First Observation of B 20 and B 21 Leblond, S.; Marqués, F. M.; Gibelin, J. ...
Physical review letters,
12/2018, Letnik:
121, Številka:
26
Journal Article
Direct proton-knockout reactions of Sc-55 at similar to 220 MeV/nucleon were studied at the RIKEN Radioactive Isotope Beam Factory. Populated states of Ca-54 were investigated through -ray and ...invariant-mass spectroscopy. Level energies were calculated from the nuclear shell model employing a phenomenological intemucleon interaction. Theoretical cross sections to states were calculated from distorted-wave impulse approximation estimates multiplied by the shell model spectroscopic factors, which describe the wave function overlap of the Sc-55 ground state with states in Ca-54. Despite the calculations showing a significant amplitude of excited neutron configurations in the ground-state of Sc-55, valence proton removals populated predominantly the ground state of Ca-54. This counterintuitive result is attributed to pairing effects leading to a dominance of the ground-state spectroscopic factor. Owing to the ubiquity of the pairing interaction, this argument should be generally applicable to direct knockout reactions from odd-even to even-even nuclei.
Spectroscopy of an unbound nucleus
17
C was performed using the SAMURAI spectrometer at RIBF of RIKEN. Six resonances were observed for the
16
C+n system with relative energies of 0.52, 0.77, 1.36, ...1.91, 2.22 and 3.20 MeV. The excitation energies (
E
x
) of the observed resonances were deduced, by taking into account the states of the
16
C fragments identified by coincident
γ
rays, as
E
x
=(3.02), 1.51, (3.86), 2.65, (4.72) and 3.94 MeV. The orbital angular momenta of the two observed states in
17
C at
E
x
=2.65 and 3.94 MeV were determined as 1 by comparing parallel momentum distributions with theoretical predictions.
An inelastic
α
-scattering experiment on the unstable
N
=
Z
, doubly-magic
56
Ni nucleus was performed in inverse kinematics at an incident energy of 50 A.MeV at GANIL. High multiplicity for
α
...-particle emission was observed within the limited phase-space of the experimental setup. This observation cannot be explained by means of the statistical-decay model. The ideal classical gas model at
kT
= 0.4 MeV reproduces fairly well the experimental momentum distribution and the observed multiplicity of
α
particles corresponds to an excitation energy around 96 MeV. The method of distributed
m
α
-decay ensembles is in agreement with the experimental results if we assume that the
α
-gas state in
56
Ni exists at around
113
-
17
+
15
MeV. These results suggest that there may exist an exotic state consisting of many
α
particles at the excitation energy of
113
-
17
+
15
MeV.
The tetraneutron has attracted the attention of nuclear physicists during the past decades, but there is still no unambiguous confirmation of its existence or non-existence. A new experiment based on
...8
He(
p
, 2
p
)
7
H{
t
+
4
n
} reaction, with direct detection of the four neutrons, has been carried out at RIBF, which can hopefully help to draw a definite conclusion on the tetraneutron system.
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