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    User name Ole Hansen

    Log entry time 13:22:45 on June 29, 2010

    Entry number 329137

    keyword=VDC 1uA vs 32uA high-rate running observations

    The first plot below shows the u1 drift time spectra (first hit only) for run 1336 from last night. Beam current was 32 uA on lead, resulting in a roughly 8 MHz track rate in the chamber.

    Aside from the obvious low signal/background, one sees some evidence of multiple hits affecting drift times: larger drift times are reduced (slope down in the signal), presumably because there is a high probability for noise hits to pre-empt them.

    The next two plots show a comparision of the detector x and y distributions for the 1 uA run 1325 (~250 kHz) and the high rate run 1336 (~8 MHz). These are coordinates in the VDC detector frame (not TRANSPORT). One sees a very unexpected drop toward small x and y. This may be indicative of the clustering algorithm not working well in the presence of many noise hits and/or clusters. Clustering starts at large positive coordinates. In the presence of noise, clusters may start overlapping, causing clusters at smaller coordinate values to be more and more incomplete as parts of good clusters are absorbed into mixed clusters at larger position.

    The integral of the spectra from the two runs in each plot differ because of the apparent lower track yield at higher rate (37997 vs. 21762 tracks found for 100,000 events analyzed).

    Figure 1

    Figure 2

    Figure 3