The interaction of an E/A=57.6-MeV 17 Ne beam with a Be target was used to populate levels in 16 Ne following neutron knockout reactions. The decay of 16 Ne states into the three-body 14 O+p+p continuum was observed in the High Resolution Array (HiRA). For the first time for a 2p emitter, correlations between the momenta of the three decay products were measured with sufficient resolution and statistics to allow for an unambiguous demonstration of their dependence on the long-range nature of the Coulomb interaction. Contrary to previous measurements, our measured limit Γ < 80 keV for the intrinsic decay width of the ground state is not in contradiction with the small values (of the order of keV) predicted theoretically.PACS numbers: 25.10.+s, 23.50.+z, 21.60.Gx, 27.20.+n Introduction -Two-proton (2p) radioactivity [1] is the most recently discovered type of radioactive decay. It is a facet of a broader three-body decay phenomenon actively investigated within the last decade [2]. In binary decay, the correlations between the momenta of the two decay products are entirely constrained by energy and momentum conservation. In contrast for three-body decay, the corresponding correlations are also sensitive to the internal nuclear structure of the decaying system and the decay dynamics providing, in principle, another way to constrain this information from experiment. In 2p decay, as the separation between the decay products becomes greater than the range of the nuclear interaction, the subsequent modification of the initial correlations is determined solely by the Coulomb interaction between the decay products. As the range of the Coulomb force is infinite, its long-range contribution to the correlations can be substantial, especially, in heavy 2p emitters.

The single-neutron properties of the N = 83 nucleus 137 Xe have been studied using the 136 Xe(d,p) reaction in inverse kinematics at a beam energy of 10 MeV/u. The helical-orbit spectrometer, HE-LIOS, at Argonne National Laboratory was used to analyze the outgoing protons, achieving an excitation-energy resolution of <100 keV. Extraction of absolute cross sections, angular distributions, and spectroscopic factors has led to a more complete understanding of the single-neutron strength in 137 Xe. In particular, the centroids of the νh 9/2 and νi 13/2 strengths appear to evolve through the N = 83 isotones in a manner consistent with the action of the tensor force.

Neutron transfer reactions with fast secondary beams of 17 Ne, 15 O, and 9 C have been studied with the HiRA and CAESAR arrays. Excited states of 18 Ne, 16 O, and 10 C in the continuum have been identified using invariant-mass spectroscopy. The best experimental resolution of these states is achieved by selecting events where the decay fragments are emitted transverse to the beam direction. We have confirmed a number of spin assignments made in previous works for the negative-parity states of 18 Ne. In addition we have found new higher-lying excited states in 16 O and 18 Ne, some of which fission into two ground-state 8 Be fragments. Finally for 10 C, a new excited state was observed. These transfer reactions were found to leave the remnant of the 9 Be target nuclei at very high excitation energies and maybe associated with the pickup of a deeply-bound 9 Be neutron.

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An upper limit to the mass of the 3-proton unbound nucleus 17 Na was measured to be ∆M ≤ 34.72 (6) MeV. The excitation energies, widths and decay modes for one-and two-proton decaying states of 8 B, 9 B, 9 C, 16 F, and 17 Ne were also measured. The energy-angular decay correlations in the 2p decay of 17 Ne are presented.

The first-excited, J π = 2 + state of 16 Ne at E * = 1.69(2) MeV is well populated in neutron knockout reactions with a 17 Ne beam and the correlations between the momenta of the three final fragments following 2p decay were measured. The correlation pattern showed aspects of both sequential and diproton-like decay, which were reproduced in three-body 14 O+p+p calculations. These calculations suggest that interference between these processes is responsible for the observed features which can be described in terms of a "tethered decay mechanism". The intrinsic width of this state was constrained to be from 100 to 250 keV. Higher excited states populating the 13 N+p+p+p exit channel were found at E

The sequential two-proton decay of the second excited state in 17 Ne, produced by inelastic excitation at intermediate energy, is studied. This state is found to be highly spin aligned, providing another example of a recently discovered alignment mechanism. The fortuitous condition that the second decay step is slightly more energetic than the first, permits the lifetime of the one-proton daughter, the ground-state of 16 F, to be determined from the magnitude of the final-state interactions between the protons. This new method gave a consistent result [Γ=20.6(57) keV] to that obtained by directly measuring the width of the state [Γ=21.3(51) keV]. This width allows one to determine the continuum coupling constant in this mass region. Real-energy continuum-shell-model studies yield a satisfactory description of both spectra and widths of low-energy resonances in 16 F and suggest an unusual large ratio of proton-proton to proton-neutron continuum couplings in the vicinity of the proton drip line.

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