National Superconducting Cyclotron Laboratory

<title>Abstract</title> This study presents a high-precision, non-destructive method for determining the areal density of thin graphenic carbon (GC) foils using alpha-particle energy loss. This approach avoids the need for destructive mass and area measurements—such as laser cutting—thereby preserving the foil for further applications. Two types of GC foils with areal densities around 0.2 mg/cm², sourced from KETEK GmbH and Applied NanoTech Inc., were investigated using a three-isotope mixed alpha source emitting particles in the (5.0–5.8) MeV energy range. The developed method demonstrated that the areal-density estimates agreed within 2% for KETEK foils and within 0.5% for Applied NanoTech foils. Experimental stopping powers were extracted by combining energy-loss data with independently measured foil masses and areas. These values were benchmarked against theoretical models, highlighting the influence of the chosen stopping-power model on measurement accuracy. A modified Bethe formalism incorporating Barkas and Bloch corrections, along with an empirically adjusted mean excitation energy Iadj, provided the most consistent results—yielding agreement within 2% across the studied energy range. The results reveal a clear difference in stopping power between the two GC foils, attributed to differences in their electronic structure, as reflected in distinct Iadj values: (85 ± 1) eV for Applied NanoTech and (73 ± 2) eV for KETEK. The study emphasizes the role of the stopping-power model in determining measurement accuracy and how the structure of GC affects the (effective) stopping power. While the method is robust for thin foils, angular straggling and the non-linear energy dependence of the stopping power may limit accuracy when applied beyond the thin-target approximation. This work provides stopping-power data relevant for benchmarking Monte Carlo codes and modeling energy deposition in carbon materials—of particular interest for accelerator related technology and radio-pharmaceutical applications. In medical physics, stopping power relates directly to the concept of linear energy transfer (LET), which governs the biological effectiveness of alpha-emitting isotopes in targeted therapies. The precision achieved in this study allows for reliable, non-destructive areal density determination, offering valuable insights into the structure-dependent stopping power in emerging carbon materials.

In this article, &lt;em&gt;β&lt;/em&gt;&lt;sup&gt;-&lt;/sup&gt; decay studies of neutron-rich isotopes &lt;sup&gt;43,45&lt;/sup&gt;S performed at the National Superconducting Cyclotron Laboratory are reported. &lt;em&gt;β&lt;/em&gt;-delayed $\\gamma$ transitions were detected by an array of 16 clover detectors surrounding the Beta Counting Station, which consists of a 40×40 double-sided silicon strip detector followed by a single-sided silicon strip detector. &lt;em&gt;β&lt;/em&gt;-decay half-lives were extracted for &lt;sup&gt;43,45&lt;/sup&gt;S by correlating implants and decays in the pixelated implant detector and by considering further coincidences with $\\gamma$ transitions in the daughter nuclei. Further, the level structures of &lt;sup&gt;43,45&lt;/sup&gt;Cl are expanded by the addition of 20 and 8 new $\\gamma$ transitions in &lt;sup&gt;43&lt;/sup&gt;Cl and &lt;sup&gt;45&lt;/sup&gt;Cl respectively, and core-excited negative-parity states were observed in both nuclei for the first time. For &lt;sup&gt;45&lt;/sup&gt;S, a large fraction of the &lt;em&gt;β&lt;/em&gt;-decay strength was observed feeding neutron-unbound states in &lt;sup&gt;45&lt;/sup&gt;Cl, which, decaying by delayed neutrons, populated excited states in the &lt;em&gt;β&lt;/em&gt;⁢1⁢&lt;em&gt;n&lt;/em&gt; daughter, &lt;sup&gt;44&lt;/sup&gt;Cl. Experimental observations were compared to detailed shell-model calculations using the SDPFSDG-MU interaction to highlight the role of the diminished &lt;em&gt;N&lt;/em&gt; = 28 neutron shell gap and the near degeneracy of the proton &lt;em&gt;s&lt;/em&gt;&lt;sub&gt;1/2&lt;/sub&gt; and &lt;em&gt;d&lt;/em&gt;&lt;sub&gt;3/2&lt;/sub&gt; orbitals in the structure of the neutron-rich Cl isotopes. The current work also provides further support to a ground-state spin-parity assignment of 3/2&lt;sup&gt;+&lt;/sup&gt; in &lt;sup&gt;45&lt;/sup&gt;Cl.

Background: The appearance of isomeric states in neutron-rich nuclei can provide a wealth of information on the underlying nuclear configurations as a progression is made from stable to exotic nuclei. These states can be characterized by measuring the half-lives and transition probabilities associated with their deexcitations. Recent studies on neutron-rich Si isotopes near $N=20$ and approaching $N=28$ have revealed states with intruder configurations resulting from multiple-particle, multiple-hole excitations across the closed shell gaps. These intruder states appear at low excitation energies along the Si isotopic chain, pointing to a diminished shell gap at $N=28$.Purpose: The experiment was set up to directly measure the half-lives of isomeric states in neutron-rich nuclei populated via $\ensuremath{\beta}$ decay.Methods: $^{37}\mathrm{Al}$ and $^{38}\mathrm{Al}$ isotopes were produced in the projectile fragmentation reaction of a $^{48}\mathrm{Ca}$ primary beam at NSCL. The isotopes were implanted into a ${\mathrm{CeBr}}_{3}$ detector, populating excited states of interest in $^{37}\mathrm{Si}$ following their ${\ensuremath{\beta}}^{\ensuremath{-}}$ and ${\ensuremath{\beta}}^{\ensuremath{-}}\mathrm{n}$ decay, respectively. Ancillary arrays of HPGe and ${\mathrm{LaBr}}_{3}(\mathrm{Ce})$ detectors were used to detect $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays. The half-lives of populated isomeric states were measured using the $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ timing method.Results: The half-life of the 68-keV $(7/{2}_{1}^{\ensuremath{-}})$ state in $^{37}\mathrm{Si}$ was measured for the first time as 9.1(7) ns, while that of the 156-keV $(3/{2}_{1}^{\ensuremath{-}})$ state was 3.20(4) ns, consistent with a previously reported value. The reduced transition probabilities and transition matrix elements associated with the $\ensuremath{\gamma}$-ray decay from these excited states to the $(5/{2}_{1}^{\ensuremath{-}})$ ground state were extracted and agree with results from shell model calculations.Conclusions: The characterization of the $(7/{2}_{1}^{\ensuremath{-}})$ and $(3/{2}_{1}^{\ensuremath{-}})$ states in $^{37}\mathrm{Si}$ validates shell model predictions of low-lying nanosecond isomers in neutron-rich odd-$A$ Si isotopes approaching the $N=28$ shell gap.

The level structure of $^{36}\mathrm{Al}$ has been studied via $\ensuremath{\beta}$ decay of $^{36}\mathrm{Mg}$ at the Facility for Rare Isotope Beams (FRIB) and the National Superconducting Cyclotron Laboratory (NSCL). A long-lived isomer in $^{36}\mathrm{Al}$ was identified which decays by $\ensuremath{\beta}$ to an excited state of $^{36}\mathrm{Si}$. The ground state and the isomeric state of $^{36}\mathrm{Al}$ were found to populate different energy levels of $^{36}\mathrm{Si}$. The results from the two data sets in the present work complement each other. Configuration interaction calculations performed with the FSU shell-model Hamiltonians provide reasonable descriptions to the experimental observations and offer insight into future improvements of the theoretical interpretation.

Nucleon knockout experiments using beryllium or carbon targets reveal a strong dependence of the quenching factors, i.e., the ratio (hbox {R}_{textrm{s}}) of theoretical to the experimental spectroscopic factors (hbox {C}^2{textrm{S}}), on the proton-neutron asymmetry in the nucleus under study. However, this dependence is greatly reduced when a hydrogen target is used. To understand this phenomenon, exclusive ^{1}text {H}(^{17}text {Ne},2hbox {p}~^{16}text {F}) and inclusive ^{12}text {C}(^{17}text {Ne},2hbox {p}~^{16}text {F})hbox {X}, ^{12}text {C}(^{17}text {Ne},^{16}text {F})hbox {X} as well as ^{1}text {H}(^{17}text {Ne},^{16}text {F})hbox {X} (X-denotes undetected reaction products) reactions with ^{16}text {F} in the ground and excited states were analysed. The longitudinal momentum distribution of ^{16}{textrm{F}} and the correlations between the detached protons were studied. In the case of the carbon target, there is a significant deviation from the predictions of the eikonal model. The eikonal approximation was used to extract spectroscopic factor values hbox {C}^2{textrm{S}}. The experimental hbox {C}^2hbox {S} value obtained with C target is markedly lower than that for H target. This is interpreted as rescattering due to simultaneous nucleon knockout from both reaction partners, ^{17}text {Ne} and ^{12}text {C}.

Deuteron elastic scattering from C15 and inelastic scattering reactions to the first excited state of C15 were studied using a radioactive beam of C15 in inverse kinematics. The scattered deuterons were measured using HELIOS. The elastic scattering differential cross sections were analyzed using the optical model. A matter deformation length δd=1.04(11)fm has been extracted from the differential cross sections of inelastic scattering to the first excited state. The ratio of neutron and proton matrix elements Mn/Mp=3.6(4) has been determined from this quadrupole transition. Neutron effective charges and core-polarization parameters of C15 were determined and discussed. Results from no-core configuration interaction calculations were also compared with the experimental observations. This result supports a moderate core decoupling effect of the valence neutron in C15 similarly to its isotone O17, in line with the interpretation of other neutron-rich carbon isotopes.

The discovery of short-lived, exotic nuclei has opened new horizons in subatomic physics and allowed many novel applications in other fields, e.g., the understanding of the evolution of matter and production of the atomic elements in the universe. The effective and highly selective separation of the exotic nuclei produced in these reactions from other more abundant products and the primary beam is critical for the study of exotic species. The high sensitivity of separators can reach single ions produced with sub-picobarn cross sections. Depending on the type of production reaction and the kinematics of the products, different accelerator facilities and separation methods have been constructed and applied. A reaction-product separator is the heart of each such facility. The presentation of fragment separators and their application have been divided here in two parts. A brief overview of the relevant production reactions for modern-day separation techniques and then the ion-optical features of fragment separator design and operation were presented in Part 1. These devices can consist of pure dispersive electromagnetic elements or can incorporate specific atomic interactions of the ions in additional matter along their path through the separator. Extension to systems that incorporate atomic interactions and combinations of these devices with additional high-resolution ion-optical systems to provide unique opportunities to study exotic nuclei immediately after they pass through the system is presented here in Part 2. Nuclear physics laboratories around the world have built reaction-product separators, have plans for new separation facilities, and are constructing new machines with enhanced capabilities that rely on these devices. Finally, the whole presentation concludes with a discussion of key goals, achievements, and limitations in the separation and science of exotic nuclei.

The β-delayed one- and two-neutron emission probabilities (P_{1n} and P_{2n}) of 20 neutron-rich nuclei with N≥82 have been measured at the RIBF facility of the RIKEN Nishina Center. P_{1n} of ^{130,131}Ag, ^{133,134}Cd, ^{135,136}In, and ^{138,139}Sn were determined for the first time, and stringent upper limits were placed on P_{2n} for nearly all cases. β-delayed two-neutron emission (β2n) was unambiguously identified in ^{133}Cd and ^{135,136}In, and their P_{2n} were measured. Weak β2n was also detected from ^{137,138}Sn. Our results highlight the effect of the N=82 and Z=50 shell closures on β-delayed neutron emission probability and provide stringent benchmarks for newly developed macroscopic-microscopic and self-consistent global models with the inclusion of a statistical treatment of neutron and γ emission. The impact of our measurements on r-process nucleosynthesis was studied in a neutron star merger scenario. Our P_{1n} and P_{2n} have a direct impact on the odd-even staggering of the final abundance, improving the agreement between calculated and observed Solar System abundances. The odd isotope fraction of Ba in r-process-enhanced (r-II) stars is also better reproduced using our new data.