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Identification of Element 107 by α Correlation Chains

Authors:
Z. Phys. A - Atoms and Nuclei 300, 107-108 (1981)
Short Note
Zeitschrift
A~#'~m~
f~r Physik A /~l,l.~,tl ! I~.~
and Nuclei
Springcr-Verlag 1981
Identification of Element 107 by 9 Correlation Chains
G. Miinzenberg, S. Hofmann, F.P. Hegberger, W. Reisdorf,
K.H. Schmidt, J.H.R. Schneider, and P. Armbruster
Gesellschaft fdr Schwerionenforschung, D-6100 Darmstadt,
Federal Republic of Germany
C.C. Sahm
Institut
ftir Kernphysik, Technische Hochschule,
D-6100 Darmstadt, Federal Republic of Germany
B. Thuma
II. Physikalisches Institut, Universitfit Giegen,
D-6300 Giel3en, Federal Republic of Germany
Received April 3, 1981
The production of nuclei at the upper end of the
periodic table in heavy ion fusion reactions is
limited by the high fissility of the compound
system. Formation cross-sections decrease
rapidly with increasing excitation energy. With
targets near the double magic 2~ and neutron
rich metal ions like 5OTi or 54Cr as projectiles,
fusion close to the barrier yields weakly excited
compound systems with excitation energies of less
than 25 MeV. The asymmetry of the target-projec-
tile combination is still sufficient to expect no
dynamic effects on the barrier. Consequently,
the irradiation of 2O9Bi with 54Cr as proposed by
Oganessian et al /1/ is up to our present knowl-
edge the most appropriate reaction to synthesize
element 107 by heavy ion fusion.
With the velocity filter SHIP /2/ for separation
and surface barrier detectors for identification
/3/ of the fusion products we can investigate the
predominant decay modes, spontaneous fission and
decay over the entire expected halflife range
down to us with low background from other
reactions.
Up to now in experiments to produce element 107
only spontaneous fission was observed. Oganessian
et al. /I/ found two spontaneous fission activ-
ities
in bombardments of 2O9Bi with 54Cr. One
with a halflife of (1-2) ms, which was assigned
to the 2n channel. The other activity with a
halflife of 5 s was explained by an unobserved
decay of 261107 leading to 257105 which was
regarded to undergo spontaneous fission with the
corresponding halflife.
The aim of our experiment was to produce element
107 in the same reaction and to identify the iso-
topes unambiguously by correlated a decay chains
to known transitions. We also investigated the
reaction 5OTi on 209Bi to find the unknown
daughter decays of element 105 and Lr.
Experimental Setup
The S4Cr beam from the UNILAC linear accelerator
had an average intensity of 6 x 1011 particles/s.
The ion source was operated with isotopically
enriched material. 209Bi targets with an average
thickness of 660 ug/cm 2 had been evaporated on a
30 ug/cm 2 carbon foil and covered with a 30
2
~g/cm carbon film for radiative cooling. They
were mounted on a rotating wheel to stand the
high beam intensities /4/. Target thickness was
monitored by Rutherford scattering.
After in-flight separation by SHIP, the evapor-
ation residues passed two large area detectors
for time-of-flight measurement and were im-
planted into a bench of 7 position sensitive
surface barrier detectors. The point of the
identification method is that all decays belong-
ing to a decay chain which starts from any inci-
dent evaporation residue have to occur at its
position of implantation within a window given by
the detector resolution. Correlation times are
limited by the rates of random events within this
position window. The detectors are cooled to
268 K and have an energy resolution of 27 keY
FWHM, the position resolution is 0.3 mm FWHM.
The evaporation residues are separated with
efficiencies of about 20%. The response of the
surface barrier detector for ~ decays is 50%, as
the evaporation residues are implanted close to
the detector surface and about half of the
particles escape. For an average beam current
of 6 x 1011 particles per second, 20 events per
nanobarn and day are implanted into the detector
array. The expected formation cross sections
for isotopes of element 107 range in the order of
0.1 nb /1/.
From preceding experiments with 4~ on 208pb,
5OTi on 2~ and 2~ we estimated an effec-
tive Coulomb radius parameter of (1.42• fm,
therefore we irradiated with 4.85 MeV/u and
4.95 MeV/u. To ensure that our projectile
energies were above the interaction barrier, we
observed the symmetric component of the mass
distribution in a scattering chamber at 45 o with
a time-of-flight measurement using surface
barrier detectors /5/.
0340-2193/81/0300/0107/$01.00
108 G. Mfinzenberg et al.: Identification of Element 107
Experimental Results
A group of six distinguished ~ decays was found.
Five of them in the interval between 10,350 keV
and 10,400 keV are observed (1-13) ms after im-
plantation of the evaporation residue. One decay
has an energy of 9,704 keV and a correlation time
of 165 ms.
One of the measured decay chains leading to the
known decay of 25~ is shown in fig. I. The
(4-.85
MeV/u) 10,367 keel 3 ms '
CN:
9 176 keV I ~ual
B,43S
keV/~3"8 s
~2.1 s
7 457
~
Fig. i*
2,030
8
others are partly incomplete because of the 50%
detector response. Another chain ends in 2S0Md
decay. Consequently, the chains start from
262107 formed by In evaporation. The decay chain
of fig. i follows an evaporation residue within a
position of 0.3 mm on the position sensitive
dectector. The evaporation residue has been im-
planted with an energy of 18.6 MeV, in good
agreement with the value of about 17 MeV calcu-
lated for the reaction and our detector system.
This energy is about 30% higher than the energy
of implanted Bi-recoils which is (14• MeV.
Moreover, the velocity has been checked twice: by
SHIP as well as by the time-of-flight. Except
the decay of 284Lr all decays of the chain were
observed in the background free beam pause of the
accelerator. The table shows a summary of our
data from 54Cr on 2~ In addition, we ob-
served one spontaneous fission with a correlation
time of 4.3 s.
The ~ decays of
252107
and also 251107 lead to
unknown isotopes of 105 and Lr. 258105 can be
produced in S~ on 2~ irradiations by evap-
oration of one neutron (Fig.2). At 4.75 MeV/u we
observed decays of (9,18g_+35) keV and (9,066-+35)
keV with (4.0~+{:~) s halflife and (8,468• keY
..........
L" CN ~3.1 s CN
7,761
keV~ ~&2 s
7,148 k~ V"
542 s
Fig. 2*
with (18~I~) s halflife corresponding to 258105
and 25~Lr respectively in good agreement to the
data from 262107 shown in the table. We observed
9 spontaneous fission events with a halflife of
(1.6~81~) s at (4.65-4.85) MeV/u irradiations.
At 4.85 MeV/u and 4.95 MeV/u decay chains from
257105 ending in the sequence 249Md- 245Es
appeared, while the 258105 and 254Lr decays dis-
appeared.
Discussion
Our results show the discovery of the m decay of
element 107. The ~ chains end in known transi-
tions of 25~ and 25~ respectively, indi-
cating the observation of the isotope 262107
formed by the In channel from the compound nu-
cleus 263107. This is also most likely from the
excitation energy of the compound system of less
than 18 MeV for 4.85 MeV/u specific irradiation
energy /6/. The step in m decay energies from
258105 to 262107 seems to be unusally high. So we
may speculate that the 10,376 keV decay corre-
sponds to an isomeric state. The 9,704 decay
which fits better to ~ decay systematics then
would be the ground state transition of 262107.
The possibility that the observed m decays may
origin from transfer products can be excluded:
The observed decay chain does not fit to known
transitions, the evaporation residue is well de-
fined to be heavier than a transfer nucleus near
Bi and the length of the decay chain would re-
quire a transfer of at least 4 ~ particles,
whereas no ~ decay of 21~p0, 212At, or 213Rn in-
dicating at least a transfer of few nucleons has
been found. The ms fission activity of (I-2) ms,
assigned to the 2n channel by Oganessian /1/ has
not been observed at our projectile energies.
No. of
Isotope Energy/keV • TI/2 events
262107 10,376• (4.7 t~.~) ms 5
9,704• (115 +23t~ ms I
-
75~
258105 9,181• (1.8 +~'~) s 3
9,104• - I
284Lr 8,446• (10 C~) s 3
2S~ 7,435• (i 386 +900~
'
-600J
S 2
2S~ 7,740• (152 +3o~ s 1
-109~
Observed decays of 54Cr on 2~ irradiations
(x• keY error for absolute calibration included)
1. Oganessian, Yu.Ts., et al.: JETP Lett. 23
No. 5, 277 (1976)
2. MUnzenberg, G., et al.: Nucl. Instrum. Meth.
161, 65 (1979)
3. lq-6-i~mann, S., et al.: Z. Physik A291, 53
(1979)
4. Marx, D, et al.: Nucl. Instrum. Meth. 163,
15 (1979)
5. Sahm, C.-C. et al.: Z. Physik A297, 241(1980)
6. Wapstra, A.H., Bos, K.: Atomic~a Nuclear
Data Tables 19, 177 (1977); Liran, S. Zeldes,
N.: Univ. Je~alem, Int. Rep. (1976)
*Examples of sequences of ~ decays with time
intervals between observed decays and
corresponding energies.
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... The difference between the single frequency ratios obtained by applying the linear interpolation method and the weighted mean frequency ratio is shown in Fig. 6.7 a. The solid line indicates the mean frequency ratio which is R = 0.96724017 (52) and the shaded band represents the 1σ uncertainty. In Fig. 6.7 b, the temporal evolution of the cyclotron frequencies of 133 Cs + and 257 Rf 2+ is shown. ...
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Atomic Data Nuclear Data Tables19
  • A H Wapstra
  • K Bos
  • S Liran
  • N Zeldes
  • C.-C Sahm
Sahm, C.-C. et al.: Z. Physik A297, 241(1980)
  • G Munzenberg
MUnzenberg, G., et al.: Nucl. Instrum. Meth. 161, 65 (1979)
Atomic~a Nuclear Data Tables 19
  • A H Wapstra
  • K Bos
  • S Liran
  • N Zeldes
Wapstra, A.H., Bos, K.: Atomic~a Nuclear Data Tables 19, 177 (1977); Liran, S. Zeldes, N.: Univ. Je~alem, Int. Rep. (1976)