Journal of Nuclear and Radiochemical Sciences, Vol. 9, No.1, pp. 27-31, 2008

Performance of the Gas-jet Transport System Coupled to the RIKEN Gas-filled Recoil Ion
Separator GARIS for the 238U(22Ne, 5n)255No Reaction
H. Haba,a,* H. Kikunaga,a,b D. Kaji,a T. Akiyama,a,c K. Morimoto,a K. Morita,a T. Nanri,dK. Ooe,b N. Sato,a,e A. Shinohara,b D. Suzuki,d T. Takabe,b I. Yamazaki,d A. Yokoyama,d andA. YonedaaaNishina Center for Accelerator Based Science, RIKEN, Wako, Saitama 351-0198, Japan
Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japanc
Department of Physics, Saitama University, Sakura, Saitama 338-8570, JapandFaculty of Science, Kanazawa University, Kanazawa, Ishikawa 920-1192, Japane
Department of Physics, Tohoku University, Aoba, Sendai 980-8578, Japanb

Received: May 1, 2008; In Final Form: June 27, 2008
The performance of the gas-jet transport system coupled to the RIKEN gas-filled recoil ion separator GARIS wasinvestigated using 255No produced in the 238U(22Ne, 5n) 255No reaction. Alpha particles of 255No separated withGARIS and transported by the gas-jet system were measured with a rotating wheel apparatus for α spectrometry
under low background condition. The high gas-jet efficiencies of about 75% were independent of the recoil rangesof 255No in the gas-jet chamber. The present results suggest that the GARIS/gas-jet system is a promising tool forthe next-generation superheavy element chemistry: (i) the background radioactivities of unwanted reaction products
are strongly suppressed, (ii) the intense beam is absent in the gas-jet chamber and hence the high gas-jet efficiency
is achieved, and (iii) the beam-free condition also allows for investigations of new chemical systems

1. Introduction
Chemical studies of superheavy elements (SHEs) withatomic numbers Z ≥ 104 have become one of the most exciting
and challenging research subjects in nuclear and radiochemistry.1,2 The extremely low production yields and short half-livesof SHEs force us to conduct rapid and efficient on-line chemical experiments with “single atoms”. Using a gas-jet transporttechnique, the experimental studies on the chemical propertiesof SHEs have been performed for elements 104 (Rf) to 108(Hs) and recently element 112.1–3 At the same time, many ofthese successful experiments have clearly demonstrated thelimitations of the applied techniques. Large amounts of background radioactivities from unwanted reaction products
become unavoidable with increasing Z of SHEs of interest.
High-intensity beams from advanced heavy-ion accelerators
also give rise to a problem in that the plasma formed by thebeam in the target chamber significantly reduces the gas-jet
transport efficiency. To overcome these limitations, the concept of physical preseparation of SHE atoms has been proposed.1, 4 With this method, background radioactivities
originating from unwanted reaction products are largely
removed. The high and stable gas-jet efficiencies are achieved
owing to the condition free from plasma. Furthermore, thisbeam-free condition allows us investigations of new chemicalsystems that were not accessible before.4 The pioneeringexperiments with the recoil transfer chamber (RTC) coupled to
the Berkeley Gas-filled Separator (BGS) were very successful.5,6 The isotope of 257Rf physically separated from the large
amount of β-decaying products was identified with a liquid
scintillator after a liquid-liquid solvent extraction. Thereafter,
the BGS/RTC system was used in the model experiments of
Rf 7-9 and Hs.10 At Gesellschaft für Schwerionenforschung(GSI), a new gas-filled separator, the TransActinide Separator
and Chemistry Apparatus (TASCA), is under commissioning

as a preseparator for chemical studies.4,11
In the RIKEN Linear Accelerator (RILAC) facility, a gas-jet
transport system was installed at the focal plane of the RIKEN
gas-filled recoil ion separator GARIS to start the SHE chemistry.12 The performance of the system was first appraised using
20 6
Fr and 2 45 Fm produced in the 169 Tm(4 0 Ar, 3n) 20 6 Fr and208
Pb(40Ar, 3n) 245Fm reactions, respectively.12 Alpha particles
of 206 Fr and 245Fm separated with GARIS and transported by
the gas-jet system were measured with a rotating wheel apparatus for α spectrometry under low background condition. The
high gas-jet efficiencies of over 80% were found to be independent of the beam intensity up to 2 particle µA (pµA).
In order to produce SHE nuclides with long half-lives forchemical experiments, further asymmetric fusion reactions
based on actinide targets such as 238U, 244Pu, and 248Cm should
be considered (hot fusion reactions). However, very small
recoil velocities of evaporation residues (ERs) produced bysuch asymmetric reactions cause serious problems in the operation of a gas-jet system coupled to a gas-filled separator. The
transport efficiency of the gas-filled separator drasticallydecreases with decreasing recoil velocity due to the multiple
small-angle scattering in the filling gas. A vacuum window
foil, which separates the gas-jet chamber from the gas-filled
separator, should be thin enough (≈1 µm as Mylar) to allow
ERs to pass though and has to withstand a pressure differenceof about 100 kPa. In this work, the performance of the GARIS/
gas-jet system for the hot fusion reactions was investigated forthe first time using 255No produced in the 238U(22Ne, 5n) 255No
reaction. The 255No atoms preseparated with GARIS were successfully extracted by the gas-jet system to a distant site where
the rotating wheel apparatus for α spectrometry was equipped.
The setting parameters such as the magnetic field of the separator and the gas-jet conditions were optimized to obtain thehighest yield of 255No.

*Corresponding author: haba@riken.jp© 2008 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2008

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Haba

J. Nucl. Radiochem. Sci., Vol. 9, No. 1, 2008(c) Gas-jet chamber(a) GARIS
Differential pumping section
SD or gas-jet
chamber at the
Evaporation
Residues(ERs)
from RILAC
Gas inlet focal plane
Beam dump

He/KCl22Ne beam

238

Mylar
window

38–92 kPa
Gas out

ERs

U rotatingtarget38 Pa
Elastic scattering
beam monitor

D1

Q1

Q2

0

1

Manometer

D2
0

50 mm

2m

(d) MANON(b) SD chamber

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

Mylarwindow

12-strip Si
detector
(SD)

ERs
38 Pa

0

Si PIN photodiodes

Mylar foil

50 mm

0.1 PaFigure 1. A schematic of the experimental setup: (a) RIKEN gas-filled recoil ion separator GARIS; (b) 12-strip Si detector (SD) chamber; (c)Gas-jet chamber; (d) Rotating wheel apparatus MANON for α spectrometry.

2. Experimental
A schematic of the experimental setup is shown in Figure 1.
The 22 Ne7+ ion beam was extracted from RILAC. A 238U3O8target of 370 µg cm–2 thickness was prepared by electrodeposition in 2-propanol onto a 1.27 mg cm –2 titanium backing foil.
The details of the target preparation are reported elsewhere.13
Sixteen targets were mounted on a rotating wheel of 30 cm indiameter. The wheel was rotated during the irradiation at 3000
rpm. The beam energy was 113.8 MeV at the middle of thetarget. At this incident energy, the excitation function for the
238
U(22Ne, 5n) 255No reaction exhibits the maximum cross section of 90 nb.14 The beam intensity was monitored by measuring elastically scattered projectiles with a Si PIN photodiode(Hamamatsu S1223) mounted at 45° with respect to the beamaxis. The typical beam intensity was 4 pµA. GARIS was
filled with helium at a pressure of 37 Pa. The other details of
GARIS are given elsewhere.15
As shown in Figures 1a and 1b, the evaporation residues ofinterest were separated in-flight from beam particles and transfer 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 foil which was supported with acircular-hole (4.0 mm diameter) grid with 71.6% transparency
and of 60 mm diameter. The thickness of the Mylar foil was
determined to be 1.1±0.1 µm based on the energy loss of 5.486MeV α particles from 241Am and on the stopping powers calculated with the SRIM code.16 The cycle of the beam-on (300 s)
and beam-off (600 s) measurements was performed, because
no α peaks of 255No were observed in the beam-on spectrum
due to large amounts of background events. The α-particle

energy resolutions of the 12 SDs were 20–60 keV FWHM. All
events were registered in an event-by-event mode using the
VME LIST/PHA module (Iwatsu A3100). The magnetic rigidities of 1.73, 1.82, 1.93, and 2.04 T m were examined to optimize the transport efficiency of GARIS for 255No.
In the gas-jet transport experiments, the reaction products
separated with GARIS were guided into the stainless-steel gasjet chamber of 60 mm depth as shown in Figure 1c. The magnetic rigidity was set at 1.93 T m. The 255 No atoms were
stopped in the helium gas, attached to aerosol particles generated by sublimation of the KCl powder, and were continuouslytransported through a Teflon capillary (1.59 mm i.d., 4 mlength) to the rotating wheel apparatus MANON for α spectrometry (Figure 1d) which was the compact one of the
Measurement system for the Alpha-particle and spontaneous
fissioN events ON-line developed at Japan Atomic Energy
Agency (JAEA).17 The temperature of the KCl aerosol generator was fixed to 620 °C based on the previous 169 Tm(40 Ar,3n) 206Fr experiment.12 The flow rates of the helium gas werevaried at 1.0, 2.0, 3.0, 4.0, and 5.0 L min –1, which resulted in
the inner pressures of the gas-jet chamber of 38, 54, 69, 81, and
92 kPa, respectively. In MANON, the aerosol particles were
deposited on 200-position Mylar foils of 0.68 µm thickness
placed at the periphery of a stainless steel wheel of 420 mm
diameter. The wheel was stepped at 90-s interval to position
the foils between seven pa i rs of Si PI N photodiodes(Hamamatsu S3204-09). Each detector had an active area of
18 × 18 mm 2 and a 38% counting efficiency for α particles.
The α-particle energy resolution was 60 keV FWHM for thedetectors which look at the sample from the collection side (topdetectors).

100

40
20
0
100

Counts / 100 s

60

8.376 213Po

10

2

10

1

10

0

0

200

400

Time / s

600

1.5

1.6

2.3

1.7

(b)

1.9

2.1

2.2

80

20

7

8

9

10

11

12

�-particle energy / MeV
Figure 2. (a) α-Particle spectra measured in the 12-strip Si detector(SD) under the beam-off condition for 36000 s. The 22Ne beam doseof 4.3×1017 was accumulated during the cycles of the beam-on (300 s)
and beam-off (600 s) measurements. The magnetic rigidity of
GARIS was set at 1.93 T m. The inset shows a decay curve of the
7.620–8.312-MeV α peaks of 255 No. (b) Background α-particle
spectrum measured with SD for 82000 s before the beam experiment.

C o u nt s / 4 0 k e V

60

0

50
40
30
20
10

As mentioned in the previous section, no α peaks were identified in the α spectrum measured with SD under the beam-on
condition (the total count rate of all strips of SD at > 1 MeV isabout 2 k counts per second for the 1-pµA beam intensity).
Figure 2a shows a sum of α-particle spectra measured in all the
strips of SD for 36000 s under the beam-off condition. The
beam dose of 4.29×1017 was accumulated during the cycles ofthe beam-on (300 s) and beam-off (600 s) measurements. The
magnetic rigidity of GARIS was 1.93 T m. As indicated in
Figure 2a, α peaks of 255No were identified in the energy regionof interest (E α = 7.620–8.312 MeV),18 though many other α
peaks are seen in the spectrum. Compared in Figure 2b is abackground spectrum measured with SD for 82000 s before thebeam experiment. Since SD was contaminated with long-lived225
Ac (T1/ 2 = 10.0 d) and 227 Th (T1/ 2 = 18.72 d) which were
implanted into SD as transfer reaction products from 232Th inthe previous experiment, α lines of their daughter nuclides of
221
Fr, 211Bi, 219Rn, 217At, 215Po, and 213Po are clearly identified.
Thus, the α peaks in Figure 2a except those for 255No are unambiguously assigned as the background components, thoughsmall contributions of the daughter nuclides of 255 No, 255 Md(T1/2 = 27 min, E α = 7.326 MeV) and 255Fm (T1/2 = 20.07 h, E α =7.016 MeV), are seen in the spectrum. The decay curve of the
7.620–8.312 MeV α peaks of 255 No is shown in the inset of
Figure 2a. The half-life of 255No was determined to be 3.8±0.7min, which is in agreement with the literature value of 3.1±0.2

10

2

10

1

10

0

0

200

400

Time / s

600

06

3. Results and Discussion

Counts / 90 s

70

6

2.0

Figure 3. Yield variation of 255No produced in the 238U(22Ne, 5n) 255No
reaction as a function of the magnetic rigidity of GARIS. The dashedcurve represents the result of the least-squares fitting with theGaussian curve with a maximum yield at Bρ = 1.89±0.02 T m and a
resolution of ∆Bρ/Bρ = 12±2%.

8.376 213Po

40

7. 6 20 - 8. 31 2

6.341 221Fr
6.623 211Bi
6.819 219Rn
7 . 0 6 7 21 7A t
7.386 215Po

60

1.8

Magnetic rigidity, B� / T m

80

Counts / 40 keV

29

7.620-8.312 255No

Counts / 40 keV

80

7.620-8.312 255No

6.341 221Fr
6.623 211Bi
6.819 219Rn
7.016 255Fm, 7.067 217At
7.326 255Md, 7.386 215Po(a)

J. Nucl. Radiochem. Sci., Vol. 9, No. 1, 2008

Yield / arbitrary unit

Performance of the Gas-jet Transport System Coupled to the RIKEN

7

8

9

10

11

12

�-particle energy / MeV
Figure 4. Sum of α-particle spectra measured in the seven top
detectors of the rotating wheel apparatus MANON for 630 s after the
90-s aerosol collection. The 90-s aerosol collection was repeated 207
times. The beam dose of 2.8×1017 was accumulated. The helium
flow rate was 1.0 L min –1 and the inner pressure of the gas-jet
chamber was 38 kPa.

min.18 The decay curve also suggests that the contribution of
8.093-MeV α peak of 254No (T1/2 = 55 s), which is producible inthe 238U(22Ne, 6n) 254No reaction, is negligible within the error
limit.
In Figure 3, relative yields of 255No are shown as a functionof the magnetic rigidity (Bρ) of the separator. As shown by adashed curve in Figure 3, a least-squares fitting with theGaussian curve gives a maximum yield at Bρ = 1.89±0.02 T m,
where the transport efficiency of GARIS was evaluated to beabout 5% for the focal plane of 60 mm diameter, assuming thecross section of the 238U(22Ne, 5n) 255No reaction to be 90 nb.14
The resolution of ∆Bρ/Bρ = 12±2% suggests that the transport
efficiency of GARIS would be increased by a factor of about 2
using the larger focal plane window of 100 mm diameter.
Figure 4 shows the sum of α-particle spectra measured in
the seven top detectors of MANON. In this measurement, thebeam dose of 2.81×1017 was accumulated. The 7.620–8.312MeV α peaks of 255No are clearly seen in the spectrum, indicating that the gas-jet transport of 2 55 No to MANON was
successfully conducted after the physical separation with

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J. Nucl. Radiochem. Sci., Vol. 9, No. 1, 2008
255

ciency of No is achieved at the lowest chamber pressure of
38 kPa. This enables us to safely handle the thinner Mylar
windows of < 1 µm thickness that should be required for SHE
nuclides such as 261Rf and 262 Db produced in the 248 Cm(18 O,5n)261Rf and 248Cm(19F, 5n)262Db reactions, respectively.

Gas-jet transport efficien cy / %100
80
38 kPa
54 kPa60

81 kPa
69 kPa4. Summary

92 kPa

40
20
0
0.0

1.0

2.0

3.0

4.0

5.0

6.0

-1

Helium flow rate / L min

Figure 5. Variation of the gas-jet transport efficiency of 255No as afunction of the helium flow rate. The inner pressures of the gas-jet
chamber are indicated for each data point.

GARIS. Background radioactivities such as 213Po (8.376-MeVα) and 212m Po (11.650-MeV α), which are largely produced in
the transfer reactions on the lead impurity in the target,19 are
fully removed by the present system. The decay curve for the
7.620–8.312-MeV component is shown in the inset of Figure
4a. The half-life was determined to be 3.4±0.8 min. Since
MANON was placed in the target room in this experiment,
some background events caused by large amounts of neutrons
during the irradiation are seen in the spectrum. Recently, we
have constructed a chemistry laboratory, which is isolated witha 50-cm concrete shield from the target room, just behind the
focal plane of GARIS. In the future, this kind of experimentwill be conducted under improved background conditions.
In the conventional gas-jet system in that the beam passes inthe target chamber, the gas-jet efficiency decreases due to theincreasing plasma induced by the beam. As an example, wemeasured the gas-jet efficiencies of 173 W produced in thenat
Gd( 22 Ne, xn) reaction without the beam separation by
GARIS.14 It was found that the gas-jet efficiency of 173W drastically decreases from 40% at 6.6 pnA to 25% at 0.5 pµA. In
Figure 5, the gas-jet transport efficiencies of 255No are shown
as a function of the helium flow rate. In this work, the highgas-jet efficiencies of about 75% were achieved even at > 1
pµA owing to the plasma-free condition. It is also found in
Figure 5 that the gas-jet efficiencies are independent of thehelium flow rate, i.e., the inner pressure of the gas-jet chamber,though a slight decreasing trend is seen. The recoil ranges of
255
No in helium at 38–90 kPa are calculated to be 16.0–6.6mm, respectively, using the LISE++ code. 20 These recoil
ranges are short enough as compared with the depth of the gasjet chamber (60 mm). In our previous study,12 the high gas-jet
efficiencies of over 80% were obtained both for 206 Fr and245
Fm. The recoil ranges of 206Fr and 245Fm in helium at 90 kPa
were 30 and 18 mm, respectively. In the conventional gas-jet
system, where a beam dump is placed in the bottom of thechamber, the gas is swept out through the capillary outlet to thevertical direction of the beam axis. Therefore, the position of
the capillary outlet in the chamber should be exactly adjusted
to the recoil ranges of the product nuclei to obtain their highest
gas-jet yields. In the present system, the beam is separated
with GARIS and hence we can put the capillary outlet in thebottom of the chamber (see Figure 1c). Thus, the gas is fed
into the chamber through the four inlets directed to the surfaceof the Mylar window and is swept out thoroughly from the bottom of the chamber. This helium flow path in the gas-jet
chamber would be advantageous to the range-independent gasjet efficiencies. It is finally pointed out that the highest effi-

We have successfully produced 255No in the hot fusion reaction of 238U(22Ne, 5n) 255No using the gas-jet transport system
coupled to GARIS. The α particles of 255 No separated withGARIS and transported by the gas-jet were clearly observedwith a rotating wheel apparatus for α spectrometry. The highgas-jet efficiencies of about 75% were obtained at the beam
intensities of over 1 pµA, and they were independent of therecoil ranges of 255 No in the gas-jet chamber. These results
suggest that the GARIS/gas-jet system is promising to explore
new frontiers in SHE chemistry: (i) the background radioactivities originating from unwanted reaction products are strongly
suppressed, (ii) the intense primary heavy-ion beam is absent
in the gas-jet chamber and hence the high gas-jet efficiency isachieved, and (iii) the beam-free condition also makes it possible to investigate new chemical systems that were not accessible before. In the next phase, we plan to investigate theproduction of SHE nuclides with long half-lives for chemical
experiments such as 261Rf, 262Db, 265Sg, and 269Hs based on the248
Cm target.
Acknowledgement. The authors express their gratitude to thecrew of the RIKEN Linear Accelerator for their invaluableassistance in the course of these experiments. This research
was partially supported by the Ministry of Education, Science,Sports and Culture, Grant-in-Aid for Young Scientists (B),16750055, 2004–2006 and (B), 17740179, 2005–2006.
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