In the upper left quadrant just above the midline there is a left curving track which seems
to disappear, then reappears a short distance away as two oppositely
curving tracks that begin in the same direction.
This must have been a positron which annihilated with a (nearly)
stationary electron in the liquid. One of the two gamma-ray photons
(which don't leave tracks since they are electrically neutral)
re-materializes as an electron-positron pair a short distance away.
Since the initial particle was a positron, the current is in the
direction of motion (thumb direction), the force is along the
radius of curvature pointing toward the center of curvature (nearly
downward in the picture - palm facing direction), so the magnetic
field must be into the picture (fingers direction).
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