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Upon looking at a decent number of bubble chamber images I noticed an x shape fermilab bubble chamber trail. What particles are producing these trails?

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They’re “stereo fiducials”: marks used to calibrate from the distances on the (somewhat deformable film image) to accurate 3D positions during the scanning process. On the chambers I'm familiar with, they were precision scribe marks on the inner side of the windows.

In the chamber in the Question photo they're found in pairs, with one on the front window and a nearby one on the back. They’ve been very accurately measured independently of the camera and scanning optics.

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    $\begingroup$ :) . scanning is the discovery of interesting events process, which was done by scanning girls with the instructions for the sought topology by physicists, on scanning tables. Then the film went to the measuring machine, where the chosen events were measured, ie the fiducial marks and points on the tracks were measured and recorded on magnetic tapes, for further analysis. $\endgroup$
    – anna v
    Jun 29, 2018 at 3:57
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    $\begingroup$ @annav Depends on location and era, I think. The Berkeley convention was to call the device "Scanning and Measuring Projector" (SMP); I was trained to set up fiducials at the beginning. These eventually became the flying spot scanners that led to automated kinematics (which is where I came in). See asprs.org/wp-content/uploads/pers/1964journal/sep/… Berkeley ended up making an undergrad lab out of this, see experimentationlab.berkeley.edu/sites/default/files/pdfs/… but we decommissioned that a couple years ago because we needed the space. $\endgroup$ Jun 29, 2018 at 5:53
  • $\begingroup$ I am talking Europe 1960~70s . We had an ENETRA meauring machine and 6 scanning tables given by CERN. Maybe that is why they gave us the machines, they were getting automated :) $\endgroup$
    – anna v
    Jun 29, 2018 at 6:40

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