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Development of a cryogenic gas target system for intense radioisotope beam production at CRIB
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Nuclear Instruments and Methods in Physics Research A 589 (2008) 150–156 www.elsevier.com/locate/nima
Development of a cryogenic gas target system for intense radioisotope beam production at CRIB H. Yamaguchia,, Y. Wakabayashia, G. Amadioa, S. Hayakawaa, H. Fujikawaa, S. Kubonoa, J.J. Heb, A. Kimc, D.N. Binhd,a a
Center for Nuclear Study (CNS), University of Tokyo, RIKEN Campus, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b School of Physics, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK c Department of Physics, Ewha Womans University, 11-1, Daehyun-dong, Seodaemun-gu, Seoul 120-750, Korea d Institute of Physics and Electronics, Vietnam Academy of Science and Technology, 8 Hoang Quoc Viet St., Nghia do, Hanoi, Vietnam Received 26 October 2007; received in revised form 7 February 2008; accepted 8 February 2008 Available online 21 February 2008
Abstract A cryogenic gas target system was developed for the radioisotope (RI) beam production at CNS Radio Isotope Beam separator (CRIB). Hydrogen gas was cooled to 85–90 K using liquid nitrogen and used as a secondary beam production target having a thickness of 2:3 mg=cm2 . An intense 7Be beam (2 108 particles per second) was successfully produced using this target. We observed a density-reduction effect at the gas target for high-current primary beams with about 7.5 W heat deposition. One main feature of the target system is forced circulation of the target gas. We have found that the circulation of the target gas at a rate of 55 standard liters per minute (slm) was effective in eliminating the density reduction. The extent to which the forced flow can prevent the density reduction had not been known well. In this work, the relation between the density reduction and the forced circulation rate was quantitatively studied. r 2008 Elsevier B.V. All rights reserved. PACS: 29.25.t; 29.27.a Keywords: RI beam production; Gas target
1. Introduction Light element gases, such as H2 , D2 , 3He, and 4He, have often been used as a radioisotope (RI) beam production target at CNS Radio Isotope Beam separator (CRIB) [1–3], and water-cooled gas cells have been used to keep the target gas at a finite volume. The water-cooled gas cells were 20–80 mm long, and have 2.2–2.5-mm-thick Havar foils as beam windows. A thick and thermostable gas target is necessary for the production of intense RI beams applicable to measurements of low cross-section reactions. However, the gas cell cannot be elongated much longer than 80 mm, because we will have significantly worse collection efficiency and larger beam images in the downstream focal planes. We have also experienced breakage of the foils of the water-cooled gas Corresponding author.
E-mail address: [email protected] (H. Yamaguchi). 0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.02.013
target by high power beams. For example, irradiation of 40Ar beam with 4.5 MeV/u and 200 pnA, which deposited heat of about 16 W on the target, broke the window foils immediately. To solve these problems, we constructed a cryogenic gas target system equipped with gas recirculation ability, which is not implemented in cryogenic gas targets for the RI beam production at other facilities, such as the Momentum Achromat Recoil Spectrometer (MARS) [4,5]. The design and basic test results of the new cryogenic gas target system are described below. 2. Design The main features of the cryogenic gas target system are as follows. (1) Liquid-nitrogen cooling: The target gas and window foils are cooled by liquid nitrogen. If the target is
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ideally cooled to the liquid nitrogen temperature (77 K), it is nearly four times thicker than a room temperature target at the same pressure. The low temperature is also desirable for stability against high power beams. (2) Forced gas circulation: The target gas can be forced to flow by circulating it with a pump in an enclosed system that includes the target cell. It is known that the effective target thickness is reduced by irradiation with a high heat depositing beam. That is, when the beam deposits heat greater than 10 mW/mm, the gas around the beam track is heated and the density of the target decreases [6]. We may be able to avoid the densityreduction effect to some extent, by maintaining the gas flow of the order of 10 standard liters per minute (slm). (3) Oxygen concentration monitoring: When we circulate the hydrogen gas in the closed system, we have to be concerned with the possibility of an explosion, which can occur if air leaks into the system. Considering the explosive limits of hydrogen, the concentration of oxygen in the hydrogen gas must be lower than 5%. We have installed an oxygen analyzer for monitoring oxygen concentration during the experiment.
250 l self-pressurized storage bottle, which can be easily disconnected from the system for refilling. A liquidnitrogen level meter was installed in the top Dewar to provide an automatic supply from the storage bottle. The liquid nitrogen is filled in the vertical duct in between the top Dewar and the gas cell, to cool the cell directly. There is a gas circulation line through the target cell and the top Dewar, most of which consists of 3/8-in.-diameter pipes. The target gas can be injected into the circulation line with a gas inlet system, which has valves for controlling the injection speed, and a Baratron gauge (MKS Instruments Inc.) for pressure monitoring. A diaphragm pump (Iwaki Co., Ltd., APN-P450NST) was installed in the line to maintain a gas circulation rate of 30 slm (55 slm was possible for the hydrogen gas). The circulation rate can be controlled by a tunable valve on the circulation line. The circulation rate is monitored by a mass-flow meter (Kofloc Co., Ltd., model 8300). A heat exchanger (coiled-pipe type) in the top Dewar cools the injected gas down to near 77 K, as the gas passes through it. The cooled gas is transmitted along the vertical duct and reaches the cell. There the gas serves as the production target, and the heated gas exits from the cell, eventually leaving the vacuum chamber. Because the pump cannot circulate low-temperature gas, a heat bath was installed in the return line. If the measured gas temperature is low (below 0 1C), the heater is turned on to heat the gas to room temperature. In most cases, however, the gas is already warmed to room temperature while transporting through the long line from the vacuum chamber to the heat bath. The temperature of the target was measured by a thermocouple attached to the pipe at the gas exit of the cell. An oxygen analyzer (GE sensing Japan Co., Ltd., O2X1 with OX-3) was installed in a bypass of the
Fig. 1 shows the entire structure of the target system. The target gas cell located in a vacuum chamber is shown at the bottom of the figure. The cell is 80-mm-long and has a 20-mm-diameter cylindrical hole to contain the gas inside. On the beam direction sides of the cell, flanges with 2.5-mmthick Havar foils were attached to seal the cell, using thin indium wires. Above the target cell, there is a Dewar-type vessel (referred to as ‘‘top Dewar’’) filled with liquid nitrogen.The top Dewar is covered by thermal-insulating plastic materials. Liquid nitrogen is supplied from a 120 or
N2 out
Liquid N2 level meter Liquid N2 in
bypass to the chamber isolation vacuum oxygen analyzer
heat exchanger (coiled pipe) evacuation port
target gas in
900
gas flow
flow control valve pump for gas flow (30 l/min)
gas cell with Havar foil windows
φ20
heat bath
151
80
heavy-ion beam
Fig. 1. Design of the cryogenic gas target system.
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circulation line to monitor the concentration of oxygen in the gas.
We performed some basic tests of the cryogenic target system producing an RI beam at CRIB. For details of CRIB and its magnets and detectors, see Refs. [1–3]. In the test measurement, hydrogen gas at 760 Torr (at maximum) was used as the RI beam production target. The primary beam used for production was 7 Li2þ at 5.6 MeV/u, with the maximum current of 2:7 emA. The cryogenic target worked stably with this maximum current, which proves that the target can accept a heat load of 7.4 W (5.2 W in the hydrogen gas, the rest in the Havar window foils). The cross-section of the beam at the target was about 5 mm2 . The RI beam produced in this measurement was 7 Be4þ at 4.0 MeV/u. The results of the test experiment are described in this section. 3.1. Liquid nitrogen consumption The measured consumption rate of the liquid nitrogen at the top Dewar was 50 ml/min, when there was neither gas circulation nor irradiation of the heavy-ion beam. This intrinsic consumption rate is mainly due to the small amounts of heat conduction into the top Dewar, although the Dewar is well covered by thermal-insulating materials. Liquid nitrogen is lost also during the transfer from the storage bottle to the top Dewar (transfer efficiency is 70–80%). Considering this efficiency, the total consumption rate is 60 ml/min (about 100 l/day). The consumption due to heat deposition of the heavyion beam is typically much smaller than the intrinsic consumption. The theoretical consumption rate due to heat deposition in this measurement (7.4 W) is 1.3 ml/min. However, the consumption rate increased when the gas circulation is turned on, as the gas is cooled and warmed repeatedly in its cycles. The measured consumption rate due to circulation was nearly linear with the circulation rate, being 13 ml/min per circulation rate of 10 slm. 3.2. Target thickness The kinetic energy of the heavy-ion beam at CRIB can be measured precisely by a dipole magnet, whose magnetic field strength is monitored with a nuclear magnetic resonance (NMR) probe. The target thickness was determined by measuring the energy loss of the beam in the target, and comparing it with a calculation using Ziegler’s formulation [7]. Fig. 2 shows the target thicknesses measured for various target pressures and circulation rates. The pressure was monitored at the gas injection system. As seen in Fig. 2, the thickness is proportional to the pressure, indicating that the target temperature stayed nearly constant, independent of the circulation rate.
Thickness (mg/cm2)
3. Test of the basic features of the target system using lowenergy heavy-ion beams
2.5 2
1.5 1
0.5
55 slm 30 slm 17 slm 0 slm
0 0
100
200
300 400 500 Pressure (Torr)
600
700
800
Fig. 2. Target thicknesses measured for various target pressures and circulation rates.
Because the circulation rate was controlled by a tunable valve on the circulation line while the pump maintains a constant gas flow, the gas pressure between the valve and the pump depends on the circulation rate. We observed a small deviation of the monitored pressure that depended on the circulation rate. Nevertheless, the observed thickness was nearly proportional to the monitored pressure, implying that the monitored pressure is sufficiently close to the pressure in the target cell. The corresponding temperature, calculated from the measured thickness and pressure, was 85–90 K. This temperature agrees with the measurement with a thermocouple near the target cell, which was about 88–90 K. The maximum target thickness in this measurement was 2:3 mg=cm2 , 3.3 times thicker than that at room temperature with the same pressure (760 Torr). 3.3. Momentum distribution of the secondary beam We produced 7Be as the secondary beam from p( Li,7Be)n reaction and its momentum distribution was measured using the CRIB dipole magnet. Fig. 3a shows the distribution of the energy and the time of flight (TOF) of the detected particles. Two kinds of particles were detected and identified as 7 Be4þ and 7 Li3þ , the latter of which came from the primary beam. Other particles were not detected in this measurement. Fig. 3b shows the counting rates of 7 Be4þ against the relative momentum offset Dp=p (in %) from the center of the distribution. The center corresponds to 608 MeV/c (4.0 MeV/u in energy) for the 7Be beam. The counting rates of 7 Li3þ detected in the same measurement are also plotted. The result shows that the momentum distribution of 7 Be4þ has a width (full width at half maximum) of 6.4%, whereas the momentum distribution of 7 Li3þ is nearly flat. Such a broadening of the secondary beam can be understood by energy straggling in the target and the finite size of 7
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153
70 30
Beam energy (MeV)
25
7
Be
Beam counting rate (arb. u.)
7Be4+
4+
20 15
7Li3+
10 5 0
60
7
Li3+
50 40 30 20 10 0
20
40 60 TOF (ns)
80
-6
-4
-2 0 Momentum offset (%)
2
4
Fig. 3. (a) Contour map of the particle identification from the energy and time of flight. (b) Momentum distribution of the counting rate of 7 Be4þ , together with the contaminant, 7 Li3þ . Note that the TOF shown here has an arbitrary time offset for each particle. In (b), the rate of 7 Be4þ was fitted with a convolution of a Gaussian and a box function.
3.4. Intense secondary beam production and target gas circulation rate Fig. 4 shows the production rates of the 7 Be4þ beam, measured for various primary-beam currents and circulation rates of the target gas. The production rates were normalized for the momentum acceptance of 3%, and we confirmed that we can obtain an intense 7Be beam of 2 108 particles per second (pps) in the best cases. The beam current was measured with a Faraday cup on the beam line. We used the following two different methods to measure the production rates. The first method is a direct measurement using a parallel-plate avalanche counter (PPAC) [8]. The PPAC can count the beam with a rate up to around 106 pps. For counting beams with a rate of 106 2107 pps, we narrowed the momentum acceptance of the beam separator. During this measurement, we determined the ratios of the counting rates for different momentum acceptances (3%, 0:6%,
production rate (pps)
108
7Be
the target thickness (i.e. 7 Be4þ particles have different energies depending on the position they are created in the target). The spectral shape broadened because of these effects was approximated by a convolution of a Gaussian and a box function (f ðxÞ ¼ 1 if jxjow=2, 0 otherwise), shown as the curve in Fig. 3. The possible source of the observed 7 Li3þ distribution is the low-energy tail of the primary beam, scattered at the inner wall of the beam separator. The beam purity of 7 Be4þ is defined as the ratio of the measured number of 7 Be4þ particles to the total number of particles. The maximum beam purity in this measurement was 75%. The beam purity is an important index when we discuss the reduction of the target density by high-intensity beams.
107 106 105 104
55 slm 17 slm 5.4 slm 0 slm (scattering) 0 slm (direct)
103
10-2
10-1
1 102 10 Beam current (nA)
103
104
Fig. 4. Production rates of the 7 Be4þ beam measured for various beam currents and target gas circulation rates. For beams with currents less than 10 nA, the rates were directly measured with the PPAC (indicated as ‘direct’). For higher current beams, the rates were determined by counting particles scattered elastically with a gold foil.
and 0:3%) with the same beam current, and used them for the normalization. This normalized production rate, which is shown in Fig. 4, should be the same as the actual counting rate what we have for the acceptance of 3%, if the momentum distribution of the beam is conserved. The beam purity was also determined by the measurement, as the two beams were identified by their difference in TOF. For beams with a much higher rate, we placed a thin ð1 mmÞ gold foil in the beam line, and detected elastically scattered beams with a silicon detector. The ratio of the measured elastic scattering events to the total number of the particles in the beam was obtained by performing measurements under the same condition as the direct method. The silicon detector was located at the angle of 201 from the beam line, and 22 cm distant from the foil. As the
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scattered 7 Be4þ and 7 Li3þ have different energies that could be clearly identified by the silicon detector, the beam purity was determined with this method as well. The production rate, beam purity and the condition of the measurement are summarized in Table 1. The results show a good linearity between the production rate and a primary beam current less than 1 emA. The beam purity remained the same for this current region. However, when the beam current exceeded 1 emA, the production rate deviated from the linearity, and the beam purity started to decrease. This is considered as the density-reduction effect caused by heat. As we observed the beams with a fixed central momentum, the counting rate of 7 Be4þ decreases because of the following two effects, both arising when the target density is reduced. The first effect is simply the reduction of the number of the beam-production reactions, as it is nearly proportional to the target density. The second effect is the momentum shift of the produced beam. The central momentum of the beam becomes higher than that for normal density, and significant portion of the beam will fall out of the momentum acceptance, optimized for low-current beam conditions. We evaluated the second effect (momentum shift) using the measured counting rate, purity, and the relationship of the momentum and counting rate, shown in Fig. 3. The momentum distribution deviates slightly when the target density is reduced. This effect was theoretically evaluated and taken into the consideration. In Table 1, we refer to the production rate without the second effect as the maximum production rate, because it represents the maximum possible production rate when we tuned the dipole magnet to the center of the momentum. The maximum production rate of the 7Be beam can also be roughly estimated from the counting rate of the
7
Li beam. As 7Li is not a reaction product, its counting rate is not affected by the first effect. By comparing the actual counting rate of 7Li and the counting rate expected from the linearity against the beam current, we can estimate how much the measured 7Be rate was suppressed by the second effect. The momentum distribution of 7 Li was also taken into account, although it was nearly flat. The maximum production rate estimated in this way was consistent with the values obtained by the above evaluation within the error bars. The effective target thickness (or target density) could be determined by the maximum production rate. Fig. 5 shows the reduction of the effective thickness against the original thickness, measured with the low-current beam, for several circulation rates. The primary beam current was around 2:7 emA. As mentioned above, there was a pressure variation related to the circulation rate, independent of the beam current. The maximum variation was 6%, therefore, it cannot be the main cause of the reduction of the counting rate. In the results shown in this figure, the density reduction due to this effect is excluded, and the reduction is considered to be purely from the heat. These results illustrate that there is a density-reduction effect, which could be minimized by circulating the target gas. The observed dependence of the density reduction R on the circulation rate could be roughly approximated with the function, R¼
548 % s þ 19:2
(1)
where s is the gas circulation rate in slm. The heat deposited from the beam was 5.2 W (corresponding to 65 mW/mm) in the gas and 2.2 W at the window foils.
Table 1 The production rate and purity of the 7Be beam for various primary beam currents and circulation rates Beam current (nA)
Production rate ðs1 Þ Normalized
0.1 1 3 10 45 5:8 102 1:78 103 2:61 103 2:67 103 2:80 103 2:73 103 2:60 103
4:54 103 1:05 105 2:75 105 1:38 106 5:15 106 5:37 107 6:42 107 3:60 107 7:29 107 2:04 108 2:33 108 2:61 108
7
Be purity (%)
Circulation rate (slm)
Target pressure (Torr)
Momentum acceptance (%)
0 0 0 0 0 0 0 0 5.4 17.3 29.5 54.5
754 754 754 754 754 754 754 754 694 711 728 734
3 0:6 3 0:3 0:3 0:3 0:3 0:3 0:3 0:3 0:3 0:3
Maximum
5:3ð2Þ 107 1:35ð5Þ 108 1:87ð7Þ 108 1:83ð7Þ 108 2:45ð10Þ 108 2:64ð12Þ 108 2:65ð13Þ 108
73.3 75.4 72.9 75.6 75.5 72.8 62.5 57.9 54.3 69.1 73.3 74.6
The two production rate types (normalized and maximum) are the counting rates of the 7Be beam normalized for the momentum acceptance of 3%, but the former is measured at a fixed momentum, and the latter represents the maximum achievable rate when the dipole magnet is tuned to the optimum setting (see text). The momentum acceptance and counting method were altered according to the production rate. The counting rates marked with stars (*) were measured by the scattering method, while the others were directly counted by the PPAC.
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The gas density r deviates according to the following equation:
0 Effective thickness reduction (%)
155
5
V
10 15
dr ¼ fQ þfF þ fT dt pffiffiffiffiffiffiffi ¼ Ar þ ðr0 rÞðrF þ rT Þ.
(6)
25
Here A is a constant which indicates the magnitude of the heat deposited. At the steady-state condition, this equation is equal to zero, and
30
Ar ¼ ðr0 rÞ2 ðrF þ rT Þ2 .
35
By rewriting this with R ¼ 1 r=r0 , we obtain
20
40 -10
0
10 20 30 40 Target gas circulation rate (slm)
50
60
Fig. 5. Effective thickness (or density) reduction observed at the hydrogen gas target with the original thickness of 2:3 mg=cm2 irradiated by the 7 2þ Li beam of 2:7 emA. The reduction is denoted as the relative deviation from the original thickness, measured with a low-current beam. The error is mostly systematic.
As shown in the figure, the target density decreased by about 30%, when there was no gas circulation. This amount of reduction agrees with the measurement reported in Ref. [6], which resulted in a reduction by 30% for a beam of 60 mW/mm. The dependence on the circulation rate assumed in Eq. (1) can be explained by the following simplified model. Here we assume that the length of the cell, and the current and profile of the primary beam are all fixed. The heat at the cell deposited by the beam Q is a function of the target gas density on the beam axis r, and approximately, Q / r.
(2)
The molecular velocity of the target gas is increased by the heat deposition Q, and gas flows from the beam position to the outer region. Therefore, the outgoing flow from the volume V, in which the target gas is irradiated by the beam, can be written as pffiffiffiffi fQ / Q pffiffiffi / r (3) where the negative sign indicates that the flow is outgoing. There are incoming flows due to the forced circulation f F and thermal diffusion f T , both of which act as the forces to restore the density r to the original (no-heat) value, r0 . f F and f T should be proportional to the gradient of the gas density, and here we write them as f F ¼ ðr0 rÞrF
(4)
f T ¼ ðr0 rÞrT
(5)
using constants rF and rT . rF is proportional to the forced circulation rate.
(7) the
Að1 RÞ ¼ r0 ðrF þ rT Þ2 R2 . This equation can be solved as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Að1 þ 1 þ 4r0 ðrF þ rT Þ2 =AÞ . R¼ 2r0 ðrF þ rT Þ2
density
reduction (8)
(9)
In our case, R is 30% at maximum and r0 ðrF þ rT Þ2 must be sufficiently larger than A. Then, it follows that sffiffiffiffiffi A 1 R’ (10) r0 ðrF þ rT Þ which is essentially the same form as Eq. (1). Note that the density reduction R has a square-root dependence on the magnitude of the heat deposition A in this model. In summary, we obtained a very intense ð2 108 ppsÞ 7 Be beam, having the energy of ð4:0 0:2Þ MeV=u. We have observed a reduction of the target density, probably due to the high heat deposition of the beam. The reduction was about 30% at maximum, when the beam deposited heat of 65 mW/mm in the gas. We succeeded in reducing this reduction effect to about 5% by circulating the gas at 55 slm. 3.5. Comparison of the 7Be production rate with calculations The results shown above indicate that a 7Be beam of 2:8 108 pps can be produced by the 7Li beam of 2:7 emA and the hydrogen gas target with a thickness of 2:3 mg=cm2 , when there is no density reduction effect. Here we compare the measured production rate with a calculated rate using a reaction cross-section measured previously. The differential cross-section of the 7Li(p,n)7Be reaction is known by the previous measurements (e.g. [9]), and it is about 42 mb/sr for the reaction energy in this study at 01. The solid angle of CRIB is 5.6 msr, according to the specification [2]. Considering the kinematics of the p(7Li,7Be)n reaction and the solid angle, the effective total cross-section was calculated as 9.3 mb. The acceptance of the momentum slits in the separator was evaluated from the measured momentum distribution of the 7Be beam described above. The slits accepted 75% of
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the 7Be beam when the momentum acceptance was 3%. There should be no significant decrease in the yield due to the selection of the charge state, as we selected the dominant 4þ state. Considering the effective cross-section, momentum acceptance, beam current ð2:7 emAÞ, and target thickness ð2:3 mg=cm2 Þ, the expected counting rate was calculated as 1:8 108 pps, which is of the same order but lower than the measured value. The main reason for the difference can be attributed to the solid angle of CRIB, that is, the effective solid angle is known to be nearly double the specification value of 5.6 msr. To have the consistency between the measured and calculated values, the effective solid angle should be about 9 msr, which agrees with our experience. This result also proves that there was no large unexpected effect that decreased the yield of 7Be. 4. Conclusion A cryogenic gas target system was developed for RI beam production at CRIB. The basic features were tested using heavy-ion beams at low energies. The target system was used for the production of 7Be and we obtained a beam of 2 108 pps, with a target of 2:3-mg=cm2 -thick hydrogen gas at 85–90 K. The target gas circulation at a rate of 55 slm can eliminate most of the target densityreduction effect, which was observed for a primary beam depositing heat of 65 mW/mm at the target gas.
Acknowledgments We are grateful to RIKEN and CNS accelerator staff for their help. This work was supported by a Grant-in-Aid for Young Scientists (B) (Grant no. 17740135) of JSPS. References [1] S. Kubono, Y. Yanagisawa, T. Teranishi, S. Kato, T. Kishida, S. Michimasa, Y. Ohshiro, S. Shimoura, K. Ue, S. Watanabe, N. Yamazaki, Eur. Phys. J. A 13 (2002) 217. [2] Y. Yanagisawa, S. Kubono, T. Teranishi, K. Ue, S. Michimasa, M. Notani, J. He, Y. Ohshiro, S. Shimoura, S. Watanabe, N. Yamazaki, H. Iwasaki, S. Kato, T. Kishida, T. Morikawa, Y. Mizoi, Nucl. Instr. and Meth. 539 (2005) 74. [3] T. Teranishi, S. Kubono, S. Shimoura, M. Notani, Y. Yanagisawa, S. Michimasa, K. Ue, H. Iwasaki, M. Kurokawa, Y. Satou, T. Morikawa, A. Saito, H. Baba, J. Lee, C. Lee, Z. Fulop, S. Kato, Phys. Lett. B 556 (2003) 27. [4] D. Semon, M. Allen, H. Dejbakhsh, C. Gagliardi, S. Hale, J. Jiang, L. Trache, R. Tribble, S. Yennello, H. Xu, X. Zhou, B. Brown, Phys. Rev. C 53 (1996) 96. [5] A. Azhari, V. Burjan, F. Carstoiu, C. Gagliardi, V. Kroha, A. Mukhamedzhanov, F. Nunes, X. Tang, L. Trache, R. Tribble, Phys. Rev. C 63 (2001) 055803. [6] J. Go¨erres, K. Kettner, H. Kra¨winkel, C. Rolfs, Nucl. Instr. and Meth. 177 (1980) 295. [7] J.F. Ziegler, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [8] H. Kumagai, A. Ozawa, N. Fukuda, K. Su¨mmerer, I. Tanihata, Nucl. Instr. and Meth. 470 (2001) 562. [9] S. Elbakr, I. Heerden, W. McDonald, G. Neilson, Nucl. Instr. and Meth. 105 (1972) 519.
Nuclear Instruments and Methods in Physics Research A 589 (2008) 150–156 www.elsevier.com/locate/nima
Development of a cryogenic gas target system for intense radioisotope beam production at CRIB H. Yamaguchia,, Y. Wakabayashia, G. Amadioa, S. Hayakawaa, H. Fujikawaa, S. Kubonoa, J.J. Heb, A. Kimc, D.N. Binhd,a a
Center for Nuclear Study (CNS), University of Tokyo, RIKEN Campus, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b School of Physics, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK c Department of Physics, Ewha Womans University, 11-1, Daehyun-dong, Seodaemun-gu, Seoul 120-750, Korea d Institute of Physics and Electronics, Vietnam Academy of Science and Technology, 8 Hoang Quoc Viet St., Nghia do, Hanoi, Vietnam Received 26 October 2007; received in revised form 7 February 2008; accepted 8 February 2008 Available online 21 February 2008
Abstract A cryogenic gas target system was developed for the radioisotope (RI) beam production at CNS Radio Isotope Beam separator (CRIB). Hydrogen gas was cooled to 85–90 K using liquid nitrogen and used as a secondary beam production target having a thickness of 2:3 mg=cm2 . An intense 7Be beam (2 108 particles per second) was successfully produced using this target. We observed a density-reduction effect at the gas target for high-current primary beams with about 7.5 W heat deposition. One main feature of the target system is forced circulation of the target gas. We have found that the circulation of the target gas at a rate of 55 standard liters per minute (slm) was effective in eliminating the density reduction. The extent to which the forced flow can prevent the density reduction had not been known well. In this work, the relation between the density reduction and the forced circulation rate was quantitatively studied. r 2008 Elsevier B.V. All rights reserved. PACS: 29.25.t; 29.27.a Keywords: RI beam production; Gas target
1. Introduction Light element gases, such as H2 , D2 , 3He, and 4He, have often been used as a radioisotope (RI) beam production target at CNS Radio Isotope Beam separator (CRIB) [1–3], and water-cooled gas cells have been used to keep the target gas at a finite volume. The water-cooled gas cells were 20–80 mm long, and have 2.2–2.5-mm-thick Havar foils as beam windows. A thick and thermostable gas target is necessary for the production of intense RI beams applicable to measurements of low cross-section reactions. However, the gas cell cannot be elongated much longer than 80 mm, because we will have significantly worse collection efficiency and larger beam images in the downstream focal planes. We have also experienced breakage of the foils of the water-cooled gas Corresponding author.
E-mail address: [email protected] (H. Yamaguchi). 0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.02.013
target by high power beams. For example, irradiation of 40Ar beam with 4.5 MeV/u and 200 pnA, which deposited heat of about 16 W on the target, broke the window foils immediately. To solve these problems, we constructed a cryogenic gas target system equipped with gas recirculation ability, which is not implemented in cryogenic gas targets for the RI beam production at other facilities, such as the Momentum Achromat Recoil Spectrometer (MARS) [4,5]. The design and basic test results of the new cryogenic gas target system are described below. 2. Design The main features of the cryogenic gas target system are as follows. (1) Liquid-nitrogen cooling: The target gas and window foils are cooled by liquid nitrogen. If the target is
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ideally cooled to the liquid nitrogen temperature (77 K), it is nearly four times thicker than a room temperature target at the same pressure. The low temperature is also desirable for stability against high power beams. (2) Forced gas circulation: The target gas can be forced to flow by circulating it with a pump in an enclosed system that includes the target cell. It is known that the effective target thickness is reduced by irradiation with a high heat depositing beam. That is, when the beam deposits heat greater than 10 mW/mm, the gas around the beam track is heated and the density of the target decreases [6]. We may be able to avoid the densityreduction effect to some extent, by maintaining the gas flow of the order of 10 standard liters per minute (slm). (3) Oxygen concentration monitoring: When we circulate the hydrogen gas in the closed system, we have to be concerned with the possibility of an explosion, which can occur if air leaks into the system. Considering the explosive limits of hydrogen, the concentration of oxygen in the hydrogen gas must be lower than 5%. We have installed an oxygen analyzer for monitoring oxygen concentration during the experiment.
250 l self-pressurized storage bottle, which can be easily disconnected from the system for refilling. A liquidnitrogen level meter was installed in the top Dewar to provide an automatic supply from the storage bottle. The liquid nitrogen is filled in the vertical duct in between the top Dewar and the gas cell, to cool the cell directly. There is a gas circulation line through the target cell and the top Dewar, most of which consists of 3/8-in.-diameter pipes. The target gas can be injected into the circulation line with a gas inlet system, which has valves for controlling the injection speed, and a Baratron gauge (MKS Instruments Inc.) for pressure monitoring. A diaphragm pump (Iwaki Co., Ltd., APN-P450NST) was installed in the line to maintain a gas circulation rate of 30 slm (55 slm was possible for the hydrogen gas). The circulation rate can be controlled by a tunable valve on the circulation line. The circulation rate is monitored by a mass-flow meter (Kofloc Co., Ltd., model 8300). A heat exchanger (coiled-pipe type) in the top Dewar cools the injected gas down to near 77 K, as the gas passes through it. The cooled gas is transmitted along the vertical duct and reaches the cell. There the gas serves as the production target, and the heated gas exits from the cell, eventually leaving the vacuum chamber. Because the pump cannot circulate low-temperature gas, a heat bath was installed in the return line. If the measured gas temperature is low (below 0 1C), the heater is turned on to heat the gas to room temperature. In most cases, however, the gas is already warmed to room temperature while transporting through the long line from the vacuum chamber to the heat bath. The temperature of the target was measured by a thermocouple attached to the pipe at the gas exit of the cell. An oxygen analyzer (GE sensing Japan Co., Ltd., O2X1 with OX-3) was installed in a bypass of the
Fig. 1 shows the entire structure of the target system. The target gas cell located in a vacuum chamber is shown at the bottom of the figure. The cell is 80-mm-long and has a 20-mm-diameter cylindrical hole to contain the gas inside. On the beam direction sides of the cell, flanges with 2.5-mmthick Havar foils were attached to seal the cell, using thin indium wires. Above the target cell, there is a Dewar-type vessel (referred to as ‘‘top Dewar’’) filled with liquid nitrogen.The top Dewar is covered by thermal-insulating plastic materials. Liquid nitrogen is supplied from a 120 or
N2 out
Liquid N2 level meter Liquid N2 in
bypass to the chamber isolation vacuum oxygen analyzer
heat exchanger (coiled pipe) evacuation port
target gas in
900
gas flow
flow control valve pump for gas flow (30 l/min)
gas cell with Havar foil windows
φ20
heat bath
151
80
heavy-ion beam
Fig. 1. Design of the cryogenic gas target system.
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circulation line to monitor the concentration of oxygen in the gas.
We performed some basic tests of the cryogenic target system producing an RI beam at CRIB. For details of CRIB and its magnets and detectors, see Refs. [1–3]. In the test measurement, hydrogen gas at 760 Torr (at maximum) was used as the RI beam production target. The primary beam used for production was 7 Li2þ at 5.6 MeV/u, with the maximum current of 2:7 emA. The cryogenic target worked stably with this maximum current, which proves that the target can accept a heat load of 7.4 W (5.2 W in the hydrogen gas, the rest in the Havar window foils). The cross-section of the beam at the target was about 5 mm2 . The RI beam produced in this measurement was 7 Be4þ at 4.0 MeV/u. The results of the test experiment are described in this section. 3.1. Liquid nitrogen consumption The measured consumption rate of the liquid nitrogen at the top Dewar was 50 ml/min, when there was neither gas circulation nor irradiation of the heavy-ion beam. This intrinsic consumption rate is mainly due to the small amounts of heat conduction into the top Dewar, although the Dewar is well covered by thermal-insulating materials. Liquid nitrogen is lost also during the transfer from the storage bottle to the top Dewar (transfer efficiency is 70–80%). Considering this efficiency, the total consumption rate is 60 ml/min (about 100 l/day). The consumption due to heat deposition of the heavyion beam is typically much smaller than the intrinsic consumption. The theoretical consumption rate due to heat deposition in this measurement (7.4 W) is 1.3 ml/min. However, the consumption rate increased when the gas circulation is turned on, as the gas is cooled and warmed repeatedly in its cycles. The measured consumption rate due to circulation was nearly linear with the circulation rate, being 13 ml/min per circulation rate of 10 slm. 3.2. Target thickness The kinetic energy of the heavy-ion beam at CRIB can be measured precisely by a dipole magnet, whose magnetic field strength is monitored with a nuclear magnetic resonance (NMR) probe. The target thickness was determined by measuring the energy loss of the beam in the target, and comparing it with a calculation using Ziegler’s formulation [7]. Fig. 2 shows the target thicknesses measured for various target pressures and circulation rates. The pressure was monitored at the gas injection system. As seen in Fig. 2, the thickness is proportional to the pressure, indicating that the target temperature stayed nearly constant, independent of the circulation rate.
Thickness (mg/cm2)
3. Test of the basic features of the target system using lowenergy heavy-ion beams
2.5 2
1.5 1
0.5
55 slm 30 slm 17 slm 0 slm
0 0
100
200
300 400 500 Pressure (Torr)
600
700
800
Fig. 2. Target thicknesses measured for various target pressures and circulation rates.
Because the circulation rate was controlled by a tunable valve on the circulation line while the pump maintains a constant gas flow, the gas pressure between the valve and the pump depends on the circulation rate. We observed a small deviation of the monitored pressure that depended on the circulation rate. Nevertheless, the observed thickness was nearly proportional to the monitored pressure, implying that the monitored pressure is sufficiently close to the pressure in the target cell. The corresponding temperature, calculated from the measured thickness and pressure, was 85–90 K. This temperature agrees with the measurement with a thermocouple near the target cell, which was about 88–90 K. The maximum target thickness in this measurement was 2:3 mg=cm2 , 3.3 times thicker than that at room temperature with the same pressure (760 Torr). 3.3. Momentum distribution of the secondary beam We produced 7Be as the secondary beam from p( Li,7Be)n reaction and its momentum distribution was measured using the CRIB dipole magnet. Fig. 3a shows the distribution of the energy and the time of flight (TOF) of the detected particles. Two kinds of particles were detected and identified as 7 Be4þ and 7 Li3þ , the latter of which came from the primary beam. Other particles were not detected in this measurement. Fig. 3b shows the counting rates of 7 Be4þ against the relative momentum offset Dp=p (in %) from the center of the distribution. The center corresponds to 608 MeV/c (4.0 MeV/u in energy) for the 7Be beam. The counting rates of 7 Li3þ detected in the same measurement are also plotted. The result shows that the momentum distribution of 7 Be4þ has a width (full width at half maximum) of 6.4%, whereas the momentum distribution of 7 Li3þ is nearly flat. Such a broadening of the secondary beam can be understood by energy straggling in the target and the finite size of 7
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153
70 30
Beam energy (MeV)
25
7
Be
Beam counting rate (arb. u.)
7Be4+
4+
20 15
7Li3+
10 5 0
60
7
Li3+
50 40 30 20 10 0
20
40 60 TOF (ns)
80
-6
-4
-2 0 Momentum offset (%)
2
4
Fig. 3. (a) Contour map of the particle identification from the energy and time of flight. (b) Momentum distribution of the counting rate of 7 Be4þ , together with the contaminant, 7 Li3þ . Note that the TOF shown here has an arbitrary time offset for each particle. In (b), the rate of 7 Be4þ was fitted with a convolution of a Gaussian and a box function.
3.4. Intense secondary beam production and target gas circulation rate Fig. 4 shows the production rates of the 7 Be4þ beam, measured for various primary-beam currents and circulation rates of the target gas. The production rates were normalized for the momentum acceptance of 3%, and we confirmed that we can obtain an intense 7Be beam of 2 108 particles per second (pps) in the best cases. The beam current was measured with a Faraday cup on the beam line. We used the following two different methods to measure the production rates. The first method is a direct measurement using a parallel-plate avalanche counter (PPAC) [8]. The PPAC can count the beam with a rate up to around 106 pps. For counting beams with a rate of 106 2107 pps, we narrowed the momentum acceptance of the beam separator. During this measurement, we determined the ratios of the counting rates for different momentum acceptances (3%, 0:6%,
production rate (pps)
108
7Be
the target thickness (i.e. 7 Be4þ particles have different energies depending on the position they are created in the target). The spectral shape broadened because of these effects was approximated by a convolution of a Gaussian and a box function (f ðxÞ ¼ 1 if jxjow=2, 0 otherwise), shown as the curve in Fig. 3. The possible source of the observed 7 Li3þ distribution is the low-energy tail of the primary beam, scattered at the inner wall of the beam separator. The beam purity of 7 Be4þ is defined as the ratio of the measured number of 7 Be4þ particles to the total number of particles. The maximum beam purity in this measurement was 75%. The beam purity is an important index when we discuss the reduction of the target density by high-intensity beams.
107 106 105 104
55 slm 17 slm 5.4 slm 0 slm (scattering) 0 slm (direct)
103
10-2
10-1
1 102 10 Beam current (nA)
103
104
Fig. 4. Production rates of the 7 Be4þ beam measured for various beam currents and target gas circulation rates. For beams with currents less than 10 nA, the rates were directly measured with the PPAC (indicated as ‘direct’). For higher current beams, the rates were determined by counting particles scattered elastically with a gold foil.
and 0:3%) with the same beam current, and used them for the normalization. This normalized production rate, which is shown in Fig. 4, should be the same as the actual counting rate what we have for the acceptance of 3%, if the momentum distribution of the beam is conserved. The beam purity was also determined by the measurement, as the two beams were identified by their difference in TOF. For beams with a much higher rate, we placed a thin ð1 mmÞ gold foil in the beam line, and detected elastically scattered beams with a silicon detector. The ratio of the measured elastic scattering events to the total number of the particles in the beam was obtained by performing measurements under the same condition as the direct method. The silicon detector was located at the angle of 201 from the beam line, and 22 cm distant from the foil. As the
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scattered 7 Be4þ and 7 Li3þ have different energies that could be clearly identified by the silicon detector, the beam purity was determined with this method as well. The production rate, beam purity and the condition of the measurement are summarized in Table 1. The results show a good linearity between the production rate and a primary beam current less than 1 emA. The beam purity remained the same for this current region. However, when the beam current exceeded 1 emA, the production rate deviated from the linearity, and the beam purity started to decrease. This is considered as the density-reduction effect caused by heat. As we observed the beams with a fixed central momentum, the counting rate of 7 Be4þ decreases because of the following two effects, both arising when the target density is reduced. The first effect is simply the reduction of the number of the beam-production reactions, as it is nearly proportional to the target density. The second effect is the momentum shift of the produced beam. The central momentum of the beam becomes higher than that for normal density, and significant portion of the beam will fall out of the momentum acceptance, optimized for low-current beam conditions. We evaluated the second effect (momentum shift) using the measured counting rate, purity, and the relationship of the momentum and counting rate, shown in Fig. 3. The momentum distribution deviates slightly when the target density is reduced. This effect was theoretically evaluated and taken into the consideration. In Table 1, we refer to the production rate without the second effect as the maximum production rate, because it represents the maximum possible production rate when we tuned the dipole magnet to the center of the momentum. The maximum production rate of the 7Be beam can also be roughly estimated from the counting rate of the
7
Li beam. As 7Li is not a reaction product, its counting rate is not affected by the first effect. By comparing the actual counting rate of 7Li and the counting rate expected from the linearity against the beam current, we can estimate how much the measured 7Be rate was suppressed by the second effect. The momentum distribution of 7 Li was also taken into account, although it was nearly flat. The maximum production rate estimated in this way was consistent with the values obtained by the above evaluation within the error bars. The effective target thickness (or target density) could be determined by the maximum production rate. Fig. 5 shows the reduction of the effective thickness against the original thickness, measured with the low-current beam, for several circulation rates. The primary beam current was around 2:7 emA. As mentioned above, there was a pressure variation related to the circulation rate, independent of the beam current. The maximum variation was 6%, therefore, it cannot be the main cause of the reduction of the counting rate. In the results shown in this figure, the density reduction due to this effect is excluded, and the reduction is considered to be purely from the heat. These results illustrate that there is a density-reduction effect, which could be minimized by circulating the target gas. The observed dependence of the density reduction R on the circulation rate could be roughly approximated with the function, R¼
548 % s þ 19:2
(1)
where s is the gas circulation rate in slm. The heat deposited from the beam was 5.2 W (corresponding to 65 mW/mm) in the gas and 2.2 W at the window foils.
Table 1 The production rate and purity of the 7Be beam for various primary beam currents and circulation rates Beam current (nA)
Production rate ðs1 Þ Normalized
0.1 1 3 10 45 5:8 102 1:78 103 2:61 103 2:67 103 2:80 103 2:73 103 2:60 103
4:54 103 1:05 105 2:75 105 1:38 106 5:15 106 5:37 107 6:42 107 3:60 107 7:29 107 2:04 108 2:33 108 2:61 108
7
Be purity (%)
Circulation rate (slm)
Target pressure (Torr)
Momentum acceptance (%)
0 0 0 0 0 0 0 0 5.4 17.3 29.5 54.5
754 754 754 754 754 754 754 754 694 711 728 734
3 0:6 3 0:3 0:3 0:3 0:3 0:3 0:3 0:3 0:3 0:3
Maximum
5:3ð2Þ 107 1:35ð5Þ 108 1:87ð7Þ 108 1:83ð7Þ 108 2:45ð10Þ 108 2:64ð12Þ 108 2:65ð13Þ 108
73.3 75.4 72.9 75.6 75.5 72.8 62.5 57.9 54.3 69.1 73.3 74.6
The two production rate types (normalized and maximum) are the counting rates of the 7Be beam normalized for the momentum acceptance of 3%, but the former is measured at a fixed momentum, and the latter represents the maximum achievable rate when the dipole magnet is tuned to the optimum setting (see text). The momentum acceptance and counting method were altered according to the production rate. The counting rates marked with stars (*) were measured by the scattering method, while the others were directly counted by the PPAC.
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The gas density r deviates according to the following equation:
0 Effective thickness reduction (%)
155
5
V
10 15
dr ¼ fQ þfF þ fT dt pffiffiffiffiffiffiffi ¼ Ar þ ðr0 rÞðrF þ rT Þ.
(6)
25
Here A is a constant which indicates the magnitude of the heat deposited. At the steady-state condition, this equation is equal to zero, and
30
Ar ¼ ðr0 rÞ2 ðrF þ rT Þ2 .
35
By rewriting this with R ¼ 1 r=r0 , we obtain
20
40 -10
0
10 20 30 40 Target gas circulation rate (slm)
50
60
Fig. 5. Effective thickness (or density) reduction observed at the hydrogen gas target with the original thickness of 2:3 mg=cm2 irradiated by the 7 2þ Li beam of 2:7 emA. The reduction is denoted as the relative deviation from the original thickness, measured with a low-current beam. The error is mostly systematic.
As shown in the figure, the target density decreased by about 30%, when there was no gas circulation. This amount of reduction agrees with the measurement reported in Ref. [6], which resulted in a reduction by 30% for a beam of 60 mW/mm. The dependence on the circulation rate assumed in Eq. (1) can be explained by the following simplified model. Here we assume that the length of the cell, and the current and profile of the primary beam are all fixed. The heat at the cell deposited by the beam Q is a function of the target gas density on the beam axis r, and approximately, Q / r.
(2)
The molecular velocity of the target gas is increased by the heat deposition Q, and gas flows from the beam position to the outer region. Therefore, the outgoing flow from the volume V, in which the target gas is irradiated by the beam, can be written as pffiffiffiffi fQ / Q pffiffiffi / r (3) where the negative sign indicates that the flow is outgoing. There are incoming flows due to the forced circulation f F and thermal diffusion f T , both of which act as the forces to restore the density r to the original (no-heat) value, r0 . f F and f T should be proportional to the gradient of the gas density, and here we write them as f F ¼ ðr0 rÞrF
(4)
f T ¼ ðr0 rÞrT
(5)
using constants rF and rT . rF is proportional to the forced circulation rate.
(7) the
Að1 RÞ ¼ r0 ðrF þ rT Þ2 R2 . This equation can be solved as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Að1 þ 1 þ 4r0 ðrF þ rT Þ2 =AÞ . R¼ 2r0 ðrF þ rT Þ2
density
reduction (8)
(9)
In our case, R is 30% at maximum and r0 ðrF þ rT Þ2 must be sufficiently larger than A. Then, it follows that sffiffiffiffiffi A 1 R’ (10) r0 ðrF þ rT Þ which is essentially the same form as Eq. (1). Note that the density reduction R has a square-root dependence on the magnitude of the heat deposition A in this model. In summary, we obtained a very intense ð2 108 ppsÞ 7 Be beam, having the energy of ð4:0 0:2Þ MeV=u. We have observed a reduction of the target density, probably due to the high heat deposition of the beam. The reduction was about 30% at maximum, when the beam deposited heat of 65 mW/mm in the gas. We succeeded in reducing this reduction effect to about 5% by circulating the gas at 55 slm. 3.5. Comparison of the 7Be production rate with calculations The results shown above indicate that a 7Be beam of 2:8 108 pps can be produced by the 7Li beam of 2:7 emA and the hydrogen gas target with a thickness of 2:3 mg=cm2 , when there is no density reduction effect. Here we compare the measured production rate with a calculated rate using a reaction cross-section measured previously. The differential cross-section of the 7Li(p,n)7Be reaction is known by the previous measurements (e.g. [9]), and it is about 42 mb/sr for the reaction energy in this study at 01. The solid angle of CRIB is 5.6 msr, according to the specification [2]. Considering the kinematics of the p(7Li,7Be)n reaction and the solid angle, the effective total cross-section was calculated as 9.3 mb. The acceptance of the momentum slits in the separator was evaluated from the measured momentum distribution of the 7Be beam described above. The slits accepted 75% of
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the 7Be beam when the momentum acceptance was 3%. There should be no significant decrease in the yield due to the selection of the charge state, as we selected the dominant 4þ state. Considering the effective cross-section, momentum acceptance, beam current ð2:7 emAÞ, and target thickness ð2:3 mg=cm2 Þ, the expected counting rate was calculated as 1:8 108 pps, which is of the same order but lower than the measured value. The main reason for the difference can be attributed to the solid angle of CRIB, that is, the effective solid angle is known to be nearly double the specification value of 5.6 msr. To have the consistency between the measured and calculated values, the effective solid angle should be about 9 msr, which agrees with our experience. This result also proves that there was no large unexpected effect that decreased the yield of 7Be. 4. Conclusion A cryogenic gas target system was developed for RI beam production at CRIB. The basic features were tested using heavy-ion beams at low energies. The target system was used for the production of 7Be and we obtained a beam of 2 108 pps, with a target of 2:3-mg=cm2 -thick hydrogen gas at 85–90 K. The target gas circulation at a rate of 55 slm can eliminate most of the target densityreduction effect, which was observed for a primary beam depositing heat of 65 mW/mm at the target gas.
Acknowledgments We are grateful to RIKEN and CNS accelerator staff for their help. This work was supported by a Grant-in-Aid for Young Scientists (B) (Grant no. 17740135) of JSPS. References [1] S. Kubono, Y. Yanagisawa, T. Teranishi, S. Kato, T. Kishida, S. Michimasa, Y. Ohshiro, S. Shimoura, K. Ue, S. Watanabe, N. Yamazaki, Eur. Phys. J. A 13 (2002) 217. [2] Y. Yanagisawa, S. Kubono, T. Teranishi, K. Ue, S. Michimasa, M. Notani, J. He, Y. Ohshiro, S. Shimoura, S. Watanabe, N. Yamazaki, H. Iwasaki, S. Kato, T. Kishida, T. Morikawa, Y. Mizoi, Nucl. Instr. and Meth. 539 (2005) 74. [3] T. Teranishi, S. Kubono, S. Shimoura, M. Notani, Y. Yanagisawa, S. Michimasa, K. Ue, H. Iwasaki, M. Kurokawa, Y. Satou, T. Morikawa, A. Saito, H. Baba, J. Lee, C. Lee, Z. Fulop, S. Kato, Phys. Lett. B 556 (2003) 27. [4] D. Semon, M. Allen, H. Dejbakhsh, C. Gagliardi, S. Hale, J. Jiang, L. Trache, R. Tribble, S. Yennello, H. Xu, X. Zhou, B. Brown, Phys. Rev. C 53 (1996) 96. [5] A. Azhari, V. Burjan, F. Carstoiu, C. Gagliardi, V. Kroha, A. Mukhamedzhanov, F. Nunes, X. Tang, L. Trache, R. Tribble, Phys. Rev. C 63 (2001) 055803. [6] J. Go¨erres, K. Kettner, H. Kra¨winkel, C. Rolfs, Nucl. Instr. and Meth. 177 (1980) 295. [7] J.F. Ziegler, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [8] H. Kumagai, A. Ozawa, N. Fukuda, K. Su¨mmerer, I. Tanihata, Nucl. Instr. and Meth. 470 (2001) 562. [9] S. Elbakr, I. Heerden, W. McDonald, G. Neilson, Nucl. Instr. and Meth. 105 (1972) 519.