Activation cross sections for some isotopes of Zr,Mo, Br,Ta and Ce at 14.7 MeV neutrons

The (n,p), (n, ), (n, 2n) and (n, ) reaction cross sections on^90,92Zr,^92,97,98,100Mo,^78,81Br,¹⁸¹Ta,¹⁴⁰Ce target nuclei have been measured forE _n=14.73±0.05 MeV neutrons using the activation technique. The induced -ray activities of the irradiated samples and their monitor foils were measured by using HPGe detector. The cross sections are compared with the existing results. The experimental uncertainties are between 3% and 10% in most cases.     

Activation cross sections and isomeric cross section ratios for 184Os(n,2n)183m,gOs, 190Os(n,p)190m,gRe and 86Sr(n,2n)85m,gSr reactions from 13.5 to 14.8 MeV

The cross sections for the ¹⁸⁴Os(n,2n)^183m,gOs, ¹⁹⁰Os(n,p)^190m,gRe and ⁸⁶Sr(n,2n)^85m,gSr reactions and their isomeric cross section ratios σ _m /σ _g have been measured in the neutron energy range of 13.5–14.8 MeV using the activation technique. Nuclear model calculations using the code HFTT, which employs the Hauser-Feshbach (statistical model) and exciton model (precompound effects) formalisms, were undertaken to describe the formation of the products. The total cross sections for ¹⁸⁴Os(n,2n)¹⁸³Os, ¹⁹⁰Os(n,p)¹⁹⁰Re and ⁸⁶Sr(n,2n)⁸⁵Sr reactions are compared with the experimental data found in the literature, with results of published empirical formulae, with values of model calculations including the pre-equilibrium contribution, and with the evaluation data of JEFF-3.1/A or ENDF/B-VII.     

Determination of the prompt k<Subscript>0,H</Subscript> factors and partial γ-ray production cross sections for B,N, Si,P, S and Cl

Summary The prompt k_0,H factors for B, N, Si, P, S and Cl were determined relative to the 2223.25 keV gamma-line from the ¹H(n,γ) reaction. The measurements were performed at the SNU-KAERI Prompt Gamma Activation Analysis facility, of which the background was greatly reduced recently by upgrading the detection system to a Compton suppressed g-ray spectrometer with a BGO/NaI(Tl) guard detector and by improving the shield geometry. From the measured prompt k_0,H factors, the partial γ-ray production cross sections were determined using the latest cross section for H. The measured prompt k_0,H factors were tabulated and compared with the other reported data.     

Mass Distribution in 12C Induced Fission of 232Th

Formation cross sections of several fission products have been determined using recoil catcher technique followed by gamma-ray spectrometry in ¹²C induced fission of ²³²Th at E _lab = 72 MeV, corresponding to E _cm just above the Coulomb barrier. The measured formation cross sections were used to get the mass distribution by using known charge distribution systematic. Critical data analysis was carried out to look for the signatures of transfer induced fission. However, within the experimental uncertainty of about 10%, no clear indication of transfer induced fission could be seen at this energy level. The mass distribution shows a single peaked broad Gaussian distribution with the most probable mass of 119.5±1.1 and FWHM of 40.6 mass units. The total fission cross section computed from the mass distribution curve is 771±50 mb.     

The updated NAA nuclear data library derived from the Y2K k0-database

Values of 2200 m^.s^-1 cross sections, together with the associated nuclear data, are tabulated for 128 (n,g) reactions of interest in NAA. The values are derived from the Y2K database of experimentally measured k ₀-factors.     

Activation cross section measurements of 16O(n,t) above 18.1?MeV up to 33.1?MeV

We have measured the cross sections of the ¹⁶O(n,t) reactions above 18.1 up to 33.1 MeV in an neutron activation method. H₂O (water) as an ¹⁶O target was irradiated with semi-monoenergetic neutrons generated from the ⁹Be(p,n)⁹B reaction with 25–35 MeV protons. The neutron flux was obtained with the aid of previous study by Uwamino et al. (Nucl Instr Methods A 271:546, 1988). The tritium activities were measured by using the liquid scintillation counting (LSC) method. The present value for the cross section of ¹⁶O(n,t) reaction agrees with previous values measured by using the same LSC method at similar neutron energy ranging from 18.1 up to 33.1 MeV.     

Dissociative Excitation of Lead Atom 6pnd Levels in Electron Collisions with Lead Dichloride Molecules

Dissociation excitation of the 6pnd levels of the lead atom in e–PbCl₂ collisions resulting in the population of metastable PbI levels via cascade transitions was experimentally studied. Cross sections for the cascade population of the 6p2 levels at an incident electron energy of 100 eV were determined. The dissociative excitation cross sections measured were compared with the excitation cross sections in e–Pb collisions.     

Complete thermal and epithermal neutron self-shielding corrections for NAA using a spreadsheet

An analytical expression has been developed to calculate the neutron self-shielding in a cylindrical sample using the elemental thermal neutron absorption cross sections, σ _abs , and the newly-defined epithermal neutron absorption cross-sections, σ _abs,ep . The σ _abs,ep were measured experimentally for 13 nuclides and calculated from resonance parameters for 76 nuclides. Agreement between the two was good to about 20% in most cases. A spreadsheet program was written to use these nuclear parameters to perform iterative self-shielding corrections of concentrations measured by NAA. In cases with up to 30% self-shielding, the correction factors had uncertainties varying from 2% to 3%.     

Fission neutron spectrum averaged cross sections for threshold reactions on arsenic

Summary We have measured the cross sections, averaged over a ²³⁵U fission neutron spectrum, for the two high threshold reactions: ⁷⁵As(n,p)^75mGe and ⁷⁵As(n,2n)⁷⁴As. The measured averaged cross sections are 0.292±0.022 mb, referred to the 3.95±0.20 mb standard for the ²⁷Al(n,p)²⁷Mg averaged cross section, and 0.371±0.032 mb referred to the 111±3 mb standard for the ⁵⁸Ni(n,p)^58m+gCo averaged cross section, respectively. The measured averaged cross sections were also evaluated semi-empirically by numerically integrating experimental differential cross section data extracted for both reactions from the current literature. The calculations were performed for four different representations of the thermal-neutron-induced ²³⁵U fission neutron spectrum. The calculated cross sections, though depending on analytical representation of the flux, agree with the measured values within the estimated uncertainties.     

Semiempirical method for extracting electron molecule cross sections from experimental data: CF4 as an example

A semiempirical method is described for calculating electron-molecule interaction potential, electron density, and elastic and vibrational inelastic cross sections using experimental data.CF ₄ was considered as un example. Experimental data from IR and Raman spectra, molecular property calculations (dipole matrix elements, polarizability, distance between atoms in molecule), beam measurements of total, momentum transfer, and differential cross sections, and swarm data (diffusion and drift veolcity) are used for extracting the interaction potential, the molecule electron density, avid the differential cross sections. Electron energy distribution functions, drift velocity, and characteristic energy are calculated with the obtained differential e-CF ₄ cross sections lierAr/CF ₄ mixtures.     

Differential Cross Sections for the F + H2 Reaction: A Comparison of Classical Trajectory Results for Two Potential Energy Surfaces

Classical trajectories were calculated for the F + H₂ reaction over two potential energy surfaces: (1) the well known Muckerman 5 surface (Theoretical Chemistry: Advances and Perspectives 1981 , 6A, 1) and (2) the recent surface of Stark and Werner (J. Chem. Phys. 1996 , 104, 6515). Integral cross sections, state specified cross sections, differential cross sections and product energy distributions were calculated for the two surfaces. Since the methods for calculating the trajectories and expressing differential cross sections were identical for both surfaces, the rather substantial differences in the results are clearly due to differences in the potential surfaces. The results are discussed in terms of the special characteristics of the two surfaces.     

Determination of absolute photoionization cross‐sections of alkanes and cyclo‐alkanes

Absolute photoionization and dissociative photoionization cross‐sections of eleven n‐alkanes (n‐pentane, n‐hexane, n‐heptane, n‐nonane, n‐decane, n‐undecane, n‐dodecane, n‐tridecane, n‐tetradecane, n‐pentadecane and n‐hexadecane), three cyclo‐alkanes (cyclopentane, methylcyclohexane and trans‐decahydronaphthalene) and iso‐octane were measured for photon energies from the ionization thresholds to 11.5 eV. The measurements were performed with the binary‐liquid‐mixture method utilizing the photoionization cross‐sections of benzene as a calibration standard. The ionization energies of n‐alkanes and cyclo‐alkanes were also calculated at the B3P86/6‐31 + +G(d,p) level and by the G3B3 method. Copyright © 2010 John Wiley & Sons, Ltd.     

L X-ray production cross sections, average L shell fluorescence yield and intensity ratios in heavy elements

L X-ray fluorescence cross sections, and intensity ratios were measured for elements in the 70￡Z￡92 atomic range at the excitation energy 59.5 keV using a Si(Li) detector. Furthermore, L X-ray fluorescence cross sections and intensity ratios were calculated for elements in the same range. The average L shell fluorescence yields were derived using experimental L X-ray fluorescence cross sections and theoretical photoionization cross sections. The obtained results were compared with other experimental and theoretical values. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measurement of 14.7 MeV neutron cross sections for several isotopes

The (n,p), (n,), (n,2n), (n,np), and (n,n) reaction cross sections on²⁴Mg, 2²⁷Al,^28,29,30Si,^35,37Cl,⁴⁵Sc,⁴⁶Ti,⁵⁶Fe,^58,61Ni,⁹²Mo,¹⁰⁷Ag,^113,115In and¹⁹⁷Au target nuclei have been measured for E_n=14.73±0.05 MeV neutrons using the activation technique. The induced -ray activities of the irradiated samples and their monitor foils were measured using a HPGe detector. The cross sections are compared with existing results. The experimental uncertainties between 3% and 10% in most cases are generally smaller than those given by other authors.     

Highly Efficient Multiphoton‐Pumped Frequency‐Upconversion Stimulated Blue Emission with Ultralow Threshold from Highly Extended Ladder‐Type Oligo(p‐phenylene)s

A series of highly extended π‐conjugated ladder‐type oligo(p‐phenylene)s containing up to 10 phenyl rings with (L)‐Ph(n)‐NPh (n=7–10) or without diphenylamino endcaps (L)‐Ph(n) (n=7 and 8) were synthesized and investigated for their multiphoton absorption properties for frequency upconverted blue ASE/lasing. Extremely large two‐photon absorption (2PA) cross‐sections and highly efficient 2PA ASE/lasing with ultralow threshold were achieved. (L)‐Ph(10)‐NPh exhibits the highest intrinsic 2PA cross‐section of 3643 GM for a blue emissive organic fluorophore reported so far. The record‐high 2PA pumped ASE/lasing efficiency of 2.06 % was obtained by un‐endcapped oligomer, (L)‐Ph(8) rather than that with larger σ₂, suggesting that a molecule with larger σ₂ is not guaranteed to exhibit higher η₂. All of these oligomers exhibit exceptionally ultralow 2PA pumped ASE/lasing thresholds, among which the lowest 2PA pumped threshold of circa 0.26 μJ was achieved by (L)‐Ph(10)‐NPh.     

Electron scattering differential cross sections for C2H2, C2H4, and C2H6 by gas phase electron diffraction - binding effects

Experimental differential cross sections for 40 keV electrons scattered by C₂H₂, C₂H₄ and C₂H₆ molecules were measured using the gas electron diffraction method in the range of the scattering variable s from s = 1 A^?1 to s = 30 A^?1. The differential cross sections for neon were also measured and compared with calculated differential cross sections to calibrate the diffractograph. Experimental differential cross sections show significant deviations with respect to theoretical differential cross sections calculated from the Debye-Ehrenfest model, mainly in the range of small scattering angles. The observed differences are connected to chemical binding effects. From the experimental data, an estimation of the binding energy was carried out. The deduced values: ?0.58 ± 0.20 au for C₂H₂, ?0.94 ± 0.30 au for C₂H₄ and ?1.23 ± 0.40 au for C₂H₆ are in agreement with those obtained by thermochemical methods.     

Theoretical study of positron scattering by group 14 tetra hydrides: A quantum mechanical approach

The present work provides a comprehensive set of positron impact scattering cross sections for group 14 tetrahydrides, namely, SiH₄, GeH₄, SnH₄, and PbH₄. The well‐established spherical complex optical potential and complex scattering potential‐ionization contribution methods are modified to incorporate positron scattering in the present work to calculate various cross sections. The positronium formation channel is adequately included through an improved inelastic threshold. The energy range chosen for the direct ionization cross section is from the respective ionization potential (I) of the molecule to 5 keV. Likewise, positronium formation and total ionization cross sections are reported from the positronium formation threshold to 300 eV and 5 keV, respectively, and the total cross section is computed for an extensive energy range of 1 eV to 5 keV. The positron impact total cross section for stannane molecule is computed for the first time. A characteristic valley is observed in the total cross sections with minima close to the positronium formation threshold. Further increase of cross section signifies the opening of inelastic channels especially positronium formation. In general, a reasonable agreement is found between the present results and other comparisons, wherever available. Furthermore, this is the first report of the inelastic cross sections (direct ionization, positronium formation, and total ionization) for the present set of targets.     

Cross sections for rotational excitations of CH4, SiH4, GeH4, SnH4 and PbH4 by electron impact

We report differential and integral cross sections for rotational excitation of XH₄ molecules (X: C, Si, Ge, Sn, Pb) from 7.5–30 eV by electron impact. These cross sections were derived from fixed-nuclei scattering amplitudes (Bettega et al. 1995) obtained using the Schwinger Multichannel Method with Pseudopotentials (SMCPP) (Bettega et al. 1993). Our results represent the first rotational excitation cross sections for molecules as large as GeH₄, SnH₄ and PbH₄ using entirely ab initio procedures. The cross sections for CH₄ and SiH₄ obtained with pseudopotentials are in very good agreement with all-electron calculations and with other theoretical results. A comparison between our calculated cross sections and experimental data for CH₄ is in general encouraging, but some discrepancies remain. We found inelastic rotational cross sections and momentum transfer cross sections to be larger for SiH₄, GeH₄, SnH₄ and PbH₄ than for CH₄. We could explain this feature.     

Dissociative Excitation of Even Doublet Levels of the Cobalt Atom in Electron Collisions with Cobalt Dichloride Molecules

The dissociative excitation of the even doublet levels of the cobalt atom upon the collisions of cobalt dichloride molecules with 100-eV monoenergetic electrons was experimentally studied. Dissociative excitation cross sections for transitions from these levels turned out to be greater than the corresponding cross sections of direct excitation in electron-atom collisions (except for the e ² P _1/2 level). The total excitation cross sections (including the contribution of cascade population) were determined for ten levels.     

Low-energy electron scattering by methane,arsine and phosphine

We present cross sections for the elastic scattering of low-energy electrons by phosphine (PH₃) and arsine (AsH₃), and for electron-impact excitation of the (1t ₂ 3sa ₁)³ T ₂ and¹ T ₂ states in methane (CH₄). These results were calculated using the Schwinger multichannel method as implemented on distributed-memory parallel computers. The PH₃ and AsH₃ cross sections show a pronounced low-energy shape resonance which may provide a pathway to dissociative attachment. The^1,3 T ₂ cross sections for CH₄ correlate fairly well with recent measurements of CH₂ production via electron-impact dissociation of methane.Contribution No. 8587