## 4 Discussion  
Considering the above-described experiment and its interpretation, a possible and very probable explanation was found for the large scatter of magnetic parameters as functions of transition temperature, experienced in our previous works [11; 13] and shown in Figures 3 and 5. It is not the error of the magnetic measurement itself, but it is originated in the inhomogeneity of the material, which results in different initial microstructural, as well as mechanical conditions resulting in different magnetic behaviours. It is an important observation that this inhomogeneity also exists for samples cut from the same block. The differences in the initial material conditions lead to different trajectories of the irradiation embrittlement, and to different final states, accordingly. To clarify that we did not use backward reasoning to decide which data points fit best to our hypothesis, we shall briefly detail the process: (1) We observed a large scatter of MAT descriptors (see Figure 6) measured on the reference and irradiated samples. (2) Regardless of the result of the MAT evaluation, we compared the magnetic behaviour of reference samples, and it was found that there were four samples that behaved similarly from a magnetic point if view. (3) The MAT evaluation was repeated, taking into account only the selected samples. This selection did not give any information about the behaviour of the irradiated samples; by taking this selection, we did not know how the irradiated samples would behave. Irradiation ageing, in practice, is the sum of the following mechanism: occurance of stable and unstable defects, precipitations, segregations, thermal ageing, and thermal recovery. The ratio of the effects of the different mechanisms depends not only on the alloy type (including the effect of polluting elements), the irradiation fluence, flux, and temperature, but also on the initial local microstructure state, as can be interpreted from our experiments. The destructively measured mechanical parameters were measured on a set of samples but not individually, in contrast to the magnetic measurements. DBTT always involves multiple specimens; it cannot be derived from one single specimen. This is the reason for the large error of the DBTT values, as shown in Figure 3. Magnetic measurements characterize better the structural changes of the specific Charpy sample than the measured transition temperature values. From this point of view, it can be stated that they follow the neutron irradiation-generated material degradation very precisely. This is a highly  
Figure 6: Ratio (normalized to the reference condition) of the optimally chosen MAT descriptor of selected irradiated and not-irradiated samples as a function of the transition temperature. important and serious conclusion of our work, which should be carefully checked in future research work. Another important message of this work is that neutron irradiation-generated embrittlement highly depends on the initial microstructure state of the given material, and even different parts of the same reactor pressure vessel can behave in rather different ways. We have shown that the observed phenomenon is _not_ due to the experimental implementation. We carefully analyzed the possible sources of uncertainty in our magnetic measurement (see Appendix A), and it was found that the scatter of the points due to possible measurement error is much less (see Figure A4) than the observed scatter of magnetic descriptors (see Figure 4). The error of the measurement is less than 1%, while the scatter in the reference samples is around 5-6%. Based on this analysis, only local material conditions can be responsible for the different embrittlement of material caused by the same dosage neutron irradiation. On the other hand, even with the existence of a large amount of scatter of the points (as can be seen in Figure 5), the _average_ correlation between the MAT parameters and the DBBT is more than satisfactory.