It is not just the C-11 decay. The neutrons produced in the side reactions during operation will have been captured by various isotopes of all the materials that make up the device and surrounding structures. Such as Cu-63 +n -> Cu-64 which then decays to nickel or zinc with a half life of 12.7hrs. There will be hundreds of combinations of activation & decay reactions going on, each insignificant on its own but when modelled as a whole can add up to an appreciable contribution. Particularly the cumulative contribution from slightly longer lived isotopes after a few years of operation – hence why careful choice of materials down to every nut & bolt is needed.
Also care should be taken when saying the level is back to ‘background’. Background in New Jersey is different to Colorado. The contribution from the device needs to get down to ‘As Low As Reasonably Achievable’ (ALARA) http://www.nrc.gov/reading-rm/basic-ref/glossary/alara.html , which could be lower than the natural background.
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