## 2 Materials and Methods
### Magnetic Adaptive Testing  
The magnetic adaptive testing (MAT) technique measures systematically the series of minor magnetic permeability loops. During the measurement, the amplitude of the magnetizing current is increased step-by-step, starting from the demagnetized state. The magnetizing current has a triangular waveform. The time-dependent increase of the magnetizing field is linear. It means that the voltage, which is generated in the pick-up coil, is proportional to the differential permeability of the measured specimen. Series of the detected permeability loops, measured on different blocks, can be seen in Figure 3. In this figure, all minor loops of all measured samples are shown. The figure is a bit overcrowded, but the main purpose is to illustrate the draw measured data in one graph. The points of these loops are the input for further data evaluation. Each point represents a kind of information about the specimen's magnetic behaviour. It is also seen in this figure, especially when the magnified part is considered, that there are significant differences in the magnetic properties of the different samples. This is the reason for the large scatter of reference samples (having zero neutron level) when the MAT descriptor as a function of the irradiated neutron fluence is considered in the Result and Discussion section. The phenomenon is surprising and still not understood, considering that the samples were cut from the same large block. A very similar scatter of magnetic parameters of not-irradiated Charpy geometry samples were observed very recently [9]. A systematic and careful further analysis of this phenomenon is necessary. Such research will be conducted, but is currently out of scope in this work. A magnetizing yoke (a half transformer core, made of laminated Fe-Si sheets) put on the top of the sample is used for magnetizing the sample. The exciting coil and the pick-up coil are wound on the yoke legs. The cross-section of the leg of the yoke was 19 \(\times\) 16 mm\({}^{2}\) in our experiments. The probe was designed for usage in a hot cell. The replacement of the samples can be done easily by a manipulator. An example of measurement of the cladded RPV steel block is shown in Figure 4. The magnetizing yoke is put on the top of the block (on the cladding side). Figure 1: Geometry and type of the measured blocks without and with cladding [12]. Figure 2: Photograph and IDs of all investigated blocks [12]. The probe itself (without sample), designed for hot cell measurements, can be seen in the left side of Figure 5, while the probe with the sample on the top, inside the hot cell, is shown in the right side of Figure 5.  
Figure 4: The magnetizing yoke, placed on the top of a cladded RPV steel block. Figure 5: Probe, designed for hot cell MAT measurements (**left**), and the same probe inside the hot cell, with the sample to be measured (**right**). Figure 3: The measured permeability loops of the investigated, not-irradiated samples, measured on the bottom side. Input: magnified part of the loops. As presented in Figure 3, regular loops with large amplitudes are experienced when the highly ferromagnetic base material is measured. The situation is drastically different on the top side, if it is covered by cladding. Cladding is an austenitic, highly non-magnetic material. This layer means a large, practically not magnetic gap between the magnetizing yoke and the ferromagnetic part of the sample. As a consequence, a significant part of the exciting magnetic flux closes outside of the ferromagnetic sample; only a very small part of this flux can be pumped into the material. In this case, a very weak signal is experienced, as shown in Figure 6 (note that the vertical scale is different from Figure 3). Nevertheless, a kind of permeability loop with a detectable maximum can be measured, even in this case. The second step of MAT technique is the evaluation of the measured permeability loops. Matrices are calculated from raw data.