Project 1

Double radioactive decay

Part I for both PHYS460 and PHYS660 students

Consider a radiactive decay problem involving two types of nuclei, A and B, with populations $N_{A}(t)$ and $N_{B}(t)$. Suppose that type A nuclei decay to form type B nuclei, which then also decay, according to differential equations:

${\frac {dN_{A}}{dt}}=-{\frac {N_{A}}{\tau _{A}}}$,

${\frac {dN_{B}}{dt}}={\frac {N_{A}}{\tau _{A}}}-{\frac {N_{B}}{\tau _{B}}}$,

where $\tau _{A}$ and $\tau _{B}$ are the decay time constants for each type of nucleus. Use the Euler method to solve these coupled equations numerically for $N_{A}(t)$ and $N_{B}(t)$ as a function of time.

Note that this problem can also be solved analytically either by using ``paper-and-pencil method or Mathematica. Obtain the analytic solutions for and and compare them with your numerical results. Present your results as graphs, with different plot for each of the three cases

• (a) $\tau _{A}>\tau _{B}$
• (b) $\tau _{A}=\tau _{B}$
• (c) $\tau _{A}<\tau _{B}$

Each plot should contain numerical solutions (using different values for the time step $\Delta t$ in the Euler algorithm) contrasted with the exact analytic solution. To avoid having to assign too many numerical values, use $N_{A}=100$, $N_{B}$ =0, and $\tau _{A}=1$ as the unit of time. In particular, try to interpret the short and long time behaviors for different value of this ratio.

Part II for PHYS660 students only

Consider again the same problem as in Part I, but now suppose that nuclei of type A decay into the ones of type B, while nuclei of type B decay into the ones of type A. Strictly speaking, this is not a "decay," since it is possible for the type B nuclei to turn back into type A nuclei. A better analogy would be a resonance in which a system can tunnel or move back and forth between two states A and B which have equal energies. The corresponding rate equations are:

${\frac {dN_{A}}{dt}}={\frac {N_{B}}{\tau }}-{\frac {N_{A}}{\tau }}$,

${\frac {dN_{B}}{dt}}={\frac {N_{A}}{\tau }}-{\frac {N_{B}}{\tau }}$

where for simplicity we assume that two types of decay are characterized by the same constant $\tau$. Solve this system of equations numerically for the numbers of nuclei $N_{A}(t)$ and $N_{B}(t)$, with initial condictions $N_{A}(0)=100$ and $N_{B}(0)=0$, and take $\tau =1$ as the unit of time. Show that your numerical results are consistent with the idea that the system reaches a steady state in which $N_{A}$ and $N_{B}$ are constant. In such a steady state, the time derivatives $dN_{A}/dt$ and $dN_{B}/dt$ should vanish.