274 Applied Biophysics 



radon concentration of 10"^^ curie per liter in the at- 

 mosphere produces only 0.00008 X 10*^ ions per cubic centi- 

 meter per second. 

 4. The air of the Joachimstal mines contains from 20 X 10'^^ 

 to 60 X 10"^^ curie of radon per liter, and occasionally as 

 much as 200 X 10"^-* curie per liter has been measured. Yet 

 lung carcinoma among the miners is attributed to the dusts 

 of arsenic and chromium, and not to the radon. 



These conflicting facts indicate that much more evidence is 

 required before the tolerance doses for radium in the body and 

 for radon and radium dust in the air of the workshop can be 

 regarded as satisfactory. 



Neutrons 



There is another type of ionizing radiation, the neutron, 

 against which adequate protection must be found. The neutron 

 is approximately the same size as the proton (the nucleus of the 

 hydrogen atom), and if the two collide, the neutron surrenders 

 a large part of its energy to the proton, which recoils along a 

 short path. Neutrons are thus effectively slowed down in 

 hydrogenous material, such as tissue. The recoiling protons 

 produce ions in the tissue, the ion density along the proton track 

 being far more intense than along the tracks of the electrons 

 which are liberated in tissue by the passage of X- or gamma rays. 



Comparisons have l)een made of the biological effects of X-rays, 

 alpha rays, gamma rays, and neutrons.'^' ^^ These raise the prob- 

 lem of the measurement of neutron doses. Since neutrons liberate 

 far more ions in tissue than in the same mass of air, it is not 

 possible to measure neutron doses directly in rontgens. The 

 accepted practice is to define an "equivalent rontgen" of neutrons 

 as the dose which produces the same number of ions per unit 

 volume of tissue as a dose of 1 rontgen of X- or gamma radiation. 

 On this basis, it is found that the ratio of gamma-ray energy to 

 the neutron energy required to produce a biological reaction 

 varies from about 1.5 to 9, according to the reaction studied. 



