Geiger counter readings from a radioactive source consisting of a small number of nuclei. The radiation from background sources grows larger than the signal from the decaying nuclei toward the very end of the process. That's why it can be tricky to do radiometric dating when most of the nuclei have decayed. Carbon dating, for example, is not useful beyond about 60,000 years ago, because not enough of the nuclei linger past that point to get good data.
The time units on the horizontal axis are arbitrary, so this could represent pretty much any type of nucleus. The numbers could be seconds, minutes, years, or centuries (though obviously it's not practical to run Geiger counters for that long!).
<a rel="nofollow" class="external text" href="https://www.youtube.com/user/GalileosPendulum">Matthew Francis</a>
CC BY 3.0
<bdo lang="en" dir="ltr"><a href="//commons.wikimedia.org/wiki/Commons:YouTube_files" title="Commons:YouTube files">YouTube</a>: <a rel="nofollow" class="external text" href="https://www.youtube.com/watch?v=VuepySONpo0"><bdi lang="">Nuclear decay: small population</bdi></a> – View/save archived versions on <a rel="nofollow" class="external text" href="https://web.archive.org/web/*/https://www.youtube.com/watch?v=VuepySONpo0">archive.org</a> and <a rel="nofollow" class="external text" href="https://archive.today/https://www.youtube.com/watch?v=VuepySONpo0">archive.today</a></bdo>