1. Radioactivity
Dr. Raad Al-Bdeery
12/27/2018

2. There are about 2,450 known isotopes of the one hundred odd elements in the Periodic Table. You can imagine the size of a table of isotopes relative to that of the Periodic Table! The unstable isotopes lie above or below the Nuclear Stability Curve. These unstable isotopes attempt to reach the stability curve by splitting into fragments, in a process called Fission, or by emitting particles and/or energy in the form of radiation. This latter process is called Radioactivity.
Dr. Raad Al-Bdeery
12/27/2018

3. RADIATION SOURCES
Radioactivity may be defined as spontaneous nuclear transformations in unstable atoms that result in the formation of new elements. These transformations are characterized by one of several different mechanisms, including alpha-particle emission, beta-particle and positron emission, and orbital electron capture. Each of these reactions may or may not be accompanied by gamma radiation. Radioactivity and radioactive properties of nuclides are determined by nuclear considerations only and are independent of the chemical and physical states of the radionuclide.
Dr. Raad Al-Bdeery
12/27/2018

4. The term radioactive refers to the emission of particles and/or energy from unstable isotopes. Unstable isotopes for instance those that have too many protons to remain a stable entity are called radioactive isotopes - and called radioisotopes for short. The term radionuclide is also sometimes used.
Dr. Raad Al-Bdeery
12/27/2018

5. Finally about 300 of the 2,450-odd isotopes mentioned above are found in nature. The rest are man-made (or person-made!), that is they are produced artificially. These 2,150 or so artificial isotopes have been made during the last 100 years or so with most having been made since the second world war.
Dr. Raad Al-Bdeery
12/27/2018

6. Radioactive Decay
We saw in the last topic that radioactivity is a process used by unstable nuclei to achieve a more stable situation. It is said that such nuclei decay in an attempt to achieve stability. So, an alternative title for this chapter is Nuclear Decay Processes. W
We also saw in the previous topic that we can use the Nuclear Stability Curve as a means of describing what is going on. So a second alternative title for this chapter is Methods of Getting onto the Nuclear Stability Curve.
Dr. Raad Al-Bdeery
12/27/2018

7. We are going to follow a descriptive or phenomenological approach to the topic here by describing in a fairly simple fashion what is known about each of the major decay mechanisms. Once again you may have already covered this material in high school physics. But bear with us because the treatment here will help us set the scene for subsequent topics.
Dr. Raad Al-Bdeery
12/27/2018

8. Methods of Radioactive Decay
Rather than considering what happens to individual nuclei it is perhaps easier to consider a hypothetical nucleus that can undergo many of the major forms of radioactive decay. This hypothetical nucleus is shown below:
Dr. Raad Al-Bdeery
12/27/2018

9. Dr. Raad Al-Bdeery
12/27/2018

10. Firstly we can see two protons and two neutrons being emitted together in a process called alpha-decay. Secondly, we can see that a proton can release a particle in a process called beta-plus decay, and that a neutron can emit a particle in a process called beta-minus decay. We can also see an electron being captured by a proton. Thirdly we can see some energy being emitted which results from a process called gamma-decay as well as an electron being attracted into the nucleus and being ejected again. Finally there is the rather catastrophic process where the nucleus cracks in half called spontaneous fission. We will now describe each of these decay processes in turn.
Dr. Raad Al-Bdeery
12/27/2018

11. Spontaneous Fission
This is a very destructive process which occurs in some heavy nuclei which split into 2 or 3 fragments plus some neutrons. These fragments form new nuclei which are usually radioactive. Nuclear reactors exploit this phenomenon for the production of radioisotopes. Its also used for nuclear power generation and in nuclear weaponry. The process is not of great interest to us here and we will say no more about it for the time being.
Dr. Raad Al-Bdeery
12/27/2018

12. Alpha Decay Where X and X’ represent different nuclear species.
In this decay process two protons and two neutrons leave the nucleus together in an assembly known as an alpha particle. Note that an alpha particle is really a helium-4 nucleus.
Where X and X’ represent different nuclear species.
Dr. Raad Al-Bdeery
12/27/2018

13. But notice that the radiation really consists of a helium-4 nucleus emitted from an unstable larger nucleus. There is nothing strange about helium since it is quite an abundant element on our planet. So why is this radiation dangerous to humans? The answer to this question lies with the energy with which they are emitted and the fact that they are quite massive and have a double positive charge. So when they interact with living matter they can cause substantial destruction to molecules which they encounter in their attempt to slow down and to attract two electrons to become a neutral helium atom.
Dr. Raad Al-Bdeery
12/27/2018

14. An example of this form of decay occurs in the uranium-238 nucleus
An example of this form of decay occurs in the uranium-238 nucleus. The equation which represents what occurs is:
Here the uranium-238 nucleus emits a helium-4 nucleus (the alpha particle) and the parent nucleus becomes thorium-234. Note that the Mass Number of the parent nucleus has been reduced by 4 and the Atomic Number is reduced by 2 which is a characteristic of alpha decay for any nucleus in which it occurs.
Dr. Raad Al-Bdeery
12/27/2018

15. From the conservation of energy, it shows which decays are energetically possible and enables us to calculate the rest energies Q or kinetic Energies K of decay products. A nucleus X will decay into a lighter nucleus X’ with the emission of one or more particles such as x only if the rest energy of X is greater than the total rest energy of X’+x. The excess rest energy is known as Q,
Dr. Raad Al-Bdeery
12/27/2018

16. Decay processes of this sort liberates energy since the decay products are more tightly bound than the initial nucleus. The liberated energy which appears as the kinetic energy of the α particle and the “daughter” nucleus X’, can be found from the masses of the nuclei involved according to Eq. 2:
Note that in equation (3) atomic masses are used since the electron masses cancel in it.
Dr. Raad Al-Bdeery
12/27/2018

17. naturally occurring alpha emitters are found only among elements of atomic number greater than 82. The explanation for this is twofold: First is the fact that the electrostatic repulsive forces in the heavy nuclei increase much more rapidly than the cohesive nuclear forces and the magnitude of the electrostatic forces, consequently, may closely approach or even exceed that of the nuclear force; the second part of the explanation is concerned with the fact that the emitted particle must have sufficient energy to overcome the high potential barrier at the surface of the nucleus resulting from the presence of the positively charged nucleons. This potential barrier may be graphically represented by the following curve
Dr. Raad Al-Bdeery
12/27/2018

18. Dr. Raad Al-Bdeery
12/27/2018

19. As derived in the lecture, the relation between the kinetic energy of the emitted alpha particle Kα and the total energy release associated with the radioactive transformation Q, is given as:
(4)
Where m and M are the masses of α-particle and daughter nucleus (recoil nucleus) respectively.
Dr. Raad Al-Bdeery
12/27/2018

20. Beta Decay Electron Emission
There are three common forms of beta decay:
Certain nuclei which have an excess of neutrons may attempt to reach stability by converting a neutron into a proton with the emission of an electron. The electron is called a beta-minus particle - the minus indicating that the particle is negatively charged.
We can represent what occurs as follows:
Electron Emission
Dr. Raad Al-Bdeery
12/27/2018

21. (5)
where a neutron converts into a proton and an electron. Notice that the total electrical charge is the same on both sides of this equation. We say that the electric charge is conserved. We can consider that the electron cannot exist inside the nucleus and therefore is ejected.
Dr. Raad Al-Bdeery
12/27/2018

22. In calculating Q, it must be noted that the electron masses may not cancel out in our previous calculations using atomic masses. This is because the initial and final nuclei have different Z values, and therefore different numbers of electrons. Therefore for neutron beta decay, mp and me can be grouped together to give the atomic mass , and so :
Dr. Raad Al-Bdeery
12/27/2018

23. All of this energy Q should appear as kinetic energy of the electron, and all emitted electrons should have exactly this energy. It was found by experiments that all the emitted electrons have less than this energy. In fact, they have a continuous range of energy, from 0 to up to their maximum.
The fact that beta radiation is emitted with a continuous energy distribution up to a definite maximum seems to violate the established energy–mass conservation laws. This is explained by the simultaneous emission of a second type of particle, called a neutrino, whose energy is equal to the difference between the kinetic energy of the accompanying beta particle and the maximum energy of the spectral distribution.
Dr. Raad Al-Bdeery
12/27/2018

24. Phosphorous-32 beta spectrum
Dr. Raad Al-Bdeery
12/27/2018

25. The neutrino, as postulated, has no electrical charge and a vanishingly small mass. Although these two characteristics make detection of the neutrino difficult, neutrinos have been measured and the neutrino hypothesis has been experimentally verified. Equation (5) should therefore be modified to
Dr. Raad Al-Bdeery
12/27/2018

26. The Q value for this decay is:
The process for negative beta decay is given by the following equation:
The Q value for this decay is:
The electron masses cancel in Eq. (8), so it is atomic masses that appear in it. The neutrino does not appear in the calculation of Q value because its mass is either zero or negligibly small.
Dr. Raad Al-Bdeery
12/27/2018

27. Once again there is nothing strange or mysterious about an electron
Once again there is nothing strange or mysterious about an electron. What is important though from a radiation safety point of view is the energy with which it is emitted and the chemical damage it can cause when it interacts with living matter
An example of this type of decay occurs in the iodine-131 nucleus which decays into xenon-131 with the emission of an electron, that is +
Dr. Raad Al-Bdeery
12/27/2018

28. (b) Positron Emission
In those instances where the neutron-to-proton ratio is too low and alpha emission is not energetically possible, the nucleus may, under certain conditions, attain stability by emitting a positron. A positron is a beta particle whose charge is positive. Its mass is amu and its charge is +1.6 × 10 C. Because of the fact that the nucleus loses a positive charge when a positron is emitted, the daughter product is one atomic number less than the parent. The mass number of the daughter remains unchanged, as in all nuclear transitions involving electrons.
Dr. Raad Al-Bdeery
12/27/2018

29. Positrons occur in nature only as the result of the interaction between cosmic rays and the atmosphere and disappear in a matter of microseconds after formation. The manner of disappearance is of interest and great importance to the health physicist. The positron combines with an electron and the two particles are annihilated giving rise to two gamma-rays whose energies are equal to the mass equivalent to the positron and electron.
Dr. Raad Al-Bdeery
12/27/2018

30. Protons in nuclei can undergo such decay processes:
The positron is not thought to exist independently within the nucleus. Rather, it is believed that the positron results from a transformation, within the nucleus, of a proton into a neutron according to the following reaction:
Protons in nuclei can undergo such decay processes:
And the Q value for this process, using atomic masses, is:
Dr. Raad Al-Bdeery
12/27/2018

31. An example of this type of decay occurs in the sodium-22 nucleus which decays into neon-22 with the emission of a positron and a neutrino, that is:
Dr. Raad Al-Bdeery
12/27/2018

32. (c) Orbital Electron Capture
Equation (10) shows that if a neutron-deficient atom is to attain stability by positron emission, it must exceed the weight of its daughter by at least two electron masses. If this requirement cannot be met, then the neutron deficiency is overcome by the process known as orbital electron capture or, alternatively, as electron capture or as K capture. In this radioactive transformation, one of the extranuclear electrons is captured by the nucleus and unites with an intranuclear proton to form a neutron according to the equation:
Dr. Raad Al-Bdeery
12/27/2018

33. The electrons in the K shell are much closer to the nucleus than those in any other shell. The probability that the captured orbital electron will be from the K shell is therefore much greater than that for any other shell; hence, the alternate name for this mechanism is K capture. The electron capture process does not occur for free protons, but in nuclei the process is:
Dr. Raad Al-Bdeery
12/27/2018

34. And the Q value, using atomic masses, is:
An example of this type of decay occurs in the thulium-170 nucleus which decays into erbium-170 with the emission of a neutrino, that is:
Dr. Raad Al-Bdeery
12/27/2018

35. where Eγ is the energy of the gamma ray, Ee is the kinetic energy of the conversion electron, and φ is the binding energy of the electron.
Here the excess energy of an excited nucleus is given to an atomic electron, e.g. a K-shell electron.
Dr. Raad Al-Bdeery
12/27/2018

36. Isomeric Transitions Gamma Rays
Gamma rays are monochromatic electromagnetic radiations that are emitted from the nuclei of excited atoms following radioactive transformations; they provide a mechanism for ridding excited nuclei of their excitation energy without affecting either the atomic number or the atomic mass number of the atom. Just as an atom does, the nucleus will reach its ground state or unexcited state after emitting one or more photons, known as nuclear gamma rays. Since the health physicist is concerned with all radiations that come from radioactive substances and since X-rays are indistinguishable from gamma rays, characteristic X-rays that arise in the extranuclear structure of many nuclides must be considered by the health physicist in evaluating radiation hazards.
Dr. Raad Al-Bdeery
12/27/2018

37. An example of this type of decay is that of technetium-99m - which by the way is the most common radioisotope used for diagnostic purposes today in medicine. The reaction can be expressed as:
Here a nucleus of technetium-99 is in an excited state, that is it has excess energy. The excited state in this case is called a metastable state and the nucleus is therefore called technetium-99m (m for metastable). The excited nucleus looses its excess energy by emitting a gamma-ray to become technetium-99.
Dr. Raad Al-Bdeery
12/27/2018

38. Internal Conversion
Internal conversion is an alternative isomeric mechanism to radiative transition by which an excited nucleus of a gamma-emitting atom may rid itself of excitation energy. It is an interaction in which a tightly bound electron interacts with its nucleus, absorbs the excitation energy from the nucleus, and is ejected from the atom. Internally converted electrons appear in monoenergetic groups. The kinetic energy of the converted electron is always found to be equal to the difference between the energy of the gamma-ray emitted by the radionuclide and the binding energy of the converted electron of the daughter element.
Dr. Raad Al-Bdeery
12/27/2018

39. Since electrons in the L level of high-atomic-numbered elements are also tightly bound, internal conversion in those elements results in two groups of electrons that differ in energies by the difference between the binding energies of the K and L levels. Internal conversion may be thought of as an internal photoelectric effect, that is, an interaction in which the gamma ray collides with the tightly bound electron and transfers all of its energy to the electron. The energy of the gamma ray is divided between the work done to overcome the binding energy of the electron and the kinetic energy imparted to the electron. This may be expressed by the equation:
Dr. Raad Al-Bdeery
12/27/2018