hi doug,
that is why i insist that the proper way to determine the k-factors is
by actually doing a linear fit of at least an entire superlayer at a
time. that way you are actually matching the cells up against each
other (using intercellular tracks) as opposed to just using looking at a
single raw tdc spectrum from one wire and trying to infer the boundaries.
of couse, one must also take into account that the t2d relationship
gives different velocities depending on the track angle, into the
analysis. it would be real nice if we had some cosmic data to
facilitate this process.
--chris
Douglas Hasell wrote:
> Hi,
>
> I have been thinking about this k factor and just realised that we
> CAN NOT directly measure the size of the cell and adjust the k factor
> accordingly. The time to position algorithm converts the drift time as
> a function of wire, superlayer, B, and track angle to a position in
> the plane of that sense wire but that position is not limited to be
> within the cell as obviously an angled track can cross the sense wire
> plane outside of the cell and be tangential to the isochrone inside
> the cell. Thus the electron drift is inside the cell but the distance
> from the sense wire, within the plane of the sense wire, is outside
> the cell. Thus the distance from the sense wire which we use in
> tracking is not limited to the cell size. I expect this could explain
> the range of k factors Wang found changing from large angles relative
> to the line normal to the sense plane where this affect is greatest to
> back angles where the tracks where the tracks are closer to the normal
> vector.
>
> The k factor is still an important adjustment to the drift
> velocity but to determine it we must specifically select events which
> cross the cell boundary and line up the hits in one cell with the hits
> in the neighbouring cell. This is not easy as at most there are two
> hits in one cell and one in the neighbouring cell.
>
> Hope this is clear to everyone. Otherwise we should meet and I
> can draw a few pictures to explain the problem.
>
>
> Cheers,
>
> Douglas
>
> 26-415
> M.I.T. Tel:
> +1 (617) 258-7199
> 77 Massachusetts Avenue Fax: +1 (617)
> 258-5440
> Cambridge, MA 02139, USA E-mail:
> hasell@mit.edu
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