Does firing a proton beam on water make it acidic? Lets say I fire a proton beam upon the surface of water. Lets assume I do the experiment in an air-free environment and that the proton beam doesn't reflect off the surface of water. 

Now, protons, which are simply $\ce{H+}$, should enter the bulk of water and probably combine with a $\ce{H2O}$ molecule and form $\ce{H3O+}$. As per the definition of acidity and pH:

$$\ce{pH}=-\log [H^+]$$

If the pH reduces, the solution would become acidic as seen here. But at the same time, the water becomes charged, so I am not sure whether this is applicable. 

So, in short, does water become acidic due to a proton beam? 

Edit: I do not have access to proton guns and it's certainly not on sale on eBay. I only attempt to theorize on what may happen.
Yes, the concentration of $\ce{H3O+}$ will increase which will lower the pH value of the water. This is theorie based.

In reality you won't be able to measure the change. The mount of protons you need to significantly change the pH will also effect the charge of the water as you mentioned. The charged water will affect your method of measuring AND the protons will leave the water because of it's positive charge.

To neutralize the water you could add elections, which will cause the protons to form hydrogen gas. Or you could add a negative ion like chloride, which would have been the same as adding $\ce{HCl}$ to the water.