A thin aluminum target is illuminated with photons of wavelength . A detector is placed at 90.0

Question:

A thin aluminum target is illuminated with photons of wavelength λ. A detector is placed at 90.0 ° to the direction of the incident photons. The scattered photons detected are found to have half the energy of the incident photons.
(a) Find λ.
(b) What is the wavelength of backscattered photons (detector at 180°)?
(c) What (if anything) would change if a copper target were used instead of an aluminum one?