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Physics News Update
Number 729, April 27, 2005 by Phil Schewe and Ben Stein

Pyrofusion: A Room-Temperature, Palm-Sized Nuclear Fusion Device

A room-temperature, palm-sized nuclear fusion device has been reported by a UCLA collaboration, potentially leading to new kinds of fusion devices and other novel applications such as microthrusters for MEMS spaceships.

The key component of the UCLA device is a pyroelectric crystal, a class of materials that includes lithium niobate, an inexpensive solid that is used to filter signals in cell phones. When heated, a pyroelectric crystal polarizes charge, segregating a significant amount of electric charge near a surface, leading to a very large electric field there. In turn, this effect can accelerate electrons to relatively high (keV) energies (see Update 564).

The UCLA researchers (Brian Naranjo, Jim Gimzewski, Seth Putterman) take this idea and add a few other elements to it. In a vacuum chamber containing deuterium gas, they place a lithium tantalate (LiTaO3) pyroelectric crystal so that one of its faces touches a copper disc which itself is surmounted by a tungsten probe. They cool and then heat the crystal, which creates an electric potential energy of about 120 kilovolts at its surface.

The electric field at the end of the tungsten probe tip is so high (25 V/nm) that it strips electrons from nearby deuterium atoms. Repelled by the positively charged tip, and crystal field, the resulting deuterium ions then accelerate towards a solid target of erbium deuteride (ErD2), slamming into it so hard that some of the deuterium ions fuse with deuterium in the target.

Each deuterium-deuterium fusion reaction creates a helium-3 nucleus and a 2.45 MeV neutron, the latter being collected as evidence for nuclear fusion. In a typical heating cycle, the researchers measure a peak of about 900 neutrons per second, about 400 times the "background" of naturally occurring neutrons.

During a heating cycle, which could last from 5 minutes to 8 hours depending on how fast they heat the crystal, the researchers estimate that they create approximately 10-8 joules of fusion energy. [To provide some perspective, it takes about 1,000 joules to heat an 8-oz (237 ml) cup of coffee one degree Celsius.]

By using a larger tungsten tip, cooling the crystal to cryogenic temperatures, and constructing a target containing tritium, the researchers believe they can scale up the observed neutron production 1000 times, to more than 106 neutrons per second. (Naranjo, Gimzewski, Putterman, Nature, 28 April 2005).

The experimental setup is strikingly simple: "We can build a tiny self-contained handheld object which when plunged into ice water creates fusion," Putterman says. (More information at http://rodan.physics.ucla.edu/pyrofusion .)

Nickel-78, The Most Neutron-Rich Of The Doubly-Magic Nuclei

Nickel-78, the most neutron-rich of the doubly-magic nuclei, has had its lifetime measured for the first time, which will help us better understand how heavy elements are made. Indeed, where do gold atoms come from? Physicists believe gold and other heavy elements (beyond iron) were built from lighter atoms inside star explosions billions of years ago. In the “r-process” (r standing for rapid) unfolding inside the explosion, a succession of nuclei bulk up on the many available neutrons.

This evolutionary buildup is nicely captured in a movie simulation showing all the species in the chart of the nuclides being made one after the other (http://www.jinaweb.org/html/movies.html). In some models the buildup can slow down at certain strategic bottlenecks. Nickel-78 is one such roadblock. This is because Ni-78 is a “doubly magic” nucleus. It has both closed neutron and proton shells; it is “noble” in a nuclear sense in the way that a noble gas atom is noble in the chemical sense owing to its completely filled electron shell.

Knowing more about this crucial nuclide is made difficult by the fact that it is, in our modern era, very rare, and hard to make artificially. Nevertheless, scientists at the National Superconducting Cyclotron (NCSL) at Michigan State University have now culled 11 specimens of Ni-78 from among billions of high-energy collision events recorded. In effect, the NCSL is a factory for reproducing supernova conditions here on Earth. Hendrik Schatz (schatz@ncsl.msu.edu, 517-333-6397), speaking at last week’s American Physical Society meeting in Tampa, reported that from the available Ni-78 decays recorded, a lifetime of 110 milliseconds could be deduced.

This is some 4 times shorter than previous theoretical estimates, meaning that the bottleneck nucleus lived shorter than was thought, which in turn means that the obstacle to making heavier elements was that much less. So far the exact conditions and site for the r-process are still unknown. With the new measurement model conditions have to be readjusted to produce the observed amounts of precious metals in the universe. This will provide a better idea of what to look for when searching for the site of the r-process. (See also Hosmer et al., Physical Review Letters, 25 March 2005)

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