Search Results (17 total)

Electron-capture delayed fission was observed in 37-s ²⁴⁴Es produced via the ²³⁷Np(¹²C,5n)²⁴⁴Es reaction at 81 MeV (on target) with a production cross section of 0.31±0.12 μb. The kinetic energies of coincident fission fragments were measured with our rotating wheel detection system and the average pre-neutron-emission total kinetic energy of the fragments was found to be 186±19 MeV. The mass-yield distribution of the fission fragments is predominantly asymmetric. Based on the ratio of the number of fission events to the measured number of α decays from the electron-capture daughter ²⁴⁴Cf (100% α branch), the probability of delayed fission was determined to be (1.2±0.4)×10^-4. This value for the delayed fission probability fits the experimentally observed trend of increasing delayed fission probability with increasing Q value for electron capture.

New neutron rich isotopes ₁₀₇²⁶⁷Bh and ₁₀₇²⁶⁶Bh were produced in bombardments of a ²⁴⁹Bk target with 117-MeV and 123-MeV ²²Ne ions at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. Identification was made by observation of correlated α-particle decays between the Bh isotopes and their Db and Lr daughters using a rotating wheel system. ²⁶⁷Bh was produced with a cross section of ≈70 pb and decays with a 17_-6⁺¹⁴ s half life by emission of α particles with an average energy of 8.83±0.03 MeV. One atom of ²⁶⁶Bh was observed, decaying within 1 s by emission of a 9.29-MeV α particle.

The excitation function for the ²³³U (³He,4n)²³²Pu reaction was measured. The maximum production cross section is 6.2±1.5 μb at 32.5±2.2 MeV. The plutonium fraction was chemically separated from interfering activities. The ²³²Pu half-life was determined to be 33.1±0.8 min using the interference-free data obtained from α-α correlations of daughter activities in equilibrium with the ²³²Pu decay.

Electron-capture delayed fission of ²⁴²Es produced via the ²³³U(¹⁴N,5n)²⁴²Es reaction at 87 MeV (on target) was observed to decay with a half-life of 11±3 s, consistent with the reported α-decay half-life of ²⁴²Es of 16_-4⁺⁶ s. The mass-yield distribution of the fission fragments is highly asymmetric. The average pre-neutron emission total kinetic energy of the fragments was measured to be 183±18 MeV. Based on the ratio of the measured number of fission events to the measured number of α decays from the electron-capture daughter ²⁴²Cf (100% α branch), the probability of delayed fission was determined to be 0.006±0.002. This value for the delayed fission probability fits the experimental trend of increasing delayed fission probability with increasing Q value for electron capture.

Following a prediction by Smolańczuk [Phys. Rev. C 59, 2634 (1999)], we searched for superheavy element formation in the bombardment of ²⁰⁸Pb with 449-MeV ⁸⁶Kr ions. We have observed three decay chains, each consisting of an implanted heavy atom and six subsequent α decays, correlated in time and position. In these decay chains, a rapid (ms) sequence of high energy α particles ( E_α≥10 MeV) indicates the decay of a new high- Z element. The observed chains are consistent with the formation of ²⁹³118 and its decay by sequential α-particle emission to ²⁸⁹116, ²⁸⁵114, ²⁸¹112, ²⁷⁷110, ²⁷³Hs ( Z  =  108) and ²⁶⁹Sg ( Z  =  106). The production cross section is 2.2_-0.8^+2.6 pb.

The new plutonium isotope, ²³¹Pu, was produced by the ²³³U(³He,5n)²³¹Pu reaction. After chemical separation, the ²³¹Pu decay modes were studied using α-spectrometry. The isotope, ²³¹Pu, was unequivocally identified by the α-decay of the chain members from its α- and electron-capture daughters using the α-α-correlation technique. The half-life of ²³¹Pu was determined to be (8.6±0.5) min. An α-group with an energy of (6.72±0.03) MeV was assigned to ²³¹Pu.

Energy levels in ⁶⁸Cu are calculated using the core-coupling model. Levels in the odd nuclei ⁶⁵Cu, ⁶⁷Cu, and ⁶⁹Zn are first calculated to find the appropriate core-coupling parameters. These parameters are then used along with a zero-range proton-neutron interaction to calculate levels in ⁶⁸Cu. In the model the second excited state is predicted to be a 3⁺ level rather than the (3^-) level currently assigned in the nuclear data sheets. The ordering of the quartet formed by coupling the p_{3 / 2} proton to the g_{9 / 2} neutron is predicted to be 6^-, 3^-, 4^-, 5^-.

NUCLEAR STRUCTURE Core-coupling model calculation of energy levels in ⁶⁸Cu, ⁶⁵Cu, ⁶⁷Cu, ⁶⁹Zn, ⁶⁷Ni.

The situation of two closely spaced resonance levels of the same spin and parity in a single channel is investigated. The conventional R-matrix theory is compared to a generalized R-matrix theory which allows for different boundary conditions for each level. The two 3 / 2⁺ resonances near 3 MeV in the ¹²C(n,n)¹²C reaction and the 1 / 2⁺ resonance dip at 2.4 MeV in the ¹⁶O(n,n)¹⁶O reaction are investigated. With no background term R₀ equivalent fits to the data can be found using a wide range of boundary conditions and corresponding reduced widths. This shows that there are ambiguities in the reduced widths of levels, depending on the values of other parameters used in fitting the levels. The conventional R-matrix theory fits the data well and allows for inclusion of distant levels through an R₀ term. The inclusion of an R₀ term for unequal boundary conditions introduces new parameters and becomes quite complicated. Thus, we conclude that the conventional R-matrix theory is not only adequate to fit closely spaced resonances of the same spin and parity, but it is preferred.

The ¹¹⁵In(p,p)¹¹⁵In reaction for E_p=6.6 to 7.6 MeV has been analyzed. The target spin of ¹¹⁵In is 9 / 2⁺. This means that there can be a maximum of 10 partial waves in each analog resonance. However, there are usually one or two dominant single-particle states which contribute to a resonance. In this reaction the s_{1 / 2} and d_{3 / 2} channels are dominant in the energy region from 6.6 to 7.6 MeV. Fits to the differential cross-section data of Thompson et al. were found to be quite good when only these two channels were included for the positive-parity states. The s_{1 / 2} and d_{3 / 2} channels can couple to the 9 / 2⁺ target spin to give total spins and parities of 4⁺, 5⁺, and 3⁺, 4⁺, 5⁺, 6⁺. All of these states were seen and analyzed. A sum rule is derived which involves the total spins of the resonances. The total spins assigned are found to be consistent with this sum rule. There is a strong amount of mixing in the 4⁺ and 5⁺ states.

The R-matrix theory of nuclear scattering is used to consider the meaning of spectroscopic factors for unbound states. Potential scattering is explicitly calculated by constructing an R function which is equivalent to the potential. The compound effects are then parametrized in the usual R-matrix fashion with the spectroscopic factors being explicitly included as parameters. The spectroscopic factors are found by fitting data. The spectroscopic factors are very sensitive to the choice of the various resonance parameters, and the validity of spectroscopic factors as a meaningful quantity for unbound states is discussed in the light of several calculations. Several types of resonances found in the reactions ¹²C(n,n)¹²C and ¹⁶O(n,n)¹⁶O are considered. These include the cases of very narrow and very broad resonances and a double-resonance situation.

The polarization P(θ) and differential cross section σ(θ) for the scattering of neutrons by ¹¹B have been measured for 0.075<~E_n<~2.2 MeV. Both σ(θ) and P(θ) are simultaneously fitted reasonably well by R-matrix parameters for broad states in ¹²B with assignments 2^- (l=0) and 4^- (l=2) at excitation energies E_x=4.37 and 4.54 MeV, respectively. The 2^- level has not previously been observed, while the 4^- level has previously been assigned to be 3^-. These results, together with a previous 3^- assignment for a state at E_x=3.39 MeV, give experimental evidence for proton-hole-neutron-particle shell-model configurations (1p_{3 / 2})^-1 (1d_{5 / 2}) and (1p_{3 / 2})^-1(2s_{1 / 2}) in ¹²B. Shell-model calculations were made with δ-function residual interactions and include configuration mixing of the resulting 2^- states. Work on T=1 states in ¹²C^* has been compared with the present work.

The polarization of protons elastically scattered from ⁸⁹Y has been observed at a laboratory angle of 125° in the incident energy range 7.1-7.6 MeV. Differential cross sections were also measured in this energy range at laboratory angles of 90°, 125°, 150°, and 170°. The isobaric analogs of the 2^- and 1^- states at 2.48 and 2.63 MeV in ⁹⁰Y were observed at resonance energies of 7.29 and 7.46 MeV. Polarization angular distributions were taken at 7.28 and 7.45 MeV. An optical-model-plus-resonance analysis of all the data showed that these analog states could be described by a d_{3 / 2} state coupling with the ½^- target spin. The amount of d_{5 / 2} and s_{1 / 2} admixture possible in the 2^- and 1^- states, respectively, was shown to be quite small. This indicates that there is very little mixing between the various 2^- and 1^- single-particle configurations in ⁹⁰Y.

Polarization and differential cross-section measurements have been made for ⁸⁹Y(p,p)⁸⁹Y in the isobaric analog regions of the 2^- and 1^- states at 2.48 and 2.63 MeV in ⁹⁰Y. An optical-model-plus-resonance analysis of the data indicates that these states can be described by a d_{3 / 2} state coupled to the 1 / 2^- target spin. The admixture of d_{5 / 2} and s_{1 / 2} configurations in the 2^- and 1^- states, respectively, is found to be small. The spin sequence 2^-, 1^- with the higher spin lower in energy is strongly favored.

The spectroscopic factors S_n of bound neutron states are usually found from (d,p) stripping reactions. An alternative method of finding S_n for medium-to-heavy nuclei is to analyze isobaric analog resonances observed in (p,p) scattering from these nuclei. The present analysis uses a modified R-matrix theory in which boundary matching is done within the optical-model potential region rather than directly onto the Coulomb potential region. A resonance mixing phase and an optical penetrability are introduced. Both single- and multilevel resonances are treated. The effects of compound elastic scattering and the energy dependence of the level shift are investigated. Formulas for the spreading width are obtained. The variation of S_n with the value of the matching radius and the best choice of this radius are discussed. As examples of the method, analyses of the s-wave resonance in ⁹²Zr(p,p)⁹²Zr near 6.0-MeV bombarding energy and of s- and d-wave resonances in ⁹⁰Zr(p,p)⁹⁰Zr near 5.8 and 6.8 MeV are presented. The values of S_n obtained are compared with those from (d,p) experiments, and the reliability of the two methods is discussed.