Halo Structure of the Neutron-Dripline Nucleus

The heaviest bound isotope of boron ^{19}B has been investigated using exclusive measurements of its Coulomb dissociation, into ^{17}B and two neutrons, in collisions with Pb at 220 MeV/nucleon. Enhanced electric dipole (E1) strength is observed just above the two-neutron decay threshold with an integrated E1 strength of B(E1)=1.64±0.06(stat)±0.12(sys) e^{2} fm^{2} for relative energies below 6 MeV. This feature, known as a soft E1 excitation, provides the first firm evidence that ^{19}B has a prominent two-neutron halo. Three-body calculations that reproduce the energy spectrum indicate that the valence neutrons have a significant s-wave configuration and exhibit a dineutronlike correlation.

First Observation of

The most neutron-rich boron isotopes ^{20}B and ^{21}B have been observed for the first time following proton removal from ^{22}N and ^{22}C at energies around 230 MeV/nucleon. Both nuclei were found to exist as resonances which were detected through their decay into ^{19}B and one or two neutrons. Two-proton removal from ^{22}N populated a prominent resonancelike structure in ^{20}B at around 2.5 MeV above the one-neutron decay threshold, which is interpreted as arising from the closely spaced 1^{-},2^{-} ground-state doublet predicted by the shell model. In the case of proton removal from ^{22}C, the ^{19}B plus one- and two-neutron channels were consistent with the population of a resonance in ^{21}B 2.47±0.19 MeV above the two-neutron decay threshold, which is found to exhibit direct two-neutron decay. The ground-state mass excesses determined for ^{20,21}B are found to be in agreement with mass surface extrapolations derived within the latest atomic-mass evaluations.

Nucleus

The unbound nucleus ^{26}O has been investigated using invariant-mass spectroscopy following one-proton removal reaction from a ^{27}F beam at 201 MeV/nucleon. The decay products, ^{24}O and two neutrons, were detected in coincidence using the newly commissioned SAMURAI spectrometer at the RIKEN Radioactive Isotope Beam Factory. The ^{26}O ground-state resonance was found to lie only 18±3(stat)±4(syst) keV above threshold. In addition, a higher lying level, which is most likely the first 2^{+} state, was observed for the first time at 1.28_{-0.08}^{+0.11} MeV above threshold. Comparison with theoretical predictions suggests that three-nucleon forces, pf-shell intruder configurations, and the continuum are key elements to understanding the structure of the most neutron-rich oxygen isotopes beyond the drip line.

Induced morphological changes in human small cell lung carcinoma cells.

Several theories suggest that lung carcinomas are not totally separate entities, but are derived from a common precursor, probably of endodermal origin. The histological classification of lung cancers is complex, with much overlap between groups broadly designated as small cell (SCLC), squamous cell, adenocarcinoma and all others simply termed non-small cell. It is shown here that in vitro exposure of classic, non-adherent SCLC lines to 10 microM 5' bromodeoxyuridine (BrdU) results in a rapid cell-line dependent change to a morphology consistent with an adherent, non-small cell phenotype. Accompanying this morphological shift is a decreased expression of the amplified N-myc protooncogene. These induced changes underline the morphological relatedness of lung carcinoma cell lines.

Human lymphoma differentiation concepts correlate with in vitro action of immunoglobulin gene-specific transcription factors.

Lymphoid tissue-specific expression of immunoglobulin genes is regulated (in part) by gene-specific trans-acting factors. Whereas different classes of human B cell lymphoid neoplasms produce immunoglobulins in amounts that correlate with the stages of normal B cell differentiation, the pattern of expression of putative regulatory trans-acting factors in human lymphoid neoplasia is unknown. Nuclear extracts made from human lymphoid neoplasms were screened for their ability to enhance transcription of the unrearranged Kappa light chain immunoglobulin gene (V Kappa) in a whole cell in vitro transcription assay. Extracts from plasmacytomas, large noncleaved cell lymphomas, and Burkitt's lymphomas specifically enhance V Kappa gene transcription up to 22-fold, whereas no enhancement was achieved using extracts from lymphoid neoplasms corresponding to the earlier stages of normal B cell maturation. We suggest that these findings mean that the production of immunoglobulins by human lymphoid neoplasms correlates with the expression of immunoglobulin gene specific trans-acting factors.