Development of pixel hybrid photon detectors ...



LHCb 2000-064 RICH

27 June 2000

a SUPPORT NOTE for the use of

PIXEL HYBRID PHOTON DETECTORS

IN THE RICH COUNTERS OF LHCB

T. Gys(

On behalf of the LHCb-RICH group

CERN, Geneva, Switzerland

Abstract

This document is a support note for the use of a hybrid photon detector with integrated silicon pixel readout in the ring imaging Cherenkov detectors of the LHCb experiment. The photon detector is based on a cross-focussed image intensifier tube geometry where the image is de-magnified by a factor of 5. The anode consists of a silicon pixel array, bump-bonded to a binary readout chip with matching pixel electronics. The document starts with the general specification of the baseline option, followed by a summary of the main results achieved so far during the R&D phase. A future R&D programme and its related time table is also presented. The document concludes with the description of a photon detector production scheme and time schedule.

–––––––––––––––––––––––––

1 General specification

1.1 INTRODUCTION

The baseline photon detector of the LHCb RICH system [1] is the hybrid photon detector (HPD) which uses a silicon detector anode inside a vacuum envelope. A photo-cathode is deposited on an optical input window in the envelope, and the photoelectron released by an incident photon is accelerated onto the silicon detector by an applied high voltage of ~20 kV (corresponding to ~5000 e- released in the silicon). Commercially available examples of HPDs exist, but do not meet fully the specific LHCb requirements, in particular the large area coverage (~2.9 m2) with high active-to-total area ratio (~70 %), small granularity (2.5(2.5 mm2 at the photo-cathode level) and high speed (25 ns timing resolution).

The present HPD development [2] is being carried out in close collaboration with the company Delft Electronic Products (DEP) [3]. It is based on a cross-focussing tube design, de-magnifying by a factor of ~5 the photo-cathode image onto a small detector array with O(500 μm) pixels, bump-bonded to a binary readout chip with matching pixel electronics integrated inside the vacuum envelope of the tube. The feasibility of this approach was demonstrated in 1994 by the successful realization of the “Imaging Silicon Pixel Array (ISPA) tube” [4]. The first ISPA-tubes, based on magnetically focussed electron optics, had a one-to-one mapping geometry resulting in a total active surface of 4.8(8.0 mm2. They were initially developed to read out small diameter scintillating fibres for particle tracking [5] and have also been shown to be an excellent detection tool for biomedical applications [6].

The performance of cross-focussed, or first generation, image intensifiers is well known [7]. In particular, these devices can reach a limiting spatial resolution of up to 100 line pairs per mm (lp/mm) (or equivalently, the full width at half maximum of the point spread function (PSF) is 10 μm). Their image distortion, caused by variations of the linear de-magnification along the radial distance, does not generally exceed 10 % at the edge, and can be corrected off-line. This tube geometry is robust to external electric field perturbations and allows for the shielding of low magnetic fields [8]. Additionally, small pixels with bump-bond connections present little load capacitance to the front-end electronics (giving low noise and high speed) and a compact anode structure with a limited number of feed-throughs. Binary electronics have low power consumption (~50 μW per channel) and are consequently well adapted to implementation in a vacuum tube. In addition, they have been shown to be compatible with the demanding bake-out cycles (typically 350 ºC for 5 hours) needed for high-quality photo-cathodes.

1.2 Pixel-HPD description

The baseline pixel-HPD for the LHCb RICH counters is shown in Figure 1. It is based on an electrostatically-focussed tube design, with a tetrode structure, de-magnifying by a factor of ~5 the photo-cathode image onto a small silicon detector array with 1024 pixels each 500 (m × 500 (m in size and arranged as a matrix of 32 rows and 32 columns. The nominal operating voltage is 20 kV corresponding to ~5000 electron-hole pairs released in the silicon. The voltage difference between photo-cathode and first electrode is adjustable and defines the precise value of the de-magnification factor. The silicon pixel detector array is bump-bonded to a binary readout chip (described in section 1.4). This assembly is mounted and wire-bonded onto a Pin Grid Array (PGA) ceramic carrier.

Cherenkov photons can be detected over an active diameter of 75 mm. Since the overall tube diameter is 83 mm, the tube active area fraction is (75/83)2=0.817. The photo-cathode is of the “thin-S20” multi-alkali type. Quantum efficiency (QE) values measured from a prototype tube are listed in table 1 (see also section 2.2). They include light reflection losses (~4 %) at the entrance face of the 7 mm-thick quartz window and its transmission (~35 % at 200 nm), and correspond to a (QE(E)dE integral value of 0.77 eV.

The tube window is made of quartz. It has a spherical shape, with 7 mm thickness and 55 mm inner radius of curvature. Light refraction at the window results in a radial coordinate correction factor of ~1.047. The baseline de-magnification law of the electron optics is:

rp = 0.200(rc - (0.4 10-3 mm-1)(rc2

rc and rp are the radial coordinates on the photo-cathode and the pixel array, respectively, and are expressed in mm. This de-magnification is achieved by polarizing the photo-cathode at -20kV, the first electrode at -19.7kV and the second electrode at -15.8kV (see bottom part of Figure 1). Consequently, the photo-cathode image (a portion of a sphere of 36 mm active radius) is de-magnified as a disk ~6.7 mm in radius on the pixel array. This value is smaller than the half size of the pixel array (8 mm) and allows some overhead in coverage, given the estimated image distortions due to stray magnetic fields (see section 2.2 for details). Taking into account the window lens effect, the pixel size at the window input is:

0.5 mm((1.047/0.200)=2.62 mm on the tube axis and

0.5 mm((1.047/(0.200-(0.4 10-3 mm-1(36 mm)))=2.82 mm at the edge.

The values for the PSF standard deviations (at the window input) are approximately constant over the tube radius and equal to 400 μm. All the above figures are valid in absence of magnetic field.

1.3 Integration

The hexagonal close-packing factor is 0.907, so theoretically, the active area fraction is (75/83)2(0.907=0.741. With a 0.9 mm-thick mu-metal shield, and considering the mechanical tolerance, a gap of at least 4 mm between tubes is needed, resulting in a packing factor of (75/86)2(0.907=0.674. Improvement of this factor by the use of a thinner shield with higher saturation induction and a thicker window, ie an increased lens effect, is envisaged. With a tube pitch of 87 mm, a total of 430 tubes are needed to cover the RICH photon detection surface (see Table 2). In this number estimate, a pointing geometry is assumed for the RICH 1 counter, ie the tube axes are parallel to the average angle of incidence of the Cherenkov photons. For the RICH 2 counter, the tubes lie in a plane corresponding to the photon detection surface, the tube axes being normal to this plane. Detailed mechanical studies for the mounting of the tubes are being carried out, and take into account the specific environmental and space constraints of the two RICH counters (see the two RICH mechanics support notes for details [9,10]).

1.4 Binary front-end electronics

A front-end binary pixel chip optimized for photo-electron detection in the LHCb RICH must meet the following requirements. Firstly, the chip must correctly discriminate hits and time-tag them with a specific bunch crossing. This requires that the front-end amplifier has a shaping time of ( 25 ns, and that the discriminator applies a threshold of ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download