Number 540, May 23, 2001 by Phil Schewe,
James Riordon, and Ben Stein
Photodetectors From DNA
The nucleoside deoxyguanosine (DG) is one of four compounds that serve
as bases to encode genetic information in DNA. It's also turning out
to be an excellent alternative to conventional semiconductor material
in some experimental photodetectors.
Ross Rinaldi (ross.rinaldi@unile.it,
39-0832-320238) and coworkers at the National Nanotechnology Laboratory
of the Instituto Nazionale per la Fisica della Materia in Italy fabricated
the new detectors by placing a tiny droplet of DG nucleosides dissolved
in chloroform at the juncture of two electrodes. As the chloroform evaporated,
the DG molecules self-assembled into an array of ribbon-like structures
between the electrodes.
The DG-based photodetectors are roughly twice as sensitive to light
as commercially available detectors, and are potentially both cheap
and simple to manufacture. The research devices are built around bits
of DG crystal a little over a tenth of a micron long, roughly the length
of the ribbons that the DG naturally forms. The researchers, however,
are working to double the length of the DG portion to about a quarter
of a micron because larger dimensions are easier to duplicate with conventional
semiconductor fabrication techniques. If the device can be enlarged,
the chloroform solution could be deposited with a modified inkjet printer
nozzle, while the rest of the device would be simple to manufacture
in a modern semiconductor lithography facility.
The researchers point out that the outstanding semiconducting properties
of biomolecular compounds like DG may ultimately lead to a plethora
of electrical components that rely on a surprisingly small quantity
of molecules. Eventually, individual biomolecules may replace entire
doped semiconductors sections in some devices. (Rinaldi et al, Applied
Physics Letters, 28 May 2001; text at Physics
News Select)
Anti-Molecules
Several experimental groups at CERN have been attempting to form anti-hydrogen
(antiH) atoms (see, for example, the
ATRAP site) by combining positrons with antiprotons in the chill
environment of a trap. AntiH has been made before at CERN and Fermilab
in high energy collisions (Updates 253,
366)
but not in a way that allowed detailed study.
In the new experiments one possible way of cooling newly-made antiH
atoms is to mingle them with ultracold hydrogen atoms. Of course one
possible outcome of mixing an atom with its antimatter counterpart is
mutual annihilation. But before that happens, a short-lived antiH-H
molecule might form.
At last week's APS atomic physics meeting in London, Ontario, Bernard
Zygelman (University of Nevada, Las Vegas, 702-895-1321, bernard@physics.unlv.edu)
explored, through simulations, what happens H and antiH stick together.
One likely scenario is as follows: first they join together as a molecule
and emit a telltale photon. After a short life together, the constituents
of the molecule would resort themselves so as to form an electron-positron
pair (positronium) and a proton-antiproton pair (protonium). Both of
these unstable twosomes will quickly annihilate to create still more
characteristic photons, signifying the fleeting existence of the molecule.
(Lay
language report here; also Zygelman et al., Physical Review A,
May 2001; text at Physics
News Select)
A Lighter-Than-Air Plasma Can Deflect an Intense Electron Beam
A lighter-than-air plasma can deflect an intense electron beam, new
experiment has shown, opening possibilities for magnet-free particle-storage
rings and plasma-based "vapor wires" which would guide electric
current. We are very familiar with matter refracting light, but scientists
have not thoroughly explored how matter can refract a charged beam.
A California-based research collaboration (Patrick Muggli, USC, 213-821-1801,
muggli@usc.edu) prepared a 40-micron-radius
beam of approximately 20 billion electrons with 28.5 GeV energy. Sent
into a plasma with a density of between 1-2 x 1014 particles/cubic
centimeter--about a million times less dense than air--the latter part
of the electron beam was deflected by about 1 milliradians as it emerged
from the plasma. This is remarkable given the fact that the electron
beam is intense enough to bore through a few millimeters of steel.
The researchers explain how the deflection occurs: When the electron
beam enters the plasma, it first repels plasma electrons, creating a
positively charged channel through which the latter part of the beam
travels. The channel first focuses the beam; but when the beam nears
its exit from the plasma, the ion channel becomes asymmetric, resulting
in a deflecting force which bends the electrons in the latter part of
the beam.
Practical applications of the technique may require a laser for pre-forming
the ion channel which would serve to deflect all, rather than part,
of the beam. The researchers envision that such a pre-treated plasma,
coupled with a hundredfold greater plasma density, might compare favorably
with a 50-kilogauss SLAC magnet in guiding charged particles; but such
a "plasma waveguide" would have a much shorter turn-on time
of about 200 femtoseconds. (Muggli et al, Nature,
3 May 2001; figures at Physics
News Graphics)
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