P. Bourgeois

TEST OF A LARGE SCALE PROTOTYPE OF THE DIRC, A CHERENKOV IMAGING DETECTOR BASED ON TOTAL INTERNAL REFLECTION FOR BABAR AT PEP-II

Abstract The principles of the DiRC ring imaging Cherenkov technique are briefly explained and its choice for the B a B ar detector particle identification system is motivated. A large scale prototype of the DIRC for the B a B ar experiment is then described. Details of the design of this prototype and its test in a hadronic particle beam at the CERN-PS are presented, and results from various prototype and test configurations are given. For example, after correcting for geometrical acceptance and estimated collection effects, the number of photoelectrons was measured to be 146 ± 1.8 ± 9 cm −1 , for a track angle of 20° at zero photon transmission distance. The effective attenuation loss was measured to be 4.1 ± 0.7% per meter of bar length, and the observed single photon resolution was 10.0 ± 0.2 mrad. This performance is consistent with what was expected from earlier tests and Monte Carlo simulations, and will be fully adequate for the physics demands of the B a B ar experiment.

Quantum efficiency and ageing studies for bulk and porous CsI photocathodes in a MWPC

Abstract The quantum efficiency of CsI photocathodes operating in a proportional gas chamber is measured in the wavelength range 185–220 nm. The tests are carried out with methane gas at atmospheric pressure using a pulsed UV lamp. Two different techniques are used to produce the photocathodes. The CsI is either evaporated under high vacuum or under low pressure of argon. The quantum efficiency of the photocathodes produced by the first technique is measured to be 9% at 190 nm while it is enhanced by a factor of 1.3 and 2 at 190 nm and 200 nm respectively for the photocathodes prepared with the second method. No significative drop of the quantum efficiency is observed over a total period of one month.

A gas secondary electron detector

Abstract A new Secondary Electron gas Detector (SED) is under development to be used in conjunction with an emissive foil to detect low energy heavy ions as an alternative to micro-channel plates. It could measure position and time of flight. Secondary electrons are accelerated to 10 keV so that they can cross through the 0.9 μm Mylar entrance window. The electrons then are multiplied in the isobutane gas of the detector at 4– 10 Torr . A time resolution of 150 ps and a spatial resolution of 3 mm have been obtained by using californium fission fragments on a prototype detector of 7×7 cm 2 . The advantage of the SED against MCP is that its size is not limited. Our final goal is to build a large size detector (15×40 cm 2 ) that will operate at the focal plane of the VAMOS magnetic spectrometer at GANIL.

Measurements of CsI photocathode quantum efficiency in methane

Abstract The Cherenkov light produced by 2.8 GeV c protons in a solid NaF radiator has been detected with a CsI photocathode coupled to a multiwire proportional chamber. The measurement of the number of photoelectrons gives an estimation of the photocathode quantum efficiency.

An internally reflecting Cherenkov detector (DIRC): properties of the fused silica radiators

The DIRC, a new type of ring-imaging Cherenkov detector that images internally reflected Cherenkov light, is being constructed as the main hadronic particle identification component of the BABAR detector at SLAC. The device makes use of 5 meter long fused silica (colloquially called quartz) bars, which serve both as the Cherenkov radiators and as light pipes for transmitting the light to an array of photo-multiplier tubes. This paper describes a program of research and development aimed at determining whether bars that meet the stringent requirements of the DIRC can be obtained from commercial sources. The results of studies of bulk absorption of fused silica, surface finish, radiation damage and bulk inhomogeneities are discussed.

Measurement of photomultiplier quantum efficiency

Abstract The absolute quantum efficiency of two Philips XP2020Q photomultipliers and one Hamamatsu R2059 photomultiplier are measured in the VUV range for three wavelength values, (193, 201, and 253 nm). We used a deuterium lamp for which the absolute radiance has been calibrated; the wavelength range is selected by means of optical filters.

SIMULATION AND MEASUREMENT OF THE FRINGE FIELD OF THE 1.5 T BABAR SOLENOID

In the context of the SLAC PEP-II asymmetric e{sup +}e{sup {minus}} collider and the BABAR detector with its 1.5 Tesla solenoid, we have calculated and measured the fringe field at the nearby beam elements and at the position of the photomultipliers external to the return iron but within a specially designed iron shield. The comparisons of these measurements with the simulations based on finite element analysis are remarkably good, within about 5 Gauss at the most critical beam element. The field at the photomultipliers is less than 1 Gauss, in agreement with the simulation. With a simple method of demagnetization of the shield, a maximum field of 0.6 Gauss is obtained.

Measurement of the quantum efficiency of CsI, amorphous silicon and organometallic reflective photocathodes

Abstract We performed a systematic investigation of the quantum efficiency of some solid reflective photocathodes in the spectral range 140–240 nm. The measurements were made without gaseous amplification in vacuum and in methane. No significant difference was found among CsI photocathodes prepared by vacuum deposition at different institutes, either from powders or from crystals of different origins, and measured either in vacuum or in methane. Amorphous silicon photocathodes were prepared by the plasma enhanced chemical vapor deposition technique. We present the results for several doping conditions of amorphous silicon and for p-n junctions. Some organometallic photocathodes, containing iron or other transition metals (cerium), were evaporated and measured. Among them decamethylferrocene exhibits the highest quantum efficiency in the range 190–240 nm.

Realization of a permanent magnet undulator for the CLIO infrared free electron laser at Orsay

Abstract The purpose of CLIO (Collaboration pour un Laser Infra-rouge a Orsay) is to provide a tunable high-power infrared source in Orsay, at LURE. The permanent magnet undulator for the linear accelerator (linac) free electron laser is described here. This undulator consists of two independent parts of equal length and equal period, with independently adjustable magnetic gaps. Moreover, the second section can be tapered, to optimize both the gain and the energy extraction efficiency for a wide range of wavelengths. The realization of the undulator is described: mechanical system, the magnet sorting, measurements of the magnetic field for several cases of operation. The results of the modelling of the corresponding electron trajectory and spontaneous emission spectra are taken as tests of the quality of the undulator. Finally, the beam diagnostics associated with the undulator are presented: two secondary emission profile monitors and three optical transition radiation screens for electron and optical beam alignment.

Operational experience with the DIRC detector

The DIRC, a novel type of Cherenkov ring imaging device, is the primary hadronic particle identification system for the BABAR detector at the asymmetric B-factory, PEP-II at the Stanford Linear Accelerator Center. It is based on total internal reflection and uses long, rectangular bars made from synthetic fused silica as Cherenkov radiators and light guides. BABAR began taking data with colliding beams in late spring 1999. This paper describes the performance of the DIRC during the first 2.5 years of operation.