Experimental study for the production cross sections of positron emitters induced from 12C and 16O nuclei by low-energy proton beams

Abstract

In proton therapy, positron emitters are induced from 12C and 16O nuclei by protons on the beam path in the patient. Many studies for monitoring positron emitters with beam-induced PET technique have been performed by various groups to verify the proton beam range and the dose in the patient for quality assurance (QA). The QA methods proposed by some groups require accurate production cross sections of the positron emitters produced by protons, especially in the low-energy region. The aim of this study was to develop a method for measuring the production cross sections of positron emitters using standard equipment for proton therapy, and to measure the cross sections of positron emitters produced by low-energy protons and verify them in comparison with data of previous experiments. An 80-MeV proton beam was produced by a synchrotron, and the energy was degraded by polyethylene blocks to obtain various beam energies. The number of protons was estimated from the charge induced in a parallel-plate ionization chamber by protons. Low-energy protons of 14–70 MeV were used to bombard 12C-rich and 16O-rich target materials: namely, polyethylene and gelatinous water. The time-activity curve was then measured by a high-sensitivity PET scanner to extract the number of positron emitters produced in the target. The production cross sections for four reaction channels: 16O(p, pn)15O, 16O(p, 3p3n)11C, 16O(p, 2p2n)13N, and 12C(p, pn)11C were then measured. The cross sections for the 16O(p, pn)15O reaction channel were consistent with data of previous experiments within the uncertainties, while those of 12C(p, pn)11C were generally lower than data of previous experiments. These results suggested that further measurements of the production cross sections will be necessary.