From: John K Clark (johnkc@well.com)
Date: Tue Mar 03 1998 - 23:41:31 MST
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>> Me:
>>Why not use radiation pressure?
>rrandall6@juno.com
>Because they had no good way to make high-energy X-rays turn a right
>angle? The geometry of the device seems to forbid anything except
>good X-ray mirrors,
It's true that true that there are no good X ray mirrors, except for some
very recent ones and they must be designed to work only in a very narrow
bandwidth, and that wouldn't be of much use for blackbody radiation in a bomb.
As I said in my other post only, 1 or 2 percent of the X rays would be
reflected, and you'd only get that if the angel of reflection was less than
180 degrees like what an optical mirror can achieve, and even less that your
right angle, but that would still be useful, however there is a more important
point, if you don't have a good mirror the next best thing is a good radiator.
If the X rays are not reflected by the Black Body chamber then they will be
absorbed, that will heat up the chamber to a very high temperature and then
re-radiate.
I doubt the following has anything to do with H bombs, but X ray optics has
always fascinated me for some reason. As I said X ray mirrors have been made,
Dr. Nat Ceglio at Livermore made an X ray mirror using many thin layers of
Molybdenum and Silicon. Each layer doesn't reflect much but if the thickness
of the layers are chosen properly at one narrow wavelength all the reflected
waves coincide. The reflection is about 60 % efficient. Useless in a bomb but
great in a LASER
X ray lens seemed impossible but very recently there was a breakthrough there
also. The trouble was that when you go beyond the near ultraviolet there
didn't seem to be any material you could make a lens out of. Aluminum is
almost as transparent to hard (over 14 kev) X rays as glass is to light,
but the index of refraction is so tiny that the focal length of a simple
lens would be huge, far too long to be practical. Heavier elements like Gold
have a higher index of refraction, but they strongly absorb X rays, your
lens would be almost completely opaque.
In the November 7 1996 issue of Nature is an article called "A Compound
Refractive Lens For Focusing High Energy X Rays" by Snigirev, Kohn, Snigireva
and Lengeler. They found a easy and cheap way to focus very Hard X Rays, up
to about 40 kev. Their lens is simply an Aluminum block with holes of radius
.3 millimeter drilled into it, it looks like this:
_____________________
---------------> | o o o o o o o o o |
X Rays | o o o o o o o o o | . <--- Focal Point
---------------> | o o o o o o o o o |
---------------> |-------------------|
Each hole drilled into the Aluminum block is a lens, one lens may give you a
ridiculously long focal length, but 2 will cut that distance in half, this
first device has 30 lenses. The focal length of their compound lens is
R/ 2NI, R is the radius of the holes, N is the number of holes, and I is the
index of refraction in the X ray region of the material. In this lens the
holes are cylinders so the X rays are focused in only 2 dimensions, for 3
you'd need spheres, or another block oriented at right angles. The authors
don't seem to think it would be very difficult to make a device with hundreds,
perhaps thousands of such lenses. The authors also calculate that if they
could make the holes in a parabolic shape they could reduce the focal length
by a further factor of 5. Although they haven't tried it yet they also think
the same lens could also focus something else that was thought to be
impossible, a beam of neutrons.
Aluminum was used in this case because it's easy to machine, but boron,
carbon, plastic and even water should work too. It occurs to me that it might
not be too difficult to make an array of small spherical bubbles in water,
they wouldn't stay in place for long but they wouldn't need to, the X rays
from a laser would be in a VERY short burst.
This could be very important for X rays in photolithography to make very tiny
semiconductor devices. If you don't have any optical elements then you can't
optically reduce the size of your image, your mask must be as small as the
very microprocessor you're trying to make, and you must place it right on
top of the silicon, as in a contact sheet in photography. None of this seemed
very practicable to most people so Xray photolithography has been stuck in a
rut. Maybe now things will change.
John K Clark johnkc@well.com
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