This weekend, CALICE has opened a new chapter of calorimeter testing: Our first full physics prototype of a digital hadron calorimeter saw its first pions at Fermilab! So, what is this about?
As you know from many previous posts here, the CALICE collaboration develops calorimeter technologies for future experiments based on particle flow event reconstruction. On the hadron calorimeter side, so far we’ve been looking at a highly granular analog hadron calorimeter: Many small plastic scintillator tiles, each read out with a tiny silicon-based photon sensor, and all that sandwiched between layers of steel absorbers, 38 of them. The “analog” in the name means that each of these cells gives out a signal that is (approximately) proportional to the energy deposited in it by throughgoing particles. By summing up the energy of all cells that saw something, we get the energy of the particle that hit the calorimeter. Of course there are very sophisticated techniques how the “summing up” is done precisely, which results in better measurements. In total, this device has close to 8000 electronics channels, all packed into one cubic meter.
Now, a digital calorimeter does things a bit differently: Instead of an energy measurement for each cell, you only get a “0” or a “1”, depending on if the cell was hit by at least one particle or not. The total energy you now get by just counting the number of “1” in your detector, again with the possibility for more sophisticated techniques. At first glance this might look like a bad deal, since you loose information. However, with the digital readout you can use simpler detectors and electronics, and thus get many more readout channels. The current 1 cubic meter prototype, that uses exactly the same steel as our analog calorimeter, has 350 000 readout channels, quite a bit more than the analog detector. And it uses so-called Resistive Plate Chambers (RPCs), simple gas-based detectors. So you get a more precise spatial image of the particle showers, for the price of sacrificing the cell-by-cell energy measurement.
What will work better? That is exactly what we are trying to find out. The first CALICE Digital HCAL, constructed in the US under the leadership of Argonne National Lab, has now started to take data at Fermilabs Meson Test Beam Facility. With those data, compared to the ones we already have for the analog calorimeter, we can hopefully make a significant step in the direction of answering that question. The first event images show amazing detail, quite a bit beyond what we had previously (for a shower picture from the analog HCAL take a look here).
I, for one, am very excited to see what the results from this new detector will look like!