NEUTRON EFFECTS ON ANIMALS 17 



creases with particle energy, and depends almost entirely on absorber 

 density. For high energy beta radiation the range is several grams per 

 sq. cm. of absorbing material. 



The products of beta particle absorption are chiefly electrons of lower 

 energy, which in turn produce electrons and ions of successively lower 

 energies down to magnitudes of one electron volt or fraction thereof. 



Positrons also produce gamma rays of 511,000 electron volts (two quanta 

 per particle). 



D. Alpha radiation consists of alpha particles (helium nuclei — hence 

 doubly ionized helium). These particles have a mass of 6.6 X 10~-* grams. 

 The term is generally applied to high energy particles (4 to 7 million volts) 

 of the radioactive elements. These particles have a very short range, but 

 produce heavy ionization along their path. The specific ionization is al- 

 most constant along the path of the particles, and much heavier than that 

 along the path of X-rays or beta particles. 



E. Neutrons are uncharged particles with a unit mass of 1.66 X 10~^* 

 grams. Fast neutrons are generally considered to be those with energies 

 from 0.1 to approximately 15 million volts. As neutrons have no charge, 

 they can lose energy only by direct impact on nuclei. High energy neu- 

 trons pass through considerable masses of material without much absorp- 

 tion or energy loss. Their penetration is comparable to that of very high 

 energy X-rays or gamma rays. However, the nature of neutron absorption 

 differs radically from the absorption of X (and gamma) rays. While the 

 absorption of X-rays increases with density of the absorber, that of neu- 

 trons increases with density of nuclei in the absorber. Materials such as 

 water and paraffin with their large concentration of hydrogen nuclei are 

 good neutron absorbers. 



When fast neutrons lose some of their energy in a relatively thin ab- 

 sorber, they do so mainly by the production of recoil nuclei. These are 

 charged nuclei of the irradiated material which have received a large por- 

 tion of the energy of the incident neutron. The recoil nuclei in turn pro- 

 duce heavy ionization along their short paths. In animal tissues, these 

 nuclei are mainly hydrogen, oxygen, and carbon ions. Their behavior is 

 therefore comparable to that of alpha particles. In materials predominat- 

 ing in heavy elements, the recoil nuclei will be heavier and have shorter 

 paths with denser specific ionization. 



