246 research outputs found
Grossflaechiger Szintillationszaehler mit ortsunabhaengiger Lichtausbeute und seine Benuetzung als Schauerzaehler
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Experimental measurements of the 15O(alpha,gamma)19Ne reaction rate and the stability of thermonuclear burning on accreting neutron stars
Neutron stars in close binary star systems often accrete matter from their companion stars. Thermonuclear ignition of the accreted material in the atmosphere of the neutron star leads to a thermonuclear explosion which is observed as an X-ray burst occurring periodically between hours and days depending on the accretion rate. The ignition conditions are characterized by a sensitive interplay between the accretion rate of the fuel supply and its depletion rate by nuclear burning in the hot CNO cycle and the rp-process. For accretion rates close to stable burning the burst ignition therefore depends critically on the hot CNO breakout reaction {sup 15}O({alpha}, {gamma}){sup 19}Ne that regulates the flow between the hot CNO cycle and the rapid proton capture process. Until recently, the {sup 15}O({alpha}, {gamma}){sup 19}Ne reaction rate was not known experimentally and the theoretical estimates carried significant uncertainties. In this paper we perform a parameter study of the uncertainty of this reaction rate and determine the astrophysical consequences of the first measurement of this reaction rate. Our results corroborate earlier predictions and show that theoretically burning remains unstable up to accretion rates near the Eddington limit, in contrast to astronomical observations
Respiration-averaged CT versus standard CT attenuation maps for correction of the 18F-NaF uptake in hybrid PET/CT
BACKGROUND: To evaluate the impact of respiratory-averaged computed tomography attenuation correction (RACTAC) compared to standard single-phase computed tomography attenuation correction (CTAC) map, on the quantitative measures of coronary atherosclerotic lesions of (18)F-sodium fluoride ((18)F-NaF) uptake in hybrid positron emission tomography and computed tomography (PET/CT). METHODS: This study comprised 23 patients who underwent (18)F-NaF coronary PET in a hybrid PET/CT system. All patients had a standard single-phase CTAC obtained during free-breathing and a 4D cine-CT scan. From the cine-CT acquisition, RACTAC maps were obtained by averaging all images acquired over 5 seconds. PET reconstructions using either CTAC or RACTAC were compared. The quantitative impact of employing RACTAC was assessed using maximum target-to-background (TBR(MAX)) and coronary microcalcification activity (CMA). Statistical differences were analyzed using reproducibility coefficients and Bland-Altman plots. RESULTS: In 23 patients, we evaluated 34 coronary lesions using CTAC and RACTAC reconstructions. There was good agreement between CTAC and RACTAC for TBR(MAX) (median [Interquartile range]): CTAC= 1.65[1.23–2.38], RACTAC= 1.63[1.23–2.33], p=0.55), with coefficient of reproducibility of 0.18, and CMA: CTAC= 0.10 [0–1.0], RACTAC= 0.15[0–1.03], p=0.55 with coefficient of reproducibility of 0.17 CONCLUSION: Respiratory-averaged and standard single-phase attenuation correction maps provide similar and reproducible methods of quantifying coronary (18)F-NaF uptake on PET/CT
Proton Unbound States in T=3/2 and T=2 Nuclei and the Reaction Rates for Sequential Two-proton Capture Reactions in the rp-Process
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
Old Voters on New Dimensions:Why Do Voters Vote for Pensioners' Parties? The Case of the Netherlands
This article analyzes the electoral support of the Dutch pensioners’ party 50Plus. Due to its open electoral system and aging population, the Netherlands is a key case to study pensioners’ parties. Our study shows that this pensioners’ party appeals to voters who are characterized by their age and their dependence on the welfare state as well as their policy positions on new lines of political conflict. In particular, their position on the new economic dimension (which concerns welfare state reform) and the new cultural dimension (which concerns immigration and EU integration) is distinct. Moreover, even when the majority of voters for this new party once supported the larger mainstream parties, they are now dissatisfied with the established politics. With rapidly aging populations across established democracies, this study is not just relevant for those studying pensioners’ parties, but rather gives an important insight into the electoral dynamics and popular support for mainstream politics, the welfare state, and social security
The Cross Section of 3He(3He,2p)4He measured at Solar Energies
We report on the results of the \hethet\ experiment at the underground
accelerator facility LUNA (Gran Sasso). For the first time the lowest
projectile energies utilized for the cross section measurement correspond to
energies below the center of the solar Gamow peak (=22 keV). The
data provide no evidence for the existence of a hypothetical resonance in the
energy range investigated. Although no extrapolation is needed anymore (except
for energies at the low-energy tail of the Gamow peak), the data must be
corrected for the effects of electron screening, clearly observed the first
time for the \hethet\ reaction. The effects are however larger than expected
and not understood, leading presently to the largest uncertainty on the quoted
value for bare nuclides (=5.40 MeV b).Comment: 18 pages, 10 postscript figures, Calculations concerning hypothetical
resonanz added, Submitted to Phys. Rev. C., available at this URL:
HTTP://www.lngs.infn.it/lngs/htexts/luna/luna.htm
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