Precision measurement of lifetimes in 74, 76Kr using the Recoil-Distance method
Precision measurement of lifetimes in 74, 76Kr using the Recoil-Distance method

Coincidence spectra showing the 2+ → 0+ (left column), the 4+ → 2+ (central column), and 6+ → 4+ (right column) transitions in 74Kr at various distances between the target and stopper foils, as indicated in the panels.

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.  
Example spectra for the first three yrast transitions in 74Kr are shown in Fig. 1. First results point to lifetimes of the first 2+ and 4+ states in both 74Kr and 76Kr that differ significantly from the published values and that are more in agreement with the recent data from Coulomb excitation. It can be speculated that the early singles measurements suffered from contaminations from other reaction channels and radioactive decay products. The data is currently being analyzed and the new and more reliable lifetimes will give important constraints on the analysis of the Coulomb excitation data, and will help to disentangle the rich shape effects that are found in these nuclei. [1] E. Bouchez et al, Phys. Rev. Lett. 9 0, 082502 (2003) [2] W. Korten et al., Nucl. Phys. A 746, 90c (2004) [3] J. Roth et al., J. Phys. G: Nucl. Phys. 10, L25 (1984) [4] B. Wörmann et al., Nucl. Phys. A 431, 170 (1984)  
Precision measurement of lifetimes in 74, 76Kr using the Recoil-Distance method

The Köln Plunger device coupled to the GASP spectrometer at LNL (Legnaro)

#587 - Màj : 04/03/2005

 

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