In 1975 I announced a unique particle that had left tracks in a balloon flight Lexan-stack particle detector launched two years earlier by PB Price et al. The suggestion I made at that time was that it fit the description of what a postulated magnetic monopole would leave in traversing the stack (http://prl.aps.org/abstract/PRL/v35/i8/p487_1). A highly improbable alternative explanation was presented of a 'doubly-fractionating' normal nucleus, in traversing the stack, that it would have approximately mimicked the tracks, and the event in question was left in limbo, and the article by Price on the event as having been caused by a magnetic monopole was subsequently retracted. At that time (1975-1978), the idea of strangelets (Strange Quark Matter; or SQM) had not been theorized, which idea did not come about until circa 1984 [E. Witten, Phys. Rev. D, 272-285 (1984)], and that idea was accordingly not considered as a possibility by myself or Price in 1975 (or 1978 when he retracted). A brief review of the particle track-etch technique is presented: Most (if not all) materials will obtain chemical bond breaking when fast-moving electrically charged particles traverse through the material. Taking advantage of that fact, Price et al. developed a technique of utilizing several layers (typically 32) of thin, clear plastic sheets of Lexan polycarbonate as charged-particle detectors. As a particle moves through such a stack of thin sheets (typically 1/4 mil, or 250 um), it will leave a series of ionization damages (chemical bond breaking) along the course of the particle track. By immersing the thin sheets of plastic in a solution of warm caustic NaOH (typically 6.25 N at 40 C), the ionization damage can be dissolved away, leaving an 'etch pit' where the particle had entered, and another one where the particle had exited. These 'etch pits' take the shape of cones, as the undamaged surface dissolves away at a much slower rate than the damaged plastic bonds. If the particle were to traverse at constant speed through the stack, the etch-pits would be essentially identical from sheet to sheet. But that almost never happens, because the particles slow down as they lose energy to their causing the ionization damage in the plastic. The rate of ionization damage is controlled by two factors: 1) the speed, and 2) the total charge on the particle. Empirically, over a wide range of charge and speed, that relationship is as follows: 1) As the charged particle (typically an atomic nucleus, or cosmic ray) slows, the ionization rate increases as the inverse square of the speed. In other words, at half the speed, the ionization damage would be four times as great. 2) If the charge is increased, the ionization rate increases as the square of the increase in charge. In other words, at twice the charge, the ionization rate would be four times as great. Normally, the charge does not change on the particle as it traverses the stack, with some occasional exceptions (charge fractionation, typically when the moving atomic nucleus strikes a nucleus in the plastic, losing a few charges). The etch pits can then be measured under high-power microscope (typically 1600X oil-immersion), and an ionization rate plotted verses depth through the stack (taking into consideration the angle relative to perpendicular through the stack, requiring a precise measurement of angle of passage through each sheet). Most particles with a Z of about 26 (Iron) would pass through the stack with a large change in speed. Many would stop in the stack. Low Z particles only become 'visible' at the tail end of their range, as they are nearly stopped and thus travelling with very low speed. At near-relativistic speed, particles with Z << 20 do not leave visible tracks (the etch rate is comparable to the base etch rate of the unaffected plastic). The anomalous particle of 1975 left a track-etch rate that was nearly uniform through the entire stack, with only very slight variations from sheet to sheet that were in full accord with the accepted standard deviations. Superficially, it resembled a relativistic (speed = c, and accordingly barely slowing, hence having a uniform ionization) nucleus with Z = 137. However, that is in a range on nuclear charge that is a highly unstable nucleus (outside the 'island of stability'). Accordingly, I suggested it might have been a magnetic monopole, which would have mimicked a relativistic Z = 137. This appeared to be confirmed when the Cerenkov detector showed the speed well below light speed through clear Lexan plastic (light speed in clear plastic is about 2/3 c). However, it has recently been suggested that SQM might be produced from the surface of pulsars and injected into space, becoming a component of the cosmic rays (http://prd.aps.org/abstract/PRD/v74/i12/e127303). If so, an alternative explanation for a near-constant ionization rate is now also available. Specifically, if the mass is huge (so loss of energy due to ionization through the stack does not significantly effect the momentum, and hence the speed), and the speed much slower than c (below the Cerenkov threshold), then a much lower charge of around 35 would also have produced a non-changing ionization through the stack that mimicked a relativistic Z = 137. This fits the theoretical description of a strangelet, which has high A, and Z approximately the square root of A. So it appears that there is some evidence for existence of high-speed stable strangelets in the cosmic ray background. The recently launched AMS-2 might help resolve this. The AMS-1 reportedly had two possible strangelet candidates, not published because they were also statistically within background variation.
