Michael Longo

Born 1935 in Philadelphia. B.A from La Salle College, 1956. He attended graduate school at the University of California, Berkeley 1956-1961. His PhD thesis was on the total cross section for π⁺-nucleus scattering at "high" energies. In those days, the 6 BeV Berkeley Bevatron was the highest energy machine in the world and there was little data for positive pions because there were only internal targets, and the positive particles were bent inward toward the center of the ring.

At that time, the main thrust of the Moyer group was to try to detect antineutrons, which had not yet been seen experimentally. That meant that he could have the pion total cross-section experiment pretty much to himself. He was always a hands-on physicist, and this was a great opportunity for him to develop his experimental skills. He was able to design the entire experiment, assemble the apparatus and electronics, take the data, and complete the analysis with the help of a very competent Bevatron staff. The apparatus had to be set up inside the Bevatron ring since positive particles were deflected inward by the Bevatron magnetic field. The stray magnetic field also meant that the phototubes had to be carefully shielded. The "long article" with the results of the experiment was published in the Physical Review with Longo and Burton Moyer, his thesis advisor, as the only authors. This was one of the first experiments to study π⁺-total cross sections.

He also was able to study π^- cross sections using the same apparatus piggybacking behind an experiment by Larry Jones and Martin Perl from the University of Michigan. These were later published in Physical Review Letters: This acquaintance with Jones and Perl led to an offer of an assistant professorship at the University of Michigan after he had finished an NSF fellowship at Saclay. He joined the UM faculty in 1962 and remained there until his retirement. At Michigan, he initiated a series of novel experiments:

-An experiment to measure the polarization in proton-proton elastic scattering at the 3 GeV Brookhaven Cosmotron. This was Homer Neal's thesis experiment.

-An experiment at the Bevatron to test for time-reversal violation in the decay K⁰ à π^- µ⁺ v . The result was a limit 4 times better than previous experiments and it was almost 14 years before a measurement with comparable accuracy was accomplished by the Yale group.

-An experiment at the Berkeley 184" cyclotron to test for time-reversal violation in the reaction n + p à ү + d. The idea is to compare the cross sections for this reaction with the inverse re-action, photodisintegration of the deuteron, which had been well measured at comparable energies. The result agreed with time reversal invariance and is still one of the best tests in nuclear interactions.

Early on, Longo had the idea of using high-energy neutron beams to study neutron scattering. This doesn't work well at lower energies because the beams have a large energy spread, and it is hard to determine the neutron energy accurately. Most of the lower energy neutron cross-section data had been obtained using a deuterium target with a (proton-deuteron) (proton-proton) subtraction. At high energies, it is easy to produce a clean, well-collimated neutron beam by forward scattering of protons on an internal target and sweeping away the protons in the secondary beam. Then, the energies of the neutrons can be determined using a hadron calorimeter. The calorimeter consists of a stack of iron plates alternating with scintillators and wire chambers to sample the ionization produced in the cascade. These had been used by Larry Jones and others in cosmic ray experiments, but not yet in accelerators. With a measurement of the neutron's energy and angle in the calorimeter and a measurement of the recoil proton, he realized you could effectively separate elastic scattering from inelastic. This led to a series of neutron elastic scattering experiments at most of the accelerators in the US.

The first was a neutron-proton elastic scattering measurement at the Bevatron. It used optical spark chambers embedded in a steel plate neutron calorimeter to determine the angle and energy of the neutrons elastically scattered at small angles and a spark chamber proton spectrometer for the recoil protons. This was the first measurement of neutron elastic scattering at GeV energies. At about the same time, he did a neutron-proton "charge exchange" measurement where the neutrons are scattered at large angles and the recoil proton goes nearly forward. The low energy scattered neutrons were detected in large scintillator blocks, and the forward-going protons in an optical spark chamber spectrometer.

Not too long thereafter, he took the show to the Brookhaven AGS to use essentially the same apparatus to measure n-p elastic scattering up to 30 GeV/c (Bruce Gibbard's thesis).

He also did a series of n-p and n-nucleus total cross-section experiments with Mike Kreisler's group, which was then at Princeton. Another advantage with neutron beams for total cross-section measurements is that you don't have to worry about Coulomb scattering at small angles, which is a problem for charged particles. Neutron-nucleus total cross sections at the AGS were Tom McCorriston's thesis.

In 1972(?), he proposed neutron total cross section as one of the first experiments at the new National Accelerator Laboratory (now Fermilab) 200 GeV "Main Ring". This was done in the beam enclosure just upstream of the not-yet-completed Meson Lab building. This was probably the first published cross-section result from the new laboratory.

The neutron beam experiments were very successful, and even now, many of the neutron cross-sections are better measured than the corresponding ones for protons. However, in the 1980's it became clear that "soft" hadron scattering data were really not that interesting because they were too complicated to understand. "Hard" scattering and weak interactions were the wave of the future.

He was the UM spokesman for Sam Ting's L3 experiment, where he designed the wire chambers for the Uranium plate calorimeter. His group produced the first complete prototype of a module. However, he was expelled from the collaboration after a disagreement over the chamber construction.

He then joined the MACRO collaboration at the Gran Sasso laboratory. His group built the 12-meter-long liquid scintillator tanks that covered the sides of the detector. These were built on time and under budget and had excellent light collection.

He was always proud that all the experiments he designed worked very well.