Simple Gas Ionization Detector Problem

[jumbogala]

Homework Statement

I'm not sure if this is really introductory physics... tell me if you think I'd have more luck in the advanced physics section.

The problem: You have a gas filled counter, in the form of a parallel plate capacitor. It has capacitance 9.1 x 10^-9 F. It takes 20 eV of energy for each ionization in the capacitor to occur.

2 x 10⁶ eV of energy is deposited between the plates by a particle. What is the size of the voltage pulse produced?

Homework Equations

C = Q/V

The Attempt at a Solution

I took a guess, but I have no idea if it's right.

I said that 2E6 / 20 = # of ionizations = 100 000

Each ionization produces an electron and an ion, with charge e. The electrons move to the positive plate and the ions move to the negative plate. Because the charges on the plates will counteract each other, we're lowering Q by (100 000*e) = 1.6E-14.

Then (change in charge) / C = (change in voltage)

(1.6E-14) / 9.1E-9 = 1.8E-6 is the voltage pulse seen (as a decrease).

Is that right? Does it make sense that the voltage decreases - doesn't pulse imply increase?

 

[Delphi51]

"Pulse" can be either up or down.
Very convincing - looks correct.