A new method for monitoring beam range by measuring low energy photons

We studied a new method to monitor a beam range in heavy-ion radiation therapy by measuring low energy photons emitted from a track of the ion beam. A 290 MeV/u carbon beam was injected into a cylindrical water phantom. A CdTe semiconductor detector with a lead slit having a width of 2mm was placed at a side of the phantom. In order to measure the position dependence of the low energy photon count, the beam range was varied by changing the injection energy using a binary energy degrader placed about 100 cm upstream of the beam focal point. The measured photon count decreased when the detector got closer to the end point of the beam range and the derivative of the photon count clearly changed in front of the range position. This was explained by our theoretical study assuming the photons were secondary electron bremsstrahlung. These results indicate that this new method could estimate the range position from the observation of bremsstrahlung with an accuracy of a few mm.