Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Study on detector response to spontaneous fission events ofheavy nuclides using the 206 Pb+48 Ca reaction
M. Takeyama,∗1,∗2 D. Kaji,∗1 K. Morimoto,∗1 Y. Wakabayashi,∗1 and M. Asai∗3
Detector response to spontaneous fission (SF ) ofproducts by the 206 Pb+48 Ca reaction was studied using a new focal plane detector that has a Si-Ge array1)
installed at a focal plane of the gas-filled recoil ion separator GARIS.
By using GARIS and GARIS-II, we plan to study
the production and decay properties of the superheavy
element (SHE) produced via actinide-based fusion reaction (hot fusion). It is reported that SHE nuclides
produced by the hot fusion are radioactive and decay
by α-particles emission or SF , and all decay chains are
terminated in SF 2) , which emit γ-rays. Therefore, it isimportant to measure the γ-rays of the SF of heavy nuclides. In 2013, we newly installed the Si-Ge detector
array3) for studying the production and decay properties of reaction products by including 248 Cm+48 Ca4) .
Thus far, we have searched for SF using Si detectors,
however, the Si-Ge array can carry out a more accurate identification than the Si detector because theSi-Ge array is expected as a probe for the detectionof prompt γ-ray coincided with SF . Before the experiment, the Si-Ge array was caliblated using a wellknown 206 Pb(48 Ca,2n)252 No reaction. The 252 No decays by 73.1% α-particle emission and 26.9% SF 5) .
We assigned this reaction based on its branching andhalf-life.
A 48 Ca11+ beam was extracted from the 18-GHz
ECR ion source and accelerated up to 218.5 MeV by
the RILAC. The typical beam intensity was 1.0×1012
particle/s (0.17 pµA). The metallic 206 Pb (enrichmentof 99.3%) target was prepared by vacuum evaporation
on a backing of 60 µg/cm2 carbon foil. The target
thicknesses had a mean value of 353 µg/cm2 . Sixteen
frames of the sector targets were mounted on a ϕ30 cm
rotating wheel, which was rotated at 3300 rpm. The
reaction products were separated in flight from projectiles and other by-products by GARIS, which wasfilled with helium gas at a pressure of 73 Pa, and thenthe products were transported into the focal plane detection system after passing through the time-of-flight(TOF) detectors. The detector system comprised twoTOF detectors, a PSD box1–3) , which is composed of aposition-sensitive detector (PSD) and four solid state
detectors (SSDs), and a planer typed Ge-detector forcounting low-energy photon (CANBERRA, BE6530).
The Ge-detector was separated from the other detector
vacuum by a 1-mm thick aluminum window. Magnetic
rigidity was set to 2.064 T·m for 252 No. Gamma rays
∗1∗2∗3RIKEN Nishina Center
Department of Physics, Yamagata University
Advanced Science Research Center, JAEA

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emitted in coincidence with SF events registered by
the PSD box were measured by the Ge-detector.
Figure 1 (A) shows a two-dimensional plot of energy
measure between PSD and SSD. SF fragment energy
is measured by SSD based on the implantation depthin PSD. When the recoil energy of evaporation residues
is low, recoil ions are stopped at the surface of the detector. Then, SF fragments are detected in both PSD
and SSD (region a). Conversely, both SF fragments
are either stopped in the detector or one of fragment
escapes in the backward direction if the recoil energy
is high, the implantation depth is deep (region b).
Figure 1 (B) shows a two-dimensional energy plotof SF - and γ-rays observed in prompt coincidence.
The probability of coincidence is 52.6% because an SF
event emits some γ-rays. From this probability, the SiGe array is considered to be useful for the identificationof SF fragments.

Fig. 1. (A) Two-dimensional plot of energy measure between PSD and SSD. (B) Two-dimensional plot of SF -γcoincidence.

References
1) D. Kaji et al., Contribution in this report.
2) Yu. Ts. Oganessian et al., J. Phys. G: Nucl. Part. Phys.
34, p.R165 (2007).
3) D. Kaji et al., Contribution in this report.
4) K. Morita et al., Contribution in this report.
5) Table of isotope 8th ed. (John Wiley & Sons, New York,1998 update).