Re-Os Cosmo chronometer

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^<187>Re-^<187>Os pair is known as one of cosmochronometers, since ^<187>Re is produced by only r-process and its half life is quite long 42.3±1.3Gyr. ^<187>Os is produced by s-process of ^<186>Os. In addition, there is a problem originating from the excited state at 10 keV in ^<187>Os, which could be populated at stellar temperature, and therefore ^<187>Os is depleted by the (n,γ)process the through the state. In order to correct for the loss rate we measured both the (n,γ) cross sections and neutron inelastic cross sections of ^<186>Os, ^<187>Os and ^<189>Os using a newly developed experimental method. Pulsed neutrons were bombarded on enriched samples of ^<186>Os, ^<187>Os and ^<189>Os and ^<197>Au. The (n,γ)cross section of Os isotopes was measured using an anti-Compton NaI(Tl) spectrometers by detecting prompt gamma rays feeding from a neutron capture state of Os to low lying states. For the measurement of the ^<189>Os(n,n')^<189>Os reaction cross section we have developed a new detection system with use of four Li-glass detectors with a diameter of 50 mm and a thickness of 10 mm. Neutron energy scattered elastically and/or inelastically by ^<189>Os is determined by employing a TOF method. The discrimination between scattered neuron by ^<189>Os and gamma ray background can be made by their pulse heights. In order to obtain the inelastic scattering cross section of ^<189>Os accurately, it was crucial to prevent direct neutrons from the ^7Li(p,n) ^7Be reaction entering into the Li-glass detector. To find proper arrangement of these shield materials of borated polyethylene, cadmium and Boron, an extensive calculation has been made using a Monte Carlo Code. In addition, through many R & D works we have finally succeeded to construct a high sensitive measuring system of the inelastic scattering cross section of a nucleus. We succeeded to measure the ^<187>Os(n,n')^<187>Os cross section with high accuracy.