ScienceWeek

APPLIED PHYSICS: A SIMPLE NEUTRON GENERATOR

The following points are made by Michael J. Saltmarsh (Nature 2005 434:1077):

1) New work[1] reports the successful demonstration of a simple neutron generator that produces neutrons possessing an energy of 2.5 mega-electronvolts (MeV) from reactions involving the fusion of two nuclei of deuterium. This device, it must be stressed, will not generate net energy, and is not related to past controversies about "cold fusion".

2) Neutrons can penetrate significant quantities of matter, and interact primarily with the nucleus rather than the electronic structure of an atom. As a result, portable neutron generators have found a wide range of applications, including well-logging for oil exploration, and the screening of baggage for airline security. Several commercial devices are available that use fusion reactions of deuterium (D) and tritium (T), whose nuclei contain one and two neutrons respectively (ordinary hydrogen nuclei have none). The reactions generate helium and a single neutron that carries away most of the reaction energy:

3) These neutron generators rely either on an ion beam from a miniature accelerator producing reactions in a solid target loaded with deuterium and/or tritium, or on the electrostatic confinement of a D-D or D-T plasma. In both cases high-voltage power is required, and the apparatus is fairly complex.

4) The device reported by Naranjo et al[1] falls into the solid-target category, only without much of the complexity. Indeed, in some ways it is remarkably low-tech -- the only input is a few tens of volts, to bias an electron-suppression grid, and some gentle heat (around 2 watts). A minute or two after the heat is applied, neutron emission starts, reaching a peak of about 1000 per second; once the heat source is removed, the device gradually switches itself off. The key to the device's simplicity lies in the replacement of the miniature ion-source and accelerator in existing generators by a system based on a combination of two well-known phenomena -- the pyroelectric effect and field ionization.

5) The pyroelectric effect -- the fact that some materials become charged when heated -- was probably first recorded in 314 BC by Theophrastus[2], Aristotle's student and successor, from his studies of the gemstone tourmaline. More recently, various man-made materials have been investigated, and potentials of around 100,000 volts reported for crystals such as lithium tantalate (LiTaO3), with the emission of energetic electrons under suitable conditions. This effect was used by Brownridge[3,4] to produce a small pyroelectric X-ray generator, of which a commercial version, powered by a 9-volt battery, is now available[5].

6) Field ionization of gases occurs when a potential difference of a few volts exists over atomic distances -- equivalent to a field greater than 10 gigavolts (10^(10) volts) per meter. The effect is widely used as the basis of field-ion microscopy. Modest voltages applied to electrodes of very small radius can produce these extremely high fields near the electrode tips, ensuring the ionization of essentially all gas molecules entering the high-field region.

References (abridged):

1. Naranjo, B. , Gimzewski, J. K. & Putterman, S. Nature 434, 1115-1117 (2005)

2. Donnay, G. Acta Crystallogr. A 33, 927-932 (1977)

3. Brownridge, J. D. Nature 358, 287-288 (1992)

4. Brownridge, J. D. & Raboy, S. J. Appl. Phys. 86, 640-647 (1999)

5. http://www.amptek.com/coolx.html

Nature http://www.nature.com/nature

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NUCLEAR PHYSICS: ON NEUTRON-RICH ATOMIC NUCLEI

The following points are made by D. Hinde and M. Dasgupta (Nature 2004 431:748):

1) Accelerator facilities that can supply beams of highly unstable, radioactive isotopes are making it possible to study nuclear reactions that occur naturally only in violent cosmic events such as supernovae, but which determine many of the elemental and isotopic abundances found on Earth. Some of the most neutron-rich of these nuclei have a diffuse neutron cloud that extends to large distances beyond the compact nuclear core. Based on the lessons learned from the fusion of stable nuclei, this halo might be expected to enhance, many times over, the probability of nuclear fusion at low energies. But recent measurements(1) demonstrated no such enhancement, indicating that the behavior of the neutron halo is more unusual than expected.

2) Atomic nuclei, which are made up of protons and neutrons, exist because of the attractive nuclear force between their constituents. This force depends on many variables, but in particular favors equal numbers of protons and neutrons. Thus, for example, the most common isotopes of the elements helium and nitrogen, having two and seven protons respectively, are 4He and 14N. If more neutrons were added to a nucleus, either one by one, or in pairs, the nucleus would become less strongly bound. Eventually, at the point where the energy of the new nucleus is greater than that of its "ingredients", the nucleus would become unbound. The transition from bound to unbound neutron-rich nuclei occurs at the so-called "neutron drip-line", which marks the boundary of the existence of nuclei. For the examples of helium and nitrogen, the drip-line isotopes are 8He and 23N. These and other neutron-rich nuclei are not found on Earth because they undergo beta-decay, in which a neutron is transformed into a proton, improving the balance between neutrons and protons.

3) Nuclei close to the neutron drip-line can have one or two extremely weakly bound neutrons. The energy required to separate the two weakly bound neutrons from 6He is only 0.973 MeV (megaelectronvolts), compared with typically 8 MeV to remove a single neutron from a stable nucleus. The low binding energy of these neutrons results in quantum-mechanical tunnelling to large distances, allowing their wavefunctions (or probability distributions) to extend well beyond the tightly bound core, forming a diffuse neutron cloud or halo.

4) The effect that this halo might have on nuclear fusion has been the subject of some controversy. Because the nuclear force has a short range, the attractive force between two nuclei is closely related to the degree of overlap of their matter distributions. The potential barrier (fusion barrier), which must be overcome in fusion, occurs at the radial separation where this force balances the repulsive Coulomb force between the positively charged protons in the two colliding nuclei. According to this picture, the neutron halo might be expected to contribute a longer-range attractive force, which would reduce the energy of the fusion barrier. Evidently, this does not happen.

References (abridged):

1. Raabe, R. et al. Nature 431, 823-826 (2004)

2. Timmers, H. et al. Phys. Lett. B 399, 35-39 (1997)

3. Dasgupta, M. et al. Phys. Rev. Lett. 82, 1395-1398 (1999)

4. Trotta, M. et al. Phys. Rev. Lett. 84, 2342-2345 (2000)

5. Di Pietro, A. et al. Phys. Rev. C 69, 044613 (2004)

Nature http://www.nature.com/nature

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APPLICATIONS OF NEUTRON BEAMS

The following points are made by F. Pfeiffer et al (Phys. Rev. Lett. 2002 88:055507):

1) Neutron diffraction, spectroscopy, and imaging using beam sizes in the submicrometer range are rapidly evolving fields of research. Progress is fueled not only by the availability of novel neutron optics, such as microcollimators, supermirrors, or focusing monochromators, but also by the urgent need for characterization tools that meet the demands of the advances in biochemical and semiconductor nanoscience. During recent years, planar thin-film x-ray waveguide structures have been developed and have been proved to deliver coherent beams efficiently with cross sections in the submicrometer range and with precisely defined properties of divergence and coherence (1 5). They have been used for submicrometer resolved one-dimensional projection phase-contrast microscopy, microdiffraction, enhanced diffuse scattering, depth profiling of incorporated nanocomposites, and for providing complex far-field diffraction patterns.

2) However, for many applications, the achieved x-ray based results (in particular for effects related to magnetism) are limited by the x-ray optical properties of materials. Thus the use of appropriate neutron investigation techniques combined with neutron optical devices with the ability to prepare submicrometer beam sizes would be desirable or even indispensable. The simple use of slits to define a submicrometer neutron beam width in the range of 100 5000 angstroms is, however, both difficult and inefficient for various reasons.

3) The authors report an experimental demonstration that planar neutron waveguides can be used as resonant beam couplers to efficiently produce a coherent neutron line source with cross sections in the submicrometer range. The Fraunhofer far-field diffraction pattern of the first three resonance modes was measured and found to be in excellent agreement with the theoretical model. The authors report their measurements confirm that an excited exiting mode is fully coherent in the direction perpendicular to the surface of the thin-film coupler and may therefore be used for applications of interest to a broad user community in biochemical and semiconductor nanosciences, such as static and time-resolved coherent speckle experiments or phase-contrast imaging.

References (abridged):

1. Y. P. Feng, H. W. Deckmann, and S. K. Sinha, Appl. Phys. Lett. 64, 1 (1993)

2. Y. P. Feng et al., Phys. Rev. Lett. 71, 537 (1993)

3. M. J. Zwanenburg et al., Phys. Rev. Lett. 82, 1696 (1999)

4. W. Jark, A. Cedola, S. Di Fonzo, and M. Fiordelisi, Appl. Phys. Lett. 78, 1192 (2001)

5. F. Pfeiffer, T. Salditt, P. Hoeghoej, I. Anderson, and C. David, SPIE Int. Soc. Opt. Eng. 4145, 193 (2001)

Phys. Rev. Lett. http://prl.aps.org

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