5.5.4.2  Overview

Each High Resolution Spectrometer detector package includes a single silica aerogel Cerenkovcounter of the compact reflection mirror design, which was dictated by the available space (36.3 cm along the incident particle direction). In addition, the high singles rates expected in Hall A are better handled with segmented detectors covering the focal plane, which requires short pulse decay times. Even though the diffusion length in silica aerogel can be quite short for low light generated in the SiO₂ radiator [11], enough directionality remains in the visible region, where the selected PMTs have good quantum efficiency, to make light collection with mirrors an attractive and practical alternative.

An effective segmentation of the aerogel Cerenkov counter, matching the segmentation of the trigger scintillators, can be used to separate multiple tracks through the focal plane and will allow an additional element of selectivity and track sensitivity in the focal plane instrumentation. This means that specific sections of the focal plane can be physically disabled from the trigger, if the experimental conditions require it. It will also provide the capability of identifying and separating pions and protons traversing the focal plane trigger scintillators and the vertical drift chambers (VDCs) within the resolving time of the system (double hits). For example, in the offline analysis, the aerogel counter PMT with the highest number of photoelectrons can be matched with the trigger counter and VDC information to identify the actual path of a pion, thus separating it from a simultaneously detected proton, which has no Cerenkovsignature. Such a capability of double hit resolution is not possible with diffusion Cerenkovcounter designs, because the photon collection efficiency does not have a strong correlation with the incident particle track within the aerogel material.

The requirement for segmentation, in addition to supplementing the information on the individual particle position along the focal plane, also couples well with the desirability of increasing the active solid angle viewed by the PMTs in the counter. Although the photon detection probability is not as directly proportional to the solid angle covered by PMTs as in the case of a diffusion box, clearly, the larger the effective coverage, the higher the probability will be that a photon will end up on a PMT. Given the divergence of the beam envelope incident on the aerogel, and the diffusion of the light in the low region by the aerogel material, an increase in the area covered by PMTs results in an increase in the number of photons detected. As a result, a total of 26 PMTs are used in the counter, as shown in figure, with minimal spacing between their -metal shields (2.8 mm). The total area covered by the PMT photocathode windows comprises 72% of the area of the counter opposite the planar parabolic mirrors. A cross sectional schematic of the detector is shown in figure , clearly illustrating the planar parabolic design of the mirror surfaces and their relative orientation with respect to the PMTs, and the orientation of the counter relative to the central axis of the spectrometers.

The close spacing of the -metal shields, which is also shown in the photograph of figure , creates dielectric breakdown problems. The -metal shields are at cathode potential (-2950 V) to avoid the capacitive discharge from a grounded -metal shield to the glass of the photocathode, which would contribute to the noise level in the PMT, and adversely affect their performance at high operating voltages. This necessitates extra precautions, in order to avoid dielectric breakdown between adjacent shields, and between the shields and the aluminum structure of the counter, which is at ground potential. The solution was to wrap the outer surfaces of the -metal shields with a high dielectric value (12,000 V/mm), thin (0.254 mm) Teflon film [12]. In addition, the PMT housings consist of fiberglass-epoxy composites, with added inner and outer skins of 0.0254 mm thick Tedlar [12], with a further combined insulating value of 3,000 V. Such a combination of insulating materials eliminates any breakdown or small leakage current induced noise and, at the same time, satisfies all safety requirements.

The final construction of the counter, described in this report, is built around the two sides of the main (PMT) section, each consisting of two pieces of aircraft quality aluminum alloy, with stiffening aluminum rods formed integrally on the top and bottom. The openings for the PMT housings were machined on these structures using CNC milling machines to keep tolerances to fine levels. The double walled structure, on both sides of the enclosure, further increases the rigidity of the exoskeleton by forming a second ``outer'' wall on each side, very similar in configuration to the inner one, and attached to the latter with crossbolt braces, as shown in the photograph of figure. Each end plate is made out of the same aluminum alloy as the side walls, and also incorporates stiffening lips folded integrally to each plate, one at the top and one at the bottom. Each end plate has been provided with inlet and outlet gasline connections, which will be used to fill the counter enclosure with dry CO₂ gas to protect the silica aerogel from water vapour absorption. figuresand show the bottom (tray) sections, and main plus upper (mirror) sections, respectively. The main (middle or PMT) section, in figure, contains the PMTs and provides the strength and rigidity for the whole counter. The one piece aluminum end plates are also shown in both photographs.

All internal surfaces of the detector, except the planar parabolic mirrors, themselves, are lined with aluminized mylar [13] to increase the overall reflectivity of the counter. The mirrors are made in moulded surfaces, formed in one rigid structure. The rigidity is provided by two layers of carbon fiber epoxy composite backing, with a combined thickness of 0.28 mm, and a single sheet of mylar with thickness 0.127 mm. The special mylar material was obtained from exposed negative film used in the cartographic industry, and is of high smoothness and uniformity. One side was aluminized at CERN, while the other side remains in its exposed negative (black) state, further adding to the successive light penetration barriers into the enclosure. A representative reflectivity curve, as a function of for these mirrors, is shown in figure.

The upper section of the counter containing the mirrors is mounted on its own aluminum subframe, which is bolted to the main frame that houses the PMTs. The upper section, on its own, is shown in the photograph of figure, while its configuration when mounted on the main section is shown in figure. The light and gas sealing action is provided by continuous twin parallel rubber strips along the joint area, and by Tedlar film of 0.025 mm thickness covering the top of the outer planar parabolic area.

The third major component of the counter consists of a removable tray where the silica aerogel is placed. The tray occupies the bottom part of the counter and has inside dimensions of where the SiO₂ silica aerogel is placed. It is formed by a frame with twin aluminum panels, which, in turn, secure the removable frame strung with fishing line in a criss-cross pattern to hold the aerogel panels in place. This ``fishnet'' frame is secured by screws and is easily removed without disturbing the aerogel panels or requiring restringing. The bottom of the tray is formed out of a single layer of carbon fiber epoxy skin (0.127 mm thick) and a layer of aluminized mylar of equal thickness. Externally, it is covered by a single layer of Tedlar film to assure integrity from light penetration; further environmental isolation is provided by two parallel strips of rubber gasket seals enclosing the circumference of the tray and containing the feed-through spacers for the retaining bolts. The tray is equipped with SMA-type fiber optic feed through connectors for the gain and timing monitor system, which utilizes fiber optic cables. Each fiber illuminates two adjacent PMTs, except the last PMT on either side (13T and 13B in figure), which have their own dedicated fiber. The light is generated in a gas plasma discharge unit [14] and duplicates the spectrum expected from Cerenkov radiation. In addition, the fibers terminate beneath the silica aerogel, thus, the light reaching the PMTs will have the absorption characteristics of real Cerenkov light produced in the aerogel radiator.

Due to the nature of Cerenkovdetectors, where few photoelectrons (PEs) are emitted by the photocathodes in the PMTs, any extraneous light entering the enclosure is very troublesome. As a result of the small number of PEs expected, the PMTs operate either near to, or at, maximum high voltage, and, thus, at maximum gain. As such, they can suffer damage if a sudden light leak develops. In testing, we verified the extreme sensitivity to minute light leaks, even across the whole length of the structure, because of the mirrored surfaces inside the enclosure. With 26 PMTs operating at maximum gain, and viewing, effectively, a giant mirror, sealing the enclosure against single photon penetration requires extra care during initial testing and operations.

The PMTs chosen for the counter were Burle model number 8854, 127 mm photocathode diameter [15]. The PMT amplification electronics have been described in Refs. [16,17]. The dynode chain incorporated a 600 resistance between the cathode and first dynode, instead of the nominal 300 . This generates a V_dyn=885 V across the cathode to dynode gap, thus, increasing the photoelectron collection efficiency and peak to valley (P/V) ratio. This modification has been proven successful in increasing the PE collection efficiency and the single PE resolution. The dynode amplification chain also incorporates a 11 resistor in series with the -metal shield to eliminate the possibility of electric shock through careless handling; this high impedance also limits the current drawn, in the unlikely event of a complete dielectric breakdown between the shields and the aluminum parts of the detector. A schematic diagram of the electronic amplification chain is shown in figure.