Drop and Run - Radioactive Cobalt-60 (Co-60) Source

Radiation protection aspects of the Drop and Run Co-60 source

We need some additional data.

Be careful where you obtain your data! You could try the Ionactive Co-60 (Cobalt-60) Radiation Safety Data resource. What we are after is a specific gamma ray constant that relates the Co-60 source specification to radiation dose rate. Using the linked resource we find the following:

• 0.00386 mSv/h (gamma dose rate), or 3.86 micro Sv/h, for 1 MBq of Co-60 at 30 cm.

We need to do some conversions to turn this data into something more suitable for the type of source we are considering. Let's start with dose date, relative to radioactivity.

• 142.82 mSv/h at 30 cm for 37 GBq (1 Ci) of Co-60 [this is just manipulating numbers and ratios].

Next, let's consider the dose rate at 1 m (assumed to be arms length, the importance of this will be obvious shortly). We do this by considering inverse square law. So we will convert the dose rate at 30 cm (0.3 m) to that at 1 m.

• 142.82 mSv/h x (0.3²/1²) = 12.85 mSv/h at 1 m for 37 GBq (1 Ci) of Co-60.

Finally we can convert the above to the actual activity of the source in 1963.

• 12.85 mSv/h (for 1 Ci) x 3540 Ci = 45.5 Sv/h.

So we see that the baseline (1963) radiation dose rate from this source is 45.5 Sv/h at 1 m.

[Ionactive comment: Do not get too hung up on the exact value at this point. For example, one well known online calculator will report 40.2 Sv/h at 1 m for this activity of Co-60. Some publications online and trusted paper texts will report values a little above or a little below our own offered value. There are other factors at play which are beyond consideration in this particular article. However, if you are itching for more content you might want to consider the following Ionactive resource : Formula for calculating dose rates from gamma emitting radioactive materials - this will open in a new tab as it is beyond our goals for this article].

We should now spend a few moment considering units of radiation exposure.

The unit of Sievert (Sv) expresses excess radiation induced cancer risk - so is mostly used at dose rates (and doses) way below what we are considering here. The unit is used when evaluating stochastic radiation risks. You will often see values in micro Sv/h, mSv etc.

The unit of Gray (Gy) (absorbed dose) is the correct unit to consider if we are looking at radiation induced clinical effects (such as nausea, vomiting, reddening of the skins etc). This is also known as Acute Radiation Syndrome (ARS). To a reasonable approximation (for this article), whole body doses of less than 1 Sv (1 Gy), received over hours / days, will most likely lead to stochastic effects (risk of excess induced cancer), as opposed to higher doses which may lead to clinically observable deterministic effects (e.g. radiation sickness).

[Ionactive comment: Since we have been using the curie (Ci) for historical context, we probably should mention the rad and rem, still widely used in the US. The direct conversion is that 100 rad = 1 Gy and 100 rem = 1 Sv ].

Whole body dose rate and accumulation

Let's imagine you see this source laying on the ground - you first spot this where the trunk of the body is 1 m from it (we will ignore approach dose / dose rates). This scenario could be depicted as follows (not to scale).

The dose rates experienced by our intrepid character at 1 m are as follows - with some rounding (we will keep them in Sv for time being).

• 45.5 Sv/h
• 0.76 Sv / min
• 0.0126 Sv / second

These dose rates are survivable at short duration (but exposure not advised !). Text books will tell you that deterministic effects (tissue reactions) can be observed (by studying changes in the blood) from about 0.1 Gy (0.1 Sv). For this article we are considering whole body deterministic effects - so assume that 4 Gy received in minutes / hours will be the LD50/60 (meaning would cause death to 50% of those exposed within 60 days without medical attention). Doses received at levels higher then this will expediate the clinically observable effects, and lower the survivability (even with medical intervention).

[Ionactive comment. Note in the above paragraph we are using the Gy, for good reason. However, as suggested earlier you can think of 1 Gy = 1 Sv for whole body exposure to gamma radiation. This is not valid when you start to consider exposures to specific organs and tissues in the body where the risk of excess radiation induced cancer needs to be considered. Additionally, exposure to alpha particles and neutrons will alter the conversion between the two where equivalent dose and effective dose then needs to be considered].

At 1 m we can consider whole body exposure over short duration as follows:

• 4 Gy (4 Sv) is reached at about 5 minutes and 16 seconds - outlook not good.
• 10 Gy (10 Sv) is reached in about 13 minutes and 9 seconds - death more or less assured.

These are quite long durations to be simply standing at 1 m and staring at the source! So now let's assume you take a closer look.

Accumulation of dose to hand and whole body

You go over and pick up the source. To keep the maths simple we will assume for the moment that you are holding the source with arm out stretched (maybe you have presbyopia - age related farsightedness). We can assume therefore that the trunk of the body is still 1 m from the source (and so the whole body dose rates / doses noted above are still valid). This is depicted as shown below.

This article uses approximations which do not detract from the message. We are going to use another approximation and assume the Co-60 source is a point source. We will now use the inverse square law to predict the dose rate to the hand. It is assumed that the average distance between the hand and the source at any point is 1 cm. We will write the derivation in Gy since this is really the right unit for the hand dose we are about to calculate.

• Dose rate at 1 m from source is 45.5 Gy/h (from earlier)
• Distance from source to hand is 1 cm
• Hand dose rate is: (1²/0.01²) = 45.5 x 10,000 = 455,000 Gy/h

That is a huge dose rate. Let's work that out over shorter time periods.

• 455,000 Gy/h
• 7583 Gy/minute
• 126 Gy/second

This dose rate to the extremity (hand) is going to damage it irrecoverably. If you pick the source up in order to read the warning, by the time your mind has registered "drop", it is too late to avoid damage. It is hard to say exactly what the damage will be, but it will be unpleasant.

The following information and pictures are taken from the IAEA publication "The Radiological Accident in Samut Prakarn" (IAEA, 2002) [opens in a new tab]. The incident involved a Co-60 teletherapy head which was partially dismantled, taken from an unsecured storage location, and then sold as scrap metal. Ten people received very high radiation doses from the source. Three of them, all workers at a junkyard, died within two months of the accident as a consequence of their exposure. The source activity at the time of the incident was estimated to be 425 Ci (15.7 TBq) which is 12% of our own drop and run source (in 1963). The image is taken from person (P1) who received a substantial radiation dose to the hands (the actual dose is not reported in the publication, but it can be assumed that our own calculated exposure to the hand must be at least as much).

Remember that the hand dose rate we have calculated for our source is 126 Gy/second (the dose rate from the IAEA source in similar geometry would be 15 Gy/second). What we do not know is handling time. However, it is a reasonable assumption that both incidents will produce similar clinical symptoms - this being (for example) : Initial erythema → dry desquamation → advanced erythema → moist desquamation → ulceration & necrosis → possible amputation. The time scale for the onset of each of these conditions will be from days (initially) towards weeks for the most severe later injury. The overall speed and severity of these stages will increase with dose received above the deterministic threshold. Typically 3 Gy to the skin will induced reddening after a few days.

It is clear that handling the 1963 Co-60 source is a bad idea and will lead to significant injury to the hand (understatement).

What if our demonstrator holds the source and needs to read it at 10 cm from the eyes (assume they have forgotten their reading glasses). To do this they probably hold the source about 30 cm from the trunk of the body.

As before we will start with a dose rate at 1 m from the source stated as 45.5 Gy/h (from earlier).

The eye dose rate will be calculated as follows : (1²/0.1²) = 45.5 x 100 = 4550 Gy/h.

The trunk body dose rate will be calculated as follows: (1²/0.3²) = 45.5 x 11.1 = 505 Gy/h.

For a 10 second exposure, we can calculate the following:

• Dose rate to the eyes is 4550 Gy/h , so the accumulated exposure over 10 seconds is 12.6 Gy.
• Dose rate to the whole body is 505 Gy/h , so the accumulated exposure over 10 seconds is 1.4 Gy.

Note we have already calculated likely hand dose rate at 126 Gy/second, so accumulated dose could be in the order of 1.26 kGy (for 10 seconds).

However you look at this, the Co-60 radioactive source is going to deliver significant radiation injury to all parts of the body.

The "drop and run" instruction is totally valid and needed, but in many cases will not be good enough! It might be survivable if you are no nearer than 1 m. It might also be survivable nearer the source, but if your hands (or any other part of the body) are in close proximity, you are going to be severally injured.

Some further thoughts on the Co-60 Drop and Run source

In this final section we provide further information and also some reaction and comment to how Drop and Run has been discussed on the internet.

No - it's not like this

• Some sites / blogs (etc) have reported the Co-60 source being used as part of an x-ray machine (not it's not).
• Some sites have suggested the Co-60 source would cause wide spread contamination (like Cs-137 potentially can when uncontrolled in a dispersible form). No it would not. Particularly with Co-60, the source material is essentially irradiated solid metal cobalt rods, singularly or doubly encapsulated in steel. They will not be a significant producer of radioactive contamination.
• The radiation hazards from the Drop and Run source are similar to that experienced by workers and members of the public at the Chernobyl and Fukushima incidents. Probably not! Several Chernobyl workers may have got close to these dose rates, but overall the intensity of radiation emitted from the deliberately constructed Co-60 source is likely to be much more than the average (external) dose rates present at the two incidents. Public exposures from both incidents would be absolutely trivial / negligible compared to the potential exposures from the drop and run source.

IAEA / world wide initiatives

The above mentioned (and linked) 'Samut Prakarn' incident and other similar incidents prompted IAEA to more closely consider the risks from orphan / abandoned radioactive sources. One initiative was to come up with new signage that could be located on the outer shielding of high activity sealed sources (but generally inside cosmetic covers). The idea would be to scare off scrap hunters (and others) who might come across the source. An example of this signage is shown below.

The IAEA conducted a survey in 11 countries around the world to determine what a new supplementary sign might look like. The conclusions were that there needed to be a sign which would:

• Consist of a red triangle which depicts radiation waves (in white).
• Display a skull and a running human figure to depict danger (also in white).

The newer IAEA symbol is not a replacement for the well known radiation trefoil symbol discussed earlier. The IAEA advised that the symbol should not be used on the outer surface of source containers, transport containers or similar. Rather it should be used on the outer surface of radiation safety shielding of high activity source equipment, and so underneath the outer cosmetic covers. Therefore it would be seen before attempting to dismantle the shielding around a sealed radioactive source, as a final warning to prevent exposure to ionizing radiation.

We have seen recent misuse of this supplementary safety sign, on the access door to a high energy accelerator facility.