Experimental measurement and Monte Carlo assessment of Argon-41 production in a PET cyclotron facility

In a medical cyclotron facility, 41Ar (t1/2 = 109.34 m) is produced by the activation of air due to the neutron flux during irradiation, according to the 40Ar(n,γ)41Ar reaction; this is particularly relevant in widely diffused high beam current cyclotrons for the production of PET radionuclides. While theoretical estimations of the 41Ar production have been published, no data are available on direct experimental measurements for a biomedical cyclotron. In this work, we describe a sampling methodology and report the results of an extensive measurement campaign. Furthermore, the experimental results are compared with Monte Carlo simulations performed with the FLUKA code. To measure 41Ar activity, air samples were taken inside the cyclotron bunker in sealed Marinelli beakers, during the routine production of 18F with a 16.5 MeV GE-PETtrace cyclotron; this sampling thus reproduces a situation of absence of air changes. Samples analysis was performed in a gamma-ray spectrometry system equipped with HPGe detector. Monte Carlo assessment of the 41Ar saturation yield was performed directly using the standard FLUKA score RESNUCLE, and off-line by the convolution of neutron fluence with cross section data. The average 41Ar saturation yield per one liter of air of 41Ar, measured in gamma-ray spectrometry, resulted to be 3.0 ± 0.6 Bq/µA*dm3 while simulations gave a result of 6.9 ± 0.3 Bq/µA*dm3 in the direct assessment and 6.92 ± 0.22 Bq/µA*dm3 by the convolution neutron fluence-to-cross section.