[d2n-analysis-talk] VDC t_0 Check

[David Flay]

Hey Brad,

Attached are four plots of the drift time (of all active wires) for planes
U1, U2, V1, and V2.

Utilizing the variable 'L.vdc.u1.time[i],' we can examine the TDC values
of the ith active wire.  If we use no index, we look at all (active)
wires.

t_0 would be the amount we need to shift the main peak to zero.  In these
plots, we can see that there is no need to shift the main peak, so such
plots indicate that a t_0 correction is not needed.

Each plot has a structure to it, which is closely correlated to how close
to the wire the track is.  There are four distinct regions: a 'shoulder'
region, a 'plateau' region, a 'rise' region, and a 'peak' region.

The 'shoulder' region is due to large-angle tracks that intersect with a
small portion of a 'drift cell' (defined as a region of +/- 2.12mm
centered on a sense wire).  This leads to less ionization of the gas,
leading to less number of events per time bin (= dN/dt).

dN/dt may be written as:

dN/dt = (dN/ds)(ds/dt)

where ds = an element of distance along the E-field lines; dN/ds = the
linear density of counts along ds; ds/dt = mean drift velocity of
ionization in an element ds.

The 'plateau' region is due to a 'flat response' -- the electric field
lines are parallel in a particular section of a drift 'cell,' which leads
to a constant dN/dt.

The 'rise' region is due to the section in the drift cell where the
E-field lines become quasi-radial.  The mean drift velocity of the
ionization is approximately constant, whereas dN/ds increases => dN/dt
increases.  In the plots, this corresponds to the section of each
histogram where we see a transition from the plateau to the main peak.

The 'peak' region is due to the E-field becoming elliptical, where dN/ds
goes through a maximum, and ds/dt increases as one gets closer to the
wire.

Dave


-------------------------------------------------
David Flay
Physics Department
Temple University
Philadelphia, PA 19122

office: Barton Hall, BA319
phone: (215) 204-1331

e-mail: flay at jlab.org
            flay at temple.edu

website: http://www.jlab.org/~flay
              http://quarks.temple.edu
-------------------------------------------------


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