In the commissioning phase of the DANCE project (Detector for Advanced Neutron Capture Experiments) measurements have been performed with special emphasis on the identification and suppression of possible backgrounds for the planned (n,γ) experiments. This report describes several background sources, observed in the experiment or anticipated from simulations, which will need to be suppressed in this and in similar detectors that are planned at other facilities. First successes are documented in the suppression of background from scattered neutrons captured in the detector as well as from the internal radiation. Experimental results and simulations using the GEANT code are compared.
The neutron-capture cross sections for '"'""""'~'"Os have been measured by the neutron-time-of-flight technique from 0.5 eV to 150 keV. Nuclear level spacings were extracted from the low-energy (resonance-regionj data below -1 keV, and average cross sections from the data above 1 keV. The ratio of the Maxwellian-weighted average cross sections for '"Os and '"Os near 30 keV is a vital parameter for the determination of the duration of nucleosynthesis prior to the formation of the solar system, and thus for the determination of the age of the universe by the nuclear-dating technique. The present result of 10.8+, ",F10' yr for the duration of nucleosynthesis is in concordance with the value obtained from U-Th dating. The result for the age of the universe {17~3 &(10' yrj agrees with the value obtained from the globular-cluster method, but clearly exceeds the most recent determination of the Hubble time.NUCLEAB REACTIONS~~~~~Qs(yg p), E= 2-j.5& &0 sured 0(n, y); Maxwelban-averaged 0(k7= 30 keV); extracted average level spacing", deduced age of universe for Be-Os clock. I~INTRODUCTIONBecause the half-life of '8'He is very long (-43 x 109 yr), because both 'MOs and '8'Os are pure s-process nuclei (i.e. , they are shielded by '@W and '8'He, respectively, from the r-process), and because these heavy metals are present in a relatively undisturbed state in primordial meteorites, the ' YHe -' 'Os nuclear p-decay clock constitutes probably the best basis for a radiogenic determination of the age of the elements.This method was suggested originally by Clayton, ' who pointed out its advantages, particularly over those methods that depend upon a knowledge of the r-process production rates, Clayton also pointed out the key role played by the neutron-capture cross sections for '@Os and '8'Os (or rather, by their ratio) in this determination. Such measurements, however, would have been beset with formidable experimental difficulties until the sufficiently massive and isotopicaQy pure samples of these rare osmium isotopes were manufactured for the measurements reported here. ' Subsequent measurements using these enriched samples have been. reported by Browne et al. and by Winters et al. 4 The results of all three measurements are compared in Sec. IV.In addition to the required ratio of Mamvellianaveraged 30-keg cross sections, a correction factor is needed in order to account for neutron capture by ' 'Os nuclei in their 9.8-keg first excited state, which also is populated significantly in. a stellar environment. A Hauser-Feshbach calculations of this correction factor has been made by Woosley and Fowler. 8 Among the important input parameters for this calculation are the average level spacings for '8'Os ('@Os+n) and '880s ('8'Os + n). We have been able to extract improved values for these level spacings as well from our low-energy (&1 keV) data.These results, together with values for the halflife of '8'He and the elemental abundance ratio of osmium and rhenium in carbonaceous chxondites and the use of an expon. ential model of the ...
The neutron-induced fission cross section of Am has been measured over the energy region from 10 eV to -20 MeV in a series of experiments utilizing a linac-produced "white" neutron source and a monoenergetic source of 14.1 MeV neutrons. The cross section was measured relative to that of U in the thermal (0.001 to -3 eV) and high energy (1 keV to -20 MeV) regions and normalized to the ENDF/B-V 2 5U(n, f) evaluated cross section. In the resonance energy region (0.5 eV to 10 keV) the neutron flux was measured using thin lithium glass scintillators and the relative cross section thus obtained was normalized to the thermal energy measurement. This procedure allowed a consistency check between the thermal and high energy data. The cross section data have a statistical accuracy of -0.5% at thermal energies and in the 1-MeV energy region, and a systematic uncertainty of -5%. We confirmed that Am has the largest thermal fission cross section known with a 2200 m/sec value of 6328 b. Results of a Breit-Wigner sum-of-single-levels analysis of 48 fission resonances up to 20 eV are presented and the connection of these resonance properties to the large thermal cross section is discussed. Our measurements are compared with previously reported results.
The beta spectrum of free molecular tritium has been measured in order to search for a finite electron-antineutrino mass. The final-state effects in molecular tritium are accurately known and the data thus yield an essentially model-independent upper limit of 27 eV on the v e mass at the 95% confidence level.
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