# Interaction of radiation with matter

by Catty
 P: 4 1. Problem statement i am trying to calculate the momentum and energy of the products in the reaction 7Be4 + e -----> 7Li3 + neutrino 2. Relavant equations p = mv mass of 7Be = 7.016929 u mass of 7Li = 7.016004 u 3. The attempt at a solution i know that after the electron capture, the neutrino and the daughter nucleas will move in opposite directions with the same momentum magnitude, and so form momentum coservation mv(Li) = mv(neutrino) but how do i go about findig the 'v' of Lithium in order to calculate its momentum, mv. the question does not give any info on 'v'. ? Do i even need the value of 'v' or not. So that i can then put that 'v' into the equation of finding the energy of the daughter nucleas as: 1/2 * m(Li)*v^2(Li) = ( m(neutrino)/m(Li) ) * E(neutrino) am i actually in the right path, i would greatly appreciate any directions?
Mentor
P: 9,585
Where do you use energy conservation? It is important here, together with momentum conservation.

 the question does not give any info on 'v'.
It does not have to.

The neutrino is ultrarelativistic, you cannot use the nonrelativistic momentum. Treat it like photons instead. Do you know the energy-momentum relation for photons?
 P: 496 * The momentum of the neutrino is not going to be mv because it will be a relativistic particle. * Did you apply energy conservation?
P: 4

## Interaction of radiation with matter

ohhh, Thank you, it's E^2 = (pc)^2 + (m0*c^2)^2

will apply it , hadn't considered the relativistic issue....

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