Discoveries: Nuclear and Particle Futures

Los Alamos is the premier laboratory in the US for “all things nuclear.”

By integrating nuclear experiments, theory, and simulation, we are working to understand and engineer complex nuclear phenomena.

The Nuclear and Particle Futures pillar is composed of four major thrusts:

• Nuclear, Particle, Astrophysics, and Cosmology,
• Applied Nuclear Science and Engineering,
• High Energy Density Plasmas and Fluids, and
• Accelerators and Electrodynamics.

Its capabilities are grounded in its Los Alamos Neutron Science Center (LANSCE) and Dual-Axis Radiographic Hydrotest (DARHT) facilities, its leadership in critical assembly work, and extensive capabilities in nuclear experiment, theory, and simulation and fundamental research probing scales from the quark to the cosmos.

Neutrons

Ultracold neutrons, cooled nearly to absolute zero, open new windows in physics. A UCN measurement at LANSCE determined the neutron lifetime with unprecedented accuracy.

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:

1. Measuring the Neutron Lifetime with Record-Breaking Precision, Hoogerheide, Shannon F. Physics 14 (2021): 142.
2. A new method for measuring the neutron lifetime using an in situ neutron detector, Morris, C. L., E. R. Adamek, L. J. Broussard et al, Review of Scientific Instruments 88, no. 5 (2017): 053508.
3. Improved Neutron Lifetime Measurement with UCN𝜏, Gonzalez, F. M., E. M. Fries, C. Cude-Woods et al. Physical Review Letters 127 (2021): 162501.

Neutrinos

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.

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:

1. Detection of the Free Neutrino: A Confirmation, Cowan, C. L., Jr., F. Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire. Science 124 (1956):103–104.

Tritium Beta Decay Experiment:

2. Limit on Electron Antineutrino Mass from Observation of the beta decay of Molecular Tritium, Robertson, R. G. H., T. J. Bowles, G. J. Stephenson, Jr., et al. Physical Review Letters 67 (1991): 957.

SAGE confirmation of neutrino oscillations:

3. Results from SAGE (The Russian-American gallium solar neutrino experiment), Abdurashitov, J. N. et al. Physical Letters B 328 (1994): 234–248.

Evidence of neutrino oscillations from LSND:

4. Evidence for Neutrino Oscillations from the Observation of ν ̄e Appearance in a ν ̄μ Beam, Aguilar, A. et al. (LSND Collaboration), Physical Review D 64 (2001): 112007.

Refinement of neutrino oscillation result with miniBoone:

5. Updated MiniBooNE neutrino oscillation results with increased data and new background studies, Aguilar-Arevalo, A. A. et al. Physical Review D 103 (2021): 052002.

Sudbury Neutrino Observatory detection of neutrino oscillations as they travel from the Sun to the Earth:

6. Measurement of the Total Active 8B Solar Neutrino Flux at the Sudbury Neutrino Observatory with Enhanced Neutral Current Sensitivity, Ahmed, S. N. et al. Physical Review Letters 92 (2004): 181301.

Nuclear Applications

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.

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:

1. Evaporation-condensation heat transfer device, Grover, G., M., US patent 3,229,759 (filed 1963).

A review of the Rover nuclear rocket program, and the technical challenges that it overcame, can be found at:

2. The Rover Nuclear Rocket Program, Spence, R. W., Science 160, 953 (1968).

Measurement of the DT fusion cross-section from thick target measurements were first reported in the archival literature in:

3. Low Energy Cross Section of the D-T Reaction and Angular Distribution of the Alpha Particles Emitted. Bretscher, E. and A. P. French. Physical Review Journal 75, no. 8 (1949): 1154.

The prospects for fusion power, with a summary of magnetic fusion work at Los Alamos, can be found in:

4. Outlook for Controlled Fusion Power. Tuck, J. L. Nature 233 (1971): 593–598.

A significant step in understanding the dynamics of fission was:

5. Nuclear fission modes and fragment mass asymmetries in a five-dimensional deformation space, Möller, P., D. Madland, A. Sierk, et al. Nature 409 (2001): 785–790.

Reactor developments in the postwar period are reviewed in:

6. Early Reactors: From Fermi’s Water Boiler to Novel Power Prototypes, Bunker, M. E., Los Alamos Science Spring 1983, 124 (1983). 

The MCNP code is a workhorse for nuclear modeling, and can be found at:

7. The Monte Carlo N-Particle Code, Los Alamos National Laboratory, (accessed February 12, 2025).

The go-to reference for nuclear data is the ENDF/B-VII data base:

8. ENDF/B-VII. 1 nuclear data for science and technology: cross sections, covariances, fission product yields and decay data, Chadwick, M.B., Herman, M., Oblozinsky, P., et al., Nuclear Data Sheets 112, no.12 (2011): 2887-2996 (2011)

Cosmic Explosions

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.

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.

Contributing Authors

Ed Fenimore, Chris Fryer

References

1. The Hydrodynamic Behavior of Supernovae Explosions, Colgate, Stirling A. and Richard H. White. Astrophysical Journal 143 (1966): 626.

The Los Alamos discovery of gamma-ray bursts was reported in:

2. Observations of Gamma-Ray Bursts of Cosmic Origin, Klebesadel, Ray W., Ian B. Strong, and Roy A. Olson. Astrophysical Journal 182 (1973): L85.

Further gamma-ray bursts discoveries included:

3. Discovery of the short γ-ray burst GRB 050709, Villasenor, J., D. Lamb, G. Ricker, et al. Nature 437 (2005): 855–858.

The revolutionary Swift satellite, which followed up gamma-ray bursts in seconds thanks to Los Alamos software, is described at:

4. The Swift Gamma-Ray Burst Mission, N. Gehrels G. Chincarini, P. Giommi, et al. Astrophysical Journal 611 (2004): 1005. 

Los Alamos scientists developed the current standard paradigm to the supernova engine, developing the first 2-dimensional models:

5. Inside the Supernova: A Powerful Convective Engine, Herant, Marc, Willy Benz, Raphael W. Hix, Chris L. Fryer, Stirling A. Colgate. Astrophysical Journal 435 (1994): 339.

They also developed the first 3-dimensional models:

6. Modeling Core-Collapse Supernovae in Three Dimensions, Fryer, Chris L., and Michael S. Warren.  Astrophysical Journal 574 (2002): L65–L68.

And carried out the first studies on what it took for a supernova to leave behind a black hole:

7. Mass Limits For Black Hole Formation, Fryer, Chris L. Astrophysical Journal 522 (1999): 413–418.