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Re: [Phys-l] Transparency

On Oct 10, 2008, at 4:27 PM, Edmiston, Mike wrote:

Slightly off topic, but related... and ending with a question...

One way to get charges "injected" into a material is by radiation. Of
course the ion track from gamma or beta radiation going through diamond
would be both positive and negative charges with no net charge.

When I designed, built, and tested nuclear detectors based on
ionization, I routinely made measurements on the properties of the ion
pairs (such as the mobility of the ions) produced in various materials
by gamma or beta radiation, and sometimes by alpha radiation.

If the detector material is silicon or germanium, both the negative and
positive charges in the ion track are mobile. An applied electric field
causes the electrons to drift one way while the "holes" drift the other
way. This gives rise to a strong signal from these detectors because
your electronics detect the movements of both the positive and negative
charges drifting in the applied field.

One type of detector I worked with was liquid argon (and also liquid
xenon). In these materials the electrons are quite mobile; so we can
say liquid argon and liquid xenon are conductors with respect to
injected electrons. On the other hand, the positive ions (Ar+ or Xe+)
are not mobile, so they do not act the same way as holes in Si and Ge.
This not only reduces the signal from the detector, it also severely
limits the usefulness of liquid argon and xenon for ionization detectors
because after a period of operation with reasonably high radiation flux,
the liquid is building up a positive space charge because the immobile
positive charges are not swept from the detector by the applied field.

After reading John D.'s comments about diamond, I am curious if diamond
is known to conduct positive charges as holes such as silicon and
germanium do. Could we make an ionization detector from diamond that
would operate similarly to ionization detectors made from silicon and
germanium? Of course there is no good reason to make diamond detectors
because diamond costs too much. I never investigated it. The idea
behind liquid argon was very large detectors can be inexpensive, at
least in terms of the cost of the material. I haven't kept up to date
on this. How much is known about diamond in this regard?

1) I remember reading about diamond detectors, this was years before Si commercial detectors became available.

Here is something more recent, found by googling.

2) John D. pointed to a common textbook error. Perhaps it can be fixed by being clear about what changes when the kT is no longer negligible in comparison with the gap size.

Ludwik Kowalski, a retired physics teacher
5 Horizon Road, Apt. 2702, Fort Lee, NJ, 07024, USA
Also an amateur journalist at