Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Development of multiwire drift chambers and the readout system for
focal-plane tracking detectors in BigRIPS
S. Y. Matsumoto,∗1,∗2 H. Fujioka,∗1,∗2 K. Itahashi,∗1 T. Kawabata,∗2 T. Nishi,∗1 A. Sakaue,∗1,∗2M. Takaki,∗3 Y. K. Tanaka,∗1,∗5 T. Uesaka,∗1 Y. N. Watanabe,∗1,∗4 K. Yako,∗3 and J. Zenihiro∗1
We are developing a new focal-plane detector
and readout system, aiming at searching for double Gamow–Teller giant resonance (DGTGR) via
the 12 C, 12 Be 0+reaction and systematic high2
precision
spectroscopyof pionic atoms (piAF) via theSn d, 3 He reaction.
Both experiments use the same detector conﬁguration for missing-mass spectroscopy. The particles emitted in the nuclear reactions are momentum-analyzed in
F0–F5 of BigRIPS and detected at the F5 dispersive
focal plane. The tracking detector at F5 (i) should
detect light ions and (ii) should work under high rate
background conditions (on the order of megahertz fortritons in the DGTGR experiment and protons in thepiAF experiment). In order to satisfy these conditions,
we adopt multiwire drift chambers (MWDCs) insteadof the standard tracking detector, PPACs.
We decided to develop low-pressure MWDCs thatcan be operated in vacuum1) . In the preceding experiment of pionic atom spectroscopy in 2014, MWDCs
with a gas pressure of 1 atm were operated in air.
A vacuum window (50-μm-thick stainless) located upstream caused multiple scattering. While the intrinsic tracking resolution was found to be ∼ 0.1 mm(FWHM), the eﬀect of multiple scattering resulted inthe deterioration of the position resolution up to 4.1mm (FWHM) at maximum. This deterioration is suppressed by using the low-pressure MWDC.
Our new MWDCs have conﬁgurations with sets of1/3-cell staggered three-layer structures, (XX’X”(0◦ ),UU’U”(+30◦ ), VV’V”(−30◦ )), as schematically shownin Fig. 1. The previous MWDCs had sets of 1/2-cellstaggered two-layer structures and showed a nonnegligible inhomogeneity in the spectra originating in ananalytic bias in the proximity of both the sense and potential wires. The asymmetric probability distributionof the estimated drift length near the wires due to theboundary condition (0 ≤ drift length ≤ wire interval)caused a bias for the reconstructed track, disfavoring
the close proximity of the wires. This eﬀect is expected
to be exhibited even with an exactly known drift-timeto-length conversion with a ﬁnite resolution, as demonstrated by a simple Monte Carlo simulation2) and becomes prominent in extremely high-statistics data. For
each set of layers in the new MWDCs, we use the drift

length information of two of the three layers by neglecting the data of the layer where the trajectory isclosest to a wire. On the basis of the previous experiment, we expect a high-rate capability (0.1–1MHz)and suﬃcient plane resolution (0.3 mm (FWHM)) for
the detector.
An upgrade of the readout system for the newMWDCs is necessary to realize high-speed data handling. As for the hardware of the system, we are developing a new all-in-one board3) that handles a higher
triggering rate (∼10kHz). The board reads 64 channelsof MWDC analog signals and ampliﬁes, discriminates,and digitizes them by FADC and TDC. The board
also receives trigger tag information from an event tagging system and transfers data by an SiTCP4) network
processor implemented by an FPGA. There are twomodes for reading data, raw and suppress modes. The
raw mode is an option for recording all of the dataof FADC, and the suppress mode, which is used inour experiments, is prepared for lowering the data sizeby collecting the data of channels with more than one
TDC hit. We have been working on building a DAQ
system with eight boards and combining this systemwith the standard RIBF DAQ5) .
                   

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Fig. 1. Schematics of the old (top) and new (bottom)
MWDC wire conﬁgurations. The new (old) MWDC
has sets of 1/3-cell (1/2-cell) staggered three(two)-layerstructures. The new MWDC has a redundant design
to avoid the analytical bias originating from the asymmetric probability density distribution of the estimateddrift length in the proximity of the wires.

References∗1∗2∗3∗4∗51) H. Miya et al., Nucl. Instr. Meth. B 317, 701 (2013).
2) Y. K. Tanaka: Ph. D. thesis, Univ. Tokyo (2016).
3) H. Yamakami: Master’s thesis, Kyoto Univ. (2015).
4) T. Uchida, IEEE Trans. Nucl. Sci. 55, 3, 1631 (2008).
5) H. Baba et al., Nucl. Instr. Meth. A 616, 65 (2010).

RIKEN Nishina Center
Department of Physics, Kyoto University
Center for Nuclear Study, University of Tokyo
Department of Physics, University of Tokyo
GSI Helmholtzzentrum für Schwerionenforschung GmbH- 205 -