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2011 Faculty & Staff News
Longwood physics professor part of international team of scientists doing experiments at national laboratory
November 21, 2011
Dr. Tim Holmstrom, assistant professor of physics, has collaborated on eight experiments over the last eight years at the Thomas Jefferson National Accelerator Facility, commonly called the Jefferson Lab, in Newport News. Six current or former Longwood students also have participated in the research since Holmstrom joined the Longwood faculty in fall 2008.
"All of these experiments are finished collecting data, and a number have published results," said Holmstrom, whose field is experimental nuclear physics. "I am currently doing analysis on two of the experiments, both of which, ‘Small Angle GDH' and ‘Transversity,' are part of the experiments with Helium-3. Most of these experiments have 50 to 80 collaborators. On some experiments, I contributed a lot, and on some a little. It can vary a lot."
"What we're looking at in these experiments is the structure of the neutron and the structure of the proton," he said. "The lab has an accelerator that accelerates electrons up to near the speed of light. We produce massive amounts of energy and use the electron as a probe, essentially as a giant microscope, to examine the neutrons and protons."
The accelerator, officially the Continuous Electron Beam Accelerator Facility, directs an electron beam into one of three experimental halls, each located in a different part of the lab. Holmstrom has collaborated on experiments in Hall A, the largest of the three staging areas (174 feet across and 80 feet tall from the floor to the highest spot on its domed ceiling). Hall A has two high-resolution magnetic spectrometers, which are connected and are virtually identical, that measure the momentum of the scattered particles. Each spectrometer weighs about 450 tons.
"We run the electron into a target - an object such as a gas, solid or liquid; sometimes a piece of carbon or hydrogen gas - trying to hit and neutrons and protons," Holmstrom said. "The electron hits the target, then scatters either right or left into the two spectrometers. You measure primarily the momentum - usually mass times velocity - of the scattered electron. The rate at which the electrons scatter for different targets, angles, and momentums tells us a lot about the structure of the protons and neutrons in the targets."
The accelerator, shaped like a racetrack, is seven-eighths of a mile long. As the electron beam makes up to five successive orbits, its energy is increased up to a maximum of six billion electron volts, known technically as 6 GeV. The electron beam circulates the track in 30 millionths of a second, helped by magnets in the arcs on each end, which steer the beam from one straight section of the tunnel to the next, and a refrigeration plant that provides liquid helium for ultra-low-temperature, superconducting operation. The accelerator and all three halls are underground, to avoid any possible exposure to radiation (the walls of the accelerator tunnels are two feet thick), though the researchers and lab staff monitoring the experiments are above-ground."
"After one to five loops, the electron beam splits and goes into one, or two, or all three halls," Holmstrom said. "Hall B has a detector that surrounds the target and detects particles, and Hall C, which is more like Hall A, has two spectrometers, though unlike Hall A, where the spectrometers are the same size, one is large and the other small. Other experiments take place in Hall B and Hall C."
"Jefferson Lab is unique; no other lab does exactly what we do. Nobody has an electron beam at our intensity and energy. Our advantage is this intensity, the number of electrons per second. Our experiments are done with high statistical precision. The lab is doing an upgrade, which will add a fourth hall and will double the energy capacity from 6 to 12 GeV within the next two years."
Holmstrom has worked on two types of experiments. He has worked on four experiments related to Helium-3 and four involving what is called parity.
"In the experiments with polarized Helium-3, we're doing measurements of the neutron-spin structure. This spin structure is heavily dependent on the properties of the ‘strong force.' One of the main focuses of the Jefferson Lab is studying the strong force, which binds particles called quarks together to form the nucleons themselves. We really understand electromagnetism very well, but we know far less about the strong force. We have a theory for it, but we can't do the mathematics. It's too hard."
"You fill up glass containers with Helium-3 - two protons and one neutron, whereas normal helium, Helium-4, is two protons and two neutrons - and you polarize the gas with a laser. Each atom is like a magnet, and we align all the little magnets so they're pointing in the same direction and get them pointing up, then you flip them so they're pointing down, scatter the electrons, and see what changes. You're studying the neutron; the protons cancel each other out. There is a lot of experimental data on the proton but not as much on the neutron. We're very interested in how the neutron behaves, and we're measuring neutrons to prove the proton data."
"The parity experiments are related to something called the ‘weak force,' though we're not studying the weak force. We're using it as a probe to study nuclear structure. Most of the parity experiments are looking at the structure of ‘strange quarks' inside of protons. Quarks are the components inside protons and neutrons. The proton always has at least three quarks - two up quarks and one down quark - and the neutron also has three quarks: two down and one up. Also, sometimes extra quarks and anti-quarks, called ‘sea quarks,' come in for brief periods, and these sea quarks make a contribution to the structure of the proton."
"In these experiments, we usually use just protons - hydrogen gas or a liquid - and we're probing the structure of the proton, specifically due to the sea quarks. You get all the electrons in the beam, which spin either right-handed or left-handed, and run them into the target and compare the scattering of right-handed versus left-handed electrons. Differences between them will tell us about how the sea quarks are arranged inside the proton."
"Everything is recorded on a computer and analyzed later in full detail. Plus, we check the data in real time to make sure nothing is broken. If something breaks during an experiment, we have to fix it. The beam usually runs from one to three months at a time, 24 hours a day, seven days a week, though sometimes experiments are stopped for maintenance. The beam typically runs from four to six months out of the year. It's not running right now but soon will start again. There are three shifts a day, manned by undergrads, graduate students, people doing postdoctoral fellowships, college faculty, lab staff."
"Several Longwood students have taken shifts there, checking data, and also have worked in my lab on the ground floor of Chichester. The students, all physics majors, are senior Ben Babineau; sophomore Garrett Josemens; James Bittner and Alvin Palmer, both of whom graduated in May 2011; Wolfgang Troth, who graduated in 2010; and Jeremy St. Johns, who transferred to Clemson. In the lab downstairs, we measure the thickness of the handmade glass targets used for Helium-3. During the school year, I go to the Jefferson Lab two or three times a month, including the one day a week I don't have classes or labs, and I also go there over winter break and summer break."
Holmstrom's involvement with the Jefferson Lab dates to 2003 when, on a postdoctoral fellowship, he started working there as a research associate for the College of William & Mary. He did that for three years, then continued his collaboration during the two years he was a visiting professor at Randolph-Macon College before coming to Longwood.
The primary mission of the Jefferson Lab, one of 17 national laboratories funded by the U.S. Department of Energy, is to "conduct basic research on the atom's nucleus using the lab's unique particle accelerator." More than 1,300 users are currently engaged in research at the lab, which also has more than 800 employees and contractors who share work space and research facilities with users on the lab's 206-acre campus. Hall A, where Holmstrom works, has collaborators from more than 70 institutions and 18 countries.
In the Chichester lab used by Holmstrom and physics students, an optics table contains a tunable diode laser that shines a laser onto a nearby glass tube suspended horizontally. "You measure the thickness of the glass by changing the frequency of the laser, which is a precise measurement," Holmstrom said. "We know that some energy is lost going into the tube and going out of the tube; it's crucial to know how much energy is lost. When the Helium-3 experiments start again, handmade cells will be brought here where measurements will be done, then the cells will go to the Jefferson Lab. Similar measurements are being done elsewhere, but there's always a need for measurements. It's neat that students have an opportunity to work on lasers, and optics, and electronics, and a little bit of everything."
A native of Marquette, Mich., Holmstrom is a graduate of the University of Michigan (B.S., physics and material science & engineering) and has graduate degrees from the University of Illinois (M.S., material science & engineering) and the University of Virginia (Ph.D., physics).