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Evaluation of photoneutron production at high energy LINACS

Description: This report describes an estimate of neutron production at a 9 MeV LINAC, and the potential for photoactivation of materials present at the LINAC facility. It was found that only isotopes of U, W, Ta, and Pb had daughters whose activities might be measurable. The LINAC was found to be capable of producing in the neighborhood of 10{sup 10} neutrons/second from these heavy metals, and that subsequent neutron activation might be more of a concern. Monte Carlo simulation of neutron transport and capture in the concrete and steel found in the LINAC vault indicates that {sup 55}Fe may be produced in measurable quantities.
Date: April 24, 1995
Creator: Bell, Z.W.
Partner: UNT Libraries Government Documents Department

Recent studies of the gamma-ray laser problem. Addendum to LA-UR-80-96

Description: The possibility of extending the laser principle into the hard x-ray region above a few keV depends upon the ability of a pump to create the critical density of population inversion for which gain overcomes loss by absorption. Although this critical density decreases with the wavelength of the radiation to be stimulated, the power required to generate it depends upon the lifetime of the state being pumped. The lifetimes of inner-shell vacancies of atoms are very short. Nuclear states, on the other hand, have much longer lifetimes, ranging from fractions of picoseconds to millennia. Moreover, in the so-called recoilless or Moessbauer transitions of nuclear isomers, it was observed that the resonance cross section often exceeds the nonresonant absorption cross section by several orders of magnitude: just the condition for lasing in an inverted population. If, other things being equal, the absorber foil of a Moessbauer experiment contained an excess of excited states, then, instead of the absorption dip normally observed at resonance, there would be an increase of intensity, and amplification by stimulated emission would be achieved. The problem in making a gamma-ray laser is, therefore, simply that of obtaining an inverted population without inhibiting the Moessbauer effect. Research on this problem is reviewed.
Date: January 1, 1980
Creator: Baldwin, G.C.; McNeil, L.E.; Solem, J.C. & Suydam, B.R.
Partner: UNT Libraries Government Documents Department

Influences of the astrophysical environment on nuclear decay rates

Description: In many astronomical environments, physical conditions are so extreme that nuclear decay rates can be significantly altered from their laboratory values. Such effects are relevant to a number of current problems in nuclear astrophysics. Experiments related to these problems are now being pursued, and will be described in this talk. 19 refs., 5 figs.
Date: September 1, 1987
Creator: Norman, E.B.
Partner: UNT Libraries Government Documents Department

Neutron capture cross sections of /sup 178/,/sup 179/,/sup 180/Hf and the origin of nature's rarest stable isotope /sup 180/Ta

Description: The neutron capture cross sections of /sup 178/,/sup 179/,/sup 180/Hf were measured in the energy range 2.6 keV to 2 MeV. The average capture cross sections were derived and fitted in terms of strength functions. Resonance parameters for the observed resonances below 10 keV were determined by shape analysis. Maxwellian-averaged capture cross sections were computed for thermal energies with kT between 5 and 100 keV. The cross sections for kT = 30 keV were used to determine the population probability of the 8- isomeric level in /sup 180/Hf by neutron capture as (1.24 +- 0.06)% and the r-process abundance of /sup 180/Hf as 0.0290 (Si = 10/sup 6/). These quantities served to analyze s- and r-process nucleosynthesis of /sup 180/Ta, nature's rarest stable isotope.
Date: January 1, 1982
Creator: Beer, H. & Macklin, R.L.
Partner: UNT Libraries Government Documents Department

Long-lived heavy mass elements half-lives (A > 125)

Description: Reported values of half-lives of intermediate mass and heavy elements are evaluated. The evaluation analysis estimates the systematic error the resulting standard deviation. Recommended values are then presented for /sup 128/Te, /sup 130/Te, /sup 129/I, /sup 138/La, /sup 144/Nd, /sup 145/Nd, /sup 146,147,148/Sm, /sup 152/Gd, /sup 154/Dy, /sup 176/Lu, /sup 174/Hf, /sup 180/Ta, /sup 187/Re, /sup 186/Os, /sup 190/Pt, /sup 204,205/Pb, and /sup 230,232/Th. 103 refs., 21 tabs. (WRF)
Date: January 1, 1985
Creator: Holden, N.E.
Partner: UNT Libraries Government Documents Department

Heavy mass elements total half-lives for selected long-lived nuclides

Description: In the past, many compilations and evaluations of half-lives have been made which have uncritically accepted authors' values and uncertainties. They have merely recommended weight-averaged reported results. This evaluation attempts to reanalyze each experiment in the literature including an estimate of the standard deviation utilizing, where possible, an estimate of the systematic error. This paper constitutes a preliminary step in the process of recommending values. The long-lived nuclides of heavy mass elements are of interest in determining geological ages using the Re-Os or the Lu-Hf dating methods, in supplying information on the p-process (proton capture) of nucleo-synthesis, in providing information on lepton number conservation and the rest mass for the electron neutrino from double ..beta.. decay processes and in the case of tantalum because it represents the first long-lived state which is actually an isomer. Experimental data on the half-lives of selected nuclides have been evaluated and recommended values and uncertainties are presented for the following nuclides: /sup 128/Te, /sup 130/Te, /sup 129/I, /sup 138/La, /sup 144,145/Nd, /sup 146,147,148/Sm, /sup 152/Gd, /sup 154/Dy, /sup 176/Lu, /sup 174/Hf, /sup 180/Ta, /sup 187/Re, /sup 186/Os, /sup 190/Pt, /sup 204,205/Pb and /sup 230,232/Th. It is shown that /sup 204/Pb, which was previously thought to be radioactive, is stable. For /sup 205/Pb, the L electron capture x-rays have been revised for the M and higher x-ray yields. The resulting half-life for /sup 205/Pb is 1.9 +- 0.3 x 10/sup 7/ years. /sup 146/Sm with a half-life of 1.03 +- 0.05 x 10/sup 8/ years is the longest-lived extinct natural nuclide. 21 tabs.
Date: January 1, 1985
Creator: Holden, N.E.
Partner: UNT Libraries Government Documents Department