The reaction mechanism of projectile fragmentation at intermediate energies has been investigated by observing the target dependence of the production cross sections of very neutron rich nuclei. Measurement of longitudinal momentum distributions of projectile-like fragments in a broad range of fragment mass and charge was performed using a 100 MeV/nucleon ${}^{40}\mathrm{Ar}$ beam incident on Be and Ta targets. By measurement of the fragment momentum distribution, a parabolic mass dependence of the momentum peak shift was observed in the results of both targets, and light-fragment acceleration was found only in the Be-target data. The analysis of production cross sections revealed an obvious enhancement of the target dependence except for a target size effect when the neutron excess is increased. This result implies the breakdown of factorization of production cross sections for very neutron rich nuclei near the drip line.

The electric quadrupole moment Q for the ground state of ${}^{17}\mathrm{B}$ was determined by the $\beta$-NMR method applied to spin-polarized projectile fragments. From the measured quadrupole coupling constant, $|{eqQ/h(}^{17}\mathrm{B}\mathrm{in}\mathrm{}\mathrm{Mg})|=137.4\mathrm{}\pm 4.5\mathrm{kHz},$ for ${}^{17}\mathrm{B}$ in a Mg single crystal stopper the Q moment was deduced as $|{Q(}^{17}\mathrm{B})|=38.6\mathrm{}\pm 1.5\mathrm{mb}.$ As was the case in the previous ${}^{15}\mathrm{B}$ result, the obtained Q moment is very close to that for the $N=8\mathrm{}$ isotope ${}^{13}\mathrm{B},$ in sharp contrast to the results of conventional shell-model calculations using constant effective charges. Comparison of the shell-model calculations indicates substantial reduction of the $E2\mathrm{}$ effective charges for isotopes with large $\left(N-Z\right)/A$ ratios. The results were well accounted for in terms of $\left(N-Z\right)/A$ dependent $E2\mathrm{}$ polarization charges as predicted by Bohr and Mottelson and by incorporating the decoupling feature of loosely bound particles.

The magnetic moments of ${}_{}{}^{17}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^{17}{}_{}{}^{}\mathrm{B}$ were measured by using spin-polarized radioactive nuclear beams which were obtained from the projectile fragmentation reaction. The observed magnetic moment of ${}_{}{}^{17}{}_{}{}^{}\mathrm{N}$, ‖μ${(}^{17}$N)‖=(0.352±0.002)${\mathrm{\mu }}_{\mathit{N}}$, where ${\mathrm{\mu }}_{\mathit{N}}$ is the nuclear magneton, falls outside the Schmidt lines. By virtue of a simplifying feature of nuclear structure inherent in a ${\mathit{p}}_{1/2}$ valence nucleus, the deviation from the Schmidt value is attributed on firm ground to admixing of the configurations in which two neutrons in the sd shell are coupled to ${\mathit{J}}^{\mathrm{\pi }}$=$2^{+}$. This interpretation is confirmed in standard shell-model calculations. The calculations reproduce fairly well the experimentally inferred amount of $2^{+}$ admixture, as well as the experimental magnetic moment itself. The magnetic moment for ${}_{}{}^{17}{}_{}{}^{}\mathrm{B}$ was determined as ‖μ${(}^{17}$B)‖=(2.545±0.020)${\mathrm{\mu }}_{\mathit{N}}$. The result is substantially smaller than the π${\mathit{p}}_{1/2}$ single-particle value, and the shell-model calculations indicate that the quenching of μ largely stems from ${\mathit{J}}^{\mathrm{\pi }}$=$2^{+}$ configurations of the sd neutrons. The observed amount of quenching, however, is larger than the shell-model predictions, suggesting an enhanced contribution of the $2^{+}$ neutron configurations. This result is explained if the pairing energy for neutrons in the sd shell of a neutron-rich nucleus is assumed to diminish by about 30%. We also find that the use of the reduced pairing energy improves agreements in the magnetic moment and low-lying energy levels of ${}_{}{}^{17}{}_{}{}^{}\mathrm{N}$ as well. © 1996 The American Physical Society.

We investigated the $\beta$ decay of the neutron drip line nucleus ${}^{14}\mathrm{Be}$ by means of double and triple coincidence measurements of $\beta$ rays, delayed neutrons, and $\gamma$ rays. The decay sequences were established and new values of the half-life and branching ratios were obtained. The decay scheme obtained is characterized by a single dominant branch leading to a $1^{+}$ state in ${}^{14}\mathrm{B}$ with a branching ratio of $(91\mathrm{}\pm 9)\%,$ which subsequently emits a delayed neutron with an energy of $288\pm 8\hspace{0.5em}\mathrm{keV}.$ The excitation energy and width of the $1^{+}$ state were determined to be $1.28\pm 0.02\hspace{0.5em}\mathrm{MeV}$ and $49\pm 2\hspace{0.5em}\mathrm{keV},$ respectively. The excitation energy and width of this level as well as the $\log \mathrm{ft}$ value were treated in terms of a simple shell model and found to be strongly related to the $N=8\mathrm{}$ shell gap quenching ${\mathrm{in}}^{13,14}\mathrm{B}$ and ${}^{12}\mathrm{Be}.$