Calorimeter, Ionization

an instrument for determination of the particle energy of cosmic rays (~ 10¹¹ electron volts [eV] and higher). In an ionization calorimeter the energy of a cosmic-ray particle is absorbed by a thick layer of material (in a fashion similar to the way in which heat is absorbed in an ordinary calorimeter). During interaction with matter, as a result of nuclear reactions, high-energy cosmic-ray particles generate a large number of secondary particles or photons, which in turn form new particles. The ultimate result of these events is the formation of an avalanche of charged particles, which moves within the material, ionizes its atoms, and thereby loses its energy. If the thickness of the layer of absorbent material is sufficiently great, and if the avalanche of charged particles is completely retained by the material, then the number of ions generated is proportional to the energy of the primary cosmic-ray particle. To measure the total number of ions, the absorber, which is made of a dense material (usually iron or lead), is divided into a number of layers several centimeters thick, and ionization chambers are placed between the layers.

The ionization calorimeter was invented in 1954 in the USSR, and it has come to be widely used, both in the USSR and abroad, in studies of the interaction of high-energy cosmic-ray particles (10¹¹—1013 eV) with atomic nuclei. In this case, the calorimeter is usually combined with instruments that make possible observation of the results of the interaction, such as cloud chambers, nuclear photographic emulsions, and spark chambers.

Typical dimensions of ionization calorimeters are as follows: height, 1.5-2.0 m; area of cross section, ~ 1 sq m; weight, 10-20 tons. A unique ionization calorimeter with an area of 10 sq m and a weight of 70 tons was constructed at the high-altitude station on Mount Aragats (Armenia, USSR) in 1964. Ionization calorimeters have also been used by the USSR in the large Proton space stations.