Laser Compton scattering is a method to obtain high-energy photons by laser photons backscattered off energetic GeV electrons. In the case of head-on collision

between relativistic electrons and laser photons, the energy of scattered photons is given by

where y = Ee/me is the Lorentz factor of the electron beam with energy Ee, me is the rest mass of the electron, El is the energy of the laser photon, and в is the scattering angle. From Eq. (1.4), the energy of the scattered photon is maximum at в = 0, and it depends on the energy of incident electrons and photons. The minimum energy of the scattered photon can be fixed by controlling в with collimators.

The scattering cross section of laser Compton scattering is given by the Klein— Nishina formula:

dNY do

Ny dEY dEY const.

Y Y dEY Y dEY

(1.5)

To have a situation in which (y, n) reactions occur, the photon beam with energy at B(n) < Ey < B(2n) is desired. In case of the free electron laser, we may assume/ expect to get the total photon flux NY « 2 x 1012/s/500mA for Ee = 1.2 GeV [1]. From Eq. (1.4), with Ee = 1.2 GeV and El = 0.7 eV, we obtain the maximum photon energy of Ey = 15 MeV at в = 0, which is equal to B(2n) for 137Cs. Figure 1.3 shows the calculated y-ray spectrum generated by laser Compton scattering using Eq. (1.5), where the total photon flux with energy from 0 to B(2n) is NY « 2 x 1012/s.

From Fig. 1.3, we can see that about half the total scattered photons are in B(n) < Ey < B(2n) and contribute to generate the (y, n) reactions for 137Cs. In contrast, for the Bremsstrahlung that is usually used to generate high-energy photons, the photon intensity decreases rapidly as the photon energy increases, and only a small part of the high-energy tail is available for (Y, n) reactions [11].