TOPIC 3: Unstable and Emissions

We’ll continue with Radiation Fundamentals my discussing Unstable atoms and their emissions.

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An atom becomes unstable when it has excess energy in its nucleus.  The atom wants to lose that energy so it can become stable.  When conditions are right in the nucleus, the energy is released.

“Unstable” means “radioactive.”

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This pent-up energy in the nucleus can be any intensity.  If there is a lot of energy to release, the nucleus can expel pieces of itself – such as the protons and neutrons in the nucleus.  If there is not that much energy, it can just release the energy. This concept of emitting energy from an atom defines radiation.

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As we stated, when a radioactive atom needs to release a large amount of energy, it can do so by throwing out large pieces of matter from the nucleus.  But there is still residual energy which is released as pure energy.

Let’s look at this through an atom.

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Let’s look at our first decay mode – the alpha particle.  The alpha particle is a type of emission from a radioisotope having a high atomic number or high “Z” number.

The alpha particle that is emitted from the nucleus consists of two protons and two neutrons, making it a nucleus of a helium atom. However, it does not have any electrons.  Since it has two protons, it has a positive charge. It will easily ionize atoms by pulling away orbital electrons from other atoms to become a complete helium atom. Because of its massive size, alpha particles do not travel far. After the Alpha is emitted from the nucleus, the resulting atom has 2 less protons making it a different element.  This new element is referred to as a “daughter.” The element  that emitted the alpha particle is the “parent.”

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Let’s take a look at an example of a radioactive element that decays through alpha emission….

We know that a U-235 atom has 92 protons; thus, there must be 143 neutrons.  So, we know the atomic weight of this uranium atom is 235.  Since the alpha has 2 protons and 2 neutrons, its atomic weight is 4.  So when the uranium decays and releases the alpha particle, the resulting atomic weight will be 231.  But what is the element?  We know there were 92 protons in uranium but the alpha that was discharged from the uranium atom took 2 protons.  So the math says our resulting atom must have 90 protons.  If we look at a periodic table, we will see that Thorium has 90 protons.  The end result of an alpha decay of U-235 is the isotope Th-231 with a helium atom floating out there somewhere.

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Since the alpha particle does not go far, it is easily stopped by anything in its path.  An alpha cannot penetrate the skin or go through a piece of paper before it strips electrons from the nearest atoms it encounters.  The problem with the alpha particle is its high ionizing potential if in direct contact with vulnerable substances.  So, if the alpha emitter is inhaled into the lungs from dust carrying the radioisotope and is deposited into the lungs, the alpha can have direct ionizing effect on the atoms that make up the al-ve-o-li.  These are extremely important cells that are instrumental in the exchange of oxygen to our blood.

Alphas are not a problem external to our bodies. The outer layer of dead skin cells is sufficient to block Alphas.  For protection from Alpha radiation, all that is needed is a Tyvex paper suit and a respirator.

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Actually, alpha particles are 20 times more dangerous when they are emitted internally due to their high energy.  The most susceptible organs are our lungs.  But alphas can also enter our bodies through injuries, such as cuts. To protect ourselves from inhalation of substances that emit Alpha radiation, we use masks.  To protect ourselves from getting radioactivity through wounds, we want to flush the wound.

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Beta particles can be both positively or negatively charged particles and are also emitted from the nucleus.  Generally, the beta particle is negatively charged, with the same mass as an electron The beta particle is much smaller than an alpha particle and has less probability of interaction with other atoms which means it can usually go farther in air than an alpha particle.

A beta particle is basically the same as an electron.  The only difference is where it originated.  An electron orbits the atom and has a negative charge.  The Beta comes from the nucleus and also has a negative charge.  When we discussed the Neutron, we stated the neutron is generally the same weight as a Proton and Electron…. and since the Proton is positively charged and the Electron is negatively charged, the combination creates a Neutron having no charge. The Neutron routinely splits into the Proton and Electron.  Often the Electron just hooks up with another Proton.  But sometimes, it is emitted from the nucleus. When it leaves the nucleus it is called a Beta particle.

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As we stated, the beta particle is much smaller than an alpha particle and has less probability of interaction with other atoms.  The distance a beta particle travels is directly proportional to the energy of the emission.  Some radioisotopes are considered “soft” beta emitters since they have very low energies and thus create less damage to tissue.  Other radioisotopes are considered “hard” beta emitters and thus emit very energetic beta particles.  “Soft” betas cannot penetrate a piece of paper or be detected more than a few inches from the surface; however, “hard” betas can go a few feet in the air and can create potential exposures to the lens of the eye, it not protected.  In addition, some “hard” betas have sufficient energy to create secondary radiations called X-rays if the betas impact heavier atoms with a lot of electrons.  If that is the case, these resultant x-rays may need to be shielded for protection.

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Many radioisotopes with “soft” betas are beneficial to the life sciences.  Elements that make up plants and animals, such as carbon, hydrogen, oxygen, sulfur, phosphorus, potassium, and strontium, all have isotopes that are beta emitters. Radioactive isotopes “behave” or metabolize the same as their stable isotope in a living being.  Thus, if administering radioactive iodine, it will go to the thyroid just like stable iodine.  Other organs have an affinity for certain elements and chemicals.  The researcher can use the radioactive isotope of the element to identify the quantity of the element or chemical that is normally taken up by the respective organ.

An extremely common radionuclide that exists in nature is carbon-14 which is a beta emitter.  It has a well-known abundance in the atmosphere and is in the tissues of plants and animals.  By measuring the concentration of carbon-14 in organic matter, it is possible to determine when plants or animals (including humans) died.   This concept will be further explored when the concept of “half-life” is discussed.

In this slide, C-14 is a radioactive material used in research as “tracers” to be a surrogate for stable carbon isotopes which are tagged to a herbicide which contains carbon.  It is used to identify the efficacy of certain herbicides and pesticides.  The compound labelled with C-14 herbicide is applied to the plants to see the uptake of the compound in the plant, thus the effectiveness of the chemical for use commercially.

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Gamma rays are electromagnetic radiations of pure energy with no mass that travel close to the speed of light.  Gamma rays do not have a positive or negative charge and do not interact easily with matter.  Gamma rays are the most penetrating of the types of radiation from a radionuclide and usually require some type of shielding to protect the worker.  Gamma rays can go a relatively far distance when compared to the alpha and beta particle and that distance is proportional to energy. In fact, terrestrial gamma radiation intensity can be measured and mapped using planes equipped with very sensitive radiation detectors. Gamma radiation intensity decreases dramatically when you back away from the source. This is because the gamma radiation beam comes out of the housing more like a flashlight than a laser, so more of the rays miss you as you back away.  Additionally, the air acts as a shield to stop or deflect more of the radiation.

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Now sometimes, the release of extra energy, above what was released by the Alpha or Beta discharge, leave an atom in an excited, or “metastable”, state. This is similar to the metastable state of an atom which still has energy that needs to be released. It could take some time before that energy is released as Gamma radiation. It does not affect the atomic weight or atomic number so it creates no new element. The metastable state is just an excited or higher energy state of the same atom.

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The x-ray is an emission from a high-speed electron that is accelerated through a vacuum to a positively charged target.  This target is usually made of tungsten or other relatively high “Z” number.  Having a large number of protons, the tungsten also has the same high number of electrons.   Since like charges repel, the electron will try to avoid an electron in the outer shell of the tungsten.  It will “curve” away from it and decelerate rapidly.  When this occurs, energy is conserved via the formation of an x-ray.   This is also called “Bremstrahlung” meaning “breaking radiation” in German. X-rays can be generated from “high energy” betas acting like electrons.

There will be a spectrum of X-ray energies from these interactions because of the degree of “bending” from the target electrons and the amount of acceleration and deceleration.  The energy of the X-ray is directly proportional to the energy of the electron impacting onto the target.

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As we stated, X-rays can also be generated naturally from “hard” betas interacting with dense materials.

Since these betas come from the nucleus of the atom, they are considered a nuclear reaction.

X-rays that come from the outer shells of an atom, not the nucleus, such as what is created by x-ray machines, are considered an atomic action, not a nuclear action.

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As we learned earlier about Roentgen, X-rays changed medicine.  It quickly changed from a “clinical” observation of the injury, to a diagnostic principle.  The x-ray enables the physician to “see” the broken bone instead of simply observing a malformation from the outside.

The photon that is emitted as an x-ray from the Crookes tube is directed toward the patient. The number of photons that go through the patient and hit the screen (or photographic film) is directly proportional to the density of the material between the screen and the X-ray source.  Since bone fractures have small gaps, this means there is less density.  The photographic film will be darker because more x-ray photos are hitting the film.  Where the bone is dense, more photons are “absorbed” and less impact the photograph film.  The image will be lighter.  This contrast between light and dark gives us the x-ray which an experienced technician can read as a bone fracture.

X-rays behave like Gamma rays.  The only difference is the source of its generation.

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If X-rays can be used to check fractures in bones, it would seem we should be able to use it in other industries.  Industrial radiography uses the same principal to check for imperfections in the welds of two pieces of metal.

We cannot see the inside of a weld with the naked eye.  But if we use very high energy gamma rays or x-rays we can penetrate welds and recorded on x-ray film.  Air bubbles have less density and would show darker on the film.  This could represent a weakness in the metal that could lead to early failures.  The most common radioactive sources for industrial radiography is iridium-192 or Cobalt-60.  The most common industries to use industrial radiography are those that cannot afford a failure of a pipe.

Industrial Radiography is a form Non-Destructive Testing.  In other words, we can observe flaws inside material without destroying the material.

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Neutrons are emitted from the nucleus of atoms of only a few radionuclides.  They do not travel far but they can cause problems.  If a neutron hits a Uranium-235 atom, it immediately unbalances the entire atom making it split almost in half.  This is called fission.  Two new elements are created, along with a lot of heat, and the release of more neutrons.  Those neutrons then hit other U-235 atoms which generate more heat and more neutrons.  Unless its controlled, it can literally explode.  It’s the source for creating the heat generated in nuclear reactors.

Also, there are those radioisotopes that spontaneously emit neutrons and are man-made. There are very few of these types of radioisotopes.  One of them californium-252,  which is a gamma and a neutron emitter.  These are used in industries to determine the feed material, like a cement plant, going into the kiln.

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Cf-252 has a high number of protons (98) compared to uranium, which has 92.  Thus, it is considered a “transuranic” and man-made.

It is radioisotope that spontaneously emits neutrons without the need of an outside flux of neutrons.  It does not have a sufficient flux of neutrons to create “criticality”, thus it cannot be used to make a nuclear bomb.  Cf-252 is routinely used in industry as a neutron source to bombard elements on a conveyor system to change the elements temporarily by irradiating them to neutrons; and, when absorbed the atoms spontaneously emit a characteristic gamma ray unique to that element.  The detection system then identifies these elements and able to quantify the amount of these elements on the conveyor system.

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Neutron emissions create much more damage to tissues due to the water content in the body.  The water quickly absorbs the neutron, but translates the energy into a large amount of cellular damage. Neutron sources have special shielding and radiation protection techniques to minimize exposures.

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We mentioned Fissioning is the process of splitting atoms.  For certain species of uranium, thorium, and plutonium isotopes, they can fission into smaller radioactive atoms upon the absorbing of a neutron.  The neutron creates excessive imbalance to the binding energy, or the energy holding the atom together.  The nucleus can no longer maintain its integrity and splits into fragments.  These fragments are called “fission fragments” or “fission products”.  They are also radioactive but with relatively short half-lives.  Upon fissioning into fragments, the original uranium, thorium or plutonium atom also emit additional neutrons…and the cycle continues which creates the heat that creates the electricity we use.   If there are a sufficient number of atoms to create these reactions in a sustained manner, it is called “criticality.”

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The prime radioisotope in nuclear reactors is the uranium-235 atom.  This isotope of uranium has a particular behavior in that since it is an odd numbered atomic weight, the nucleus will readily accept a neutron, and then literally fissions into fragments.

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The fissioning of the U-235 nucleus gives off a tremendous amount of energy in the form of heat, more neutrons, and highly radioactive fission fragments, such as Cs-137, I-131, Co-60, I-131 and Mn-54.   These radioisotopes are considered “fission fragments” . When the rods holding these fission fragments are removed from a nuclear reactor, they are considered as radioactive waste from the nuclear power industry known as “spent fuel”.  Radioisotopes that fission other than U-235 in the presence of neutrons are U-233 and isotopes of plutonium (Pu).  The neutrons emitted are “slowed down” by the use of moderators in the reactor to create “thermal neutrons” which are more readily captured by other U-235 atoms.  This enables the sustained reaction to occur.