Development of a beam trajectory monitoring system using e⁺/e^- pair production events - 6447

In particle therapy, it is important to monitor the Bragg-peak position. A simulation using GEANT4 Monte Carlo code shows the distribution of secondary generated gamma rays on the carbon beam therapy and the proton beam therapy. This simulation shows that gamma rays whose energy is 10 MeV or more are intensively generated at the Bragg-peak position. We are developing a system to monitor the Bragg-peak position which can measure pair production events occurred in the detector by gamma rays from irradiation points. The momentum direction of the gamma ray can be determined by measuring passing points and energy of e⁺ and e^- generated by pair production. This system has 5 parts. The first part is the conversion part. This part consists of several layers. Each layer is composed of a La-GPS ((Gd_0.75La_0.24Ce_0.01)₂Si₂O₇) scintillator plate and wavelength-shifting fibre (WLSF) sheets. The scintillator plate is sandwiched between sheets, where the directions of the sheets are in orthogonally x and y directions. In this part, gamma rays are converted to e⁺, e^- pairs and the position where the conversion occurred is determined. The second part is the tracking part. This part consists of 2 layers of scintillating fibre tracker. Each layer has 6 scintillating fibre sheets for x, x', u, u', v, and v'. The third part is the energy measurement part. It measures the energy of e⁺ and e^- by scintillator array and Silicon Photomultipliers. The fourth part is the veto counter for Bremsstrahlung gamma rays from e⁺ and e^-. The fifth part is the beam monitor. Experimental data show that the number of photoelectrons of La-GPS with a WLSF (B-3(300)MJ, Kuraray) sheet and scintillating fibre (SCSF-78, Kuraray) are 9.7 and 7.6 respectively. (authors)