Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Commissioning the BRIKEN array in parasitic mode using exotic Ni-Ga
secondary beam
*1*2*4 䠆4*51
䠆1 *1 J. Agramunt,
䠆2,3H. Baba,*1䠆
G. G.䠆Kiss,
S. Nishimura,
Ahn,䠆*11 A.
A. Algora,Algora,*2,*3
Bae,
T.N.Brewer,G. G. Kiss,
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Ahn,
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Bae,N.
T. Brewer, 䠆5*6*7*6*8*1*9*7
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䠆9J. Grifﬁn,
R. Caballero
T. Davinson,
I. Dillmann,
Go,T. Davinson,
I. Dillmann,
A. Estrade,
N. Fukuda, 1 S.
Go,C.
C. J. Griffin, 䠆7
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R. K. Grzywacz,
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J. Ha,J. Ha,
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H. Suzuki, 䠆1
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P. Vi,
L. Tain,M. M.
Takechi H.H.
Takeda, A.
A.Tarifeno-Saldivia,
Tarifeno-Saldivia, A.
P. Vi

Very neutron-rich nuclei can emit neutrons after ȕ--decay
when the decay Q-value is larger than the (one/two/three…)
neutron separation energy of the daughter nucleus (Qȕ>Sxn).
For neutron-rich nuclei away from the valley of stability,ȕ-delayed one/two/three-… neutron emission is thedominant decay process. These ȕ-delayed neutrons play akey role in the formation of heavy elements in the so-called
astrophysical r process1,2). Furthermore, the emission
probability (Pn) provides information about the distributionof the nuclear levels above the separation energy populated
in the ȕ--decay. A common approach to measure Pn values isto implant the isotope of interest into a silicon detector,
register its ȕ-decay and count the delayed neutrons using aneutron detector in coincidence with the ȕ--decay events.
In the last year, the largest and most efficient neutron
counter has been built at the RIKEN Nishina Center3). The
neutron detector, surrounding the AIDA silicon implantationarray4), has a flexible design. The so-called compact
BRIKEN array consists of 166 3He gas-filled proportionalcounters, embedded into a 90 cm x 90 cm x 75 cm highdensity polyethylene neutron moderator. The hybrid
BRIKEN detector configuration contains 148 proportionalcounters and two Eurisol-type clover HPGe detectors.
Figure 1 shows the schematic view of the experimental
setup.
The detection system was commissioned in November
2016 in parasitic mode during the NP1412-RIBF123R1
main experiment. A high intensity 238U beam wasaccelerated up to an energy of 345 MeV/nucleon at theRIKEN cyclotron accelerator complex before hitting a 5000
micron thick beryllium target to produce secondary beams
by in-flight fission.

Fig. 1.: Schematic view of the experimental setup.
Particle identification was carried out using the ionizationchamber (IC) and the plastic scintillator (Pl) located at F11.
The energy of the isotopes of interest was reduced using theadjustable degrader (AD) and then they were implanted into
the silicon detectors (Sid) and their decay was measured
using 166 3He counters (3HeC) and the clover type HPGe
detectors (HPGe1 and 2). In order to reduce the neutron
background the BRIKEN array is shielded by apolyethylene wall (PES) equipped with active vetodetectors (VPu and VPd). Furthermore, at the rear side of
the array, a veto detector (VPl) and a beamdump (BD) arelocated.
The secondary radioactive beam was filtered through theBigRIPS and ZeroDregree spectrometer and implanted in
the AIDA array containing 6 layers of highly segmented
silicon detectors. The emitted neutrons and Ȗ-rays following
the ȕ-decay were recorded by the BRIKEN array configured
in hybrid geometry. Some of the implanted isotopes areidentified as ȕ-delayed neutron emitters with well-known Pnvalues, which allowed us to determine the efficiency of thearray and compare it with the results of the Monte-Carlosimulations3) and a good agreement was found. In the latter
phase of the analysis, new Pn values will be obtained forisotopes with scarce or uncertain from previousexperiments.

1

RIKEN Nishina Center
Instituto de Física Corpuscular3
Institute for Nuclear Research (Atomki)
4
Department of Physics, Seoul National University
5
Oak Ridge National Laboratory
6
TRIUMF
7
School of Physics and Astronomy, University of Edinburgh
8College of Science and Engineering, Central Michigan University
9
Department of Physics and Astronomy, University of Tennessee
10
STFC Daresbury Laboratory
11
Department of Physics, University of Hong Kong
12
Graduate School of Science and Technology, Niigata University
13
Universitat Politècnica de Catalunya (UPC)
2References
1) R. M. Mumpower, R. Surman, G. C. McLaughlin, A.
Aprahamian, Prog. Part. Nucl. Phys. 86, 86 (2016).
2) A. Arcones and G. Martinez Pinedo, Phys. Rev. C 83,045809 (2011).
3) A. Tarifeno-Saldivia et al., Jour. of Instr. 12, P04006(2016).
4) C. Griffin et al., POS (NIC-XIII) 097 (2014).

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