Coulomb excitation

The potential energy due to the Coulomb interaction between a heavy ion and a nucleus can be written as  

Because of the strong, long-range electric field between projectile and target nuclei, it is possible for the incident heavy ion to excite the target nucleus 

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In addition, Coulomb excitation can be used to populate the Mossbauer levels of I77,i78,i80j j [165-167]. The experimental line width using these sources is only slightly larger than the natural line width (e.g., the thickness corrected line width of Hf in a tantalum foil is in good agreement with the natural line width Texp = 1.90 0.07 mm s T at = 1-99 0.04 mm s [168]). 

Wender and Hershkowitz [237] used the sensitivity of the recoil-free fraction in tungsten Mossbauer spectroscopy to deduce the effect of irradiation of tungsten compounds by Coulomb excitation of the resonance levels (2 states of I82,i84,i8 y with 6 MeV a-particles. While no effect of irradiation on the/-factors could be observed for tungsten metal in agreement with [233], a decrease of/was measured for WC, W2B, W2B5, and WO3 after irradiation. 

Dr. Herber We were interested in seeing whether we could get the 23.8-kv. level in tin-119 by Coulomb excitation. The B E2) for this is quite small, but it can be done. However, the experiment is not... 

Figure 10.23 Schematic illustration of the l dependence of the partial cross section for compound nucleus (CN), fusionlike (FL), deep inelastic (D), quasi-elastic (QE), Coulomb excitation (CE), and elastic (EL) processes. [From Schroeder and Huizenga (1984, p. 242).]...

Parent nuclides produced by the processes mentioned above can all be used for several half-lives. In contrast, one can also populate the Mossbauer excited state directly via Coulomb excitation (84). In this technique, a beam of high-energy ( 10 MeV) charged particles (e.g., O4+, Cl7 +) is directed onto the Mossbauer isotope and the electromagnetic field generated by these particles induces nuclear transitions, which can include transitions to the Mossbauer excited state. Subsequent decay to the nuclear ground state then provides the appropriate y radiation. The half-life of a source created in this manner is the half-life of the Mossbauer excited state (e.g., several nanoseconds), and thus Coulomb excitation is necessarily an in situ technique, i.e., the Mossbauer effect experiment must be performed at the location of the charged particle beam. 

In the above pairs of elements the first is the Mossbauer isotope and the second the parent nuclide, the pairs in parentheses are possible additions to the classes, and represents y-ray production using nuclear reaction or coulombic excitation. (See Appendix I for alternative sources.)... 

Q and n data from 1-7, 30) A data from (1-7. 85)-, G refers to ground state M refers to excited state CE refers to Coulomb excitation a source enclosed in parentheses refers to a possible y-ray source, but one that has not been referred to widely in the literature. 

Experimental Tests of Boson-Fermion Symmetries and Supersymmetries Using Coulomb Excitation with Heavy Ions... 

The 7 Lu target is unique because this material has an extremely low natural isotopic abundance (2.61Z). A sample of several tens of milligrams of LU2O3 was obtained from J he Oak Ridge National Laboratory and had an isotopic enrichment of 70Z in i/8Lu. An additional isotope separation step was done on this sample at LLNL By this procedure we were able to make a target of isotopically pure 176Lu ( 99.9Z) that had a thickness of 22 mg/cm2 and which was supported on a Th substrate. This target was subsequently used for i80 Coulomb-excitation experiments. 

The 130 keV State. The decay of the 130 keV state has been studied extensively, and several inconsistencies are being resolved. The results of different measurements of the mean life and decay mode of the 130 keV state are discussed by Fink and Benczer-Koller (8). The half-life of the state has been measured electronically, and the transition matrix element for excitation has been derived from Coulomb excitation data (12). The combination of the Coulomb excitation yield, the internal conversion coefficient (8) a = 1.76 =t= 0.19, and the branching ratio (8) PCo = 0.060 zb 0.008 for the crossover decay to ground, yields a half-life ti/2 = (0.414 0.014) ns in excellent agreement with a recent (15) Mossbauer determination of the line width, r = (4.4 zb 0.4) mm/sec, equivalent to t1/2 = (0.49 0.05) ns. Wilenzick et al. (15) do not indicate the thickness of the Pt absorber used. 

Different types of interaction are distinguished, as illustrated in Fig. 8.23. (The spherical form is a simplification which is only applicable for nuclei with nuclear spin / = 0.) On path 1 the nuclei are not touching each other elastic scattering and Coulomb excitation are expected. On path 2 the nuclei are coming into contact with each other and nuclear forces become effective inelastic scattering and transfer reactions... 

The interactions with a medium of charged particulate radiations such as protons, (p-particles, and y-rays consist predominately of electrostatic coulomb excitation and ionization caused by ejection of atomic and molecular electrons in the medium. According to Bethe s semiclassic treatment, the energy lost to the medium, per unit length of path, by a heavy particle of charge Ze and velocity v is ... 

Similar situations arise, for example, in Coulomb excitation reactions. In the Ge case, the low Debye temperature of the Ge metal produces a very low recoil-free fraction. As mentioned in more detail later (p. 109), it is possible to displace the excited atoms completely out of the target material and implant them into a new matrix with a high Debye temperature, thereby obtaining a considerable improvement in the quality of the spectra. 

An upper limit can be set to the duration time of several nuclear processes and their consequences such as Auger cascades in the case of electron capture reactions, or the thermal displacement of atoms from one site to another in a-emitting or Coulombic-excitation processes. The subject has recently been reviewed [20]. 

Germanium-73 has several low-lying excited levels, of which the 13-5-keV first excited state suffers from excessive internal conversion (ax > 1000) and a long lifetime (4 us), as does the 66-8-keV second excited state (0-S3 s). The 67 03-keV third excited state has convenient properties for Mbssbauer spectroscopy but is only weakly populated by the decay of Ga. All Mossbauer experiments have therefore used a direct population of this level by Coulomb excitation in an ion beam. The ground state has I — f+, while the 67-03-keV level is probably / = J-H and decays directly to the ground state. 

Very similar results were obtained using Coulomb excitation of a nickel foil target by 25-MeV oxygen ions [3], and the Coulomb-recoil implantation technique has also been demonstrated [4]. 

Coulombic excitation of Hf and °Hf by 6-MeV a-particles in targets of HfC and HfN at 78 K has shown considerable line broadening which is not a feature of these materials used as absorbers [34]. It may be presumed that the effect is a result of radiation damage associated with the recoil of the excited nuclei. The latter come to rest at lattice sites with one or more lattice vacancies nearby which generate an electric field gradient at the nucleus. 

Coulomb excitation of the 44-5-keV level has been achieved using a... 

MeV proton beam and a tungsten target foil at 77 K [54]. The Ufetime obtained was = 0-194(10) ns. The target recoilless fraction was lower than expected, either because of radiation damage or localised heating effects. The 182W, and levels have been simultaneously Coulomb-excited by... 

Coulombic excitation by 3-MeV protons has been used for several of the gadolinium levels [75, 87]. In each case the target was GdaOs at 4-2 or 77 K enriched in the appropriate isotope. The ise.iss.ieoQ j resonances showed unresolved quadrupole splitting which was analysed to give the ratios i56g /i55gg 1.04(2), = M4(2), and °Ge/ Gg = M8(2). 

The 43-8-keV Dy transition was not recorded until 1969 when it was observed following Coulomb excitation of Dy in a Dy203 target at 80 K with 3-3-MeV a-particles [124]. Dy203 and DylG absorbers showed paramagnetic relaxation broadening at low temperatures, but no analysis for the excited-state nuclear parameters was attempted on the preliminary data. 

Coulombic excitation of a Dy203 target gave well-resolved hyperfine structure in Dy203 at 4-2 K [127, 128], but as the velodty range scanned did not include all the component lines no calculation of the magnetic moment was made. 

The 79-3-keV resonance in Er was the last to be reported [149]. Coulomb excitation of an ErjOa target by 3-meV protons populates the first excited level, and the spectrum of an EraOa absorber at 30 K is a single line. The linewidth of 33-4 mm s corresponds to a lower limit to the excited-state lifetime of 0-103 ns. No hyperfine effects have been reported. 

The use of Tm to populate Er has already been mentioned. Coulomb excitation of the 79-8-keV resonance has also been used [150,151]. An ErjOs target gave a single emission line, and enabled the five-line magnetic spectrum to be resolved in Er metal at 4-2 K. Assuming a field of 7460 kG the data gave = 0-66(4) n.m. 

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