丰中子核145,147La的八极形变

通过对²⁵²Cf自发裂变产生的瞬发γ谱的实验研究,建立与扩展了丰中子核^145,147La的高自旋态能级图,最高自旋态达到(41/2⁺)与(43/2^－).测得的交叉相联的相反宇称带及带间的强化E1跃迁说明在^145,147La中形成强的八极形变.对¹⁴⁵La的研究表明,具有轴对称形状的四极形变与具有反演不对称形状的八极形变在同一核中产生共存与竞争.在πh_11/2带中观测到的带交叉位于ω≈0.26—0.30MeV附近,由推转壳模型计算表明,它是由一对i_13/2中子的顺排所引起的.     

The reaction 48Ca + 248Cm → 296116* studied at the GSI-SHIP

The synthesis of element 116 in fusion-evaporation reactions of a ⁴⁸Ca beam with radioactive ²⁴⁸Cm targets was studied at the velocity filter SHIP of GSI in Darmstadt. At excitation energies of the compound nuclei of 40.9MeV, four decay chains were measured, which were assigned to the isotope ²⁹²116, and one chain, which was assigned to ²⁹³116. Measured cross-sections of (3.4 _?1.6 ^+2.7 ) pb and (0.9 _?0.7 ^+2.1 ), respectively, and decay data of the chains agree with data measured previously at the Flerov Laboratory of Nuclear Reactions in Dubna. As a new result, one ??-decay chain was measured, which terminates after four ?? decays by spontaneous fission. The ?? energies of the second-to-fourth decay are considerably higher than those measured for the ?? decays of ²⁸⁹114, ²⁸⁵Cn, and ²⁸¹Ds and the spontaneous fission half-life is significantly longer than that of ²⁷⁷Hs measured in previous experiments. A possible assignment is discussed in the frame of excited quasiparticle states of nuclei populated in the decay chain from ²⁹³116. Also other possible assignments were considered and are discussed. At an excitation energy of 45.0MeV no events were observed resulting in a one-event cross-section limit of 1.6 pb. The technical aspects related with the use of radioactive target material at SHIP are described in detail. The experience gained in this experiment will serve as a basis for future experiments aiming to study still heavier elements at the velocity filter SHIP.     

Identification of gamma transitions from He and Be ternary fission fragments

He and Be ternary fission processes of ²⁵²Cf have been studied in two experiments with the Gammasphere detector array with light charged particle detectors surrounding the source. From α-γ double gated spectra, neutron multiplicity distributions were determined for related α ternary fission pairs. In going from binary to α ternary SF for approximately the same mass splittings (A ≈ 104–146) the average neutron multiplicity decreases about 0.7 AMU. In the first light charged particle (LCP) γ-γ experiment, the ¹⁰Be spectrum was cutoff below 27 MeV and in the recent experiment, below 18 MeV. For high energy (E > 27 MeV) ¹⁰Be ternary fission, the data indicate that the largest yields go via the cold process (zero neutron evaporation). In the recent experiment with E cutoff of 18 MeV, the ¹⁰Be ternary SF was observed for zero to 4n emissions. It seems that in some cases like ¹³⁶Te, the On channel is the strongest and in the other cases like ¹⁰⁰Zr the 1n or 2n channel dominates. Clearly, there is a shift to lower average number of neutrons emitted for ¹⁰Be compared to α ternary SF. The ¹⁰⁴Zr and ¹³⁶Te cases where zero neutron emission occurs may be related to the fact that these nuclei are near the limits of the more neutron rich Zr and Te nuclei observed. The ¹³⁶Te is more spherical than the heavy partners in the other pairs and this may influence the 0n channel. Finally, the 0n channel may be more enhanced in the first data with the higher ¹⁰Be energy cutoff, leading to lower excitation energy. Also, we confirmed the 3368 keV peak with the FWHM of 60 keV emitted from the moving Be particles in the Doppler effect corrected spectrum.     

Angular momenta of fission fragments in the α-accompanied fission of 252Cf

For the first time, average angular momenta of the ternary fission fragments ^{100, 102}Zr, ¹⁰⁶Mo, ^{144, 146}Ba and ^{138, 140, 142}Xe from the α-accompanied fission of ²⁵²Cf were obtained from relative intensities of prompt γ-ray transitions with the use of the statistical model calculation. Average values of the angular momenta were compared with the corresponding values for the same fission fragments from the binary fission of ²⁵²Cf. Results indicate the presence of a decreasing trend in the average values of angular momenta induced in ternary fission fragments compared to the same binary fission fragments. On the average, the total angular momentum extracted for ternary fission fragments is ∼1.4 ℏ lower than in binary fission. Consequently, results indicate that the mechanism of the ternary α-particles emission may directly effect an induction of angular momenta of fission fragments, and possible scenarios of such mechanisms are discussed. Further, the dependence of the angular momenta of ¹⁰⁶Mo and ¹⁴⁰Xe on the number of emitted neutrons from correlated pairs of primary fragments was obtained also showing a decreasing dependence of average angular momenta with increasing number of emitted neutrons. Consequences are briefly discussed.     

Synthesis of superheavy nuclei in 48Ca+244Pu interactions

This article reports the results of experiments aimed at producing hypothetical long-lived superheavy elements located near the spherical-shell closures with Z≥114 and N≥72. For the synthesis of superheavy nuclei, we used a combination of neutron-rich reaction partners, with a ²⁴⁴Pu target and a ⁴⁸Ca projectile. The sensitivity of the present experiment exceeded by more than two orders of magnitude previous attempts at synthesizing superheavy nuclides in reactions of ⁴⁸Ca projectiles with actinide targets. We observed new decay sequences of genetically linked alpha decays terminated by spontaneous fission. The high measured alpha-particle energies, together with the long decay times and spontaneous fission terminating the chains, offer evidence for the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive alpha decays originating from the parent nuclides ^288,289114, produced in the 3n-and 4n-evaporation channels with cross sections of about a picobarn. The present observations can be considered experimental evidence for the existence of the “island of stability” of superheavy elements and are discussed in terms of modern theoretical approaches.     

Detection of99Tc by accelerator mass spectrometry: Preliminary investigations

Accelerator mass spectrometry (AMS) is an established technique for the detection of long-lived radionuclides at environmental levels. At LLNL, planned facility upgrades and advances in detection techniques are allowing us to explore the applicability of AMS to isotopes not previously pursued. One such isotope is⁹⁹Tc. We have performed a number of preliminary tests to examine the technical feasibility of AMS for the detection of⁹⁹Tc. The questions addressed were negative ion production in the cesium sputter source, transport efficiency for the ions through the spectrometer, and detection efficiency for⁹⁹Tc ions after the spectrometer. Based on the positive results of these tests, we have begun to develo measurement protocol.     

Nuclear rotational population patterns in heavy-ion scattering and transfer reactions

A model of²³⁹Pu with decoupled neutron is used for theoretical calculations of rotational population patterns in heavy ion inelastic scattering and one-neutron transfer reactions. The system treated is⁹⁰Zr on²³⁹Pu at the near-barrier energy of 500 MeV and backscattering angles of 180° and 140°. The influence of the complex nuclear optical potential is seen to be very strong, and the Nilsson wave function of the odd neutron produces a distinctive pattern in the transfer reaction.