# Homework Help: Calculating how many neutrinos passed through a body

1. Jan 19, 2012

### mattdespat

Supernova Shelton 1987A, located about 170 000 ly from the Earth, is estimated to have emitted a burst of neutrinos carrying energy ~1046 J. Suppose the average neutrino energy was 6 MeV and your body presented cross-sectional area 5 000 cm^2. To an order of magnitude, how many of these neutrinos passed through you?

My first step was to convert both energies to J, once this was done I was able to find out how many neutrinos were present in the original burst. My issue is at this point, how to connect the amount of neutrinos originally to how many would pass through a body with cross sectional area 5 000cm^2.
Thanks for the help.

2. Jan 19, 2012

### zhermes

Good start!

This is basically the same problem as trying to figure out how many darts randomly thrown at a dart board will hit a given region. In the dart-board case: what fraction of darts would hit the top half of the board?

Can you generalize this idea to your current problem?
Hint: the surface-area of a sphere is $$A = 4\pi r^2$$

3. Jan 19, 2012

### mattdespat

Thanks for the help.

So I took the area of the earth, by using the radius and using the equation above. Then like in the dartboard example half the darts would hit the top half of the dart board, so whatever percentage that the body is of the total area of the earth would be the same percentage of neutrinos that pass through the body.

My one remaining question is whether it would be appropriate to say that all neutrinos from the original burst would hit earth?

4. Jan 19, 2012

### zhermes

You're exactly on the right track!
The question is the fraction of the total area of the burst which is covered by a person. And your end question is also exactly on the right track---the earth would definitely not receive all of the neutrinos. Assume that the neutrinos are emitted 'isotropically' (equally in all directions). What spherical surface should you consider then?