## 3 Results  
Magnetic measurements and mechanical tests were performed on the same samples before and after neutron irradiaton. As has been reported presently [13], irradiaton caused measurable changes in the material properties. The correlation between the transition temperature and neutron fluence is not linear: relatively low neutron dosage causes a relatively large increase of \(\Delta\)DBBT, as shown in Figure 2 (or \(\Delta\)T\({}_{4\text{JJ}}\) in the Figure), and later, a larger dosage causes smaller changes in material embrittlement due to the accumulation of the induced damages. The sample series 15 Kh2 NMFA in Figure 2 is the same as was measured in the present work. Good, monotonic correlation was found between nondestructively measured magnetic parameters (regardless on the used measurement technique) and mechanical hardening (determined by DBBT, Vickers hardness, and uniaxial tensile tests). However, a large scatter of magnetic parameters was found again, similarly as reported in [11]. To clarify this problem, the results of the MAT measurements on 15 Kh2 NMFA type material are analyzed in this section. The result of the MAT evaluation as function of the transition temperature is shown in Figure 3. \"Optimally chosen MAT descriptor\" is the parameter that describes in the most adequate way the embrittlement of the material due to neutron irradiation. In other words, this is the parameter that ensured the highest sensitivity at good reproducibility. In our case (plotted in Figure 3), this descriptor is characterized by h\({}_{\text{a}}=-30\) mA and h\({}_{\text{b}}=1080\) mA magnetic field values. The tendency of increasing embrittlement due to neutron irradiation is clearly seen, but the large scatter of points is also evident. In this figure, all measured results are plotted together, and the influence of neutron irradiation on individual samples is not seen. Figure 3: Optimally chosen MAT descriptors as a function of the transition temperature for the material 15 Kh2 NMFA. Figure 2: Embrittlement of RPV steel materials after neutron irradiation as a function of the neutron fluence. The dashed lines are added to guide the eye (power law fit) [13]. In spite of the fact that Charpy samples were cut from the same block of reactor steel material, they are magnetically rather different, even before irradiation, as can also be seen in Figure 3 (reference samples). In order to study the magnetic behaviour that makes the difference between the samples more visible, the measured permeability loops of all not-irradiated (reference) samples are shown in Figure 4. In Figure 4a, total loops (including all minor loops as well) are shown, while the magnified part of the graph is shown in Figure 4b. In this latter case, only the envelope of the loops (minor loops with the largest amplitude) can be seen. Another way of plotting was used in Figure 5 to show the influence of the neutron irradiation on the individual samples. In this way of presentation, each point represents one sample, and the MAT descriptor is normalized in each case with the same descriptor measured on the same sample before irradiation. This is why only one point shows the reference case. Neutron irradiation caused a 10-24% increase of magnetic parameters compared with the same parameter of a virgin sample. Evidently, this way of plotting did not reduce the scatter of points. Nevertheless, the tendency is clearly seen. A more or less linear correlation was found between the transition temperature and the MAT parameter modification. The scatter of points is demonstrated also by the R = 0.783 regression factor of the linear fit. The possible influence of the magnetic behaviour by the neutron irradiation-generated degradation of the reactor pressure vessel steel is shown in Figure 6. A subset of samples was chosen from the measured set of samples, which behave similarly from a magnetic  
Figure 4: The measured permeability loops of the investigated, not-irradiated samples. (**a**) Full loops, (**b**) magnified parts of the loops.