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. 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 – β+ 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+ + νe
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.
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 × 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.
Figure 6: Antiproton Decelerator schematics
Penning Trap
When one can create antimatter, there is little point unless one can store it. Clearly any casing made of matter used to hold the antimatter into place would be pointless: the antimatter would simply annihilate with the matter case in which we keep it.
Figure 7: Penning Trap
The penning trap uses a combination of a magnetic field (represented by the upward ‘B’ arrows in Figure 7) and an electric field (represented by V) in order to prevent the antiparticles from colliding with the matter. By combining these, we are able to cause axial movement of the particle.
Parallel to the top of this paper, we know that motion must be circular. The electric field only exerts an upward or downward force, so the magnetic field is the only one needed to be taken into account. By using Flemming’s Left Hand Rule, if the field is up, and the current is the direction of motion of the antihydrogen ions, then the direction of the force, must be perpendicular to the direction of motion. This creates a system in which circular motion is observed. However, if the electric field were not there, then the particle would fall under its own weight and annihilate with the bottom of the Penning Trap. Therefore, there must be another field to maintain equilibrium when one is considering the vertical position of the atom.
In the situation above, there is a positive area in the middle, where the antihydrogen particles are, and negative areas above and below that.
Figure 8: Electric Fields within a Penning Trap
As the antihydrogen atoms fall, the electron field will exert an upward force and cause them to ender the negative area above, which will again exert a downward force. This would repeat, and cause an oscillation up and down, which would allow the antiparticles to stay suspended in the middle, and allow the scientists to maintain the position of the antiparticles without needing to put them in a cage crafted from matter.
Below, Figure 9 plots the path of an antihydrogen atom in one of these traps.
Figure 10: Path of an antihydrogen atom in a Penning Trap
Uses of antimatter?
Energy Production
As I was discussing earlier, there is an ability to create energy harnessed through matter-antimatter annihilation. In the example I was using, I was talking about electron-positron decay and showed that one collision between the two particles would released 0.511MeV of energy or 0.819fJ. Therefore, in order to produce just 1J of this energy, we would need 1.22 × 1013 collisions. Although this might seem enough, using the fact that the mass of an electron is 9.1 × 10-31 g means that to release 1J of energy we require 1.11 × 10-17g of electrons, and similar amounts of positrons to release 1J of energy.
Rewriting this in a different form (amount of energy one gram can produce) we can compare it to other fuel sources, which we have compared on the graph on the next page.
As well as the obvious benefits involving the production of more energy, there is also the advantage of it being a totally clean fuel. By annihilating matter and antimatter there are no toxins or pollutants released into the atmosphere - in fact, no mass is left at all!
Thus, clearly if we could harness the full power of the energy of antimatter cheaply and effectively, it will be a feasible energy production. However, the one problem with antimatter is the difficulty in creating it in the first place, and storing it. Because both of these require large amounts of energy in most situations antimatter acts more as energy storage. (Although you could argue that combustible fuels are merely releasing energy ‘stored’, the energy originally put into these was not created by other energy sources, which the antimatter would have to be.)
So, if we can find a feasible means of either creating, or discovering and harnessing antimatter, there will be no future in antimatter energy production. However, it is worth investing money in the research of antimatter. If one could potentially discover a cheap and low-energy way to produce usable antimatter, this would be one of the most powerful energy production methods ever.
Figure 11: Graph of Energy Density
Military Weaponry
Re-consider: http://gltrs.grc.nasa.gov/reports/1996/TM-107030.pdf
Military Weaponry is aided by antimatter in two major ways. The first is using it as the actual cause of the explosion, and the second is using it as a fuel, to help power demanding and needy equipment. However, both of these uses require the same properties – easily obtainable in large quantities; combustible and releasing significant amounts of energy.
There are three major advantages of using antimatter as a way of releasing energy, and three significant difficulties.
Advantages
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The sheer volume of fuel the antimatter fuel would generate is massive, and considerably larger than other fuels. When one considers antimatter energy, and chemical energy then one is ten orders of magnitude greater! This is clearly a significant advantage when one considers the amount of a spacecraft’s weight consists of the fuel. If one is able to reduce this amount by 1010, or carry that much more fuel, then space travel or exploration becomes considerably more feasible.
- The efficiency of antimatter is unrivalled. Even if one were to consider the 50% of the energy released being carried away by neutrinos, there is still 50% efficiency in the conversion of energy. If one were to consider the most comparable H-Bomb, only 0.7% of its potential energy is converted to its explosive power.
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Although if one is considering military weaponry and the desire cause damage to civilisations, there is still an environmental concern with the effects of the nuclear fallout. Antimatter has no such problem, as all of the matter involved is destroyed in the reaction, and as such potential antimatter bombs are often called safe or clean bombs because of the lack of dangerous fallout they leave.
Disadvantages
- Again, one must consider the problems that come with the storage of the antimatter. Although a penning trap is used to store antimatter in other situations, firstly there are problems involving the size of the instruments involved. Also, there would be problems involving the containment of such a large magnetic and electric field on board a space vessel with small, and vulnerable equipment.
- Even if one is able to store the component parts separately, which would require significant cost, there is also the problem of later reuniting them at exactly the right time. The volatility of the components means that were they not combined at exactly the right time, there would be significant damage.
- This is, unfortunately, the biggest nail in the coffin: the sheer cost of obtaining significant amounts of antimatter means that producing usable amounts of energy is ridiculously space-consuming and expensive.
Medical Imaging
One of the major uses of antimatter in modern society is through the technique of PET Scanning, or Positron Emission Tomography which enables scientists to create highly accurate 3D mappings of the inside of the body.
Production of Positrons
First of all, one needs positrons in the body, in order to monitor their activity. This is the job of a positron emitter, as I discussed when talking about beta-plus decay. It is unfeasible to get a box of positrons and open it in someone’s stomach, so instead they use small amounts of a positron-emitting source and require that to enter the blood stream in order to monitor its activity.
The emitter of choice is normally. There are three reasons for this.
- It is cheap, and easily obtainable.
- It is contained in a useful sugar, Fluorodeoxyglucose, which leaves a digestible sugar after the radioactive decay
- The decay constant is of a very usable size: one must wait approximately an hour before you can scan for the annihilation.
Imaging
When one is considering the production of the gamma photons involved in positron-electron annihilation, we can assume that they are travelling very close to c, the speed of light in a vacuum. On the contrary electrons in your body aren’t travelling very close to c, and the positrons are not able to be released at this speed, so one can consider the speed, and hence momentum, of the electrons as negligible when one monitors the photons produced as a result. This means that in order to conserve linear momentum, the photons will be released at an angle of very slightly under 180o from each other. As the body part is inserted into a circle of scanners, one is able to monitor the activity of the photon, and by measuring the amount by which the wavelength of the photon has decayed at each end and can find the exact point at which the decay occurred, allowing us to see the volume of blood distributed to each area around the body.
Figure 12: Photon Emission in PET Scanning
Conclusion
What is Antimatter?
Antimatter, contrary to popular belief, is just simply ordinary matter with a line over it! The difference between matter and antimatter has puzzled scientists, and all they know is that for each of the elementary particles in the matter classification, there exists a reverse, or antiparticle to that. One possible theorem was that which Feynman suggested, namely that antimatter is merely matter going backwards in time. The charge is opposite but apart from that, they are almost identical. Why this type of matter over the other type has survived is an interesting question which has stumped physicists and, indeed, there is still no answer!
What physicists do know, however, is the vast amounts of energy that could be used were antimatter harnessed properly. Its production, through the beta-plus decay process or pair production of a photon is uncontrolled, as is the annihilation energy, but could it be harnessed then there would be significant advances in physics.
Is it worth creating antimatter?
Is it worth creating antimatter now?
Clearly, no.
Although the antimatter is a very useful substance, it is simply worth too much to be efficient. When it was first created, less than one trillionth of a gram was seen as remarkable, and when creating this amount of the material is abnormal or strange, then there is little point in investing the money or energy into created a large amount of antimatter using the techniques we have at the moment. Although antimatter could be a valid energy production source (see Figure 13 for evidence of this,) first a valid method of creating or harnessing the antimatter fuel must be created.
Is it worth funding research into antimatter?
However, I would definitely advocate the research into antimatter. During the cold war, when scientific progress was much enhanced, antimatter was being vehemently researched because scientists knew of its sheer energy density, and there is clearly every reason to attempt to harness it. Although some people may not agree with the sentiments behind all of the motives to creating this new matter, it is clear that it will do more good than evil. Nuclear energy has had a large amount of good and the energy that could be produced outweighs the potential evil that it could be used to create weaponry. All science could be used rightly, or wrongly and funding research into antimatter allows the use of it in such brilliant scientific schemes such as solving the world’s renewable energy crisis; medical imaging or even just furthering research into astrophysics.
Appendices
Appendix A: Table of Figures
Figure 1: Image from http://athena-positrons.web.cern.ch/ATHENA-positrons/wwwathena/anderson.html
Figure 2: Image from Uppsala website: http://www.linnaeus.uu.se/online/phy/microcosmos/different_forces.html
Figure 3: Image from Physics Revealed by F.A. Scott, page 48
Figure 4: Adapted from Image from Physics Revealed by F.A. Scott, page 48
Figure 5:
Figure 6: CERN press release,
Figure 7:
Figure 8: http://livefromcern.web.cern.ch/livefromcern/antimatter/factory/ADpictures/trap-big.jpg
Figure 9: http://www.physik.uni-mainz.de/werth/g_fak/penning.htm
Appendix B: Bibliography
Appendix C: Synoptic Links
Encyclopaedia of Surface and Colloid Science, Ponisseril Somasundaran (pg. 5103)
Data and Information gathered from http://en.wikipedia.org/wiki/Energy_density
