338 



CORNELIUS A. TOBIAS 



RECORDING EEG 



PRE-AMP 



HILAC. 

 beam 



PRE-AMP 



H.V 



o 



^A Oscilloscope 



Beam focusing magnet 



Foil monitor Absorbers Rabbit (in holder) 



INTEGRATING 

 DOSIMETER 



Fig. 6. Method of exposure of rabbit cornea to pulsed heavy ion beams. One or 

 more radiation pulses of about 2 millisec duration are allowed to fall on the cornea. 

 The beam pulse as well as contraction of the auricular muscle are simultaneously re- 

 corded, and the blinking directly observed by closed circuit television. 



was exposed to one or more pulses of accelerated alpha particles, over an 

 area 5 mm in diameter. The response of the auricular muscle was followed 

 by recordino- the potential between the electrodes inserted into it. (Tobias 

 et at., 1961). It was found that single or multiple pulses of beam did not 

 cause blinkino- if less than about 20,000 rad was delivered within 1 sec to the 

 substantia propria. Pidses larger than the threshold of about 30,000 rad per 

 pulse did cause reflex muscle action with a delay of about 0.2-0.3 sec. By 

 adjusting the range of the alpha particles it was also found that maximum 

 stimulation occ tared when the Bragg ionization peak is at depth 100-200 fi 

 under the corneal siuface; local anesthesia by a few drops of tetracaine 

 abolished the effect. Such blinking reflex cotild occur if light, secondary to 

 radiation, falls on the retina. That this was not a major part of the effect, 

 was demonstrated by showing that the response is still present when the optic 

 nerve is cut. The energy required for stimidation is sufficient to raise the 

 local temperatiu'e (at the Bragg ionization peak) by about 0.1°C; it is 

 known that absorption of infrared rays of about the same energy content 

 can also initiate the blinking reflex (Dawson, 1961). Figure 7 reproduces 

 some typical electrical records obtained in the course of radiation stimula- 

 tion. The dose in a single beam pulse is sufficient to cause permanent patho- 

 logic changes; these are presently being studied by S. Kimura. 



The corneal fibers that respond to such stimulation are believed to be pain 

 fibers: it is possible that other type nerve endings or synapse will exhibit dif- 

 ferent and perhaps greater radiation sensitivity. At the present time one 



