J . Phys. Chem. 1990,94, 8494-8494
8494
COMMENTS Comment on Cluster-Impact Fusion Sir: The new phenomenon of cluster-impact fusion, reported' by Beuhler, Friedlander, and Friedman, has now been described2 in considerably greater detail. In their experiments singly charged clusters of 25-1300 D 2 0 molecules were accelerated in the range 200-325 keV and then impinged on a titanium deuteride target. The 3-MeV protons and 1 -MeV tritons were detected coming from the target. The observed thermonuclear D-D fusion reactions were attributed to the compression and heating caused by the impact of the cluster. Although extensive tests were performed by Beuhler et al. to exclude artifacts as the cause of the observed D-D fusions, the possibility that accelerated deuterons or single DzO+ molecules were actually responsible for the results may not have been completely ruled out. The purpose of this Comment is to suggest that there is a test not yet carried out by Beuhler et al. that could positively associate the reaction products with the impact of the cluster. This consists of pulsing the beam and observing the time distribution of the emitted protons and tritons relative to the time of the pulse. As an example, assume that a singly charged cluster of 100 D 2 0 molecules is accelerated to 300 keV, starting from rest. It is easily calculated that the time taken for traversal from the ion source to the target over the I-m length of their accelerator column is about 12 ps. On the other hand, a deuteron would traverse that distance in only 0.37 ps and a single D20+ molecule in about 1 ps. Thus, if the beam is pulsed, a two-dimensional display of detected proton or triton energy versus time after the pulse should show a 12-ps time delay if the fusion is indeed caused by the cluster impact. Directly accelerated deuterons or single D20+molecules at 300 keV would have enough energy to produce the observed events via the well-understood D-D reaction, without invoking new mechanisms. Such particles could come from several sources. Under pulsed conditions those emitted by the ion source would be pulsed along with clusters but would arrive at very short times after the pulse. Stray deuterons or D20+ not coming from the ion source would be expected to produce continuous events independent of the pulser. Finally, there is a possibility that deuterons or single D20+molecules could be stripped from the molecular cluster by impact with the accelerator tube or by collisions in the vacuum and then accelerated. All such events would produce protons and tritons earlier than the main cluster. The closer (in distance) to the target that stripping would occur, the nearer (in time) to the main cluster the protons would be produced, but such deuterons or D 2 0 +molecules would have lower energy and, therefore, a smaller cross section for forming the D-D reaction. One might expect in the above example that stripping would produce a continuum of events with delays in the range 0.37-12 p s . For the suggested pulsed-beam tests the timing is technically in a convenient range by using standard methods such as electrostatic deflection or mechanical beam chopping. Because of the breakup of clusters in such arrangements, there might be difficulties in maintaining a good vacuum. As in any pulsing measurement there will be a loss of yield due to the duty cycle. However, what is sought is essentially a "yes-no" answer as to the association of reaction products with clusters, and this might be possible in spite of the poorer statistics due to the duty cycle. One might imagine that pulses of 5-ps duration every 25 ps (20%
duty cycle) could lead to definitive results in the above example. The capability of pulsing the beam would seem to be a useful feature of any accelerator arrangement to be used for studying cluster-impact fusion. Acknowledgment. Research has been performed under contract DE-AC02-76CH00016 with the United States Department of Energy. Department of Physics Brookhaven National Laboratory Upton, New York 11 973 Received: June 25, I990
Reply to "Comment on Cluster-Impact Fusion" Sir: In response to Alburger's Comment, we note that we have been aware of his suggestion and have considered the possibility of a time-of-flight (TOF) experiment. We concluded that, with the experimental facilities available to us, such an experiment presented serious difficulties and at best would give marginal results. In view of these difficulties, we chose to devote our limited resources to the development of higher energy experiments, which are now in progress. Future improvements in fusion yields may justify TOF experiments. Investigators considering TOF experiments with cluster beams should be aware of the problems associated with "bunching" pulses of low-velocity heavy molecular ions which are very sensitive to collision-induced dissociation. Electrical pulsing with low-velocity ions must be carried out with "gates" open for times sufficient to permit transit of the heavy ions. Electrical pulsing of accelerated cluster ions presents problems associated with high-voltage pulse techniques. Mechanical chopping is complicated by high sputtering rates and pressure buildup associated with the decomposition of cluster ions on the chopper surface. In both cases duty cycle losses demand a margin of safety in beam intensity not, in our opinion, available in our experiments to date. TOF experiments with fragile polyatomic ions are not as simple and clean as with very stable atomic or molecular ions. It is not clear to us that an experiment, in which a large fraction of cluster ions were destroyed by a chopper, would give definitive results. Acknowledgment. This research was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the U S . Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. Brookhaven National Laboratory Upton, New York 11973 Received: August I , I990
( I ) Beuhler, R. J.; Friedlander, G.; Friedman, L. Phys. Reo. Lett. 1989, 63, 1292. ( 2 ) Beuhler, R. J.; Chu,Y. Y.; Friedlander, G.; Friedman, L.; Kunnmann, W .J . Phys. Chem. 1990, 94, 7 6 6 5 .
0022-3654/90/2094-8494$02.50/0
D. E. Alburger
0 1990 American Chemical Society
R. J. Beuhler G . Friedlander L. Friedman*
