Comparative study of PMT- and SiPM-based detectors for a few MeV gamma-ray diagnostic
Report by Valeria Perseo. Powered by FuseNet

My internship at NCBJ (Swierk, Poland) has been interesting from a lot of different points of view. It has been thought to be a part and an enrichment of my master thesis about the European project of the JET Gamma-ray Camera Upgrade (GCU). The gamma ray camera is an important part of the diagnostics of JET because it is meant to be the instrument for studying the alpha particles, which are one of the fusion products. The alpha particles are important because they can contribute in the heating process of the plasma: their kinetic energy (order of MeV) is much higher than the one of the reactants (order of keV), so a significant exchange in energy can take place. The problem of measuring and understanding some properties of the alpha particles is linked to their position in the tokamak: due to the fact that they are charged, they remain in the centre of the machine, so a direct study is impossible. The upgrade gamma ray camera will measure nineteen line integrated gamma ray emission spectra associated with specific reactions among fast ions or fusion alphas with impurities, and thus reconstruct the energy resolved gamma ray spatial emissivity from the plasma. From these data it will be possible to measure the spatial distribution/redistribution of the fast ions and fusion alphas in selected ranges of their energy distributions and to follow their evolution in time. So, the aim of the GCU project is the improvement of the existing gamma-ray spectroscopic and count rate capability by replacing the 19 detector used heretofore with new faster and better energy resolution detector modules. Due to the limitation on the space available for installing at JET, the detector device should be small: the chosen detector is a SiPM (silicon photomultiplier).

My work has been mostly about comparison between different sizes of CeBr3 and between a classical fast photomultiplier tube (PMT) and a silicon-photomultiplier (SiPM) in order to understand what is the better crystal and what are the main differences between PMT and SiPM. The used SiPM is produced by Hamamatsu and it's called MPPC (multi pixel photon counter).

Enough measurements have been done to understand that CeBr3 is a good kind of crystal because it is fast, bright, stable and proportional in the energy range of interest. With the efficiency measurements it has been pointed out that the bigger is the scintillator, the better is the efficiency, so in the final prototype the scintillator should be as big as possible. The comparison between PMT and MPPC shows that the MPPC has to be improved in terms of stability and linearity by the producers, even if the measured energy resolution is acceptable. For the project, it is important to focus on shortening the length of the signal in order to make possible to measure gamma rays also with high count rate because it is expected a high gamma-ray back- ground produced by fusion neutrons.

It has been very satisfying to work in a big centre of research with a lot of well prepared researchers. I spent my time mostly with Slawek, Lukasz and Pawel, but also with Marek, Marci, Grzegorz and Iza. All of them helped me with my work and made my internship more pleasant.

Another interesting thing for me is that this internship has been my first one abroad, so I learnt and understood how it is to live for some time in a different country (and it's great! especially if one finds a cool roommate as I did). Warsaw is nice, mainly in spring and summer that is when I lived there. There are a lot of great places to rest during hot days, like the Lazienki park, or the shores on the Vistula river, or the Multimedialny Park Fontann near the Old Town. I also enjoyed some free open air concerts (jazz ones in the Old Town and Chopin's music concerts in the Lazienki park) and some Polish food, like pierogi (the best are the "ruskie" ones!), a lot of different soups (the most interesting is the "chlodnik", a cold soup made with beetroot and it is surprisingly fuchsia) or a sweet cake called sekacz.

For this experience, I would like to thank FuseNet, NCBJ and University of Milano Bicocca for the opportunity that they have given to me. Special thankfulness goes to Marco Tardocchi for having proposed the traineeship and to Izabella Zychor for having accepted it.