05 rad/hr*24 hr= 1.2 rad/day. Sounds kind of high to be just background. Did you calibrate your meter against a standard source recently?
the real test would be to measure the radioctivity of the tuna itself. cs137 decays via beta. Your meter wouldn't be able measure beta through a can anyway.
Your reply is incomplete and incorrect. Although Cs137 does decay thru beta, it emits gamma rays. A standard gamma detector will detect Cs 137.
From wikipededia:
Caesium-137 has a small number of practical uses. In small amounts, it is used to calibrate radiation-detection equipment. It is used as a gamma emitter for oilfield wireline density measurements. It is also sometimes used in cancer treatment, and it is also used industrially in gauges for measuring liquid flows and the thickness of materials.
Again, a standard gamma detector will detect Cs 137.
This article does not specify the amounts of contamination, and is therefore good for little besides fear mongering.
I had my meter repaired and calibrated by www.KI4U.com last September.
BTW, that is one great website. You can download Kearney’s book, Nuclear War Survival Skills for free. It covers a lot more than nuke war.