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Terrestrial

If strange matter has a cosmological origins, and is stable, then it is possible that some has survived and is now present on Earth. If strangelets are released from collisions of strange stars, some of this fallout may also be present. Searches for superheavy isotopes of normal elements on earth have placed limits on strangelet abundances. One experiment used Rutherford backscattering of heavy ions to obtain such limits.

The experiment consisted of a fixed target of a sample of natural material 1-2 mm thick. The target was bombarded by a beam of tex2html_wrap_inline594 U at an energy of 1.4 MeV/A. The backscattered Uranium passed through a series of multi-wire proportional counters, which measured the time-of-flight, position, and deposited energy ( tex2html_wrap_inline596 ). From this information it is straightforward to extract the charge and scattering angle of the backscattered particle; the energy loss provides additional particle identification information. This information was used to separate the backscattered Uranium from target nuclei (e.g. Iron) that were rescattered after recoiling from a collision with the Uranium. Uranium ions with too long a time of flight were not included in the data, as they might have been multiply scattered.

From a knowledge of the Rutherford cross section and the scattering angle, it is possible to determine the mass of the struck target particle. This determination is limited to a mass of tex2html_wrap_inline598 , since higher mass particles can no longer be described as point-like objects in the Rutherford formula. The lowest mass the experiment was sensitive to was A = 400, corresponding to directly backscattered Uranium at the maximum allowed time-of-flight. No events were found in this allowed region. A knowledge of the target thickness provides limits on the abundance of strangelets ranging from tex2html_wrap_inline602 at A=400 to tex2html_wrap_inline606 at tex2html_wrap_inline608 . The increase in sensitivity with mass corresponds to an increase in the Rutherford cross section. These limits will be somewhat model dependent, as the Rutherford cross section depends on Z/A. For the strangelet model described above, varying the parameters can change the limits on strangelet abundance by as much as a factor of 20. This experiment measured samples of Iron from a meteorite, as well as terrestrial samples of garnet and manganese, and several lanthanide elements extracted from various minerals.

Other experiments, designed to look for relics from the Big Bang, have placed applicable limits on naturally occurring strangelets in the mass range tex2html_wrap_inline612 . These experiments yield much lower limits for these lighter strangelets.


next up previous
Next: Artificial sources Up: Detection of natural sources Previous: Astronomical

Joshua Holden
Sun May 17 15:37:00 EDT 1998