Computers at CERN

The popular picture of a biologist shows him wearing a white coat, peering through a microscope. If it is at all possible to draw a popular picture of an experimental high-energy physicist, he will probably be sitting at his desk and looking at his output from the computer. For the computer is increasingly becoming the tool by which raw experimental results are made intelligible to the physicist.

By one of those apparent strokes of luck that occur so often in the history of science, electronic digital computers became available to scientists at just that stage in the development of fundamental physics when further progress would otherwise have been barred by the lack of means to perform large-scale calculations. Analogue computers, which had been in existence for a number of decades previously, have the disadvantage for this work of limited accuracy and the more serious drawback that a more complicated calculation needs a more complicated machine. What was needed was the equivalent of an organized team of people, all operating desk calculating machines, so that a large problem in computation could be completed and checked in a reasonable time. So much was this need felt, that in England, for example, before digital computers became generally available, there was a commercial organization supplying just such a service of hand computation.

At CERN the requirement for computing facilities in the Theory Division was at first largely satisfied by employing a calculating prodigy, Willem Klein. Mr. Klein is one of those rare people who combine a prodigious memory with a love of numbers, and it was some time before computers in their ever-increasing development were able to catch up with him. He is still with us in the Theory Division, giving valuable help to those who need a quick check calculation. It is of interest to note, however, that he has now added a knowledge of computer programming to his armoury of weapons for problem solution!

The first computer calculations made at CERN were also done in the very earliest days of the Organization. Even before October 1958, when the Ferranti Mercury computer was installed, computer work had been sent out to an English Electric Deuce in Teddington, an IBM 704 in Paris and Mercury computers at Harwell and Manchester. Much of this work was concerned with orbit calculations for the proton synchrotron, then being built.

We have now reached a stage at which there is hardly a division at CERN not using its share of the available computer time. On each occasion that a new beam is set up from the accelerator, computer programmes perform the necessary calculations in particle optics as a matter of routine; beam parameters are kept in check by statistical methods; hundreds of thousands of photographs from track-chamber experiments are ‘digitized’ and have kinematic and statistical calculations performed on them; the new technique of sonic spark chambers, for the filmless detection of particle tracks, uses the computer more directly. In addition, more than 80 physicists and engineers use the computer on their own account, writing programmes to solve various computational problems that arise in their day-to-day work.

There are now two computers at CERN, the original Ferranti Mercury and the IBM 7090, a transistorized and more powerful replacement for its predecessor, the IBM 709. The 7090, in spite of its great speed (about 100 000 multiplications per second!), is rapidly becoming overloaded and is to be replaced towards the end of this year by a CDC 6600, which at present is the most powerful computing system available in the world.

It is hoped that this new machine will satisfy the computing needs of CERN for upwards of five years. The Mercury computer is now being used more and more as an experimental machine and there is, for instance, a direct connexion to it at present from a spark-chamber experiment at the proton synchrotron. Calculations are performed and results returned to the experimental area immediately, giving great flexibility.

Track-chamber photographs

Among the biggest users of computer time are the various devices for converting the information on bubble-chamber and spark-chamber photographs, usually on 35-mm film, into a form in which the tracks of the particles can be fitted with curves and the entire kinematics of an event subsequently worked out. To this end, from the earliest days of CERN, IEPs (instruments for the evaluation of photographs) [rumour once had it that IEP stood for ‘instrument for the elimination of physicists’!] have been built and put into use. These instruments enable accurately measured co-ordinates of points on a track, together with certain identifying information, to be recorded on punched paper tape. Their disadvantage is that measuring is done manually, requires skill and, even with the best operator, is slow and prone to errors. The paper tapes produced have to be copied on to a magnetic tape, checking for various possible errors on the way, and the magnetic tape is then further processed to provide in turn geometric, kinematic and statistical results.

It was recognized at an early stage, both in Europe and in the United States, that for experiments demanding the digitization of very large numbers of pictures, for example those requiring high statistical accuracy, some more-automatic picture-reading equipment would be needed. Two such devices are now coming into use at CERN. One, the Hough-Powell device (known as HPD), developed jointly by CERN, Brookhaven, Berkeley and the Rutherford Laboratory, is an electro-mechanical machine of high precision, which still requires a few pilot measurements to be made manually, on a measuring table named ‘Milady’, when used for bubble-chamber pictures. It has already been used in one experiment, for the direct processing of 200 000 spark-chamber photographs (for which the pilot measurements are unnecessary). The other device is called ‘Luciole’, a faster, purely electronic machine, although of lower precision, specially developed at CERN for digitizing spark-chamber photographs.

With the sonic spark chamber, the position of the spark between each pair of plates is deduced from the time intervals between its occurrence and the detection of the sound by each of four microphones. Arrays of such devices can be connected directly to the computer, thus dispensing with the taking, developing and examination of photographs.

The study of dynamics of particles in magnetic and electric fields gives rise to another important family of computer programmes. The electron storage ring, or beam-stacking model, required the writing of a programme that followed the motion of individual batches of particles during their acceleration and stacking in the ring. A previous study by the same group resulted in a series of programmes to examine the behaviour and stability of a proposed fixed-field, alternating-gradient stacking device. Various aspects of the performance of the linac (the linear accelerator that feeds the PS) have been studied and improved using the computer, and the later stages of the design of the PS itself involved a detailed computer simulation of the beam in the ring, including the various transverse or ‘betatron’ oscillations to which it is subjected.

There also exists a series of particle-optics programmes used for the design and setting up of particle beams, particularly the ‘separated’ beams producing particles of only one kind. These are ‘production’ calculations, in the sense that the programme is run with new parameters every time there is a major change in beam layout in any of the experimental halls.

Computer language

At first, a major bar to the use of computers for small, but important, calculations was the difficulty of programming them in their own special ‘language’ to solve the specific problem in hand. What is the use of a machine that can perform a particular calculation in a minute, the would-be-user asks, if it will take a month to provide the programme for the calculation? Given a hand calculating machine, a pencil and paper, and a quiet room, I can do it myself in three weeks! This valid argument limited the use of computers to two kinds of calculation: those too long to be performed by hand, and those that had to be carried out so often that the original effort of producing the programme was justified.

This situation was rectified by the use of ‘programming languages’, which make it possible to express one’s problem in a form closely resembling that of mathematics. Such languages, if defined rigorously enough, express the problem unambiguously, and they can be translated automatically (by the computer) into the instructions for a particular computer. Two such languages have been used at CERN: Mercury Autocode, and Fortran. The use of Mercury Autocode has recently been discontinued, but until a short time ago many physicists used both languages with great success to express their computational problems in a form directly comprehensible by a computer. Courses in the Fortran language, given both in English and French, are held regularly, and usually last about three weeks. Such ‘compiler languages’, as they are called, used to be considered a rather inferior method for programming computers, as the translations obtained from them often used the machine at a low efficiency, but it is now recognized that the advantage of writing programmes in a language that can be translated mechanically for a number of different computers far outweighs a small loss in programme efficiency.

In the Theory Division, computer programmes are often written by individual theorists to check various mathematical models. Having devised a formula based on a novel theory, the physicist computes theoretical curves for some function that can be compared with experimental results.

Operation

The Data Handling Division, which actually has CERN’s central computers under its charge, contains a number of professional programmers, mostly mathematicians by training. Some of these are responsible for the ‘systems programmes’, that is, the Fortran compiler and its associated supervisor programme. Some have the job of disseminating programming knowledge, helping individual users of the computer and writing programmes for people in special cases. Others are semi-permanently attached to various divisions, working on particular experiments as members of the team.

A small number of mathematicians are also engaged in what might be called ‘specialist computer research’, covering such things as list-programming languages and methods of translating from one programme language into another. Such work might be expected to yield long-term profits by giving increases in computing power and efficiency.

As at all large computing installations, computer programmers at CERN do not operate the machine themselves

As at all large computing installations, computer programmers at CERN do not operate the machine themselves. Data and programmes are submitted through a ‘reception office’ and the results are eventually available in a ‘computer output office’, leaving the handling and organizing of the computer work-load, and the operating of the machine, to specialist reception staff and computer operators in the Data Handling Division.

What of the future of computers at CERN? In a field as new as this, predictions are even more dangerous than in others, but it is clear that the arrival of the new computer at the end of the year will cause a great change in the way computers are used. Having ten peripheral processors, each of which is effectively an independent computer, the machine may have many pieces of equipment for data input and output attached to it ‘on-line’. The old concept that a computer waiting for the arrival of data is standing idle, and that this is therefore wasteful and expensive, need no longer be true. With the new system, a computer that is waiting for new data for one problem is never idle, but continues with calculations on others. Every moment of its working day is gainfully employed on one or other of the many problems it is solving in parallel. Even so, there will still be a need for a number of smaller computer installations forming part of particular experiments.

As M.G.N. Hine, CERN’s Directorate Member for Applied Physics, pointed out at a recent conference, even with the growth of such facilities, the amount of computing time available may one day dictate the amount of experimental physics research done at CERN, in much the same way as the amount of accelerator time available dictates it now.