Design and simulation

As I presented in previous entries, we develop two different detector arrays for our research projects at ELI-NP. The first step of the development is planning, which means we sketch our idea on paper according to the requirements and the scientific goals of the project. We have to decide the dimensions, the geometry and the materials. During the planning we also have take our financial possibilities into consideration. It also a great advantage if the tolerance of the mechanical and electric parts allows to use the infrastructure of our institute (e.g.: the mechanical and electronic workshop).

A very important step is to estimate the response of the detector to the investigated phenomenon and also to estimate the possible background signals having particular physical conditions. For these estimations we perform simulations with a software environment developed at CERN (GEANT4).

It is especially important to perform such simulations for the Bragg ionization chamber. For this we have to know all the relevant environmental conditions of the experiments. The time structure of the photon beam seems to be particularly important. And we are going to have serious technological challenges due to the time structure of the beam at ELI-NP: photons will come in little bunches and not as a continuous beam. We are going to have 100 big bunches in one second each having 32 small bunches. The time interval of one small bunch is 1-2 picosecond (!!), in which we get 100000 photons. The problem is obvious: reactions induced by individual photons will "overlap" since the time resolution of our detector limited resulting in a distortion of the energy measurement. This background we have to minimize.

According to our very recent GEANT4 simulations we can minimize the effect down to 1% of the effective signal if we design a chamber taking some consideration on the materials and dimensions. For example we have to use entrance and outgoing window on the wall of the chamber for the beam covered by a very thin (Kapton) foil in order to minimize the scattering of photons.