120 research outputs found
One-neutron removal reactions on neutron-rich psd-shell nuclei
A systematic study of high energy, one-neutron removal reactions on 23
neutron-rich, psd--shell nuclei (Z=5-9, A=12-25) has been carried out. The
longitudinal momentum distributions of the core fragments and corresponding
single-neutron removal cross sections are reported for reactions on a carbon
target. Extended Glauber model calculations, weighted by the spectroscopic
factors obtained from shell model calculations, are compared to the
experimental results. Conclusions are drawn regarding the use of such reactions
as a spectroscopic tool and spin-parity assignments are proposed for 15B, 17C,
19-21N, 21,23O, 23-25F. The nature of the weakly bound systems 14B and 15,17C
is discussed.Comment: 11 pages + 2 figure
B(E1) Strengths from Coulomb Excitation of 11Be
The (E1;) strength for Be has been extracted from
intermediate energy Coulomb excitation measurements, over a range of beam
energies using a new reaction model, the extended continuum discretized coupled
channels (XCDCC) method. In addition, a measurement of the excitation cross
section for Be+Pb at 38.6 MeV/nucleon is reported. The (E1)
strength of 0.105(12) efm derived from this measurement is consistent
with those made previously at 60 and 64 MeV/nucleon, i n contrast to an
anomalously low result obtained at 43 MeV/nucleon. By coupling a
multi-configuration description of the projectile structure with realistic
reaction theory, the XCDCC model provides for the first time a fully quantum
mechanical description of Coulomb excitation. The XCDCC calculations reveal
that the excitation process involves significant contributions from nuclear,
continuum, and higher-order effects. An analysis of the present and two earlier
intermediate energy measurements yields a combined B(E1) strength of 0.105(7)
efm. This value is in good agreement with the value deduced
independently from the lifetime of the state in Be, and has a
comparable p recision.Comment: 5 pages, 2 figures, accepted for publication in Phys. Lett.
Single-Proton Removal Reaction Study of 16B
The low-lying level structure of the unbound system B has been
investigated via single-proton removal from a 35 MeV/nucleon C beam. The
coincident detection of the beam velocity B fragment and neutron allowed
the relative energy of the in-flight decay of B to be reconstructed. The
resulting spectrum exhibited a narrow peak some 85 keV above threshold. It is
argued that this feature corresponds to a very narrow (100 keV)
resonance, or an unresolved multiplet, with a dominant + configuration which decays by d-wave neutron
emission.Comment: 16 pages, 5 figures, 1 table, submitted to Phys. Lett.
Helium Clustering in Neutron-Rich Be Isotopes
Measurements of the helium-cluster breakup and neutron removal cross sections
for neutron-rich Be isotopes A=10-12,14 are presented. These have been studied
in the 30 to 42 MeV/u energy range where reaction measurements are proposed to
be sensitive to the cluster content of the ground-state wave-function. These
measurements provide a comprehensive survey of the decay processes of the Be
isotopes by which the valence neutrons are removed revealing the underlying
alpha-alpha core-cluster structure. The measurements indicate that clustering
in the Be isotopes remains important up to the drip-line nucleus 14^Be and that
the dominant helium-cluster structure in the neutron-rich Be isotopes
corresponds to alpha-Xn-alpha.Comment: 5 pages, 2 tables and 3 figure
Single-neutron transfer from 11Be gs via the (p,d) reaction with a radioactive beam
The 11Be(p,d)10Be reaction has been performed in inverse kinematics with a
radioactive 11Be beam of E/A = 35.3 MeV. Angular distributions for the 0+
ground state, the 2+, 3.37 MeV state and the multiplet of states around 6 MeV
in 10Be were measured at angles up to 16 deg CM by detecting the 10Be in a
dispersion-matched spectrometer and the coincident deuterons in a silicon
array. Distorted wave and coupled-channels calculations have been performed to
investigate the amount of 2+ core excitation in 11Be gs. The use of "realistic"
11Be wave functions is emphasised and bound state form factors have been
obtained by solving the particle-vibration coupling equations. This calculation
gives a dominant 2s component in the 11Be gs wave function with a 16% [2+ x 1d]
core excitation admixture. Cross sections calculated with these form factors
are in good agreement with the present data. The Separation Energy prescription
for the bound state wave function also gives satisfactory fits to the data, but
leads to a significantly larger [2 x 1d] component in 11Be gs.Comment: 39 pages, 12 figures. Accepted for publication in Nuclear Physics A.
Added minor corrections made in proof to pages 26 and 3
Re-examining the transition into the N=20 island of inversion: structure of 30Mg
Intermediate energy single-neutron removal from Mg has been employed
to investigate the transition into the N=20 island of inversion. Levels up to
5~MeV excitation energy in Mg were populated and spin-parity assignments
were inferred from the corresponding longitudinal momentum distributions and
-ray decay scheme. Comparison with eikonal-model calculations also
permitted spectroscopic factors to be deduced. Surprisingly, the 0
level in Mg was found to have a strength much weaker than expected in
the conventional picture of a predominantly intruder configuration
having a large overlap with the deformed Mg ground state. In addition,
negative parity levels were identified for the first time in Mg, one of
which is located at low excitation energy. The results are discussed in the
light of shell-model calculations employing two newly developed approaches with
markedly different descriptions of the structure of Mg. It is concluded
that the cross-shell effects in the region of the island of inversion at Z=12
are considerably more complex than previously thought and that
configurations play a major role in the structure of Mg.Comment: Physics Letters B, Volume 779, 10 April 2018, Pages 124-12
Shell evolution approaching the N=20 island of inversion : Structure of 26Na
The levels in 26Na with single particle character have been observed for the first time using the d(25Na, pγ) reaction at 5 MeV/nucleon. The measured excitation energies and the deduced spectroscopic factors are in good overall agreement with (0+1)hω shell model calculations performed in a complete spsdfp basis and incorporating a reduction in the N=20 gap. Notably, the 1p3/2 neutron configuration was found to play an enhanced role in the structure of the low-lying negative parity states in 26Na, compared to the isotone 28Al. Thus, the lowering of the 1p3/2 orbital relative to the 0f7/2 occurring in the neighbouring Z=10 and 12 nuclei - 25,27Ne and 27,29Mg - is seen also to occur at Z=11 and further strengthens the constraints on the modelling of the transition into the island of inversion
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
D* Production in Deep Inelastic Scattering at HERA
This paper presents measurements of D^{*\pm} production in deep inelastic
scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The
data have been taken with the ZEUS detector at HERA. The decay channel
(+ c.c.) has been used in the study. The
cross section for inclusive D^{*\pm} production with
and is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region
{ GeV and }. Differential cross
sections as functions of p_T(D^{*\pm}), and are
compared with next-to-leading order QCD calculations based on the photon-gluon
fusion production mechanism. After an extrapolation of the cross section to the
full kinematic region in p_T(D^{*\pm}) and (D^{*\pm}), the charm
contribution to the proton structure function is
determined for Bjorken between 2 10 and 5 10.Comment: 17 pages including 4 figure
Observation of hard scattering in photoproduction events with a large rapidity gap at HERA
Events with a large rapidity gap and total transverse energy greater than 5
GeV have been observed in quasi-real photoproduction at HERA with the ZEUS
detector. The distribution of these events as a function of the
centre of mass energy is consistent with diffractive scattering. For total
transverse energies above 12 GeV, the hadronic final states show predominantly
a two-jet structure with each jet having a transverse energy greater than 4
GeV. For the two-jet events, little energy flow is found outside the jets. This
observation is consistent with the hard scattering of a quasi-real photon with
a colourless object in the proton.Comment: 19 pages, latex, 4 figures appended as uuencoded fil
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