Particle Detectors

I

Introduction

Particle Detectors, instruments used to detect and study fundamental subatomic particles (see Atom; Nuclear Energy). These detectors range in complexity from the well-known portable Geiger counter to installations several storeys high, consisting of several kinds of device.

II

Ionization Chamber

One of the first detectors to be used in nuclear physics was the ionization chamber, which consists essentially of a closed vessel containing a gas and equipped with two electrodes at different electrical potentials. The electrodes, depending on the type of instrument, may consist of parallel plates or coaxial cylinders, or the walls of the chamber may act as one electrode and a wire or rod inside the chamber as the other. When ionizing particles or radiation enter the chamber they ionize the gas between the electrodes. The ions that are thus produced migrate to the electrodes of opposite sign (negatively charged ions move towards the positive electrode, and vice versa), creating a current that may be amplified and recorded by means of electronic circuits.

Ionization chambers adapted to detect individual ionizing particles of radiation are called counters. The Geiger-Müller counter is one of the most versatile and widely used instruments of this type. It was developed by the German physicist Hans Geiger from an instrument first devised by Geiger and the British physicist Ernest Rutherford; it was improved in 1928 by Geiger and by the German-American physicist Walther Müller. The counting tube is filled with a gas or a mixture of gases at low pressure, the electrodes being the thin metal wall of the tube and a fine wire, usually made of tungsten, stretched lengthwise along the axis of the tube. A strong electric field maintained between the electrodes accelerates the ions; these then collide with atoms of the gas, detaching electrons and thus producing more ions. When the voltage is raised sufficiently, the rapidly increasing current produced by a single particle sets off a discharge throughout the counter. The pulse caused by each particle is amplified electronically and then actuates a loudspeaker or a mechanical or electronic counting device.

III

Track Detectors

Detectors that enable researchers to observe the tracks that particles leave behind are called track detectors. Spark and bubble chambers are track detectors, as are the cloud chamber and nuclear emulsions. Nuclear emulsions resemble photographic emulsions but are thicker and not as sensitive to light. A charged particle passing through the emulsion ionizes silver grains along its track. These grains become black when the emulsion is developed and can be studied with a microscope.

A

Cloud Chamber

The fundamental principle of the cloud chamber was discovered by the British physicist C. T. R. Wilson in 1896, although an actual instrument was not constructed until 1911. The cloud chamber consists of a vessel several centimetres or more in diameter, with a glass window in the top and a movable piston forming the lower side. The piston can be dropped rapidly to expand the volume of the chamber. The chamber is usually filled with dust-free air saturated with water vapour. Dropping the piston causes the gas to expand rapidly and causes its temperature to fall. The air is now supersaturated with water vapour, but the excess vapour cannot condense unless ions are present. Charged nuclear or atomic particles produce such ions, and any such particles passing through the chamber leave behind them a trail of ionized particles (see Ionization) upon which the excess water vapour will condense. This makes the course of the charged particle visible as a line of tiny water droplets, like the vapour trail left by an aeroplane. These tracks can be photographed and the photographs then analysed to provide information on the characteristics of the particles.

Because the paths of electrically charged particles are bent or deflected by a magnetic field, and the amount of deflection depends on the energy of the particle, a cloud chamber is often operated within a magnetic field. The tracks of negatively and positively charged particles will curve in opposite directions. By measuring the radius of curvature of each track, its velocity can be determined. Heavy nuclei such as alpha particles form thick and dense tracks, protons form tracks of medium thickness, and electrons form thin and irregular tracks. In a later refinement of Wilson’s design, called a diffusion cloud chamber, a permanent layer of supersaturated vapour is formed between warm and cold regions. The layer of supersaturated vapour is continuously sensitive to the passage of particles, and the diffusion cloud chamber does not require the expansion of a piston for its operation. Although the cloud chamber has now been supplanted almost entirely by later devices, it was used in making many important discoveries in nuclear physics.