Do you mind specifying the title of the paper? It appears there's quite a few papers[1][2][3] published concerning Therac-25 by an author named Leveson.
[1] http://sunnyday.mit.edu/papers/therac.pdf
[1] http://sunnyday.mit.edu/papers/therac.pdf
#1 is a later version that was an appendix to her book Safeware (which I have not read), and [3] is a nice second read that follows up on [2] many years later but isn't quite the relentless engineering detective story that makes the original so poignant.