145 research outputs found
Delayed neutrons measurement at the MEGAPIE target
In the framework of the Neutronic and Nuclear Assessment Task Group of the
MEGAPIE experiment we measured the delayed neutron (DN) flux at the top of the
target. The measurement was proposed mainly for radioprotection purposes since
the DN flux at the top of the target has been estimated to be of the same order
of magnitude as the prompt neutron flux. Given the strong model-dependence of
DN predictions, the measurement of DN contribution to the total neutron
activity at the top of the target was thus desired. Moreover, this measurement
is complementary to the DN experiments performed at PNPI (Gatchina) on solid
lead and bismuth targets. The DN measurement at MEGAPIE was performed during
the start-up phase of the target. In this paper we present a detailed
description of the experimental setup and some preliminary results on decay
spectra.Comment: 4 pages, 4 figures, Proceedings of International Conference on
Nuclear Data for Science and Technology (ND 2007), Nice, France, 22-27 Apr
200
Metal-Enriched Plasma in Protogalactic Halos: A Survey of N V Absorption in High-z Damped & Sub-Damped Lyman-alpha Systems
We continue our recent work to characterize the plasma content of
high-redshift damped and sub-damped Lyman-alpha systems (DLAs/sub-DLAs), which
represent multi-phase gaseous (proto)galactic disks and halos seen toward a
background source. We survey N V absorption in a sample of 91 DLAs and 18
sub-DLAs in the redshift range 1.67<z<4.28 with unblended coverage of the N V
doublet, using data from VLT/UVES, Keck/HIRES, and Keck/ESI. In DLAs, we find
eight secure N V detections, four marginal detections, and 79 non-detections.
The detection rate of N V in DLAs is therefore 13^{+5}_{-4}%. Two sub-DLA N V
detections are found among a sample of 18, at a similar detection rate of
11^{+15}_{-7}%. We show that the N V detection rate is a strong function of
metallicity, increasing by a factor of ~4 at [N/H]=[NI/HI]>-2.3. The N V and
CIV component b-value distributions in DLAs are statistically similar, but the
median b(N V) of 18 km/s is lower than the median b(O VI) of 25 km/s. Some ~20%
of the N V components have b<10 km/s and thus arise in warm photoionized plasma
at log (T/K)<4.92; local sources of ionizing radiation (as opposed to the
extragalactic background) are required to keep the cloud sizes physically
reasonable. The nature of the remaining ~80% of (broad) N V components is
unclear; models of radiatively-cooling collisionally-ionized plasma at
log(T/K)=5.2-5.4 are fairly successful in reproducing the observed integrated
high-ion column density ratios and the component line widths, but we cannot
rule out photoionization by local sources. Finally, we identify several unusual
DLAs with extremely low metallicity (<0.01 solar) but strong high-ion
absorption [log N(N V)>14 or log N(O VI)>14.2] that present challenges to
either galactic inflow or outflow models.Comment: 26 pages, accepted by Astronomy and Astrophysics, updated with proof
change
Wideband tunable microwave signal generation in a silicon-based optoelectronic oscillator
Si photonics has an immense potential for the development of compact and
low-loss opto-electronic oscillators (OEO), with applications in radar and
wireless communications. However, current Si OEO have shown a limited
performance. Si OEO relying on direct conversion of intensity modulated signals
into the microwave domain yield a limited tunability. Wider tunability has been
shown by indirect phase-modulation to intensity-modulation conversion,
requiring precise control of the phase-modulation. Here, we propose a new
approach enabling Si OEOs with wide tunability and direct intensity-modulation
to microwave conversion. The microwave signal is created by the beating between
an optical source and single sideband modulation signal, selected by an
add-drop ring resonator working as an optical bandpass filter. The tunability
is achieved by changing the wavelength spacing between the optical source and
resonance peak of the resonator. Based on this concept, we experimentally
demonstrate microwave signal generation between 6 GHz and 18 GHz, the widest
range for a Si-based OEO. Moreover, preliminary results indicate that the
proposed Si OEO provides precise refractive index monitoring, with a
sensitivity of 94350 GHz RIU and a potential limit of detection of only 10-8
RIU, opening a new route for the implementation of high-performance Si photonic
sensors
Metallicities and dust content of proximate damped Lyman alpha systems in the Sloan Digital Sky Survey
Composite spectra of 85 proximate absorbers (log N(HI)>20 and velocity
difference between the absorption and emission redshift, dv<10,000 km/s) in the
SDSS are used to investigate the trends of metal line strengths with velocity
separation from the QSO. We construct composites in 3 velocity bins: dv<3000
km/s, 30006000 km/s, with further sub-samples to
investigate the metal line dependence on N(HI) and QSO luminosity. Low (e.g.
SiII and FeII) and high ionization (e.g. SiIV and CIV) species alike have
equivalent widths (EWs) that are larger by factors of 1.5 -- 3 in the dv<3000
km/s composite, compared to the dv>6000 km/s spectrum. The EWs show an even
stronger dependence on dv if only the highest neutral hydrogen column density
(log N(HI)>20.7) absorbers are considered. We conclude that PDLAs generally
have higher metallicities than intervening absorbers, with the enhancement
being a function of both dv and N(HI). It is also found that absorbers near
QSOs with lower rest-frame UV luminosities have significantly stronger metal
lines. We speculate that absorbers near to high luminosity QSOs may have had
their star formation prematurely quenched. Finally, we search for the signature
of dust reddening by the PDLAs, based on an analysis of the QSO continuum
slopes relative to a control sample and determine a limit of E(B-V)<0.014 for
an SMC extinction curve. This work provides an empirical motivation for
distinguishing between proximate and intervening DLAs, and establishes a
connection between the QSO environment and galaxy properties at high redshifts.Comment: Accepted for publication in MNRA
Casting light on the 'anomalous' statistics of Mg II absorbers toward Gamma-Ray Burst afterglows: the incidence of weak systems
We revisit echelle spectra (spectral resolution R ~ 40000) of 8 Gamma-Ray
Burst afterglows to obtain the incidence (dN/dz) of weak intervening Mg II
systems at a mean redshift of = 1.5. We show that dN/dz of systems having
restframe equivalent widths 0.07 A < W_r(MgII) < 1 A toward GRBs is
statistically consistent with the incidence toward QSOs. Our result is in
contrast to the results for Mg II systems having W_r > 1 A, where dN/dz toward
GRBs has been found to be larger than toward QSOs by a factor of ~ 4. We
confirm the overdensity albeit at a factor of ~ 3 only. This suggests that any
explanation for the GRB/QSO discrepancy, be it intrinsic to the absorbers or a
selection effect, should be inherent only to the galaxies that host strong
absorbers in the line-of-sight to GRBs. We argue that, of all scenarios that
have been proposed, lensing amplification is the one that could explain the
strong Mg II enhancement while allowing for no significant enhancement in the
weak absorbers.Comment: Accepted version for publication in the ApJ, results unchanged. New
figure added (equivalent width distribution). Discussion extended. 19 pages,
3 tables, 4 figure
Higher-order multipole amplitudes in charmonium radiative transitions
Using 24 million decays in CLEO-c, we have searched
for higher multipole admixtures in electric-dipole-dominated radiative
transitions in charmonia. We find good agreement between our data and
theoretical predictions for magnetic quadrupole (M2) amplitudes in the
transitions and ,
in striking contrast to some previous measurements. Let and
denote the normalized M2 amplitudes in the respective aforementioned decays,
where the superscript refers to the angular momentum of the . By
performing unbinned maximum likelihood fits to full five-parameter angular
distributions, we determine the ratios and , where
the theoretical predictions are independent of the charmed quark magnetic
moment and are and .Comment: 32 pages, 7 figures, acceptance updat
Dalitz Plot Analysis of Ds to K+K-pi+
We perform a Dalitz plot analysis of the decay Ds to K+K-pi+ with the CLEO-c
data set of 586/pb of e+e- collisions accumulated at sqrt(s) = 4.17 GeV. This
corresponds to about 0.57 million D_s+D_s(*)- pairs from which we select 14400
candidates with a background of roughly 15%. In contrast to previous
measurements we find good agreement with our data only by including an
additional f_0(1370)pi+ contribution. We measure the magnitude, phase, and fit
fraction of K*(892) K+, phi(1020)pi+, K0*(1430)K+, f_0(980)pi+, f_0(1710)pi+,
and f_0(1370)pi+ contributions and limit the possible contributions of other KK
and Kpi resonances that could appear in this decay.Comment: 21 Pages,available through http://www.lns.cornell.edu/public/CLNS/,
submitted to PR
Charmonium decays to gamma pi0, gamma eta, and gamma eta'
Using data acquired with the CLEO-c detector at the CESR e+e- collider, we
measure branching fractions for J/psi, psi(2S), and psi(3770) decays to gamma
pi0, gamma eta, and gamma eta'. Defining R_n = B[ psi(nS)-->gamma eta ]/B[
psi(nS)-->gamma eta' ], we obtain R_1 = (21.1 +- 0.9)% and, unexpectedly, an
order of magnitude smaller limit, R_2 < 1.8% at 90% C.L. We also use
J/psi-->gamma eta' events to determine branching fractions of improved
precision for the five most copious eta' decay modes.Comment: 14 pages, available through http://www.lns.cornell.edu/public/CLNS/,
published in Physical Review
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