The Ewen-Purcell Horn at Green Bank

(click on any picture for a large version)

This horn antenna, now displayed in front of the Jansky Lab at NRAO in Green Bank, WV, was used by Harold Ewen and Edward Purcell, then at the Lyman Laboratory of Harvard University, in the first detection of the 21 cm emission from neutral hydrogen in the Milky Way.  The emission was first detected on March 25, 1951.

Edward Mills Purcell:

Born in Taylorville, IL, August 30, 1912; Died March 7, 1997
Graduated Purdue University 1933, PhD Harvard, 1938
Worked on radar research during World War II
Shared the 1952 Nobel Prize with Felix Bloch for development of the nuclear magnetic resonance method of measuring magnetic fields in the nuclei of atoms.

Harold Irving Ewen:

Born in Chicopee, MA, March 5, 1922
Graduated Amherst College 1943, MA (1948) and PhD (1951) from Harvard Univ.
Later was President of Ewen Knight Corp (1952-) and Ewen Dae Corp. (1958-)

H.I. ("Doc") Ewen pictured with his horn on the occasion of a visit to NRAO-Green Bank May 22, 2001. He had not seen his horn antenna since 1956!

The Discovery of Hydrogen Radio emission by Ewen and Purcell

The original paper by Van de Hulst predicting the existence of the 21 cm line expressed doubt that the line would be detectable. A paper by Shklovski in 1948 was more optimistic. Ewen and Purcell assumed that the Dutch were probably not working on detecting the line and that the Russians might soon be. They could probably make the first sensitive detection attempt. It seemed likely that the experiment would yield a negative result, but the experience would be worth the trouble and a well defined upper limit to the 21 cm line emission strength would be important.

Purcell asked for, and received, a grant of $500 from the Rumford Fund of the American Academy of Arts and Sciences for the costs of materials for building the antenna (schematic at left), waveguide, and electronic components (schematic at right).

Ewen proceeded to design the horn antenna and the mixer and receiver, consulting with experts in these fields, including Sam Silver on antenna design and Bob Pound on mixers. The receiver used a frequency switching technique to cancel out background noise, a novel technique for astronomy at the time.

Ewen installed the completed horn antenna just outside a window on the fourth floor of the Lyman Lab at Harvard, with the waveguide leading in through the window to the receiver and recorder.

He had to contend with some unexpected hazards. In a heavy rain, the horn antenna acted as a funnel, flooding the lab, when the drain became plugged up. Also, during the winter, passing students found the horn a tempting target for snowballs.

The whole project, from receipt of the $500 to detection of the line, took one year. Since the work was done on the weekends, the total time spent working on the project was (2/7) x 12 = 3.4 months. The image at far right shows Ewen with the completed receiver at the time of detection.

After detecting the 21 cm line, Ewen learned that Prof. Van de Hulst was spending a sabbatical year at Harvard. Ewen wrote:

At the suggestion of Purcell, I went to the Harvard Observatory, in mid April, to meet and talk to Van de Hulst for the first time, and to tell him about the discovery. Ed told me that Van de Hulst was teaching a course at Harvard during the spring term. It was the same course that he taught at Leiden, during the prior fall term. When I met with Van de Hulst at the Observatory he called Oort, and I talked with Oort for nearly an hour describing the switch frequency technique. This was the first time that we learned the Dutch had been searching for the line for several years. Had we known, we would not have tried.
...... Doc Ewen

The Dutch group adopted Ewen's frequency switching technique and succeeded in detecting the line on May 11.

A further confirmation of the detection was made by Australian radio astronomers. This came about because Frank Kerr was on a fellowship at Harvard that year, on leave from the CSIRO Division of Radiophysics in Australia. On hearing about Ewen and Purcell's discovery, he wrote to J.L. Pawsey describing the experiment. Christiansen and Hindman, members of Pawsey's radiophysics group, built a receiver that used frequency switching, and were able to first detect the line on July 12.

The American and Dutch results were published in the same issue of Nature: Ewen and Purcell, Nature v.168, p.356, 1951; and Muller and Oort, Nature v.168, p.357, 1951. These were followed by a brief note in the same issue from Pawsey mentioning that Christiansen and Hindman had also detected the line. The Australian results were published more fully later: Australian J. Sci. Res., vol. A5, p. 437, 1952.

In the following years, observations of the neutral hydrogen line were done with increasing accuracy by numerous astronomers, primarily Dutch, American, and Australian. The technique proved to be successful in mapping the large-scale distribution of matter in the galaxy. Observations of the 21 cm line remains a very important branch of radio astronomy to this day, resulting in ever more detailed understanding of the interstellar medium in our galaxy, and in studies of external galaxies.

Based on notes and photographs courtesy of H.I."Doc" Ewen.

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.