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!

Detector development II.

In the previous blog a newly developed detector array was presented that devoted to the measurement of the cross-section of photofission and the angular distribution of the fission fragments. However, measuring the atomic and mass number as well as the energy of the fragments can also serve very important information on the fission process and the structure of the fissioning nucleus. Thus, we started the development of a more complicated detector system which will be dedicated to measure all these quantities.

Gaseous ionization chambers are ideal devices to measure the total kinetic energy (TKE) of the fission fragments with high resolution and large solid angle when compared to solid state detectors. The measurement of fragment kinetic energies allows to deduce the mass distribution using the 2E technique. Moreover, the so-called cold fission events (the fragments are in the ground state) can be selected by setting a proper lower limit on the energy.

The proposed design (Figure) is based on a twin gridded ionization chamber. Frisch-gridded, twin-ionization chambers permit simultaneous measurements of the TKE and the emission angles of both fission fragments in coincidence. The energy of the fragment is determined from the anode pulse height, while the sum of the grid and anode signals is used to deduce the fragment emission angle with respect to the symmetry axis of the chamber. Thus, a precise angle-dependent energy loss correction can be performed. The proposed multi-target detector layout, 5 small area (1 cm2) and thin (200-300 μg/cm2) actinide targets will be placed into the center of each cathode to increase the effective target thickness, and thus the fission yield of the (γ,f) reaction at deep sub-barrier energies.

This chamber will be a versatile tool which will allow measurements of mass distribution of the fission fragments induced by brilliant γ beams on Th, U and other actinide isotopes. Due to its capability of determining the particle’s specific ionization and localization of their traces by using a digitizer and digital signal processing (DSP) techniques, the chamber can provide also information about charge distribution of the fission fragments. In order to increase the capability of the chamber to detect also the process of nuclear fission accompanied by emission of light charge particles (LCP) ΔE-E telescopes will be placed inside the chamber in a transversal position between the electrodes.

The first test experiment of the prototype ionization chamber, which was  performed in November 2013 at MTA Atomki using a 252Cf fission source, was devoted to acquire knowledge on the digital signal processing of fission events in
particular extracting atomic number related information of the signal. 

Schematic design of the proposed detector system