Photon with negative energy, Compton Scattering

[zellwwf]

Hello PF people,

Homework Statement

In a Compton scattering event, after the collision, the Photon has an energy of 0.12 MeV
and the Electron has an energy of 0.04 MeV. Find the following:

i) The Wavelength of the photon before the collision.
ii) The scattering angle for the photon after the collision.
iii) The scattering angle for the electron after the collision.

Homework Equations

1 . Conservation of Energy
2 . Conservation of Momentum
3 . Compton's Scattering Δλ = \frac{h}{mc}(1-cos(θ))

The Attempt at a Solution

I tried to equate the conservation of energy,
i get a negative energy for the photon before it collides:
E_p + E_e = E_p + E_e
Then substituting (in MeV):
X + 0.511 = 0.04 + 0.12 --> X = -0.351 MeV

I did not copy anything wrong, the problem sheet gives these numbers, what do i do? is this wrong or should i continue?

 

[voko]

The given energy for the electron obviously does not include its rest energy.

 

[zellwwf]

Hello again voko, ur my savior! :)
Yea, what you are saying makes total sense.. thank you a lot