Search Results (70 total)

Production of nuclei above ¹⁰⁰Sn in fusion-evaporation reactions between ⁵⁸Ni and ⁵⁴Fe ions was studied at Oak Ridge National Laboratory by means of the recoil mass spectrometer and charged particle detection. The beam energy was varied to optimize the yields for the two-, three- and four-particle evaporation channels. Experimental results verified the predictions of the statistical model code HIVAP. The optimum energy for the ⁵⁴Fe(⁵⁸Ni,4n)¹⁰⁸Xe reaction channel that allows one to study the ¹⁰⁸Xe-¹⁰⁴Te-¹⁰⁰Sn α decay chain is deduced as 240 MeV.

An α-decay branch of (1.4±0.4)×10^-4 has been discovered in the decay of ¹⁰⁹I, which predominantly decays via proton emission. The measured Q_α value of 3918±21  keV allows the indirect determination of the Q value for proton emission from ¹⁰⁵Sb of 356±22  keV, which is approximately of 130 keV more bound than previously reported. This result is relevant for the astrophysical rapid proton-capture process, which would terminate in the ¹⁰⁵Sn(p,γ)¹⁰⁶Sb(p,γ)¹⁰⁷Te(α decay)¹⁰³Sn cycle at the densities expected in explosive hydrogen burning scenarios, unless unusually strong pairing effects result in a ¹⁰³Sn(p,γ)¹⁰⁴Sb(p,γ)¹⁰⁵Te(α decay)¹⁰¹Sn) cycle.

Two new α emitters ¹⁰⁹Xe and ¹⁰⁵Te were identified through the observation of the ¹⁰⁹Xe→¹⁰⁵Te→¹⁰¹Sn α-decay chain. The ¹⁰⁹Xe nuclei were produced in the fusion-evaporation reaction ⁵⁴Fe(⁵⁸Ni,3n)¹⁰⁹Xe and studied using the Recoil Mass Spectrometer at the Holifield Radioactive Ion Beam Facility. Two transitions at E_α=4062±7  keV and E_α=3918±9  keV were interpreted as the l=2 and l=0 transitions from the 7/2⁺ ground state in ¹⁰⁹Xe (T_1/2=13±2  ms) to the 5/2⁺ ground state and a 7/2⁺ excited state, located at 150±13  keV in ¹⁰⁵Te. The observation of the subsequent decay of ¹⁰⁵Te marks the discovery of the lightest known α-decaying nucleus. The measured transition energy E_α=4703±5  keV and half-life T_1/2=620±70  ns were used to determine the reduced α-decay width δ². The ratio δ_¹⁰⁵Te²/δ_²¹³Po² of ∼3 indicates a superallowed character of the α emission from ¹⁰⁵Te.

Results from the study of the β decays of ^40,42S and ⁴³Cl, produced in the fragmentation of a 70-MeV/nucleon ⁴⁸Ca beam, are presented. The half-lives for ⁴²S and ⁴³Cl have been measured to be 1.03±0.03 s and 3.13±0.09 s, respectively. On the basis of γ-ray singles and γγ coincidence data, decay schemes for each of these decays have been established. Only subtle changes in low-energy nuclear structure, including switching of the order of the 2^- and 3^- states in the odd-odd Cl isotopes, were observed when passing the midpoint of the νf_7/2 subshell. These results agree well with previous shell-model calculations using restricted model spaces suggesting that deformation and shell-breaking effects in the midshell region are small. However, excitations across the N=20 shell gap are important to explain the low-lying positive parity states.

The systematics of the πh_11/2⊗νh_11/2 and πh_11/2⊗νs_1/2 isomeric configurations was studied for the odd-Z N=77 isotones near the proton drip line. The isomeric decays in ¹⁴⁰Eu, ¹⁴²Tb, ¹⁴⁴Ho, and ¹⁴⁶Tm were measured by means of x-ray, γ-ray, and charged particle spectroscopy at the Recoil Mass Spectrometer at the Holifield Radioactive Ion Beam Facility (ORNL). The spin and parity of I^π=8⁺ and 5^- were deduced for the isomers in ¹⁴⁰Eu and ¹⁴²Tb. New decay schemes were established, and the half-lives of the 8⁺ isomers were measured to be 302(4) ns for ^140m2Eu and 25(1) μs for ^142m2Tb. No evidence for the expected 1⁺ ground-state was found in the ¹⁴⁴Ho decay data. The proton-emission from ¹⁴⁶Tm was restudied. Five proton transitions were assigned to two proton-emitting states. The half-lives of 198(3) ms and 68(3) ms and the spin and parity values of I^π=10⁺ and 5^- were established for ^146mTm and ^146gsTm, respectively. For the first time for an odd-odd nucleus, the interpretation of the observed decay properties and structure of the proton-emitting states was made by accounting for deformation and proton and neutron coupling to the core excitations. A complex wave-function structure was obtained, with dominating components of πh_11/2⊗νh_11/2 for the 10⁺ isomer and πh_11/2⊗νs_1/2 for the 5^- ground state.

A new isomer in ¹¹⁶Ag with a half-life of 20(1) s has been discovered through the use of conversion electron, β, and γ spectroscopy of on-line mass-separated radioactivities at the Holifield Radioactive Ion Beam Facility at ORNL. The observed electron peaks at 22.5, 44.42, and 47.33 keV were interpreted as the K,L, and M conversion electron lines resulting from a 47.9-keV E3 transition associated with the decay of a second isomeric level in ¹¹⁶Ag. A new level structure of ¹¹⁶Ag is proposed, with the levels identified as the 0^- ground-state and isomers at 47.9 and 128.8 keV having spin/parities of 3⁺ and 6^- respectively.

An unexpected strong interaction between normal deformed and triaxial strongly deformed levels at high spin, ∼32ℏ, is observed in ¹⁶⁷Lu. This constitutes the first observation of accidental degeneracy, at such high spins, causing cross talk between levels in the normal deformed and in the second potential well in any known mass region. Furthermore, evidence of quasi-particle excitations highly competing with the wobbling excitation mode in a triaxial superdeformed well has been established in ¹⁶⁷Lu.

Fine structure in proton emission from the 3.1(3)  μs activity of ¹⁴⁵Tm was discovered by using a novel technique of digital processing of overlapping recoil implantation and decay signals. Proton transitions to the ground state of ¹⁴⁴Er and to its first excited 2⁺ state at 0.33(1) MeV with a branching ratio I_p(2⁺)=9.6±1.5% were observed. The structure of the ¹⁴⁵Tm wave function and the emission process were analyzed by using particle-core vibration coupling models.

A new 7 μs isomer in the drip line nucleus ¹⁴⁰Dy was selected from the products of the ⁵⁴Fe (315 MeV)+⁹²Mo reaction by a recoil mass spectrometer and studied with recoil-delayed γ-γ coincidences. Five cascading γ transitions were interpreted as the decay of an I^π=8^- {ν9/2^-[514]⊗ν7/2⁺[404]} K isomer (T_1/2=7.0(5) μs) via the ground-state band. The probability of proton emission from ¹⁴¹Ho to the 0⁺ ground state and to the 2⁺ excited state in ¹⁴⁰Dy is discussed.

Results from the study of the β decay of ⁴⁰P, produced in the fragmentation of a ⁴⁸Ca beam, are presented. The ⁴⁰P half-life and delayed-neutron emission probability have been measured to be 153±8 ms and 15.8±2.1%, respectively. On the basis of γ-ray singles and γγ coincidence data, 21γ rays have been assigned to the decay. Three γ rays are assigned to excited states in ³⁹S populated in the delayed-neutron branch while a total of 15 γ rays were placed in a level scheme for ⁴⁰S with nine excited states up to 5 MeV. The structure of low-energy states in ⁴⁰S has been fit using the geometrical collective model that indicates the structure to be consistent with that of an anharmonic oscillator with a soft potential.

The enhancement in the production of even-Z nuclei observed in nuclear fission has also been observed in fragments produced from heavy ion collsions. Beams of ⁴⁰Ar, ⁴⁰Cl, and ⁴⁰Ca at 25 MeV/nucleon were impinged on ⁵⁸Fe and ⁵⁸Ni targets. The resulting fragments were detected using the MSU 4π detector array, which had additional silicon detectors for better isotopic resolution. Comparison of the ratios of yields for each element showed enhancement of even-Z fragment production. The enhancement was more pronounced for reactions with a greater difference in the N/Z of the compound system. However, this effect was less for systems that were more neutron rich. The average N/Z for fragments also displayed an odd-even effect with a lower average N/Z for the even-Z fragments. This is related to the greater availability of neutron-poor isotopes for even-Z nuclei.

X-ray bursts have been suggested as a possible site for the astrophysical rp-process. The time scale for the process is governed by β-decay half-lives of several even-even N  =  Z waiting point nuclei, in particular, N  =  Z  =  40 ⁸⁰Zr. A 4.1({+0.8}{-0.6})-s β⁺/EC half-life for ⁸⁰Zr was determined by observing delayed 84-keV γ rays depopulating a T_1/2  =  4-μs isomer at 312 keV in the daughter ⁸⁰Y. As this half-life is lower than many previously predicted values, the calculated excessive production of A  =  80 nuclides in astrophysical x-ray burst scenarios is reduced, and less extreme conditions are necessary for the production of heavier nuclides.

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.

Isotopically resolved intermediate-mass fragments and light charged particles have been detected from the reactions ⁴⁰Ar and ⁴⁰Ca with ⁵⁸Fe and ⁵⁸Ni at E_beam=33 and 45 MeV/nucleon. There is an angular dependence to the isotopic ratios. A moving source analysis shows that fragments emitted at Θ_lab=40° can be attributed primarily to a composite source while the fragments emitted at backward angles are primarily from a targetlike source. The results are compared to predictions of QMD, BUU, and GEMINI. QMD generally reproduces the charge distribution and energy spectra and has partial success with the isobaric ratios when the system is chemically equilibrated. All of the models have difficulty reproducing the isotopic ratios when the system is not chemically equilibrated.

Reaction cross sections have been measured at energies ranging from 50 to 70 MeV/nucleon for a variety of exotic neutron rich isotopes of Ar, K, Ca, and Sc. A method where Si detectors are used also as targets has been utilized for the measurements. The strong absorption radii r₀² have been determined and compared to the results of Glauber-type calculations which used density distributions extracted from the relativistic mean field theory. The isospin dependence of the radii was investigated and a trend similar to the one for lighter nuclei has been observed. The possible existence of halo or skin nuclei has also been addressed.

Photodisintegration of ¹¹Li was accomplished by sending a beam of ¹¹Li at 28 MeV/nucleon through the equivalent photon field of a lead target. By measuring the complete kinematics of the disintegration products, ⁹Li+n+n, we constructed the correlation of the angle between the two neutrons in the rest frame of the ¹¹Li. The correlation is independent of angle. This result argues against the existence of a bound dineutron in the ground state of ¹¹Li. © 1996 The American Physical Society.

We modify the method of Albergo et al. for determining the temperature of an excited nucleus from double ratios of isotope yields and present a statistical model which accounts for the population and decay of excited states of the emitted fragments. Nuclear temperatures are extracted using experimental ratios of isotopic yields of fragments from helium through carbon for the reactions ⁴⁰Ca + ⁵⁸Ni, ⁴⁰Ar + ⁵⁸Ni, ⁴⁰Ca + ⁵⁸Fe, and ⁴⁰Ar + ⁵⁸Fe at 33 MeV/nucleon projectile energy. Using the model we obtain consistent values for the temperature from various isotope combinations within the experimental error when accounting for the population and decay of the excited fragments.

To gain a better understanding of the production of exotic isotopes and provide information on the stability of nuclei along the path of the rapid-proton capture process, isotopic cross sections from the reaction ⁷⁸Kr + ⁵⁸Ni at 75 MeV/nucleon were measured at 0° using the A1200 fragment separator. Most notably the particle stability of ⁶⁹Br was thoroughly probed during this experiment and it appears to be particle unstable. The experimental production cross section data are compared to previous krypton isotope fragmentation data to explore the dependence of the N/Z ratio of the projectile on the observed isotopic distributions (‘‘memory effect’’) as well as with an intranuclear cascade code developed for higher energies (≳ 200 MeV/nucleon) and a semiempirical parametrization derived from high energy systematics. © 1996 The American Physical Society.

The isotopic production cross sections and momenta of all residues with nuclear charge greater than 39 from the reaction of 26, 40, and 50 MeV/nucleon ¹²⁹Xe+Be, C, and Al were measured. The isotopic cross sections, the momentum distribution for each isotope, and the cross section as a function of nuclear charge and momentum are presented here. The new cross sections are consistent with previous measurements of the cross sections from similar reaction systems. The results are compared to a geometric incomplete fusion model and a Boltzmann-Nordheim-Vlasov model. Agreement between the models and the data is fair. The most proton-rich nuclei observed in this study are predicted to have less than 50 nb production cross sections by both of these models but are observed to have much larger cross sections.

The inclusive parallel momentum distributions of ⁹Li fragments from the breakup of a secondary ¹¹Li beam have been measured at 66 MeV/nucleon over a wide range of targets (⁹Be to ²³⁸U). The measurements were performed using a zero-degree fragment separator as an energy-loss spectrometer operating in a dispersion matched mode. Earlier measurements have been extended here to the case of breakup by ²³⁸U where a distribution with a width of FWHM=38.1±1.9 MeV/c was observed. Together with a remeasurement of breakup on ⁹³Nb (FWHM)=42.8±2.6 MeV/c) a weak dependence of distribution width on target nucleus was observed. A discussion of the nature of the momentum distributions and relationship to the structure of the halo is presented in the light of recent calculations. It is concluded that the ⁹Li fragment parallel momentum distributions are relatively insensitive to the reaction and reflect the extended neutron distribution of the halo.

The neutron spectroscopy of the β-delayed neutron decay of ¹⁴Be has been investigated. Two neutron groups were observed, and energy limits are given for a third (unobserved) group that receives most of the β-decay strength. Branching ratios and logft values deduced for all three groups are compared with the results of a shell-model calculation.

Projectilelike residue yields and momentum distributions from the reaction of 70 MeV/nucleon ⁸⁶Kr with ²⁷Al at 0° were studied. The data is compared with a model developed originally for higher energies (>200 MeV/nucleon), data from prior experiments performed at both higher and lower bombarding energies, and a semiempirical parametrization derived from high energy systematics.

The three-body decay ¹²O→2p+¹⁰C was studied following production via single-neutron stripping from a radioactive ¹³O projectile. This is the first observation of two-proton emission from an unbound ground state where the one-proton emission channel is energetically closed beyond the lightest case of ⁶Be. No evidence for ²He emission is seen, despite predictions for a large diproton branching ratio. An upper limit of 7% (95% C.L.) is established for this decay branch. The implications of the small diproton branching ratio observed here and seen previously in ⁶Be are discussed.

Distributions of the parallel momentum of ¹⁰Be fragments from the breakup of 63A MeV ¹¹Be have been measured for ⁹Be, ⁹³Nb, ¹⁸¹Ta, and ²³⁸U targets. The distributions have similar narrow widths with a mean value of 43.6 ± 1.1 MeV/c FWHM, and agree with a theoretical momentum distribution for the valence neutron in ¹¹Be. The breakup mechanisms do not appear to distort the parallel momentum distribution of the ¹⁰Be core. These findings support the existence of a one-neutron halo in ¹¹Be and the use of the parallel momenta of the heavy breakup products as a reliable probe of halo neutron momenta.

Measurements of differential cross sections at 0° for projectilelike fragments from ¹⁸O and ⁴⁰Ar projectiles at E/A=80 MeV are presented and are compared with the predictions of the abrasion-ablation model, the internuclear cascade model, a high-energy empirical parametrization, and a simple cluster breakup model. The data for both projectiles are in good agreement with the high-energy systematics. The abrasion-ablation and internuclear cascade models can adequately describe the cross sections of fragments from ⁴⁰Ar with large mass losses, whereas the cluster breakup model can describe the relative cross sections of few-nucleon removal products from both projectiles.