RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEAR

New isotope candidates, 215 U and 216U
Y. Wakabayashi,∗1 K. Morimoto,∗1 D. Kaji,∗1 H. Haba,∗1 M. Takeyama,∗1,∗2 S. Yamaki,∗1,∗3 K. Tanaka,∗1,∗4K. Nishio,∗5 M. Asai,∗5 M. Huang,∗1 J. Kanaya,∗1 M. Murakami,∗1,∗6 A. Yoneda,∗1 K. Fujita,∗7 Y. Narikiyo,∗7T. Tanaka,∗7 S. Yamamoto,∗7 and K. Morita∗1,∗7Daughter alpha energy

Theory1) predicts that nuclei with N = 126 exist
up to Fm(Z = 100) because of the appearance of thefission barrier originating from the ground-state shellcorrection. The heaviest N = 126 nuclei reported so far
is 218 U(Z = 92). In this paper, we attempt to produce
heavier nuclei such as 220 Pu. In our experiment, weobserved a new isotope, 216 U, which is the daughter
nucleus of 220 Pu.
We performed an experiment at the RIKEN Linear
Accelerator (RILAC) facility. We used 82 Kr beams of
372 and 387 MeV to bombard a rotating BaCO3 target
foil having a thickness of approximately 400 µg/cm2 .
To determine the efficient reaction for the productionof 216 U, we studied the reaction 82 Kr + 136,137,138 Ba
leading to the same nucleus 216 U with different neutron evaporation channels. Each 136,137,138 BaCO3 target was prepared by sputtering on 0.8–2.3-µm-thickaluminum foils, and they were also covered with 40µg/cm2 of aluminum by sputtering. Several 0.8-µmand 1.1-µm-thick aluminum foils were prepared as thedegraders. The beam energies at the center of the target were changed from 344 to 374 MeV by combining backings and degraders to obtain the excitation
function. Evaporation residues (ERs) were separated
from the beam particles and other reaction products
using a gas-filled recoil ion separator (GARIS), and(keV)

215

7800
7600

Pa

216

7400

UU

Pa

215

Pa

217

215

Th

211,212

7200
7000

Th

213,214

217

U 216

7500

8000

U

U

6800
7000

8500

Parent alpha energy

9000 (keV)

Fig. 1. α-α correlation spectrum. The time difference between the implanted ERs and the parent α-decay, andbetween the parent and the daughter α-decays werewithin 150 ms and 2.2 s, respectively. The horizontaland vertical position windows in the PSD were within
the same strip (∼3.6 mm width) and ±1.5 mm, respectively.
∗1
∗2
∗3
∗4
∗5
∗6
∗7Table 1. α-α correlated events of 215 U and 216 U. The timeand position difference between the implanted ERs andthe α-decay are ∆T and ∆X, respectively. Ebeam represents the 82 Kr beam energy at the center of the target.

215, 216

U217

they were implanted into a position-sensitive strip detector (PSD; 58 × 58 mm2 ) at the focal plane. Twotiming detectors were set in front of the PSD to determine the time-of-flight (TOF) of the ERs. Time
information was also used to distinguish between theα-decay events in the PSD and the recoil implantations. A Ge-detector was placed 6 mm behind thePSD for the α-γ coincidence measurement. In this experiment, 1.7 × 1017 and 2.7 × 1017 beam doses wereaccumulated at 372 MeV and 387 MeV, respectively.
Isotope identification was performed by using an αdecay chain with the help of known α-decay properties(energies and half-lives) of the descendants and theposition correlations between the implanted ERs in thePSD and the subsequent α-decays. Figure 1 shows anα-α correlation spectrum obtained in this experiment.
In Fig. 1, the candidates of the new isotopes, 215 U and216
U, were observed. These α-decay properties andthe obtained cross sections are summarized in Table 1.
The decay energies and half-lives of these descendants
agree well with those of the references. In the future,
an additional irradiation experiment will be performed
to confirm the production of 215 U and 216 U.

RIKEN Nishina Center
Department of Physics, Yamagata University
Department of Physics, Saitama University
Tokoy University of Science
Japan Atomic Energy Agency
Department of Chemistry, Niigata University
Department of Physics, Kyushu University

Eα∆T∆X
Reaction (Ebeam )(keV)(mm) Cross section
216
137
6.98 ms 0.15
U8408
Ba+82 Kr (365)
212
43.4
ms
0.12
Th 7811→ 216 U + 3n
208
2.23s
1.12
Ra 7144
0.19+0.44−0.16 nb
204
34.7 s0.14
Rn 6424215
1365.82 ms 1.02
U8436
Ba + 82 Kr (374)211
29.1 ms 0.72
Th 7807→ 215 U + 3n
207773
ms0.36
Ra 7124
0.34+0.49−0.22 nb20345.6 s
2.71
Rn 6474215635 µs
0.35U8230
21159.9 ms 0.99
Th 7799207
1.06 s
0.39
Ra 7145Eα (keV) ref. T1/2 ref.
α-decay branch212
30+2099.7%
Th 7802±10−10 ms
208
1.3±0.2s
95%
Ra 7133±520474.4±1.8 s
72.4%
Rn 6418.9±0.4
211
37+28
∼100%
Th 7792±14−11 ms
207
1.2±0.1s≤100%
Ra 7131±4
20344±2 s66%
Rn 6.499±2References1) H. Koura and T. Tachibana: Butsuri (in Japanese), 60,717 (2005).

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