Silica aerogels in high energy physics

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Abstract

Low-refractive-index silica aerogel is the most convenient radiator for threshold Cherenkov counters, used for particle identification in high energy physics. For the BELLE detector at the KEK B-Factory we have produced about 2 m3 of hydrophobic silica aerogels of n=1.01–1.03. Particle identification capability of the aerogel Cherenkov counters was tested and 3 σ pion/proton separation has been achieved at 3.5 GeV/c. Radiation hardness of the aerogels was confirmed to 9.8 Mrad. Thanks to the improved transparency, aerogels prepared by the two-step method can be used as radiators for not only threshold type but also for Ring Imaging type Cherenkov counters.

Introduction

One of the most intriguing puzzles of nature is why the Universe is composed only of matter in contradiction with cosmological theory, which suggests that an equal amount of particles and antiparticles have been produced in the `big-bang'. A simple explanation of this phenomenon requires the violation of matter–antimatter symmetry (so-called CP symmetry). In order to elucidate this interesting physics, several B-factories have been proposed and are being constructed around the world 1, 2, 3, 4, 5, where large numbers (∼107–8/year) of B-meson decays will be examined for the study of CP violation. In such B-factories, separation of pions from kaons is vital for the identification of B or mesons and the selection of rare decays.

One method for such particle identification involves detection of the Cherenkov light emitted by charged particles passing through transparent materials [6]. This light is produced only when the velocity of the particle is faster than the velocity of light in the material. Since each particle has a characteristic mass, any particle can be identified by a simultaneous measurement of its velocity and momentum. Hence information of the velocity obtained by the Cherenkov light is directly translated into mass information.

There are two methods that make use of Cherenkov light for particle identification; one is known as the `threshold' type and the other is the `Ring Imaging (RICH)' type. The former method identifies particle species according to whether Cherenkov light is emitted or not in the material, while the latter measures the Cherenkov angle by observing the position of light on a detector. A material having a refractive index less than 1.03 is necessary for pion/kaon separation at a few GeV/c region by a threshold-type Cherenkov detector. However, it is very difficult to attain such a low refractive index with most materials. For this reason, a large number of experiments [7]have used silica aerogels as the radiator for threshold type Cherenkov counters.

Before the invention of the two-step method [8], it was very difficult to produce silica aerogels having a refractive index lower than 1.02. Recent B-physics experiments, however, require good particle identification on the momentum range up to 4 GeV/c, and a low refractive index material (n=1.007 to 1.03) becomes vital for such experiments. Thanks to the two-step method, silica aerogel with a very low refractive index can easily be produced.

We report a production method and describe quality of silica aerogels prepared.

Section snippets

Experiments

Fig. 1 shows the configuration of the aerogel Cherenkov counter (ACC) in a central part of the BELLE detector [5]at the KEK B factory. The ACC consists of 960 counter modules for the barrel part and 228 modules for the end-cap part of the detector. In order to obtain good pion/kaon separation for the whole kinematical range, refractive indices of aerogels are selected to be between 1.01 and 1.03, depending on their polar angle region. A typical single ACC module looks as shown in Fig. 2. Five

Results

All the aerogel tiles have been checked for optical transparency, refractive index, density, size, etc. Fig. 4 shows a typical transmittance curve obtained by a photo-spectrometer for aerogels of four different refractive indices. The n=1.028 aerogels have better transmittance than others. Their average transmission length (Λ) at 400 nm is 46 mm, while others are around 25 mm. Here we define Λ as: T/T0=exp(−d/Λ), where T/T0 is the transmittance, and d is the thickness of the aerogels. These

Discussion

Performance of single ACC modules was tested using a 3.5 GeV/c negative pion beam at KEK PS (π 2 beam line). The number of photoelectrons obtained for 3.5 GeV/c pions are 18.2, 20.3 and 20.3 for n=1.01, 1.015 and 1.02 silica aerogels, respectively. Typical pulse height distributions for 3.5 GeV/c pions and protons observed by an aerogel counter (n=1.015 with two 2.5″ PMTs) are shown in Fig. 6. Pions (above threshold) and protons (below threshold) are clearly separated by more than 3σ. This

Conclusion

For the BELLE detector at KEK B-Factory we have produced about 2 m3 of silica aerogels of n=1.01–1.03 using a new method. The particle identification capability of the aerogel Cherenkov counters was tested by using real beams. Pion/proton separation of three standard deviations has been achieved. Radiation hardness of aerogels was tested up to 9.8 Mrad. Neither deterioration of transparency nor change in the refractive index was observed, which give us confidence in particle identification with

Acknowledgements

This work was partially supported by a collaborative research program between Matsushita Electric Works and KEK. We are indebted to S. Iwata, F. Takasaki and M. Kobayashi at KEK for their continuous encouragement. We are grateful to H. Koike and S. Hirano at Matsushita Electric Works for their support. We thank all the staff in Central Research Laboratory at Matsushita Electric Works and all members of the BELLE Collaboration. T. Yoshida of the BESS Collaboration is also acknowledged for

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