So how do you measure something that’s invisible? Something that weighs nothing, that we can’t smell or taste, that we can’t feel – how can we know how much is there? And just as important – how can we know how much is dangerous? Somehow we need to find a way to measure it – whatever it is – and then we have to be able to put a number to that measurement. That means that we have to come up with a system of units.
Think for example if you’re impressed by an athlete and ask your friend how fast they are. “Nine-five!” is the enthusiastic reply. And you’re left wondering what in the world that means. Is it 9.5 minutes to run a mile or two? Is it 95 seconds to run a half-mile? Does your friend mean 9.5 seconds to run 40 yards, or maybe to run 100 yards? Some of these times are genuinely impressive, others are remarkably ordinary or unimpressive. Without knowing the units (seconds, minutes, yards, miles) you have no way to evaluate the number your buddy gave you.
So now think about the early days of studying and learning about radiation – there was this phenomenon that scientists knew was important, something that could injure or even kill, but that we had no way to sense.
This was a problem in the early days of using radiation. Early workers found out that x-ray machines could burn the skin and that carrying radium around could cause skin burns as well. There had to be a link between, say, the amount of radium somebody was holding, the strength of an x-ray beam, and the ability of both of these to burn skin.
Part of the problem was finding a unit for radioactivity that made sense, and the committee that was assembled to find a way to quantify radioactivity decided to let Marie Curie solve this problem for them, and to honor her by naming the unit of radioactivity after her. Since radioactivity is characterized by the decay of unstable atoms, it seemed reasonable that the Curie should measure the rate at which these atoms were decaying – Curie eventually decided that the decay rate of one gram of Ra-226, her discovery of which led to her first Nobel Prize, would be an appropriate basis for a unit of radioactivity. Thus, one curie (Ci) of radioactivity is the amount of any material that undergoes 37 billion decays every second – the decay rate of Ra-226.
With time the curie has become the American unit of radioactivity and has been replaced with the Becquerel as the SI unit of radioactivity. The Bq was defined as the amount of radioactive material that undergoes a decay rate of 1 decay per second; since 1 Ci undergoes 37 billion decays per second, 1 Ci = 37 billion Bq (or 37 GBq). But what both of these units have in common is that they’re both a measure of the decay rate, which we can only measure with our instruments.
It’s also important to mention that we can’t look at a radioactive source and tell by looking at it how quickly it’s decaying away. One gram of Ra-226 is about 1 Ci of radioactivity, one gram of Co-60 has more than 1000 Ci of activity, one gram of tritium (H-3) contains about 10,000 Ci of activity. On the other extreme, 1 Ci of depleted uranium (U-238) weighs about three tons. A source we can barely see can be deadly while a source the size of a steamer trunk poses only a minor radiological risk.
But simply quantifying the decay rate is only part of the issue of determining the risk a radioactive source poses – we also have to figure out how much those radioactive decays will damage our bodies. This gets into radiation dose.
Ionizing radiation can strip the electrons from atoms, creating two charged particles (the negatively charged electron and the positively charged atom) called an ion pair. Measuring this ionization – the amount of electrical charge created in the air by a source of radiation – was the first way that scientists used to measure radiation dose, resulting in a unit called the Roentgen. One problem with the Roentgen, though, was that it measured the electrical charge produced in air, but we were interested in what was happening to organisms – especially in people – who were exposed to radiation.
This led to still another unit, the rad (the gray in the SI system), which simply measured the amount of energy that was deposited in any substance by the radiation – the rad. A rad was defined as depositing 100 ergs of energy for every gram of the material absorbing the radiation. This made it much easier to determine radiation dose, although the details are beyond the scope of this particular piece.
But there was one more piece of the puzzle that needed to be solved – scientists realized that some types of radiation were more effective at damaging our DNA than were other types. Alpha radiation, for example, turned out to cause about 20 times as much DNA damage as did beta or gamma radiation for the same amount of energy deposited, and neutrons caused anywhere from 5-20 times as much damage. The amount of energy deposition is important, but the amount of DNA damage is even more so.
Consider, for example, a bowling ball versus a ping pong ball – even if they have the same amount of kinetic energy the bowling ball is going to do much more damage to the pins while it’s going to be hard to roll a strike with the ping pong ball. By the same token, an alpha particle that goes zipping through the DNA is more likely to cause the sort of damage that might one day lead to cancer. This factor is known as the Quality Factor (QF) or the Relative Biological Effectiveness (RBE). And this takes us to the final unit – the rem (or the sievert in the SI system). To find out the dose in rem we just measure (or calculate) the energy deposition – rads – and multiply by the RBE. The RBE for beta and gamma radiation is equal to one, so one rad of beta or gamma radiation gives a biological dose of one rem. The RBE of alpha radiation is 20 so one rad of alpha radiation gives a biological dose of 20 rem.
The units we use for measuring radiation, radioactivity, and the biological damage from radiation exposure were developed over the course of several decades, evolving as we’ve learned more about radiation and its effects and growing somewhat more sophisticated at the same time. And, in fact, our units today (rem and sievert) are measuring the amount of damage inflicted on DNA, a molecule that hadn’t even been discovered at the time Marie Curie was defining the unit named in her honor. We’ve come quite a long ways.
 Although, with more precise measuring technology we’ve found out that one gram of Ra-226 actually has a slightly different decay rate; the specific activity of one gram of Ra-226 turns out to be 0.986 Ci.