We present a laser-target scaling model which permits approximate prediction of the dependence of ablation pressure, mechanical coupling coefficient, and related parameters in vacuum upon single-pulse laser intensity (I), wavelength (λ), and pulse width (τ) over extremely broad ranges. We show that existing data for vacuum mechanical coupling coefficient for metallic and endothermic nonmetallic, surface-absorbing planar targets follows this empirical trend to within a factor of 2 over 7 orders of magnitude in the product (Iλ(τ)1/2). The comparison we present is valid for intensity equal to or greater than the peak-coupling intensity Imax, where dense plasma formation mediates laser-target coupling. Mechanical coupling coefficients studied ranged over two orders of magnitude. The data supporting this trend represent intensities from 3 MW/cm2 to 70 TW/cm2, pulse widths from 1.5 ms to 500 ps, wavelengths from 10.6 μm to 248 nm, and pulse energies from 100 mJ to 10 kJ. With few exceptions, data approximating one-dimensional or planar expansions were selected. Previously, meaningful scaling of ablation pressure parameters with I, λ, τ was not possible because existing data concentrated in a small range of these parameters. Our own data, obtained in the low- and midrange of (Iλ(τ)1/2), completes the experimental picture. Since this new data was derived from five separate experiments with specialized character and purpose, detailed accounts of this work will appear separately. In this paper, we summarize the experimental conditions and select only those data which are relevant to the scaling issue. We find that laboratory-scale laser experiments can often give impulse coupling data which agree with results from much higher-energy experiments without much error, and at much lower cost. We review a theory of vacuum laser ablation, specialize it to a quantitative description of mechanical coupling, and show that the resulting model provides a simple physical description which comes quite close to the observed empirical trend. This is accomplished with minor elaborations of the theory as originally presented to account for the temperature dependence of plasma ionization states, while adhering to the premise that a simple and generally applicable treatment of laser impulse production should be available. The theoretical model can quantitatively predict vacuum ablation pressure for opaque targets without adjustable parameters to the factor-of-2 accuracy in which we are interested. Other published scaling models omit one or more of the important variables, lack broad applicability, or deviate more noticeably from the observed trend.

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Within the stack, consisting of 35 Lexan detectors and three nuclear emulsions. in which the unusual event was found, we have measured tracks of -300 cosmic-ray nuclei with 2 6 1 % 1 8 3 , which provide an internal calibration of the response of the detectors. Our measurements in Lexan and in emulsion together show that the unusi~al particle produced a knockon-electron energy distribution incompatible with any known nucleus. 'The track etch rate and its gradient in Lexan give the quantity lZliP and, if the particle was a nucleus. a lower limit on its velocity. We found lZI/P = 114 at each of 6 6 positions in the Lexan stack extending over a range of -1.4 g/cm2 . The best fit to the Lexan data alone would be for a hypothetical superheavy element with Z 2 108 to 114 and 0 such that ZIP -r 114. A known nucleus with 9 0 I Z l 9 6 would also give an acceptable fit to the Lexan data if it fragmented once in the stack with a loss of about 2 units of charge, keeping ZIP -114.A nucleus with Z < 9 0 could maintain Z;P = 114 only by a properly spaced set of fragmentations. A nucleus with fl as low as 0.6 could fit the Lexan data only if it fragmented at least eight times in succession, with a probabilitylo-". In the 200-pm G-5 emulsion, visual measurements of the track "cores" produced by relativelylow-energy electrons (s 10 keV) are consistent with the Lexan result that the unusual particle had iZ i/P= 114.However, measurements of the density of silver grains at radial distances greater than -10 ,urn show that the particle produced far fewer high-energy (2 50 keV) knockon electrons in each of the three emulsions than would a known nucleus with Z / p = 114. If it were a known, long-lived nucleus with Z< 96, and therefore having 0.84 2 P> 0.6 in order to fit the Lexan data, its signals in the three emulsions would imply a very low ZIP of only -8 5 instead of 114. The abnormally small production rate of long-range electrons observed in all three emulsions is the essential evidence that we have found a unique particle. A monopole does not provide an acceptable fit to all of the data. A slow particle (0 0.4) could fit all of the observations, provided its mass were so great (> l o 3 amu) that it did not slow appreciably in the 1.4-g/cm2 stack. A fast (0.7 5 P i 0.9) antinucleus with Z/p = -1 14, because of its low Mott cross section for production of high-energy knockon electrons, could fit the data, especially if it fragmented once with loss of 1 or 2 units of charge. An ultrarelativistic (p > 0.99) superheavy element with Z = +I 10 to +I 14 can also account for the data and is not in conflict with any negative searches. Our knowledge of the Z and 0 dependence of the response of Lexan appears sufficient to preclude values of lZ/B I less than -110. An explanation of the weak distant energy deposition in terms of fluctuations by a normal nucleus or locally insensitive emulsion regions appears to be unlikely. Freak occurrences s~i c h as a 1020-eV jet or an upward moving nucleus d o not tit the data. Having achieved only an incom...

The crew members on the last seven Apollo flights observed light flashes that are tentatively attributed to cosmic ray nuclei (atomic number >/= 6) penetrating the head and eyes of the observers. Analyses of the event rates for all missions has revealed an anomalously low rate for transearth coast observations with respect to translunar coast observations.