128 RADIATION GENETICS 



suitable for isolated investigators. Anyone anticipating research activity in the field of 

 mammalian radiation genetics is well advised to enlist the consulting services of a 

 radiological physicist and a radiobiologist. 



RADIATION PARAMETERS 



As intimated above, there is a wide array of radiation sources available for experi- 

 mental use; and the source or sources should be carefully chosen for their general 

 applicability and, in some studies, for their use as a simple means of inducing injury. 

 The methodology of exposure is also of extreme importance. This involves the correct 

 selection of total doses, dose rates, and other temporal factors that will be discussed 

 below. Before enumerating radiation parameters, it should be stated that accurate 

 and uniform measurement of the absorbed dose is essential. The techniques and 

 instrumentation of modern radiation dosimetry are sometimes quite specialized and the 

 experimental biologist should not hesitate to ask for outside assistance. The best bio- 

 logical measures are of little value when the physical parameters are uncertain or 

 inaccurate. Considerable insight into the questions of dosimetry and radiation 

 sources may be obtained by referring to Hine and Brownell, 582 Glasstone, 438 Fano, 346 

 and Marinelli and Tavlor. 850 



SOURCES OF RADIATION 



A' and gamma radiations. — These need little description as they have been the 

 principal sources of radiation for most genetic studies. A wide range of energy levels 

 is available for both radiations, but, for mammalian studies, it is advisable to avoid 

 energies below 1 20 kev in order to avoid the problem of nonuniformity of absorbed dose. 476 



When energies appreciably above 500 kev are employed, the specific ionization or 

 linear energy transfer per unit length of the ionization track declines ; and a factor of 

 relative biological effectiveness, the RBE, may be needed to provide comparative 

 analysis with data obtained from more conventional deep therapy X-ray units of 200- 

 300 kev. The most economical gamma-ray sources available are in the higher energy 

 class: cesium-137 and cobalt-60 with photon energies of 0.66 Mev and 1.25 Mev, 

 respectively. 



Neutrons. — Neutron energies vary from 0.025 ev for thermal neutrons to over 20 

 Mev for fast neutrons. Linear accelerators can provide reasonably monochromatic 

 energies, whereas the neutron-energy spectrum from the fissioning of uranium-235 is 

 extremely heterogeneous and has an average energy of about 2.5 Mev. Details of the 

 spectrum can be obtained in Lapp and Andrews. 758 The RBE for neutrons is extremely 

 variable and for most biological effects reaches a maximum value at energies of 2-4 

 Mev. The RBE itself may vary with the biological endpoint and all too little is known 



