Question

A radioactive material P first decays into Q and then Q decays to non-radioactive material R. Which of the following figure represents time dependent mass of P, Q and R?

• A.
• B.
• C.
• D.

Hint:

The decay of a radioactive material follows an exponential decay law, and the mass of the decay products can be determined using the decay constants for each material.

Answer 1

The Correct Option is B

Given that material P decays into Q, and then Q decays into R, we need to determine the time-dependent mass of P, Q, and R. 
1. Decay of P to Q: Let the initial mass of P be \( m_0 \). The mass of P at time \( t \) is given by the exponential decay equation: \[ m_P(t) = m_0 e^{-\lambda_1 t} \] where \( \lambda_1 \) is the decay constant for P. This describes how P decreases over time. 
2. Decay of Q to R: The mass of Q at time \( t \) is determined by the difference between the mass of P that has decayed and the mass of Q that has decayed into R. 
The mass of Q at any given time is given by: \[ m_Q(t) = \frac{\lambda_1}{\lambda_2 - \lambda_1} m_0 \left(e^{-\lambda_1 t} - e^{-\lambda_2 t}\right) \] where \( \lambda_2 \) is the decay constant for Q. 
This equation represents how Q evolves as it decays from P to R. 
3. Mass of R: The mass of R at time \( t \) is the remaining mass, which can be expressed as the sum of the mass lost from P and Q: \[ m_R(t) = m_0 - m_P(t) - m_Q(t) \] Substituting the expressions for \( m_P(t) \) and \( m_Q(t) \): \[ m_R(t) = m_0 \left(1 - e^{-\lambda_1 t} - \frac{\lambda_1}{\lambda_2 - \lambda_1} \left(e^{-\lambda_1 t} - e^{-\lambda_2 t}\right)\right) \] This shows how R increases as both P and Q decay. 
The correct figure representing the time-dependent mass of P, Q, and R is (2), where: 
- The mass of P decreases exponentially. 
- The mass of Q initially increases and then decreases as it decays into R. 
- The mass of R increases as Q decays.