From: tjroberts137@sbcglobal.net   
      
   On 10/11/17 6:46 PM, John Heath wrote:   
   > At Fermilab they needs to focus a beam of anti protons. [done with a lithium   
   > lens ...] How can an anti proton make it through 4 inches of solid lithium   
   > without hitting a proton ?   
      
   More than 90% of the anti-protons get through the Li lens with only multiple   
   scattering, and the focusing is much stronger than the multiple scattering. As   
   the moderator said, "solid" lithium is mostly intra-atomic space as the   
   anti-protons see it.   
      
   > The anti proton negative charge only increases the chances of a proton hit.   
      
   These are 8 GeV anti-protons, and their negative charge has only a tiny effect   
   on the cross-section to interact with a nucleus [#]. At 8 GeV the total   
   cross-sections for protons and anti-protons differ by about 1% with an errorbar   
   that is larger than the difference.   
      
   	[#] You are probably thinking that the negative anti-proton will   
   	be attracted to the positive Li nucleus, and thus be deflected   
   	to hit it more often than a positive proton (which would be   
   	repelled). In fact, these 8 GeV anti-protons are so stiff it is   
   	easier to move the nucleus than deflect the track, and that is   
   	quite small because the anti-proton track is near each nucleus   
   	for such a short time that there is essentially no time for the   
   	EM force to accelerate it closer to the track. (This is a rather   
   	loose, classical description.)   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)