setting scintillator frisker 43-92 with model 3277/1 alarm

[Gracie J]

I'm trying to convert what would be equivalent to 0.4bq into CPS on the Ludlum 3277/1 with 43-92 scintillator that has an efficiency of 25.5%..

everyone around me is getting differing answers,

the calibration unit has set the alarm to 10CPS but I don't think this is correct? can someone break this down for me?

Source: Pu-239, 4337
Activity: 30972.44 dpm

Meter reading: 7960,7873,7780
count time: 1 min
Distance source to detector: surface

average efficiency 25.5% 4pi

?? would an alarm at 10CPS be equivalent to 0.4Bq?

[bigjoemonger]

Pu-239 is a straight alpha emitter, emitting one alpha per decay. So should be a 1 to 1 relationship for a standard efficiency conversion.
Average meter reading of 7871 cpm?
Average 4pi efficiency of 0.255

7871 / 0.255 = 30,867 dpm.

You state an activity of 30,972.44 dpm

That's a difference of 0.3% so I'd say your calibration is pretty spot on.

Knowing that you can apply the same logic to your alarm setpoint.

10 cps / 0.255 = 39.2 dps

If you're trying to set the alarm setpoint at 0.4 Bq = 0.4 dps then you'd do
0.4 * 0.255 = 0.102 cps

I dont think that instrument is capable of setting an alarm setpoint that low, is it?