Observaciones a las clases

With a maximum proton energy of 28GeV and a circumference of628m, the Proton Synchrotron was the world leading accelerator machine by the time it was commissioned at CERN in 1959. 17 years later from 1976 on, the Super Proton Synchrotron with a circumference of7km could deliver a proton energy of up to400GeV (later450GeV) to

Hadron Data Energy

Test Beam

PS SPS AHCAL SPS SDHCAL

10GeV 5.3M Events - -

60GeV - 4.6M Events 1.6M Events

80GeV - 5.1M Events 2.0M Events

180GeV - 1.6M Events 1.2M Events Muon Data

Energy

Test Beam

PS SPS AHCAL SPS SDHCAL

6GeV 0.02M Events - -

180GeV - 2.3M Events 13.4M Events Table 4.2: Acquired statistics of the T3B experiment at the various test beam phases for hadronic particle shower events and for muon data.

the experiments using the PS as a preaccelerator. Today the SPS acts as an injector to the Large Hadron Collider which will eventually be capable to accelerate the protons to the design energy of7TeV. Additionally, the PS and SPS deliver protons to fixed target experiments such as the CALICE calorimeter prototypes and the T3B experiment. The test beam experiments are located at different beam lines within experimental halls (East Area for the PS, North Area for the SPS). Each beam line is consecutively served by the accelerator following a certain periodicity that is called supercycle and that is decided on by the PS and SPS committees before the respective run periods.

The monoenergetic proton beam stored within the accelerators is not delivered to the respective beam lines directly. Instead the protons are steered on a target creating secondary particles of various momenta and masses. A magnetic deflection system steers particles of the desired momentum through a collimator. This is also called wobbling. One can easily choose between a positive or negative particle beam by changing the polarity of the magnets. A positive particle beam as arriving at the experiment consists primarily of protons,π+ andµ+, whereas a negative beam comprises mainly π−,e− and

µ−. The exact composition depends on the target type, the selected beam momentum and various other factors.

In many beam lines a set of two Cerenkov counters allows for an event-by-event identification of the different particle types. Here, a Cerenkov counter is realized as an airtight tube positioned along the beam line and filled with a gas (e.g. CO2, Nitrogen

or Helium) at a certain pressure. If a beam particle traverses the tube with a velocity faster than the speed of light within the medium, a flash of light caused by the localized polarization of the gas is emitted in forward direction. The inside of the tube is covered with reflective mirror foil and a photomultiplier tube (PMT) attached to one side of the Cerenkov counter detects the light signal. Increasing the gas pressure results in a

Extraction ~400 ms Super Cycle ~45 s

Proton Synchrotron

Super Proton Synchrotron

Waiting Time ~35 s

Figure 4.10: Sketch of the spill delivery sequence to the CALICE/T3B experimental area for one supercycle of the PS and SPS at CERN. During the extraction (green) the machine delivers particles to the experiments followed by a waiting time (red) in which other experimental areas are served.

decrease of the speed of light within the medium. Since all particles within the beam have the same momentum after the wobbling, heavier particles have a lower velocity and vice versa. Now the pressure of the gas can be adjusted such that the speed of light is slightly higher than the velocity of e.g. the protons within the beam. Therefore, the protons cannot induce Cerenkov radiation whereas all lighter particles can. If one adjusts the pressure of a second Cerenkov counter such that all particles lighter than pions create light, one can efficiently identify pions. The pion identification signature would be: Cerenkov A: On, Cherenkov B Off. Note that the gas pressures necessary to identify e.g. pions varies with the beam momentum. If the required pressures are out of the operation range of the used Cerenkov counter, the particle ID can only be determined with low efficiency or not at all.

In case the Cerenkov counters cannot provide a reliable particle ID, the CALICE calorimeters have further options. In the offline analysis, one can cut all events that do not start a shower before the last active layer of the calorimeter. This removes all non-interacting hadrons - so-called punch-throughs - but also all muons from the analyzed data set. The radiation length of electrons or positrons is significantly smaller than the nuclear interaction length of hadrons (see Section 3.1). Therefore, one can cut events that deposit a large fraction of their energy within the first few layers of the calorimeter to efficiently reject pure electromagnetic showers.

Very long and dense shutters are located along the beam line to stop the beam if access to the test beam area is required. The only measurable particles that traverse all obstacles with high probability are muons. By closing all shutters and defocussing the beam one can irradiate the calorimeter almost exclusively with minimum ionizing particles. Such muon runs are very valuable for the calibration procedure of the T3B and CALICE AHCAL cells and have been carried out at all test beam phases (see Chapter 5).

Figure 4.10 (left) shows the supercycle of the PS and SPS as experienced by the T3B experiment during its test beam phases. The PS delivered up to three spills of particles

within a supercycle of 45s to the T9 beam line in which CALICE/T3B was located. The particle energy could be adjusted in a range of 1−10GeV. Each spill had a length of∼400ms in which T3B recorded on average ∼600 particle events synchronously to the CALICE AHCAL. The variable waiting time of5−20s was a challenge for the T3B DAQ. The operation mode of T3B had to be optimized to be capable of processing the acquired events and the consecutive intermediate run mode in less than5s to avoid the loss of spills. During the SPS phases (see Figure 4.10, right), the supercycle was more constant. After a waiting time of ∼35s, a spill with a length of ∼10s was extracted by the machine and delivered to the H8 beam line. In H8, the particle energy could be steered in the range of10−300GeV. When triggering particle events synchronously to the CALICE AHCAL, the T3B experiment could record up to ∼2500 events per spill. The number of collected events per spill was limited by the finite event buffer size of the AHCAL electronics. Triggering standalone, the T3B DAQ is capable of processing up to 6500events per spill and finishing the readout before the next supercycle starts (find more details on the trigger system in the next Section). In this mode, up to ∼500.000

events can be accumulated per hour under stable beam conditions.