PHYS 403 :: Physics Illinois :: University of Illinois at Urbana-Champaign

Nuclear/Particles experiments

1

Alpha range experiment.

    The range of a charged particle in an absorber provides a measure of its energy. In this experiment, the range in air, and energy, of the alpha particles emitted from 241Am is determined using a solid-state detector in a chamber in which the air pressure can be varied, and different gases can be introduced. The gas-dependence of alpha particle range is a principle used in smoke detectors.

    Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. As the alpha particles travel through a gas, they ionize electrons in the gas and lose some of their kinetic energy. Eventually, the alpha particles stop, and this distance corresponds to their range in the gas. The alpha particles are detected by a silicon detector. When a charged particle travels through silicon, it loses energy and creates a free electron-hole pair at a rate of one electron-hole pair for an energy loss of 3.62 eV at a temperature of 300K. Knowing this allows us to determine the energy of alpha particles detected in this experiment.

To perform the data collection and analysis, we use NIM, which stands for nuclear instrumentation modules. The detected signals go to a multichannel analyzer, which creates a histogram of the voltage pulses versus pulse-height, which corresponds to the energy. We can then look at the resulting spectrum and use it determine the alpha particle energy and range in various gases.

2

Gamma spectroscopy experiment

      Gammy rays are extremely high-frequency electromagnetic radiations that are emitted during nuclei decaying from excited states. By studying the energy and distribution of gamma rays, we can gain understanding of the energy levels and spin structure of nuclei. Gamma ray spectroscopy finds applications in many different fields such environmental monitoring, homeland security, and nuclear non-proliferation.

     In this lab, we focus on understanding different detection methods for gammas rays.  We compare different gamma ray detectors: plastic scintillator, NaI detector and Ge detector. The first two detectors consist of a scintillator which generates photons in response to incident radiation, a sensitive photomultiplier tube which converts the light to an electrical signal and electronics to process this signal. The Ge detector detects the electron-hole pairs created by the gamma rays. After we have gained a good understanding of the detectors and measured their performances, we will move on to two applications of gamma ray spectroscopy:  measuring Rn content in air using charcoal canister and studying residual radioactivity inside a trinitite rock sample.

3

“Muon telescope” experiment

When high-energy galactic cosmic rays from space, predominantly protons, enter the atmosphere, they interact with nuclei present in the air (nitrogen, oxygen, argon) to produce neutrons, protons, and pions. Due to various interactions and decay processes, at sea level, the main constituent of cosmic rays are muons with a fluence rate of ~1.9 x 10-2 cm-2 s-1 for the United States. The energy and angular distributions of muons are very complex and, in fact, they cannot be separated. However, roughly speaking, most muons occur in the energy range 0.2–20 GeV, with a median value of 2 GeV. The goal of this experiment is to use three plastic scintillator detectors and form a “telescope” that can be pointed at different angles with respect to the angle to the surface normal to earth. Using the current setup, can we determine the absolute muon flux? What is the angle dependence of the muon flux? Can we determine that muons come from above and not below?

4

Cosmic Rays Muons Experiments

   This experiment spans the entire semester, with each group performing a different part of the experiment. The goal is to measure the lifetime and magnetic moment of the positive muon using the muon flux arising due to high-energy galactic cosmic rays from space, with a fluence rate of ~1.9 x 10-2 cm-2 s-1 for the United States.

    On absorption, muons decay into subsequent detectable particles. We set up 14 thin plastic scintillators that absorb the muons and measure coincidences between the detectors to signal when a muon has been captured and probe its decay. We determine the ratio of positive to negative muons in the sample of muons stopping in the apparatus and change the stopping medium to test the sensitivity to the capture rate law.

   Using this apparatus, we trap muons unchanged and measure their lifetime, spin precession and g factor (magnetic moment). The lifetime will allow us to determine the Fermi constant, which is a measure of the weak interaction strength, just as the fine-structure constant, α, is a measure of the electromagnetic strength. This lab also involves coding in Root, an object-oriented program and library developed by CERN, to collect and analyze the data.

5

gamma-gamma correlations experiment

   Some nuclear isotopes decay into pairs of high-energy photons, called gamma rays. Here we detect the angular correlation of the decay events using scintillating detectors placed on a horizontal stand that rotates. We use a multichannel analyzer to look at the spectrum of the gamma rays and set up a circuit to measure coincidences between the detectors as a function of angle. We use this setup to look at two materials: 

  1. Angular correlation of 22Na.  This is a simple back-to-back coincidence of two 0.511 MeV gammas from e+ e- annihilation. This provides a system with high signal-to-noise ratio for first measurements and allows us to determine the angular resolution of the detectors.

  2. Angular correlation of 60Co. This is more subtle, as the non-isotropic rate of counts only varies by about 16% from 90 to 180-degree separation. Here we use three detectors to make three simultaneous measurements with a clever measuring plan. We compare the results to the quantum mechanical theoretical prediction.

 

6

Mössbauer Spectroscopy

 This experiment is based on the Nobel-prize winning work of Rudolf Mössbauer. In 1958 he discovered that it is possible eliminate the recoil experienced by an atom as it emits a gamma ray by embedding it in a crystal structure that absorbs the recoil energy. This allows us to study nuclear energy levels by measuring the absorption of gamma radiation. The absorption profile is affected by the surroundings of the absorbing atom.

  Here we use an 57Fe sample and the Doppler effect to probe 57Fe hyperfine structure in a variety of samples, including samples you decide on and create from materials containing iron, such as rust and vitamins. The collected spectra give information on the nuclear interactions present, including: the isomer shift, arising from differences in nearby electron densities; quadrupole splitting, due to atomic-scale electric field gradients; and magnetic Zeeman splitting from non-nuclear magnetic fields. Due to gamma rays’ high energy and extremely narrow energy spread, Mössbauer spectroscopy is a very sensitive technique for measuring changes in energy (and hence frequency), with a resolution of a few parts in 1011.

AMO/Optics/Quantum Optics Experiments

7

Quantum Optics Experiments. Entanglement

   Erwin Schrödinger called entanglement "...the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought." Einstein, Podolsky and Rosen were disturbed by the nonlocal nature of interactions between entangled particles and suggested the existence of hidden variables not included in quantum mechanics. In 1964, John Bell came up with a way to test the hidden variable theory, and such tests have been the focus of experiments through to the present day. In this experiment, you will perform Bell’s test on two entangled photons created through interaction of a laser with a crystal, learning the basics of optics, single-photon detection, and statistical analysis in the process. Do the nonlocal correlations that Einstein referred to as ‘spooky action at a distance’ in fact exist?

8

Quantum Optics Experiments. Quantum Erasure

   In this experiment, you will study several unintuitive consequences of quantum mechanics by constructing an interferometer of the type used in the famous Michelson-Morley experiment. The Michelson interferometer can be tuned so that there is complete destructive interference in one output port and complete constructive interference in the other output port. If one uses wave plates to prepare a path-dependent polarization for the photon, the quality of the interference will degrade. The “which-path” information provided by different polarizations in each arm makes the underlying physical processes distinguishable. Remarkably, a polarizer can be used at the output of the interferometer to “erase” the which-path information and recover the interference fringes

9

Quantum Optics Experiments. Berry’s Phase

A Sagnac interferometer is a loop where a clockwise and counterclockwise path interfere. Because these paths overlap in space, the interferometer is very stable. We will use a Sagnac to study Berry’s phase – if the polarization state of light is cycled through a closed path on the Poincaré sphere (whose surface represents all possible pure polarization states, i.e., linear, circular, etc.), the photons will acquire an additional phase that depends only on the net solid angle subtended by the path of the polarization trajectory on the sphere.

10

Optical Pumping

Optical pumping is a widely used and powerful technique for exploring atomic energy states, atomic transitions, and atomic collisions using electromagnetism in the form of light, radio frequency, and uniform constant magnetic fields. Among the many applications of optical pumping is quantum information, in which precise control over the occupation of atomic energy states is critical to state preparation. This experiment explores the atomic physics of both isotopes of natural rubidium.

   The rubidium atom is an ideal model system to study: its energy states, in an externally applied uniform magnetic field, can be understood using a semi-classical model. This model describes the coupling of a single electronic orbital and spin angular momentum with the nuclear spin angular momentum and of the coupled system to the external field. The experimental determination of these atomic energy states can be compared to the theoretical predictions of the Breit-Rabi equation. The two isotopes of rubidium, 85Rb and 87Rb, with different nuclear magnetic moments, make the experimental data even richer.

   In this experiment we can explore a wealth of atomic physics, including temperature dependent cross-sections for photon absorption, zero magnetic field transitions, spin-spin collision processes, field inversion measurements, Rabi oscillation of the atomic magnetic moment, optical pumping times, and other atomic physics experiments.

11

Fluorescence

  In fluorescence, a material emits light after it has absorbed electromagnetic radiation. The emitted light is typically at a lower energy than the absorbed radiation, and the time over which the light is emitted is directly related to the average time for which the illuminated material is in the excited state. Different materials show different lifetimes, and many of them have lifetimes in the nanosecond range. The fluorescence lifetime can provide information about the composition of the material and its local environment. In our experiment we are investigating fluorescence in different ruby crystals with rather long fluorescence lifetimes – a couple of milliseconds. For measurement of the fluorescence lifetime we use two techniques – one in the time domain and the other in the frequency domain. Lifetime also depends on the temperature of the crystal; the fluorescence experiments can be done in temperatures ranging from 100 K to -400 K.

Condensed Matter Experiments

12

Superconductivity 1 (resistivity and I-V measurements)

Superconductivity is the quantum mechanical phenomenon of disappearance of the electrical conductivity at temperatures below a critical temperature. For most known metals the critical temperature is rather low (<18 K); to measure the resistivity of these superconductors we need to use a liquid He cryogenic setup. In this experiment we perform measurements of the resistivity of thin films of several metals (In, Al, Sn, Pb). From the resistivity vs temperature data we can extract the critical temperature of the superconducting transition (Tc) and how Tc depend on the thickness of the film. Another experiment  with these films is investigating the current-voltage (I-V) characteristic of the metallic film samples at different temperatures in vicinity and below Tc. The outcome from this experiment is the dependence of the superconducting critical current on the temperature. These laboratory experiments require preparation of thin film samples using vacuum deposition techniques. 

13

Superconductivity 2 (measuring the critical magnetic field)

This experiment is similar to “Superconductivity 1”; it is performed using a different cryogenic setup equipped with magnetic field control. We can measure the properties of the superconductors (thin films) in magnetic field. The result of these measurements is the dependence of the critical magnetic field on temperature and comparison of the results with theory.

14

Superconductivity 3 (Mutual inductance experiment)

In this experiment we are exploring the superconducting properties of thin films using a contactless technique. We apply a small AC magnetic field to the thin film sample and analyze the propagation of the magnetic field through the sample. As the material goes into the superconducting state it repels the magnetic field, which results in a significant detector response in measuring the value of the transmitted magnetic field. These results provide information on the critical temperature of the studied sample. Also from the raw data we can extract information on the penetration depth of the magnetic field into the superconductor and its temperature dependence (Meissner effect).

15

Tunneling spectroscopy of the superconductors.

This is a classical tunneling experiment based on analysis of the current (I) and its two first derivatives (dI/dV and d2I/dV2) generated by a Normal Metal-Insulator-Superconductor (N-I-S) sandwich under application of a DC bias voltage. Analysis of dI/dV in frames of BSC (Bardin-Schrieffer-Cooper) theory can be used to extract the value of the superconducting energy gap (D) of the studied material and finally determine its temperature dependence D(T). This experiment requires the preparation of samples using a vacuum deposition technique.

16

Pulsed Nuclear Magnetic Resonance (pNMR)

In this experiment we study spin-lattice (T1) and spin-spin relaxation times of different fluids containing hydrogen atoms. Small samples are placed in a uniform magnetic field. Radio-frequency magnetic field pulses tuned to the Larmor frequency of hydrogen atoms are applied to move the spin system out of equilibrium. We analyze how the system returns to equilibrium. Possible samples for investigation include various mixtures of organic fluids (glycerol-water, water-ethanol etc.) and water solutions containing paramagnetic or ferromagnetic impurities (water solution of CuSO4 or FeCl3). We can conduct an experiment analyzing the spin-lattice relaxation parameters of curing epoxy.

17

The Speed of the Second Sound in Superfluid 4He

Below 2.17 K the liquid 4He shows very interesting quantum mechanics phenomenon – superfluidity. Superfluid component has no entropy and has zero viscosity. It was predicted theoretically and then shown experimentally that in superfluid helium it is possible to excite the special kind of heat propagation which works as waves but not based on the diffusion. This phenomenon is named as the second sound and in this experiment we investigating the propagation of the second sound waves as the function of temperature.  

18

Ferroelectrics. Ferro1

This experiment investigates the ferroelectric and pyroelectric properties of different ferroelectric materials. This includes measuring of the complex dielectric susceptibility as the function of temperature, DC electrical field, and time (aging experiment). The spectrum of materials offered for measurements is very broad, including some classical ferroelectrics BaTiO3, KH2PO4 (KDP), KD2PO4; disordered ferroelectrics (relaxors) Pb(Mg1/3Nb2/3)O3 (PMN), Pb(Mg1/3Nb2/3)O3-PbTiO3; and some other lead-containing and lead-free compositions. Some of these materials show nonequilibrium glass-like properties and are interesting as subjects for fundamental research. This experiment includes sample preparation using crystal and polishing equipment and vacuum film deposition techniques. All measurements are controlled by an advanced data acquisition program. The Ferro1 experiment is also associated with a lot of data analysis.

19

Ferroelectrics. Ferro2.

Most ferroelectrics entering the ferroelectric region at temperatures below the critical temperature form so-called domain states. The ferroelectric begins with uniform polarization but below the critical temperature domain patterns arise – the polarization of the sample is broken into smaller macroscopic (usually micron size) domains of uniform polarization. Within the sample the domains are aligned in various directions to keep the net polarization of the material close to the zero. The exact pattern depends on the crystallographic structure of the studied material. Using polarizing microscopy, the domain structure of ferroelectrics can be visualized. This is based on the idea of the variation of the polarization of the light traveling through the polarized object. In this experiment we are using a Leica DM2700M polarizing microscope equipped with INSTEC temperature-controlled stage which can cover the temperature range 100 K - 800 K. Ramping the temperature across the critical temperature of the ferroelectric phase transition we can observe the entering of the material into the ferroelectric state and also further development of the domain structure of the material. In this experiment we are using transparent single crystals of several ferroelectrics like BaTiO3 and others listed in the Ferro1 description.

20

Ferroelectrics. Ferro3.

In this experiment we are investigating the polarization properties of the ferroelectrics. By applying a strong DC electrical field to the ferroelectric material we can modify the domain structure of the ferroelectric. This finally leads to uniform alignment of the domains in the direction of the electric field (saturation state). In this experiment we are measuring the polarization versus electrical field dependencies (P-E). P-E dependencies show the hysteretic behavior of the material and from these hysteresis loops we can extract some important material parameters like saturation polarization, coercive field and remnant polarization. The measurements can be performed at different temperatures (200 K- 450 K), and the mentioned parameters can be represented as functions of temperature. For P-E measurements we use the ferroelectric tester RT66B from Radiant Technologies Inc.  The choice of materials for the Ferro3 experiment is not restricted only to single crystals – we can also use ceramic materials.

 

21

Scanning Probe Microscopy (SPM)

This experiment includes working with scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The goal of this experiment is not to investigate the properties of materials specifically but rather to become familiar with these techniques, which already include some very sophisticated physics. In this experiment we offer the chance to investigate the structure of several microscale and nanoscale objects – different optical recording media (CD, DVD, blue-ray), pyrolytic graphite, blood cells, etc.