Neutron resonance capture analysis: Improvements for resonances above 3 keV and new applications

In neutron resonance capture analysis (NRCA), the detection of resonances in the neutron cross section as a function of neutron energy with the time of flight (TOF) technique is used to identify and quantify isotopes. In this paper the improvements of the measuring conditions are discussed in order to reduce the high background, especially in the keV range. The detection efficiency was generally improved, especially for resonances above 3 keV, which allowed the detection of lighter elements, usually having resonances only in the keV region. This broadened the number of applications that could be targeted with NRCA.     

Non-destructive bulk analysis of the Buggenum sword by neutron resonance capture analysis and neutron diffraction

Two neutron based techniques, neutron resonance capture analysis (NRCA) and time-of-flight neutron-diffraction (TOF-ND) have been used to determine the elemental composition and structure of a precious and very well preserved all-metal sword from the Bronze Age. This Buggenum sword was on loan from the National Museum of Antiquities (NMA) in Leiden (NL). NRCA and TOF-ND experiments have been carried out at a number of more or less identical positions of the sword. The tin-bronze ratio and the relative amounts of some minor elements (Sb, As, Ag, In) have been determined. The results of neutron diffraction measurements showed considerable tin-segregation, and clear indications of hardening on the edges of the blade. In addition, radiographs using Bremsstrahlung revealed the construction of the hilt–blade connection. The work was carried out at the EC Joint Research Centre IRMM in Geel (B) and at the ISIS facility of the Rutherford Appleton Laboratory (UK).     

Characterization of 235U targets for the development of a secondary neutron fluence standard

The MetroFission project, a Joint Research Project within the European Metrology Research Program, aims at addressing a number of metrological problems involved in the design of proposed Generation IV nuclear reactors. As part of this project a secondary neutron fluence standard is being developed and tested at the neutron time-of-flight facility GELINA of the JRCs Institute for Reference Materials and Measurements. Such a secondary standard will help to arrive at the neutron cross section measurement uncertainties required for the design and safety assessment of new generation power plants and fuel cycles. Such a neutron fluence device contains targets for which the neutron induced cross section is considered to be a standard. A careful preparation and characterization of these samples is an essential part of the development of the secondary standard. In this framework a set of ²³⁵U targets has been produced by vapour deposition of UF₄ on aluminium backings by IRMMs target preparation laboratory. These targets have been characterized for both their total mass and mass distribution over the sample area.     

Improving neutron activation analysis accuracy for the measurement of gold in the characterization of heterogeneous catalysts using a TRIGA reactor

To measure the gold content of a catalyst accurately, neutron activation analysis (NAA) is one of the methods of choice. NAA is preferred for such heterogeneous catalysts because: (1) it requires minimal sample preparation; (2) NAA provides consistent and accurate results; and (3) in most cases results are obtained much quicker than competing methods. NAA is also used as a referee for the other elemental techniques when results do not fall within expected statistical uncertainties. However, at very high gold concentrations, applying NAA to determine the gold in a heterogeneous catalyst is more challenging than a routine NAA procedure. On the one hand, the neutron absorption cross section for gold is very high, resulting in significant self-shielding related errors. On the other hand, gold exhibits low energy resonance neutron absorptions. In this application the self-shielding minimization effort was handled more rigorously than the classic suppression of neutron flux within a specimen. This non-routine approach was used because: (1) for most applications, high accuracy, <3 % relative, is desired, (2) the low energy resonances of gold make its neutron reaction rate complex and (3) the TRIGA reactor flux profile used in this study contains both thermal and significant epithermal neutron fluxes. Accuracy and precision, using this new approach, are expected to improve from 15 % to better than 3 % relative uncertainty. This has been accomplished through a rigorous assessment of the observed effects of low energy resonance on the neutron flux spectral shape within the sample and designing an experiment to minimize the effects.     

Determination of neutron temperature in irradiation channels of reactor

The neutron temperature is a characteristic parameter in irradiation channels of reactor. For nuclides which have resonances in the thermal neutron energy range, their Westcott g-factors are different from unity. The values of g-factors and cross-sections of (n, γ) reaction of these nuclides are temperature dependence. The standard energy for tabulation of thermal neutron cross-section (σ₀) is that of room temperature (293.59 K or 20.43 °C), corresponding to a neutron energy 0.0253 eV or to a neutron velocity of 2200 m/s. However, in the irradiation channels of reactor, the temperature is not exact at 20.43 °C. Thus, the temperature at the irradiation position must be known to convert σ₀ to σ(T). A method for determination of the neutron temperature in irradiation channels of Dalat reactor is presented by fitting the thermal neutron spectrum obtained from the calculation using MCNP code.     

Evaluation of nuclear effects in the analysis of plastics by neutron activation analysis

Validation of a method for the determination of trace elements in plastic using neutron activation analysis (NAA) required evaluation of nuclear effects. Since plastic is a moderator of neutrons, it is possible that the neutron flux may be affected in such a way as to effectively increase or decrease the measured activity of radionuclides within the plastic. Such effects are likely to depend on the thickness of the sample and so thicknesses of plastic ranging from 1 to 20 mm were used to quantify the effects. A standard containing vanaldium (a thermal neutron absorber) and silver (with epithermal neutron resonances) was used in a sandwich of the plastic. The sandwich was then irradiated and the standard counted. The activity of each radionuclide for each thickness was evaluated. The results showed that a variation of only 10% maximum was observed for thicknesses up to 20 mm. An increase in measured activity was observed for both the thermal and epithermal absorbers.     

两亲型杯芳烃诱导ZnO微纳结构的合成及光催化性能

回流条件下将两亲型杯芳烃,即丙基间苯二酚杯[4]芳烃(PRCA)、己基间苯二酚杯[4]芳烃(HRCA)和壬基间苯二酚杯[4]芳烃(NRCA),应用于诱导制备ZnO微纳结构,采用X射线衍射、扫描电镜、紫外可见漫反射光谱、红外光谱、X射线光电子能谱等分析了所制备ZnO样品的组成、形貌与微观结构,并以罗丹明B为模型污染物考察了所制备杯芳烃保护的ZnO样品的光催化性能。表征结果显示当杯芳环下缘所连直链烷基中含较多碳原子个数时(HRCA和NRCA),可调控ZnO粒子的尺寸和形貌;而无保护剂和下缘连接较短直链烷基的间苯二酚杯[4]芳烃(PRCA)调控ZnO形貌的能力较弱。模拟太阳光照射下,HRCA-ZnO和NRCA-ZnO的光催化效率相近且均高于无保护剂制备的ZnO和PRCA-ZnO。     

Vibrational and electronic excitation in p-benzoquinone by electron impact

Vibrational and electronic excitation by electron impact in p-benzoquinone was studied using a trochoidal electron spectrometer. Two distinct patterns of vibrational excitation were observed. First, low quanta of a few selected vibrations are specifically excited at incident electron energies corresponding to shape resonances. Some resonances excite mainly the CO stretch, others the CH stretch vibration, and this selectivity is used in the discussion of the assignment of the resonances. A second pattern is an unspecific excitation of a quasi-continuum where no structure due to individual vibrational levels can be discerned. This feature peaks at threshold, large amounts of vibrational energy can be deposited in the molecule, and the excitation also proceeds via shape resonances. Electronic excitation spectra in the valence and Rydberg regions are also presented and discussed. A band observed at 4.37 eV with low residual energies has been tentatively assigned to the second π — π* triplet state ³B_3g.     

The calculation of neutron self-shielding factors of a group of isolated resonances

The resonance neutron self-shielding factor, G _res, is required in neutron metrology and activation data analysis. In a previous paper, the authors have shown that a dimensionless variable can be introduced which converts the dependence of G _res on the physical and nuclear properties of the material samples into an universal curve, valid for the isolated resonances of any nuclide. This work presents a methodology based on the universal curve, which enables to calculate G _res for a group of isolated resonances by weighting its individual contributions. A good agreement was reached with results calculated by the MCNP code and with experimental values for Mo foils and wires. This revised version was published online in July 2006 with corrections to the Cover Date.     

Erratum

A semi-empirical method for infrared, Raman and neutron inelastic scattering AH stretching band shape analysis in strong hydrogen bonded systems is proposed. The fast (n) and the slow (N) stretching modes are supposed to be adiabatically separable. In the fundamental state of the fast mode the adiabatic potential function for the slow mode is assumed to be harmonic and wavefunctions are known exactly. Profiles are assumed to be dominated by the short-time effects on the time dependent formulation for spectral intensities which are then determined by the overlap between the adiabatic wavefunction in the fundamental state and an Airy function depending on the slope of the upper potential function at the classical turning point, multiplied by the transition moment. Infrared, Raman and neutron inelastic scattering transition probabilities are derived including high order terms of the n,N electrical coupling. This method does not contradict earlier theories but it is believed to provide a more realistic band profile determination. Examples of computed spectra at low temperature are given which show that short-time effects and electrical coupling may account for typical band shapes of strong hydrogen bonded systems (ABC bands for instance) without supposing Fermi resonances. Evans-type interactions or a double minimum potential function for the proton motions.     

An alternative convention describing the (n, γ)-reaction rate suited for use in the k0-method of NAA

An alternative convention for use in the k₀-method describing the (n, )-reaction rate upon reactor neutron irradiation has been derived by dividing the cross-section in a (v)=₀v₀/v part and a pure resonance integral, instead of splitting up the neutron spectrum. It describes the (n, )-reactions with the Westcott factor g(T)1 but without resonances below 0.35 eV, and should yield better results for those with resonances below this limit. The resulting formulas are simpler than the ones currently used. An important practical aspect of this new convention is that no irradiations under Cd-cover are needed to determine the parameters to be used in the k₀-method. The parameters determined previously for (n, )-reactions with g(T)=1 can still be used.     

Resonance activation analysis of iodine

Resonance neutron activation and gamma-ray spectrometry were used to determine traces of iodine in biological materials. The method developed is purely instrumental and fairly rapid. The major interfering activities of²⁴Na,³⁸Cl,³⁶Mn and⁸²Br were significantly reduced by irradiating the samples inside a shield of Cd, NaCl, MnBr₂ and MnO₂. Neutrons with energies close to the resonances of Na, Cl, Mn and Br were absorbed in the shield and did not activate the sample while neutrons with energies close to the resonances of iodine were not absorbed appreciably. Thus the activity of¹²⁸I was enhanced relative to the interfering activities. The 442 keV gamma of¹²⁸I (T=25 min) was measured using a high-resolution Ge(Li) detector and a multichannel analyzer. Sensitivity of 0.05 ppm was obtained.     

DNA fragment conformations. I—methods for the assignment of DNA fragment proton resonances. 1H NMR spectra of dApTpGpT and dApCpApTpGpT

A general method for the assignment of DNA fragment proton resonances, especially for the sugar protons, has been presented and used to interpret the 400 MHz proton spectra of dApTpGpT and dApCpApTpGpT in neutral aqueous solution. Only fine splittings of about 3 Hz are observed in the H-2″ resonances, and the total splitting is larger for the H-2′ (≈29 Hz) than for the H-2″ (22–23 Hz) multiplets. The purine and pyrimidine resonances can be distinguished on the basis of the H-2″ and H-2″ chemical shifts. The resonances of the H-2′ and H-2″ protons (above and below the sugar plane, respectively) of dA and dG exhibit chemical shifts of 2.65—2.80 ppm, while those of dC and dT residues are located at higher fields between 1.95 and 2.40 ppm. At high temperature (≥60°C), δH-2′>YδH-2″ for the purine family, while δH-2′ « δH-2″ in the case of the pyrimidine family. Except for the terminal residue, the H-3′ resonances of dA and dG are located at lower fields compared with those of the dC and dT residues. The same is true for the H-4′ resonances. In general δA_1′>δG_1′ and in the case of self complementary duplexes the H-1′ and H-2′ chemical shift variations versus temperature are found to be larger for the dC than for the dT residues.     

Characterization of some boron minerals against neutron shielding and 12 year performance of neutron permeability

In this study, the nuclear radiation permeability properties of various boron minerals are evaluated because of their high neutron absorption and lowest transmission properties. Because of these properties boron minerals can be used at the area of neutron shielding. X-ray diffraction (XRD) analyses are done for the identification of the minerals, and then their B₂O₃ contents are determined experimentally. In addition, X-ray florescence (XRF) analyses are made for quantitative determination of calcium, iron, zinc and arsenic contents. The methods of Differential Thermal Analysis, Thermal Gravimetry (TG/DTA) and Differential Scanning Calorimeter (DSC) are used for obtaining the enthalpy and weight changes with temperature. Additionally, neutron permeability experiments are conducted. From the experimental results, the highest boron oxide content was found in clay containing colemanite. Iron, zinc and arsenic contents were not affecting the neutron shielding. The lowest permeability is provided by the kurnakovite mineral. Also it is observed that all of the minerals show an increase in their permeability in 12 years. It can be stated that boron minerals, specifically kurnakovite, is determined to yield the lowest neutron permeability value and therefore, the use of these materials for neutron shielding would be suitable.     

Design and evaluation of a TNA explosive-detection system to screen carry-on luggage

Thermal neutron analysis (TNA) technology has been used for the non-destructive detection of explosives. The system uses a relatively weak ²⁵²Cf neutron source (1.03·10⁷ n/s) and two 3"×3" NaI(Tl) detectors. The presence of explosives is confirmed via detection of the 10.83 MeV prompt gamma-ray associated with nitrogen decay. The MCNP4A code was used to simulate the neutron and gamma transport through the system. The thermal neutron flux in the activation position was measured using gold and indium foils. The measured thermal neutron flux was lower, by not more than 9.5%, than that of simulation. In this report the results of the preliminary tests on the system are described.     

Photon-stimulated desorption of F(-) ions from CF(3)Cl adsorbed on Si(111)-7x7

We report the photon-stimulated desorption of negative ions induced by direct dipolar dissociation and dissociative electron attachment. The photon-stimulated desorption of F(-) ions from CF(3)Cl physisorbed on a Si(111)-7x7 surface at 30 K in the photon energy range 12-35 eV was studied. The F(-) ion yield exhibits four resonances, at 12.8, 16.2, 19.5, and 22.3 eV, quite unlike the gas phase photodissociation cross section. The intensities of these resonances depend strongly on the CF(3)Cl coverage in a manner which varies from peak to peak. The resonances at 19.5 and 22.3 eV, which have a significant enhancement in the monolayer regime, are due to electron mediated dipolar dissociation of adsorbed CF(3)Cl molecules. The enhancement is attributed to surface electron attachment following molecular excitation. A significant enhancement in the monolayer regime has also been observed for the resonances at 12.8 and 16.2 eV. These two resonances are ascribable to a combination of electron mediated dipolar dissociation and dissociative electron attachment driven by photoelectrons generated in the neighboring molecules.     

Resonant Formation of Strand Breaks in Sensitized Oligonucleotides Induced by Low‐Energy Electrons (0.5–9 eV)

Halogenated nucleobases are used as radiosensitizers in cancer radiation therapy, enhancing the reactivity of DNA to secondary low‐energy electrons (LEEs). LEEs induce DNA strand breaks at specific energies (resonances) by dissociative electron attachment (DEA). Although halogenated nucleobases show intense DEA resonances at various electron energies in the gas phase, it is inherently difficult to investigate the influence of halogenated nucleobases on the actual DNA strand breakage over the broad range of electron energies at which DEA can take place (<12 eV). By using DNA origami nanostructures, we determined the energy dependence of the strand break cross‐section for oligonucleotides modified with 8‐bromoadenine (^8BrA). These results were evaluated against DEA measurements with isolated ^8BrA in the gas phase. Contrary to expectations, the major contribution to strand breaks is from resonances at around 7 eV while resonances at very low energy (<2 eV) have little influence on strand breaks.     

Epithermal neutron activation analysis using a boron carbide irradiation filter

The use of boron carbide as a thermal neutron filter in epithermal neutron activation (ENAA) analysis has been investigated. As compared to the use of a cadmium filter, boron provides a greater reduction of activities from elements relatively abundant in terrestrial rocks and fossil fuels, such as Na, La, Sc and Fe. These elements have excitation functions which follow the 1/v law in the 1 to 10 eV lower epithermal region. This enhances the sensitivity of ENAA for elements such as U, Th, Ba and etc. which have strong resonances in the higher epithermal region above 10 eV. In addition, a boron carbide filter has the advantages over cadmium of acquiring a relatively low level of induced activity which poses minimal radiation safety problems, when used for ENAA.     

Electron dynamics at polyacene/Au(111) interfaces

Two-photon photoemission (2PPE) spectroscopy is used to examine the excited electronic structure and dynamics at polyacene/Au(111) interfaces. Image resonances are observed in all cases (benzene, naphthalene, anthrathene, tetracene, and pentacene), as evidenced by the free-electron like dispersions in the surface plane and the dependences of these resonances on the adsorption of nonane overlayers. The binding energies and lifetimes of these resonances are similar for the five interfaces. Adsorption of nonane on top of these films pushes the electron density in the image resonance away from the metal surface, resulting in a decrease in the binding energy (-0.3 eV) and an increase in the lifetime (from <20 to approximately 110 fs). The insensitivity of the image resonances to the size of polyacene molecules and the absence of photoinduced electron transfer from the metal substrate to molecular states both suggest that the unoccupied molecular orbitals are not strongly coupled to the delocalized metal states or image potential resonances.     

Depth distribution of light elements in heavy matrices with charged-particle-induced reactions

Two different methods are described for measuring the depth distribution of carbon, nitrogen and oxygen in metals: Proton activation makes use of resonances in the excitation function of the (p, γ) reactions for depth determination, and with the aid of the different half-lives for for β⁺-decay provides information on the different constituents present. The other method makes use of the shape of neutron groups in the time-of-flight spectra from deuteron-induced reactions. Depths of some tens of microns with a resolution of fractions of a micron are studied. The detection limit is of the order of 100 ppm.