Recoil Separator Research Articles

Discovery of New Isotopes ^{160}Os and ^{156}W: Revealing Enhanced Stability of the N=82 Shell Closure on the Neutron-Deficient Side.

Using the fusion-evaporation reaction ^{106}Cd(^{58}Ni,4n)^{160}Os and the gas-filled recoil separator SHANS, two new isotopes _{76}^{160}Os and _{74}^{156}W have been identified. The α decay of ^{160}Os, measured with an α-particle energy of 7080(26)keV and a half-life of 201_{-37}^{+58} μs, is assigned to originate from the ground state. The daughter nucleus ^{156}W is a β^{+} emitter with a half-life of 291_{-61}^{+86} ms. The newly measured α-decay data allow us to derive α-decay reduced widths (δ^{2}) for the N=84 isotones up to osmium (Z=76), which are found to decrease with increasing atomic number above Z=68. The reduction of δ^{2} is interpreted as evidence for the strengthening of the N=82 shell closure toward the proton drip line, supported by the increase of the neutron-shell gaps predicted in theoretical models.

Multinucleon transfer in 48Ti + 48Ti collisions at 11.5 MeV/nucleon

In this work, experimental data of projectile fragment distributions from a preliminary experimental study of the reaction of 11.5 MeV/nucleon 48Ti on 48Ti analyzed with the MARS recoil separator at the Cyclotron Institute of Texas A&M University are presented. Production cross sections, momentum distributions and excitation-energy distributions are extracted and compared with calculations with the Deep Inelastic Transfer (DIT) model and the Constrained Molecular Dynamics model (CoMD). Despite the limited extent of the present dataset, an overall agreement of the models with the data is found pointing at the prevailing nucleon-exchange character of the collisions at this energy.

Multinucleon Transfer in 25 MeV/nucleon 86Kr + 64Ni, 124Sn Peripheral Collisions

In this work we investigate multinucleon transfer reaction channels in peripheral collisions between a 86Kr projectile at 25 MeV/nucleon with 64Ni and 124Sn targets. The experimental data of these reactions were obtained with the Momentum Achromat Recoil Separator (MARS) at Texas A&M University in previous works of our group. We extracted and compared experimental mass and momentum per nucleon distributions of ejectiles with a two step model calculation. For the interaction of the projectile with the target, the phenomenological Deep Inelastic Transfer model (DIT) and the microscopic Constrained Molecular Dynamics model (CoMD) were used, both followed by the GEMINI model for the de-excitation of the primary excited projectile-like fragments. Generally, both DIT and CoMD appear to describe the trend of the data for both systems. However, possible further improvements in the models may be necessary. In conjunction with our recent work with a 86Kr beam at 15 MeV/nucleon with a 64Ni target, this study provides further insights into the reaction mechanisms of peripheral collisions in the Fermi energy regime and the production of neutron rich nuclides.

Decay spectroscopy at the two-proton drip line: Radioactivity of the new nuclides 160Os and 156W

The radioactivity of 76160Os84 and 74156W82 that lie at the two-proton drip line has been measured in an experiment performed at the Accelerator Laboratory of the University of Jyväskylä. The 160Os nuclei were produced using fusion-evaporation reactions induced by a beam of 310 MeV 58Ni ions bombarding a 106Cd target. The 160Os ions were separated in flight using the recoil separator MARA and implanted into a double-sided silicon strip detector, which was used to measure their decays. The α decays of the ground state of 160Os (Eα = 7092(15) keV, t1/2 = 97−32+97 μs) and its isomeric state (Eα = 8890(10) keV, t1/2 = 41−9+15 μs) were measured, allowing the excitation energy of the isomer to be determined as 1844(18) keV. These α-decay properties and the excitation energy of the isomer are compared with systematics. The α decays were correlated with subsequent decays to investigate the β decays of the ground state of 156W, revealing that unlike its isotones, both low-lying isomers were populated in its daughter nuclide, 156Ta. An improved value for the half-life of the proton-decaying high-spin isomeric state in 73156Ta83 of 333−22+25 ms was obtained in a separate experiment using the same experimental systems with a 102Pd target. This result was employed to improve the precision of the half-life determined for 156W, which was measured as 157−34+57 ms.

Using (α, xn) reaction rates and abundance ratios to constrain the weak r-process

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 and using new, constrained (α, xn) reaction rates based on the Atomki-V2 αOMP. We have identified a list of relevant reactions that affect elemental abundance ratios that can be compared to abundances from metal-poor stars. Our results show how when reducing the nuclear physics uncertainties, we can use abundance ratios to constrain the astrophysical conditions/environment. This will be possible with the planned experiments to measure key (α, xn) reaction rates using the SECAR recoil separator at FRIB that will also be briefly discussed.

Feasibility study of a compact and multi-gas supersonic plasma jet for nuclear astrophysics and space research

Supersonic plasma jet targets can improve the present knowledge on low energy nuclear reaction cross sections allowing to study electron screening effects. They could also be employed both in the preliminary tests of space vehicles thermal protection systems and in the validation of computational fluid dynamics (CFD) evaluations of hot exotic gases. This common interest between nuclear astrophysics and space research led to a joint effort between the European Recoil Separator for Nuclear Astrophysics collaboration of the Italian National Institute for Nuclear Physics and the Italian Aerospace Research Center (CIRA) to develop an innovative, compact and multi-gas supersonic plasma jet setup. In this work we present the characterization of a test supersonic cold jet target, performed with ion beam analysis techniques, as benchmark for the validation of the CFD code NExT developed by CIRA. Being NExT outputs in agreement within 5% with experimental data, we employed it together with the software CEA of the United States National Aeronautics and Space Administration to design setups to be employed in both nuclear astrophysics and space re-entry research fields. We finally provide an overview of the plasma technologies to be employed in the construction of this supersonic plasma jet, aiming for a good balance of costs and performances. A 200 kW plasma arch torch, being also commercially available, was found to be a good option for both nuclear astrophysics and space re-entry experiments in small laboratories: for the first, a 20% of ionization may be reached with still an adequate target thickness (∼1017nuclei/cm2), while for the latter the most energetic scenario could be reproduced.

Properties of the new α -decaying isotope At190

The $\ensuremath{\alpha}$ decay of a new isotope $^{190}\mathrm{At}$ has been studied via the $^{109}\mathrm{Ag}(^{84}\mathrm{Sr},3\mathrm{n})^{190}\mathrm{At}$ fusion-evaporation reaction by employing a gas-filled recoil separator. An $\ensuremath{\alpha}$-particle energy of 7750(20) keV and a half-life of $1.{0}_{\ensuremath{-}0.4}^{+1.4}$ ms were measured. The measured decay properties correspond to an unhindered $\ensuremath{\alpha}$ decay, suggesting the same spin and parity of (${10}^{\ensuremath{-}}$) as those of the final state of the decay. The systematics of the nearby nuclei and the predictions of selected atomic mass models were compared with the measured decay properties.

Simulated and Experimental Characteristics of a Gas-Filled Recoil Separator DGFRS-2

Simulated and Experimental Characteristics of a Gas-Filled Recoil Separator DGFRS-2

An innovative Superconducting Recoil Separator for HIE-ISOLDE

The ISOLDE Scientific Infrastructure at CERN offers a unique range of post-accelerated radioactive beams. The scientific program can be improved with the “Isolde Superconducting Recoil Separator” (ISRS), an innovative spectrometer able to deliver unprecedented (A, Z) resolution. In this paper we present an overview of the physics and ongoing technical developments.

In-beam gamma -ray spectroscopy of ^{94}Ag

A recoil-beta-tagging experiment has been performed to study the excited T=0 and T=1 states in the odd–odd N=Z nucleus ^{94}Ag, populated via the ^{40}Ca(^{58}Ni,1p3n)^{94}Ag reaction. The experiment was conducted using the MARA recoil separator and JUROGAM3 array at the Accelerator Laboratory of the University of Jyväskylä. Through correlating fast, high-energy beta decays at the MARA focal plane with prompt gamma rays emitted at the reaction target, a number of transitions between excited states in ^{94}Ag have been identified. The timing characteristics of these transitions confirm that they fall within decay sequences that feed the short-lived T=1 ground state of ^{94}Ag. The transitions are proposed to proceed within and between the sets of states with T=0 and T=1. Possible correspondence between some of these transitions from analog states in ^{94}Pd has been discussed, and shell-model calculations including multipole and monopole electromagnetic effects have been presented, in order to enable predictions of the decay patterns between the T=0 and T=1 states and to allow a theoretical set of Coulomb energy differences to be calculated for the A = 94T=1 analog states.

A gas-filled recoil separator, SHANS2, at the China Accelerator Facility for Superheavy Elements

A gas-filled recoil separator, SHANS2 (Spectrometer for Heavy Atoms and Nuclear Structure-2), with five magnets arranged in a Qv-D-Qh-Qv-D configuration is constructed at the China Accelerator Facility for Superheavy Elements (CAFE2) of the Institute of Modern Physics (IMP) in Lanzhou. It is dedicated to the study of the heavy nuclei produced in heavy-ion-induced fusion evaporation reactions, especially to the experiments on the superheavy elements with cross-sections lower than 1 pb. The maximum magnetic rigidity of the main dipole magnet is designed to be 2.5 T m. The detailed design of the SHANS2 and the results of several test experiments are presented.

Reaction 55Mn + 159Tb : preparation for the synthesis of new elements* *Supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDB34010000), the Guangdong Major Project of Basic and Applied Basic Research (2021B0301030006), the National Key R&D Program of China (2018YFA0404402), the CAS Project for Young Scientists in Basic Research (YSBR-002), the Youth Innovation Promotion Association CAS

The complete fusion reaction of Mn + Tb was studied on the gas-filled recoil separator SHANS2. Nineteen ER - α - α decay chains from Th produced from the 4n evaporation channel were observed. The α-particle energy and half-life of Th were determined as 7922(14) keV and 14(4) ms, respectively. In addition, the decay properties of = 7788(14) keV and = 36 ms were obtained for Th. The measured α decay properties of Th and Th were consistent with literature data. The cross sections were measured to be 0.59 nb and 0.19 nb for Th and Th, respectively. The equilibrium charge state of the recoiled nucleus Th was determined experimentally. The new data were helpful for estimating the equilibrium charge states of elements 119 and 120, which could be produced via the Pu(Mn, 3n)119 and Am(Mn, 3n)120 reactions, respectively.

Recoil mass separators for nuclear astrophysics: The role of ERNA

The measurements of radiative capture reactions can be performed in inverse kinematic detecting directly the recoil produced in the nuclear reaction using a recoil mass separator (RMS). The development of RMS allows the possibility to overtake both the problems of gamma background signal and purity and production of target. The European Recoil Separator for Nuclear Astrophysics (ERNA) is a RMS designed with the main goal of determining the 12C(α, γ)16O, presently hosted at Center for Isotopics Research and Cultural hEritage laboratory of Department of Mathematics and Physics, University of Campania Luigi Vanvitelli, Caserta Italy (CIRCE-DMF). A general discussion on measurement techniques with recoil mass separator will be presented in this contribution with a focus on the ERNA one.

Determination of the Plasma Delay Time in PIPS detectors for fission fragments at the LOHENGRIN spectrometer

The VElocity foR Direct particle Identification spectrometer (VERDI) is a 2E-2v fission spectrometer that allows the measurement of the total mass distribution of secondary fission fragments with a resolving power of 1-2 u. It consists of two time-of-flight (ToF) arms, with one Micro Channel Plate (MCP) detector and up to 32 Silicon PIPS (Passive Implanted Planar Silicon) detectors per arm. The MCPs provide the start timing signals and the PIPS detectors provide both the energy and the stopping ToF signals. In real conditions, the PIPS signals are affected by the formation of plasma from the interaction between the heavy ions and the detector material. The plasma contributes to a reduction in signal amplitude, resulting in a Pulse Height Defect (PHD), and introduces a signal delay, known as Plasma Delay Time (PDT). An experiment to characterize the PDT and PHD was performed at the LOHENGRIN recoil separator of the Institut Laue Langevin (ILL). Characteristic fission fragments from the 239Pu(n,f) reaction were separated based on their A/Q and E/Q ratios, allowing the measurement of a wide range of energies from 21 to 110 MeV and masses between 80 and 149 u. Six PIPS detectors were characterized to study their individual responses to the PDT and PHD effects. The signals were recorded in a digital acquisition system to completely exploit the offline analysis capabilities. Achieved combined timing and energy resolutions for fission fragments varied between 72(2) ps and 100(4) ps and 1.4% - 2% (FWHM), respectively. Preliminary PHD and PDT data are presented from the masses A=85, 95, 130 and 143. The PHD trends are strongly correlated with both the ion energy and mass. The PDT, on the other hand, shows a strong variation as a function of the ion kinetic energy but a smaller dependence on the ion mass.

Fission hindrances in transfermium nuclei

Very heavy nuclei owe their stability against spontaneous fission to quantum shell effects, which depend on the local density of single-particle states. The height but also the width and the structure of the barrier in multi-dimensional deformation space determine the fission half-lives. Other effects come into play, such as the conservation of quantum numbers and superfluidity or stiffness of the system in the fission process. This is why odd nuclei have longer fission partial half-lives with respect to their even neighbours and also why multi-quasi-particle states, such as high-K states, are thought to be more stable against fission than the ground state. We will report here on two different fission studies carried out with the GABRIELA detector array at the focal plane of the recoil separator SHELS. The first study concerns the fission properties of 253Rf, the most neutron deficient Rf isotope known to date. The second study focusses on a new measurement of the fission hindrance of the known 8- isomer in 254No.

Examining the impact of α -decay energies on the odd-even staggering in half-lives: α -decay spectroscopy of Ac207–209

The α decays of Ac207–209, located near the proton drip-line, were reinvestigated by means of α-decay spectroscopy. They were produced in the fusion reaction Ar36+Hf176 and separated in flight using the gas-filled recoil separator SHANS. A new α-decay line at 7483(15) keV was observed and assigned as the decay from the ground state of Ac208 to the excited (2+) or (4+) state in Fr204. In addition, the previously known α-decay properties of Ac207–209 were measured with improved precision. The half-lives obtained, together with the existing decay data of other N≤126 actinium isotopes, were compared with the values calculated with the Wentzel-Kramers-Brillouin approximation and without considering the α-preformation factors. It is shown that the calculated half-lives exhibit a distinct odd-even staggering (OES) following the same trend as the experimental data, whereas the amplitudes of the exhibited OES are 30–60 % of the real ones. This implies that apart from the α-preformation factors, the α-decay energies also contribute significantly to the OES of α-decay half-lives.Received 20 September 2022Accepted 10 November 2022DOI:https://doi.org/10.1103/PhysRevC.106.064311©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNuclear structure & decaysProperties190 ≤ A ≤ 219Nuclear Physics

Direct Measurement of Resonances in ^{7}Be(α,γ)^{11}C Relevant to νp-Process Nucleosynthesis.

We have performed the first direct measurement of two resonances of the ^{7}Be(α,γ)^{11}C reaction with unknown strengths using an intense radioactive ^{7}Be beam and the DRAGON recoil separator. We report on the first measurement of the 1155 and 1110keV resonance strengths of 1.73±0.25(stat)±0.40(syst) eV and 125_{-25}^{+27}(stat)±15(syst) meV, respectively. The present results have reduced the uncertainty in the ^{7}Be(α,γ)^{11}C reaction rate to ∼9.4%-10.7% over T=1.5-3 GK, which is relevant for nucleosynthesis in the neutrino-driven outflows of core-collapse supernovae (νp process). We find no effect of the new, constrained reaction rate on νp-process nucleosynthesis.

First inverse kinematics measurement of resonances inBe7(α,γ)C11relevant to neutrino-driven wind nucleosynthesis using DRAGON

A possible mechanism to explain the origin of the light $p$ nuclei in the Galaxy is the nucleosynthesis in the proton-rich neutrino-driven wind ejecta of core-collapse supernovas via the $\ensuremath{\nu}p$ process. However, this production scenario is very sensitive to the underlying supernova dynamics and the nuclear physics input. As far as the nuclear uncertainties are concerned, the breakout from the $pp$ chains via the $^{7}\mathrm{Be}(\ensuremath{\alpha},\ensuremath{\gamma})^{11}\mathrm{C}$ reaction has been identified as an important link which can influence the nuclear flow and, therefore, the efficiency of the $\ensuremath{\nu}p$ process. However, its reaction rate is poorly known over the relevant temperature range, $T$ = 1.5--3 GK. We report on the first direct measurement of two resonances of the $^{7}\mathrm{Be}(\ensuremath{\alpha},\ensuremath{\gamma})^{11}\mathrm{C}$ reaction with previously unknown strengths using an intense radioactive $^{7}\mathrm{Be}$ beam from the Isotope Separator and Accelerator (ISAC-I) Center facility and the DRAGON recoil separator in inverse kinematics. We have decreased the $^{7}\mathrm{Be}(\ensuremath{\alpha},\ensuremath{\gamma})^{11}\mathrm{C}$ reaction rate uncertainty to $\ensuremath{\approx}9.4\text{--}10.7$% over the relevant temperature region.

Attempts to produce new americium isotopes near N=126

Searches for new americium isotopes with neutron number around 126 are made in bombardments of $^{191,193}\mathrm{Ir}$ targets with $^{40}\mathrm{Ar}$ ions by employing the gas-filled recoil separator SHANS and the recoil-$\ensuremath{\alpha}$ correlation technique. No evidence for nuclides of interest is obtained, and the upper limits of the cross sections producing the evaporation residues are estimated to be 31.4 pb and 5.4 pb for the reactions of $^{40}\mathrm{Ar}+^{193}\mathrm{Ir}$ and $^{40}\mathrm{Ar}+^{191}\mathrm{Ir}$, respectively. We have performed microscopic finite-temperature Skyrme Hartree-Fock$+$ (Bardeen- Cooper- Schripffer theory) BCS calculations, and found that the fission barriers of compound nuclei in this region decrease rapidly with increasing proton number. The non-observation of new americium isotopes would be attributed to small survival probabilities of the $^{231}\mathrm{Am}$ and $^{233}\mathrm{Am}$ compound nuclei due to their much reduced fission barriers at high excitations.

Superconducting Curved Canted–Cosine–Theta (CCT) for the HIE-ISOLDE Recoil Separator Ring at CERN

The ISOLDE facility at CERN delivers the largest range of low-energy radioactive beams, exploited by several detector systems to investigate nuclear properties from the stable isotopes to the very exotic systems close to the neutron or proton drip lines. These studies can largely benefit from the use of a high-resolution fragment separator. To achieve this goal, an innovative spectrometer based on a compact superconducting (SC) ring, the Isolde Superconducting Recoil Separator (ISRS), is being studied. The ring will operate as an isochronous non-scaling fixed-field alternating-gradient (FFAG) system based on Canted-Cosine-Theta (CCT) magnets. These multifunction magnets have two alternating-gradient quadrupoles nested inside an outer dipole. According to preliminary beam dynamics studies, the dipole will need to generate a maximum field of 2.2 T. A maximum quadrupole gradient of approximately 14 T/m will guarantee orbit stability for heavy ions with a maximum kinetic energy of 10 MeV/u. Fine tuning of the CCT magnets and the FFAG optics will provide very large solid angles > 100 msr and momentum acceptances Δp/p > 20%. In this paper we present, the magnet designs and their optimisation. A cost-effective active stray field superconducting coil shield design has been introduced to be able to remove approximate 4000 kg of iron yoke and the complexity of building a tightly curved yoke.