On Sat, 31 Oct 2020 15:45:55 -0700 (PDT), wrote:

Half life of Am is over 400 years but I suspect they are
intrinsically "fail safe" as it's the current flow between plates
due
to the ionisation of the air that holds the alarm off. Take away
the
ionising radiation, current stops and the alarm goes off.

I believe it's the other way round.

Nope, alpha radiation from the Am ionises the air, the plates have a
voltage between them so the generated ions whizz off to their
opposite charge plate causing a current to flow in the circuit
between the plates. Smoke absorbs/slows some of the ions, reducing
the current triggering the alarm.

So removing, or having much reduced radiation from, the Am would
trigger the alarm.

Don't know how the test button on an optical one works.

it doesn't test the detector

So not a real test is it? B-)

--
Cheers
Dave.