by Madeleine Needles, Haystack Observatory
An article to be published in the 1 September issue of Astrophysical Journal Letters will announce the first radio detection of deuterium by a team of scientists and engineers from MIT's Haystack Observatory. The team, led by Dr Alan E.E. Rogers, has made the detection using a radio telescope array designed and built at the MIT research facility in Westford, MA. This array, which is electronically steered and looks in several directions at one time, was built to detect and measure signals from the deuterium atom at 327 MHz. After gathering data for almost one year, a solid detection was obtained on May 30, 2005, with a deuterium-to-hydrogen ratio of 23 parts per million. The other members of the team working with Dr. Rogers are Kevin Dudevoir, Joe Carter, Brian Fanous, and Eric Kratzenberg - all of Haystack Observatory, and Tom Bania of Boston University.
The detection of deuterium is of interest because the amount of deuterium can be related to the amount of dark matter in the universe, but accurate measurements have been elusive. Because of the way deuterium was created in the Big Bang, an accurate measurement of deuterium would allow scientists to set constraints on models of the Big Bang, Also, an accurate measurement of deuterium would be an indicator of the density of cosmic baryons, and that density of baryons would indicate whether ordinary matter is dark and found in regions such as black holes, gas clouds, or brown dwarfs, or is luminous and can be found in stars. This information helps scientists who are trying to understand the very beginning of our universe.
Until now the deuterium atom has been extremely difficult to detect with instruments on Earth. Emission from the deuterium atom is weak since it is not very abundant in space. There is approximately one deuterium atom for every one hundred thousand hydrogen atoms, thus the distribution of the deuterium atom is diffuse. Also, at optical wavelengths the hydrogen line is very close to the deuterium line which makes it subject to confusion with hydrogen; but at radio wavelengths, deuterium is well separated from hydrogen and measurements can provide more consistent results.
In addition, our modern lifestyle, filled with gadgets that use radio waves, presented quite a challenge to the team trying to detect the weak deuterium radio signal. Radio frequency interference (RFI) bombarded the site from cell phones, power lines, pagers, fluorescent lights, TV, and even in one case where the doors had been left off of a telephone equipment cabinet. To locate the interference, a circle of yagi antennas was used to indicate the direction of spurious signals, and a systematic search for the RFI sources began. At times, Dr. Rogers actually asked for help from Haystack's neighbors, and in several instances replaced a certain brand of answering machine that was sending out a radio signal with one that did not interfere with the experiment. The interference caused by one person's stereo system was solved by having a part on the sound card replaced by the factory.
The Deuterium Array at Haystack is a soccer field size installation with 24 stations spread out across the sight. Each station has 24 crossed dipole yagi antennas for a total of 1152 receiver ports. The stations were completed in June of 2004, and started taking data immediately. This project was conceived and built at the Haystack facility at a cost of less than $1 million with support from the National Science Foundation, MIT and TruePositition Inc. The array is currently pointed at the anticenter of the galaxy and it is planned that the stations will be redirected toward the center of the galaxy at a later date.
Dr. Alan E.E. Rogers is currently a Senior Research Scientist and Associate Director of the Haystack Observatory located in Westford, MA. He received his PhD in Electrical Engineering from MIT in 1967.