Search Results (27 total)

The masses of very neutron-deficient nuclides close to the astrophysical r p- and ν p-process paths have been determined with the Penning trap facilities JYFLTRAP at JYFL/Jyväskylä and SHIPTRAP at GSI/Darmstadt. Isotopes from yttrium (Z=39) to palladium (Z=46) have been produced in heavy-ion fusion-evaporation reactions. In total, 21 nuclides were studied, and almost half of the mass values were experimentally determined for the first time: ⁸⁸Tc, ^90-92Ru, ^92-94Rh, and ^94,95Pd. For the ⁹⁵Pd^m, (21/2⁺) high-spin state, a first direct mass determination was performed. Relative mass uncertainties of typically δm/m=5×10^-8 were obtained. The impact of the new mass values has been studied in ν p-process nucleosynthesis calculations. The resulting reaction flow and the final abundances are compared with those obtained with the data of the Atomic Mass Evaluation 2003.

The masses of six neutron-deficient rare holmium and thulium isotopes close to the proton drip line were determined with the SHIPTRAP Penning trap mass spectrometer. For the first time the masses of the proton-unbound isotopes ^144,145Ho and ^147,148Tm were directly measured. The proton separation energies were derived from the measured mass values and compared to predictions from mass formulas. The new values of the proton separation energies are used to determine the location of the proton drip line for holmium and thulium more accurately.

The new neutron-deficient isotopes ^193,194Rn have been identified in the complete fusion reaction ⁵²Cr+¹⁴⁴Sm→¹⁹⁶Rn^* at the velocity filter SHIP. The α-decay energy and half-life value of ¹⁹⁴Rn were determined to be E_α=7700(10) keV and T_1/2=0.78(16) ms, respectively. For ¹⁹³Rn the half-life of T_1/2=1.15(27) ms and two α lines at E_α1=7685(15) keV, I_α1=74(20)% and E_α2=7875(20) keV, I_α2=26(12)% were found. The decay pattern of ¹⁹³Rn, which is substantially different from that of the heavier odd-A Rn isotopes, provides first experimental evidence for the long-predicted deformation in the very neutron-deficient Rn nuclei.

The new neutron-deficient isotope ¹⁸⁷Po has been identified in the complete fusion reaction ⁴⁶Ti+¹⁴⁴Sm→¹⁸⁷Po+3n at the velocity filter SHIP. Striking features of the ¹⁸⁷Po α decay are the strongly-hindered decay to the spherical ground state and unhindered decay to a surprisingly low-lying deformed excited state at 286 keV in the daughter nucleus ¹⁸³Pb. Based on the potential energy surface calculations, the ¹⁸⁷Po ground state and the 286 keV excited state in ¹⁸³Pb were interpreted as being of prolate origin. The systematic deviation of the α-decay properties in the lightest odd-A Po isotopes relative to the smooth behavior in the even-A neighbors is discussed. Improved data for the decay of ¹⁸⁷Bi^m,g were also obtained.

Decay properties of the new neutron-deficient nuclide ¹⁹²At have been studied in the complete fusion reaction ¹⁴⁴Sm(⁵¹V,3n)¹⁹²At at the velocity filter SHIP. Two isomeric states with half-lives of 88(6) ms and 11.5(6) ms, respectively, and with complex α-decay schemes were identified in ¹⁹²At. The decay pattern of one of the isomers suggests that it is based on the oblate-deformed π2f_7/2⊗ν1i_13/2 configuration, which confirms the expected onset of deformation in the At isotopes by approaching the neutron midshell at N=104.

We report on a set of in-beam studies of excited states in ²⁵⁰Fm. We detected prompt γ rays by using the JUROSPHERE IV array and conversion electrons by using the SACRED spectrometer. Both devices were used in conjunction with the RITU gas-filled recoil separator located at the University of Jyväskylä. ²⁵⁰Fm nuclei were produced through the ²⁰⁴Hg(⁴⁸Ca,2n)²⁵⁰Fm fusion-evaporation reaction. An experimental excitation function gave a maximum reaction cross section of (980 ± 160) nb at an energy of 209 MeV in the center of the target. Tagging techniques were employed, and a number of E2 transitions were observed that connected the ground-state band levels from spin 4⁺ up to 18⁺. The highly converted 4⁺→2⁺ transition is observed only by use of conversion electron spectroscopy. The observed ground-state band transitions indicate a rotational structure. We deduce a quadrupole deformation parameter of β₂=0.28 ± 0.02. A low-energy background of apparent nuclear origin is observed in conversion electrons and postulated to arise from the decay of high-K bands. The half-life of ²⁵⁰Fm is measured to be 30.4 ± 1.5 min.

The determination of the 0^°-180^° asymmetry (A_γ), which arises because of the parity nonconserving matrix element, in the 751-keV γ decay of polarized 17/2^- isomers in ⁹³Tc with respect to the direction of polarization is reported. A combined analysis of the present results together with those from our earlier work yields an effect of two standard deviations.

The production of the very neutron-deficient nuclides ^184-192Bi and ^186-192Po in the vicinity of the neutron midshell at N = 104 has been studied by using heavy-ion-induced complete fusion reactions in a series of experiments at the velocity filter SHIP. The cross sections for the xn and pxn evaporation channels of the ⁴⁶Ti+¹⁴⁴Sm→¹⁹⁰Po^*,⁹⁸Mo+⁹²Mo→¹⁹⁰Po^*,^50,52Cr+¹⁴²Nd→^192,194Po^*, and ^94,95Mo+⁹³Nb→^187,188Bi^* reactions were measured. The results obtained, together with the previously known cross section data for the heavier Bi and Po nuclides, are compared with the results of statistical model calculations carried out with the HIVAP code. It is shown that a satisfactory description of the experimental data requires a significant (up to 35%) reduction of the theoretical fission barriers. The optimal reactions for production of the lightest Bi and Po isotopes are discussed.

Proton and α decay of the proton-rich nuclide ¹⁸⁵Bi has been restudied in more detail in the complete fusion reaction ⁹³Nb(⁹⁵Mo,3n)¹⁸⁵Bi at the velocity filter SHIP. The observed decay pattern of ¹⁸⁵Bi and of the heavier odd-mass isotopes ^{187,189,191,193}Bi are interpreted based on potential-energy surface calculations. It is shown that the experimental systematics of the particle decays and of the excited states in these nuclei (where known) can be explained by the prolate-oblate shape co-existence at low excitation energy. The observed state in ¹⁸⁵Bi is proposed to be of prolate nature, which is in contrast with the previously proposed oblate interpretation.

The α decay of ^188,192Po has been reexamined in order to probe the 0⁺ states in the daughter nuclei ^184,188Pb that can be associated with coexisting spherical, oblate, and∕or prolate configurations. Improved values were measured for the excitation energy and the feeding α-decay intensity of the 0₂⁺ state in ^184,188Pb and conflicting results on the 0₃⁺ state in ¹⁸⁸Pb were clarified. All known cases of fine structure in the α decay of the even-even Po nuclei are reviewed. The reduced α-decay width systematics combined with potential-energy-surface calculations confirm the onset of deformation in the ground state of the polonium nuclei around the neutron midshell. An isomeric state with a half-life of 580(100)ns has been identified in ¹⁹²Po.

Near-barrier excitation functions have been measured for evaporation-residue production and fission in the ¹²C+^204,206,208Pb and ⁴⁸Ca+^168,170Er systems that lead to the compound nuclei ^216,218,220Ra^*. A pronounced suppression of evaporation-residue production is observed for the more symmetric combinations, ⁴⁸Ca+^168,170Er. We relate this to the significant quasifission components already observed for these systems.

Pair correlation effects in the transfer processes have been investigated through an odd-even staggering in the measured yields of Q-value integrated multi-neutron transfer channels in the ⁶²Ni+²⁰⁶Pb system at three bombarding energies close to the Coulomb barrier. The experimental results for these inclusive transfer data suggest a dominance of a sequential mechanism in the transfer process.

In-beam γ rays have been observed in the neutron-deficient isotopes ^170,171,172Pt using the recoil-decay tagging technique. The yrast transition sequence proposed for ¹⁷²Pt indicates that the 0⁺ bandhead of the deformed intruder band is situated about 900 keV above the weakly deformed ground state, i.e., its excitation energy has risen by about 300 keV compared to ¹⁷⁴Pt. The measured energy of the 2⁺→0⁺ transition in ¹⁷⁰Pt supports an even larger increase in the excitation energy of the intruder configuration with the departure from the middle of the 82–126 major neutron shell. Furthermore, a band with transition energies almost identical to those found in ¹⁷²Pt has been assigned to ¹⁷¹Pt and was interpreted as corresponding to a rotationally aligned i_13/2 neutron orbital coupled to the core excitations.

A Reply to the Comment by R. R. Chasman.

High-spin states in ₉₄²⁴⁰Pu and ₉₆²⁴⁸Cm have been populated with Coulomb excitation at beam energies slightly above the Coulomb barrier. In each nucleus a new side band has been identified in γ-γ coincidence measurements and has been interpreted as the lowest-lying octupole excitation. The alignments and Routhians of these bands, as well as the branching ratios of E1 decays to the ground-state bands, are in good agreement with results from cranked RPA calculations.

Quadrupole moments of the six known superdeformed (SD) bands of ¹⁹³Hg and the yrast SD band of ¹⁹²Hg have been determined by a Doppler-shift-attenuation-method measurement utilizing the gammasphere array. The quadrupole moments of all ¹⁹³Hg SD bands were found to be similar, suggesting the active single-particle orbitals in the mass-190 region exhibit only small shape-driving effects. Additionally, there is evidence for an unexpected difference in the quadrupole moments of SD bands in ¹⁹²Hg and ¹⁹³Hg.

The single-particle character of states outside the doubly magic (radioactive) nucleus ⁵⁶Ni has been determined through a measurement of the (d,p) neutron transfer reaction using inverse kinematics. From the spectroscopic factors of the low-lying states in ⁵⁷Ni, the astrophysically interesting yield for the ⁵⁶Ni(p,γ) reaction to the mirror nucleus ⁵⁷Cu has been calculated, utilizing charge symmetry. The rate for this reaction in the temperature range typical of novae, supernovae, and x-ray bursts is found to be more than 10 times higher than previously assumed.

An excited superdeformed band in ¹⁹⁴Hg, observed to decay directly to both normal-deformed and superdeformed yrast states, is proposed to be a K^π  =  2^- octupole vibrational band, based on its excitation energies, spins, and likely parity. The transition energies are identical to those of the yrast superdeformed band in ¹⁹²Hg, but originate from levels with different spins and parities. The evolution of transition energies with spin suggests that cancellations between pairing and particle alignment are partly responsible for the identical transition energies.

Excited states have been observed for the first time in the neutron-deficient ^176,178Hg nuclei using the recoil-decay tagging (RDT) technique in which prompt γ rays are associated with a particular isotope through a correlation with the characteristic ground state α decay. Below N  =  102, the excitation energy of a rotational band built on a prolate shape (β₂∼0.25) increases with decreasing mass to the point where there is no longer any evidence for its presence at low spin in ¹⁷⁶Hg. The data are in qualitative agreement with recent mean field calculations.

High-precision lifetime measurements have been performed in superdeformed (SD) bands of ^192,194Hg with the Doppler-shift attenuation method. Intrinsic quadrupole moments Q₀ were extracted for three SD bands in ¹⁹⁴Hg and for the yrast SD band in ¹⁹²Hg. Within experimental uncertainties, all four SD bands have equal Q₀ values. These results provide constraints on differences in Q₀ values between the “identical” SD bands ¹⁹⁴Hg(3) and ¹⁹²Hg(1).

Four new superdeformed bands have been observed with the Gammasphere array and have been assigned to the ¹⁹⁵Hg nucleus. Two of the bands are interpreted as signature partners most likely based on N_osc=6 neutron quasiparticles coupled to a superdeformed core, while the other two appear to be based on a j_15/2 intruder orbital. These four bands do not exhibit a simple, “identical bands” relationship to other superdeformed bands in this mass region.

The presence of ΔI=2 staggering in the three known superdeformed (SD) bands of ¹⁹⁴Hg has been reexamined in a new experiment with Gammasphere. A relative precision of better than 60 eV was achieved for most transition energies. Staggering plots were extracted and their statistical significance was analyzed. No clear evidence was found for an extended regular ΔI=2 staggering in the three SD bands of ¹⁹⁴Hg. However, statistically significant deviations from a smooth reference were observed in the two excited SD bands. Different scenarios are discussed but no firm conclusion about the origin of the observed deviations can be drawn.

Angular and Q-value distributions for a large variety of charge and mass partitions populated in the reaction ⁴⁰Ca + ¹²⁴Sn have been measured at 170 MeV with a new time-of-flight magnetic spectrometer. Reaction channels involving the net transfer of up to six protons and six neutrons have been detected. Population patterns, cross sections, and energy-loss distributions are compared with results of theoretical calculations based on independent single-nucleon transfer modes. The overall agreement is good, but more complex mechanisms may be needed to account for the larger drift towards neutron stripping revealed by the experimental data. © 1996 The American Physical Society.

Discrete γ rays directly connecting states of a superdeformed (SD) band in ¹⁹⁴Hg to the yrast states have been discovered. Thus, the excitation energies and spins of all members of the lowest SD band are established for the first time, together with their likely parity. The SD band decays from its 10⁺ and 12⁺ states, which lie 4204.8 and 4407.4 keV above the normal-deformed yrast states of the same spins.

Fusion-evaporation cross sections have been measured for ^32,36S+¹¹⁰Pd to high accuracy in the energy range around the Coulomb barrier. The two extracted fusion barrier distributions display very similar structures with two well-resolved peaks below the nominal barrier, which are consistent with a model calculation including the coupling of the one-, two-, and three-phonon collective vibrations of ¹¹⁰Pd. A third lower-energy barrier, together with a strong relative cross section enhancement, is observed for ³²S+¹¹⁰Pd, and is reproduced by coupling the two-neutron pickup channel which has a large and positive ground state Q value (+5.1 MeV). For ³⁶S+¹¹⁰Pd, the calculated effect of the corresponding transfer channel (with negative Q value) is much smaller.