10 research outputs found
Closed-system behaviour of the intra-crystalline fraction of amino acids in mollusc shells
When mollusc shells are analysed conventionally for amino acid geochronology, the entire population of amino acids is included, both inter- and intra-crystalline. This study investigates the utility of removing the amino acids that are most susceptible to environmental effects by isolating the fraction of amino acids encapsulated within mineral crystals of mollusc shells (intra-crystalline fraction). Bleaching, heating and leaching (diffusive loss) experiments were undertaken on modern and fossil Corbicula fluminalis, Margaritifera falcata, Bithynia tentaculata and Valvata piscinalis shells. Exposure of powdered mollusc shells to concentrated NaOCl for 48 h effectively reduced the amino acid content of the four taxa to a residual level, assumed to represent the intra-crystalline fraction. When heated in water at 140 °C for 24 h, only 1% of amino acids were leached from the intra-crystalline fraction of modern shells compared with 40% from whole shell. Free amino acids were more effectively retained in the intra-crystalline fraction, comprising 55% (compared with 18%) of the whole shell after 24 h at 140 °C. For fossil gastropods, the inter-shell variability in D/L values for the intra-crystalline fraction of a single-age population was reduced by 50% compared with conventionally analysed shells. In contrast, analysis of the intra-crystalline fraction of C. fluminalis does not appear to improve the results for this taxon, possibly due to variability in shell ultrastructure. Nonetheless, the intra-crystalline fraction in gastropods approximates a closed system of amino acids and appears to provide a superior subset of amino acids for geochronological applications
‘Hertwig Effect’ in plants: induced parthenogenesis through the use of irradiated pollen
Measurement of Prompt Photons with Associated Jets in Photoproduction at HERA.
The photoproduction of prompt photons, together with an accompanying jet, has
been studied in ep collisions at a centre-of-mass energy of 318 GeV with the
ZEUS detector at HERA using an integrated luminosity of 77 pb-1. Cross sections
were measured for the transverse energy of the photon and the jet larger than 5
and 6 GeV, respectively. The differential gamma+jet cross sections were
reconstructed as functions of the transverse energy, pseudorapidity and
x_gamma^obs, the fraction of the incoming photon momentum taken by the
photon-jet system. Predictions based on leading-logarithm parton-shower Monte
Carlo models and next-to-leading-order (NLO) QCD generally underestimate the
cross sections for the transverse energies of prompt photons below 7 GeV, while
the kT-factorisation QCD calculation agrees with the data better. When the
minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD
and the kT-factorisation calculations are in good agreement with the data.Comment: 22 pages, 7 eps figues, 2 table
Study of the effective transverse momentum of partons in the proton using prompt photons in photoproduction at HERA
The photoproduction of prompt photons, together with an accompanying jet, has
been measured with the ZEUS detector at HERA using an integrated luminosity of
38.6 pb^{-1}. A study of the effective transverse momentum, k_T, of partons in
the proton, as modelled within the framework of the PYTHIA Monte Carlo, gives a
value of k_T = 1.69+/-0.18 ^{+0.18}_{-0.20} GeV for the photon-proton
centre-of-mass energy range 134 < W < 251 GeV. This result is in agreement with
the previously observed trend in hadron-hadron scattering for k_T to rise with
interaction energy.Comment: 20 pages including 4 figures; accepted by Physics Letters