NRC Science 101: Understanding Ionizing Radiation – It’s Not That Bohr-ing!

Harry Anagnostopoulos
Health Physicist
 

science_101_squeakychalkIn this post, we will be discussing ionizing radiation. But to do that, we first have to talk about radiation, in general, and then build up to the concept of ionization.

In previous NRC Science 101 posts, we’ve talked about the composition of an atom, including electrons, protons and neutrons. In 1913, physicist Niels Bohr made adjustments to an earlier model which imagined that the structure of an atom was similar to a solar system: electrons in circular orbits around a “sun” otherwise known as an atomic nucleus.

While modern atomic science has a more accurate understanding of the atom, Bohr’s model is still useful. It is easy to visualize and helps us to think about the relationship between electrons and energy. So, for the purposes of this post, let’s use Bohr’s atomic model.

Radiation is simply the transfer of energy through a medium. The medium can be anything: water, air or even the vacuum of outer-space. The transfer of energy can be carried out by particles or by electromagnetic waves.

Let’s conduct a small experiment. Imagine putting your face close to (but not touching) a bare 100-watt light bulb in a lamp. If you did this, and closed your eyes, could you still tell if the light was on? Could you feel the heat on your face, even though you are not touching the bulb?

Of course you could. That’s radiation! Light, heat, pressure waves in the air (sound), radio signals, and x-rays are all forms of radiation.

atom2As noted in prior NRC Science 101 posts, the core of an atom (the nucleus) is surrounded by orbiting electrons, like planets or comets around a sun. The number of electrons (each with one negative electric charge) usually equals the number of positive charges in the center (from an equal number of protons). These charges cancel out. However, if an orbiting electron is pushed out of its orbit (due to it absorbing energy from an outside source), the charges are now unequal.

The result? An “ion pair” has been formed. The creation of an “ion pair” is called . . . ionization.

Ionizing radiation is radiation with enough energy to create ion pairs in atoms. It is ionizing radiation that is of particular interest to the NRC because of its potential to cause health effects (as will be discussed in a future post).

cometearthTo help you visualize this, think again about Bohr’s model. Imagine a comet (standing in place of an electron) passing through our solar system. As the comet approaches the sun, it feels an intensifying push as light from the sun imparts more and more energy to the comet. Eventually, there is so much “push” that the comet either changes speed or changes direction. Now where will it go? Will it now be on course to strike a planet or will it veer out of our solar system? It’s exactly what could happen to an electron in the subatomic universe it occupies.

But this example is nothing compared to the bizarre realm of atomic physics where a solar system (an atom) might spit out a mini-version of itself, split into two, or where two twin comets (electrons) might appear out of nothing! And there’s more! However, you will have to wait until a later post.

NRC Science 101 – Quantities and Units of Measure

Suzanne Schroer
Reliability and Risk Analyst
 

science_101_squeakychalkIn the last Science 101 blog post, we discussed measurements made in various units of measure, particularly meters, grams and liters. You might be wondering where these units of measure come from and how they relate to one another. If so, you’re in luck, as that is the topic for today’s post.

Quantities are characteristics or properties we are trying to measure, such as the length of an object. Units of measure are how we express measurements of quantities. For length, the unit we would use in science is meters. A unit is really only a particular amount of some quantity used as a reference point for measurements of that quantity. Put differently, units of measure are chosen and accepted by the people who use them.

Often, units of measure were agreed upon many years ago. One meter is as long as it is because that’s what scientists agreed to use as a base unit for length. A meter could have been some other length. For the sake of establishing well-defined and easily-accessible units of measure, the General Conference of Weights and Measures (a collection of scientists from multiple countries) created in 1960 the Sytème International d’Unités (otherwise known as the SI¬). This system relies upon the base units of measure listed in the following table.

Table 1

Often times, however, the base unit of measure can be either too large or too small to be useful in describing a particular measurement. For example, while we could talk about the distances between cities in meters, we would be using very large (and, as such, cumbersome) numbers. For example, from Portland, Maine, to St. Louis, Missouri, is 2,060,000 m or 2,060 km. Similarly, if we are talking about the size of atoms, the basic building blocks of matter, speaking in terms of meters would be difficult as the diameter of a hydrogen atom is only 0.000000000120m. So, instead, for the sake of convenience, we often use prefixes to modify the size of the base unit. The following table lists a number of such prefixes using meter for the base.

Table2 Update again

The above table lays out conversion factors, ratios that can be used to convert one unit to another. For example, one such ratio, expressing the relationship between kilometers and meters, would be 1 km / 1000m. Using this ratio, we can convert 12348m into 12.348km (12348m x 1km / 1000m).

The NRC uses these same ideas when measuring radiation. These measurements will be discussed in an upcoming Science 101 post. As always, thank you for reading the NRC’s Science 101 blog series.

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NRC Science 101: What is Matter?

Suzanne Schroer
Reliability and Risk Analyst
Office of New Reactors
 

science_101_squeakychalkIn the last Science 101 post, we talked a little about the law of conservation of mass. Again, that law states that mass can neither be created nor destroyed as part of an ordinary chemical change (or, for that matter, a physical change). In this post, we’ll talk more in depth about what specifically “mass” is.

Everything that exists is made up of matter. It has two fundamental properties: volume and mass. Volume simply refers to the space an object takes up. Depending on the physical state of an object, there are a couple ways to measure volume. If we are trying to measure the volume of a box, for instance, we would multiply the length of the box by its height and by its width.

Let’s say that we have a box with the following dimensions: length = 3 meters (“m”), height = 4m, and width = 5m. Based on those dimensions, our box would have a volume of 60m3 (3m x 4m x 5m = 60m3). That is, again, a measure (in cubic meters) of how much three-dimensional space our box takes up.

If, on the other hand, our object was a liquid, we could use a graduated cylinder (a scientific measuring cup) to measure the volume of our object. This measure would be reported in liters. Again, a liter is just a measure of how much space a liquid takes up. For example, you can purchase soda in 2-liter bottles.soda

Since we’ve been talking a little about measurements, it might make sense at this point to distinguish between quantity and units. Thinking again about our example, the quantity we are trying to measure is volume. The unit we use to report this measurement is in liters or cubic meters.

Now, let’s now talk about the other fundamental property of matter — mass. When we talk about mass, we are referring to how much “stuff” is in an object. To illustrate this, think about two pieces of candy, both of the same kind and both the same size, however one of them is hollow. The candy that isn’t hollow has more mass compared to the hollow candy. Given that we often use scales to measure mass, you might think that mass and weight are the same thing. But they aren’t. Mass is the measure of matter in a particular object. No matter where that object is in the vast universe, it will have the same mass.

scaleWeight, on the other hand, is a measure of how much gravitational force is exerted on an object. While the weight of an object is proportional to its mass (the more mass of an object the more it will weigh), gravity varies according by where you are in the universe or even where you are on Earth—you actually weigh more, because there is a higher gravitational force, on the poles than you would at the equator. So, while an object will have a particular weight here on Earth, it will not have the same weight on the moon. It would, however, have the same mass both places.

Now that we can determine if something is matter (if it has volume and mass), we can use another measurement, density, to determine what kind of matter a substance is. Density is the ratio of how much mass is in an object compared to the volume of that object. Density is calculated by dividing an object’s mass by its volume.

Think back to our box with a volume of 60m3. Let’s say that our box has a mass of 240 grams (g). If that were the case, the density of our box would be 4 g/m3 (240g / 60m3). Density is nothing more than a way of stating how much matter fits within a particular volume. Using the two pieces of candy, while both have the same size (or volume), the solid one has more mass when compared to the hollow one and, as such, the solid candy is more dense (more matter in a particular volume) than the hollow candy.

Because the density of a particular substance (something with a defined composition, such as pure copper), is the same for all pieces of that substance, regardless of size, density is often useful in determining the identity of a particular object. Once we’ve calculated the density of an object, we can compare that value to the known densities for substances to determine what substance we believe the object is.

The author has a bachelor’s degree in Nuclear Engineering and a master’s in Reliability Engineering.

NRC Science 101 – What are Chemicals?

Suzanne Schroer
Reliability and Risk Analyst
Office of New Reactors
 

science_101_squeakychalkIn the last Science 101 blog post, we talked about the atoms, the basic building blocks of matter, and molecules. In this post, we’ll talk about chemicals, which are made up of a collection of molecules.

A chemical is any substance that has a defined composition. In other words, a chemical is always made up of the same “stuff.” Some chemicals occur in nature, such as water. Other chemicals are manufactured, such as chlorine (used for bleaching fabrics or in swimming pools). Chemicals are all around you: the food you eat, the clothes you wear. You, in fact, are made up of a wide variety of chemicals.

A chemical reaction refers to a change in a chemical. More generally, a chemical reaction can be understood as the process by which one or more substances change to produce one or more different substances. Chemical changes are different from physical changes, which don’t result in a change in substances. One example of a physical change is when water freezes into ice. While ice may have different physical properties, it is still just water. Another example is when you dissolve salt into a cup of water. While the salt may appear to disappear into the water, you still have water and salt—no substance changed into a completely new substance.

Here is one example of a chemical reaction: Iron + Oxygen → Iron Oxide

Iron oxide, also known as rust, cannot become iron or oxygen again. It is a completely new substance. In the equation, the substances on the left-hand side of the arrow are considered reactants (the substances that participate in a chemical reaction). The substance on the right-hand side of the arrow is considered a product (a substance that results from a chemical reaction). It’s important to note from this example that no material is “lost” in the reaction. On one side of the equation you have iron and oxygen; on the other you still have iron and oxygen (now just combined into one chemical).

In that sense, this example illustrates what is known as the law of conservation of mass. By “law,” we mean a general rule of how something works or how something occurs. This description is considered to be extremely reliable due to a large amount of supporting experimental testing and observation. Considering the given example, the law states the products of a chemical reaction have the same mass (“stuff”) as the reactants. In other words, while things are rearranged, nothing is created or destroyed.

chemistrylab2Here are some ways to tell if a chemical change is occurring:

1. You might notice bubbling or a change in odor, indicating the production of a gas. Such is the case when baking soda is mixed with vinegar.

2. When two clear solutions are mixed together and the resulting mixture is cloudy (due to the presence of some solid substance now in the liquids). This is known as the formation of a precipate.

3. A change of color (like in our rust example).

4. A change in temperature or if light is produced, such as with fire.

While any of the above may be evidence of a chemical change, physical changes can have some of the same effects. One way to determine the difference between the two is to think about whether the new substance could be physically separated back into its original parts—in other words, if the involved matter could “go back” to how it originally was.

The author has a bachelor’s degree in Nuclear Engineering and a master’s degree in Reliability Engineering.

NRC Science 101 – What is an Atom?

Suzanne Schroer
Reliability and Risk Analyst
Office of New Reactors

science_101_squeakychalkWelcome to the NRC’s new blog series, Science 101. Over the course of this series, NRC experts will discuss various scientific principles, with some of the later posts relying on principles and ideas discussed in earlier ones. So, in that sense, this blog series will play out much like a textbook, with each post (or chapter) building upon the previous one.

We hope the information in this series will be helpful to teachers, students and the public who want to better understand the science behind the NRC.

So where do we begin? The topic for today’s post is the atom. It’s considered the basic building block of matter.

Anything that has a mass—in other words, anything that occupies space—is composed of atoms. While its name originally referred to a particle that couldn’t be divided any more—the smallest thing possible—we now know that each atom is generally made up of smaller particles. Given that these particles make up atoms, they are often referred to as subatomic particles. There are three subatomic particles: protons, neutrons and electrons.

Two of the subatomic particles have electrical charges: protons have a positive charge while electrons have a negative charge. Neutrons, on the other hand, don’t have a charge. A fundamental rule is that particles with the same charge are repulsed from each other, while particles with opposite charges are attracted to each other. So, much like opposite ends of a magnet, protons and electrons are attracted to each other. Likewise, just as when you experience resistance trying to push the same ends of two magnets together, protons are repelled from other protons and electrons are repelled from other electrons.

atom1The nucleus (or center) of an atom is made up of protons and neutrons. The number of protons in the nucleus, known as the “atomic number,” primarily determines where that atom fits on the Periodic Table. The number of protons in the nucleus also defines in large part the characteristics of an atom—is it a gas or a metal, for example.

Two atoms with an identical number of protons in their nuclei belong to the same element. An element, like hydrogen, oxygen or iron, is a substance that cannot be broken down—outside of a nuclear reaction—into anything else. In other words, one element cannot be transformed into another (again, with the exception of nuclear reactions).

Now, while the protons are the same in an element, the number of neutrons may vary from atom to atom. The number of neutrons determines what isotope an atom is. This is important to the NRC because the number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay. While radioactive decay can occur in a variety of ways, it is, simply put, the process by which unstable atoms break down, releasing particles (and energy).

Generally speaking, atoms with roughly matching numbers of protons and neutrons are more stable against decay. Given how important it is to the work of the NRC, the concept of radioactive decay will be taken up as part of a future blog post.

The nucleus of an atom is surrounded by a cloud of electrons. Remember, electrons are negatively-charged and are attracted to the positively-charged protons in the nucleus. An atom is considered to be electrically neutral if it has an equal number of protons and electrons. If an atom has a different number of electrons and protons, it is called an ion.

An important principle to know is electrons may be transferred from one atom to another or even shared between atoms (allowing atoms to bind together). These bonds allow for the formation of molecules, combinations of atoms (including those of different elements). Just as several atoms make up a molecule, many molecules make up a chemical. Chemicals will be the subject of our next Science 101 post.

The author has a bachelor’s degree in Nuclear Engineering and a master’s in Reliability Engineering.