Focus on High-Energy Detection
THE Department of Energy’s Stockpile Stewardship Program is central to Livermore’s national security mission. Underlying all of the Laboratory’s activities in stockpile stewardship is a commitment to ensuring that nuclear weapons will continue to be safe and reliable. As part of these efforts, Livermore researchers work to improve the ability to model the behavior of energetic materials (chemical high explosives) and metals at extreme conditions. In particular, they are focusing on how nuclear weapons materials, such as plutonium and uranium, behave under high pressures and temperatures.
The explosion process of a nuclear weapon produces interesting phase changes. The only other environment where such extreme pressure and temperature conditions occur is in the center of big planets, such as Jupiter and Neptune. At Livermore, our stockpile stewardship responsibility requires that our researchers have the tools they need to duplicate such conditions in a laboratory setting.
As the article Putting the Squeeze on Materials describes, the diamond anvil cell (DAC) requires only a microgram of material and allows Livermore researchers to study plutonium under extreme pressures without an explosion. Fifteen years ago, geologists used laser-heated diamond anvils to study the phase changes that occur in iron when it is under the temperatures and pressures at Earth’s core. Livermore researchers believed the same technology could help them understand where and when plutonium melts in a nuclear weapon. The importance of knowing when these state changes occur can be illustrated with an ice-skating analogy. Before ice skaters venture onto a lake on a warm day, they must determine whether the lake is frozen or the ice is melting. Likewise, in taking care of a stockpile, we must know when plutonium is liquid and when it is solid.
Until recently, it has been very difficult, even with a DAC, to perform electrical conductivity and magnetic susceptibility studies because of the challenge involved in placing diagnostic equipment close enough to the tiny samples to record accurate measurements. To solve this problem, Livermore researchers developed a new type of designer diamond anvil by embedding tiny tungsten coils into the diamonds. The coils are microcircuits that allow scientists to measure material properties under static pressure and then vary pressures and temperatures to simulate conditions occurring in a nuclear weapon.
Today, we can develop simulation codes that more accurately reflect changes in a nuclear weapon because the designer DAC provides direct access to what is going on inside the weapon. When weapon scientists relied on underground nuclear testing to supply experimental data on plutonium, they could only infer the metal’s state changes. Our understanding of these processes now is so radically different from what we thought occurred in a weapon 15 years ago that I could never then have imagined that we would be where we are today.
Livermore’s advances with DACs also illustrate the importance of the Laboratory Directed Research and Development (LDRD) Program. Applying DACs to study plutonium was beyond the scope of the ongoing weapons program, but LDRD support helped launch their use for this weapons-related application. That risk taken 15 years ago turned out to have a huge payoff.
This work is a classic example of how research within the context of the nuclear weapons program produces fundamental science that is remarkable in its own right. Usually, fundamental scientific discoveries help to advance applied science. In this case, applied science produced a tool for pure science. The result is improved precision of computer codes to model weapon performance, which helps ensure the safety and reliability of the nation’s aging nuclear weapons stockpile. In addition, that same applied science advances basic science in many other areas, helping scientists improve their understanding of the universe.