J . Phys. Chem. 1990, 94, 7665-7671 (3)O, values respectively obtained in evacuated (CuOH+)2(NH4+) The proximity of N ( l ) to a logical site (at ca. 0.305, 0.305, 0.305) for a fifth ligand to complete a commonly seen trigonal bipyramidal geometry about Ni2+ does not allow this work to conclude unambiguously that the novel trigonal pyramidal coordination of Ni2+ has been found. Nonetheless, this work does not indicate the presence of such fifth ligands, and the vacuum conditions used in the preparation of this material provide a basis for accepting a result that no neutral ligands remain in the structure. The location of the Ni2+ ion near the center of the 6-ring is consistent with Klier's spectroscopic result^.^-^ Presumably, the axial ligand of the Ni2+ ion would be removed upon full desolvation leaving the Ni2+ ion in a trigonal configuration on the plane of the 6-ring. Solvated Structure. Attempts to determine the solvated structure of Ni2+,NH4+-exchangedzeolite A yielded only approximate results (Table Ib) which, nevertheless, parallel those of the solvated Cu2+,NH4+-exchangedzeolite A structureg quite closely. A Ni2+ position is located at the origin where it is octahedrally coordinated by solvent molecules. Eight NH4+ ions are found on threefold axes in the large cavity hydrogen bonded to 6-ring oxygens. Two more NH4+ ions are associated with the 8-ring. Another Ni2+ ion position is located deep in the large cavity opposite one of the 8-rings where it is coordinated to four solvent molecules in a near square-planar manner. As in solvated Cu2+,NH4+-exchangedzeolite A, this Ni2+ ion has a very large thermal parameter. Its inclusion in least-squares refinement significantly lowered the error indices. The NH4+ions in the large
7665
cavity are bridged by H 2 0 molecules at O(4) located on a mirror plane near a 4-ring. Although there are 12 4-rings in the unit cell, only 8 H 2 0 molecules are at O(4); the remaining 4 solvent molecules at X(2) have moved closer to the Ni(2) position to be within bonding distance of it. All solvent molecules (20 of them were located) are within hydrogen-bonding distance of the framework and one another. Unfortunately this structure determination could not be carried to completion, perhaps because of the limited number of observations (only 197 with I > 341)) and the large number of variables associated with a solvated structure. The final error indices of R l = 0.1 11 and R2 = 0.093 are still relatively high. Moreover, there are uncertainties in the number (but not the positions) of Ni2+ ions present per unit cell: only 0.75 Ni2+ ions were located at the origin (by occupancy refinement) and attempts to increase this occupancy to 1.O increased the error indices. Because separate exchange preparations for the evacuated and solvated crystals were done, incomplete ion exchange may have occurred in this case. In the final refinement, 10 NH4+ ions and only 1.75 Ni2+ ions are located. Nonetheless, the similarity of the positions of the cations and solvent molecules in this structure to those observed for solvated Cu2+,NH4+-exchanged zeolite A supports this interpretation. Acknowledgment. This work was supported by the National Science Foundation (Grant No. CHE77- 12495). We are indebted to the University of Hawaii Computing Center. Supplementary Material Available: Listings of the observed and calculated structure factors of the structures (Tables I and 11) (3 pages). Ordering information is given on any current masthead page.
Deuteron-Deuteron Fusion by Impact of Heavy-Water Clusters on Deuterated Surfaces R. J. Beuhler, Y. Y. Chu, G. Friedlander, L. Friedman,* and W. Kunnmann Chemistry Department, Brookhaven National Laboratory, Upton, New York 1 I973 (Received: April 10, 1990; In Final Form: April 25, 1990)
The apparatus and techniques for producing, accelerating, and measuring beams of 200-325-keV singly charged cluster ions containing up to hundreds of D20 molecules are described. The diagnostics used to ascertain beam quality via secondary electron distributions are discussed. Results on DD fusion obtained when (C2D4)#,TiD, and ZrD1,65targets are bombarded with D 2 0 cluster ions are presented, including the dependence of fusion rates on target, beam energy, and cluster size. DD fusion events are also reported for H 2 0 clusters impinging on (C2D4)", but the rate is only 5% of that found with D20 clusters of the same size and energy. Extensive tests performed to exclude artifacts as the cause of the observed DD fusions are described and discussed.
I. Introduction The creation of transient assemblies of atoms at extremely high energy densities by cluster ion impact on surfaces has led to speculation that it might be possible to ignite thermonuclear fusion reactions in such assemblies.' Cluster ion impacts generate systems with unique properties of translationally "hot" atoms and relatively "cold" electrons. Production of such translationally hot atomic assemblies by impact of low-velocity atomic ion beams is precluded by space charge limitations.2 The chemical "bunching" of many atoms in a cluster with a single charge serves to overcome space charge limits on the density of particles that can be accelerated in a beam. If, as a specific illustration, we consider a cluster ion containing 100 atoms traveling with a velocity of IO7 cm/s, the projectile will impact an element of surface area of approximately cm2 (1) Beuhler, R.; Friedman, L. G e m . Rev. 1986.86, 521-37. ( 2 ) Keefe, D. Annu. Rev. Nucl. Pur?. Sci. 1982, 32, 428-431.
0022-3654/90/2094-7665$02.50/0
in a time of roughly s. This is equivalent to a particle current density for atomic ions of the order of 1030cm-2 s-I or about 10" A/cmZ, orders of magnitude greater than can actually be attained with atomic ions because of space charge limitatiom2 The 1Watom cluster, accelerated to lo5eV, thus deposits 10l6 W/cm2 of target surface. If the cluster is stopped in 10 atomic layers of target, the resulting energy density is approximately 100 eV per target atom, corresponding to a translational temperature of lo6 K. Electron microscopic examination of the morphology of holes and craters produced by cluster impact on thin films' shows that a major fraction of the cluster energy is consumed in the formation of these holes and craters, with only small amounts of energy dissipated in secondary electron emission and thermal conduction. (3) Matthew, M.; Ledbctter, M.; Beuhler, R. J.; Friedman, L. Nucf. Imrrum. Methods Phys. Res. 1986, B14,448-460. Matthew, M.W.; kuhler,
R. J.; Ledbetter, M.; Friedman, L. J . Phys. Chem. 1986, 90, 3152-59.
0 1990 American Chemical Society
7666 The Journal of Physical Chemistry, Vol. 94, No. 19, 1990
Beuhler et al.
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Figure 1. Schematic diagram of the apparatus. A mixture of D20and He gas is ionized in a corona discharge in the ion source. The weakly ionized plasma is expanded through a supersonic nozzle in the ion source into a differentially pumped region between the nozzle and the skimmer. Cluster ions are extracted from the skimmer with the draw-out lens and mass analyzed in the quadrupole mass spectrometer. A low-frequency (292-kHz) quadrupole power supply capable of mass analysis up to an m / e of about 200000 was used. The ions are then focused and accelerated in a Cockcroft-Walton column. Ions traverse an aperture and impact on a deuterated target. A monitor grid covers the aperture and is used to measure the cluster ion current. A silicon solid-state detector approximately 1 cm from the target is used to detect energetic products of fusion reactions
These observations support the conclusion that the hot atom assembly generated by cluster impact is, to a large extent, inertially confined, with confinement times calculable from the velocities of the constituent atoms and the size of the assembly. Under these circumstances one could reasonably expect that, in such a hot atom assembly containing deuterium atoms, some of these atoms might have sufficient energy to effect barrier penetration and thus to produce thermonuclear deuterium-deuterium (DD) fusion reactions. In a previous publication4 we reported on the observation of DD fusion events in the bombardment of TiD with singly charged clusters of D20molecules. The 3-MeV protons and 1-MeV tritons detected in a surface barrier silicon detector served as signatures of fusion. Clusters of 20-1300 D20molecules were found to induce fusion, with yields of about 0.05 fusion per second per cluster nanoampere for clusters of 100-500 DzO, dropping off to smaller values at smaller and larger cluster sizes. Accelerating voltages between 200 and 325 keV were used, with an increase in fusion yield of about an order of magnitude observed over that range. Also reported were several auxiliary experiments designed to establish whether the observed fusions could be caused by small-ion impurities in the beam rather than by the cluster ions. The conclusion was that cluster ion impact was indeed responsible for the fusions. In this paper we report additional data on cluster-induced fusion, including the observation of fusion events with several different deuterium-containing target materials. Also presented for the first time is evidence for DD fusion events from light-water cluster impacts on a deuterated target. Several further experiments designed to prove conclusively that our results are due to cluster impact rather than to artifacts are reported. 11. Experimental Section The experimental techniques used in this work for the production, analysis, and acceleration of cluster ions have been described in previous publications.5 They will be reviewed briefly here with emphasis on those aspects that are particularly relevant to experiments on cluster impact fusion. The apparatus used for fusion studies is shown schematically in Figure 1. Helium (or other) carrier gas saturated with D20 vapor is introduced into the ion source at pressures ranging from 150 to 760 Torr. A corona discharge generates gaseous ions, and these react rapidly with neutral water molecules in the source. The solvated deuterons and deuteroxyl ions are stabilized by the carrier gas and carried in the gas stream out of the source through the supersonic nozzle. Cluster ions grow in the nozzle expansion, with cluster size determined by the number density of neutral water molecules in the expansion volume element.5 Cluster size distributions can be controlled by the nozzle geometry, the arc (4) Beuhler, R. J.; Friedlander, G.; Friedman, L. Phys. Reu. Left. 1989, 63, 1292-5. ( 5 ) Beuhler, R. J.; Friedman, L. J . Chem. Phys. 1982,77,2549-57; Ber. Bunsen-Ges. Phys. Chem. 1984,88, 265-70.
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Figure 2. Mass spectrum of water molecule ion clusters showing a typical distribution of cluster ions peaked at about 63 water molecules per cluster ion. The spacing between peaks corresponds to 18 mass units for these light water ions.
current, the choice of carrier gas, and the partial pressures of water and carrier gas in the ion source. Lower carrier gas velocity produces larger clusters; therefore, as was established in earlier studies,' the generation of larger clusters is facilitated by the use of higher molecular weight carrier gases such as nitrogen or argon. The growth of cluster ions in the nozzle expansion virtually eliminates low molecular weight ions from the ion beam entering the skimmer. To illustrate this point, a mass distribution of water clusters generated in the ion source was scanned. The peak intensity was a t 15 water molecules, and the mass peaks corresponding to 9 and 21 water molecules had intensities less than 1% of the (H20)15 peak. No peaks were discernible at lower and higher masses. In the region of m / e = 20 (D20+ and D30+)a conservative upper limit of 0.1% of total beam intensity can be set. (See section IV for other experiments leading to lower limits.) To produce mass distributions peaked at between 15 and 500 water molecules, nozzles of 0.4-mm diameter and source pressures between 150 and 760 Torr were used. Some additional details of the ion source and its characteristics will be presented in a separate publication.6 Mass analysis was carried out using a low-frequency (292-kHz) quadrupole power supply capable of focusing singly charged ions with mass as large as 250000 Da. The quadrupole mass analysis served to provide a crude identification of the mass distribution of cluster ions when generated under conditions of low resolution. The system is capable of resolving individual cluster species as illustrated in Figure 2 for an HzO cluster distribution peaking at (H20)63. However, low resolution was necessary to maximize (6) Beuhler, R. J.; Friedman, L. To be submitted for publication in J. Appl. Phys.
The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 7667
Deuteron-Deuteron Fusion I
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Figure 3. Secondary electron pulse.distributionsobtained from impact of 275-keV cluster ions containing 133 D20molecules impacted on a clean copper surface. The peak at approximately 70 electrons was produced by a signal from an electronic test pulse introduced into the experiment. The secondary electron pulse distribution is peaked between 90 and 100 secondary electrons.
the total beam intensity of the mass distribution, which was -38% full width at half-height. Such operation was sufficient to eliminate any traces of low molecular weight ions, D+, D20+,D30+, D2+ etc., that might possibly exit from the ion source or the skimmer or draw-out apertures. Whenever a specific cluster size such as (D20)loois referred to in this paper, it is to be understood as the distribution peaked at that cluster size. Critical for cluster impact fusion experiments is the maintenance of optimum vacuum and focusing conditions. In the system sketched in Figure 1, the region between nozzle and skimmer was pumped with a Rootes blower of 400 cfm pumping speed through a IO-cm line. Beyond the skimmer, the system was pumped with two stages of liquid nitrogen trapped 15-cm oil diffusion pumps to achieve pressures of 1 X lo4 Torr or less in the region of the quadrupole mass analyzer, focusing lenses, and acceleration column. The target chamber, separated from the acceleration column by a 1-cm aperture, was independently pumped through a 15-cm column by a liquid nitrogen trapped 15-cm oil diffusion pump. Doubling the acceleration column and target chamber pressures led to some reduction in primary beam intensity but produced no significant changes in rates of cluster impact fusion per nanoampere of primary beam. This indicates that the vacuum conditions were adequate and that small ions produced by collision-induced dissociation did not significantly contribute to fusion events. (See section IV.) The quality of cluster beam focus conditions was established by measurements of secondary electron pulses: these measurements had to be done with primary beams of greatly reduced intensity. The intensity of the primary cluster beam could be varied under constant ion source and focusing conditions by variation of the resolution control of the quadrupole mass analyzer. In this manner the beam intensity could be reduced to sufficiently low values (on the order of IOs ions/s) to permit pulse counting of secondary electrons with a Chevron multichannel array electron detector. The channel plate was located below the target. Evidence that most of the clusters are intact after the acceleration process was obtained through measurement of the secondary electron distribution ejected when the cluster beam strikes a metallic surface. Figure 3 shows the electron distribution obtained when a positively charged cluster beam of 133 D20s strikes a copper dynode after acceleration to 275 keV. Clusters striking the dynode eject secondary electrons which are then deflected into the Chevron multichannel array electron detector. Each electron that enters a channel in the array is amplified by a factor of at least 10’. The amplified pulses are sent through pulse-shaping circuitry, encoded onto optical fibers, and sent to a multichannel analyzer at ground potential. Impact events that eject small numbers (
