Profiling and modeling of wide range irradiation field of NIRS cyclotron

The proton beam from the cyclotron (NIRS-930) is used for irradiation for biological and physical experiments in the all-purpose irradiation room C-8. The irradiation field is widened by control of scattering material Al and Wobbler magnet. This report describes the actual lateral profiling of the proton beam and its fitting to the equation of distribution to see the validity of the modeling. For measurement of the profile, a scintillator array was prepared where seven 1-cm scintillators were arranged in a cruciform (13 scintillators in total). Each scintillator was connected to each 16-channel multi-anode photomultiplier by the optical fiber for processing the photon count. The array coordinate was consisted of horizon (x), vertical (y) and relative intensity/dose (z) and this array was placed on the automatically moving stage at measurement done for every 1 minute. The beam was monitored with an ionization chamber AE-175 (Applied Engineering Inc.) in front of the irradiation field, which controlled the particle number <10⁵ with the cyclotron beam current of several pA. Measured data were fitted to the data analysis framework ROOT developed by the European Organization for Nuclear Research (CERN), which finally gave a cubic function of irradiation field feature with a precision of about 2%, indicating that the modeling was valid and could give the dose of irradiation with this precision. The field profile was found to be formed by sigma in Gaussian distribution, horizontal/vertical swing radii of the beam center and of Wobbler magnet. The beam map corresponding to each target form can be calculated in future. (T.T.)