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The four proton-hole nucleus, 204 Pt, was populated in the fragmentation of an E/A = 1 GeV 208 Pb beam. The yrast structure of 204 Pt has been observed up to angular momentum I = 10 by detecting delayed γ-ray transitions originating from metastable states. These long-lived excited states have been identified to have spin-parities of I π = (10 + ), (7 − ) and (5 − ) and half-lives of T 1/2 = 146(14) ns, 55(3) µs and 5.5(7) µs, respectively. The structure of the magic N = 126 204 Pt nucleus is discussed and understood in terms of the spherical shell model. The data suggests a revision of the two-body interaction for N = 126, Z < 82, which determines the evolution of nuclear structure towards the r-process waiting point nuclei.PACS numbers: 29.30. Kv, 23.20.Lv The evolution of the properties of atomic nuclei with respect to neutron and proton numbers is a key question of nuclear physics. The study of unstable, neutron-rich nuclei represents one of the foremost pursuits of modern nuclear physics. Over the coming decade new radioactive ion beam facilities are being built with the main objectives being to probe neutron-rich nuclei. Within recent years surprising phenomena have been observed in neutron-rich nuclei such as neutron skins, halos and dramatic changes in the ordering and spacing of energy levels [1].While the stability of the N = 82 shell gap is an active topic of research [2,3], an open question is whether or not there is a quenching of the N = 126 shell gap as protons are removed from doubly magic 208 Pb. The proton dripline has been experimentally reached up to heavy elements [4], our present knowledge of the neutron dripline is limited to light species. The part of the nuclear chart with the least information on neutron-rich nuclei is the 76 Os to 82 Pb region, with experimental knowledge on only a few isotopes. This mass region is however an ideal testing ground of nuclear theories. With the removal of just a few protons and neutrons the landscape evolves from spherical to elongated prolate through disk shaped oblate and triaxial forms [5]. Consequently the information gained on neutron-rich, N = 126 nuclei is essential for the understanding of nuclear structure in heavy nuclei. From a longer-term perspective, experi-

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Time-resolved Schottky mass spectrometry has been applied to uranium projectile fragments which yielded the mass value for the 208 Hg (Z = 80, N = 128) isotope. The mass excess value of ME=-13265(31) keV has been obtained, which has been used to determine the proton-neutron interaction strength in 210 Pb, as a double difference of atomic masses. The results show a dramatic variation of the strength for lead isotopes when crossing the N=126 neutron shell closure, thus confirming the empirical predictions that this interaction strength is sensitive to the overlap of the wave-functions of the last valence neutrons and protons.Atomic nuclei are many-body systems in which the strong, weak and electromagnetic fundamental interactions play a major role by acting between the nucleons. The sum effect of these interactions is reflected in the total binding energy of the nucleus, which is directly connected with its mass [1]. Therefore, nuclear masses often provide hints to new nuclear structure effects. Indeed, shell structure and pairing correlations have been discovered through nuclear masses. Nuclear masses and binding energies are important, however, in a much wider domain. Examples occur in the studies of various nucleosynthesis processes in stars, in weak interaction physics relating to the unitarity of the CabibboKobayashi-Maskawa matrix, in tests of QED and in the determination of fundamental constants [2].Dedicated filters can be constructed from mass differences to isolate specific nucleonic interactions. One such filter is the average interaction strength of the last proton(s) with the last neutron(s) denoted δV pn . The proton-neutron interaction-being of fundamental interest for nuclear structure-has been intensively discussed in the last decades [3][4][5][6][7][8][9][10][11][12][13][14]. It has been shown that it is essential for the development of configuration mixing and for the onset of collectivity and deformation in nuclei [6], for changes of the underlying shell structure [7], and for the microscopic origins of phase transitional behavior in nuclei [7][8][9]. Being the largest along the N=Z line, the p−n interaction can be related to Wigner's SU(4) symmetry [3]. The average value of the p−n interaction strength can also be related to the nuclear symmetry energy [10]. Moreover, treatment of the nucleon-nucleon correlations finds similarities in interpreting other many-body systems: for example, odd-even staggering effects were observed in ultrasmall superconducting metallic grains [15]. This Letter will present a mass measurement for 208 Hg isotope, that provides one of the most important δV pn values in the entire nuclear chart.For even-even nuclei, the average p − n interaction of the last two protons with the last two neutrons can be defined as [13]:where B is the binding energy of the nucleus. By assuming that the nuclear core remains essentially unchanged, δV pn , by its definition, largely cancels the interactions of the last nucleons with the core. A given δV pn value for an even-even nucleus ref...

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