The digitization of streamer chamber tracks using a standard 625 line T.V. camera

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T H E D I G I T I Z A T I O N OF STREAMER CHAMBER TRACKS USING A STANDARD 625 LINE T.V. CAMERA D . J . MARTIN, A. M. MacLEOD, K. M. SMITH,

University of Glasgow, Glasgow, Scotland and M . D . ROUSSEAU

Daresbury Laboratory, Daresbury, Nr. Warrington, Cheshire, England Received 23 August 1976 A description is given of a simple device for digitizing streamer chamber tracks which have first been video-recorded using an unmodified high sensitivity T.V. camera.

The possibility of using a T.V. camera to digitise track chamber events has led to the development of sophisticated computer-controlled systems, such as the Omega spectrometer systemt). An alternative and less expensive approach to digitising simple views, e.g. oscilloscope traces, is suggested by Paiss e t al.2), using a slightly modified television camera interfaced to a mini-computer. We describe here a system originally designed to digitise streamer chamber tracks, using a standard 625-1ine T.V. camera of high sensitivity which is interfaced to a mini-computer3). This system had the distinct advantage of using an unmodified camera; further, it was possible to use a standard video-tape

recorder to store permanently the camera output for subsequent reprocessing. Tracks of cosmic rays were produced in a streamer chamber which had an active volume of 230 mm diameter and 50 mm inter-electrode gap. The chamber was filled with 70% neon and 30% helium to a total pressure of one atmosphere. To detect cosmic rays, the chamber was mounted with the electrodes in a vertical plane, and a coincidence between pulses from scintillation counters placed above and below the chamber triggered the high voltage pulse from a Marx generator and Blumlein pulse shaping line. The chamber was viewed by an image isocon camera, type t

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Fig. 2. Block diagram of the synchronisation separator and streamer chamber digitizing circuitry. P880X, manufactured by the English Electric Valve Co., and the camera output was recorded on a conventional video recorder, the sound track being used to mark those frames with cosmic ray tracks. In order to use an unmodified T.V. camera, it is necessary that the interlace pattern of the T.V. scan remain constant, i.e. that the period of the vertical scan be exactly 625/2= 312.5 times as long as the horizontal (or line) scan period. Under this condition, the position of the scanning beam is specified in our system by the line number and the time since the beginning of the current line scan. The digitiser thus has to extract three features from the composite T.V. waveform (fig. 1), which contains vertical and horizontal synchronisation pulses as well as amplitude information, namely 1) the centres of the track video signals, 2) the beginning of each line, and 3) the line number. A block diagram of the digitiser is shown in fig. 1. The composite waveform is decomposed in the 'waveform separator' into the synchronisation pulses, and the amplitude information (the track signals). Standard T T L components are used in the 'line detector' to recognise the beginning of each line scan and to determine the line number from a count of the synchronisation pulses. The field synchronisation pulse, marking the beginning of each vertical scan, is detected as a 'low' in the synchronisation signal which is maintained for longer than the line synchronisation pulse length. By requiring that the first such low level occurred during the second half of a horizontal scan, then the first line detected was 'line 3', at which time the rest of the counting circuit was enabled. The beginning of the line scan, detected as a 'low' to 'high' transition in the synchronisation signal, started

the clock count: counting was stopped on the detection of the track signal by a zero cross-over circuit, or at the end of the line scan. As in other digitizing circuits (e.g. those of refs. 2 and 3), the track position is obtained from the number of clock pulses counted in the interval between the start of the line scan and the detection of the track centre (see fig. 2). The clock used is a 15 MHz, crystalcontrolled oscillator, and the clock pulses are counted in a 12-bit scaler. The contents of the scaler are transferred at the end of each line to a 12-bit latch, ready to be read by the mini-computer during the next line scan. After a delay of 400 ns, the scaler is reset. An assembler-language programme was written for the mini-computer, which transferred one coordinate per scan line from the latch into sequential locations in the computer memory. The programme was designed to read the digitisations from up to 625 lines and then to punch the coordinates onto paper tape. In the present tests, 256 lines were sufficient to cover the chamber image, at a demagnification of 10. The coordinates were not corrected for either pin-cushion distortions or optical distortions in the lens system, as was done by Paiss et al.2).However, most of the tracks passed through the centre of the chamber, and the expected distortions are small. On lines where no track signal was detected, a count equal to the total number of clock pulses during that line scan was recorded. By observing the variation of this number for a set of blank lines, an estimate can be made of the stability of all signals other than the track signals. Using the digitiser connected directly to the T.V. camera, two different counts were recorded, viz. n and n + 1, where n ~ 900, depending on which camera was used. This result is consistent with the use of an asynchronous clock. With output from the video-tape recorder, three successive counts ( n - 1, n and n + 1),

DIGITIZATION OF STREAMER CHAMBER TRACKS were obtained. This is equivalent to an error of ~ 0 . 1 % of the width of the scanned area. Straight-line fits to track segments produced a m e a n residual of 1.5 clock counts, or 1.2 m m in the streamer chamber4). I n conclusion, it is d e m o n s t r a t e d that simple views of d u r a t i o n 1 ms can be digitised using an unmodified T.V. camera. The use of videotape for the storage of the camera o u t p u t permits the recording o f data in one location a n d its analysis in another, a n d avoids possible problems due to limited availability of a suitable mini-computer. Additionally, image storage provides the possibility of multiple processing of more complex

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images (for example with more t h a n one track), if suitable modifications are made in the electronics.

References 1) j. Garvey et al., in Proc. Conf. on Instrumentation for high energy physics, Frascati (1973). 2) y. Paiss, H. Szichman and M. Rosmann, Nucl. Instr. and Meth. 131 (1973) 323. 3) Data General Corp. NOVA. 4) Similar results have been reported by J. Badier et al. (Internal Report, LPNHE, Ecole Polytechnique, Paris), using a more sophisticated s2~stem.The use of T.V. cameras to view streamer chamber tracks has also been described by F. Cesarc,ni, U. Florean and M. Severi (University of Rome internal report), and by A. Huber and A. Ladage (DESY, Hamburg) in unpublished work.