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Register a new userStudy of the ABC Enhancement in the d d -> alpha X reaction
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The polarized d d -> alpha X reaction at beam energies close to the eta threshold shows very strong structure in the missing mass corresponding to the ABC enhancement. The deuteron tensor analysing power A_yy, and the slope of the vector analysing power A_y with respect to angle, have been measured for this reaction around the forward direction. Both signals are small, and their variations with the alpha-particle momentum are in broad agreement with a theoretical model in which each pair of nucleons in the projectile and target deuterons undergoes pion production through the NN -> d pi reaction.
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A novel method is proposed to measure eta(958) meson bound states in 11C nuclei by missing mass spectroscopy of the 12C(p,d) reaction near the eta production threshold. It is shown that peak structures will be observed experimentally in an inclusive measurement in case that the in-medium eta mass reduction is sufficiently large and that the decay width of eta mesic states is narrow enough. Such a measurement will be feasible with the intense proton beam supplied by the SIS synchrotron at GSI combined with the good energy resolution of the fragment separator FRS.
The extremely neutron-rich system $^{6}$H was studied in the direct $^2text{H}(^8text{He},{^4text{He}})^{6}$H transfer reaction with a 26 $A$ MeV secondary $^{8}$He beam. The measured missing mass spectrum shows a broad bump at $sim 4-8$ MeV energy relative to the $^3$H+$3n$ decay threshold. This bump can be interpreted as a broad resonant state in $^{6}$H at $6.8(5)$ MeV. The population cross section of such a presumably $p$-wave state (or may be few overlapping states) in the energy range from 4 to 8 MeV is $dsigma/dOmega_{text{c.m.}} simeq 190(40)$ $mu$b/sr in the angular range $5^{circ}<theta_{text{c.m.}}<16^{circ}$. The obtained missing mass spectrum is practically free of the $^{6}$H events below 3.5 MeV ($dsigma/dOmega_{text{c.m.}} lesssim 5$ $mu$b/sr in the same angular range). The steep rise of the $^{6}$H missing mass spectrum at $sim 3$ MeV allows to derive the lower limit for the possible resonant state energy in $^{6}$H of $4.5(3)$ MeV. According to the paring energy estimates, such a $4.5(3)$ MeV resonance is a realistic candidate for the $^{6}$H ground state (g.s.). The obtained results confirm that the decay mechanism of the $^{7}$H g.s. (located at 2.2 MeV above the $^{3}$H+$4n$ threshold) is the true (or simultaneous) $4n$ emission. The resonance energy profiles and the momentum distributions of fragments of the sequential $^{6}$H$ ,rightarrow , ^5$H(g.s.)+$n, rightarrow , ^3$H+$3n$ decay were analyzed by the theoretically-updated direct four-body-decay and sequential-emission mechanisms. The measured momentum distributions of the $^{3}$H fragments in the $^{6}$H rest frame indicate very strong dineutron-type correlations in the $^{5}$H ground state decay.
The vector and tensor analysing powers, $A_y$ and $A_{yy}$, of the $vec{p}d to n{pp}_{s}$ charge-exchange reaction have been measured at a beam energy of 600 MeV at the COSY-ANKE facility by using an unpolarised proton beam incident on an internal storage cell target filled with polarised deuterium gas. The low energy recoiling protons were measured in a pair of silicon tracking telescopes placed on either side of the target. Putting a cut of 3 MeV on the diproton excitation energy ensured that the two protons were dominantly in the $^{1}S_{0}$ state, here denoted by ${pp}_{s}$. The polarisation of the deuterium gas was established through measurements in parallel of proton-deuteron elastic scattering. By analysing events where both protons entered the same telescope, the charge-exchange reaction was measured for momentum transfers $qgeq 160$ MeV/$c$. These data provide a good continuation of the earlier results at $qleq 140$ MeV/$c$ obtained with a polarised deuteron beam. They are also consistent with impulse approximation predictions with little sign evident for any modifications due to multiple scatterings.
The transfer of neutrons onto 24Ne has been measured using a reaccelerated radioactive beam of 24Ne to study the (d,p) reaction in inverse kinematics. The unusual raising of the first 3/2+ level in 25Ne and its significance in terms of the migration of the neutron magic number from N=20 to N=16 is put on a firm footing by confirmation of this states identity. The raised 3/2+ level is observed simultaneously with the intruder negative parity 7/2- and 3/2- levels, providing evidence for the reduction in the N=20 gap. The coincident gamma-ray decays allowed the assignment of spins as well as the transferred orbital angular momentum. The excitation energy of the 3/2+ state shows that the established USD shell model breaks down well within the sd model space and requires a revised treatment of the proton-neutron monopole interaction.
The cross section for the $^3$He(e, e$$d)p reaction has been measured as a function of the missing momentum $p_m$ in q$omega$ -constant kinematics at beam energies of 370 and 576 MeV for values of the three-momentum transfer $q$ of 412, 504 and 604 mevc. The L(+TT), T and LT structure functions have been separated for $q$ = 412 and 504 mevc. The data are compared to three-body Faddeev calculations, including meson-exchange currents (MEC), and to calculations based on a covariant diagrammatic expansion. The influence of final-state interactions and meson-exchange currents is discussed. The $p_m$-dependence of the data is reasonably well described by all calculations. However, the most advanced Faddeev calculations, which employ the AV18 nucleon-nucleon interaction and include MEC, overestimate the measured cross sections, especially the longitudinal part, and at the larger values of $q$. The diagrammatic approach gives a fair description of the cross section, but under(over)estimates the longitudinal (transverse) structure function.
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