A dose badge is a passive crystal, read by an accredited laboratory, and it cannot be faked or fooled. Badge 41 hangs on a hook in a corridor as our control and returns about 0.09 millisieverts a quarter.
I am a radiation protection adviser to two hospitals. It is a statutory appointment and my signature goes on things.
The monitoring is the whole of this so I will be exact.
Staff who work with radiation wear a badge. Ours are thermoluminescent: a small chip of lithium fluoride in a holder. Ionising radiation knocks electrons into traps in the crystal, and they stay there. The laboratory heats the chip and the trapped electrons fall back and emit light, and the amount of light is the dose.
It is passive. There is no battery, no electronics and nothing to interfere with. A badge cannot report a dose it did not receive, because the dose is physically stored in the crystal.
The badges go to an approved dosimetry service, which is a laboratory accredited to do this and nothing else, and the results come back as a formal record that has to be kept for fifty years.
Every batch includes a control badge.
The control does not go to a person. It hangs somewhere with no radiation, so that whatever it reads is the background: cosmic rays, the granite in the building, the natural potassium in everybody's body walking past.
It is subtracted from everybody else's result. Without a control you cannot tell a genuine occupational dose from the dose everybody in the county gets for being alive.
Ours is badge 41 and it hangs on a hook in the corridor outside the imaging office. Ground floor, internal wall, no windows, nothing above it but an office.
It reads high.
Background in this part of the country is about 0.06 millisieverts a quarter for a badge hanging still. That is what the laboratory expects and it is what our other sites return.
Badge 41 comes back at 0.09.
Half again is not noise. The laboratory's uncertainty at that level is a few per cent, and a badge that reads half again over expected background is a badge that has been near something.
So I did what an RPA does, which is find the something.
The imaging department is fifty metres away down the corridor and round a corner, through two structural walls. Every room in it is shielded to a specification I signed off, and the shielding is verified by survey, and the surveys are in my files.
I re-surveyed. Twice, in 2019 and 2022, with a calibrated instrument, in the corridor, in the office, and against the wall the hook is on.
Nothing above background. Not slightly above. At background, everywhere in that corridor, including six inches from the hook.
An instrument survey and a badge measure different things, and there is a good reason they might disagree, so it is worth taking seriously.
A survey meter measures dose rate now. A badge integrates over three months. So a source that was there for an hour on a Tuesday would show on the badge and not on my survey.
There is no source that could be. Mobile equipment is logged, tracked and stored in the department. Nuclear medicine is in a different building with its own controls and a transport log I audit.
And whatever it is would have to happen every quarter, for eleven years, at almost exactly the same magnitude.
I moved the badge.
That is the obvious experiment and it took me until 2020 to think of it, which I am not proud of.
I put the control in the office instead of the corridor, four metres away, for two quarters.
It read 0.06. Ordinary background, twice running.
I put it back on the hook. 0.09.
I then hung a second control on the same hook alongside it for a year, so two badges from two different batches were on the same nail.
Both read 0.09, all four quarters.
The hook.
It is a brass hook screwed into the plaster at about shoulder height, and it was there before the corridor was refurbished in 2011, because I have a photograph from a survey in 2009 with it in the background.
The wall behind it is a structural wall. On the other side is a lift shaft.
I have surveyed inside the lift shaft, in a hard hat, with the engineer, at the level of that hook. Background.
I have had a sample of the plaster and a sample of the brick analysed, because a naturally radioactive aggregate is a real and boring explanation for a hot spot in a building.
The report says both are unremarkable.
What the number means for anybody.
Nothing. That is the answer I have given three times to three managers and it is true.
Nought point nought nine millisieverts a quarter is about what you get on two flights to Spain. Nobody stands in that corridor for a quarter. Nobody is at risk and nothing in this hospital is out of compliance.
The only real consequence is arithmetic: because the control is subtracted from everybody's badge, a control that reads high makes every member of staff's recorded occupational dose lower by 0.03 a quarter than it truly is.
That matters to me more than it matters to anybody else, because the dose record is the thing that would be looked at in thirty years if somebody got ill, and it should be right.
So I moved the control to the office in 2023 and left it there.
That is the correct decision and I would make it again. The office is a proper control location, the numbers are right now, and every staff dose since is a better record than the ones before it.
Badge 41's hook is empty.
I still walk that corridor most weeks and I have got into the way of glancing at it, and it is an empty brass hook on a plaster wall, and last April there was a badge on it.
Not one of ours. The holder is the old pattern, the grey ones we stopped using in about 2014, and it has a number on it that is not in the series we have ever been issued.
I sent it to the laboratory in the next batch, as an unknown, which is the correct thing to do.
It came back at 0.09.