Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 183–186, 2002

183

Preparation of Thick Americium Targets and Synthesis of 259 Db∗
J. S. Guo,† Z. Qin, Z. G. Gan, H. M. Fan, Y. B. Xu, J. J. He, X. G. Lei, X. L. Wu, H. Y. Liu,X. H. Zhou, S. G. Yuan, and G. M. Jin
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China
Received: October 30, 2001; In Final Form: March 25, 2002
A new isotope of 259 Db has been produced via the reaction 241 Am(22 Ne, 4n) at Elab = 118 MeV, and its half-life of
0.51 ± 0.16 s and α-decay energy of 9.47 MeV were determined in this work. The americium targets with thickness
ranged from 0.7 to 1.0 mg/cm2 were prepared by molecular plating method through a single electrodeposition cycle.
The reaction products were transported and collected using a helium-jet technique and a rotating wheel apparatus.
The α decays of both the products and their daughter nuclides were detected by a series of Si(Au) surface barrier
detectors arranged ingeniously. The identification of 259 Db was performed based on the genetic relationship between
the new activity and the known 255 Lr established by α-recoiled milking measurement. The Qα value of 259 Db derived
from the present experiment is in good agreement with theoretical prediction.

Since the discovery of the element 105, named Db, many isotopes from 255 Db through 263 Db, with the exception of 259 Db,
have been identified based on the studies of genetic α-links topreviously known daughter nuclides of Lr1–5 and the measurement of characteristic x ray of element 103 in coincidence withthe α-particle groups of Db isotopes.6 According to systematics and theoretical predictions by Möller et al.7 and Wapstra etal.,8 the isotope 259 Db should be an α emitter with Qα value of9.61 MeV and half-life of 0.14 s with respect to α decay. Therefore it is the aim of our experiment to close this gap and to study
the decay properties of 259 Db nuclide.
With the purpose to produce the expected nuclide by heavyion fusion evaporation reaction, it is of importance to prepare
the thicker actinide targets. The molecular plating method is
the preferred method for preparing targets of actinide elements.
Its deposition efficiency is very high and the equipment neededfor deposits is simple and can be handled in glove box or hoods.
The molecular plating has been used for preparation of relatively
thick targets. However, the deposits were very adherent as longas not more than 100 µg/cm2 of the elements were precipitatedat one time. To obtain thicker layers, the backing material withthe layers was calcinated in a muffle furnace and the deposition
procedure repeated several times.9 In this work, 241 Am targets
on aluminum foil with thickness ranged from 622 to 910 µg/cm2
were obtained through a single molecular plating cycle.
The 259 Db was produced via the reaction 241 Am(22 Ne, 4n), andthe half-life and α-decay energy of 259 Db were determined. Its
identification was performed by recoil-milking the 21-s 255 Lrdaughter. The Qα value of 259 Db derived from the present experiment is in good agreement with theoretical prediction. Comparing with the trend for the other Z = 105 isotopes, it fits quite
well for the general trend.
2. Experimental Procedure
2.1. Preparation of Thicker Americium Targets. Enriched 241 Am was purchased in the oxide form of a dark brownpowder. 241 AmO2 (8 mg) was dissolved completely in 5 mL of
0.16 N nitric acid, and the solution of Am(NO3 )3 was produced
as a pink colored stock solution of 241 Am, with a concentrationof 1.6 mg/mL. Since 241 Am has a characteristic γ ray of energy of59.5 keV, it is convenient to measure the γ activities of the sample for determining the amount of americium. The concentra-tion and isotopic composition of stock solution were assayed bymeasuring γ activity with HPGe detector. It was demonstrated
that only the γ ray of energy of 59.5 keV could be observed,
and other γ activities which could contaminate the sample were
not found in the γ spectrum. Especially, the γ ray of energyof 312 keV which might arise from 237 Np (daughter nuclide of241
Am α decay) did not appear.
The molecular plating was carried out in the cell with coolingwater jacket made of Pyrex glass. A rubber “O” ring of 10 mm
inner diameter was used to define the area of deposition. To
stir the solution, a platinum wire (anode, 1 mm diameter) wasrotated continuously by a motor. An aluminum foil (cathode,1.7 mg/cm2 thickness) as the backing material for electrodeposition was put between the cell and the stainless-steel base. The
cell was filled with 15 mL isopropyl alcohol and 300 ∼ 500 µLof the 241 Am stock solution. A typical electrolysis was carriedout with a current of 5 mA/cm2 at the voltage of ∼500 V, andthe distance between cathode and anode was 3 cm. The efficiency of deposition of americium by molecular plating method
was investigated as a function of duration of electrodeposition
as shown in Figure 1. One can find that the main part (ca.
85%) of 241 Am had already deposited within 5 min. For plating the remaining fraction a total time of 1 h was necessary.
1009590

Electrodeposition efficency (%)

1. Introduction

8580757065600∗This work

is supported by NSFC Project number 19775053,
19905012 and the Foundation of the Chinese Academy of Sciences forthe researcher return from abroad.
†Corresponding author. E-mail: JSGuo@impcas.ac.cn.

102030405060Duration of molecular plating (min.)Figure 1. Efficiency of deposition of americium by molecular plating
method as a function of duration of electrodeposition.

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

184 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002
After electrodeposition, the electrolysis solution in the cell waspoured into a receptacle from which any remaining Am could
be recovered later. The layer on the aluminum foil was rinsed
with 5 mL of isopropyl alcohol, and then placed in a muffle furnace at 773 K for 15 min. Since americium is deposited as hydrolytic species of americium which has a chemical formula ofAm(OH)3 ·nH2 O, it is necessary to convert it into an oxide form
as AmO2 which has a good thermal stability under 1273 K. Finally, three pieces of 241 Am targets with thickness ranged from
622 to 910 µg/cm2 had been obtained through a single molecular plating. The procedure for preparing the thicker americium
targets has been described in detail elsewhere.10
2.2. Experimental Setup. 259 Db was produced at the SFC(Sector Focus Cyclotron) of HIRFL (Heavy Ion Research Facility at Lanzhou) via the reaction of 241 Am(22 Ne, 4n). The beamenergy was about 118 MeV in the center of target material, corresponding to the maximum of the excitation function for the241
Am(22 Ne, 4n) reaction based on the statistical evaporation calculations using the Alice Code.11 The beam of 132-MeV 22 Ne
ions delivered from cyclotron was degraded to 118 MeV after
passing through a 1.94 mg/cm2 Havar window, 1.5 cm heliumgas at 740 torr, and a 1.70 mg/cm2 Al target backing. The typical beam current of 22 Ne was 0.8–1.0 µA. Three pieces of americium targets prepared above were changed at regular irradiationinterval in order to avoid the irradiation damage of the target
foils.
The reaction products recoiling out of the target were stopped
in helium gas which loaded with NaCl aerosols. The products attached to aerosols were swept out of the target chamber with the helium gas, and then transported through a 1.27mm-diameter and 20-cm-length capillary into a rough vacuum
chamber to impinge upon the periphery of a vertically mounted
wheel. The wheel was periodically rotated by the preset interval
to place the collected recoil atoms to the position in front of aseries of peripherally mounted Si(Au) surface-barrier detectors
in order to measure their α-particle spectra.
In the present experiment, the detectors were divided into fourgroups and each group included three Si(Au) surface-barrier detectors. They were arranged around the wheel according to unequal intervals. The typical α-energy resolution for the mostof detectors was 30–40 keV (FWHM) calibrated by a RdThsource. A schematic representation of the detectors arrangement is shown in Figure 2. The wheel-stepping interval is incorrespondence with the arrangement of the detectors and controlled by a computer. The first collected source on the wheel
was rotated 45◦ to the No.1-1 detector, the α decays of products
and their daughters were measured, while the second source wascollected. The second source was rotated 90◦ to the No.2-1 de-Figure 2. Scheme of the arrangement of the detectors around therotating wheel.

Guotector, while the first source was moved to the front of No.1-2
detector. Meanwhile, No.1-1 detector faced to no product sourceon the wheel, thus only records the α-particle activities fromdaughters which have recoiled off the wheel into the surface of
No.1-1 detector. Successively the third source was rotated 75◦
to the No.3-1 detector, simultaneously the 2nd source is rotatedto No.2-2 detector, and the 1st source goes to No.1-3 detector.
In this time detectors No.1-1, No.1-2, and No.1-3 recorded thesame source (1st source), thus the time sequential spectra could
be obtained. When 4th source was rotated 150◦ to the front ofNo.4-1 detector, 3rd source is sent to No.3-2 position; the 2ndsource is sent to No.2-3 position, and the 1st source is returnedto the collection position and the 5th source will be collected
there. After a cycle of the wheel a new sequential measurement
will be started. By this way the measurement procedure will berepeated and continued up to the end of the experiment. In theprocess, the measuring time of each detector for every source isequal to the collecting time of this source. In order to measure
the longer and shorter half-lives of the products, the two runs experiments, the collection time of 3 s and 10 s, were performed,respectively. The total of 9-s and 30-s time-sequential decays
in each cycle for 3-s and 10-s collection, respectively, could beobtained.
In the measurement process one can see that there are one
time-interval facing the source and three time-intervals facing nosource for each detector. During the detector facing no source, it
is recording only the decay of α-recoils (daughters) on the surface of the detectors. Furthermore, the detectors of each groupare recording the time sequential decays of the products andtheir daughters when facing the source on the wheel. When thedata are processed and analyzed we can accumulate the recorded
counts for each detector in accordance with the wanted timeinterval so as to obtain the time sequential spectra, such as themother and daughter as well as the pure daughter decay spectra.
In other words, the time sequential decay could be obtained bothfrom the recorded counts of separated time intervals of each detector and from the recorded counts of the ordinal detectors ofeach group. This arrangement of the detectors is much simple
than that with a shuttle system of detector used by Ghiorso etal.,2 but our detector arrangement without shuttle system has alow efficiency for recording daughter decay.
3. Results and Discussion
The α-particle spectra shown in Figure 3 resulted from bombardment of the 241 Am target with 118 MeV 22 Ne ions in 3-s collection and measurement time interval by No.1 detector of eachFigure 3. The α-particle spectra produced by bombardments of 241 Amwith 118-MeV 22 Ne ions. It is the sum counts recorded by No.1 detectorof each group in 3-s collection and measurement time for each cycle.
(a) The α-particle spectrum recorded by the detectors facing the productsource. (b) The α-particle spectrum recorded by the detectors not facing
the product source (the α spectrum of the recoil daughter).

Preparation of Thick Americium Targets and Synthesis of 259 Dbgroup. The α spectra recorded by the detectors in facing theproduct source on the wheel, combined with the α spectra fromthe same detectors but not facing the product source, are shownin Figure 3(a) and (b), respectively. In other words, Figure 3(a)represents the α spectra of the products (including mother anddaughter nuclides) on the wheel, and Figure 3(b) is the α spectra from the decay of α-recoil-daughter nuclides embedded in
the surface of detectors.
An α peak with the energy of 9.47 MeV observed clearly in
Figure 3(a) is assigned to 259 Db in the present work. Its half-life
is measured to be 0.51 s (as shown in Figure 4). In addition, anα peak with an energy of 8.36 MeV has a measured half-life of
21 s and thus could be assigned to the previous known nuclide255
Lr. Its second α peak of 8.40 MeV is just overlapped withanother peak of 8.41 MeV. We believe that the nuclide 255 Lr is αdecay daughter of 259 Db nuclide, and it could exclude the contribution of 255 Lr produced directly in the bombardment due torather small production cross section of (α4n) reaction according to Alice Code calculation.
A complex group of peaks with the energies of 9.08, 9.17, and9.30 MeV in the Figure 3(a) could be assigned to 258 Db basedon the whole complex group decays with a measured half-lifeof 4.3 s. This nuclide arose from the reaction 241 Am(22 Ne, 5n).
The α-decay daughter 254 Lr of 258 Db nuclide presented in Figure 3(a) has an α peak with an energy of 8.46 MeV. And an-

10J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

185

other 8.41 MeV α peak of 254 Lr nuclide is inter-overlapped withan 8.40 MeV α peak of 255 Lr. The decay curve of 254 Lr has a
growth-decay phenomenon, thus it proved that the 254 Lr nuclide
arose from the decay of the mother nuclide 258 Db not from thereaction directly. The half-lives of 4.3 and 13.4 s deduced fromthe decay curves of 254 Lr are in coincidence quite well with theknown values of 258 Db and 254 Lr, respectively. Therefore, thenuclide 258 Db and its daughter 254 Lr have also been clearly observed from the same projectile-target combination, thus, proving the reliability of our assignment of 259 Db.
Figure 3(b) represented only α spectra of the recoil-daughter
nuclides when the detectors were not facing the products source.
In these “daughter” spectra the presented α peaks have the sameenergies of 8.36, 8.41, and 8.46 MeV and same half-lives of 13.4
and 21.3 s, respectively, as the values of 254 Lr and 255 Lr in the“parent” spectra (Figure 3(a)), thus confirming the assignmentof the 254 Lr and 255 Lr, and hence their precursors of 258 Db and259
Db.
The production cross sections are estimated from the yields ofthe 259 Db and 258 Db α decays to be 1.6 ± 1.2 nb and 3.6 ± 1.8 nb,
respectively. These values result from an assumed 60 ± 10%
He-jet transport efficiency, a detection efficiency of 40 ± 10%,and a transport time of 0.2 s for the products from the target chamber to the collection wheel system. The Qα valueof 9.62 MeV for new isotope 259 Db derived from the present
experiment is in good agreement with the values of 9.61 and9.60 MeV theoretically predicted by Möller et al.7 and Wapstra
et al.8 We compared the Qα of new isotope 259 Db to the valuesof the known isotopes in a “Qα systematics” for isotopes withZ ≥ 98, and it shows that the Qα value of 259 Db fitted well to thegeneral trend.

Counts4. Conclusion
Three pieces of 241 Am targets with thickness ranged from 622
to 910 µg/cm2 had been obtained by a single molecular plating.
A new nuclide 259 Db has been produced by bombarding 241 Am
with 22 Ne ions. The identification of this nuclide has been performed by measuring the α-particle emission of the mother anddaughter nuclides. 259 Db has a 0.51 ± 0.16 s half-life and decays by α emission with the energy of 9.47 MeV. The Qα value
for new isotope 259 Db derived from the present experiment is ingood agreement with the theoretical prediction.

259Db
Eα=9.47 MeVT1/2=0.51â 0.16s

10.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Acknowledgments. We wish to express our thanks to theoperators and crew of the IMP SFC Cyclotron for providing theintense 22 Ne ion beams used in this experiment. The authors
are also grateful to the staffs of the detector group in IMP for
preparing detectors.
10Counts

References255

LrEα=8.36 MeV
T1/2=21â 8s
051015202530Time(sec)Figure 4. The decay curves for α decays of 259 Db with α energy of
9.47 MeV and 255 Lr with α energy of 8.36 MeV when the detectors werefacing the product source. For 259 Db decay (upper portion) the counts
were obtained from the sum counts by No.1 detector of each group for
3-s collection and measurement time in each cycle, and for 255 Lr (lowerportion) the decay was obtained from the sequential detectors of eachgroup for 10-s collection and measurement time in each cycle.

(1) G. N. Flerov, J. Phys. Soc. Jpn. Suppl. 24, 237 (1968).
(2) A. Ghiorso, M. Nurmia, K. Eskola, J. Harris, and P. Eskola,
Phys. Rev. Lett. 24, 1498 (1970).
(3) G. Münzenberg, S. Hofmann, F. P. Heßberger, W. Reisdorf,K. H. Schmidt, and P. Armbruster, Z. Phys. A 300, 107(1981).
(4) G. Münzenberg, P. Armbruster, S. Hofmann, F. P.
Heßberger, H. Folger, J. G. Keller, V. Ninov, K.
Poppensicker, A. B. Quint, W. Reisdorf, K.-H. Schmidt, J.
H. R. Schneider, H.-J. Schött, K. Sümmerer, E. Hanelt, W.
Morawek, D. Vermeulen, Y. Fujita, and T. Schwab, Z. Phys.
A 333, 163 (1989).
(5) F. P. Heßberger, G. Münzenberg, S. Hofmann, Y. K.
Agarwal, K. H. Schmidt, J. H. R. Schneider, W. F. W.
Schneider, H.-J. Schött, and P. Armbruster, Z. Phys. A 322,557 (1985).

186 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002
(6) C. E. Bemis, Jr., P. F. Dittner, R. J. Silva, R. L. Hahn, J. R.
Tarrant, L. D. Hunt, and D. C. Hensley, Phys. Rev. C 16,1146 (1977).
(7) P. Möller, J. R. Nix, and K. L. Kratz, At. Data Nucl. Data
Tables 66, 315 (1997).
(8) A. H. Wapstra and G. Audi, Nucl. Phys. A 432, 55 (1985).

Guo(9) J. E. Evans, R. W. Lougheed, M. S. Coops, R. W. Hoff, andE. K. Hulet, Nucl. Instrum. Methods 102, 389 (1972).
(10) Qin Zhi, Guo Junsheng, and Gan Zaiguo, Appl. Radiat. Isot.
54, 741 (2001).
(11) M. Blann, University of Rochester Report, UR NSRL-181(1978).