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NUSTAR-SHE-07 GSI SCIENTIFIC REPORT 2009
Towards a direct mass measurement of Lr at SHIPTRAP
M. Dworschak 1 , D. Ackermann 1 , K. Blaum 2,3 , M. Block 1 , C. Droese 4 , S. Eliseev 2 , E. Haettner 1,5 ,
F. Herfurth 1 , F.P. He berger 1 , S. Hofmann 1 , J. Ketelaer 6 , G. Marx 4 , D. Nesterenko 7 , Yu. Novikov 1,7 ,
W.R. Pla 1,5 , A. Popeko 8 , D. Rodríguez 9 , C. Scheidenberger 1,5 , L. Schweikhard 4 , P. Thirolf 10 , and
C. Weber 10
1 GSI, Darmstadt; 2 MPI-K Heidelberg; 3 Universität Heidelberg; 4 Universität Greifswald; 5 Universität Gie en;
6 Universität Mainz; 7 Petersburg Nuclear Physics Institute; 8 JINR, Dubna; 9 Universidad de Granada; 10 LMU München
The mass of a nuclide is a fundamental parameter. It
provides information about the nuclear binding energy and
is thus crucial for nuclear structure and reaction studies.
Compared to the indirect mass determination via decay energies,
a method in which the mass can be directly obtained
is favorable since it is independent of a detailed knowledge
of nuclear level schemes. Especially in the region of
transuranium nuclides direct mass measurements are desirable
to check the information on the binding energy which
is presently based on spectroscopic data.
The Penning-trap mass spectrometer SHIPTRAP [1]
is presently the only facility where direct measurements
on transfermium nuclides can be performed. After the
first successful experiments on three nobelium isotopes
( =102) [2] the focus was set on extending direct mass
measurements to higher . For the next step the isotope
Lr was selected, which is of specific interest since its
mass, as well as that of its daughter nuclide Md, are
presently only estimated from systematic trends. Furthermore,
two low-lying levels decaying by emission have
been observed. The excitation energy of the isomeric
state could not be determined so far but has to be settled
at . A direct mass determination would
thus provide more information on ground state masses and
would hence be another anchorpoint in an odd-even -
decay chain.
The nuclei investigated at SHIPTRAP were produced by
fusion-evaporation reactions and were separated from the
primary beam by the velocity filter SHIP [3]. Lr was
produced in the reaction Bi( Ca,2n) Lr at a primary
beam energy of 4.55 MeV/u with a cross section of about
200 nb. This corresponds to a rate of about 0.3 ions per
second measured at a position-sensitive silicon detector in
front of SHIPTRAP. The reaction products with kinetic energies
of about 40 MeV were decelerated in degrader foils
and were stopped in a gas cell at 50 mbar helium. The
lawrencium atoms were extracted from the gas as singly
and doubly charged ions.
After the extraction from the gas cell the ions were
cooled and accumulated in an RFQ structure and then
ejected as a short ion bunch. Subsequently, the ions were
injected into a double Penning trap system, which is consisting
of a preparation trap and a measurement trap, placed
in a 7T magnet. In the latter the mass was determined by
measuring the cyclotron frequency of the ions using a
176
time-of-flight cyclotron-resonance-detection technique.
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Figure 1: Time-of-flight resonance of Lr . The solid
line is a fit of the theoretical line shape to the experimental
data points.
Altogether around 30 Lr ions were detected in about
30 hours measurement time and the time-of-flight resonance
shown in Fig. 1 was obtained. Lr is thus both,
the heaviest radionuclide and the one with the lowest production
rate ever studied in a Penning trap.
The SHIPTRAP measurements on nobelium and lawrencium
are first steps away from the well-studied region of
known masses towards direct mass measurements of superheavy
nuclides. Further improvements of the experimental
setup will pave the way to address nuclides with even
lower production rates. One goal is to fix the endpoints
of -decay chains originating from superheavy nuclei that
currently are not connected to the area of known nuclei.
This will open up new perspectives for the identification of
long-lived elements on the predicted island of stability.
References
[1] M. Block et al., Eur. Phys. Jour. D 45 (2007) 39.
[2] M. Block et al., Nature, in print (2010).
[3] G. Münzenberg et al., Nucl. Instr. Meth. 161 (1979) 65.