Question

A radioactive substance decays from 90 mg to 12.6 mg in 29 years according to the exponential decay model y=ae−bx , where a is the initial amount and y is the amount remaining after x years.
Find the b -value.

Use the EXACT b -value to write the exponential decay model for this substance with initial amount 90 mg, then use that model to find the half-life.



Find the half-life.

Answer 1

The half life of the radioactive substance is 10.22 years.

What is Half Life ?

Half Life is the amount of time needed by the radioactive substance to reduce to its half concentration (as compared to the initial concentration).

It is given that

y = ae⁻ᵇˣ

here a is the initial amount , y is the amount remaining after x years

a = 90mg at x =0

Value of y = 12.6 mg at x = 29 years

On substitution of value

12.6 = 90 e⁻²⁹ᵇ

0.14 = e⁻²⁹ᵇ

b = 0.0678

The equation is

$\rm y = 90 e^{-0.0678 * x}$

Half life is when the concentration is reduced to half

On 1st half life , the concentration = 90/2 = 45 mg

$\rm 45 = 90 e^{-0.0678 * x}$

x = 10.22 years

Therefore the half life of the radioactive substance is 10.22 years.

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