Herbert Hwa-sen Chen
Born(1942-03-16)March 16, 1942
Chungking, China
DiedNovember 7, 1987(1987-11-07) (aged 45)
Irvine, CA United States
NationalityAmerican
Alma materCalifornia Institute of Technology (BS)
Princeton University (PhD)
SpouseCatherine Li (1969 - 1987)
Scientific career
FieldsParticle physics
InstitutionsUniversity of California, Irvine
ThesisElectromagnetic simulation of time reversal violation (1968)
Doctoral advisorSam Treiman

Herbert Hwa-sen Chen (Chinese: 陈华生) (March 16, 1942 – November 7, 1987) was a theoretical and experimental physicist at the University of California at Irvine known for his contributions in the field of neutrino detection. Chen's work on observations of elastic neutrino-electron scattering provided important experimental support for the electroweak theory of the standard model of particle physics.[1] In 1984 Chen realized that the deuterium of heavy water could be used as a detector that would distinguish the flavors of solar neutrinos.[2] This idea led Chen to develop plans for the Sudbury Neutrino Observatory that would eventually make fundamental measurements demonstrating that neutrinos were particles with mass.

Education and early life

Chen (left) and Professor Tadayoshi Doke (right) during a visit to Waseda University in Tokyo in 1986. In the background is a cryostat, probably a liquid argon based calorimeter in Professor Doke's laboratory.

Born in Chunking, China in 1942, Chen had an early childhood of wartime instability and insecurity. He immigrated to the United States with his family in 1955[3] under the Eisenhower Refugee Relief Act of 1953.[4] He graduated high school from Cushing Academy, Massachusetts in 1960.[3] With an education supported almost entirely by scholarships, he subsequently earned a Bachelor of Science degree in physics from the California Institute of Technology in 1964.[4] Chen then earned his doctorate in theoretical physics from Princeton University in 1968, writing his thesis on "Electromagnetic simulation of time reversal violation" under the supervision of Sam Treiman.[5][6]

Chen joined the newly formed physics department at University of California, Irvine as a postdoctoral theorist in 1968.[7][8] He was an early addition to Frederick Reines' Neutrino Group. Reines had worked for the wartime Manhattan Project, and had discovered the neutrino in 1956, which would earn him a Nobel Prize in 1995,[9] and had helped found the new university at Irvine in 1966.[8] Though trained in theoretical physics, Chen began a long-term experimental program for the development of methods to measure the properties of neutrinos.[7]

Chen was promoted to Associate Professor of Physics at U.C. Irvine in 1974,[10] and to Professor of Physics in 1980.[11]

Neutrino physics at LAMPF

Chen began a research program to exploit the dense flux of neutrinos created at the Los Alamos Meson Physics Facility (LAMPF), now called the Los Alamos Neutron Science Center. While the LAMPF accelerator was designed primarily to accelerate a high-intensity beam of protons to energies high enough to produce unbound pions, by-products of LAMPF operation were intense pulses of neutrinos with kinetic energies between 10 and 55 million electron volts (MeV).[12] In 1971, even before LAMPF began operation, K. Lande, F. Reines, and others including Chen proposed to exploit these neutrinos.[12][13] By 1981 Chen was chair of the working group on neutrino facilities and on the Technical Advisory Panel of the LAMPF User's Group.[14]

One focus of Chen's work at LAMPF was an experiment, E-225 begun in 1975 and headed by Chen, to measure electron neutrino-electron elastic scattering,


ν
e +
e−
 →
ν
e +
e−
.[12]

This seemingly simple interaction is, in fact, a weak force interaction mediated by either the neutral
Z0
 or the charged
W+
,
W−
 weak interaction bosons.[15] In the latter interaction, the electron is converted to a neutrino (and vice versa) by the virtual particle exchange. Measurement of the elastic scattering was therefore a means to determine properties of the bosons, first detected at the particle physics laboratory CERN in 1983. Measurements of this cross section, final results published in 1993, were in excellent agreement with Standard Model predictions. By verifying the quantum mechanical interference effects of the two modes of interaction, LAMPF experiment E-225 was an important test of Standard Model theory.[1]

Liquid Argon Time Projection Chamber

In 1976 Chen with collaborators at U.C. Irvine and the California Institute of Technology proposed one of the earliest uses of liquid argon in a time projection chamber (liquid Ar TPC).[16][17] This proposal was independent of, and almost simultaneous with, Carlo Rubbia's proposal to construct such a device at CERN for rare event particle physics experiments.[18] Chen's initial goals with such a detector were to study neutrino-elecron scattering, but the goals evolved to measure solar or cosmic neutrinos or proton decay.[16][18][19]

Computing for particle physics by network

In 1984 Chen chaired an ad hoc committee sponsored by the National Science Foundation (NSF) to examine the problem of how particle physicists could obtain remote access to the few NSF Supercomputing Centers around the United States for their computations.[20] As described by John Cramer, a professor of physics at the University of Washington in Seattle, the final report of the committee was compiled by Chen. The submitted report contributed to congressional action sponsored by Senator Al Gore. Eventually five new NSF Supercomputer Centers around the United States were created with the NSFNET designed to connect them to universities and other users.[20] NSFNET was soon merged with ARPANET, and this network eventually became the Internet.

The solar neutrino problem

The Sun is powered by nuclear fusion via the proton–proton chain reaction, which converts four protons into alpha particles, neutrinos, positrons, and energy.[21] The energy of the fusion process is released in the form of electromagnetic radiation, gamma rays, and the kinetic energy of both the charged particles and the neutrinos. The neutrinos travel from the Sun's core to Earth without any appreciable absorption by the Sun's outer layers. The expected number of solar neutrinos arriving at the earth could be computed using the standard solar model.[21] The model gives a detailed account of the Sun's internal operation.

In the late 1960s, Ray Davis and John N. Bahcall designed the Homestake Experiment to measure the flux of neutrinos from the Sun. Within the Homestake Gold Mine in Lead, South Dakota, Davis placed a 380 cubic meter (100,000 gallon) tank of perchloroethylene 1,478 meters (4,850 feet) underground as a neutrino target.[22] The experiment would measure neutrino interactions with chlorine, since perchloroethylene is a common dry-cleaning fluid rich in that element. A target deep underground was needed to reduce the noise from cosmic rays, while a large target was needed since the probability of successful neutrino capture was very small. A very low effective detection rate was expected, even with the huge mass of the target. The experiment measured far fewer neutrino interactions than expected, thus indicating a deficit in the neutrino flux. Many subsequent radiochemical and water Cherenkov detectors confirmed the deficit, which became known as the solar neutrino problem. The result seemed to imply that neutrinos were changing their properties as they traveled from the sun to the earth.

In 2002, Ray Davis and Masatoshi Koshiba won part of the Nobel Prize in Physics for experimental work which found the number of solar neutrinos to be around a third of the number predicted by the standard solar model.[23]

Heavy water for a neutrino detector

In 1984, Chen proposed the use of a large heavy water detector as a means of observing neutrinos from the sun to resolve the solar neutrino problem.[2][24] The use of the deuterium of heavy water had the property that neutrino interactions could be observed by both neutral current and charged current reactions:


ν
 + d+ →
ν
 +
p+
 +
n
 (neutral current)

ν
e + d+ →
e−
 +
p+
 +
p+
 (charged current)

where on the left
ν
,
ν
e, and d refer to generic neutrino, electron neutrino, and deuterium, respectively, while on the right
p+
,
n
, and
e−
 refer to proton, neutron, and electron.[25][26] Their electric charges are indicated. There are three different neutrino types or flavors, electron, muon, or tau. The neutral current reaction involves all neutrino types, while the charged current reaction involves just the electron neutrino type. The charged current is mediated by the charged
W+
 and
W−
 bosons, while the neutral current is mediated by the neutral
Z0
. The reactions above could be distinguished in the detector by their different properties, e.g., the gamma radiation from the capture of the neutron in the first reaction, and the Cherenkov radiation of the electron in the second reaction. The relative rates of these reactions would be very different if neutrinos did or did not change flavor as they traveled from the sun to the earth.

Chen and others formed the research team that designed the Sudbury Neutrino Observatory (SNO) to exploit the idea of his seminal paper.[2][27] The observatory was to be located 2100 m underground in a nickel mine near Sudbury, Ontario, Canada. Chen was the U.S. leader and spokesman for this project, while George Ewan led the Canadian team.[2][28] While one focus of the research was on the solar neutrino question, the use of the term "Observatory" was to emphasize the intent to use the facility to record neutrino pulses produced by astronomical events, neutrino astronomy.[29] The astronomical observatory argument proved compelling after neutrino bursts were detected from supernova SN 1987A in February 1987.[30] The initial problem Chen and the collaboration addressed was the acquisition of 1000 tones of heavy water from the Canadian nuclear power company Atomic Energy of Canada Limited that would be used as the detector.[2] The principal problem with neutrino observations is that the chance of an interaction is so slight that huge numbers of possible targets are required to be able to observe the small number of interactions that occur.

Death

During the intensive planning and development phase for SNO, Chen was diagnosed with leukemia. After a year-long battle with the disease, Chen died in November 1987.[4] In January 1988, a symposium on neutrino physics was held at U.C. Irvine to honor Chen's contributions, moderated by Frederick Reines. A keynote speaker was Nobel laureate and astrophysicist William Fowler, who led a discussion on "Herb Chen and Solar Neutrinos."[31]

The University of California, Irvine, Physical Sciences established the Herbert H. Chen Award "given to an outstanding junior level physics student."[32]

The Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory was completed in the 1990s, and its first director was Chen's collaborator, Arthur B. McDonald.[2][33][34] The observations by SNO would demonstrate that neutrinos oscillated between neutrino flavors (electron, muon, and tau), thus demonstrating that the neutrino was not massless.[29] For this fundamental discovery in physics, McDonald and the Sudbury Neutrino Observatory Collaboration were awarded the 2015 Nobel Prize in Physics jointly with Japanese physicist Takaaki Kajita and the Super-Kamiokande Collaboration.[35]

See also

References

  1. 1 2 Zuber, K. (2004). Neutrino Physics. New York, London: Taylor and Francis Group. p. 56. ISBN 978-0-7503-0750-5.
  2. 1 2 3 4 5 6 Ewan, G.T.; Davidson, W.F. (2005). "Early Development of the Underground SNO Laboratory in Canada" (PDF). Physics in Canada. Vol. 61. pp. 339–346, 347–350. Archived from the original (PDF) on December 4, 2016. Retrieved December 13, 2016.
  3. 1 2 "Herb Chen '60: Groundbreaking Contributions to 2015 Nobel-Winning Research". Cushing Academy Magazine: Cushing Yesterday & Today. Cushing Academy. Spring 2016. Retrieved May 30, 2017.
  4. 1 2 3 Bander, M.; Reines, F.; Shaw, G. (1987). "Herbert H. Chen, Physics: Irvine". In Memoriam. University of California. Retrieved October 13, 2015.
  5. ↑ "Electromagnetic simulation of time reversal violation". Princeton University. Retrieved January 25, 2017.
  6. ↑ Chen, H.H. (1969). "Electromagnetic Simulation of Time-Reversal Violation in Mirror Spin-3/2 Beta Decays". Physical Review. 185 (5): 2003–2006. Bibcode:1969PhRv..185.2003C. doi:10.1103/PhysRev.185.2003.
  7. 1 2 Allen, R.; Doe, P.; Reines, F. (1988). "Herbert H. Chen (Obituary)". Physics Today. 4 (9): 128. Bibcode:1988PhT....41i.128A. doi:10.1063/1.2811575.
  8. 1 2 Kropp, W.; Schultz, J.; Sobel, H. (2009). Frederick Reines 1918-1998 A Biographical Memoir (PDF). Washington D.C.: National Academy of Sciences. Retrieved March 17, 2010.
  9. ↑ Reines, Frederick (December 8, 1995). "The Neutrino: From Poltergeist to Particle" (PDF). Nobel Foundation. Retrieved February 20, 2015. Nobel Prize lecture
  10. ↑ "Appointments and Promotions: Irvine: To Associate Professor or Equivalent". University Bulletin: A publication for faculty and staff of the University of California. University of California. December 16, 1974. {{cite web}}: Missing or empty |url= (help)
  11. ↑ "University of California, Irvine, 1980-81 General Catalogue" (PDF). Department of Physics. 1980. Retrieved June 6, 2017.
  12. 1 2 3 Garvey, G. (1997). "A Brief History of Neutrino Experiments at LAMPF". Los Alamos Science. 25: 8 pp. Retrieved January 21, 2017.
  13. ↑ Lande, K.; Reines, F. (1971). "LAMPF Neutrino Facility Proposal". Los Alamos Scientific Laboratory Report. LA-4842-MS: 51 pp. Retrieved January 21, 2017.
  14. ↑ Cochran, D.R.F. (1982). "Proceedings of the Fifteenth LAMPF Users Group Meeting". LAMPF User's Group Proceedings, los Alamos National Lab., NM (USA): 136 pp. Retrieved January 20, 2017.
  15. ↑ Allen, R.C.; Chen, H.H.; et al. (1993). "Study of electron-neutrino–electron elastic scattering at LAMPF". Physical Review D. 47 (1): 11–28. Bibcode:1993PhRvD..47...11A. doi:10.1103/PhysRevD.47.11. PMID 10015375.
  16. 1 2 Chen, H.H.; Condon, P.E.; Barish, B.C.; Sciulli, F.J. (1976). "A Neutrino detector sensitive to rare processes. I. A Study of neutrino electron reactions" (PDF). Fermi National Accelerator Laboratory. Proposal P-496: 42 pp. Retrieved January 28, 2017.
  17. ↑ Chen, H.H.; Lathrop, J.F. (1978). "Observation of ionization of electrons drifting large distances in liquid argon". Nuclear Instruments and Methods in Physics Research. 150 (3): 585–588. Bibcode:1978NucIM.150..585C. doi:10.1016/0029-554x(78)90132-5.
  18. 1 2 Doke, T. (1993). "A historical view on the R&D for liquid rare gas detectors". Nuclear Instruments and Methods in Physics Research. A327 (1): 113–118. Bibcode:1993NIMPA.327..113D. doi:10.1016/0168-9002(93)91423-K.
  19. ↑ "The time projection chamber turns 25". CERN: CERN Courier. December 27, 2004. Retrieved January 29, 2017.
  20. 1 2 Cramer, J.G. (2013). "How Al Gore and I Invented the Internet". Analog Science Fiction and Fact. March, Alternate View Column AV-166: 113–118. Retrieved January 28, 2017.
  21. 1 2 Serenelli, A. (2008). "Standard Solar Models". In Soler, F.J.P.; Froggatt, C.D.; Muheim, F. (eds.). Neutrinos in Particle Physics, Astrophysics and Cosmology. Boca Raton, FL: CRC Press. p. 119. ISBN 9781420082395.
  22. ↑ Cleveland, B. T.; et al. (1998). "Measurement of the solar electron neutrino flux with the Homestake chlorine detector". Astrophysical Journal. 496 (1): 505–526. Bibcode:1998ApJ...496..505C. doi:10.1086/305343.
  23. ↑ "The Nobel Prize in Physics 2002". Retrieved July 18, 2006.
  24. ↑ McDonald, A.B.; Klein, J.R.; Wark, D.L. (2003). "Solving the Solar Neutrino Problem". Scientific American. 288 (4): 40–49. Bibcode:2003SciAm.288d..40M. doi:10.1038/scientificamerican0403-40. PMID 12661314.
  25. ↑ Chen, H.H. (1985). "Solar Neutrinos and Neutrino Astronomy (Homestake, 1984)". AIP Conf. Proc. 126: 249–276. doi:10.1063/1.35156.
  26. ↑ Chen, H.H. (1985). "Direct Approach to Resolve the Solar Neutrino Problem". Phys. Rev. Lett. 55 (14): 1534–1536. Bibcode:1985PhRvL..55.1534C. doi:10.1103/PhysRevLett.55.1534. PMID 10031848.
  27. ↑ Sinclair, D.; Carter, A.L.; Kessler, D.; et al. (1986). "Proposal to build a neutrino observatory in Sudbury, Canada". Il Nuovo Cimento C. 9 (2): 308–317. Bibcode:1986NCimC...9..308S. doi:10.1007/BF02514850. S2CID 122544471.
  28. ↑ Chen, H.H.; for the Sudbury Neutrino Observatory Collaboration (1988). "The Sudbury Neutrino Observatory: Solar and supernova neutrino studies with a large heavy water Cherenkov detector". Nuclear Instruments and Methods in Physics Research. A264 (1): 48–54. Bibcode:1988NIMPA.264...48C. doi:10.1016/0168-9002(88)91101-1.
  29. 1 2 "The Sudbury Neutrino Observatory – Canada's eye on the universe". CERN: CERN Courier. December 4, 2001. Retrieved December 15, 2016.
  30. ↑ Arnett, W.D.; et al. (1989). "Supernova 1987A". Annual Review of Astronomy and Astrophysics. 27: 629–700. Bibcode:1989ARA&A..27..629A. doi:10.1146/annurev.aa.27.090189.003213.
  31. ↑ "UCI Seminar to Honor Chen's Neutrino Work". Los Angeles Times. December 31, 1987. Retrieved January 20, 2017.
  32. ↑ "UCI Physical Sciences, Honors and Awards". University of California, Irvine. Retrieved January 30, 2017.
  33. ↑ Boger, J.; et al. (2000). "The Sudbury Neutrino Observatory". Nuclear Instruments and Methods in Physics Research. A449 (1–2): 172–207. arXiv:nucl-ex/9910016. Bibcode:2000NIMPA.449..172B. doi:10.1016/S0168-9002(99)01469-2.
  34. ↑ "Interview with Arthur B. McDonald". Archived from the original on November 17, 2007. Retrieved November 2, 2007.
  35. ↑ "The Nobel Prize in Physics 2015". Retrieved January 24, 2017.
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