R. Coussement

The theory of nuclear level mixing resonant spectroscopy

In atomic spectroscopy, crossings and anticrossings of electronic levels are well studied, and are the source of a considerable amount of information. In the hyperfine splitting of nuclear levels, such crossings and anticrossings can occur as well. Until now, however, these phenomena have been used for hyperfine investigations in only a few cases [1,2]. In this paper, we study anticrossings (we call them level mixings) which occur in the hyperfine energy level scheme of nuclei experiencing an axially symmetric electric quadrupole interaction and a magnetic dipole interaction, the axes of which are slightly misaligned by an angleΒ. In an experiment in which the initial orientation can be produced by any means (very low temperature, nuclear reactions, surface interaction, etc.), the angular distribution of radiation emitted by such nuclei shows resonances under the influence of these mixings. In this paper, a qualitative description of the behaviour of these resonances as a function of several (mostly geometrical) parameters is given. The observation of these resonances allows very accurate measurements of hyperfine parameters (especially of the quadrupole frequency) of nuclei with lifetimes shorter than the spin-lattice relaxation time. The less accessible lifetime range between microseconds and minutes is covered by this method.

Intermediate state perturbation in low temperature nuclear orientation

This paper deals with the perturbation of low temperature nuclear orientation in an intermediate state by an interaction not axially symmetric around the orientation axis. The directional distribution of the emitted radiation is factorized into initial orientation tensors and perturbation factors known from perturbed angular correlation theory. Low temperature nuclear orientation coefficients have been calculated numerically for111Cd in a polycrystalline sample with axially symmetric electric field gradient. Nuclear orientation experiment on111In in indium host confirms theoretical perturbation predictions and demonstrates the sensitivity of the method on different parameters. Moreover this orientation allows the determination, in magnitude and sign of111In (ground state) nuclear magnetic moment: μ = +5.48(10)μn.

Quadrupole-moments of high-spin isomers studied by level mixing spectroscopy (lems)

This paper intends to introduce the reader into Level Mixing Spectroscopy (LEMS), and to the results obtained with it so far. LEMS is a rather recent method used to study the quadrupole interaction of isomeric states in solid hosts, and was developed at Leuven. After an introduction dealing with both the theoretical background and the experimental set-up, a detailed comparison will be made with the Time Differential Perturbed Angular Distribution (TDPAD)-method. It turns out that LEMS is well suited for very high spin states in the ns-ms lifetime region. In the second part, the results in isotopes of Bismuth (10−-isomers in202-204-206Bi and the 21/2+-state in207Bi), Astatine (211At, 29/2+-and 39/2−-states;210At, 15−-and 19+-isomers;209At, 29/2+-isomer and208At, 16−-level) and Francium (213Fr, 29/2+-and 65/2−-levels;212Fr, 15−-and 27−-states and211Fr, 29/2+-and 45/2−-isomers) are discussed. The spin values range from 10 up to 65/2 and the lifetime region extends from 70 ns up to 13 ms, which proves already the applicability of LEMS. The results will also be compared to other data known so far in the208Pb-region.

Higher order level mixing resonances on oriented nuclei

Abstract Higher order level mixing resonance have been measured on the 40 s half-life 109m Ag in Zn. It is shown that the line-width is tunable with the misalignment angle of the electric to magnetic axes, and enables precise quadruple coupling determinations. For 109 m Ag Zn eQV zz /μ = 3.08(2) T has established.

g-factor measurements of μs isomeric states in neutron-rich nuclei around 68Ni produced in projectile-fragmentation reactions

We report the first g factor measurement on microsecond isomers of neutronrich nuclei produced in projectile-fragmentation reactions at intermediate energies. The nuclides in the vicinity of 68 Ni were produced and spin oriented following the fragmentation of a 76 Ge, 61.4 MeV u −1 beam at GANIL. The LISE spectrometer was used to select the nuclei of interest. The time-dependent perturbed angular distribution (TDPAD) method was applied in combination with th eh eavy-ion–gamma correlation technique to measure the g factors of 69m Cu (J π = 13/2 + , T1/2 = 350 ns) and 67m Ni (J π = 9/2 + , T1/2 = 13.3 µs). Specific details of the experimental technique and the comparison of the results (|g( 69m Cu) |= 0.225(25) and |g( 67m Ni) |= 0.125(6)) with theoretical calculations are discussed. These results provide another indication of the importance of proton excitations across the Z = 28 shell gap.

Extension of the level mixing resonance (LMR) method to study the alignment and the quadrupole moment of light exotic projectile fragments

The level mixing resonance (LMR) method has proven to be a very powerful method to measure the quadrupole interaction frequency of long-lived low-spin nuclear states which decay through γ-radiation [R. Coussement et al., Hyperfine Interactions 23 (1985) 273, G. Scheveneels et al., ibid., 52 (1989) 257, 179]. In this paper the concept of LMR has been extended for β-decaying nuclei produced in a fragmentation reaction at a recoil mass spectrometer. The idea is to apply the LMR method to study some features of light exotic nuclei (e.g. initial alignment, radiation parameter, quadrupole moment, magnetic moment, spin, etc.). It is found that in a level mixing resonance, an appreciable amount of initial alignment is transferred to polarization due to a combined electromagnetic interaction. This transfer of alignment to polarization induces a resonant change in the β-anisotropy as a function of the applied magnetic field. From the amplitude of the resonance, the initial alignment of the projectile fragments can be derived immediately if the radiation parameter A1 is known (and vice versa). The position of the resonance as a function of an externally applied magnetic field is extremely sensitive to the quadrupole interaction frequency of the nucleus in a host. The quadrupole frequency can be derived if the magnetic moment is known. If not, the ratio μνQ is determined. The LMR method applied to recoil separated exotic nuclei produced in a fragmentation reaction provides thus a very powerful tool to study several aspects of the recoil fragments.

Quadrupole moments of isomeric states in 212Ra, 213Ra and 214Ra

Abstract The static-quadrupole-interaction frequency of the 8 + isomers in 212 Ra and 214 Ra and of the case17 2 − isomer in 213 Ra has been measured with the level-mixing-spectroscopy (LEMS) method. Measurements have been performed both at low and high target temperatures (10 and 420 K) and on polycrystalline as well as single-crystalline targets. Shell-model calculations have been performed to demonstrate the influence of configuration mixing on the B (E2, 8 + → 6 + ) values and the quadrupole moments of N = 126 isotones.

In-beam study of the level crossing resonance on the 4 μs 2 9+ isomer of69Ge in a Zn single crystal

The time-integral angular distribution was measured as a function of an external magnetic field for γ-rays from the 398 keV29+ state of69Ge populated by bombarding a Zn single crystal with a natural α-beam from a cyclotron. Clear resonances of dispersion as well as Lorentz type having a narrow width consistent with the natural width (τ=4 μs) were observed at Bres=9.15 kG corresponding to the first level-crossing resonance of the29+ state due to the combined collinear magnetic and quadrupole interactions. The effects of noncollinearity were also demonstrated and compared with theory. As an application of this resonance method, the temperature dependence of the quadrupole interaction frequency was measured.

Magnetic moments and mixing ratios in the γ-decay of 125Te

Abstract Angular correlations in the γ-decay of 125Te have been investigated with a Ge(Li) and NaI (Tl) detector combination. The results for the character of the observed transitions are: two pure M1 transitions (172 and 209 keV), two pure E1 transitions (117 and 320 keV), one pure E2 transition (379 keV) and finally one mixed transition (204 keV), −1.36 μ 9 2 − = −(0.66 ± 0.09) n.m. (321 keV state ) , μ 9 2 + = + (0.79 ± 0.30) n.m. (463 keV state ) and the precession angle ωτ = +0.107 ± 0.024 rad for the 525 keV state are determined. All these data, together with the results of Coulomb excitation and (d, p) reaction work, are used to explain the level structure in the 125Te decay scheme. The experimental results are compared with the predictions of the Kisslinger-Sorensen calculations and a striking agreement is found. Comments are also made about the experimental conditions of the earlier measurements of the anticipated three quasi particles state at an excitation energy of 321 keV.

Gain without inversion for gamma radiation

Gain without inversion for a level-mixing scheme is studied for gamma-optics. In this scheme nuclear level mixing is created by misalignment of a dc magnetic field with respect to the c-axis of a noncubic crystal. Axially symmetric electron nuclear coupling and nuclear quadrupole interaction with an electric field gradient produce two electro-nuclear levels that are equally mixed and split, with the energy gap dependent on the tilting angle of the magnetic field. By laser excitation of an electron transition, nuclear spin coherence can be created between these two levels. A condition is found for the predominant population of the dark state by spontaneous emission. This state is a particular superposition of the mixed states, such that, by selection rules, the transition from it cannot be excited by γ-radiation. If all absorbing nuclei are in the dark state, resonant γ-absorption is suppressed in the sample. At the same condition, γ-emission of the excited nuclei is allowed because the corresponding transition terminates in another component of the mixed states superposition. The constraints on the tilting angle and reciprocal gap between two mixed electro-nuclear levels are found, setting the limits to these values beyond which the gain without inversion becomes impossible.