CsI–RICH Detectors

1. Introduction

CsI photocathodes have quantum efficiency in the UV region as high as TMAE vapour and thus represent a viable alternative in many applications. The main advantage of the CsI photo convertor is that it can be exposed to normal gases and even to dray air without loss in its quantum efficiency. This tremendously simplifies its handling such as transfer from the evaporation system to the gas chamber and also allows using cheap construction materials which strongly reduces the detector price.

Note also that in the case of solid photocathodes all photoelectrons are extracted from an extremely thin surface layer whereas in the case photosensitive vapour they are create within an orders of magnitude thicker gas volume. This causes a considerable jitter in arrival time of the primary electrons to the region of gas amplification. Therefore, CsI-based photosensitive detectors offer much better time resolution.

The first application of CsI photocathodes in a high-energy physics experiment was the CsI-MWPCs of the Threshold Imaging Cherenkov detector (TIC) at the CERN-NA44 heavy ion experiment (Braem, 1998). The schematic view of this device is shown in Figure 1.

Figure 1.

Schematics of the threshold imaging Cherenkov detector in the NA44 experiment at CERN

From Braem, 1998.

The TICs main components are: the gas container which serves as a Cherenkov radiator where the UV photons are created, two V-shaped mirrors (inclined on 45°with respect to the chamber axis) and two CsI-MWPCs detecting the Cherenkov photons.

A particle of mass m_p, traversing a radiator with refractive index n, above the Cherenkov momentum threshold p_t, given byp_t = m_p[{1/(√n{1-(1/n²)}], (1) will result in a number of photons being created in the radiator.

In Figure 1, two particle tracks are shown for the illustration, one below and the other one (pion) above the Cherenkov threshold and emitting photons along its path. The Cherenkov light is reflected by the mirror and creates a well localized image of the Cherenkov photons emitted. The image is in the shape of a fiducial disc, which is the projection of the cone of Cherenkov light on the detector plane.

The V-shaped mirror geometry and the division of the photosensitive area in two parts minimizes the variation of the photon yield due to the different path lengths of particles and the Cherenkov photons through the gas radiator.

CsI-TIC were in use in the NA44 experiment during the 1995 and1996 physics runs with lead-lead collisions, allowing track-by-track identification of up to five simultaneous particles in the 3-8 GeV/c momentum range. No deterioration of the photocathode’s quantum efficiency were observed during the experiment even though they were continuously ion bombarded corresponding to the total accumulated charge of about 100 µC/cm².Thesefirst results demonstrated that CsI photo cathodes are well suited for long term recording of Cherenkov photons.

2. Csi-Mwpc Rich Detectors

Several high-energy physics experiments have been using CsI-MWPC operated with CH₄ at normal temperature and pressure, see Table 1.

From Nappi, 2005.