Soil deposition density maps of gamma-ray emitting radioactive nuclides from the Fukushima Dai-ichi Nuclear Power Plant (NPP) accident were constructed on the basis of results from large-scale soil sampling. In total 10,915 soil samples were collected at 2168 locations. Gamma rays emitted from the samples were measured by Ge detectors and analyzed using a reliable unified method. The determined radioactivity was corrected to that of June 14, 2011 by considering the intrinsic decay constant of each nuclide. Finally the deposition maps were created for (134)Cs, (137)Cs, (131)I, (129m)Te and (110m)Ag. The radioactivity ratio of (134)Cs-(137)Cs was almost constant at 0.91 regardless of the locations of soil sampling. The radioactivity ratios of (131)I and (129m)Te-(137)Cs were relatively high in the regions south of the Fukushima NPP site. Effective doses for 50 y after the accident were evaluated for external and inhalation exposures due to the observed radioactive nuclides. The radiation doses from radioactive cesium were found to be much higher than those from the other radioactive nuclides.
We have measured the asymmetric emission of protons from the nonmesonic decay of polarized (5)(Lambda)He produced by the (pi(+), K+) reaction. (5)(Lambda)He is an s-shell hypernucleus and its polarization is due to the Lambda. One expects to obtain direct information on the elementary weak Lambda-->p-->np process. The asymmetry parameter has been determined to be 0.24+/-0.22. The implication of the result is discussed.
We have measured lifetimes of L hypernuclei over a broad mass range explicitly identifying L hypernuclear production in the ͑p 1 , K 1 ͒ reaction. The obtained results are t͑ 12 L C͒ 231 6 15 ps, t͑ 28 L Si͒ 206 6 12 ps, and t͑ L Fe͒ 215 6 14 ps. The lifetimes of L hypernuclei over the mass region from carbon to iron are found almost constant at about 80% of that of free L within the statistical uncertainties. The short-range nature of the nonmesonic weak decay process, which is dominant in heavy L hypernuclei, is possibly responsible for the observed weak hypernuclear mass dependence.[S0031-9007(98)07604-2] PACS numbers: 21.80. + a, 13.30.Eg, 13.75.Ev, 21.10.Tg A L hyperon bound in a nucleus eventually decays from the ground state either through a p-mesonic or a nonmesonic weak decay (NMWD) process. Through a p-mesonic process, a L in a hypernucleus decays to a nucleon and a pion, i.e., L ! Np, as a free L does. However, the p-mesonic decay is strongly suppressed in hypernuclei, except very light ones, due to the small energy release in the process and the Pauli blocking. Instead, the NMWD process, which is the strangeness changing baryon-baryon weak interaction process ͑L 1 N ! N 1 N͒, becomes dominant in most hypernuclei due to the much larger momentum of the final nucleon, ϳ400 MeV͞c, than the typical Fermi momentum, ϳ270 MeV͞c.The total decay width of a L hypernucleus, G, is the sum of the partial decay widths; 1 n, respectively, and G 2p2h is that of two-nucleon-induced NMWD ͑LNN ! NNN͒. The branching ratio for the NMWD process already reaches to about 3͞4 by 12 L C [1,2]. Therefore, the decay of heavy L hypernuclei provides us with valuable and clean information on the NMWD process, henceforth the baryon-baryon weak interaction and the short-range correlations in nuclear matter.However, the understanding of the NMWD process is still in a primitive stage. Although there have been several microscopic model calculations for NMWD processes, which are based on either various meson exchange interactions [3][4][5] or quark-hadron hybrid interactions [6,7], none of them have successfully reproduced both the total ͑G͒ and partial decay widths (G n and G p ) simultaneously and particularly their ratio ͑G n ͞G p ͒.So far the experimental data for the partial decay widths had large errors and provided only limited constraints. Among the weak decay observables, the lifetime can be determined most accurately free from nuclear final state interactions and material effects. Therefore its accurate data for heavy L hypernuclei are very much awaited to understand the NMWD mechanism.The most reliable measurements of hypernuclear lifetimes were carried out for relatively light hypernuclei from 4 L H [8] to 12 L C [9] by direct measurements of the production and decay times of the L hypernuclei. There have been a few other lifetime measurements for heavier L hypernuclei [10][11][12][13]. However, in these measurements the produced hypernuclei were not explicitly identified. The reliable measurement for heavier hypernuclear l...
We have measured the energy spectra of pions and protons emitted in the weak decay of 12 C, 28 Si, and Fe hypernuclei produced via the (π + , K + ) reaction. The decay widths of the π − mesonic decay ( → pπ − ) and the nonmesonic decay ( N → NN) were extracted. The present results demonstrate an increase of the mesonic decay width due to a distortion of the pion wave function in nuclear medium for the first time. The ratios of the neutron-to proton-induced nonmesonic decay widths, n ( n → nn)/ p ( p → np), were evaluated by a direct comparison of the measured proton energy spectra with the calculated ones. No theoretical calculation which has been proposed so far can simultaneously account for both the nonmesonic decay widths and the n / p ratios in the present data.
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