Hint1 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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