That's a good link, but not quite right for the detector here. On this one, pressing the switch closes a contact, rather than opening one. So perhaps the circuit has a reverse bias to that on the link you found.

The nice thing about this kind of detector, is that it is all solid state, apart from the radioactive emitter bit. So nothing to gum up or fail. If the radioactive emitter itself fails, the alarm should go off. But with a half life of hundreds of years, I think this detector has quite a bit of life in it yet.

Cheers