I am investigating the effect of the magnetic field on the tube gains/instabilities.  We see about 50% of the tubes with some degree of instability when we turn up the STAR field (Full, Polarity B) and the DX magnet (145 Amps = injections, 1422 Amps = 250 GeV store)

Oleg Tsai suggested that the 'instability' could be due to the LEDs.  Here is a map of the led system: www.star.bnl.gov/protected/spin/trent/fms/ledmap.gif

Run 12017047 Large Cell HV=-1600 V Examine time stability for 4 cells which are each attached to the same LED.  Two of the PMTs are stable, while the two others are unstable in different ways:www.star.bnl.gov/protected/spin/trent/fms/compare_led15.gif

During the evening of January 19 2011, I started a run while the DX magnet current was falling from 1118 Amps to 145 Amps.  Here is that data for a column of cells closest to the centerline.  I plot the ADCs of cells in a row above and below the beamline.  The series belowthe beamline illustrates the effect is a monotonic increase in the gain as you get farther from the beampipe.  The effect of the DX magnet is approximately uniform and about 20-30% at the highest DX field.  The DX effect is of opposite sign below the beamline as above it.

Below:www.star.bnl.gov/protected/spin/trent/fms/compare_2C0_bottom_gain_change_DX_1100_145.gif

Above:www.star.bnl.gov/protected/spin/trent/fms/compare_2C0_top_gain_change_DX_1100_145.gif