Gamma beams with lasers vs. Bremsstrahlung

By going through the previous entries, I have just realized that no explanation was given on the reason of our enthusiasm about the γ beam of ELI-NP... So here, in a few sentence, I will give a short comparison on the two ways of producing energetic fotons.

In the past decades many experiments have been performed on photon-induced reactions all over the world by using the so-called bremsstrahlung beams of electron accelerator facilities. These energetic photons were produced by the deceleration of the accelerated electrons in a gold target foil/plate. The photon spectrum is continuous, having a maximum energy which is equal to the energy of the electron beam. The difficulty of these experiments is to give the cross section of a photon-induced reaction at a given energy (E0). The procedure was the following: 1) measuring the reaction yield at E1=E0+dE than 2) measuring the yield at E2=E0-dE and finally 3) subtract the second from the first. If we do the same procedure on the energy spectrum of the two bremsstrahlung beams (E1 and E2) then we get the energy spectrum of the "effective" beam which is "responsible" to the measured yield, cross section at E0. However, as you see, this effective beam is not well-defined, a long tail is present in the spectrum at low energies, which is not negligible. So due to the experimental procedure described here, a very large uncertainty is present in the extracted results of such an experiment.

Energy spectra of the bremsstrahlung beam at two energies (top) and the difference "effective" beam

 

The situation is getting better, if we measure the energy of the decelerated electron so we can "tag" the photon and assign an energy to the bremsstrahlung photon. Of course we have to be sure in this case that we produce only one photon by the deceleration of the electron. This can be assured by choosing the right gold-target (so-called radiator) thickness. On one hand the problem of the energy determination is solved, on the other hand we face to a new problem: due to the coincidence technique involved in such an experiment, there is a strong limitation on the photon flux we can use... This is typically very low, too low for us. We want to measure very low photofission cross sections....

This is the reason for our interest in Compton backscattered γ beam facilities. In such a facility a conventional, high power, optical laser beam is pointed to a relativistic electron beam. The photons backscatters on the electrons resulted in an up-shifted energy of the photons. This up-shift is very effective: we can have photons in the γ (MeV) region! The beam has a Gaussian energy profile and the intensity is not limited. We can measure the cross sections directly having low uncertainty. This is what we were waiting for!