This paper intends to introduce the reader to a recent method of determining the static quadrupole interaction of isomers in solids: LEMS (level mixing spectroscopy). First of all, the basic principle of level mixing is explained, and it is shown how this phenomenon can be observed in the angular distribution of radiation emitted by the decaying isomer. Furthermore, LEMS is compared to level mixing resonances and time differential perturbed angular distribution. It is shown that LEMS is a very attractive method in cases where high spin states are involved, and for isomers with lifetimes up to several milliseconds.

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Quadrupole moments of six high-spin isomers in the At isotopes have been measured with the level-mixing-spectroscopy method:'At(16 ), At( -+ ), ' At(19+, 15 ), "At( -',-+ ). The results show that level mixing spectroscopy is a promising technique to determine quadrupole moments of isomers that are dificult to measure by other in-beam hyperfine interaction methods. A large increase of the quadrupole moment is observed if neutrons are excited across or removed from the X= 126 shell closure. This behavior is explained in terms of an enhanced core softness for fewer core neutrons; the aligned valence particles, moving in equatorial orbits, then easily polarize the core towards oblate deformation.

The level mixing spectroscopy method has been applied to measure the static quadrupole moments of six isomeric states in "Fr, " Fr, and "Fr (Z = 87). For isomers with the proton configuration mh 9/2l13/2 a large increase of the quadrupole moment was observed with the removal of neutrons from the closed %=126 core: Q( -+, ' Fr)= -70 (7) e fm, Q(15, ' Fr)= -80(12) e fm', and Q ( -',9, 2 "Fr) = -107(18) e fm'. Quadrupole moments for very high spin states were also measured: Q( -, ',' "Fr)= -219(53) e fm', Q(27, '"Fr)= -152(31) e fm, and Q( -, ',"' Fr)= -198(56) e fm . The observed values are in good agreement with shell-model calculations, but are lower than that predicted using the deformed-independent particle model. The implantation behavior of Fr in a Tl host is also discussed.