Ultracold neutrons, cooled nearly to absolute zero, open new windows in physics. A UCN measurement at LANSCE determined the neutron lifetime with unprecedented accuracy.
A trap for ultracold neutrons with a calibration probe used to map its magnetic field. Measurements with the trap set a record for the most precise measurement of neutron lifetime.
Summary
Fundamental neutron science is key to understanding how elements are formed and why the Universe is made of matter, not antimatter.
Neutrons cooled to “walking speed” (Ultracold neutrons [UCN]) are essential to these studies because they can be channeled and trapped. The Laboratory UCN source holds the world’s intensity record. Thanks to that source, we measured neutron lifetime (877.75 seconds) with unprecedented accuracy.
In the LANL UCNA experiment, UCNs are used to measure the correlation between the spin of a decaying neutron and the resulting electron. Its 0.55% precision provides the accuracy needed to test the Standard Model of elementary particles.
The LANL neutron Electric Dipole Moment (nEDM) experiment, also using UCNs, is on track to improve limits on the EDM by three times, thus probing time reversal in fundamental physics, i.e., that the laws of physics change if the arrow of time is reversed.
Contributing author
Tom Bowles
References
Measurement of the neutron lifetime with record accuracy:
Los Alamos scientists detected the neutrino in 1956. Subsequent studies revealed its tiny mass and its oscillation between electron, muon, tau, and possible sterile states.
Frederick Reines was awarded the 1995 Nobel Prize in physics for his co-detection of the neutrino with Clyde Cowan, using a detector developed at Los Alamos and fielded at a Savannah River nuclear reactor.
Summary
The elusive neutrino was detected by LANL’s Cowan and Reines at a nuclear reactor (1956), yielding Reines’ 1995 Nobel Prize in physics. In the 1980s, Russian claims of a 30-eV electron neutrino, heavy enough to close the universe, were disproved by a LANL tritium beta decay experiment. In the 1980–90s, a US-Soviet solar neutrino detector proved that neutrinos oscillate between their electron, muon, and tau states, and drove the mass limit below 1 eV.
The LSND detector at LAMPF and miniBoone detector at Fermilab confirmed neutrino oscillations that suggest a new “sterile” neutrino yet more elusive than regular neutrinos. LANL was the first US institution to join the Sudbury Neutrino Observatory, which showed that neutrinos oscillate on their way from the Sun to the Earth, requiring a mass in the milli-eV range. We currently collaborate on a Ge double beta decay experiment (LEGEND) to test whether neutrinos are their own antiparticles.
Corresponding author
Tom Bowles
References
Detection of the neutrino that led to the 1995 Nobel Prize:
From Manhattan Project fusion discoveries to modern fission data, applied nuclear science is everywhere at Los Alamos. Applications range from nuclear safeguards to nuclear deterrence.
Chi-Nu experiments at LANSCE were essential to understanding the prompt neutron spectrum from nuclear fission.
Summary
LANL is the world’s leading laboratory in applied nuclear science. First-ever nuclear reactors were developed here; the elements einsteinium and fermium were discovered after the Laboratory’s 1952 Ivy Mike test. Deuterium-tritium fusion is only possible because of a resonance discovered by Bretsher (1945); a burning fusion plasma was first demonstrated in the Greenhouse George test in 1951. Nuclear-thermal rockets more powerful than the biggest power reactors were built during Project Rover (1955–1973). The National Criticality Experiments Research Center is based on LANL systems. We lead the US in Evaluated Nuclear Data Files that integrate experiment and theory. At the LANSCE accelerator, advanced detectors deliver nuclear cross-sections with unprecedented accuracy.
On the international stage, LANL has been pivotal in establishing and supporting International Atomic Energy Agency (IAEA) safeguards, training all IAEA inspectors, and developing advanced detectors. Satellite systems monitor for nuclear explosions, ensuring compliance with international treaties from 1963 to the present day.
Contributing authors
Mark Chadwick and Bill Priedhorsky
References
A US patent for the Los Alamos invention of the heat pipe, essential to compact nuclear reactors, was granted in 1966:
A new field of astrophysics — gamma-ray bursts — began at Los Alamos. The Laboratory has led the way in modeling the cosmic explosions that cause them.
Artist’s conception of the 1963 launch of the Vela satellites, which were flown to monitor for nuclear explosions but also made major astrophysical discoveries.
Summary
As the space age dawned, Los Alamos launched a gamma-ray detector on a satellite called Vela, intended to detect any surreptitious nuclear test in space. Instead of nuclear gamma rays, the detectors saw gamma-ray bursts from beyond the solar system (1973), opening a new field of astrophysics. Decades later, thanks to Los Alamos contributions in robotic telescopes and gamma-ray imaging, we have learned that these bursts come from the births of black holes and the collisions of neutron stars in far-away galaxies.
Some gamma-ray bursts arise from the explosive deaths of stars. For years, just how supernovae worked was a mystery. Colgate and collaborators performed the first numerical simulation of stellar collapse into a supernova (1961). LANL scientists have continued to be at the forefront of theory and simulation of these phenomena, producing the first 2D and 3D models of supernovae and studying their impact on the universe from their compact remnants to their transient displays.
Slice of the first 3D simulation of a core-collapse supernova, circa 2002, coupling detailed neutrino and equation of state physics into a smooth particle hydrodynamics code running on LANL’s early Beowful cluster space simulator.
