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.
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.
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