire periodic table. CRYRING@ESR is able to de- or accelerate ions and cool and store beams of isotopically pure species in a desired charge state. While the analysis is still largely ongoing, the first experimental data already show that the machine reached its expected performance level, and our high expectations regarding achievable resolution in spectroscopy experiments have been fulfilled. With access to new classes of ions available through ESR injection and a new generation of experimental instrumentation, CRYRING@ESR is a unique facility for experiments with heavy, highly charged ions. Here, we will review our present setup and machine performance, discuss the data from our first commissioning experiments and briefly preview the upcoming new installations for the coming years.

J/ψ yields in low energy nuclear collisions at SPS and FAIR: A baseline estimation

The yield of J/ψ mesons, produced in proton-nucleus (p+A) and nucleus-nucleus (A+A) collisions are estimated within a Glauber model ansatz for the upcoming low energy heavy-ion collision experiments at SPS and FAIR. A data driven parametrization is employed to incorporate the effects of Cold Nuclear Matter (CNM) on the J/ψ production cross-section.

Response of a baryon-charged medium to energetic partons

We explore the response to energetic partons of a baryon-charged medium produced in low energy heavy-ion collisions in which the partonic energy loss rate is expected to depend on both temperature and baryon chemical potential. The energy and momentum deposited by the partons are described by dynamical sources added to hydrodynamic equations of motion. We study the distortions of various hydrodynamic quantities, especially the energy and baryon densities, induced by energetic partons plowing through the medium. By studying the contribution from this medium response to the emission spectra of several identified hadron species we identify qualitative differences between the jet-induced modifications for mesons and net protons.

Development of a beam energy adjustment system after a Radio-Frequency-Quadrupole

The Low Energy heavy-ion Accelerator Facility (LEAF) is a low-energy, high-intensity, heavy ion accelerator for multidisciplinary research developed at the Institute of Modern Physics (IMP). Beam commissioning of the facility began in 2018, but until recently, only a fixed beam energy of 0.5 MeV/u was available because the main accelerator structure is a Radio-Frequency Quadrupole (RFQ), which hinders LEAF to meet the requirements of the experimental terminals, for example, 12C+12C fusion reaction investigation at astrophysical energies requires beam energies adjustable from 0.3 MeV/u to 0.7 MeV/u. To expand the beam energy range, a “constant-beta” structure Drift Tube Linac (DTL) was developed providing an adjustable beam energy from 0.3 to 0.7 MeV/u. To control the beam quality, specifically the energy spread, two re-bunchers are employed, one upstream and one downstream of the DTL. Here we present the development of the key device: an Interdigital H-mode DTL (IH-DTL). First, we discuss the beam dynamics design for the beam energy adjustment system, then we present details of the test bench, and finally, we report measurements performed with the finished hardware. The measurements of energy adjustment and energy spread agree well with the simulations, demonstrating the feasibility of this design.