1 Introduction
Figure 1-1 a) View of the prototype along the beam direction looking downstream, b) side view of the prototype. Three slabs of aerogel (4.5x4.5x1.5 cm3) were used. The WLS thickness was 0.3 cm.
The prototype was positioned to a particle beam of 10 GeV/c negative pions. The beam definition was provided by 5 scintillators[1], two positioned upstream from the prototype, and three positioned downstream. These scintillators provided a beam definition of 2x2 cm2. The discriminated coincidence signal of the five scintillators was forming the QVT trigger pulse. The intensity of the beam was reduced to several hundreds of particles per spill (the spill duration at CERN-PS is 1.5 sec). The amplified PMT signal was measured by the QVT, in V mode (i.e. peak measurement). Using the setup mentioned above we performed two measurements: a) with the setup in-line with the beam but without aerogel, and b) with the setup in-line with the beam filled with 3 slabs of aerogel of total thickness 9 cm. The spectra of the two measurements are shown in fig. 1-2, and fig. 1-3, respectively. In the first case (without aerogel) we observe 6.4 photoelectrons, by counting the peak-to-peak distance between the pedestal and the signal peak. We have, also, measured the performance of the prototype using four aerogel samples from another manufacturer, Airglass S.A. (Sweden). We have placed 4 blocks of Airglass aerogel one on top of the other in order to create an effective thickness of 8 cm. The lateral dimensions of the radiator were ~6x6 cm2. These samples have the same index of refraction n=1.055 (at 500 nm). Although the Airglass material had been previously baked in order to remove humidity, its transparency remained low. We have observed a very poor performance from the Airglass radiator due to the poor quality of the material. In the first run we observe six photoelectrons (coming from the WLS), and when we introduce the aerogel we observe, in addition to the photoelectrons coming from the WLS, approximately. 12 photoelectrons.
Figure 1-2 Spectrum of detected photons with detector in-line with the beam but containing no aerogel. The amount of photons observed is due to Cerenkov light produced in the WLS. The first peak corresponds to the pedestal.
Figure 1-3 Spectrum of detected photons, produced in a 9 cm aerogel Cerenkov radiator, coupled to a 0.3 cm thick WLS. We estimate that the total number of photons detected, per event is ~18.