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Thanks a lot for your reply!!!!

Thanks a lot! I get it now.

Thank you so much for the explanations!! Could you also tell me why a tiny electron hitting a massive atoms gives a large acceleration? What does "larger mass and lower speed" tell us about momentum? Thank you so much for the extra information!!!!! Photons have a finite path in matter, and so gamma rays can be completely blocked? Why does stopping the protons create gamma rays? What does an individual pellet basis mean? Why does a strong electric field near a heavy nucleus favour the creation of electron-positron pairs? I'm sorry I have so many questions...

1) Why can hitting a heavy metal target with high-speed electrons produce X-rays? 2) Alpha particles have a larger mass and lower speed. Thus, they are more likely to knock electrons out of air molecules. Why? Is it about momentum? I thought particles with high speed can knock electrons out of air molecules easily. 3) Can we completely block gamma radiation? My teacher said that we can't. A piece of lead of 25mm can only reduce the strength by 2 times. If you add one more lead, then the strength will be reduced by 4 times. This goes on and on and then eventually the strength is still not zero. Is he incorrect? I think we can come up the number of leads that are used to block gamma radiation. (Am I right?) 4) I found on the Internet that "lead is good for shielding against X-rays, gamma rays because lead has lots of electrons the rays can interact with". Then what about those elements that have high atomic numbers? Are they really good at shielding? Thank you so much for your help.