New Moves Introduced for Monte Carlo Technique
A research collaboration involving Livermore biophysicist Jerome Nilmeier has developed a class of candidate moves based on nonequilibrium dynamics for Monte Carlo simulations—a breakthrough that allows the methodology to more efficiently simulate biological processes. Widely used to model systems, the Monte Carlo technique harnesses the power of computers to calculate the probable outcomes of equations with hundreds or thousands of variables.
Scientists working on the Manhattan Project first developed the sampling procedure to figure out how far neutrons might pass through various shielding materials. However, different types of systems have numerous variables that form a wide range of relationships. What works well for measuring how far a neutron will pass through different radiation shields may not function at all when applied to a biological system in which millions of molecules are moving rapidly in many directions but for very short distances.
To test the revised approach, the research team, which included scientists from Lawrence Berkeley and Argonne national laboratories and the University of California at Berkeley, used a chemical compound with two identical or similar subunits. Called a dimer, this compound served as a proxy for a reactive system where molecules are allowed to collide and form new molecules but can also dissociate into free atoms. “With the new technique, we can bias our simulation to sample the collision event more frequently and obtain better statistics,” says Nilmeier. Results from the team’s research appeared in the November 8, 2011, issue of Proceedings of the National Academy of Sciences.
Contact: Jerome Nilmeier (925) 423-9218 (firstname.lastname@example.org).
Separating Climate Signal and Noise
A study led by scientists at Livermore shows that temperature records must be at least 17 years long to discriminate between internal climate noise and the signal of human-caused changes in the atmosphere’s chemical composition. The research team also found that climate models can and do accurately simulate short, 10- to 12-year periods with minimal warming, even when the models are run with historical increases in greenhouse gases and sulfate aerosol particles. The team’s results appeared in the November 17, 2011, online edition of Journal of Geophysical Research (Atmospheres).
When the scientists analyzed satellite measurements of the temperature in the lower troposphere (the region extending from Earth’s surface to about 8 kilometers into the atmosphere), they saw a clear signal of human-induced warming. Satellites use microwave radiometers to measure atmospheric temperature, and these recordings are independent from surface thermometer measurements. The satellite data indicate that since 1979, when satellites first began to record temperatures, the lower troposphere has warmed by about 0.9°F. This increase is consistent with the warming of Earth’s surface estimated from thermometer records.
“Looking at a single, noisy 10-year period is cherry picking,” says Livermore climate scientist Benjamin Santer. Focusing on such a short period does not provide reliable information about differences across multiple decades, such as the presence or absence of human effects on climate. Santer adds that shorter periods generally have a small signal-to-noise ratio, making it difficult to identify a human-caused signal with high
By analyzing multi-decadal records, scientists can eliminate the large year-to-year temperature variability caused by natural weather patterns such as El Niño and La Niña. According to Santer, this approach makes it easier to identify a slowly emerging signal arising from gradual, human-caused changes in atmospheric levels of greenhouse gases.
Contact: Benjamin Santer (925) 423-2253 (email@example.com).
Improved Focus for Proton-Beam Experiments
An international collaboration has discovered an approach for focusing protons with curved surfaces. Developed by scientists from Lawrence Livermore and Los Alamos national laboratories, the University of California (UC) at San Diego, Helmholtz-Zentrum Dresden-Rossendorf and Technische Universität Darmstadt of Germany, and General Atomics, the new method could be adapted to heat materials, create new types of matter, develop medical applications, and better understand planetary science.
Working with the Trident subpicosecond laser at Los Alamos, the team used a cone-shaped target to generate and focus a proton beam. The sheath electric field generated in this closed geometry effectively channels the proton beam through the cone tip, substantially improving beam focus. The results, which appeared in the December 4, 2011, online issue of Nature Physics, provide insights into the physics of proton focusing.
Lead author Teresa Bartal from UC San Diego says, “The ability to generate high-intensity, well-focused proton beams can open the door to new regimes in high-energy-density science.” For example, focusing a proton beam on a solid density or compressed material creates the extreme pressures required to examine the properties of warm dense matter, similar to that found in the interior of giant planets such as Jupiter. Laser-produced proton beams could also improve medical applications such as isotope production for positron emission tomography and proton oncology.
Contact: Mark Foord (925) 422-0990 (firstname.lastname@example.org).