Contrastes en la obra y percepción del lector

A dedicated measurement was performed in September 1999 at the M2 beam line at the SPS at CERN with the same rates as the ones expected in COMPASS. The main goals of the tests were to measure the efficiency and the occupancy as a function of flux rates. The tests were done with a prototype built in Dubna consisting of 6 mm and 10 mm straws. The prototype has an insensitive area in the centre of the chamber for the beam. (The prototype is described in more detail in section 3.3). The setup was as follows: the chamber was situated after SM1, corresponding to its nominal position. Downstream of the straw chamber, a large scintillator was placed horizontally and further downstream two small scintillators were placed forming a cross (vertically and horizontally, see figure 3.18).

Beam

SM1 Straw double layer

Small Scintillators Large Scintillator

Figure 3.18: Setup for the rate measurements performed in September 1999 at the M2 beam line at the SPS at CERN.

The chamber was running with Ar/CO2/CF4 at a flow of 6 l/h, the total volume of the chamber being 40 l. The straws in the chamber were read out exclusively by scalers. The first measurement’s goal was to obtain the efficiency versus high voltage and its dependence on the rates. The efficiency was obtained using the following method: three straws are chosen, two from one layer and one from the other layer, that were labelled 2, 3 and 4, see figure 3.19.

The efficiency was computed by taking the coincidence between the straw 2 and the straw 3, then the coincidence between the straw 3 and the straw 4. The rates given by these coincidences were added. The result of this addition was divided by the free rate obtained in straw 3. This efficiency was measured for different high voltages. These measurements were repeated for a low (2 · 107 particles/spill) and high (2 · 108 particles/spill) muon beam intensity, and for a pion beam (2.5 · 107 particles/spill). Figure 3.20 shows the

2 4 3

Figure 3.19:Straws configuration for the efficiency measurements.

efficiency measurements with the muon beam for high and low intensity. From the results it can be concluded that no significant drop of the efficiency is visible.

0 10 20 30 40 50 60 70 80 90 100 1550 1650 1750 1850 1950 HV [V] Efficiency [%]

Low muon intensity High muon intensity

Figure 3.20:Efficiency versus high voltage for high and lowintensity muon beam.

Measurements of the rates and measurements of straw chamber characteristics that might be influenced by the rates were performed. To obtain the beam profile it was enough to measure the free rates on the straws.

The normalised rate, obtained as the free rate on a straw divided by the beam intensity (which was provided by a ionization chamber) was measured in the following three different conditions: muon beam with target in/out, muon beam of high/low intensity and muon beam with magnet on/off. A measurement was also performed with a hadron beam.

The figure 3.21.a shows that the rate in the straws is higher when the target is in (the target was 2.6 interaction lengths). The result agrees with the expectation. When the target is in, the hits in the straw chamber correspond to particles from the beam and from the interaction of the beam in the target.

The figure 3.21.bshows that the rate is lower when the magnet is on. The explanation for this effect is the following: when the magnet is on, the particles with low momentum are strongly deflected by the magnet. Therefore, they do not reach the straw chamber. Thus the rate in the chamber will be lower than when the magnet is off.

The same kind of measurements were performed with a hadron beam, ofπ− with an energy of 225 GeV. The only parameter which was changed during the measurement was whether the target was in or out (see figure 3.22). The target was made of polyethylene with 10% interaction length.

The tests were performed using 10 mm straws. The main reason for this choice was the easier alignment of the chamber in the beam. The 10 mm are situated at the outer

3.2. R&D 39 0 0.001 0.002 0.003 0.004 0.005 0.006 Target in Target out 0 0.001 0.002 0.003 0.004 0.005 0.006

Distance from the beam [cm]

Distance from the beam [cm]

Normalized Rate Normalized Rate Magnet off Magnet on 7 12 17 22 7 12 17 22

Figure 3.21: The normalized rate i.e., the rate in the straws divided by the beam rate. The figures showthe measurement of the normalized rate for a high intensity muon beam in two different cases: a) target in and target out (the SM1 magnet is switched off). b) SM1 magnet is switched off or on (the target is in).

part of the chamber, thus only vertical alignment was needed.

The occupancy is defined as the number of tracks passing through the straws in a time gate corresponding to the drift time of the straw. The maximum occupancy for a 10 mm straw can be calculated as:

M aximum occupancy = (maximum measured normalized rate)

×(highest expected beam rate)

×(maximum drif t time)

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 6 1 1 1 6 2 1

Distance from beam [cm]

Normalized Rate

Target out Target in

Figure 3.22:The figure shows the measurement of the normalized rate for the hadron beam for two different conditions, when the target is in or out (the SM1 magnet is switched off).

To extract the occupancy for the 6 mm straws from the occupancy measurements for the 10 mm straws, the measurements have to be multiplied by a factor (₁₀6)2. The reason for this factor is that the tracks are traversing a straw of a diameter 10₆ smaller than the one for the 10 mm straws. The other factor ₁₀6 comes from the fact that the drift time is shorter by this factor for the case of the 6 mm straws. The maximum occupancy for 6 mm straws closest to the beam corresponds to the case where the target is in. The results for this case are:

• for the hadron beam, the maximum occupancy with the magnet off is 2% (Beam: 220 GeV π−, beam intensity: 5 · 107 s−1).

• for the muon beam, the maximum occupancy with the magnet off is 3,6% (Beam: 100 GeV µ, beam intensity: 2 · 108 s−1).

• for the muon beam, the maximum occupancy with the magnet on is 2% (Beam: 100 GeV µ, intensity: 2 · 108 s−1).

These results are consistent with the proposal and with Monte Carlo simulations. The Monte Carlo simulations were performed using as event generator called FRITIOF [37]. It is an event generator which simulates the interactions between hadrons and nuclei.

The figure 3.23 shows thex andy distribution for particles at the same distance from the target as the straw chamber in COMPASS. As beam particles π− were chosen.

The occupancy is computed dividing the entries for an x or y position by the total amount of generated events, and multiplying this quantity by the beam intensity, the interaction length of the target material (to simulate the number of interactions that will occur in the target) and the drift time. In figure 3.24, the different values for the occupancy are computed for different distances to the beam centre. A value of 2.2% occupancy is obtained for the straw nearest to the beam, considering that there will be a physical hole for the beam. This value agrees with the 2% occupancy measured in M2 for the hadron beam.