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The basic concepts behind positron research aren't difficult
to understand, but they do require some background information.
--So where do I find a positron? What's this positronium
stuff I've been hearing about?
The positron is the antiparticle of the electron, meaning it
has the same mass but an opposite, positive charge. In the the
Standard Model of modern physics, every particle has an antiparticle
and the electron is no exception. Like the electron, positrons
are fundamental particles, meaning they cannot be broken into
smaller parts (as opposed to the proton, which is made of quarks).
Positrons are emitted by radioactive sources through beta-plus
decay, a process that converts a proton into a neutron and a positron.
We use the radioactive isotope sodium-22 because of its reasonable
cost, long half-life, and high rate of positron emission.
A positron in the presence of an electron will often form an
exotic atom called positronium (Ps). One way to visualize positronium
is to consider the orbital model for hydrogen, where an electron
travels around a much heavier proton. In contrast, Ps would consist
of an electron and the positron (a particle of equal mass), making
Ps about a thousand times less massive than hydrogen. While that
small relative mass is very interesting, the most exciting feature
of positronium is the fact that it can spontaneously explode.
Because positronium's constituent particles are antiparticles
of each other, Ps can self-annihilate, converting the mass of
the positron and electron into pure energy in the form of gamma
rays. This self-annihilation feature of Ps means it is extremely
short-lived, with an average lifetime on the order of nanoseconds
(billionths of a second). As an interesting note, if you had one
milligram of Ps, the energy released from its annihilation would
be equal to 25 kilotons of dynamite!
--Wow, positronium sounds really cool! Why is it important
for research?
As far as we are concerned, positronium is great for two things.
First, its small mass and simple structure means it interacts
with other gases in a unique way. The scattering of Ps with even
the lightest gas atoms is comparable to a ping-pong ball bouncing
off of bowling balls. This difference in mass means Ps doesn't
lose much energy from each collision and thus takes a long time
to come to equilibrium with the gas (for a demonstration of Ps
scattering in gases, check out this java
applet). For these and other reasons, theoretical physicists
like positronium as a tool to model atomic interactions and test
quantum theories.
Unfortunately, there is widespread disagreement between current
theoretical models and the experimental results. The first step
towards resolving this conflict is to obtain reliable and extensive
measurements on Ps scattering for the physics community. Our interest,
then, is to construct an apparatus that will provide such precise
scattering cross-sections.
A second application of Ps is rooted in materials science and
a technique known as PALS (Positron Annihilation Lifetime Spectroscopy).
The concept revolves around detecting premature annihilations
when positrons are "picked off" by free electrons from
an enclosing surface. As an example, imagine two Ps atoms trapped
in an arbitrary material, one in a large hole (or pore) and one
in in a small pore. As the Ps atoms move in their pores, the positron
within the atom has a chance to pick off electrons from the material,
causing a premature annihilation. The likelihood of the pick-off
effect is determined by the characteristics of the pore, so the
atom in the small pore should have, on average, a shorter lifetime
than the one in the large pore. From the lifetime data, you can
infer information about the surroundings of the atom.
Our plan is to extend traditional probing techniques into new
domains, beginning with the structure of DNA as it denatures (or
melts). By looking at annihilation spectra before and after the
denaturation, we hope to see some quantitative change that corresponds
to the altered structure.
--OK, I'm convinced that positronium is a useful research
tool. But how exactly will you study Ps?
We study positronium through a combination of the aforementioned
PALS technique and another technique called Doppler Broadening.
Doppler Broadening allows us to measure the energy of the gamma
rays using a high-purity Germanium detector (HPGe). The two gammas
emitted from Ps annihilation should have an energy of 511 keV,
the energy predicted by Einstein's E=mc2 (the eV, or
electron volt, is a unit of energy).
The Doppler effect describes how the frequency of a wave emitted
from a moving source changes depending on the relative motion
between the source and the observer. For example, an ambulance’s
siren appears to increase in pitch as it moves towards you and
decrease in pitch as it moves away. Since the gamma rays emitted
from Ps annihilations behave as waves, gamma rays also exhibit
the Doppler effect. The final energy spectra consists of the sum
of the individual shifts from many annihilations, so we see broadened
energy peaks.
This Doppler broadening winds up being very important, because
we can infer useful information from the width of the peak using
statistical analysis software. This software, combined with some
custom electronics, will allow us ascertain information about
the lifetime and the momentum of the Ps. We'll cover this more
thoroughly later, but the ability to measure age and momentum
will prove more than sufficient to accomplish our ultimate goals.
--Give me a summary of the essential information I need
to take away from this.
To summarize, our research centers around the positron (the antimatter
version of the electron) and its combining with electrons to form
positronium. Positronium's unstable makeup of matter and antimatter
means it quickly self-annihilates into gamma rays. Measurement
of this annihilation radiation gives us a measurement of the positronium's
lifetime and momentum. After the raw data is processed by our
electronics and software, we can correlate data for age and momentum
of the Ps. The data we collect will provide information to:
(1) obtain reliable data on the process by which Ps scatters
off of other gas atoms;
(2) probe the microscopic structure of objects surrounding the
positronium.
To find out exactly how we do this, take a look at the pages
for the individual experiments.
Take me to the gas-scattering
page... --->
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