Chapter Eight:
Nuclear physics
Intended Learning Outcomes:
In this chapter we will
• Define the Radioactive decay and the radiation. • Count the different types of radiation.
• Discuss the radioactive decay law. • Define the half life time constant.
8.1Radioactive decay and radiation
What is the radioactive decay?
It is the spontaneous disintegration of the unstable nucleus to more stable nucleus by emitting radiation (particles or/and electromagnetic waves).
What is Radiation?
It is the energy emitted from the unstable nuclei in form of emitted particles or emitted electromagnetic waves (or both).
The most common types of radiation are
1. Alpha particle (𝛼 ) 2. Beta particle ( 𝛽 ) 3. Gamma ray ( 𝛾 )
Nuclear Notation
The standard nuclear notation shows the chemical symbol, the mass number, and the atomic number of the element.
Relative Atomic Mass (RAM) = No. of protons + No. of neutrons
Alpha radiation ( 𝜶 )
If a nucleus emits an alpha particle ( 𝐻𝑒 2
4 ), it loses two protons and two neutrons. Therefore, the mass number A decreases by 4 and the atomic number Z decreases by 2. The decay can be written symbolically as
Electric Charge: +2
Relative Atomic Mass: 4
Where X is called the parent nucleus and Y is known as the daughter nucleus. As examples, 238 U and 226 Ra are both alpha emitters and decay according to the schemes
Description: 2 neutrons, 2 protons (Helium nucleus)
Penetration power: stopped by paper or a few cm of air
Ionisation effect: strong ionisation effect, so it transfers energy in very short distances
Effects of Magnetic/Electric Field: weakly deflected
Gamma ray ( 𝜸 )
Very often a nucleus that undergoes radioactive decay is left in an excited energy state. The nucleus can then undergo a second decay to a lower energy state-perhaps even to the ground state-by emitting one or more high-energy photons. The process is similar to the emission of light by an atom. An atom emits radiation to release some extra energy when an electron “jumps” from a state of high energy to a state of lower energy. Likewise, the nucleus uses essentially the same method to release any extra energy it may have following a decay or some other nuclear event. In nuclear de-excitation, the “jumps” that release energy are made by protons or neutrons in the nucleus as they move from a higher energy level to a lower level. The photons emitted in the process are called gamma rays, which have very high energy relative to the energy of visible light.
Description: High energy electromagnetic radiation
Electric Charge: 0
Relative Atomic Mass: 0
Penetration power: Reduced by several cm’s of lead or several metres of concrete
Ionisation effect: very weak ionisation effect
Effects of Magnetic/Electric Field: no deflection
The effect of electric and magnetic fields on radiation
o Beta radiation has a -1 charge and a small mass so is strongly deflected o Alpha radiation has a +2 charge but a RAM* of 4 so is only weakly deflected o Gamma radiation has no mass or charge so it is not deflected
*Relative Atomic Mass (RAM) = No. of protons + No. of neutrons
The Penetration Power of Radiation
The penetration power of different radiations is shown in the figure below.
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12 окт 2024
