Chapter 2 



COMPARISON OF PHYSICAL PROPERTIES OF FAST NEUTRONS 

 WITH THOSE OF SOME OTHER FORMS OF RADIATION 



By T. ENNS 



A. Ultraviolet rays are electromagnetic waves travelling at the velocity 

 of light. The photon energy* is several electron volts. For any homo- 

 geneous material placed in their path, electromagnetic waves are absorbed 

 exponentially: if a given thickness of material absorbs half of the incident 

 radiation, an additional equal thickness of material absorbs half of the 

 remaining radiation. However, the relative absorption differs greatly with 

 the chemical composition of the absorber, as the energy of the photons 

 falls within the range of organic and inorganic molecular binding energies. 

 For the same reason the relative absorption differs greatly when the photon 

 energy (and hence the wave length) of the incident light is changed. 



B. X-rays and gamma rays are also electromagnetic waves, but have 

 photon energies much greater than those of ultraviolet rays. High energy 

 rays may have photon energies of several million electron volts. In recent 

 terminology the names X-rays and gamma rays more often indicate the 

 source of radiation rather than its nature. In general, gamma rays have a 

 lower energy limit of approximately 100,000 electron volts. X-ray ab- 

 sorption, for energies below 100,000 electron volts, depends on the exact 

 energy as well as the elements (but not the chemical composition) of the 

 absorber. The absorption of higher energy X-rays and gamma rays de- 

 pends almost entirely on density of the material and radiation energy. 

 Radiation of higher energy requires more material to effect a given reduc- 

 tion in intensity. The processes of absorption are quite complex, tut most 

 of the energy is lost in the production of X-rays and electrons of lower 

 energies which in tiun produce electrons and ions of still lower energies, 

 the process being continued down to energies of one volt or less. 



C. Beta radiation consists of beta particles. These may be either elec- 

 trons or positrons. Their rest mass is 9 X 10~-^ grams and their velocity 

 increases with their energy, but is always less than the ^•elocity of light. 

 Beta particle spectra from radioactive elements have peak energies between 

 0.05 and 20 million electron volts. Beta particles are not absorbed ex- 

 ponentially as is electromagnetic radiation, but have a definite absorber 

 range beyond which no beta radiation may be detected. This range in- 



12343 . . ° 



* Photon energy in volts = where X is the wave length in A. 



A 



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