by Geoff Greene (Aug, 2022)
Disclaimer: The author was engaged in a variety of experiments at ILL between 1975 and the mid 1990’s, many of which involved collaboration with Norman Ramsey. The following has been written informally and relies entirely on personal memories, some of which are more than 40 years old. I apologize for any omissions or errors of fact.
Norman Ramsey began his association with the ILL in the mid 1970’s with a cold neutron beam, neutron Electric Dipole Moment (EDM) experiment that ran on beam H18. This project was a continuation of a series of neutron EDM searches that he had led and which were carried out at Oak Ridge National Laboratory. That program included the first measurement done at the ORNL Graphite Reactor in the early 1950’s which was, in fact, the first experiment specifically designed to search for parity violation. Ramsey induced Mike Pendlebury and Ken Smith from Sussex University to join his ORNL collaborators (Phil Miller and Bill Dress) in adapting the ORNL apparatus and installing it at ILL. (It is noteworthy that this experiment was the first neutron activity by Mike Pendlebury and thus represented the first step in his illustrious neutron career).
Benefiting from the cold neutron beam’s lower velocity and high intensity, the experiment provided the most sensitive “beam type” neutron EDM result and it was not eclipsed for several years until the first successful Ultra Cold Neutron (UCN) EDM results became available.
All the carrier of Geoffrey (Geoff) Greene was devoted to the neutron. He arrived at the ILL in 1975 as a PhD student of Norman Ramsey and worked with him on the accurate measurement of the magnetic moment of the neutron. Then he worked as a postdoc on the nEDM experiment. When he moved to the USA he continued working on fundamental neutron physics (neutron mass, neutron lifetime, etc.). He was the 2021 recipient of the Tom W. Bonner Prize in Nuclear Physics for "developing experimental techniques for in-beam measurements of the neutron lifetime and other experiments, and for realizing a facility for the next generation of fundamental neutron physics measurements."
The neutron is made of three quarks with negative and positive fractional electric charges the sum of which is zero and, hence, the neutron is electrically neutral to the outside. However, if negative and positive charge distributions inside the particle do not coincide the neutron would exhibit a non zero - but incredibly small - Electric Dipole Moment (EDM). Current theories say that this can only occur if both parity symmetry (P) and time reversal symmetry (T) are violated, and this is also one of the necessary conditions to explain the observed matter anti-matter asymmetry of our Univers. That’s why, since Ramsey’s first EDM experiment, scientists are working so hard to try to observe the EDM of the neutron.
It was initially assumed that an experiment conducted in a mirror should be the same one conducted in our world. Norman Ramsey was the first to check that at the particle level by searching for an Electric Dipole Moment (EDM) of the neutron which can only exist if that symmetry does not exist (parity violation).
Most solide state physics spectrometers use hot to cold neutrons (temperature: 1500K to 2,5K, wavelength: 2,5Å to 20Å) but Ultra Cold Neutrons (temperatures 800Å) are much praised for particle physics due to their very special properties. E.g., as photons are reflected by mirrors, UCNs are reflected by material surfaces under any angle and cannot cross them. Thus they can be stored in bottles which gives particle scientists the possibility to measure their properties for a very long time. This bottle storage is todays best technique to measure the neutron lifetime (about 15 minutes), a crucial parameter to current particle physics theories and to explain the isotope abundance of the Universe.
The magnetic moment gives the magnetic strength and orientation of an object that produces a magnetic field. The neutron has a magnetic moment and hence behaves as a tiny compas sensitive to its magnetic environment. The accurate knowledge of the magnitude of this magnetic moment is crucial to the many solid state physics and fundamental physics experiments and techniques where magnetism is involved.
The neutron is like a tiny compas the needle of which (the spin direction) rotates as a function of the magnetic field direction and intensity it crosses. Hence the spin of the neutron is not expected to rotate when it crosses a non-magnetic material. However, unlike other fundamental forces, the weak interaction which controls the interactions between the quarks inside the neutron is known to be chiral since 1957. In other words this force does not acts the same on neutrons with opposite spin directions. In 1998, Ramsey and collaborators observed this tiny effect for the first time in the isotope 117Sn and then in others isotopes. This offered physicists a new way for the study of parity violation.
Following this work, Ramsey’s efforts bifurcated into two different projects. In the first, Ramsey recognized that the high resolution of the ORNL EDM apparatus could be employed to allow an accurate measurement of the neutron magnetic moment through the use of a clever flowing water NMR calibration procedure suggested by Edward Purcell. Graduate student Geoff Greene, along with Walter Mampe and Mike Pendlebury were successful in modifying the ORNL EDM apparatus and obtained a x100 improvement in accuracy. This was, by the way, Walter Mampe’s first project involving fundamental neutron physics.
In the second effort, Ramsey, along with many others, recognized the enormous benefits that Ultra Cold Neutrons (UCN) could provide for neutron EDM experiments and a new collaboration developed to support a UCN based project. New personal included Bob Golub, who had been working on atomic physics at Sussex, Walter Mampe (local contact) and Paul Ageron from ILL, as well as Keith Green and Tony Baker from the Rutherford Laboratory. This effort, of course, continues into the present. [E.g., by 2022, panEDM is an experiment being installed on SuperSUN, a new UCN source]
In the late 1970’s the question of the existence of Weak neutral currents became a priority in particle physics. This led to major High Energy (HE) physics efforts at CERN and elsewhere. But it was also recognized that Weak neutral currents would lead to interesting phenomena at low energies. This included the possibility of coherent parity violation, a phenomenon analogous to optical activity in handed molecules. While most interest focused on atomic physics, a few people observed that there could be an observable “neutron optical activity” in ordinary matter due to the nucleon-nucleon Weak interaction. This possibility was actually first pointed out by F. Curtis Michael in the early 1960’s, but it was Ramsey who recognized that, while the effect is very small, it might be observable using ILL’s cold neutron beams. Ramsey and the Sussex-Rutherford group began exploring experimental possibilities and about this time, graduate student Blayne Heckel joined the collaboration.
Independently, Mario Forte, then at Euratom, suggested that there could be significant enhancements due to collective nuclear effects and suggested that the isotope 124Sn could have an observable effect. An apparatus to detect this parity violating spin rotation, built largely by Forte, was installed at the ILL, a new polarized beam line built by Blayne Heckel and Geoff Greene (instrument S43 on the H142 guide tube).
The following personal recollection discusses a particularly amusing aspect of that project.:
Ramsey used his influence to obtain the world’s supple of single isotope 124Sn which was cast into a sample block ~10x10x40 mm3. To help account for possible systematic effects, a similar block of natural isotopic tin (which has only ~6% abundance of 124Sn) was prepared to provide a “null check.” No effect was observed in the 124Sn, but, surprisingly, the natural Tin showed a small effect. With extended running, the effect did not disappear. After some clever “detective” work, Blayne Heckel suggested that the effect might come from 117Sn (~8% abundance) which was known to have an enhanced neutron capture parity violation. Despite the small abundance of 117Sn, if the optical activity was enhanced as much as the parity violating capture asymmetry, one might find an observable effect even in natural Tin. Ramsey had been planning a visit to ILL and was, in short order, able to persuade ORNL to provide the world supply of 117Sn to be cast into a suitable sample. Today, the paperwork associated with acquiring this essentially irreplaceable sample would probably take years and transporting it abroad would be practically impossible. Nonetheless, Ramsey was able to use his influence to get the sample in a matter of weeks and brought it with him in his hand luggage when he flew into Lyon. Greene and Heckel met Ramsey at the airport and the three of them drove directly to the ILL and installed the sample in the apparatus that evening (S43 on the H18 cold neutron guide tube). By two or three in the morning, they had already seen a positive signal. When they returned in the morning, there was an obvious, statistically significant, signal. It was a very exciting day and was, in fact, the first observation of coherent parity violation.
The spin rotation studies continued for several more years.
Ramsey’s influence on the course of fundamental neutron physics at the ILL was profound. It should be recalled that the initial nuclear physics program at ILL was focused on nuclear structure and reactions and was focused at fixed instruments like GAMS and LOHENGRIN. There were no explicit plans for a “Fundamental Neutron Physics” program. Initially, the format of the nuclear physics research program was envisioned to be similar to the neutron scattering program, with users bringing samples and running distinct experiments of relatively short duration. Ramsey’s EDM experiment was the paradigm for subsequent experiments in highly successful ILL fundamental physics program that involved the installation of specialized equipment for experiments of extended duration. In this, Ramsey brought something new and significant to the ILL.
Ramsey’s influence was also profound in the number of future leaders in the field of fundamental neutron research researchers whom he encouraged to participate in the field. The names listed above represent only a limited subset of these.
- "Search for an electric dipole moment of the neutron." Dress W.B., Miller P.D., Pendlebury J.M., Perrin P. and Ramsey N.F., Physical Review D 15, 9-21 (1977), DOI: 10.1103/PhysRevD.15.9
- "A new measurement of the magnetic moment of the neutron." Greene G.L., Ramsey N.F., Mampe W., Pendlebury J.M., Smith K., Dress W.D., Miller P.D. and Perrin P., Physics Letters B 71, 297-300 (1977). DOI: 10.1103/PhysRevD.15.9
- "Measurement of the neutron magnetic moment." Greene G.L., Ramsey N.F., Mampe W., Pendlebury J.M., Smith K., Dress W.B., Miller P.D. and Perrin P., Physical Review D 20, 2139-2153 (1979). DOI: 10.1016/0370-2693(77)90220-9
- "An improved derived value for the neutron magnetic moment in nuclear magnetons." Greene G.L., Ramsey N.F., Mampe W., Pendlebury J.M., Smith K., Dress W.B., Miller P.D. and Perrin P., Metrologia 18, 93 (1982). DOI: 10.1088/0026-1394/18/2/005
- "Electric dipole moment of the neutron." Ramsey N.F., Annual Review of Nuclear and Particle Science 40, 1-14 (1990). DOI: 10.1146/annurev.ns.40.120190.000245
- "A search for the electric dipole moment of the neutron." Smith K.F., Crampin N., Pendlebury J.M., Richardson D.J., Shiers D., Green K., Kilvington A.I., Moir J., Prosper H.B., Thompson D., Ramsey N.F., Heckel B.R., Lamoreaux S.K., Ageron P., Mampe W., Steyerl A, Physics Letters B 234, 191-196 (1990). DOI: 10.1016/0370-2693(90)92027-G
- "The electric dipole moment of the neutron." Ramsey N.F., Physica Scripta T22, 140-143 (1988). DOI: 10.1088/0031-8949/1988/T22/021
- "Search for a neutron electric dipole moment." Ramsey N.F., In: "Weak and electromagnetic interactions in nuclei", Klapdor H.V. (Eds.)(Springer Verlag, 1986) pp.861-865. DOI: 10.1007/978-3-642-71689-8_171
- "Search for a neutron electric dipole moment." Pendlebury J.M., Smith K.F., Golub R., Byrne J., McComb T.J.L., Sumner T.J., Burnett S.M., Taylor A.R., Heckel B., Ramsey N.F., Green K., Morse J., Kilvington A.I., Baker C.A., Clark S.A., Mampe W., Ageron P. and Miranda R., Physics Letters B 136, 327-330 (1984). DOI: 10.1016/0370-2693(84)92013-6
- "Electric-dipole moments of elementary particles." Ramsey N.F., Reports on Progress in Physics 45, 95-113 (1982). DOI: 10.1088/0034-4885/45/1/003
- "Measurement of parity nonconserving neutron spin rotation in lanthanum." Heckel B., Forte M., Schaerpf O., Green K., Greene G.L., Ramsey N.F., Byrne J. and Pendlebury J.M., Physical Review C 29, 2389-2391 (1984). DOI: 10.1103/PhysRevC.29.2389
- "A measurement of parity non-conserving neutron spin rotation in lead and tin." Heckel B., Ramsey N.F., Green K., Greene G.L., Gähler R., Schaerpf O., Forte F., Dress W.B., Miller P.D., Golub R., Byrne J. and Pendlebury J.M., Physics Letters B 119, 298-302 (1982). DOI: 10.1016/0370-2693(82)90674-8
- "First measurement of parity-nonconserving neutron spin-echo rotation: The tin isotopes." Forte M., Heckel B.R., Ramsey N.F., Green K., Greene G.L., Byrne J. and Pendlebury J.M., Physical Review Letters 45, 2088-2092 (1981). DOI: 10.1103/PhysRevLett.45.2088