A Unique Institution: The National Bureau of Standards 1950-1969

Among the division's various programs, the best known in the scientific world was its program in low-temperature physics. The Bureau had been involved in cryogenic research since 1904 when it obtained a hydrogen liquefier, but it was not until 1948, when it obtained a helium liquefier, that low-temperature physics research began in earnest. Built up by Brickwedde and supported by Condon, the low-temperature physics laboratory became one of the best in the world. It attracted a number of outstanding scientists. First was Emanuel Maxwell with his work on the isotope effect in superconductivity. He was followed by John R. Pellam, who did pioneering work in the determination of the speed of second sound in helium.'35 He in turn was followed by two outstanding young scientists from Oxford University, Ralph P. Hudson and Ernest Ambler, both students of Nicholas Kurti. Hudson in due course became chief of the division, and Ambler became the Bureau's eighth director in 1978. This cryogenics capability was the reason for the AEC's choice of the Bureau to build up their cryogenic engineering program...

In the early fifties, adiabatic demagnetization was a "hot" scientific area. Brickwedde hired first Hudson and then Ambler to work in the area, and by 1956, the Bureau was recognized as one of the foremost laboratories in the world for research at "very low temperatures." Now fate was to conspire to bring about one of the most famous—if not the most famous—experiments in the Bureau's history: the experimen- tal demonstration of the nonconservation of parity in "weak interactions."...

Now, the Cryogenic Physics Section of the Bureau, whose chief in 1956 was Hudson and in which Ambler was a principal scientist, knew how to orient radioactive nuclei. Both earned their doctorates in the Clarendon Laboratory at Oxford University where, under the leadership of Sir Francis E. Simon and Nicholas Kurti, there was a major research program in the physics of very low temperatures produced by magnetic cooling, coupled with the work of Brebis Bleaney, Maurice H. L. Pryce, and later with others on the techniques of nuclear orientation. Ambler and Hudson brought these techniques to the Bureau and, working with Georges M. Temmer of the Carnegie Institution, had published two papers on nuclear alignment in cerium-141, cerium-139, and neodymium-147, all radioactive nuclei.'40 Moreover, while still a graduate student at Oxford, Ambler, working with six others, had polarized cobalt-60 nuclei, and measured the anisotropic emission of the gamma radiation. But there had been no good reason at that time to tackle the experimentally difficult task of measuring the asymmetry of the beta radiation as was now being suggested by Lee and Yang.'4'...

The Lee-Yang work was not immediately known to Ambler and Hudson, but their own work and capabilities were generally known to most of the physics community. Consequently, on June 4, 1956, before the publication of the Lee-Yang paper, Ambler received a telephone call from Professor Chien-Shiung Wu, a colleague of Lee at Columbia University and herself an expert in beta decay. Ambler recalls, "I didn't know who she was, although I'd heard of the name. She said that Lee and Yang had had this idea that with beta particles from cobalt-60, more will come up in one direction of the field than the other. I said, 'Are you sure you mean up and down?' She said, 'Yes, up and down, that's the difference.' I said, 'Is there a preprint of that paper?' She said, 'Yes.' I said, 'Send me one.' So she sent me one. The first thing I did was to check with our radioactivity people and discovered that she was tops in her field, so it was a request to be taken very seriously." Having the request from Wu to carry out the Lee-Yang experiment, and knowing that it was a very serious request, Ambler checked with "some of the senior physicists at the Bureau, and they all shook their heads and said, 'It's a very, very, very long shot.' Ralph [Hudson] and I talked about it and we sort of decided, and I became convinced, that it was one of those things that is a risk you've absolutely got to take, because it was clear that the whole thing would be absolutely revolutionary. So I went to see Brick [Ferdinand Brickwedde] and explained it and told him that I thought we could do it with the budget we had. Damn if old Brick said, 'Well, Ernie, if it's not going to cost any more money, you go right ahead and do it.' I called her and said, 'Sure."

After several weeks of preparatory work, two nuclear physicists, Raymond W. Hayward and Dale D. Hoppes—the experts on beta radiation from the Bureau's Atomic and Radiation Physics Division—were asked to join the effort. Prof. Wu had been coming down from Columbia periodically with two graduate students, but it had become clear that beta radiation experts were needed on the spot. The objective of the experiment was the measurement of the forward-backward asymmetry of the electron emission from the polarized cobalt-60 sample.'43 Because the range of the beta rays is very short, the radioactivity had to be confined to the very surface layers of the para- magnetic salt used for cooling. And a serious concern was whether the surface layers would stay cold long enough to do the experiment. There clearly was only one place to do the counting of the emitted electrons, namely inside the experimental chamber just above the sample of cobalt-60. With this limitation, there was only one way to determine if there was any asymmetry of the electron emission with respect to the direction of the nuclear spin. First, the spins are oriented in one direction, say "up," and the electron counting rate determined. Then the spins are oriented in the opposite direction, say "down," and the counting rate determined again. If the counting rates are different in the two cases, the electrons are emitted preferentially along (or against) the spin direction, hence the emission is asymmetric with respect to the direction of the nuclear spin and parity is not conserved.

In more detail, to do the counting a small thin disk of anthracene was placed just above the sample and a Lucite light pipe carried the scintillations to a phototube outside the cryostat. The sample itself was a single crystal of cerium magnesium nitrate with a thin layer containing the cobalt-60 grown on its upper surface. Equatorial and polar sodium iodide photomultiplier counters placed well outside the cryostat mon- itored the gamma emission, hence the nuclear polarization of the sample.

Many operational difficulties had to be overcome as the work progressed and, in fact, at one point an entirely redesigned cryostat was constructed. Conclusive results were first obtained in late December 1956, some six months after Prof. Wu's first telephone contact. The experiment was carried out as follows: First the sample was cooled by adiabatic demagnetization using a large electromagnet. Then the latter was removed and a small solenoid magnet placed around the cryostat. Because of the interaction of the nuclear spins with the very strong field caused by the electron spins, a relatively small current passing through the solenoid oriented the electron spins and through them the nuclear spins along the magnetic field in the solenoid, either up or down depending on the direction of the current through the solenoid. Due to small but inevitable heat leaks, the sample continued to warm, and in about eight minutes warmed up to such a temperature that the nuclei were no longer oriented. During this time the electron counting rate dropped for one orientation of the field as the orientation in the sample decreased as monitored by the gamma emission. The counting rate reached a constant value when the sample was sufficiently warm that no orientation existed. Now the experiment was repeated for the opposite direction of current in the polarizing solenoid. The behavior of the electron counting rate with time was now the opposite of what it had been with the field in the other direction. If it previously decreased with time, it now increased, and vice versa, eventually reaching the same warm-temperature value. This experimental result proved conclusively that the emission of the electrons is preferentially along the spin, and further analysis shows that it is preferentially in the direction opposite to the spin. Hence there is an asymmetry of the electron emission with respect to the direction of orientation of the nucleus, and parity is not conserved in the weak interaction.

The demonstration of the nonconservation of parity in the weak interaction stunned the theoretical physics community, which immediately became concerned with the violation of other symmetry principles. The situation became more complex. Of particular interest were the symmetries of charge conjugation, (C-inversion of charge, or changing from particle to antiparticle), and time reversal (T). Work by Lee, Yang, and Reinhard Oehme published after the original cobalt-60 paper—but known to the Bureau group via communication with Yang—showed, as was already suggested in the original paper, that not only was parity not conserved in weak interactions, but chargeconjugation invariance was also not obeyed, although under certain conditions the combination of the two was. It then became very important to see if invariance under time reversal was also violated, for a very fundamental theorem due to Wolfgang Pauli and Gerhart Lüders states that the triple operation of charge conjugation (C), space inversion (P) and time reversal (T), or CPT, will always be conserved. Hence the Bureau group continued to work in the area, carrying out essentially the same experiments with cobalt-58 (a positron emitter and hence important because of charge conjugation), and later on manganese-52 specifically to see if time-reversal invariance could be proved. Within the limits permitted by the data, T was conserved.'45 Further work continued on yet other nuclei to obtain data on the parameters in the theory and the work became more and more nuclear physics.'44 In 1969 the final Bureau work on the conservation laws was carried out by Russell C. Casella, theoretician member of the Radiation Theory Section. By an analysis of experimental data on the decay of the neutral K mesons, for which it was known that CP invariance is violated, Casella showed that the CP violation is connected with a violation of time-reversal invariance, but that there was no evidence of CPT violation. 47 "In Nature, past and future are thus distinguishable even on a microscopic level."

In retrospect it is difficult to find a better example of what the Kelly Committee had in mind when it insisted that the Bureau get back to doing research in its unique mission. A group formed to work on the production, measurement, and application of low temperatures, and given some latitude to follow their own interests were, admittedly by a happy combination of circumstances, brought face to face with some of the most fundamental questions in all of physics. And they were able to answer at least one of them.

41 E. Ambler, M. A. Grace, H. Halban, N. Kurti, H. Durand, C. E. Johnson. and H. R. Lemmer, "Nuclear Polarization of Cobalt 60," Philosophical Magazine 44 (1953): 216-218. The possibility of observing the beta emission had been discussed often at Oxford, but it was not done for two reasons. First, because of the limited range of the beta radiation (compared to the gamma, which had been observed), the electrons could only get out of the surface layers of the paramagnetic salt used for cooling, and second, could not pass out of the cryostat. Most important, of course, before the Lee and Yang paper, accepted theory predicted no unusual effects, so that the scientific spur to do the difficult experiment was lacking.