Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV (J^{π}=1^{+}, T=1) state→ground state (J^{π}=0^{+}, T=0) and the isoscalar magnetic dipole 18.15 MeV (J^{π}=1^{+}, T=0) state→ground state transitions in ^{8}Be. Significant enhancement relative to the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of >5σ. This observation could possibly be due to nuclear reaction interference effects or might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70±0.35(stat)±0.5(syst)  MeV/c^{2} and J^{π}=1^{+} was created.

A rotational band of nineteen transitions with a moment of inertia 3$ nA of 84£ 2 MeV" 1 has been observed in 152 Dy. The band feeds into the oblate yrast states between 19" and 25" and it is proposed that the lowest member of the band has a spin of 22 + and thus the band extends up to 6Qfr. It is identified as the yrast superdeformed band and its intensity accounts for the whole of the ridge structure seen previously in continuum E y -E r correlations.PACS numbers: 21.10. Re, 23.20.Lv, 27.70.+q The nucleusDy has been extensively studied and three different structures have been identified. The low-spin yrast levels have a pseudovibrational structure 1 which develops into a low-deformation (fi = 0.15) prolate rotational band 2 extending up to 40fr. This band, in the spin region between SR and 38£, lies between 0.5 and 1.5 MeV above the yrast states which have a weak oblate structure formed by particles in equatorial orbits. 3 " 5 At higher spins the y-ray continuum is dominated by a collective E2 bump. 6 Part of this bump has been shown to arise from superdeformed (/J^O^) bands from the existence of ridges with a moment of inertia3 (2) = (85 ±2)H 2 MeV" 1 in E y 'Ey correlation spectra. 7,8 In this Letter we present data showing a discrete-line rotational band extending over nineteen transitions from 602 to 1449 keV with an almost constant energy separation of 47 keV which corresponds to the superdeformed moment of inertia. The major -y-ray decay deexciting the band feeds into the yrast oblate structure between the 19"" and 25"" states and then proceeds via the 60-ns 17 + isomer. Additionally 25% of the decay intensity bypasses this isomer. We propose that the decay process from the bottom of the band is essentially statistical, involving several transitions, and we assign the spin at the bottom of the band to be 22£, thus establishing the spin at the top of the band to be 60fr. This is the first observation of a discrete-line superdeformed band and it extends the spin at which discrete states have been seen from about 46* (e.g., 158 Er, Tj0m etaL 9 ) to 60T.The experiment was carried out on the tandem accelerator at the Daresbury Laboratory using the TES-SAS spectrometer, which consists of a 50-element bismuth germanate (BGO) crystal ball similar to that used in TESSA2 10 with twelve escape-suppressed germanium detectors. 11 The states in 152 Dy were populated by the reaction 108 Pd( 48 Ca,4>7) at 205 MeV with a target consisting of two 500-/ig-cm~2 self-supporting foils isotopically enriched at 95% in 108 Pd. A 15-mgcm" 2 gold catcher foil was positioned 5 cm downstream of the targets such that it was outside the focus of the germanium detectors but within the full detection efficiency of the BGO ball. A total of over 150 million double (Ge-Ge) coincidences were recorded together with the sum energy and number of hits (fold) in the BGO ball. The time difference between the BGO ball and the second-coincidence germanium detector was recorded and enabled most of the neutron-induced events in the germanium detec...

3The general phenomenon of shell structure in atomic nuclei has been understood since the pioneering work of Goeppert-Mayer, Haxel, Jensen and Suess [1].They realized that the experimental evidence for nuclear magic numbers could be explained by introducing a strong spin-orbit interaction in the nuclear shell model potential.However, our detailed knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), the unique nature of the atomic nucleus as an object composed of two distinct types of fermions can be expressed as enhanced correlations arising between neutrons and protons occupying orbitals with the same quantum numbers. Such correlations have been predicted to favor a new type of nuclear superfluidity; isoscalar neutron-proton pairing [2][3][4][5][6], in addition to normal isovector pairing (see Fig. 1). Despite many experimental efforts these predictions have not been confirmed. Here, we report on the first observation of excited states in N = Z = 46 nucleus 92 Pd. Gamma rays emitted following the 58 Ni( 36 Ar,2n) 92 Pd fusionevaporation reaction were identified using a combination of state-of-the-art highresolution -ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutronproton coupling scheme, different from the previous prediction [2][3][4][5][6]. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling [7,8]) in the ground and low-lying excited states of the heaviest N = Z nuclei. The strong isoscalar neutron-proton correlations in these N = Z nuclei are predicted to have a considerable impact on their level structures, and to influence the dynamics of the stellar rapid proton capture nucleosynthesis process.For all known nuclei, including those residing along the N = Z line up to around mass 80, a detailed analysis of their properties such as binding energies [9] and the spectroscopy of the excited states [10] strongly suggests that normal isovector (T = 1) pairing is dominant at low excitation energies. On the other hand, there are long standing predictions for a change in the heavier N = Z nuclei from a nuclear superfluid dominated by isovector pairing to a structure where isoscalar (T = 0) neutron-proton (np) pairing has a major influence as the mass number increases towards the exotic doubly magic nucleus 100 Sn [2-6], the heaviest N = Z nucleus to be bound. N = Z nuclei with mass number > 90 can only be produced in the laboratory with very low The two-neutron (2n) evaporation reaction channel following formation of the 94 Pd compound nucleus, leading to 92 Pd, was very weakly populated with a relative yield of less than 10 −5 of the total fusion cross section. Gamma rays from decays of excited states in 92 Pd were identified by comparing γ-ray spectra in coincidence with two emitted neutrons and no charged particles with γ-ray spectra in coincidence with oth...