The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

CHARGE STRIPPING SCHEME IN THE RIKEN RI-BEAM FACTORY
H . R y u t o , N . I n a b e ，N . F u k u n i s h i , H . H a s e b e ' M . K a s e a n d Y . Y a n o
C y c l o t r o n C e n t e r ，R I K E N , 2 - 1 H i r o s a w a ，W a k o , J a p a nA b stra c t
A charge stripping scheme in the RIKEN Rl-beamfactory is described. Four stripper sections in the RIKEN
Rl-beam factory at five typical kinds of energies for theintense heavy-ion beams are under investigation. Anoverview of these four stripper sections is presented.
Charge-state distributions of 136Xe stripped by carbon
foils and polymer films measured at 11 MeV/nucleon
and 39 MeV/nucleon are also presented.
IN T R O D U C T IO N

The RIKEN Rl-beam factory (RIBF) is an accelerator
complex for the acceleration of heavy-ion beams up touranium to a final energy of 350 MeV/nucleon with anobject of producing a great variety of Rl-beam [1]. Insuch heavy-ion accelerators that form the RIBF, a charge
stripper plays an essential role because high charge-statesallow small machines that require low construction costs
or allow extended application of existing accelerators.
In order to design accelerators or plan acceleration
schemes,
accurate predictions of charge-state
distributions for heavy-ions behind the charge strippers
are indispensable, because i) the design of magnets andRF systems depend wholly on the charge-state of the ion,ii) the beam current on target is roughly proportional tothe product of the charge-state fractions behind thestrippers, iii) the emittance superfluously blows up if we
use unnecessarily thick stripper foils. A number ofexperiments to measure the equilibrium charge-states ofvarious ions at various energies were performed fordecades. By fitting these data several semi-empiricalformulae were made (e.g., refs. [2-6]). Semi-empiricalformulae, which provide equilibrium carbon foil or gas
thicknesses, were also developed [7, 8]. In the last half
decade of twentieth century, numerical calculation codes
GLOBAL [9] and ETACHA [10] which calculate pre­equilibrium charge-state distributions were developed.
However, the predictions of the formulae and the codes
vary significantly.
1st STRIPPER丨丨，■_『丨M

RILAC
ION SOURCE— — —|ベ

4th STRIPPER
RRC )…11 1 j i (^ ^IRC j—|SRCV

ノCSM

J, [ fRC }\ ----------- ----ノ 3rd STRIPPER

2nd STRIPPER VFigure 1:Schematic of RIBF.
Present plan for the charge-stripping scheme of theRIBF investigated using experimental data, semiempirical formulae and calculation codes are presented.
*SHI Accelerator Service, Ltd.

Some experimental data on pre-equilibrium charge-state
distributions are also presented comparing withcalculations.
S T R IP P IN G S C H E M EFigure 1 shows the schematic of the RIBF. In the RIBF,
four stripper sections are under investigation at present.
The first section is located between the accelerator anddecelerator of the charge-state multiplier (CSM) [11].
The second section is located between the RIKEN ring
cyclotron (RRC) and the fixed-frequency ring cyclotron(fRC). The third section is located between the fRC and
the intermediate-stage ring cyclotron (IRC). The fourth
section is located between the RRC and the IRC.
Uranium and xenon beams at five kinds of energies are
considered as typical beams that incident on strippers.
Three energies out of five energies correspond to thecase that the beam is accelerated by all the cyclotrons,
the RRC, the fRC, the IRC and the superconducting ring
cyclotron (SRC) to 350 MeV/nucleon using strippers at
the first, second and third stripper sections. Remaining
two energies correspond to the case that the beam isaccelerated to 276 MeV/nucleon without the fRC using
strippers at first and fourth stripper sections. The firststripper is not used when an ion source supplies asufficiently intense ion beam at the required charge-state.
In the following discussion, the incident beam currentson the strippers are estimated as to achieve a 1 p|iiA
uranium beam and 1.5 pjiA xenon beam at 350
MeV/nucleon, and a 0.2 p\iA xenon beam at 276
MeV/nucleon on target. The beam loss caused by othersources than charge strippers are neglected here. The
parameters of the strippers for the RIBF underconsideration are tabulated in Table 1 .An overview ofthese four stripper sections is presented in the following
subsections.
F irst Stripper Section
The current of 238U35+ beam behind the first stripper inthe case with the fRC is expected to be 15 pjnA in orderto achieve 1 pfiA at the exit of the SRC. The energy of
the incident uranium beam is 0.9 MeV/nucleon.
According to the table of equilibrium charge-state
fractions in ref. [4], the incident beam current should be
as nigh as 90 pjnA. Similarly, the incident xenon beam
current in the case without the fRC is 10 pjiA. The
distinctive features of the first stripper section are thus
the high beam intensity and the low energies. Bothfeatures cause the short lifetime of carbon foils that are
most commonly used charge strippers. Some semiempirical formulae for carbon foil lifetime were made byfitting rather scattered experimental data [7,12,13]. For
example, when a 90 pjnA uranium beam at 0.9

The N th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Table 1:Parameters of the strippers for the RIBF.
with the fRC
without the fRC
Ion
136Xe
238u
135Xe
3rd
3rd
2nd
2nd
Stripper section
4th
1st
1stEnergy (MeV/nucleon)
0.9
1146
1146
2.9
39
Required charge-state
72+
35+
42+
51+
38+
45+
88+
Thickness (mg/cm2)
C foil 0.025 [7] 0.5 [9]0.061[7] 0.3 [9]14 [9]
0.15
20 [9]
Expecting charge-state C foil
36+
72+
44+
52+
38+
45+
88+
Fraction
C foil 17% [4] 19% [9]34% [9]52% [9]9% [4]
26% [9]30%
Thickness (mg/cm2)
N gas
0.08 [8]0.09 [8],14 [9] 0.03 [8] 0.09 [8]Expecting charge-state N gas
52+
51+
88+
43+
Fraction
N gas
28% [3]2% [3], 30% [9] 20% [3] 30% [3]MeV/nucleon is focused on a carbon foil with 5-mmcarbon plate, so again a rotating stripper is indispensable.
diameter circular shape, using Livingston’s formula [12] At the same time, a relatively large energy loss struggling
it is predicted that the lifetime of the carbon foil is about and multiple scattering are caused by such a thick stripper.
20 seconds. A rotating carbon foil may have a greater A rotating carbon disk stripper may cause anotherlifetime because it decreases the energy density deposited problem. If the thickness of the carbon disk varies 10%,to the carbon foil by the beam. For example, when we the energy of the beam downstream oscillates about 0.8%
bombard a 5_mm-diameter beam at the spot between 15 in magnitude. The requirement to the uniformity of themm and 20 mm radiuses of the rotating carbon foil, the carbon disk thickness is closely related to the runningenergy density deposited to the carbon foil by the beam is costs of the strippers at this section. A rotating carbon disk
1/28 of that of still carbon foil. Another possible method stripper is under development now.
of improving the resistance to the beam for the xenon
The possibility to use other solid strippers thinner thancase is to use a liquid film stripper [14] because the carbon stripper foils was also examined by the GLOBAL
temperature of the carbon foil is well below the calculation. According to the calculation, for example, weevaporating temperature, thus breakage is not expected to can obtain 238U83+ at 46 MeV/nucleon with 18% fraction
be caused by the heat [15]. When a xenon beam is by a 1 mg/cm2 thick tantalum foil receiving a lower
accelerated to 350 MeV/nucleon with the fRC, the first amount of heat deposit than a 14 mg/cm2 thick carbon
stripper is not required because a sufficiently intense plate. However, it is necessary to use another stripper
xenon beam of the required charge-state is supplied from before injecting into the SRC because the SRC requires
the ion source.
88+. Multiplying the fractions at the strippers downstreamof the fRC and the IRC, we obtain one-sixth fraction ofS e c o n d Stripper Section
the case stripped by a 14 mg/cm2 thick carbon plate
Behind the second stripper section, 3 pjnA 238U72+ beam downstream of the fRC, so we will not discuss the otherand 136Xe42+ beam at 11 MeV/nucleon are expected. In the materials further here.
uranium case an experimental data [9] exists, that theThe problem caused by the intense heat may be solved
equilibrium charge-state at 11.4 MeV/nucleon stripped by if we would use a gas stripper. We can obtain xenon ions
a 0.49 mg/cm2 thick carbon foil is 73+ with a fraction of at a sufficiently high charge-state using a gas stripper
19%. When a 15 pfiA uranium beam is stripped by a 0.5 according to the semi-empirical formulae by Sayer andmg/cm2 thick carbon fo il,1 kW power is deposited to the Schiwietz et al. However, it is a delicate situation in thefoil. Again, a rotating carbon foil is expected to facilitate uranium case. The predicted equilibrium charges are 86+
the situation. We cannot use a gas stripper for uranium by Schiwietz's formula and 88+ by the GLOBAL
here in order to obtain a sufficiently high charge-state that calculation, but 84+ by Sayer’s formula, so the fractions
can be accelerated by the fRC. On the other hand, a semi- of 88+，the required charge-state by the SRC, are fifteen
empirical formula [3] predicts a 20% fraction of 136Xe43+.
times different as 30% by the GLOBAL calculationcompared with 2% by Sayer’s formula. An experiment
Third Stripper Section
with a uranium beam will clarify the uncertainty, but weBehind the third (and last) stripper section,1 p|j,A cannot obtain a uranium beam at present.
238U88+ and 1.5 pjnA 136Xe51+ at 46 MeV/nucleon areexpected as same as the currents at the exit of the SRC. F ourth Stripper Section
The GLOBAL calculation predicts that 34% and 52%
At the fourth stripper section, a 0.2 p|a,A 136Xe45+ beam
fractions of 238U88+ and 136Xe52+ can be obtained by 14 at 39 MeV/nucleon is expected. The thickness of themg/cm2 and 20 mg/cm2 thick carbon plates, respectively.
stripper at the fourth section is required to be thin in order
Such a thick stripper causes a serious energy loss of the to avoid a large energy loss aiming at the incident energyions. The fRC accelerate to a higher energy than the range of the IRC. The thickness of the carbon stripper foil
injection energy of the IRC taking the energy loss caused will be thus 0.3 mg/cm2, at which thickness the fraction ofby the third stripper into consideration. About 8% of xenon ions at 45+ is maximized (see Figure 3). Theenergies, 30 kW in the uranium case，are deposited to the energy loss is about 0.2%. Another solution to obtain a

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

higher charge than 45+ without suffering a large energy
loss is the use of a gas stripper.

'0 .0 50 .100 .2 00 .5 01.00

thi ckness ( m g / c m 2)

Figure 2: Charge-state fractions of 136Xe at 11MeV/nucleon stripped by carbon foils, aramid films, anda polyimide film. Horizontal and vertical axes indicate thethickness of the strippers and the charge-state fractions,respectively. Numbers joined with solid lines indicate thelowest digit of measured charge-state numbers between
42+ and 50+. The charge-state fractions are normalized
by the area of the Gaussian fitted to the measured chargestate distribution. Dashed lines indicate the calculation byETACHA.

sections respectively, were measured with strippers ofvarious thicknesses. The xenon beam was bombarded oncarbon foils, aramid films, and a polyimide film withthicknesses from 40 |ig/cnT to 7 mg/cm' Figure 2 shows
the charge-state fractions of 136Xe at 11 MeV/nucleon. We
can use, for example, a 0.15 mg/cm2 thick foil to obtain
136Xe44+ with 30% fraction. The ETACHA calculation is
also drawn on the figure, however the calculation rather
poorly reproduced the data. The GLOBAL code does notcover this energy range. Figure 3 shows the charge-state
fractions of 136Xe at 39 MeV/nucleon. Solid and dashedlines indicate the GLOBAL and ETACHA calculations,respectively. Comparing the calculations to the data, theGLOBAL calculation rather well reproduces the data thanthe ETACHA calculation. According to the GLOBAL
calculation, we can get 136Xe45+ by a 0.3 mg/cm" thickcarbon foil with a 26% fraction.
C O N C L U S IO NA charge stripping scheme in the RIKEN Rl-beamfactory is investigated. Due to the highness of the beam
intensity, rotating strippers are indispensable. It is likely
to be effective to strip xenon ions by a gas stripper.
Charge-state distributions of 136Xe stripped by carbonfoils and polymer films were measured at 11
MeV/nucleon and 39 MeV/nucleon.
REFERENCES[ 1 ] Y. Yano et al., Proc. of the 13th Sympo. onAccelerator Science and Technology, Osaka, 20(2001).

フン、ノ ゾ メ CXグ、
X W V 、
\ >o<0.10 .20 .5

1.0

2 .0

5 .0

10.0

十hick.ness ( m g / c m 2)
Figure 3: Charge-state fractions of 136Xe at 39
MeV/nucleon stripped by carbon foils, aramid films, anda polyimide film. The definition of the axes is the same as
Figure 2. Numbers without line indicate the lowest digitof measured charge-state numbers between 47+ and 53+.
The charge-state fractions are normalized by the area ofthe Gaussian fitted to the measured charge-state
distribution. Solid and dashed lines indicate thecalculations by GLOBAL and ETACHA, respectively.
M E A S U R E M E N T O F C H A R G E -S T A T E
D IS T R IB U T IO N S

The charge-state distributions of 136Xe at 11
MeV/nucleon and 39 MeV/nucleon, which correspond tothe incident energy of the second and fourth stripper[2] H.D. Betz, Rev. of Mod. Phys. 44,465 (1972).
[3] R.O. Sayer, Rev. Phys. Appl.12 ,1543 (1977).
[4] K. Shima et al., At. Data Nucl. Data Tables 51,173(1992).
[5] A. Leon et al., At. Data Nucl. Data Tables 69, 217(1998).
[6] G. Schiwietz, P.L. Glande, Nucl. Instrum. Meth.
B175-177,125 (2001).
[7] E. Baron, IEEE Trans. Nucl. Sci. NS-26, 2411 (1979).
[8] M.A. McMahan, Handbook of Accelerator Physics
and Engineering, ed. by A.W. Chao and M. Tigner(World Scientific, 1999).
[9] C. Scheidenberger et al., Nucl. Instrum. Methods
B142, 441(1998).
[10] J.P. Rozet et al., Nucl. Instrum. Methods B107, 67(1996).
[ 11 ]O. Kamigaito et al., Proc. of the 12th Sympo. on
Accelerator Science and Technology, Wako, 233(1999).
[12] A.E. Livingston et al., Nucl. Instrum. Methods 148,125 (1978).
[13] F. Nickel, Nucl. Instrum. Methods 195, 457 (1982)。
[14] H. Akiyoshi et al., Proc. of the 13th Sympo. on
Accelerator Science and Technology, Osaka, 331(2001 ).

[15] G. Dollinger and P. Maier-Komor, Nucl. Instrum.
Methods A282, 223 (1989).