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### How a calorimeter works – part 2

Portuguese version below…

So, I am finally back from vacation, with the email list almost cleaned up and a list of tasks ready to start piling up before I can do anything.. In a nutshell : the usual working life. The last time I posted something, the idea was to explain how a particle, like an electron, a photon or a neutron enters in the detector called calorimeter, hits the material named the absorber and gets a part of its original energy (speed!) converted into a little shower of particles. Part of this energy however gets sampled by a material called (see how physicists can be very creative sometimes) the sampling material. So, what happens in the sampling material of the two ATLAS calorimeters and how we can use this information to “measure” the final energy of the incoming particle?

The sampling material uses some basic physics process to convert the energy it receives into some other physical quantity. The smart thing physicists try to do is to make this “other physical quantity” something easy to measure. Let’s start with a very simple example. In your home, you probably still have either in your wall or in your medical box an old style thermometer. I mean, not a digital one. It is built with a small scale and a little pipe containing either artificially colored alcohol (the wall thermometer) or mercury (the medical box one). When it receives heat from air or a person’s body, the atoms of such “sampling” materials get very agitated. Due to that, the same number of molecules now occupy a much larger space, increasing the necessary volume to contain them. This translates into a longer column which we can easily readout.

In here, we can identify all the elements of the sampling process. First, the heat modifies some inner property of the sampling material considered (their internal heat or movement of the material). Then, the material responds with some global change in a larger scale (its volume increases) and, in consequence, you can now measure with light and some light receiver (your eyes) the shift in the top position of the alcohol or mercury column.

Now, let’s see the case of the liquid argon (electromagnetic calorimeter) of ATLAS. You might want to review the discussion about the absorption process on the previous post. Again, to illustrate, let’s see an extract of the ATLAS Episode II movie, that you can see completely in youtube (part 1 and part 2). There, we followed an electron as it enters the electromagnetic calorimeter and looses its energy by producing a huge shower of particles thanks to the lead absorbers. When the electron is not in the absorber material, it is walking through argon cooled to -185 oC in order to be kept liquid. The electron, or the particles that come out of it, crosses many atoms of Argon, giving so much energy to the argon electrons, that many of of them get free from the argon nuclei in a process called ionization (see the white little dots in the movie!). The argon electron is now a little negative free charge and the rest of the atom is a positive free charge (yellow). Remember that between one accordion plate and the next one, there are copper plates, the electrodes. Between the electrodes, there is a very high voltage (2000V – almost 10 times the voltage in an electric wall plug in an European home). Such high voltage in a very small gap (~2mm) and a bunch of free electrons is a shocking combination, a bit like throwing a small wire on an electric fence. Usually, the high voltage would not cause electrons to travel from one plate to the other, but now that they were freed by the particle’s energy, they will be collected by the positive electrode, generating an electric current. The intensity of this current relates to the energy that the particle lost in the region around the electrode (this region, we call a cell). The current sparks quite quickly (around 400 ns – you will later see that this is actually not as quick as we would love to!) and this is the “other physical quantity” that we can measure. Using this information, we can calculate back the energy of the original particle that entered the calorimeter.

Those interested in seeing a nice, live example of the ionization process are invited to check how to build a cosmic ray detector (sorry for the “idiot”!) and a little video I recorded during a conference (CHEP2010) of a little device called Cloud Chamber (no relation to Harry Potter!).

I would also like to show an example of a particle entering the hadronic calorimeter of ATLAS called Tile Calorimeter. We will use another extract of the ATLAS Episode II Movie to see it. In this video, we start by seeing the iron absorbers (gray) and the plastic scintillating plates (the “Tiles”, drawn in Violet). The absorption part is very similar to the Liquid Argon calorimeter (particle hits, particle looses energy, shower forms) with the difference that particles hit the nuclei and NOT the atoms as in the EM calorimeter case. The sampling material is an special plastic. The particles passing inside this plastic excite the electrons of the atoms that compose it. Not enough to ionize it as in the argon (the electron stays attached to the nucleus but a bit farther away than its usual “orbit”). When the electron return to its natural position, the energy is released back in the form of photons, or, to simplify, light of a very specific frequency (or a specific color, coincidence : Violet). Some of these photons will be collected by an optical fiber and send to a special device that converts light into an electrical signal, a photomultiplier (some of you may have played with that if your school ever had a project to build cosmic ray detectors!). See a photo of Cíbran Santamarina Rios, a colleague from Galicia beside a plastic scintillator detector (the photomultiplier I hope I am saying this right, is in the bottom of the plastic plate – protected by a black cover!).

Photo with Ph.D Cibran Santamarina Rios assembling a cosmic rays detector

In the next post, I will discuss a bit the outcome of the sampling process : the electrical signal and how it can be used to calculate the energy of a particle, specially when multiple particles hit the calorimeter in different collisions. This concludes our session on “How the detector works”. Then, we will discuss how the trigger works to select particles for a discovery!!!!

Portuguese version :

Finalmente de volta das férias, com uma lista de emails quase limpa e uma lista de tarefas começando a crescer antes que eu possa tentar fazer algo… Em resumo, uma semana normal de trabalho. Na última vez que postei algo, a idéia era explicar como uma partícula tal como um elétron, um fóton ou um nêutron entra num detetor chamado de calorímetro, bate num material denominado absorvedor e perde parte de sua energia, que estava acumulada na forma de velocidade, na forma de uma cascata de partículas. Parte dessa energia, entretanto, sensibiliza um material chamado “material de amostragem” (o termo vem de amostra, parte de um todo que o representa). Assim sendo, como podemos usar o material de amostragem no caso dos dois calorímetros do ATLAS para medir a energia das partículas que entram nestes detetores?

O material de amostragem usa alguns princípios básicos de física para converter a energia que recebe em algum outro valor com significado físico. A esperteza está em tentar fazer esse “outro valor” ser algo fácil de medir. Comecemos com um exemplo bem simples. Em sua casa, Vocês ainda devem ter em sua parede ou na caixinha de remédios um termômetro antigo (não digital). Esse termômetro é constituído de uma pequena escala com as temperaturas e um tubinho contendo álcool colorido (termômetro de parede) ou mercúrio (o da caixinha de remédios). Quando o termômetro recebe calor do ar ou do corpo de uma pessoa, os átomos desse material de “amostragem” ficam muito agitados. Desta forma, um dado número de moléculas passa a ocupar um volume bem maior. Isso se traduz numa coluna mais longa que pode ser facilmente lida.

Neste exemplo, podemos identificar todos os elementos do processo de amostragem. Primeiro, o calor modifica uma propriedade interna do material de amostragem (o seu “calor interno” ou o movimento de seus átomos). Depois, o material responde com uma mudança global numa escala mais larga (seu volume cresce) e, em conseqüência, pode-se medir com luz e um receptor de luz (os olhos do observador) a variação da posição do topo da coluna de mercúrio ou álcool.