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INTERNAL CONVERSION COEFFICIENTS - HOW GOOD ARE THEY NOW?

Description: Internal conversion coefficients involving atomic electrons (ICC) and electron-positron pairs (IPC) are often required to determine transition multipolarities and total transition rates. A new internal conversion coefficient data base, BrIcc has been developed which integrates a number of tabulations on ICC and IPC, as well as {Omega}(E0) electronic factors. To decide which theoretical internal conversion coefficient table to use, the accurately determined experimental {alpha}{sub K}, {alpha}{sub L}, {alpha}{sub Total} and {alpha}{sub K}/{alpha}{sub L} values were compared with the new Dirac-Fock calculations using extreme assumptions on the effect of the atomic vacancy. While the overall difference between experiment and theory is less than 1%, our analysis shows preference towards the so called ''Frozen Orbital'' approximation, which takes into account the effect of the atomic vacancy.
Date: April 22, 2007
Creator: KIBEDI,T.; BURROWS, T.W.; TRZHASKOVSKAYA, M.B.; NESTOR, JR., C.W. & DAVIDSON, P.M.
Partner: UNT Libraries Government Documents Department

Improving the Efficiency of Solid State Light Sources

Description: This proposal addresses the national need to develop a high efficiency light source for general illumination applications. The goal is to perform research that would lead to the fabrication of a unique solid state, white-emitting light source. This source is based on an InGaN/GaN UV-emitting chip that activates a luminescent material (phosphor) to produce white light. White-light LEDs are commercially available which use UV from a GaN chip to excite a phosphor suspended in epoxy around the chip. Currently, these devices are relatively inefficient. This research will target one technical barrier that presently limits the efficiency of GaN based devices. Improvements in efficiencies will be achieved by improving the internal conversion efficiency of the LED die, by improving the coupling between the die and phosphor(s) to reduce losses at the surfaces, and by selecting phosphors to maximize the emissions from the LEDs in conversion to white light. The UCSD research team proposes for this project to develop new phosphors that have high quantum efficiencies that can be activated by the UV-blue (360-410 nm) light emitted by the GaN device. The main goal for the UCSD team was to develop new phosphor materials with a very specific property: phosphors that could be excited at long UV-wavelengths ({lambda}=350-410 nm). The photoluminescence of these new phosphors must be activated with photons emitted from GaN based dies. The GaN diodes can be designed to emit UV-light in the same range ({lambda}=350-410 nm). A second objective, which is also very important, is to search for alternate methods to fabricate these phosphors with special emphasis in saving energy and time and reduce pollution.
Date: March 31, 2003
Creator: McKittrick, Joanna
Partner: UNT Libraries Government Documents Department

New Results on Fission Cross Sections in Actinide Nuclei using the Surrogate Ratio Method and on Conversion Coefficients in Triaxial Strongly Deformed Bands in 167Lu from ICE Ball and Gammasphere

Description: The surrogate ratio technique is described. New results for neutron induced fission cross sections on actinide nuclei, obtained using this technique are presented. The results benchmark the surrogate ratio technique and indicate that the method is accurate to within 5% over a wide energy range. New results for internal conversion coefficients in triaxial strongly deformed bands in {sup 167}Lu are also presented.
Date: January 30, 2007
Creator: Beausang, C; Lesher, S; Burke, J; Bernstein, L; Phair, L; Ai, H et al.
Partner: UNT Libraries Government Documents Department

Low energy gamma-transitions in Pu{sup 238} and Pu{sup 240}

Description: We investigated electron and gamma spectra of mixtures of isotopes of curium. This was done on a beta spectrometer with double focusing at an angle of {pi}{radical}2 and on a scintillation gamma spectrometer with a 40 {times} 40 mm NAI(Tl) single crystal detector. The observed electron spectrum are interpolated and presented. Also, experimental results of the ratio of conversion at L and M subshells are presented and compared to theoretical values.
Date: March 27, 1967
Creator: Akalaev, G. G.; Vartanov, N. A. & Samoilov, P. S.
Partner: UNT Libraries Government Documents Department

Photofragment Translational Spectroscopy of Propargyl Radicals at 248 nm

Description: The photodissociation of propargyl radical, C{sub 3}H{sub 3}, and its perdeuterated isotopolog was investigated using photofragment translational spectroscopy. Propargyl radicals were produced by 193 nm photolysis of allene entrained in a molecular beam expansion, and then photodissociated at 248 nm. photofragment time-of-flight spectra were measured at a series of laboratory angles using electron impact ionization coupled to a mass spectrometer. Data for ion masses corresponding to C{sub 3}H{sub 2}{sup +}, C{sub 3}H{sup +}, C{sub 3}{sup +}, and the analogous deuterated species show that both H and H{sub 2} loss occur. The translational energy distributions for these processes have average values <E{sub T}> = 5.7 and 15.9 kcal/mol, respectively, and are consistent with dissociation on the ground state following internal conversion, with no exit barrier for H loss but a tight transition state for H{sub 2} loss. The translational energy distribution for H atom loss is similar to that in previous work on propargyl in which the H atom, rather than the heavy fragment, was detected. The branching ratio for H loss/H{sub 2} loss was determined to be 97.6/2.4 {+-} 1.2, in good agreement with RRKM results.
Date: December 21, 2007
Creator: Goncher, S.J.; Moore, D.T.; Sveum, N.E. & Neumark, D.M.
Partner: UNT Libraries Government Documents Department

Ultrafast Structural Rearrangements in the MLCT Excited State for Copper(I) bis-Phenanthrolines in Solution

Description: Ultrafast excited state structural dynamics of [Cu{sup I}(dmp){sub 2}]{sup +} (dmp = 2,9-dimethyl-1,10-phenanthroline) have been studied to identify structural origins of transient spectroscopic changes during the photoinduced metal-to-ligand-charge-transfer (MLCT) transition that induces an electronic configuration change from Cu(I) (3d{sup 10}) to Cu(II) (3d{sup 9}). This study has important connections with the flattening of the Franck-Condon state tetrahedral geometry and the ligation of Cu(II)* with the solvent observed in the thermally equilibrated MLCT state by our previous laser-initiated time-resolved x-ray absorption spectroscopy (LITR-XAS) results. To better understand the structural photodynamics of Cu(I) complexes, we have studied both [Cu{sup I}(dmp){sub 2}]{sup +} and [Cu{sup I}(dpp){sub 2}]{sup +} (dpp = 2,9-diphenyl-1,10-phenanthroline) in solvents with different dielectric constants, viscosities and thermal diffusivities by transient absorption spectroscopy. The observed spectral dynamics suggest that a solvent-independent inner-sphere relaxation process is occurring despite the large amplitude motions due to the flattening of the tetrahedral coordinated geometry. The singlet fluorescence dynamics of photoexcited [Cu{sup I}(dmp){sub 2}]{sup +} were measured in the coordinating solvent acetonitrile, using the fluorescence upconversion method at different emission wavelengths. At the bluest emission wavelengths, a prompt fluorescence lifetime of 66 fs is attributed to the excited state deactivation processes due to the internal conversion and intersystem crossing at the Franck-Condon state geometry. The differentiation between the prompt fluorescence lifetime with the tetrahedral Franck-Condon geometry and that with the flattened tetrahedral geometry uncovers an unexpected ultrafast flattening process in the MLCT state of [Cu{sup I}(dmp){sub 2}]{sup +}. These results provide guidance for future x-ray structural studies on ultrafast time scale, as well as for synthesis towards its applications in solar energy conversion.
Date: October 5, 2006
Creator: Shaw, G B; Grant, C D; Shirota, H; Castner Jr., E W; Meyer, G J & Chen, L X
Partner: UNT Libraries Government Documents Department

A microdosimetric study of {sup 10}B(n,{alpha}){sup 7}Li and {sup 157}Gd(n,{gamma}) reactions for neutron capture therapy

Description: This paper presents the microdosimetric analysis for the most interesting cell survival experiment recently performed at the Brookhaven National Laboratory (BNL). In this experiment, the cells were first treated with a gadolinium (Gd) labeled tumor-seeking boronated porphyrin (Gd-BOPP) or with BOPP alone, and then irradiated with thermal neutrons. The resulting cell survival curves indicate that the {sup 157}Gd(n,{gamma}) reactions is very effective in cell killing. The death of a cell treated with GD-BOPP were attributed to either the {sup 10}B(n,{alpha}) {sup 7}Li reactions or the {sup 157}Gd(n,{gamma}) reactions (or both). However, the quantitative relationship between the two types of reaction and the cell survival fraction was not clear. This paper presents the microdosimetric analysis for the BNL experiment based on the measured experimental parameters, and the results clearly suggest a quantitative relationship between the two types of reaction and the cell survival fraction. The results also suggest new research in Gadolinium neutron capture therapy (GDNCT) which may lead to a more practical modality than the boron neutron capture therapy (BNCT) for treating cancers.
Date: December 31, 1996
Creator: Wang, C.K.C.; Sutton, M.; Evans, T.M. & Laster, B.H.
Partner: UNT Libraries Government Documents Department

The positron peak puzzle - recent results from APEX

Description: Results are presented from a new experiment, APEX, designed to study the previously reported sharp lines in sum-energy spectra of positrons and electrons produced in collisions of very heavy ions. Data have been collected for {sup 238}U+{sup 181}Ta and {sup 238}U+{sup 232}Th. No evidence is found for narrow structures similar to those previously reported. For the specific case of the isolated decay of a neutral particle of mass 1.4-2.1 MeV/c{sup 2}, the upper limits on cross sections obtained are significantly less than previously reported. Data are also presented for internal pair conversion in {sup 206}Pb. These results are used to set limits for the possible contribution to the pair yield of a 1780 keV transition in {sup 238}U observed in heavy-ion gamma-ray coincidence measurements.
Date: December 31, 1995
Creator: Ahmad, I.; Back, B.B. & Kutschera, W.
Partner: UNT Libraries Government Documents Department

Effect of material environment on a class of nuclear lifetimes

Description: The connection between internal conversion of a nuclear transition and EXAFS is pointed out. A prediction is made of sizable variations of lifetimes of nuclear states depending on the surrounding material environment, provided that the transition energy is just above threshold and the internal conversion coefficient is appreciable. 12 refs., 2 figs.
Date: January 1, 1988
Creator: Perlow, G.J.
Partner: UNT Libraries Government Documents Department

Predicted changes of the internal conversion rates in /sup 119/Sn due to admixtures of lower multipole order

Description: One conceivable basis for interlevel transfer from a long-lived isomer to a graser state might be to alter the multipolarity of the transfer step. As an example, in the case of /sup 119/Sn any mechanism that introduces angular momentum to admix M3 radiation into the normally M4 transition from the 89.6 keV, 11/2/sup -/ level to the 23.9 keV, 3/2/sup +/ state could greatly shorten its normal 293 day half-life. We have calculated the K-shell internal conversion rate for the 89.6 keV level in /sup 119/Sn for small admixtures of M3 radiation using Moszkowski formulas for the radiative transition rates and tabulated values of the internal conversion coefficients. We find that even very small admistures of lower multipole order can produce very large changes in the internal conversion coefficient. This offers the basis for a simple and definitive test of the hypothesized mechanism for creating population inversion by introducing angular momentum. 4 refs., 1 fig.
Date: January 1, 1985
Creator: Wender, S.A.; Baldwin, G.C.; Talbert, W.L. & Reiss, H.R.
Partner: UNT Libraries Government Documents Department

Ionization by nuclear transitions

Description: A phenomenological description of ionization events is given that applies to both shaking transitions of the composite nucleus-atom system and the process in which the charged particles emitted in a nuclear decay transfer energy by a relatively slow ''direct collision'' final state interaction with atomic electrons. Specific areas covered include shakeoff in internal conversion, shakeoff in electron capture, shakeoff in beta decay, and inner electron ejection in alpha decay. (5 figs., 1 table) (GHT)
Date: January 1, 1975
Creator: Freedman, M.S.
Partner: UNT Libraries Government Documents Department

Effects of Irradiation on the Mechanical Properties of Tantalum

Description: Tensile, bend ductility, and hardness tests were performed at room temperature on irradiated tantalum sheet to determine the effect of irradiation on the strength and ductility. Sheet tensile specimens were irradiated in an attempt to produce corversions of tantalum to tungsten of approximately 1.5 and 3.0 wt.%. Unirradiated tantalum and arc-melted alloys of tantalum-1.5 and -3 wt.% tungsten were tested for comparison with the irradiated material. The tensile and yield strengths of tantalum were found to increase appreciably as a result of irradiation whereas the tensile properties of unirradiated Ta-W alloys prepared by arc melting showed that small additions of tungsten do not signicantly increase the strength of tantalum. These results indicate that the major part of the increase in strength resulting from irradiation of tantalum can be attributed to fast-neutron damage and that any contribution produced by the conversion of tantalum to tungsten is a minor one. (auth)
Date: November 18, 1960
Creator: Franklin, C. K.; Stahl, D.; Shober, F. R. & Dickerson, R. F.
Partner: UNT Libraries Government Documents Department

The Decay of Neptunium-238

Description: >A study was made of the energy levels of Pu/sup 238/ which are populated by Np/sup 238/ beta decay, by an examination of the Np/sup 238/ conversion electron spectrum in high-resolution beta spectrographs. The general features of the level scheme as previously given were unchanged but several new transitions were observed, with energies of 119.8, 871, 943, 989, and 1034 kev. Two new levels are postulated at 915 and 1034 kev which accommodate all but the 943-kev transition. A possible assignment of the 943-kev transition to the (0+.0) state of the beta vibrational band is discussed. In addition, the weak 885-kev transition from the 2+ state of the gamma -vibrational band to the 4+ state of the ground band was seen and its relative intensity determined. Comparisons were made of the experimental relative transition intensities of the three photons depopulating this band with those predicted from the rules of Alaga et al.; only fair agreement was noted. A discussion is given of the beta decay branchings and log ft values of Np/sup 238/ decay in terms of the postulated characters of the Pu/sup 238/ states and the measured spin of Np/sup 238/. (auth)
Date: December 1, 1960
Creator: Albridge, R. G. & Hollander, J. M.
Partner: UNT Libraries Government Documents Department