Radiative Capture on Nuclear Isomers: Direct Measurement of the

We present the first direct measurement of an astrophysical reaction using a radioactive beam of isomeric nuclei. In particular, we have measured the strength of the key 447-keV resonance in the ^{26m}Al(p,γ)^{27}Si reaction to be 432_{-226}^{+146} meV and find that this resonance dominates the thermally averaged reaction rate for temperatures between 0.3 and 2.5 GK. This work represents a critical development in resolving one of the longest standing issues in nuclear astrophysics research, relating to the measurement of proton capture reactions on excited quantum levels, and offers unique insight into the destruction of isomeric ^{26}Al in astrophysical plasmas.

Direct Measurement of Resonances in

We have performed the first direct measurement of two resonances of the ^{7}Be(α,γ)^{11}C reaction with unknown strengths using an intense radioactive ^{7}Be beam and the DRAGON recoil separator. We report on the first measurement of the 1155 and 1110 keV resonance strengths of 1.73±0.25(stat)±0.40(syst) eV and 125_{-25}^{+27}(stat)±15(syst) meV, respectively. The present results have reduced the uncertainty in the ^{7}Be(α,γ)^{11}C reaction rate to ∼9.4%-10.7% over T=1.5-3 GK, which is relevant for nucleosynthesis in the neutrino-driven outflows of core-collapse supernovae (νp process). We find no effect of the new, constrained reaction rate on νp-process nucleosynthesis.

First Direct Measurement of an Astrophysical p-Process Reaction Cross Section Using a Radioactive Ion Beam.

We have performed the first direct measurement of the ^{83}Rb(p,γ) radiative capture reaction cross section in inverse kinematics using a radioactive beam of ^{83}Rb at incident energies of 2.4 and 2.7A MeV. The measured cross section at an effective relative kinetic energy of E_{cm}=2.393 MeV, which lies within the relevant energy window for core collapse supernovae, is smaller than the prediction of statistical model calculations. This leads to the abundance of ^{84}Sr produced in the astrophysical p process being higher than previously calculated. Moreover, the discrepancy of the present data with theoretical predictions indicates that further experimental investigation of p-process reactions involving unstable projectiles is clearly warranted.

Direct Measurement of the Key E_{c.m.}=456 keV Resonance in the Astrophysical

We have performed a direct measurement of the ^{19}Ne(p,γ)^{20}Na reaction in inverse kinematics using a beam of radioactive ^{19}Ne. The key astrophysical resonance in the ^{19}Ne+p system has been definitely measured for the first time at E_{c.m.}=456_{-2}^{+5} keV with an associated strength of 17_{-5}^{+7} meV. The present results are in agreement with resonance strength upper limits set by previous direct measurements, as well as resonance energies inferred from precision (^{3}He, t) charge exchange reactions. However, both the energy and strength of the 456 keV resonance disagree with a recent indirect study of the ^{19}Ne(d, n)^{20}Na reaction. In particular, the new ^{19}Ne(p,γ)^{20}Na reaction rate is found to be factors of ∼8 and ∼5 lower than the most recent evaluation over the temperature range of oxygen-neon novae and astrophysical x-ray bursts, respectively. Nevertheless, we find that the ^{19}Ne(p,γ)^{20}Na reaction is likely to proceed fast enough to significantly reduce the flux of ^{19}F in nova ejecta and does not create a bottleneck in the breakout from the hot CNO cycles into the rp process.

Investigation of the

This Letter reports a study of the highly debated ^{10}Li structure through the d(^{9}Li,p)^{10}Li one-neutron transfer reaction at 100 MeV. The ^{10}Li energy spectrum is measured up to 4.6 MeV and angular distributions corresponding to different excitation energy regions are reported for the first time. The comparison between data and theoretical predictions, including pairing correlation effects, shows the existence of a p_{1/2} resonance at 0.45±0.03 MeV excitation energy, while no evidence for a significant s-wave contribution close to the threshold energy is observed. Moreover, two high-lying structures are populated at 1.5 and 2.9 MeV. The corresponding angular distributions suggest a significant s_{1/2} partial-wave contribution for the 1.5 MeV structure and a mixing of configurations at higher energy, with the d_{5/2} partial-wave contributing the most to the cross section.

Nuclear Force Imprints Revealed on the Elastic Scattering of Protons with

How does nature hold together protons and neutrons to form the wide variety of complex nuclei in the Universe? Describing many-nucleon systems from the fundamental theory of quantum chromodynamics has been the greatest challenge in answering this question. The chiral effective field theory description of the nuclear force now makes this possible but requires certain parameters that are not uniquely determined. Defining the nuclear force needs identification of observables sensitive to the different parametrizations. From a measurement of proton elastic scattering on ^{10}C at TRIUMF and ab initio nuclear reaction calculations, we show that the shape and magnitude of the measured differential cross section is strongly sensitive to the nuclear force prescription.

Direct Measurement of the Astrophysical

We have performed the first direct measurement of the ^{38}K(p,γ)^{39}Ca reaction using a beam of radioactive ^{38}K. A proposed ℓ=0 resonance in the ^{38}K+p system has been identified at 679(2) keV with an associated strength of 120_{-30}^{+50} meV. Upper limits of 1.16 (3.5) and 8.6 (26) meV at the 68% (95%) confidence level were also established for two further expected ℓ=0 resonances at 386 and 515 keV, respectively. The present results have reduced uncertainties in the ^{38}K(p,γ)^{39}Ca reaction rate at temperatures of 0.4 GK by more than 2 orders of magnitude and indicate that Ar and Ca may be ejected in observable quantities by oxygen-neon novae. However, based on the newly evaluated rate, the ^{38}K(p,γ)^{39}Ca path is unlikely to be responsible for the production of Ar and Ca in significantly enhanced quantities relative to solar abundances.

Measurement of 23Na(α,p)26Mg at Energies Relevant to 26Al Production in Massive Stars.

26Al is an important radioisotope in astrophysics that provides evidence of ongoing nucleosynthesis in the Galaxy. The 23Na(α, p)26Mg reaction has been identified by a sensitivity study as being one of the most important reactions for the production of 26Al in the convective C/Ne burning shell of massive stars. Owing to large uncertainties in previous experimental data, model calculations are used for the reaction rate of 23Na(α, p)26Mg in this sensitivity study. Current experimental data suggest a reaction rate a factor of ∼40 higher than model calculations. However, a new measurement of this reaction cross section has been made in inverse kinematics in the energy range E(c.m.)=1.28-3.15 MeV at TRIUMF, and found to be in reasonable agreement with the model calculation. A new reaction rate is calculated and tight constraints on the uncertainty in the production of 26Al, due to this reaction, are determined.

Inverse Kinematic Study of the (26g)Al(d,p)(27)Al Reaction and Implications for Destruction of (26)Al in Wolf-Rayet and Asymptotic Giant Branch Stars.

In Wolf-Rayet and asymptotic giant branch (AGB) stars, the (26g)Al(p,γ)(27)Si reaction is expected to govern the destruction of the cosmic γ-ray emitting nucleus (26)Al. The rate of this reaction, however, is highly uncertain due to the unknown properties of key resonances in the temperature regime of hydrogen burning. We present a high-resolution inverse kinematic study of the (26g)Al(d,p)(27)Al reaction as a method for constraining the strengths of key astrophysical resonances in the (26g)Al(p,γ)(27)Si reaction. In particular, the results indicate that the resonance at E(r)=127 keV in (27)Si determines the entire (26g)Al(p,γ)(27)Si reaction rate over almost the complete temperature range of Wolf-Rayet stars and AGB stars.

Evidence of soft dipole resonance in

The first conclusive evidence of a dipole resonance in ^{11}Li having isoscalar character observed from inelastic scattering with a novel solid deuteron target is reported. The experiment was performed at the newly commissioned IRIS facility at TRIUMF. The results show a resonance peak at an excitation energy of 1.03±0.03 MeV with a width of 0.51±0.11 MeV (FWHM). The angular distribution is consistent with a dipole excitation in the distorted-wave Born approximation framework. The observed resonance energy together with shell model calculations show the first signature that the monopole tensor interaction is important in ^{11}Li. The first ab initio calculations in the coupled cluster framework are also presented.

Measurement of radiative proton capture on 18F and implications for oxygen-neon novae.

The rate of the 18F(p,γ)19Ne reaction affects the final abundance of the γ-ray observable radioisotope 18F, produced in novae. However, no successful measurement of this reaction exists and the rate used is calculated from incomplete information on the contributing resonances. Of the two resonances thought to play a significant role, one has a radiative width estimated from the assumed analogue state in the mirror nucleus, 19F. The second does not have an analogue state assignment at all, resulting in an arbitrary radiative width being assumed. Here, we report the first successful direct measurement of the 18F(p,γ)^19Ne reaction. The strength of the 665 keV resonance (Ex=7.076 MeV) is found to be over an order of magnitude weaker than currently assumed in nova models. Reaction rate calculations show that this resonance therefore plays no significant role in the destruction of ^{18}F at any astrophysical energy.

Measurement of the 18Ne(α,p0) 21Na reaction cross section in the burning energy region for x-ray bursts.

The 18Ne(α,p) 21Na reaction provides one of the main HCNO-breakout routes into the rp process in x-ray bursts. The 18Ne(α,p0) 21Na reaction cross section has been determined for the first time in the Gamow energy region for peak temperatures T∼2 GK by measuring its time-reversal reaction 21Na(p,α) 18Ne in inverse kinematics. The astrophysical rate for ground-state to ground-state transitions was found to be a factor of 2 lower than Hauser-Feshbach theoretical predictions. Our reduced rate will affect the physical conditions under which breakout from the HCNO cycles occurs via the 18Ne(α,p) 21Na reaction.

Cloning of a beta integrin subunit cDNA from an embryonic cell line derived from the freshwater mollusc, Biomphalaria glabrata.

A cDNA encoding an integrin subunit was cloned and structurally characterized from an embryonic cell line derived from Biomphalaria glabrata, snail intermediate host of the human blood fluke Schistosoma mansoni. Cells of the B. glabrata embryonic (Bge) snail cell line were initially tested for their sensitivity to the integrin-specific tetrapeptide inhibitor Arg-Gly-Asp-Ser (RGDS). Washed Bge cells when exposed to 0.5 to 2.0mM of RGDS were significantly inhibited in their ability to spread on a glass substrate. Spreading inhibition was specific, since a control peptide Arg-Gly-Glu-Ser (RGES) did not have the same effect. RT-PCR was performed using previously reported degenerate oligonucleotide primers to the ligand binding domain (LBD) of known beta integrin subunits and Bge cDNA. A 137 bp fragment was amplified, TA-cloned, sequenced, and the na and deduced aa sequences were compared with other beta integrins. Databank analysis showed that the 137 bp product shared >/=55.6% aa similarity to other beta integrin LBDs. Southern and northern blot analyses using the 137 bp sequence as a probe revealed binding to Bge genomic DNA restriction fragments and to an approximately 8 kb poly-(A)+RNA transcript, respectively. An exact 5' primer synthesized to the 137 bp product and an oligo-d(T) primer then were used to amplify from Bge cDNA, a partial beta integrin sequence of 2285 bp that contained a 1971 bp ORF. The remaining upstream coding region was obtained using 5' RACE methods. The complete ORF, consisting of 2364 bp, encoded a 788 aa sequence with shared similarity to other known beta integrins (44.6-61.5%). Sequence and structural comparisons, which include a characteristic LBD, a series of three homologous cysteine-rich repeats, membrane proximal sequence (LLTFIHD), cytoplasmic NPXY motifs, and predicted domain lengths of the molluscan protein, clearly identifies it as an integrin homologue. This report represents the first cloning of a cDNA putatively encoding an integrin subunit from molluscan cells, and establishes the Bge cell line as a model for studying cellular adhesion in molluscs at the molecular level.

Dependence of Giardia lamblia encystation on novel transglutaminase activity.

Earlier, we found that three protein disulfide isomerases (PDI) from Giardia lamblia (gPDI) also have transglutaminase (TGase) activity in vitro. We now show that differentiating Giardia cells contain isopeptide bonds (epsilon(gamma-glutamyl)lysine), the biological product of TGase activity that results in irreversible crosslinking of proteins in vivo. HPLC analyses showed the highest isopeptide bond content in cells encysting for 21 h, indicating an important role for TGase early in encystation. We were not able to detect isopeptide bonds in water-resistant cysts, possibly because they could not be extracted. One of the hallmarks of early encystation is the formation of encystation secretory vesicles (ESV) that transport nascent cyst wall proteins (CWPs) to the outer cell surface. ImmunoEM and live-cell immunofluorescence assays of encysting parasites revealed that gPDIs 1-3 are located in ESV and that gPDI-2 is also novel in that it is localized on the cell surface. Cystamine, a widely used TGase inhibitor, caused a dose-dependent inhibition of ESV formation by 21 h, thereby preventing development of trophozoites into cysts. Since cystamine (0.5-1 mM) inhibited the TGase activity of recombinant gPDIs 1-3 in vitro, PDIs appear to be the physiologic targets of cystamine. We found that when parasites were treated with cystamine, CWPs were not processed normally. These data suggest that TGase-catalyzed reactions may be needed for either the machinery that processes CWP precursors or their recruitment to ESV.

Integrin-like RGD-dependent binding mechanism involved in the spreading response of circulating molluscan phagocytes.

Circulating phagocytic cells (hemocytes) of the snail Biomphalaria glabrata, intermediate host of the human blood fluke Schistosoma mansoni, were treated with the tetrapeptide, arg-gly-asp-ser (RGDS), an integrin-specific adhesion inhibitor, and assessed for their ability to adhere and spread on uncoated and snail plasma protein-coated glass slides. Although cells were capable of adherence, RGDS significantly inhibited the spreading ability of hemocytes in both a time and RGDS concentration-dependent fashion regardless of plasma protein coating. The inhibition of hemocyte spreading by RGDS was a specific response, since treatment of cells with a glutamic acid-substituted control peptide (RGES) did not exert the same inhibitory effect. A comparison of RGDS-responses between hemocytes of two strains of B. glabrata, one resistant (R; 13-16-R1 strain) and the other susceptible (S; NMRI strain) to infection by S. mansoni, revealed several snail strain-specific differences. At concentrations of 0.5 mM RGDS, R snail hemocyte spreading was unaffected, whereas a significant depression of spreading was seen in cells of the S snail. Moreover, we observed that R strain hemocytes spread more rapidly on homologous plasma-coated surfaces than the S snail strain following peptide pretreatment and removal. These data suggest that hemocytes from S and R snails may differ either in the number of RGDS-binding receptors or in their affinity for the RGDS peptide. In order to identify the type(s) of integrin-like RGD-binding receptors that may be present on the surface of snail immunocytes, washed hemocytes were placed on various mammalian extracellular matrix proteins and evaluated for their spreading function in the presence of specific or non-specific peptides. Hemocyte aggregation or clumping was observed on all test protein substrates, and this aggregation behavior was specifically inhibited by RGDS. Thus, RGD-binding receptors appear to play a critical role in cellular motility on matrix-coated surfaces and/or cell-cell binding. Our data provide functional evidence for an integrin-like receptor on circulating phagocytes of snails, and for an RGD-binding mechanism involved in cell-substrate interactions.

One-neutron knockout from individual single-particle states of 11Be

The structure of the halo nucleus 11Be has been studied using the reaction 9Be(11Be,10Be+gamma)X at 60 MeV/nucleon. The ground state structure of 11Be is determined by comparing the experimental cross sections to a calculation combining spectroscopic factors from the shell model with l-dependent single-particle cross sections obtained in an eikonal model. This experiment shows the dominant 1s single-particle character of the 11Be ground state and indicates a small contribution of 0d admixture in the wave function. After correction for the approximately 22% intensity to excited levels, a clean and precise distribution of parallel momentum for knockout from the 1s halo wave function is obtained for the first time.

Magnetic resonance brain imaging in women with obsessive-compulsive disorder and trichotillomania.

Magnetic resonance imaging of the brain was undertaken in women with obsessive-compulsive disorder (n = 13), trichotillomania (n = 17), and healthy controls (n = 12). Caudate volume and ventricular-brain ratio (VBR) (variables that have previously been highlighted as abnormal in studies of OCD) were compared in the three subject groups and were correlated with neuropsychological and neurological soft sign findings. No significant differences were found between women with OCD, trichotillomania and normal controls on caudate volume or VBR. Decreased left caudate volume was significantly correlated with impairment on neuropsychological testing and with increased neurological soft signs. The negative findings here may indicate that in women with OCD and related disorders structural brain abnormalities are less obvious or less common than in men with these conditions, or they may reflect inadequate sensitivity of the brain measures employed.

An atypical proprotein convertase in Giardia lamblia differentiation.

Proteolytic activity is important in the lifecycles of parasites and their interactions with hosts. Cysteine proteases have been best studied in Giardia, but other protease classes have been implicated in growth and/or differentiation. In this study, we employed bioinformatics to reveal the complete set of putative proteases in the Giardia genome. We identified 73 peptidase homologs distributed over 5 catalytic classes in the genome. Serial analysis of gene expression of the G. lamblia lifecycle found thirteen protease genes with significant transcriptional variation over the lifecycle, with only one serine protease transcript upregulated late in encystation. The translated gene sequence of this encystation-specific transcript was most similar to eukaryotic subtilisin-like proprotein convertases (SPC), although the typical catalytic triad was not identified. Epitope-tagged gSPC protein expressed in Giardia under its own promoter was upregulated during encystation with highest expression in cysts and it localized to encystation-specific secretory vesicles (ESV). Total gSPC from encysting cells produced proteolysis in gelatin gels that co-migrated with the epitope-tagged protease in immunoblots. Immuno-purified gSPC also had gelatinase activity. To test whether endogenous gSPC activity is involved in differentiation, trophozoites and cysts were exposed to the specific serine proteinase inhibitor 4-(2-aminoethyl)-benzenesulfonyl fluoride hydrochloride (AEBSF). After 21 h encystation, a significant decrease in ESV was observed with 1mM AEBSF and by 42 h the number of cysts was significantly reduced, but trophozoite growth was not inhibited. Concurrently, levels of cyst wall proteins 1 and 2, and AU1-tagged gSPC protein itself were decreased. Excystation of G. muris cysts was also significantly reduced in the presence of AEBSF. These results support the idea that serine protease activity is essential for Giardia encystation and excystation.

One-neutron removal measurement reveals 24O as a new doubly magic nucleus.

The first measurement of the momentum distribution for one-neutron removal from (24)O at 920A MeV performed at GSI, Darmstadt is reported. The observed distribution has a width (FWHM) of 99 +/- 4 MeV/c in the projectile rest frame and a one-neutron removal cross section of 63 +/- 7 mb. The results are well explained with a nearly pure 2s_{1/2} neutron spectroscopic factor of 1.74 +/- 0.19 within the eikonal model. This large s-wave probability shows a spherical shell closure thereby confirming earlier suggestions that (24)O is a new doubly magic nucleus.