Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 48 (2015)

Simulations for experimental studies of breakup reactions on
neutron-deficient isotopes relevant to the astrophysical rp-process
V. Panin,∗1 K. Yoneda,∗1 T. Uesaka,∗1 M. Kurokawa,∗1 T. Motobayashi,∗1 H. Otsu,∗1 Z. Elekes,∗2A. Saastamoinen,∗3 and B. C. Rasco∗4
Our experimental program aims to investigate explosive nucleosynthesis in the rp-process occurring intype I X-ray bursts related to the most frequent typeof stellar explosions in the Galaxy1) . Indispensable
experimental information related to resonant structures in most sensitive proton-capture reactions withextremely neutron-deficient isotopes will result in theadvancement of the existing X-ray burst models by
providing a reliable nuclear-data input2) . A particularfocus of this research is to obtain spectroscopic information about proton-unbound excited states in 66 Se
and 58 Zn, which are expected to be involved in thebreakout from the waiting-point nuclei 64 Ge and 56 Ni
through the sequential two-proton capture3) . Singleneutron removal reactions from 67 Se and 59 Zn at 250
MeV/nucleon beam energy will be deployed to populate the states above one- and two-proton separationenergy in 66 Se and 58 Zn, respectively. Proton decay
in-flight of these highly excited states will then be measured in complete kinematics using High-Resolution(HR) 90◦ mode of the SAMURAI magnetic spectrometer4) in combination with the existing tracking systems
and the custom-designed Si-strip detectors for simultaneous detection and tracking of heavy ions and protons.
The conceptual design of the setup was examinedby detailed Geant4 simulations, aiming to achieve anexperimental resolution of about 100 keV (sigma) fora single resonance and a total detection efficiency
of at least 20%. The multidimensional parameterization and fit algorithm was developed to reconstruct
the total momentum of every particle and eventually
the proton-fragment relative energy Erel in their restframe. Compromising the resulting Erel resolution, detection efficiency, and construction cost, an appropriate experimental configuration was worked out after
systematical variation of the setup parameters such as
the geometry and location of the tracking detectors,
target thickness, magnetic field strength, etc. In addition, specific simulations were performed to study
the effect of δ-electrons on the measurements by theSi trackers. In particular, the interference with a proton signal is studied together with various options tosuppress this effect by placing compact kick-off magnets between the Si-trackers, by adjusting the rela∗1∗2∗3∗4RIKEN Nishina Center
Institute for Nuclear Research, MTA Atomki
Cyclotron Institute, Texas A&M University
Department of Physics and Astronomy, Louisiana State University

tive distance between the detectors, and/or by track
discrimination using track information in the protontracking system after the magnet. Based on thesestudies, the minimum requirements for the design ofa vacuum chamber for the Si trackers (i.e., its size, position relative to the target, and internal locations ofthe detectors) were specified and the following conclusions were drawn:(1) The HR-mode is suitable for the heavy-ionproton (HI-p) experiments with beam energies
ranging from 200 to 250 MeV/nucleon.
(2) The value of acceptance×efficiency in the HRmode can be comparable to that of the standard“Day one” configuration (40% for one-proton
decay with Erel =1 MeV and beam energy of 250
MeV/nucleon).
(3) The HR-mode provides a better mass (momentum) resolution (1/1500) for heavy ions.
(4) The momentum resolution for protons and Erelresolution can be determined mainly by thestraggling in the target and by the position resolutions of the proton-tracking detectors.
(5) Erel resolution (sigma) of approximately 100
keV can be achieved at Erel =1 MeV alongwith the beam energies between 200 and 250
MeV/nucleon.
These results together with the LISE++ simulations
were used in the experimental proposal that was reviewed and approved by the 14th NP-PAC in June
2014. In a similar way, feasibility studies and experimental optimizations were performed for other potential HI-p experiments involving one- and two-proton
breakup of 9 C, 28 S, 32 Ar, 34 Ca, 35 K, 36 K and 36 Ca. A
common setup configuration for all HI-p experiments
was figured out and presented by the collaborators in
the 15th NP-PAC. Thus, we intend to perform a totalof four HI-p experiments in the next two years, ideallyas campaign-type measurements, which will provide
a wealth of experimental data that can significantly
enrich our knowledge and understanding of nucleosynthesis in type I X-ray bursts.
References
1) S. E. Woosley and R. E. Taam: Nature, Vol. 263 (1976),
101-103
2) A. Parikh, J. Jose, G. Sala, and C. Iliadis: Prog. Par.
and Nucl. Phys., Vol. 69 (2013), 225-223
3) O. Forstner et al.: Phys. Rev. C, Vol. 64 (2001), 045801
4) T. Kobayashi et al.: Nucl. Inst. Meth. B 317 (2013),
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