Excitation Function Data Research Articles

O(1D) Production Following Electron Impact on Oxygen-Containing Molecules

A solid Ne matrix detector is used to study electron-impact dissociation pathways in CO2 and N2O. This matrix is selectively sensitive to the metastable oxygen species, O(1D). O(1D) is an important constituent in the atmospheres of Earth and other extra-terrestrial objects. Kinetic energy and excitation function data for production of this species will be presented.

A three-dimensional finite element model and image reconstruction algorithm for time-domain fluorescence imaging in highly scattering media

In this work, development and evaluation of a three-dimensional (3D) finite element model (FEM) based on the diffusion approximation of time-domain (TD) near-infrared fluorescence light transport in biological tissue is presented. This model allows both excitation and fluorescence temporal point-spread function (TPSF) data to be generated for heterogeneous scattering and absorbing media of arbitrary geometry. The TD FEM is evaluated via comparisons with analytical and Monte Carlo (MC) calculations and is shown to provide a quantitative accuracy which has less than 0.72% error in intensity and less than 37 ps error for mean time. The use of the Born–Ratio normalized data is demonstrated to reduce data mismatch between MC and FEM to less than 0.22% for intensity and less than 22 ps in mean time. An image reconstruction framework, based on a 3D FEM formulation, is outlined and simulation results based on a heterogeneous mouse model with a source of fluorescence in the pancreas is presented. It is shown that using early photons (i.e. the photons detected within the first 200 ps of the TPSF) improves the spatial resolution compared to using continuous-wave signals. It is also demonstrated, as expected, that the utilization of two time gates (early and latest photons) can improve the accuracy both in terms of spatial resolution and recovered contrast.

DIFFUSENESS OF WOODS–AXON POTENTIAL AND SUB-BARRIER FUSION

We have proposed an energy-dependent parametrization scheme for determining the diffuseness parameter of Woods–Saxon potential which when used in conjunction with the coupled channel code CCFULL explains very well the fusion excitation function data around the barrier for various systems.

Status of vibrational structure inNi62

Measurements consisting of $\ensuremath{\gamma}$-ray excitation functions and angular distributions were performed using the ($n,{n}^{\ensuremath{'}}\ensuremath{\gamma}$) reaction on $^{62}\mathrm{Ni}$. The excitation function data allowed us to check the consistency of the placement of transitions in the level scheme. From $\ensuremath{\gamma}$-ray angular distributions, the lifetimes of levels up to $~3.8$ MeV in excitation energy were extracted with the Doppler-shift attenuation method. The experimentally deduced values of reduced transition probabilities were compared with the predictions of the quadrupole vibrator model and with large-scale shell model calculations in the $\mathit{fp}$ shell configuration space. Two-phonon states were found to exist with some notable deviation from the predictions of the quadrupole vibrator model, but no evidence for the existence of three-phonon states could be established. $Z=28$ proton core excitations played a major role in understanding the observed structure.

Quasi-elastic scattering in theLi6+Th232reaction

Quasi-elastic scattering measurements at large backward angles have been carried out for the $^{6}\mathrm{Li}+^{232}\mathrm{Th}$ system at energies around the Coulomb barrier. Barrier distribution has been obtained from the excitation function data. Coupled channel calculations using a double-folding potential as the bare potential have been performed. The disagreement between data and theoretical predictions shows a large breakup effect in the quasi-elastic scattering of a weakly bound projectile on a deformed target.

Incomplete fusion mechanism in 16O + 93Nb reaction at 6 MeV/nucleon: Recoil range measurements

Summary Recoil range distribution of evaporation residues formed in complete and incomplete fusion of 96 MeV 16O with 93Nb has been measured by recoil catcher technique followed by off-line gamma-ray spectrometry. The cross sections for complete and incomplete fusion were delineated from the earlier measured excitation function data at the same beam energy. The results show that incomplete fusion is associated with angular momentum values close to the critical angular momentum for complete fusion of the projectile and target.

Linear momentum transfer in incomplete fusion reactions of 12C and 13C with 181Ta

Recoil range distribution of evaporation residues formed in complete and incomplete fusion of 82.5 MeV 12C and 13C with 181Ta have been measured by recoil catcher technique followed by off-line gamma-ray spectrometry. The recoil range distributions of incomplete fusion products were found to be in good agreement with that simulated on the basis of direct surface transfer reaction model. The cross-sections for complete and incomplete fusion were delineated from the measured excitation function data at the same beam energy. The fraction of incomplete fusion in the total cross-section was found to be more in the case of 12C than that in the case of 13C indicating the dependence of incomplete fusion probability on the cluster structure of the projectile.

Excitation functions of nuclear reactions induced by 3He-particles on cobalt

Activation cross-sections for production of 54Mn, 56,57,58,60Co and 60,61Cu radioisotopes in 3He-induced reactions on Co were measured from the respective thresholds up to 26 MeV. The stacked foil irradiation method was used to produce the activation products and the emitted gamma photons were detected by high-resolution gamma-spectroscopy. The obtained new cross-sections are compared with literature data. The possible applications of the available excitation function data are discussed.

A Study of 16O +24Mg Reaction Using a Coupled Channel Approach

Different interpretations are introduced to describe the uprising oscillatory structures of 16O +24Mg reaction. The gross resonant structures to the ground- and first-excited states have been studied successfully in terms of both the DWBA and coupled channel calculations. The DWBA results introduce a reasonable description of the angular distributions and excitation function data. The coupled channel calculations provide a better agreement with the experimental forward and backward angle data than the DWBA calculations.

Reaction dynamics of O(1D)+HD. II. Effects of excited surfaces

To explore the effects of excited surfaces on the title reaction, an in-depth crossed-beam investigation was carried out at two collision energies, 3.7 and 4.6 kcal/mol. In combination with the results obtained at the lower energy, 2.05 kcal/mol (the preceding paper), the extent of abstraction contribution to total reactivity was deduced from the doubly differential cross-section measurements and its dominant dynamical features were revealed for the first time. The former was found to be in reasonable agreement with our previous estimate based on the excitation function data. We also present quasiclassical trajectory results based on accurate potential energy surfaces for the 1 1A′ and 1 1A″ surfaces. The trajectory results give approximately the same breakdown into insertion and abstraction components as in the experiments, and some aspects of the product distributions, such as the angular distributions, are in rough agreement. However, other aspects of the product distributions, such as the abstraction component of the product translational distributions, are in serious disagreement. Possible deficiencies in the potential surfaces and dynamics that could be responsible for these results are considered.

Fluctuations of the second order observables for dissipative processes in 19F+27Al system

The excitation function (EF) data for dissipative processes in 19F + 27Al system in the incident energy interval from 113.5 to 130 MeV are used to obtain the dependence of the charge variance and of the interaction time as a function of the incident energy. Fluctuations are observed in the EFs of both these secondary observables. Their correlation is supported by a mechanism based on stochastic exchange of nucleons.

Excitation function experiments at storage rings with the ultimate goal to probe the liquid-gas phase transition

The use of storage rings, operating in slow ramping mode to obtain excitation function data, is presented. Such experiments could provide a new way of probing the liquid-gas phase transition. We also discuss “isotopic effect ratios” which may act both as thermometer and as clock for the fragmentation process. Recent data on π +, p and d emission in the first slow ramping experiments at the CELSIUS storage ring are presented.

Crossed-beam studies of radical reaction dynamics:

The title reaction has been studied in a crossed molecular beam apparatus. Both the product state distributions and the translational energy dependence of the reaction cross sections were measured under single collision conditions. Excellent agreement was found over a wide temperature range (26–3800 K) between rate constants deduced from the translational excitation function and recent thermal kinetic data. The rotational state distribution was found to be very cold compared to the reaction exothermicity, and could be described by a Boltzmann temperature of 110 K for all K-doublet levels. The vibronic state distribution was also found to be cold, with 70% of the products formed in the vibrational ground state. By comparing the molecular beam results for vibronic state distributions with those obtained from recent bulb experiments, it was conjectured that there appears to be a strong correlation between rotation in the reactants and bending excitation in the products.

Systematic variation of fission barrier heights for symmetric and asymmetric mass divisions.

The excitation functions and the mass yield curves for proton-induced fissions of $^{233}\mathrm{U}$, $^{235}\mathrm{U}$, $^{236}\mathrm{U}$, $^{237}\mathrm{Np}$, $^{239}\mathrm{Pu}$, $^{242}\mathrm{Pu}$, $^{244}\mathrm{Pu}$, $^{241}\mathrm{Am}$, and $^{243}\mathrm{Am}$ targets were studied. The excitation functions for near-symmetrically divided fragments were found quite different in shapes and threshold energies from those of asymmetrically divided ones in all the fissioning systems studied. In order to obtain the fission barrier heights for symmetric and asymmetric mass division, the conventional Bohr-Wheeler type calculations for the competition between fission and neutron emission were performed. In the calculation, two independent fission channels, symmetric and asymmetric, were assumed, and fission barrier heights and level density parameters that could best reproduce the shapes of the excitation functions (incident energy dependence) of typical symmetric and asymmetric products were deduced. Barrier heights for symmetric and asymmetric mass divisions both decrease as the mass of fissioning nuclide ${\mathit{A}}_{\mathit{f}}$ increases, and they also exhibit slight dependence on the atomic number. Difference between the two barrier heights is 2.0--2.5 MeV in the region of ${\mathit{A}}_{\mathit{f}}$=236--245. From similar analysis of excitation function data reported in literature, it is found that the difference changes from a large positive value for N\ensuremath{\sim}150, neutron number of the fissioning nuclide, to a large negative value for N\ensuremath{\sim}126, and the two barrier heights become comparable at N=135--138.

Nucleus-nucleus potential in the range -2<S<2.5 fm

The nucleus-nucleus potentials (${\mathit{V}}_{\mathit{N}}$) over a range of interaction distances (-2S2.5 fm) have been obtained from the analyses of the fusion excitation function data spanning a large range of energies. A three-parameter Fermi function adequately represents the observed variation of ${\mathit{V}}_{\mathit{N}}$/R\ifmmode\bar\else\textasciimacron\fi{}[R=${\mathit{R}}_{1}$${\mathit{R}}_{2}$/(${\mathit{R}}_{1}$+${\mathit{R}}_{2}$), with ${\mathit{R}}_{1}$ and ${\mathit{R}}_{2}$ the radii of the interacting nuclei] with S in the above-mentioned range.

High-resolution inelastic gamma-ray measurements with a white neutron source from 1 to 200 MeV

Measurements of prompt gamma rays following neutron-induced reactions have recently been made at the spallation neutron source at the WNR target area of LAMPF using germanium detectors. These experiments provide extensive excitation function data for inelastic neutron scattering as well as for other reactions such as (n, α), (n, nα), (n, p), (n, np), (n, nnp) and (n, xn) for 1 ≤ x ≤ 11. The continuous energy coverage available from 1 MeV to over 200 MeV is ideal for excitation function measurements and greatly extends the energy range for such data. The results of these measurements will provide a database for interpretation of gamma-ray spectra from the planned Mars Observer mission, aid in radiation transport calculations, allow verification of nuclear reaction models, and improve the evaluated neutron reaction database.

Superdeformed bands in194Tl

Two superdeformed (SD) bands have been found in194Tl using the reactions176Yb(23Na,5n) and181Ta(18O,5n). The mass assignments were made from excitation function data. These are the first SD bands found in the mass-190 region that are not in Hg nuclei. The energies of many of the transitions of one of these new SD bands are similar in energy to those in the SD band of192Hg.

Orientational effects in the energy dependence of the reaction cross-section from model calculations

A dynamical model to calculate the kinetic-energy dependence of the reaction cross-section is presented. The most important aspects of the present treatment area) an orientational dependence of the reaction barrier,b) the onset of a recrossing mechanism andc) the operation of alignment forces during the reaction time. Application to several reactions of oriented or nonoriented reactants’ geometry gives a satisfactory fit of the excitation function data providing a clear insight into the diagnosis of the reaction dynamics that governs the reactive collision.

Vector polarized 6Li scattering from 12C and 16O.

The elastic scattering vector analyzing powers for /sup 6/Li+/sup 16/O at 25.7 MeV and /sup 6/Li+/sup 12/C at 30 MeV have been measured. In addition, measurement of the vector analyzing power for inelastic /sup 6/Li scattering has been made for the 2/sup +/ (4.44 MeV) state in /sup 12/C. Excitation function data for /sup 6/Li+/sup 16/O elastic and inelastic scattering have also been measured at five angles for /sup 6/Li lab energies through the range 15--28 MeV in 375 keV steps. These data show the system to be resonance free at 25.7 MeV. The data have been analyzed in the coupled channels framework, employing the double folding model for the real part of the nuclear scattering potential. The coupling strengths in the coupled channels calculations were determined from inelastic scattering cross-section data directly. The elastic scattering vector analyzing powers arise from a complicated interference between channel coupling and spin-orbit contributions, whereas the /sup 12/C, 2/sup +/ inelastic vector analyzing power arises from the spin-orbit potential only. Inelastic scattering analyzing powers may allow the shape and strength of heavy-ion spin-orbit potentials to be investigated more clearly than do the elastic scattering analyzing powers. The normalization of the double-folded real potential wasmore » found to be in the range 0.85--1.00 in the coupled channel calculations, thus bringing the normalization of /sup 6/Li scattering into agreement with other projectiles.« less

Measurement and statistical model analysis of pre-fission neutron multiplicities

The average number of neutrons preceding fission ( ν pre) was measured for the compound systems 168YB, 178W, 188Pt, 192Pb, 198Pb, 200Pb, 210Po, 213At and 251Es formed by reactions induced by 16O, 18O, 19F, 28Si or 30Si projectiles with energies ( E) between 4.9 and 7.2 MeV/ A In some cases ν pre is seen to increase with increasing E above a threshold energy ( E th) whereas the statistical model indicates that it should decrease. For a given projectile, this threshold decreases with increasing fissility, becoming equal to the Coulomb barrier around A CN∼213 for 16O projectiles. Below E th the variation of ν pre with E is consistent with statistical model predictions. The deviations above E th have been attributed to dissipative effects not included in the model. Extensive statistical model and Xχ 2 analyses of the pre-fission data below E th and of fission and fusion excitation function data, previously measured, were made. The diffuseness parameters of the fusion spin-distributions agreed reasonably well with those suggested by the zero-point motion model. The ratios of level densities at saddle and equilibrium deformations ( a f / a v ) were found to be consistent with a value of unity, and the fission barriers ( E f) consistent with the predictions of the finite-range rotating liquid-drop model. However these values for a f/ a ν and E f may not represent the true values. Inclusion of dissipation requires higher values, whilst inclusion of the temperature dependence of E f in statistical model calculations is shown to result in a reduction in the value of a f / a ν . Since reliable theoretical calculations are unavailable for either effect the consistency of the data with the finite range fission barriers can only be demonstrated to within 10–15% and values for a f / a ν , have an uncertainty of at least 5%.