Radioactive Ion Beam Facility Research Articles

Nuclear structure aspects of the heaviest N∼Z nuclei south of 100Sn

Ever-lasting interest in the structure of 100Sn and neighbouring nuclei is still well justified by the fact that it is the heaviest doubly-magic nucleus with N=Z. State-of-the-art experimental techniques involving stable and radioactive beam facilities have enabled access to these exotic nuclei. In particular, the analysis of experimental data obtained in two DESPEC experiments at GSI Darmstadt extends the information on the shell structure and its evolution towards N = Z = 50, and allows the study of seniority conservation and proton-neutron interaction in the g9/2 orbit. Several theoretical approaches for shell-model investigations are discussed and their predictive power assessed. The calculated systematics of the reduced transition probabilities for high- to medium-spin states in N∼Z isotopes with active g9/2 orbit is presented for the first time.

Experimental setup for Weak Interaction Studies with Radioactive ion-beams WISArD

The article describes the commissioning and technical development of the Weak Interaction Studies with 32Ar Decay (WISArD) experiment, installed at the radioactive ion-beam facility ISOLDE/CERN. The experiment aims to extend the present limits on scalar and tensor currents in the weak interaction and hence search for physics beyond the Standard Model. The evaluation of these limits relies on measuring the proton energy in beta-delayed proton emission, sensitive to both the beta-neutrino angular correlation coefficient aβν and the Fierz interference term b. The method tries to improve previous studies by considering the positron-proton coincidences when determining the kinematic shift in the energy of the emitted protons. Using this coincidence technique, the aβν and b coefficients will be measured at the per mil level. Simulations were employed to optimize the ion beam transport efficiency and validate proof-of-principle results obtained in November 2018 (Nov2018). Upgrades are ongoing, and we are looking into improvements to the overall performance of the setup.

TriSol: A major upgrade of the TwinSol RNB facility

We report here on the recent upgrade of the TwinSol radioactive nuclear beam (RNB) facility at the University of Notre Dame. The new TriSol system includes a magnetic dipole to provide a second beamline and a third solenoid which acts to reduce the size of the radioactive beam on target.

Outlook on software framework in low and intermediate energy nuclear physics

Over the past decades nuclear physics experiments have seen a drastic increase in complexity. With the arrival of second generation radioactive ion beam facilities all over the world, the run for exploring more and more exotic nuclei is raging. The low intensity of RI-beams requires more complex setup, larger solid angle coverage and detection of a wider variety of charged and neutral particles. Design, construction and operation of a variety of complex instruments used in such experiments require many software developments. The short lifetime of experimental setups and multiple combinations of instruments demand a strong methodology. As the community is shifting to this new paradigm, the quest for the optimum framework is becoming central in the field. In this outlook we will introduce the specificity of the nuclear physics community, technical needs of such frameworks, and give an overview of the existing ones, with an emphasize on the difficult balance between computing performances, versatility and integration with other frameworks.

Constraining nucleosythesis in neutrino-driven winds using the impact of (α, xn) reaction rates

The lighter heavy elements of the first r-process peak, between strontium and silver, can be synthesized in the moderately neutron-rich neutrino–driven ejecta of either core–collapse supernovae or neutron star mergers via the weak r–process. This nucleosynthesis scenario exhibits uncertainties from the absence of experimental data from (α, xn) reactions on neutron–rich nuclei, which are currently based on statistical model estimates. We have performed a new impact study to identify the most important (α, xn) reactions that can affect the production of the lighter heavy elements under different astrophysical conditions using new, constrained (α, xn) reaction rates based on the Atomki-V2 αOMP. Our results show how when reducing the nuclear physics uncertainties, we can use abundance ratios to constrain the astrophysical conditions/environment. This can be achieved in the near future, when the key (α, xn) reaction rates will be measured experimentally in radioactive beam facilities.

Total Absorption Spectroscopy of Fission Fragments Relevant for Reactor Physics and Nuclear Structure and Astrophysics

The accurate determination of reactor antineutrino spectra remains a very hot research topic, where new questions have emerged in recent years. Indeed, after the “reactor anomaly” – a deficit of measured antineutrinos at short baseline reactor experiments with respect to spectral predictions – the three international reactor neutrino experiments Double Chooz, Daya Bay and Reno have evidenced spectral distortions in their measurements with respect to the same spectral predictions. This puzzle is called the “shape anomaly”. Recently summation calculations of reactor antineutrino spectra based on the use of nuclear data have obtained the best agreement to date with the reactor neutrino flux measurements at the level of 2% thanks to a decade of Total Absorption Gamma-ray Spectroscopy (TAGS) measurements at the radioactive beam facility of the University of Jyväskylä in two experimental campaigns. A selection of the results obtained so far is presented.

Radioactive ion beam opportunities at the new FRAISE facility of INFN-LNS

At the Laboratori Nazionali del Sud of INFN (INFN-LNS) in Catania, the construction of the new Radioactive Ion Beams (RIBs) facility FRAISE (FRAgment In-flight SEparator) has reached its ending phase. The facility uses the in-flight technique based on a primary beam fragmentation impinging on light Be or C targets. FRAISE makes use of light and medium mass primary beams, having power up to ≈ 2–3 kW, leading to RIBs, whose intensities vary in the range of ≈ 103–107 pps, for nuclei far from and close to the stability valley, respectively. FRAISE aims at providing high-intensity and high-quality RIBs for nuclear physics experiments, also serving to interdisciplinary research areas, such as medical physics. Critical aspects for high-quality beams are the tuning and transport, representing time-consuming processes and requiring dedicated diagnostics and tagging devices measuring many features of RIBs. Some of these devices should be capable to operate in radioactively activated environments because of the expected 2 kW beam lost in the dipole after the production target. Due to its peculiar robustness to radioactive damage, Silicon Carbide (SiC) technology has been considered for the detection layer. In this view, an R&D campaign has been started aiming at developing the FRAISE facility, the new diagnostics system, and a new tagging device, the latter of which will be useful for the CHIMERA multidetector beamline. In this paper, we discuss the status and the perspectives of the facility with a focus on the RIBs opportunities.

β decay of neutron-rich Cu76 and the structure of Zn76

The $\ensuremath{\beta}$ decay of $^{76}\mathrm{Cu}$ to the levels of $^{76}\mathrm{Zn}$ was studied at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Lab (ORNL). A purified $^{76}\mathrm{Cu}$ beam was developed and data were recorded for the decay of the $A=76$ decay chain using four high-purity germanium (HPGe) clover detectors at the Low-energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS). In this measurement, data on \ensuremath{\gamma}-ray emission following $\ensuremath{\beta}$ decay, including $\ensuremath{\beta}\ensuremath{\gamma}$ and $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidences, were collected and $\ensuremath{\gamma}\ensuremath{\gamma}$ spectra were analyzed to identify the statistically significant coincidences. From this analysis, we propose a level scheme for $^{76}\mathrm{Zn}$ which contains a total of 59 energy levels up to 6.0 MeV containing 105 $\ensuremath{\gamma}$ rays. We have identified an additional 53 $\ensuremath{\gamma}$ rays associated with this decay which could not be place in the decay scheme due to insufficient coincidence information or no energy match to identified levels. No $\ensuremath{\gamma}$ rays from states in $^{75}\mathrm{Zn}$ fed in the delayed-neutron branch were observed even though other $\ensuremath{\gamma}$ rays in the $A=75$ decay chain were observed. Spin and parity assignments are proposed for some levels based on comparison to systematics and shell model calculations.

Collision-Induced Dissociation at TRIUMF's Ion Trap for Atomic and Nuclear science

The performance of high-precision mass spectrometry of radioactive isotopes can often be hindered by large amounts of contamination, including molecular species, stemming from the production of the radioactive beam. In this paper, we report on the development of Collision-Induced Dissociation (CID) as a means of background reduction for experiments at TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN). This study was conducted to characterize the quality and purity of radioactive ion beams and the reduction of molecular contaminants to allow for mass measurements of radioactive isotopes to be done further from nuclear stability. This is the first demonstration of CID at an ISOL-type radioactive ion beam facility, and it is shown that molecular contamination can be reduced up to an order of magnitude.

Development and testing of 37.8 MHz, 3.0 kW RF amplifier system for Radioactive Ion Beam facility at VECC Kolkata

This paper reports the design, development and testing of a 3.0 kW, 37.8 MHz radio frequency (RF) amplifier system that has been built by combining four solid-state amplifier modules of 1.0 kW each. The design philosophy and test results of individual components such as driver amplifier, final amplifier modules, power divider/combiner, dual directional coupler (DDC) with system control and interlocks are discussed in detail. Compact 4-way Wilkinson power divider and combiner have been implemented using combination of 50 Ω coaxial cables at this lower frequency of 37.8 MHz. It provides better isolation between the ports and also have high power handling capability. The amplifier has been tested with existing 37.8 MHz quarter-wave resonator type re buncher in the Radioactive Ion Beam (RIB) facility. The performance test results of developed RF system with Re buncher cavity have been presented in this report.

Solid-phase isotope harvesting of 88Zr from a radioactive ion beam facility

During routine operation of the Facility for Rare Isotope Beams (FRIB), radionuclides will accumulate in both the aqueous beam dump and along the beamline in the process of beam purification. These byproduct radionuclides, many of which are far from stability, can be collected and purified for use in other scientific applications in a process called isotope harvesting. In this work, the viability of 88Zr harvesting from solid components was investigated at the National Superconducting Cyclotron Laboratory. A secondary 88Zr beam was stopped in a series of collectors comprised of Al, Cu, W, and Au foils. This work details irradiation of the collector foils and the subsequent radiochemical processing to isolate the deposited 88Zr (and its daughter 88Y) from them. Total average recovery from the Al, Cu, and Au collector foils was (91.3 ± 8.9) % for 88Zr and (95.0 ± 5.8) % for 88Y, respectively, which is over three times higher recovery than in a previous aqueous-phase harvesting experiment. The utility of solid-phase isotope harvesting to access elements such as Zr that readily hydrolyze in near-neutral pH aqueous conditions has been demonstrated for application to harvesting from solid components at FRIB.

Constraining Nucleosynthesis in Neutrino-driven Winds: Observations, Simulations, and Nuclear Physics

A promising astrophysical site to produce the lighter heavy elements of the first r-process peak (Z = 38 − 47) is the moderately neutron-rich (0.4 < Y e < 0.5) neutrino-driven ejecta of explosive environments, such as core-collapse supernovae and neutron star mergers, where the weak r-process operates. This nucleosynthesis exhibits uncertainties from the absence of experimental data from (α, xn) reactions on neutron-rich nuclei, which are currently based on statistical model estimates. In this work, we report on a new study of the nuclear reaction impact using a Monte Carlo approach and improved (α, xn) rates based on the Atomki-V2 α optical model potential. We compare our results with observations from an up-to-date list of metal-poor stars with [Fe/H] < −1.5 to find conditions of the neutrino-driven wind where the lighter heavy elements can be synthesized. We identified a list of (α, xn) reaction rates that affect key elemental ratios in different astrophysical conditions. Our study aims to motivate more nuclear physics experiments on (α, xn) reactions using the current and new generation of radioactive beam facilities and also more observational studies of metal-poor stars.

The polysiloxane-based scintillator for measurements of fast neutron spectra in nuclear physics experiments

Next generation radioactive ion beam (RIB) facilities among which the SPES facility in construction at INFN-LNL require selective detection apparatuses of high resolution and efficiency as well as high granularity and then the highest covering of the solid angle. On this line the fast neutron detection still represents an uneasy task. Among the organic scintillators, the new polysiloxane-based scintillators have appealing features for measurements of neutron spectra in RIBs applications. In this work a prototype of polysiloxane-based scintillator was fully tested to gamma, alfa and neutron radiation. The scintillator calibration was studied by comparing more than one method and the pulse shape discrimination capabilities were appropriately assessed. Three fast neutron spectra were produced at the INFN-LSN tandem accelerator by means of the 7Li(p,n)7Be reaction at different energies of the incident protons. The measurements of the produced neutron yield together with theoretical estimations based on FLUKA simulations allowed first measurements of the detection efficiency. The fast response, non-toxicity, non-flammability, cheapness and high radiation resistance together with good efficiency and pulse shape capabilities make these scintillators an interesting detector for fast neutron spectroscopy in applications with RIBs.

Construction and commissioning of the collinear laser spectroscopy system at BRIF

We have constructed a collinear laser spectroscopy (CLS) system installed at the Beijing Radioactive Ion-beam Facility (BRIF), aiming to investigate the nuclear properties of unstable nuclei. The first on-line commissioning experiment of this system was performed using the continuous stable (39K) and unstable (38K) ion beams produced by impinging a 100-MeV proton beam on a CaO target. Hyperfine structure spectra of these two isotopes are reasonably reproduced, and the extracted magnetic dipole hyperfine parameters and isotope shift agree with the literature values. The on-line experiment demonstrates the overall functioning of this CLS system, opening new opportunities for laser spectroscopy measurement of unstable isotopes at BRIF and other radioactive ion beam facilities in China.

Trends in the Structure of Nuclei near 100Sn

Inevitable progress has been achieved in recent years regarding the available data on the structure of 100Sn and neighboring nuclei. Updated nuclear structure data in the region is presented using selected examples. State-of-the-art experimental techniques involving stable and radioactive beam facilities have enabled access to those exotic nuclei. The analysis of experimental data has established the shell structure and its evolution towards N = Z = 50 of the number of neutrons, N, and the atomic number, Z, seniority conservation and proton–neutron interaction in the g9/2 orbit, the super-allowed Gamow–Teller decay of 100Sn, masses and half-lives along the rapid neutron-capture process (r-process) path and super-allowed α decay beyond 100Sn. The status of theoretical approaches in shell model and mean-field investigations are discussed and their predictive power assessed. The calculated systematics of high-spin states for N = 50 isotopes including the 5− state and N = Z nuclei in the g9/2 orbit is presented for the first time.

Neutron transfer reactions on the ground state and isomeric state of a Sn130 beam

The structure of nuclei around the neutron-rich nucleus 132Sn is of particular interest due to the vicinity of the Z = 50 and N = 82 shell closures and the r-process nucleosynthetic path. Four states in 131Sn with a strong single-particle-like component have previously been studied via the (d,p) reaction, with limited excitation energy resolution. The 130Sn(9Be,8Be)131Sn and 130Sn(13C,12C)131Sn single-neutron transfer reactions were performed in inverse kinematics at the Holifield Radioactive Ion Beam Facility using particle-gamma coincidence spectroscopy. The uncertainties in the energies of the single-particle-like states have been reduced by more than an order of magnitude using the energies of gamma rays. The previous tentative Jpi values have been confirmed. Decays from high-spin states in 131Sn have been observed following transfer on the isomeric component of the 130Sn beam. The improved energies and confirmed spin-parities of the p-wave states important to the r-process lead to direct-semidirect cross-sections for neutron capture on the ground state of 130Sn at 30 keV that are in agreement with previous analyses. A similar assessment of the impact of neutron-transfer on the isomer would require significant nuclear structure and reaction theory input. There are few measurements of transfer reaction on isomers, and this is the first on an isomer in the 132Sn region.

Charge breeders: Development of diagnostic tools to probe the underlying physics.

Charge breeders were developed more than 20 years ago in the context of radioactive ion beam (RIB) production. The main goal is to boost the charge state of a singly charged RIB to a higher value matching the A/Q of a post-accelerator. In that way, the RIB produced at some tens of keV can efficiently be accelerated to energies in a range of few MeV/u up to several tens of MeV/u, which is of interest for nuclear structure and nuclear astrophysics experiments. Two families of charge breeders have been developed: one based on Electron Cyclotron Resonance Ion Sources (ECRISs) suitable for Continuous Wave (CW) operation and another built around Electron Beam Ion Sources (EBISs) used for pulsed post-accelerators. For many years, experimental studies have been carried out to enhance the charge breeding process and improve the extracted beam quality, i.e., purity, intensity, emittance, and time structure. The evolution of the charge breeders is also closely related to the emergence of new facilities delivering even more exotic beams. Diagnostic tools, such as Faraday cups, beam profile monitors, and emittance scanners, are mandatory to tune the machines and evaluate their performances. However, to go beyond that, a better understanding of plasma physics (in ECRIS) and electron-ion interactions (in EBIS), as well as the 1+ beam characteristics, for instance, energy spread transverse emittance, is needed. Therefore, simulation codes modeling the physics phenomena in both type of charge breeders were developed to support the understanding of experimental outputs and to gain insights into non-observable parameters. Nevertheless, more sophisticated diagnostic tools are required to improve the charge breeding performance and to extend the potential applications toward new RIB facilities. This Review will present the two kinds of charge breeders and their technical evolutions. It will review the diagnostic tools and simulation codes employed for operation and evaluation of the internal physics processes.

Two-proton emission from excited states of proton-rich nuclei

The effective liquid drop model (ELDM) and the generalized liquid drop model (GLDM) are extended to the case of studying the two-proton (2p) radioactivity from the excited states of proton-rich nuclei. It is shown that the experimental 2p decay half-lives are reproduced well by the ELDM and the GLDM. Then, the 2p decay half-lives of excited states of some nuclei that are not yet available experimentally are predicted by the two models, which are useful for searching for the new 2p decay candidates in future. Meanwhile, the above predicted half-lives are analyzed and compared with those given by the unified fission model (UFM). Next, the influence of the uncertainties of the decay energy and the angular momentum on the half-lives are analyzed in the frame of the two models by taking the 2p radioactivity of the 21&lt;sup&gt;+&lt;/sup&gt; isomeric state of &lt;sup&gt;94&lt;/sup&gt;Ag for example. It is found that the half-lives go up with the increase of the angular momentum, following the law of the quadratic function. Furthermore, the strong dependence of the half-lives on the decay energy suggests that it is important and necessary to measure accurately the mass value of the parent nucleus and the daughter nucleus and the excitation energy. Finally, it is necessary to point out that the existence of the 2p radioactivity in the 21&lt;sup&gt;+&lt;/sup&gt; isomeric state of&lt;sup&gt; 94&lt;/sup&gt;Ag remains to be a mystery. Moreover, although the 2p radioactivity is observed from the higher excited states of &lt;sup&gt;17&lt;/sup&gt;Ne and &lt;sup&gt;18&lt;/sup&gt;Ne, the relevant hypotheses have not yet been further tested experimentally. The construction of a new generation of radioactive ion beam facilities, such as the high intensity heavy-ion accelerator facility (HIAF), is expected to be used to uncover the nature of the 2p radioactivity in the 21&lt;sup&gt;+&lt;/sup&gt; isomeric state of &lt;sup&gt;94&lt;/sup&gt;Ag and further test the hypotheses of the 2p decay from the higher excited states of &lt;sup&gt;17&lt;/sup&gt;Ne and &lt;sup&gt;18&lt;/sup&gt;Ne. On the other hand, some microscopic models, such as the shell model, need to be further developed by including some necessary physical factors, such as the tensor force, three-body force and accurate pairing force, to describe the mechanism of the 2p emission of the excited states more reasonably. In summary, more nuclear structure information can be extracted by studying the 2p radioactivity of the excited states. It is worth studying further although it is rather difficult to observe.

On the Feasibility of Rovibrational Laser Cooling of Radioactive RaF+ and RaH+ Cations

Polar radioactive molecules have been suggested to be exceptionally sensitive systems in the search for signatures of symmetry-violating effects in their structure. Radium monofluoride (RaF) possesses an especially attractive electronic structure for such searches, as the diagonality of its Franck-Condon matrix enables the implementation of direct laser cooling for precision experiments. To maximize the sensitivity of experiments with short-lived RaF isotopologues, the molecular beam needs to be cooled to the rovibrational ground state. Due to the high kinetic energies and internal temperature of extracted beams at radioactive ion beam (RIB) facilities, in-flight rovibrational cooling would be restricted by a limited interaction timescale. Instead, cooling techniques implemented on ions trapped within a radiofrequency quadrupole cooler-buncher can be highly efficient due to the much longer interaction times (up to seconds). In this work, the feasibility of rovibrationally cooling trapped RaF+ and RaH+ cations with repeated laser excitation is investigated. Due to the highly diagonal nature between the ionic ground state and states in the neutral system, any reduction of the internal temperature of the molecular ions would largely persist through charge-exchange without requiring the use of cryogenic buffer gas cooling. Quasirelativistic X2C and scalar-relativistic ECP calculations were performed to calculate the transition energies to excited electronic states and to study the nature of chemical bonding for both RaF+ and RaH+. The results indicate that optical manipulation of the rovibrational distribution of trapped RaF+ and RaH+ is unfeasible due to the high electronic transition energies, which lie beyond the capabilities of modern laser technology. However, more detailed calculations of the structure of RaH+ might reveal possible laser-cooling pathways.

Examination of n − T 9 conditions required by N = 50, 82, 126 waiting points in r-process

We examined the conditions of neutron density (n) and temperature (T 9) required for the N = 50, 82, and 126 isotopes to be waiting points (WP) in the r-process. The nuclear mass based on experimental data presented in the AME2020 database (AME and AME ± Δ) and that predicted using FRDM, WS4, DZ10, and KTUY models were employed in our estimations. We found that the conditions required by the N = 50 WP significantly overlap with those required by the N = 82 ones, except for the WS4 model. In addition, the upper (or lower) bounds of the n − T 9 conditions based on the models are different from each other due to the deviations in the two-neutron separation energies. The standard deviations in the nuclear mass of 108 isotopes in the three N = 50, 82, and 126 groups are about rms = 0.192 and 0.434 MeV for the pairs of KTUY-AME and WS4-KTUY models, respectively. We found that these mass uncertainties result in a large discrepancy in the n n − T 9 conditions, leading to significant differences in the conditions for simultaneously appearing all the three peaks in the r-process abundance. The newly updated FRDM and WS4 calculations can give the overall conditions for the appearance of all the peaks but vice versa for their old versions in a previous study. The change in the final r-process isotopic abundance due to the mass uncertainty is from a few factors to three orders of magnitude. Therefore, accurate nuclear masses of the r-process key nuclei, especially for 76Fe, 81Cu, 127Rh, 132Cd, 192Dy, and 197Tm, are highly recommended to be measured in radioactive-ion beam facilities for a better understanding of the r-process evolution.