MEDICAL USES OF THE CYCLOTRON — SPEAR 149 



trating radiations, both at Berkeley and elsewhere (24). There 

 is, however, as yet no accepted physical unit by which neutron and 

 X- or gamma-ray dosage can be directly compared. If intensity 

 of radiation is measured in ion pairs per centimeter, then neutron 

 intensity is a thousand times that of gamma rays. 



At Berkeley the unit dose of neutrons is measured by means of a 

 Victoreen ionization chamber and the dose is described as 1 "n-unit" 

 when the Victoreen (calibrated for X-rays) records 1 roentgen. 7 

 Measured in these terms, it is reckoned that In is equivalent to about 

 2.5 r. (25). The crucial question, however, is whether neutrons can 

 bring about desirable biological reactions which cannot be obtained 

 with other types of radiation. Investigations on the effects of neu- 

 trons on cancerous growths in animals as well as human beings were 

 being made, up to the outbreak of war, to test this point. It is 

 possible that best results will eventually be obtained by combining 

 neutron irradiation with other forms of treatment. It is already 

 clear that there are differences in the biological action of neutrons 

 and X-rays, and effort must now be directed to exploit these dif- 

 ferences to the advantage of radiotherapeutic treatment. 



Neutrons are very penetrating. When "cross-fire" methods are 

 used 30 to 40 percent of the dosage comes from the opposite side. 

 Much work needs still to be done on the comparative effects of slow 

 and fast neutrons (26) and the exclusion of neutrons from regions 

 where they are not w T anted. 



In spite of their penetrating abilities neutrons are more easily 

 absorbed by such light elements as water, wax, and especially borax 

 than by heavy elements such as lead or platinum, though blocks of 

 lead are used in canalizing the beam for medical and biological 

 experiments (27). The demand for cyclotron time was so heavy 

 when apparatus 26 (table 1) was in full running, that it was decided 

 to build a new and bigger machine (No. 3 of table 1) to be reserved 

 for medical and biological work. Before this aparatus was completed 

 plans were begun for the giant cyclotron on the hills behind the Uni- 

 versity campus. It was decided that protection would be a serious 

 problem with this machine and that it would be best to isolate it as 

 far as possible from other laboratories. Even with cyclotron 2& 

 there was unavoidable interference by penetrating neutrons with 

 delicate physical apparatus in nearby laboratories, particularly in 

 the chemical department adjacent to the cyclotron building. The 

 cyclotron team arranged a special signal to warn their neighbors that 

 neutrons were about to be liberated in their neighborhood; it was 



7 A roentgen is the unit of dosage used in X- and gamma-ray therapy and is defined as the 

 quantity of X- or gamma radiation such that corpuscular emission per 0.001293 g. of air 

 produces in air ions carrying 1 e. s. u. of electricity of either Bign. 



