Lesson 2 Reading

Radioactive Decay, the Natural Decay Series, and Half-Lives

As we explore the nature of half-lives and radioactive decay series, we’ll be reviewing, enriching, and expanding our knowledge of atoms and radiation. Some material may be familiar to you from Lesson 1 of this unit.

Isotopes

Carbon-12: 6 protons + 6 neutrons

The nucleus in every atom of a particular element always has the same number of protons. However, the number of neutrons may vary. Atoms that contain the same number of protons, but different numbers of neutrons, are called isotopes of the element.

The “atomic number” of an atom is equal to the number of protons in its nucleus. The “atomic weight” is the sum of the protons and neutrons in the nucleus. For instance, the most common form of carbon has 6 protons and 6 neutrons. Its atomic number is 6 — the atomic number of any isotope of carbon — and its atomic weight is 12 (6 protons + 6 neutrons = 12).

Isotopes of a given element have the same chemical properties, but they may differ in their nuclear properties. Isotopes of any given element always have the same number of protons, even when the number of neutrons varies.

All atoms are isotopes of their particular element. To show which isotope of an element we are talking about, the method is always the same. We total the number of protons and neutrons. Then we write the sum after the chemical symbol for the element.

Uranium-235: 92 protons + 143 neutrons (not all shown)	Uranium-238: 92 protons + 146 neutrons (not all shown)
	^Back to top

For example, in the nucleus of one of the isotopes of uranium there are 92 protons and 143 neutrons. We refer to this isotope as uranium-235 or U-235 (92 protons + 143 neutrons = 235). A second uranium isotope, which contains three additional neutrons, is uranium-238 or U-238 (92 + 143 + 3 = 238). (The number may also be written in superscript before the symbol: ²³⁵U or ²³⁸U.)

Stable and Unstable Atoms

Some atomic nuclei, i.e., the proton-neutron combinations at the center of atoms, are stable. Others are unstable. Unstable (radioactive) atoms (nuclides) stabilize themselves over time by emitting radiation. In doing so, they change their proton-to-neutron ratio, in the process becoming a different nuclide. (Remember, the number of protons in an atom’s nucleus determines the chemical element.) Unstable (radioactive) atoms are called radionuclides.

The words isotope and nuclide are often used interchangeably in this curriculum and in practice. When referring to neptunium-237, for example, we can call it either a radionuclide or a radioisotope, either would be correct. The difference in the meaning of the term comes in its usage. For example, neptunium-237 is an isotope of neptunium. Neptunium has several isotopes. “Nuclide” would not be appropriate in either of the two previous sentences — since all isotopes of neptunium are radioactive.

Adding “radio” in front of either word simply means it is a radioactive nuclide or isotope. Several elements have both a stable and one or more radioactive isotopes. Carbon-12 is a stable nuclide. Carbon-11 (C-11 or ¹¹C) are radionuclides. They are both radioisotopes of carbon.

Carbon-11 is a short-lived radionuclide used in medical applications. Carbon-14 is a longer-lived isotope used for geologic, environmental, and anthropological age-dating studies. In the latter two sentences the words nuclide and isotope could have been interchanged because no reference is being made to the stable atom of which it is an isotope, although it is obvious that the atom or nuclide is Carbon.

When an atom is radioactive, it’s almost always the nucleus that spontaneously emits energetic particles and, sometimes, highly energetic electromagnetic rays (gamma rays) that escape the bounds of the atom. Because the emitted particles and rays come from the nucleus, they are referred to as “nuclear” radiations.

This process of becoming stable over time is called radioactive decay — because the radioactivity of the radionuclide is actually decaying away, or declining, over time. In the process, the radionuclide changes into other species, a nuclide that is either radioactive (another radionuclide) or stable. Eventually, the radionuclide will lose all its radioactivity, becoming a stable, non-radioactive atom.

Depending on the radionuclide, the complete decay process takes from a few seconds to billions of years. Sometimes it takes more than one radioactive decay for a radionuclide to turn into a stable decay product. This process of turning into stable nuclides through a series of decays is called the radioactive decay series or decay chain.

In all cases, radioactive decay always releases an electrically charged particle; in some cases, gamma rays are emitted at the same time. The energetic particles and any rays released by this process are all forms of ionizing radiation.

Radioactive Decay and Nuclear Transformation: Alpha Particles

An alpha particle, emitted by certain radionuclides, comprises two protons and two neutrons: the same thing as a helium nucleus.

The nuclei of some radioactive atoms, generally the heavier or larger atoms, spontaneously expel a part of themselves as a particle composed of two neutrons and two protons.

Two neutrons and two protons combined are the exact equivalent of a helium nucleus, helium being the second lightest of all the elements. This two-neutron, twoproton combination — essentially a helium atom minus any electrons — is called an alpha particle. Relative to the nuclei of most isotopes, a helium nucleus is very light. So an alpha particle travels very fast.

By expelling an alpha particle, the atomic nucleus changes into the nucleus of an element with an atomic number that is lower by 2 (it has lost two protons) and an atomic weight lower by 4 (it has lost a total of four protons and/or neutrons). By losing one or more protons, the atom has actually changed to a different chemical element.

Uranium-238: An Alpha Emitter Decays

Let’s consider an example of a specific radionuclide changing to a different element by losing protons. When the uranium-238 nucleus emits an alpha particle (two protons and two neutrons), it becomes the nucleus of a thorium-234 atom. Following the alpha particle emission, the remaining nucleus is different (its number of neutrons is reduced by two and its number of protons is also reduced by two).

In other words, by losing two protons, a uranium-238 atom (which has 92 protons, as do all uranium isotopes) becomes a thorium atom because it now has 90 protons. It is now a nucleus of a different chemical element altogether. With the loss of two protons and two neutrons, the atom has chemically become a different element than what it was before the emission of the alpha particle. This is the natural radioactive decay process of uranium-238 at work.

Radioactive Decay and Nuclear Transformation: Beta Particles

Some radionuclides change by gaining a proton, when they emit a negatively charged particle from their nuclei. This happens when one of the neutrons in the nucleus emits a negatively charged particle, the equivalent of an electron. This negatively charged particle let loose is called a beta particle.

By emitting the beta particle, the neutron changes into a positively charged proton and is no longer neutral. It has become positively charged and, thus, a proton in its own right. This increases by one the number of protons existing in the nucleus.

Through this process, the atom becomes an atom of a different element, with an atomic number higher by one. (Remember, the number of protons in a nucleus determines what the chemical element is.) The atom becomes an isotope of whatever element has that particular number of protons in its nuclei.

Strontium-90: A Beta-Emitter Decays

Now let’s consider an example of a specific radioinuclide changing to a different element by emitting an electron from a neutron in its nucleus.

When a strontium-90 nucleus emits a negative beta particle from one of its neutrons, that neutron loses its “neutrality” and becomes positively charged — i.e., the neutron becomes a proton. By gaining a proton, the atom changes its element, in this case from strontium-90 to yttrium-90, because yttrium has one more proton than strontium. (The atomic weight doesn’t change in such cases because the total number of neutrons and protons hasn’t changed: there is one less neutron, but it gained a positive charge and became a proton.)

Again, in the process of nuclear transformation, what was one element became a different element. Any radioactive decay series goes on until the atom becomes stable, or non-radioactive. This may take an extremely long time (in some cases, billions of years), but at that point of stability, the atom remains the same element forever.

Radioactive Decay Series

There are three naturally occurring radioactive decay series: the uranium-238 (U-238) series, the uranium-235 (U-235) series, and the thorium-232 (Th-232) series. The very long-lived uranium and thorium isotopes were present in the Earth from the beginning, and the many radionuclides resulting from their decay are called “decay products.”

It is important to understand these naturally occurring decay series (or decay chains) and their decay products because we need to realize that radioactivity is and always has been a fact of life on Earth. Radioactivity is not just the product of the so-called “nuclear age.”

At the same time, it is also important to distinguish between the naturally occurring radioactive decay products like radium-226, radon-222, and polonium-210 and those radioactive “fission products” produced by the fissioning (or splitting) of uranium isotopes in a nuclear reaction. The fission products, such as iodine-131, cesium-137, and strontium- 90, result from the splitting of uranium atoms in a nuclear reaction. They are not the products of the natural radioactive decay of uranium. They can be thought of as “manmade” radionuclides, although some amount of these materials was produced in natural “reactors” in the Gabon, Africa, two billion years ago when the earth was more radioactive.

The natural radioactive decay chain of uranium-238 can be thought of as a series of steps downward, until the final nuclide in the chain, lead-206, which is stable.

Natural Alpha Decay: Uranium-238

During alpha decay, a uranium-238 nucleus (consisting of 92 protons and 146 neutrons) emits an alpha particle, consisting of two protons and two neutrons. As a result, the nucleus now contains 90 protons and 144 neutrons. Because it now has a different number of protons, it is a different chemical element. Having 90 protons, it is now the nucleus of a thorium atom. This thorium isotope has an atomic weight of 234 (90 + 144 = 234), so it is thorium-234.

Natural Beta Decay: Thorium-234

During beta decay, a neutron in a thorium-234 nucleus (which consists of 90 protons and 144 neutrons) becomes a proton because it emits a negatively charged beta particle. As a result, the nucleus now consists of 91 protons and 143 neutrons. This means it is an atom of protactinium. This protactinium isotope has an atomic weight of 234 (91 + 143 = 234), so it is protactinium-234.

Gamma Emissions

Some radioactive materials also emit gamma rays. Gamma rays are electromagnetic rays that are similar to X-rays but are even more energetic. Gamma rays are emitted when a nucleus changes from a higher to a lower energy state; with most radionuclides, this gamma emission accompanies alpha or beta decay. For example, cesium-137, a beta emitter, and radium-236, an alpha emitter, both also emit strong gamma rays.

Detecting Radioactive Decay

In their study of radioactive materials, scientists use a variety of extremely sensitive instruments. Various instruments are used to measure radioactivity in natural sources such as rocks and soils, as well as in man-made sources, including spent (or used) nuclear fuel.

Geiger counters and other instruments are helpful in cleaning up research and work facilities where radioactive materials have been present. People at these work locations often wear badges that contain a small bit of a special photographic film. These film badges record the amount of ionizing radiation to which a worker has been exposed.

Radionuclides and Half-Lives

A half-life is the amount of time required for a radioactive substance to lose one-half of its radioactivity. Put another way, the half-life of a radionuclide is the time for one-half of any given quantity of that radionuclide to decay. Each radionuclide has its own unique half-life. This is true whether the radionuclide is occuring naturally, e.g. is part of one of the three naturally occurring decay chains, or whether it is one of the radionuclides created by fissioning uranium atoms in a manmade nuclear reaction.

For example, the half-life of the natural and widely abundant uranium- 238 is 4.5 billion years. This means that 100 grams of uranium-238 atoms will decay to only 50 grams of uranium-238 over the course of 4.5 billion years. After another 4.5 billion years, those 50 grams of uranium-238 will have decayed to only 25 grams of the radionuclide. The very long half-life of uranium-238 partially explains why there is so much of it still in the rocks and soil of planet Earth, which itself is only about 4.6 billion years old.

Of course, it’s not that one-half of the original physical substance has vanished into nothing over the course of the half-life. Being a radionuclide, the uranium-238 has simply gone through nuclear transformations into different nuclides within the uranium-238 decay chain. Eventually, most of the original atoms of uranium-238 will lose all their radioactivity to become a stable isotope of lead, lead-206.

The radioactive decay process is both random and statistically predictable. This means that it’s not possible to know which particular atom in a sample of a radioactive substance will decay and when. Nevertheless, when a great number of the atoms of the same radionuclide are together in a sample, the half-life of the radionuclide is predictably constant. Moreover, the half-life of any given radionuclide is uniquely characteristic of that radionuclide. No other radionuclide has that same half-life, a fact that can be very helpful when applied in radiation health protection and other scientific applications.

The Importance of Knowing About Half-Lives

The radiation emitted by radionuclides is always ionizing, and thus could increase health risks. Alpha and beta particles are indeed physical particles, even though they are even smaller than the atom that emitted them. As such, once in the body, they can travel through the body and transfer their energy to molecules within the body, potentially knocking electrons out of their atoms, thus increasing the chance of cellular damage.

The shorter the half-life of the radionuclide, the more atoms of this radionuclide will decay per unit time for a given mass of radionuclide.

Knowledge of a radionuclide’s half-life is important if a person is accidentally exposed to an especially high level of that radionuclide. Likewise, knowledge of a radionuclide’s half-life is important for doctors and other health professionals when using radioactive substances to diagnose or treat diseases. If a patient’s exposure to a radionuclide for diagnosis or treatment is not carefully controlled, the risks may outweigh the benefits.

Understanding half-lives of particular radionuclides is also very important in other scientific endeavors. Specific knowledge of the half-lives of various radionuclides in various kinds of industrial radioactive materials is essential in safely handling and managing such materials. An obvious example of this would be the situation with spent nuclear fuel (see Lesson 4 of this unit). Understanding half-lives is also an important key to dating many different materials, e.g., organic materials that are as much as 50,000 years old (see Lesson 3 of this unit, especially the student activity entitled “Using Radionuclides to Date Materials”).

Relative Differences Between Particles and Rays

Being an electron, a beta particle is much smaller and lighter than an alpha particle. Both an alpha particle (positively charged with its two protons and two neutrons) and a beta particle (negatively charged) are ionizing and travel very fast. But they don’t travel as fast as gamma rays or X-rays nor do they penetrate matter as effectively. Gamma rays and X- rays are part of the electromagnetic spectrum and, thus, travel at the speed of light.

Gamma rays and X-rays are different from alpha and beta particles in a couple of important ways. These highly energetic, extremely smallwavelength electromagnetic rays pass right through most physical substances at the speed of light. As ionizing rays, they can cause molecular and cellular damage as they pass through the body. They can only be stopped by sufficient amounts of dense substances, such as water, concrete, and lead. By contrast alpha and beta particles can be stopped by lighter materials.

Neutrons and Induced Radiation

As we’ve discussed throughout this unit, on our planet the most common forms of ionizing radiation are alpha and beta particles, along with gamma and X-rays.

These radiations may cause changes in the substances they contact, and the changes may be serious, particularly in living cells and tissues.

Many people, however, mistakenly believe that these common radiations actually cause the substances they contact to become radioactive too. This is not true. Alpha and beta particles, and gamma rays and X-rays, can cause damage, but they do not create additional radioactivity.

On the other hand, neutrons, if loosed from the bonds of a nucleus, can in fact cause many other substances to become radioactive — by changing the nuclear structure of atoms they hit and get absorbed into.

Neutrons can be freed from the bond of a nucleus during nuclear reactions inside certain radioisotopes. And some radioisotopes lose a neutron at the same time they emit an alpha or beta particle. We’ve also discovered ways of controlling and beaming neutrons at substances.

Fast and Slow (Thermal) Neutrons

Because neutrons have no charge, when they are freed from a nucleus, they are not attracted to other particles and thus move very fast. Until they slow down considerably, they bounce off of other atoms in their pathway, sharing some of their kinetic energy (motion-related) as they go. These are called “fast neutrons.” They are very penetrating of substances, but they do not get involved in nuclear reactions within individual atoms.

Eventually such loose neutrons lose enough of their kinetic energy to be able to interact with atoms in their path. They are then considered “slow” or “thermal” neutrons — meaning they have about the same thermal energy level as atoms of the surrounding material.

Once a neutron loses enough kinetic energy, becoming a thermal neutron, it can break through the nuclear bond of certain other atoms. In such cases, the neutron can be absorbed by the nucleus of the atom it hits. This causes the absorbing atom to change into a different isotope of the same element or into another element. The change can cause a previously stable atom to become radioactive and start emitting alpha or beta particles and sometimes gamma rays or X-rays as well.

This process of an atom absorbing a “slow” neutron is called induced radiation, neutron activation, or neutron capture.

With many elements, when neutron capture does occur, the resulting radioisotope is a gamma emitter. Since gamma rays are part of the electromagnetic spectrum, their wave frequencies can be measured by highly sensitive devices (much as FM radiowaves and microwaves can be measured). Because we know very precisely the frequency of the gamma rays given off by many radioisotopes, we can determine the presence of those particular elements in a sample of material.

Neutron capture happens constantly in outer space and even in the higher levels of our atmosphere. For example, when a neutron traveling at the right speed hits an atom of stable nitrogen-14 in the Earth’s atmosphere, the atom becomes the radioisotope carbon-14, which emits beta particles.

This is how carbon-14 gets into our air and subsequently into the entire food chain on our planet. (See the enrichment reading and activity entitled “Using Radioisotopes to Date Materials” in Lesson 3 of this unit.) Similarly, when a stable chlorine-37 atom absorbs a neutron, it becomes the radioisotope chlorine-38, which emits both beta particles and gamma rays.

Engineering Induced Radiation

Induced radiation also takes place inside of manmade nuclear reactors. Many substances can be bombarded with neutrons inside a nuclear reactor for purposes of inducing radioisotopes in the material so that any associated gamma rays can be measured. By this process scientists can identify many of the elements in the material. In fact, about sixty elements can now be identified by this method.

More recently, with continuing advances in electronics (particularly miniaturization), a number of portable neutron activation devices have become available. These smaller, more portable devices allow us to induce radioisotopes, measure the gamma rays, and identify elements in many more circumstances.

Applications of Neutron-induced Radiation

Neutron activation of radioisotopes in materials for purposes of measuring the associated gamma rays is called neutron activation analysis. The technique has become very useful in industry, commerce, crime-solving, agriculture, environmental cleanup, and medical diagnosis and treatments (nuclear medicine).

Scientists have come to understand the process of neutron capture very well, and how to make it happen under controlled situations. Applying this knowledge has led to many practical applications in industry, commerce, and medicine:

• Determining the purity of silicon used in computer chips
  
  
• Analyzing blood samples from a crime scene for the presence of substances that would indicate foul play
  
  
• Determining chemicals present in a Superfund clean-up site
  
  
• Detecting the presence of certain explosives in a suspicious package
  
  
• Determining the thickness of welds
  
  
• Producing radioisotopes for treating cancer

Using Neutrons to Split Atoms

Neutrons are also used, of course, in nuclear power plants. We use neutrons to split the nuclei of highly fissionable atoms (enriched uranium-235 and/or plutonium-239) to create a nuclear criticality (a self-sustaining nuclear reaction). The reaction sustains itself because when one neutron splits the nucleus of a fissionable atom, additional neutrons are freed, which then go on to split other fissionable nuclei, and the process continues. Splitting the atoms releases great amounts of energy, much of it in the form of heat. The heat produced turns water into steam, which turns the turbines that create the electricity.

Measuring Dose

The total amount of ionizing radiation received by our bodies over a given time frame is what counts the most in considering potential health risks. For a basic understanding of the potential risks, we must use a standard system for measuring radiation absorbed by the different cells, tissues, and organs of our bodies.

In the United States, a widely used unit for measuring radiation absorbed by the human body is the rad (radiation absorbed dose). One rad equals the absorption of 100 ergs* of energy in every gram of tissue exposed to radiation.

To better represent biological risk, the rad is converted to rem. Rem stands for roentgen-equivalent-man. The roentgen, named after William Roentgen, who discovered X-rays, has become the international unit for measuring x-radiation and gamma radiation. Subdivisions of the rem have come to be used as a measurement of ionizing radiation doses in humans.

Because almost all human exposures are mere fractions of a rem, to arrive at a precise unit to measure human dose, the rem is further subdivided by 1,000 to get the millirem or 1/1,000 of a rem. The millirem has become the standard in America for measuring dose in human tissue. (Elsewhere in the world, scientists use units called grays and sieverts, which are 100 times greater than rads and rems, respectively.) To make its application reliably consistent, the measurements of doses in humans account for the type of radiation absorbed, i.e., the differences in likelihood of damage from the different types of radiation, and for the types of exposed tissues.

As you read, study, and discuss the rest of this unit, millirem will be an important concept to remember. It’s probably easier to grasp simply by understanding how many millirem the average American receives every year from various sources. The exercises that follow will help you understand these concepts. As you work through them, keep one key fact in mind: Every year, the average American absorbs a dose of about 360 millirem from all sources of radiation, both natural and manmade. And, in terms of effects, it makes no difference whether the ionizing radiation is natural or manmade. It’s the dose that counts, not the source.