Shape Coexistence at Zero Spin in

The low-spin structure of the semimagic ^{64}Ni nucleus has been considerably expanded: combining four experiments, several 0^{+} and 2^{+} excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0^{+} excitation is located at a surprisingly high energy (3463 keV), with a collective 2^{+} state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N=40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.

Erratum: Refractive Index of Silicon at γ Ray Energies [Phys. Rev. Lett. 108, 184802 (2012)].

This corrects the article DOI: 10.1103/PhysRevLett.108.184802.

Polarization delivery in heterodyne interferometry.

Optically heterodyned laser interferometry, as applied to measuring linear displacements, requires different optical frequencies to be encoded onto unique polarization states. To eliminate non-linear contributions to the interferometer signal, the frequency difference must be introduced after beam splitting and the interfering beams must be recombined via spatially separated paths. The polarization jitter of the frequency-shifted beams still originates a noise in the beat-signal phase. A formula is given expressing the noise amplitude in terms of the illuminating beam's extinction ratio.

Refractive index of silicon at γ ray energies.

For x rays the real part of the refractive index, dominated by Rayleigh scattering, is negative and converges to zero for higher energies. For γ rays a positive component, related to Delbrück scattering, increases with energy and becomes dominating. The deflection of a monochromatic γ beam due to refraction was measured by placing a Si wedge into a flat double crystal spectrometer. Data were obtained in an energy range from 0.18 MeV to 2 MeV. The data are compared to theory, taking into account elastic and inelastic Delbrück scattering as well as recent results on the energy dependence of the pair creation cross section. Probably a new field of γ optics with many new applications opens up.

Neutron interference from a split-crystal interferometer.

The first successful operation of a neutron interferometer with a separate beam-recombining crystal is reported. This result was achieved at the neutron interferometry setup S18 at the ILL in Grenoble by a collaboration between TU Wien, ILL, Grenoble, and INRIM, Torino. While previous interferometers have been machined out of a single-crystal block, in this work two crystals were successfully aligned on nanoradian and picometre scales, as required to obtain neutron interference. As a decisive proof-of-principle demonstration, this opens the door to a new generation of neutron interferometers and exciting applications.

Ultrahigh-resolution γ-ray spectroscopy of 156Gd: a test of tetrahedral symmetry.

Tetrahedral symmetry in strongly interacting systems would establish a new class of quantum effects at subatomic scale. Excited states in 156Gd that could carry the information about the tetrahedral symmetry were populated in the 155Gd(n,γ)156Gd reaction and studied using the GAMS4/5 Bragg spectrometers at the Institut Laue-Langevin. We have identified the 5(1)- → 3(1)- transition of 131.983(12) keV in 156Gd and determined its intensity to be 1.9(3)x10(-6) per neutron capture. The lifetime τ=220(-30)(+180) fs of the 5(1)- state in 156Gd has been measured using the GRID technique. The resulting B(E2)=293(-134)(+6) Weisskopf unit rate of the 131.983 keV transition provides the intrinsic quadrupole moment of the 5(1)- state in 156Gd to be Q0=7.1(-1.6)(+0.7) b. This large value, comparable to the quadrupole moment of the ground state in 156Gd, gives strong evidence against tetrahedral symmetry in the lowest odd-spin, negative-parity band of 156Gd.

Simulations of Gamma Cascades and Modelling Atomic Collision Chains.

A new method for simulation of nuclear γ cascades by the Monte Carlo technique is described. It makes it possible to generate artificially individual events of the γ-cascade decay of neutron capturing states in heavy nuclei. For the purpose of determination of lifetimes of nuclear levels by the GRID method the previously developed Mean Free Path Approach for modelling atomic collisions and their interplay with cascades has been generalized. Examples of the use of this generalization for determination of lifetimes of selected (150)Sm and (158)Gd levels are given and discussed.

The GRID Technique: Current Status and New Trends.

In the GRID technique one measures Doppler-broadened line profiles of γ transitions using the high resolution crystal spectrometers GAMS, which are installed at the high flux reactor of the ILL Grenoble. One of the essential applications of this technique is the measurement of nuclear state lifetimes. In the present contribution the precision and the principal limits of the technique are discussed.

Study of Interatomic Potentials in ZnS-Crystal-GRID Experiments Versus Ab Initio Calculations.

Crystal-GRID measurements have been performed with ZnS single crystals. For the first time, an asymmetric Crystal-GRID line shape could be observed. The preliminary data evaluation indicates that the reported lifetime of the 3221 keV level in (33)S is too short. A value of about 60 fs has been found. Due to this "long" lifetime the line shape is much less structured than calculated with the reported lifetime.

Study of Interatomic Potentials Using the Crystal-GRID Method on Oriented Single Crystals of Ni, Fe, and Cr.

The Crystal-GRID method is used to study interatomic collisions at low energy in metals and such to probe the repulsive interatomic potential. Line shapes of gamma rays, emitted by the recoiling (59)Ni isotope after thermal neutron capture in Ni single crystals, were measured and compared to results obtained by molecular dynamics simulations of the slowing down. The same procedure is also used for recoiling (57)Fe and (54)Cr atoms in Fe and Cr single crystals, respectively. Different potentials (including several from the embedded atom method) are investigated using the observed fine structure of the line shape which depends on the crystal orientations. From the detailed study of the lineshapes measured in two different orientations, a new potential is then derived for each element. Nuclear state lifetimes for the excited isotopes are also deduced with a higher precision than obtained with standard nuclear techniques.

Application of GRID to Foreign Atom Localization in Single Crystals.

The application of GRID (Gamma Ray Induced Doppler broadening) spectroscopy to the localization of foreign atoms in single crystals is demonstrated on erbium in YAP. By the investigation of the Doppler broadened secondary γ line for two crystalline directions, the Er was determined to be localized on the Y site. Conditions for the nuclear parameters of the impurity atoms used for the application of GRID spectroscopy are discussed.

GAMS5.

The construction of the double-crystal γ-spectrometer GAMS5 was finished recently and the instrument is now operational. Measurements with double flat crystals were carried out and we will report here on the progress concerning the characteristics of the spectrometer.

Lifetime Measurements in (178)Hf.

Lifetimes of levels from K(π) = 2(+), K(π) = 4(+) and several K(π) = 0(+) bands have been measured in the (178)Hf nucleus using the GRID technique. Lifetimes of the 2(+) and 3(+) levels were measured within the K(π) = 2(+) γ band. A lower limit was established for the lifetime of the 4(+) level of the K(π) = 4(+) band. The resulting upper limits for the absolute B(E2) values exclude collective transitions from the K(π) = 4(+) to the ground state band but not to the K(π)= 2(+) band. Level lifetimes were also measured for several states within three separate K(π)= 0(+) bands. Evidence is presented for a previously unobserved case of two excited K(π)= 0(+) bands being connected via collective E2 transitions.