cooling medium (paramagnetic crystal) and also on the surface, so that the beta particles could get out of the crystal. Could the contact and refrigeration be made adequate? Would back-scattering be, in consequence, a major drawback? How to count the B-rays? In situ? Remotely? And either way-exactly how? What of the external magnetic field necessary for polarizing the cobalt nuclei without heating up the refrigerating salt? What was the optimum activity of the B-source-large being best for detection sensitivity, small for minimizing local heating?

Polarization of the nuclei was achieved by cooling a paramagnetic crystal containing "Co to within 0.003 K and subjecting it to a magnetic field. At this temperature the effects of thermal agitation are so small that atomic nuclei can line up in a given direction within the crystal lattice when a magnetic field is applied. The magnetic polarity of the nucleus is determined by its direction of spin and, under the influence of a magnetic field, most of the Co nuclei align themselves so that their spin axes are parallel to the field. If parity is conserved in beta decay, then the intensity of the beta emission should be the same in either direction along the axis of spin. This, of course, was the critical question in the "Co experiments. It was resolved by measuring the intensity of beta emission in both directions, i.e., along and against the field direction.

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The Co was located in a thin (50 μm) surface layer of a single crystal of cerous magnesium nitrate (CMN). The crystal was placed in an evacuated flask which, in turn, was immersed in liquid helium within a Dewar flask surrounded by liquid nitrogen. An inductance coil on the surface of the inner flask was used to measure the temperature of the crystal in terms of its magnetic susceptibility. CMN is extremely anisotropic: the trivalent sites in its plate-like natural form are almost non-magnetic along the (out-of-plane) c-axis, but are uniformly magnetic in the plane. Co ions, however, would go into divalent sites which are contrary magnetically, i.e., most easily magnetized along the crystallographic c-axis. Thus in magnetic anisotropy, CMN should be ideal for the experiment: major (magnetic cooling) field in the plane, small polarizing (solenoid) field perpendicular to the plane, with negligible temperature-raising effect. But might not that polarizing field exert a torque on the crystal of sufficient strength to break a typically fragile thermally-isolating mounting?

A major experimental problem was the location of a radiation counter within the evacuated flask for detection of beta particles. This problem was solved by placing a thin anthracene crystal inside the chamber. to serve as a scintillation counter. The anthracene crystal was located about 2 cm above the "Co source. Scintillations caused by beta particles striking the

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crystal were transmitted through a glass window and a 120 cm lucite rod acting as a light pipe to a photomultiplier at the top of the flask. The resulting pulses were counted on a 10-channel pulse-height analyzer. It proved possible to design the light pipe so as to hold the resultant contribution to the liquid-helium loss rate to a tolerable level.

In addition to the beta counter within the vacuum chamber, two sodium iodide gamma scintillation counters were used externally to measure the directional intensity of the more penetrating gamma radiation. In this way the investigators were able to determine the degree of polarization of the "Co nuclei. The two gamma counters were biased to accept only the pulses from the photopeaks in order to discriminate against pulses from Compton scattering.

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Close to midway through the six-month work-up period, the team reached the conclusion that problems arising from outgassing within the crucial chambers of the apparatus would never be surmounted, and the entire assembly was re-designed from stainless steel to glass and a new version quickly constructed and assembled.

Cooling to the low temperature necessary for nuclear alignment was accomplished by the process of adiabatic demagnetization using a magnetic field of about 2.3 T (23 kilogauss). This process involved isothermal magnetization and subsequent isentropic demagnetization of the paramagnetic salt, CMN, which supported the "Co specimen. The heat produced by magnetization was removed by transfer through helium "exchange gas" and the boiling off of liquid helium in the surrounding dewar. The specimen was then thermally isolated by pumping out the exchange gas and upon demagnetization the temperature fell to about 0.003 K.

Next, a vertical solenoid was raised around the lower end of the outer dewar to provide a magnetic field for polarization of the Co nuclei. After the beta emission had been measured for this condition, the direction of the magnetic field was reversed and the beta emission again measured for the nuclei now polarized in the opposite direction. It was found that the emission of beta particles is greater in the direction opposite to that of the nuclear spin. Thus, a spinning "Co nucleus has a beta emission distribution that is not the same as that of its mirror image. This result unequivocally demonstrated that parity is not conserved in the emission of beta particles by Co.

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Beyond the primary question resolved by this experiment, another matter of great interest was "how large was the effect" since, in principle, the asymmetry-if observed at all-could have turned out to be anywhere from zero to maximum (asymmetry parameter from 0 to 1); it was, in fact, maximum. Thus the general opinion (largely derisory!) about the likelihood of the

proposal of Lee and Yang bearing fruit changed overnight, and the nuclear physics community scrambled to try out other tests that would now be quite feasible. In fact, colleagues of C. S. Wu were able to design an experiment at the Columbia University cyclotron and demonstrate within a day or two the "parity effect" in a π-u-e decay experiment [6], long before the NBS-Wu team could carry out all the "check experiments" they anticipated would now be demanded by the skeptics!

After those checks had been completed, a second experiment was performed [7] using 58Co, which is a positron emitter. In this case the positrons were emitted preferentially in the opposite direction to that of the electrons, that is, B* particles are preferentially emitted along the direction of the nuclear spins. This provided

additional confirmation of the conjecture of Lee and Yang and supported the new theory that was being developed at the time to explain parity non-conservation.

While the general U.S. physics community reacted rapidly with great interest and excitement to these momentous events, culminating in extraordinary jampacked sessions at the New York meeting of the American Physical Society in January 1957, other sentiments intruded upon the otherwise euphoric scene: Skepticism leveled at the original Lee-Yang proposal was replaced, in some minds, by disbelief in the results of these validating experiments, for parity conservation was an article of faith not to be discarded lightly. The NBS team's colleague from Washington's Carnegie Institution Department of Terrestrial Magnetism,

Georges M. Temmer, was on a laboratory odyssey in Western Europe at the time of the first news outbreak. At one point, Temmer found himself in the presence of eminence grise Wolfgang Pauli, who asked for the latest news from the United States. Temmer told him that parity was no longer to be assumed "conserved." "That's total nonsense" averred the great man. Temmer: "I assure you the experiment says it is not." Pauli (curtly): "Then it must be repeated!" [8].

Not long after this, the world settled down to the realization that Lee and Yang had been right. Interestingly, though, a reluctance to believe that NBS staff had played a significant role (indeed, any role for some minds) in the crucial experiment began to spread in the less-informed parts of the U.S. scientific community, and elsewhere. Even as early as that European tour of Temmer's, he encountered this. At a colloquium, also in Switzerland, and present when the proceedings were interrupted for an announcement of the "triumph at Columbia U.," Temmer spoke from the floor to make the correction that the work had been carried out at the National Bureau of Standards; he was-more or less politely-"hooted down." And despite his international standing even especially in that particular community he was pitied as being extraordinarily "mistaken." Later on, and forever afterward, hardly a speaker or writer referred to the event in any other term than "the Wu experiment" and only C. N. Yang himself and Chief Cryogenic Notable Nicholas Kurti went out of their way to try to set the record straight.

The further developments of the theory, together with a large number of follow-up experiments, have led to the unification of the weak and electromagnetic interactions. A description of both the history and the physics is available in the Nobel lectures by Weinberg, Salam, and Glashow [9].

In 1957, NBS moved rapidly to include those of its staff in the "parity experiment team" in its Honors & Awards for that year, presenting them with the Commerce Department's Award for Exceptional Service (the Gold Medal). In 1964 it added its own highest recognition, the Samuel Wesley Stratton Award. In 1962, the Franklin Institute of Philadelphia awarded its John Price Wetherill Medal to the full team.

C. S. Wu resumed her full-time preoccupation with B-decay research at Columbia University and the concomitant training of graduate students there. Over the years she received many honors, including the National Medal of Science (1975), the Wolf Prize in Physics (1978), and election to the Presidency of the American Physical Society (1975), the first woman to achieve that distinction. Wu died in New York in February 1997 at the age of 84 [10].

Ralph Hudson was appointed Chief of the Heat Division at NBS in 1961, and Ernest Ambler moved up to take his place as Chief of the Cryogenic Physics Section. For several years Ambler continued to carry out research, in collaborative efforts on oriented nuclei and superconductivity. He then went on to occupy a series of positions of increasing responsibility at NBS, culminating in Director of the agency-after several years as Acting Director-a post he held from 1978 to 1988 [11]. During this period, he received the President's Award for Distinguished Federal Civilian Service. Prior to his retirement from NIST in 1989, at the age of 65, he was Acting Undersecretary for Technology in the Department of Commerce.

Hudson continued to do research, as administrative preoccupations would permit, on cryothermometry and low-temperature magnetism. A review article coauthored by him received NBS's Condon Award in 1976 for distinguished authorship [12]. In the NBS reorganization of 1978, the Heat Division was abolished and Hudson became Deputy Director, under Karl G. Kessler, of the Center for Absolute Physical Quantities, with additional responsibility for managing the standards activity in Mass and Length. He resigned in 1980 and went to work at the International Bureau of Weights & Measures in Sèvres, France, as Director of Publications and editor of the international journal Metrologia. Upon retirement therefrom in 1989, at the age of 65, he returned to the Washington area and took a three-year temporary post at the National Science Foundation as Program Director for Low-Temperature Physics.

Raymond Hayward, after involvement with his colleagues in several follow-up experiments in the Low-Temperature Laboratory, returned to duty in the Radioactivity Section (Wilfrid B. Mann, Chief). When a separate Nuclear Spectrometry Section was created he was appointed Chief. He wrote a monumental treatise on the dynamics of particles of higher spin (>H2) [13], after which he devoted himself to the study of gravitation. He retired in 1980 at the age of 59.

Dale Hoppes continued experimental studies of betaparticle distributions from oriented nuclei [14] in the Nuclear Spectrometry Section, earning a Ph.D. from The Catholic University of America in 1961. He later returned to the Radioactivity Section, where he was involved in activity and gamma-ray-probability measurements. When Mann retired in 1981, Hoppes took over as Radioactivity Group Leader until his retirement from NIST in 1992 at the age of 64.

Prepared by Ralph P. Hudson.

Bibliography

[1] C. S. Wu, E. Ambler, R.W. Hayward, D. D. Hoppes, and R. P. Hudson, Experimental test of parity conservation in beta decay, Phys. Rev. 105, 1413-1415 (1957).

[2] See, for example, D. de Klerk and R. P. Hudson, Installation for Adiabatic Demagnetization Experiments at the National Bureau of Standards, J. Res. Natl. Bur. Stand. 53, 173-184 (1954); E. Maxwell, Isotope effect in the superconductivity of mercury, Phys. Rev. 78, 477 (1950); D. de Klerk, R. P. Hudson, and J. R. Pellam, Second sound propagation below 1 °K, Phys. Rev. 93, 28-37 (1954).

[3] Low-Temperature Alinement of Radioactive Nuclei, NBS Tech. News Bull. 40, 49-51 (1956); see also E. Ambler, R. P. Hudson, and G. M. Temmer, Alignment of Cerium-141 and Neodymium-147 Nuclei, Phys. Rev. 97, 1212-1221 (1955); E. Ambler, R. P. Hudson, and G. M. Temmer, Alignment of Three Odd-A Rare-Earth Nuclei, Phys. Rev. 101, 196-200 (1956).

[4] T. D. Lee and C. N. Yang, Question of parity conservation in weak interactions, Phys. Rev. 104, 254-258 (1956).

[5] J. M. Daniels, M. A. Grace, and F. N. H. Robinson, An Experiment on Nuclear Alignment: the Anisotropy of y-Radiation from Oriented Cobalt-60 Nuclei, Nature 168, 780-781 (1951); E. Ambler, M. A. Grace, H. Halban, N. Kurti, H. Durand, C. E. Johnson, and H. R. Lemmer, Nuclear Polarization of Cobalt 60, Philos. Mag. 44, 216-218 (1953).

[6] R. L. Garwin, L. M. Lederman, and M. Weinrich, Observations of the failure of conservation of parity and charge conjugation in meson decays: the magnetic moment of the free muon, Phys. Rev. 105, 1415-1417 (1957).

[7] E. Ambler, R. W. Hayward, D. D. Hoppes, R. P. Hudson, and C. S. Wu, Further Experiments on ẞ Decay of Polarized Nuclei, Phys. Rev. 106, 1361-1363 (1957).

[8] G. M. Temmer-private communication. Temmer, a gifted scientist of infectious enthusiasm, had played an important part in establishing the solid reputation of the NBS Cryogenic Physics Section's nuclear orientation program, first while employed in the NBS Radioactivity Section, later after moving to the Department of Terrestrial Magnetism.

[9] Steven Weinberg, Conceptual foundations of the unified theory of weak and electromagnetic interactions, Rev. Mod. Phys. 52, 515-523 (1980); Abdus Salam, Gauge unification of fundamental forces, ibid., pp. 525-538; Sheldon Lee Glashow, Towards a unified theory: Threads in a tapestry, ibid., pp. 539-543.. [10] R. L. Garwin and T. D. Lee, Obituaries: Chien-Shiung Wu, Phys. Today 50 (10), 120,122 (1997).

[11] E. Ambler, Historical Perspective: 1973-1989, in NBS/NIST: A Historical Perspective, A Symposium in Celebration of NIST"s Ninetieth Anniversary, March 4, 1991, NIST Special Publication 825, National Institute of Standards and Technology, Gaithersburg, MD (1992) pp. 31-40.

[12] R. P. Hudson, H. Marshak, R. J. Soulen, Jr., and D. B. Utton, Recent Advances in Thermometry below 300 mK, J. Low Temp. Phys. 20, 1-102 (1975).

[13] Raymond W. Hayward, The Dynamics of Fields of Higher Spin, NBS Monograph 154, National Bureau of Standards, Washington, DC (1976).

[14] A. T. Hirshfeld and D. D. Hoppes, Transition Mixing Ratios Determined from a Study of the Electron and Gamma-Ray Distributions from Oriented Iridium-192, Phys. Rev. C 2, 23412349 (1970).

One of the enduring goals of scientific work at the National Institute of Standards and Technology (NIST) has been the expression of measurements in terms related directly to natural phenomena of an invariant and absolute character.

For example, the unit of time, the second, is now defined as exactly 9,192, 631, 770 periods of oscillation of the radiation associated with a specified quantum transition between states of the 133Cs atom. This makes it possible, in principle, for any laboratory to realize the value of the second by preparing a sample of 133Cs in conditions that make it resemble a group of identical atoms unperturbed by their immediate environment.

Of course, such an ideal realization is not attainable in practice. This permanent fact of life has provided steady stream of work over the years for theoretical physicists at NIST: there is always a need for models that can provide better quantitative links between realistic and ideal situations.

Ugo Fano (1912-) is believed to be the first theoretical physicist hired by NIST, and he has certainly been one of the most influential to date. His 1961 paper Effects of Configuration Interaction on Intensities and Phase Shifts [1] is one of the most frequently referenced journal articles by a NIST author, having been cited over 3200 times in the scientific literature. This paper treats a subject of fundamental interest to metrology and physics: the excitation spectra of quantum-mechanical systems. Its key result, the simple formula given in Eq. (3) below, is now well known to physicists as the "Fano profile" or "Fano line shape." It addresses the challenge of expressing observed phenomena in a concise manner that can be derived from first principles. The celebrity enjoyed by this formula derives from the basic importance of the systems it describes, its wideranging practical utility, and the historical context in which it emerged. These aspects are discussed in turn below, though they cannot be entirely disentangled.

Among the phenomena key to the early development of quantum mechanics were atomic spectra, i.e., the colors of light absorbed or emitted by free atoms. Such light was found to consist largely of discrete frequencies whose distribution is a characteristic property of the atomic species involved (see Fig. 1). The existence of these discrete frequencies led Niels Bohr [2] to postulate a model of atomic structure in which the atom can subsist only in certain states of well-defined energy,

Fig. 1. Visible emission spectrum of the hydrogen atom (spectral lines of the Balmer series). The atomic radiation has been dispersed by a spectral grating and projected onto a photographic plate, to show the discrete frequency components of the radiation. The frequency increases to the right, with white indicating higher intensity (i.e., this is a photographic positive). If viewed by the eye, these lines would appear blue-violet. Figure courtesy of Prof. C. R. Vidal.

although transitions between such states can be induced. Transitions between two states may be associated with accompanying optical radiation, of angular frequency w。 = 2π▲E/h, where AE is the difference in energies of the two states, and h is Planck's constant. At this level of detail, the modern concept of atomic structure is the same as Bohr's. Thus, accurate data on intrinsic atomic frequencies has great fundamental and practical value. For example, as noted above, such frequencies now provide the legal definition of the second. NIST has for many years maintained a program to generate, evaluate, and maintain a database of relevant atomic spectral properties; that effort is summarized elsewhere in this volume.

Bohr's basic idea, that transitions between atomic states are associated with radiation of a definite frequency, needs to be broadened somewhat to describe phenomena actually encountered in the laboratory. For example, if one prepares a sample of atoms in a highenergy state, they may make transitions to a lowerenergy state by emission of optical radiation. This process will take place over some period of time. Under simplifying but widely applicable assumptions, Viktor Weisskopf and Eugene Wigner [3] showed that quantum mechanics describes the time dependence of the intensity of emitted radiation, I(t), as following the law of exponential decay,