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That is, after creating a quantity of a radioactive isotope with a half-life of several seconds or minutes, could the sample be seen to change into decay products with the naked eye? (Excluding nuclear bombs!)

I understand it may not be possible to create a large sample of radioactive material with a short half-life in the first place, nor to safely observe it's decay.

In any case, I think it would be fascinating to watch as a lump of solid material transmutes or vanishes before your eyes in a matter of seconds.

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    $\begingroup$ You seem to assume that a radioactive substance disappears when it decays. That is not the case: It turns into some other substance, and depending on chemistry and I don't know what all else, the other substance will not necessarily look any different from the original. (E.g., cobalt-60 changes to nickel when it decays.) $\endgroup$ – Solomon Slow May 22 '18 at 17:09
  • $\begingroup$ @jameslarge Understood - so can a large lump of cobalt-60 (or another radioactive material) be seen to visibly shrink or become lighter as it decays? $\endgroup$ – RobertF May 22 '18 at 17:12
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    $\begingroup$ Don't think that you would want to be close enough to observe with your "naked eye" any macroscopic sample which is so intensely radioactive that it would transform into decay products in a time period of seconds to minutes. $\endgroup$ – Samuel Weir May 22 '18 at 17:13
  • $\begingroup$ @SamuelWeir Good point - let's say observing through heavy glass is allowed. :-) $\endgroup$ – RobertF May 22 '18 at 17:16
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    $\begingroup$ Also, don't loose track of what @SamuelWeir said: The radioactivity of different substances tends to be inversely proportional to their half-lives. Substances with half-lives in the tens of years are dangerous. You want a visible amount of something with a half-life measured in minutes? I can't even comprehend... Also, where are you going to get it? Pull it fresh from the core of an operating nuclear reactor? $\endgroup$ – Solomon Slow May 22 '18 at 17:50
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There's no practical way to do it, but if you had a quantity of Hg$_{195}$, it would decay over the course of a day or so (half-life = 10 hours) into isotopes of gold and platinum. Not the "seconds to minutes" that you asked about, but somewhat visible.

Most other transitions would be less dramatic to the eye. Solid -> gas is almost invisible unless the gas is very dense. Gas -> solid might create a bit of dust on the vessel, but it's hard to get a lot of mass in as a gas.

The real problem is that anything with a half-life short enough to have a visible decay would have to be created/isolated immediately before the viewing. Since we can only create such substances at tiny rates, the decay overwhelms the production and you never have any sizeable quantity.

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