Protection by reducing exposure time - is it always valid?
Published: Sep 29, 2021
The short answer is No! This rule of thumb is perhaps a little more abstract, but it might get you thinking. Reducing exposure by minimising time is a valid radiation safety principle in many circumstances. However, this is not always the case as these three examples will show.
Very high gamma dose rates
We will not dwell on the circumstances leading to a high dose rate, only the implication of using time as a protection measure. Consider the Co-60 activity in an industrial irradiation facility. Typical activity in such a facility might be 37 PBq (1 MCi). The dose rate at 1 m from the fully exposed sources would be in the order of 11400 Sv/h. This is therefore about 3 Sv / second. Given that the LD50-60 for survival from a high dose of ionising radiation (i.e. 50% exposed will die within 60 days without medical intervention) is about 5 Gy, then the 3 Sv / second exposure will be catastrophic. The reduction of time in these circumstances is completely irreverent - there is no thinking time to even consider protection by time. [For the purposes of this example we will assume that 1 Gy = 1 Sv whole body dose from gamma rays].
A vial containing 1 GBq of finely powered Po-210
Many years ago we did a blog article hosted on our old website which looked at the Alexander Litvinenko Po-210 event. We calculated early on in the incident that his death could have been induced by an intake of as little as 115.5 MBq (7 ug of pure Po-210). This web article is a reminder of this 2006 event (via Wikipedia) : Alexander Litvinenko ).
So consider 1 GBq of Po-210 in a sealed glass vial on an open bench. What about time protection in these circumstances? The emissions from Po-210 are a 5.3 MeV alphas with a probability of 100%, and a gamma ray of 0.8 MeV with < 0.001% probability, Essentially we say that Po-210 is an alpha emitter, but technically is also a gamma ray emitter as indicated. The gamma ray dose can be shown to be about 0.001 micro Sv/h at 1 m, and on the surface of the vial would be of the order of a few micro Sv/h at most.
Therefore, as long as the vial is intact and the Po-210 stays where it is (contained) then protection by reducing exposure time is irrelevant.
Work around an airport x-ray screening unit at departures
You queue up and wait your turn to put your cabin baggage, laptop, liquids etc into a grey tray and send it through the x-ray system. Those around the x-ray unit will include yourself (generally at the in-feed or out-feed) and other security personnel including the operator who will be located to the side of the unit looking at the display screen. The unit will contain one or more x-ray tubes typically operating at 140 kV and 1mA. Some of the latest x-ray units are based on CT (computed tomography) technology with similar kV but higher current. Whilst NOT a legal limit, the internationally recognised external instantaneous dose rate for x-ray cabinet systems will not exceed 1 micro Sv/h. Depending what you read, this will be measured at the surface, at 5 cm or 10 cm from surface. If you dig into the origin of the 1 micro Sv/h (which is based on exemption from notifying your local regulator of your use of ionising radiation), then its 10 cm from the surface. These exemptions are not always granted (they are not available in the UK for x-ray systems). However they are meant to suggest that the exposure potential is so low the the system may not need further regulatory control.
In practice most manufacturers will measure this at the surface. In the competitive world of x-ray security no company is going to supply x-ray equipment that does not meet this dose rate 'limit'. Ionactive finds that in most cases dose rates around such units are so near background that there is effectively no exposure from the x-ray unit. Sometimes we will find slightly elevated dose rates at the in-feed and out-feed, but by the time dose averaging is calculated exposure potential in these areas is no greater than background.
Therefore, operators of this type of x-ray equipment, and the security personnel near by, do not need to wear dosimetry (personal radiation monitor) and there is no restriction on working time (with respect to radiation safety). The rule of reducing exposure by minimising time does not apply.