Neutrons weighting factor

In hadron therapy, a similar approach and symbolism are presented to take into account the RBE differences. RBE values for neutrons are well documented, and reliable data are now becoming available for protons and heavy ions, on which safe weighting factors IErbe can be based. Besides radiobiological determinations, microdosimetry brings independently additional information on the radiation quality and improves confidence in both sets of data. Microdosimetry provides an objective description of the radiation quality at the point of interest under the actual irradiation conditions. Correlation between this set of information and the experimental RBE results is of great scientific and clinical value in hadron therapy [24,25]. 

Weighting factors recommended are approximately 1 for x-rays, gamma rays, beta particles, and electrons and 20 for conventional neutrons ( >0.1 to 2MeV), protons, alpha particles and charged particles of unknown energy. 

Figure 22.2. Weighting factor wr for neutrons as a function of their energy.

Weighting factors are assessed on the basis of the LET value. The influences of spatial ionization density and of temporal distribution of ionization have to be taken into account separately. Weighting factors for various particles and for y-ray photons are plotted in Fig. 22.1 and those for neutrons in Fig. 22.2 as a function of the energy. 

This table is mainly based on a similar table in BNL 325 with some additions. (See the comments preceding the references to the experimental work.) The basic theory is given in the text of this article. Fy, etc. are the partial level width for decay by neutron, gamma ray, etc. emission. The factor gF appears in the theory, where g is a spin weight factor J if two compound spin states are possible. In most cases g is not known, but is taken to be j. The quoted uncertainties in F and F = F include estimated uncertainties in Fy, but not in g, in the analysis for F. ... 

Both the step function and the continuous function in O Fig. 47.2 had been introduced in the former recommendation (in ICRP 1991). The recently recommended continuous function of radiation weighting factors for neutrons reflects only practical considerations and does not imply the availability of more precise data. Mainly due to the continuous energy spectrum of the neutrons, in calculations the continuous function of the Wr has been used rather than the step function. 

Radiation weighting factors (Wr) for neutrons versus neutron energy... 

The neutron weight below which neutrons were kiUed by Russian Roulette was a function of the distance in the reflector from the core. This function was inversely proportional to the adjoint flux, obtained from Sy) transport theory calculations of a uranium-metal sphere surrounded by an 18-in. carbon reflector. Thiis function was used for all assemblies. The standard deviations on the multiplication factors are between 0.9 andT.3%. 

The tetrachloride liquids XCiti (X = C, Si, Ti, V, Ge, Sn etc) make an interesting series for studying aspects of orientational correlation in relation to molecular contours. The various diffraction studies have been surveyed in a paper [253 which uses the reference interaction site model (RISM) to evaluate the partial distribution functions. Data for carbon tetrachloride are particularly instructive as they Illustrate the complementary nature of X-ray and neutron scattering. There are three partial functions gcc( )> weighting factors... 

Carbon disulphide has been studied by X-ray [353 and neutron scattering [36]. The resulting dL(r) functions are shown super-imposed in Figure 9. This is another good example of the complementary nature of the two measurements since sulphur has a small b-value for neutron scattering and the weight factors... 

Fig. 11.3 Total simulated neutron-weighted structure factor Sj-fk) for three selected glassy compounds, compared to experimental data GeOa [85], GeSes [86], and As2Sc3 [87, 88], The broken curve (for As2Se3) is a result from a classical force field [89]...

We examine now structural anomalies that correlate to those detected previously, and follow the total neutron weighted structure factor SN(k) of glassy NS2 with pressure. In Fig. 11.22 is represented the total computed structure factor SN(k), obtained from a linear combination of the partial structure factors Sij (fe), defined by ... 

Unfortunately, in the case of B2O3 only the (neutron and X-ray) total stracture factors have been measured and no partials are available. We provide however in Fig. 14.12 a comparison of the neutron weighted Faber-Ziman partials obtained from our models. It is seen that the differences are very tiny and essentially limited to a small frequency shift in the B-B partial. Clearly, a quantitative assessment of / is unlikely to be made from these measurements. 

Here Pyj is the structure factor for the (hkl) diffiaction peak and is related to the atomic arrangements in the material. Specifically, Fjjj is the Fourier transform of the positions of the atoms in one unit cell. Each atom is weighted by its form factor, which is equal to its atomic number Z for small 26, but which decreases as 2d increases. Thus, XRD is more sensitive to high-Z materials, and for low-Z materials, neutron or electron diffraction may be more suitable. The faaor e (called the Debye-Waller factor) accounts for the reduction in intensity due to the disorder in the crystal, and the diffracting volume V depends on p and on the film thickness. For epitaxial thin films and films with preferred orientations, the integrated intensity depends on the orientation of the specimen. 

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