7 Search Results

We present the first measurements with a new collinear laser spectroscopy setup at the Argonne Tandem Linac Accelerator System, utilizing its unique capability to deliver neutron-rich refractory metal isotopes produced by the spontaneous fission of Cf 252 . We measured isotope shifts from optical spectra for nine radioactive ruthenium isotopes Ru 106 – 114 , reaching deep into the mid-shell region. The extracted charge radii are in excellent agreement with predictions from the Brussels-Skyrme-on-a-Grid models that account for the triaxial deformation of nuclear ground states. We show that triaxial deformation impacts charge radii in models that feature shell effects, inmore » contrast to what could be concluded from a liquid drop analysis. This indicates that this exotic type of deformation should not be neglected in regions where it is known to occur, even if its presence cannot be unambiguously inferred through laser spectroscopy.« less

The r-process abundance peaks, particularly near mass number A ∼ 130, reflect underlying nuclear structure effects such as closed neutron shells, yet modeling the nucleosynthesis in this region remains hindered by uncertain neutron-capture rates. These rates are especially sensitive to nuclear level densities (NLDs) and γ-ray strength functions of neutron-rich nuclei, where experimental data are scarce. We present the first experimental constraint on the NLD of ¹⁴¹Cs using the β-Oslo method, extending sensitivity to the neutron-rich regime near the N = 82 closed shell. Our data allow for critical calibration of microscopic NLD models and reveal that ¹⁴¹Cs lies nearmore » the limit of statistical model applicability. Using this experimental input, we evaluate radiative neutron-capture rates across neighboring isotones using both Hauser–Feshbach (HF) and High Fidelity Resonance (HFR) models. Our results show order-of-magnitude rate increases for nuclei along the N = 86 line, signaling a transition to resonance-dominated capture in this region. These findings underscore the importance of constraining NLDs to improve r-process reaction network predictions, particularly in environments where the validity of statistical models breaks down.« less

Observed abundances of Z ∼ 40 elements in metal-poor stars vary from star to star, indicating that the rapid and slow neutron capture processes may not contribute alone to the synthesis of elements beyond iron. The weak r-process was proposed to produce Z ∼ 40 elements in a subset of old stars. Thought to occur in the ν-driven ejecta of a core-collapse supernova, (α, xn) reactions would drive the nuclear flow toward heavier masses at T = 2−5 GK. However, current comparisons between modeled and observed yields do not bring satisfactory insights into the stellar environment, mainly due to themore » uncertainties of the nuclear physics inputs where the dispersion in a given reaction rate often exceeds 1 order of magnitude. Involved rates are calculated with the statistical model where the choice of an α-optical-model potential (αOMP) leads to such a poor precision. The first experiment on ⁸⁷Rb(α, xn) reactions at weak r-process energies is reported here. Total inclusive cross sections were assessed at E_c.m. = 8.1−13 MeV (3.7−7.6 GK) with the active target MUlti-Sampling Ionization Chamber. With an N = 50 seed nucleus, the measured values agree with statistical model estimates using the αOMP Atomki-V2. A reevaluated reaction rate was incorporated into new nucleosynthesis calculations, focusing on ν-driven ejecta conditions known to be sensitive to this specific rate. These conditions were found to fail to reproduce the lighter heavy element abundances in metal-poor stars.« less

New astronomical observations point to a nucleosynthesis picture that goes beyond what was accepted until recently. The intermediate “i” process was proposed as a plausible scenario to explain some of the unusual abundance patterns observed in metal-poor stars. The most important nuclear physics properties entering i-process calculations are the neutron-capture cross sections and they are almost exclusively not known experimentally. In this report we provide the first experimental constraints on the ¹³⁹Ba(n,γ)¹⁴⁰Ba reaction rate, which is the dominant source of uncertainty for the production of lanthanum, a key indicator of i-process conditions. This is an important step towards identifying themore » exact astrophysical site of stars carrying the i-process signature.« less

The nuclear level density (NLD) is a fundamental measure of the complex structure of atomic nuclei at relatively high energies. Here, in this study, we present the first model-independent measurement of the absolute partial NLD for a short-lived nucleus. For this purpose we adapt the recently introduced “shape method” for β-decay experiments, providing the shape of the γ-ray strength function for exotic nuclei. In this work, we show that combining the shape method with the β-Oslo technique allows for the extraction of the NLD of the populated states without the need for theoretical input. This development opens the way formore » the extraction of experimental NLDs far from stability with major implications in astrophysical and other applications. We benchmark our approach using data for the stable ⁷⁶Ge nucleus, finding excellent agreement with previous experimental results. In addition, we present new experimental data and determine the absolute partial level density for the short-lived ⁸⁸Kr nucleus. Our results suggest a fivefold increase in the NLD for the case of ⁸⁸Kr, compared to the recommended values from semimicroscopic Hartree-Fock Bogoliubov calculations recommended by the RIPL3 nuclear data library. However, our results are in good agreement with other semimicroscopic level density models. We demonstrate the impact of our method on the ⁸⁷Kr(n, γ) neutron capture rate and show that our experimental uncertainties for NLDs fulfill the requirements needed for astrophysical calculations predicting r-process abundances.« less

${}_{19}\mathrm{Ne}$ is an important isotope in nuclear astrophysics due to its role in both the ${}_{18}\mathrm{F}(p,\alpha ){}_{15}\mathrm{O}$ and ${}_{15}\mathrm{O}(\alpha ,\gamma ){}_{19}\mathrm{Ne}$ reactions in novae and Type I x-ray bursts, respectively. The energy levels of ${}_{19}\mathrm{Ne}$ near the $\alpha$ and proton thresholds ( $S_{\alpha }=3529more »$ keV, $S_p=6410$ keV) correspond to resonances in both of these reactions. Previous measurements to study the structure of ${}_{19}\mathrm{Ne}$ have focused on both regions in an effort to constrain these reaction rates. Discrepancies in the energies, spins, and parities for levels in ${}_{19}\mathrm{Ne}$ from previous measurements contribute to the reaction-rate uncertainties. Gamma rays from the depopulation of excited states in ${}_{19}\mathrm{Ne}$ were measured to reduce the level-energy uncertainties and inconsistencies in previous spin-parity assignments.The ${}_{19}\mathrm{F}({}_3\mathrm{He},t){}_{19}\mathrm{Ne}$ reaction was used to elucidate the structure of ${}_{19}\mathrm{Ne}$ levels up to $E_x=6.9$ MeV. The reaction products were measured using Gammasphere ORRUBA: Dual Detectors for Experimental Structure Studies—a coupling of the Oak Ridge Rutgers University Barrel Array and Gammasphere at Argonne National Laboratory. Tritons produced in the reaction were measured in coincidence with $\gamma$ rays from the deexcitation of ${}_{19}\mathrm{Ne}$ energy levels. Previously unobserved transitions allowed for discrepancies in the resonance properties relevant to these two reactions to be resolved. In total, 41 transitions from 21 energy levels were measured in ${}_{19}\mathrm{Ne}$, with 21 of those transitions being previously unobserved. Of particular importance, transitions from two $3/2^{+}$ states with energies of 6423(3) and 6441(3) keV, crucial for accurate estimations of the ${}_{18}\mathrm{F}(p,\alpha ){}_{15}\mathrm{O}$ reaction rate, were found. Energies and spin-parities of important energy levels near the proton and $\alpha$ thresholds were measured and some of the discrepancies in previous measurements were resolved. Overall, measurement of the two near-threshold $3/2^{+}$ states reduced the calculated upper limit of the ${}_{18}\mathrm{F}(p,\alpha ){}_{15}\mathrm{O}$ reaction rate by factors of 1.5–17 in the nova temperature range.« less

Purpose: To demonstrate robustness of thermooacoustic range verification to acoustic heterogeneity and discrepancies between assumed and true propagation speed, i.e., soundspeed errors. Methods: A beam sweeper was used to deliver 250 ns pulses that deposited 0.26 Gy of 16 MeV protons and 2.3 Gy of 60 MeV helium ions into water and oil targets, respectively. Thermoacoustic signals were detected by a 96-channel ultrasound array with a 1-4 MHz sensitivity band (-6 dB), bandpass filtered and backprojected to create thermoacoustic images in the plane of the ultrasound array. The same soundspeed and transducer array were used to estimate range and generatemore » the ultrasound images onto which Bragg peak locations were overlaid. An air-gap phantom that displaced the Bragg peak by 6.5 mm demonstrated accuracy. Robustness to soundspeed errors was demonstrated in a waterbath as the assumed propagation speed scanner setting was altered by +/- 5%. Tissue-mimicking gelatin and a bone sample were introduced to demonstrate robustness to acoustic heterogeneity relative to ultrasound images of the underlying morphology. Results: Single ion pulse measurements sufficed during the helium run, but signal averaging was required for protons. Moreover, range and entry point into the target were estimated from data collected by transducers placed at least 6 cm distal to the Bragg peak. When ultrasound images depicted the air-target interface where the beam enters, estimates of the entry point agreed with ultrasound images and range estimates agreed with Monte Carlo simulations to within 300 mu m, even when thermoacoustic emissions traveled through a strongly scattering bone sample. Estimated Bragg peak locations were translated 6.5 mm by the air-gap phantom and correctly identified scenarios when the beam stopped inside the bone. Conclusions: Soundspeed errors dilate and acoustic heterogeneities deform ultrasound images. When thermoacoustic receivers are co-located with the ultrasound imaging array, the same transformations shift thermoacoustic range estimates. Hence, thermoacoustic range verification is robust relative to ultrasound images of underlying anatomy. When the treatment target is visible in ultrasound, e.g., prostate, online thermoacoustic range estimates could verify that the treatment spot is inside the target.« less