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Nuclear magnetic dipole and electric quadrupole moments and changes in mean square charge radii for the neutron-rich 155-159Eu isotopes have been measured using resonance ionization spectroscopy at the IRIS facility. It has been found that the isotopes with N > 92, unlike the isotopes with 89 < N < 92, have an ordinary character of odd-even staggering in nuclear charge radii. This means that the octupole deformation attributed previously to the europium nuclei around 154Eu does not display itself in the charge radii of heavier europium isotopes.PACS: 21.10.Ft, Ky; 31.30.GsThe odd-even staggering (OES) of nuclear-charge radii is a very remarkable effect. It consists in the fact that the mean square charge radius (r z) of an odd-N nucleus is always (with a few exceptions) smaller than the average value of the radii for its even-N neighbours (the normal OES).The staggering effect is expressed by the parameterwhere A (r2)a_l,A = (r2)A -(rZ)A _ ~ with A being the atomic number of an odd-N nucleus. For the normal OES, 7,~ < 1. The OES is understood qualitatively as an effect of pairing giving rise to an enlarged zero-point vibration amplitude in even-N nuclei [1]. It has been shown also [-2] that three-and four-body correlations may be essential in forming the normal staggering. However, there are a few instances with YA > 1 (the inverted OES). Many of them are considered to be due to a shape instability, when a sudden onset of nuclear quadrupole deformation occurs as, for example, in the cases of the neutron deficient 7] isotopes the inverted OES is likely to be connected with the octupole degree of freedom. The invertion takes place exactly in those nuclei for which reflection-asymmetric shapes were theoretically predicted and suggested also by some nuclear spectroscopic evidence. It was found also [6], that the normal staggering is restored towards the isotopes which are supposed to be reflection-symmetric. A marked inverted OES was observed also in the vicinity of 154Eu in the course of isotope shift (IS) investigations at the IRIS facility [8]. The conception of quadrupole deformation instability can not be used to interpret the found inversion because all the nuclei involved (153,154,155Eu ) have stable and strong quadrupole deformation, as it is well known from the quadrupole moments [8] and rotational bands for these nuclei [9]. Recently, as a result of thorough analysis of a good many spectroscopic information, Sheline [10] has come to the conclusion that there is a sizeable stable octupole deformation in 153,154,155Eu ' the deformation parameter/13 for the odd-neutron a54Eu being greater than for the adjacent even-N nuclei. According to [10] this difference in the values of f13 is responsible for the inverted OES observed in Eu isotopes. In this connection a question of importance arises: whether this unusual behaviour of A (r 2) (the inverted OES) will remain for the more neutron-rich Eu isotopes; or, whether the region of stable octupole deformation comprises these nuclei too.To answer this question we ha...

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