About the miniTRASGO
Some ideas on RPCs
Resistive Plate Chambers, or RPCs, are particle detectors with high resolution in time based on, using high voltage, the amplification of the charge generated by an ionization caused by the scaterring of a particle inside of a gap (that is filled with some gas) between those resistive plates. The multiplication in charge in RPCs is usually 10^6 from ionization to the collection. RPC detectors have several unique and important practical features, such as good spark protection and excellent time resolution, even down to few tens of picoseconds.
One of the fundamental parameters of RPC detectors is the Townsend first parameter: alpha = 1/lambda.
The thicker the gap, the higher the efficiency, since there is more distance for the avalanche to grow and become noticeable. The problem: bigger gaps require larger times for the ions to be collected (microseconds to travel a 1-2 mm gap), and also, since there is more charge involved in the processes, more charge could acumulate on the glass layers with no time to get automatically dispersed/evacuated through the surface, and hence reaching the initial state, so there would be a nominal electric field affected by these charges that would be smaller than the one desired. In other words: great time precision requires thinner gaps. This motivated the idea of the multiple gap in an attemp to reach the efficiency of a thick gap without losing other properties. Actually minGO has 2 gaps that would allow to reach the 90% in efficiency.
There are two types of RPCs: those with 1-2 mm gaps, that are useful only for triggering and have around 1 ns of time resolution (minGO is one of those) and those called timing RPCs that have 50 ps of time resolution: these are more complicated because require gas mixing and very high electric fields, which make them delicate detectors.
One of the plates must necessarily be resistive (also both could be), and approximately 10^12 ohms/cm to stop undesired avalanches that would create sparks that could kill the electronics. A resistivity that is too big would not allow any detection at all. The resistivity comes from the glass, the conductivity (to establish the field) comes from the paint (an acrilic, artistic, painter, paint: for some reason the manufacturer gives the resistivity as technical specification).
The gas that miniTRASGO uses is F134a, usually found in refrigerator units. The name is 1,1,1,2-Tetrafluoroetane and it does not hurt the Ozone layer, but it is a greenhouse effect gas. The new european(?) laws discourage its use and require special protocols for its treatment, which is making the refrigerator manufacturers to stop its use and therefore creating a surpluss that is making it cheaper for us to buy.
Some RPCs at Coimbra are said to be gasless: this means that it has ionizing gas, of course, but that it is a closed cage that does not need to be refilled in a constant gas flux. Some voices claim that the gas properties might be lost in high particle flux regime due to creation of free radicals, etc. but the fact is that for now (July 14th, 2023) they have used a really big RPC (more than 1 m * 1 m) for Cosmic Ray detection and no efficienty loss is seen. Maybe in very high luminosity (e.g. at CERN) the gas actually will lose properties.
The TRASGO family and the CASTRO colaboration
The original concept of the TRASGO (TRAcK reconStructing bOx) is the use of RPC layers with some mechanism of position determination over the plane to create a cosmic ray telescope, since a track can be reconstructed with the information of a particle trigger passing through each of the layers. The beginning of the TRASGO project (the -1 step, actually) comes from the HADES RPC ToF Wall, around the 2000's. LIP (Coimbra), GSI (Darmstad), IFIC (Valencia) and LabCAF (Santiago de Compostela) participated. Since them, both the detectors and the people behind them has grown into the CASTRO (Cosmic Ray Survey Trasgo Network).
The TRASGO family
The concept of the telescope has been used to create a variety of telescopes that we call the TRASGO family. These cosmic ray detectors gave information in solar and Earth weather, as well as some insights on fundamental physics. It is essential, though, to set the path for future work, which requires getting more colleagues involved to take profit on these new datastream, developing new analysis techniques, but also new ideas to test more physical problems.
The TRASGO features: - High granularity and tracking (granularity is measured in particles/event) - Sensitive to bundles of particles - Muon / Electron software PID - Rough estimation of the electron energy distribution
To these jumble of detectors the miniTRASGO is added: a compact, cheap, version of the original concept that can be used to get results in both new physics and instrumental methods. The techniques developed for miniTRASGO could also be extrapolated to the rest of the TRASGO family.
Participating groups
There have been already two TRASGO meetings and two TRAGALDABAS meetings, and the new born CASTRO colaboration is integrated by people all around the globe: - Warsaw University of Technology (WUT), Poland. - Benemérita Universidad Autónoma de Puebla (BUAP), Mexico. - Tunja, Colombia. - Madrid, Spain. - Valencia, Spain. - Santiago, Spain. - LIP Coimbra, Portugal.
The idea is to deploy a global network of cosmic ray detectors working in common terms and trying to colaborate is giving support, software creation and also new experimental configurations and ideas. The miniTRASGO will be key in this development.