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[en] Full text: Supernovae are a key site for heavy element synthesis. Despite much recent progress, significant uncertainty remains concerning the underlying mechanism that allows a collapsing massive star to successfully explode. A satellite based gamma-ray observation of the isotope 44Ti may hold the key to resolving this problem, as the amount ejected is thought to depend on the explosion mechanism. However, to allow such a deduction, the key 44Ti(α,p)47V reaction rate must be better known. A direct measurement of this reaction has recently been performed at the ISOLDE facility at energies within the Gamow window of core collapse supernovae, employing a beam of 44Ti derived from highly irradiated components of the SINQ spallation neutrons source of the Paul Sherrer Institute. Impinged on a helium-filled cell, protons were detected in segmented silicon detectors. Details of the experimental preparations and set up and provisional results will be presented.
[en] A study of the production of a 44 Ti target was carried out aiming the determination of its thermal neutron capture cross-section. With this purpose, the cross-section of the reaction 45 Sc(p,2 n) 44 Ti was determined in the energies 16-, 18-, 20-22- and 45 MeV. The cross-section of the reactions (p,n) 45 Ti, (p,pn) 44m Sc, (p,pn) 44g Sc and (p,p2n)43 Sc were also measured. The results in the low energy region are in good agreement with a previous work by McGee et al. On the other hand, the cross-section at 45 MeV is different from McGee's result and indicates the existence of an abnormal behavior of the excitation function at higher energies. Furthermore, a radiochemical separation method was developed in order to eliminate Sc from the 44 Ti target which was irradiated with neutrons. It was possible to determine an upper limit for the cross-section of the reaction 44 Ti (n, γ) of 4 x 103 b. At last, it is presented a discussion of the results obtained and their possible astrophysical implications. (author)
[en] New laboratory measurements have probed what happens inside an exploding supernova. Three independent research teams have provided new and more accurate measurements of the half-life of 44Ti, a radioactive isotope of titanium produced in supernovae. Previous measurements had suggested a half-life of about 50 years, but with a large variation in values that was not well understood. Moreover, this half-life could not explain the large amount of 44Ti thought to exist in the remnant of the Casseopeia (Cas) A supernova, which exploded in our galaxy about 320 years ago. The improved measurements suggest that the half-life of 44Ti is longer, about 60 years, in better agreement with models of supernova nucleosynthesis. (UK)
[en] The measurement of the 44Ti half-life, started 3 years ago, is still continuing. The goal of this measurement is to determine the half-life of 44Ti, which is ∼52 y, to a precision of ∼5%. An accurate value of this half-life is of interest to cosmologists who need it to determine the production of heavy elements in supernova. Three sets of samples - a pure 200-nCi 44Ti sample, a pure 300-nCi 60Co source, and a mixed 44Ti-60Co source of similar strength - were prepared and their spectra are being measured with Ge spectrometers at Argonne, Torino and Jerusalem. Each sample is counted for a period of 2 days, at approximate intervals of 4 months. The room background is also measured for the same length of time. We hope to start data analysis at the end of summer and obtain a value for the 44Ti half-life
[en] Experimental limits on the branching ratio for charged-particle decay of the lowest T=2 state in 44Ti are obtained. The implications for recent γ-decay studies of this state are discussed
[en] The fabrication of 68Ge and 44Ti positron sources which can withstand temperatures up to 1,0000C is described. The sources are designed for use in the measurement of thermally-generated vacancy concentrations by the positron trapping technique. Measurements of positron lifetimes and Doppler-broadened annihilation photon lineshapes indicate that if suitable corrections are made the contribution of source matrix annihilations to experimental data can be reduced to less than 0.5% of the total annihilation events. (orig.)
[de]Es wird die Herstellung von 68Ge- und 44Ti-Positronenquellen beschrieben, die bis zu Temperaturen von 1.0000C betrieben werden koennen. Diese Quellen werden bei der Messung von durch thermische Anregung erzeugten Leerstellenkonzentrationen nach der Positronentrappingtechnik benutzt. Messungen der Positronenlebensdauer und der Doppler-verbreiteten Linien der Zerstrahlungsquanten weisen darauf hin, dass, bei passenden Korrekturen, der Beitrag der Quellenmatrixzerstrahlungen zu den experimentellen Werten auf weniger als 0,5% der totalen Zerstrahlungen reduziert werden kann. (orig./HPoe)
[en] We evaluate two dominant nuclear reaction rates and their uncertainties that affect 44Ti production in explosive nucleosynthesis. Experimentally we develop thick target yields for the 40Ca(α, γ)44Ti reaction at E α = 4.13, 4.54, and 5.36 MeV using γ-ray spectroscopy. At the highest beam energy, we also performed an activation measurement which agrees with the thick target result. From the measured yields a stellar reaction rate was developed that is smaller than current statistical-model calculations and recent experimental results, which would suggest lower 44Ti production in scenarios for the α-rich freezeout. Special attention has been paid to assessing realistic uncertainties of stellar reaction rates produced from a combination of experimental and theoretical cross sections. With such methods, we also develop a re-evaluation of the 44Ti(α, p)47V reaction rate. Using these two rates we carry out a sensitivity survey of 44Ti synthesis in eight expansions representing peak temperature and density conditions drawn from a suite of recent supernova explosion models. Our results suggest that the current uncertainty in these two reaction rates could lead to as large an uncertainty in 44Ti synthesis as that produced by different treatments of stellar physics.