Thermal capture

Thru 1967, emphasis was given to the use of neutrons as the bombarding source of radiation. Almost all possible neutron reactions were considered including moderation of fast neutrons by hydrogen in the expl, thermal capture reactions, elastic and inelastic scattering of neutrons and neutron activation reactions. These neutron reactions are listed as follows ... 

A description of the emission and capture processes at a trap will be useful before discussing the various experimental methods. Figure 1 depicts the capture and emission processes that can occur at a center with electron energy ET. The subscripts n and p denote electron and hole transitions, and the superscripts t and differentiate between thermally and optically stimulated processes. It is assumed here that only thermal capture processes are occurring. 

As can be seen from Eqs. (18) and (21), the initial amplitude of the current or capacitance transient is proportional to nT(0). Therefore, if the bias has been kept at zero for a time sufficiently long to fill all traps with electrons, then nT(0) = NT and the trap concentration can be determined from the initial amplitude of the transient. The thermal capture rate is measured by restoring the reverse bias before the traps are completely filled by electrons. Adjusting conditions (low temperature) such that inequality (15) holds, then for a width tf of the filling pulse, the initial amplitude of the trap-emptying transient is given by... 

Provided that microscopic reversibility is properly accounted for, thermal averaging of specific rate constants leads to thermal capture rate constants such that a relation between /rigid and /ngid( ,7) can be established. As the charge-dipole potential is particularly simple, the behavior is transparent such that a reference for comparison with other types of interaction potentials is available. 

It is easily shown that, in the classical limit, Eqs. (41) and (42) are consistent with the thermal capture rate constants for the oscillator model of charge-permanent dipole capture. The relevant part of the activated complex partition function, instead of Eq. (11), can be written as... 

In other words, the thermal capture rate constant of CVTST exceeds that of SACM/PST by a factor e - 2.718. The result may be somewhat improved by ICVTST, that is, by replacing Q = G( ) by Q(r, V(j, m, l, r ) > 0). Again the calculation is straightforward, giving... 

Thermal averaging of /xVTST or I/xVTST results, via Eq. (43), leads to thermal capture rate constants, with... 

The cross sections for (n,y) reactions common in reactor thermal neutron activation generally decrease with increasing neutron energy with the exception of resonance-capture cross section peaks at specific energies. This reaction is, therefore, not important in most 14 MeV activation determinations. However, some thermalization of the 14 MeV flux may always be expected due to the presence of low Z elements in the construction materials of the pneumatic tubes, sample supports, sample vial, or the sample itself (particularly when the sample is present in aqueous solution). The elements Al, Mn, V, Sn, Dy, In, Gd, and Co, in particular, have high thermal neutron capture cross sections and thermal capture products have been observed in the 14 MeV neutron irradiation of these elements in spite of care taken to reduce the amount of low Z moderating materials in the region of the sample irradiation position 25>. 

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. 

WITH 18 Neutral Reactants at Thermal Energies, and also the Ionization Energies of the Reactants, the Thermal Capture Rate Constants k, and the Exoergicity for the Respective Charge-Transfer Reactions Using the Adiabatic RE(KrJ) = 12.85 eV... 

Excitation of bound states by radiative capture of slow neutrons. Radiative capture is mainly important for thermal neutrons for which there is the technical advantage that very large intensities are available from reactors. The energy spectmm of thermal capture radiations has been observed by Kinsey and his collaborators [37] for a large number of nuclei. In this work the neutron capturing sample was placed in a high flux near the core of a nuclear reactor, and collimated... 

A very similar situation exists in the T = — i nucleus AP where extensive information on the level structure up to about 6 MeV is available from the (dp) reaction. The lower levels are also confirmed and defined by differences between gamma ray lines observed in the (ny) thermal capture reaction. The virtual... 

The total capture of cold neutrons per c.c. in the metal is found by multiplying the thermal capture cross-section by the cold neutron density, and similarly for the hot neutrons. In this computation the densities must be reduced to the normalization, Pq = 0.5647T, P2 = 7.557T. The total thermal capture divided by the total neutron production is thus found to be. 850 hence = 1.176. This may be compared with 1.185 for the two region comparison system. 

Scaling the rate to water and assuming that all neutrons produced by muons within the fiducial volume of the detector thermalize, capture on gadolinium, and escape a muon veto, we obtain an upper bound on the uncorrelated rate of 147,000 neutrons per day in the fiducial volume. Of course, this rate is greatly reduced by the requirement of spatial and time coincidence with the positron signal. 

Comparison with (71) shows that the motion is strongly tender-damped by the delayed neutrons. This situation is fairly unusual, and here stems primarily from the extraordinarily low thermal capture cross section of D2O, which yields an unusually large value for Z. 

Neutrons with energies greater than 0.1 MeV are called fast neutrons. The fission spectrum of a light-water-moderated reactor provides as many fast neutrons as thermal neutrons. Therefore, fast neutron activation of certain elements via (n,p) reactions is a very selective technique, complementary to thermal and epithermal NAA. A typical example is Fe, for which the Fe(n,p) Mn activation reaction produces a better gamma-ray emitter than the thermal capture reaction. 

The reaction rate parameters measured are ratio of therinat-to-thermal captures, (pu) ratio of qiithermal-to-aiermai fissions (6 ratio of U captures to U fissions (C ) ratio of U fissions to U fissfams (On) and ratio id Lu-to- CU subcadmium captures in the fuel normalized to the same ratio in a Maxwellian thermal flux (R). Measurement and analysis techniques used in this study were Identical to those described In Ref. 3. 

Trombe Wall A two-part wall structure consisting of a transparent thermal capture outer wall and a dense, massive thermal storage inner wall. 

Markovic N, Nordhohn S. (1989) Simple estimation of thermal capture rates for ion-dipole colhsions by canonical effective potential methods. Chem. Phys. 135 109-122. 

Pu239 u235 Qjj other hand, because the thermal capture-to-... 

Constituent Free energy of formation at 1000°K (kCal/mol °F) Cation thermal capture cross-section (bams)... 

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