Alpha Decay Definition
Alpha decay or α-decay refers to any decay where the atomic nucleus of a particular element releases 42He and transforms into an atom of a completely different element. This decay leads to a decrease in the mass number and atomic number, due to the release of a helium atom.
To understand this entirely, consider this alpha decay example. Suppose element Z has mass number ‘a’ and atomic number ‘b’. During α decay, this element changes to X. Take a look at the equation below.
abZ → a-4b-2X + 42He
Thus, you can see that the mass number and the atomic number balances out on both sides of this equation.
Alpha Decay Example Problems
Now, using the same concept, solve the following problem. A Uranium nucleus, 23892U undergoes alpha decay and turns into a Thorium (Th) nucleus. What would be the mass and atomic number for this resulting nucleus after the decay?
Solution –
23892U → 238-492-2Th + 42He → 23490Th + 42He
Therefore, the resulting Thorium nucleus should have 234 mass numbers and 90 atomic numbers.
Alpha Decay Equation
Alpha decay formula can be written in the following way –
AZX → A-4Z-2Y + 42α
In this equation, AZX represents the decaying nucleus, while A-4Z-2Y is the transformed nucleus and 42α is the alpha particle emitted.
Understanding Q Value of Alpha Decay
In Physics and Chemistry, Q-value is defined as the difference between the sum of the rest masses of original reactants and the sum of final product masses. In simpler terms, you can say that the Q-value is the difference between the final and initial mass energy of the decayed products.
For alpha decay equations, this Q-value is,
Q = (mX – mY – mHe) c2
The energy Q derived from this decay is divided equally into the transformed nucleus and the Helium nucleus.
Gamow Theory of Alpha Decay
Gamow's Theory of Geiger-Nutall law defines the relationship between the energy of an alpha particle emitted with the decay constant for a radioactive isotope. It was derived by John Mitchell Nutall and Hans Geiger in 1911, hence the name for this law.
With this rule, it becomes abundantly clear that shorter-lived isotopes emit greater energy when compared to isotopes with longer lives. However, α decay is just one type of radioactive decay. A nucleus can undergo beta and gamma decay as well.
What is Beta Decay?
In beta decay, the radioactive isotope emits an electron or positron. This decay occurs by following the radioactive laws, just as alpha decay does. An example of beta decay is –
3215P → 3216S + e- + v-
What is Gamma Decay?
The last form of radioactive decay is gamma decay. Here, a high-energy radioactive nucleus can lower its energy state by emitting electromagnetic radiation. Gamma decay is common for the daughter nucleus formed after α decays and ß decays.
This happens because daughter nuclei in both these forms of decay are in a heightened state of energy. To return to a stable state, these nuclei emit electromagnetic radiation in the form of one or multiple gamma rays.
What are the Major Components of the Equation that Represents Alpha Decay?
The general equation of alpha decay contains five major components like the parent nucleus which is the starting nucleus, the total number of nucleons present in the nucleus (that is, the total number of neutrons and protons present in the nucleus), the total number of protons in an atom, the daughter nucleus which is the ending nucleus and the alpha particle that is released during the process of alpha decay.
What is the Safety Level of Alpha Decay?
Though the alpha particles are not very penetrating, the substance that undergoes alpha decay when ingested can be harmful as the ejected alpha particles can damage the internal tissues very easily even if they have a short-range. This is basically due to the contact of emitted particles with membranes and living cells.
The major health effects of alpha particles depend on the time and reason due to exposure to alpha particles. If in case the alpha particles are swallowed, inhaled, or absorbed into the bloodstream which can have long-lasting damage on biological samples. The damage caused due to alpha particles increases a persons’ risk of cancer like lung cancer. Radon which is an alpha emitter, when inhaled by individuals can cause related illnesses in humans.
Exercise
14964Gd undergoes α decay to form one nucleus of Sm. Calculate the atomic and mass number of the daughter nucleus.
Solution –
14964Gd → 149-464-2Sm + 42He → 14562Sm + 42He
As per the alpha decay equation, the resulting Samarium nucleus will have a mass number of 145 and an atomic number of 62.
The isotope element that emits radiation is known as the Radioactive Element. This element is also the object that undergoes radioactivity.
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FAQs on Alpha Decay
1. What is alpha decay and how is it represented by a general equation?
Alpha decay is a type of radioactive decay where an unstable atomic nucleus emits an alpha particle to become more stable. An alpha particle is identical to a helium nucleus, containing two protons and two neutrons. This process transforms the original element (parent nucleus) into a new element (daughter nucleus) with a mass number reduced by 4 and an atomic number reduced by 2. The general equation for alpha decay is: AZX → A-4Z-2Y + 42α.
2. Can you provide a common example of alpha decay as seen in the CBSE syllabus?
A classic example of alpha decay, often studied in Class 12 Physics, is the decay of Uranium-238 into Thorium-234. In this process, the Uranium nucleus ejects an alpha particle. The decay equation is written as: 23892U → 23490Th + 42He. Here, Uranium (atomic number 92) transforms into Thorium (atomic number 90), demonstrating the change in element and the conservation of mass and atomic numbers.
3. Why do only heavy nuclei typically undergo alpha decay?
Alpha decay occurs primarily in heavy nuclei because of the interplay between two fundamental forces. The strong nuclear force, which holds protons and neutrons together, is very powerful but has a very short range. In contrast, the electromagnetic force causes repulsion between positively charged protons and has a much longer range. In very large nuclei (typically with more than 210 nucleons), the cumulative long-range repulsion between the many protons starts to overpower the short-range strong force holding the nucleus together. Emitting an alpha particle is an efficient way for the nucleus to reduce its size and proton number, thus reaching a more stable, lower-energy state.
4. How does alpha decay differ from beta and gamma decay?
Alpha, beta, and gamma decay are all processes by which an unstable nucleus releases energy, but they differ in what they emit and how they affect the nucleus:
- Alpha (α) Decay: Emits an alpha particle (a helium nucleus). The mass number decreases by 4, and the atomic number decreases by 2.
- Beta (β) Decay: Emits an electron or a positron. The mass number remains unchanged, but the atomic number increases or decreases by 1.
- Gamma (γ) Decay: Emits a high-energy photon (gamma ray). It does not change the mass number or atomic number; it only lowers the energy of the nucleus, often occurring after an alpha or beta decay has left the nucleus in an excited state.
5. What are some important real-world applications of alpha decay?
The properties of alpha-emitting isotopes are used in several important applications. Key examples include:
- Smoke Detectors: Many household smoke detectors use a small amount of Americium-241, an alpha emitter. The alpha particles ionise the air, creating a small electric current. Smoke particles disrupt this current, triggering the alarm.
- Space Exploration: Radioisotope Thermoelectric Generators (RTGs) use the heat generated from the alpha decay of elements like Plutonium-238 to produce electricity for spacecraft and rovers on long missions.
- Medical Treatments: In a procedure called Targeted Alpha Therapy (TAT), alpha-emitting isotopes are attached to molecules that target cancer cells. Their high energy and short range allow them to destroy tumour cells with minimal damage to surrounding healthy tissue.
6. What does the Q-value of an alpha decay represent?
The Q-value in an alpha decay represents the total energy released during the reaction. This energy originates from the conversion of mass into energy, as described by Einstein's mass-energy equivalence equation. It is calculated as the difference between the rest mass of the parent nucleus and the sum of the rest masses of the daughter nucleus and the alpha particle, multiplied by the speed of light squared (c²). This released energy primarily appears as the kinetic energy of the daughter nucleus and the emitted alpha particle.
7. Is the alpha particle just a helium atom? What are its key properties?
No, an alpha particle is not a helium atom; it is a helium nucleus. This is a critical distinction. A neutral helium atom has two electrons orbiting its nucleus, whereas an alpha particle has no electrons, giving it a +2 electric charge. Its key properties are:
- It has a relatively large mass (4 atomic mass units).
- It carries a positive charge of +2e.
- It has very high ionising power, meaning it can easily knock electrons off other atoms.
- It has very low penetrating power and can be stopped by a sheet of paper or the outer layer of human skin. However, it can be very harmful if an alpha-emitting substance is inhaled or ingested.
