TOPIC 1: Units for Disintegrations
Now we are ready to discuss radioactivity and half-life. First, let’s talk about the units of the disintegrations from radioactive atoms.
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Now we are going to discuss activity in the radiation field.
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We have electromagnetic radiation activity in the form of gamma rays and x-rays. These are pure energy.
We also have activity from the particulate radiation which comes in the form of alpha particles, beta particles and neutrons. Alpha particles have an atomic mass of 4 – 2 protons and 2 nutrons. The 2 protons means an alpha has a plus 2 charge. Beta particles have a small mass and negative 1 charge. Neutrons have an atomic mass of 1 and no charge. All of these radiation types are emitted from the nucleus during radioactive transformation (also called “decay” or “disintegration”).
This emission is called its activity.
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As discussed previously, ionizing radiation is from an unstable atom releasing energy. The radiation may be in the form of particles with a known mass but different energies. The radiation could also be electromagnetic (pure energy) and have no mass and no charge. The radiation dose a person receives depends on the activity of the radionuclide and also on whether the radionuclide is inside or outside the body as well as the types and energies of the radionuclide.
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The “amount” of radioactivity that we have is NOT measured in gallons, kilograms or other typical quantifying measurements. We cannot predict when a specific atom will discharge particles and energy, and change from the Parent isotope to the Daughter isotope. We can, however, statistically determine how a large population – millions and billions – of these atoms, will “decay” over some set period of time. We call this decay “disintegration” and we commonly use one second as a reference of time.
So, for radioactive materials, then, we don’t define the quantify, such as gallons or kilograms, but we define its “activity”. In other words, we define the number of disintegrations occurring per unit of time.
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So one unit of measurement is the number of disintegrations per second. The scientific community adopted this basic unit of one disintegration per second and gave it the name becquerel.
As mentioned earlier, Marie Curie discovered the new element radium. She also created a benchmark for quantifying radio-activity. She determined that one gram of radium emitted 37 billion disintegrations per second. The scientific community adopted this amount of disintegrations per second and it is still referred today as the Curie.
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Since the becquerel is a small quantity of radiation, equal to one disintegration per second, and then we have the extreme of the curie which indicates 37 billion disintegrations per second, which do we use? We use them all.
More importantly, in order to understand how much radioactivity we have, we need to know:
- What radio-nuclide is present
- Which tells us what type of radiation is emitted
- How much alpha, beta or gamma radiation is being detected
Because, ultimately, we are concerned with the damage the radiation can do to our bodies.
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If we use both Becquerel, which is the number of disintegrations per second, and the Curie, which is billions of disintegrations per second, how can we define the activity without using the words millions, billion, trillions, quadrillions, quintillions, and so on?
We use what is called a “metric prefix”. Just like what you see on the screen, instead of saying thousands, we can say Kilo. Instead of saying millions we can say Mega.
We also do this for very small numbers too. One-one-thousandth is a milli. One-one-millionth is a micro, one-one-billionth is a nano, and one one-trillionth is a pico. Because you will be using these prefixes often, they will become familiar to you. More information about these prefixes, and how to use them in math, can be found in the Math Primer.
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Let’s take a brief tour around these Metric Prefixes. With the exception of “centi” which is 100, the other prefixes are in increments of 1,000. We use scientific notation or orders of magnitudes of 10.
For example, When something has the prefix Mega, that number is multiplied by 1 million. 1 million is 10 X 10 X 10 X 10 X 10 X 10. There are 6 of these 10s, so we can shorten that with scientific notation writing 10 to the power of 6. Put another way, take our number and move the decimal point to the right 6 places.
When dealing with a kilo that is equal to 10 to the third, or one thousand.
What happens with numbers that are less than one but still greater than zero? There is the Milli. The milli is one over a thousand or one over ten to the 3rd power of just ten to the minus 3rd power.
The micro is equal to one over a million or one over ten to the 6th power of just ten to the minus 6th power.
The Nano is one over one billion or one over ten to the 9th power of just ten to the minus 9th power.
The Pico is one over one trillion or one over ten to the 12th power of just ten to the minus 12th power.
It may take a little getting used to, but in the end it will make talking about radioactivity a lot easier.
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Now let’s work a problem of radioactivity where we want to move between Milli and Micro
So let’s start off, I have a 100 millicuries. “How many microcuries do I have?”
Let’s put it in numbers
One way to do this is put everything we can in a factor of 10
100 is 10 to the factor of 2, or 10 to the positive 2 power
A milli is 1 over 1000 or 10 the factor of minus 3
A micro is 1 over 1,000,000 or 10 to the factor of minus 6
When dealing with factors, we can add them. So 10 to the power of positive two and 10 to the power of minus 3 is 10 to the power of positive 2 minus 3 or just 10 to the power of minus 1.
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Continuing on with the math, we can always divide both sides by the same thing. In this case we wants the ??? By themselves so we divide both sides by 10-6 Ci.
Ci divided by Ci is one.
We have 10 to the minus 1 times 10 to the minus minus 6.
That means our questions marks are 10 to the positive 5.
Going back to our original question from the previous slide, 100 milli curies is equal to 100 thousand micro curie.
If you are completely lost, you may want to take break and review the Math Primer to work through some more of these type problems.
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We can annotate these values in different ways. Instead of always writing the number 10 and putting the power value in a superscript, it common to use the letter “E” to tell the reader the next number is a factor or “exponent”. So 10 to the power f minus 12 would be written “E-12”.
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In review, for large or small numbers, we express the number in scientific notation. Scientific Notation is using one integer followed by a decimal with additional two significant numbers multiplied by the number of places the decimal has been moved. This is expressed as a multiple of 10 with an exponent. For any number greater than 1, the exponents will be positive. For numbers less than one, the exponents will be negative.
Refer to your math primer if you want more explanation.
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Lets go back to the value of the Curie. We said it was 37,000,000,000 disintegrations per second. In scientific notation that is 3.7 times 10 to the power of 10.
1 dps is the same as 1 Becquerel
So 1 Ci is equal to 3.7 times 10 to the power of 10 Becquerel.
Or it can be written 1 Ci = 3.7 E+10 Bq.
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3.7 X 1010 disintegrations per second is a lot of radioactivity. Most of our uses of radioactivity are far less than 1 Ci. So we use the prefixes. For a simple example, let’s say we have 3.7 X 107 dps. We can see this number is exactly 1/1000 or one thousandth of a Curie. Earlier we learned one thousandth, or 10-3, is a “milli”. So our activity of 3.7 X 107 dps can be called a “millicurie”.
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The international scientific community began to adopt a more basic unit of measurement, the disintegration per second. This basic unit of measurement was named in honor of Henri Becquerel. One “becquerel” is one disintegration per second.
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Since the curie is the quantitative measure of 3.7 X 1010 disintegrations per second, we can also express it in disintegrations per minute.
We take 3.7 X 1010 dis/sec X (60 sec/min) = 2.22 X 1012 dis/min (or dpm)
Since a milli- is 10-3. The millicurie would then be 2.22 X 1012 X 10-3 = 2.22 X 10(12 – 3) = 2.22 X 109 disintegrations per minute, which we can shorten to DPM
Please refer to the Math Primer for more assistance and problems to work to understand this concept better.
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The international scientific community began to adopt a more basic unit of measurement, the disintegration per second. This basic unit of measurement was named in honor of Henri Becquerel. One “becquerel” is one disintegration per second.
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Since the curie is the quantitative measure of 3.7 X 1010 disintegrations per second, we can also express it in disintegrations per minute.
We take 3.7 X 1010 dis/sec X (60 sec/min) = 2.22 X 1012 dis/min (or dpm)
Since a milli- is 10-3. The millicurie would then be 2.22 X 1012 X 10-3 = 2.22 X 10(12 – 3) = 2.22 X 109 disintegrations per minute, which we can shorten to DPM
Please refer to the math primer for more assistance and problems to work to understand this concept better.
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Activity concentration is the radioactivity per mass or unit volume. For a liquid the most typical units are the milliliter or liter. For a solid, they are the gram or kilogram. For air the typical units of radioactivity are per cubic meter or centimeter (cm3). The activity concentration does not require an exact knowledge of the chemical makeup of that material. It can simply be some mass or volume of air, soil, food, equipment, etc. The radiotoxicity of a radioactive material depends on the concentration of the chemical properties of the radionuclide. For example, the activity concentration of plutonium that would make it toxic is much smaller than the activity concentration of tritium (radioactive Hydrogen). This is due to the inherent toxicity of the chemical that could be breathed by a person.
Specific Activity (S.A.) is the activity per mass or volume of a pure radionuclide in a chemically pure media. Typically, specific activity is also expressed in activity per mass or unit volume.
Total Activity is the activity concentration multiplied by the total volume or mass.
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Now, let’s solve a problem:
We have a drum that contains 220 liters of liquid (approximately a 55 gallon drum). We know we have radioactive Iodine-131 mixed in this drum. If we took a one milli-liter sample, can we calculate the total radioactivity in the drum? Sure.
We multiply the amount of the activity in the one milli-liter by the number of milli-liters in the drum. 220 liters has 220,000 milli-liters., then we know our TOTAL Activity.
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Radioactive contamination is simply radioactivity in some location that you don’t want it. The word “contamination” carries with it a certain stigma that can lead to its misuse. For example, you may read in news articles about wells contaminated with radium, when in fact the radium is a natural part of the groundwater system. Scientists look beyond the concentrations. They first must know what abundance the radionuclide is in nature. This would be the concentration of the radioactive element before man made any impact.
If there is an increase in the concentration of radioactivity by man’s interference, then it could be said there is contamination.
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Like I said, radioactive contamination has a certain stigma to it. When we hear about Three Mile Island or Chernobyl or even about “dirty bombs” spreading radioactive contamination, the fear of radioactive materials is real and affects how society views radiation. But there has always been radiation from the dawn of time. That is called BACKGROUND radiation, and there is nothing we can do about it.
To most people, the word radiation is synonymous with contamination. Simply hearing the word “nuclear” creates a knee-jerk reaction:
NIMBY (Not in my backyard); or,
BANANA: (Build absolutely nothing anywhere near anybody); or,
NOPE: (Not on planet earth).
Having these strong feelings in our society makes it extremely difficult to find locations for new Nuclear Power Plants.
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Let’s look at measuring emissions from a radiative decay. The basic unit used to describe the energy of a radiation particle or photon is the electron volt (eV). An electron volt is equal to the amount of energy gained by an electron passing through a potential difference of one volt. The total energy of the radiation emitted is a characteristic of the radionuclide. For example, the total energy of Ra-226 decaying to Rn-222 by throwing off an alpha is 4.785 MeV (MeV is mega-electron volts, which means million eV). The energy from the gamma of cesium-137 has 663 KeV energy or 0.663 MeV. Many radionuclides have more than one decay route meaning a different energy alpha emissions combined with different gamma energies. Remember, the goal of the radioactive atom is to shed its excess energy.
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This Gamma Ray Spectrum shows a typical sample analysis with three distinct peaks. Almost every natural radionuclide emits low energy gammas, so the peak to the left, near 0 keV, doesn’t help us. But the other three can. Each emission from a radioisotope has a different energy of that particular emission. We can look up these energies and define what radioactive material is in our sample.
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These different energies help us determine how much shielding we may need. For example, the energy of the gamma rays from cobalt-60 is almost double the energy of the gamma rays from cesium-137. Thus, cobalt-60 will have more extensive shielding parameters than the same amount of activity from cesium-137 because of the increased energy.
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Since the energies of the radioactive materials emissions are well known, our technology allows us to distinguish the different types of radionuclides that may be in a substance by evaluating the energies of the respective emissions. This measurement is done through sophisticated portable and laboratory instrumentation. Unfortunately our portable beta “Geiger counter” cannot differentiate these energy differences effectively. They just count the number of ionizations.