Physics 352: Experimental Modern Physics
Spring 2022
The labs meet: Thursdays, 2.00 – 4.50 p.m. in Small 143 and Small 133
Instructor: Irina Novikova
Office: Small 251 |
E-mail: ixnovi@wm.edu |
Office hours: by appointment |
Telephone: (757) 221-3693 |
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Web-site: http://www.physics.wm.edu/~inovikova/phys352/phys352.htm |
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Teaching assistants: Carlos Pernas |
E-mails: cpernas@email.wm.edu |
Office hours: by appointment (grading questions only) |
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Course structure
Expectations: given that this is an upper-level lab course, it is expected that everyone has basic knowledge in experimental techniques, data analysis and lab report preparation. Some helpful materials are linked below, but feel free to ask your instructor if you need any help. This course will be about improving your lab techniques and professional skills as a scientist.
Some potentially helpful information:
Lab report preparation guidelines
Acceptable
Overleaf template.
Experimental
uncertainties and error propagation
Matlab
tutorials from PHYS251
Introductory meeting (week 1): this meeting is mandatory. We will go over the schedule, syllabus, etc. Most likely it will be fairly short meeting (but everyone still needs to attend). The specific assignments will be discussed.
Experiments (next 12 weeks): Students will work on each experiments in groups of two, spending 4 weeks on each experiment. There are 7 possible experiments to choose from, and everyone will complete three experiments. The individual schedules will be decided during the first week, with students’ preferences in mind.
Final presentation (last week of classes): During the exam week each lab group will present a 15 minute talk based on the last experiment they have completed. All group members have to participate in the presentation preparation and delivery, and the grade will be based on the quality of the presentation, accuracy of the presented theoretical and experimental data and the ability to answer audience questions. Part of the presentation grade for each student will also be based on the questions asked after presentations of other class members.
Lab reports: All students will be expected to submit a written individual lab report after each lab (three total). The lab report grade will provide the major component of the final grade. Lab reports are due one week after completing the lab. Unexcused late submissions will normally result in 5% late fee per day. If there is a reason you won’t be able to complete the report on time, please contact the instructor for extension before the due date. The lab reports will be submitted in electronic format via Blackboard.
Professional development activity: There will be additional activities throughout the semester, such as peer review of the report drafts, elevator pitches for on-going experiments, short experiment descriptions for a broad audience. Each lab will have one of these activities, as shown in the course schedule.
Lab books: while there is no formal grading for the lab book, all students are expected to have one and keep careful records of all activity relevant to the experiments (settings on the equipment, values of the parameters for each measurements, name of the files or measured date, relevant calculations, etc.) In case any questions arise during grading of the reports, the log book will be used to resolve them. The log book can be kept either in a dedicated physical notebook or electronically (backed up!), and can be shared by the lab partners
Missed lab meetings: While students are expected to make any effort to attend every lab meeting, it is possible that they will have to miss a day due to illness or quarantine. If you have to miss a class, please contact the instructor and your partner immediately. Due to the multi-week nature of each experiment, it most likely be possible to make up the missed work, or contribute remotely. In case of prolonged absence it may be necessary to make adjustments in the lab schedule or alter the lab tasks to be completed remotely.
Office hours: Because of a small size of the class and social distancing settings, I will not schedule a designated time for office hours. Instead, any student willing to discuss any aspect of the course will be able to schedule time for a face-to-face or zoom discussion.
Course Schedule
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Date |
Experiment |
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Jan 27 |
Organizational meeting (Small 133) |
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Feb 3 |
Experiment 1 |
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Feb 10 |
Experiment 1 (check for experiment plan, lab report plan) |
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Feb 17 |
Experiment 1 (check of lab books, theory report section) |
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Feb 24 |
Experiment 1 (broad audience abstracts due) |
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March 3 |
Experiment 2 (reports for Experiment 1 due) |
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March 10 |
Experiment 2 |
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March 17 |
Spring break |
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March 24 |
Experiment 2 |
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March 31 |
Experiment 2 |
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April 7 |
Experiment 3 (reports for Experiment 2 due for peer review) |
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April 14 |
Experiment 3 (peer review reports due) |
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April 21 |
Experiment 3 (revised reports for Experiment 3 due) |
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April 28 |
Experiment 3 (elevator pitch talks are due) |
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May 5 |
Final presentations (reports for Experiment 3 due) |
Other dates to note: the add/drop deadline - Friday Feb. 4; the withdraw deadline -Monday March 28
Grading
Lab reports: 25% each (75% total)
Final presentation: 15%
Additional activities: 10%
General Lab safety rules
· No food or drinks are allowed.
· Check all your electrical connections before powering the equipment, especially if the experiment involves high voltage.
· Be careful with the lab equipment and handle it with care: almost any piece of equipment is hard to fix or replace. Please let the instructor or TA know if there is any problem with the equipment, either by your own doing or by someone else’s.
· Avoid touching any transparent surfaces of any optical component. A worthless looking pieces of glass may actually cost more than your smartphone, and a simple fingerprint can ruin them. If you did touch it by accident, notify the instructor or TA right away, so that the piece can be cleaned.
· Students are expected to follow all the mandated COVID-related safety policies, such as mask wearing, isolation and quarantine, etc.
Brief Description of the Experiments
Muon Lifetime (Nuclear/Particle computational physics): The muon (µ) is a member of the lepton family (electron, muon, and tau), all interacting or decaying either electromagnetically or weakly with other particles. Important numbers: massµ = 105 MeV, mean-life time 2.2 x 10−6 s (in the rest frame of the muon). Muons are a component of the cosmic flux reaching the surface of earth, resulting from the interaction of a high energy nucleon or light nuclei with the air in the upper terrestrial atmosphere. At an energy of typically 10 GeV/nucleon, the primary cosmic flux consists of 79% protons and 15% alphas and the remainder heavier nuclei all the way to lead.
The earth’s atmosphere can be considered to be a 15 km thick layer of air, with a constant pressure (and density) equal to that on the earth surface. In this model the muon flux decreases to 1/e its original intensity after one mean life-time. Assuming the muons moves at near the speed of light, the flux decreases to 1/e over (3 x 108 m/s) x (2.2 x 10−6)s = 660 m. The model atmosphere is thus 23 times thicker than the decay length of the muon, and the muon flux would be (1/e)23 = 10−10 times the initial (proton + alpha + ...) flux. Special relativity changes all that, as the lifetime in the earth reference frame system is boosted by the relativistic boost factor γ = (1-β2)1/2, β = v/c ≈ 1-ε. For a 1 GeV/c muon (rest mass 0.105 GeV/c2), γ = 10.5, β = 0.995 and thus ε = 0.005. You can now evaluate how much greater the muon survival probability is.
Compton (γ) Scattering (Nuclear/Particle physics): study response of doped NaI crystal to gamma rays, then measure the Compton scattering cross section. Compton scattering is scattering of a photon by a free electron: 𝛾𝛾 + 𝑒𝑒 → 𝛾𝛾′ + 𝐸𝐸′. Compton scattering (discovered in 1922) provided one of the earliest proofs of the duality of electromagnetic waves: wave and/or particle, depending on the means of observation and energy.
Another process of interest here is the photoelectric effect (PE): photon absorbed on an electron bound in an atom, with subsequent emission of an electron. In PE, the energy of the emitted electron is equal to that of the incident photon, minus the binding energy of the electron in the medium; in good approximation Ee = Eγ. Photoelectric effect is what is observed to measure the energy of the incident photon in the NaI crystal used to detect the scattered photon; but Compton scattering in the NaI produces a continuum on the low energy side of the PE peak, with an observable shoulder; it can also be used to obtain the energy of the incident photon. You will first observe both effects (Compton and PE) in the NaI, then proceed with measuring the scattering angular distribution and corresponding cross section, as well as the angular distribution of the scattered photon energy, from Compton scattering of the 662 keV gamma ray from 137Cs source on an aluminum target.
Pulsed Nuclear Magnetic Resonance (Condensed Matter physics): The proton in the hydrogen atom has a spin and magnetic moment. When placed in a magnetic field the spin orients itself along the magnetic field direction; in a water or mineral oil sample the protons become polarized. The time it takes to do that is a characteristic of the proton interaction with its surrounding; this time is T1 for spin-lattice relaxation.
By suitably applied radio frequency (RF) power, it is possible to make the spin of the protons rotate by 90 degrees, and thus lie in a plane perpendicular to the main B- field. Now the spins are going to diffuse in that plane with a characteristic time T2, called the spin-spin relaxation time. The amount of depolarization is measured by applying a second, 180 degree pulse and measuring the signal, called the echo, detected in a pickup coil wrapped around the probe. T1 and T2 are characteristics of the material of great interest in condensed matter studies.
Optical Pumping (Atomic/Optical physics): Optical pumping produces spin polarization in a gas of an alkali metal like rubidium, following absorption of circularly polarized photons. In the presence of an external magnetic field, B, the transition from the ground state populates some of the Zeeman split levels of an excited state, which then de-excite to some of the original states. In hydrogen-like atoms like Rubidium (and Potassium), one of the ground state levels cannot be depopulated by the polarized photons because of the selection rules for absorption of electromagnetic waves (∆M=+1 only, or ∆M= -1 only, depending on the polarization of the photon), resulting in an overall polarization of the medium (meaning that all spins are oriented preferably in a given direction, that of the external magnetic field, B). The gas used in this experiment is Rubidium, with two isotopes 85Rb and 87Rb, with different nuclear spins (but the same atomic spin). You will measure first the ground state gyro-magnetic factors (gF) of these two isotopes from the resonance or Larmor frequencies ω=gFµ0B/h, with gF the gyro-magnetic factor of the state with total quantum number F=S+L+I; S and L (J=S+L) are the spin and angular momentum of the electronic state, I is the spin of the nucleus. You will observe the de-polarization of the medium when the frequency of an applied radio frequency (RF) field matches the Zeeman splitting of either isotopes, and obtain the gFfactor for the 4 lowest hyperfine-structure states of Rb.
Mossbauer Effect (Condensed Matter physics): The energy of a photon emitted in the transition from an excited state to another state of a nucleus does not have the energy corresponding to the mass difference between these two states. The nucleus recoils, and momentum is conserved. This photon therefore is unable to resonantly excite the reverse transition in another nucleus of the same species. In fact the photon loses the same amount of energy in the absorption process too, as the target nucleus recoils. Many attempts were made in the past to compensate for this energy loss, for example by using supercentrifuges. What R. Mossbauer discovered in 1958 is that it is possible for the transitions (emission and absorption) to occur without energy loss to recoil: the whole matrix of the material of the source (a radioactive source of 57Co, which decays to 57Fe), and of the target (a sample of iron enriched in 57Fe), takes the recoil. The energy losses are then many orders of magnitude smaller, allowing for resonant absorption. The technique allows extremely accurate measurements of energy differences, and the determination of magnetic dipole and electric quadrupole fields inside an atom.
Noise Fundamentals (Electronics, Experimental methods): Noise in experiments usually refers to a randomly fluctuating voltage signal. Extrinsic sources of noise are a nuisance and their contribution to measurements should be minimized. However, there are also intrinsic sources of noise that are inherent in the system being studied. These intrinsic sources of noise are sometimes called fundamental sources of noise and represent the physics-based limit on the degree to which you can measure in a given experiment.
Two fundamental sources of noise are Johnson noise that arises from thermal fluctuations at finite temperature, and shot noise whose origin is the quantization of electric charge. The former allows you to determine the Boltzmann constant (kB), and the latter allows you to determine the magnitude of the charge (e) of the electron. Note that both kB and e are fundamental constants of nature.
Quantum analogues (Quantum physics): The experiment exploits the mathematical analogy between Schrödinger-Equation eigenstates on the one hand, and, on the other, the resonant modes of sound waves in air inside confined structures. Quantum Analogs uses sound waves in cylinders and spheres to model the quantum states in semiconductors, hydrogen atoms, and hydrogen molecules. The apparatus includes precisely machined aluminum cylinders, hemispheres and irises. The controller facilitates interfacing the speakers and microphones that generate and detect the sound. Such setup allows to see not just the frequency spectrum of resonant modes inside a sphere, but also map out the angular structure of each such mode — the spherical harmonics can be encountered experimentally. Using cylinders, it is also possible to model the location and width in frequency of a band of states, and the number of states in a band.
Mental and Physical Well-Being
William & Mary recognizes that students juggle different responsibilities and can face challenges that make learning difficult. There are many resources available at W&M to help students navigate emotional/psychological, physical/medical, material/accessibility concerns. Asking for help is a sign of courage and strength. If you or someone you know is experiencing any of these challenges, we encourage you to reach out to the following offices:
For psychological/emotional stress, please consider reaching out to the W&M Counseling Center https://www.wm.edu/offices/wellness/counselingcenter/; or (757) 221-3620, 240 Gooch Dr., 2nd floor. Services are free and confidential.
For physical/medical concerns, please consider reaching out to the W&M Health Center at https://www.wm.edu/offices/wellness/healthcenter/; or (757) 221-4386, 240 Gooch Drive.
For additional support or resources, please contact the Dean of Students by submitting a Care Report at https://www.wm.edu/offices/deanofstudents/services/caresupportservices/index.php; or by calling 757-221-2510, or by emailing deanofstudents@wm.edu.
For a list of many other resources available to students, see Health and Wellness Resources for Students
As your professor, I also ask you to reach out to me if you are facing challenges inside or outside the classroom; I will guide you to appropriate resources on campus.