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Beta-radiation, at a basic level, is simply the decay of a down quark to an up quark. However, this breaks some of the conservation laws! Since an up quark is slightly lighter than a down quark, there must be another constituent particle released, and for years, scientists thought this was it: d –> u + e- Charge is clearly conserved, as is mass. However, when one actually observes the decay, there is a difference in mass that one would not expect to see:

Figure 3: Energy Released in Beta-Decay This is seemingly unexplained! Clearly there cannot be energetic photons released just like in alpha-emission, because otherwise the energy would come in steps, energy being quantized. The answer: Neutrinos! Latin for ‘little neutral particle,’ these were first suggested by Pauli and by releasing these particles with differing amounts of energy, this accounts for the seemingly unexplained difference in the energy levels.

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This is what Figure 3, the Feynman diagram shows with one slight exception – the anti-neutrino is there to maintain lepton number, as otherwise there is two leptons on one side and none on the other; there must be an antilepton to cancel out the ‘leptonicity’ electron. Positive Beta Radiation There is another type of beta radiation – i?? + decay. This is different from the previous example because it does not occur, naturally releasing energy; instead, it requires energy put into the system to occur in most cases. However, there are some isotopes that are able to do it in normal situations.

d –> u + e+ + ? e Figure 4: Beta-plus decay Although this does not occur normally in isolation, as the mass of the down quark is greater than that of the up quark there are rare instances where this can occur. This is normally when there is an abnormally high proton to neutron ratio, and there exists a more energetically favourable configuration of the particles. Examples of this situation for lighter nuclei include and . Despite these being ‘light’ elements, they’re definitely not common! This is one of the sources of antimatter.

By using elements that naturally produce antimatter like this we have a feasible source of antimatter which is used in medical imaging. The most common element used for this situation is . Positron-Electron Annihilation The other most commonly occurring type of interaction between matter and antimatter is when two of the corresponding types of particle collide: the two will annihilate and produce energy by producing at least two photons of the total amount of energy the two particles had. Figure 5: Feynman diagram showing the annihilation of an electron-positron pair

As you can see in the Feynman diagram (Figure 5), the two particles, in this case an electron and a positron meet, and are reduced simply to two energetic photons: all the rest mass has been converted to energy. Using e = mc2, we can work out that the rest energy of the electron is equal to approximately 0. 511 MeV; and thus the photons must have this energy also. 1 The other facet of this type of annihilation is the reverse: the production of electron-positron pair from energy; or similarly for very high energy photons, one can produce other particles and their corresponding antiparticles.

However, since the electron has a significantly lower mass than the other particles, it is more likely to be created than, for example a proton-antiproton pair. As long as the photon has sufficient energy (2 i?? 0. 511 MeV as discovered above) it will be able to produce an electron-positron pair. In many cases, the two simply recombine shortly, and this has little net effect. However, in rare cases the two split apart and produce two disparate particles. Although the positron will quickly annihilate with another electron, it in theory could be possible to capture this positron, and use this pair production as a method of creating antimatter.

How do we store and create antimatter? Current Antimatter Production Production of antiprotons The production of antiprotons first occurred about 20 years ago. A particle accelerator accelerated protons to very high speeds, and fired them at a stationary, fixed material, of which Iridium is the most commonly used. By hitting this metal, the protons decelerate very quickly, and the release of this energy produces photons more than capable of creating proton-antiproton pairs. The antiprotons are captured using a magnetic field, and then forced around a ring using similar magnetic fields, which allowed the researches to manage the rings.

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