Ludwik made a good point (alpha source is not a point source) and asked a question (alpha energy spectrum). Bernard asked about dead-time.
It turns out that the alpha source currently being used in the vacuum chamber I built is smaller than Ludwik imagined, although he is correct that it is not a point. Some smoke detectors have sources that are roughly 5 mm in diameter, as Ludwik suggestion. However, the one used for the data I provided is actually 2.5 mm diameter. Not a point, but pretty small.
The best alpha sources I am aware of have been diffused into a metal surface. I frequently used stainless steel substrates, but many people use gold. You deposit a small drop of source material onto the metal substrate, and then heat the substrate and source on a hotplate that is just less than dull-red from heat. (Use a dark room to set the hotplate temp to just barely see the dull red.) At that temperature the source atoms diffuse into the surface of the metal substrate. The result is amazing. It is a very thin source and you get full energy alphas from it. Yet, if you rub it fairly vigorously with a cotton Q-tip, the source does not rub off. I am aware of a lot of people who make thin sources this way. It's a common nuclear science technique. Some smoke detector sources are made this way. They are typically about 5 mm in diameter, and these are what you want if you want to have nearly a "line spectrum" at the published alpha energy for that nuclide.
A much cheaper way of making a 241-Am source is to press americium oxide into a pellet. This is the source I used for the data you have seen. It is 2.5 mm diameter and 1 mm thick. Since it is so thick the alphas coming from this source run the full range of energy from zero to 5.29 MeV. From a nuclear science viewpoint it is a lousy source because it is too thick. From a smoke detector viewpoint it is a good source because it is way less costly than making a diffused source.
As for dead time, the data in the table are counts per minute, so the dead time is being shown in minutes. The number listed 4e-6 minutes. In seconds this would be 2.4e-4 s, which is indeed, as Bernard suggested, a couple hundred microseconds. Dead time was measured in the standard way of counting a split source. In this case a beta source. I don't recall for sure, but think each half of the source was about 10,000 cpm, so you count each source alone, then together, and notice that 10,000 plus 10,000 only yields perhaps 17,000 (when counted simultaneously), so dead time is calculated from this. Note, these count rates for dead-time measurement are made-up numbers, but I think they are in the correct ballpark.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu