8 



3. The curve for this cross-section versus ion density can usually be 

 fitted by a theoretical relation, and the theoretical relation rests 



on the random production of a definite minimum number of ion pairs 

 in a molecule of definite thickness. These measurements enable 

 you to get an independent measure of thickness. 



4. Both these quantities, cross-section and volume, vary with temper- 

 ature during irradiation. If the material is cooled to dry ice or liq- 

 uid air temperature, it is likely, although not guaranteed, that the 

 sensitive volume will be smaller. If you want to get a most dramat- 

 ic variation you can get it every timie by irradiating just below the 

 temperature where you would inactivate thermally. If you hold the 

 material about 20 C below that for thermal inactivation and irradi- 

 ate at the same time, the volume and cross-section will both be of 

 the order of 3 to 5 times larger than normal. It is not a small ef- 

 fect. It is definite. 



5. You can have partial damage due to ionizing radiation. This shows 

 up in the case of hemoglobin. If you irradiate hemoglobin and then 

 look for any change in it by any method you like, the first thing you 

 need to do is to put it into solution. If this is attempted at an ad- 

 verse pH, the irradiated material will not go into solution. How- 

 ever, hemoglobin is soluble at pH 4 or 5 and once in solution it will 

 not appear to be damaged. Since there is a change in the solubility 

 at high pH, partial damage of some kind has occurred (8). 



6. Radiation action can migrate. It can migrate across an enzyme in- 

 hibitor or an enzyme substrate bond. We have measured the effect 

 of trypsin and soybean trypsin inhibitor separately and combined and 

 the effect of hyaluronic acid and hyaluronidase separately and com- 

 bined. In both cases we conclude that energy can migrate. We are 

 now studying this in the case of antigen antibodies. 



7. On the other hand, radiation action does not readily migrate from 

 one molecule to another in a dry solid. 



We have a rather simple experiment to show this, being done at the 

 moment by Hutchinson. If you take electrons of finite range, e.g. , 

 200-volt electrons, and you bombard a layer of invertase, you can- 

 not burn off more than one monolayer no matter how long the radia- 

 tion is applied. You only eliminate from this invertase preparation 

 the top layer that corresponds to one molecule. This means that the 

 transfer of radiation energy from the top layer to the second layer 

 is very difficult. 



More recently, Hutchinson has shown that this is difficult even if the 

 temperature of invertase is increased. So that the transfer from 

 one molecule to another in dry solid is actually difficult in the case 

 of invertase. 



8. Previous treatment of a molecule, e.g., by heat, can condition its 

 radiosensitivity. 



9. Loss of solubility is an important response to radiation. It is not 

 necessarily the most sensitive index, although on occasion, this is 

 the case. For example, the main effect of irradiation of bovine 

 serum albumin in bulk is the loss of solubility. If it is put on a 



