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Nuclear radiation can be incredibly dangerous, but it can also be incredibly useful to us. The Shedding Light on Nuclear Radiation series teaches students what nuclear radiation is and how humans have harnessed its awesome power.
In Episode 9, Radiation Dose, we take an in-depth look at the different types of radiation that can cause atoms to ionize. We explain that since different types of ionizing radiation have different ionizing abilities and that different tissues of the body have different susceptibilities to ionizing radiation, scientists have developed three closely related ways of expressing radiation dose: absorbed dose, equivalent dose, and effective dose.
Video coming soon.
Contents:
Part A: Introduction
Part B: Ionizing Radiation
Part C: What is Radiation Dose?
The Episode 9 Question Sheet for Students:
The PDF version.
Google The Google Doc version. Google
Get the answers. (COMING SOON)
Transcript (More or Less)
Part A: Introduction
This is Alexander Litvinenko, a former Russian secret service operator and defector.
In London on November 1st, 2006, he met with some other Russian operatives in a café. A few hours later he started vomiting, he developed diarrhea and he found himself unable to walk without help. His whole body was in severe pain.
Two days later he went to hospital and doctors with the help of nuclear scientists eventually determined that he had polonium-218 in his body.
This radioisotope is a very strong alpha-emitter. The tea he had drunk at the meeting had been laced with polonium-218. Litvinenko’s conditioned worsened in the following weeks and he died on November 23rd 2006 only 22 days after his meeting at the cafe.
He had received a huge dose of alpha radiation from the polonium-218 that he had ingested which led to multiple organ failures and eventually to death.
So, how big of a radiation dose did Litvinenko receive and how does it compare to what the average person receives… from the natural environment or from, for example, a PET scan? Well, in the next few lessons, we’re going to look at this very important concept of radiation dose:
- How much radiation do workers who work with radioisotopes receive compared to the average person? This might include workers who, for example, work in nuclear power stations or in hospitals or industries that use radioisotopes.
- In what units is the radiation that we receive measured? … and…
- how much radiation can we be exposed to before our health is affected?
Let’s begin by taking a quick look again at the concept of ionizing radiation.
Part B: Ionizing Radiation
Nuclear radiation can harm cells because it can ionize atoms, that is, it can rip electrons off atoms. This affects the chemistry that’s going on in our cells. Any type of radiation that can ionize atoms is called ionizing radiation.
Ionizing radiation can either kill cells or cause cancer when exposed cells start dividing and reproducing at a really high rate. Cancer IS in fact this rapid unrestrained reproduction of cells in the body. However, our bodies can repair themselves if any damage done is not too severe.
Alpha, beta, and gamma radiation are all forms of ionizing radiation as are cosmic rays AND X-rays and the higher-energy forms of ultraviolet (or UV) rays, which are called UV-B and UV-C rays. I haven’t mentioned X-rays and UV rays in this series because they aren’t forms of nuclear radiation. Nuclear radiation comes from atomic nuclei as the name suggests, whereas X-rays and UV rays come from electrons jumping from higher-energy electron shells to lower-energy electron shells. Since they can still ionize atoms though, they can still be classified as ionizing radiation.
UV-A rays, visible light, infrared light, microwaves, and radio waves are forms of non-ionizing radiation. They still carry energy obviously, but they don’t generally cause ionizations that lead to chemical changes. The different forms of ionizing radiation have different ionizing abilities and different penetrating abilities and this changes how much damage they can do to our cells.
Looking at the first three for now, alpha particles have a very high ionizing ability, because of their double positive charge and their relatively large mass, but they have a low penetrating ability. Beta particles have a medium ionizing ability and a medium penetrating ability. Gamma rays have a low ionizing ability but they have a high penetrating ability. So how much damage do they do?
Well, if we compare just beta and gamma radiation, it turns out that they both cause about the same amount of damage as each other. The higher penetrating ability of gamma rays compared to beta particles, is balanced out by their lower ionizing ability. This assumes equal amounts of both of course.
However, the ionizing ability of alpha particles is so high that despite their low penetrating ability, they can do the most damage to our cells, 20 times as much damage, in fact, as beta and gamma rays if they’re being emitted from inside our bodies. If the alpha particles are being emitted from outside our bodies though, they can barely penetrate a few centimetres of air, so they’re not really as dangerous.
Now when I say damage, that can be a good thing or a bad thing. For the workers at Chernobyl in 1986 when the nuclear power plant exploded, the damage to their cells from the massive radiation leak was obviously a bad thing as was the damage caused to Alexander Litvinenko’s body by his ingestion of polonium-218, but for patients receiving radiotherapy which kills cancer cells, it’s obviously a good thing, despite the fact that some healthy cells are inevitably killed at the same time.
Back to our list, cosmic rays do about 10 times as much damage as beta and gamma rays but the atmosphere shields us from most of them. X-rays and UV rays can do about the same amount of damage as gamma rays and beta particles. However, nothing naturally on Earth really creates X-rays in high numbers. Cosmic rays and lightning produce tiny tiny amounts of X-ray radiation, but our biggest source of exposure, by far, is from getting X-ray images taken. Ultraviolet rays can cause ionizations in our skin cells but they’re easily stopped by a bit of clothing.
So though UV rays are the easiest to stop, the fact that we are potentially exposed to so much UV radiation compared to all other forms of ionizing radiation, means that we should be a little cautious.
We all need t? get out in the sun because sunlight is good for us (it helps our skin produce vitamin D), but you never want to overdo it. Sunburn is caused by the production of too many ions in your skin by UV rays and way too many people develop skin cancer because of overexposure to UV.
So, what exactly is radiation dose and how do we measure the radiation dose that we receive from all the different sources of ionizing radiation that we’re exposed to? Let’s take a look.
Part C: What is Radiation Dose?
Radiation dose to a human body and its effects are very difficult to measure. Different organs are affected in different ways and the different forms of radiation do different amounts of damage. Also, our cells and organs continuously repair themselves if they’ve suffered damage caused by low-level exposure over a long period of time, but they might not be able to repair themselves if the same amount of ionizing radiation is received over a shorter time frame. After extensive research over the past 100 years or so, scientists have come up with three ways of quantifying the radiation dose that we receive from both natural and from artificial sources of radiation.
The first measure is what’s called the Absorbed Dose. This is a measure of the energy absorbed per kilogram of tissue. The international unit for Absorbed Dose is the gray (Gy), named after English scientist Louis Gray who was a major pioneer in the study of how ionizing radiation affects living things. Louis Gray 1905 – 1965
Now it’s a little bit more complicated because of the fact that different types of ionizing radiation have a different effect on our bodies. As I said, alpha particles emitted inside our bodies cause much more biological damage than beta-minus particles or gamma rays.
So, to take into account the different types of radiation, scientists use what’s called the Equivalent Dose, which represents the probability of suffering harm… like developing cancer or suffering organ damage as a result of radiation exposure. The unit for equivalent dose is the sievert (Sv), named after Swedish scientist Rolf Sievert, who like Louis Gray was a major pioneer in the study of how radiation affects living things. An absorbed dose of 1 Gray of alpha radiation produces an equivalent dose of 20 sieverts, but beta and gamma radiation both produce an equivalent dose of only 1 sievert. Alpha particles can do more damage.
Now to take into account the different susceptibilities of different organs to radiation, scientists use what’s called the Effective Dose. It’s the third (and final) measurement.
It also represents the probability of suffering harm as a result of radiation exposure, but different organs are treated differently in the calculations.
To calculate effective dose, a weighting factor for different organs is used.
These are the figures for the weighting factors. The tissues of these 5 organs are particularly susceptible to radiation and they’re given a weighting factor of 0.12. Breast tissue here applies equally to both sexes, because both sexes have prominences of fatty and other tissue in the chest region, though they’re typically more obvious in women of course.
The tissues that make up gonads, that is, ovaries in women and testes in men, have been given a weighting factor of 0.08. The tissues of these four organs are given a weighting factor of 0.04, while the tissues that make up these four organs are given a weighting factor of 0.01. All of the rest of the tissues in our bodies are given a combined weighting factor of 0.12. Now all of these numbers add up to 1, which covers our whole body. So, for example, if 100 grams of stomach tissue is exposed to exactly the same amount of, say, alpha radiation as 100 grams of liver tissue, the stomach tissue will suffer more damage so this is taken into account when the effective dose is calculated.
The unit of effective dose is the same as the unit for equivalent dose: the sievert (Sv). Effective dose allows for a standardized way of estimating the overall risk of suffering harm associated with radiation exposure. It’s calculated by considering the amount of radiation, the type of radiation, and the varying sensitivities of different tissues and organs to radiation. The harm might be immediately noticeable for larger doses or it might lead to cancers months or years later.
So, unlike, say, medicine, where you might take a dose of 500 milligrams of something in a pill, equivalent dose and effective dose, I’ll say again, are measures of the probability of developing cancer or suffering other harm. They’re not physical quantities of something. The higher the dose of radiation, the higher the chance of developing cancer or suffering other harm, but predicting the exact outcome for an individual is impossible, because it depends on many factors like genetics and overall health.
In Episode 11 of this series, I’ll show you how equivalent dose and effective dose are calculated mathematically. For now though, all you really need to know is that the effective dose is the most widely used concept in assessing and managing radiation exposure in, for example, medical procedures or workplace settings where ionizing radiation is present.
The average effective dose that we in Australia receive from natural radioactive sources in the environment is about 1,700 microsieverts per year. This is obviously not harmful and there’s not much we could do about it anyway if it was.
Getting X-rays or CT scans, or undergoing radiotherapy exposes us to more radiation and some workers who work in industries that use radiation also receive more than the average person.
However, it’s estimated that Alexander Litvinenko received an effective dose of upwards of about 5,000,000 microsieverts when he was poisoned with polonium-210, which was enough to kill him in only 22 days.
So, between the effective dose of 1,700 microsieverts per year that we on average receive from natural background radiation and Litvinenko’s 5 million plus microsieverts, there’s a huge gap. How much danger is there to our health as the amount of radiation we’re exposed to increases? Well, that’s what we’ll be looking at in our next episode. See you then.
CREDITS:
Written and directed by Spiro Liacos
“Chernobyl Part 1 The Mother of all Nuclear Reactor Disasters 1986 | A Brief History of Documentary” by Plainly Difficult. https://youtu.be/IkBJU8BbrUs Creative Commons License.
“How radiopharmaceuticals help diagnose cancer and cardiovascular disease” by IAEA. https://youtu.be/mQjCTTKWOFU. Creative Commons License.
Tour the Department of Radiation Oncology UMMCVideos https://www.youtube.com/watch?v=UhJnNGgTsrQ&ab_channel=UMMCVideos
Images of Alexander Litvinenko taken from Wikipedia. Three days before his death, photographs were taken of Litvinenko and released to the public. “I want the world to see what they did to me,” he said.
https://en.wikipedia.org/wiki/Poisoning_of_Alexander_Litvinenko