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Christine Nattrass | USLHC | USA

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Tour of the the EMCal test beam

I briefly mentioned the EMCal test beam – the reason for my latest post – but here’s some more details and some gratuitous cool pictures.  I am now back in the US and have more time for blog posts.

The goal of our time at the test beam was to calibrate our detector, an electromagnetic calorimeter, by measuring its response to known particles with a known momentum.  Then when we put in the rest of ALICE we can use it to help identify unknown particles and measure their energy.  Here’s a rough diagram of the test beam set up for the EMCal:

We use the beam from the Super Proton Synchrotron (SPS) to get our test beam.  The SPS is also used as a source of protons for the LHC.  The proton beam from the SPS is aimed at a target – a thin block of material.  When it hits this target it creates a lot of secondary particles – mostly pions and protons.  This beam is then directed through a magnet.  A particle with a charge q with a momentum p in a magnetic field B will move in a circle with a radius r=p/qB.  We aim the beam through an aperture, which fixes the radius.  Then we can change the magnetic field, which changes the momentum of the particles which pass through to our detector.  First we passed the beam through two multiwire proportional chambers, which let us measure the position of the incoming particles.  (These work because of the same physical principles as the Time Projection Chamber.  Charged particles moving through a gas ionize the gas and we can collect the electrons knocked out of gas molecules to see where the particles went.  The details of how multiwire proportional chambers work are different, however.)  Then the particles hit our detector.  We only have a few towers of the calorimeter in the test beam – enough to be representative of the whole detector.

The dashed line in the bending magnet shows an optional converter that we could put in or take out.  When a photon hits the converter, it changes into an electron and a positron.  The converter is only part of the way through the bending magnet, so electrons will not be bent as much as hadrons.  This meant we could choose between a beam with hadrons (pions and protons) or electrons.  We’re particularly interested in the difference between the response of our detector to hadrons and electrons.

Here you can see the set up from above:

I’ve labeled our EMCal towers and the multiwire proportional chambers.  The large cement blocks arranged like giant Legos are for shielding.  There are several cables leading from the detectors up to the barracks where we sat when taking data.  Some of these were used to carry data from the detectors up to us.

Here you can see the back of our detector and see our data acquisition system:

The big green object labeled “DESY” is a movable table that we used to move our detector around in the beam.  We could control it remotely.  Despite what it may look like, all of those cables are meticulously arranged.  We had to arrange the cables so that they stayed plugged in even when we moved the table around.  The computer is for data acquisition.  Here’s a different view:

And that’s me next to the set up – smiling, even though it’s about 1:30 AM and my shift just started.  I have my finger on the outside of one of the EMCal towers.  There’s one wire chamber to my left and one to my right, right in front of the EMCal towers.  The chain around my neck has a dosimeter on it, measuring how much radiation I get exposed to to make sure I’m not getting a dangerous dose of radiation.  (The risk is very low.  There was no beam when we took these pictures and a physical barrier preventing the beam from coming in anyways.)

And here you can see the beam pipe delivering beam to our set up:

With Betty, a post doc at Laurence Livermore National Laboratory.  Betty and I took the midnight to 8 AM shifts for the test beam together.

We got lots of valuable data on the EMCal response to electrons and hadrons from a momentum of 6 GeV/c to 225 MeV/c.  Now we have to analyze the data.  We’ll be able to use it to determine how good our detector is at separating electrons from hadrons.  We have an idea of how well it should work from simulations, but nothing beats a measurement.