What is radioactivity? This is the spontaneous decay or disintegration of unstable nuclei of atoms of radioactive elements by the emission of alpha (α-) particle, beta (β-) particle and gamma (γ-) radiation with the production of energy.
What are radioactive elements? These are the unstable elements that can decay or disintegrate spontaneously to emit alpha particles or beta particle or gamma radiation.
Examples of radioactive elements are, radium, uranium, thorium, protactinium, radon, polonium and many others
Alpha decay by radioactive elements
The alpha particle which is helium, when it is being emitted by a decaying radioactive element, we say that element has undergone alpha decay. During alpha decay , the neutron number and proton number decrease each by two, and nucleon number also decrease by four, moving the element closer to a stable state. That is:
Properties of alpha particles
- The mass of alpha particle equal to 6.64 x 10-27Kg
- The energy produced in this decay =9.0 x 10-13Jss
- Alpha particle can only travel a few centimeters in air
- Alpha can only move through solid for about 10-3cm, before being brought to rest by collision
Beta decay by radioactive elements
In beta decay, the neutron number and proton number decrease by 0ne each, while, nucleon does not decrease. The beta particle can be best described as electron, with a speed of 99.5% the speed of light, which is about 2.99 x 108m/s,
Gamma decay by radioactive elements
When a nucleus of atom is placed in an excited state, either by bombardment with high energy particle or by radioactive transformation, it may decay to ground state by emission of one or more photons, called gamma ray . The gamma ray has energy range of 10kev-5mev.
In gamma rays there is no change in neutron number or proton number, and also the nucleon, remains the same.
The characteristic of gamma ray
1.it travels in a straight lines
2.It has negligible mass
3.It has little ionizing power
It has high penetrating power
Comparison between the emitted particles of radioactive elements (α-,β-) and γ-radiation.
|Radiation||Alpha particle (α-)||Beta (β-)||Gamma (γ) ray|
|Nature||Helium nuclei||High speed electrons||Electromagnetic waves of a very short of wavelength|
|Velocity||(0.15-0.21) x 108m/s||2.9 x 108m/s||Speed of light|
|Charge||+Ze(3.2 x 10-19C)||-e(-1.6 x 10-19C)||No charge|
|Mass||6.65 x 10-27kg||9.1 x 10-31kg||Negligible|
|Effect of magnetic field||Slight deflection||Strongly deflected||Little or no effect|
|Ionizing power||Heavy ionization||Medium ionization||Little ionization|
|Penetrating power||Little penetrating power||Medium penetrating power||High penetrating power|
Half life of radioactive elements
What is half life of a radioactive element? The half life of radioactive elements can be defined as the time it take the radioactive element nuclei to reduce to half of its original value
Mathematically half life of a radioactive element is given as:
The half life, decay constant of radioactive isotope
|Isotope||Half life(T1/2)||Decay constant (λ)|
|Uranium-238||4.5 x 109 years||5.10 x10-18|
|Carbon-14||5.57 x 103 years||3.90 x 10-12|
|Radium||1.62 x 103 years||1.35 x 10-4|
|Helium||6 x 10-20s||1.20 x 1019|
Two type of radioactivity; Natural and artificial
How do you describe natural radioactivity? It can be described as the spontaneous ( that is without external cause) disintegration of an unstable nuclide by emission of α-particle or β-particle or γ-radiation, or a combination of any two or all the three with energy released
What is artificial radioactivity? This is the type of radioactivity that does not occur naturally but they are being produced by artificial means. It occur by bombarding the stable nuclides with small nuclear particles such as γ-radiation, β-particle , α-particles or even neutron and protons
In artificial radioactivity , a non radioactive elements, are made radioactive elements , by bombarding it with small nuclear particles (α-,β-particles)
There are two types of nuclear reaction – there are nuclear fission and nuclear fusion
This can be defined as a process in which an unstable nucleus split into two parts of comparable mass.
Fusion was discovered by Hann and strassman in 1938, when they bombarded uranium (Z=96) with neutrons. They found the resulting radiation to be a radioactive isotope of barium (Z=56) and krypton(Z=36). These results were interpreted as showing that uranium nuclei were splitting into two parts (fragments) called the fission fragments. Also in this nuclear reaction, two or three neutrons also appear along with the fission fragments.
This can be define as a process whereby two or more light nuclei join together to form a heavy nucleus with release of enormous energy
The neutrons and protons in a stable nucleus by nuclear forces , so to pull them apart energy is needed and this energy needed is called the binding energy of nucleus. The greater the binding energy the more stable is the nucleus
The Einstein mass energy relation
In 1905, Albert Einstein published his special theory of relativity, one of the conclusion from this is that mass and energy are equivalent and they are related by the equation E=∆mc2
The difference between the masses before and after reaction, ∆m, correspond to the reaction energy, E, where C is the speed of electromagnetic radiation. For ∆m to equal 1 kg, the amount of energy produced is 9×1016J. Conversely, a 1Mev gamma ray can be converted into a given mass of 1.78 x10-30Kg
If one of the neutrons produced by the first fission hits a second uranium nucleus the letter will also split and the process will continue causing a very number of fission reactions. This reaction is called chain reaction.
A chain reaction occurs on the fission of uranium only when the quantity of uranium is large.
If uranium is small and undergoes fission reaction the secondary neutrons escape from the sides before they cause fission. So to sustain chain reaction there must be sufficient pure uranium-235 known as the critical mass to avoid neutrons from being lost from the sides before they cause further fission
Atomic bomb chain reaction
Atomic bomb explosion occurred when two pieces of uranium with both masses below the critical mass whose sum is greater than the critical mass are brought together at the right time. Then there will be an increase in uncontrolled chain reaction and massive explosion with extensive destructive effects
Nuclear reactor is a system in which a controlled nuclear chain reaction is used to produce energy. In a nuclear power plant the energy is used to generate steam which operate a turbine and rotate electric generator( dynamo) . The main application of nuclear reactors is for generation of electric power.
Other application include
(a) production of artificial radioactive elements or isotopes
(b) for medical research
(c) for production of high intensity neutron beams and fissionable nuclides
Advantages of fusion over fission
- Funsionis easily achieved with hydrogen
2. The raw materials required for fusion are more readily available
3. Fusion produces less dangerous by-products
4. Fission reaction produces much larger release of nuclear energy