Dose is dose rate multiplied by time. If the rate is uniform, the time available before a chosen dose allowance is spent is simply t = D_allowed / D_rate. When both are expressed per hour the answer comes out in hours; multiplying by 60 gives the working figure in minutes.

Stay time is the third leg of the classic protection triad – time, distance, shielding – and it is the one under direct operator control at the moment of an intervention. It is used when someone must enter a field that cannot be eliminated: recovering a jammed source, retrieving a cassette from a hot bay, or working alongside another crew’s exposure. The dose allowance is not the annual limit. It is a small operational budget agreed in advance, typically a few tens of microsievert for a routine task, set well below any statutory figure so that the intervention leaves the yearly total essentially untouched.

The calculation assumes a constant rate over the whole period. Real interventions move through a steep gradient, since the field falls as the inverse square of distance. Where the person will be approaching and retreating, use the rate at the closest approach point for the whole duration – the error is on the safe side, and the alternative is an integral that nobody will do under pressure.

Stay time is a planning tool, not a substitute for an electronic personal dosimeter with a dose alarm. The alarm is what actually stops the job.

Worked example

Dose allowance for the task	100 uSv
Dose rate at the work position	2000 uSv/h
Number of people entering	1
Stay time	0.05 h
Stay time	3 min
Stay time	180 s
Dose accrued per minute	33.333 uSv

t = 100 uSv / 2000 uSv/h = 0.0500 h. In minutes that is 0.05 x 60 = 3.00 min, or 180 s. The field delivers 2000/60 = 33.333 uSv every minute.

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