RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-1. Nuclear Physics

Production of 215U and 216U and attempt to produce 219Np and 220Pu
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 Y. Komori,∗1 M. Murakami,∗1,∗6 T. Tanaka,∗7 A. Yoneda,∗1 and K. Morita∗1,∗7Theory1) predicts that nuclei with N = 126 exist upto Fm (Z = 100) because of the fission barrier arisingfrom the ground-state shell effect. The heaviest N =126 nuclei reported so far is 218 U (Z = 92). In thisprogram to study nuclei with N = 126, we attempt
to produce heavier nuclei such as 220 Pu. In previousexperiments, a new isotope 216 U, which is the daughter
nucleus of 220 Pu and 215 U, were observed2) .
We performed an experiment at the RIKEN LinearAccelerator (RILAC) facility. We used 82 Kr ion as anincident beam and 136,137,138 BaCO3 , N at La2 O3 , andN atCeO2 as targets to study the 82 Kr + 136,137,138 Ba,139
La, and 140 Ce reactions. Each target materialwas prepared by sputtering on 0.8–1.1-µm-thick aluminum foils so as to achieve a thickness of 300–500µg/cm2 , and it was also covered with 40 µg/cm2 ofaluminum.The 82 Kr beams with energies of 365, 381
and 386 MeV were used to bombard these target foils
mounted on a rotating target.
Evaporation residues (ERs) were separated from thebeam particles and other products using a gas-filled recoil ion separator (GARIS), and they were implanted
into a position-sensitive strip detector (PSD; 58 × 58mm2 ). The PSD was boxed in four Si detectors (SSD)
to catch α particles escaping from the PSD. Two timing detectors were set in front of the PSD to measure

2Previous(a)1∆T(ER - αp)

T1/2 (exp.) of Eα1(exp.)= 2.6 +3.6
ms
-1.0024∆T(αp - αd)T1/2 (exp.) = 18+11-5 ms+20

T1/2 (ref.) = 30 -10 ms+0.5

T1/2 (exp.) = 0.7 -0.2 sec
T1/2 (ref.) = 1.3 (2) sec ∆T(αd - αgd)

101Previous(b)216

U (αp)
Eα3Eα1 (exp.)= 8390 (33) keVEα1Table 1. α-decay events of 215 U and 216 U. The time andposition difference between the implanted ERs and theα-decay are ∆T and ∆X, respectively. Eb represents
the 82 Kr beam energy at the center of the target.

Eα20

102

2

the time-of-flight (TOF) of the ERs. Time information was also used to distinguish between the α-decay
events in the PSD and the recoil implantations. A Gedetector was placed 6 mm behind the PSD to measure
the γ-rays coinciding with the α-decays. The isotopes
were identified by using an α-decay chain with known
α-decay properties of the descendants and the position
correlations between the implanted ERs in the PSD
and the subsequent α-decays.
In this experiment, we confirmed the production of215
U and 216 U by observing one chain and six chains,respectively, including the candidates of new transitions. For the decay chains of 216 U, the α-decay energies and decay times are shown in Fig. 1. Thesedecay events and cross sections are summarized in Table 1, and we labeled the decay energies with Eα1 ,Eα2 , and Eα3 temporarily. For a new transition, Eα2of 216 U may be a transition from isomer-state in 216 U
to ground-state in 212 Th as well as an isomer state of218
U with the α-decay energy of 10678 keV3) . In theattempt to produce 219 Np and 220 Pu using the 82 Kr+ 139 La and 140 Ce reactions, cross section upper limits of 28 pb and 46 pb, respectively, were obtained.
Further discussion, such as the interpretation of newtransitions, is ongoing.

-1 0

1

2

3

4

log10(T1/2)(sec)5212

3Th (αd)2Eα (exp.) = 7810 (21) keV

216

U(Eα1 )Eα (ref.) = 7802 (10) keV

103208Ra (αgd)

2Eα (exp.) = 7124 (23) keV

1Eα (ref.) = 7133 (5) keV

0216

6000 7000 8000 9000 10000

Energy(keV)

Fig. 1. α-decay time (a) and energy (b) spectra for 216 U.
The previous results are indicated by dotted arrows.
Each ∆T indicates the time difference between each decay generation (αp , αd , and αgd ). The labeled energies
Eα1 , Eα2 , and Eα3 for 216 U are specified. Observed αdecay energies and half-lives are written with reportedones except for 216 U.

U(Eα2 )
216
U(Eα3 )

Eα(keV)
84082)
8371
8379*
10518
10459*
8254
8265*∆T(ms)
6.98
3.95
0.43
2.50
0.43
1.81
3.40

|∆X|(mm)
0.2
0.2
0.5
0.2
2.3
0.1
0.2

U84362)5.82
1.0(Eα1 )
215
U82302)0.64
0.4(Eα2 ) 8283
2.100.2* Sum energy of PSD and SSD.
215

Reaction (Eb ) &Cross section
137
Ba+82 Kr (366)→ 216 U + 3n
22+14−9 pb
136

Ba+82 Kr (350)→ 216 U + 2n
58+77−38 pb

136

Ba+82 Kr (373)→ 215 U + 3n31+28−18 pb

References∗1∗2∗3∗4∗5∗6∗7

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

1) H. Koura: Prog. Theor. Exp. Phys. 2014, 113D02(2014).
2) Y. Wakabayashi et al.: RIKEN Accel. Prog. Rep. vol.47,xxii (2014).
3) A.P. Leppänen et al.: Eur. Phys. J. A25, Supplement 1,183 (2005).

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