Therac-25 (was Gamma 11)

[Bob Armstrong]

>The software was custom grown PDP-11 software from the AECL company. 
>The problem was that the instrument could deliver a massive 25,000 rad 
>blast over a 1 second interval due to the fact that the beam control 
>mask was incorrectly positioned.

  It's worth mentioning that the machine's predecessor, the Therac-20, had a
hardware interlock that would prevent the beam from being turned on if the
collimator was out of position.  The collimator blocks were mounted on a
rotating turntable and this interlock was nothing more complicated than a
microswitch that disabled the HV supply unless the table was rotated to the
right orientation.

  AECL removed the microswitch and several other safety interlocks from the
Therac-25 in the name of cost reduction, the plan being that the software
would perform the equivalent safety checks.

Bob