Question

A Decay Chain

When the daughter nucleus produced in a radioactive decay is itself unstable, it will eventually decay and form its own daughter nucleus. If the newly formed daughter nucleus is also unstable, another decay will occur, and the process will continue until a nonradioactive nucleus is formed. Such a series of radioactive decays is called a decay chain.

A good example of a decay chain is provided by 232 90Th, a naturally occurring isotope of thorium.

What is the energy Q released in the first step of the thorium-232 decay chain? The atomic mass of 232 90Th is 232.038054 u and the atomic mass of 228 88Ra is 228.0301069 u.

Answer 1

Answer : The energy released in first step of thorium-232 decay chain is
`7.974* 10^(-13)J`

Explanation :

First we have to calculate the mass defect
`(\Delta m)`.

The balanced reaction is,

`^(232)Th\rightarrow ^(228)Ra+^(4)He`

Mass defect = Sum of mass of product - sum of mass of reactants

`\Delta m=(\text{Mass of Ra}+\text{Mass of He})-(\text{Mass of Th})`

`\Delta m=(228.0301069+4.002602)-(232.038054)=5.34* 10^(-3)amu=8.86* 10^(-30)kg`

conversion used :
`(1amu=1.66* 10^(-27)kg)`

Now we have to calculate the energy released.

`Energy=\Delta m* (c)^2`

`Energy=(8.86* 10^(-30)kg)* (3* 10^8m/s)^2`

`Energy=7.974* 10^(-13)J`

The energy released is
`7.974* 10^(-13)J`

Therefore, the energy released in first step of thorium-232 decay chain is
`7.974* 10^(-13)J`