Multiple Mechanisms in Proton-Induced Nucleon Removal at ∼100 MeV/Nucleon.

We report on the first proton-induced single proton- and neutron-removal reactions from the neutron-deficient ^{14}O nucleus with large Fermi-surface asymmetry S_{n}-S_{p}=18.6 MeV at ∼100 MeV/nucleon, a widely used energy regime for rare-isotope studies. The measured inclusive cross sections and parallel momentum distributions of the ^{13}N and ^{13}O residues are compared to the state-of-the-art reaction models, with nuclear structure inputs from many-body shell-model calculations. Our results provide the first quantitative contributions of multiple reaction mechanisms including the quasifree knockout, inelastic scattering, and nucleon transfer processes. It is shown that the inelastic scattering and nucleon transfer, usually neglected at such energy regime, contribute about 50% and 30% to the loosely bound proton and deeply bound neutron removal, respectively. These multiple reaction mechanisms should be considered in analyses of inclusive one-nucleon removal cross sections measured at intermediate energies for quantitative investigation of single-particle strengths and correlations in atomic nuclei.

Observation of the Exotic 0_{2}

We report here the first observation of the 0_{2}^{+} state of ^{8}He, which has been predicted to feature the condensatelike α+^{2}n+^{2}n cluster structure. We show that this state is characterized by a spin parity of 0^{+}, a large isoscalar monopole transition strength, and the emission of a strongly correlated neutron pair, in line with theoretical predictions. Our finding is further supported by the state-of-the-art microscopic α+4n model calculations. The present results may lead to new insights into clustering in neutron-rich nuclear systems and the pair correlation and condensation in quantum many-body systems under strong interactions.

Extended p_{3/2} Neutron Orbital and the N=32 Shell Closure in

The one-neutron knockout from ^{52}Ca in inverse kinematics onto a proton target was performed at ∼230 MeV/nucleon combined with prompt γ spectroscopy. Exclusive quasifree scattering cross sections to bound states in ^{51}Ca and the momentum distributions corresponding to the removal of 1f_{7/2} and 2p_{3/2} neutrons were measured. The cross sections, interpreted within the distorted-wave impulse approximation reaction framework, are consistent with a shell closure at the neutron number N=32, found as strong as at N=28 and N=34 in Ca isotopes from the same observables. The analysis of the momentum distributions leads to a difference of the root-mean-square radii of the neutron 1f_{7/2} and 2p_{3/2} orbitals of 0.61(23) fm, in agreement with the modified-shell-model prediction of 0.7 fm suggesting that the large root-mean-square radius of the 2p_{3/2} orbital in neutron-rich Ca isotopes is responsible for the unexpected linear increase of the charge radius with the neutron number.

Pairing Forces Govern Population of Doubly Magic

Direct proton-knockout reactions of ^{55}Sc at ∼220 MeV/nucleon were studied at the RIKEN Radioactive Isotope Beam Factory. Populated states of ^{54}Ca were investigated through γ-ray and invariant-mass spectroscopy. Level energies were calculated from the nuclear shell model employing a phenomenological internucleon interaction. Theoretical cross sections to states were calculated from distorted-wave impulse approximation estimates multiplied by the shell model spectroscopic factors, which describe the wave function overlap of the ^{55}Sc ground state with states in ^{54}Ca. Despite the calculations showing a significant amplitude of excited neutron configurations in the ground-state of ^{55}Sc, valence proton removals populated predominantly the ground state of ^{54}Ca. This counterintuitive result is attributed to pairing effects leading to a dominance of the ground-state spectroscopic factor. Owing to the ubiquity of the pairing interaction, this argument should be generally applicable to direct knockout reactions from odd-even to even-even nuclei.

Long-term Survival Results following Endobronchial RF Ablation in a Healthy-Porcine Model.

This paper presents results from long-term survival study where healthy swine were ablated with a novel technology designed for treating early-stage non-small cell lung cancer using an endobronchial flexible catheter.Methods - The radiofrequency ablation (RFA) system has been presented previously and consisted of an ablation catheter, radiofrequency generator, irrigation pump for infusion of hypertonic saline (HS) and a laptop. The catheter carried an occlusion balloon, a 5 mm long RF electrode, with irrigation holes, and a 1 mm long electrode for bipolar impedance measurements. The outer diameter (OD) was 1.4 mm for compatibility with current bronchoscopes, navigation systems and radial EBUS. Nine swine were treated in this study with survival times of 1, 4 and 12 weeks (N=3 at each time point). In all animals, the treatment sites consisted of one location in the upper right lung (RUL) and another one in the lower right lung (RLL). CTs were taken pre-op, immediately post-op and at every 2 weeks post treatment. Ablation times ranged from 6 to 8 min and average applied power was 68 W (range 63 - 72 W).Results - At 1-week survival, large zones of necrotic tissue were observed in all respective 6 ablations. Ablation volumes had an average diameter of 3.2 cm at RUL locations and 3.8 cm in RLLs (likely due to longer RLL ablation durations). As time progressed, the necrotic tissue was gradually replaced with fibrotic tissue. At 4-week survival, the replacement was almost complete in all respective 3 animals. As a result, ablation volumes decreased to an average diameter of 1.3 cm at RUL locations and 2.3 cm in RLLs (likely due to longer RLL ablation durations). At 12-week survival, as the replacement process continued, histopathology revealed zones of residual necrotic tissue that were further reduced in size. Ablation zones had been resorbed and contracted by fibrous scar tissue. The average volume of the treatment effect decreased to 1.1 cm (RUL) and to 1.6 cm (RLL) in equivalent diameter. There were no complications in any of the nine animals.Conclusion - In healthy swine lungs, RFA with a 1.4-mm OD, radial-EBUS-sheath-compatible, endobronchial catheter was effective and safe. This system and therapeutic approach may be considered for further evaluation in minimally invasive treatment of tumorous lung nodules.

Sequential Nature of (p,3p) Two-Proton Knockout from Neutron-Rich Nuclei.

Twenty-one two-proton knockout (p,3p) cross sections were measured from neutron-rich nuclei at ∼250 MeV/nucleon in inverse kinematics. The angular distribution of the three emitted protons was determined for the first time, demonstrating that the (p,3p) kinematics are consistent with two sequential proton-proton collisions within the projectile nucleus. Ratios of (p,3p) over (p,2p) inclusive cross sections follow the trend of other many-nucleon removal reactions, further reinforcing the sequential nature of (p,3p) in neutron-rich nuclei.

Quasifree Neutron Knockout from

Exclusive cross sections and momentum distributions have been measured for quasifree one-neutron knockout reactions from a ^{54}Ca beam striking on a liquid hydrogen target at ∼200 MeV/u. A significantly larger cross section to the p_{3/2} state compared to the f_{5/2} state observed in the excitation of ^{53}Ca provides direct evidence for the nature of the N=34 shell closure. This finding corroborates the arising of a new shell closure in neutron-rich calcium isotopes. The distorted-wave impulse approximation reaction formalism with shell model calculations using the effective GXPF1Bs interaction and ab initio calculations concur our experimental findings. Obtained transverse and parallel momentum distributions demonstrate the sensitivity of quasifree one-neutron knockout in inverse kinematics on a thick liquid hydrogen target with the reaction vertex reconstructed to final state spin-parity assignments.

78Ni revealed as a doubly magic stronghold against nuclear deformation.

Nuclear magic numbers correspond to fully occupied energy shells of protons or neutrons inside atomic nuclei. Doubly magic nuclei, with magic numbers for both protons and neutrons, are spherical and extremely rare across the nuclear landscape. Although the sequence of magic numbers is well established for stable nuclei, experimental evidence has revealed modifications for nuclei with a large asymmetry between proton and neutron numbers. Here we provide a spectroscopic study of the doubly magic nucleus 78Ni, which contains fourteen neutrons more than the heaviest stable nickel isotope. We provide direct evidence of its doubly magic nature, which is also predicted by ab initio calculations based on chiral effective-field theory interactions and the quasi-particle random-phase approximation. Our results also indicate the breakdown of the neutron magic number 50 and proton magic number 28 beyond this stronghold, caused by a competing deformed structure. State-of-the-art phenomenological shell-model calculations reproduce this shape coexistence, predicting a rapid transition from spherical to deformed ground states, with 78Ni as the turning point.

Prominence of Pairing in Inclusive (p,2p) and (p,pn) Cross Sections from Neutron-Rich Nuclei.

Fifty-five inclusive single nucleon-removal cross sections from medium mass neutron-rich nuclei impinging on a hydrogen target at ∼250 MeV/nucleon are measured at the RIKEN Radioactive Isotope Beam Factory. Systematically higher cross sections are found for proton removal from nuclei with an even number of protons as compared to odd-proton number projectiles for a given neutron separation energy. Neutron removal cross sections display no even-odd splitting, contrary to nuclear cascade model predictions. Both effects are understood through simple considerations of neutron separation energies and bound state level densities originating in pairing correlations in the daughter nuclei. These conclusions are supported by comparison with semimicroscopic model predictions, highlighting the enhanced role of low-lying level densities in nucleon-removal cross sections from loosely bound nuclei.

How Robust is the N=34 Subshell Closure? First Spectroscopy of

The first γ-ray spectroscopy of ^{52}Ar, with the neutron number N=34, was measured using the ^{53}K(p,2p) one-proton removal reaction at ∼210 MeV/u at the RIBF facility. The 2_{1}^{+} excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with N>20. This result is the first experimental signature of the persistence of the N=34 subshell closure beyond ^{54}Ca, i.e., below the magic proton number Z=20. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured 2_{1}^{+} systematics of neutron-rich Ar isotopes, and support a N=34 subshell closure in ^{52}Ar.

Early Preclinical Experience with a Novel Endobronchial Radiofrequency Ablation System for Lung Cancer Treatment.

This paper introduces a novel technology for treating early-stage non-small cell lung cancer using an endobronchial approach via a flexible radiofrequency ablation (RFA) catheter. Methods - The RFA system consisted of an ablation catheter, radiofrequency generator, irrigation pump for infusion of hypertonic saline (HS) and a laptop. The catheter carried an occlusion balloon, a 5 mm long RF electrode, with irrigation holes, and a 1 mm long electrode for bipolar impedance measurements. The outer diameter was 1.4 mm for compatibility with current bronchoscopes, navigation systems and radial EBUS. The RFA system was extensively bench tested on fresh heart, liver and lung animal tissues using power levels of 30 - 60 W, RF energy delivery durations of 3 - 15 min and HS concentrations of 5% and 23.4%. Two swine were then treated at 60 W for 15 min per bronchus. Several bronchi were involved. For both animals and for all treatment sites, 20% HS was used. Animals were survived for six weeks. Results - Bench studies showed that 60 W, 7 - 15 min ablations can produce large ablation volumes, in excess of 3 - 4 cm diameter. In the chronic animal study, no clinically adverse events occurred. There was no evidence of hemorrhage. Animals vital signs, breathing patterns and their behavior were normal throughout the six-week period. Their appetite was normal and they gained weight according to expectations. The RF ablation created discrete volumes of thermal coagulative necrosis which were subsequently encapsulated ("walled off") by zones of organized fibrosis. The dimensions of coagulative necrotic sequestra met expectations, as at six weeks they exceeded volumes corresponding to 2 cm nodules, the size of tumors normally addressed in the peripheral lung by localized therapy. Conclusion - This therapy showed promise. Appropriate energy settings combined with suitable treatment locations safely produced large ablation volumes of uniform thermal coagulative necrosis. Further studies and optimization of treatment parameters can develop it into a mainstream therapy for treating early-stage lung tumors in humans.

Erratum: Persistence of the Z=28 Shell Gap Around

This corrects the article DOI: 10.1103/PhysRevLett.119.192501.

First Observation of

The most neutron-rich boron isotopes ^{20}B and ^{21}B have been observed for the first time following proton removal from ^{22}N and ^{22}C at energies around 230 MeV/nucleon. Both nuclei were found to exist as resonances which were detected through their decay into ^{19}B and one or two neutrons. Two-proton removal from ^{22}N populated a prominent resonancelike structure in ^{20}B at around 2.5 MeV above the one-neutron decay threshold, which is interpreted as arising from the closely spaced 1^{-},2^{-} ground-state doublet predicted by the shell model. In the case of proton removal from ^{22}C, the ^{19}B plus one- and two-neutron channels were consistent with the population of a resonance in ^{21}B 2.47±0.19 MeV above the two-neutron decay threshold, which is found to exhibit direct two-neutron decay. The ground-state mass excesses determined for ^{20,21}B are found to be in agreement with mass surface extrapolations derived within the latest atomic-mass evaluations.

Persistence of the Z=28 Shell Gap Around

In-beam γ-ray spectroscopy of ^{79}Cu is performed at the Radioactive Isotope Beam Factory of RIKEN. The nucleus of interest is produced through proton knockout from a ^{80}Zn beam at 270 MeV/nucleon. The level scheme up to 4.6 MeV is established for the first time and the results are compared to Monte Carlo shell-model calculations. We do not observe significant knockout feeding to the excited states below 2.2 MeV, which indicates that the Z=28 gap at N=50 remains large. The results show that the ^{79}Cu nucleus can be described in terms of a valence proton outside a ^{78}Ni core, implying the magic character of the latter.

Nucleus

The unbound nucleus ^{26}O has been investigated using invariant-mass spectroscopy following one-proton removal reaction from a ^{27}F beam at 201 MeV/nucleon. The decay products, ^{24}O and two neutrons, were detected in coincidence using the newly commissioned SAMURAI spectrometer at the RIKEN Radioactive Isotope Beam Factory. The ^{26}O ground-state resonance was found to lie only 18±3(stat)±4(syst) keV above threshold. In addition, a higher lying level, which is most likely the first 2^{+} state, was observed for the first time at 1.28_{-0.08}^{+0.11} MeV above threshold. Comparison with theoretical predictions suggests that three-nucleon forces, pf-shell intruder configurations, and the continuum are key elements to understanding the structure of the most neutron-rich oxygen isotopes beyond the drip line.

Extension of the N=40 Island of Inversion towards N=50: Spectroscopy of (66)Cr, (70,72)Fe.

We report on the measurement of the first 2(+) and 4(+) states of (66)Cr and (70,72)Fe via in-beam γ-ray spectroscopy. The nuclei of interest were produced by (p,2p) reactions at incident energies of 260 MeV/nucleon. The experiment was performed at the Radioactive Isotope Beam Factory, RIKEN, using the DALI 2γ-ray detector array and the novel MINOS device, a thick liquid hydrogen target combined with a vertex tracker. A low-energy plateau of 2(1)(+) and 4(1)(+) energies as a function of the neutron number was observed for N≥38 and N≥40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modified Lenzi-Nowacki-Poves-Sieja (LNPS) interaction in the pfg(9/2)d(5/2) valence space reproduce the observations. Interpretation within the shell model shows an extension of the island of inversion at N=40 for more neutron-rich isotopes towards N=50.

Low-Lying Structure of (50)Ar and the N=32 Subshell Closure.

The low-lying structure of the neutron-rich nucleus (50)Ar has been investigated at the Radioactive Isotope Beam Factory using in-beam γ-ray spectroscopy with (9)Be((54)Ca,(50)Ar+γ)X, (9)Be((55)Sc,(50)Ar+γ)X, and (9)Be((56)Ti,(50)Ar+γ)X multinucleon removal reactions at ∼220 MeV/u. A γ-ray peak at 1178(18) keV is reported and assigned as the transition from the first 2(+) state to the 0(+) ground state. A weaker, tentative line at 1582(38) keV is suggested as the 4(1)(+)→2(1)(+) transition. The experimental results are compared to large-scale shell-model calculations performed in the sdpf model space using the SDPF-MU effective interaction with modifications based on recent experimental data for exotic calcium and potassium isotopes. The modified Hamiltonian provides a satisfactory description of the new experimental results for (50)Ar and, more generally, reproduces the energy systematics of low-lying states in neutron-rich Ar isotopes rather well. The shell-model calculations indicate that the N=32 subshell gap in (50)Ar is similar in magnitude to those in (52)Ca and (54)Ti and, notably, predict an N=34 subshell closure in (52)Ar that is larger than the one recently reported in (54)Ca.

Observation of a p-wave one-neutron halo configuration in (37)Mg.

Cross sections of 1n-removal reactions from the neutron-rich nucleus (37)Mg on C and Pb targets and the parallel momentum distributions of the (37)Mg residues from the C target have been measured at 240 MeV/nucleon. A combined analysis of these distinct nuclear- and Coulomb-dominated reaction data shows that the (37)Mg ground state has a small 1n separation energy of 0.22(-0.09)(+0.12) MeV and an appreciable p-wave neutron single-particle strength. These results confirm that (37)Mg lies near the edge of the "island of inversion" and has a sizable p-wave neutron halo component, the heaviest such system identified to date.