Physics Projects Seek To Advance Particle Experiments
By Eric Gorton, JMU Public Affairs
The task is somewhat like trying to improve on an award-winning chili recipe, but in this case, the results won't be a matter of taste. The success of this recipe will be determined by aiming gamma rays at the creation, plastic disks doped with a special compound so they can be polarized and serve as both targets and detectors in particle acceleration experiments.
Swiss researchers pioneered the doping technique in the mid-1990s, producing polarized scintillating targets. The technology makes it possible to examine pion production, the production of sub-atomic particles that carry the nuclear force, and Compton scattering, the process of scattering light from a particle, at low energies. Physics department head Steve Whisnant and his students are working on a doping process they hope will not only replicate the work of the Swiss lab, but also improve on it. The disks they are working on will be put to the test in experiments at Duke University starting in fall 2008.
A big challenge so far, Whisnant said, has been finding the right type of compound and the right amount of compound to mix with the plastic to get the desired results. The multi-step process includes sending sample disks to the University of Virginia where they can be polarized in an apparatus that subjects them to temperatures within .1 degree of absolute zero (-459.67 degrees Fahrenheit) and a strong magnetic field.
A pending challenge will be finding a way to create disks that will enable research on interactions with neutrons, Whisnant said. The plastic that is being used to create the disks for the initial experiments contains regular hydrogen atoms that have one proton and no neutrons in their nuclei. To study interactions with neutrons, disks containing deuterium, a hydrogen isotope with a proton and a neutron in the nucleus, will have to be produced.
Creating the scintillating targets is one of two projects Whisnant and his students are working on for experiments at Duke, home of a free-electron laser that produces high-intensity gamma rays to study low-energy properties of neutrons and protons. Students also are busy testing large gamma-ray detectors that will be used to detect photons that are scattered from the targets. Whisnant received a $667,675 grant from the National Science Foundation to purchase eight annular detectors that will be installed at Duke's High Intensity Gamma Source.
The new annular detectors will enclose existing photon detectors that contain single crystals of sodium iodide that are 10 inches in diameter and 12 inches long. The addition of the new detectors will effectively make the old detectors about four inches larger in diameter, greatly improving their efficiency to detect gamma rays, Whisnant said.
"By doing it in two pieces like this, we can design the electronics to read this out in such a way that we can say, if I see something here and something here at the same time, that can't be real," Whisnant said while sitting amidst a menagerie of electronic equipment in one of two labs being used for the projects. "That must have been because of a gamma ray that did not scatter from the target. And so right now, what I have students doing is setting up the electronics. They turn all this stuff on and we can determine then with what efficiency can we reject all those cosmic rays. What fraction of the time does a cosmic ray hit the annular detector twice, and it looks like it's essentially 100 percent. I think it probably won't be quite that high when we start doing this for real, but it's going to be very high."
Whisnant said the students working on the projects are gaining experience that will be valuable when applying for graduate school. "This is exactly the type of equipment we would set up to do a real experiment. So once we get a little bit further in this and we're actually testing one of these detectors, then students will be using the real system, they will be writing software to do the data analysis and I think they will be well prepared to go down to Duke, to go to graduate school and just walk right in knowing what's going on."
In a third project, students are distilling hydrogen gas for experiments that will take place at Jefferson National Laboratory in Newport News. For those experiments, a purified form of hydrogen, which cannot be purchased, will be cooled to its liquid form and then frozen and polarized to create targets for experiments using much higher energy than the experiments at Duke.
"What we want, to be able to make our targets, is a particular molecule that has one regular H and one heavy H. So we have H and we have D, hydrogen deuteride," Whisnant said. "So we buy this gas from a company in Massachusetts, but they can't provide it to us in the purity we need. So I have a system for distilling the gas, separating out the isotopes, and what I have students working on over here is measuring with gas chromatography the composition of that gas."
The Strongly Polarized Hydrogen and deuterium ice (SPHICE) target will provide a unique opportunity to study polarized nucleons, in particular the neutron, in detail, Whisnant said. Using deuterium as a neutron target, researchers will be able to make simultaneous measurements on polarized protons and neutrons with a polarized photon beam.
“There have been some measurements on the proton. That’s the thing that’s been mostly studied," Whisnant said. "We have some pretty good numbers for a few of those parameters, but on the neutron, very little is known. A few measurements have been made, but the errors are large … the data is scattered pretty badly and so we’re really hoping that we can learn something about the neutron.”
Like the projects for Duke, the gas distilling operation is providing valuable experience for students, Whisnant said. "The students get experience handling liquid helium, they get some experience using a system that really does show them what thermodynamics works like and how we have to wait what seems like forever for anything to happen, we can't do anything quickly. Once the system is disturbed, it must return to equilibrium. They get a little experience analyzing the data, collecting that. They get a lot of experience, Travis Kelley's getting a lot of experience understanding the gas chromatograph system upstairs. Neither he nor I had any idea what we were doing back in May and now we have a system that he understands how to make it work and what to do if it doesn't. He's really learned a lot about this and taught me a lot about it too."
The chance to do hands-on research as an undergraduate is one of main reasons Kelley, a senior who will graduate in May, chose JMU over other programs. "I was impressed that undergraduate students do the research as opposed to ODU or George Mason, where you have to be a graduate student to work with this equipment. ... Not only is it allowed here, it's required," he said.
Another student, Ryan Burke, programmed a computer to monitor the temperature and the level of liquid nitrogen, Whisnant said.
Published February 2008