1,316 research outputs found
Time and Amplitude of Afterpulse Measured with a Large Size Photomultiplier Tube
We have studied the afterpulse of a hemispherical photomultiplier tube for an
upcoming reactor neutrino experiment. The timing, the amplitude, and the rate
of the afterpulse for a 10 inch photomultiplier tube were measured with a 400
MHz FADC up to 16 \ms time window after the initial signal generated by an LED
light pulse. The time and amplitude correlation of the afterpulse shows several
distinctive groups. We describe the dependencies of the afterpulse on the
applied high voltage and the amplitude of the main light pulse. The present
data could shed light upon the general mechanism of the afterpulse.Comment: 11 figure
Composition and structure of the RuO2(110) surface in an O2 and CO environment: implications for the catalytic formation of CO2
The phase diagram of surface structures for the model catalyst RuO2(110) in
contact with a gas environment of O2 and CO is calculated by density-functional
theory and atomistic thermodynamics. Adsorption of the reactants is found to
depend crucially on temperature and partial pressures in the gas phase.
Assuming that a catalyst surface under steady-state operation conditions is
close to a constrained thermodynamic equilibrium, we are able to rationalize a
number of experimental findings on the CO oxidation over RuO2(110). We also
calculated reaction pathways and energy barriers. Based on the various results
the importance of phase coexistence conditions is emphasized as these will lead
to an enhanced dynamics at the catalyst surface. Such conditions may actuate an
additional, kinetically controlled reaction mechanism on RuO2(110).Comment: 12 pages including 8 figure files. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Neutron beam test of CsI crystal for dark matter search
We have studied the response of Tl-doped and Na-doped CsI crystals to nuclear
recoils and 's below 10 keV. The response of CsI crystals to nuclear
recoil was studied with mono-energetic neutrons produced by the
H(p,n)He reaction. This was compared to the response to Compton
electrons scattered by 662 keV -ray. Pulse shape discrimination between
the response to these 's and nuclear recoils was studied, and quality
factors were estimated. The quenching factors for nuclear recoils were derived
for both CsI(Na) and CsI(Tl) crystals.Comment: 21pages, 14figures, submitted to NIM
Channeling in direct dark matter detection I: channeling fraction in NaI (Tl) crystals
The channeling of the ion recoiling after a collision with a WIMP changes the
ionization signal in direct detection experiments, producing a larger signal
than otherwise expected. We give estimates of the fraction of channeled
recoiling ions in NaI (Tl) crystals using analytic models produced since the
1960's and 70's to describe channeling and blocking effects. We find that the
channeling fraction of recoiling lattice nuclei is smaller than that of ions
that are injected into the crystal and that it is strongly temperature
dependent.Comment: 37 pages, 35 figures, Accepted for publication in JCAP on 27 October
2010, Minor revisions: added an appendix, updated references, updated Fig. 9,
corrected a few typo
Enhanced production of tropane alkaloids in transgenic Scopolia parviflora hairy root cultures over-expressing putrescine N-methyl transferase (PMT) and hyoscyamine-6ÎČ-hydroxylase (H6H)
Scopolia parviflora adventitious roots were metabolically engineered by co-expression of the two gene putrescine N-methyl transferase (PMT) and hyoscyamine-6ÎČ-hydroxylase (H6H) cDNAs with the aid of Agrobacterium rhizogenes. The transformed roots developed into morphologically distinct S. parviflora PMT1 (SpPMT1), S. parviflora PMT1 (SpPMT2), and S. parviflora H6H (SpH6H) transgenic hairy root lines. Consequent to the introduction of these key enzyme genes, the production of the alkaloids hyoscyamine and scopolamine was enhanced. Among the transgenic hairy root lines, SpPMT2 line possessed the highest growth index. The treatment of transgenic hairy roots with growth regulators further enhanced the production of scopolamine. Thus, the results suggest that PMT1, PMT2, and H6H genes may not only be involved in the metabolic regulation of alkaloid production but also that these genes may play a role in the root development
Exceptionally Slow Rise in Differential Reflectivity Spectra of Excitons in GaN: Effect of Excitation-induced Dephasing
Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) and
four-wave mixing (FWM) experiments were performed simultaneously to study the
initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats
between the A-B excitons were found \textit{only for positive time delay} in
both PP and FWM experiments. The rise time at negative time delay for the
differential reflection spectra was much slower than the FWM signal or PP
differential transmission spectroscopy (DTS) at the exciton resonance. A
numerical solution of a six band semiconductor Bloch equation model including
nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS
results from excitation induced dephasing (EID), that is, the strong density
dependence of the dephasing time which changes with the laser excitation
energy.Comment: 8 figure
Composition, structure and stability of RuO_2(110) as a function of oxygen pressure
Using density-functional theory (DFT) we calculate the Gibbs free energy to
determine the lowest-energy structure of a RuO_2(110) surface in thermodynamic
equilibrium with an oxygen-rich environment. The traditionally assumed
stoichiometric termination is only found to be favorable at low oxygen chemical
potentials, i.e. low pressures and/or high temperatures. At realistic O
pressure, the surface is predicted to contain additional terminal O atoms.
Although this O excess defines a so-called polar surface, we show that the
prevalent ionic model, that dismisses such terminations on electrostatic
grounds, is of little validity for RuO_2(110). Together with analogous results
obtained previously at the (0001) surface of corundum-structured oxides, these
findings on (110) rutile indicate that the stability of non-stoichiometric
terminations is a more general phenomenon on transition metal oxide surfaces.Comment: 12 pages including 5 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Resolution Tests of CsI(Tl) Scintillators Read Out by Pin Diodes
This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440
Search for solar axions in XMASS, a large liquid-xenon detector
XMASS, a low-background, large liquid-xenon detector, was used to search for
solar axions that would be produced by bremsstrahlung and Compton effects in
the Sun. With an exposure of 5.6ton days of liquid xenon, the model-independent
limit on the coupling for mass 1keV is
(90% C.L.), which is a factor of two stronger than the existing experimental
limit. The bounds on the axion masses for the DFSZ and KSVZ axion models are
1.9 and 250eV, respectively. In the mass range of 10-40keV, this study produced
the most stringent limit, which is better than that previously derived from
astrophysical arguments regarding the Sun to date
Carrier-mediated ferromagnetic ordering in Mn ion-implanted p+GaAs:C
Highly p-type GaAs:C was ion-implanted with Mn at differing doses to produce
Mn concentrations in the 1 - 5 at.% range. In comparison to LT-GaAs and
n+GaAs:Si samples implanted under the same conditions, transport and magnetic
properties show marked differences. Transport measurements show anomalies,
consistent with observed magnetic properties and with epi- LT-(Ga,Mn)As, as
well as the extraordinary Hall Effect up to the observed magnetic ordering
temperature (T_C). Mn ion-implanted p+GaAs:C with as-grown carrier
concentrations > 10^20 cm^-3 show remanent magnetization up to 280 K
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