Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Development Of Multiple-Particle Tracking Algorithm For Forward
Drift Chamber In SAMURAI
M. Kurata-Nishimura,∗1 H. Otsu,∗1 and T. Isobe∗1∗1

RIKEN Nishina Center

X [mm]of degree of freedom (NDF) is within 10%, the track
information is stored as a candidate. Then, a precise
hit position on the plane is calculated from the drift
time for the candidate tracks. Two possibilities, i.e.,
whether the particle passed through in the left side orright side of the wire, are resolved by calculating theminimum χ2 configuration.
The track reconstructed in the X-Z plane is projected onto the U and V planes, and the wire positions in the Y axis are evaluated. By combining the Yposition on these planes, a linear fitting is applied forthem with spatial resolution of the wire pitch size. If
the χ2 /NDF is less than 1, the ambiguity of the leftor right side path is resolved in the same manner as inX-Z plane.
As a result, three-dimensional multiple tracks in
FDC1 were reconstructed as shown in Fig.1. In
this event, two tracks were found in both FDC1 andFDC2 independently. Tracks reconstructed withoutbias caused by other detectors enable us to estimate
the intrinsic tracking efficiency. This code is applicable for any experiments performed in the SAMURAIspectrometer.
150
100500-50-100150

Y [mm]The Superconducting Analyzer for MUlti-Particles
from RadioIsotope Beam (SAMURAI)1) has beenplaying an important role in studying unstable nuclei in RIBF since 2012. In April 2013, 16 C(α, α′ )experiment2) was carried out to investigate a degreeof freedom for an exotic cluster which appears above
an α emission threshold in high excited states. In theSAMURAI spectrometer, four momenta of α and aresidue were measured simultaneously to reconstruct
the invariant mass of the excited states. Thus far, nocomputer code has been developed for multiple charged
particle tracking, the primary issue being that trackreconstruction takes a long CPU processing time tofind a true hits combination from numerous hits combinations. With this background, a new algorithm for
multiple charged particle tracking was developed in theanaROOT3) framework and is described here.
In the SAMURAI spectrometer, two Forward Drift
Chambers (FDC1 and FDC2) are installed to reconstruct the tracks of scattered reaction residues in theforward direction. FDC1 and FDC2 are located upstream and downstream of the SAMURAI dipole magnet, respectively. Each FDC consists of three kindsof wire orientation planes referred to as X, U, and V
planes, and two planes of the same type are placednext to each other. This pair of two planes is hereafter
called a super pair plane. In the X plane, the wires
are parallel to the Y axis and, in the U and V planes,the wires are tilted +30o and −30o , respectively to resolve the three-dimensional flight path. The planes areassembled in the order of X, X’, U, U’, V, V’, X, X’,U, U’, V, V’, X, and X’ in FDC2 and in the opposite
order in FDC1.
In the beginning, reliable hits on each plane are selected. When a charged particle passing through ionizing gases around a wire, a δ-ray or X-ray would generate signals on adjacent wires. Since a real hit signal
reaches the wire faster than fake ones, it can be chosen
as the fastest timing signal delivered from a multiple
hit TDC on a common stop mode.
Using these candidates, two hits on the super pairplane are coupled if the difference between the hit wire
positions is within the pitch size of the wires. Multiple
use of a hit is allowed. Even if a hit is not associatedin the super pair plane, it can be used with the same
method for coupled hits in the following analysis.
With considerat all combinations of coupled hits
among X planes, a track in X-Z plane is evaluatedby a linear fitting. If the number of hits included in a
track is more than 4 and if χ2 divided by the number

100500-50-100-150-100 -80-60-40-20020406080100Z [mm]Fig. 1. Two reconstructed tracks in FDC1. Top and bottom figures show the track in the X-Z plane and Y-Zplane, respectively. The color of the linear fitting function indicates correspondences.

References
1) T. Kobayashi, et. al: Nucl. Instr. and Meth. B317 294304 (2013)
2) H. Otsu, et. al.: In this report3) https://ribf.riken.jp/RIBFDAQ/index.php?Tools%2FAnalysis%2FANAROOT- 176 -