Energy measurement

Calorimeters in particle physics measure the energy of incoming particles. Originally "calorimeter" is derived from the latin word calor (heat). In a wider sense heat is just one form of energy. Since the amount of heat produced by the energy depositions high energy physicists are interested in is too small to be measured, we have to determine the energy with different methods. Therefore, the interaction of an incoming particle with the detector material has to be converted into an electric signal. How this can be done is sketched below.

working principle of a sandwich calorimeter

 

Sandwich-Calorimeters consist of passive absorber layers (e.g. steel plates), red in the plot, interleaved with active layers (e.g. scintillators), indicated in blue in the plot. An incoming particle loses its energy in the absorber layers, initiating a shower of secondary particles. Interleaved active material converts the deposited energy into light.
In case of incoming electrons, or photons this cascade is a simple a multiplication of particles:
The initial electron interacts with the detector material by emitting a photon via Bremsstrahlung. The photon carries away part of the initial particle energy, leaving twice as much particles with roughly half energy each. After a while, the photon converts into an electron-positron-pair, and the initial electron radiates another photon. This makes 4 particles with about quarter of the initial energy for each of them. This cascade continues until the newly created particles don't have enough energy left to produce further particles. The sum of all these particles is then called a shower of secondary particles.
For hadronically interacting particles like protons or pions these showers can look much more complicated, since they have many more possibilities for interacting with the detector material.
The light produced in the active layers is proportional to the incident particles energy. It can be detected with photo detectors, that convert light into an electric signal. The more energy the incoming particle has, the more secondary particles it will produce, and the bigger the measured signal will be.

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A calorimeter for the ILC

In order to fulfill the high precision aims of the ILC, the calorimetric system needs to have a very good resolution. It should be able to distinguish between near by jets from incoming hadrons. This can be done by calorimeters with unprecedented high granularity in combination with new reconstruction methods, following the particle flow approach.
The CALICE collaboration is examining possibilities how to reach the ambitious goals of the ILC. Therefore, various prototypes based on innovative technologies are build and tested at test-beam facilities. These data will also help to validate hadronic shower models for simulations, as well as to test existing, and new cluster models.

Within our working group we are exploring the possibilities to build a hadronic calorimeter for the ILC. In the CALICE framework we have developed small prototypes:

• the MiniCal was meant to choose the readout technology: classical photomultiplier tubes, avalanche photodiodes and novel silicon based pixel detectors, the so called SiPMs were tested
• the physics prototype is a proof of principle. It will establish the chosen technology, and help to improve our understanding of hadronic showers.
• the engineering prototype is presently tested. It is meant to be scalable to a fullsize ILC calorimeter. It has fully integrated read-out electronics, including 23'000 latest-generation SiPMs.