Measurement of absolute activation cross sections from carbon and aluminum for proton therapy

This experimental study aims to measure nuclear cross sections from C-nat and Al-27 which serve as reference targets regarding radioactivation in proton therapy. Graphite and aluminum foils were activated in broad, quasi-monochromatic proton fields generated with the pencil-beam scanning technique. The gamma-emissions were analyzed subsequently in a nearby gamma-ray spectrometry facility designed for low-level radioactivity measurements. The number of incident protons was measured with a Faraday Cup. A detailed evaluation of the uncertainties was performed. The cross section for the C-nat(p,x)C-11 reaction at 100 MeV is (70.2 +/- 1.6 (sys.) +/- 0.2 (stat.)) mb. The production cross sections from Al-27 at 100 MeV are (19.9 +/- 0.4 +/- 0.3) mb for Na-22 and (11.5 +/- 0.3 +/- 0.1) mb for Na-24. The systematic uncertainties contribute 2.0%-2.4% of the quoted uncertainties (k = 1) of cross sections. Concluding, radioactivation cross sections were measured with proton fields in a clinically commissioned environment. The measured activation cross sections from aluminum used for monitoring of the proton fluence agree with the averaged values compiled in nuclear reaction data bases. For proton-induced radioactivation from carbon the chemical purity of the target strongly affects the experimental results. The cross section for the production of C-11 derived from a measurement with a high-purity graphite target is in line with the largest cross section obtained in prior studies.