Experiment on the Synthesis of Element 113 in the Reaction 209Bi(70Zn,n)278113

Abstract

The convincing candidate event of the isotope of the 113th element, 278113, and its daughter nuclei, 274111 and 270Mt, were observed, for the first time, in the 209Bi + 70Zn reaction at a beam energy of 349.0 MeV with a total dose of 1.7 × 1019. Alpha decay energies and decay times of the candidates, 278113, 274111, and 270Mt, were (11.68 ± 0.04 MeV, 0.344 ms), (11.15 ± 0.07 MeV, 9.26 ms), and (10.03 ± 0.07 MeV, 7.16 ms), respectively. The production cross section of the isotope was deduced to be 55+150-45 fb (10-39 cm2).

Full text

Experiment on the Synthesis of Element 113 in the Reaction
209
Bi(
70
Zn,n)
278
113
Kosuke MORITA
1
!
, Kouji MORIMOTO
1
, Daiya KAJI
1
, Takahiro AKIYAMA
1;2
, Sin-ichi GOTO
3
,
Hiromitsu HABA
1
, Eiji IDEGUCHI
4
, Rituparna KANUNGO
1
, Kenji KATORI
1
, Hiroyuki KOURA
5
,
Hisaaki K
UDO
6
, Tetsuya OHNISHI
1
, Akira OZAWA
7
, Toshimi SUDA
1
, Keisuke SUEKI
7
,
HuShan X
U
8
, Takayuki YAMAGUCHI
2
, Akira YONEDA
1
, Atsushi YOSHIDA
1
and YuLiang ZHAO
9
1
RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-0198
2
Department of Physics, Saitama University, Sakura-ku, Saitama 338-8570
3
Center for Instrumental Analysis, NiigataUniversity,Ikarashi,Niigata950-2181
4
Center for Nuclear Study, University of Tokyo Wako Branch, Wako, Saitama 351-0198
5
Advanc e d S c ie n ce Resea rc h Cente r, Japa n A t omi c Energy Research Institute, Tokai, Ibaraki 319-1195
6
Department of Chemistry, Niigata University, Ikarashi, Niigata 950-2181
7
University of Tsukuba, Tsukuba, Ibaraki 305-8571
8
Institute of Modern Physics, Chinese Academy of Science, Lanzhou 730000, China
9
Institute of High Energy Physics, Chinese Academy of Science, Beijing 100039, China
(Received July 30, 2004)
The convincing candidate event of the isotope of the 113th element,
278
113, and its daughter nuclei,
274
111 and
270
Mt, were observed, for the first time, in the
209
Bi þ
70
Zn reaction at a beam energy of
349.0 MeV with a total dose of 1:7 # 10
19
. Alpha decay energies and decay times of the candidates,
278
113,
274
111, and
270
Mt, were (11:68 $ 0:04 MeV, 0.344 ms), (11:15 $ 0:07 MeV, 9.26 ms), and
(10:03 $ 0:07 MeV, 7.16 ms), respectively. The production cross section of the isotope was deduced to
be 55
þ150
%45
fb (10
%39
cm
2
).
KEYWORDS: new element
278
113, new isotopes
274
111 and
270
Mt, gas-filled recoil separator
DOI: 10.1143/JPSJ.73.2593
Finding the new isotopes of very heavy elements,
including new eleme nts, and studying their decay properties
are interesting subjects in both nuclear physics and nuclear
chemistry. Since 2002, we have investigated the production
and decay of
271
Ds (Z ¼ 110) and
272
111 using
208
Pb(
64
Ni,n)
and
209
Bi(
64
Ni,n) reactions, respectively,
1,2)
at the RIKEN
Linear Accelerator Facility (RILAC). Subsequently, we
studied the isotope of the 112th element using the
208
Pb(
70
Zn,n)
277
112 reaction.
3)
Our results clearly confirmed
the production of the isotopes reported by Hofmann and
coworkers,
4–9)
and provided new spectroscopic information
on the isotopes and their daughter nuclei.
As an extension of our previous work, we performed
experiments aimed at synthesizing an isotope of larger
atomic number, Z ¼ 113, using the
209
Bi þ
70
Zn reaction.
During irradiation, we observed one !-decay chain that can
be assigned to subse quent decays from
278
113, using the
genetic correlation of !-decays connected to the known
nuclides,
266
Bh and
262
Db.
The production of element 113 was first reported by
Oganessian et al.
10)
in 2004 usin g the
243
Am(
48
Ca,xn) (x ¼
3; 4) reaction at the Flerov Laboratory of Nuclear Reaction
(FLNR) of Joint Institute of Nuclear Research (JINR),
Russia. They reported that the observed decay chains were
consistent with the subsequent decays from
288
115 produced
by the 3n evaporation channel and from
287
115 produced by
the 4n evaporation channel. In these chains, three atoms of
284
113 and one atom of
283
113 were consequently assigned
to be !-decay daughters of
288
115 and
287
115, respectively.
The chains ended by spontaneous fission of previously
unknown dubnium isotopes,
268
Db and
267
Db. To confirm
both the atomic number and mass number of these isotopes,
several types of further experiments such as measurement of
the excitation function, chemical separation of long-lived
dubnium isotopes, and direct mass measurement of the
products using an isotope separator on-line system, are
scheduled at FLNR.
11)
The results of the present work strongly indicate the
synthesis of the 113th element, even though the number of
chains observed was only one, because the chain ended with
known nuclides. More chains are expected to be observed in
further experiments.
The present experiment, which started on September 5,
2003, was interrupted on December 29, 2003, and then
restarted on July 8, 2004 and continued until August 2, 2004.
The net irradiation time was 79 days.
A
70
Zn ion beam of 352.6 MeV was extracted from
RILAC. The beam energy was determined by measuring
magnetic rigidity in a 90
'
bending mag net and by a time-of-
flight method. The absolute accuracy was $0.6 MeV. The
drift in the beam energy during the whole beam time was
$0.3 MeV. The beam intensity was monitored by measuring
projectiles elastically scattered by the targets with a PIN
photodiode mounted at 45
'
with respect to the incident beam
direction at a distance of 1.28 m from the target position. The
typical beam intensity on the target was 2:4 # 10
12
s
%1
.
Targets were prepa red by vacuum evaporation of metallic
bismuth onto carbon backing foils of 30
mg/cm
2
thickness.
The thickness of the bismuth layer was about 450
mg/cm
2
.
The targets were covered by 10-
mg/cm
2
-thick carbon to
protect them from sputtering. The energy loss of the beam in
the target was estimated to be 5.4 MeV using range and
stopping power tables.
12)
Beam energy at the half-depth of
the targets was est imated to be 349.0 MeV. Sixteen targets
were mounted on a rotating wheel of 30 cm diameter. The
wheel was rotated during irradiation at 2000 rpm.
The reaction products were separated in-flight from the
beam using a gas-filled recoil ion separator, GARIS,
1)
and
LETTERS
!
E-mail: morita@rarfaxp.riken.go.jp
Journal of the Physical Society of Japan
Vol. 73, No. 10, October, 2004, pp. 2593–2596
#2004 The Physical Society of Japan
2593

were guided into a detector box placed at the focal plane of
GARIS. The separator was filled with helium gas at a
pressure of 86 Pa. The value of the magnetic rigidity (B") of
GARIS for evaporation residue measurement was set at
2.09 Tm.
The focal plane detection system consists of two sets of a
timing detector and a silicon semiconductor detector box
(SSD box). The timing detector is an assembly of micro-
channel plates (MCP) that detect secondary electrons
emitted from a thin foil by impact of ions passing through
the foil. The details of the timing detector were described
elsewhere.
2)
The SSD box placed downstream of the timing
detectors consists of five silicon detector plates. The
dimensions of each detector are 60 mm # 60 mm . One of
the silicon detectors, which faces the direction of incoming
particles, is placed at the bottom of the SSD box and consists
of 16 strip detectors (PSD). The dimensions of each strip
detector are 3:75 mm # 60 mm. The strip detectors are
position sensitive along the longer dimension. Four other
detectors (SSDs) are set to detect decaying particles from the
reaction products implanted in the PSD.
Evaporation residues are implanted in the PSD after
passing through the two timing counters. The timing signal
was used for two purposes. One is to measure the time of
flight (TOF) of incoming particles for rough estimation of
their mass number together with energy signals from the
PSD. The second purpose is to identify decay events
originating from implanted nuclei in the PSD. The events
without signals from either timing detector are regarded as
the decay events.
The detection system was periodically checked by
measuring the several !-decay lines of the transfer-reaction
products, such as
211
Po,
212
At, and
213
Rn, setting the B" of
GARIS at 1.67 Tm, and keeping all the other conditions the
same as those for the actual measurement. Energy calibra-
tion of the detectors for decay !-par ticles was performed
simultaneously.
We observed one event of implantation of an evaporation
residue (ER) in the PSD followed by four consecutive !-
decays terminated by a spontaneous fission decay. The
measured positions of all six sequential events were within
the spatial resolution of the PSD.
1)
The observed energies,
time differences between events, and positions of each decay
are summarized in Table I, together with those of the
implantation event. The energies have been calibrated for !-
particles. Therefore, the listed energies for the ER and
spontaneous fission are not accurate because possible effects
of pulse height defect are not taken into account. The events
were registered in strip #12 of the 16 strips in the PSD.
For the !
1
and !
4
, the ! particles were detected only by
the PSD. The energy resolution measured using only the
PSD was 39 keV in full width at half maximum (FWHM).
For the !
2
and !
3
, the decaying ! particles were ejected from
the PSD and implanted to the SSD. Therefore, the energies
were measured partly by the PSD and the residual ones by
the SSD. The resolution in these cases was 66 keV FWHM.
For the fifth decay (hereafter, we use SF to represent the fifth
decay), the energy was measured using only the PSD. For all
the five decay events, no TOF signal registered from the
timing detectors was associated.
The probabilities of accidental coincidence between the
implantation of ER and individual decays were estimated as
follows. The counting rate of decay events, which yielded no
TOF signal for a decay energy greater than 8 MeV in the run
(Run #196) and the strip (strip #12) where the decay chain
was observed, was 8:5 # 10
%4
s
%1
. That for a decay energy
greater than 100 MeV was 9:6 # 10
%6
s
%1
. With the position
window of 0.6 mm/60 mm (¼ 0:01) and time differences of
0.344 ms for !
1
, 9.6 ms for !
2
, 16.8 ms for !
3
, 2.5 s for !
4
,
and 43.4 s for SF, the probabilities of accidental coincidence
were evaluated to be 2:9 # 10
%9
, 8:2 # 10
%8
, 1:4 # 10
%7
,
2:1 # 10
%5
, and 9:5 # 10
%6
, for !
1
, !
2
, !
3
, !
4
, and SF,
respectively. The singles counting rate of the PSD was 2 s
%1
at the typical beam intensity.
For the ER event, TOF between the timing counters was
44.6 ns. The time resolution of the system was measured to
be 0.5 ns FWHM for 5.5 MeV !-particles. The resolution is
expected to be better for ER, where the energy measured by
the PSD was 36.75 MeV. Figure 1 shows a scat ter plot of
energy and TOF for the run (Run #196) in which the decay
chain was observed. In the figure, only events detected by
strip #12 of the PSD are plotted. The point corresponding to
the implantation event is indicated by a circle with an arrow.
Loci corresponding to projectile-like particles (A ( 70) and
Table I. Observed events. !E
sum
: Energy resolution in FWHM. See text.
!T: Time differences between events. Position: measured from the
bottom of the detector.
E
PSD
(MeV)
E
SSD
(MeV)
E
sum
(MeV)
!E
sum
(MeV)
!T
TOF
(ns)
Position
(mm)
ER 36.75 — 36.75 — — 44.6 30.33
!
1
11.68 — 11.68 0.04 0.344 ms — 30.49
!
2
6.15 5.00 11.15 0.07 9.26 ms — 30.40
!
3
1.14 8.89 10.03 0.07 7.16 ms — 29.79
!
4
9.08 — 9.08 0.04 2.47 s — 30.91
SF 204.1 — 204.1 — 40.9 s — 30.25
TOF/ ns
0 20 40 60 80 100
E/ MeV
120
100
80
60
40
20
0
projectile-like particles
target-like particles
evaporation residue
Fig. 1. Two-dimensional plot of energy measured by PSD vs TOF
measured by timing counters for the run (run #196, time period = 12.7 h),
in which the decay chain was observed. Events detected only by strip #12
of PSD are shown. A point corresponding to the implantation event is
shown by a circle with an arrow. Loci corresponding to projectile-like
particles (A ( 70) and target-like particles (A ( 209) are also shown.
2594 J. Phys. Soc. Jpn., Vol. 73, No. 10, October, 2004 L
ETTERS K. MORITA et al.

target-like particles (A ( 209) (target recoil and transfer
reaction products) are seen in the figure. The implantation
event is well separated from the locus of the target-like
particles that may contribute to the background of the
measurement. The figure indicates that the mass of the
implanted particle is higher than that of the target-like
particles. The mass number of ER was roughly estimated to
be 280 $ 15 using a method similar to that described in
ref. 2.
The total dose of the
70
Zn projectile was 1:7 # 10
19
. The
production cross section of this specific event was deduced
to be 55
þ150
%45
fb (10
%39
cm
2
) using the transmission efficiency
of GARIS of 0.8.
2)
Indicated uncertainty in the cross section
is only statistical one with 1# (68% confidence level).
The experiment was designed to produce the isotope,
278
113, by the one-neutron evaporation channel in the
209
Bi þ
70
Zn complete fusion reaction. On the basi s of a
systematic study of the most probable reaction energies for
the one-neutron evaporation channel in the
208
Pb(
64
Ni,n)
271
-
Ds,
1)
209
Bi(
64
Ni,n)
272
111,
2)
and
208
Pb(
70
Zn,n)
277
112
3)
reac-
tions, a reaction energy of 349.0 MeV at the half-depth of the
targets was adopted to maximize the relevant cross section
for producing
278
113. The corresponding energy of the
center of mass frame is 261:4 $ 2:0 MeV, where $2.0 MeV
indicates the range of reaction energy due to the energy loss
of the beam in the bismuth targets. The excitation energy
(E
!
CN
) of the compound nucleus,
279
113, is calculated to be
14:1 $ 2:0 MeV using the predicted mass for a compound
nucleus
13)
and the experimental masses of the beam and
target.
If we assume that the implanted nucleus followed by the
decay chain observed in the present study is the product of
the
209
Bi(
70
Zn,n)
278
113 reacti on, the members of the decay
chain can be assigned as
278
113,
274
111,
270
Mt,
266
Bh, and
262
Db. The probability that the decaying ! particle escapes
in the backward direction and gives no signal below the
threshold of PSD is 15%.
1,2)
Therefore, the members can
also be possibly assigned as further !-decay products, such
as
258
Lr. However, this possibility is excluded in the present
case on the basis of the decay property of
258
Lr.
The isotope
262
Db is known to decay by ! and sponta-
neous fission and branching ratios of (64% and (33%,
respectively, with a half-life (T
1=2
) with a 1# error of
34 $ 4 s.
14)
The measured decay time of 40.9 s for SF,
corresponding to T
1=2
¼ 28
þ140
%13
s, is well consistent with the
reported value.
The !-decay of
266
Bh produced by the
249
Bk +
22
Ne
reaction was reported by Wilk et al.
15)
This decay was
followed by the !-decays of
262
Db and
258
Lr. The decay
energy (E
!
) and decay time for
266
Bh were 9.29 ($)0:10)
MeV and 0.87 s, respectively. The decay time, 0.87 s,
corresponds to T
1=2
¼ 0:6
þ2:9
%0:3
s. The measured decay time
of !
4
in the present work, 2.47 s, corresponding to T
1=2
¼
1:7
þ8:2
%0:8
s, agrees within the 1# error with the value reported
by Wilk et al. Considering the rather wide distribution of !-
decay energies from odd-odd nuclei, as shown in the case of
272
111,
2)
our present !
4
(E
!
¼ 9:08 $ 0:04 MeV) does not
contradict the value reported in ref. 15. It should be noted
that only one !-decay event was reported in this reference.
The possibility of a radiative capture process (zero-
neutron evaporation channel) to form the observed decay
chain could not be excluded purely on the basis of the
property of the measured SF. The isotope
263
Db, which is a
decay product of
279
113 after four sequential !-decays, is
also known to decay by spontaneous fission ((57%) and !-
decay ((43%) with a half-life of 27
þ10
%7
s,
14)
consistent with
the measured time for SF. However, this channel is excluded
by considering the excitation energy of the compound
nucleus. The calculated value, E
!
CN
¼ 14:1 $ 2:0 MeV, is
too high to populate the ground state of the compound
nucleus by only $-ray emission, because the calculated one-
neutron separation energy of the compound nucleus is only
7.5 MeV, using the same mass prediction.
13)
The two-neutron separation energy of the compound
nucleus is calculated to be 13.8 MeV using also the same
theoretical prediction.
13)
The free energy for two-neutron
emission is calculated to be only 0.3 MeV in this case.
Therefore, the two-neutron evaporation channel is also
excluded in this excitation energy because the phase volume
of this channel is much smaller than that of the one-neutron
evaporation channel.
The possibility of a one-proton evaporation channel
leading to the product of
278
112 is excluded by comparing
the observed decay time of SF with that of
262
Rf which
decays by 100% spontaneous fission with a half-life of
47 $ 5 ms,
16)
greatly shorter than the observed value, T
1=2
¼
1:7
þ8:2
%0:8
s.
In conclusion, the reaction product, followed by the decay
chain observed in our experiment, was considerd to be most
probably due to the
209
Bi(
70
Zn,n)
278
113 reaction. As a result,
the members of the decay chain were consequently assigned
as
278
113,
274
111,
270
Mt,
266
Bh, and
262
Db. The decay chain
is shown in Fig. 2, together with decay energies (E
!
and/or
E
SF
) and decay times.
In summary, a convincing candidate event of the isotope
of the 113th element,
278
113, and its daughter nuclei,
274
111
and
270
Mt, were produced, for the first time, by the
209
Bi þ
70
Zn reaction at a beam energy of 349.0 MeV with a total
278
113
274
111
270
Mt
266
Bh
CN
11.68 0.04 MeV
344
µs
11.15
0.07 MeV
9.26 ms
10.03
0.07 MeV
7.16 ms
9.08
0.04 MeV
2.47 s
36.75 MeV
TOF 44.6 ns
E (
70
Zn) = 349.0 MeV
23-July-2004 18:55 (JST)
209
Bi +
70
Zn
278
113 + n
α
1
Strip #12
262
Db
204.1 MeV
40.9 s
α
2
α
3
α
4
SF
Fig. 2. Decay chain observed in irradiation of
209
Bi targets by
70
Zn
projectiles. Measured energies and decay times are indicated in the figure.
J. Phys. Soc. Jpn., Vol. 73, No. 10, October, 2004 L
ETTERS K. MORITA et al. 2595

dose of 1:7 # 10
19
. The result led to the identification of an
isotope of the 113th element for the first time. The
production cross section of the isotope by the studied
reaction was deduced to be 55
þ150
%45
fb (10
%39
cm
2
).
Acknowledgements
The authors would like to thank Dr. Y. Yano, Professors
M. Ishihar a, and H. Kamitsubo for continuous support,
encouragement, and useful suggestions. We also would like
to thank accelerator operation group head, M. Kase for
support and beam time arrangement. Many thanks are due to
all accelerator staff members for excellent operation during
the long period of this experiment. The authors were also
greatly encouraged by all the members of the Cyclotron
Center, RIKEN. The authors also would like to thank
Professor S. Kobayashi for warm support. This program is
partly supported by RIKEN Strategic Programs for R&D.
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