Sessions - DPG-Tagungen

Nuclear Physics Tuesday
been installed into the beamline. In August 2003, first backscattered
photons have been detected.
This talk will present the current status of operation and the challenges
encountered, and give an outlook at the expected performance of
the system.
HK 12.33 Tue 13:30 Foyer
Low energy proton detection — •Dong Chen, Johannes
Bröcker, Walter Carli, F. Joachim Hartmann, Stephan
Paul, Gerd Petzoldt, Rüdiger Picker, Wolfgang Schott,
and Oliver Zimmer — Physik-Department, Technische Universität
München
The neutron life time can be determined in an ultra cold neutron trap
experiment by measuring the protons from the neutron β decay( maximum
p energy 752 eV) with a large area p detector. Different methods,
where the p are measured by scintillators and photon detectors,either
directly or via p- induced δ electrons, are being investigated. A complete
signal- background separation was obtained by exposing 20 keV
p to a bulk Tl dotted CsI crystal connected to a photomultiplier tube.
More interesting are thin scintillator layers connected to photo diodes or
avalanche photo diodes, which are insensitive to γ rays and can be used
in a large magnetic field.
HK 12.34 Tue 13:30 Foyer
The COMPASS Online Filter — •Roland Kuhn, Thiemo Nagel,
Stephan Paul, and Lars Schmitt for the COMPASS collaboration
— TU München, Physik-Department E18, 85747 Garching
COMPASS is a high rate fixed target experiment, taking data at the
SPS at CERN. Due to the stringent time constraint at the first trigger
level an acceptable trigger rate can only be achieved at the cost of a
lower purity. The physics selectivity can be greatly enhanced by allowing
higher first level rates and enriching the interesting data after event
assembly, which saves storage space and reconstruction time. This also
leads to a reduction of trigger dead time, if hardware vetoes are partly
replaced by software filtering. The concept of the filter software along
with first performance results from the 2003 run of COMPASS will be
presented.
HK 12.35 Tue 13:30 Foyer
Beam-induced Depolarisation in the HERMES Transversely
Polarised Hydrogen Target * — •Phil Tait, Davide Reggiani,
and Erhard Steffens for the HERMES collaboration — Physikalisches
Institut, Universität Erlangen-Nürnberg
The Hermes polarised Hydrogen target is situated in the HERA electron
storage ring in Hamburg. For the transverse spin program at HER-
MES, the magnetic holding field of the target is perpendicular to the Hera
positron beam. An unwanted consequence is that the beam-induced resonance
between Hydrogen hyperfine states with ∆mF = 0, which was
previously forbidden, can occur. The shape and spacing of these resonances
will be shown. In order to prevent these resonances from reducing
the nuclear polarisation in the target cell, an additional coil has been
added to improve the field homogeneity. The success of these measures
will be demonstrated.
*) Supported by BMBF, project 06-ER-127
HK 12.36 Tue 13:30 Foyer
Interactive Parallel Analysis with PROOF — •Schwarz Kilian
— GSI, Planckstr. 1, 64291 Darmstadt, Germany
To be prepared for the parallel analysis of distributed datasets the different
options of setting up a PROOF cluster are investigated at GSI and
GridKa.
The Parallel ROOT Facility, PROOF, is an extension of the ROOT
system. It enables physicists to analyse large sets of ROOT files in parallel
on remote computer clusters.
PROOF consists of a 3-tier architecture, the ROOT client session, the
PROOF master server and the PROOF slave servers. PROOF daemons
are assigned on demand which start according to a predefined config-file
the masterserver to which the user connects. The master server in turn
creates, again via the PROOF daemons, slave servers on the nodes in the
cluster, which ask the master for work packets by using a pull protocoll.
The results can be merged at the end of the session. Such an environment
has been set up and tested. Hereby we compared the possibility
of setting up a dedicated PROOF cluster with the option to integrate
PROOF into the local batch system.
Parallel analysis of worldwide distributed data sets is possible by com-
bining PROOF with Grid technology. This has been tested by using the
ALICE Grid implementation AliEn, where the PROOF client connects
to a PROOF master server running on an AliEn core service machine.
The PROOF master connects to PROOF daemons in distributed sites
through an AliEn TCP routing service. This enables connectivity and
connection control to computing farms on private networks.
HK 12.37 Tue 13:30 Foyer
A VME module for Digital Pulse Processing — •Martin
Lauer 1 , Vinzenz Bildstein 1 , Hans Boie 1 , Frank Köck 1 , Ian
Lazarus 2 , Oliver Niedermaier 1 , Uttam Pal 1 , Heiko Scheit 1 ,
and Dirk Schwalm 1 — 1 MPI für Kernphysik, Heidelberg, Germany
— 2 Daresbury Laboratory, Warrington, UK
An algorithm to determine the energy of a γ ray from digitized preamplifier
signals was implemented on a four channel VME card with 14
bit, 80 MHz digitizers and XILINX Spartan 2 [1] Field Programmable
Gate Arrays (FPGA) for signal processing. In addition the firmware of
the GRT4 card (Gamma-Ray Tracking 4 channel) [2] was extended by a
module to trigger on the leading edge of the signal. The Moving Window
Deconvolution (MWD) algorithm [3] was used, which transforms the
preamplifier signal into a trapezoidal shape, the preferred shape for highrate
and high-resolution γ ray spectroscopy. The implementation also
includes the digital equivalent of a baseline restorer to remove any offset
from the spectra. The free XILINX WebPACK [1] software was used
to generate the programming information for the FPGA from a VHSIC
(Very High Speed Integrated Circuit) Hardware Description Language
(VHDL) design. The pulse processing capabilities of the GRT4 module
and the MWD implementation will be presented, as well as results
obtained with HPGe detector signals as input to the GRT4.
[1] www.xilinx.com
[2] http://nnsa.dl.ac.uk/GRT/grt4 brochure.pdf
[3] J. Stein et al. NIM B 113 (1996) 141-145
HK 12.38 Tue 13:30 Foyer
Effects of N2 in the ALICE TRD and TPC gases — •Chilo
Garabatos for the ALICE collaboration — Gesellschaft für Schwerionenforschung,
Darmstadt, Germany
It is inevitable that nitrogen impurities build up in detectors operated
in a closed-loop gas circulation mode. Therefore, we have studied the
effect of N2 contamination on the drift and amplification properties of
mixtures with noble gases and CO2.
For the Xe-CO2 [85-15] mixture foreseen for the ALICE Transition
Radiation Detector (TRD), the admixture of up to 20 % N2 marginally
affects the drift velocity at the operating drift field of 700 V/cm, whereas
the gain slightly decreases.
On the other hand, for the Ne-CO2 [90-10] drift gas of the ALICE
Time Projection Chamber (TPC), the addition of 5 % N2 results in a
decrease of the drift velocity of only 5 % at 400 V/cm, but substantially
extends the efficiency plateau of the Readout Chambers. Measurements
and simulations of drift velocities and gains in these mixtures will be
shown, and a picture describing the detector stability involving Penning
effects in the avalanche will be discussed.
HK 12.39 Tue 13:30 Foyer
Background in the ALICE TRD and in the TPC electronics
based on FLUKA calculations. — •Georgios Tsiledakis 1 , A.
Fassò 1,2 , P. Foka 1,2 , A. Morsch 2 , and A. Sandoval 1,2 for the AL-
ICE TRD collaboration — 1 GSI, Darmstadt — 2 CERN, Geneve
The main focus of the ALICE experiment at LHC is the study of
Pb–Pb collisions at nucleon-nucleon center-of-mass energy of 5.5 TeV.
Due to the high luminosity (10 27 cm −2 s −1 ) and the high particle multiplicities
anticipated in these collisions, a high background of thermal
neutrons is expected to build up as the particles shower and get stopped
in the material of the detectors, magnets, support structures, and in particular
in the concrete walls of the experimental cavern. The Transition
Radiation Detector (TRD) contains a gas mixture of 85% Xe, 15% CO2
with a total volume of 27.2 m 3 . Especially 131 Xe (abundance 21.18%) has
a very high neutron capture cross-section. This leads to multi-gamma deexcitation
cascades which can then produce low energy electrons through
photo-effect, Compton scattering, and pair production, thus resulting in
an uncorrelated background. We estimate the level of this random background
during the 3 µs gating time of the TRD chambers. In addition,
neutron fluences, doses, and the induced Xe radioactivity in the gas of the
TRD are calculated. We also estimate the radiation level in the region
where the electronics of the Time Projection Chamber (TPC) is located,