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Antimatter

Antimatter can be bought “online”, perhaps even directly on ebay! But do not expect to receive an antimatter can as the one depicted in the photo above.
What you get instead is a capsule containing an infinitesimal quantity of the isotope Sodium 22. This radioactive isotope decays by emitting a positron (the electron antiparticle!) plus a certain amount of gamma radiation. The amount of sodium 22 contained in the plastic capsule is very small, however, this isotope has a rather high intrinsic activity, this means that the capsule has an activity of approximately 1μCi that corresponds to 37KBq that is 37000 disintegrations per second! Our antimatter capsule thus produces the beauty of about 37000 antielectrons per second.
Do not be scared by these numbers, these radioactive sources can be bought and kept at home without any special license because they still have a very low value and therefore do not constitute a health hazard, anyway they have to be handled with caution. Keep in mind that even our body is weakly radioactive, mainly because of the content of isotopes Potassium 40 and Carbon 14. It is estimated that the inherent radioactivity of the human body is about 9KBq, a considerable value!

In the image below we show our antimatter source:

sodio

The isotope Na-22 decays  (in 99.95% of cases) with half-life of 2.6 years for positron emission or electron capture to the first excited state of 22-Ne at 1.274 MeV (which subsequently relaxes with gamma photon emission). The positron is emitted with maximum energy of 544KeV.

In the picture below you can see the scheme of the Sodium 22 decay in which is shown the maximum energy of the emitted positron, that is 544KeV.

Of course, the beta decay also provides for the emission of a neutrino that capture part of the energy, for this reason, the energy of the emitted positron can be between a minimum and a maximum of 544KeV and is spread over a continuous spectrum of values. With our DIY Alpha Spectrometer we got the spectrum shown in the image below, where we see the distribution of the energy values around 200-300KeV and mainly less than 500KeV.

The positrons have however not an easy life! After they have been slowed inside the material they are going through they recombine with an electron and disappear transforming into pure energy : it is the phenomenon of the annihilation of matter and anti-matter.

The process of electron-positron annihilation is a reaction that takes place when an electron meets a positron (the electron antiparticle, that is a particle of antimatter): the subsequent collision process triggers the production of 2 annihilation photons or, more rarely, 3 photons or other particles.

e^{{+}}+e^{{-}}\longrightarrow 2\gamma

This process must follow some conservation laws, among which:

  • The conservation of electric charge: the total final and initial charge is equal to zero.
  • The conservation of momentum and the total energy: it prohibits the creation of a single photon.
  • The conservation of angular momentum.

At low energies the annihilation results do not have a wide variety of cases; the most common involves the creation of two or more annihilation photons; the conservation of energy and momentum prohibits the creation of a single photon. In the most common case, two photons are created each having energy equal to the rest energy of electron or positron (511 keV). Since the system initially has a total momentum equal to zero, the gamma rays are emitted in opposite directions.

If the electron and / or the positron have high kinetic energy,  several massive particles (for example mesons) can be produced, provided that the energy of the two particles is sufficient to be transformed in the corresponding rest energy of the produced particles. One of the objectives of the International Linear Collider is the production of the Higgs boson starting right from electron-positron annihilation.

Our DIY Gamma spectrometer can verify experimentally the process of electron-positron annihilation !

In the graph below, obtained with a gamma scintillation probe and with the application Theremino MCA, are apparent peaks at 1274KeV, coming from the decay of the isotope Na22, and especially at 511KeV, coming from the reaction of electron-positron annihilation.

spettrosodio

With the Gamma spectrometer we have verified experimentally that in the annihilation process the energy is conserved. It is also possible to verify experimentally that the momentum is conserved: in fact, given that the system is initially at rest, the two photons at 0.511 MeV are emitted at 180 degrees from each other, so that the total momentum remains null. This fact can be verified with angular correlation and coincidence measurements.

The principle of the experiment is the following:

positronPSoC

The image below shows the setup:

PSoCSetup4

The two detectors are operated in “coincidence mode” to detect only the  photon pairs generated by the same annihilation. There have been two measurements, the first with the detectors aligned (left image) and the second with the detectors angled between them but both aligned to the source.

Detectors Aligned

Time = 3100 s
Coincidence Rate = 59,2 ± 1 CPM
Detector 1 Rate = 102,0 CPS
Detector 2 Rate = 109.8 CPS

Detectors Angled

Time = 3700 s
Coincidence Rate = 0,257 ± 0,064 CPM
Detector 1 Rate = 93,6 CPS
Detector 2 Rate = 109.7 CPS

In the measurement with the detectors angled, the detector 1 is little further from the source than the detector 2, and in fact the value measured by the detector 1 is slightly lower.
It is clear that the rate of coincidences count goes to virtually zero as soon as the two detectors are positioned off axis, this is a proof of the fact that the gamma photons emitted from the positron annihilation, because of conservation of momentum, are spatially phase-shifted exactly by 180°.

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