A 3-stage 80:7 mm Image Intensifier Combination for the CERN UA2 Scintillating Fibre Detector

A 3-stage 80:7 mm Image Intensifier Combination for the CERN UA2 Scintillating Fibre Detector L. BOSKMA, A. BAKKER, J. A. C. COCHRANE and K. W. J. STOOP Devt Electronische Produkten,Roden. Holland

INTRODUCTION At CERN, interactions between protons and antiprotons are observed with

a number of new systems (Ansorge, 1987). One of the main motivations for

physicists is the search for certain electrons as a signature for production of top quarks. Protons and antiprotons travel in opposite directions through a beam pipe. A 2 m long, cylindrical detector, consisting of 24 layers of 1 mm diameter scintillating-plastic fibres (a total of about 60 000 fibres) encompasses the zone of collision. A resulting electron, passing through these fibres, causes scintillations forming a track with a wavelength peaking at 440 nm. At both ends the fibres are brought together in 16 bundles, 32 in total, each bundle being coupled to an image intensifier chain. These image intensifiers transfer the light information to CCDs. This article concerns the work involved in the development at DEP of these image intensifier chains.

THESYSTEM The intensifier chains had to be gateable together with specific requirements for photon gain, resolution, decay times and frame magnification. In close cooperation with scientists from CERN, an image intensifier chain was envisaged consisting of three tubes (Fig. 1). The first stage is an 80: 16 type (that is to say, with an 80 mm input diameter and a 16 mm output diameter), electrostatically focused demagnifying tube. The second stage is an 18 : 18 second-generation tube. The third stage is an 18 :7 first-generation tube. All three tubes have fibre-optic input and output windows. The spectral response of the photocathode of the 80: 16 tube has to match that of the scintillating fibres (Fig. 2). For the coupling between the first and the second stages, and for the coupling between the second and the third stages, a combination of P-47 fast blue phosphor and an S-20 photocathode was chosen (Fig. 3). With this combination a higher photon gain is achievable

35 ADVANCES IN ELECTRONICS A N D ELECTRON PHYSICS VOL. 74

Copyright 0 1988 Academic Press Limited All rights of reproduction in any form reserved ISBN 0-12-014674-6

FIG.1. The image intensifier chain.

50

n

I

Photocathode sensitivity

-

40

I

c

I

3 30-

\

-IE U

g 2cVI

d

fibres (a u 1

L

\ 1

h

Wavelength A ( nm )

FIG.2. Spectral responses of 80: 16 intensifier photocathode and scintillating fibres.

A-

50

40

-

2 30 I

E %

H 20

t

[r

10

0

\ 3 m

c

500

700

600

800

900

Wavelength X(nrn)

FIG.3. Spectral responses of S.20 photocathode and P.47 phosphor.

2kd-Bd-k Wavelength,X(nrn)

FIG. 4. Spectral responses of

P.46 phosphor and CCD.

38

L. BOSKMA E T A L .

TABLE I

Characteristics of individual tubes and the whole chain I Tube Frame magnification Cathode quantum efficiency (%) Photon gain Phosphor type Decay time (ns) Centre resolution at 50% MTF EBI (FIX)

I1

80: 16 18: 18 0.2 1 .o > 13 16 - 10 -800 Pa47 P.47 -50 - 50 > 27 >9 < 0.2 <0.2

I11

I+II+III

18: 18

0.39 16 -6 Pa46 -100 > 30 < 0.2

0.08

48000
than with a combination of a fast green P.46 phosphor and an S.25 photocathode. Ideally, the final output should spectrally match the CERN CCD. The best compromise available was a Pa46 phosphor (Fig. 4). Typical data for the individual tubes and for the chain, are given in Table I. The frame magnification of the complete chain is 0.08 reducing from 80 mm down to 6.5 mm. (CERN uses a somewhat smalfer rectangle in this 80 mm diameter.) Shown in the table are the quantum efficiencies of the photocathodes including their fibre substrates. These are equivalent to sensitivities of 50 to 65 mA W-I for 440 nm. The MCP is not run at its full voltage as there is already enough photon gain available, thus increasing the working life of the chain. It was required that the system be fast because CERN needs to gate immediately after an interesting event, allowing time to read the CCD without the image being spoilt by incoming light from further particle interactions. SPECIAL REQUIREMENTS

The DEP range of tubes includes the 80 : 18 tube as used for medical X-ray purposes in the Delcalex system. This standard tube uses glass input and output windows, so that a redesign was necessary to allow the use of fibreoptic plates. Also to change format from 80 : 18 to 80: 16, a recomputation of the electron optics was required. The dimensions of the second stage are basically identical to a DEP standard tube. The third stage required only a special fibre output to accommodate the CCD. The S.20 Photocathode

Though sensitivities at 440 nm of 60 mA W-l for the smaller tubes were

3-STAGE SO :7 MM IMAGE INTENSIFIER COMBINATION

39

achieved without difficulty, in the 80 mm tube the target of a minimum sensitivity over the complete input area of 45 mA W-’ required a great deal of investigation. It was found that the conditions of the process environment, such as substrate preparation and the wall materials, were more critical when growing a blue-sensitive cathode for the larger tubes than for the smaller tubes. Gain

In optimizing the screen efficiency, it was found that the classic sedimentation technique gave better results than the normal “brushing” technique, but also required that the optimum thicknesses of both the phosphor screen and the aluminium backing layer had to be found. A further improvement came from the use of fibre-optic plates from Francke Optics in Giessen (Germany) that have a significantly higher transmission for blue light. High- Voltage Problems

CERN wished to have both the input and the output planes of the chains at earth potential. This meant that there was 44 kV across two fibre-optics. This problem was not eased by the additional requirement that the cathode should not be exposed to the influences of high electric fields that could lead to deterioration in cathode stability. The solution chosen was to deposit a conducting layer on the outside of the cathode fibre-optic, electrically connected to the cathode flange. Thus there remain two fibre-optics, each with 22 kV across them, and this can lead to scintillations. To prevent these scintillations, the fibre-optics were aged. Finally, CERN required tighter mechanical tolerances than normal for a production tube. For example the parallelism of the output plane of the 80: 16, referred to the input plane should be better than 0.02 mm. ACKNOWLEDGEMENT

The authors wish to thank the many DEP employees for their help, particularly J. Hofman, H. Janssen and M. Zoethout.

REFERENCE Ansorge, R.E. (1987) CERN-EP/87-63 “Performance of a scintillatingfibre detector for the UA2 upgrade”