Takayuki Sumiyoshi

A novel type of proximity focusing RICH counter with multiple refractive index aerogel radiator

Abstract A proximity focusing ring imaging Cherenkov detector, with the radiator consisting of two or more aerogel layers of different refractive indices, has been tested in 1 – 4 GeV / c pion beams at KEK. Essentially, a multiple refractive index aerogel radiator allows for an increase in Cherenkov photon yield on account of the increase in overall radiator thickness, while avoiding the simultaneous degradation in single photon angular resolution associated with the increased uncertainty of the emission point. With the refractive index of consecutive layers suitably increasing in the downstream direction, one may achieve overlapping of the Cherenkov rings from a single charged particle. The impact of the observed improvement on the π / K separation at the upgraded Belle detector is discussed.

Studies of proximity focusing RICH with an aerogel radiator using flat-panel multi-anode PMTs (Hamamatsu H8500)

Abstract A proximity focusing ring imaging Cherenkov detector using aerogel as the radiator has been studied for upgrading the Belle detector at the KEK-B-factory. We constructed a prototype Cherenkov counter using a 4×4 array of 64-channel flat-panel multi-anode PMTs (Hamamatsu H8500) with a large effective area. The aerogel samples were made with a new technique to obtain a higher transmission length at a high refractive index ( n =1.05). Multi-channel PMTs are read-out with analog memory chips. The detector was tested at the KEK-PS π 2 beam line in November, 2002. To evaluate systematically the performance of the detector, tests were carried out with various aerogel samples using pion beams with momenta between 0.5 and 4 GeV /c . The typical angular resolution was around 14 mrad , and the average number of detected photoelectrons was around 6. We expect that pions and kaons can be separated at a 4 σ level at 4 GeV /c .

Development of transparent silica aerogel over a wide range of densities

Abstract We have succeeded in developing hydrophobic silica aerogels over a wide range of densities (i.e. refractive indices). A pinhole drying method was invented to make possible producing highly transparent aerogels with entirely new region of refractive indices of 1.06–1.26. Obtained aerogels are more transparent than conventional ones, and the refractive index is well controlled in the pinhole drying process. A test beam experiment was carried out in order to evaluate the performance of the pinhole-dried aerogels as a Cherenkov radiator. A clear Cherenkov ring was successfully observed by a ring imaging Cherenkov counter. We also developed monolithic and hydrophobic aerogels with a density of 0.01xa0g/cm 3 (a low refractive index of 1.0026) as a space dust capturer for the first time. Consequently, aerogels with any refractive indices between 1.0026 and 1.26 can be produced freely.

Hydrophobic silica aerogel production at KEK

Abstract We present herein a characterization of a standard method used at the High Energy Accelerator Research Organization (KEK) to produce hydrophobic silica aerogels and expand this method to obtain a wide range of refractive index ( n =1.006–1.14). We describe in detail the entire production process and explain the methods used to measure the characteristic parameters of aerogels, namely the refractive index, transmittance, and density. We use a small-angle X-ray scattering (SAXS) technique to relate the transparency to the fine structure of aerogels.

Test of various photocathodes

Abstract For various photocathodes (CsI, CsTe, their multilayers, etc.) the quantum efficiencies were measured under a vacuum and/or after exposure to several kinds of gases.

Silica aerogel radiator for use in the A-RICH system utilized in the Belle II experiment

Abstract This paper presents recent progress in the development and mass production of large-area hydrophobic silica aerogels for use as radiators in the aerogel-based ring-imaging Cherenkov (A-RICH) counter, which will be installed in the forward end cap of the Belle II detector. The proximity-focusing A-RICH system is especially designed to identify charged kaons and pions. The refractive index of the installed aerogel Cherenkov radiators is approximately 1.05, and we aim for a separation capability exceeding 4σ at momenta up to 4xa0GeV/c. Large-area aerogel tiles (over 18×18×2xa0cm3) were first fabricated in test productions by pin drying in addition to conventional methods. We proposed to fill the large end-cap region (area 3.5xa0m2) with 124 water-jet-trimmed fan-shaped dual-layer-focusing aerogel combinations of different refractive indices (1.045 and 1.055). Guided by the test production results, we decided to manufacture aerogels by the conventional method and are currently proceeding with mass production. In an electron beam test undertaken at the DESY, we confirmed that the K/π separation capability of a prototype A-RICH counter exceeded 4σ at 4xa0GeV/c.

Development of a multi-pixel photon sensor with single photon sensitivity

Abstract A multi-pixel photon sensor with single-photon sensitivity has been developed. Based on a hybrid photo-detector (HPD) technology, it consists of a photocathode and a multi-pixel avalanche diode (MP-AD). The developed HPD has a proximity-focused structure, where the photocathode and MP-AD face each other with a small gap of 2.5xa0mm. The MP-AD, which has an effective area of 16xa0mm×16xa0mm, is composed of 8×8 pixel and has been specially designed for the HPD. The maximum gain of the HPD is 5×10 4 , sufficiently high to detect single photons with a timing resolution better than 100xa0ps. Up to four photoelectrons can be clearly identified as distinct peaks in a pulse-height spectrum, thanks to the low-noise characteristics of the HPD. It is also demonstrated that the HPD can be operated with good performance in a magnetic field as high as 1.5xa0T.

Large-area silica aerogel for use as Cherenkov radiators with high refractive index, developed by supercritical carbon dioxide drying

Abstract This study presents the development of large-area (18xa0cmxa0×xa018xa0cmxa0×xa02xa0cm), high refractive index ( n xa0∼xa01.05) hydrophobic silica aerogel tiles for use as Cherenkov radiators. These transparent aerogel tiles will be installed in a Cherenkov detector for the next-generation accelerator-based particle physics experiment Belle II, to be performed at the High Energy Accelerator Research Organization (KEK) in Japan. Cracking has been eliminated from the prototype aerogel tiles by improving the supercritical carbon dioxide (scCO 2 ) extraction procedure when drying the wet gel tiles. Finally, a method of mass-producing aerogel tiles for the actual detector was established. It was confirmed that the experimentally manufactured aerogel tiles meet the required optical and hydrophobic characteristics and have a uniform tile density.

Development of silica aerogel with refractive index gradient for RICH radiator

A proximity focusing RICH using aerogel as a Cherenkov radiator has been developed for the upgrade of the Belle detector. We have considered the idea to accumulate multiple aerogel radiators so that Cherenkov photons from each tile organize a unified ring image on a photo-detector plane. Advancing this idea, we have developed aerogel in which the refractive index varies continuously. We report the production method of the silica aerogel with refractive index gradient and he result of test beam experiment.

Studies of a proximity focusing aerogel RICH for the Belle upgrade

For future upgrade of the Belle detector at the KEK B-factory we are developing a proximity focusing ring imaging Cherenkov detector using aerogel as a radiator. We present results from recent beam tests using 4times4 array of the flat-panel multi-anode PMTs. We introduce a new design, so called dual radiator scheme to improve the performance. In this design, we image Cherenkov photons from two aerogel radiators with different refractive indices. The results of the beam tests show that the photoelectron yield can be increased without significant deterioration in the angular resolution, leading to 4.8sigma K/pi separation at 4 GeV/c. We also discuss multi-radiator RICH, which consists of more than two radiators.