KEDR ATC

Aerogel Cherenkov Counters

25 April 2002

Aerogel, pictures

Design of the KEDR ATC

In the KEDR detector at the VEPP-4M collider an identification system based on aerogel Cherenkov counters (Fig.1) shall be used. As a radiator of Cherenkov light aerogel with refraction index 1.05 was chosen giving possibility to separate pi and K mesons in the momentum range from 0.6 up to 1.5 GeV/c.

For the Cherenkov light collection reemitters of light (Wave Length Shifters) are used. This gives a possibility to diminish number of photomultipliers in the system and to enlarge size of the counters. Advatage of this method of light collection in comparison with the direct one is due to much better optical properties of light shifter compared to aerogel.

The system consists of barrel and end cap parts. The counters are arranged in two layers, rotated in phi on half of period with respect to each other (Fig.2). The sizes of counters and their position are chosen in such a way that a particle from the interaction point hits shifter in only one layer Large fraction of particles cross aerogel in both layers. These particles can be identified much better. The system is segmented in 20 sectors in p Total number of counters equals 160. In Fig.3 and Fig.4 views of barrel and end-cap parts of the systems are shown.

Aerogel, reflector, WLS

The light absorbtion length in aerogel SAN-95 as a function of wavw lengh is shown in Fig.5. The light absorbtion coefficient 1-R for the teflon PTFE, used in the KEDR ATC, is shown in Fig.6 as a function of wavw length.

Reemitters, or shifters, represent lightguides of rectangular section produced from optically transparent material, organic polimer with small luminicent dope. Light of definite wave lentgh moving through reemitter is being partly absorbed and partly reemittred, shifted in the red region of spectrum. A part of reemittered light is being captured incide the critical angle of total reflection, determined by geometry and reflection coefficient of the shifter material and latter is being transported to photodetector.

For the KEDR ATC shifters on the base of PMMA were chosen. This plastic is cheep and more simple for production, than other organic matters, its mechanical properties are well known. Moreover, this plastic can be flexed easy, that is very important to adjust the section of shifter with photokathode. The shifters done from PMMA doped by BBQ were produced by the Institute of polimers in Dzerzhinsk.

Photomultipliers

Since amount of the Cherenkov ligth is small, photodetectors are placed on the conunters and, consequenly, with all system are inside the field up to 1.8 T.

The most popular photomultipliers operating in such magnetic field are the Hamamatsu fine-mesh PMT and the PMT based on microchannel plates (MCP). The photomultipliers with MCP, produced by "Katod" firm from Novosibirsk, were chosen for the system (Fig.7).

This choice is due to relatively small decrease of amplification coefficient in magnetic field for such devices - approxiomately in 5 times at B=1.8 N, their small price and compactness. The diameter of entrance window of PMT with MCP equals 18 mm, high of this device equals 17 mm and outer diameter - 31 mm. The PMT produced by Katod have two microchannel plates. This gives a possibility to have the amplification coefficient of 10⁶. The thickness of one plate is 400 micrometers, the channel diameter is about 10 micrometers. The channels have a 5 degrees angle with plane of the plate. In Fig.8 and Fig.9 photographs of the surface and the section of the MCP, obtained with an electron microcope, are shown. A size of the photograph is 200 x 200 micrometers.

In Fig.10 the quantum efficiencies of bialcaly photocathode R6150 with quartz entrance window and PMT with MCP, having multialcaly photocathode and borosilicate glass are shown. As one can see, the photocathode of PMT with MCP is more sensitive to green spectrum region, while the R6150 to violet one. These devices have equal detection efficiency for the emission spectrum of POPOP (luminicent dope to WLS) while for the BBQ dope the multialcaly photocathode is more efficient. Moreover, use of BBQ is more attractive due to wider absorbtion spectrum.

Test of counters at beam in JINR

In 2000 a test of the end-cap aerogel counter of the KEDR at the proton and pion beams in JINR (town Dubna). This experiment was performed at the beam of secondary particles of 10 GeV proton synchrophasotron. The measurements were performed for two types of counters. Ones were filed with aerogel blocks, which are being produsec now for the KEDR, and the others were filled with aerogel crumb (5 - 20 mm size).

4 counters, placed one after another, were tested simultaneously. Layout of the experiment (view from above) is shown in Fig.11. ATC 1,3 -- filled with blocks; ATC 2,4 -- filled with crumb. The measurements were carried out with protons of 0.86 GeV/c -- 2.1 GeV/c momentum and with 0.86 GeV/c -- 1.6 GeV/c pions. For the particles separation a time of flight counters system with 30 m base was used. To determine a coordinate, with which the particle passes the counter, a godoscope of scintillation counters was used.

Data on number of photoelecterons as a function of pion momentum are shown in Fig.12. Data for kaons were obtained using protons with corresponding velocity. In Figures Fig.13 and Fig.14 the probability of misidentification of kaons and pions is shown as a function of threshold for 0.86 and 1.2 GeV/c momentum. Counter 1 is filled with blocks, counter 2 is filled with crumb. At the zero threshold for the 0.86 GeV/c momentum the coefficient of pion suppression is 860 at 94% kaon detection efficiency, this corresponds to 4.7 sigma separation. For 1.2 GeV/c momentum at the zero threshold the coefficient of pion suppression is 1300 at 90% kaon detection efficiency, this corresponds to 4.5 sigma separation. For 0.86 GeV/c pions the inhomogeneity of light collection in the counter was measured, the maximum number of photoelectrons equals 9.7, minimal -- 7.1.

Production of aerogel for RICH detectors

In cooperation with Boreskov institute of catalysis of SB RAS the studies on production of aerogel with refractive index 1.03 for use as the Cherenkov light radiator in RICH detectors are being carried out. The main requirement for this aerogel -- its high transparency. For these detectors the transparency is determined by the light scattering length. At present time samples with thickness 2 and 4 cm and area 5 x 5 cm squared were obtained. Blocks of 2 cm thickness have L_sc=5.3 cm at 400 nm, 4 cm blocks - 4.3 cm.

Last modified 25 April 2002