- Email: [email protected]
Charge-state distribution measurements using gas charge stripper toward 238U and 136Xe acceleration at RIKEN RIBF
Nuclear Instruments and Methods in Physics Research A 655 (2011) 17–20
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Charge-state distribution measurements using gas charge stripper toward 238 U and 136Xe acceleration at RIKEN RIBF H. Kuboki , H. Okuno, S. Yokouchi, H. Hasebe, T. Kishida, N. Fukunishi, O. Kamigaito, A. Goto, M. Kase, Y. Yano RIKEN Nishina Center, Wako, Saitama 351-0198, Japan
a r t i c l e i n f o
abstract
Available online 21 June 2011
The charge-state distributions of 238U and 136Xe ions at 11 MeV/nucleon were measured using a gas charge stripper to test whether the charge states in gaseous media were acceptable for acceleration at the RIKEN RI-Beam Factory (RIBF). A differential pumping system facilitated the increase of the N2 gas thickness to 1.3 mg/cm2; this is sufficient for the most probable charge state to attain equilibrium. The charge states of 238U attain equilibrium at 56.0, 56.6, and 55.7 in N2, Ar, and CO2 media with thicknesses of 125, 79, and 126 mg=cm2 , respectively, whereas those of 136Xe attain equilibrium at 40.5, 40.1, and 40.3 in N2, Ar, CO2 media with thicknesses of 163, 95, and 139 mg=cm2 , respectively. The equilibrium charge states of 136Xe are acceptable for acceleration by the subsequent cyclotron, whereas those of 238 U are not acceptable for acceleration without a major remodeling of the accelerator. & 2011 Elsevier B.V. All rights reserved.
Keywords: Uranium beam Xenon beam Equilibrium charge distribution Charge stripper Gas target
1. Introduction Charge strippers play an essential role in a heavy-ion accelerator complex because a high-charge state enables the acceleration of heavy-ion beams to a high energy with small accelerators. The RIKEN RI-beam factory (RIBF) is one such heavy-ion accelerator complex [1]. Heavy ions such as uranium (238U) and xenon (136Xe) are successively accelerated to a final energy of 345 MeV/nucleon using one linear accelerator and four cyclotrons [2]. Two charge stripper sections are placed downstream of the first two cyclotrons, namely, the RIKEN ring cyclotron (RRC) [3] and a fixed-frequency ring cyclotron (fRC) [4]. The energies of the 238U and 136Xe ions incident on each stripper are 11 and 51 MeV/nucleon, respectively. The initial charge states of 238U and 136Xe are 35þ and 20 þ, respectively. Because the lowest acceptable charge states of 238U and 136Xe for fRC acceleration are 71 þ and 41þ, respectively, carbon foils with a thickness of 300 mg=cm2 are used as the first charge stripper for enhancing the charge states of 238U and 136Xe up to 71þ and 47 þ, respectively. Carbon foils with thicknesses of 17 mg/cm2 for 238U and 20 mg/cm2 for 136Xe are used as the second charge stripper to obtain U86 þ and Xe52 þ . It should be noted that there exist two serious problems with the first charge stripper. One is its finite lifetime and the other is the energy spread owing to the nonuniformity of the foil thickness [5]. The carbon charge stripper has a lifetime of 10–12 h for 238U beam
Corresponding author. Tel.: þ 81 48 462 1503; fax: þ81 48 462 1629.
E-mail address: [email protected] (H. Kuboki). 0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.06.012
irradiation with a beam intensity of 14 pnA (0.5 W); this intensity is only one-hundredth of the desired beam intensity. An experiment involving 136Xe beam irradiation for 6 h at an intensity of 1 pmA ð18 WÞ showed that the lifetime of carbon foils subjected to 136 Xe beam irradiation is longer than that of carbon foils subjected to 238U beam irradiation. However, the lifetime is expected to drastically decrease when the irradiated beam intensity is increased by a factor of hundred using a newly developed ion source [6] and a new injector [7,8] in the near future. It is believed that a charge stripper based on gaseous materials will function better as it would have a uniform thickness and it would be able to withstand a high heat load. However, a gas charge stripper has a disadvantage: the charge state of 238U becomes lower than 71þ owing to the density-effect [9–13]. In the case of gas charge strippers, excited projectiles have sufficient time between collisions to return to their ground states before subsequent collisions. Because the ionization cross-section of projectiles in excited states is larger than that in ground states, the charge states obtained with a gas charge stripper are lower than those obtained with carbon foil strippers. Moreover, it is difficult to predict the charge states obtained with a gas charge stripper, because the data of charge states in gaseous media are insufficient as compared to those in solid materials. The available data for a uranium beam in gases are mostly for energies below 0.1 MeV/nucleon and are hence of little importance at the desired energy of 11 MeV/nucleon [2]. In this study, we developed a windowless gas charge stripper whose thickness is sufficient for attaining equilibrium charge states. Then we measured the charge-state distributions of 238U
18
H. Kuboki et al. / Nuclear Instruments and Methods in Physics Research A 655 (2011) 17–20
aperture 6mmφ
aperture 6mmφ
stage 6-2
8mmφ
stage 4
P4
10mmφ
stage 3
P3
stage 2
P2
stage 1
stage 5-2
stage 5-1
P1
P5-2
P5-1
beam stage 6-1
14cm
TMP3-2 220 L/s
TMP2-1 350 L/s
TMP2-2 550 L/s
TMP3-1 220 L/s
MBP1 MBP2 3 TMP1 600 m3/h1020 m /h 1450 L/s Fig. 1. Schematic diagram of gas charge stripper with its differential pumping system [2,14]. Gases are injected in the target cell (stage 1) located at the center. The length of the target cell is 14 cm. Other stages are also shown along with the pumping speeds of their respective attached pumps. The beam passes through four apertures (6 mm in diameter) to enter the target cell. The exits of the target cell and stage 2 are also 6-mm-diameter apertures. The exits of stages 3 and 4 are 8-mm and 10-mm-diameter apertures, respectively.
MBP2
Gas stripper chamber MBP1
Mechanical booster pumps (MBPs) Fig. 2. Overview of gas charge stripper system. The gas charge stripper chamber and two mechanical booster pumps (MBP1 and MBP2) are shown in the photograph.
and 136Xe at 11 MeV/nucleon up to the equilibrium state to test whether the equilibrium charge state is acceptable for acceleration by the fRC [2].
P2−P5, beamline (Pa)
103 102
P2
101
P3
100 10−1 10−2
P4 P5-2
vacuum limit (4x10−3)
P5-1
10−3
beamline2
10−4
beamline1 1
2
3
4
5
6
7 8 9 10
P1 (kPa) Fig. 3. Pressure at each stage (P2–P5) plotted as a function of P1 [2]. Stage 5 is divided into two separate stages, stages 5-1 and 5-2, corresponding to the areas upstream and downstream of stage 4, respectively. The pressures at stages 5-1 and 5-2 are denoted by P5-1 (blue) and P5-2 (green), respectively. The solid line indicates the limit for an interlock (4 10 3 Pa). The pressures at stages 6-1 and 6-2 are denoted by ‘‘beamline1’’ and ‘‘beamline2’’, respectively, and these are maintained lower than the interlock limit if P1 is lower than 7.7 kPa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2. Gas charge stripper system 2.1. System overview We have modified a windowless gas target [14] into a gas charge stripper system, as described below [2]. Fig. 1 shows the schematic diagram of this system along with its differential pumping system. The region in the target cell is defined as stage 1. The physical length of the target cell is 14 cm. The region outside the target cell is defined as stage 2. Similarly, the regions outside stages 2–4 are defined as stages 3–5, respectively. Stage 5 is divided into stages 5-1 and 5-2. The pressure at stages 1–5 is indicated by P1–P5, respectively. Because the gas charge stripper is operated along a beamline, the pressures P2–P5 should decrease as rapidly as possible while maintaining sufficiently high pressure P1. Stages 6-1 and 6-2, corresponding to the beamline upstream and downstream of stages 5-1 and 5-2, respectively, were newly added in order to maintain the pressures at the beamline such
that they were lower than the admissible value. Fig. 2 shows a photograph of the gas charge stripper installed in the beamline. 2.2. Offline pressure test We performed an offline test to determine the achievable pressure in the target cell with nitrogen (N2) gas injection [2]. Fig. 3 shows the results for pressures P2–P5 and the pressures at the beamline plotted against P1. These results indicate that P1 can be increased to 7.7 kPa (1.3 mg/cm2) while maintaining beamline pressures lower than 4 10 3 Pa, which is the allowable vacuum limit for beamlines. The most probable charge state attains equilibrium at a thickness of 500 mg=cm2 in the case of carbon foils for 238U at 11 MeV/nucleon [15]. With the assumption that the N2 gas thickness necessary for equilibrium is the same as the carbon foil thickness, we consider that a N2 gas stripper with its thickness
H. Kuboki et al. / Nuclear Instruments and Methods in Physics Research A 655 (2011) 17–20
adjustable up to 1.3 mg/cm2 is sufficient for attaining equilibrium charge states.
19
48μg/cm2 84 150 334 752
0.16 0.12 0.08
3. Experiment
0.04 0 0.16 Fraction
The experiments were performed at RIKEN Accelerator Research Facility [2]. Fig. 4 shows an overview of the beamline around the gas charge stripper. 238U and 136Xe beams at 11 MeV/nucleon were transported to the gas charge stripper located upstream of the fRC. The incident beam intensities were measured using a Faraday cup (FC-D16) placed upstream of the gas charge stripper to be 3002500 enA ð9214 pnAÞ and 5002600 enA ð25230 pnAÞ for 238U and 136Xe, respectively. The spot size was approximately 6 mm in diameter; this was comparable to the aperture at the entrance of the gas charge stripper. The charge state was analyzed using dipole magnets DAD4 and DMD4. The magnetic fields of these magnets were corrected by considering the energy loss in the gas calculated by ATIMA in LISEþþ [16]. The intensities of the stripped beams were measured using a Faraday cup (FC-F41) downstream of DMD4.
0.12 0.08 0.04 0 0.16
0.08 0.04
238
U charge-state distribution
Gas charge stripper
Beam from RRC
38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 Charge states
Fig. 5. Charge-state distributions of 238U beam at 11 MeV/nucleon [2]. The data of fractions calculated for (a) N2, (b) Ar, and (c) CO2 are plotted. (a) The data for the N2 gas medium with thicknesses of 48, 84, 150, 334, and 752 mg=cm2 are represented by asterisks, open triangles, open circles, open squares, and open diamonds, respectively. (b) The data for the Ar gas medium with thicknesses of 39, 106, 205, and 474 mg=cm2 are represented by open triangles, open circles, open squares, and open diamonds, respectively. (c) The data for the CO2 gas medium with thicknesses of 32, 61, 122, 225, and 516 mg=cm2 are represented by asterisks, open triangles, open circles, open squares, and open diamonds, respectively.
58 56 Most probable charge state
The charge-state distributions of 238U measured with N2, Ar, and CO2 gases at different pressures P1 are shown in Fig. 5. It was found that the most probable charge states in the case of N2, Ar, and CO2 attain equilibrium at 56.0, 56.6, and 55.7, respectively [2]. The equilibrium charge states of 238U are currently unacceptable for acceleration by the fRC, since the lowest acceptable value is 71þ . Fig. 6 shows the most probable charge states plotted as a function of the gas thickness. The thickness of the gas medium was estimated by the energy loss measured by using an alpha source and a silicon detector [2]. The data were fitted by a function f ðtÞ ¼ ab expðctÞ, where a, b, and c are fitting parameters. The equilibrium value of the most probable charge state is denoted by a. Parameter b is related to the thickness required for equilibrium. Parameter c indicates the slope of the curve before equilibrium. The obtained parameters are listed in Table 1. The thicknesses necessary for equilibrium, as calculated by f(t), are 125, 79, and 126 mg=cm2 for N2, Ar, and CO2, respectively [2]. Here, the thickness required for equilibrium is defined as the thickness required for attaining 99% of the equilibrium charge
32μg/cm2 61 122 225 516
0.12
0 3.1.
39μg/cm2 106 205 474
U
54 52 50
N2 Ar CO2
48 46 44 42 40
5
DAD4 Faraday cup D16 (FC-D16)
DMD4
Faraday cup F41 (FC-F41)
fRC
Fig. 4. Schematic view of gas charge stripper. The beam intensity upstream of the gas charge stripper was measured using a Faraday cup (FC-D16). Stripped beams were analyzed using a pair of dipole magnets, DAD4 and DMD4. The beam intensity downstream of DAD4 and DMD4 was measured using a Faraday cup (FC-F41).
50 100 20 Thickness (μg/cm2)
10
200
500
1000
Fig. 6. Most probable charge states of 238U beam at 11 MeV/nucleon plotted as a function of gas thickness [2]. The data for N2, Ar, and CO2 are represented by solid circles, solid triangles, and solid squares, respectively. Fitting functions are represented by solid curves.
Table 1 Fitting parameters of f(t) for 238U beam at 11 MeV/nucleon together with thickness required for equilibrium [2]. The equilibrium value of the most probable charge state is denoted by a. Parameter b is related to the thickness required for equilibrium. Parameter c indicates the slope of the curve before equilibrium. Gas
a
b
c
Equilibrium thickness ðmg=cm2 Þ
N2 Ar CO2
55.98 56.62 55.85
16.73 14.26 16.36
0.027 0.041 0.027
124.92 79.28 125.83
20
H. Kuboki et al. / Nuclear Instruments and Methods in Physics Research A 655 (2011) 17–20
Table 2 Fitting parameters of f(t) and equilibrium thickness for 136Xe beam at 11 MeV/nucleon [2]. The notations are the same as those used in Table 1.
47μg/cm2 47 85 150 336 752
0.20 0.15 0.10 0.05 0
27μg/cm2 94 196 462
Fraction
0.20 0.15 0.10 0
5μg/cm2 11 50 111 217 506
0.20 0.15 0.10 0.05
20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 Charge states
42
Most probable charge state
Xe
38 36 34 N2 Ar CO2
32 30 28 26 24 22
5
10
20
50
100
200
500
1000
Thickness (μg/cm2) Fig. 8. Most probable charge states of 136Xe plotted as a function of gas thickness [2]. The data for N2, Ar, and CO2 are represented by solid circles, solid triangles, and solid squares, respectively. Fitting functions are represented by solid curves.
state. The above-mentioned calculated values are considerably less than the carbon foil thickness ð 500 mg=cm2 Þ. 3.2.
136
b
c
Equilibrium thickness ðmg=cm2 Þ
N2 Ar CO2
40.56 40.08 40.26
12.14 15.09 17.04
0.021 0.038 0.027
162.75 94.85 139.37
4. Summary and conclusions We have obtained the charge-state distributions of 238U and Xe at 11 MeV/nucleon using a gas charge stripper. The most probable charge states of 238U in N2, Ar, and CO2 at equilibrium are 56.0, 56.6, and 55.7, respectively, whereas those of 136Xe in the same conditions are 40.5, 40.1, and 40.3, respectively. The equilibrium charge states of 238U are currently not acceptable for acceleration without a major remodeling of the fRC. On the other hand, the charge states of 136Xe are acceptable for fRC acceleration. For 238U, the N2, Ar, and CO2 gas thicknesses required for the most probable charge states to attain equilibrium are 125, 79, and 126 mg=cm2 , respectively; for 136Xe, the corresponding thicknesses are 163, 95, and 139 mg=cm2 , respectively. 136
Fig. 7. Charge-state distributions of 136Xe beam at 11 MeV/nucleon [2]. The data of fractions calculated for (a) N2, (b) Ar, and (c) CO2 are plotted. (a) The data for the N2 gas medium with thicknesses of 47, 85, 150, 336, and 752 mg=cm2 are represented by asterisks, open triangles, open circles, open squares, and open diamonds, respectively. (b) The data for the Ar gas medium with thicknesses of 27, 94, 196, and 462 mg=cm2 are represented by open triangles, open circles, open squares, and open diamonds, respectively. (c) The data for the CO2 gas medium with thicknesses of 5, 11, 50, 111, 217, and 506 mg=cm2 are represented by crosses, asterisks, open triangles, open circles, open squares, and open diamonds, respectively. Solid lines are curves fitted using Gaussian functions.
40
a
and CO2, respectively [2]. The obtained parameters are listed on Table 2. The equilibrium charge states of 136Xe are acceptable for acceleration by the fRC.
0.05
0
Gas
Xe charge-state distribution
The measured charge-state distributions of 136Xe with N2, Ar, and CO2 are shown in Fig. 7. It was found that the most probable charge states in the case of N2, Ar, and CO2 attain equilibrium at 40.5, 40.1, and 40.3, respectively [2]. The most probable charge states are plotted as a function of the gas thickness in Fig. 8. The thicknesses necessary for equilibrium are 163, 95, and 139 mg=cm2 for N2, Ar,
References [1] Y. Yano, Nucl. Instr. Meth. Phys. Res., Sect. B 261 (2007) 1009. [2] H. Kuboki, H. Okuno, H. Hasebe, S. Yokouchi, N. Fukunishi, O. Kamigaito, A. Goto, M. Kase, Y. Yano, Phys. Rev. ST Accel. Beams 13 (2010) 093501. [3] Y. Yano, in: Proceedings of the 13th International Cyclotron Conference, World Scientific Publishing Co., Vancouver, BC, Canada, 1992, p. 102. [4] T. Mitsumoto, N. Fukunishi, A. Goto, N. Inabe, O. Kamigaito, H. Ryuto, N. Sakamoto, Y. Yano, in: Proceedings of the 17th International Conference on Cyclotrons and Their Applications, Particle Accelerator Society of Japan, Tokyo, Japan, 2004, p. 384. [5] N. Fukunishi, M. Fujimaki, T. Fujinawa, A. Goto, H. Hasebe, Y. Higurashi, K. Ikegami, E. Ikezawa, N. Inabe, T. Kageyama, O. Kamigaito, M. Kase, M. Kidera, S. Kohara, M. Kobayashi-Komiyama, M. Nagase, K. Kumagai, T. Maie, T. Nakagawa, J. Ohnishi, H. Okuno, H. Ryuto, N. Sakamoto, M. Wakasugi, T. Watanabe, K. Yamada, S. Yokouchi, Y. Yano, in: RIKEN Accel. Prog. Rep., vol. 41, 2008, p. 81. [6] T. Nakagawa, Y. Higurashi, J. Ohnishi, T. Aihara, M. Tamura, A. Uchiyama, H. Okuno, K. Kusaka, M. Kidera, E. Ikezawa, M. Fujimaki, Y. Sato, Y. Watanabe, M. Komiyama, M. Kase, A. Goto, O. Kamigaito, Y. Yano, Rev. Sci. Instr. 81 (2010) 02A320. [7] K. Yamada, S. Arai, M. Fujimaki, T. Fujinawa, N. Fukunishi, A. Goto, Y. Higurashi, E. Ikezawa, O. Kamigaito, M. Kase, M. Komiyama, K. Kumagai, T. Maie, T. Nakagawa, J. Ohnishi, H. Okuno, N. Sakamoto, Y. Sato, K. Suda, H. Watanabe, Y. Watanabe, Y. Yano, S. Yokouchi, in: Proceedings of the First International Particle Accelerator Conference, IPAC’10 OC/ACFA, Kyoto, Japan, 2010, p. 789. [8] K. Suda, S. Arai, Y. Chiba, O. Kamigaito, M. Kase, N. Sakamoto, K. Yamada, in: Proceedings of the First International Particle Accelerator Conference, IPAC’10 OC/ACFA, Kyoto, Japan, 2010, p. 3726. [9] N. Bohr, J. Lindhard, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 28 (1954) 7. [10] H. Betz, L. Grodzins, Phys. Rev. Lett. 25 (1970) 211. [11] H. Betz, L. Grodzins, Rev. Mod. Phys. 44 (1972) 465. [12] G. Maynard, M. Chabot, D. Garde s, C. Schmelzer, B. Stadler, J. Weihrauch, Nucl. Instr. and Meth. B 164–165 (2000) 139. [13] A. Perumal, V. Horvat, R. Watson, Y. Peng, K. Fruchey, Nucl. Instr. and Meth. Phys. Res., Sect. B 227 (2005) 251. [14] T. Kishida, Y. Gono, M. Shibata, H. Watanabe, T. Tsutsumi, S. Motomura, E. Ideguchi, X. Zhou, T. Morikawa, T. Kubo, M. Ishihara, Nucl. Instr. and Meth. Phys. Res., Sect. A 438 (1999) 70. [15] H. Ryuto, H. Hasebe, S. Yokouchi, N. Fukunishi, A. Goto, M. Kase, Y. Yano, in: Proceedings of the 18th International Conference on Cyclotrons and Their Applications, INFN-LNS, Catania, Italy, 2007, p. 314. [16] J. Lindhard, A. Sorensen, Phys. Rev. A 53 (1996) 2443. ATIMA site: /http:// www-linux.gsi.de/weick/atima/S.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Charge-state distribution measurements using gas charge stripper toward 238 U and 136Xe acceleration at RIKEN RIBF H. Kuboki , H. Okuno, S. Yokouchi, H. Hasebe, T. Kishida, N. Fukunishi, O. Kamigaito, A. Goto, M. Kase, Y. Yano RIKEN Nishina Center, Wako, Saitama 351-0198, Japan
a r t i c l e i n f o
abstract
Available online 21 June 2011
The charge-state distributions of 238U and 136Xe ions at 11 MeV/nucleon were measured using a gas charge stripper to test whether the charge states in gaseous media were acceptable for acceleration at the RIKEN RI-Beam Factory (RIBF). A differential pumping system facilitated the increase of the N2 gas thickness to 1.3 mg/cm2; this is sufficient for the most probable charge state to attain equilibrium. The charge states of 238U attain equilibrium at 56.0, 56.6, and 55.7 in N2, Ar, and CO2 media with thicknesses of 125, 79, and 126 mg=cm2 , respectively, whereas those of 136Xe attain equilibrium at 40.5, 40.1, and 40.3 in N2, Ar, CO2 media with thicknesses of 163, 95, and 139 mg=cm2 , respectively. The equilibrium charge states of 136Xe are acceptable for acceleration by the subsequent cyclotron, whereas those of 238 U are not acceptable for acceleration without a major remodeling of the accelerator. & 2011 Elsevier B.V. All rights reserved.
Keywords: Uranium beam Xenon beam Equilibrium charge distribution Charge stripper Gas target
1. Introduction Charge strippers play an essential role in a heavy-ion accelerator complex because a high-charge state enables the acceleration of heavy-ion beams to a high energy with small accelerators. The RIKEN RI-beam factory (RIBF) is one such heavy-ion accelerator complex [1]. Heavy ions such as uranium (238U) and xenon (136Xe) are successively accelerated to a final energy of 345 MeV/nucleon using one linear accelerator and four cyclotrons [2]. Two charge stripper sections are placed downstream of the first two cyclotrons, namely, the RIKEN ring cyclotron (RRC) [3] and a fixed-frequency ring cyclotron (fRC) [4]. The energies of the 238U and 136Xe ions incident on each stripper are 11 and 51 MeV/nucleon, respectively. The initial charge states of 238U and 136Xe are 35þ and 20 þ, respectively. Because the lowest acceptable charge states of 238U and 136Xe for fRC acceleration are 71 þ and 41þ, respectively, carbon foils with a thickness of 300 mg=cm2 are used as the first charge stripper for enhancing the charge states of 238U and 136Xe up to 71þ and 47 þ, respectively. Carbon foils with thicknesses of 17 mg/cm2 for 238U and 20 mg/cm2 for 136Xe are used as the second charge stripper to obtain U86 þ and Xe52 þ . It should be noted that there exist two serious problems with the first charge stripper. One is its finite lifetime and the other is the energy spread owing to the nonuniformity of the foil thickness [5]. The carbon charge stripper has a lifetime of 10–12 h for 238U beam
Corresponding author. Tel.: þ 81 48 462 1503; fax: þ81 48 462 1629.
E-mail address: [email protected] (H. Kuboki). 0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.06.012
irradiation with a beam intensity of 14 pnA (0.5 W); this intensity is only one-hundredth of the desired beam intensity. An experiment involving 136Xe beam irradiation for 6 h at an intensity of 1 pmA ð18 WÞ showed that the lifetime of carbon foils subjected to 136 Xe beam irradiation is longer than that of carbon foils subjected to 238U beam irradiation. However, the lifetime is expected to drastically decrease when the irradiated beam intensity is increased by a factor of hundred using a newly developed ion source [6] and a new injector [7,8] in the near future. It is believed that a charge stripper based on gaseous materials will function better as it would have a uniform thickness and it would be able to withstand a high heat load. However, a gas charge stripper has a disadvantage: the charge state of 238U becomes lower than 71þ owing to the density-effect [9–13]. In the case of gas charge strippers, excited projectiles have sufficient time between collisions to return to their ground states before subsequent collisions. Because the ionization cross-section of projectiles in excited states is larger than that in ground states, the charge states obtained with a gas charge stripper are lower than those obtained with carbon foil strippers. Moreover, it is difficult to predict the charge states obtained with a gas charge stripper, because the data of charge states in gaseous media are insufficient as compared to those in solid materials. The available data for a uranium beam in gases are mostly for energies below 0.1 MeV/nucleon and are hence of little importance at the desired energy of 11 MeV/nucleon [2]. In this study, we developed a windowless gas charge stripper whose thickness is sufficient for attaining equilibrium charge states. Then we measured the charge-state distributions of 238U
18
H. Kuboki et al. / Nuclear Instruments and Methods in Physics Research A 655 (2011) 17–20
aperture 6mmφ
aperture 6mmφ
stage 6-2
8mmφ
stage 4
P4
10mmφ
stage 3
P3
stage 2
P2
stage 1
stage 5-2
stage 5-1
P1
P5-2
P5-1
beam stage 6-1
14cm
TMP3-2 220 L/s
TMP2-1 350 L/s
TMP2-2 550 L/s
TMP3-1 220 L/s
MBP1 MBP2 3 TMP1 600 m3/h1020 m /h 1450 L/s Fig. 1. Schematic diagram of gas charge stripper with its differential pumping system [2,14]. Gases are injected in the target cell (stage 1) located at the center. The length of the target cell is 14 cm. Other stages are also shown along with the pumping speeds of their respective attached pumps. The beam passes through four apertures (6 mm in diameter) to enter the target cell. The exits of the target cell and stage 2 are also 6-mm-diameter apertures. The exits of stages 3 and 4 are 8-mm and 10-mm-diameter apertures, respectively.
MBP2
Gas stripper chamber MBP1
Mechanical booster pumps (MBPs) Fig. 2. Overview of gas charge stripper system. The gas charge stripper chamber and two mechanical booster pumps (MBP1 and MBP2) are shown in the photograph.
and 136Xe at 11 MeV/nucleon up to the equilibrium state to test whether the equilibrium charge state is acceptable for acceleration by the fRC [2].
P2−P5, beamline (Pa)
103 102
P2
101
P3
100 10−1 10−2
P4 P5-2
vacuum limit (4x10−3)
P5-1
10−3
beamline2
10−4
beamline1 1
2
3
4
5
6
7 8 9 10
P1 (kPa) Fig. 3. Pressure at each stage (P2–P5) plotted as a function of P1 [2]. Stage 5 is divided into two separate stages, stages 5-1 and 5-2, corresponding to the areas upstream and downstream of stage 4, respectively. The pressures at stages 5-1 and 5-2 are denoted by P5-1 (blue) and P5-2 (green), respectively. The solid line indicates the limit for an interlock (4 10 3 Pa). The pressures at stages 6-1 and 6-2 are denoted by ‘‘beamline1’’ and ‘‘beamline2’’, respectively, and these are maintained lower than the interlock limit if P1 is lower than 7.7 kPa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2. Gas charge stripper system 2.1. System overview We have modified a windowless gas target [14] into a gas charge stripper system, as described below [2]. Fig. 1 shows the schematic diagram of this system along with its differential pumping system. The region in the target cell is defined as stage 1. The physical length of the target cell is 14 cm. The region outside the target cell is defined as stage 2. Similarly, the regions outside stages 2–4 are defined as stages 3–5, respectively. Stage 5 is divided into stages 5-1 and 5-2. The pressure at stages 1–5 is indicated by P1–P5, respectively. Because the gas charge stripper is operated along a beamline, the pressures P2–P5 should decrease as rapidly as possible while maintaining sufficiently high pressure P1. Stages 6-1 and 6-2, corresponding to the beamline upstream and downstream of stages 5-1 and 5-2, respectively, were newly added in order to maintain the pressures at the beamline such
that they were lower than the admissible value. Fig. 2 shows a photograph of the gas charge stripper installed in the beamline. 2.2. Offline pressure test We performed an offline test to determine the achievable pressure in the target cell with nitrogen (N2) gas injection [2]. Fig. 3 shows the results for pressures P2–P5 and the pressures at the beamline plotted against P1. These results indicate that P1 can be increased to 7.7 kPa (1.3 mg/cm2) while maintaining beamline pressures lower than 4 10 3 Pa, which is the allowable vacuum limit for beamlines. The most probable charge state attains equilibrium at a thickness of 500 mg=cm2 in the case of carbon foils for 238U at 11 MeV/nucleon [15]. With the assumption that the N2 gas thickness necessary for equilibrium is the same as the carbon foil thickness, we consider that a N2 gas stripper with its thickness
H. Kuboki et al. / Nuclear Instruments and Methods in Physics Research A 655 (2011) 17–20
adjustable up to 1.3 mg/cm2 is sufficient for attaining equilibrium charge states.
19
48μg/cm2 84 150 334 752
0.16 0.12 0.08
3. Experiment
0.04 0 0.16 Fraction
The experiments were performed at RIKEN Accelerator Research Facility [2]. Fig. 4 shows an overview of the beamline around the gas charge stripper. 238U and 136Xe beams at 11 MeV/nucleon were transported to the gas charge stripper located upstream of the fRC. The incident beam intensities were measured using a Faraday cup (FC-D16) placed upstream of the gas charge stripper to be 3002500 enA ð9214 pnAÞ and 5002600 enA ð25230 pnAÞ for 238U and 136Xe, respectively. The spot size was approximately 6 mm in diameter; this was comparable to the aperture at the entrance of the gas charge stripper. The charge state was analyzed using dipole magnets DAD4 and DMD4. The magnetic fields of these magnets were corrected by considering the energy loss in the gas calculated by ATIMA in LISEþþ [16]. The intensities of the stripped beams were measured using a Faraday cup (FC-F41) downstream of DMD4.
0.12 0.08 0.04 0 0.16
0.08 0.04
238
U charge-state distribution
Gas charge stripper
Beam from RRC
38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 Charge states
Fig. 5. Charge-state distributions of 238U beam at 11 MeV/nucleon [2]. The data of fractions calculated for (a) N2, (b) Ar, and (c) CO2 are plotted. (a) The data for the N2 gas medium with thicknesses of 48, 84, 150, 334, and 752 mg=cm2 are represented by asterisks, open triangles, open circles, open squares, and open diamonds, respectively. (b) The data for the Ar gas medium with thicknesses of 39, 106, 205, and 474 mg=cm2 are represented by open triangles, open circles, open squares, and open diamonds, respectively. (c) The data for the CO2 gas medium with thicknesses of 32, 61, 122, 225, and 516 mg=cm2 are represented by asterisks, open triangles, open circles, open squares, and open diamonds, respectively.
58 56 Most probable charge state
The charge-state distributions of 238U measured with N2, Ar, and CO2 gases at different pressures P1 are shown in Fig. 5. It was found that the most probable charge states in the case of N2, Ar, and CO2 attain equilibrium at 56.0, 56.6, and 55.7, respectively [2]. The equilibrium charge states of 238U are currently unacceptable for acceleration by the fRC, since the lowest acceptable value is 71þ . Fig. 6 shows the most probable charge states plotted as a function of the gas thickness. The thickness of the gas medium was estimated by the energy loss measured by using an alpha source and a silicon detector [2]. The data were fitted by a function f ðtÞ ¼ ab expðctÞ, where a, b, and c are fitting parameters. The equilibrium value of the most probable charge state is denoted by a. Parameter b is related to the thickness required for equilibrium. Parameter c indicates the slope of the curve before equilibrium. The obtained parameters are listed in Table 1. The thicknesses necessary for equilibrium, as calculated by f(t), are 125, 79, and 126 mg=cm2 for N2, Ar, and CO2, respectively [2]. Here, the thickness required for equilibrium is defined as the thickness required for attaining 99% of the equilibrium charge
32μg/cm2 61 122 225 516
0.12
0 3.1.
39μg/cm2 106 205 474
U
54 52 50
N2 Ar CO2
48 46 44 42 40
5
DAD4 Faraday cup D16 (FC-D16)
DMD4
Faraday cup F41 (FC-F41)
fRC
Fig. 4. Schematic view of gas charge stripper. The beam intensity upstream of the gas charge stripper was measured using a Faraday cup (FC-D16). Stripped beams were analyzed using a pair of dipole magnets, DAD4 and DMD4. The beam intensity downstream of DAD4 and DMD4 was measured using a Faraday cup (FC-F41).
50 100 20 Thickness (μg/cm2)
10
200
500
1000
Fig. 6. Most probable charge states of 238U beam at 11 MeV/nucleon plotted as a function of gas thickness [2]. The data for N2, Ar, and CO2 are represented by solid circles, solid triangles, and solid squares, respectively. Fitting functions are represented by solid curves.
Table 1 Fitting parameters of f(t) for 238U beam at 11 MeV/nucleon together with thickness required for equilibrium [2]. The equilibrium value of the most probable charge state is denoted by a. Parameter b is related to the thickness required for equilibrium. Parameter c indicates the slope of the curve before equilibrium. Gas
a
b
c
Equilibrium thickness ðmg=cm2 Þ
N2 Ar CO2
55.98 56.62 55.85
16.73 14.26 16.36
0.027 0.041 0.027
124.92 79.28 125.83
20
H. Kuboki et al. / Nuclear Instruments and Methods in Physics Research A 655 (2011) 17–20
Table 2 Fitting parameters of f(t) and equilibrium thickness for 136Xe beam at 11 MeV/nucleon [2]. The notations are the same as those used in Table 1.
47μg/cm2 47 85 150 336 752
0.20 0.15 0.10 0.05 0
27μg/cm2 94 196 462
Fraction
0.20 0.15 0.10 0
5μg/cm2 11 50 111 217 506
0.20 0.15 0.10 0.05
20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 Charge states
42
Most probable charge state
Xe
38 36 34 N2 Ar CO2
32 30 28 26 24 22
5
10
20
50
100
200
500
1000
Thickness (μg/cm2) Fig. 8. Most probable charge states of 136Xe plotted as a function of gas thickness [2]. The data for N2, Ar, and CO2 are represented by solid circles, solid triangles, and solid squares, respectively. Fitting functions are represented by solid curves.
state. The above-mentioned calculated values are considerably less than the carbon foil thickness ð 500 mg=cm2 Þ. 3.2.
136
b
c
Equilibrium thickness ðmg=cm2 Þ
N2 Ar CO2
40.56 40.08 40.26
12.14 15.09 17.04
0.021 0.038 0.027
162.75 94.85 139.37
4. Summary and conclusions We have obtained the charge-state distributions of 238U and Xe at 11 MeV/nucleon using a gas charge stripper. The most probable charge states of 238U in N2, Ar, and CO2 at equilibrium are 56.0, 56.6, and 55.7, respectively, whereas those of 136Xe in the same conditions are 40.5, 40.1, and 40.3, respectively. The equilibrium charge states of 238U are currently not acceptable for acceleration without a major remodeling of the fRC. On the other hand, the charge states of 136Xe are acceptable for fRC acceleration. For 238U, the N2, Ar, and CO2 gas thicknesses required for the most probable charge states to attain equilibrium are 125, 79, and 126 mg=cm2 , respectively; for 136Xe, the corresponding thicknesses are 163, 95, and 139 mg=cm2 , respectively. 136
Fig. 7. Charge-state distributions of 136Xe beam at 11 MeV/nucleon [2]. The data of fractions calculated for (a) N2, (b) Ar, and (c) CO2 are plotted. (a) The data for the N2 gas medium with thicknesses of 47, 85, 150, 336, and 752 mg=cm2 are represented by asterisks, open triangles, open circles, open squares, and open diamonds, respectively. (b) The data for the Ar gas medium with thicknesses of 27, 94, 196, and 462 mg=cm2 are represented by open triangles, open circles, open squares, and open diamonds, respectively. (c) The data for the CO2 gas medium with thicknesses of 5, 11, 50, 111, 217, and 506 mg=cm2 are represented by crosses, asterisks, open triangles, open circles, open squares, and open diamonds, respectively. Solid lines are curves fitted using Gaussian functions.
40
a
and CO2, respectively [2]. The obtained parameters are listed on Table 2. The equilibrium charge states of 136Xe are acceptable for acceleration by the fRC.
0.05
0
Gas
Xe charge-state distribution
The measured charge-state distributions of 136Xe with N2, Ar, and CO2 are shown in Fig. 7. It was found that the most probable charge states in the case of N2, Ar, and CO2 attain equilibrium at 40.5, 40.1, and 40.3, respectively [2]. The most probable charge states are plotted as a function of the gas thickness in Fig. 8. The thicknesses necessary for equilibrium are 163, 95, and 139 mg=cm2 for N2, Ar,
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