ELECTRON AI 1< KOSCOPY 



Fig. 10. Contamination on collodion film pro- 

 duced by electron irradiation. 



ments. Fig. 10 shows contamination on 

 collodion films produced by electron irradia- 

 tion for several minutes at liquid nitrogen 

 temperature in conventional vacuum of the 

 order of 1 X 10~^ mm Hg. This micrograph 

 was taken after the films were re warmed 

 to 200°C. It is considered that this contami- 

 nation is caused by electron irradiation, be- 

 cause the amount of deposit depends on 

 dosage of electron irradiation. It is very 

 likely that this contamination is a product 

 of hydrocarbon vapor decomposed by elec- 

 trons, which is observed in ordinary electron 

 microscope. 



In order to reduce contaminations, we 

 must improve the working vacuum as far 

 as possible. First, the inside walls of all parts 

 of the apparatus are thoroughly cleaned, 

 warmed and baked up to 70° to 150°C in 

 the dryer, before assembly. Photographic 

 plates were pre-evacuated in another vac- 

 uum system. When the vacuum was broken, 

 well-dried air was introduced into the appa- 

 ratus. Then, using a 4-inch oil diff vision 

 pump backed by a phosphorus pentoxide 

 trap and a 150 1/min oil rotary pump, the 

 vacuum of the order of 1 to 2 X 10~^ mm 

 Hg could be obtained after half an hour. 

 The refrigerant was poured into the reservoir 

 without inserting the specimen. At this stage 

 the vacuum of the specimen chamber became 

 about 1 to 2 X 10-^ mm Hg. Finally, after 

 being bombarded by the electron beam for 

 several minutes, the specimen was put into 

 the apparatus. 



This procedure efficiently reduced con- 

 taminations. Fig. 11 shows a pair of electron 

 micrographs of a collodion film of about 400 

 A thick; (a) taken before cooling and (b) at 

 liquid nitrogen tem^perature after a con- 

 tinual observation of 45 min. There is no 

 appreciable change in the contrast between 

 the film itself and its holes. This proved that 

 contaminations were completely eliminated. 



Application of Specimen Cooling De- 

 vice. Study of Mercury (5) : This study con- 

 cerned the following points: (a) The char- 



FiG. 11. Collodion film about 400°A thick, (left) at room temperature before cooling, (right) after 

 45 minute continual observation at liquid nitrogen temperature. 



264 



