Epithermal cross section

In the early days of criticality safety computations, when a two-group diffusion theory calculation in cylindrical or spherical coordinates 1 desk calculator was a tedious and somewhat formidable task and when cross-section data were more sparse, the selection of cross sections was perhaps a sinqiler task. The fbur-fitctor formula was widely used for moderated systems. A factor was used to indicate the deviation from 1/v behavior of an absorption cross section at thermal energy. - Thermal and epithermal cross sections were related to the Integral parameters, diffusion area, and neutron age. 

Electrons of still lower energy have been called subvibrational (Mozumder and Magee, 1967). These electrons are hot (epithermal) and must still lose energy to become thermal with energy (3/2)kBT — 0.0375 eV at T = 300 K. Subvibrational electrons are characterized not by forbiddenness of intramolecular vibrational excitation, but by their low cross section. Three avenues of energy loss of subvibrational electrons have been considered (1) elastic collision, (2) excitation of rotation (free or hindered), and (3) excitation of inter-molecular vibration (including, in crystals, lattice vibrations). 

It is interesting to note the structure in the total electron attachment cross section o (e) below M eV in Fig. 5, which indicates the existence of tSree NISs in this energy range. Since at thermal and epithermal energies the ions formed are long-lived and the measured attachment rates showed ( 8, ] ) no pressure... 

Carboranes in Boron Neutron Capture Therapy of Cancer (B7YC2). The stable isotope of boron, B (19.8% natural abundance), is very effective as a neutron capture agent with the effective nuclear cross section of 3837 bams, while the "B nucleus is incapable of undergoing a BNC reaction. Therefore, the B-emiched carborane and borane-substituted biomolecules and dmgs are selectively dehvered to the cancer cells in the human body and then the tumor-localized B nucleii are bombarded with either thermal or epithermal neutrons that results in a fission reaction producing the high energy alpha (a) particles as shown in equation (2). 

If activation by epithermal or fast neutrons is to be favoured over activation by thermal neutrons, the flirx of thermal neutrons can be suppressed by wrapping the samples in foils of cadmium, which has a high neutron absorption cross section for thermal neutrons (ua = 2520 b). This possibility is also used in activation analysis. 

In general, activation analysis relies on the use of standards that are irradiated under the same conditions and in the same position, and are also measured under the same conditions. Monoelement standards contain a known amount of one element. If they are applied to the evaluation of other elements the ratio of the cross sections o x/o s under the special conditions of irradiation and the ratio H /Hs of the relative abundances of the decay processes that are measured must be known (subscript x is for the sample and subscript s for the standard). Knowledge of the ratio o x/o s may cause problems, because the cross sections may vary drastically with the energy of the projectiles, for instance in the energy range of epithermal neutrons. These problems are not encountered with multielement standards that contain all the elements to be determined. However, the preparation of such multielement standards may be time-consuming. 

Samarium-153 has relatively high neutron capture cross-sections (with a thermal capture cross-section of 206 b and an epithermal capture cross-section of 3000 b), thus enabling production of high specific activity with minimal long lived radionuclidic impurities. The activities of various samarium targets obtained post-irradiation in PARR-I are given in Table 12.4. 

Because of epithermal resonance absorption of neutrons in Pa, its effective cross section in a thermal-neutron spectrum is much greater than the 2200 m/s cross section listed in Table... 

If a sample contains N nuclei of a particular stable isotope, the rate of formation of its (n,y) product nuclei is Nc in which is the neutron flux density of the thermal and epithermal neutron flux density in n-cm -s (O = Oth + epi) and a the (n.y) cross section (generally expressed in barns, one barn equalling 10 cm ) of the target nuclei in the neutron spectrum in which the sample is irradiated a = + lo epi/ ) ... 

The SDP is an average value over the epithermal neutron energy region. A good neutron moderator should divert few neutrons from the fission process, i.e. the neutron absorption cross section must be small. In this respect both heavy water (i.e. D2O) and carbon are... 

There are several approaches to the use of NAA for iodine determination in foodstuffs and other biological materials. Of the two nuclear reactions of stable the reaction of T(n, ) I with thermal and epithermal neutrons is almost exclusively used, because favorable nuclear parameters (high activation cross-section and resonance integral for thermal and epithermal neutrons, respectively, cf. Table 2.3) provide much lower detection limits compared... 

The low cross-section of the reaction of I(n,p) I with fast neutrons (cf Table 2.3) and a low abundance of neutrons with energies higher than 9MeV, which are needed for this reaction, in the neutron spectrum of a nuclear reactor result in a detection limit which is not sufficient for iodine determination in most types of foodstuffs, even if an RNAA procedure is applied. However, this reaction, which is completely independent in relation to the reaction of I(n, ) I with thermal and epithermal neutrons may be useful for cross-checking results in analysis of foodstuff samples with higher iodine contents, using the so-called self-verification principle in NAA (Byrne and Kucera, 1997). Detection limits of various NAA modes, which were achieved in the authors laboratory are compared in Table 2.4. 

Deutsch [22]. In this approach the cross sections which were well known, such as the thermal averages and scattering cross sections, were used as given by the measured cross section data. Less well known values, such as the proper epithermal capture cross section of the fuel, were determined by fitting the experiments. 

At energies higher than about 1 eV, the reaction cross sections exhibit huge variations with the incident neutron energy and are called resonances. Up to about 0.1 MeV, however, the differential flux, E), of partially thermalized neutrons is approximately proportional to 1/E. Hence, an analogous expression can be derived for epithermal neutron activation ... 

This equation, first written by Simonits et al. (1980), has the same structure as the equation used with the classical relative method, but contains additional correction factors because the analyte nuclide is different from the monitor nuclide. The subscript m refers to the monitor, /is the ratio of thermal flux to epithermal flux, and Qo is the ratio of the resonance integral to the thermal neutron cross section (see Eq. (30.9)). 

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