I just watched Kyle Hill’s Half-Life Histories video about the Therac-25, but while he explains the software error with the turntable, he never explained why it was bad.
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Plainly difficult explains it well in his video, but in short the software thought the machine was emitting less dangerous radiation than it actually was. The radiation exposure was far in excess of what is safe for a person and the effects were fatal.
The turntable shifted different parts of the system into place, depending on what mode the operator wanted to operate the machine in.
There were 3 basic modes of operation.
1. Field light – this was just a light that shined on the patient to allow the equipment to be placed correctly
2. Low power electron therapy – a low power electron beam was shot at the patient as part of their treatment.
3. high power X-ray mode – a high power electron beam was fired at a secondary target that absorbed the electrons and emitted X-rays.
The problem happened when the control computer had an issue where it would generate a full power electron beam but the turntable was not in the correct orientation to put the X-ray target between the beam and patient. Subsequently, the patient got a lethal dose of electron (beta) radiation.
Other versions of the Therac machine had hard interlocks to prevent this, but in the interest of being simpler and cheaper, the Therac25 relied strictly on software guards that didn’t work as expected.
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