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

Let's start with a picture of this infamous Co-60 radioactive source.

Drop and Run Ionactive Blog

Drop and Run Cobalt-60 (Co-60) radioactive source

Let's take a look at this from the top.

  • Danger
  • Radiation
  • Ionising radiation trefoil sign (upside down!)
  • Drop and Run
  • Co-60 (radioactive material, Cobalt-60)
  • 3540 Curies (on different lines, these go together)
  • 7 January 1963 (the reference date of the source)

Let's look at the above information in a little more detail.

Danger / Radiation - Self explanatory. As we will see below, the source was a danger in 1963 and still presents a radiation risk today (assuming the source was still available for measurement).

Ionising radiation symbol. If you do not read / understand English, then you might still understand this symbol (☢) which may be the most recognised safety related sign in the world (featured in plenty of movies etc). It is known as a trefoil, and call us pedantic, but it is the wrong way up on the source (we hear you 'so turn the source upside down'). A variety of symbols were adopted in the mid 1940's in several different colour schemes and orientations. The ISO standard 361 was created in 1975 which specified the black symbol used today (☢), normally featuring on a yellow background.

Co-60 (Cobalt-60) - this specifies the type of radioactive material contained inside the source casing. Co-60 is an artificially made radionuclide, generally created in a nuclear reactor by bombarding Co-59 (naturally occurring and stable) with neutrons. The decay of Co-60 starts from the moment it is created. For those that want to know:

\[_{27}^{59}\textrm{Co}+n\to _{27}^{60}\textrm{Co}\to _{28}^{60}\textrm{Ni}+e^{-}+2\gamma \]

We see from the above expression that Co-60 emits a beta particle and 2 gamma rays (there are other things going on but we will save them for another day). For the purpose of this article (and radiation safety) we can ignore the beta particle, so are left with the 2 gamma rays and an atom of stable Ni-60.

The two gamma rays are particularly important since both are released with near 100% probability during the decay (think of double barrel gun which shoots two bullets at once when fired). The energy of the gamma rays are 1.17 and 1.33 MeV respectively. For the purposes of this article we can say that MeV represents the penetration (impact / "punch" of the radiation), and its use is best understood by comparing with other radioactive materials. Co-60 can be a problem radiation safety wise because the gamma radiation is penetrating, difficult to shield, and a significant dose can be delivered per decay. For example, compare with (probably) the most widely publicly recognised radioactive material Cs-137 (cesium-137). Cs-137 emits one gamma ray per disintegration with 85.1% probability at an energy of 0.662 MeV. So per disintegration (radioactive decay), Co-60 is obviously more problematic than Cs-137 (although these very properties when carefully harnessed are used in industrial sterilisation which is a vital service to the medical sector). Finally - never use "Cobalt" or "Cesium", always specify their mass number (i.e. Co-60) - this makes all the difference. For example Co-57 emits gamma rays from 6-136 keV (note 1 keV = 1/1000 MeV) - so you will never find a Co-57 source with "Drop and Run" written on it!

3540 Curies - this expresses the radioactivity of the source (how radioactive it is). In 1963 most were using the unit of curies (Ci). The unit of radioactivity is about the rate of decay (how many disintegrations per unit time). 1 Ci is a significant activity and is equal to 3.7 x 1010 disintegrations per second. That is a lot of gamma rays being emitted every second! But we have 3540 Ci (in 1963), so that is then 1.31 x 1015 disintegrations a second, written long hand that is 1,309,800,000,000,000 per second (and every disintegration causes two relatively high energy gamma rays to be released). A high activity Co-60 source creates a significant high energy high dose rate radiation exposure. "Drop and Run" is making some sense.

The Curie (Ci) has been replaced with the Becquerel (Bq) under the SI (International System of Units) since the 1960s and most of the world now uses the Bq. A notable exception is the USA which still formally uses the Ci (although that appears to be slowing changing towards the metric system). Furthermore, in some areas of industry such as industrial radiography and industrial irradiation, you will still find activity referred to in curies, even if the official records are in SI units (Bq).

It can be shown that 1 Ci = 37 GBq of activity. So 3540 Ci (in 1963) can be expressed as 130.98 TBq. We use the Bq in our blog articles (at least primarily), but make an exception here given the historical context.

Be careful if you see comparisons between two different activities - know what radioactive material is being considered. If you are fairly comfortable with this article so far, you will be able to acknowledge that 1 Ci (37 GBq) of Co-60 is significantly harder to deal with than 1 Ci (37 GBq) of Cs-137 (in terms of radiation dose rate, exposure potential, extent of shielding and so on). This is despite the fact both have the same activity.

The date 7 January 1963 - this is often known as the source 'reference date'. On this date the reported activity is 3540 Ci (130.98 TBq). From this date onwards, the activity of the source is reducing, and this is where we can introduce half life. Half life is the time taken for the activity of radioactive material to reduce by half (sometimes written as T1/2). Half life is unique for every radioisotope (artificial or natural) and is fixed by the physics of the particular radioactive substances (so T1/2 cannot be slowed down, sped up or changed by external factors such as heat).

The half life of Co-60 is 5.3 years (we could be more precise than this, but not needed for this article). By comparison, the Cs-137 mentioned earlier has a half of 30 years. Half life varies greatly such that F-18 (used in PET diagnostic procedures in medical facilities) has a half life of 110 minutes, where as naturally occurring uranium U-238 (found in the ground) has a half life of about 4.5 billion years (4.468×109 years if you really need to know!).

The date of issuing this blog is 22 January 2024. Using whole years the Co-60 has been decaying for 61 years since the reference date. The number of T1/2 is then 61/5.3 = 11.3 half life. The reduction factor will then be 0.511.3 = 3.96 x 10-4. Therefore, with the information we have to date, the current activity of Co-60 today will be 3540 Ci x 3.96 x 10-4 = 1.404 Ci (51.95 GBq) - ignore small rounding errors. [Ionactive comment: online radiation protection calculators, including our own, can do these calculations in an instant. However sometimes it's nice to see how these are hand calculated].

The source today is not quite the source it used to be. Later below we shall determine how much of a problem it might still cause if presented in an uncontrolled way in 2024. For now we will look at radiation protection aspects of the source during 1963 with maximum activity and see how valid "Drop and Run" was then.

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).

Drop and Run Ionactive Blog source at 1m

The Drop and Run Co-60 source is discovered (at 1 m)

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.

Drop and Run Ionactive Blog source hand and body dose

Holding the Drop and Run Co-60 source in the hand, arm out stretched to read the warning

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.

Drop and Run Ionactive Blog Samut Prakarn hand injury and doses

IAEA - The Radiological Accident in Samut Prakarn - hand injury to person (P1)

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.

How about finding the drop and run source today -2024?

This is depicted as shown below.

Drop and Run Ionactive Blog 2024 exposures

Drop and Run Co-60 source in 2024

What we do is as follows:

  • Find the ratio of radioactivity i.e. 2024 compared to 1963.
  • We can then modify dose rates (calculated above) by multiplying by our ratio.
  • We can also modify the accumulated dose (for a specific time) by dividing by the ratio. Recall that dose accumulated is related to dose rate and time of exposure. If the dose rate is less (because the activity is less), then the time required to reach a specific level of exposure will increase.

Let's look at some examples to illustrate this.

The ratio of activity will be as follows: 1.401 Ci (2024) / 3540 Ci (1963) = 3.95 x 10-4 (0.000395). Since gamma dose rate for Co-60 is related to activity (all other things being equal), we can use this ratio to modify our calculations.

  • Dose rate 1 m from the source (1963) : 45.5 Gy/h (calculated earlier).
  • Dose rate 1 m from the source (2024) : 18 mGy/h (could be written as 18 mSv/h as we are now out of deterministic territory and into stochastic effects).

By modern public health and occupational radiation protection standards, these are still significant dose rates.

Let's look at the extremity dose rate to the hand.

  • Dose rate 1 cm from the source (1963) : 455,000 Gy/h (calculated earlier)
  • Dose rate 1 cm from the source (2024) : 180 Gy/h (with some rounding)

Let's look at that extremity dose rate over a shorter time interval.

  • Dose rate per minute = 3 Gy / minute
  • Dose rate per second = 0.05 Gy per second

by modern standards, these are significant dose rates. However, look back at our analysis for holding the source for 10 seconds. This would yield a dose of 0.5 Gy. Whilst still massive compared to public or occupational exposure legal limits, this exposure is well outside of a deterministic radiation injury. If we consider 3 Gy to the hands as being indicative of the onset of reddening of the skin, then this would require at least 1 minute of exposure. It would be expected (hoped!), that the "drop and run" information would have been well read and complied with in well under 1 minute.

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.

Drop and Run Ionactive Blog IAEA HASS symbol

IAEA - HASS Radioactive Source - supplementary Symbol

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.

Cobalt-60 (Co-60) uses today

We are conscious that some of you reading this article might assume that radioactive Co-60 is an unjustified risk - this is not the case. We have already featured some uses earlier, but here are some current uses of Co-60 which improve our world significantly. All that is required is compliant management of Co-60 like any other potential health and safety risk in the workplace.

  • Industrial radiography (less used today).
  • Gamma knife (stereotactic radiosurgery of the head and neck)..
  • Industrial sterilisation of medical products and similar.
  • Calibration of high dose rate radiation monitors / detectors for industry.
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