Radiation Sources

After reading this section you will be able to do the following:

  • Explain what a radioisotope is and what it is used for.
  • State why artificially produced radioisotopes are used by industry instead of naturally occurring radioisotopes.
  • Identify the three principle ways to produce radioisotopes.

As you know, there are a number of sources of radiation, ranging from naturally occurring radioisotopes to X-ray machines, and other forms of particle accelerators. In this section we are going to take a look at the different sources of gamma radiation commonly used today.

Radioisotopes Review

Remember from our previous discussion, that radioisotopes are elements that are atomically unstable and radioactive. Radioisotopes stabilize by releasing energy and matter. Natural radioisotopes, which have relatively low radioactive energy, have been largely replaced by artificially produced radioisotopes. Artificially produced radioisotopes are widely utilized as sources of radiation for radiography, gauging, and as tracers for a multitude of measurements that are not easily made by other methods.

How are radioisotopes produced?

Present day production of radioisotopes includes three principle categories, which are (1) neutron activation (bombardment), (2) fission product separation, and (3) charged particle bombardment. Nuclear bombardment constitutes the major method for obtaining industrially important radioisotope materials. Radioisotopes may exist in any form of matter, with solid materials comprising the largest group.

Rutherford's experiment

The study of radioactive decay led scientists to believe that transmutations from one element to another could be achieved by adding protons to the nucleus. Rutherford was the first to successfully perform a transmutation in 1919. In Rutherford's experiment, a container filled with nitrogen gas was used, along with a radioactive source that emitted alpha particles. A silver foil, thick enough to absorb the alpha particles that were not absorbed by the nitrogen gas, was placed at one end of the container. A zinc sulfide screen behind the foil recorded the scintillations of particles that had enough energy to pass through the foil. A scintillation is a flash of light produced in certain media by absorption of an ionizing particle or photon. These scintillations could then be observed by a microscope.

The following nuclear equation represents the transformation product of nitrogen when bombarded by alpha particles:

14 nitrogen atoms with 7 protons each plus 4 helium atoms with 2 protons each leads to 17 oxygen atoms with 8 protons each plus one hydrogen atom with one proton.

The validity of the equation was supported by the detection of oxygen gas in the container after the alpha bombardment of pure nitrogen. The alpha bombardment of nitrogen results in proton emission.

Chadwick's bombardment

Other bombardments produce differing particle emissions. An important bombardment was performed in 1932, by Chadwick. This experiment led to the discovery of the neutron. The following is Chadwick's nuclear equation:

24α+49Be612C+01n^{4}_{2}\alpha+^{9}_{4}Be \rightarrow ^{12}_{6}C+ ^{1}_{0}n

Numerous similar bombardments have since been performed, and found to produce neutrons. Since the neutron is not electrically charged, it penetrates atomic nuclei more readily than that of protons or alpha particles. It is known that the neutron has become a significant nuclear bullet.

What is neutron activation?

Neutron activation (bombardment) is the means in which two very important industrial radiographic sources, Cobalt-59 (Co-59) and Iridium-191 (Ir-191), are produced. Cobalt and Iridium exist in nature as stable elements. Cobalt is element 27 and Iridium is element 77 on the periodic table. In the production of a radiographic source the Cobalt and Iridium materials are usually formed into small metal pellets, typically ranging in size from 1 mm by 1 mm to 3 mm by 3 mm, or formed into 0.04 in. by 0.04 in. to 0.12 in. by 0.12 in.. The size of the radioisotope may vary significantly depending on the application.

An example of a very small radioisotope source is those utilized for medical treatment or research. The pellets are encapsulated in stainless steel containers to form radiographic sources. Exposing these elements to a large thermal neutron flux (neutrons with energies less than 0.4 eV) enables the stable element to capture a thermal neutron and thus becoming one mass unit heavier. Immediately after the neutron is absorbed by the nucleus, the energy that binds the neutron is lost as a prompt or capture gamma ray. This is not the gamma ray utilized for radiography but purely a byproduct of the production process. It is the radioactivity that the element now possesses that generates the gamma rays for radiography purposes. Remember, radioactivity is the result of radioactive decay. The following equation expresses one example of Co-59 activation:

2759Co+n2760Co2860Ni+gamma rays^{59}_{27}Co+n \rightarrow ^{60}_{27}Co \rightarrow + ^{60}_{28}Ni

Where: Co-60 is the radioisotope, symbol 103 \f "Symbol" \s 12g represents the gamma emission, and symbol 98 \f "Symbol" \s 12b represents the beta emission. Remember, both energy and matter are part of radioactive decay, and due to this decay the element transmutes to another different element. In this case Co-60 transmutes to Nickel-60, (2860Ni^{60}_{28}Ni).

What do all those symbols mean?

Bombardment reactions are commonly designated by the symbols for the incident and emitted matter and energy, such as the above representation for Co-59 The incident matter is the neutron (n), the emitted matter and energy is the beta particle (symbol 98 \f "Symbol" \s 12b), and the gamma ray (symbol 103 \f "Symbol" \s 12g). In bombardment reactions, the atomic mass number of the target nucleus may increase or decrease as illustrated by the Co-59 example. Notice that the atomic number of the isotope is the same as the stable element, but what about the atomic mass number?

Use of Fission to produce radioisotopes

Another, less common radiographic source, cesium-137 (Cs-137), is produced by a completely different means known as fission. In 1938, an Italian physicist Enrico Fermi along with a group of other scientists discovered nuclear fission. They found that when an atomic nucleus of uranium-235 is struck by a neutron, it splits into two nuclei of approximately equal mass. Nuclear fission has been defined as the splitting of an atomic nucleus into two smaller nuclei of roughly equal mass, known as intermediates. Heavy elements with atomic numbers greater than 90 are fissionable. As a result of fission, we can produce energy for electricity, produce neutrons for production of radioisotopes, and produce fission products. The latter being a stable element from an unstable nucleus.

During the fission reaction of uranium-235, the nucleus is bombarded with neutrons, which results in the fission fragments of barium-141 nucleus, and krypton-92 nucleus. In addition three neutrons are released (the original bullet neutron, and two neutrons from the U-235 nucleus) along with an energy.

his process may be represented by the following nuclear equation:

92235U+01n3692Kr+56141Ba+301n+energy^{235}_{92}U+^{1}_{0}n \rightarrow ^{92}_{36}Kr +^{141}_{56}Ba+ 3^{1}_{0}n+energy

It is this process of fission that is important to the production Cs-137 as a radioisotope. Let's look specifically at the reaction and describe what is taking place. The reaction may be represented as:

235U+nfission^{235}_{}U+n \rightarrow fission 235U+nfission^{235}_{}U+n \rightarrow fission

The two fission fragments are the previously mentioned smaller atoms into which the U-235 has been split. It has been determined that the majority of fission fragments are close to a mass of 95 and or 140. A chemical treatment is required to extract the cesium from the uranium. Normally it is recovered as a chemical form, cesium chloride (CsCl). The product is then converted to a ceramic or glass form for encapsulation purposes to be utilized as a radiographic source.

Review:

  1. Artificially produced radioisotopes are primarily used by industry because they can be produced so as to have much more radioactive energy that natural types.
  2. The three ways to produce radioisotopes are neutron activation, fission product separation, and charged particle bombardment.
  3. Elements that are atomically unstable and radioactive are called radioisotopes.