I suppose it depends on what is meant by "gone". Presumably it doesn't mean actually, physically absent, but simply below some activity threshhold, because 0.5^7 indicates .78% of the original parent should still be present. There are circumstances where one could consider that level of concentration to be equivalent to none for practical purposes, but practical purposes are not the same as reality.
Being pedantic you have 0.78125 % of the parent material. Realy it depends on the exact parent and the daughter products. 0.7% could be negligible/ignored for safety purposes or it could still be a class AAA hazard.
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After 7 half lives you're supposed to still have almost 1%...
I suppose it depends on what is meant by "gone". Presumably it doesn't mean actually, physically absent, but simply below some activity threshhold, because 0.5^7 indicates .78% of the original parent should still be present. There are circumstances where one could consider that level of concentration to be equivalent to none for practical purposes, but practical purposes are not the same as reality.
Being pedantic you have 0.78125 % of the parent material. Realy it depends on the exact parent and the daughter products. 0.7% could be negligible/ignored for safety purposes or it could still be a class AAA hazard.