Quote of the Day
Doing statistics is like doing crosswords except that one cannot know for sure whether one has found the solution.
— John Tukey, statistician and data analyst extraordinaire. If you get the chance, read his book Exploratory Data Analysis. It is a gem.
Introduction
Figure 1: Cobalt60 Use in a Gamma Knife. (Source)
I was reading the Washington Post this weekend when I stumbled upon an 22July2017 article about concerns that ISIS in Mosul had access to an old medical radiation source. This source, which contains the radioactive isotope cobalt60, is used in the treatment of cancer (Figure 1). However, cobalt60 is extremely radioactive and could be used to build a dirty bomb. Fortunately, ISIS did not touch the source, but the concerns about a terrorist being able to use one of these radiation sources for a dirty bomb are real.
There have been encounters between radiation sources and unwary people – the encounters did not end well. For example, in one case, people tried to breakdown a cesium137 radiation source for scrap. The incident ended with four people dead, twenty hospitalized, and 249 contaminated.
The Washington Post article mentioned three facts that we can easily verify using some simple math.
 The source contains 9 grams of cobalt60, which generates a radiation level of 10,000 curies (Ci) when new. (Quote)
 Person standing three feet (~1 meter) from the unshielded source would receive a fatal does in less than 3 minutes. (Quote)
 The source is 30 years old, so its radiation level is significantly diminished with respect to a new source. (Quote)
Background
Definitions
 becquerel (Bq)
 One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. (Source)
 curie (Ci)
 The curie (symbol Ci) is a nonSI unit of radioactivity, one Ci = 3.7 × 10^{10} nucleus decays per second or one Ci = 3.73.7 × 10^{10} Bq. (Source)
 sievert (Sv)
 The Wikipedia defines the Sievert (symbol: Sv) as the SI derived unit of equivalent radiation dose. The Sievert represents a measure of the biological effect, and should not be used to express the unmodified absorbed dose of radiation energy, which is a physical quantity measured in Grays.
 Dose Equivalent (H)
 Equivalent dose is a dose quantity H representing the stochastic health effects of low levels of ionizing radiation on the human body. It is derived from the physical quantity absorbed dose, but also takes into account the biological effectiveness of the radiation, which is dependent on the radiation type and energy. In the SI system of units, the unit of measure is the sievert (Sv). (Source)
 HalfLife (t_{HL})
 Halflife is the time required for a quantity to reduce to half its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo, or how long stable atoms survive, radioactive decay. (Source)
Radiation Level Given HalfLife
Equation 1 tells us the amount of a radioactive species we have remaining after time t assuming that we had a 100% pure sample at time ø.
Eq. 1 

where
 T_{HL} is the halflife of the radioactive species.
 t is elapsed time since having a pure sample.
 N_{0} is the initial amount of the substance. You can use mass or moles or even numbers of atoms.
 N is the amount of the radioactive species left after time t.
To obtain the level of radioactivity (i.e. decays per second), we need to take the derivative of Equation 1 (see Figure 3).
Analysis
Setup
Figure 2 shows how I setup the calculations in Mathcad 15.
Figure 2: Analysis Setup.
Calculations
Figure 3 shows my calculations that duplicate the results in the Washington Post article. The purple check marks indicate the specific results. Note that the Post article computes that you would get a lethal dose of radiation from a new Co_{60} source ~2.5 minutes when it is new, a calculation which I duplicate below. The 30year old source in question is shown to have its lethality reduced by a factor of over 50. This means you would get a lethal dose of radiation from this source with ~2 hours of exposure (thanks to Ronan in the comments for pointing this out).
Figure 3: Mathcad Calculations.
Conclusion
I had never thought about the potential security issues associated with medical radiation sources. I was surprised to the see how intense the radiation levels were from a cobalt source. While the cobalt source mentioned in the article is 30 years old and only 2% of its initial radiation level, it is still a very dangerous item.
Appendix A: Quotes from the Article
Radiation Level Quote
In a draft report written in November 2015, research fellow Sarah Burkhard calculated that the radioactive cores, when new, contained about nine grams of pure cobalt60 with a potency of more than 10,000 curies — a standard measure of radioactivity.
Fatal Dose Quote
A person standing three feet from the unshielded core would receive a fatal dose of radiation in less than three minutes.
Reduction in Radiation Level Quote
Because cobalt60 decays over time, the potency of the Mosul machines’ 30yearold cobalt cores would have been far less than when the equipment was new, but still easily enough to deliver a lethal dose at close range, the report said.
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