Quarks, Leptons, Fermions, and Bosons—The Subatomic World of Radiation Therapy
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《肿瘤学家》
LEARNING OBJECTIVES
After completing this course, the reader will be able to:
Describe where the commonly prescribed forms of radiation such as photons, electrons, and protons fit into the grander scheme of subatomic particles.
Discuss the categories of subatomic particles prescribed in radiation oncology such as bosons, fermions, leptons, quarks, baryons, hadrons, and mesons.
Categorize fundamental particles as fermions or bosons based on their spin.
Categorize particles as leptons or quarks based on their types of interactions.
Categorize hadrons as baryons or mesons based on their quark composition.
While it is well known that the fundamental building blocks of molecules are atoms and that atoms in turn are composed of electrons orbiting a nucleus of protons and neutrons, it is less universally understood precisely where the commonly prescribed agents of radiation therapy fit into the grand scheme of the subatomic world. Scientists in fields such as particle physics, astrophysics, and cosmology frequently allude to exotic-sounding particles such as quarks, neutrinos, mesons, gluons, etc. Radiation oncologists routinely treat cancer patients with radiation therapy in various forms—although the precise nature of the various forms of radiation commonly prescribed is not always fully appreciated.
Quarks, leptons, hadrons, and bosons may seem exotic and esoteric but, in fact, they play a very mundane role in the world of radiation oncology. The familiar components of atomic nuclei, protons and neutrons (i.e., nucleons), are composed of smaller fundamental building blocks known as quarks. Quarks come in six "flavors," which go by the somewhat capricious names of up, down, strange, charmed, bottom, and top, arranged from least to most massive. (The two most massive quarks, bottom and top, are also occasionally referred to as beauty and truth.) The six flavors of quarks can be arranged into three families (or "generations"): up and down, charmed and strange, and bottom and top (Table 1). The proton is made up of two up quarks and one down quark while the neutron is composed of two down quarks and one up quark (Fig. 1). From the net charges of the proton and neutron (+1 and 0, respectively), one can deduce that quarks must have fractional charges and that the charges are of opposite signs. The proton, with a net charge of +1, is composed of two up quarks, each with a charge of +2/3, and one down quark, with a –1/3 charge. The neutron, having no net electrical charge, is composed of two down quarks, each with a –1/3 charge, and one up quark with a +2/3 charge, canceling out to a net of zero. Particles such as protons and neutrons, which are composed of three quarks, are classified as baryons from the Greek word for "heavy" (the same root as the relatively new field of bariatric medicine). Particles made up of a quark and antiquark pair are known as mesons. Negatively charged pi-mesons, or pions, consist of pairs of down and antiup quarks and were formerly used in radiation therapy. Because all particles composed of quarks are subject to the so-called strong nuclear force, they are known as hadrons from the Greek word for "strong." The particles not subject to the strong force (such as electrons) are known as leptons (derived from the Greek word for "small" or "light"—the same root used in the term leptomeningeal). Hadron therapy is relatively rare in the U.S., with only a handful of sites offering treatment with protons or neutrons; therapy with pi-mesons and atomic nuclei has fallen out of favor due to costs, impracticality, and improvements in other technologies. On the other hand, the majority of North American radiation oncology facilities offer and routinely administer electron-beam radiotherapy. The electron, along with its antimatter counterpart the positron, belongs to the lepton family of particles and, as such, is not subject to the strong nuclear force. Like the quarks, the leptons come in six flavors arranged in three families or generations (Table 1). In ordinary matter, there are six known quarks and six known leptons. These twelve fundamental particles have twelve antimatter counterparts, bringing the total to 24. Additionally, all quarks can exist in three "color" quantum states—red, blue, and green. Together, the flavors and families of leptons and quarks (in all their colors) constitute the fundamental particles of the so-called Standard Model (Table 2).
Although all known matter is made up of particles from the Standard Model, there are other particles that do not constitute matter but rather mediate the interactions governing the behavior of matter. Such mediators belong to the category of particles known as bosons, named after the Indian physicist Satyendra Nath Bose (Table 3). While there are several categories of bosons (which mediate the fundamental forces of gravity, electromagnetism, the weak nuclear force, and the strong nuclear force), the most familiar boson is the photon, the mediator of the electromagnetic force. The overwhelming majority of patients undergoing radiation therapy are treated with high-energy photons, that is, bosons. Bosons, as a group, are characterized by the common feature of zero or integral intrinsic angular momentum or spin. This contrasts them from particles with odd half-integral spin, the fermions (named after the Italian physicist Enrico Fermi). The behaviors of bosons and fermions are vastly different and are governed by different quantum rules: bosons follow statistical laws known as Bose-Einstein statistics whereas fermions follow Fermi-Dirac statistics. An example of such differences is illustrated by the behavior of the electrons in an atom. As fermions, electrons must follow Fermi-Dirac statistics, including the well-known Pauli Exclusion Principle, which states that no two electrons in a given atom can occupy the exact same quantum state. Bosons such as photons do not obey the Pauli Exclusion Principle and actually tend to congregate in the same quantum state. This property accounts for the behavior of lasers, which produce intense, coherent beams of visible light photons. One final observation is that all matter is composed of fermion building blocks, that is, the quarks and leptons of the Standard Model are all fermions.
Returning to the clinic, bosons, in the form of high-energy photons such as gamma and x-rays, are by far the most commonly prescribed entities in radiation oncology. Electrons are classed as leptons (as they are not subject to the strong force) and fermions (as they have half-integral spin) and represent the next most commonly prescribed radiation. The antimatter counterpart of the electron, the positron, likewise is both a lepton and fermion and is now well known to oncologists through positron emission tomography or PET. Hadrons, such as protons, neutrons, pi-mesons, and heavier charged particles such as carbon and neon ions, have been successfully used in cancer treatment but are now rarely used in the U.S., largely because of technical limitations stemming from the fact that the facilities were designed for experimental high-energy particle physics research rather than radiation therapy. Several dedicated medical cyclotrons and synchrotrons intended specifically for proton therapy are planned or currently under construction. The physical and radiobiological characteristics of hadron therapy promise to make it an attractive option in the future of radiation oncology.
DISCLOSURE OF POTENTIAL CONFLICTS OF INTEREST
The author indicates no potential conflicts of interest.(James S. Welsh)
After completing this course, the reader will be able to:
Describe where the commonly prescribed forms of radiation such as photons, electrons, and protons fit into the grander scheme of subatomic particles.
Discuss the categories of subatomic particles prescribed in radiation oncology such as bosons, fermions, leptons, quarks, baryons, hadrons, and mesons.
Categorize fundamental particles as fermions or bosons based on their spin.
Categorize particles as leptons or quarks based on their types of interactions.
Categorize hadrons as baryons or mesons based on their quark composition.
While it is well known that the fundamental building blocks of molecules are atoms and that atoms in turn are composed of electrons orbiting a nucleus of protons and neutrons, it is less universally understood precisely where the commonly prescribed agents of radiation therapy fit into the grand scheme of the subatomic world. Scientists in fields such as particle physics, astrophysics, and cosmology frequently allude to exotic-sounding particles such as quarks, neutrinos, mesons, gluons, etc. Radiation oncologists routinely treat cancer patients with radiation therapy in various forms—although the precise nature of the various forms of radiation commonly prescribed is not always fully appreciated.
Quarks, leptons, hadrons, and bosons may seem exotic and esoteric but, in fact, they play a very mundane role in the world of radiation oncology. The familiar components of atomic nuclei, protons and neutrons (i.e., nucleons), are composed of smaller fundamental building blocks known as quarks. Quarks come in six "flavors," which go by the somewhat capricious names of up, down, strange, charmed, bottom, and top, arranged from least to most massive. (The two most massive quarks, bottom and top, are also occasionally referred to as beauty and truth.) The six flavors of quarks can be arranged into three families (or "generations"): up and down, charmed and strange, and bottom and top (Table 1). The proton is made up of two up quarks and one down quark while the neutron is composed of two down quarks and one up quark (Fig. 1). From the net charges of the proton and neutron (+1 and 0, respectively), one can deduce that quarks must have fractional charges and that the charges are of opposite signs. The proton, with a net charge of +1, is composed of two up quarks, each with a charge of +2/3, and one down quark, with a –1/3 charge. The neutron, having no net electrical charge, is composed of two down quarks, each with a –1/3 charge, and one up quark with a +2/3 charge, canceling out to a net of zero. Particles such as protons and neutrons, which are composed of three quarks, are classified as baryons from the Greek word for "heavy" (the same root as the relatively new field of bariatric medicine). Particles made up of a quark and antiquark pair are known as mesons. Negatively charged pi-mesons, or pions, consist of pairs of down and antiup quarks and were formerly used in radiation therapy. Because all particles composed of quarks are subject to the so-called strong nuclear force, they are known as hadrons from the Greek word for "strong." The particles not subject to the strong force (such as electrons) are known as leptons (derived from the Greek word for "small" or "light"—the same root used in the term leptomeningeal). Hadron therapy is relatively rare in the U.S., with only a handful of sites offering treatment with protons or neutrons; therapy with pi-mesons and atomic nuclei has fallen out of favor due to costs, impracticality, and improvements in other technologies. On the other hand, the majority of North American radiation oncology facilities offer and routinely administer electron-beam radiotherapy. The electron, along with its antimatter counterpart the positron, belongs to the lepton family of particles and, as such, is not subject to the strong nuclear force. Like the quarks, the leptons come in six flavors arranged in three families or generations (Table 1). In ordinary matter, there are six known quarks and six known leptons. These twelve fundamental particles have twelve antimatter counterparts, bringing the total to 24. Additionally, all quarks can exist in three "color" quantum states—red, blue, and green. Together, the flavors and families of leptons and quarks (in all their colors) constitute the fundamental particles of the so-called Standard Model (Table 2).
Although all known matter is made up of particles from the Standard Model, there are other particles that do not constitute matter but rather mediate the interactions governing the behavior of matter. Such mediators belong to the category of particles known as bosons, named after the Indian physicist Satyendra Nath Bose (Table 3). While there are several categories of bosons (which mediate the fundamental forces of gravity, electromagnetism, the weak nuclear force, and the strong nuclear force), the most familiar boson is the photon, the mediator of the electromagnetic force. The overwhelming majority of patients undergoing radiation therapy are treated with high-energy photons, that is, bosons. Bosons, as a group, are characterized by the common feature of zero or integral intrinsic angular momentum or spin. This contrasts them from particles with odd half-integral spin, the fermions (named after the Italian physicist Enrico Fermi). The behaviors of bosons and fermions are vastly different and are governed by different quantum rules: bosons follow statistical laws known as Bose-Einstein statistics whereas fermions follow Fermi-Dirac statistics. An example of such differences is illustrated by the behavior of the electrons in an atom. As fermions, electrons must follow Fermi-Dirac statistics, including the well-known Pauli Exclusion Principle, which states that no two electrons in a given atom can occupy the exact same quantum state. Bosons such as photons do not obey the Pauli Exclusion Principle and actually tend to congregate in the same quantum state. This property accounts for the behavior of lasers, which produce intense, coherent beams of visible light photons. One final observation is that all matter is composed of fermion building blocks, that is, the quarks and leptons of the Standard Model are all fermions.
Returning to the clinic, bosons, in the form of high-energy photons such as gamma and x-rays, are by far the most commonly prescribed entities in radiation oncology. Electrons are classed as leptons (as they are not subject to the strong force) and fermions (as they have half-integral spin) and represent the next most commonly prescribed radiation. The antimatter counterpart of the electron, the positron, likewise is both a lepton and fermion and is now well known to oncologists through positron emission tomography or PET. Hadrons, such as protons, neutrons, pi-mesons, and heavier charged particles such as carbon and neon ions, have been successfully used in cancer treatment but are now rarely used in the U.S., largely because of technical limitations stemming from the fact that the facilities were designed for experimental high-energy particle physics research rather than radiation therapy. Several dedicated medical cyclotrons and synchrotrons intended specifically for proton therapy are planned or currently under construction. The physical and radiobiological characteristics of hadron therapy promise to make it an attractive option in the future of radiation oncology.
DISCLOSURE OF POTENTIAL CONFLICTS OF INTEREST
The author indicates no potential conflicts of interest.(James S. Welsh)