Hello everyone,

this is Alessia again. Today I would like to tell you more about my past work, and in particular about my Master’s Thesis.

The idea behind it is simple but incredibly ingenious at the same time.

Currently, one of the most used scintillators in Nuclear Physics is Barium Fluoride. This material has a lot of interesting properties (it is hard, non-hygroscopic, radiation-resistant), but only one of its characteristics makes it the suitable candidate for the discrimination of different kinds of particles.

Barium Fluoride has two different scintillation components: the fast one, with a decay constant of about 600 ps, and the slow one, with a decay constant of about 600 ns, the latter being significantly higher than the former. The ratio between the fast component and the slow one is characteristic for every kind of particle. Hence, it is possible to tag electrons, photons, protons, muons etc. just by looking at this ratio (that has to be measured during the experiment).

But how to use this material within the field of high energy physics?

As we know, the concept behind a calorimeter for high energy experiments is that a particle impinging on it (primary particle) creates a shower. If the primary particle is an electron, a positron or a photon, the shower will be made up of electromagnetic particles only; otherwise, if the incident particle is a hadron, the shower will consist of hadronic particles, but also of electromagnetic particles, due to the neutral pions created when the primary particle has nuclear interactions with the molecules of the material.

The most tricky issue of the hadronic showers lies in the fact that the fraction of electromagnetic energy deposited in the calorimeter is highly variable from event to event, so that large fluctuations are associated with it. It has been shown that it is possible to reduce the fluctuations on the fraction of the electromagnetic energy deposited or, equivalently, improve the energy resolution, by discriminating the hadronic and the electromagnetic components of a hadronic shower.

And here is the big deal… it is absolutely possible to do that just by using a calorimeter totally made up of Barium Fluoride!

It is straightforward to see that this calorimeter can be used both for big experiments at colliders and for Astroparticle Physics experiments. In particular, in my Master’s thesis I focused on the CaloCube experiment.

CaloCube is a calorimeter projected by the University of Florence, Italy, for the detection of cosmic rays in space. Hopefully, when in orbit, it will be able to measure the cosmic ray spectrum in the region of the knee, the most interesting, but also the most difficult to deal with.

A prototype of the CaloCube calorimeter.

Just a note to continue…

A hadronic particle absorbed in a calorimeter releases both electromagnetic energy (lost in ionization processes) and “invisible” energy (that is the energy used to break the nuclear bonds, the energy carried by neutrinos). But only the former is detectable, and the energy we need to know is the sum of these two (called total energy).

From this note, you realize that it is necessary to establish a calibration line to correlate the detected ionizing energy to the total one.

To do that, I performed several simulations with FLUKA (a Monte Carlo simulation software for high energy physics) both with a prototype of CaloCube and a bulk of Barium Fluoride.

What is important to highlight is that I wrote the code of the simulation in order to associate a certain fast/slow ratio to each particle, so that I could retrieve the fast/slow ratio for each shower created in the bulk of Barium Fluoride.

The second step was the analysis of the data extracted from the simulations with the software ROOT.

All the data were divided into four different intervals of fast/slow ratio, and consequently each calibration line for each geometric configuration was split into four lines, each tagged with a specific fast/slow ratio interval.

Unfortunately, I’m not allowed to publish the results, because an article is going to be published… I just can say that, by dividing the data into different fast/slow ratios, the energy resolution improved by 40%!

So in the future we could be able to measure the energy of the cosmic rays in space with high precision in the region of the knee… and this would be a great result!

In my Master’s thesis I also showed a technique to measure the fast and the slow components for different particles. I took part in a test beam at CERN, during which protons, muons and electrons accelerated by SpS were sent into some bulks of Barium Fluoride. Also in this case I can’t publish my plots, but if you have questions or you’d like to have clearings, do not hesitate to comment or contact me. I will be pleased to answer you!

(Written by A. Saggio)