Shortly before the start of World War II, several theoretical

physicists, including Hans Bethe and Carl von Weizsacker, advanced the

idea that the sun derives it energy from nuclear reactions within its

core. C. C. Lauritsen and William Fowler, nuclear physicists at

Caltech's Kellogg Laboratory, were among the first experimentalists to

appreciate the application of nuclear physics to stellar interiors.

Post-war strategies for studying nuclear processes in the stars

included an innovative series of unofficial, weekly seminars with Mt.

Wilson astronomers at director Ira Bowen's house, the testing of

Bethe's carbon cycle in Kellogg, and the collaboration with a diverse

group of scientists ranging from cosmologist Fred Hoyle to astronomers

Margaret and Geoffrey Burbidge. The events leading up to the

publication of the 1957 paper by Fowler, Hoyle, Burbidge, and Burbidge,

in The Reviews of Modern Physics, now regarded as a watershed in the

history of nuclear astrophysics, are discussed. For his work in low-energy

nuclear astrophysics, Fowler won the 1983 Nobel Prize in

physics

Goodstein, Judith R.

Greenberg, John L.

English

Caltech Authors - Main

DIVISION OF THE HUMANITIES AND SOCIAL SCIENCES 
CALIFORNIA INSTITUTE OF TECHNOLOGY 
PASADENA, CALIFORNIA 91125 
THE ORIGINS OF NUCLEAR ASTROPHYSICS AT CAL TECH 
John L. Greenberg and Judith R. Goodstein 
Paper prepared for the History of Science 
Annual Meeting, Norwalk, October 29, 1983 
HUMANITIES WORKING PAPER 97 
-November 1983 
ABSTRACT 
Shortly before the start of World War II, several theoretLcal 
physicists, including Hans Bethe and Carl von WeizsHcker, advanced the 
idea that the sun derives it energy from nuclear reactions within its 
core. C. C. Lauritsen and William Fowler, nuclear physicists at 
Caltech's Kellogg Laboratory, were among the first experimentalists to 
appreciate the application of nuclear physics to stellar interiors. 
Post-war strategies for studying nuclear processes in the stars 
included an innovative series of unofficial, weekly seminars with Mt. 
Wilson astronomers at director Ira Bowen's house, the testing of 
Bethe's carbon cycle in Kellogg, and the collaboration with a diverse 
group of scientists ranging from cosmologist Fred Hoyle to astronomers 
Margaret and Geoffrey Burbidge. The events leading up to the 
publication of the 1957 paper by Fowler, Hoyle, Burbidge, and Burbidge, 
in The Reviews of Modern Physics, now regarded as a watershed in the 
history of nuclear astrophysics, are discussed. For his work in low-
energy nuclear astrophysics, Fowler won the 1983 Nobel Prize in 
physics. 
THE ORIGINS OF NUCLEAR ASTROPHYSICS AT CALTECH* 
John L. Greenberg and Judith R. Goodstein 
Subjects ranging from physical chemistry and geophysics at the 
turn-of-the century, to biophysics and applied mathematics in more 
recent times, have successfully bridged several disciplines and become 
independent enterprises. 1 Nuclear astrophysics, the offspring of the 
marriage of nuclear physics and astronomy, is a case in point. 
Cal tech's Kellogg Radiation Laboratory pioneered the 
application of nuclear physics to astrophysics problems. Shortly 
before the start of World War II, several theoretical physicists, 
including Hans Bethe at Cornell and Carl von WeizsHcker in Germany, 
advanced the idea that the sun derives its energy from nuclear 
reactions within its core. To Caltech nuclear experimentalists C. C. 
Lauritsen and his student William Fowler, the suggestion about where 
the energy came from in stars crowned six years of intensive study of 
the excited states in the nuclei of the light elements listed in the 
periodic table. Lauritsen's group learned about Bethe's work early in 
1938 through Robert Oppenheimer, who was then dividing his time between 
UC Berkeley's physics department and Caltech's. "For those of us in 
Kellogg," Fowler later recalled, "this was a dramatic event in our 
lives. What we were doing in the lab had something to do with the 
stars.,,2 
2 
Pearl Harbor abruptly ended Kellogg's peacetime basic research 
program, and with it any chance of Lauritsen and Fowler following up 
Bethe's theoretical ideas. Post-war strategies for studying nuclear 
processes in the stars included a series of informal, weekly seminars 
with Mt. Wilson astronomers at director Ira Bowen's house, the testing 
of Bethe's carbon-nitrogen cycle in Kellogg's laboratory, and Fowler's 
collaboration with a diverse group of scientists ranging from 
cosmologist Fred Hoyle to astronomers Margaret and Geoffrey Burbidge. 
In 1957, the four published a comprehensive account of their work in 
Reviews of ~ Physics. Still a classic paper in the field, 
"Synthesis of the Elements in Stars" at once marked the successful 
passage of nuclear astrophysics from adolescence to maturity. 
The story of Kellogg's role in this work dates back to the 
early thirties, when Cal tech's Lauritsen, Carnegie's Merle Tuve, and 
Berkeley's Ernest Lawrence were the three big names in American 
experimental nuclear physics. The Tuve-Lawrence-Lauritsen 
correspondence preserved at the Library of Congress, the Bancroft 
Library and the Cal tech Archives lends substance to local oral 
testimony that the three laboratories ran on thinly-camouflaged 
institutional rivalries, intense scientific competitiveness, and a 
healthy dose of respect. 3 Above all, the letters are a reminder that 
Cal tech was at the cutting edge of nuclear physics within six months of 
the original artificial-disintegration-of-nuclei experiments of 
Cockcroft and Walton at the Cavendish in 1932. 
"Lauritsen's qualifications in nuclear physics are obvious," 
3 
Tuve said of his Pasadena colleague in 1937, several years after the 
three laboratory leaders had carved out rather different research 
programs. By then, the scientists had largely buried their 
differences, as well as the urge to rush into print first, right or 
wrong. Lauritsen had the ability to "work his men very hard and make 
them love it," he added. 4 Tuve did not exaggerate. 
The Kellogg Laboratory, built in 1931 with funds supplied by 
Detroit cornflake magnate, W. K. Kellogg, led a double life for much of 
the thirties. By day, C. C. Lauritsen's students operated and 
maintained the high potential x-ray tube used to treat cancer patients; 
by night they studied proton, deuteron, and helium-ion interactions 
with carbon, lithium, beryllium, and boron nuclei. By 1939, the 
cancer research treatment had run its course, the doctors and medical 
technicians had left, and the nuclear research that Lauritsen had 
cultivated and protected during the depression years took over 
Kellogg. 5 
From the start, the Kellogg group concentrated on the nuclear 
disintegrations and atomic transformations of carbon and the other 
light elements. An accomplished structural and architectural engineer, 
the versatile Danish-born Lauritsen excelled in designing simple, 
straightforward, and elegant experiments. The group's initial 
experiments consisted of making transformations from one element to 
another; to do this, they had to design and build high-voltage 
accelerators, and develop ion sources and detection equipment. 
Lauritsen's sensitive electroscopes became the industry's standard. In 
4 
1934, Tuve challenged in print the results from one of Lauritsen's 
experiments; Lauritsen simply sent him one of his meters and suggested 
he measure the rate of radioactivity again. Not long after, Tuve 
apologized and published a retraction. 6 
Lawrence's equivocal attitude towards Cal tech and the Kellogg 
research group permeates the letters he wrote to Tuve, a childhood 
friend and then, in 1923, a fellow graduate student at the 
University of Minnesota. Ten years later, in 1933, Tuve and Lauritsen 
were in a race to produce artificial neutrons with accelerators. 
Lawrence knew what was going on in Pasadena, because Oppenheimer 
divided his time between Berkeley and Ca1tech. On February 9, 1933, 
Lawrence wrote to Lauritsen, congratulating him on the production of 
neutrons. "From Robert Oppenheimer's account of your work, there can 
be little doubt that you are actually detecting neutrons. I understand 
also that Tuve in Washington has gotten plenty [of] good evidence of 
neutrons produced by 600 kilovolt helium ions.,,7 While praising his 
work, Lawrence wasn't about to concede the definitive discovery to 
Lauritsen yet. 
Lawrence told Tuve as much. In a letter to the Washington 
scientist dated 18 February, nine days later, he complained about 
Millikan's influence with the press. "I have noticed a report in 
'Science Service' that Lauritsen has produced neutrons, and the usual 
Cal tech ballyhoo is set forth regarding his being the first in the 
country, etc., to do it." And he urged Tuve to publish his own 
findings quickly, saying, "Despite the Science Service report it 
5 
appears that you are the first one to accomplish it." Tuve knew 
better. At the end of March, he wrote Lauritsen that he had made an 
extensive search for neutrons, all "with negative results." B Even so, 
Lauritsen refined and repeated the experiments again and again before 
sending the paper announcing his discovery of artificial neutrons to 
Physical Review in September of 1933. 
The historical connection between Kellogg's experimental work 
and Bethe's deduction that fusion powers the sun and the stars turns on 
a discovery made by Lauritsen and his graduate student Dick Crane in 
1934. Lauritsen and Crane bombarded carbon 12, the most abundant of 
the carbon isotopes, with protons, and to their surprise, the nuclei 
did not disintegrate--or so it seemed. When the proton bombarding 
energy exceeded 650 kev, they observed radioactive nitrogen 13 and 
gamma radiation instead. But Lauritsen waged a lonely battle in the 
beginning; few physicists, Oppenheimer included, were prepared to 
believe in 1934 that a particle could be added to a nucleus without 
some other particle being spun off to carry away the excess energy. 
Tuve, in fact, had quarrelled publicly with Lauritsen's findings, 
attributing his observations to natural deuterium contamination. When 
Lauritsen pressed him to repeat the experiment, Tuve replied, II OK , 
Charlie, lend me one of your electroscopes.1I 9 This time,' Tuve and his 
collaborator Lawrence Hafsted also found the narrow peaks, called 
resonances, in the excitation curves for the proton-induced activity. 
The proton-carbon excitation curves bore little resemblance to 
the smooth and continuous reaction-rate curves associated with deuteron 
6 
energy. "This marked difference in excitation curves," Fowler later 
wrote, "convinced Lauritsen and Crane that protons did indeed produce 
nitrogen 13 in carbon bombardment.,,10 The reaction Fowler described is 
an example of a process known as radiative capture. In this process, 
the projectile is captured by the nucleus and forms an excited state of 
a new isotope, or element, which then decays to its ground state by 
radiating a gamma ray. It is "resonant" because the projectile must 
have just the right energy to form the excited state of the new 
species. 
The discovery of radiative capture of protons by carbon fixed 
Kellogg's nuclear physics program for the rest of the decade. 
Convinced that the excitation levels in the light nuclei were the key 
to understanding the structure of the nucleus, Lauritsen and his 
students undertook detailed measurements of nuclear reaction rates of 
all the light nuclei. Fowler received his doctorate in 1936, and for 
the next three years, he and Lauritsen spent much of their time 
studying excitation curves, the yield of the activity produced versus 
energy, for the carbon and nitrogen isotopes bombarded with protons. 
At the end of the decade, Bethe suggested that the 
thermonuclear reactions underlying the conversion of hydrogen into 
helium in the stars depends in a crucial way on a catalytic process 
known as the carbon-nitrogen cycle. 11 In the first of the six nuclear 
reactions involved in the transformation cycle, a nucleus of carbon 12 
fuses with a proton or hydrogen nucleus to yield a nucleus of nitrogen 
13 and a gamma ray. Bethe's first reaction in the cycle matched 
7 
exactly Lauritsen's 1934 laboratory reaction. 
The controversy over the discovery of the radiative capture of 
protons by carbon 12 was still fresh in Lauritsen and Fowler's mind 
when news of Bethe's carbon-nitrogen cycle reached them. "When Bethe 
came out with the carbon-nitrogen cycle, we kind of felt a proprietary 
interest in this group of reactions," Fowler recalls, "because we had 
been working on them ••• [itl all tied very closely together. 1I12 By 
then, the Kellogg researchers had switched from an alternating-current 
high-voltage tube to a 2-MeV direct current electrostatic accelerator, 
capable of high resolution work. With the new Van de Graaff machine, 
Lauritsen and Fowler had begun to measure very carefully all the effects 
associated with resonance phenomena. Experimental work on reaction-
rates at resonance, locations and widths of resonances, and gamma-ray 
spectra at low energy in the light elements boomed. For the first 
time, they were in a position to do very careful excitation curves 
and only by accurately measuring nuclear reaction rates could problems 
such as Bethe's application of nuclear physics to astronomy be solved. 
Kellogg launched its low-energy nuclear astrophysics research 
program in 1946. Fowler's graduate student, R. N. Ball, took as his 
topic for a Ph.D. thesis the determination of the rates of the 
reactions in the carbon-nitrogen cycle at stellar conditions. Four 
years later, Fowler and Hall published their first paper on the 
problem. 13 
Ball's problems were considerable. First, it took time to build 
a low-energy Cockcroft-Walton-like accelerator to stimulate low, 
8 
stellar energies. Even so, the terrestrial laboratory energies were 
too high, and extrapolation to lower energies unavoidable. The 
reactions, moreover, that Hall was after occur infrequently at low 
energies. Indeed, it took Hall three years just to accumulate 1000 
positron tracks in the cloud chamber. In the sun, for example, the 
effective energy for the carbon-proton interaction measures only 30 
kilovolts; the machine hall built in Kellogg was a low-energy 150 
kilovolt machine -- this was the lowest energy the physicists could get 
in the laboratory and still detect something. In the end, Fowler and 
his students showed conclusively that the carbon-nitrogen cycle is not 
the dominant process in the sun. 
To be sure, Bethe had also suggested another process, the 
proton-proton chain. 14 The measurements made in Kellogg supported the 
latter process, in which protons combine to form helium, with the 
emission of large amounts of energy. To the question: what does the 
sun shine on? Fowler's group decisively answered: the proton-proton 
chain. Kellogg researchers had conclusively resolved the first 
nuclear-astrophysical problem based on Bethe's pre-World War 
deductions. 
9 
ENDNOTES 
* This research was supported in part by a grant from the 
Haynes Foundation. 
1. Extensive discussion on this point is provided in J. L. 
Greenberg and J. R. Goodstein, "Theodore von Karman and Applied 
Mathematics in America," Science (in press). 
2. W. A. Fowler, "Phyphty Years of Ph un and Physics in 
Kellogg," Engineering and Science, 1982, 45(4): 18-21, on p. 20. 
3. Robert W. Seidel discusses in detail the work of the three 
laboratories in, "Physics Research in California: The Rise of a 
Leading Sector in American Physics (unpublished Ph.D. thesis, 
History, University of California, Berkeley, 1978), Part II. 
4. Tuve to Slepian, 7 May 1937, Library of Congress, Tuve 
Papers. 
5. Charles H. Holbrow, "The giant cancer tube and the Kellogg 
Radiation Laboratory," Physics Today, July 1981, 6-13. 
6. L. R. Hafstad and M. A. Tuve, "Artificial Radioactivity 
Using Carbon Targets," Physical Review, 1934, 45:902-903; L. R. 
Hafstad and M. A. Tuve, "Induced Radioactivity Using Carbon Targets," 
Physical Review, 1935, 47:506. 
10 
7. Lawrence to Lauritsen, 9 February 1933, Bancroft Library, 
Lawrence Papers, C 10, f 36. 
8. Lawrence to Tuve, 18 February 1933, 18 March 1933, Library 
of Congress, Tuve Papers; Tuve to Lauritsen, 29 March 1933, Cal tech 
Archives, C. C. Lauritsen Papers, File 1.8. 
9. William A. Fowler, "Nuclear Astrophysics Today and 
Yesterday," Engineering and Science, 1969, 32(9):8-13, on p. 8. 
10. Ibid., p. 9. 
11. H. A. Bethe, "Energy Production in Stars," Physical 
Review, 1939, 55:434-456. 
12. Quoted in "William A. Fowler," transcript of an oral 
interview conducted by Charles Weiner, AlP, June 8 and 9, 1972, p. 39. 
13. R. N. Hall and W. A. Fowler, "The Cross Section for the 
Radiative Capture of Protons by C12 near 100 Kev," Physical Review, 
1950, 77:197-204. 
14. H. A. Bethe and C. L. Critchfield, "The Formation of 
Deuterons by Proton Combination," Physical Review, 1938, 54:248-254. 


The origins of nuclear astrophysics at Caltech

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