1. Basic Nuclear Physics - 3
Modes of Radioactive Decay and Types of Radiation
Now let’s discuss radioactive decay, including important terms and concepts
Day 1 – Lecture 3

2. Objective
To understands modes of radioactive disintegration and types of radiation
To learn about basic atomic structure; alpha, beta, and gamma decay; positron emission; differences between gamma rays and x-rays; orbital electron capture; and internal conversion
We will discuss the types of radioactive decay and radiations emitted which are important to a health physicist in terms of exposure and dose.

3. Content Basic atomic structure and isotopes
Alpha, beta, and gamma decay
Decay spectra
Differences between gamma rays and x-rays
Positron emission
Orbital electron capture
Internal conversion
This session will focus on describing the basic structure of the atom; fundamental atomic particles; isotopes; alpha, beta, and gamma modes of disintegration; decay spectra, ; production of x-rays and the essential differences between x-rays and gamma rays; positron emission; orbital electron capture; and internal conversion.

4. Atomic Structure proton neutron electron
The atom consists of neutrons (no electrical charge) and positively charged protons in a central nucleus. Negatively charged electrons circle the atom in various orbitals which represent different energy levels. Protons and neutrons have virtually the same mass. The electron has a mass which is only 1/1820 of that of protons and neutrons. Thus, the nucleus makes up almost all of the mass of an atom.
Electrons circle the atom in atomic orbitals which represent different energy states. Electrons closest to the nucleus are the most tightly bound (this orbital is called the K-shell). Going outward from the nucleus, the other orbitals are called the L, M, N, O and P- shells.
In the electrically neutral atom, the number of electrons circling the nucleus is equal to the number of protons in the nucleus.
The number of protons in the nucleus (called the Z number or atomic number) determines the chemical element.

5. Atomic Number (Z) Hydrogen 1 Carbon 6 Cobalt 27 Selenium 34 Iridium 77
Uranium 92
This slide shows some examples of the Z number or atomic number of selected chemical elements. This value represents the number of protons in the nucleus.

6. Isotopes An isotope of an element has: the same number of protons
a different number of neutrons 1H 2H 3H
Note that isotopes are the same chemical element, with the same number of protons and electrons. H-3 (tritium) is a radioactive isotope of hydrogen. Some isotopes are stable and some are radioactive (unstable).

7. Isotopes
The number of protons determines the element. Elements with the same number of protons but different numbers of neutrons are called isotopes. Some isotopes are radioactive.
This explains the concept of isotopes and shows atomic nomenclature. “C” stands for the element carbon, the neutron number is shown in the lower right, the Z number (protons) is shown on the lower left, and the atomic mass number A (protons plus neutrons) is shown on the upper left.

8. Radioactive Decay
Spontaneous changes in the nucleus of an unstable atom
Results in formation of new elements
Accompanied by a release of energy, either particulate or electromagnetic or both
Nuclear instability is related to whether the neutron to proton ratio is too high or too low
Unstable atoms undergo radioactive decay.

9. The Line of Stability N > Z
Protons in the nucleus are positively charged and thus tend to repel each other. Were it not for the poorly understood nuclear force, the nucleus would fly apart due to this electrical repulsion. The nuclear force keeps the nucleus together.
The dotted line shows where the stable isotopes would fall if the neutron number, N, was always equal to the proton number, Z. However, as Z number increases N becomes greater than Z to offset the electrostatic repulsion of the positively charged protons (see the plot of the stable isotopes in the figure). All the stable isotopes fall on this plot. The unstable (radioactive) isotopes are not on this plot and by their disintegration, they tend to approach stability by adjusting their neutron to proton ratio.

10. Alpha Emission
Emission of a highly energetic helium nucleus from the nucleus of a radioactive atom
Occurs when neutron to proton ratio is too low
Results in a decay product whose atomic number is 2 less than the parent and whose atomic mass is 4 less than the parent
Alpha particles are monoenergetic
Alpha decay results in formation of a new chemical element. The decaying nucleus adjusts its neutron to proton ratio to try to get onto the line of stability.

11. Alpha Particle Decay Alpha particle charge +2
The alpha particle is a helium nucleus with a positive charge of 2 (two protons but no electrons). It is emitted from the nucleus of a decaying atom.

12. Alpha Particle Decay
The decay product has an atomic mass number “A” four less than that of the parent and an atomic number “Z” two less than that of the parent. The alpha particle has an atomic mass number of 4 and an atomic number of 2.

13. Alpha Decay Example 226Ra decays by alpha emission
When 226Ra decays, the atomic mass decreases by 4 and the atomic number decreases by 2
The atomic number defines the element, so the element changes from radium to radon
226Ra  222Rn He 2 86 88
This example shows alpha decay for the familiar radionuclide Ra It decays by alpha emission to a new chemical element Rn-222.

14. Beta Emission
Emission of an electron from the nucleus of a radioactive atom ( n  p+ + e-1 )
Occurs when neutron to proton ratio is too high (i.e., a surplus of neutrons)
Beta particles are emitted with a whole spectrum of energies (unlike alpha particles)
Another type of radioactive disintegration is beta emission. The decaying nucleus adjusts its neutron-to-proton ratio to try to get onto the line of stability.

15. Beta Particle Decay Beta particle charge -1
The beta particle is emitted from the nucleus and is a negative electron.

16. Beta Particle Decay
The decay product in beta decay has the same atomic mass “A” of the parent but its atomic number “Z” is one greater than that of the parent (a neutron has disappeared but has been replaced by a proton). The antineutrino is emitted to conserve mass and energy but has no significance at all from a health physics standpoint. It produces no dose!

17. Beta Decay of 99Mo
This spectrum shows an example of beta decay of the isotope Mo-99. Part of the time (about 89%) it decays to Tc-99m and part of the time (about 11%) it decays directly to Tc-99, both of which are themselves radioactive. The “m” means metastable, which is an excited energy state of Tc-99. Thus, Tc-99m has excess energy relative to Tc-99 that it must rid itself of by emission of a gamma ray. More will be said about gamma rays in this session.

18. Beta Spectrum
As opposed to alpha particles which are all emitted from the same radionuclide with the same energy, beta particles are emitted with a continuous spectrum of energies from essentially zero up to some maximum energy. The maximum energy available is shared between the beta particle and the anti-neutrino. The maximum energy of the beta spectrum is unique for each beta emitting radioisotope.
This situation is analogous to the production of Bremsstrahlung x-radiation.

19. Rule of Thumb
Average energy of a beta spectrum is about one-third of its maximum energy or:
Eav = Emax 1 3
Since beta particles are not all emitted with the same energy, a good rule of thumb is that the average energy of a beta spectrum is about 1/3 of the maximum energy of that spectrum. For dose calculation, we would use the average energy, not the maximum.

20. Positron (Beta+) Emission
Occurs when neutron to proton ratio is too low ( p+  n + e+ )
Emits a positron (beta particle whose charge is positive)
Results in emission of 2 gamma rays (more on this later)
Positrons are positively charged electrons and represent another means by which unstable radionuclides adjust their neutron to proton ratio to get onto the line of stability.

21. Positron (Beta+) Emission
In positron emission, the daughter product has the same atomic mass “A” as the parent but its atomic number “Z” is one less than that of the parent. A proton disappears but is replaced by a neutron which has the same mass. In positron emission, a neutrino is also emitted. However, the neutrino has absolutely no significance from a health physics standpoint. It produces no dose.

22. Positron Decay
In this example of positron decay, C-11 decays to B-11 with the emission of a positive electron.

23. Positron Decay
Another example of positron emission: Mg-23 decays by positron emission to Na-23. The positron has charge but virtually no mass while the neutrino has neither charge nor mass.

24. Positron Decay
In this sample spectrum, the positron emitter Na-22 decays to Ne-22. EC stands for orbital Electron Capture which is an alternate mode of disintegration which competes with positron emission. More will be said about electron capture in just a few minutes.
In this spectrum, 90% of the decays of Na-22 are by positron emission and 10% of its decays are by electron capture. Also, Na-22 emits a separate positron of a different energy a very small percentage of its decays (0.05% or 5 in 10,000 decays).

25. Positron Annihilation
Positron annihilation is a very important phenomenon associated with positron decay. A positron is antimatter. When antimatter contacts matter, annihilation of both particles occurs (i.e. both particles are completely converted to energy in the form of photons). Matter and antimatter cannot exist together.
When a positron at rest encounters an ordinary electron at rest, both annihilate and convert their mass to energy in the form of two photons each with an energy of 511 keV (the rest mass of the electron). The photons are emitted in exactly the opposite direction to satisfy the laws of angular momentum.
These photons allow positron emitters to be detected by gamma ray counting. They are also important from a health physics standpoint, since they are relatively high energy, they must be shielded and they produce dose to people.

26. Orbital Electron Capture
Also called K Capture
Occurs when neutron to proton ratio is too low
Form of decay competing with positron emission
One of the orbital electrons is captured by the nucleus: e p+1  n
Results in emission of characteristic x-rays
This is a form of radioactive decay which competes with positron emission (remember the previous slide on Na-22).

27. Orbital Electron Capture
In orbital electron capture, the decay product has the same atomic mass as that of the parent but its atomic number “Z” is one less than that of the parent, because a proton has disappeared from the nucleus. The positive proton “captures” a negative electron from the closest orbit (the K shell) and becomes a neutron.
An important associated phenomenon is that a photon (called a characteristic x-ray) is emitted as the vacancy in the orbit is filled by electrons transitioning from other orbits.

28. Orbital Electron Capture
Here is another example (Be-7) decaying by orbital electron capture.

29. Ionization ionized atom ejected electron +1 -1 radiation path
Let’s briefly discuss ionization at this point. When an orbital electron is lost from an atom either by radiation interacting with it, an ion pair is produced. The resulting atom has a +1 charge and a hole is left where the electron was. Outer orbital electrons will then fall down or cascade down into the lower orbitals to fill this vacancy.
radiation
path

30. X-Ray Production characteristic x-rays electron fills vacancy electron
ejected
characteristic
x-rays
When these orbital electrons change orbitals, they change energy states and in the process, they emit photons of specific energies which are “characteristic” of their orbitals for each element. Thus, these photons are called characteristic x-rays.
The various chemical elements can be identified by their characteristic x-rays.
Note that in orbital electron capture, the characteristic x-rays emitted are characteristic of the daughter product chemical element and not the parent!

31. Electromagnetic Spectrum
x- and -rays
Ultra-
violet
Infra-
red
Visible
This figure shows the electromagnetic spectrum. X-rays and gamma rays are photons (i.e. pure energy) and tend to have the highest energy (shortest wavelength) of any radiation on the spectrum. X-rays and gamma rays have enough energy to ionize atoms, whereas ultraviolet, visible, and infrared radiation can excite atoms and molecules but lack sufficient energy to produce ionization.
Increase in wavelength : decrease in frequency and energy

32. Gamma Ray Emission
Monoenergetic radiations emitted from nucleus of an excited atom following radioactive decay
Rid nucleus of excess energy
Have characteristic energies which can be used to identify the radionuclide
Excited forms of radionuclides often referred to as “metastable”, e.g., 99mTc. Also called “isomers”
Note that gamma ray emission does not result in a new chemical element (the number of protons does not change); rather it rids the nucleus of excess energy.

33. Gamma Ray Emission Gamma Radiation
Gamma rays are emitted from the nucleus of an excited atom.

34. Gamma Ray Emission
Here is another slide re-enforcing the point that gamma rays are emitted from the nucleus. They are NOT related to the movement of electrons between orbitals.

35. Photon Emission Difference Between X-Rays and Gamma Rays
Let’s remember the differences between x-rays and gamma rays. The principal difference is in their point of origin. Gamma rays are emitted from the nucleus of an atom, whereas x-rays are emitted when orbital electrons change orbitals (i.e. change energy states).

36. Internal Conversion
Alternative process by which excited nucleus of a gamma emitting isotope rids itself of excitation energy
The nucleus emits a gamma ray which interacts with an orbital electron, ejecting the electron from the atom
Characteristic x-rays are emitted as outer orbital electrons fill the vacancies left by the conversion electrons
Internal conversion is a competing process with gamma ray emission. In this case, the gamma ray emitted from the nucleus does not make it out of the atom. Instead all of its energy is given to an electron, no gamma is detected even though one was emitted from the nucleus.

37. Internal Conversion
These characteristic x-rays can themselves be absorbed by orbital electrons, ejecting them.
These ejected electrons are called Auger electrons and have very little kinetic energy
If the electrons ejected are from the outermost shells (those most loosely bound to the nucleus) they are termed Auger electrons.

38. Internal Conversion
In this example, the gamma ray interacts with a K shell electron, ejecting it from its orbital. Although the gamma ray could conceivably interact with an electron in any shell, it is most likely to interact with a K shell electron, since they are the closest to the nucleus.

39. Summary of Radioactive Decay Mechanisms
Mode
Characteristics
of Parent Radionuclide
Change in
Atomic Number
(Z)
Atomic Mass
Comments
Alpha
Neutron Poor -2 -4
Alphas Monoenergetic
Beta
Neutron Rich +1
Beta Energy Spectrum
Positron -1
Positron Energy Spectrum
Electron
Capture
K-Capture; Characteristic X-rays Emitted
Gamma
Excited
Energy State
None
Gammas Monoenergetic
Internal Conversion
Ejects Orbital Electrons; characteristic x-rays and Auger electrons emitted
This is a summary slide showing the various radioactive decay modes we have discussed in this session.

40. Summary Basic atomic structure was described Isotopes were defined
Modes of radioactive disintegration were discussed (including alpha, beta, gamma, positron emission, orbital electron capture, and internal conversion)
Ionization was defined
X-ray production and the differences between gamma rays and x-rays were described

41. Where to Get More Information
Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009)
International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002)