Irradiation effects number density

Under almost all the deposition conditions we have studied, the number density of nuclei first increased approximately linearly to irradiation time with a negligible induction period, and then the rate of increase was gradually slowed down due to a saturation effect (3-5). We have to properly discriminate between a true induction period and only an apparent one. The latter is connected to the detection limit and means a period during which the nuclei are too small to be observed even if they actually exist. Provided that some induction period was detected, one must be very careful in judging its origin. Fortunately, we have not encountered such a situation, which in turn implies that the minimum size of Si nuclei detectable by the chemical amplification is actually very small, though precise size evaluation has not been successful, as stated before. 

If the irradiated sample area is a and the sample thickness is t, then for a sample concentration of c mg/ml, the sample mass is ate. The total of particles present is atcN M. The total intensity received on the irradiated sample area can be written as SIP/a. In order to allow for the solvent contrast when the particles of partial specific volume v are immersed in a solvent of electron density ps, z is replaced by (z - i5 Ps/N ) to correspond to the effective number of mole-electrons per gram of solute. Thus the final expression for 7(0) is... 

Despite the uncertainties, APT experiments performed with optimised experimental conditions have provided a very significant contribution to our understanding of the effects of irradiation on RPV steels. It is the only technique that provides direct information on the composition, size and number density of solute-enriched clusters. Interpretation of the data is non-trivial and the effects of the imperfect detection efficiency and artefacts... 

The levels of bulk Cu and Ni are also important and the trend is for the number density and size to increase with increasing bulk levels. This is illustrated in Figs 9.37a and b for increasing levels of Ni in a 0.4wt%Cu model steel irradiated in IVAR. Clearly, increasing Ni increases precipitate size, the effect on number density is pronounced. 

For steels with Ni levels < 1.2wt% there are significant data on the development of Cu clusters with fluence, irradiation temperature, flux and composition. It has been demonstrated that, as the irradiation proceeds, the number density and size of the clusters build up to a plateau value. There are limited data on the microstructure formed at high fluence, but there is now some evidence for over-ageing of the clusters. The size and number density of Cu clusters formed at a given fluence before the plateau are strongly dependent on the flux and material composition, but only weakly dependent on irradiation temperature. Finally, Ni and Mn may have a profound effect on the development of CECs. At high bulk Ni levels > 1.2wt%, the volume fraction of solute clusters may not reach a plateau (in the dose range of interest). 

In LEI the matrix effects are low. This applies in particular to samples with complicated atomic spectra, as no spectral interferences are encounterd. Elements with low ionization energies may cause matrix effects. The latter could be due to a change in the ion current as a result of changes in the electron number density in the flame. This can be compensated for by measuring alternately with and without laser irradiation. Ionization of the matrix by the laser may also lead to errors in the ion currents measured. These can be eliminated by modulation of the frequency of the exciting laser radiation around v, which corresponds to E2 — = hv, where 2... 

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