# An Active Irradiation System with Automatic Beam Positioning and Focusing for a Medical Cyclotron
## 3 Results  
To predict the position and the shape of the beam on target, an accurate knowledge of the magnetic field produced by the MBL is essential. For this purpose, we performed a measurement campaign using a Hall probe. Effects due to magnetic hysteresis were studied in detail in order to implement the necessary corrections in the optimization algorithm. Since the MBL embeds two quadrupole doublets and two steering dipoles within the same structure, overlapping effects occur that influence the bending and focusing capabilities. As an example, the effect of the quadrupole field on the dipole is shown in Figure 5. It can be noted that a quadrupole current of 50 A reduces the dipole field strength of about 20%. The fit corresponds to an arc-tangent function, which is used along with the upstream profiles measured by the UniBeaM and Equations (1) and (2) to calculate the position and shape of the beam on target. To verify the capability of predicting the shape of the beam on target, a UniBeaM detector was installed at the target position and the measured profiles were compared with the calculated ones. An example is shown in Figure 6, demonstrating a very good agreement. A first series of tests of the AFS were performed to obtain a very focused beam on target starting with a broad beam. To get the smallest possible beam spot on target, we set the target region to 1 mm in \(x\) and 1 mm in \(y\). For this purpose, a \(\pi^{2}\) detector was placed at the target location to visualize and assess the beam size (Figure 1). As shown in Figure 7, a very broad beam is measured before starting the AFS procedure that is able to focus it into the requested small spot. This procedure was successfully repeated for several initial beam conditions. It is important to remark that, even starting from a worse case situation (broad beam), a spot much smaller than the one required for the production of novel radioisotopes (about 5 mm diameter) can be obtained. Figure 5: Relative strength of the upstream bending dipole as a function of the current in the upstream quadrupole. Figure 6: Beam profiles at the target position measured by the UniBeaM detector (blue) and calculated (red). A further test aims at evaluating the response of the AFS to an external perturbation of the beam that was produced by the horizontal and vertical steering magnets of the BTL, which are located upstream of the AFS. Figure 8 shows the beam profiles in the horizontal and vertical planes measured by a two-dimensional UniBeaM detector located at the position of the target. The measured profiles demonstrate the successful recovery of the beam position, width and intensity. For the production of radioisotopes, the current hitting of the target material is a key parameter. To test the performance of the AFS to recover the beam current on an about 5 mm diameter target following a perturbation, a beam dump with current read-out equipped with a 6 mm diameter collimator was installed at the target position. The current on target as a function of time is shown in Figure 9 when a \(\sim\)60% reduction of the beam current is produced by means of the steering magnets of the BTL. One can observe the evolution of the current on target all along the optimization procedure that brings to about 90% of the initial value. A full recovery was not possible since about 10% of the current was lost before the MBL. This was proved by measuring the current at the entrance of the MBL before and after the perturbation. Analogously, Figure 10 shows the optimization procedure after a perturbation induced by acting on the quadrupoles of the BTL. In this case, a \(\sim\)30% reduction in the beam current was completely recovered. Figure 8: Capability of the AFS to restore the initial beam conditions after a perturbation. Horizontal (**left**) and vertical (**right**) beam profiles measured by a two-dimensional UniBeaM located at the position of the target. The output of the photo-diode of the commercial UniBeaM is reported in Volt.