Distributed drift chamber design for rare particle detection in relativistic heavy ion collisions Page: 2 of 24
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2
for Ho production in relativistic Au + Au col-
lisions at the Brookhaven National Laboratory
(BNL) AGS. The experiment was designed to be
sensitive to weak decay channels of neutral par-
ticles, and specifically optimized to detect the
Ho -> E- + p decay channel. The main compo-
nents of the experimental setup were two topolog-
ical tracking detectors, shown in Figure 1. One,
a 15 plane silicon drift detector array, has been
reported in a previous paper [6]. The other is a
distributed drift chamber (DDC), located in a 1.6
Tesla analyzing magnet with a 6.2 Tesla sweep-
ing magnet and collimator placed just upstream
to minimize charged particles reaching the DDC.
In the present paper, we discuss the design and
implementation of the DDC.
2. Design
2.1. Designing the Tracking Detector
The primary constraints placed on the tracking
detector in the E896 experiment were that it un-
ambiguously identify the topological signature of
particle decays, provide enough track information
to reconstruct the rigidity of each charged daugh-
ter produced, and operate at high rates in a high
flux environment. Given these restrictions, a dis-
tributed drift chamber (DDC) was the optimal
detector for this experiment. A Multiwire Pro-
portional Chamber would lack the spatial reso-
lution (approximately an order of magnitude less
than a drift chamber) necessary to reconstruct the
invariant mass of the E- +p channel and thereby
distinguish it from the K- + p background pro-
duced in n + n interactions. A Time Projection
Chamber approach was not chosen due to insuffi-
cient double track resolution and because typical
recovery rates are much slower than that of a drift
chamber. A Drift Tube configuration was rejected
because its mass is an order of magnitude larger
than a DDC, as well as the fact that its assembly
would be significantly more difficult.
For the physical size of the tracking detector,
most of the outer dimensions were constrained by
the bore of the 48D48 Analyzing Magnet. The
48D48 was chosen because it provides the largest
fiducial volume of any of the standard BNL mag-
nets with a strong enough magnetic field to al-low identification of the daughter particles. The
tracking detector (including electronics, cabling,
etc.) is thereby constrained to be not more than
45cm in height, 120cm in width, and 120cm in
depth. The inner or "working" area of the track-
ing detector was determined by a combination of
the hardware constraints imposed by the Sweep-
ing Magnet[7] and the simulations performed us-
ing GEANT[8]. The inner height of 20cm is
a compromise between maximizing the available
tracking volume and allocating finite space for
structural integrity and the necessary on-board
electronics.
To reduce the number of neutral and charged
particles interacting with the DDC structure, a
50cm long lead-tungsten collimator was designed
and placed 40cm downstream of the target. A
vertical opening angle of 2.7 was determined by
GEANT simulations to be optimum for the sys-
tem, effectively cutting the vertical acceptance of
the DDC, 132cm downstream of the target, in
half. The maximum inner width of the track-
ing detector was determined by the background
Monte Carlo. The results indicated that the cen-
tral area of the chamber should be instrumented
to detect the Ho decays, while the area con-
taining the beam and lower rigidity positively
charged beam fragments must remain sparce to
reduce secondary interactions. Additionally, the
low rigidity area on the negative bend side should
also remain empty to reduce the background of
7r- particles produced in the Au + Au collisions.
Accordingly, the collimator was completely open
in the X direction to beam right. To beam left the
opening angle is drawn from the target pointing
to the last beam left active wire in the most up-
stream part of the DDC. The chamber and mag-
nets were oriented -3.2 from the 0 beamline
to maximize the acceptance around the mid-line
for neutral particles produced in the target and
to resolve charged daughter particles produced by
their decays.
In the active area of the DDC a minimum of
2mm cell radius was required to produce enough
primary electrons liberated by the passage of a
minimum ionizing particle to be detectable. Sim-
ulations determined this was sufficient granularity
transverse to the direction of the magnetic field to
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Bellwied, R.; Bennett, M. J.; Bernardo, V.; Caines, H.; Christie, W.; Costa, S. et al. Distributed drift chamber design for rare particle detection in relativistic heavy ion collisions, article, October 2, 2001; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc785144/m1/2/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.