This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fundamentals in Radiology, Subject name: Radiology
Which of the following has maximum ionisation potential? 
Select and explain. 
A. Electron, B. Helium ion, C. Proton, D. Gamma photon
B: Helium ion
Answer: B: Helium ion 
Explanation: Helium ion (alpha rays) has maximum ionisation potential Gamma rays have maximum penetration power.