UltraLo-1800 Alpha Particle Counter
XIA LLC
Page 9
All of the electrons in the track drift at the same velocity, so the initial geometry is preserved from
the moment it is created (t
0
) until the first electron reaches the electrode (t
S
), as shown in Figure II-3.
(There is some diffusion of electrons out from the track, but for our purposes this effect is negligible). As
the track continues to drift, the electrode absorbs the electrons as they arrive, until they’re all absorbed
(t
R
). The time between t
0
and t
R
is called the risetime.
3.
Signal Generation
As an electron moves toward the electrode it induces a current in the attached electronics. This may
be understood through Gauss’s Law, which states that the net electric flux through any closed surface is
proportional to the enclosed electric charge. Because the electrode is a conductor it does not support
internal electric fields, all external fields must terminate at its surface, which implies a net flux and, by
Gauss’s Law, the presence of charge on its surface. Further, any changes in the flux imply a change in the
charge present on the surface and therefore imply current in the circuitry connected to the electrode
that is proportional to the rate change of flux. This means that as the electron drifts toward the
electrode it induces a current in the attached electronics (this is also known as the Shockley-Ramo
theorem). This induction only happens while the electron is traveling; once it reaches the electrode its
electric field disappears, the changing flux goes to zero, and therefore the induced current goes to zero.
Figure II-3: The location of the electron track at the 3 important times.
