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Bates develops new experiment

Researchers working at MIT's Bates Linear Accelerator Center have developed a new and unique experimental capability that they have used to open a window into some of the fundamental properties of the proton.

Using a technique known as Virtual Compton Scattering (VCS), they have made precise measurements of how the shape of the proton responds to applied electric and magnetic fields.

"The VCS experiment has pioneered a powerful new experimental technique to study the electromagnetic structure of the proton," wrote Professor of Physics Richard G. Milner, director of Bates, in a message to the laboratory's users. "This is a major milestone for Bates."

VCS is the first nuclear physics experiment to be completed using the Bates South Hall Ring.

During the past year, two key components of the experiment were developed at Bates. They are the ability to collide nearly continuous electron beams with a proton target and the ability to observe the results of the collisions in a way previously unattainable.

Many nuclear physics experiments today are "coincidence" experiments, meaning they look simultaneously at multiple products of the collision of an electron from a particle accelerator with a nuclear target such as a proton.

To avoid accidental coincidences and maintain a good signal-to-noise ratio in the experimental data, it's necessary to keep the instantaneous current of the electron beam low. However, to achieve adequate precision in the final result, the average beam current must be as high as possible. The solution is to use nearly continuous beams, rather than the pulsed beams used in the past. Bates achieved this with a process known as "pulse stretching" by using its South Hall Ring to convert the pulsed beam from the linear accelerator to a quasi-continuous beam.

The effort to produce the necessary beams was led by Dr. Kenneth D. Jacobs, Accelerator Systems division head at Bates. "Commissioning the South Hall Ring as a pulse-stretcher was a challenging task. In the end, though, we succeeded, and we were able to complete this world-class experiment," he said.

Even with high-duty factor beams, the VCS experiment needed a new spectrometer system to be developed to detect the collision products travelling in directions not previously accessible.

Historically, fixed-target experiments such as VCS have measured the scattered particles in one plane, usually horizontal. This was due to the large size of the spectrometers, which could weigh as much as several hundred tons.

With the recently completed "Out Of Plane Spectrometer" (OOPS) system at Bates, researchers could measure scattered particles above and below the scattering plane, in addition to in-plane particles.

"We're pleased with the high-quality data we collected," said VCS experiment spokesman Dr. Jeffrey J. Shaw of the University of Massachusetts at Amherst. "Even though we've only begun preliminary analysis of the data, the worldwide nuclear physics community has already shown great interest in the experimental results. Now that the OOPS system is operational, we look forward to using it in other experiments."

Under development is the ability to deliver a nearly continuous spin-polarized electron beam, in which the spins of the individual electrons are aligned in the same direction. Use of such beams with OOPS will give the laboratory another capability not available anywhere else in the world.

"I congratulate the members of the OOPS collaboration on their farsighted scientific vision and sustained commitment over more than a decade to realizing their unique and powerful spectrometer," said Professor Milner.

"OOPS will continue as a major part of the ongoing program at Bates, exploring fundamental issues of nuclear matter," added Dr. Christoph Tschalaer, associate director of Bates, which is operated by the Laboratory for Nuclear Science. "As we speak, we are settingup for the next OOPS experiment using stretched beam, this time looking at properties of excited states of the nucleon."

The VCS experiment was conducted by a collaboration of institutions, including MIT, the University of Massachusetts, the University of Arizona, the University of Athens (Greece), the University of New Hampshire and Tohoku University in Japan.

A version of this article appeared in MIT Tech Talk on February 28, 2001.

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