Dale D. Hoppes

Dale D. Hoppes was born in Liberty, IndianaIndianaIndiana is a state in the Midwestern United States; its capital and largest city is Indianapolis. It was admitted to the Union on December 11, 1816, as the 19th state, and its population in the 2020 census (April 1, 2020) was 6,785,528.Open the full entry, on September 13, 1928.⁠[1]

In 1950 he received a B.S. from Purdue University, and from that year he worked as a nuclear physicist at the National Bureau of Standards (later the National Institute of Standards and Technology, NIST).⁠[1]

In 1956 Tsung-Dao LeeTsung-Dao LeeTsung-Dao Lee (1926–2024) was a Shanghai-born American physicist who taught for decades at Columbia University. In 1956 he and Chen Ning Yang proposed that parity might not be conserved in weak interactions, for which they shared the 1957 Nobel Prize in Physics; from the 1970s he helped advance high-energy physics and the training of physicists in China.Open the full entry of Columbia UniversityColumbia UniversityColumbia University, formally Columbia University in the City of New York, is a private research university in New York City and a member of the Ivy League. Founded in 1754 by royal charter from King George II as King's College, it is the oldest institution of higher education in New York State and the fifth oldest in the United States. It was renamed Columbia College in 1784 and adopted the university name and moved to Manhattan's Morningside Heights in 1896. It has 17 schools and four affiliated colleges, with 35,978 students enrolled in fall 2025. Legal scholar Jennifer L. Mnookin became president on July 1, 2026.Open the full entry and Chen Ning YangChen Ning YangChen Ning Yang (1922–2025) was a theoretical physicist born in Hefei, Anhui, China, who shared the 1957 Nobel Prize in Physics with Tsung-Dao Lee for their investigation of the parity laws. In 1954 he and Robert L. Mills formulated non-Abelian gauge theory (Yang–Mills theory), and he worked in turn at the Institute for Advanced Study, Stony Brook University and Tsinghua University.Open the full entry of the Institute for Advanced Study proposed that parity might not be conserved in weak interactions and suggested experiments to test the idea, among them a measurement of the directional distribution of beta emission from oriented cobalt-60 nuclei. On the suggestion of Henry Boorse and Mark W. Zemansky, Chien-Shiung Wu of Columbia University contacted Ernest Ambler at the National Bureau of Standards, and the experiment was then arranged in the bureau’s low-temperature laboratory.⁠[2][3]

Ambler and Ralph P. Hudson, chief of the cryogenic physics section, first checked the situation with the bureau’s beta-decay experts, Raymond W. Hayward and Hoppes, and then agreed to try. Magdolna Hargittai’s article in Physics World dates a meeting between Chien-Shiung Wu and Ambler in Washington to September 1956, at which the two invited three of his colleagues at the bureau to join them: Hudson, an expert in cryogenics, and the radiation-detection experts Hayward and Hoppes.⁠[4][5]

Hudson’s article for NIST adds that Chen Ning Yang and Tsung-Dao Lee had consulted Chien-Shiung Wu, a beta spectroscopist and their colleague at Columbia University; that Henry Boorse of Columbia and Mark W. Zemansky of the City College of New York, who ran a modest research program in cryophysics at Columbia University, lacked the facilities the experiment required but knew that Ambler had recently moved to the National Bureau of Standards from the cryonuclear physics program at Oxford; and that the bureau’s low-temperature laboratory was chosen because it could work in the millikelvin range and its staff had experience in orienting atomic nuclei. The 2021 article in NIST’s “Taking Measure” blog states that Tsung-Dao Lee had described the ideas to her before the paper was published.⁠[2][6]

The same article relates that when Chien-Shiung Wu looked for a low-temperature laboratory to host the test, those at Oak Ridge National Laboratory and Leiden University were busy with their own work, while the one at the National Bureau of Standards was between major projects. In the question period of a colloquium that Tsung-Dao Lee gave at NIST in 2005 for the experiment’s 50th anniversary, Hudson said that she was at the top of her field and had worked on the Manhattan ProjectManhattan ProjectThe Manhattan Project was the United States program that developed the atomic bomb during the Second World War, managed from 1942 by the Manhattan Engineer District of the Army Corps of Engineers with British and Canadian participation. It built plants and laboratories at Oak Ridge, Tennessee, Hanford, Washington, and Los Alamos, New Mexico, among other sites, carried out the first nuclear test in 1945, and produced the bombs dropped on Hiroshima and Nagasaki, Japan, later that year; after the war its functions passed to the civilian Atomic Energy Commission.Open the full entry, so that her inquiry deserved serious attention; their superior, Ferdinand Brickwedde, approved the work on condition that it cost no money. In his oral history Ambler said “We went for it very hard” and recalled that senior physicists at the bureau considered the experiment a very long shot.⁠[6]

The same article relates that at the 2005 colloquium Tsung-Dao Lee read out Chien-Shiung Wu’s published account of her reaction: she and her husband had booked passage on the Queen Elizabeth to attend a conference in Geneva and then travel to East Asia, but she realized that she had to do the experiment immediately, before others did it first. Hargittai’s article says that in the spring of 1956 Tsung-Dao Lee asked her in her office about the experimental evidence for parity conservation in beta decay, that she proposed a cobalt-60 beta source polarized by demagnetization, and that she recalled it in 1973 as a “golden opportunity for a beta-decay physicist”; that spring she began planning the experiment and cancelled a trip to China, which she had not visited for about 20 years. The experiment required combining two techniques never before used together, and she lacked the equipment and expertise for work near absolute zero.⁠[6][5]

James Schooley’s tribute to Hudson says that Chien-Shiung Wu telephoned Ambler, acquainted him with the Lee–Yang paper, which then existed only as a preprint, and asked whether the National Bureau of Standards would try the experiment. Hargittai’s article has the joint team starting measurements in October 1956; the NIST article speaks of six months of work in 1956 and says that she sent graduate students to the bureau’s headquarters, then in Washington, and travelled there herself periodically. A NIST history volume states that Hudson shared with Ambler the responsibility for the cryogenic aspects of the experiment.⁠[4][5][6][7]

Hayward and Hoppes were both members of the Radioactivity Section of the Atomic and Radiation Physics Division and were responsible for the radiation physics portion of the experiment. Hoppes was still a graduate student and Hayward’s student, and the two helped design and construct the coincidence system used in the experiment.⁠[7][6][8][9]

In a NIST oral history interview Ambler recalled that Chien-Shiung Wu, Hayward and Hoppes contributed the beta-particle detection and counting side of the experiment, while he and Hudson contributed the low-temperature side.⁠[6]

The cobalt-60 was placed in a thin surface layer of a cerous magnesium nitrate crystal and cooled by adiabatic demagnetization. A thin anthracene scintillation crystal inside the vacuum flask detected the beta particles, the light passing through a 120 cm lucite light pipe to a photomultiplier, and two external sodium iodide gamma counters measured the degree of nuclear polarization. The work-up took about six months, during which outgassing problems led to a redesign of the apparatus from stainless steel to glass.⁠[2]

Hudson’s article gives further details: the anthracene crystal sat about 2 cm above the cobalt-60 source, the pulses were counted on a 10-channel pulse-height analyzer, the adiabatic demagnetization used a field of about 2.3 tesla (23 kilogauss), and the specimen reached about 0.003 K, the figure that NIST’s exhibit page also gives; that page puts the thickness of the surface layer holding the cobalt-60 at 50 µm. The biographical memoir of Chien-Shiung Wu written by Noemie Benczer-Koller for the National Academy of Sciences describes the crystal as “cerium magnesium (cobalt) nitrate,” gives the temperature as 0.01 K and the length of the light pipe as 4 feet.⁠[2][3][9]

The article also sets out the difficulties of the design: the beta source had to be in contact with the cooling medium, a paramagnetic crystal, and yet lie at its surface so that the beta particles could escape; the external field needed for polarization must not warm the crystal; and the strength of the source had to balance sensitivity against local heating. Cerous magnesium nitrate is strongly anisotropic magnetically, which allowed a large cooling field in the plane of the crystal and a small polarizing field perpendicular to it. The crystal sat in an evacuated flask immersed in liquid helium within a jacket of liquid nitrogen, an inductance coil on the inner flask measuring the temperature through the crystal’s magnetic susceptibility, and both gamma counters were set to accept only photopeak pulses.⁠[2]

Beta emission was measured with the polarizing field first in one direction and then in the other. Hudson’s article states that more beta particles were emitted in the direction opposite to the nuclear spin and that the asymmetry was as large as it could be; the NIST article quotes Chien-Shiung Wu calling the results “reproducible and huge.” The National Academy of Sciences memoir says that a large effect had been observed by the end of December 1956.⁠[2][6][9]

On December 27, 1956, the National Bureau of Standards experimenters saw the first signs of the asymmetry. Page 90 of Ambler’s notebook records that day’s experiment; Hudson later added “PARITY NOT CONSERVED!” at the top of the page. NIST’s page says that the first of two successful runs that day began at 12:04 and that the opposite trend was seen once the polarization was reversed.⁠[5][3]

At about 2 a.m. on January 9, 1957, the team was finally convinced of the result. The paper “Experimental test of parity conservation in beta decay” appeared in the February 15, 1957, issue of the Physical Review (volume 105, pages 1413–1415), with C. S. Wu, E. Ambler, R. W. Hayward, D. D. Hoppes and R. P. Hudson as authors in that order.⁠[6][2]

Once the team was convinced, Hudson took a bottle of wine from a drawer and set it on a table with small paper cups. Tsung-Dao Lee quoted Chien-Shiung Wu’s words: “We finally drank to the overthrow of the law of parity.” Hudson said that the wine was a Chateau Lafite Rothschild of the 1949 vintage.⁠[6][5]

The NIST article says that before publishing the team had to confirm the result under pressure, because researchers at Columbia University were preparing to publish on the same question; theorists had expected very little difference when the direction of the spin was reversed.⁠[6]

Hudson’s article states that Chien-Shiung Wu’s colleagues at Columbia University designed an experiment at the Columbia University cyclotron and within a day or two demonstrated the parity effect in a meson-decay experiment, long before the National Bureau of Standards team could carry out all the check experiments it had planned; the National Academy of Sciences memoir says that Leon Lederman, Richard Garwin and Marcel Weinrich detected the asymmetry in muon decay. Hargittai’s article relates that on January 4, 1957, Tsung-Dao Lee spoke about the bureau’s result at the Columbia University physicists’ regular Chinese lunch; that Leon Lederman telephoned Richard Garwin at home at about 8 p.m. that evening with a test using muons; that the two, using apparatus that Lederman’s graduate student Marcel Weinrich had built for another project, took measurements from the early hours of January 5 until dawn on January 8 and wrote their report that day; and that Tsung-Dao Lee advised against submitting it first, arguing that this would be unfair to the National Bureau of Standards team, which had worked for months.⁠[2][5][9]

Hargittai’s article says that the National Bureau of Standards team’s paper was completed on January 10, 1957; the team and Garwin, Lederman and Weinrich submitted their papers on the same day, both were received by the Physical Review on January 15, and the two appeared back to back in the February 1957 issue; on January 15 the physics department of Columbia University held a press conference announcing the violation of parity, which members of the National Bureau of Standards team attended.⁠[5][6]

The paper lists Chien-Shiung Wu first, with Columbia University as her affiliation, followed by the National Bureau of Standards authors in alphabetical order. Hargittai writes that the order was proposed by the bureau’s team, departing from its usual alphabetical custom: Ambler wrote to her that he had put Chien-Shiung Wu first as a courtesy because she had brought the preprint of the paper by Tsung-Dao Lee and Chen Ning Yang, while the other bureau authors believed it was a courtesy to a woman. She also notes that the two-page report never says that the experiment was carried out at the National Bureau of Standards, an omission she considers misleading.⁠[5]

In the same year the five published a second paper in the Physical Review (volume 106, pages 1361–1363); the experiment used cobalt-58, a positron emitter, showed the opposite effect, the positrons being emitted preferentially along the nuclear spin, and provided additional confirmation of the conjecture of Tsung-Dao Lee and Chen Ning Yang.⁠[2][3]

In 1957 the Department of Commerce presented its Gold Medal (the Award for Exceptional Service) to the four National Bureau of Standards experimenters.⁠[2][4]

The 1957 Nobel PrizeNobel PrizeThe Nobel Prize is an international award established under the will of Alfred Nobel, comprising five prizes in physics, chemistry, physiology or medicine, literature and peace, awarded since 1901; its funds are managed by the Nobel Foundation, whose Board is based in Stockholm. The Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel, established in 1968, is not a Nobel Prize.Open the full entry in Physics was shared by Tsung-Dao Lee and Chen Ning Yang.⁠[6]

After the parity experiment he continued experimental studies of beta-particle distributions from oriented nuclei in the Nuclear Spectrometry Section, and in 1961 he received a Ph.D. from the Catholic University of America.⁠[2][1]

In August 1961 the National Bureau of Standards issued his Technical Note 93, “An experimental study of beta decay using the radiations from oriented nuclei.”⁠[10][11]

In 1962 he, J. F. Schooley and A. T. Hirshfeld published “The Gamma-Ray Distribution From Oriented Cerium-141” in the Journal of Research of the National Bureau of Standards (volume 66A, number 4, July–August, pages 317–320); the paper was received on March 2, 1962. They remeasured the gamma-ray distribution from cerium-141 nuclei aligned in neodymium ethylsulfate crystals at temperatures down to 0.0162 K, found a stronger anisotropy than earlier measurements had given, and deduced a hyperfine constant and the E2/M1 mixing ratio of the 145 keV gamma ray in praseodymium-141. The paper acknowledges the suggestions of E. Ambler of the Cryogenic Physics Section.⁠[11]

Its references include an article by R. W. Hayward and D. D. Hoppes in the IRE Transactions on Nuclear Science (1958, NS-5, number 3) and a paper by R. W. Hayward, A. T. Hirshfeld, D. D. Hoppes and R. S. Kaeser that was then still to be published.⁠[11]

In 1962 the Franklin Institute awarded its John Price Wetherill Medal to Chien-Shiung Wu and her four collaborators at the National Bureau of Standards for demonstrating that parity is not conserved in weak interactions. The notice in Physics Today places the award ceremony in Philadelphia on October 17.⁠[2][12]

In the Franklin Institute’s register of laureates, the page for Ernest Ambler files the 1962 Wetherill medal under the subject area Chemistry, with the citation “Experimental disproof of the conservation of parity.”⁠[13]

In 1964 the National Bureau of Standards gave the parity team its own highest recognition, the Samuel Wesley Stratton Award.⁠[2]

In 1970 he and A. T. Hirshfeld published “Transition Mixing Ratios Determined from a Study of the Electron and Gamma-Ray Distributions from Oriented Iridium-192” in Physical Review C (volume 2, pages 2341–2349).⁠[2]

In 1976 A. T. Hirshfeld, D. D. Hoppes and F. J. Schima published “Germanium Detector Efficiency Calibration with NBS Standards” in the proceedings of the ERDA Symposium on X- and Gamma-Ray Sources and Applications, and in 1979 F. J. Schima, D. D. Hoppes and A. T. Hirshfeld published “An Accurate On-Line Method for the Evaluation of Peak Areas in Pulse-Height-Analyzer Data” in the volume Computers in Activation Analysis and Gamma-Ray Spectroscopy.⁠[14]

After returning to the Radioactivity Section he worked on activity and gamma-ray-probability measurements. When Wilfrid B. Mann retired in 1981, he took over as Radioactivity Group Leader.⁠[2]

He was a member of the International Committee for Radionuclide Metrology.⁠[1]

Members of the committee’s alpha-, beta- and gamma-ray spectrometry group agreed in 1979 to collect the nuclear data from measurements in their laboratories that bear on the efficiency calibration of germanium spectrometers. The compilation, edited by him and F. J. Schima, was issued in January 1982 as NBS Special Publication 626, Nuclear Data for the Efficiency Calibration of Germanium Spectrometer Systems, with contributions from 14 laboratories; the editors’ affiliation is given as the Center for Radiation Research of the National Bureau of Standards.⁠[14]

In their introduction the two editors explain that complete, self-contained submissions were included unedited, while informal communications and references were covered in the compilers’ comments; they suggested submitting only data obtained or published after 1968 and documenting uncertainties fully enough for an evaluator to judge them. The data are of three kinds, half-lives, photon probabilities per decay and relative photon-emission probabilities; the summary table includes only radionuclides with data from two or more laboratories, and the volume makes no evaluation of its own. It runs to 151 pages and carries Library of CongressLibrary of CongressThe Library of Congress is the library of the United States Congress. Founded in 1800 and housed on Capitol Hill in Washington, D.C., it answers reference requests from Congress, the public and other federal agencies.Open the full entry Catalog Card Number 81-600161; in the list of contributors he is the correspondent for the National Bureau of Standards.⁠[14]

The 14 contributing laboratories are Atomic Energy of Canada Ltd. (Chalk River); the Bureau International des Poids et Mesures (Sèvres); the Central Bureau of Nuclear Measurements (Geel); the Georgia Institute of Technology; Hiroshima, Nagoya and Rikkyo Universities with the Electrotechnical Laboratory in Tanashi (one entry; the list prints “Rikko University,” the contribution itself “Rikkyo University”); the International Atomic Energy Agency (Vienna); the Institut d’Électrochimie et de Radiochimie in Lausanne; the Idaho National Engineering Laboratory; the Lawrence Livermore Laboratory; the Laboratoire de Métrologie des Rayonnements Ionisants (Gif-sur-Yvette); the National Bureau of Standards; the National Physical Laboratory (Teddington); the Physikalisch-Technische Bundesanstalt (Braunschweig); and the University of Guelph. The contribution from Atomic Energy of Canada Ltd., by A. R. Rutledge, L. V. Smith and J. S. Merritt, lists a private communication from him, dated 1979, as its first reference.⁠[14]

The contribution of the National Bureau of Standards in the volume, “Nuclear Data for X- or Gamma-Ray Spectrometer Efficiency Calibrations,” begins on page 85 and is by D. D. Hoppes, J. M. R. Hutchinson, F. J. Schima and M. P. Unterweger. Its half-lives were measured with well-type ionization chambers filled with argon at 20 atmospheres, against radium reference sources; gamma-ray probabilities per decay were determined with germanium spectrometers for selected lines of yttrium-88, molybdenum-99, antimony-125, iodine-131, europium-154 and thorium-228; the probability of the 59.5 keV gamma ray of americium-241 was measured, with a specially constructed NaI(Tl) detector, as 0.3582 ± 0.0014, and the ratio of K x-rays to 88 keV gamma rays in cadmium-109 as 27.34 ± 0.27.⁠[14]

He served as the United States delegate to section II of the Consultative Committee on Ionizing Radiation Metrology.⁠[1]

In 1982 the National Museum of American History in Washington, D.C., marked the 25th anniversary of the parity experiment with an exhibit entitled “The Fall of Parity”; much of the apparatus shown was reassembled with the help of Hayward and Bruce Dove. NIST’s online exhibit of the same name went online in July 1997, and its gallery of photographs and short biographies includes a biography of Hoppes, labelled “Experimentalist.”⁠[15][1]

NIST’s exhibit page states that the 1982 exhibit was the work of Paul Forman, curator for modern physics at the Smithsonian InstitutionSmithsonian InstitutionThe Smithsonian Institution is a museum, education and research institution created by the U.S. Congress in 1846 with the bequest of the English scientist James Smithson. Based in Washington, D.C., it runs museums such as the National Air and Space Museum and the National Museum of Natural History, as well as the National Zoo.Open the full entry’s museum, that the title, the introductory paragraph, the section “Parity: What’s Not Conserved?” and certain other material of the online exhibit are taken from or based on his exhibit, and that its descriptive account of the experiment is largely taken from an article of April 1957 in the NBS Technical News Bulletin (volume 41, number 4); the online exhibit was prepared by the NIST Physics Laboratory (now part of the Physical Measurement Laboratory) as a project of the NIST Museum Committee and designed by W. C. Martin, J. Coursey and R. A. Dragoset. The page also says that the reassembled apparatus is on long-term loan from the National Museum of American History to the NIST Museum in Gaithersburg, MarylandMarylandMaryland is a Middle Atlantic state of the United States, deeply indented by Chesapeake Bay and bordered by Pennsylvania, Delaware, Virginia, the District of Columbia and West Virginia; its capital is Annapolis and its largest city is Baltimore. One of the original 13 states, it ratified the U.S. Constitution on April 28, 1788, as the seventh state to do so, and had a population of 6,177,224 in the 2020 census (April 1, 2020).Open the full entry.⁠[15]

NIST’s certificates record that Standard Reference Material 4329a (a curium-243 radioactivity standard) was prepared in March 1985 in the Radioactivity Group of the Nuclear Radiation Division, Center for Radiation Research, and Standard Reference Material 4370d (a europium-152 radioactivity standard) in February 1987 in the Radioactivity Group of that center’s Ionizing Radiation Division; both certificates state that the work was done under the direction of D. D. Hoppes, Group Leader. The certificates now in force were issued on January 27, 2020, and June 13, 2019, respectively; the latter adds that a re-measurement was made in July 2018 in the Radiation Physics Division of the NIST Physical Measurement Laboratory under the direction of B. E. Zimmerman, Group Leader of the Radioactivity Group.⁠[16][17]

Three further certificates, for the environmental radioactivity standards SRM 4350B (river sediment; certificate first issued on September 9, 1981, and revised on November 3, 2015), Standard Reference Material 4352 (human liver; certificate first issued in June 1982 and revised on December 8, 2016) and Standard Reference Material 4354 (freshwater lake sediment; certificate first issued in November 1986 and revised on December 8, 2016), state that the technical measurements leading to their certification were coordinated by D. D. Hoppes, whom the certificates, in their revisions of 2015 and 2016, describe as “formerly of NIST.”⁠[18][19][20]

In 1989 the Journal of Research of the National Institute of Standards and Technology (volume 94, number 6, November–December, pages 363–366) published his “Special Report on Standards for Radioactivity: Report on the 1989 Meeting of the Radionuclide Measurements Section of the Consultative Committee on Standards for the Measurement of Ionizing Radiations,” accepted on August 16. The article describes him as a physicist and leader of the radioactivity measurements group in the Ionizing Radiation Division of the Center for Radiation Research, NIST National Measurement Laboratory.⁠[21]

The report covers the 10th meeting of Section II of that consultative committee (CCEMRI), held at Sèvres, France; the abstract dates the meeting to May 1989, the “Background” section to March 29–31. It explains that CCEMRI helps the Bureau International des Poids et Mesures (BIPM) ensure the uniformity of physical measurements and has three sections, of which Section II is responsible for radioactivity measurements; representatives of 11 countries and of the European Community laboratory took part in the meeting.⁠[21]

The report notes that no official international standard existed for any radionuclide, each country’s standardizing laboratory developing its own methods of measurement; that NIST had received justified requests for more than 130 radionuclides; and that 28 comparisons of activity had been completed since 1961. A 1986 comparison of cadmium-109 showed satisfactory agreement on activity; 10 of the 18 participating laboratories also measured the emission rate of the 88 keV gamma ray, and the combined result for the gamma-ray probability per decay was 0.03614 ± 0.00012. Of the 1988 comparison of iodine-125 solutions the report says that the value suggested for the comparison, (59.4 ± 0.5) days, perhaps carried so large an uncertainty because of chemical instability or problems in detecting the low-energy radiations in past measurements; three methods gave very consistent results in-house but deviated conspicuously, by 1.5 percent, from the average of the other laboratories’ results, and a later recheck with the main solution agreed with the average of the other participants. The BIPM’s International Reference System (SIR) for gamma-ray emitters had by the time of the meeting registered 373 results for 48 radionuclides from 25 laboratories. The report ends by stressing that international cooperation is especially important in the standardization of radioactivity and mentions the associated meetings of the International Committee for Radionuclide Metrology on applied measurements.⁠[21]

He retired from NIST in 1992, at the age of 64, as leader of the Radioactivity Group.⁠[2][1]

In the same year he, Michael P. Unterweger and Francis J. Schima published “New and Revised Half-Life Measurements Results” in Nuclear Instruments & Methods in Physics Research (volume A312, pages 349–352).⁠[22]

In 1999 NIST released the web publication Radionuclide Half-Life Measurements made at NIST (version 2.0), by Michael P. Unterweger, Dale D. Hoppes, Francis J. Schima and J. S. Coursey.⁠[23]

In the same year volume 59 of Physical Review C carried “Long-term Measurement of the Half-life of 44Ti” by Fred E. Wietfeldt, Francis J. Schima, Bert M. Coursey and Dale D. Hoppes, starting on page 528. NIST’s publication record classes the half-life publication under “Websites” and gives its address as physics.nist.gov/Halflife.⁠[24][23]

The 16th annual meeting of the Council on Ionizing Radiation Measurements and Standards (CIRMS), held at NIST in Gaithersburg, Maryland, on October 22–24, 2007, on the theme “Measurements and Standards for Radiation Based Imaging,” gave its Randall S. Caswell Award to Bert Coursey; a slide headed “Thanks” in his presentation lists Dale Hoppes together with Robin Hutchinson, Wilfrid Mann, Randy Caswell, Walter Bambynek (marked “EC” on the slide), Lura Powell, Curt Reimann, Ray Kammer, Katharine Gebbie, Parney Albright and George Ryan, Jr.⁠[25][26]

The American Physical Society designated the place where the 1956 experiment was done a “historic site.” It was then on the campus of the National Bureau of Standards and by 2011 was the site of the University of the District of Columbia; the commemorative plaque unveiled on November 9, 2011, names C-S Wu, E. Ambler, R.W. Hayward, D.D. Hoppes and R.P. Hudson.⁠[27]

The plaque’s inscription says that the experiment revealed that in certain nuclear processes mirror-image pairs of events occur with different probabilities, a discovery that “revolutionized our understanding of nature’s fundamental laws.” Kate Kirby, executive officer of the American Physical Society, Allen Sessoms, president of the University of the District of Columbia, and Katharine Gebbie, director of NIST’s Physical Measurement Laboratory, took part in the unveiling.⁠[27]

Nearly 50 years later Hoppes described the team’s very clumsy equipment: the glass joints of the cryostat were sealed with a mixture of glycerin and detergent (the article specifies Palmolive, and later Ivory) and kept leaking, and it took 6–8 hours to warm up, re-grease and cool the cryostat again. He slept in the laboratory near the device, both to save time and to let the other team members know when they needed to come back. His verdict was “It was an outrageous experiment to try and it’s surprising that it worked!”⁠[6]

Chien-Shiung Wu had teaching duties at Columbia University, could not be at the National Bureau of Standards all the time, and was not present on the day the first signs of the asymmetry were seen. In an e-mail to Magdolna Hargittai, Hoppes wrote that she probably regretted her absence to the last day of her life. Hargittai writes that on hearing the news Chien-Shiung Wu hurried to Washington and a few days later informed Tsung-Dao Lee and Chen Ning Yang at Columbia University; the NIST article adds that she travelled to the bureau and was very hesitant to accept the results.⁠[5][6]

Hudson’s article says that the result overturned a principle that nuclear physics had accepted for about 30 years, that lifting the restriction of parity conservation resolved the “tau-theta puzzle” in the theory of subatomic particles, and that later theory and experiment led to the unification of the weak and electromagnetic interactions, for which he refers to the Nobel lectures of Weinberg, Salam and Glashow. The NIST article cites the Massachusetts Institute of Technology physicist and historian of science David Kaiser, writing in Science on February 5, 2021, that the discovery set the stage for modern particle theory. Hargittai’s article also relates that Valentine Telegdi and Jerome Friedman began another experiment in the summer of 1956 without knowing of the work at the National Bureau of Standards, were already taking measurements in October, and had their paper received by the journal on January 17, 1957; she judges it most probable that the bureau’s team saw the first genuine signs, on December 27, but needed time to verify them.⁠[2][6][5]

Hudson’s article describes the reaction among physicists: sessions at the American Physical Society meeting in New York in January 1957 were packed, while some scientists exchanged their earlier skepticism about the Lee–Yang proposal for disbelief in the results. When the news broke, Georges M. Temmer of the Carnegie Institution’s Department of Terrestrial Magnetism, a colleague of the bureau’s experimenters, was touring Western Europe; when he informed Wolfgang PauliWolfgang PauliWolfgang Pauli (1900–1958) was a Vienna-born theoretical physicist who received the 1945 Nobel Prize in Physics for the discovery of the exclusion principle, also called the Pauli principle, and who in 1930 proposed the neutral particle later named the neutrino; he was professor of theoretical physics at ETH Zurich from 1928 until his death.Open the full entry that parity was no longer assumed to be conserved, Pauli called it nonsense and said the experiment would have to be repeated.⁠[2]

The same article says that less well-informed parts of the scientific community in the United States and elsewhere were reluctant to credit the staff of the National Bureau of Standards: at a colloquium in Switzerland Temmer corrected an announcement of the “triumph at Columbia” by pointing out that the work had been done at the bureau and was hooted down, and afterwards the experiment was almost always called “the Wu experiment,” with only Chen Ning Yang and Nicholas Kurti trying to correct the record.⁠[2]

Hargittai’s article says that the press conference at Columbia University added emphasis to Chien-Shiung Wu’s role and that the label “Wu experiment” was a convenient shorthand because she had proposed the experiment; Telegdi, Nicholas Kurti and Christine Sutton all questioned the label and stressed the cryogenic work, Ambler and Hudson both having been Kurti’s students at Oxford. A commentary by Kurti and Sutton in Nature in 1997 (volume 385, page 575) called the experiment a collaborative team effort combining nuclear and cryogenic expertise; the Nobel Prize laureate Val Fitch, interviewed by her in 2002, said that all who did the cobalt-60 experiment contributed substantially although “Ms Wu is often given the credit”; and Telegdi, interviewed the same year, held that no experimentalist deserved the prize because the experiment used known techniques and had been suggested by theorists. Her own impression from the surviving participants was that the roles of Chien-Shiung Wu and of Columbia University may have been overemphasized in early coverage.⁠[5]

The NIST article points out that the National Bureau of Standards researchers received neither a Nobel Prize nor a Wolf Prize. Hargittai cites the explanation of Anders Bárány, a former secretary of the Nobel Committee for Physics, that the prize requires work published before the year of the award, so that the three experimental papers, all published early in 1957, could not have qualified in 1957; she adds that the statutes allow at most three laureates and that the number of physicists involved in the experiments would have made any choice difficult.⁠[6][5]

Hudson’s article ends with the later careers of the other experimenters. Chien-Shiung Wu returned to full-time research on beta decay and the training of graduate students at Columbia University, received the National Medal of Science and was elected president of the American Physical Society in 1975, received the Wolf Prize in Physics in 1978, and died in New York in February 1997 at the age of 84. Hudson himself became chief of the Heat Division in 1961, was deputy director, under Karl G. Kessler, of the Center for Absolute Physical Quantities after the reorganization of 1978, resigned in 1980 to become director of publications and editor of Metrologia at the Bureau International des Poids et Mesures in Sèvres, France, retired in 1989 at the age of 65 and then held a three-year temporary post as program director for low-temperature physics at the National Science Foundation. Ambler succeeded him as chief of the Cryogenic Physics Section, was director of the National Bureau of Standards from 1978 to 1988, received the President’s Award for Distinguished Federal Civilian Service and retired from NIST in 1989 at the age of 65. Raymond W. Hayward, after several follow-up experiments, returned to the Radioactivity Section under Wilfrid B. Mann, later became chief of the newly created Nuclear Spectrometry Section, wrote what the article calls “a monumental treatise” on the dynamics of particles of higher spin, turned to the study of gravitation and retired in 1980 at the age of 59.⁠[2]

The NIST article carries a photograph of the bureau’s four experimenters with their equipment, from left Hudson, Ambler, Hoppes and Hayward, credited to the NIST Digital Archives. Hargittai’s article sets out, under the heading “Candidates for credit,” the portraits of eleven people: Ambler, Hayward, Chien-Shiung Wu, Hoppes, Hudson, Lederman, Garwin, Tsung-Dao Lee, Chen Ning Yang, Friedman and Telegdi.⁠[6][5]

National Institute of Standards and Technology: Official website: https://www.nist.gov/ (opens in a new window).⁠[28]