Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Background estimation for multi-charged particle coincidence eventswith SAMURAI
∗1,∗2
∗1,∗2
∗2 ∗2
Koyama, H.H.
Otsu, for
S.S.Koyama
andOtsu
forSAMURAI08
SAMURAI08collaboration
collaboration∗1∗2LO (a.u.)

1000

Department of Physics, University of Tokyo
RIKEN Nishina Center- 179 -(a)

(b)

800

600

400

20006070809060708090

T oF (ns)Fig. 1. T oF versus LO of a typical slat of HOD gated by(a) multiplicity of one and (b) M 2.

in Fig. 2(a) is a typical trajectory of the 16 C beam
which overlaps with the reaction points. The projection for the z−axis is shown in Fig. 2(b). Two peaks
around z = −440 mm and z = −930 mm are signiﬁcant. These values are consistent with the z positionsof the FDC2 entrance window and the charged particle
exit window of the SAMURAI magnet ,3) and 92 % of
the background is tagged as this component.
Other minor components of the background are
single events including more than two beam particles (3 %) and one particle going through neighboring
HOD slats (1 %). The other component (4 %) probably consists of reactions with materials upstream ofthe SAMURAI magnet other than the He target.
In conclusion, the origins of the background for MCP
coincidence events are identiﬁed and the main component of the background is beam fragmentation downstream of the SAMURAI magnet.
z (mm)

Several experiments have been performed with theSAMURAI spectrometer1) in recent years. Someof these experiments performed invariant-mass spectroscopy for the particle-unbound states of unstable
nuclei. For the invariant-mass spectroscopy of multicharged particles (MCPs) such as protons or α particles and residuals, it is important to select MCP coincidence events. Charged-particle events can be identiﬁedby a hodoscope (HOD)1) composed of plastic scintillator slats, which is placed downstream of the SAMURAI
magnet. Therefore, a value of more than two of HOD
slats (M 2) is able to select MCP coincidence events. In
this report, we estimate the origins of the backgroundfor MCP coincidence events.
For the estimation, we analyzed the data of
the SAMURAI08 experiment.2) In this experiment,
α-cluster levels in 16 C were investigated with a200 MeV/nucleon 16 C beam and liquid He target.
Here, we used down-scaled Beam trigger events. ABeam trigger is generated by the coincidence of twoplastic scintillators SBT1 and SBT21) and the anticoincidence of a perforated plastic scintillator SBV.
The SBV is installed to reject events in which a 16 C
beam hits the target cell frame. The averaged countrate of the Beam trigger is 100 kcps and the purity of
the 16 C is more than 99 %. An M 2 event is 2.5 % ofthe Beam trigger events for a 150 mg/cm2 -thick liquid
He ﬁlled tearget and 2.3 % for empty. Therefore, the8 % of M 2 events are caused by the reaction between
the 16 C and the He target. We deﬁne the other 92 %of M 2 events as background.
The main component of the background is found tobe fragmentation of the 16 C beam downstream of theSAMURAI magnet. The time of ﬂight (T oF ) versus
the light output (LO) plot of a typical slat of HOD
gated by a multiplicity of one and M 2 is shown inFigs. 1(a) and (b), respectively. Several isotopes canbe identiﬁed in Fig. 1(a), while loci in low LO events
occupy the main part in Fig. 1(b). These loci areidentiﬁed as light ions originating from the 16 C beam
downstream of the SAMURAI magnet as follows. The
positions and angles of these loci can be deduced bydrift chamber FDC2.1) Reaction points can be vertexreconstructed when two positions and angles are deduced in one event, as shown in Fig. 2(a). In thisﬁgure, the coordinate origin (x = z = 0) is the boxcenter of FDC2. Both the x−axis and z−axis are perpendicular to the gravity direction and the x−axis isparallel to the FDC2 entrance window. The red line0(a)(b)

50010005000500x (mm)CountsFig. 2. (a) Reaction points deduced from the positions andangles of two fragments. The red line is a typical trajectory of the 16 C beam. (b) Projection for z of (a).

References
1) T. Kobayashi et al., Nucl. Instrum. Meth. B 317 (2013).
2) H. Otsu et al., RIKEN Accel. Prog. Rep. 47, xx (2014).
3) Y. Shimizu et al., RIKEN Accel. Prog. Rep. 47,
175 (2014).