The neutron-deficient krypton nuclei at and near the N = Z line exhibit some of the best examples of shape coexistence,
i.e. states of different deformation within a small energy range. The systematics of the excited 0
+ states that are found throughout the light even-even krypton isotopes suggests that the heavier isotopes
76Kr and
78Kr are prolate in their ground state, that prolate and oblate configurations are almost degenerate in
74Kr, and that the oblate configuration becomes the ground state in self-conjugate
72Kr [1]. In order to prove this scenario, Coulomb excitation experiments using radioactive
74Kr and
76Kr beams have been performed at GANIL (
see ). Utilizing higher-order effects, diagonal matrix elements and, therefore, the intrinsic shapes are accessible in Coulomb excitation experiments at safe energies below the barrier. The results qualitatively confirm the proposed scenario of shape coexistence [2]. However, the transitional matrix elements,
i.e. the B(E2) values, which could be extracted from the Coulomb excitation data with high precision, are partly in disagreement with earlier lifetime measurements [3,4]. In order to resolve this question, a new lifetime measurement of the relevant states in
74Kr and
76Kr was performed with much superior equipment compared to the earlier measurements from 20 years ago. A recoil-distance Doppler shift experiment was performed at LNL (Legnaro) using the Köln Plunger device coupled to the GASP spectrometer. States in
74Kr and
76Kr were populated in the reaction
40Ca +
40Ca at a bombarding energy of 147 MeV, corresponding to a center-of-target energy of 124 MeV. At this energy the 4p (
76Kr) and 4p (
74Kr) are the second and third strongest reaction channels, and no further channel identification is needed. The distance between the target, which was protected against oxidation by gold layers on both sides, and the gold stopper foil was changed on average every twelve hours. Data were taken for 13 distances between 7.5 μm and 1.5 mm. The GASP array comprised 32 Compton-suppressed germanium detectors at the time of the experiment, grouped in seven rings in the so-called configuration II without the BGO elements of the inner ball. Lifetimes are extracted for each distance and for each ring of detectors using the differential-decay curve method. Side-feeding effects can be eliminated by gating from above on the transition that populates the state in question.