Table of contents:
- Helpful Webpages
- Link to inclusion in public webpage:
- Speaker notes:
- Introduction slide / General speaker notes:
- Slide 1: Beta decay
- Slide 2: Beta decay and weak force
- Slide 3: Weak force
- Slide 4: Parity conservation (analogy)
- Slide 5: Parity conservation (example)
- Slide 6: Lee and Yang theory
- Slide 7: Wu's experiment (hypothesis)
- Slide 8: Wu's experiment (setup)
- Slide 9: Wu's experiment (setup, adiabatic magnetization)
- Slide 10: Wu's experiment (observations)
- Slide 11: Wu's experiment (results)
- Slide 12: Nobel Prize
- Slide 13: Beta-NMR
- Slide 14: Parity and Cosmology
Helpful Webpages
Link to inclusion in public webpage:
Chien-Shiung WuSpeaker notes:
Introduction slide / General speaker notes:
Synopsis of work:
Chien-Shiung Wu is most well known for her work investigating and establishing beta decay.
Enrico Fermi originally theorized the existence of beta decay in 1934. He predicted that neutrons can radioactively decay by direct coupling with a proton, an electron, and a neutrino (later clarified to be an antineutrino).
In 1949, Wu confirmed Enrico Fermi's theory of beta decay despite varying result produced by earlier scientists. She discovered sources of experimental error in these experiments, and produced results consistent with Fermi's prediction.
Her most famous work investigated the behaviour of beta decay and its relation to parity conservation. Previously, the conservation of parity had always been assumed as a law in fundamental physics. To conserve parity means that a mirror image of our world would behave in an identical manner, and that 'right' and 'left' are indistinguishable in physics.
Wu conducted an experiment to test the hypothesis of Tsung-Dao Lee and Chen Ning Yang stating that parity conservation was broken in the weak interaction, the interaction that governs beta decay. By studying the direction of beta decay of radioactive cobalt-60 aligned and cooled in a magnetic field, Wu was able to observe asymmetric beta decay and prove that parity was maximally broken in the weak interaction. This was a major contribution to particle physics and the development of the Standard Model.
Researcher's background:
Chein-Shiung Wu was born in a small town near Shanghai, and grew up in a family who deeply valued women's equality and education. Wu studied physics at the National Central University in Nanjing, where she became involved in student politics and led several protests as a student leader.
After graduating, Wu worked as a research assistant under the supervision of Gu Jing-Wei, a female professor who encouraged Wu to pursue a PhD abroad at the University of Michigan, just as Jing-Wei had done herself. Inspired by her mentor, Wu applied and was accepted to the University of Michigan, and travelled to the USA shortly after. Upon arrival, Wu changed trajectory and instead enrolled at the University of California, Berkeley, partly influenced by their more advanced facilities, and partly due to the more liberal atmosphere (women at Michigan were prohibited from even using the front entrance). Wu worked formally under the supervision of Ernest O. Lawrence, a soon-to-be Nobel laureate for his invention of the cyclotron particle accelerator, and she also worked closely with physicist Emilio Segrè. Wu's 1940 PhD thesis features two sections of work: bremsstrahlung (her first work on beta decay), and radioactive isotopes of Xenon produced by nuclear fission.
After her PhD, Wu remained at the Radiation Laboratory at Berkeley as a post-doctoral fellow. During this time, Wu married fellow physicist Luke Chia-Liu Yuan. After a brief stint teaching at Smith College in Northampton Massachusetts, Wu accepted a job from Princeton University as the first female faculty in the history of the department.
As a response to the growing severity of WW2 in 1944, Wu joined the Manhattan Project at the Substitute Alloy Materials Laboratories at Columbia University alongside many other notable physicists developing radiation detector instrumentation, and models for producing enriched uranium as fuel for atomic bombs. In her later years, Wu greatly distanced herself from the project and recommended to the Taiwanese president in 1962 to never build nuclear weapons.
In 1949, Wu joined the Brookhaven National Laboratory and balanced her time between Brookhaven and Manhattan, near Columbia University where Wu was appointed the first female physics professor. She became a US citizen in 1954.
Wu was awarded the Wolf Prize in Physics in 1978, partially considering her as someone who deserved a Nobel Prize but was unfairly left out.
Wu retired in 1981, and passed away in 1997.
Societal relevance:
Wu's contributions to nuclear and particle physics shaped the way that we understand the Standard Model today. Her work on beta decay and parity conservation led directly to the discovery of Beta-NMR which has revolutionized material characterization and the study of metal-ion interactions with biomolecules. In addition, understanding parity violation in the weak force brought a deeper understanding of the origins of our universe, and why our universe is matter-dominated.
Wu also contributed greatly to nuclear and particle physics with her work, and even went on to contribute to medicine conducting research on sickle-cell disease.
Adjacent to her research contributions, Wu published a book in 1965 titled 'Beta Decay', which is now regarded as one of the most important reference texts for nuclear physicists today.
Especially in her later career, Wu was an outspoken advocate for women's equal treatment, and dedicated herself to educational programs and public speaking encouraging young women to pursue careers in the sciences. She was also a prominent critic of human rights issues following the Tiananmen Square massacre of 1989. Wu has a longstanding legacy of advocacy for social justice.
Wu was awarded the Comstock Prize in Physics in 1964, the National Medal of Science in 1975, the Wolf Prize in Physics in 1978, and she was the first woman to serve as president of the American Physical Society, among many other awards and honours.
General citations and resources:
https://en.wikipedia.org/wiki/Chien-Shiung_Wu
https://www.womenshistory.org/education-resources/biographies/dr-chien-shiung-wu
https://www.atomicheritage.org/profile/chien-shiung-wu
Slide 1: Beta decay
Science details:
When there are too many protons or neutrons in a nucleus, one will transform into the other in order to bring the nucleus into a more stable state with lower binding energy. In this transformation, a beta particle and a neutrino or antineutrino are emitted. Beta particles are electrons or positrons (also known as antielectrons: the antiparticle of an electron, it has the same mass but opposite charge). This type of radioactive decay is called beta decay, and it transforms a nuclide into an isobar of itself. A nuclide is an atom defined by its number of protons and neutrons, and an isobar of a nuclide has the same mass number, but different atomic number, meaning a different number of protons. Quarks are one of the fundamental elementary particles that make up matter. They combine in twos or threes to make up hadrons (a group of subatomic particles that includes nucleons) and are held together by the strong interaction. There are six types (flavours) of quarks (up, down, strange, charm, bottom, and top) in the Standard Model. Nucleons are composed of up and down quarks: neutrons have two down quarks and one up quark and protons have two up quarks and one down quark. For a neutron to transform into a proton (or vice versa), it must change its flavour by emitting a W boson (intermediate vector boson). This leads to the creation of a negative beta particle and a corresponding antineutrino (or a positive beta particle and corresponding neutrino in the transformation of a proton into a neutron). The exchange of a W or Z boson between fermions (either the elementary electrons and quarks or the composite protons and neutrons) is the mechanism of the weak force, one of the four fundamental interactions. Beta decay was first formally theorized by Enrico Fermi, and he predicted correctly that neutrinos and electrons (or their antiparticles) are created in the process of beta decay and not stored within the nucleus.
Citations and resources:
https://www2.lbl.gov/abc/wallchart/chapters/03/2.html
https://en.wikipedia.org/wiki/Beta_decay
https://www2.lbl.gov/abc/wallchart/chapters/03/2.html
Figures:
Top: Equation of negative beta decay: the neutron transforms into a proton and emits a negative beta particle and an antineutrino. Equation of beta decay of cobalt-60 (atomic number 27) into nickel-60 (atomic number 28), one electron, one antielectron neutrino, and two photons.
Bottom: Depiction of beta decay from an atomic nucleus (protons in red, neutrons in blue). Inset shows a depiction of beta decay of a free neutron.
https://en.wikipedia.org/wiki/Beta_decay#/media/File:Beta-minus_Decay.svg
Slide 2: Beta decay and weak force
Science details:
When there are too many protons or neutrons in a nucleus, one will transform into the other in order to bring the nucleus into a more stable state with lower binding energy. In this transformation, a beta particle and a neutrino or antineutrino are emitted. Beta particles are electrons or positrons (also known as antielectrons: the antiparticle of an electron, it has the same mass but opposite charge). This type of radioactive decay is called beta decay, and it transforms n nuclide into an isobar of itself. A nuclide is an atom defined by its number of protons and neutrons, and an isobar of a nuclide has the same mass number, but different atomic number, meaning a different number of protons. Quarks are one of the fundamental elementary particles that make up matter. They combine in twos or threes to make up hadrons (a group of subatomic particles that includes nucleons) and are held together by the strong interaction. There are six types (flavours) of quarks (up, down, strange, charm, bottom, and top) in the Standard Model. Nucleons are composed of up and down quarks: neutrons have two down quarks and one up quark and protons have two up quarks and one down quark. For a neutron to transform into a proton (or vice versa), it must change its flavour by emitting a W boson (intermediate vector boson). This leads to the creation of a negative beta particle and a corresponding antineutrino (or a positive beta particle and corresponding neutrino in the transformation of a proton into a neutron). The exchange of a W or Z boson between fermions (either the elementary electrons and quarks or the composite protons and neutrons) is the mechanism of the weak force, one of the four fundamental interactions. Beta decay was first formally theorized by Enrico Fermi, and he predicted correctly that neutrinos and electrons (or their antiparticles) are created in the process of beta decay and not stored within the nucleus.
Citations and resources:
https://en.wikipedia.org/wiki/Beta_decay
https://en.wikipedia.org/wiki/Weak_interaction
https://www2.lbl.gov/abc/wallchart/chapters/03/2.html
Figures:
Top: Equation of negative beta decay: the neutron transforms into a proton and emits a negative beta particle and an antineutrino. Equation of beta decay of cobalt-60 (atomic number 27) into nickel-60 (atomic number 28), one electron, one antielectron neutrino, and two photons.
Bottom: Animation of beta decay of a neutron via the weak interaction. The neutron is initially composed on one up quark (green) and two down quarks (blue and red) and is transformed into a proton with one down quark (red) and two up quarks (blue and green) by emitting a W- boson (red arrow) which is then split into an electron (white) and an electron antineutrino (magenta).
https://newbedev.com/what-exactly-does-the-weak-force-do
Slide 3: Weak force
Science details:
The four fundamental forces are the weak, strong, electromagnetic, and gravitational force. The weak force is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms. The weak, strong, and electromagnetic forces act by exchanging a force-carrying boson between particles. In the weak interaction, fermions exchange massive gauge bosons (the W± boson and the neutral Z⁰ boson). Their mass is much greater than that of a nucleon, and this is the reason the weak force is weaker than the strong and electromagnetic forces: its effective range is less than the diameter of a proton. As a result, weak decay lifetimes are much longer and have smaller cross sections (a particle physics term for probability of interactions (???)) than those of the other electromagnetic or strong decays.
Citations and resources:
https://warwick.ac.uk/fac/sci/physics/staff/academic/boyd/warwick_week/neutrino_physics/weak.pdf
https://en.wikipedia.org/wiki/Weak_interaction
Figures:
Left: Feynman diagram for beta decay of a neutron (one up quark, two down quarks) into a proton (one down quark, two up quarks), an electron, and an electron antineutrino via an intermediate W⁻ boson. The time axis points from bottom to time. https://en.wikipedia.org/wiki/Weak_interaction#/media/File:Beta_Negative_Decay.svg
Right: Standard model of elementary particles. Three generations of matter (fermions): quarks (up, down, charm, strange, top, bottom) are shown in purple, leptons (electron, electron neutrino, muon, muon neutrino, tau, tau neutrino) are shown in green. Interactions/force carriers (bosons): gauge bosons or vector bosons (gluon, photon, Z boson, W boson) are shown in orange, the scalar boson (higgs boson) is shown in yellow.
https://en.wikipedia.org/wiki/Standard_Model
Slide 4: Parity conservation (analogy)
Science details:
Parity conservation is a rule that dictates that in a mirror-image world, objects should behave in mirror-image ways. For example, in the real world we read left to right. In a mirror image world, where left and right are inverted, mirror world people would read from right to left. In other words, “left” and “right” have no physical definition if parity is conserved. It is also impossible to tell whether we exist in a real world or mirror world at a given moment. Imagine a lineup of people, half are right-handed and half are left-handed, each are given a ball to throw with their dominant hand. In this analogy, the left-handed people can be thought of as inhabitants of the mirror world as they are the inversion of the right-handed people (or vice versa). To test for parity conservation, one would measure the average distance the balls are thrown to either side. If parity is conserved then the distances would, on average, be the same. This is because there is no difference in throw strength between right and left handed people: the mirror world is a perfect reflection of the real world. If parity is not conserved, then the average distance the ball is thrown would be biased in one direction. Here, changing handedness when going from the real world to the mirror world somehow changes the throw strength: the right and left handed people are not perfect inversions of each other.
Citations and resources:
https://en.wikipedia.org/wiki/Parity_(physics)
https://sciencenonfiction.org/2015/07/06/the-experiment-that-taught-us-what-left-means/
https://warwick.ac.uk/fac/sci/physics/staff/academic/boyd/warwick_week/neutrino_physics/weak.pdf
Figures:
Top: Equation of parity transformation P: spatial coordinates x,y,z are taken to -x,-y,-z.
Bottom: Drawing of humans lined in single-file ready to throw tennis balls. Those throwing to the right/left are shown in red/blue, respectively.
https://sciencenonfiction.org/2015/07/06/the-experiment-that-taught-us-what-left-means/
Slide 5: Parity conservation (example)
Science details:
In particle physics, parity is quantified as a multiplicative quantum number intrinsic to particles that can take the value +1 (even parity) or -1 (odd parity) depending on the outcome of a parity transformation. A parity transformation involves flipping the sign of one or more spatial coordinates (depending on the number of spatial dimensions in a given system), reflecting objects in the real world to a mirror world. If a system is exactly the same after a parity transformation, it has even parity. Similarly, if a system is exactly its opposite after a parity transformation, it is said to have odd parity. Scalars and axial vectors (also called pseudovectors, such as angular momentum and magnetic fields) have even parity because they do not change sign under parity transformation. Vectors (such as linear momentum and current) and pseudoscalars have odd parity because they change sign under parity transformation. (If we examine why this is the case, angular momentum (L) is defined as the cross product of the position vector (r) and the linear momentum vector (p). Flipping the sign of the spatial coordinates takes r to -r and p to -p. Then taking the cross product of the parity transformed vectors leaves the sign of the angular momentum unchanged). If an electron is moving counterclockwise in a magnetic field, the field would have to point outwards in a normal world. In the mirror world, flipping the sign of the spatial coordinates keeps the electron moving counterclockwise. Because the magnetic field is an axial vector (even parity), its sign does not change after the parity transformation, so it still points outwards in the mirror world. Parity is conserved in electromagnetism - if it were not, we would sometimes find magnetic fields following a right-hand rule, and sometimes following a left-hand rule. In 1956, there was conclusive evidence for parity conservation in electromagnetic processes, but no evidence that this was the case for the weak interaction. Chien-Shiung Wu devised a beautiful experiment to test for parity conservation by measuring beta decays from polarized cobalt-60. In the real and mirror world, both the magnetic field polarizing the cobalt-60 and its spin will point in the same direction. When the cobalt-60 undergoes beta decay, it will emit an electron in a particular direction in the real world. In the mirror world, this corresponds to the electron being emitted in the opposite direction. Since the real world and the mirror world are indistinguishable from each other, parity conservation would mean that the electrons are emitted equally in both directions over the course of the decay.
Citations and resources:
https://en.wikipedia.org/wiki/Parity_(physics)
https://sciencenonfiction.org/2015/07/06/the-experiment-that-taught-us-what-left-means/
https://warwick.ac.uk/fac/sci/physics/staff/academic/boyd/warwick_week/neutrino_physics/weak.pdf
Figures:
Left: diagram of electron with counterclockwise current and corresponding magnetic field vector pointing outwards in the normal world. Under parity inversion, the current still flows counterclockwise and the magnetic field vector points outwards.
Right: cobalt-60 atom with a spin axis pointing up, a magnetic field pointing up, and an electron being deflected from the spin axis by an angle θ in the normal world. In the parity inverted world, the system is the same except that the electron is being deflected by an angle 180⁰-θ from the spin axis.
https://twiki.cern.ch/twiki/pub/Main/ICJournalClub/JC_Parity_Wu.pdf
Slide 6: Lee and Yang theory
Science details:
In the 1950’s, there was conclusive evidence that parity was conserved in both the electromagnetic force and the strong force but there was no proof that this was the case for the weak force. In 1956, Chen-Ning Yang and Tsung-Dao Lee theorized that parity was not conserved in the weak interaction after observing the puzzling weak decays of θ and τ kaons into pions. Pions are elementary particles made of one quark and one antiquark and they were experimentally determined to have odd parity (taking the value P=-1). Both kaons had the same mass, charge, and spin, but different parities. θ⁺ decayed into two pions: π⁺ and π⁰, and τ⁺ decayed into three pions: π⁺, π⁰, and π⁻. Since parity is a multiplicative quantum number, the parity of θ⁺ is equal to the product of the parity of the two pions: (-1)x(-1)=(+1) so θ⁺ has even parity. Similarly, the parity of τ⁺ is the product of the parity of the three pions: (-1)x(-1)x(-1)=(-1) so τ⁺ has odd parity. The assumption here is that parity is conserved by the weak interaction so both sides of the decay equation have the same parity, which was believed at the time to be a law of nature for all interactions. This meant that the kaons were thought to be different particles with different intrinsic parities despite having the same mass, spin, and charge. If, however, parity was not conserved by the weak interaction then the kaons would be the same particle. Yang presented the “θ-τ Puzzle” at a conference in 1956 where it was proposed that perhaps they were the same particle with two possible decay modes, implying that parity was not conserved. Following the conference, Yang and Lee proposed that parity was not conserved by the weak interaction and discussed their hypothesis with Wu, who went on to devise the elegant experiment to test the possibility of parity violation in beta decay.
Citations and resources:
https://www.secretsofuniverse.in/parity-violation-weak-experiment/
Figures:
Equation showing decay of θ⁺ (even parity) into π⁺ (odd parity) and π⁰ (odd parity). Equation showing decay of τ⁺ (odd parity) into π⁺ (odd parity), π⁰ (odd parity) and π⁻ (odd parity). https://www.secretsofuniverse.in/parity-violation-weak-experiment/
Slide 7: Wu's experiment (hypothesis)
Science details:
Wu’s experiment tested for parity conservation in the weak interaction by placing cobalt-60 in a magnetic field, cooling the nuclei to near absolute zero, then measuring beta and gamma ray emissions. The emission of photons is an electromagnetic process and thus was already known to conserve parity. This means that these emissions would not be biased in a particular direction dependent on the spin of the cobalt-60 atoms. (Spin is a polar vector therefore its direction does not change going from the real world to the mirror world, unlike the electron emission vector which would invert in the parity-transformed world). Even if the gamma rays were emitted asymmetrically along the spin axis, the anisotropy of that distribution could be used as a control to determine how well the cobalt-60 nuclei were aligned in an external magnetic field. The asymmetry of the electron emissions could then be compared to that of the gamma rays to measure the degree to which parity was (or was not) conserved in the weak interaction. Wu hypothesized that if there was symmetric parity (parity is conserved by the weak interaction) then one would observe no preference in the direction of emitted electrons in both the real and mirror world. If parity was broken (parity is not conserved by the weak interaction) then one would observe a preferred direction of emitted electrons, thus making the mirror image asymmetric.
Citations and resources:
https://en.wikipedia.org/wiki/Wu_experiment
https://journals.aps.org/pr/pdf/10.1103/PhysRev.105.1413
Figures:
Top: diagram of emission of electrons from cobalt-60 atom if parity conservation was true. The emissions are asymmetrically biased downward in both the real world (left, cobalt-60 has counter-clockwise spin) and the mirror world (right, cobalt-60 has clockwise spin). https://sciencenonfiction.org/2015/07/06/the-experiment-that-taught-us-what-left-means/
Bottom: diagram of cobalt-60 atom with spin vector (J) and direction of electron emission (e). The real world (left) and mirror world (right) have the same spin vector but different directions of electron emission.
https://warwick.ac.uk/fac/sci/physics/staff/academic/boyd/warwick_week/neutrino_physics/weak.pdf
Slide 8: Wu's experiment (setup)
Science details:
The experiment was designed to count and compare the emission rate of electrons and gamma rays from the beta decay of highly polarized cobalt-60. The smaller magnetic moments of cobalt-60 nuclei compared to those of the electrons meant that the nuclei had to be cooled to extremely low temperatures. This could be done using adiabatic demagnetization: a magneto-thermodynamic effect wherein an object’s temperature changes when placed in a magnetic field. If heat is not allowed back into the object then it will lose thermal energy to the environment. Wu’s experiment utilized the Gorter-Rose method to highly polarize cobalt-60: a layer of cobalt-60 covered a paramagnetic crystal of cerium-magnesium nitrate and was placed into a scintillator. The salt was exposed to a horizontal magnetic field then cooled to 1.2 K by pumping liquid helium to low pressure, then further cooled to 0.003 K by shutting off the magnetic field. With the field off, a solenoid was placed vertically along the scintillator housing the cobalt-60 - cerium-magnesium nitrate specimen which could be used to align the cobalt-60 nuclei upwards or downwards. Wu used two scintillators positioned in two different directions to continuously measure the gamma ray polarization as the crystal warmed up while also measuring electron emissions using a photomultiplier tube. This process was repeated by reversing the direction of the magnetic field to measure results for both possible polarizations of the nuclei.
Citations and resources:
https://en.wikipedia.org/wiki/Magnetic_refrigeration
https://journals.aps.org/pr/pdf/10.1103/PhysRev.105.1413
https://en.wikipedia.org/wiki/Wu_experiment
Figures:
Left: plot of entropy (S) against temperature (T). The curve is steeper with no applied magnetic field (red) than with an applied magnetic field (blue). Three points (A,B,C) are marked to compare entropies and temperatures along the two curves. C (red) and B (blue) mark the same entropy for lower and higher temperatures, respectively. B and A (red) mark the same temperature for lower and higher entropies, respectively. http://ecoursesonline.iasri.res.in/mod/page/view.php?id=124333
Right: Equation of beta decay of cobalt-60 (atomic number 27) into nickel-60 (atomic number 28), one electron, one antielectron neutrino, and two photons. Schematic of Wu’s experiment: cobalt-60 - cerium-magnesium nitrate specimen in scintillator below a light pipe leading to a photomultiplier. The scintillator is surrounded by a dewar containing liquid helium. The dewar is contained inside a tube of liquid nitrogen within a solenoid (for specimen polarization). A magnet (for cooling by adiabatic demagnetization) is shown to the left and right of the solenoid. Two scintillators (for measurement of gamma ray polarization) are positioned at equatorial and polar directions with the z-axis pointing upward. https://en.wikipedia.org/wiki/Wu_experiment
Slide 9: Wu's experiment (setup, adiabatic magnetization)
Science details:
The experiment was designed to count and compare the emission rate of electrons and gamma rays from the beta decay of highly polarized cobalt-60. The smaller magnetic moments of cobalt-60 nuclei compared to those of the electrons meant that the nuclei had to be cooled to extremely low temperatures. This could be done using adiabatic demagnetization: a magneto-thermodynamic effect wherein an object’s temperature changes when placed in a magnetic field. If heat is not allowed back into the object then it will lose thermal energy to the environment. Wu’s experiment utilized the Gorter-Rose method to highly polarize cobalt-60: a layer of cobalt-60 covered a paramagnetic crystal of cerium-magnesium nitrate and was placed into a scintillator. The salt was exposed to a horizontal magnetic field then cooled to 1.2 K by pumping liquid helium to low pressure, then further cooled to 0.003 K by shutting off the magnetic field. With the field off, a solenoid was placed vertically along the scintillator housing the cobalt-60 - cerium-magnesium nitrate specimen which could be used to align the cobalt-60 nuclei upwards or downwards. Wu used two scintillators positioned in two different directions to continuously measure the gamma ray polarization as the crystal warmed up while also measuring electron emissions using a photomultiplier tube. This process was repeated by reversing the direction of the magnetic field to measure results for both possible polarizations of the nuclei.
Citations and resources:
https://en.wikipedia.org/wiki/Magnetic_refrigeration
https://journals.aps.org/pr/pdf/10.1103/PhysRev.105.1413
https://en.wikipedia.org/wiki/Wu_experiment
Figures:
Top: Equation of beta decay of cobalt-60 (atomic number 27) into nickel-60 (atomic number 28), one electron, one antielectron neutrino, and two photons.
Bottom: Schematic of Wu’s experiment: cobalt-60 - cerium-magnesium nitrate specimen in scintillator below a light pipe leading to a photomultiplier. The scintillator is surrounded by a dewar containing liquid helium. The dewar is contained inside a tube of liquid nitrogen within a solenoid (for specimen polarization). A magnet (for cooling by adiabatic demagnetization) is shown to the left and right of the solenoid. Two scintillators (for measurement of gamma ray polarization) are positioned at equatorial and polar directions with the z-axis pointing upward. https://en.wikipedia.org/wiki/Wu_experiment
Slide 10: Wu's experiment (observations)
Science details:
As the specimen warmed up and lost polarization, the beta asymmetry and gamma anisotropy leveled off, which took around 6 minutes for both emissions. The control measured a gamma ray anisotropy of 0.6, meaning 60% of gamma rays were emitted in one direction and 40% were emitted in the other. If parity were conserved by the weak interaction, then the electron emissions would have shown similar distributions. Wu observed that there was in fact a strong preference in the direction of electron emission (specifically opposite to the nuclear spin direction) with a much greater asymmetry value than the gamma ray anisotropy value. This asymmetry and preferred direction opposite to nuclear spin was unchanged after reversing the direction of the magnetic field, proving that the results were not due to remnant magnetization of the cobalt-60 specimen. Wu showed maximal parity violation in the weak interaction, a discovery which shocked the physics community.
Citations and resources:
https://en.wikipedia.org/wiki/Wu_experiment
https://journals.aps.org/pr/pdf/10.1103/PhysRev.105.1413
Figures:
Top: cobalt-60 atoms with both possible spin directions and electron emissions. If parity conservation holds true, the asymmetry of emission directions does not change with spin direction (left). The experimental results (right) show that asymmetry does change with spin direction and has a preference for emission in the direction opposite to the spin axis. https://sciencenonfiction.org/2015/07/06/the-experiment-that-taught-us-what-left-means/
Bottom: plot of beta asymmetry (measured as counting rate divided by average warm counting rate) against time (measured in minutes) for the polarizing field point up (H↑, top) and down (H↓, bottom). https://journals.aps.org/pr/pdf/10.1103/PhysRev.105.1413
Slide 11: Wu's experiment (results)
Science details:
As the specimen warmed up and lost polarization, the beta asymmetry and gamma anisotropy levelled off, which took around 6 minutes for both emissions. The control measured a gamma ray anisotropy of 0.6, meaning 60% of gamma rays were emitted in one direction and 40% were emitted in the other. If parity were conserved by the weak interaction, then the electron emissions would have shown similar distributions. Wu observed that there was in fact a strong preference in the direction of electron emission (specifically opposite to the nuclear spin direction) with a much greater asymmetry value than the gamma ray anisotropy value. This asymmetry and preferred direction opposite to nuclear spin was unchanged after reversing the direction of the magnetic field, proving that the results were not due to remnant magnetization of the cobalt-60 specimen. Wu showed maximal parity violation in the weak interaction, a discovery which shocked the physics community.
Citations and resources:
https://en.wikipedia.org/wiki/Wu_experiment
https://journals.aps.org/pr/pdf/10.1103/PhysRev.105.1413
Figures:
Left: plot of beta asymmetry (in counting rate divided by average warm counting rate) against time (in minutes) for the polarizing field pointed up (H↑, upper curve) and down (H↓, lower curve). https://journals.aps.org/pr/pdf/10.1103/PhysRev.105.1413
Right: (top) plot of gamma anisotropy (in counting rate divided by average warm counting rate) against time (in minutes) measured by the equatorial (a, upper curve) and polar (b, lower curve) counters. (Bottom) plot of gamma ray anisotropy calculated using (a) and (b) against time. The curve is the same for the polarizing field pointing up and down. https://journals.aps.org/pr/pdf/10.1103/PhysRev.105.1413
Slide 12: Nobel Prize
Science details:
In 1957, C.D, Yang and T.D. Lee were awarded the Nobel Prize for their theory of parity violation in the weak interaction, and Wu was mentioned in their award speech. Many believe she was greatly overlooked for the Nobel Prize as she devised and conducted the experimental work that confirmed Yan and Lee’s theory of parity conservation in the weak interaction. Wu was not honored for her work until she was awarded the Wolf Prize in 1978. She has also received recognition, including but not limited to the National Medal of Science in Physics, and election as president of the American Physical Society in 1975.
Citations and resources:
https://en.wikipedia.org/wiki/Wu_experiment
Figures:
Top: Chien-Shiung Wu Commemorative Forever Stamp, issued on the International Day of Women and Girls in Science in February 2021. https://www.lanl.gov/discover/news-stories/2021/February/0211-wu-postage-stamp.php
Bottom: Depiction of beta decay from an atomic nucleus (protons in red, neutrons in blue). Inset shows a depiction of beta decay of a free neutron.
https://en.wikipedia.org/wiki/Beta_decay
Slide 13: Beta-NMR
Science details:
Wu’s legacy includes, but is not limited to the Beta-NMR (Nuclear Magnetic Resonance) technique. Beta-NMR is one of the most sensitive methods for determining electric quadrupole and magnetic dipole moments by detecting beta particles emitted from decaying radioactive nuclei. Since beta emissions are (an)isotropic when emitted from (un)polarized nuclei, measuring the angular distribution of emissions relative to the polarization axis of an external magnetic field reveals the spin-polarization of the decaying nuclei. Spin alignment with the applied magnetic field is due to the Zeeman effect: introducing particles with two possible spin states (±½) into a magnetic field splits the particles into distinct energy levels associated with positive and negative spin. This energy difference between nuclear spin states is proportional to the Larmor frequency, which is in turn proportional to the applied magnetic field. Initially, the nuclear spins in a substance are partially aligned with the external magnetic field. To destroy the asymmetric beta-ray distribution, one can apply radio frequencies at the resonant Larmor frequency to induce Zeeman splitting. The resonance signal is measured by counting the number of beta particles using detectors directly above and below the polarization axis. Then the magnetic dipole moment can be deduced from the resonant frequency given the known nuclear spin. Beta-NMR has wide applications due to its sensitivity of measurement, including material characterization, and detecting metal-ion interactions with biomolecules.
Citations and resources:
https://mriquestions.com/energy-splitting.html
https://groups.nscl.msu.edu/becola/bnmr.html
Figures:
Typical resonant spectrum of potassium-36 in potassium-bromide, shown as asymmetry change against frequency (in kHz). Data points with horizontal and vertical error bars are shown in red and black and a fitted curve is shown in blue. https://groups.nscl.msu.edu/becola/bnmr.html
Slide 14: Parity and Cosmology
Science details:
Following Wu’s discovery of parity-violation, charge conjugation parity symmetry (CP symmetry) was proposed to restore order, a rule which dictated that laws of physics remained the same when (1) all particles are exchanged with their antiparticles (charge symmetry) and (2) spatial coordinates are mirrored (parity symmetry). This is to say that the laws of physics should be the same for both matter and antimatter under CP symmetry. This is not the case, however, for weak interactions: experiments have shown that when particles are exchanged with their antiparticles, CP symmetry in the real and mirror world is violated. In particular, CP violation was directly observed in strange B-meson decays by LHCb in 2013. The expansion of the universe was largely driven by radiation (the dominant form of energy in the early stages of the big bang) before entering a matter-dominated stage. If CP symmetry was conserved, then the big bang should have created equal amounts of matter and antimatter, with no biases on the creation and annihilation of particles or antiparticles. Observations show that the non-dark-matter part of our Universe is mostly made up of matter, as opposed to 50-50 matter and antimatter. This imbalance has been theorized by cosmologists as a direct result of CP violation occurring in the first seconds after the big-bang. The three necessary conditions for producing matter and antimatter at different rates (Sakharov Conditions) are (1) baryon number violation: when more baryons are produced than anti-baryons, (2) charge and CP violation: such that there are a greater number of interactions that produce baryons than anti-baryons (in both the real and mirror world), and (3) interactions must occur outside of thermal equilibrium: which would allow for asymmetries. The third condition also allows for time-reversal symmetry (T symmetry). We now know that nature only conserves combined CPT symmetry, meaning when CP symmetry is broken then T symmetry must also be broken. Wu’s experiment on parity violation directly contributed to this understanding of laws of symmetries in nature and was profoundly important for the field of cosmology and nuclear and particle physics.
Citations and resources:
https://en.wikipedia.org/wiki/CP_violation
https://www.symmetrymagazine.org/article/october-2005/explain-it-in-60-seconds
https://en.wikipedia.org/wiki/Baryogenesis#GUT_Baryogenesis_under_Sakharov_conditions
Figures:
Bottom: illustration of scale with matter (left) outweighing antimatter (right). https://www.symmetrymagazine.org/article/october-2005/explain-it-in-60-seconds
Top: Depiction of CP symmetry in an electron (left) with clockwise spin and positron (right) with counterclockwise spin. CP symmetry states that the oppositely charged particle and antiparticle should also have opposite spins.
_C/w=1920,quality=90,fit=scale-down)
