material comparisons for FDC cathodes

[Elton Smith <elton@jlab.org>]

Hi Simon,

I have done a brief literature search to compare various thickensses of
cathode layers relative to our nominal FDC design. A second question of
interest is the amount of material that is required for cathodes in a
wires-only design to establish proper electrostatics.


Cathode Strips
==============

Below is a summary of some interesting comparisons. The Belle CDC is an
interesting comparison as it has comparable amounts of material per layer
as our nominal design. The BNL 787 central tracker as very thin cathode
foils (factor of 3-4 less).


Material in a single cathode layer

FDC
---

Material thickness    x/X0 (10^-4)

Rohacell     2mm      1.6
epoxy       20micron  0.5
kapton      25micron  0.9
Cu           2micron  1.4
Total                 4.4 10^-4

KEK Belle Akatsu NIMA 454 (2000) 322-333.
---------

Material thickness    x/X0 (10^-4)

Mylar     38micron    1.3
Adhesive  50micron    0.7 (obtained by subtraction)
Al         9micron    1.0
Total                 3.0 10^-4

BNL E787
--------

Material thickness    x/X0 (10^-4)

Kapton    25micron     0.9
Cu/Ni     0.15micron   0.1
Total                  1.0 10^-4


Wire-only Design
================

For a wire-only design, cathodes can be thinner as in the design by
the HERMES detector with mylar cathode foils (6.4 microns with
double-sided aluminized). The area of this chamber is relatively small
(22x68 cm), so it is likely that thicker foils are more realistic for our
chambers. We have used 25 microns of Kapton in our material budgets which
is probably reasonable as the area is almost an order of magnitude larger.

Considerations for material placement
=====================================

In terms of momentum resolution in the FDC itself, the amount of material
at the beginning and end of the measurements is unimportant.  Therefore,
if we want to have cathode planes to enhance our track-finding efficiency,
we should study how much it helps to have cathode planes only at the
beginning for the first package and at the end of the fourth package. This
might help resolve ambiguities and eliminate ghost hits which can be used
as an anchors for finding tracks segments in the other two wire-only inner
packages.

For the angular resolution (theta), material at the end of the track (i.e.
4th package) does not contribute. The material in the first package does
impact the extrapolation to the vertex. On the other hand, there is
already a significant amount of material in the CDC end-plate, so a solid
measurement in all coordinates in the first FDC plane should be very
useful if it is combined with information from the vertex/CDC.

This last discussion highlights the importance of z-information in the
CDC. We need to study how much improvement in z-position measurements in
the CDC helps reconstruction. We have discussed charged division in the
past, but also see the papers on the Belle detector that have cathodes at
the inside of their central drift chamber.

Cheers, Elton.


References:

[1] cathode-strip foils  BNL E787
http://sitka.triumf.ca/e787/pub/utc/utc_preprint.ps
Blackmore NIM A 404 (1998) 295.

[2] KEK B factory
http://linkinghub.elsevier.com/retrieve/pii/S0168900200005131
http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6TJM-41KP24T-5-1Y&_cdi=5314&_user=859303&_orig=browse&_coverDate=11%2F11%2F2000&_sk=995459997&view=c&wchp=dGLbVtb-zSkWA&md5=5270988ff2614fc4843b088f7e132e08&ie=/sdarticle.pdf

[3] Hermes FD detectors
http://linkinghub.elsevier.com/retrieve/pii/S0168900201007100






Elton Smith
Jefferson Lab MS 12H5
12000 Jefferson Ave
Suite # 16
Newport News, VA 23606
elton@jlab.org
(757) 269-7625
(757) 269-6331 fax