SCHOOL OF PHYSICS
Part III Laboratories
OUTLINE OF EXPERIMENTS
NUCLEAR PHYSICS
Level 1, Nuclear Building.
The overall purpose of the laboratory is to acquaint students with the procedures of nuclear physics. These are now in widespread use in all branches of the physical sciences. In the laboratory students use radiation detectors to observe charged particles and gamma rays, and nuclear reactions are analysed.
Coincidence Spectroscopy & Angular Correlation (2 weeks)
In this experiment the energy level structure of a nucleus is decoded by observation of the gamma decay spectrum from a radioactive source. Gamma rays which are members of the same cascade are identified by using two NaI crystals and electronic timing to measure coincidences between the gamma rays contributing to the same cascade.
When an excited nucleus decays via gamma-ray emission, the angle of emission is related to the initial and final nuclear spins and their alignments. By a careful measurement of the gamma-ray angular correlation between two successive decays of a nucleus, the spin sequence of the 3 levels involved is determined.
Accelerator Experiments (4 weeks)
There is a 200 keV proton accelerator installed and operating in the laboratory. This is used to study nuclear reactions.
The reaction B(p,

) to C is studied at bombarding energies between 150 keV and 200 keV, using a scintillation counter and a multichannel analyser. This reaction has a strong resonance at about 168 keV incident energy, which provides an interesting phenomenon to study. The experiment which includes setting up and calibrating the scintillation counter, setting up and testing a vacuum system,operating the accelerator, and analysing the data, takes 4 weeks.
The reaction B(p,

) to C is studied below 200 keV bombarding energy using a surface barrier detector to observe the alpha particles, the data being collected in a multichannel analyser. From the energy of the alpha particles, the Q-value of the reaction is deduced, and the 2 and 3 body breakup studied. From the angular distribution, information is gained about the angular momentum of the resonance(s)contributing to the reaction. The experiment, which includes setting up and calibrating the detector, setting up and testing the scattering chamber vacuum system, operating the accelerator, and analyzing the data, takes 4 weeks.
Mossbauer Effect (2 weeks)
The Mossbauer Effect is the resonant emission of a gamma ray from a source, followed by similar resonant absorption in an absorber. Small changes in the gamma ray energy can be produced by a Doppler shift. This enables small shifts or splitting in the absorber energy levels
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to be measured to a very high precision. Consequently, the electromagnetic environment of the nucleus can be investigated.
DIFFRACTION PHYSICS AND MICROSCOPY
Level 1, Nuclear Building.
The diffraction laboratory unit gives the student an appreciation of the versatility and value of the diffraction technique as a solid state information probe.
Optical Diffractometer and Image Processing (2 weeks)
The optical diffractometer (first week) is used to introduce students to diffraction algebra by analysis of the diffraction patterns produced by some relatively simple two-dimensional masks.
A combination of empirical technique, theory and computer simulations will enable them to develop an intuitive understanding of the object-reciprocal space relationship.
Principles of Image Processing (second week) will be learnt using a variety of experimental images, including high-resolution electron micrographs of crystal structures. This experiment complements the x-ray diffraction and/or the electron microscope experiments.
X-Ray Diffraction (2 weeks)
The X-ray diffractometer confronts students with the basic techniques used to determine crystal structures. It allows them to experience three-dimensional reciprocal space. The X-ray patterns and crystal structures of some basic structures like BCC, FCC, HCP etc. will become familiar.
Scanning Probe Microscopy (2 weeks)
The techniques used in Scanning Probe Microscopy (SPM) allow for the highest resolution 3D measurements of surface properties of materials to date. SPM incorporates Atomic Force
Microscopy (AFM), which is sensitive to inter-atomic Van der Waals forces as well as electromagnetic forces, and Scanning Tunneling Microscopy (STM) which utilises electron tunneling to image the surface topology of materials.
In this experiment students will examine the surface topology and magnetic properties of a hard disk as well as other materials using both AFM and STM modes of the SMEENA scanning probe microscope. Students will gain an understanding of the forces involved in AFM, an understanding of electron tunneling, and how SPM allows for the imaging of materials at high resolution in atmosphere and at room temperature.
Electron Microscope (3 weeks)
At the time of writing, it is expected that this experiment will definitely be offered in second semester. It may be offered in first semester, but probably not until about half way through.
Students should feel free to indicate an interest in doing this experiment in either semester and we will try to accommodate them.
High performance Electron Microscopes are available in the laboratory and can be used not only for electron diffraction studies of crystals, but also for the imaging of perfect and imperfect crystals and for studying the fundamental physics of the imaging process. Electron diffraction
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from both polycrystalline and single crystal specimens will be examined and basic imaging theory and microscope performance is investigated. There is ample scope for a variety of project-type investigations once the operation of the instrument and the basics of diffraction pattern and images has been mastered.
ATOMIC-OPTICAL & MAGNETISM
Level 2, Main Building, Room 260.
The experiments in the Atomic - Optical and Magnetism Laboratory centre around the study of electron energy levels in atoms and molecules. In the Atomic - Optical laboratory students observe the spectra arising from various transitions using precision optical systems and analyze their results by directly applying their knowledge of Quantum Mechanics. Four experiments are currently available:
Zeeman Effect (2 weeks)
The Zeeman Effect involves the splitting of electronic energy levels by the application of a magnetic field. A precision Fabry-Perot Etalon is used to observe the low level spectral line splitting due to the Zeeman Effect in Mercury. The Fabry-Perot etalon is a sensitive instrument capable of precise measurements of visible light wavelengths (1 in 5 x 10EXP+7 accuracy).
Students apply the fundamental principles of quantised angular momenta in their attempt to develop an understanding of the observed phenomena.
Hyperfine Structure (2 weeks)
The Bohr model of atomic structure predicts, quite accurately, the electronic energy levels of
Hydrogen atoms. Larger atoms with more electrons have more complex spectra, with energy levels dependent on electron-electron, spin-orbit and relativistic effects. These effects create the
``fine structure'' of atomic spectra. Atomic energy levels may also be affected by extremely weak interactions between the orbiting electrons and the nuclear magnetic dipole, and electric quadrupole, moments. These effects produce extremely fine splitting of spectral lines. This hyperfine structure is observed and measured by students using a Czerny-Turner spectrometer.
Students perform an extensive quantum mechanical analysis of their data based on a model of the interactions involved.
Nuclear Magnetic Resonance (2 weeks)
Nuclei placed in an external magnetic field will have their atomic energy levels split into 2s+1 sub-levels. Resonant absorption and emission of radio-frequency photons will induce transitions between the sub-levels. The phenomena of NMR are studied and observed in a variety of materials, using a RF generator and detector, interfaced to a fast Digital Storage Oscilloscope.
The form of signal detected is affected by a number of secondary effects, including spin-lattice interactions, spin-spin interactions, thermal effects, and chemical effects. These effects are observed and explained by the students. Students develop skills in the use of a modern fast
Digital Storage Oscilloscope interfaced to a laboratory computer.
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PARTICLE PHYSICS
Level 1, Nuclear Physics Building, Particle Physics Lab.
Bubble Chamber Analysis (2 weeks)
In this analysis a Bubble Chamber film measuring facility is used to study an interaction between a high energy (approx. 1 GeV) antiproton and a deuterium nucleus. In the interaction the incident particles annihilate and produce new final state particles. The task is to identify these particles and understand the interaction via the more fundamental description of the quark model.
The analysis of the data is performed on one of the Physics research computers and requires each student to write a short computer program for this purpose. The subsequent interpretation of the results requires the application of relativistic mechanics and some elementary knowledge of particle physics, also provided by the demonstrator. At the conclusion of the two weeks each student will have at least a rudimentary knowledge of FORTRAN (or other language as appropriate), have applied relativistic kinematics in an experimental situation, have learnt the basics of the quark model, and used this model to further their understanding of the strong and weak forces in nature.
In second semester, the bubble chamber experiment will be replaced with a new, computerbased laboratory:
Computer analysis of electronic data acquired by the Belle experiment at KEK (2 weeks)
In this lab students will analyze events acquired by the Belle experiment at KEK in 1999. These data were part of the data set used to discover CP violation in the B-meson system in 2001.
Students will use a 3-dimensional event display to view and understand all aspects of the Belle detector and a particular event. They will be required to reconstruct the event and obtain the particular B-meson decay chain shown in the data.
Muon Detection and Lifetime Measurement (2 weeks)
In this experiment a multi-layer system of absorber and scintillator panels is used to detect muons and measure their lifetime. A convenient source of muons is the decay of pions generated in the upper atmosphere by incident cosmic ray protons. Fast electronics and accurate timing is used to detect the passage of muons through horizontal scintillator panels connected by light guides to photomultiplier tubes. Some of these may stop in the apparatus, detected by their non-passage through lower scintillator detector panels and side veto panels.
These decay via muons decaying to (electron + neutrino + antineutrino), with a lifetime of about
2 microseconds, whereupon the emitted electron may be detected. Students build up the detector system with appropriate timing and logic connecting the various individual detectors in a fashion similar to that of the vastly larger and more complicated detector systems used in modern high energy accelerators.
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ASTROPHYSICS
Level 2, Main Building, Room 259.
Observational Astronomy (2 weeks)
This includes a set of four exercises and short projects aimed at familiarizing students with some of the techniques of practical astronomy. It does not assume any previous background in astronomy. The topics covered will include:
(i)Charge Coupled Device (CCD) Analysis of a galactic star cluster. This will enable a
Hertzsprung--Russell diagram to be established yielding information on stellar temperatures, luminosities and masses.
(ii) Measurement of the Milky Way rotation curve. Using a small radio telescope to observe the velocity of neutral hydrogen in the Milky Way, a picture of how our galaxy rotates can be obtained.
(iii) Clusters of Galaxies photographed with the 48’’ Schmidt telescope are examined to estimate the value of the Hubble constant and thus the age of the Universe.
(iv) Uncatalogued Asteroids captured on high quality photographic plates are analysed to determine the location and thickness of the asteroid belt as well as an estimate of the total number of asteroids present in the belt.
Computational Astrophysics (2 weeks)
This experiment involves astrophysical simulation exercises based on a laboratory computer.
Students use Fortran or C to write computer programs to perform numerical simulations of processes of astrophysical interest. Students can develop algorithms for the simulation of binary stars, atmospheric braking for spacecraft, the passage of a black hole through our solar system, etc.
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