Antimatter: It Matters 1. Antimatter: It Matters. Alex Elizabeth Heart. University of Cincinnati

Transcription

1 Antimatter: It Matters 1 Antimatter: It Matters Alex Elizabeth Heart University of Cincinnati

2 Antimatter: It Matters 2 Antimatter: It Matters Introduction Isaac Newton, Louis Pasteur, and William Roentgen all have something in common; they made historic scientific discoveries that changed the world. In 1932, Paul Dirac also made a discovery, a finding that has not yet made its full impact on our world, but the promise it shows is astounding. In 1932, Paul Dirac discovered antimatter (CERN, 2014). There is no inherent difference between matter and antimatter. As far as we know, the sister particles follow the same laws of physics, their only difference is that the charge is reversed. In matter, protons are positive and electrons are negative, but in antimatter, protons (antiprotons) are negative, and electrons (positrons) are positive. The exception to this rule is the neutron, whose antimatter counterpart, the antineutron, shares the same neutral charge, but opposite baryon number (Moyer, 2002). Perhaps the most important property of these two particles is what happens when they meet: annihilation. When combined, they erupt in an explosion of energy and light destroying each other. This annihilation is why antimatter is so special in our universe of matter. It is annihilation that helps us diagnose cancer in PET scanners and has great promise to help us cure cancer in the future. Currently, radiation therapy treatment is using conventional high energy x- rays to irradiate tumors. While this type of therapy does have the ability to damage cancerous cells, there are major pitfalls to this type of therapy that cannot be overlooked. The properties of antimatter compared to traditional modalities using matter, have an advantage in simulations of cancer therapy and radiation therapy.

3 Antimatter: It Matters 3 Background In order to understand why the use of antimatter particles can benefit medical imaging and radiation therapy we first need to gain a general knowledge about them. One question that has plagued scientists for years is: Why does matter exist? To answer this question, we must look back to the time of the big bang. According to Michael Moyer, 15 billion years ago, the universe was a very different place; it was inhabited by huge expanses of nothingness. All of the energy that existed was squeezed into an infinitely dense, hot fireball. The extreme heat in that hot, dense fireball created what scientists call homogeneity, or symmetry. Then, when the universe was less than one nanosecond old, it began cooling and the energy began to coalesce into perfectly symmetrical amounts of matter and antimatter (2002). This symmetry presents a problem. If there were perfectly equal amounts of matter and antimatter, and when they met they destroyed each other, then no form of matter or antimatter would exist, let alone, an abundance of matter: us. According to Hitoshi Murayama, professor at the University of California, Berkeley, One thing we know for sure is that the standard model by itself can t give enough (of a) difference between matter and antimatter for us to have survived (Moyer, 2002). Therefore, scientists have taken to researching and exploring the differences, or asymmetries between matter and antimatter. In 1964, physicists discovered that antimatter could decay more rapidly than matter. Now we experiment with B mesons, which are especially heavy particles, by studying their rate of decay to further unveil asymmetry, and to help explain why

4 Antimatter: It Matters 4 matter exists. The theory of Leptogenesis, although incredibly complicated, explains this asymmetry by using a seesaw: when one side goes up, the other must go down. Neutrinos morphed into antineutrinos, which pulled the lepton side of the seesaw down and the amount of ordinary matter had no choice but to increase (Moyer, 2002). Therefore, one of every billion antimatter particles changed into matter. Antimatter then destroyed % of matter. Then atoms such as hydrogen, deuterium, and helium began to form. Over the next 15 billion years, atoms coalesced into stars, galaxies, and us (Moyer, 2002). Progression of Studies Below is a list of discoveries and experiments that tell the story of antimatter according to CERN (2014). Each is essential to account for where our knowledge of antimatter is at today. 1905, Albert Einstein publishes his theory of Special Relativity explaining the relationship between space and time, and energy and mass, in the equation e=mc , Victor Hess discovers cosmic rays, in which a small fraction of antimatter particles are contained 1926, Erwin Schrödinger and Werner Heisenberg devise a quantum theory of mechanics to describe slowly moving particles, but objected to Einstein s theory from 1905 (Poythress, 2014)

5 Antimatter: It Matters , Dirac s equation predicts antiparticles by combining Einstein s theory of special relativity and Schrödinger and Heisenberg s quantum theory (Poythress, 2014). In his 1933 Nobel Prize lecture, Dirac said, If we accept the view of complete symmetry between positive and negative electric charge so far as concerns the fundamental laws of Nature, we must regard it rather as an accident that the Earth (and presumably the whole solar system), contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about, these stars being built up mainly of positrons and negative protons. In fact, there may be half the stars of each kind. The two kinds of stars would both show exactly the same spectra, and there would be no way of distinguishing them by present astronomical methods (CERN, 2014). 1932, Carl Anderson discovers the positron, proving the existence of antimatter as predicted by Dirac 1934, Ernest Lawrence patents the cyclotron which accelerates particles to high velocities without high voltages 1954, the Bevatron, designed to collide protons, starts up at Berkeley, California 1955 and 1966 the Bevatron discovers the antiproton and the antineutron respectively, further proving the symmetry of nature 1964, Cronin and Fitch detect a difference between matter and antimatter s decay rate

6 Antimatter: It Matters , first observations of antinulcei, further proving the symmetry of nature 1978, first storage of antiprotons 1995, first antiatoms produced: antihydrogen 1997, Antiproton Decelerator approved 2002, ATHENA and ATRAP create cold, or slow moving antimatter particles allowing them to be studied before annihilation 2011, ALPHA traps antimatter atoms for 1000 seconds 2011, ASACUSA weighs antimatter to one part in a billion allowing scientists to study nature s preference for matter over antimatter All of the studies and experiments listed above had a hand in shaping our knowledge of antimatter. Yet, it wasn t just science, but also science fiction that helped bring to light some of antimatter s practical uses. Science Fiction Influence Shortly after the first time that it was produced in a lab in 1932, antimatter began showing up in science fiction. It began with Seetee Stock (1949), or CT, which stands for contraterrene. By definition this means pertaining to antimatter. In the 1950 s it was seen again in Isaac Asimov s book I, Robot in which the cyborgs had positronic brains (Bartimo, 2002). Both Seetee Stock and I, Robot brought antimatter to life in ways that science had not yet imagined. Yet, neither compare to the legacy and influence of Star Trek. Since 1966, the starship Enterprise has been whizzing through the universe via antimatter as a fuel (Bartimo, 2002). Is this just science fiction or science fact? According to NASA, this is science fact. Most self- respecting starships in science fiction stories

7 Antimatter: It Matters 7 use antimatter as fuel for a good reason it s the most potent fuel known (NASA, 2006). Currently, NASA Institute for Advanced Concepts is funding a group of scientists to create an antimatter- powered spaceship to travel to Mars. It would work by releasing antimatter and matter particles to be collided, causing annihilation and full conversion into energy. Albert Einstein said, If an idea does not sound absurd there is no hope for it (The History Channel, 2013). His theory of relativity gave us a source of nuclear energy that gave us a way to power a spacecraft and travel the stars. Radiation Therapy The ideas that came from science fiction gave rise to further research and experimentation with antimatter. It was through this research and experimentation that scientists realized that certain unique properties of antimatter are better suited for cancer therapy than any other particle. Current particle modalities for simulations of cancer therapy and treatment rely on photons, electrons, protons, or other heavy particles. Depending on the specific reason an individual is receiving radiation; these current particle modalities carry benefits and/or problems. As a radiation therapist, one of our most imperative goals is normal tissue sparing. According to CERN, To date, particle- beam therapy has used mainly protons to destroy cancer cells. The particles are sent into a patient s body with a pre- determined amount of such a beam of heavy, charged particles enters a human body, it initially inflicts very little damage. Only in the last few millimeters of the journey, as the beam ends its gradual slow- down and comes to an abrupt

8 Antimatter: It Matters 8 cancer it does affect healthy cells along its path, so the damage to healthy tissues increases with repeat treatments (2014). Therefore, The Antiproton Cell Experiment (ACE) at CERN has been studying the use of antiproton particles vs. other particles such as protons to determine whether antiprotons are safer for patients with fewer side effects (CERN, 2014). Until recently, the practical use of antimatter was unavailable due to the inability to trap and transport the particles. Typically, when particle accelerators produce antiprotons, their energies can be measured in GeV, or gigaelectronvolt (Gabrielse, 1993). At this high energy, it was impossible to trap the particles before annihilation. So, to fix this problem, CERN began doing experiments to slow down the particles and their energies. Their Low Energy Antiproton Ring (LEAR) was the first program that was able to decelerate and store antiprotons (CERN, 2014). According to Gerald Gabrielse, Department of Physics professor at Harvard University, LEAR decelerated and cooled antiprotons to an energy of several kev, or kilo- electron volts (1993). The low energy antiprotons were then stored in a penning trap, where the charged particles were suspended by a superposition of magnetic and electric fields (CERN, 2014). The portable penning trap is capable of transporting about antiprotons, which makes medical applications practical. Therapy Simulation Before treatment can begin, patients must undergo a simulation of therapy. This simulation is how the oncologists outline the three main volumes associated with radiation planning: the gross tumor volume (GTV), the clinical treatment volume (CTV), and the planning target volume (PTV). The first volume, the GTV,

9 Antimatter: It Matters 9 represents the extent of the tumor that can be seen or imaged. The second volume, the CTV, contains the GTV plus a margin for sub- clinical disease spread, which cannot be imaged. The third volume, the PTV, contains both the GTV and the CTV and allows for uncertainties in planning or treatment delivery (Washington & Leaver, 2010, p. 445). The accuracy of these simulated volumes defines the safety and effectiveness for the duration of the treatment. Compared to protons, antiproton simulators offer a more accurate measurement of the three tumor volumes by pinpointing the precise stopping point (Kalogeropoulos & Muratore, 1989, p. 1324). When the particles annihilate, they produce on average three subatomic particles called pions, which are able to be detected external to the patient (Lewis, Smith, & Howe, 1997). According to Lewis et al., Because the pions are emitted isotropically, the stopping point can be very accurately imaged in three dimensions and in real time, allowing more accurate and rapid diagnosis of tumor development than available with proton radiography (1997). Thus, using antiprotons to plan therapy is far more accurate than using any other particle that we know of. Treatment Currently, proton therapy is the most precise and advanced form of radiation therapy available. The major clinical advantage of protons over photons is their ability to deposit the dose into the specified volume. Consider a patient with a meduloblastoma in the spinal cord that is treated with a posterior radiation beam. A photon beam will treat the target volume but then exit out anteriorly through the patient s chest and abdomen. However, a proton beam will cease at the end of its

10 Antimatter: It Matters 10 range. Therefore, the dose is deposited into the specified volume and the normal tissue of the chest and the abdomen do not receive unnecessary radiation (Washington & Leaver, 2010, p. 325). On the other hand, the antiproton provides an even more accurate form of radiation delivery. In addition to efficient deposition of energy, the annihilation offers other unique, valuable assets. Annihilation occurs when the antiproton stops. At this point, the antiproton ionization profile is much more concentrated at the end of its range than that of protons (Lewis et al., 1997). As seen in figure 1, the difference between the relative ionization by beams of protons and antiprotons is remarkable. By using antiproton therapy, a Figure 1. Variation of energy deposition by beams of protons and antiprotons with depth in an absorber (Lewis et al., 1997). higher dose of radiation can be delivered to the desired volume. Another advantage to using antiprotons in therapy is that when the pions are released from annihilation, another burst of heavy ion radiation is delivered. Therefore, not only is radiation being delivered as the antiproton slows, but the newly created ions will travel about 20 to 30 microns before depositing more

11 Antimatter: It Matters 11 energy. Due to the typical cell width of 20 microns, this allows more killing power within a few cell diameters (Jackson, 2003). Antiproton Availability As of today, there is only one facility that makes antiprotons available for consumption. According to Gerald Jackson, The present antiproton production rate at the Fermi National Accelerator Laboratory (Fermilab) is 100 billion antiprotons per hour. Given an initial consumption rate of 1% of the production rate, the flux of antiprotons for commercial applications would be approximately one billion antiprotons per hour (2003). Due to limited resources, Fermilab would be unable to meet the demands of radiation therapy without hindering PET isotope production. It is speculated that the current methods of creating, capturing, and cooling antiprotons is no longer ideal. Future changes to these methods are expected to increase the antiproton stacking- rate by two or three (Jackson, 2003). This would result in the large production numbers of antiprotons that would be needed to fulfill the requirements for cancer therapy. Conclusion There is no question that more research and resources are needed before antimatter use in radiation therapy can become feasible. Although it has been 86 years since Dirac discovered antimatter, this area of science is still in its infancy. More facilities need to focus on making antiprotons and better methods need to be identified. Once that work is completed, researchers can begin to determine the best way to apply the use of anti- protons into the field of radiation therapy. According to Kalogeropoulos & Muratore "Antiprotons are the best

12 Antimatter: It Matters 12 particles for [cancer] therapy" (1989), so there is no doubt that the field of radiation therapy will continue to evolve as we become more effective in creating and storing antimatter. After all, health care providers must provide the best care possible to cancer patients and the use of anti- protons appear to be the wave of the future.

13 Antimatter: It Matters 13 References Bartimo, J. (2002). KABOOM! kerpow! antimatter to the rescue. Popular Science, 260(4), 61. Retrieved January 6, CERN. (2014). CERN accelerating science. CERN. Retrieved February 6, 2014, from Gabrielse, G. (1993). Antiproton studies in penning traps (p. 3) (United States, Air Force Office for Scientific Research). Cambridge, MA: Harvard University. The History Channel. (2013, October 06). History channel star trek secrets of the universe. YouTube. Retrieved February 6, 2014, from Jackson, G. P. (2003). Practical uses of antiprotons. Hyperfine Interactions, 146/147(1-4), doi: /B:HYPE c4 Kalogeropoulos, T. E., & Muratore, R. (1989). Antiprotons for imaging and therapy. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 40-41, doi: / X(89) Lewis, R. A., Smith, G. A., & Howe, S. D. (1997). Antiproton portable traps and medical applications. Hyperfine Interactions, 109(1-4), Moyer, M. (2002, March 27). Antimatter. Popular Science. Retrieved January 6, 2014, from NASA. (2014). NASA. NASA. Retrieved February 6, 2014, from Poythress, V. S., Ph.D. (2014). Antimatter. Facts About Antimatter : Angels and Demons Truth. Retrieved February 20, 2014, from about- antimatter/articles/antimatter.html Washington, C. M., & Leaver, D. T. (2010). Principles and practice of radiation therapy (3rd ed., pp ). St. Louis, MO: Mosby Elsevier.