F. Hardeman

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.

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.

In-beam quadrupole interaction measurements by level mixing resonance

A new method to measure the quadrupole interaction of in-beam produced isomeric nuclear states is proposed. A simplified version of the theory, adapted to the particular experimental configuration of an in-beam experiment is described.

LMR quadrupole interaction measurement on69mGe and71mGe in Zn

A new experimental set-up to perform in-beam LMR (Level Mixing Resonance) measurements is briefly outlined. The first results are reported here. The quadrupole interaction of69mGeZn is measured and in good agreement with earlier measurements:vQ=80.6(4) MHz; for71mGeZn, the valuevQ=33.1(30)MHz is found. In addition, the relaxation behaviour and radiation damage of Ge recoil implanted in Zn is studied.

Level Mixing Resonance Spectroscopy (LEMS)— a novel probe of non-linear nuclear quadrupole resonance

Abstract LEMS is discussed for the case of a nuclear spin experiencing a static EFG ( V zz ) and an external magnetic field ( B ) applied nearly colinear to V zz ). It is shown that the nature of resonances at level crossings strikingly parallels those encountered in conventional NQR with the important difference that non-linear effects involving multiphoton transitions (Δ m > 1) are readily observed in LEMS because of large equivalent RF fields possible. The linewidth of these resonances are power-broadened and can be continuosly tuned by merely changing the angle between V zz and B .

Level mixing resonances on oriented nuclei

The level crossing and level mixing resonance methods have been developed in order to enable the measurement of quadrupole interaction frequencies of nuclei with lifetimes between microseconds and minutes in solids. Both methods are shown to be better suited for application 1n this lifetime region than the existing ones. Only the level mixing resonance method can be applied to nuclei with lifetimes up to several minutes. A review of our level crossing and level mixing experiments is also given in this paper.

Nuclear level mixing resonances (LMR)

The recently developed Level Mixing Resonance method for the measurement of hyperfine interactions turns out to be a very useful complement to the other methods. Especially in the “difficult” cases, e.g. When the lifetime of the isomer under consideration is very long or when its spin is very high, this new method can yield valuable, otherwise inaccessible, information. In this paper the concept of this method is briefly presented and some of the recent experiments are described.

Nuclear level mixing resonance spectroscopy

The existent methods for measuring quadrupole interactions are not suited to nuclei with lifetimes in the micro-seconds to minutes region. AD/NQR, a possible candidate in this lifetime gap, has not yet succeeded in overcoming its predicted difficulties. A new resonant method, recently developed and based on the principles of level mixing (cfr atomic spectroscopy) covers this less accessible lifetime range. Many other kinds of resonances can be described according to the level mixing formalism. The underlying theory of LMR and its important consequences, leading to some interesting features of the method, is briefly formulated. Two successfully performed measurements demonstrate the feasibility and the predicted characteristics of this new promising method.

NUCLEAR LEVEL MIXING RESONANCES ON NON-AXIALLY SYMMETRIC SYSTEMS

For several years, nuclear Level Mixing Resonances (LMR) have been well studied, both theoretically [1,2] and experimentally [3,4]. The experimental results show that LMR is a very powerful technique to determine the static quadrupole interaction frequencies of long living isomers (10 ns up to 100 ms). The LMR formalism has been developed for isomers implanted in a solid host, providing an axially symmetric electric field gradient (EFG). In this paper, the theory has been elaborated to non-axially symmetric systems (η≠0). It will be shown that these systems have some special features, for example the possibility to derive the asymmetry η of the EFG if its orientation (ß, γ) in the LAB system is known.