THE USE OF ACCELERATORS OF CHARGED PARTICLES 



403 



necessity to maintain vacuum in the acceleration tube (10~^ to 10~^ mm Hg). 

 To this end the following methods can be employed. 



Table 1. Comparison of the effectiveness of the reactions H^ {d,n) He^ 



AND K^{d,n) He* 



The energy of a 



deutron in MeV. 



Cross section of the 



reaction E.^{d,n)}ie^ 



in bams. 0.005 0.01 0.02 0.04 0.05 0.07 0.09 0.105 0.105 0.1 



Cross-section of the 



reaction H3(c?,re)He* 



in bams. 0.3 0.7 4 3.3 2 0.75 ~0.2 ~0.2 ~0.2 ~0.2 



Ratio of the reaction 



cross-sections 



H3(f,ra)He* and 



H2(d.re)He3 



1. Pumping out the gas using a vacuum pump 



The use of an ordinary — for instance a steam oil — vacuum pump in a well 

 generator is unsuitable for the following reasons: (a) the need for a vacuum 

 cyhnder which w^ould occupy a lot of space in the well instrument; (b) the 

 difficulty of cooling the pump. Since, as a rule, the drilling fluid in the bore- 

 hole has an elevated temperature it would be necessary either to introduce 

 a cooling device (e.g. a semiconductor) or to introduce a cold substance 

 (solid carbon dioxide, ice, etc,). However, the use of cooHng devices which 

 are inefficient demands a considerable power transmission through the cable 

 of the apparatus, whereas the use of a cold substance involves operational 

 invonvenience, such as a frequent necessity of taking apart and reassembling 

 the instrument, etc. Consequently, it is advisable to find other less cumber- 

 some means of maintaining vacuum in the acceleration tube. 



2. The use of a sealed tube 



It is possible to avoid the necessity of pumping the gas out if a sealed 

 acceleration tube — necessary for the acceleration process — is filled with 

 a suitably rarified gas. The construction scheme of such a tube is shown in 

 Fig. 4(14). 



Positive ions, which spontaneously form around the electrode 4, are 

 accelerated by the potential difference E^ existing between the electrode and 

 the grid 3 and enter the acceleration interval and then the target. As the 

 target 1 is being bombarded by the accelerated neutrons nuclear reactions- 

 occur. These depend on the nature of the target and the gas with which. 



26* 



