Neutron experiment

There are many classes of experiment that can be made with neutrons they cover a wide range of angles, wavelengths and energy transfer and it is not possible to give a full description of them all here. Some experiments are frequently used with polymers, and these will be outlined briefly in three categories after a few general remarks on neutron instrumentation. 

Contrast with isotopes Contrast variation Measurements in bulk Easy control of sample environment Measurements of molecular motion Can be surface specific Low flux Large, expensive source 

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Edwards et al. (1980) used a Cm-242 source to irradiate externally a thin film of blood. The energy of 4 9 MeV of the alpha particles were almost entirely absorbed by the blood. The dicentrics yield was linear from 0.11 to 4 2 Gy. From this resulted a RBE of 17.9 with respect to Co-60 gamma rays. It was, however, only 6.0 at the initial slope. To explain this RBE, which was low compared to that expected from neutron experiments, a model is presented taking into account cell killing and mitotic delay. 

A detailed test of dynamic RPA requires a combination of static and dynamic neutron experiments involving investigations of the respective component dynamics. All three issues addressed in the introduction were investigated. We will start with the collective dynamics of a diblock copolymer and then address the single component dynamics. Finally, we will discuss aspects of the interface dynamics. The experiments were performed on diblock copolymers of the PE-PEE and PE-PEP type. In order to access the different dynamics a series of materials with different h-d labelUng was employed (see Table 6.1). 

The X-N deformation densities are important for the study of the charge density distribution in and around hydrogen atoms. Without the extra effort required for a neutron experiment, assumptions on the hydrogen atom location and vibrations must be made which introduce a considerable uncertainty in the results. 

Reliable criteria to identify the (individual) patients suitable for high-LET radiation therapy need to be developed. At present, the available criteria are derived (mainly) from the clinical fast neutron experience. It can be expected that novel approaches based on modern techniques involving molecular biology or gene identification may provide appropriate and still missing information. They may also provide information on the susceptibility or risk for secondary radio-induced cancer. 

As we have seen, the most advanced photoelectron techniques, especially those which necessitate the use of synchrotron radiation sources, have been applied until now only to U and Th systems. Application on Pu and Am systems as well as to heavier actinides is to be expected in the future. The same development is likely to occur as for neutron experiments, where more and more these hazardous actinides are investigated at high levels of instrumental sophistication. Difficulties arising from handling and protection problems are, of course, much greater for photoelectron spectroscopy. 

Thermal or low-energy neutron scattering experiments have been most valuable in throwing light on molecular motion in plastic crystals. These experiments measure changes in the centre of mass of a molecule. Diffusion constants obtained from neutron experiments differ widely from those obtained from tracer experiments since neutron scattering is mainly determined by rotational diffusion. The scattering function has the form... 

Daoud M, Cotton JP, Famoux B, Jannink G, Sarma G, Benoit H, Dupressix R, Picot C, De-Gennes PG (1975) Solutions of flexible polymers. Neutron experiments and interpretation Macromolecules 6 804... 

It has already been pointed out that the power spectrum of this function at zero frequency determines the translational diffusion coefficient, D. The full-time dependence of this function can be obtained indirectly from inelastic slow neutron experiments.57 Unfortunately, these experiments are not yet precise enough to say anything quantitatively about this function. /(t) s memory function, K t), is defined by... 

Table 7 M—O Distances (A) in Aqueous Solution and in Hydrates, from X-Ray and Neutron Experiments ...

The results of these neutron experiments need to be compared with singlecrystal electron energy loss data and theoretical predictions before a reliable assignment can be made of the observed excitations.19-26 Low energy electron loss data is available for H2 adsorbed upon the 100 and 111 surfaces of Ni. Tables 2 and 3 correlate the results of EELS experiments with theoretical predictions for the two surfaces. For the (110) surface it has been shown that H2 adsorption is very similar to that on the (111) surface since in both cases the preferred site has three-fold co-ordination and since the coordination about a (110) site is identical to that on a facet of a (111) plane.25 Comparisons of the results of Table 1 with those of Tables 2 and 3 show that the majority of the H atoms adsorbed by Raney nickel at coverages up to saturation occupy sites of three-fold co-ordination.3 18 The results of Kelley et al.17 suggest that in addition a small proportion of the H atoms occupy four-fold sites. 

Daoud M, Cotton JP, Farnoux B, et al (1975) Solution of flexible polymers. Neutron experiments and interpretation. Macromolecules 8(6) 804—818 Dasbach TP, Manke CW, Williams MC (1992) Complex viscosity for the rigorous formulation of the multibead internal viscosity model with hydrodynamic interaction. J Phys Chem 96(10) 4118—4125 (McMahon)... 

We used the measurements of the amount of water absorbed in the laboratory experiments to give an independent check of the -values obtained in the neutron experiments we did this by assuming that the swelling is perfectly homogeneous. In these circumstances, for each volume of water that a unit volume of clay absorbs, the interlayer spacing must have increased by 19.4 A. The conditions used are listed in Table 5.3 and the results are given in Table 5.4. There is a systematic variation... 

In the characterization of samples containing >6 x 10 protons, adequate spectra can be obtained on a timescale ranging from several hours to about one day. For quantitative and semi-quantitative comparison, the INS spectra can be normalized to the amount of sample in the beam. Most catalyst characterization experiments are difference measurements the spectrum of the catalyst with and without an adsorbate present is recorded and the difference taken. After the neutron experiment, safety requires that the samples be stored on site, as a consequence of neutron activation. The radioactivity decreases to background levels within a period ranging from a few hours to several months, depending on the atomic number and isotopes of the catalyst components. 

The DAC is unsuitable for an ordinary neutron experiment, because the latter requires much larger samples. Alternative means of compression, such as liquid- and gas-pressure cells, " or even sapphire anvils (specially designed for use with He cryostats), are but poor substitutes in the magnitudes of pressure. However, encouraging results have been obtained by using DACs in conjunction with high-power neutron sources and optimizing the diffraction instrument itself... 

Optimally, X-ray crystallography requires single crystals of a few tenths of a millimetre in size, which are not intergrown and which have grown slowly (they are usually of better quality). Neutron experiments generally require larger crystals a decade ago ca. 5mm was considered a standard size, but recent improvement of the technique make considerably smaller samples acceptable (<1 mm ). 

Selection of the crystal is the crucial step of the whole study and the time spent on it is well spent. During data collection the crystal should remain within that inner part of the incident beam which has a uniform intensity distribution. Its diameter is smaller (80% at best) than the nominal diameter of the collimator. Thus in X-ray difffactometry a good working norm is for all dimensions of the crystal to be <0.5 mm, and preferably as uniform as possible. For neutron experiments,... 

Nuclides with an excess of neutrons experience P decay. In the nucleus a neutron is converted into a proton, an electron and an electron antineutrino, as indicated in Table 5.1. The atomic number increases by one unit, whereas the mass number does not change (second displacement law of Soddy and Fajans). The energy of the decay process can again be calculated by comparison of the masses according to Einstein ... 

The techniques of single-crystal diffractometry have been discussed by Arndt and WUlis (51). We should note that extinction is a very serious problem in the determination of accurate magnetic intensity data from single crystals. Although extinction must always be accounted for in conventional crystallographic studies, it is particularly important to make proper correction in polarized neutron experiments where the ratio of magnetic to nuclear structure factors is determined. 

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