The reaction117Sn (n,n′)117mSn as a primary interference in (n, γ) neutron activation analysis

The¹¹⁶Sn (n, γ)^117mSn reaction commonly used in reactor-neutron activation analysis (RNAA) turned out to be seriously interfered by the¹¹⁷Sn (n, n′)^117mSn reaction, as observed from irradiation in channels with largely different neutron thermalization. To estimate the magnitude of this primary interference an attempt was made to determine the relevant fission neutron averaged cross-section, yielding approximately σ_{n, n}, (¹¹⁷Sn)==0.09±0.01 barn. This value—believed to be the first measured and published—is remarkably high especially when compared to the 2200 m·s^?1 cross-section σ_o[¹¹⁶Sn(n, γ)^117mSn]=0.006 barn.     

Determination of the 54Fe(n, 2n)53gFe and 54Fe(n, 2n)53mFe cross sections averaged over a 235U fission neutron spectrum

The reaction cross sections averaged over a ²³⁵U fission neutron spectrum have been measured for the ⁵⁴Fe(n, 2n)^53gFe and ⁵⁴Fe(n, 2n)^53mFe threshold reactions. The values found are, respectively: (1.14±0.13) mb, and (0.52±0.16) mb. The measured cross sections are referred to the (111±3) mb standard cross section of the ⁵⁸Ni(n, p)^58m+gCo reaction. The (81.7±2.2) mb standard cross section value for the ⁵⁴Fe(n, p)⁵⁴Mn reaction, was also used as a monitor to check the results obtained with the Ni standard, leading to an excellent agreement.     

Cross-sections of (n,p) reaction at a neutron energy of 14–15 MeV

The (n,p) reaction cross-sections induced by 14–15 MeV neutrons have been analyzed on the basis of available data taken from the literature, using the multiple regression technique. An empirical formula describing the dependence of (n,p) reaction cross-sections on 5 parameters: neutron (N) and proton (Z) number, asymmetry factor ((N–Z)/A) where A=N+Z, neutron and proton binding energies (B_n, B_p) has been found. Its validity is checked and discussed.     

Determination of70Ge(n,p)70Ga and74Ge(n,p)74Ga reaction cross sections for a fission neutron spectrum

The fission neutron spectrum averaged cross-sections for the reactions⁷⁰Ge(n,p)⁷⁰Ga and⁷⁴Ge(n,p)⁷⁴Ga have been determined. The averages of four determinations are, respectively, (3.10±0.30) mb and (0.00938±0.00059) mb. The present values are, to the authors' knowledge, the first experimental data on the fission averaged cross-section for these reactions.     

Activation cross-sections for some (n, 2n), (n,p) and (n, γ)-reactions

The average fission neutron cross-sections of the reactions²³³U(n, 2n)²³²U,⁶⁰Ni(n, p)⁶⁰Co and²⁷Al(n, p)²⁷Mg and the resonance integrals of the (n, γ)-reactions of the nuclides¹⁸¹Ta,¹⁷⁶Lu,¹⁷⁵Lu,⁶⁴Ni,⁵⁹Co and²⁶Mg have been determined by the activation method following the well-known conventions. The results verify some of the existing values and present data for hitherto unknown or poorly known reactions.     

Effect of temperature on the number of active sites and propagation rate constant at ethylene polymerization over supported bis(imino)pyridine iron catalysts

The inhibition of ethylene polymerization with radioactive carbon monoxide (¹⁴CO) was used to obtain data on the number of active sites (C_P) and propagation rate constant (k_P) at ethylene polymerization in the temperature range of 35–70 °C over supported catalysts LFeCl₂/Al₂O₃, LFeCl₂/SiO₂, and LFeCl₂/MgCl₂ (L: 2,6‐(2,6‐(Me)₂C₆H₃N = CMe)₂C₅H₃N) with activator Al(i‐Bu)₃. The values of effective activation energy (E_eff), activation energy of propagation reaction (E_P), and temperature coefficients of variation of the number of active sites (E_Cp = E_eff ? E_P) were determined. The activation energies of propagation reaction for catalysts LFeCl₂/Al₂O₃, LFeCl₂/SiO₂, and LFeCl₂/MgCl₂ were found to be quite similar (5.2–5.7 kcal/mol). The number of active sites diminished considerably as the polymerization temperature decreased, the E_Cp value being 5.2–6.2 kcal/mol for these catalysts at polymerization in the presence of hydrogen. The reactions of reversible transformations of active centers to the surface hydride species at polymerization in the presence and absence of hydrogen are proposed as the derivation of E_Cp. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6621–6629, 2008     

Investigation of fast neutron shielding characteristics depending on boron percentages of MgB<Subscript>2</Subscript>, NaBH<Subscript>4</Subscript> and KBH<Subscript>4</Subscript>

The macroscopic cross-section Σ and average neutron fluence in matter Φ are usable factors to comment neutron shielding property of samples. In this paper, we have used MgB₂, NaBH₄ and KBH₄ samples including different percentages of boron. Neutron macroscopic cross-section measurements of them have been done by using a source of mono-energetic neutrons (E _eff = 4.5 MeV ²⁴¹Am–Be). Average neutron fluence values and double differential fast neutron flux distributions of each samples calculated by using FLUKA Monte Carlo code. Also half value layers (HVLs) of samples are compared to paraffin which is one of the most neutron moderators. As a result, growing boron concentration can raise neutron shielding property of materials.     

Temperature‐Dependent Electron Shuffle in Molecular Group 13/15 Intermetallic Complexes

Monovalent RAl (R=HC[C(Me)N(2,6‐iPr₂C₆H₃)]₂) reacts with E₂Et₄ (E=Sb, Bi) with insertion into the weak E? E bond and subsequent formation of RAl(EEt₂)₂ (E=Sb 1 ; Bi 2 ). The analogous reactions of RGa with E₂Et₄ yield a temperature‐dependent equilibrium between RGa(EEt₂)₂ (E=Sb 3 ; Bi 4 ) and the starting reagents. RIn does not interact with Sb₂Et₄ under various reaction conditions, but formation of RIn(BiEt₂)₂ ( 5 ) was observed in the reaction with Bi₂Et₄ at low temperature.     

Cross-sections for (n,2n), and (n,α) reactions on 55Mn isotope around neutron energy of 14 MeV

Cross sections for (n,2n) and (n,α) reactions on the Manganese isotope have been measured at the neutron energies of 14.1 and 14.7 MeV using activation method with the monitor reaction ²⁷Al(n, α)²⁴Na. This measurement was carried out by γ-detection using a coaxial HPGe detector. High-purity natural Manganese powder (99.9%) was used to produce the samples. The fast neutrons were produced by T(d, n)⁴He reaction. The results obtained are compared with the previous data.     

The determination of copper in zinc materials by neutron activation analysis using the 65Cu(n,γ)66Cu reaction

The nondestructive and destructive determinations of copper in zinc materials by neutron activation using ⁶⁵Cu(n,γ)⁶⁶Cu reaction are described. Observed cross-sections of ⁶⁴Zn(n,p)⁶⁴Cu and ⁶⁶Zn(n,p)⁶⁶Cu reactions for fission spectrum neutrons were 28 and 0.51 mb respectively. A double irradiation technique with one sample wrapped in cadmium foil and another bare, was employed for the correction of the concurrent formation of ⁶⁶Cu by (n,p) reaction. 100 p.p.m. of copper in zinc sulfide was determined nondestructively by γ-ray spectrometry using a thermal flux of 5 · 10¹¹ n/cm²/sec, while 0.5 p.p.m. of copper in zinc was determined by a thermal flux of 2.5 · 10¹⁰ n/cm²/sec, and by a rapid chemical separation involving copper(I) thiocyanate precipitation. The proposed activation procedures were compared with the spectrophotometric dibenzyldithiocarbamate procedure.     

Use of k0 concept to corrections for interferences from fissionable nuclides in reactor NAA

A neutron spectrum-independent compound nuclear constant, Ik_o, is proposed for fission interference corrections in reactor NAA by parametric method. Ik₀ values for eight major fission interference cases, belonging to three different types, have been determined in six irradiation positions (with _th/ _e 13,7–134) of three research reactors in our Institute. The general agreement among experimental Ik₀ 's from different irradiation positions and between experimental and calculated Ik₀ values for the same interference case verifies the validity of the method. Calculated Ik₀ values for all the possible fission interferences are tabulated. Interferences from²³⁸U(n,) and²³²Th(n,) reactions and reactor fast neutron induced²³⁸U(n, f) and²³²Th(n, f) reactions are discussed.     

Neue Rheniumkomplexe mit Trichalkogenido- und Tetrachalkogenido-Chelatliganden

New Rhenium Complexes Containing Trichalcogenido and Tetrachalcogenido Chelate Ligands The reactions of Cp*ReCl₄ with polychalcogenide salts such as Na₂S₄ or (NEt₄)₂Se₆ lead initially to the violet trichalcogenido chelate complexes Cp*ReCl₂(E₃) (E = S ( 3a ), Se ( 3b )) which, due to their functional chloro ligands, can be used as intermediates for further reactions. Upon hydrolysis in moist solvents or aminolysis with tert. butylamine 3a, b are converted into the tetrachalcogenido chelate complexes Cp*Re(O)(E₄) (E = S ( 4a ), Se ( 4b )) and Cp*Re(N^tBu)(E₄) (E = S ( 5a ), Se ( 5b )), respectively. X-Ray structure analyses were carried out for the three mononuclear cyclo-oligoselenido compounds 3b–5b . It appears that the size of the Se^2?_n chelate ring (n = 3 or 4) essentially depends on steric factors within the coordination sphere of rhenium.     

Factors determining the reactivity of the bromine atom toward haloalkanes

Experimental data concerning reactions of the bromine atoms with haloalkanes and carbonyl compounds (25 reactions) have been analyzed within the intersecting parabolas model. The following factors have an effect on the activation energy of these reactions: enthalpy of reaction, triplet repulsion, electronegativity of reaction center atoms, dipole–dipole and multidipole interactions of the reaction center with polar groups, and the interaction of π electrons with electrons of the reaction center. The increments characterizing the contribution from each factor to the activation energy of the reaction have been calculated. The increment ΔEμ, which characterizes the dipole–dipole interaction in the transition state, and the dipole moment of the polar group (μ) are correlated by the following empirical equation: ln(ΔE _μ/Σμ) = ?0.14 + 0.47(ΔE _μ/Σμ) ? 0.024(ΔE _μ/Σμ)².     

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.     

Cross section measurements for (n, p) and (n, 2n) reactions of rare-earth isotopes at neutron energies from 13.5 to 14.6 MeV

Cross sections for (n, p) and (n, 2n) reactions have been measured on some rare-earth isotopes at neutron energies of 13.5-14.6 MeV using activation technique. Data are reported for the following reactions:¹⁵⁰Nd(n, 2n) ¹⁴⁹Nd, ¹⁴⁸Nd(n, 2n) ¹⁴⁷Nd, ¹⁴²Nd(n, 2n) ¹⁴¹Nd, ¹⁶⁰Gd(n, 2n) ¹⁵⁹Gd, ¹⁵⁸Gd(n, p) ¹⁵⁸Eu, ¹⁴⁶Nd(n, p) ¹⁴⁶Pr, ¹⁴¹Pr(n, p) ¹⁴¹Ce and ¹³⁹La(n, p) ¹³⁹Ba. The neutron fluences are determined by the cross sections of ²⁷Al(n, a) ²⁴Na and ⁹³Nb(n, 2n) ^92mNb reactions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Charge degree of freedom as a sensitive probe for fission mechanism

The role of the charge degree of freedom in the heavy-ion-induced fission was investigated by carrying out a systematic analysis of radiochemically observed charge distribution in the fission of²³⁸U with¹²C ions of the incident energy between 85 and 140 MeV, particularly in connection with the energy given to the compound system. The charge distribution was found to follow essentially identical systematics as those which govern the light-ion fission except for the extremely weak energy dependence of the most probable chargeZ _p. That is, values of the derivative ofZ _p with respect to the energy were found to be quite small, or nearly zero, in the heavy-ion fission as compared to those of the light-ion fission. According to an analysis combining the derivatives ofZ _p and fission neutron data, it was deduced that the excess energy given to the fused system was spent completely in the form of pre-scission neutrons and hence the number of post-scission neutrons remained constant as in the case of light-ion fission. The observed charge distribution was reproduced under the conditions that the relaxation of the charge degree of freedom be very fast and that the separation between the two potential fragments at the moment when the charge degree of freedom has been frozen is determined by usingViola's systematics on the fragment kinetic energy.     

DFT Rationalization of the Diverse Outcomes of the Iodine(III)‐Mediated Oxidative Amination of Alkenes

A computational study of the mechanism for the iodine(III)‐mediated oxidative amination of alkenes explains the experimentally observed substrate dependence on product distribution. Calculations with the M06 functional have been carried out on the reaction between PhI(N(SO₂Me)₂)₂ and three different representative substrates: styrene, α‐methylstyrene, and (E)‐methylstilbene. All reactions start with electrophilic attack by a cationic PhI(N(SO₂Me)₂)⁺ unit on the double bond, and formation of an intermediate with a single C?I bond and a planar sp² carbocationic center. The major path, leading to 1,2‐diamination, proceeds through a mechanism in which the bissulfonimide initially adds to the alkene through an oxygen atom of one sulfonyl group. This behavior is now corroborated by experimental evidence. An alternative path, leading to an allylic amination product, takes place through deprotonation at an allylic C?H position in the common intermediate. The regioselectivity of this amination depends on the availability of the resonant structures of an alternate carbocationic intermediate. Only in cases where a high electronic delocalization is possible, as in (E)‐methylstilbene, does the allylic amination occur without migration of the double bond.     

Determination of reactor neutron spectra with multicomponent activation detectors

A procedure is described for the functional fitting of reactor neutron spectra with C/Eⁿ and fission spectra. The method is applied to multicomponent activation detector measurements in a Triga research reactor. In multicomponent detectors a mixture of several detector materials is irradiated as a single unit and measured simultaneously for all reaction products with a Ge(Li) gamma ray spectrometer.     

New Heptalenes Substituted with Extended π-Systems

The synthesis of π-substituted heptalenecarboxylates or -dicarboxylates, starting with the easily available dimethyl 9-isopropyl-1, 6-dimethylheptalene-4, 5-dicarboxylate ( 2b ), are described. Treatment of 2b with t-BuOK and C₂Cl₆ at ?78° leads to the chemoselective introduction of a Cl substituent in Me-C(1) (see 5b in Scheme 1). Formation of the corresponding triphenylphosphonium salt 7b via the iodide 6b (Scheme 2) allowed a Wittig reaction with cinnamaldehyde in the two-phase system CH₂Cl₂/2N NaOH. Transformation of the 4, 5-dicar-boxylate of 2b into the corresponding pseudo-ester 10b allowed the selective reduction of the carbonyl function at C(4) with DIBAH to yield the corresponding 4-carbaldehyde 11b (Scheme 3). Wittig reaction of 11b with (benzyl) triphenylphosphonium bromide led to the introduction of the 4-phenylbuta-1, 3-dienyl substituent at C(4). The combination of both Wittig reactions led to the synthesis of the 1, 4-bis(4-phenylbuta-1, 3-dienyl)-substituted heptalene-5-carboxylate (all-E)- 17b (Scheme 5). In a similar manner, by applying a Horner-Wadsworth-Emmons reaction, followed by the Wittig reaction, the donor-acceptor substituted heptalene-5-carboxylate (E;E)- 22b was synthesized (Scheme7). Most of these new heptalenes are in solution, at room temperature, in thermal equilibrium with their double-bond shifted (DBS) isomers. In the case of (all-E)- 17b and (E;E)- 22b , irradiation of the thermal equilibrium mixture with light of λ -(439 ± 10) nm led to a strong preponderance ( > 90%) of the DBS isomers 17a and (E;E)- 22a , respectively (Schemes 6 and 7). Heating of the photo-mixtures at 40° re-established quickly the thermal equilibrium mixtures. Heptalenes-carboxylates (all-E)- 17a and (E;E)- 22a which represent the off-state of a 1,4-conjugative switch (CS) system show typical heptalene UV/VIS spectra with a bathochromically shifted heptalene band III and comparably weak heptalene bands II and I which appear only as shoulders (Figs. 4 and 5). In contrast, the DBS isomers (all-E)- 17b and (all-E)- 22b , equivalent to the on-state of a 1,4-CS system, exhibit extremely intense heptalene bands I and, possibly, II which appear as a broad absorption band at 440 and 445 nm, respectively, thus indicating that the CSs (all-E)- 17a ?(all-E)- 17b and (E;E)- 22a ?(E;E)- 22b are perfectly working.     

Herstellung von fluorreichen Verbindungen der Reihe C2ClnF6−n in heterogener Katalyse

Preparation of Compounds of the Series C₂Cl_nF_6?n with High Fluor Contents by Heterogeneous Catalysis A survey is given on catalytic systems for Cl? F exchange reactions with C₂Cl₆. A catalyst is described which is formed by reaction of C₂Cl₄/Cl₂/HF on γ-Al₂O₃ in Ni reactors. Deposition of nickel proceeds by the reaction Ni(CO)₄ → Ni + 4 CO. The formation of the catalyst and the catalytic reactions which give highly fluorinated C? Cl? F compounds are discussed.