Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 121–124, 2002

121

SISAK Liquid-Liquid Extraction Experiments with Preseparated 257 Rf
Jon Petter Omtvedt,∗,a J. Alstad,a H. Breivik,a J. E. Dyve,a K. Eberhardt,b C. M. Folden III,d,e
T. Ginter,d K. E. Gregorich,d E. A. Hult,a M. Johansson,a U. W. Kirbach,d D. M. Lee,dM. Mendel,b A. Nähler,b V. Ninov,d L. A. Omtvedt,d J. B. Patin,d,e G. Skarnemark,c L. Stavsetra,a,d
R. Sudowe,d N. Wiehl,b B. Wierczinski,†,b P. A. Wilk,d,e P. M. Zielinski,d,e,f J. V. Kratz,bN. Trautmann,b H. Nitsche,d,f and D. C. Hoffmand,eaDepartment of Chemistry, University of Oslo, P.O.Box 1033, Blindern, N-0315 Oslo, Norwayb
Institut für Kernchemie, Johannes Gutenberg-Universit ät Mainz, GermanycDepartment of Nuclear Chemistry, Chalmers University of Technology, Göteborg, SwedendNuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USAe
Department of Chemistry, University of California, Berkeley, CA, USA
f
Glenn T. Seaborg Center, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
Received: November 13, 2001; In Final Form: May 9, 2002
The SISAK liquid-liquid extraction system was used to extract 4.0-s 257 Rf. The 257 Rf was produced in the reaction
208
Pb(50 Ti, 1n)257 Rf with 237-MeV beam energy on target, separated in the Berkeley Gas-filled Separator (BGS) andtransferred to a gas jet using the Recoil Transfer Chamber (RTC). The activity delivered by the gas jet was dissolvedin 6-M HNO3 and Rf was extracted into 0.25-M dibutyl-phosphoric acid in toluene. This was the first time atransactinide, i.e., an element with Z ≥ 104, was extracted and unequivocally identified by the SISAK system. Thus,
this pilot experiment demonstrates that the fast liquid-liquid extraction system SISAK, in combination with liquidscintillation detectors, can be used for investigating the chemical properties of the transactinides. The extraction
result is in accordance with the behaviour shown by the Rf group IV homologues Zr and Hf.
• Adding the scintillator chemicals after the final extraction

1. Introduction
SISAK1 is a fast chemical-separation system for liquid-liquid
extraction. It was developed to investigate short-lived nuclei recoiling out of an irradiated target and transported in a gas jet.
The system is based on small centrifuges with a continuous feed
and output.2–4
During recent years, work has been undertaken to deploy
SISAK in studies of the chemical properties of the transactinide
elements (Z ≥ 104).2, 5, 6 The only suitable detection method
found was liquid-scintillation (LS) detection.7 This was mainly
because of the rather high flow rates (0.5–2.0 mL/s) encounteredin the SISAK system. Other detection methods usually require
either thin, dry samples or very thin liquid films. No suitable
method was found to prepare such samples without an unacceptable time delay between separation and detection.
LS counting has high efficiency, but suffers from relatively
poor energy resolution (about 300-keV FWHM at 7-MeV α energy) and is sensitive to β particles and γ rays. This puts very
stringent demands on the separation system, because most of
the β- and γ-emitting contaminants are produced with orders ofmagnitude higher yield than the transactinide under study. A
huge effort was made to develop methods for LS detection ofvery low levels of α activity under these conditions. The mostimportant techniques implemented are:• Pulse-shape discrimination is used to reduce the β background in the α spectra by a factor >1000. Without this technique the β- and γ-induced background from the surroundings
and from nuclides produced in the target will interfere withthe α spectrum to such an extend that it becomes useless.8• Real-time scintillation-yield monitoring by continuous measurement of the Compton edge of 662-keV 137 Cs γ rays. The
position of the Compton edge is used to automatically adjust
the energy calibration. Based on this adjustment the event
gates used for switching the detection cells between motherand daughter mode (open or closed valve, respectively) will
always be set on the correct energy range.9∗Corresponding author. E-mail: j.p.omtvedt@kjemi.uio.no.
FAX: +47-2285-5477.
†Present address: Department of Radiochemistry, Delft Universityof Technology, 2629 JB Delft, The Netherlands.

step and flushing the mixture with an inert gas (He or Ar)
to remove O2 . Oxygen quenches the scintillation process, inparticular the time resolution necessary for the pulse-shapediscrimination is severely affected. This procedure also improves the phase separation and extraction yield.2, 10• Improving the light transfer between the detection cell and thephotomultiplier tube by improved light-guide design.10
In November 2000 a successful test experiment with the
SISAK LS detectors was performed at the Lawrence Berkeley
National Laboratory (LBNL). 257 Rf was produced in the reaction 208 Pb(50 Ti, 1n)257 Rf and separated in the Berkeley Gas-filled
Separator (BGS). By using such a preseparation technique, thebackground is reduced by more than three orders of magnitude.
This gives the experimenter two significant advantages:• The chemistry apparatus is liberated from the need to rejectunwanted activities produced in huge quantities by various
unwanted reactions between the beam and target. Thus, thechemistry can be optimized for studying the chemical properties of the given transactinide without at the same timefunctioning as an extremely efficient separation system. This
gives more freedom in designing the chemical system used toinvestigate the transactinide.
• The amount of unwanted activity is so low that the transactinide under study can be detected directly, without any chemical separation. This gives the experimenter the opportunity
to observe the transactinide in the detector system without using the chemical separation system. Furthermore, using preseparated activity enables direct measurement of the amountentering the chemistry apparatus. Because the activity in theraffinate (the aqueous phase) is not measured, this is an important number in order to calculate extraction yields withgood accuracy. The activity in the raffinate cannot be measured directly because water and acids destroy the energy resolution that is necessary for precise LS α-spectroscopy.
An overview of a typical SISAK experimental setup, withpreseparation, is given in Figure 1.

c 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

122 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Omtvedt

Extraction
Transfer to
Scintillator
aqueousphase
Ti-50 beam
feed
Preseparationin the BGS
Pb-208target

Scint. yieldmonitoring
PMT

Signalconditioning
and acquisition

PMT

PMT

Recoil transfer
to KCl gas jet

PMTValves

Detection cells
and PMTs

Figure 1. Overview of the SISAK-BGS setup.

0.25M HDBP

Gas jetMixer

He gasScintillator

Org.
Mixer
Degasser

6M HNO3

Main
Extraction

Washing

He flushingTo
detectors

2M NaNO3
Figure 2. Schematic diagram of the SISAK liquid-liquid extraction system.

2. Experimental
An experiment in which the BGS was used as a preseparator was performed. 257 Rf was produced in the reaction
208
Pb(50 Ti, 1n)257 Rf, which has a cross section of about 10 nb.
257
Rf has a metastable state with half-life = 4.0 s and a groundstate with half-life 3.5 s. About 63% of the decay is from themetastable state.11
The 237-MeV 50 Ti12+ beam was delivered by the LBNL 88Inch Cyclotron with about 200–300 pnA intensity. The 208 Pb
targets was mounted on the standard BGS rotating target-wheel.
At the exit of the BGS the separated 257 Rf-ions were transferred
to a He/KCl gas jet by the recently constructed Recoil Transfer
Chamber (RTC).12
The amount of 257 Rf entering the RTC was measured withan array of Si-strip detectors. Six to seven 257 Rf–253 No ααcorrelations per hour were detected. This corresponds to about
25 257 Rf nuclides entering the RTC per hour.13
The mean transport time from the RTC to the SISAK-LS detector cells was measured using Hf activity produced in the reaction 120 Sn(50 Ti, xn)170−x Hf. The main activity detected in an
HPGe detector facing the detection cell was from 164−166 Hf nuclides and their daughters. The half-lives of these nuclides arelarger than one minute, thus their decay should not influence theshape of the transport-time distribution to any significant degree.
A transport time of about 6 s for break-through and about 11 s
for the main part of the reaction products was observed.13
A test with the SISAK LS-detectors was then performed. Any
α-event with an energy between 8.3 and 10.5 MeV forced thecell to close, i.e., switch to daughter mode. The cell remainedin daughter mode for more than five daughter (253 No) half-lives.
The measured correlation matrix clearly shows that the SISAK
LS-detection system is able to detect nuclides at a rate of lessthan one atom per hour. Experimental details can be found in
Reference 13.
Thus having proven that the LS detection system worked,
pilot liquid-liquid extraction experiments were performed:
Rf was extracted from a 6-M HNO3 solution into 0.25-M
dibutyl-phosphoric acid (HDBP) in toluene as a nitrate-dibutylphosphate complex. Figure 2 shows a schematic diagram ofthe experimental setup. In the first section, the mixer/degasser,

the gas-jet transported activity (1.3 ± 0.1 L/min He with KCl
aerosol particles) is dissolved in the 6-M HNO3 solution(0.50 ± 0.05 mL/s flow rate at 60 ◦ C) and the gas is removed.
In the next centrifuge the main extraction is performed. Herethe Rf-nitrate complexes in the aqueous phase are thoroughly
contacted (∼ 50–100 ms) with the 0.25-M HDBP solution(0.42 ± 0.05 mL/s flow rate) and extracted as a nitrate-dibutylphosphate complex into the organic toluene solution. Then theorganic phase is washed with 2.0-M NaNO3 (0.42 ± 0.05 mL/s
flow rate) to remove dissolved HNO3 , which severely quenches
and oxidizes the scintillator ingredients otherwise. After thewashing step the scintillator ingredients, methyl-naphthalene,
dimethyl-POPOP, and trioctyl-amine (TOA), are mixed with theorganic phase in a 1 : 1 ratio. The scintillator cocktail (i.e., themixture of the organic phase and the scintillator ingredients) fed
to the detectors consisted of: 1.5-g/L dimethyl-POPOP, 25-vol%methyl-naphthalene, 3-mL/L TOA, 0.125-M HDBP, and toluene
as solvent. The TOA prevents any HNO3 traces left after thewashing step from interfering with the scintillator performance.
Together with the scintillator ingredients, He gas is also forced
into the second mixer to remove O2 , which quenches the scintillation process. In the final centrifuge the He is removed and theorganic phase/scintillator mixture is pumped to the detectors at
a flow rate of 0.84 ± 0.07 mL/s.
3. Results
The HDBP extraction experiment collected data for a total of
17.8 hours. The beam integral was 8.34 ×1016 50 Ti12+ particles,
corresponding to an average beam intensity of 208 pnA. During
this period five 257 Rf–253 No αα correlations were observed in
detector 1 and none in detector 2. Detector 3 was not used. In
Figure 3 the correlation matrix is shown for detector 1. The
analysis method is thoroughly described in Reference 13.
During an experiment in which the activity introduced into
the system was detected directly, 37 correlations were observedin the same energy region in a 31.3 hour experiment with a beam
integral of 1.86 ×1017 (Ref. 13). In this experiment the activity
was dissolved directly in the scintillator and led straight to thedetectors with a flow rate of 0.84 ± 0.06 mL/s. As three of thefour centrifuges in Figure 2 were not used in this experiment,

SISAK Liquid-Liquid Extraction Experiments

J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

the transport time was much faster. Therefore, the number ofcorrelations in the two experiments cannot be compared directly,
since corrections for decay losses must be taken into account.
It is important to be able to calculate the decay correction
factor between the extraction experiment reported in this paper
and the measurement of the activity entering the SISAK system
reported in Reference 13. The reason for this is that the distribution ratio D (or extraction yield) is calculated from the numberof correlations observed in these experiments:
D≡[Rf]org.
Rorg. /Vorg.
Rorg.
Vaq.
=
=
·
,
Rtot. − Rorg. Vorg.
[Rf]aq.
Raq. /Vaq.

(1)

where Rorg. and Raq. are the number of correlations (activity) observed in the respective phases, Rtot. = Rorg. + Raq. . Vorg. and Vaq.
are the volumes of the two phases, which for a system with continuous mixing and separation like SISAK are equivalent to theflow rates of the respective phases.
The transport through the RTC, the SISAK system, the detector cells, and the interconnecting capillaries will result in atime distribution with a definite break-through time and peaked
shape. Generally, if the volume of the entire system increases,
the distribution will become broader and more tailed and themain part of the activity will arrive later. Thus, the correction
necessary for calculating Raq. from Rtot. and Rorg. has to takethese two distributions for the different setups into account. One
way to do this is to perform Monte Carlo simulations, where themodelled distributions are based on empirical data. A computer
program, called SimSISAK, has been developed for such calculations.14
By using SimSISAK to calculate the amount of decay losses,
the activities can be normalised to a chosen mean transporttime. The SimSISAK calculations gave 91.0% decay loss for
the HDBP experiment and 73.8% for the experiment where theactivity was dissolved directly in the scintillator (referred to as
the “scintillator experiment” in the following).
The additional loss for the HDBP experiment corresponds toan average delay of 6.2 s, which is what is expected. (257 Rf has
a half-life of 4.0 s, experience suggests about 2 s avarage delay
per added centrifuge at the flow rates used.)
The amount of activity in the “scintillator experiment” would
have been only 12.7 αα-correlations if the decay loss had been
11,0
10,5

9,5

253

No -> 249Fm
correlations

8,0
7,5

257

253

Rf -> Nocorrelations

7,0
8,5

9,0

9,5

10,0

10,5

(2)

and
M(NO3 )2+
2 (aq.) + 2(HDBP)2 (org.) M(NO3 )2 · (DBP)2 · (HDBP)2(org.) + 2H+ (aq.) .
(3)In the first case one would expect an extraction curve slope(on a log-log plot) equal to one, in the second case it should be
two. In Figure 4 we observe a slope of around 1.5 for both Zr(0.008 M ≤ [HDBP] ≤ 0.03 M) and Hf (0.1 M ≤ [HDBP] ≤0.5 M). This indicates that the metal is present in about equal+amounts as M(NO3 )2+
2 and MO(NO3 ) . It should be noted thatextraction in the SISAK centrifuges is too fast to achieve equilibrium, thus the data in Figure 4 cannot be directly compared todata from, e.g., batch experiments under equilibrium conditions.
As Rf seems to behave similarly to Hf and Zr, it is reasonable
to expect Rf to also be extracted in this manner. However, moredetailed studies must be undertaken before such a conclusion
can be made.

100

8,5

8,0

MO(NO3 )+ (aq.) + (HDBP)2(org.)+ MO(NO3 ) · (DBP) · HDBP(org.) + H (aq.)

1st T1/2
2nd T1/2
3rd T1/2
4th T1/2
5th T1/2

9,0

6,5
7,5

91.0% (as in the HDBP experiment). Taking into account thebeam integrals, this number is reduced to 5.7. This last number can be directly compared to the five counts measured in the
HDBP experiment.
The transport time through the detector cells was equal forboth experiments. Thus, no corrections need to be done on behalf of correlation-detection efficiency.
Assuming Rtot. = 5.7 then Raq. = 5.7 − 5.0 = 0.7. From eq 1
the distribution ratio can then be calculated to be 8.5+∞−7.5 . The
huge uncertainty is mainly due to the small number of counts
in the organic phase. The extraction yield is 90+10−30 %, i.e., theextraction of Rf is above 50% in this system. Thus, Rf behaves
similar to its lighter homologues Hf and Zr. The 165 Hf data arefrom SISAK experiments at the Oslo Cyclotron Laboratory andthe 100 Zr data from SISAK experiments performed at the Mainz
TRIGA reactor. The Hf and Zr experiments were performed by
Dyve and coworkers.15
In toluene HDBP will mainly be present as the dimer(HDBP)2 .16 The Group IV metal ion will most likely be present
or MO(NO3 )+ . Then the extraction reactionsas M(NO3 )2+
2
might be:Distribution ratio

Daughter energy (MeV)

10,0

123

11,0

Mother energy (MeV)
Figure 3. Correlation matrix showing the expected energy ranges for257
Rf–253 No and 253 No–249 Fm αα correlations. The data are from the firstdetector cell. The energy gates used in the SISAK extraction experimentcorrespond to the plot, i.e., an α with energy between 7.5 and 11 MeV
switched the detector cell from mother to daughter mode (from open to
closed). There are no correlations hidden behind the text boxes.

10

1

100

Zr
Hf
257
Rf
165

0,1

0,005

0,05

0,5

[(HDBP)2] (M)Figure 4. On-line SISAK liquid-liquid extraction of Zr, Hf, and Rf
with HDBP into toluene from 6-M HNO3 . The lines are drawn to guidethe eye.

124 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Omtvedt

4. Conclusion and Future Work

References

The work reported here was the first to use preseparation in atransactinide chemistry experiment. As can been seen from Figure 3 and thoroughly discussed in Reference 13, this eliminates
most of the background activity. The correlations outside theindicated 257 Rf–253 No correlation box, which are not daughtergranddaughter correlations, are most likely also 257 Rf–253 No correlations, but with shifted α energies due to α-electron coincidence summing. Such events are currently under investigation,
but until their nature is clearly understood they are not includedin the calculations. Regardless of the nature of these events, it’s
clear that preseparation offers great improvement in the noiseto-background ratio and permits greater freedom in designing
the experiment.

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For SISAK type of experiments, and any experiment in which
the activity only can be measured in one of the phases, the ability
to precisely determine the amount of activity entering the chemistry apparatus, using the same detector system as in the chemistry experiment, is a great advantage. Estimating the amountof activity by more indirect methods inevitably leads to larger
uncertainties. Therefore, as has been demonstrated in this experiment, using preseparation not only makes the data analysis
easier and less uncertain, but enables more accurate measurements of extraction yield, distribution values, etc.
It is also clear from our results and the SimSISAK decay loss
calculation of 91.0%, that using such short-lived activity as 257 Rf
demands as rapid as possible transport. In this respect, the rather
low flow rates used in this experiment can be significantly increased. From this alone a yield increase by a factor of two
should be easily achievable. This has already been demonstrated
in another LBNL SISAK Rf-experiment where trioctyl-amine
was used to extract Rf from oxalic acid (work in progress). Furthermore, improvements to the RTC and gas-jet transport are expected to give another factor of two yield increase. With these
improvements, more detailed SISAK experiments for studying
the aqueous chemistry of Rf are possible and are in preparation.
It should be noted that the current experiment was designed
to give maximum extraction yield of Rf. Therefore, conditions
under which both Hf and Zr are extracted with high yield wereselected. This is generally not an advantage for deducing chemical differences from such liquid-liquid extraction experiments.
Usually one would like to be able to distinguish between a Hfand a Zr-like behaviour of Rf. This is possible at HDBP concentrations below 0.1 M. The results from this pilot experimentwill be used to design experiments, using preseparated activity,for detailed studies of the chemistry of Rf where these considerations are taken into account.
Acknowledgement. Financial support was gratefully received from: The Norwegian Research Council, projectno. 138507/410 and the Office of High Energy and Nuclear
Physics, Division of Nuclear Physics and the Office of Energy
Research, Division of Chemical Sciences, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract
DE-AC03-76SF000098.
We would also like to thank the operators of the 88-inch cyclotron at LBNL for providing high intensity and stable beams
during our experiments.