Two-dimensional position sensitive transition radiation detector

Abstract

A new transition radiation detector (TRD) prototype foreseen to fulfill the requirements of the TRD subdetector of the CBM experiment at FAIR has been designed, constructed and tested with electrons and pions of a few GeV/c. The detector prototype was build with the original TRD architecture which preserves the high conversion efficiency of transition radiation in a single TRD layer. This TRD architecture is based on two multiwire proportional chambers readout by a common double-sided pad read-out electrode. The triangular shape of the readout pads gives access to the position information in both coordinates which defines the readout electrode plane. Pion efficiency as a function of number of TRD layers and position resolution were studied using electron and pion beams delivered by PS at CERN. Dedicated front-end electronics, designed for high counting rate environment was used. An extrapolated pion efficiency of 0.5% for a six layer TRD configuration at 90% electron efficiency using a regular foil radiator was obtained. The position resolution across the pads is of the order of $320\mathrm{\mu }\mathrm{m}$ and along the pads of 5.5 mm.

Introduction

The CBM (Compressed Baryonic Matter) experiment [1] at the future Facility for Antiproton and Ion Research (FAIR) [2] is designed to measure hadrons, multi-strange hyperons, lepton pairs and charmed particles produced in nuclear collisions at up to 10 MHz; it is a large acceptance and high-rate detection system. The CBM experiment includes a transition radiation detector (TRD) for identification of high-momentum electrons with a pion efficiency better than 1% for 90% electron efficiency. At the same time it will perform intermediate tracking with a position resolution of $200\mathrm{\mu }\mathrm{m}\mathrm{–}400\mathrm{\mu }\mathrm{m}$ in order to match tracks reconstructed in the silicon tracking system (STS) to the time-of-flight (TOF) system. Being a fixed target experiment, the most forward angles of CBM have to cope with a counting rate up to 100 kHz/cm². Therefore a fast detector is required.

The original TRD architecture for electron discrimination in a high counting rate environment is based on two multiwire proportional chambers with a common double-sided pad structure read-out electrode [3], [4]. The detector maintains the timing properties of a single multiwire proportional chambers (MWPC) [5] while the gas thickness for transition radiation (TR) absorption is doubled. An extrapolated pion efficiency better than 1% for a six layer configuration [3] and position resolution of the order of $160\mathrm{\mu }\mathrm{m}$ [4] were obtained. Negligible gain drop and less than $20\mathrm{\mu }\mathrm{m}$ position resolution degradation were observed up to an average particle rate of 200 kHz/cm². In order to cope with the requirement of a reasonable geometrical efficiency, a larger size prototype using the same architecture, but increasing the size of the rectangular pads of the central double sided read-out electrode was built. With the aim to access the position information in both coordinates of the readout electrode pad-plane (across and along the pads) with a single TRD layer, the rectangular pads of the read-out electrode were split diagonally, each triangular pad being readout separately. The paper is focused on the $\mathrm{e}/\mathrm{\pi }$ discrimination and position resolution of two such TRD prototypes which were tested in a mixed secondary beam at the CERN PS.