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What is a Positron? Crash Course for Beginners

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Positron Research Group
Science Center 155
1520 St. Olaf Avenue
Northfield, MN 55057

Dr. Jason Engbrecht
engbrech@stolaf.edu
507-646-3849
507-646-3968 FAX

  [-web-]

Anna Legard '08
legard@stolaf.edu

Dan Endean '09
endean@stolaf.edu

David Green '09
greend@stolaf.edu

 

 

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?

Standard Model
particle
(charge)
antiparticle
(charge)
mass (kg)
     
electron
(-)
positron
(+)
9.1 x 10^-31
proton
(+)
antiproton
(-)
1.7 x 10^-27
neutron
(0)
antineutron
(0)
1.7 x 10^-27

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.

Hydrogen
Positronium
Hydrogen, with an electron and a heavy nucleus
Positronium, with a positron and electron
Orbital Models of H and Ps

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!

Graph of the differing results for scattering data (He)

A sample of some of the differing results for scattering data of Ps in helium gas. Note the large disagreement between various results.

[Theoretical calculations]
[Experimental results]

 

--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.

Ps in differing pore sizes

Ps in differing pore sizes. The "happy" Ps is less likely to pick off an electron before it annihilates because it lives in a larger pore. The "sad" Ps will have a shorter lifetime because it is more likely to find an electron.

 

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?

Doppler Broadening
Stationary Ps, with narrow energy spectrum
For stationary Ps, we would expect the energy spectrum to consist of a narrow peak at 511 keV.
Energetic Ps, with Doppler-broadened peak
Since the Ps is moving, the detector reads energies slightly higher and lower than 511 keV. A Doppler-broadened peak results.

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... --->