[Physics Class Notes] on Alpha Decay Pdf for Exam

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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