Correct Answer - Option 3 : 73.11
Concept:
Radioactive decay
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Radioactivity: The atoms having a number of neutrons much more than proton are unstable. The nuclei of such atoms exhibit radioactivity.
- An example of Such an Atom is U - 238, where the number of Neutrons is 146, and the number of protons is 92.
- Radium is an important radioactive atom.
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Radioactive decay: The spontaneous breakdown of such an unstable atomic nucleus causes radioactivity.
- The process of radioactive decay as a function of time is represented by
\(ln (\frac{N}{N_0}) = - λ t\)
N is the number of radioactive nuclei present in a sample, No is the number of radioactive nuclei present in the sample at t = 0, λ is disintegration constant.
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Disintegration constant: The disintegration constant is the unique constant related to the radioactive nature of the given nucleus.
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Half-Life Period: The time in which the number of nuclei reached to half of its initial value is called the half-life period.
It is the time period when N = N0 / 2
The half - Life period is given as
\(ln (\frac{N_0 /2}{N_0}) = - λ t_{1/2}\)
⇒ \(t_{1/2} = \frac{ln2}{λ}\)
Calculation:
Given half-life is 22 years
\(\implies 22 = \frac{ln2}{λ}\)
\(\implies \lambda = \frac{ln2}{22}\) -- (1)
Now, we got the value of disintegration constant.
It is given that the substance is decreased to 10 %
So, N = 10 % of No
Or
\(\frac{N}{N_0} = \frac{1}{10}\) --- (2)
We know that
\(ln (\frac{N}{N_0}) = - λ t\)
Using (1) and (2) in the above equation
\(\implies ln (\frac{1}{10}) = - \frac{ln 2}{22} t\)
\(\implies - ln \ 10 = - \frac{ln 2}{22} t\)
\(\implies t = \frac{ln 10}{ln 2} \times 22\)
⇒ t = 73.11 years.
So, the correct option is 73.11 years