Measurement of K absorption edge energies of silver and tin targets using a weak beta source

K absorption edge energies of Ag and Sn elements have been determined by using a weak beta source. In this method, the beta particles from a ⁹⁰Sr? ⁹⁰Y beta source interact with an iron foil to produce the external bremsstrahlung (EB) photons. The spectrum of EB photons is passed through the elemental target and the spectrum of transmitted photons is recorded with a high resolution HPGe detector spectrometer coupled to 16K multichannel analyzer. The recorded transmitted EB spectrum shows a sharp decrease in intensity at the K shell binding energy of elemental target. Such a sharp decrease region, which is corrected for Kβ′₂ contribution, has been used to determine the K absorption edge energies of Ag and Sn elemental targets. The measured values have been compared with theoretical and experimental values. Copyright © 2010 John Wiley & Sons, Ltd.     

External bremsstrahlung spectra of the Sr source in some lead compounds measured using NaI detector

The spectral distributions of external bremsstrahlung (EB) excited by beta particles from a ⁹⁰Sr/⁹⁰Y source in thick target compounds PbCl₂, PbF₂, Pb(NO₃)₂ and CdO were measured using a 3.8 cm × 3.8 cm NaI(Tl) crystal. The spectra, unfolded using the Liden-Starfelt procedure, showed fairly good agreement with theory (Tseng and Pratt) at low energies and some deviation (less than 15%) at higher energies. The discrepancy between theory and experiment increases with modified atomic number of the target compound and photon energy. The application of the measured/theoretical spectrum to estimate the bremsstrahlung dose is also discussed.     

Theoretical evaluation of the probability of success of the quasi-anomalous method

Theoretical expressions for the probability of success of the quasi-anomalous method have been derived for triclinic, monoclinic and orthorhombic crystals containing a few (1, 2 or 3) heavy atoms per asymmetric unit besides a large number of light atoms. The results derived take into account data-truncation due to unobserved reflections. Using the theoretical expressions, tables of probability values for the success of the quasi-anomalous method are obtained as a function of the relevant parametersk andδ ₁ ² . Corresponding results for triclinic crystals containing many heavy atoms (i.e.P = MN and MC cases) have also been obtained. It is seen that, using suitable heavy atoms to prepare the heavy-atom derivative, probability of success as high as 0.7 could be obtained in the case of proteins containing 1000 to 1500 atoms. Contribution No. 702.     

A validity test of E = m ? c2

The comparison of mass and energy variation in a nuclear reaction allows an experimental verification of Einstein’s energy - mass equivalence principle. Mass measurements are performed in a high precision Penning trap and yield values in unified atomic mass units. The energies of emitted gamma radiation are determined via Laue-diffraction with perfect crystals. The according values of the gamma ray wave lengths are expressed in units of the crystal lattice constant. The comparison of masses and wave lengths requires a conversion factor, which represents the unified atomic mass unit within the SI unit system. The latter is given by the molar Planck constant N _A h, which itself is known via its relation to the fine structure constant. In the present paper we report on measurements carried out until 2003 with an uncertainty level of 4 ⋅ 10^-7. We discuss the main limitations of these experiments and outline the possibilities for future measurements at the 10^-8 level. Such measurements would allow a direct representation of the unified atomic mass unit in terms of a Compton frequency and are of utmost importance for a future re-definition of the kilogram mass unit.     

Measurement of the effective atomic number of some transition and rare earth compounds using the Rayleigh to Compton scattering ratio of gamma radiation

The effective atomic numbers of some transition and rare earth compounds have been determined by measuring the ratio of Rayleigh to Compton scattering signal using 59.5?keV gamma radiation from americium-241 radioactive source. The scattered gamma photons from the elements and compounds at an angle of 90° were detected using an ORTEC high-purity germanium detector coupled with 16K multi-channel analyzer. By measuring the ratio of Rayleigh to Compton scattered signal (momentum transfer 3.38?Å^?1), the effective atomic numbers of the transition and rare earth compounds have been determined and compared with theoretical values predicted by AutoZeff, power law, and direct method.     

Dependence of the Compton to Rayleigh intensity ratio on the scatterer atomic number in the range of 4(Be) to 31(Ga)

Compton to Rayleigh scattering intensity ratios (I_C/I_R) have been measured using X-rays with energy 17.44 keV for single-component materials with atomic number Z from 4 (Be) to 31 (Ga) and binary compounds of stoichiometric composition. The measurements have been performed using two optical schemes: an energy-dispersive X-ray fluorescence scheme with a molybdenum secondary target and wavelength-dispersive X-ray fluorescence one. The processing of the spectra was carried out by fitting with Pearson VII functions. For single-component and binary standards, the experimental dependence of the scattering intensity ratio on the atomic number was found to be the same. This confirms the additivity of the contribution of different atoms to the scattering. The dependence has a complex shape but is well described by the theoretical relationship for I_C/I_R with correction on the difference between Compton and Rayleigh radiation absorption coefficients. Two ranges of atomic number values are defined, in which the effective atomic number Z_eff can be determined by the calibration method using this dependence: for Z from 4 to 7 with low error of ΔZ_eff =±0.15 and for Z_eff from 10 to 18 with low error of ΔZ_eff =±0.69. A change in the shape of the Compton peak and an overestimated value of the of the Compton and Rayleigh peak intensity ratio when passing from a single-component scatterer (Al or Si) to their oxides Al₂O₃ or SiO₂, respectively, have been revealed.     

Inertial deposition of nanoparticle chain aggregates: Theory and comparison with impactor data for ultrafine atmospheric aerosols

Nanoparticle chain aggregates (NCAs) are often sized and collected using instruments that rely on inertial transport mechanisms. The instruments size segregate aggregates according to the diameter of a sphere with the same aerodynamic behavior in a mechanical force field. A new method of interpreting the aerodynamic diameter of NCAs is described. The method can be used to calculate aggregate surface area or volume. This is useful since inertial instruments are normally calibrated for spheres, and the calibrations cannot be directly used to calculate aggregate properties. A linear relationship between aggregate aerodynamic diameter and primary particle diameter based on published Monte-Carlo drag calculations is derived. The relationship shows that the aggregate aerodynamic diameter is independent of the number of primary particles that compose an aggregate, hence the aggregate mass. The analysis applies to aggregates with low fractal dimension and uniform primary particle diameter. This is often a reasonable approximation for the morphology of nanoparticles generated in high temperature gases. An analogy is the use of the sphere as an approximation for compact particles. The analysis is applied to the collection of NCAs by a low-pressure impactor. Our results indicate the low-pressure impactor collects aggregates with a known surface area per unit volume on each stage. Combustion processes often produce particles with aggregate structure. For diesel exhaust aggregates, the surface area per unit volume calculated by our method was about twice that of spheres with diameter equal to the aerodynamic diameter. Measurements of aggregates collected near a major freeway and at Los Angeles International Airport (LAX) were made for two aerodynamic cutoff diameter diameters (d _a,50), 50 and 75 nm. (Aerodynamic cutoff diameter refers to the diameter of particles collected with 50% efficiency on a low-pressure impactor stage.) Near-freeway aggregates were probably primarily a mixture of diesel and internal combustion engine emissions. Aggregates collected at LAX were most likely present as a result of aircraft emissions. In both measurements, the aggregate aerodynamic diameters calculated from the primary particle diameter were fairly close to the stage cutoff diameter. The number of primary particles per aggregate varied one order of magnitude for particles depositing on the same stage. The average aggregate surface area per unit volume was 2.41 × 10⁶ cm⁻¹ and 2.59 × 10⁶ cm⁻¹ (50 nm d _a,50) and 1.81 × 10⁶ cm⁻¹ and 1.68 × 10⁶ cm⁻¹ (75 nm d _a,50) for near-freeway and LAX measurements, respectively. These preliminary measurements are consistent with values calculated from theory.     

Experimental evaluation of effective atomic number of composite materials using back-scattering of gamma photons

A method has been presented for calculation of effective atomic number (Z_eff) of composite materials, by using back-scattering of 662?keV gamma photons obtained from a ¹³⁷Cs mono-energetic radioactive source. The present technique is a non-destructive approach, and is employed to evaluate Z_eff of different composite materials, by interacting gamma photons with semi-infinite material in a back-scattering geometry, using a 3^″?×?3^″ NaI(Tl) scintillation detector. The present work is undertaken to study the effect of target thickness on intensity distribution of gamma photons which are multiply back-scattered from targets (pure elements) and composites (mixtures of different elements). The intensity of multiply back-scattered events increases with increasing target thickness and finally saturates. The saturation thickness for multiply back-scattered events is used to assign a number (Z_eff) for multi-element materials. Response function of the 3^″?×?3^″ NaI(Tl) scintillation detector is applied on observed pulse-height distribution to include the contribution of partially absorbed photons. The reduced value of signal-to-noise ratio interprets the increase in multiply back-scattered data of a response corrected spectrum. Data obtained from Monte Carlo simulations and literature also support the present experimental results.     

Improvement of the Applicability, Efficiency, andPrecision of the Penning Trap Mass Spectrometer ISOLTRAP

With the Penning trap mass spectrometer ISOLTRAP, close to 200 nuclides have already been investigated and their masses determined with a typical relative precision of δm/m=10⁻⁷. Recently, ISOLTRAP's beam preparation system was replaced by an RFQ ion beam cooler and buncher. The principle and the characteristics of this new beam preparation system will be presented. It is planned to use ions of various carbon clusters C⁺ _n (n>1) as reference ions for mass measurements. Apart from negligible molecular binding energies, these clusters have masses that are exact multiples of the unified atomic mass unit. This will allow ISOLTRAP to carry out absolute mass measurements as well as to investigate possible mass-dependent systematic errors. The results of tests of the production, transport, and trapping of such carbon clusters will be presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photon mass attenuation coefficients,effective atomic numbers and electron densities of some thermoluminescent dosimetric compounds

Photon mass attenuation coefficients of some thermoluminescent dosimetric (TLD) compounds, such as LiF, CaCO₃, CaSO₄, CaSO₄.2H₂O, SrSO₄, CdSO₄, BaSO₄, C₄H₆BaO₄ and 3CdSO₄.8H₂O were determined at 279.2, 320.07, 514.0, 661.6, 1115.5, 1173.2 and 1332.5 keV in a well-collimated narrow beam good geometry set-up using a high resolution, hyper pure germanium detector. The attenuation coefficient data were then used to compute the effective atomic number and the electron density of TLD compounds. The interpolation of total attenuation cross-sections of photons of energyE in elements of atomic numberZ was performed using the logarithmic regression analysis of the data measured by the authors and reported earlier. The best-fit coefficients so obtained in the photon energy range of 279.2 to 320.07 keV, 514.0 to 661.6 keV and 1115.5 to 1332.5 keV by a piece-wise interpolation method were then used to find the effective atomic number and electron density of the compounds. These values are found to be in agreement with other available published values.     

The Visible Intensified Cameras for Plasma Imaging in the TJ‐II Stellarator

Visible cameras are widely used in fusion experiments for diagnosis and for machine safety issues. They are generally used to monitor the plasma emission, but are also sensible to surface Blackbody radiation and Bremsstrahlung. Fast or high speed cameras capable of operating in the 10⁵ frames per second speed range are today commercially available and offer the opportunity to plasma fusion researchers of two‐dimensional (2D) imaging of fast phenomena such as turbulence, ELMs, disruptions, dust, etc. The tracking of these fast phenomena requires short exposure times down to the μ s range and the light intensity can be often near the signal to noise ratio limit especially in low plasma emission regions such as the far SOL Additionally, when using interference filters to monitor, e.g. impurity line emission, the photon flux is strongly reduced and the emission cannot be imaged at high speed. Therefore, the use of image intensifiers that amplify the light intensity onto the camera sensor can be of great help. The present work describes the use of intensifiers in the visible fast cameras of TJ‐II stellarator. We have achieved spectroscopic plasma imaging of filtered impurity atomic line emission at short exposure times down to the 10 μ s range depending on atomic line and concentration. Additionally, plasma movies at velocities of 2x10⁵ frames per second near the camera operation limit can be recorded with exposure times well below 1 μ s with sufficient signal to noise ratio. Although an increasing degradation of the image quality appears when raising the light amplification, an effective gain of up to two orders of magnitude of the light intensity is feasible for many applications (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relation between the activation parameters of low-temperature slip and the mean-square amplitude of atomic vibrations in cubic metals

The available data on various activation parameters of low-temperature slip in 9 body-centred cubic and 5 face-centred cubic elements have been examined as a function of a single microscopic parameter, namely mean-square amplitude of atomic vibrations <u ²>, specific to the material. It is found that for a given crystal structure, the microscopic parameters of the unit activation process of yielding, e.g. the initial length of dislocation segment, the critical height of the kink-pair nucleated, the associated activation volume, the binding energy per interatomic spacing along the glide dislocation on the slip plane etc. correlate well with the mean-square amplitude of atomic vibrations <u ²> through a power regression formula.     

Z-dependence of the intensity of |Δn|=2 pionic X-rays

The intensity per stopped π^?of|Δn|=2 pionic X-rays are observed to have larger variations with atomic number Z than do the |Δn|=1. The 6–4 intensity has a well-defined maximum at Z=34 with a FWHM of ΔZ ～10.     

Experimental measurement of effective atomic number of composite materials for Compton effect in the <Emphasis Type="Italic">γ</Emphasis>-ray region 280–1115 keV by a new method

In this paper, we report a new method to determine the effective atomic number, Z _eff, of composite materials for Compton effect in the γ-ray region 280–1115 keV based on the theoretically obtained Klein–Nishina scattering cross-sections in the angular range 50°–100° as well as a method to experimentally measure differential incoherent (Compton) scattering cross-sections in this angular range. The method was employed to evaluate Z _eff for different inorganic compounds containing elements in the range Z = 1–56, at three scattering angles 60°, 80° and 100° at three incident gamma energies 279.1 keV, 661.6 keV and 1115.5 keV and we have verified this method to be an appropriate method. Interestingly, the Z _eff values so obtained for the inorganic compounds were found to be equal to the total number of electrons present in the sample as given by the atomic number of the elements constituting the sample in accordance with the chemical formula of the sample. This was the case at all the three energies.     

24 electron cluster formulas as the ‘molecular’ units of ideal metallic glasses

It is known that ideal metallic glasses fully complying with the Hume-Rothery stabilization mechanism can be expressed by a universal cluster formula of the form [cluster](glue atom)_{1 or 3}. In the present work, it is shown, after a re-examination of the cluster-resonance model, that the number of electrons per unit cluster formula, e/u, is universally 24. The cluster formulas are then the atomic as well as the electronic structural units, mimicking the ‘molecular’ formulas for chemical substances. The origin of different electron number per atom ratios e/a is related to the total number of atoms Z in unit cluster formula, e/a?=?24/Z. The 24 electron formulas are well confirmed in typical binary and ternary bulk metallic glasses.     

Effect of Tb3+ co-doping and particle size on K2Ca2(SO4)3:EU phosphor

K₂Ca₂(SO₄)₃ doped with Eu, and co-doped with Tb were prepared by the solid state diffusion method. The nanoparticles of these phosphors were also prepared by the chemical co-precipitation method. The formation of the compounds was confirmed by XRD. The particle size was calculated by the broadening of the XRD peaks using Scherrer's formula. The particle size was found to be around 20?nm. Thermoluminescence (TL) was studied to see the effect of co-doping and particle size. Tb³⁺ co-doping decreases the intensity in the Eu²⁺ doped phosphor due to the energy transfer and multiple de-excitations through various radiative and non-radiative processes. The K₂Ca₂(SO₄)₃:Eu,Tb phosphor is found to be 0.33 times more sensitive than TLD-700H, but around 15 times more than LiF-TLD 100, and 7 times more than CaSO₄:Dy. The effective atomic number Z_eff is around 15, which is again comparable to CaSO₄:Dy.

However, very low sensitivity was observed in the case of nanoparticles. The decrease in the sensitivity is attributed to the particle size effect i.e., the volume-to-surface ratio. Study of photoluminescence (PL) of the material is also carried out.     

Multiple scattering features in argon and helium Ion Scattering Spectra (ISS) from polycrystalline materials

Multiple ion scattering from polycrystalline materials was studied by ion scattering spectrometry using ⁴He⁺ and ⁴⁰Ar⁺ at a scattering angle of 90° over an energy range of 0.1–2.5 keV. It was found that the high energy shoulder on the single scattering peak caused by multiple collisions increased in intensity with increasing energy, generally following the same trend as found earlier for neon scattering. The separation of the shoulder from the single scattering peak decreased linearly with increasing target atomic number and increased in direct proportion to the atomic number of the bombarding ion. Linear extrapolation of data from ⁴He, ²⁰Ne, and ⁴⁰Ar allows calculation of shoulder position for heavier gases such as krypton and xenon.     

Edge electroluminescence of single-crystal silicon at 80 K: Structures based on a high-efficiency solar cell

The spectral, kinetic, and energy characteristics of edge luminescence of silicon light-emitting diodes (LEDs) with radiating surface area of 0.055 cm² are studied at a temperature of 80 K over a wide range of pulsed currents. The LEDs are fabricated by cutting a high-efficiency solar cell. In contrast to a number of less effective silicon LEDs studied earlier, the external quantum efficiency of our LEDs at a fixed current at 80 K is higher than that at 300 K and its maximum value is about 0.4%. Despite the occurrence of Auger recombination, a record-high emissive power per unit surface area P = 0.2 W/cm² is attained at a pulsed current of 12 A. It is shown that this record value is achieved largely because the mechanism of radiative recombination is changed at large currents. The conditions are analyzed under which the free-exciton luminescence is changed to electron-hole plasma luminescence.     

Sum rules including first order quantum corrections for neutron scattering experiments on isotopic mixtures of monatomic fluids

The sum rules for neutron scattering experiments on monatomic isotopic mixtures are studied. The scattering is separated into a self part and a distinct part rather than into an incoherent part and a coherent part. Exact expressions for the moments are derived in terms of polynomials in ? ². The coefficients in these polynomials are sums of averages of hermitean operators that have a classical analogue. For the interpretation, the coefficients are approximated including first order quantum corrections.

It is argued that the separation into coherent and incoherent scattering is not justified in the presence of ‘isotopic incoherence’. Approximate expressions for the sum rules are proposed, in which combined averages (over the isotopic composition) appear of the scattering amplitudes and masses of the scattering atoms. These expressions, including first order quantum corrections to the sum rules, are not available in the literature.

The importance of correct averaging over the isotopic composition of expressions that involve scattering amplitudes and masses is discussed for the case of a maximum incoherent mixture of ³⁶Ar and ⁴⁰Ar.     

Structural and optical studies of the discontinuous behaviour of Te thin films subjected to He implantation

Electron microscopy investigations of tellurium thin films implanted with singly ionized He ions have revealed the appearance of a large number of surface structures when N _s, the number of implanted ions per unit area within the films, exceeds 0.2×10¹⁵ ions/cm² at a beam energy of 32 keV. This coincides with an observed discontinuity in the optical properties of the Te thin films and with a sudden decrease in the degree of orientation of the thin films as measured by X-ray diffraction. The same type of behaviour is observed for implantations with variable ion-beam energies and a fixed N _s of 0.5×10¹⁵ He ions/cm², with a discontinuity apparent near a value of 30 keV.