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