39 research outputs found
Fusion-Fission of 16O+197Au at Sub-Barrier Energies
The recent discovery of heavy-ion fusion hindrance at far sub-barrier
energies has focused much attention on both experimental and theoretical
studies of this phenomenon. Most of the experimental evidence comes from
medium-heavy systems such as Ni+Ni to Zr+Zr, for which the compound system
decays primarily by charged-particle evaporation. In order to study heavier
systems, it is, however, necessary to measure also the fraction of the decay
that goes into fission fragments. In the present work we have, therefore,
measured the fission cross section of 16O+197Au down to unprecedented far
sub-barrier energies using a large position sensitive PPAC placed at backward
angles. The preliminary cross sections will be discussed and compared to
earlier studies at near-barrier energies. No conclusive evidence for
sub-barrier hindrance was found, probably because the measurements were not
extended to sufficiently low energies.Comment: Fusion06 - Intl. Conf. on Reaction Mechanisms and Nuclear Structure
at the Coulomb Barrier, San Servolo, Venezia, Italy, March 19-223, 2006 5
pages, 4 figure
Hindrance of Heavy-ion Fusion at Extreme Sub-Barrier Energies in Open-shell Colliding Systems
The excitation function for the fusion-evaporation reaction 64Ni+100Mo has
been measured down to a cross-section of ~5 nb. Extensive coupled-channels
calculations have been performed, which cannot reproduce the steep fall-off of
the excitation function at extreme sub-barrier energies. Thus, this system
exhibits a hindrance for fusion, a phenomenon that has been discovered only
recently. In the S-factor representation introduced to quantify the hindrance,
a maximum is observed at E_s=120.6 MeV, which corresponds to 90% of the
reference energy E_s^ref, a value expected from systematics of closed-shell
systems. A systematic analysis of Ni-induced fusion reactions leading to
compound nuclei with mass A=100-200 is presented in order to explore a possible
dependence of the fusion hindrance on nuclear structure.Comment: 10 pages, 9 figures, Submitted to Phys. Rev.
Experimental limits on nucleon decay and ΔB=2 processes
Results from the IMB collabration to detect possible proton decay in a salt mine near Cleveland, Ohio are presented. Detection apparatus is described.(AIP)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87900/2/1_1.pd
The search for proton decay
Following a very brief description of the theoretical developments which motivated the search for proton decay, I shall describe one of these experiments (the IMB experiment) in some detail. Then I shall compare recent results from that experiment with those from other detectors.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87399/2/321_1.pd
IMB Detector‐The first 30 Days
A large water Chernekov detector, located 2000 feet below ground, has recently been turned on. The primary purpose of the device is to measure nucleon stability to limits 100 times better than previous measurements. The properties of the detector are described along with its operating characteristics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87428/2/138_1.pd
Fusion hindrance for a positive Q-value system
An excitation function for the fusion reaction Si28 + Si30 (Q=14.3MeV) has been measured down to 40μb. Deviations from the behavior predicted by the optical model and standard coupled-channels calculations have been observed in this system. The fusion cross sections can be reproduced by a shallow potential model well, which was originally developed to explain the hindrance of heavy-ion fusion for systems with negative Q-values
Hindrance of heavy-ion fusion at extreme sub-barrier energies in open-shell colliding systems
The excitation function for the fusion-evaporation reaction 64Ni + 100Mo has been measured down to a cross section of ∼5 nb. Extensive coupled-channels calculations have been performed, which cannot reproduce the steep falloff of the excitation function at extreme sub-barrier energies. Thus, this system exhibits a hindrance for fusion, a phenomenon that has been discovered only recently. In the S-factor representation introduced to quantify the hindrance, a maximum is observed at Es = 120.6 MeV, which corresponds to 90% of the reference energy Esref, a value expected from systematics of closed-shell systems. A systematic analysis of Ni-induced fusion reactions leading to compound nuclei with mass A = 100-200 is presented in order to explore a possible dependence of fusion hindrance on nuclear structure
New determination of the astrophysical S factor SE1 of the C12(α,γ)O16 reaction
A new measurement of the β-delayed α decay of N16 has been performed using a set of high efficiency ionization chambers. Sources were made by implantation of a N16 beam, yielding very clean α spectra down to energies as low as 400 keV. Our data are in good agreement with earlier results. For the S factor SE1, we obtain a value of 74±21keVb. In spite of improvements in the measurement, the error in SE1 remains relatively large because of the correlations among the fit parameters and the uncertainties inherent to the extrapolation
First evidence of fusion hindrance for a small Q-value system
The excitation function for the fusion-evaporation reaction 28Si + 64Ni has been measured down to a cross section of 25 nb. This is the first observation of fusion hindrance at extreme sub-barrier energies for a system with a small, negative Q-value (- 1.78 MeV). This result is further proof that heavy-ion fusion hindrance, reported earlier only for systems with large, negative Q-values, is a general phenomenon. The measured behavior can be reproduced by coupled-channels calculations with a modified ion-ion potential incorporating the effects of nuclear incompressibility
Descriptions of new fossils, from the coal measures of Missouri and Kansas
by B.F. Shumard and G.C. SwallowExt. from Trans. Aca. Sci., St. Louis, Vol. I., No.