Squeeze a thing violently and generally it heats up. So when a shock wave races through a solid, the material squeezed by that wave of compression has a higher temperature than the stuff lying ahead of the wave or in its wake. Now, physicists have taken the temperature of a fleeting shock wave by shooting neutrons through it. Described in the 1 April PRL, the new technique could be a boon to fields ranging from weapons research, to planetary science, to condensed matter physics.
For decades, physicists have used compressive shocks to probe matter at extreme conditions. Vital to weapons design, shock experiments can also simulate the hot, dense cores of planets and can even reveal details of fundamental processes, such as the formation of ice from liquid water. But measuring the temperature of a shock wave remains a challenge. Researchers can deduce the temperature of shocks in translucent materials from the intensity and color of light emitted by the squeezed material, a technique known as optical pyrometry. When applied to opaque materials, however, optical pyrometry reveals the temperature only at the sample’s surface, which is generally lower.
A burst of low energy neutrons can probe deep within a material and sense the temperature of a shock wave, report Vincent Yuan, David Funk, Charles Ragan, and colleagues, at Los Alamos National Laboratory in New Mexico. They created a shock wave in a sample of molybdenum by smacking it with an aluminum plate accelerated by an explosion to 3.6 kilometers per second–a whopping 11 times the speed of sound. The molybdenum plate contained a thin layer of a tungsten-molybdenum alloy. The neutrons passed through that layer just as it was squeezed by the shock wave, and the tungsten nuclei acted as the thermometers. The nuclei absorbed neutrons with energies in a certain range, and the extent of the range revealed the temperature.
That’s because as the temperature of the metal plate increases, the tungsten nuclei rattle about with greater speed. Those rushing into the neutrons add their energy to the collisions, enabling them to absorb slightly slower neutrons. On the other hand, tungsten nuclei rushing away from the beam only absorb slightly faster neutrons, as the neutrons need the extra speed to catch up to the retreating nuclei. So the range of speeds of absorbed neutrons increases as the temperature of the tungsten alloy increases. The researchers tracked the speeds of the unabsorbed neutrons simply by timing their arrival in a detector some 23 meters behind the sample.
The team compared neutron data from the shocked plate with data from an unshocked sample, and, as anticipated, the tungsten in the shocked sample absorbed neutrons over a wider range of energy. They deduced that the temperature of the shock wave was a toasty 875 degrees Kelvin. The technique could potentially help researchers to determine the mathematical relation between pressure, density, and temperature for a given material, the “equation of state” that describes how a material will behave in a wide range of conditions.
“The method is elegant, no question,” says Yogendra Gupta, director of the Institute for Shock Physics at Washington State University in Pullman. However, he says, the researchers should perform additional checks to ensure that their method is accurate. Sarah Stewart of Harvard University in Cambridge, Massachusetts, agrees. “It’s very promising,” she says, “but it must be validated on another material.”