Time-of-propagation Cherenkov counter for particle identification

doi:10.1016/S0168-9002(99)00819-0

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 440, Issue 1, 21 January 2000, Pages 124-135

https://doi.org/10.1016/S0168-9002(99)00819-0 Get rights and content

Abstract

We describe here a new concept of a Cherenkov detector for particle identification by means of measuring the Time-of-Propagation (TOP) of Cherenkov photons.

Introduction

Particle-identification (PID) capability plays an essential role in experiments at B-factories. Especially, π/K identification in the momentum range up to 4 GeV/c over a wide angular region is crucially important for the primary physics goals to measure CP violation. In the Belle detector at KEK-B, a combination of Aerogel Cherenkov counters, TOF counters and dE/dX measurement in a central drift chamber (CDC) provides PID information for charged particles.¹ Although the present detector system covers most of the momentum region of the charged particles, the PID power for a particle with momentum above 3 GeV/c is not sufficiently satisfactory. Concerning the future upgrade of PID devices, we discuss here the effectiveness of measuring both the Time-of-Propagation (TOP) and the emission angle of the Cherenkov photons.

The time resolution of the Time-of-Flight (TOF) counter using a plastic scintillator is inherently limited by the following effects besides the transit-time spread (TTS) of a phototube: (1) a finite decay time of photon emission, and (2) a different photon propagation length or propagation time in the scintillator to a phototube, depending on the photon emission angle. Thus, the conventional TOF counter measures the arrival time of the earliest photons out of many, whereby the remaining large amount of photons are not effectively used for the time measurement. While effect (1) cannot be reduced as far as the scintillation is concerned, effect (2) can be resolved by measuring the arrival times of the earliest photons as a function of the photon emission angle, since, in the total-reflection-type scintillator bar, most photons propagate in the scintillator while keeping their original emission angles. This two-dimensional information could improve the time original emission angles. This two-dimensional information could improve the time resolution, depending on the number of angular segmentations of the photon detector and on the number of photons collected at each segment.

When Cherenkov radiation is utilized, although its number of produced photons is much smaller than in the scintillation case, effect (1) can, in principle, be disregarded. In addition, since the Cherenkov photon emission angle is uniquely determined by the particle velocity (β) and since the propagation time of photons in a light guide of an internal-total-reflection type can be calculated as a function of the photon emission angle, a measurement of a correlation between TOP and the photon emission angle could provide PID information by itself, as we propose below.

In the DIRC concept [3], [4], [5], [6], [7], [8], Cherenkov photons produced by a quartz radiator are transported to an end of the radiator by means of internal total reflection, and its ring image is magnified and projected onto a photon-detector plane placed at the bar end. A basic study of this concept clearly proved that a photon is effectively transported in the long quartz bar, preserving its original photon direction. The Babar detector at SLAC adopted this concept for PID, and introduced a large stand-off photon detection system in order to ineffectuate the finite size of the quartz cross section onto the ring image [6], [7], [8]. On the other hand, Kamae et al. [9], [10], [11] proposed a compact focussing type of DIRC using a small mirror instead of a large stand-off system. This approach suits particles incident normal to the quartz bar, but for inclined particles the ring image is distorted due to the finite quartz cross section. To make this image-fusing effect insignificant, the size and geometry of the focussing mirror become unfeasible [12].

We propose here a new concept for the Cherenkov-ring image detector, the TOP detector. The two-dimensional information of the ring image is represented by the TOP of the Cherenkov photon and its horizontal angle (Φ; see Fig. 1). The use of TOP information with an appropriate focussing mirror makes the image-fusing effect be disregarded and compactification can be realized while maintaining the high enough PID ability. We describe below the basic concept, characteristic features, practical configurations and simulation results on the TOP detector, and discuss some issues for making it a realistic device.

We should mention a paper by Honsheid et al. [13], which we noted after our detailed analysis had been completed. It discusses the Cherenkov Correlated Timing (CCT) detector: CCT [13], [14], [15] is a kind of TOF counter with a single phototube readout for detecting early arriving photons of Cherenkov radiation. In Ref. [6] a similar concept as ours can be found, although no detailed study is presented and the focussing mirror approach is not introduced.

Section snippets

Conceptual design and expected properties

The principal structure of the TOP counter in the $(x, y, z)$ coordinate system is illustrated in Fig. 1. When a charged particle passes through the radiator bar, Cherenkov photons are emitted in a conical direction defined by the emission angle (θ_C), where $cos θ_{C} =1/nβ$ , n=refractive index. Then, photons propagate to both or either ends by means of total reflection on the internal bar surface. Photons propagating backward are reflected by a flat mirror at the end. At the forward end, photons are

Simulation study

Based on the basic concepts considered in the preceeding section, we carried out a simulation study on the realistic environment. Photons are generated following to the Cherenkov spectrum (dN(λ)/dλ) convoluted by the quantum efficiency QE(λ) of the phototube. The Cherenkov angle is determined by using the λ-dependent quartz refractivity (n(λ)). The effect of the quartz thickness is naturally implemented in the simulation procedure. The effective TTS of the phototube is therefore set to be σ_TTS

Discussion and summary

We have shown that a high PID capability is attainable by using a $(t_{p}, Φ)$ measurement of the Cherenkov ring image. Although R&D is necessary, especially, on a high-quality position-sensitive phototube operable in a magnetic field and on an optimum focussing mirror, a good π/K separation is well expected at the Belle barrel detector.

Some remarks are given below:

•
The development of a position-sensitive phototube is most important to realize the separability so far presented. It has twofold issues.

Acknowledgements

We greatly thank Prof. Yoshitaka Kuno for informing us about the paper on the CCT counter. We also thank Prof. Katsumi Tanimura for his consultation on optics relating to our materials and devices. This work was supported by Grant-in-Aid for Scientific Research on Priority Areas (Physics of CP violation) from the Ministry of Education, Science, and Culture of Japan.

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Time-of-propagation Cherenkov counter for particle identification

Abstract

Introduction

Section snippets

Conceptual design and expected properties

Simulation study

Discussion and summary

Acknowledgements

Nucl. Instr. and Meth. A

Nucl. Phys. B

Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. A