From the June 1973 issue

Scanning tables at CERN

Photographs of tracks made by high-energy charged particles passing through a bubble chamber constitute one of our most direct means to access the sub-nuclear world. No other detection technique gives such a satisfying portrayal of particle interactions.

The full measurement of an “event” – track images recorded on stereoscopic photographs – consists of geometrical and kinematic analyses, determining the number, type, momentum and energy of the particles in the interaction.

This involves reconstructing the spatial geometry of the tracks and applying the laws of conservation of energy and momentum to the initial and final particles. First, however, the film needs to be examined (scanned) to see whether any events of interest occurred.

Scanning tables consist, essentially, of an optical-projection system. They allow an operator to select interesting events and to record data (on magnetic tape, for example) to be fed to measuring devices.

Scanning can be difficult because what is seen of a track on the film does not often, by itself, identify the particle that caused it, and because uncharged particles leave no tracks and yet can be an essential part of the information required. It is a relatively straightforward operation for a human being. However, it has proved very difficult to implement an exclusively automated process.

• Compiled from texts on pp186–188.

Compiler’s Note

A la recherche du temps perdu!

Mechanical gadgets are intrinsically fascinating – witness the popular appeal of the kinetic art of Swiss sculptor Jean Tinguely – and physicists with an engineering bent, a flair for computing and unbridled ingenuity found the challenge of constructing scanning machines irresistible. Given the dearth of female physicists in labs at the time and the fact that most scanners were “girls”, computer-aided data analysis often gave rise to computer-aided dating. Many a romance began in the purlieu of a scanning room, sometimes blossoming into a lifelong relationship, sometimes ending in heartbreak. There must be several senior readers who can confirm this!

About the author

Compiled by Peggie Rimmer.