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Ultralow-background photon-detection system for collinear laser spectroscopy of high-energy highly charged ion beams
Nuclear Instruments and Methods in Physics Research A 419 (1998) 50—59
Ultralow-background photon-detection system for collinear laser spectroscopy of high-energy highly charged ion beams M. Wakasugi *, T. Ariga, K. Ikegami , N. Inabe , M. Kase , T. Katayama, T. Kubo , H. Okuno , S. Ozawa, I. Tanihata , Y. Watanabe , A. Yoshida , Y. Yano RIKEN, The Institute of Physical and Chemical Research, wako, Saitama 351-01, Japan Department of Physics, Saitama University, Urawa, Saitama 338, Japan CNS, Center for Nuclear Study, School of Science, University of Tokyo, Tanashi, Tokyo 188, Japan Department of Physics, Rikkyo University, Toshima, Tokyo 175, Japan Received 21 May 1997; received in revised form 25 June 1998
Abstract An ultralow-background fluorescence-photon detection system has been successfully developed for the collinear laser spectroscopy of highly charged ion beams having a relativistic velocity. We applied the technology of a cryopump to a photon-detection system. A charcoal panel, optical filters and four movable collimators strongly reduced the background due to scattered photons of the incident laser beam. The background due to collisions between the ion beam and residual gas was also suppressed by improving the vacuum using a cryopump technique. With this system, we clearly observed the laser-induced fluorescence spectra of the 2S -2P transitions in an Li-like B> ion beam with a signal-to-noise ratio of 200. The ion-beam energy was 11.25A MeV and the intensity was 10 ions/bunch. We also demonstrated that the system has the capability of measuring the lifetimes of excited states. 1998 Elsevier Science B.V. All rights reserved. PACS: 39.30.#w; 32.30.Jc; 32.10.Fn Keywords: Ultralow-background fluorescence-photon detection system; Collinear laser spectroscopy
1. Introduction Systematic measurements of isotope shifts and hyperfine structures by means of standard collinear fast ion-beam laser spectroscopy [1—3] and its variants [4—6] have been highly successful in studying
* Corresponding author. Tel.: 81 48 467 9461; 81 48 461 5301; e-mail: [email protected].
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the electromagnetic properties of the nuclear ground states and isomeric states [7]. In these experiments, singly charged ion beams or neutral atomic beams having energies less than 60 keV were used. On the other hand, laser spectroscopy has also been applied to measuring the Lamb shifts, finestructure splittings, and transition probabilities in highly charged ions [8—18]. Some of those measurements [14—18] were performed by means
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 9 1 5 - 2
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
of collinear fast ion-beam laser spectroscopy. The purpose of these experiments was to check precise calculations [19—28] of the higher-order relativistic and QED contributions to the electronic states in relatively simple electronic systems, e.g. H-like and He-like ions. We plan to make a collinear laser (or X-ray) spectroscopy system that will be applied to highly charged (Li-like) radioactive isotope beams produced with a projectile-fragment separator [29]. The aim is to carry out a systematic study of the mean-square nuclear-charge radii of radioactive isotopes by means of measurements of the isotope shifts. In our future facility [30—33], a fragment separator will provide about 3000 radioactive-isotope beams of all elements. The combination of a collinear laser (X-ray) spectroscopy technique with a fragment separator will allow us to make quick measurements of the isotope shifts and to extend the measurable region of the isotopes in the nuclear chart toward the neutron-rich side and toward shorter lifetimes. As the first step of the experiment, we have developed a standard collinear laser spectroscopy system which includes an ultralow-background photon-detection system. The performance of this system was studied in the present experiment. We observed the resonance fluorescence spectra of the 2S -2P (D1 and D2) transitions in the Li-like B> ion accelerated up to 11.25A MeV with the RIKEN ring cyclotron. The ion beam, which we treat here, had a much higher-energy and highercharge state than those used in the previous experiments for isotope-shift measurements [1—7]; also, it was a bunched beam with a bunch length of about 1 ns. We had to detect the fluorescence photons from the same transition as that induced by the laser beam. That was the difference between our experiment and the previous experiments for highlycharged ions [8—18]. The ion-beam bunch took about 2 ns to pass through the detection region viewed by a photon-counting unit including a photomultiplier. During this interval, we had a chance to detect the fluorescence photons. At the same time, however, there was a large number of scattered photons from the laser beam in the detection region, and additional photons were produced
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by collisions between the ion beam and residual gas. There was much background and screen fluorescence photons. It was hard to distinguish the fluorescence photons from the background. An ultralow-background photon detection system was, therefore, required to detect a small number of fluorescence photons. Since in the present case the difference in wavelength between the laser beam and the fluorescence was about 40 nm, because of the Doppler effect, we used optical filters to isolate them. It was, however, insufficient to completely suppress the scattered light of the laser beam. We introduced movable collimators in order to reduce the scattered light reaching the detection region. Moreover, we pasted charcoal chips on the inner surface of the vacuum chamber at the detection region in order to make the reflection coefficient of light smaller. Because the background due to collisions between the ion beam and residual gas can be reduced by improving the vacuum in the detection region, we cooled the charcoal panel down to 18 K. This is exactly the cryopump technique. In this paper, the results of the first experiment using this system and the performance of the system are reported.
2. Experimental An Li-like B> ion beam, accelerated up to 11.25A MeV with the RIKEN ring cyclotron, was transported to the RIPS-beam line [34]. Our experimental setup, which is schematically shown in Fig. 1, was connected to the RIPS beam line. The system consisted of a photon-detector region, a laser-beam transport region and beam monitors (five ZnS plates and two plastic scintillators). The ion beam was collided with a counter-propagating laser beam at the collision point. The laser beam was provided by the combination of a secondharmonic generator (SHG) including a BBO crystal, with a pulsed dye laser (Lambda Physics SCANMATE 2E) excited by an excimer laser (Lambda Physics LPX240i). The spatial overlap and time overlap between both beams were confirmed with ZnS plates and plastic scintillators, respectively. The excited optical transitions were the 2S -2P (D1) and the 2S -2P (D2)
Fig. 1. Experimental setup and fluorescent photon detection system. The upper part shows the entire system and an expanded view of the photon detector is shown in the lower part.
52 M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
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transitions. The excited states emitted fluorescence photons during flight and the fluorescence photons were counted by the photon detector as a function of the laser wavelength. 2.1. Photon detector We applied the technology of a cryopump system to the photon detector in order to overcome the background problems described above. The use of a material having a small reflection coefficient for light on the inner surface of the vacuum chamber is effective for reducing any background due to scattered light of the laser. A charcoal chip, which is usually used in the cryopump system, was made of nearly pure carbon and had a lot of micro-holes. We measured the reflection rate of the laser light from the surface of a charcoal chip and compared the results with those for the cases of black paint and a black Al O coating on the aluminum surfa ces shown in Fig. 2. The scattered light intensity integrated over a 2p solid angle was about 13% of the incident laser beam intensity for black paint, 39% for black Al O coating, and 8% for the charcoal chip. This means that the charcoal chip is much better than the others as an absorber of scattered light. The inner surfaces of the other chambers made of stainless steel were coated with MoS . We could reduce the background produced by collisions between the ion beam and residual gas by improving the vacuum. Since charcoal chips absorb residual gas (e.g. H O, O , N , CO , etc.) when they are cooled down to less than 30 K a higher vacuum could be obtained at the detection region. Actually, the pressure at the detection region was 8;10\ Torr and 8;10\ Torr at 298 and 18 K, respectively. The photon detector had two tubes inside the vacuum chamber, as shown in Fig. 1. The inner tube was made of copper and painted black; charcoal chips were pasted at the central region of the tube. The inner tube was connected to the second stage of the refrigerator of the cryopump and cooled down to 18 K. The size of the inner tube was 112 mm in inner diameter and 960 mm in length. The outer tube was a heat shield which protected the inner tube from any thermal radiation. This was
Fig. 2. Reflection rate of laser light for three kinds of surface materials. The data are given in units of % per steradian. The measurement was performed using a 480 nm laser. The open squares are for matt-black paint on an aluminum surface, the open triangles for a black Al O coating on an aluminum surface, and the closed circles for a charcoal chip.
also painted black and cooled down to 80 K by connecting it to the first stage of a refrigerator. These tubes had 22 holes at the detection region in order to mount ten photon-counting units (two holes for one unit) and two vacuum gauges. The total-radiation power coming into the inner tube was estimated to be 9.5 W. The cooling power of the cold head driver used here was 10.5 W for the second stage and 68 W for the first stage. The photon-counting units were arranged in three rows; four units were set along the beam axis, as shown in the lower part of Fig. 1. The distance between them was 85 mm. Each photon-counting unit consisted of a dichroic mirror, two UV-quartz lenses, two dichroic filters, a collimator, and a photomultiplier (HAMAMATSU R3197) having a photocathode of CsTe. The fluorescence photons emitted from the ion beam were collected with the mirror and the lenses. A collimator having a 3 mm width and 50 mm length was placed in the focal plane for the 206 nm fluorescence light. The transmission of the two dichroic filters was less than 1% at the incident-laser wavelength of 241.5 nm and more than 95% at the fluorescence wavelength of 206 nm. The geometrical photon-collection efficiency (e ) was estimated to be 5%, and the
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M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
quantum efficiency (e ) of the photomultiplier was .+ 13.5%. The detection efficiency for fluorescence photons emitted in the detection region was e e .+ "0.68%. 2.2. Laser beam transport One more important component for reducing the background due to scattered laser light was the laser beam-transport region comprising four movable-collimator units, as shown in Fig. 1. Each unit consisted of a movable collimator and a ZnS viewer. The position of each collimator could be moved in a plane perpendicular to the beam axis; and the size of the collimator could be continuously changed from 1 to 30 mm in diameter. The distance between the two collimators was 400 mm. When the laser beam was collimated by the collimator before the entrance window, a large amount of scattered light was produced at the edge of the first movable collimator. The second movable collimator reduced the intensity of the scattered light produced by the first collimator by a factor of about 300, as shown in Fig. 3. Since there were four movable collimators in our system, the scattered
Fig. 3. Angular distribution of scattered light after collimators. The laser wavelength was 480 nm. The open squares show the distribution after the first collimator, and the closed circles that after the second collimator. The data at 70° for the first collimator have been normalized to unity.
light produced at the first collimator was reduced by a factor of 10\ at the detection region. It was important that the position and size of the collimators was carefully adjusted one by one along the laser-beam axis. The size of the first collimator was 2 mm in diameter; also, the size of each collimator had to be slightly larger than that of the previous one in order to remove only scattered components of the laser beam. The size of the last collimator was about 3 mm in diameter. We adjusted the position and size of each collimator while monitoring the laser beam profile with a ZnS plate (ZnS(2)-(5) in Fig. 1). The optimum settings of the four collimators were finally found with fine corrections, so that the count rate of laser noise with the photomultipliers could be minimized. The thickness of the collimators was about 10 lm in order to reduce the scattered light from the edge of the collimators. 2.3. Experimental procedure First, we adjusted the parameters of all magnets in the RIPS beam line, so that the Li-like B> ion-beam axis was on the central orbit of the RIPS beam line. Then, the beam velocity was derived to be b"0.15394(7) from the magnetic fields of the bending magnets of the RIPS. The ion beam was chopped at a repetition rate of 100 Hz. The chopped beam included two micro-bunches, of which the bunch length was about 1 ns. The ion beam intensity was about 10 ions/bunch. According to the NBS data [35], the excitation energies of the D1 and D2 transitions are 48358.5 cm\ (206.789 nm) and 48392.6 cm\ (206.643 nm), respectively, in the Li-like B> ion. Because of the Doppler shift, the resonance wavelength of the counter propagating laser was expected to be 241.501 and 241.330 nm, respectively. The output laser power from the SHG was about 100 lJ/pulse and the laser power injected into the beam line was reduced to be about 10 lJ/pulse by the first collimator. The laser was operated at a repetition rate of 50 Hz in order to maintain stable operation for a long time. The laser pulse width was about 15 ns. The laser was collided with the first bunch in the chopped beam at the collision point.
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
The spatial overlap between the ion beam and the laser beam was confirmed using two viewers, ZnS(1) and ZnS(2) (see Fig. 1), placed at the entrance and exit of the photon detector, respectively. Since ZnS emits fluorescence for both the ion beam and the UV laser beam, the positions of both beams on the ZnS plates could be monitored at the same time. The ion beam path was slightly adjusted using steering magnets and overlapped the laser beam. The ion-beam size in the detection region was about 2 mm in diameter and the size of the laser was also 2 mm in diameter. The time overlap between the two beams is essential in this experiment. This was controlled as follows. A fast signal from the master oscillator (MO) of the cyclotron was used as the origin of time. Using the MO signal and some delay, an external trigger for the excimer laser was made, so that both beams collided with each other at the collision point. A plastic scintillator (1) placed upstream of the photon detector was used in order to choose the delay time for the laser trigger. Since the plastic scintillator (1) was a destructive monitor, another plastic scintillator (2) placed at the end of the beam line was used as an on-line timing monitor during the measurement. The plastic scintillator (2) had a hole at the center, so that it did not intercept the laser beam. When the delay time was optimized, the time difference between the ion beam and the laser beam at the plastic scintillator (2) was !90 ns. This time difference was kept within $5 ns during the measurement.
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Fig. 4. Timing spectra measured at temperatures of the charcoal panel of 298 K (closed circles) without a laser, and 18 K with a laser (open squares).
second bunch completely disappeared, and a sharp peak was observed at the position of the first bunch, indicating that the noise from collisions between the ion beam and the residual gas could be greatly suppressed by improving the vacuum with the cryopump technique, and that the peak from the first bunch at 18 K contained only the fluorescence photon signal. The fluorescence peak was well separated from the background due to scattered laser light. This means that the noise due to scattered laser light was reduced to the same level as that of the fluorescence signal in our photondetection system. 3.2. Fluorescence spectrum
3. Results 3.1. Background suppression The timing spectra measured with the photomultiplier at charcoal-panel temperatures of 298 and 18 K are shown in Fig. 4. The laser beam was injected and the wavelength was scanned over the resonance in the case of 18 K, but the laser beam was absent in the case of 298 K. Signals from both the first and second bunches were clearly observed at 298 K. These were noise due to collisions between the ion beam and the residual gas. In the case of 18 K, however, the signal from the
Taking a timing gate at the first bunch in Fig. 4, the fluorescence spectra were obtained as shown in Fig. 5. Spectrum (a) shows a wide-range spectrum including both the D1 and D2 transitions, and (b) and (c) show expanded views around both transitions measured with smaller scanning steps of the laser wavelength. As can be seen in Fig. b, the relative intensity of the resonance peak and the average background are about 80 and 0.4, respectively. The signal-to-noise ratio is about 200. The ratio has been improved by a factor of more than 200 compared with our previous result [36]. If the signal-to-noise ratio of 1.0 is assumed to be the
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M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
Fig. 5. Observed fluorescence spectra of the 2S -2P transitions in the B> ion beam. Fig. 5a shows the spectrum obtained with a wide scanning range of the laser wavelength; (b) and (c) show expanded views obtained with smaller scanning steps around the 2S -2P (D2) and the 2S -2P (D1) transitions, respectively.
limit of sensitivity, the lower limit of the ion-beam intensity for the present experiment is estimated to be about 50 ions/bunch. The detection efficiency for fluorescence photons has been found to be e "3.3;10\ counts/ion from the spectrum. On the other hand, the expected efficiency is as follows. The excitation efficiency was estimated to be e "2.5% by taking into account the momentum spread of the ion beam and the population of the hyperfine sublevels in the ground state. The survival efficiency of the excited state is e "10% because of the time-of-flight from the collision point to the detection region. As described above, the photon-collection efficiency is e e "0.68%. Finally, the expected detection effi .+ ciency is e "e ;e ;e ;e "1.7;10\. .+
The difference between the estimation and the experimental value may come from the uncertainty in the absolute ion-beam current, incompleteness of the spatial overlap and fluctuation of the time overlap. The absolute excitation energies of the D1 and D2 transitions were derived to be 48346.8(33) cm\ and 48380.0(33) cm\, respectively. Both values are about 12 cm\ smaller than the NBS values (D1"48358.5 cm\ and D2"48392.6 cm\) [35]. These deviations may be due to systematic errors in the ion-beam velocity and the calibration of the laser wavelength. The fine-structure splitting obtained from our measurement is *E(P P )"33.2(47) cm\. That is the relative value between the D1 and the D2 transitions, and is in
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
good agreement with the NBS value of *E(P P ) "34.1 cm\. To make a precision measurement of the absolute excitation energies, the following improvements are required in the next step. An absorption spectrum of Te or I and a fluorescence spectrum should be observed simultaneously for laser wavelength calibration. To suppress the uncertainty in the absolute ion velocity, two laser beams, which propagate in the parallel and the anti-parallel directions to the ion beam, should be used in the measurement. According to the Doppler-shift formula, the excitation energy (E ) can be determined independently of the ion-beam velocity from E "(E E , where E and E are the resonance energies obtained with the two lasers. From the observed fluorescence spectrum, the momentum spread of the ion beam was also derived. The resonance line width is 130(30) GHz (0.025(6) nm). This value corresponds to a momentum spread of *P/P"6(1);10\. The line width includes the natural line width and the laser power broadening. These contributions, however, are negligible compared with the Doppler broadening due to the momentum spread of the ion beam.
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Fig. 6. Different timing spectra measured with photomultipliers (a)—(d). Labels (a)—(d) indicate the photomultiplier position along the ion-beam axis, as shown in Fig. 1. The data in the figure have been normalized by the total counts of laser noise.
3.3. Lifetime of the excited state This fluorescence photon-detection system could also measure the lifetime of the excited P state. The count rate of fluorescence photons for each photomultiplier depends on the time of flight of the ion beam from the collision point to the detection region. Fig. 6 shows timing spectra measured with photomultipliers (a—d) for the D1 transition; here, (a—d) indicates the photomultiplier position along the ion beam axis, as shown in Fig. 1. Assuming that the timing of the laser noise signals is the same for every photomultiplier, it can be seen in Fig. 6 that the position of the fluorescence peaks is shifted according to the time of flight. In fact, the time difference between (a) and (d) is found to be about 5.5 ns from the spectra; this is in good agreement with the estimated time of flight of 5.6 ns. The ratio of the fluorescence intensity to the noise due to the scattered laser light decreases with increasing time of flight. Assuming that the scattered laser light is
Fig. 7. Decay curve of the 2P state obtained from the data shown in Fig. 6. The relative fluorescence yield for the photomultiplier (a) has been normalized to unity.
uniform in the detector chamber, integrated fluorescence signals can be normalized by the total counts of the laser noise. The relative yields of the fluorescence for photomultipliers (a—d) are plotted as a function of time of flight in Fig. 7. This figure shows a decay curve of the excited state. The lifetime of the 2P state was estimated to be q"3.6(#2.0/!0.6) ns by making a least-squares
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M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
fit to the experimental data. The lifetime agrees well with the expected value of 5 ns from a MCDF (Multi-Configuration Dirac-Fock) calculation [37]. 4. Summary A low-background fluorescence photon-detection system has been successfully developed. The performance has been demonstrated by collinear laser spectroscopy on an Li-like B> ion beam having relativistic velocity. The combination of cryopump technology with a movable collimator system drastically suppressed the background due to the scattered photons of the laser and collisions between the ion beam and residual gas. A signalto-noise ratio of 200 was obtained at an ion-beam intensity of 10 ions/bunch, and we estimate the lower limit of the ion-beam intensity for our measurement to be 50 ions/bunch. From the present experiment, we conclude that this system is quite useful for the collinear laser spectroscopy of high-energy highly charged ion beams. References [1] A.C. Mueller, F. Buchinger, W. Klempt, E.W. Otten, R. Neugart, C. Ekstro¨m, J. Heinemeier, Nucl. Phys. A 403 (1983) 234. [2] G. Ulm, S.K. Bhattacherjee, P. Dabkiewicz, G. Huber, H.-J. Kluge, T. Ku¨hl, H. Lochmann, E.W. Otten, K. Wendt, S.A. Ahmad, W. Klempt, R. Neugart, Z. Phys. A 325 (1986) 247. [3] J. Eberz, U. Dinger, G. Huber, H. Lochmann, R. Menges, R. Newgart, R. Kirchner, O. Klepper, T. Ku¨hl, D. Marx, G. Ulm, K. Wendt, Nucl. Phys. A 464 (1987) 9. [4] D.A. Eastham, P.M. Walker, J.R.H. Smith, D.D. Warner, J.A.R. Griffith, D.E. Evans, S.A. Wells, M.J. Fawcett, I.S. Grant, Phys. Rev. C 36 (1987) 1583. [5] R. Neugart, W. Klempt, K. Wendt, Nucl. Instr. and Methods B 17 (1986) 354. [6] E. Arnold, J. Bonn, R. Gegenwart, W. Neu, R. Neugart, E.W. Otten, G. Ulm, K. Wendt, Phys. Lett. B 197 (1987) 311. [7] E.W. Otten, Treaties on heavy-ion science, vol. 8 in: D.A. Bromly (Ed.), Plenum, New York, 1989, p. 517. [8] H.W. Kugel, M. Leventhal, D.E. Murnick, C.K.N. Patel, O.R. Wood, Phys. Rev. Lett. 35 (1975) 647. [9] O.R. Wood, C.K.N. Patel, D.E. Murnick, E.T. Nelson, M. Leventhal, H.W. Kugel, Y. Niv, Phys. Rev. Lett. 48 (1982) 398.
[10] P. Pellegrin, Y. El Masri, L. Palffy, R. Prieels, Phys. Rev. Lett. 49 (1982) 1762. [11] H.D. Stra¨ter, L. von Gerdtell, A.P. Georgiadis, D. Mu¨ller, P. von Brentano, J.C. Sens, A. Pape, Phys. Rev. A 29 (1984) 1596. [12] E.G. Myers, Nucl. Instr. and Meth. B 9 (1985) 662. [13] H.-J. Pross, D. Budelsky, L. Kremer, D. Platte, P. von Brentano, J. Gassen, D. Mu¨ller, F. Scheuer, A. Pape, C. Sens, Phys. Rev. A 48 (1993) 1875. [14] T.P. Dinneen, N. Berrah-Mansour, H.G. Berry, L. Young R.C. Pardo, Phys. Rev. Lett. 66 (1991) 2859. [15] T.J. Scholl, R. Cameron, S.D. Rosner, L. Zhang, R.A. Holt, C.J. Sansonetti, J.D. Gillaspy, Phys. Rev. Lett. 71 (1993) 2188. [16] I. Klaft, S. Borneis, T. Engel, B. Fricke, R. Grieser, G. Huber, T. Ku¨hl, D. Marx, R. Neumann, S. Schro¨der, P. Seeling, L. Vo¨lker, Phys. Rev. Lett. 73 (1994) 2425. [17] E.G. Myers, J.K. Thompson, E.P. Gavathas, N.R. Claussen, J.D. Silver, D.J.H. Howie, Phys. Rev. Lett. 75 (1995) 3637. [18] E.G. Myers, D.J.H. Howie, J.K. Thompson, J.D. Silver, Phys. Rev. Lett. 76 (1996) 4899. [19] G.W. Erickson, J. Phys. Chem. Ref. Data 6 (1977) 831. [20] P.J. Mohr, Phys. Rev. A 26 (1982) 2338. [21] G.W.F. Drake, Adv. At. Mol. Phys. 18 (1982) 399. [22] G.W.F. Drake, Can. J. Phys. 66 (1988) 586. [23] M.H. Chen, K.T. Cheng, W.R. Johnson, Phys. Rev. A 47 (1993) 3692. [24] K.T. Cheng, M.H. Chen, W.R. Johnson, J. Sapirstein, Phys. Rev. A 50 (1994) 247. [25] Z.-C. Yan, G.W.F. Drake, Phys. Rev. Lett. 74 (1995) 4791. [26] T. Zhang, Z.-C. Yan, G.W.F. Drake, Bull. Am. Phys. Soc. 40 (1995) 1284. [27] Z.-C. Yan, G.W.F. Drake, Phys. Rev. Lett. 74 (1995) 4791. [28] H. Persson, S.M. Schneider, W. Greiner, G. Soff, I. Lindgren, Phys. Rev. Lett. 76 (1996) 1433. [29] M. Wakasugi, N. Inabe, T. Katayama, Proc. 5th European Particle Accelerator Conf., Institut of Physics Publ., Bristol, Philadelphia 1996, p. 611. [30] Y. Yano, A. Goto, Y. Miyazawa, M. Hemmi, T. Chiba, T. Kageyama, K. Ikegami, M. Takanaka, T. Kawaguchi, S. Kohara, M. Kase, T. Tominaka, Y. Batygin, T. Kubo, T. Nakagawa, Y. Tanaka, E. Ikezawa, M. Wakasugi, T. Mitsumoto, N. Inabe, O. Kamigaito, H. Okuno, S. Fujishima, A. Ohtomo, T. Ohkawa, T. Katayama, S. Watanabe, M. Tomozawa, J. Xia, Y. Rao, Y. Yuan, Proc. 14th Int. Conf. on Cyclotron and Their Application, World Science, Singapore 1995, p. 596. [31] Y. Yano, A. Goto, T. Katayama, Proc. 5th European Particle Accelerator Conf. Institut of Physics Publ., Bristol, Philadelphia 1996, p. 536. [32] T. Katayama, S. Watanabe, Y. Batygin, N. Inabe, K. Ohtomo, T. Ohkawa, M. Takanaka, M. Wakasugi, Y. Yano, K. Yoshida, J. Xia, Y. Rao, Y. Yuan, Proc. 5th European Particle Accelerator Conf. Institut of Physics Publ., Bristol, Philadelphia 1996, p. 563.
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59 [33] I. Tanihata, Nucl. Phys. A 588 (1995) 253c. [34] T. Kubo, M. Ishihara, N. Inabe, H. Kumagai, I. Tanihata, K. Yoshida, T. Nakamura, H. Okuno, S. Shimoura, K. Asahi, Nucl. Instr. and Meth. B 70 (1992) 309. [35] C.E. Moor, ‘Atomic Energy Levels’, NBS, Washington DC, 1949, p. 19.
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[36] M. Wakasugi, N. Inabe, T. Kubo, H. Okuno, I. Tanihata, Y. Watanabe, Y. Yano, A. Yoshida, M. Hies, T. Katayama, S. Ozawa, Physica Scr. T 73 (1997) 70. [37] K.T. Cheng, Y.-K. Kim, J.P. Desclaux, Atom. Data Nucl. Data Tables 24 (1979) 111.
Ultralow-background photon-detection system for collinear laser spectroscopy of high-energy highly charged ion beams M. Wakasugi *, T. Ariga, K. Ikegami , N. Inabe , M. Kase , T. Katayama, T. Kubo , H. Okuno , S. Ozawa, I. Tanihata , Y. Watanabe , A. Yoshida , Y. Yano RIKEN, The Institute of Physical and Chemical Research, wako, Saitama 351-01, Japan Department of Physics, Saitama University, Urawa, Saitama 338, Japan CNS, Center for Nuclear Study, School of Science, University of Tokyo, Tanashi, Tokyo 188, Japan Department of Physics, Rikkyo University, Toshima, Tokyo 175, Japan Received 21 May 1997; received in revised form 25 June 1998
Abstract An ultralow-background fluorescence-photon detection system has been successfully developed for the collinear laser spectroscopy of highly charged ion beams having a relativistic velocity. We applied the technology of a cryopump to a photon-detection system. A charcoal panel, optical filters and four movable collimators strongly reduced the background due to scattered photons of the incident laser beam. The background due to collisions between the ion beam and residual gas was also suppressed by improving the vacuum using a cryopump technique. With this system, we clearly observed the laser-induced fluorescence spectra of the 2S -2P transitions in an Li-like B> ion beam with a signal-to-noise ratio of 200. The ion-beam energy was 11.25A MeV and the intensity was 10 ions/bunch. We also demonstrated that the system has the capability of measuring the lifetimes of excited states. 1998 Elsevier Science B.V. All rights reserved. PACS: 39.30.#w; 32.30.Jc; 32.10.Fn Keywords: Ultralow-background fluorescence-photon detection system; Collinear laser spectroscopy
1. Introduction Systematic measurements of isotope shifts and hyperfine structures by means of standard collinear fast ion-beam laser spectroscopy [1—3] and its variants [4—6] have been highly successful in studying
* Corresponding author. Tel.: 81 48 467 9461; 81 48 461 5301; e-mail: [email protected].
fax:
the electromagnetic properties of the nuclear ground states and isomeric states [7]. In these experiments, singly charged ion beams or neutral atomic beams having energies less than 60 keV were used. On the other hand, laser spectroscopy has also been applied to measuring the Lamb shifts, finestructure splittings, and transition probabilities in highly charged ions [8—18]. Some of those measurements [14—18] were performed by means
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 9 1 5 - 2
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of collinear fast ion-beam laser spectroscopy. The purpose of these experiments was to check precise calculations [19—28] of the higher-order relativistic and QED contributions to the electronic states in relatively simple electronic systems, e.g. H-like and He-like ions. We plan to make a collinear laser (or X-ray) spectroscopy system that will be applied to highly charged (Li-like) radioactive isotope beams produced with a projectile-fragment separator [29]. The aim is to carry out a systematic study of the mean-square nuclear-charge radii of radioactive isotopes by means of measurements of the isotope shifts. In our future facility [30—33], a fragment separator will provide about 3000 radioactive-isotope beams of all elements. The combination of a collinear laser (X-ray) spectroscopy technique with a fragment separator will allow us to make quick measurements of the isotope shifts and to extend the measurable region of the isotopes in the nuclear chart toward the neutron-rich side and toward shorter lifetimes. As the first step of the experiment, we have developed a standard collinear laser spectroscopy system which includes an ultralow-background photon-detection system. The performance of this system was studied in the present experiment. We observed the resonance fluorescence spectra of the 2S -2P (D1 and D2) transitions in the Li-like B> ion accelerated up to 11.25A MeV with the RIKEN ring cyclotron. The ion beam, which we treat here, had a much higher-energy and highercharge state than those used in the previous experiments for isotope-shift measurements [1—7]; also, it was a bunched beam with a bunch length of about 1 ns. We had to detect the fluorescence photons from the same transition as that induced by the laser beam. That was the difference between our experiment and the previous experiments for highlycharged ions [8—18]. The ion-beam bunch took about 2 ns to pass through the detection region viewed by a photon-counting unit including a photomultiplier. During this interval, we had a chance to detect the fluorescence photons. At the same time, however, there was a large number of scattered photons from the laser beam in the detection region, and additional photons were produced
51
by collisions between the ion beam and residual gas. There was much background and screen fluorescence photons. It was hard to distinguish the fluorescence photons from the background. An ultralow-background photon detection system was, therefore, required to detect a small number of fluorescence photons. Since in the present case the difference in wavelength between the laser beam and the fluorescence was about 40 nm, because of the Doppler effect, we used optical filters to isolate them. It was, however, insufficient to completely suppress the scattered light of the laser beam. We introduced movable collimators in order to reduce the scattered light reaching the detection region. Moreover, we pasted charcoal chips on the inner surface of the vacuum chamber at the detection region in order to make the reflection coefficient of light smaller. Because the background due to collisions between the ion beam and residual gas can be reduced by improving the vacuum in the detection region, we cooled the charcoal panel down to 18 K. This is exactly the cryopump technique. In this paper, the results of the first experiment using this system and the performance of the system are reported.
2. Experimental An Li-like B> ion beam, accelerated up to 11.25A MeV with the RIKEN ring cyclotron, was transported to the RIPS-beam line [34]. Our experimental setup, which is schematically shown in Fig. 1, was connected to the RIPS beam line. The system consisted of a photon-detector region, a laser-beam transport region and beam monitors (five ZnS plates and two plastic scintillators). The ion beam was collided with a counter-propagating laser beam at the collision point. The laser beam was provided by the combination of a secondharmonic generator (SHG) including a BBO crystal, with a pulsed dye laser (Lambda Physics SCANMATE 2E) excited by an excimer laser (Lambda Physics LPX240i). The spatial overlap and time overlap between both beams were confirmed with ZnS plates and plastic scintillators, respectively. The excited optical transitions were the 2S -2P (D1) and the 2S -2P (D2)
Fig. 1. Experimental setup and fluorescent photon detection system. The upper part shows the entire system and an expanded view of the photon detector is shown in the lower part.
52 M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
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transitions. The excited states emitted fluorescence photons during flight and the fluorescence photons were counted by the photon detector as a function of the laser wavelength. 2.1. Photon detector We applied the technology of a cryopump system to the photon detector in order to overcome the background problems described above. The use of a material having a small reflection coefficient for light on the inner surface of the vacuum chamber is effective for reducing any background due to scattered light of the laser. A charcoal chip, which is usually used in the cryopump system, was made of nearly pure carbon and had a lot of micro-holes. We measured the reflection rate of the laser light from the surface of a charcoal chip and compared the results with those for the cases of black paint and a black Al O coating on the aluminum surfa ces shown in Fig. 2. The scattered light intensity integrated over a 2p solid angle was about 13% of the incident laser beam intensity for black paint, 39% for black Al O coating, and 8% for the charcoal chip. This means that the charcoal chip is much better than the others as an absorber of scattered light. The inner surfaces of the other chambers made of stainless steel were coated with MoS . We could reduce the background produced by collisions between the ion beam and residual gas by improving the vacuum. Since charcoal chips absorb residual gas (e.g. H O, O , N , CO , etc.) when they are cooled down to less than 30 K a higher vacuum could be obtained at the detection region. Actually, the pressure at the detection region was 8;10\ Torr and 8;10\ Torr at 298 and 18 K, respectively. The photon detector had two tubes inside the vacuum chamber, as shown in Fig. 1. The inner tube was made of copper and painted black; charcoal chips were pasted at the central region of the tube. The inner tube was connected to the second stage of the refrigerator of the cryopump and cooled down to 18 K. The size of the inner tube was 112 mm in inner diameter and 960 mm in length. The outer tube was a heat shield which protected the inner tube from any thermal radiation. This was
Fig. 2. Reflection rate of laser light for three kinds of surface materials. The data are given in units of % per steradian. The measurement was performed using a 480 nm laser. The open squares are for matt-black paint on an aluminum surface, the open triangles for a black Al O coating on an aluminum surface, and the closed circles for a charcoal chip.
also painted black and cooled down to 80 K by connecting it to the first stage of a refrigerator. These tubes had 22 holes at the detection region in order to mount ten photon-counting units (two holes for one unit) and two vacuum gauges. The total-radiation power coming into the inner tube was estimated to be 9.5 W. The cooling power of the cold head driver used here was 10.5 W for the second stage and 68 W for the first stage. The photon-counting units were arranged in three rows; four units were set along the beam axis, as shown in the lower part of Fig. 1. The distance between them was 85 mm. Each photon-counting unit consisted of a dichroic mirror, two UV-quartz lenses, two dichroic filters, a collimator, and a photomultiplier (HAMAMATSU R3197) having a photocathode of CsTe. The fluorescence photons emitted from the ion beam were collected with the mirror and the lenses. A collimator having a 3 mm width and 50 mm length was placed in the focal plane for the 206 nm fluorescence light. The transmission of the two dichroic filters was less than 1% at the incident-laser wavelength of 241.5 nm and more than 95% at the fluorescence wavelength of 206 nm. The geometrical photon-collection efficiency (e ) was estimated to be 5%, and the
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M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
quantum efficiency (e ) of the photomultiplier was .+ 13.5%. The detection efficiency for fluorescence photons emitted in the detection region was e e .+ "0.68%. 2.2. Laser beam transport One more important component for reducing the background due to scattered laser light was the laser beam-transport region comprising four movable-collimator units, as shown in Fig. 1. Each unit consisted of a movable collimator and a ZnS viewer. The position of each collimator could be moved in a plane perpendicular to the beam axis; and the size of the collimator could be continuously changed from 1 to 30 mm in diameter. The distance between the two collimators was 400 mm. When the laser beam was collimated by the collimator before the entrance window, a large amount of scattered light was produced at the edge of the first movable collimator. The second movable collimator reduced the intensity of the scattered light produced by the first collimator by a factor of about 300, as shown in Fig. 3. Since there were four movable collimators in our system, the scattered
Fig. 3. Angular distribution of scattered light after collimators. The laser wavelength was 480 nm. The open squares show the distribution after the first collimator, and the closed circles that after the second collimator. The data at 70° for the first collimator have been normalized to unity.
light produced at the first collimator was reduced by a factor of 10\ at the detection region. It was important that the position and size of the collimators was carefully adjusted one by one along the laser-beam axis. The size of the first collimator was 2 mm in diameter; also, the size of each collimator had to be slightly larger than that of the previous one in order to remove only scattered components of the laser beam. The size of the last collimator was about 3 mm in diameter. We adjusted the position and size of each collimator while monitoring the laser beam profile with a ZnS plate (ZnS(2)-(5) in Fig. 1). The optimum settings of the four collimators were finally found with fine corrections, so that the count rate of laser noise with the photomultipliers could be minimized. The thickness of the collimators was about 10 lm in order to reduce the scattered light from the edge of the collimators. 2.3. Experimental procedure First, we adjusted the parameters of all magnets in the RIPS beam line, so that the Li-like B> ion-beam axis was on the central orbit of the RIPS beam line. Then, the beam velocity was derived to be b"0.15394(7) from the magnetic fields of the bending magnets of the RIPS. The ion beam was chopped at a repetition rate of 100 Hz. The chopped beam included two micro-bunches, of which the bunch length was about 1 ns. The ion beam intensity was about 10 ions/bunch. According to the NBS data [35], the excitation energies of the D1 and D2 transitions are 48358.5 cm\ (206.789 nm) and 48392.6 cm\ (206.643 nm), respectively, in the Li-like B> ion. Because of the Doppler shift, the resonance wavelength of the counter propagating laser was expected to be 241.501 and 241.330 nm, respectively. The output laser power from the SHG was about 100 lJ/pulse and the laser power injected into the beam line was reduced to be about 10 lJ/pulse by the first collimator. The laser was operated at a repetition rate of 50 Hz in order to maintain stable operation for a long time. The laser pulse width was about 15 ns. The laser was collided with the first bunch in the chopped beam at the collision point.
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
The spatial overlap between the ion beam and the laser beam was confirmed using two viewers, ZnS(1) and ZnS(2) (see Fig. 1), placed at the entrance and exit of the photon detector, respectively. Since ZnS emits fluorescence for both the ion beam and the UV laser beam, the positions of both beams on the ZnS plates could be monitored at the same time. The ion beam path was slightly adjusted using steering magnets and overlapped the laser beam. The ion-beam size in the detection region was about 2 mm in diameter and the size of the laser was also 2 mm in diameter. The time overlap between the two beams is essential in this experiment. This was controlled as follows. A fast signal from the master oscillator (MO) of the cyclotron was used as the origin of time. Using the MO signal and some delay, an external trigger for the excimer laser was made, so that both beams collided with each other at the collision point. A plastic scintillator (1) placed upstream of the photon detector was used in order to choose the delay time for the laser trigger. Since the plastic scintillator (1) was a destructive monitor, another plastic scintillator (2) placed at the end of the beam line was used as an on-line timing monitor during the measurement. The plastic scintillator (2) had a hole at the center, so that it did not intercept the laser beam. When the delay time was optimized, the time difference between the ion beam and the laser beam at the plastic scintillator (2) was !90 ns. This time difference was kept within $5 ns during the measurement.
55
Fig. 4. Timing spectra measured at temperatures of the charcoal panel of 298 K (closed circles) without a laser, and 18 K with a laser (open squares).
second bunch completely disappeared, and a sharp peak was observed at the position of the first bunch, indicating that the noise from collisions between the ion beam and the residual gas could be greatly suppressed by improving the vacuum with the cryopump technique, and that the peak from the first bunch at 18 K contained only the fluorescence photon signal. The fluorescence peak was well separated from the background due to scattered laser light. This means that the noise due to scattered laser light was reduced to the same level as that of the fluorescence signal in our photondetection system. 3.2. Fluorescence spectrum
3. Results 3.1. Background suppression The timing spectra measured with the photomultiplier at charcoal-panel temperatures of 298 and 18 K are shown in Fig. 4. The laser beam was injected and the wavelength was scanned over the resonance in the case of 18 K, but the laser beam was absent in the case of 298 K. Signals from both the first and second bunches were clearly observed at 298 K. These were noise due to collisions between the ion beam and the residual gas. In the case of 18 K, however, the signal from the
Taking a timing gate at the first bunch in Fig. 4, the fluorescence spectra were obtained as shown in Fig. 5. Spectrum (a) shows a wide-range spectrum including both the D1 and D2 transitions, and (b) and (c) show expanded views around both transitions measured with smaller scanning steps of the laser wavelength. As can be seen in Fig. b, the relative intensity of the resonance peak and the average background are about 80 and 0.4, respectively. The signal-to-noise ratio is about 200. The ratio has been improved by a factor of more than 200 compared with our previous result [36]. If the signal-to-noise ratio of 1.0 is assumed to be the
56
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
Fig. 5. Observed fluorescence spectra of the 2S -2P transitions in the B> ion beam. Fig. 5a shows the spectrum obtained with a wide scanning range of the laser wavelength; (b) and (c) show expanded views obtained with smaller scanning steps around the 2S -2P (D2) and the 2S -2P (D1) transitions, respectively.
limit of sensitivity, the lower limit of the ion-beam intensity for the present experiment is estimated to be about 50 ions/bunch. The detection efficiency for fluorescence photons has been found to be e "3.3;10\ counts/ion from the spectrum. On the other hand, the expected efficiency is as follows. The excitation efficiency was estimated to be e "2.5% by taking into account the momentum spread of the ion beam and the population of the hyperfine sublevels in the ground state. The survival efficiency of the excited state is e "10% because of the time-of-flight from the collision point to the detection region. As described above, the photon-collection efficiency is e e "0.68%. Finally, the expected detection effi .+ ciency is e "e ;e ;e ;e "1.7;10\. .+
The difference between the estimation and the experimental value may come from the uncertainty in the absolute ion-beam current, incompleteness of the spatial overlap and fluctuation of the time overlap. The absolute excitation energies of the D1 and D2 transitions were derived to be 48346.8(33) cm\ and 48380.0(33) cm\, respectively. Both values are about 12 cm\ smaller than the NBS values (D1"48358.5 cm\ and D2"48392.6 cm\) [35]. These deviations may be due to systematic errors in the ion-beam velocity and the calibration of the laser wavelength. The fine-structure splitting obtained from our measurement is *E(P P )"33.2(47) cm\. That is the relative value between the D1 and the D2 transitions, and is in
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59
good agreement with the NBS value of *E(P P ) "34.1 cm\. To make a precision measurement of the absolute excitation energies, the following improvements are required in the next step. An absorption spectrum of Te or I and a fluorescence spectrum should be observed simultaneously for laser wavelength calibration. To suppress the uncertainty in the absolute ion velocity, two laser beams, which propagate in the parallel and the anti-parallel directions to the ion beam, should be used in the measurement. According to the Doppler-shift formula, the excitation energy (E ) can be determined independently of the ion-beam velocity from E "(E E , where E and E are the resonance energies obtained with the two lasers. From the observed fluorescence spectrum, the momentum spread of the ion beam was also derived. The resonance line width is 130(30) GHz (0.025(6) nm). This value corresponds to a momentum spread of *P/P"6(1);10\. The line width includes the natural line width and the laser power broadening. These contributions, however, are negligible compared with the Doppler broadening due to the momentum spread of the ion beam.
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Fig. 6. Different timing spectra measured with photomultipliers (a)—(d). Labels (a)—(d) indicate the photomultiplier position along the ion-beam axis, as shown in Fig. 1. The data in the figure have been normalized by the total counts of laser noise.
3.3. Lifetime of the excited state This fluorescence photon-detection system could also measure the lifetime of the excited P state. The count rate of fluorescence photons for each photomultiplier depends on the time of flight of the ion beam from the collision point to the detection region. Fig. 6 shows timing spectra measured with photomultipliers (a—d) for the D1 transition; here, (a—d) indicates the photomultiplier position along the ion beam axis, as shown in Fig. 1. Assuming that the timing of the laser noise signals is the same for every photomultiplier, it can be seen in Fig. 6 that the position of the fluorescence peaks is shifted according to the time of flight. In fact, the time difference between (a) and (d) is found to be about 5.5 ns from the spectra; this is in good agreement with the estimated time of flight of 5.6 ns. The ratio of the fluorescence intensity to the noise due to the scattered laser light decreases with increasing time of flight. Assuming that the scattered laser light is
Fig. 7. Decay curve of the 2P state obtained from the data shown in Fig. 6. The relative fluorescence yield for the photomultiplier (a) has been normalized to unity.
uniform in the detector chamber, integrated fluorescence signals can be normalized by the total counts of the laser noise. The relative yields of the fluorescence for photomultipliers (a—d) are plotted as a function of time of flight in Fig. 7. This figure shows a decay curve of the excited state. The lifetime of the 2P state was estimated to be q"3.6(#2.0/!0.6) ns by making a least-squares
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fit to the experimental data. The lifetime agrees well with the expected value of 5 ns from a MCDF (Multi-Configuration Dirac-Fock) calculation [37]. 4. Summary A low-background fluorescence photon-detection system has been successfully developed. The performance has been demonstrated by collinear laser spectroscopy on an Li-like B> ion beam having relativistic velocity. The combination of cryopump technology with a movable collimator system drastically suppressed the background due to the scattered photons of the laser and collisions between the ion beam and residual gas. A signalto-noise ratio of 200 was obtained at an ion-beam intensity of 10 ions/bunch, and we estimate the lower limit of the ion-beam intensity for our measurement to be 50 ions/bunch. From the present experiment, we conclude that this system is quite useful for the collinear laser spectroscopy of high-energy highly charged ion beams. References [1] A.C. Mueller, F. Buchinger, W. Klempt, E.W. Otten, R. Neugart, C. Ekstro¨m, J. Heinemeier, Nucl. Phys. A 403 (1983) 234. [2] G. Ulm, S.K. Bhattacherjee, P. Dabkiewicz, G. Huber, H.-J. Kluge, T. Ku¨hl, H. Lochmann, E.W. Otten, K. Wendt, S.A. Ahmad, W. Klempt, R. Neugart, Z. Phys. A 325 (1986) 247. [3] J. Eberz, U. Dinger, G. Huber, H. Lochmann, R. Menges, R. Newgart, R. Kirchner, O. Klepper, T. Ku¨hl, D. Marx, G. Ulm, K. Wendt, Nucl. Phys. A 464 (1987) 9. [4] D.A. Eastham, P.M. Walker, J.R.H. Smith, D.D. Warner, J.A.R. Griffith, D.E. Evans, S.A. Wells, M.J. Fawcett, I.S. Grant, Phys. Rev. C 36 (1987) 1583. [5] R. Neugart, W. Klempt, K. Wendt, Nucl. Instr. and Methods B 17 (1986) 354. [6] E. Arnold, J. Bonn, R. Gegenwart, W. Neu, R. Neugart, E.W. Otten, G. Ulm, K. Wendt, Phys. Lett. B 197 (1987) 311. [7] E.W. Otten, Treaties on heavy-ion science, vol. 8 in: D.A. Bromly (Ed.), Plenum, New York, 1989, p. 517. [8] H.W. Kugel, M. Leventhal, D.E. Murnick, C.K.N. Patel, O.R. Wood, Phys. Rev. Lett. 35 (1975) 647. [9] O.R. Wood, C.K.N. Patel, D.E. Murnick, E.T. Nelson, M. Leventhal, H.W. Kugel, Y. Niv, Phys. Rev. Lett. 48 (1982) 398.
[10] P. Pellegrin, Y. El Masri, L. Palffy, R. Prieels, Phys. Rev. Lett. 49 (1982) 1762. [11] H.D. Stra¨ter, L. von Gerdtell, A.P. Georgiadis, D. Mu¨ller, P. von Brentano, J.C. Sens, A. Pape, Phys. Rev. A 29 (1984) 1596. [12] E.G. Myers, Nucl. Instr. and Meth. B 9 (1985) 662. [13] H.-J. Pross, D. Budelsky, L. Kremer, D. Platte, P. von Brentano, J. Gassen, D. Mu¨ller, F. Scheuer, A. Pape, C. Sens, Phys. Rev. A 48 (1993) 1875. [14] T.P. Dinneen, N. Berrah-Mansour, H.G. Berry, L. Young R.C. Pardo, Phys. Rev. Lett. 66 (1991) 2859. [15] T.J. Scholl, R. Cameron, S.D. Rosner, L. Zhang, R.A. Holt, C.J. Sansonetti, J.D. Gillaspy, Phys. Rev. Lett. 71 (1993) 2188. [16] I. Klaft, S. Borneis, T. Engel, B. Fricke, R. Grieser, G. Huber, T. Ku¨hl, D. Marx, R. Neumann, S. Schro¨der, P. Seeling, L. Vo¨lker, Phys. Rev. Lett. 73 (1994) 2425. [17] E.G. Myers, J.K. Thompson, E.P. Gavathas, N.R. Claussen, J.D. Silver, D.J.H. Howie, Phys. Rev. Lett. 75 (1995) 3637. [18] E.G. Myers, D.J.H. Howie, J.K. Thompson, J.D. Silver, Phys. Rev. Lett. 76 (1996) 4899. [19] G.W. Erickson, J. Phys. Chem. Ref. Data 6 (1977) 831. [20] P.J. Mohr, Phys. Rev. A 26 (1982) 2338. [21] G.W.F. Drake, Adv. At. Mol. Phys. 18 (1982) 399. [22] G.W.F. Drake, Can. J. Phys. 66 (1988) 586. [23] M.H. Chen, K.T. Cheng, W.R. Johnson, Phys. Rev. A 47 (1993) 3692. [24] K.T. Cheng, M.H. Chen, W.R. Johnson, J. Sapirstein, Phys. Rev. A 50 (1994) 247. [25] Z.-C. Yan, G.W.F. Drake, Phys. Rev. Lett. 74 (1995) 4791. [26] T. Zhang, Z.-C. Yan, G.W.F. Drake, Bull. Am. Phys. Soc. 40 (1995) 1284. [27] Z.-C. Yan, G.W.F. Drake, Phys. Rev. Lett. 74 (1995) 4791. [28] H. Persson, S.M. Schneider, W. Greiner, G. Soff, I. Lindgren, Phys. Rev. Lett. 76 (1996) 1433. [29] M. Wakasugi, N. Inabe, T. Katayama, Proc. 5th European Particle Accelerator Conf., Institut of Physics Publ., Bristol, Philadelphia 1996, p. 611. [30] Y. Yano, A. Goto, Y. Miyazawa, M. Hemmi, T. Chiba, T. Kageyama, K. Ikegami, M. Takanaka, T. Kawaguchi, S. Kohara, M. Kase, T. Tominaka, Y. Batygin, T. Kubo, T. Nakagawa, Y. Tanaka, E. Ikezawa, M. Wakasugi, T. Mitsumoto, N. Inabe, O. Kamigaito, H. Okuno, S. Fujishima, A. Ohtomo, T. Ohkawa, T. Katayama, S. Watanabe, M. Tomozawa, J. Xia, Y. Rao, Y. Yuan, Proc. 14th Int. Conf. on Cyclotron and Their Application, World Science, Singapore 1995, p. 596. [31] Y. Yano, A. Goto, T. Katayama, Proc. 5th European Particle Accelerator Conf. Institut of Physics Publ., Bristol, Philadelphia 1996, p. 536. [32] T. Katayama, S. Watanabe, Y. Batygin, N. Inabe, K. Ohtomo, T. Ohkawa, M. Takanaka, M. Wakasugi, Y. Yano, K. Yoshida, J. Xia, Y. Rao, Y. Yuan, Proc. 5th European Particle Accelerator Conf. Institut of Physics Publ., Bristol, Philadelphia 1996, p. 563.
M. Wakasugi et al./Nucl. Instr. and Meth. in Phys. Res. A 419 (1998) 50—59 [33] I. Tanihata, Nucl. Phys. A 588 (1995) 253c. [34] T. Kubo, M. Ishihara, N. Inabe, H. Kumagai, I. Tanihata, K. Yoshida, T. Nakamura, H. Okuno, S. Shimoura, K. Asahi, Nucl. Instr. and Meth. B 70 (1992) 309. [35] C.E. Moor, ‘Atomic Energy Levels’, NBS, Washington DC, 1949, p. 19.
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[36] M. Wakasugi, N. Inabe, T. Kubo, H. Okuno, I. Tanihata, Y. Watanabe, Y. Yano, A. Yoshida, M. Hies, T. Katayama, S. Ozawa, Physica Scr. T 73 (1997) 70. [37] K.T. Cheng, Y.-K. Kim, J.P. Desclaux, Atom. Data Nucl. Data Tables 24 (1979) 111.