Search Results (65 total)

The neutron unbound ground state of ²⁵O (Z=8, N=17) was observed for the first time in a proton knockout reaction from a ²⁶F beam. A single resonance was found in the invariant mass spectrum corresponding to a neutron decay energy of 770_-10⁺²⁰  keV with a total width of 172(30) keV. The N=16 shell gap was established to be 4.86(13) MeV by the energy difference between the ν1s_1/2 and ν0d_3/2 orbitals. The neutron separation energies for ²⁵O agree with the calculations of the universal sd shell model interaction. This interaction incorrectly predicts an ²⁶O ground state that is bound to two-neutron decay by 1 MeV, leading to a discrepancy between the theoretical calculations and experiment as to the particle stability of ²⁶O. The observed decay width was found to be on the order of a factor of 2 larger than the calculated single-particle width using a Woods-Saxon potential.

We have observed a resonance in neutron-fragment coincidence measurements that is presumably the first excited state of ²³O at 2.8(1) MeV excitation energy which decays into the ground state of ²²O. This interpretation is consistent with theory. The reaction mechanism supports the assignment of the observed state as the 5/2⁺ hole state. This assignment and the recently observed 3/2⁺ particle state advance the understanding of ²³O.

The results of measurements of the production of neutron-rich nuclei by the fragmentation of a ⁴⁸Ca beam at 142 MeV/nucleon are presented. Evidence was found for the production of a new isotope that is the most neutron-rich silicon nuclide, ⁴⁴Si, in a net neutron pickup process. A simple systematic framework was found to describe the production cross sections based on thermal evaporation from excited prefragments that allows extrapolation to other weak reaction products.

We have searched for particle-bound ²⁶O and ²⁸F isotopes in the reaction products of secondary ²⁷F and ²⁹Ne beams, respectively. No events have been observed. Upper limits of 3.3 mb and 1.2 mb for the respective production cross sections of ²⁶O and ²⁸F by one p-stripping reactions are established. Since in the case of ²⁸F, this upper limit is sufficiently small compared to common estimates, we conclude that ²⁸F is most likely unbound. For the case of ²⁶O, our result is not statistically significant, thus, the question cannot be answered whether ²⁶O is a particle-unbound nucleus or not.

A search for ¹⁹Mg was performed using projectile fragmentation of a 150  MeV∕nucleon ³⁶Ar beam. No events of ¹⁹Mg were observed. From the time of flight through the fragment separator an upper limit of 22  ns for the half-life of ¹⁹Mg was established.

The cross sections of the single proton knock-out reactions from ²⁴F, ²⁵F, and ²⁶F on a ¹²C target were measured at energies of about 50  MeV∕nucleon. Ground state populations of 6.6±0.9  mb, 3.8±0.6  mb for the reactions ¹²C(²⁴F,²³O) and ¹²C(²⁵F,²⁴O) were extracted, respectively. The data were compared to calculations based on the many-body shell model and the eikonal theory. In the reaction ¹²C(²⁶F,²⁵O) the particle instability of ²⁵O was confirmed.

In a recent paper, the giant dipole resonance width was studied as a function of angular momentum in the nucleus ⁸⁶Mo. The width of the resonance was found to be constant over a spin range of (0–40)ħ. It was concluded that the angular momentum dependence for ⁸⁶Mo differs from that of Sn isotopes. We compared both datasets with a phenomenological formula based on the thermal fluctuation theory. The ⁸⁶Mo data are inconsistent with the formula in contrast to the previously analyzed Sn data, which seems to indicate that the angular momentum dependence of the phenomenological model is not universally applicable.

The ground state and first excited state of ¹⁵F were measured by the method of elastic resonance scattering in inverse kinematics. A secondary beam of 115 MeV/nucleon ¹⁴O was slowed down to 8 MeV/nucleon and energy bunched before stopping in a C₂H₄ target. The ¹⁵F excitation energy spectrum was extracted from elastically scattered protons at 0°. The 1/2⁺ ground state resonance of ¹⁵F was determined to be unbound with respect to single-proton emission by 1.51±0.11 MeV, corresponding to a mass excess of 16.81±0.11 MeV. The 5/2⁺ first excited state resonance is unbound by 2.853±0.045 MeV leading to an excitation energy of 1.34±0.15 MeV. A comparison with systematics of single-nucleon separation energies and theoretical models suggests that ¹¹N should be unbound by about 1.5±0.15 MeV.

Intermediate energy Coulomb excitation at 100 MeV/nucleon was used to probe the low-lying level structure of both ¹⁸O and ²⁰O in the region between 1 and 8 MeV. Discrete 1^- states with energies of 5.35(10) and 6.85(5) MeV were observed in ²⁰O. The strong direct excitation and subsequent γ-ray decay of these states, along with B(Eλ) predictions for ^18,20O levels in this energy region, established their dipole character. The extracted B(E1)↑ values of 0.062(16)e² fm² and 0.035(9)e² fm² for the 5.35- and 6.85-MeV states, respectively, are significantly larger than shell model calculations, though modification of the single-particle energies, in particular, the p-sd shell gap, improves the agreement. The summed B(E1)↑ value for these levels (in Weisskopf units) is consistent with that for other nuclei that have been discussed recently in the literature as potential Pygmy dipole resonance hosts.

In order to determine the position of the neutron dripline for Z=4, a primary beam of ⁴⁰Ar was accelerated to 140 MeV/nucleon using the newly completed Coupled Cyclotron Facility at the National Superconducting Cyclotron Laboratory at Michigan State University. Neutron-rich fragmentation products emerging from a beryllium production target were separated with the A1900 fragment separator. The isotopes ^6,8He, ^9,11Li, ^12,14Be, ^17,19B, and ²⁰C were identified using time-of-flight and energy-loss information, but no events of ¹⁶Be were recorded.

The knockout of protons from ⁸B and ⁹C on a carbon target has been studied at average energies of 76 and 78 MeV/nucleon, respectively, with beams from the A1900 fragment separator incident on a stack of silicon detectors. The following cross sections were obtained: σ_-1p(⁸B→⁷Be)=130(11) mb, σ_-1p(⁹C→⁸B)=54(4) mb, and σ_-2p(⁹C→⁷Be)=98(7) mb. The results are discussed within the framework of an eikonal approach and compared with measurements performed at higher energies. From this analysis, a consistent picture emerges that gives evidence for the validity of the eikonal approach at energies below 100 MeV/nucleon. Knockout reactions at intermediate energy can thus be used to deduce absolute shell occupancies. We find the spectroscopic factors to be reduced by R_s of 0.86(7) and 0.82(6) for ⁸B and ⁹C, respectively, relative to shell-model predictions. The ⁹C result provides an accurate measurement of the asymptotic normalization coefficient of 1.27(10) fm^-1. A new technique is reported for determining separately the contributions from stripping and diffractive breakup.

The decay of the first two excited states of ¹⁷Ne (I^π=3 / 2^-, 5 / 2^-) has been studied via intermediate energy Coulomb excitation of a radioactive ¹⁷Ne beam on a ¹⁹⁷Au target using a particle detector setup allowing a kinematically complete detection of the reaction products. Despite the first excited state being bound to single proton emission but unbound with respect to two proton emission, no evidence for a simultaneous two proton decay competing with the known γ decay of the 3 / 2^- state could be observed. The 5 / 2^- state decays via sequential two-proton emission to the ground state of ¹⁵O. The transition matrix elements B(E2,1 / 2^-→3 / 2^-)=66_-25⁺¹⁸ e² fm⁴ and B(E2,1 / 2^-→5 / 2^-)=124(18)e² fm⁴ have been deduced. From the nonobservation of the simultaneous two-proton emission of the 3 / 2^- state a lifetime limit τ_2p>26 ps could be deduced in agreement with recent theoretical calculations.

We agree with the preceding Comment that the evaporation residue cross-section measurement is indeed a sensitive tool to study nuclear viscosity. The CASCADE calculations with the improved level-density description used by Diószegi reproduce our evaporation residue measurement without nuclear viscosity. However, our CASCADE calculations were not used to reproduce the absolute magnitude of the evaporation residue cross section but rather the shape of the spin distribution. The calculations show that the spin distribution can be fitted to the standard calculation without dissipation in agreement with the Comment by Diószegi. We conclude that the need to include nuclear viscosity effects in previous measurements must be due to strong viscosity outside of the saddle point.

The particle-unbound nucleus ¹³Be was populated in fragmentation reactions using the method of sequential-neutron-decay-spectroscopy at 0°. The observed central peak in the relative velocity spectrum is most likely first evidence for low lying s-wave strength with a scattering length of a_s<-10 fm. This virtual state as the ground state of ¹³Be would make it unbound with respect to ¹²Be and a neutron by <200 keV.

Evaporation residue (ER) excitation function and γ-ray multiplicity in coincidence with ER’s have been measured for the ¹⁹F+¹⁷⁵Lu system for studying the spin dependence of dissipation effects in fission. A combined analysis of total ER cross section and the spin distribution data confirms that there is no onset of dissipation in the presaddle region of the fission process.

The level structure of the unbound nucleus ¹¹N has been studied by ¹⁰C+p elastic resonance scattering in inverse geometry with the LISE3 spectrometer at GANIL, using a ¹⁰C beam with an energy of 9.0 MeV/nucleon. An additional measurement was done at the A1200 spectrometer at MSU. The excitation function above the ¹⁰C+p threshold has been determined up to 5 MeV. A potential-model analysis revealed three resonance states at energies 1.27_-0.05^+0.18 MeV (Γ=1.44±0.2 MeV), 2.01_-0.05^+0.15 MeV (Γ=0.84±0.2 MeV), and 3.75±0.05 MeV (Γ=0.60±0.05 MeV) with the spin-parity assignments I^π=1 / 2⁺,1 / 2^-,5 / 2⁺, respectively. Hence, ¹¹N is shown to have a ground state parity inversion completely analogous to its mirror partner ¹¹Be. A narrow resonance in the excitation function at 4.33±0.05 MeV was also observed and assigned spin parity 3 / 2^-.

The absolute γ-ray/fission multiplicities from hot rotating ²⁴⁰Cf, populated at seven bombarding energies using the reaction ³²S+²⁰⁸Pb, are reported. Statistical model calculations including nuclear dissipation have been performed to extract the dependence of the nuclear viscosity on temperature and/or nuclear deformation. The extracted nuclear dissipation coefficient is found to be independent of temperature. Large dissipation during the saddle to scission path provides a good fit to the γ-ray spectra.

The strength functions of the hot giant dipole resonance (GDR) in ¹²⁰Sn obtained within the phonon damping model (PDM) have been included in the complete statistical calculations and compared with their experimental divided spectra as a function of temperature T. The entire experimental shape of the hot GDR can be reproduced reasonably well using the PDM strength functions. The effect of superfluid pairing at T<1 MeV is important to obtain a better agreement between theory and data.

Energy-integrated reaction cross sections have been measured at energies ranging from 38 to 80 MeV/nucleon for various exotic neutron-rich isotopes of Al, Si, P, S, Cl, Ar, K, Ca, Sc, and Ti stopping in Si. An experimental technique is employed where Si detectors are used for both particle identification and to serve as the target material. The reduced strong absorption radii r₀² are deduced and compared with other experimental results. The radius dependence on the neutron number was studied and a trend of increasing reduced radius with neutron excess was found. This behavior is similar to that seen in lighter systems, although less pronounced than found there. The implications of this result on the conjectured existence of neutron halo or skin nuclei is discussed.

The decay structure of the particle-unstable nucleus ¹⁰Li was studied using the method of sequential neutron decay spectroscopy (SNDS) at 0°. The decay energies of ¹⁰Li can be derived from the relative velocity spectrum of the ⁹Li daughter and the neutron measured in coincidence. Evidence for low-lying s-wave strength was observed with a scattering length of <-20 fm, corresponding to a peak energy of <50 keV.

The previously measured decay of the ground state of ¹²O was reanalyzed based on new experimental and theoretical results for the ground state of ¹¹N. In the previous analysis no evidence for diproton emission was found and the measured large decay width was inconsistent with sequential proton decay via the intermediate system of ¹¹N. The recent results on ¹¹N show evidence that the ground state of ¹¹N is at substantially lower energy allowing for a consistent explanation of the two-proton decay of ¹²O in terms of sequential proton emission.

Complete statistical model calculations including temperature- and spin-dependent theoretical strength functions of the giant dipole resonance (GDR) have been performed for the decay of excited ¹²⁰Sn for the first time. Previous analyses of GDR data with theoretical models compared the centroid and full width at half maximum of the theoretical strength functions with the extracted GDR parameters. In the new approach presented, the entire shape of the strength functions is considered and the theoretical spectra obtained can be directly compared with the experiment. This analysis does not rely on the accuracy of extracting the GDR parameters and/or the nuclear temperature of one data point. The nature of the temperature dependence of the GDR in the hot ¹²⁰Sn nucleus within the thermal fluctuation and collisional damping model is discussed in this new perspective.

The disintegration of a secondary beam of ¹¹Be has been studied in the Coulomb plus nuclear field of Au and Be targets. The vector momentum (longitudinal and transverse components) was measured with a magnetic spectrograph with sufficient precision to detect previously reported differences in the velocity of the ¹⁰Be fragments and the incident beam. Coincident γ rays, detected by an array of BaF₂ scintillators surrounding the target, were used to identify events in which the target was simultaneously excited. No evidence for the previously suggested velocity difference was observed.

The β-unstable nuclei ^32,34,36,38Si have been produced by projectile fragmentation and studied by in-beam Coulomb excitation. Excited states at 1399±25 keV and 1084±20 keV have been identified for the first time in ³⁶Si and ³⁸Si, respectively, and tentatively assigned J^π  =  2⁺. The B(E2;0₁⁺→2₁⁺) values leading to these states and the previously identified 2₁⁺ states in ^32,34Si have been measured, and are compared to shell model calculations. Our results indicate that the 2₁⁺ state in ³⁴Si has a large fp-shell intruder component, and that the 2₁⁺ states in the N>20 silicon isotopes can be reproduced assuming an N  =  20 shell closure.

The proton-unbound nucleus ¹¹N has been studied via kinematic reconstruction of the emitted proton in coincidence with the residual ¹⁰C daughter nucleus. Resonances in ¹¹N were populated by using a 40 MeV/nucleon radioactive beam of ¹²N to induce the reaction ⁹Be(¹²N,¹¹N), followed by the proton decay of ¹¹N. The decay energy spectrum was constructed from the energies and separation angle of the ¹⁰C and the proton. In addition to protons from the known 1/2^- state, at 2.24 MeV above the proton decay threshold, another peak is seen near 1.45 MeV. This peak could potentially be due to the predicted 1/2⁺ ground state and/or due to the decay of the 3/2^- state to the first excited state of ¹⁰C