# Help needed in calculating Flux of point source (NucEng)

1. Apr 5, 2012

### stvoffutt

1. The problem statement, all variables and given/known data

A 2 $\mu$g sample of ${}^{47}Ca$ is encased in a lead sphere. What is the photon intensity 5 cm from the source? Use some of the tables in your book to find the required information.

2. Relevant equations

Flux/Intensity of photons from a point source:
$\phi(r)=\frac{S_p}{4\pi r^2} e^{-\mu r}$
Where $\mu$ is the total microscopic cross section, r is the radius(distance) from point source and $S_p$ is the particles per unit time.

3. The attempt at a solution

I can and have done these problems when the energy of the sample is given. However I have never done one where the weight of the sample is given. I am lost on this problem. My main question here is: How can I go from weight to photons/second?

I tried calculating $S_p$ by using 1047.1 keV/decay listed in the textbook but I still don't know where to go from there. Ci:=decays/second. How many photons per keV?? Or am I over thinking this?

2. Apr 6, 2012

### Staff: Mentor

Radioactive calcium 47 has a half-life of 4.7 days and emits a high-energy gamma ray (photon) of 1.3 Mev as well as a beta particle in its decay to scandium 47. - reference

If you know the sample's mass you can calculate the number of atoms present, and based on its half-life you can determine the expected number of photons the sample is emitting per unit of time.