Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 55-58, 2007

Development of Gas-jet Transport System Coupled to the RIKEN Gas-filled Recoil Ion
Separator GARIS for Superheavy Element Chemistry
H. Haba,*,a D. Kaji,a H. Kikunaga,a T. Akiyama,a,b N. Sato,a,c K. Morimoto,aA. Yoneda,a K. Morita,a T. Takabe,d and A. ShinoharadaNishina Center for Accelerator Based Science, RIKEN, Wako, Saitama 351-0198, Japan
Department of Physics, Saitama University, Sakura, Saitama 338-8570, Japanc
Department of Physics, Tohoku University, Aoba, Sendai 980-8578, Japand
Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japanb

Received: January 27, 2007; In Final Form: April 3, 2007
A gas-jet transport system for the superheavy element chemistry was coupled to the gas-filled recoil ion separator
GARIS at the RIKEN Linear Accelerator. The performance of the system was investigated using 206Fr and 245Fm
produced in the 40Ar-induced reactions on 169Tm and 208Pb targets, respectively. Alpha particles of 206Fr and 245Fm
separated with GARIS and transported by the gas-jet were measured with a rotating wheel system for α spectrometry under low background condition. The high gas-jet efficiency of over 80% was found to be independent of thebeam intensity up to 2 particle µA. These results suggest that the GARIS/gas-jet system is a promising tool for
future superheavy element chemistry at RIKEN.

1. Introduction
The chemistry of the superheavy elements (SHEs) withatomic numbers Z ≥ 104 has become one of the most exciting
and challenging research subjects in nuclear and radiochemistry.1,2 Experimental studies on chemical properties of SHEs
have been performed for elements 104 to 108 and very recently
element 112.1,2 The production rates of the SHE nuclides, however, are extremely low, i.e., atoms per minute for elements 104(Rf) and 105 (Db) down to atoms per hour or day for elements
106 (Sg) to 108 (Hs),2 and their half-lives are less than ~1 min.
This situation forces us to perform rapid and efficient on-line
chemical experiments with “single atoms” using heavy-ion
accelerators. The gas-jet transport technique has been commonly used to transfer SHE atoms from a target chamber tovarious chemistry apparatuses. Here, nuclear reaction products
recoiling out of a target are stopped in the chamber filled withhelium gas, and are often attached to aerosol particles such as
potassium chloride (KCl) and carbon. The gas and activityladen aerosol particles are swept out through a capillary to a
distant site where chemistry apparatuses and detectors areequipped.
SHE atoms are produced among large amounts of background radioactivities which hinder the detection of decay signals of SHE nuclides. Recently available high-intensity beams
of more than 1 particle µA (pµA) also give rise to a serious
problem in that the plasma formed by the beam in the target
chamber significantly reduces the gas-jet transport efficiency.
To overcome these situations, it has been proposed that a recoil
separator for nuclear physics research on SHEs should be coupled to the chemistry system with the aid of the gas-jet transpor t technique mentioned above.1, 3 With this method,background radioactivities originating from other reaction
products are largely removed. The high and stable gas-jet efficiencies are obtained owing to the condition free from plasma.
Furthermore, chemical reactions of various compounds can bestudied by directly feeding complexing reagents into the gas-jet
chamber without aerosol materials. The first experiment with*Corresponding author. E-mail: haba@riken.jp. Fax: +81-48-4615301.

the recoil transfer chamber (RTC) coupled to the Berkeley Gasfilled Separator (BGS) was very successful.4,5 The isotope of257
Rf physically separated from the large amount of β -decaying
products was identified with a liquid scintillator after a liquidliquid solvent extraction. Thereafter, the BGS/RTC system
was used in the model experiments of Rf (References 6–8) andHs (Reference 9). At Gesellschaft für Schwerionenforschung(GSI), the components of the former HElium Charge-exchange
Kaleidoscope (HECK) separator are being used to set up a
dedicated separator for the SHE chemistry.3
In the RIKEN Linear Accelerator (RILAC) facility, the gasfilled recoil ion separator GARIS has been used to search forthe heaviest SHE nuclides.10–13 The isotopes of 271Ds, 272Rg, and277
112 found at GSI were confirmed with better statistics andwith new spectroscopic information,10–12 and a new isotope ofelement 113, 278113, was successfully synthesized.13 GARIS is
expected to give us low background condition and high transport efficiencies for SHEs. In this work, we have installed agas-jet transport system at the focal plane of GARIS to start
the SHE chemistry in RIKEN. The performance of the system
was appraised using 2 0 6 Fr and 2 4 5 Fm produced in the169
Tm(40Ar,3n) 206Fr and 208Pb(40Ar,3n) 245Fm reactions, respectively.
2. Experimental
A schematic of the experimental setup is shown in Figure 1.
The 40Ar9+ ion beam was extracted from RILAC. The metallic
169
Tm and 208Pb targets of 120 and 420 µg cm –2 thicknesses,
respectively, were prepared by vacuum evaporation on 30 µg
cm–2 carbon backing foils. Sixteen targets were mounted on arotating wheel of 30 cm in diameter. The wheel was rotatedduring the irradiation at 3000 rpm. The beam energies were169.7 MeV for 169Tm and 198.6 MeV for 208Pb at the middle ofthe targets. At these incident energies, the cross section for the169
Tm(40Ar,xn)209–xFr reactions (x = 2 + 3) is 376 ± 7 µb,14 whilethat for 208Pb(40Ar,3n) 245Fm is 15 ± 5 nb.15 The beam intensity
was monitored by measuring elastically scattered projectiles
with a Si PIN photodiode (Hamamatsu S1223) mounted at 45°
with respect to the beam axis. The typical beam intensity was
2 pµA. GARIS was filled with helium at a pressure of 88 Pa.

© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

56

Haba

(c) Gas-jet chamber

400

He / KC l

350

target

88 Pa
D1

Q1

Q2

0

1

Manometer

D2

0

50 mm

2m

( d ) R o ta ti n g w h e el s y s te m , M AN O N(b) SD chamber

12-strip Si
detector
(SD)

ER s

88 Pa

0

Si PIN photodi odes

Mylar foil

Figure 1. A schematic of the experimental setup: (a) RIKEN gasfilled recoil ion separator GARIS; (b) 12-strip Si detector (SD) chamber; (c) Gas-jet chamber coupled to GARIS; (d) Rotating wheel
system MANON for α spectrometry.

150
100

ERs and TRs

0
400

(b)

350

50 mm

0.1 Pa200

50

Teflon capillary(1.59 mm i.d. and 4 m length)

Mylarwindow

Counts / 10 keVElas tic sca ttering
beam monitor

250

300
250
200
150
100

Counts per 5 s

Gas out

ERs

169Tm or
208Pb rotating

(a)

300

6.790 206Fr
6.915 205Fr

Mylar
window

90 kPa6.135 202At
6.228 202At
6.344 201At

SD or G as - jetfrom RILAC Evaporation Residues (ERs) chamber at theGas inlet focal plane
Beam dump
40Ar beam

6.135 202At
6.228 202At
6.344 201At

Differential pumping section

Counts / 10 keV( a) G AR I S

6.790 206Fr
6.915 205Fr

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

10

4

10

3

10

2

10

1

10

0

0

50

100

Time / s

15050

3. Results and Discussion
206

Figure 2(a) shows an α -particle spectrum measured in the
Fr experiment for 2600 s in the 6th strip (middle) of SD

(c)

350
300

6.135 202At
6.228 202At
6.344 201At

250
200
150
100

6.915 205Fr

6.790 206Fr

0400

Counts / 10 keV

The magnetic rigidities were set at 1.64 Tm for 206Fr and at 2.01
Tm for 245 Fm. The other details of GARIS are given elsewhere.11
As shown in Figures 1(a) and 1(b), the evaporation residues
of interest were separated in-flight from beam particles andtransfer reaction products with GARIS, and were implanted
into a 12-strip Si detector (SD) of 60 × 60 mm 2 (Hamamatsu
12CH PSD) through a Mylar window of 3.5 ± 0.1 µm thickness
which was supported with a stainless-steel honeycomb gridwith 92.5% transparency and of 60 mm diameter. Alpha-particle measurements of 206 Fr were conducted for 2600 s underthe beam-on condition and for 300 s after a 30-s beam irradiation. In the 245Fm experiment, the cycle of the beam-on (5 s)and beam-off (15 s) measurements was repeated 1265 times,
because no α peak of 245Fm was observed in the beam-on spectrum due to large amounts of background events. The α-particle energy resolution of SD was 50 keV FWHM. All eventswere registered in an event-by-event mode.
In the gas-jet transport experiments, 206 Fr and 245Fm separated by GARIS were guided into the gas-jet chamber of 60
mm length as shown in Figure 1(c). The reaction products
were stopped in the helium gas at 90 kPa, attached to KCl aerosol particles generated by sublimation of the KCl powder at 620°C, and were continuously transported through a Teflon capillary (1.59 mm i.d., 4 m length) to the rotating wheel system
MANON for α spectrometry (Figure 1(d)) which was the compact one of the Measurement system for the Alpha-particle andspontaneous fissioN events ON-line developed at Japan Atomic
Energy Agency (JAEA).16 The flow rate of the helium gas was
5 L min–1. In the 206Fr experiment, the temperature of the KClaerosol generator was varied from 540 to 640 °C in steps ofabout 20 °C to optimize the gas-jet transport efficiency. In
MANON, aerosol particles were deposited on Mylar foils of
0.68 µm thickness and 20 mm diameter placed at the peripheryof a 40-position stainless steel wheel of 420 mm diameter. The
wheel was stepped at 30-s and 2-s intervals for 206Fr and 245Fm,
respectively, to position the foils between seven pairs of Si PIN
photodiodes (Hamamatsu S3204-09). Each detector had an
active area of 18 × 18 mm2 and a 38% counting efficiency for α
particles. The α-particle energy resolution was 60 keV FWHM
for the detectors which look at the sample from the collection
side (top detectors).

50
0

5

6

7

8

9

10

α-particle energy / MeV
Figure 2. (a) Alpha-particle spectrum measured in the 206Fr experiment for 2600 s in the 6th strip (middle) of the 12-strip Si detector(SD) under the beam-on condition. The 40Ar beam intensity was
0.017 pµA. (b) Alpha-particle spectrum measured in the 6th strip of
SD for 300 s after the 30-s beam irradiation. The inset shows a decay
curve of the 6.790-MeV α peak of 206Fr measured in all the strips ofSD. The beam intensity was 1.9 pµA. (c) Alpha-particle spectrum
measured in the first top detector of the rotating wheel system
MANON for 30 s after the 30-s aerosol collection. The beam intensity was 1.8 pµA. The helium flow rate was 5 L min–1 and the chamber pressure was 90 kPa. The temperature of the KCl aerosol
generator was 620 °C.

under the beam-on condition. Alpha peaks of 206Fr (T1/2 = 15.9
s, E α = 6.790 MeV)17 and 205Fr (3.85 s, 6.915 MeV)17 and of theirdaughter nuclides 202At (182 s, 6.135 MeV; 184 s, 6.228 MeV)17
and 201At (89 s, 6.344 MeV)17 were identified. The broad component above 7 MeV is associated with the implantation of
evaporation residues (ERs) and target recoils (TRs). Figure2(b) shows an α-particle spectrum measured in the 6th strip of
SD for 300 s after the 30-s beam irradiation. The decay curveof the 6.790-MeV α peak of 206Fr is shown in the inset of Figure2(b). The half-life of 206Fr was determined to be 15.3 ± 0.2 s,
which is in agreement with the literature value of 15.9 ± 0.2 s(Reference 17) within 2σ. The contribution of the α particles
of 207Fr (14.8 s, 6.768 MeV)17 produced in the 169Tm(40Ar,2n)207Fr
reaction to the 6.790-MeV peak of 206Fr is found to be negligible from the absence of the 6.087-MeV peak of its daughter
nuclide 203At (7.4 min, 6.087 MeV)17 in Figure 2(a). On theother hand, the α -particle spectrum measured in the first top
detector of MANON for 30 s after the 30-s aerosol collection iscompared in Figure 2(c). The α peak of 206Fr is clearly seen inJ. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
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80

104

Counts / 20 keV

100

60
40
20

101

580

600

620

640
o

Temperature of KCl aerosol generator / C

660

2

10

(b)

100
80
60
40
20

100

Counts per 2 s

560

Figure 3. Variation of the gas-jet transport efficiency of Fr as afunction of the temperature of KCl aerosol generator. The helium
flow rate was 5 L min–1 and the chamber pressure was 90 kPa. The
40
Ar beam intensity was 1.7 pµA.

1

10

0

2

4

6

8

Time / s

10

12

0
80

80

(c)

70

Counts / 20 keV

Gas-jet transport efficiency / %102

7.342 241Cf
7.699, 7.730 245Es
8.150 245Fm

540

206

60
40
20
0

103

60

7.342 241Cf
7.699, 7.730 245Es
8.150 245Fm

0
520

57

(a)

100
120

Counts / 20 keV

Gas-jet transport efficiency / %Development of Gas-jet Transport System Coupled to the RIKEN

50
40
30
20
10
0

-2

-1

10

10
40

0

10

0

2

4

6

8

10

12

14

16

18

α-particle energy / MeV

Ar beam intensity / pµA
206

Figure 4. Variation of the gas-jet transport efficiency of Fr as afunction of the 40Ar beam intensity. The helium flow rate was 5 L
min–1 and the chamber pressure was 90 kPa. The temperature of the
KCl aerosol generator was 620 °C.

the spectrum, indicating that the gas-jet transport of 206 Fr to
MANON was successfully conducted after the physical separation by GARIS. The transport efficiency of the gas-jet system
was evaluated using the 6.790-MeV α peak of 206Fr.
In Figure 3, the gas-jet transport efficiencies of 206 Fr are
shown as a function of the temperature of the KCl aerosol generator. The efficiency increases smoothly with an increase of
the temperature and attain to over 90% at 620 °C. The variation of the gas-jet efficiencies of 206Fr is shown in Figure 4 as afunction of the 40Ar beam intensity. The efficiencies are constant irrespectively of the beam intensities up to 2 pµA. In theconventional gas-jet system in that the beam passes through thetarget chamber, the gas-jet efficiency decreases due to theincreasing plasma condition induced by the intense beam. As
an example, we previously measured the gas-jet efficiencies of173
W produced in the natGd(22Ne,xn) reaction without the beam
separation by GARIS. It was found that the gas-jet efficiencyof 173W drastically decreases from 40% at 6.6 pnA to 25% at
0.5 pµA with increasing beam intensity. Since the beam isseparated by GARIS in the present experiment, such a decreaseof the gas-jet efficiency is not observed for 206 Fr as shown in
Figure 4.
Alpha-particle spectra of 245 Fm under the beam-on andbeam-off conditions are shown in Figures 5(a) and 5(b),respectively. Although no α peaks are seen in the beam-on

Figure 5. Sum of α-particle spectra measured in the 245Fm experiment in the 12-strip Si detectors (SD) under (a) the beam-on and (b)beam-off conditions. The cycle of the beam-on (5 s) and beam-off(15 s) measurements was repeated 1265 times. The 40Ar beam doseof 6.55 × 1016 was accumulated. The inset shows a decay curve of the
8.150-MeV α peak of 245Fm. (c) Sum of α-particle spectra measured
in the seven top detectors of the rotating wheel system MANON for
14 s after the 2-s aerosol collection. The 2-s aerosol collection was
repeated 7289 times. The beam dose was 9.76 × 1016. The helium
flow rate was 5 L min–1 and the chamber pressure was 90 kPa. The
temperature of the KCl aerosol generator was 620 °C.

spectrum (Figure 5(a)), α peaks of 245Fm (4.2 s, 8.15 MeV)17
and its daughter 241Cf (3.78 min, 7.342 MeV)17 are clearly seenin the beam-off spectrum (Figure 5(b)). The decay curve of
the 8.15-MeV α peak is shown in the inset of Figure 5(b). The
half-life of 245 Fm was determined to be 6.3 ± 1.3 s, which
agrees with the literature value of 4.2 ± 1.3 s.17 The α peaks
around 7.7 MeV would be those of 245 Es (1.1 min, 7.699 and7.730 MeV)17 produced in the 208Pb(40Ar,p2n) 245Es reaction or
the EC decay of 245Fm. Background α-particles of, e.g., 211Bi
and 211m,212m Po which are produced in the transfer reactions on
the 208Pb target15 are completely removed by GARIS. The separation factors in excess of 10 4 were evaluated for 211Bi and
211m,212m
Po based on their cross sections reported in Reference
15.
Compared in Figure 5(c) is the sum of α -particle spectra
measured in the seven top detectors of MANON. The 8.15MeV α peak of 245Fm is clearly seen in the spectrum, and thegas-jet transport efficiency is determined to be 83 ± 9%.
Despite of the physical separation by GARIS, one can see a lotof background events in Figure 5(c), especially below 4 MeV.

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J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

Since MANON was placed in the target room in this experiment, these background events were caused by large amountsof neutrons and/or γ rays during the irradiation. Very recently,
we have constructed a chemistry laboratory isolated with a 50cm concrete shield from the target room, where the backgroundlevel is two orders of magnitude lower than that in the targetroom.
In this work, the high gas-jet efficiencies over 80% wereobtained both for 206Fr and 245Fm. The recoil range of 206Fr in
helium at 90 k Pa is calculated to be 30 mm based on the SRIM
2006 code,18 and that of 245Fm is extrapolated to 18 mm fromthose of the lower Z ions (Z = 68–92) of A = 245. These recoil
ranges are short enough as compared with the helium thicknessof the gas-jet chamber used in this work (60 mm at 90 kPa). In
the conventional gas-jet system in that the beam passes in therecoil chamber, the helium gas is swept out through the capillary outlet to the vertical direction of the beam axis. Therefore,
the position of the capillary outlet in the chamber should be
adjusted to the recoil ranges of the product nuclei of interest.
Since the beam is separated by GARIS in the present system,
we can put the capillary outlet at the center end of the chamber(see Figure 1(c)). Thus, helium gas is fed into the chamber
through the four inlets directed to the surface of the Mylar
window and is swept out thoroughly from the end of the chamber. In addition to the beam-free condition, this smooth helium
flow in the chamber would be advantageous to the high gas-jet
efficiency.
4. Summary and Perspectives
We have developed the gas-jet transport system coupled to
GARIS as a preseparator for the SHE chemistry. The performance of the system was demonstrated using 206Fr and 245Fm
produced in the 169 Tm(40Ar,3n) 206 Fr and 208Pb(40Ar,3n) 245 Fm
reactions, respectively. The α particles of 206Fr and 245Fm separated with GARIS and transported by the gas-jet were clearlyobserved with a rotating wheel system for α spectrometry. The
high gas-jet efficiency of over 80% was found to be independent of the beam intensity up to 2 pµA. These results suggestthat the GARIS/gas-jet system is a promising tool for future
SHE chemistry at RIKEN. Recently, the gas-jet transport of255
No produced in the 238U(22Ne,5n) reaction was also successful with the gas-jet efficiency over 80%. In the future, productions of SHE nuclides with long half-lives for chemical
experiments such as 261Rf, 262Db, 265Sg, 269Hs, and 283112 will be
studied with the present system using 238U and 248Cm targets.
Acknowledgement. The authors express their gratitude to
the crew of the RIKEN Linear Accelerator for their invaluableassistance in the course of these experiments. This research
was partially supported by the REIMEI Research Resources ofJapan Atomic Energy Research Institute, 2003, and by the
Ministry of Education, Science, Sports and Culture, Grant-inAid for Young Scientists (B), 16750055, 2004–2006.
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