Conventional neutron sources

It is also interesting to note that the limitations of most neutron systems lie not in the neutron source(s) but rather in the (gamma ray) detectors and subsequent data acquisition system. The obvious need is for development of detectors with higher rate capability and improved energy resolution. All modern digital data processing techniques have allowed significant improvement in the performance of conventional scintillation detectors with respect to rate and pileup rejection. 

The conventional method for determining cation ordering and site populations within a crystal structure is by diffraction techniques using X-ray, electron and neutron sources. For determining site occupancies of transition metal ions, these methods have been supplemented by a variety of spectroscopic techniques involving measurements of Mossbauer, electron paramagnetic resonance (EPR or ESR), X-ray absorption (EXAFS and XANES), X-ray photoelectron (XPS), infrared and optical absorption spectra. 

Historically, all the early neutron sources were of the conventional reactor type, and a large monochromating crystal was commonly employed (for single-crystal diffraction experiments) to select neutrons with a small wavelength spread in order to produce a monochromatic neutron beam. The rest... 

In this section, we are concerned with a powder diffraction experiment, which consists of a single pattern (profile). The Rietveld technique may also be used to conduct refinement of the crystal structure employing multiple patterns collected from the same material. For example, powder diffraction data collected using conventional x-ray sources with different wavelengths, conventional and synchrotron x-rays, conventional or synchrotron x-rays and neutron source may be used simultaneously in a combined Rietveld refinement. The fundamentals of the combined Rietveld refinement are briefly considered in section 7.3.8. 

There are several types of neutron sources one can use for NAA, but megawatt nuclear reactors with their intense flux of 10 to 10 neutrons m s from uranium fission, offer the highest available sensitivities for most elements. Neutron energy distribution is quite broad, but for conventional NAA, low-energy neutrons (energies below 0.5 eV) are chosen that represent 90—95 per cent of the neutron flux. Sufficient levels of activation are reached in a few minutes, even if the isotope formed has a long half-life. The procedure imposes that the sample to be treated must be thermally stable. It is enclosed in a tube with a comparator standard of known concentration, before being introduced in a reactor beam port. 

Within the last few years high resolution data collected either from synchrotron or neutron sources has allowed new structures to be solved from powder samples. Powder data from these new sources are superior to that collected from a conventional x-ray source both in resolution and peak to background ratio these improvements in data quality have the capability of making the determination of new molecular sieve topologies easier but not necessarily routine. Synchrotron radiation has possible future capabilities of being used to collect data from crystals that are only a few microns in size. At that time most newly synthesized molecular sieve materials will be solved by single crystal techniques instead of powder techniques. At the present time an acceptable structure refinement can be obtained from a crystal with a... 

Small angle neutron diffraction method permits the observation of large scale structures with clusters (particles) size from 1 nm to 100 nm. From the properties of the Fourier transform, [4], it follows that the diffraction intensity from objects of this size is concentrated in a small angle region, 0.2 < (9 < 10 radians, in the so-called zero peak. In conventional diffractometers the zero peak is inseparable from the instrumental broadening of the incident neutron beam. To make the measurement possible, SANS method applies cold neutron sources and filtration of the incident neutron flux by mirror guides or Be filters. In SANS, the angular interval Q < I sin QR/QR) in (2) is practically constant... 

Reactor Neutrons. Nuclear reactors are the largest and most often used neutron sources. Neutrons formed in the reactor have a continuous energy spectrum extending from nearly 0 to 15MeV. Conventionally the neutron spectrum is divided in three components ... 

Sources of Neutrons.—Isotopic Neutron Sources. Conventional isotopic sources of neutrons utilize the production of neutrons by the (a,n) reaction (e.g, Am-Be) or the (y,n) reaction (e.g. Sb-Be). Typical sources give a neutron output of 10 —10 s Ci S depending upon the composition the usable neutron fluxes are of the order of 10 —10 n cm" s" The neutron energies are of the order of 1—10 MeV energy for (a,n) sources (26 keV for a Sb-Be source) and in many cases neutron moderation will be necessary, reducing the usable neutron flux. The low fluxes available preclude their use... 

The fission-chamber signals are fed to conventional preamplifiers, A-1 linear amplifiers, logarithmic count-rate meters and a dual-pen recorder. For initial reactor startup before gamma-neutron reactions provided a sizable neutron source, the fi.ssion-chamber output was u.scd to drive a low-range pulse counter. 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

The XRD method can be successfully apply to detect the /-/" phase boundary by analysing the (112) and (400)/(004) reflections respectively at about 42 and 61-64 20 (Fig. 6.3.) [22,23], even though, due to the low intensity of these peaks, confirmation by using a synchrotron radiation or even neutron scattering source may be desirable [45,46]. Due to the low intensity of the conventional XRD technique to the oxygen atoms, the / -c phase transition is best detected by using... 

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