Investigation of the nuclear matter density distributions of the exotic 12 Be, 14 Be and 8 B nuclei by elastic proton scattering in inverse kinematics
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Abstract
The proton-nucleus elastic scattering at intermediate energies is a
well-established method for the investigation of the nuclear matter
distribution in stable nuclei and was recently applied also for the
investigation of radioactive nuclei using the method of inverse
kinematics. In the current experiment, the differential cross sections
for proton elastic scattering on the isotopes $^{7,9,10,11,12,14}$Be
and $^8$B were measured. The experiment was performed using the
fragment separator at GSI, Darmstadt to produce the radioactive
beams. The main part of the
experimental setup was the time projection
ionization chamber IKAR which was simultaneously used as hydrogen
target and a detector for the recoil protons. Auxiliary detectors for
projectile tracking and isotope identification were also installed.
As results from the experiment, the absolute differential cross
sections d$sigma$/d$t$ as a function of the four momentum transfer
$t$ were obtained. In this work the differential cross sections for
elastic p-$^{12}$Be, p-$^{14}$Be and p-$^{8}$B scattering at low $t$
($t leq$~0.05~(GeV/c)$^2$) are presented. The measured cross sections
were analyzed within the Glauber multiple-scattering theory using
different density parameterizations, and the nuclear matter density
distributions and radii of the investigated isotopes were determined.
The analysis of the differential cross section for the isotope
$^{14}$Be shows that a good description of the experimental data is
obtained when density distributions consisting of
separate core and
halo components are used. The determined {it rms} matter radius is
$3.11 pm 0.04 pm 0.13$~fm. In the case of the $^{12}$Be nucleus the
results showed an extended matter distribution as well. For this
nucleus a matter radius of $2.82 pm 0.03 pm 0.12$~fm was
determined. An interesting result is that the free $^{12}$Be nucleus
behaves differently from the core of $^{14}$Be and is much more
extended than it. The data were also compared with theoretical
densities calculated within the FMD and the few-body models. In the
case of $^{14}$Be, the calculated cross sections describe the
experimental data well while, in the case of $^{12}$Be there are
discrepancies in the region of high momentum transfer.
Preliminary experimental results for the isotope
$^8$B are also presented. An extended matter distribution was
obtained (though much more compact as compared to the neutron
halos). A proton halo structure was observed for the first time with
the
proton elastic scattering method. The deduced matter radius is
$2.60pm 0.02pm 0.26$~fm. The data were compared with microscopic
calculations in the frame of the FMD model and reasonable agreement
was observed.
The results obtained in the present analysis are in most cases
consistent with the previous experimental studies of the same isotopes
with different experimental methods (total interaction and reaction
cross section measurements, momentum distribution measurements).
For future investigation of the structure of exotic nuclei a universal
detector system EXL is being developed. It will be installed at the
NESR at the future FAIR facility where higher intensity beams of
radioactive ions are expected. The usage of storage ring techniques
provides high luminosity and low background experimental
conditions. Results from the feasibility studies of the EXL detector
setup, performed at the present ESR storage ring, are presented.