Tracy Johnson writes:

> What bright propellerhead gave rise to the thought that X-Rays can harm
>  magnetic media anyway?  The creator of this urban legend obviously took
>  biology as his "hard science" required course and not physics.
>
>  We need an authoritive answer from Wirt.  Wirt?

Well, it's nice to be an authority on something, but Bruce Toback gave the
correct answer earlier. X-rays, at least at the level used in an x-ray
machine, will not harm the magnetic information stored on a hard drive or a
floppy. There is simply no interaction of any consequence between the x-rays
photons and the magnetic domains created in the oxide layer of the disc.

The only exception to this statement would be if the x-ray flux density was
intense enough to cause localized heating. Magnetic domains (the 1's and 0's)
are "reset" at sufficiently high temperatures. The reset temperature varies
from material to material, but you can be sure that at sub-molten
temperatures, some or all of the magnetic information will be lost. However,
as the coatings on discs become increasingly thinner films and the information
density on the disc becomes progressively higher and higher, localized heating
due to locally focussed x-rays cannot be completely dismissed.

For a short tutorial on extremely thin-film, superfine magnetic domains,
please see:

     http://info.ntt.co.jp/RD/ACT/RD_act_3/super.html

Nonetheless, the bottom line is: airport x-ray machines should never be a
problem -- unless they turn up the power significantly on the machines and we
push on towards very large drives in extremely tiny configurations.

As Bruce also said, I believe, the far greater danger lies from getting your
laptop too near an improperly shielded motor or a handheld magnetic wand.
These devices may as well be called degaussers.

Wirt Atmar