Nuclear orientation as a tool for studying the structure of very unstable nuclei

With the availability of modern isotope separator on-line systems it has become possible to make broad and systematic studies of low-energy low-spin nuclear structure. A vital ingredient in such a program is unique spin-parity assignments to all low-lying levels. A most desirable complement to spin-parity information is detailed spectroscopic information. Obtaining such information far from stability is difficult because of low activity production. Nuclear orientation provides a means for obtaining spin assignments usingsingles measurements. This is less demanding on source intensities than - angular correlation coincidence measurements. Further, nuclear orientation can provide information on magnetic moments and on multipole mixing ratios. A number of structural problems are discussed: the need for unique spin assignments in systematics schemes; the need to distinguish between E2+E0 and M1 transitions; the importance of measuring E2-M1 mixing ratios; and the value of magnetic moment information. Particular emphasis is placed on the desirability of obtaining such information in the neutron-deficient Pt, Au, Hg, Tl, Pb and Bi isotopes, based upon the experimental program at the UNISOR facility.Work supported in part by U.S.Dept. of Energy, Contract No. DE-AS05-80ER10599.     

Nuclear orientation and nuclear structure

The purpose of this paper is to demonstrate how recent experimental results of¹⁵¹Eu and⁵⁷Fe high pressure Mössbauer studies in 4f and 3d metallic magnetic systems can contribute to a deeper understanding of the nature of local moment (4f) and itinerant (3d) magnetism in these systems. Special emphasis is given on the comparison of the experimental results with related theoretical models.     

Nuclear structure of neutron-rich nuclei near closed shells from excited-state g-factor measurements

New techniques to measure the g factors of picosecond-lived excited states of neutron-rich nuclei produced as radioactive beams are discussed along with their applications to study nuclear structure near ¹³² ₅₀Sn₈₂ and in the region between N=20 and N=28.     

Nuclear orientation following muon capture by spin zero nuclei

It is shown that in the proposed experimental set-up of the Louvain-Saclay collaboration, the nuclei recoiling into the forward (or backward) hemisphere in the reaction $\text{μ}{}^{\mathrm{?}}\text{+ A(J}{}_{\mathrm{<mtext>i</mtext>}}\text{= 0) → B(J}{}_{\mathrm{<mtext>f</mtext>}}\text{≧ 1) + ν}{}_{\mathrm{\mu }}$ will not only be polarized but also aligned. A general expression for the alignment of the recoiling nucleus is derived in terms of the other observables and it is shown to be independent of nuclear structure and the dynamics of the muon capture interaction. Numerical results are presented for muon capture by ¹²C and ¹⁶O.     

Nuclear structure investigations in the region of superheavy nuclei

Radioactive decay from the ground state or isomeric states has been investigated for a series of nuclei in the region of Z = 100–106 by means of α–γ—or evaporation residue-(γ, conversion electron)—measurements in prompt and delayed coincidence. Systematic trends in single-particle level energies in N = 145–151 odd-even isotones could be extended up to Z = 104, while an energy systematics of lowlying Nilsson levels in odd-mass einsteinium isotopes was established. Information on nuclear levels at E* > 500 keV was obtained from the decay study of isomeric states in ^251−255No. The text was submitted by the author in English.     

Nuclear structure and formation mechanism of heavy shell-stabilized nuclei

In a series of experiments, ⁴⁸Ca induced cold fusion reactions on Pb targets were employed to populate states in ^253,254No isotopes. The groundstate band of ²⁵⁴No is identified up to spin 20⁺. The deduced quadrupole deformation is β=0.27(2). Evidence for decays from a rotational band built on a 7/2[624] configuration was obtained in ²⁵³No. The initial angular momentum and excitation energy of ^253,254No residues, after neutron emission, was determined. States up to spin 22 ? and excitation energy of E ^*=8.5 MeV were observed. The data provide direct information on the fission barrier at high spins B _f ≥5 MeV, and on the shell-correction energy E _shell ^exp ≥4.1 MeV.     

Nuclear structure effects in the isotope shift with halo nuclei

Corrections to atomic energy levels due to nuclear structure effects are discussed. These are the finite nuclear size combined with relativistic and recoil corrections, and the nuclear polarizability. Good understanding of these effects is necessary for interpretation of high-precision measurements of the isotope shifts with neutron-rich nuclei ^6,8He, ¹¹Li and ¹¹Be. The summary of the results of the accurate atomic structure calculations is presented also.     

Mass measurements of neutron-deficient nuclei and their implications for astrophysics

During the years 2005-2010 the double-Penning-trap mass spectrometer JYFLTRAP has been used to measure the masses of 90 ground and 8 isomeric states of neutron-deficient nuclides with a typical precision of better than 10keV. The masses of 14 nuclides -- ⁸⁴Zr , ^{88, 89}Tc , ^90-92Ru , ^92-94Rh , ^{94, 95}Pd , ^{106, 108, 110}Sb -- have been experimentally determined for the first time. This article gives an overview on these measurements and their impact on the modeling of the astrophysical rp -process.     

Nuclear magnetic resonance measurements and the energy band structure of PtSn

Nuclear magnetic resonance measurements of the Knight shift and spin-lattice relaxation time for ¹⁹⁵Pt and ¹¹⁹Sn in PtSn are reported. The energy band structure as determined by the relativistic orthogonalized plane wave method is also presented. The band model developed has holes in the Pt d-band but does not have a large density of states associated therewith.     

Nuclear structure studies of187Ir via on-line nuclear orientation

A more complete level scheme is presented here for the decay of¹⁸⁷Pt, incorporating many new lines and levels up to 2.4 MeV. Emphasis has been placed on the extraction of multipole mixing ratios from nuclear orientation data. Analysis of that data, however, required a more complete understanding of the level structure (including, in particular, branching ratios, conversion coefficients, and level feedings), prompting the collection of new spectroscopy data. The low-lying, positive-parity levels are described in terms of (odd-proton) single-particle Nilsson states coupled to a triaxial core. Multipole mixing ratios are compared to those calculated in the Particle-Plus-Triaxial-Rotor Model.     

COMPASS and HERMES contributions to our understanding of the nucleon spin

An update is given on the ongoing experimental investigation of the spin structure of the nucleon, with particular emphasis on the results from the COMPASS and HERMES experiments. Both longitudinal and transverse spin phenomena are covered. In the first case, the hot topic is the direct measurement of the gluon polarization. Evidence is presented for ΔG/G being small around x_g?0.1, and the first moment of ΔG should not be larger than 0.2-0.3. About transverse spin effects, evidence is given for new phenomena, associated with transverse-momentum-dependent distribution and fragmentation functions, which might explain the transverse spin phenomena observed for a long time in pp scattering.     

Mesonic contribution to the Compton scattering amplitude for heavy nuclei

The contribution of mesonic exchange currents to nuclear Compton scattering is investigated within the framework of a Fermi gas model of nuclear matter in the non-relativistic limit. The additional interaction between the nucleons is accounted for by including two- and three-body diagrams. As a test of this model, the enhancement constant κ is calculated. The full correlators for the central and tensor parts of the nucleon-nucleon interaction due to pion exchange are obtained and the energy dependence of the amplitude is investigated. The contribution of the Δ-excitation to the mesonic part of the Compton amplitude is calculated explicitly using an effective Hamiltonian in the static limit.     

Manifestation of the structure of heavy nuclei in their alpha decays

Low-lying one- and two-quasiparticle states of heavy nuclei are predicted. Alpha-decay chains, including those that proceed through isomeric states, are examined on the basis of the predicted properties of superheavy nuclei.     

Nuclear orientation and quadrupole interaction

In quadrupole-interaction nuclear-orientation experiments (QI-NO) the quadrupole interaction frequency V_Q=e²qQ/h is determined from the angular distribution of -rays emitted in the decay of radioactive nuclei oriented at low temperatures as impurities in noncubic single-crystal matrices. This means that either the spectroscopic quadrupole moment Q of radioactive isotopes or the crystal electric field gradient eq can be determined from QI-NO experiments. Recent developments in QI-NO are discussed, especially with regard to the investigation of nuclei far from stability, with the following main topics: (1) Experimental aspects; (2) Quadrupole moments of short-lived nuclei; (3) Systematics of electric field gradients; (4) QI-NO with new host lattices, such as dichalcogenides; (5) Resonance techniques.     

Time-resolved and time-integral on-line nuclear orientation measurements of neutron-deficient Hg−Au−Pt−Ir nuclei

The methods of time-resolved and time-integral on-line nuclear orientation have been applied to study short lived nuclei with the NICOLE facility (Nuclear Implantation into Cold On-Line Equipment) at ISOLDE-3 in CERN using beams of^182–186Hg. The half-lives in these decay chains are of the order of seconds and therefore comparable to the spin-lattice relaxation times of the nuclei in iron. As the relaxation rate depends strongly on the g-factor, g-factors of nuclei in the decay chains can be deduced from the observation of the time evolution of γ-ray anisotropy. Using this technique the existence of an isomer in¹⁸⁴Au has been found and the g-factors of¹⁸⁴Au,^184mAu and¹⁸²Au have been determined. Accurate half-lives have been extracted from the data. Time-integral nuclear orientation has been observed for short lived as well as longer lived isotopes of the Hg decay chains. From these measurements, after proper correction for incomplete relaxation, the magnetic moments of^183mPt,¹⁸³Ir and¹⁸²Ir have been derived. The applicability of the time-resolved nuclear orientation technique for nuclei far from stability and its possible limitations is discussed.