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Radioactivity (also known as Radioactive decay, radioactive disintegration, nuclear decay, or nuclear disintegration)
What is Radioactivity?
The balance of protons and neutrons in a nucleus determines whether a nucleus will be stable or unstable. Too many neutrons or protons can disturb this balance making the nucleus unstable. Atoms found in nature are either stable or unstable. An atom is stable if the forces among the particles that make up the nucleus are balanced. An atom is unstable (radioactive) if these forces are unbalanced; if the nucleus has an excess of internal energy. The instability of an atom’s nucleus may result from an excess of either neutrons or protons. A radioactive atom will attempt to reach stability by ejecting nucleons (protons or neutrons), as well as other particles, or by releasing energy in other forms.
So, What is the cause of Radioactivity?
In a nucleus of an atom, two types of forces are acting there. One is the attractive strong nuclear force between the nucleons and the other is repulsive electrostatic force between the protons. If attractive force dominates the repulsive force, the nucleus becomes stable but if repulsive force is too much greater than the attractive force, the nucleus becomes unstable. such an unstable nucleus emits radioactive radiations. Hence, unstable is the main cause of radioactivity.
The process by which an unstable atomic nucleus loses energy by radiation and as the nucleus emits radiation or disintegrates, the radioactive atom (radionuclide) transforms to a different nuclide. This process is called radioactive decay or radioactivity. It will continue until the forces in the nucleus are balanced. For example, as a radionuclide decays, it will become a different isotope of the same element if it gives off neutrons or a different element altogether if it gives off protons.
“एक अस्थिर आणविक न्यूक्लियसले (Unstable atomic nucleus) ऊर्जा विकिरणद्वारा गुमाउँछ र न्यूक्लियसले (nucleus) विकिरण (radiation) निकाल्दा वा विघटनको (disintegration) प्रक्रियामा जाँदा रेडियोएक्टिभ एटम (radionuclide) फरक न्यूक्लाइडमा (nuclide) परिवर्तन हुन्छ। यस प्रक्रियालाई रेडियोएक्टिभ क्षय वा रेडियोएक्टिभिटी (Radioactive decay or Radioactivity)भनिन्छ। न्यूक्लियसका शक्तिहरू सन्तुलित नभएसम्म यो जारी रहनेछ। उदाहरणको लागि, एक रेडियुन्युक्लाइड डिके हुँदा, यदि यसले विकिरणद्वारा न्यूट्रन निकालछ भने तत्वको भिन्न आइसोटोप बन्छ र यदि यसले विकिरणद्वारा प्रोटोनहरू निकालछ भने यो भिन्न तत्व बन्छ ।”
It is important to note that radioactive nuclei disintegrate:
- and randomly
This means that the process of radioactive decay can not be speeded up or slowed down by artificial means and it happens spontaneously (spontaneous decay). It also means that we cannot tell when a particularly unstable nucleus will decay (random decay). Some examples of radioactive elements are Uranium, Polonium, Radium, Thorium, etc.
Natural and Artificial Radioactivity
Nuclear reactions which occur spontaneously are said to be an example of natural radioactivity. There are three naturally occurring radioactive series among the elements in the periodic table. These are known as the uranium series, the actinium series, and the thorium series, each named after the element at which the series starts (except the actinium series which starts with a different uranium isotope). Each series decays through a number of unstable nuclei by means of alpha and beta emission, until each series end on a different stable isotope of lead.
Not all nuclear reactions are spontaneous. These reactions occur when stable isotopes are bombarded with particles such as neutrons. This method of inducing a nuclear reaction to proceed is termed artificial radioactivity. This meant new nuclear reactions, which wouldn’t have been viewed spontaneously, could now be observed. Since about 1940, a set of new elements with atomic numbers over 92 (the atomic number of the heaviest naturally occurring element, Uranium) have been artificially made. They are called the transuranium elements.
Types of Radioactive Decay:
Three of the most common types of decay are alpha decay, beta decay, and gamma decay, all of which involve emitting one or more particles or photons.
Radioactive decay is a stochastic (i.e., random) process at the level of single atoms. According to quantum theory, it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed.
The decaying nucleus is called the parent radionuclide (or parent radioisotope), and the process produces at least one daughter nuclide. Except for gamma decay or internal conversion from a nuclear excited state, the decay is a nuclear transmutation resulting in a daughter containing a different number of protons or neutrons (or both). When the number of protons changes, an atom of a different chemical element is created.
- Alpha decay occurs when the nucleus ejects an alpha particle (helium nucleus).
- Beta decay occurs in two ways;
- (i) beta-minus decay, when the nucleus emits an electron and an antineutrino in a process that changes a neutron to a proton.
- (ii) beta-plus decay, when the nucleus emits a positron and a neutrino in a process that changes a proton to a neutron, this process is also known as positron emission.
- In gamma decay, a radioactive nucleus first decays by the emission of an alpha or beta particle. The daughter nucleus that results is usually left in an excited state and it can decay to a lower energy state by emitting a gamma ray photon.
- In neutron emission, extremely neutron-rich nuclei, formed due to other types of decay or after many successive neutron captures, occasionally lose energy by way of neutron emission, resulting in a change from one isotope to another of the same element.
- In electron capture, the nucleus may capture an orbiting electron, causing a proton to convert into a neutron in a process called electron capture. A neutrino and a gamma ray are subsequently emitted.
- In cluster decay and nuclear fission, a nucleus heavier than an alpha particle is emitted.
Discovery of radioactivity
Like Thomson’s discovery of the electron, the discovery of radioactivity in uranium by French physicist Henri Becquerel in 1896 forced scientists to radically change their ideas about atomic structure. Radioactivity demonstrated that the atom was neither indivisible nor immutable. Instead of serving merely as an inert matrix for electrons, the atom could change form and emit an enormous amount of energy. Furthermore, radioactivity itself became an important tool for revealing the interior of the atom.
Soon after the discovery of natural radioactivity, a large number of experiments were conducted to determine the nature of radiations emitted by the radioactive substances.
Ernest Rutherford’s experiments:
(a) With Electric Field
(b) With the Magnetic Field
The above figures show a simple experiment to determine different types of radiations emitted by a radioactive substance. A small hole is drilled in a lead block inside which a piece of a radioactive substance such as radium is placed and a narrow beam of radiation emerges out of the hole. The nature of the radiation is studied by applying electric field as in figure (a) or magnetic field as in figure (b). In both cases, the narrow beam splits into the following three components.
- Alpha particles, which are attracted to the negative plate and deflected by a relatively small amount, must be positively charged and relatively massive.
- Beta particles, which are attracted to the positive plate and deflected a relatively large amount, must be negatively charged and relatively light.
- Gamma rays, which are unaffected by the electric field, must be uncharged.
We now know that α particles are high-energy helium nuclei, β particles are high-energy electrons, and γ radiation composes high-energy electromagnetic radiation. We classify different types of radioactive decay by the radiation produced.
Three types of emission from a radioactive nuclide are observed as discussed above. In addition to these, electron capture and positron emission are also considered radioactive decay.
Alpha (α) decay is the emission of an α particle from the nucleus. For example, polonium-210 undergoes α decay:
Alpha decay occurs primarily in heavy nuclei (A > 200, Z > 83). Because the loss of an α particle gives a daughter nuclide with a mass number four units smaller and an atomic number two units smaller than those of the parent nuclide, the daughter nuclide has a larger n:p ratio than the parent nuclide. If the parent nuclide undergoing α decay lies below the band of stability, the daughter nuclide will lie closer to the band.
Beta (β) decay is the emission of an electron from a nucleus. Iodine-131 is an example of a nuclide that undergoes β decay:
Beta decay, which can be thought of as the conversion of a neutron into a proton and a β particle, is observed in nuclides with a large n:p ratio. The beta particle (electron) emitted is from the atomic nucleus and is not one of the electrons surrounding the nucleus. Such nuclei lie above the band of stability. The emission of an electron does not change the mass number of the nuclide but does increase the number of its protons and decrease the number of its neutrons. Consequently, the n:p ratio is decreased, and the daughter nuclide lies closer to the band of stability than did the parent nuclide.
Gamma emission (γ emission) is observed when a nuclide is formed in an excited state and then decays to its ground state with the emission of a γ ray, a quantum of high-energy electromagnetic radiation. The presence of a nucleus in an excited state is often indicated by an asterisk (*). Cobalt-60 emits γ radiation and is used in many applications including cancer treatment:
There is no change in the mass number or atomic number during the emission of a γ ray unless the γ emission accompanies one of the other modes of decay.
Positron emission (β+ decay) is the emission of a positron from the nucleus. Oxygen-15 is an example of a nuclide that undergoes positron emission:
Positron emission is observed for nuclides in which the n:p ratio is low. These nuclides lie below the band of stability. Positron decay is the conversion of a proton into a neutron with the emission of a positron. The n:p ratio increases and the daughter nuclide lies closer to the band of stability than did the parent nuclide.
Electron capture occurs when one of the inner electrons in an atom is captured by the atom’s nucleus. For example, potassium-40 undergoes electron capture:
Electron capture occurs when an inner shell electron combines with a proton and is converted into a neutron. The loss of an inner shell electron leaves a vacancy that will be filled by one of the outer electrons. As the outer electron drops into the vacancy, it will emit energy. In most cases, the energy emitted will be in the form of an X-ray. Like positron emission, electron capture occurs for “proton-rich” nuclei that lie below the band of stability. Electron capture has the same effect on the nucleus as does positron emission: The atomic number is decreased by one and the mass number does not change. This increases the n:p ratio, and the daughter nuclide lies closer to the band of stability than did the parent nuclide. Whether electron capture or positron emission occurs is difficult to predict. The choice is primarily due to kinetic factors, with the one requiring the smaller activation energy being the one more likely to occur.
Properties of Radioactive Radiation
Properties of α-particles
- α-particles are the nuclei of a helium atom
- They are positively charged particles having charge +2e = 3.2 x 10-19 C.
- Their rest mass is equal to 6.4 x 10-27 Kg.
- They move with high velocities which depend on the source emitting the particles.
- They are deflected in an electric and magnetic field.
- They affect the photographic plate.
- The energy of α particles emitted from the radioactive substance is about 6MeV.
- They produce fluorescence and phosphorescence on some materials such as zinc sulfide.
- They ionize the gas through which they pass. The ionizing power is higher than the β-particle.
- They get scattered while passing through the thin metal foil.
- They penetrate through matter, but the penetrating power is lower than that of β-particle.
- They can induce artificial radioactivity.
Properties of β-particles
- β-particles are electrons of nuclear origin
- They are negatively charged particles having charge -1.6 x 10-19 C.
- Their rest mass is equal to 9.1 x 10-31 Kg.
- They move with the velocity of the order of 108 ms-1
- Their energy ranges from 2 to 3 MeV.
- They are deflected by an electric and magnetic field.
- They affect photographic plates.
- They can easily pass through a few mm of aluminum. Their penetrating power is 100 times more than that of α-particles.
- They produce fluorescence and phosphorescence on some materials such as zinc sulfide.
- They can induce artificial radioactivity.
Properties of γ-rays
- γ-rays are electromagnetic waves of very short wavelength.
- They are chargeless particles.
- They move with a velocity equal to that of light.
- They are not deflected by the electric and magnetic field
- They can affect the photographic plate.
- They produce fluorescence on some materials
- They ionize the gas through which they pass, but it is about only 1/100th of that of β-particles.
- They penetrate through matter and the penetrating power is very high. It is about 100 times that of β-particles.
- They knock out the electrons from the surface on which they fall.
- They produce heat on the surface exposed to them.
- They can produce nuclear reactions.