γ transitions in A=7 hypernuclei and a possible derivation of hypernuclear size

On the basis of the He-7(Lambda)+ N + N three-body model which has successfully been applied to a systematic study of the energy and nucleon halo structure of the A=7 isotriplet hypernuclei, strengths of gamma transitions in Li-7(Lambda) and He-7(Lambda) are calculated. The new model confirms that the B(E2;5/2(+) --> 1/2(+)) value in Li-7(Lambda) is reduced remarkably in comparison with the corresponding B(E2;3(+) --> 1(+)) in the core nucleus Li-6. This is due to the gluelike role of the Lambda particle which induces a contraction of the core nuclear size. It is suggested that a measurement of the 5/2(+)-->1/2(+)E2 transition rate in Li-7(Lambda) (ongoing at KEK as E419) provides a unique opportunity to derive the hypernuclear size and hence to confirm the size contraction experimentally. The E2 and M1 transition strengths are also predicted for low-lying states in the hypernucleus He-7(Lambda) whose core nucleus He-6 is known to have a neutron halo. Another prediction is made of much enhanced E2 transitions in Li-7(Lambda) from the 5/2(+) and 3/2(+)(T=1) states which are expected to have a proton halo structure. [S0556-2813(99)00705-0].