Take the example of a radioisotope with thalf = 13.9 ms. This time has been chosen to give a nice round number for the decay constant of 50 s−1 or, in different units, 0.05 ms−1. Suppose we begin the second with a thousand of this isotope’s short-lived nuclei. Then by the end of that second 50000 of these nuclei would have had time to decay if the high activity at the start of the second could have been maintained— clearly far more than are actually present in the sample. By the same token, 50 would decay in one millisecond if the activity at the start of the millisecond could have been maintained. In fact only 49 (approximately) decay in the millisecond.