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...
We have identified cosmic rays with Z = 40, 44, 52, 71, 77, 78, and 92 slowing in a balloon-borne stack. The values of Z assigned by both Lexan plastic detectors and nuclear emulsions were the same to within two units. Both low-and high-energy heavy cosmic rays are synthesized in the rapid neutron-capture process.We have found that nuclei with Z up to 92 are present in the low-energy cosmic radiation. Their abundance relative to Fe is similar to that previously observed at energies above ~1 GeV/ amu. 1,2 From their large rate of change of ionization as they slow down in a detector, their atomic number and energy can be much better determined than if they were relativistic. These observations are important as a basis for understanding the origin and propagation of the cosmic rays and as a standard of comparison with results obtained from the analysis of tracks of ancient cosmic rays left in meteorites 3 and moon rocks. 4
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