Ⅱ-9. Instrumentation
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Installation of the NEBULA-Plus neutron array
N. A. Orr,∗1 J. Gibelin,∗1 Y. Kondo,∗2 and H. Otsu∗3
on behalf of the NEBULA-Plus Collaboration∗1,∗2,∗3,∗4
The direct investigation, through high-energy reactions and invariant mass spectroscopy, of neutron
unbound states and systems requires the detectionof one or more neutrons. In the case of the RIBF,
the principal permanently available fast neutron detector is the two-wall 120-element NEBULA array,1)which has been in operation since the commissioningof the SAMURAI setup in 2012. In order to augment
the detection eﬃciency, in particular for multineutronchannels, an upgrade comprising two supplementarywalls to be located just forward of NEBULA (Fig. 1)
has been built within the context of the French ANR
funded project EXPAND.2)

After detailed acceptance testing of all the bars andPMTs at LPC-Caen and optimization of the operationof the FASTER electronics (in particular the timingand charge measurements) NEBULA-Plus was shippedto the RIBF in summer 2022, following the easing ofcovid travel restrictions. The mechanical installationand cabling were completed in early October (Fig. 2)
and were followed by the installation of the high voltage system and electronics. Extensive testing wasthen performed during October and early November
using cosmic rays and γ-ray sources with NEBULAPlus running in a stand alone mode with FASTER.
Subsequently the eﬀort has been focused on the coupling of the FASTER electronics and DAQ with thatof SAMURAI. At the time of writing (January 2023)
this work and that required to merge the NEBULAPlus data with that from the SAMURAI detectors has
been completed and tested with cosmic ray data acquired with NEBULA and NEBULA-Plus operated intandem. Initial in-beam commissioning is planned tobe undertaken using the well-known 7 Li(p, n) reaction
which will also enable the single-neutron detection efﬁciency of NEBULA-Plus to be determined.4)

Fig. 1. Schematic view of the SAMURAI setup including
the NEBULA-Plus and NEBULA arrays.

The NEBULA-Plus walls are composed of a totalof 90 plastic scintillator bars (Eljen EJ200), with dimensions identical to those of NEBULA,1) read out
by Hamamatsu H11284 PMT assemblies. Each wallis composed of two layers - with each layer in the ﬁrst
wall comprising 22 bars and in the second wall 23 bars.
As for NEBULA, each of the walls is preceded by alayer of thin (1 cm thick) large area charged-particle
veto detectors. The read out of the PMT signals is
performed using the FASTER digital electronics andacquisition system3) developed at LPC-Caen. Whilst
FASTER has been conceived as a triggerless system,
it has been modiﬁed for use with NEBULA-Plus togenerate a trigger (derived from the coincidence between the signals from the two PMTs of each bar) soas to allow for its integration into the SAMURAI dataacquisition system (DAQ).
∗1∗2∗3∗4Fig. 2. View of the NEBULA-Plus array, including the vetodetectors, prior to the installation of the electronics.

References
1) T. Nakamura et al., Nucl. Instrum. Methods Phys. Res.
B 376, 156 (2016).
2) EXPAND: “Explorations Beyond the Neutron Dripline,”
ANR-14-CE33-0022-02.
3) http://faster.in2p3.fr/ .
4) Y. Kondo et al., Nucl. Instrum. Methods Phys. Res. B
463, 178 (2020).

LPC-Caen, France
Department of Physics, Tokyo Institute of Technology
RIKEN Nishina Center
Department of Physics, Tokyo City University- 91 -