The measurement of the energy of cosmic rays I—The electro-magnet and cloud chamber

Abstract

1-T h e C o n s t r u c t io n a n d P e r f o r m a n c e o f t h e E l ec t r o -M a g n etThe energy o f the cosmic ray particles has been determined from the curvature o f their tracks in a strong magnetic field by Kunze,* and by Anderson.f Kunze used a power of 500 kw in a copper solenoid weighing 1100 kg to give a magnetic field of 18,400 gauss over a chamber 16-4 cm in diameter. Anderson used an electro-magnet with heavy watercooled copper coils and a relatively light iron yoke. A power of 440 kw gave a field o f 15,000 gauss over a chamber 16*5 cm in diameter, the actual length o f the tracks photographed being about 12 cm.In order to obtain a similar performance without the use of such a very large amount o f electric power, an electro-magnet has been con structed o f a more conventional design, that is with an iron yoke which is heavy compared with the weight of the copper coils. The iron yoke weighs about 8000 kg and the copper coils 3000 kg. Figs. 1 and 1 show the detail o f the design, and fig. 2 shows a photograph of the magnet in use with the cloud chamber and subsidiary apparatus. The diameter of the pole face is 25 cm, and the gap can be varied from 5 to 20 cm by sliding one pole piece along the baseplate by means o f a screw.The magnet is very efficient from the point o f view of power consumption as it enables tracks 17 cm long to be photographed in a field of 14,000 gauss using a power of only 25 kw. Apart from the measurement of the cosmic ray energies, the main object o f the magnet and chamber was to study the showers, and in particular, to try to obtain showers originating in the gas. Since showers are of rare occurrence the apparatus must be run for long periods, and so the low power consumption of such a magnet, compared with that o f a solenoid, becomes a very great advantage.The three parts of the magnet itself, the two pole pieces and the endplate, were made of cast steel with the magnetic properties shown in Table I.