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?
Alpha decay is the process of transformation of a radioactive nucleus by emitting helium. During this transformation, the initial element changes to another completely different element, undergoing a change in mass and atomic number as well. The radioactive disintegration of alpha decay is a phenomenon in which the atomic nuclei which are unstable dissipate excess energy by ejecting the alpha particles in a spontaneous manner. Since the alpha particles have a mass of four units and two units of positive charges, their emission from nuclei results in daughter nuclei that have a positive nuclear charge. The atomic number of such nuclei has a mass that is four units less than the parent and an atomic number that is two units less than the parent.
2. Which elements can undergo alpha decay?
All elements heavier than lead can undergo alpha decay. However, lighter elements do not exhibit radioactive decay of any kind. Alpha decay is a commonly found principle in elements that are heavier than bismuth, which has an atomic number 83. The phenomenon of alpha decay is also found in rare earth elements ranging from neodymium, which has atomic number 60, to lutetium, which has atomic number 71.
3. What is the use of the Geiger-Nuttall Law?
Geiger-Nutall law establishes a relation between the decay constant of a radioactive isotope and the energy of the emitted alpha particle. Geiger-Nuttall law is used in nuclear physics and it relates the energy of the alpha particle emitted to the decay constant of a radioactive isotope. According to this law, those isotopes which are short-lived emit more energetic alpha particles as compared to those isotopes which are long-lived. Also, according to the law, the half-lives of isotopes are exponentially dependent on the decay energy because of which very large changes in the half-life result in a very small difference in decay energy.
4. What are the applications and importance of alpha decay?
There are a lot of applications of alpha decay occurring in radioactive elements. The major application of alpha decay in radioactive elements is:
Smoke detectors (for example, Americium) use the alpha decay property of radioactive elements. The radioactive elements release alpha particles that ionize the air present inside the detector. These alpha radiations are absorbed by the smoke in the detector, therefore, if the smoke is available the ionization is altered and the alarm gets triggered.
Alpha particles are also used in APXS, that is, Alpha Particle X-Ray Spectroscopy. APXS is a process that is used to determine the elemental composition of rocks and soil. This method was used by NASA for its mission to Mars.
It was also used in Pathfinder missions for determining the elements that existed in Martian rocks.
Alpha particles are also used in the medical field, like for the treatment of cancer through targeted alpha therapy (TAT) for killing cancer cells. In this procedure, lead-212 is used that is ingested into the body and travels to the site of the tumour where it gives off alpha radiation and kills all the cells in the area.
5. What is the mechanism behind the phenomenon of alpha decay?
The nuclear force that holds an atomic nucleus is even stronger than the repulsive electromagnetic forces between the protons. The nuclear force is a short-range force that drops quickly in strength beyond 1 femtometer whereas the electromagnetic force has a very vast range. The strength of the nuclear force that keeps the nucleus together is directly proportional to the number of nucleons. The electromagnetic force is a disruptive force that breaks the nucleus apart. This disruptive electromagnetic force is proportional to the square of its number. In the case of the nucleus that has more than 210 nucleons, the nuclear force that binds the nucleus together cannot counterbalance the electromagnetic repulsion between the protons it contains. Therefore, such nuclei accelerate the stability by reducing their size results in alpha decay.