698 TEXTBOOK OF ZOOLOGY 



Radioactive Atoms. — The nucleus of radium is unstable and in 

 changing to a more stable condition it emits radiation of two kinds. 

 These are particle and wave radiation. Particle radiations have mass 

 and vary in velocity. Alpha particles are positively charged (2 plus) 

 and have a mass of four. They are produced by nuclear disintegra- 

 tions. Beta particles are high speed electrons that have been removed 

 from atomic orbits. They carry a negative charge and have very 

 little mass. 



Neutrons have a mass equal to that of the nucleus of heavy hy- 

 drogen but carry no charge. These are produced by nuclear dis- 

 integrations. 



The cyclotron is a machine which swings particles, such as the 

 nuclein of heavy hydrogen, through an electric field increasing the 

 velocity of the particle at each cycle until it finally emerges with 

 a charge of several million volts. Such particles are able to remove 

 neutrons from metals such as berillium and are able to produce 

 artificial radioactivity in atoms such as phosphorus, sodium, carbon, 

 etc. Particles may also be accelerated to high velocities by static 

 generators such as the Van de Graaf type. 



Wave radiation or electromagnetic radiation is different from par- 

 ticle radiation in several respects. The units, photons, have no mass, 

 no charge, and travel at a constant velocity (equal to that of light). 

 The wave lengths, and some other characteristics, of the various radi- 

 ations are shown in Fig. 369. The frequency of the vibration is in- 

 versely proportional to the wave length. The amount of energy 

 associated with a photon is termed its quantum. The quanta of short 

 wave length radiations are large whereas those of long wave length 

 are relatively small. 



Roentgen radiation is produced by bombarding a metal target 

 with high speed electrons in a vacuum. If the voltage of the elec- 

 trons is high enough, it will remove electrons from an orbit near 

 the nucleus of the target atom {tungsten, for example). As an 

 outer electron drops into the vacated orbit near the nucleus it gives 

 up its excess energy in the form of a photon of roentgen radiation. 

 High voltage roentgen radiation will penetrate iron and copper to 

 a depth of several millimeters but low voltage x-rays will be stopped 

 by even thin sheets of aluminum. 



Enough energy can be given to atoms by heat or electricity to 

 cause them to emit radiation resulting from rearrangement of outer 



