Activation measurement for material selection of OpenPET components in particle therapy

In-beam positron emission tomography (PET) is expected to be suitable as an in situ dose verification technique in particle therapies. We have developed the world's first open-type PET, OpenPET, to monitor radiation field in heavy ion radiation therapy. In using in-beam PET, the number of annihilation radiation particles to be measured is generally low. On the other hand, OpenPET devices can be activated by secondary particles such as neutrons generated through the nuclear reactions between primary particles and irradiated objects. This causes an increase of single counts and that could possibly decrease the signal-to-noise ratio. Thus we should carefully select materials for devices with less activation. We are going to use aluminum or stainless steel for the body of the equipment in our next development of a human size OpenPET. To that goal, in this work, we measured activation of an aluminum plate and a stainless plate to estimate the influence on PET scanning. The sample plates were placed at downstream of a water phantom and was irradiated by a
¹²
C beam with 290 MeV/u. to activate with effect, we shoot higher intensity beam than clinical usage. After the irradiation, we measured radioactivity of the both samples using a germanium detector. We evaluated the radioactivity especially gamma rays with 511 keV and isotopes created by activation. Both aluminum sample and stainless sample had not less than 400 Bq and 1300 Bq of radioactivity immediately after the irradiation respectively. Comparing activity of the both sample, we found that aluminum is thus one of the promising candidates for the better material of OpenPET components. We also evaluated the activation of Gd
₂
SiO
₅
scintillator in the same way.