MAMMALIAN RADIATION GENETICS 129 



about this factor for induction of mutation in mammalian systems, although consider- 

 able work has been done with Tradescantia and Drosophila. 222 - 317 More accurate 

 knowledge of the nonlinear relation between RBE and energy is critical, since the 

 maximum RBE is in the region of the mean energy for fission neutrons. 



Unfortunately, neutron sources are generally expensive and require careful 

 monitoring; thus, they are not widely available. On the other hand, the increasing 

 number of training reactors on college campuses may improve neutron source avail- 

 ability for genetic studies. A complete discussion of neutron physics and neutron 

 sources is given in Glasstone 438 and Lapp and Andrews. 758 



Cosmic radiation. — As man prepares to enter the new and rather exciting scientific 

 era of space flight, considerable increase in interest in the estimation of the genetic 

 effects of cosmic radiation can be expected. There are two radiation belts, the Van 

 Allen belts, held in the earth's magnetic field. The outer belt is composed of com- 

 paratively low-energy electrons, predominantly in the 20 to 100 kev range, which can 

 be easily shielded out with aluminum. However, the production of soft secondary 

 X rays may still present some difficulties. The inner belt, which comes to within 600- 

 700 miles of the earth's surface, contains extremely high-energy protons. The energy 

 spectrum is not fully known, but ranges up to at least 700 Mev. Present data suggest 

 that the dose rate can reach 10 r/hour at altitudes of about 1,600 miles and 10,000 miles 

 for the inner and outer belts. 



In addition to this trapped radiation, there are the primary cosmic particles. 

 These are composed of about 85 per cent protons, 15 per cent helium atoms, and less 

 than 1 per cent heavy nuclei such as carbon, calcium, iron, and oxygen. The cosmic 

 primaries go into the billion-electron-volt energy range and can penetrate deep into 

 tissue with a very dense ionization track. The iron nucleus, for example, has a maxi- 

 mum ionization density of 100,000 ion pairs per micron and can produce a dose to an 

 individual cell of over 1,000 rep. A comprehensive recent review of space radiation 

 has been presented by Schaefer. 1161 



Certainly, very few humans will be subjected to these radiations, but their ability 

 to induce severe damage all along the ionizing pathway, their unknown RBE values, 

 and our inability to duplicate the very high-energy heavy nuclei with earthbound 

 machines present an intriguing set of problems. Some progress is being made with 

 high-energy linear-accelerator beams of stripped nuclei and deuterons at the Lawrence 

 Radiation Laboratory, University of California, and at Brookhaven National Labora- 

 tory. 201 ' 234 



Internal emitters or internally deposited radioisotopes. — The vast majority of radio- 

 nuclides present no unique genetic problem. Those nuclides with an affinity to bone, 

 for example, are of little genetic concern. Gamma-emitting isotopes that seek the soft 

 tissues, such as cesium- 137, can be treated as any typical external radiation source. 

 The more difficult problems arise from those isotopes that can become incorporated 

 into the genetic materials. Tritium, carbon- 14, and phosphorous-32 are good ex- 

 amples and all emit a beta particle without an associated gamma-photon emission. 



