- Email: [email protected]
Development of a helium cryostat for laser spectroscopy of atoms with unstable nuclei in superfluid helium
Nuclear Instruments and Methods in Physics Research B 317 (2013) 595–598
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of a helium cryostat for laser spectroscopy of atoms with unstable nuclei in superfluid helium Kei Imamura a,⇑, Takeshi Furukawa b,c, Takashi Wakui d, Xiaofei Yang c,e, Yasuhiro Yamaguchi a, Hiroki Tetsuka f, Yosuke Mitsuya a, Yoshiki Tsutsui f, Tomomi Fujita g, Yuta Ebara f, Miki Hayasaka f, Shino Arai a, Sosuke Muramoto a, Yuichi Ichikawa c,h, Yoko Ishibashi c,i, Naoki Yoshida c,h, Hazuki Shirai c,h, Atsushi Hatakeyama j, Michiharu Wada c, Tetsu Sonoda c, Yuta Ito c,i, Hitoshi Odashima a, Tohru Kobayashi k, Hideki Ueno c, Tadashi Shimoda g, Koichiro Asahi h, Yukari Matsuo c a
Department of Physics, Meiji University, 1-1-1 Higashi-Mita, Tama, Kawasaki, Kanagawa 214-8571, Japan Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan c RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan d Cyclotron and Radioisotope Center, Tohoku University, 6-3 Aoba, Aramaki, Aoba, Sendai, Miyagi 980-8578, Japan e School of Physics, Peking University, Chengfu Road, Haidian District, Beijing 100871, China f Department of Physics, Tokyo Gakugei University, 4-1-1 Nukuikitamachi, Koganei, Tokyo 184-8501, Japan g Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan h Department of Physics, Tokyo Instutute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan i Department of Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan j Department of Applied Physics, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan k RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b
a r t i c l e
i n f o
Article history: Received 21 March 2013 Received in revised form 9 August 2013 Accepted 12 August 2013 Available online 23 August 2013 Keywords: Superfluid helium Laser spectroscopy Nuclear structure
a b s t r a c t We are developing a new nuclear laser spectroscopic technique for the study of nuclear structure that can be applied to short-lived low-yield atoms with unstable nuclei. The method utilizes superfluid helium (He II) as a trapping medium for high-energy ion beams. A liquid helium cryostat with optical windows is a key apparatus for this type of experiment. We describe the design and the performance of the cryostat which is developed for the present project. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction We have been developing a novel technique in nuclear laser spectroscopy using superfluid helium (He II) for the study of structure of unstable nuclei. We name the method OROCHI, which stands for the Optical Radioisotope Observation in Condensed Helium as Ion-catcher. In the OROCHI, He II works as a trapping medium to stop accelerated ion beams and as a host matrix for laser spectroscopy of trapped atoms. We are planning to measure Zeeman splittings and hyperfine structure splittings to determine nuclear spins and moments using laser-radio frequency/microwave (RF/MW) double resonance spectroscopy of stopped atoms in He II. The most prominent characteristic of He II as host matrix for laser spectroscopy is the large spectral shift between emission ⇑ Corresponding author. Tel.: +81 48 467 8548. E-mail address: [email protected] (K. Imamura). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.08.024
and absorption of the embedded atoms [1]. Thanks to this characteristic, we can perform highly sensitive detection of the signal form introduced atoms by making use of this characteristic. So far, Zeeman splittings and hyperfine structure splittings measurements using double resonance method have been performed for stable isotopes of Cs and Rb [2,3]. We also have successfully demonstrated that precision spectroscopy of atoms in He II is applicable to alkali and alkali-like atoms using the combination of optical pumping and double resonance method [4]. A high degree of spin polarization is achieved for stable species of Rb (50%), Cs (90%), Ag (80%), and Au (80%) atoms [5]. It is also shown that the nuclear spins and moments can be deduced from the Zeeman splitting and hyperfine splitting measurements using stable isotopes of these elements. In order to apply the OROCHI method to atoms with unstable nuclei generated in accelerator facilities, we need to construct a liquid helium cryostat with optical windows specially designed for the experiments to be installed in the beam line.
596
K. Imamura et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 595–598
Compared with the cryostat being used for the stable atoms in the laser laboratory, the design and the operation of the cryostat for beam line experiments are different in many aspects. For instance, all the liquid helium operation processes need to be remote controlled, the location of the optical detection window is restricted to the bottom of the cryostat to avoid interference with the beam-line, and so forth. It is inevitable to establish the operation procedure of such a cryostat for the success of the experiment because the cryostat is a key apparatus to realize superfluid helium condition (<2 K) that is necessary for both stable beam injection and highly sensitive laser spectroscopy. In this paper, we describe the details of the cryostat such as the design, performance, and operating method. 2. Design and characteristic 2.1. Basic structure of a helium cryostat The schematic layout of the helium cryostat with optical windows is shown in Fig. 1. Basically, the cryostat consists of four baths. The role of each part is described below. (1) The outer vacuum chamber which is to prevent thermal radiation from the outside. The pressure of the outer vacuum chamber is continuously monitored by a cold cathode pirani gauge. (2) Liquid nitrogen bath (77 K bath) which is filled with liquid nitrogen and suppresses evaporation of liquid helium. (3) Liquid helium bath (4.2 K bath) which is filled with liquid helium which is supplied to the superfluid helium bath. The 4.2 K bath has a diaphragm pressure gauge and a liquid helium level sensor. These instruments enable us to monitor the pressure variation and the amount of liquid helium remaining. (4) Superfluid helium bath (1.5 K bath) which is the most important part of our cryostat as an observation region. The 1.5 K bath has a thermometer, a diaphragm pressure gauge and a liquid helium level sensor. We can monitor the temperature, pressure and liquid helium level inside of the 1.5 K bath. To stabilize the 1.5 K bath, a needle valve and a butterfly valve, which are used to control the flow rate of liquid helium and pumping speed, respectively, are indispensable. They are controlled by LabVIEW with a GPIB interface. 2.2. Superfluid helium bath As mentioned above, the most important part of the cryostat is the superfluid helium bath (1.5 K bath). In this section, we will describe it more in detail. The 1.5 K bath plays two important roles.
Fig. 2. Schematic diagram of the 1.5 K bath.
One is to maintain the superfluid helium condition steadily. The other is to realize the environment suitable for laser spectroscopy. Fig. 2 shows the schematic diagram of the 1.5 K bath. In the 1.5 K bath, a pair of Helmholtz coils which are important to produce atomic spin polarizations and double resonance method are placed in the parallel direction along the laser path. A pair of RF coils for measuring Zeeman splittings is placed in the perpendicular direction to the laser path. A MW loop antenna that is necessary to measure hyperfine structure splittings is installed upward of the laser path. These instruments are carefully located at the position where they do not disturb the laser path. If these instruments interfere with the laser path, intense scattered light would be induced, which disturbs the observation of laser-induced fluorescence (LIF) emitted from atoms in the observation region. Special care should be taken to use quartz windows at the bottom of the 1.5 K bath to detect LIF efficiently. 3. Cooling procedure The cooling procedure is as follows: (i) Pump out the 1.5 K and 4.2 K baths (24 h). (ii) Pre-cool the 1.5 K, 4.2 K and 77 K baths using liquid nitrogen (24 h). (iii) Remove liquid nitrogen from the 1.5 K and 4.2 K baths (3 h). (iv) Pump out the 1.5 K and 4.2 K baths again (2 h). (v) Fill the 1.5 K and 4.2 K baths with helium gas (two times) (2 h). (vi) Transfer liquid helium to the 4.2 K bath (1 h). (vii) Transfer liquid helium from the 4.2 K bath to the 1.5 K bath slowly (1 h). (viii) Pump out the 1.5 K bath slowly (1 h). (ix) Finally, liquid helium in the 1.5 K bath becomes superfluid (He II). After we achieve the He II condition of the 1.5 K bath, we stabilize the height of the liquid surface and the pressure inside the 1.5 K bath using the GPIB controlled instruments such as stepping motors to move the butterfly valve and the needle valve. 4. Results
Fig. 1. Schematic layout of the helium cryostat. The helium cryostat has four baths. Each part has a different role. The outer vacuum chamber is pumped continually using a rotary pump and a turbo molecule pump. We can connect the 1.5 K bath to a rotary pump and a mechanical booster pump. After introducing liquid helium in the 1.5 K bath, we pump out the 1.5 K bath to decrease the temperature of liquid helium using vaporization heat.
In September 2012, we performed a beam-line experiment using the cryostat. Fig. 3 shows variations of parameters during the experiment. As a consequence of our development, we evaluated the continuous operation time with a single helium transfer as 19 h and we could stabilize the liquid helium level of the 1.5 K
K. Imamura et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 595–598
597
Fig. 3. Results of the cryostat monitoring. (a) The temperature and pressure variation of the superfluid helium bath (1.5 K bath) during an experiment. The dark blue line corresponds to the time variation of the temperature. The pink line corresponds to the time variation of the pressure. Upper window is the enlarged view of the temperature variation. Lower window is the enlarged view of the pressure variation. (b) Time variation of the pressure of the outer vacuum chamber. The inset figure is the enlarged view of a part of the experiment. (c) Time variation of the liquid helium level of the superfluid helium bath (1.5 K bath) and the liquid helium bath (4.2 K bath) during experiment. The black line shows the helium level of the 1.5 K bath and the red line shows the helium level of the 4.2 K bath relative to the total capacity of each bath respectively. Upper window shows the enlarged view of the the liquid helium level variation of the 1.5 K bath. A sudden increase of the helium level in the 4.2 K bath corresponds to a transfer of liquid helium. The liquid helium level of the 1.5 K bath is kept constant, because liquid helium is supplied from the 4.2 K bath through a needle valve. While liquid helium is not supplied to the 4.2 K bath continuously, the liquid helium level gradually decreases. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bath at 90% of the total capacity. In terms of continuous operation time, in this work, we successfully elongated twice as long as that in the previous work [6]. We attribute the reason of the successful extension of the continuous operation time to the establishment of the efficient cooling procedure described in Section 3. In the previous experiment, such a cooling procedure was not established yet, thus the protocol changed time by time. In this experiment, we could finally optimize the procedure after a series of cooling tests. We could also confirm that the outer vacuum chamber pressure was stably maintained at 10 7 Torr. In Fig. 3(b), a sudden increase of the outer vacuum chamber pressure was observed every 3–4 h. We can understand these peaks as the consequence of outgassing from the inner wall of the outer vacuum chamber. When low temperature liquid is introduced to the cryostat, it acts as a cryo-pump so that the pressure of the outer vacuum chamber is lowered by one or two orders of magnitude because atoms and molecules in the chamber stick to the inner wall. Then, such atoms and molecules may evaporate when the inner wall temperature of the chamber increases due to the decrease of the amount of the low temperature liquid such as liquid helium and liquid nitrogen. Because the period of the increase is limited to a few seconds, it does not affect the condition of the 1.5 K bath. We could achieve stable conditions of the temperature and the pressure as 1.79 0.02 K, 12 2 Torr of the 1.5 K bath, respec-
tively. It is clearly seen that the correlation between the temperature and the pressure of the 1.5 K bath is quite reasonable. We have also confirmed that the temperature evaluated from the vapor pressure of liquid helium is in agreement with the measured temperature within 0.1 K. In addition, although the high-energy ion beams, microwave, radio-frequency wave, and laser light were irradiated to the 1.5 K bath, no influence caused by the irradiation was observed. The result will validate the stable condition of the superfluid environment for the laser-RF/MW double resonance spectroscopy. Finally, we successfully observed a laser-RF double resonance signal for the measurement of Zeeman splittings for 85;84 Rb atoms using this cryostat [7]. 5. Conclusion We are developing the OROCHI method as a new method for nuclear laser spectroscopy of atoms with unstable nuclei that are produced at accelerator facilities. The OROCHI uses superfluid helium as a stopping medium of high energetic ion beams as well as a host matrix of laser spectroscopy. We have constructed a helium cryostat with optical windows for the experiments at the accelerator facility. As characteristics of the cryostat, it has to fulfill two demands. One is to produce the stable He II condition. The
598
K. Imamura et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 595–598
other is to realize the environment suitable for laser spectroscopy. As a result of the development and careful operation of the cryostat, we have successfully achieved a very stable environment in the 1.5 K bath of the cryostat. The evaluated continuous operation time with a single transfer of liquid helium is 19 h. We have also successfully measured the Zeeman splittings of 85;84 Rb atoms with this cryostat. As further plans, we are going to develop an automatic control system for the cryostat and liquid helium transfer. In the present system, we can supply just 30 L liquid helium from a 100 L liquid helium tank to the 4.2 K bath by a single transfer. After we finish developing the automatic control system, we will be able to keep the liquid helium level of the 4.2 K bath and fully consume liquid in a tank with a single transfer. Calculating from liquid helium decrease time of the liquid helium level in the 4.2 K bath, continuous operation time with a single transfer is estimated to be 2 days. Thus it will contribute to a more reliable and stable system.
Acknowledgment This work was partly supported by KAKENHI (Grant-in-Aid for Scientific Research) 19204029 and 19740170. References [1] [2] [3] [4] [5] [6] [7]
Y. Takahasi et al., Phys. Rev. Let. 71 (1993) 7. Y. Takahasi et al., Phys. Rev. B49 (1993) 5. Y. Takahasi et al., Z. Phys. B98 (1995) 391. T. Furukawa et al., Hyp. Int. 196 (2010) 191. T. Furukawa et al., Physica E43 (2011) 843. T. Furukawa et al., J. Phys. Conf. Ser. 302 (2011) 012054. T. Furukawa et al., Nucl. Instrum. Method. B, 317 (2013) 590–594.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of a helium cryostat for laser spectroscopy of atoms with unstable nuclei in superfluid helium Kei Imamura a,⇑, Takeshi Furukawa b,c, Takashi Wakui d, Xiaofei Yang c,e, Yasuhiro Yamaguchi a, Hiroki Tetsuka f, Yosuke Mitsuya a, Yoshiki Tsutsui f, Tomomi Fujita g, Yuta Ebara f, Miki Hayasaka f, Shino Arai a, Sosuke Muramoto a, Yuichi Ichikawa c,h, Yoko Ishibashi c,i, Naoki Yoshida c,h, Hazuki Shirai c,h, Atsushi Hatakeyama j, Michiharu Wada c, Tetsu Sonoda c, Yuta Ito c,i, Hitoshi Odashima a, Tohru Kobayashi k, Hideki Ueno c, Tadashi Shimoda g, Koichiro Asahi h, Yukari Matsuo c a
Department of Physics, Meiji University, 1-1-1 Higashi-Mita, Tama, Kawasaki, Kanagawa 214-8571, Japan Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan c RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan d Cyclotron and Radioisotope Center, Tohoku University, 6-3 Aoba, Aramaki, Aoba, Sendai, Miyagi 980-8578, Japan e School of Physics, Peking University, Chengfu Road, Haidian District, Beijing 100871, China f Department of Physics, Tokyo Gakugei University, 4-1-1 Nukuikitamachi, Koganei, Tokyo 184-8501, Japan g Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan h Department of Physics, Tokyo Instutute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan i Department of Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan j Department of Applied Physics, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan k RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b
a r t i c l e
i n f o
Article history: Received 21 March 2013 Received in revised form 9 August 2013 Accepted 12 August 2013 Available online 23 August 2013 Keywords: Superfluid helium Laser spectroscopy Nuclear structure
a b s t r a c t We are developing a new nuclear laser spectroscopic technique for the study of nuclear structure that can be applied to short-lived low-yield atoms with unstable nuclei. The method utilizes superfluid helium (He II) as a trapping medium for high-energy ion beams. A liquid helium cryostat with optical windows is a key apparatus for this type of experiment. We describe the design and the performance of the cryostat which is developed for the present project. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction We have been developing a novel technique in nuclear laser spectroscopy using superfluid helium (He II) for the study of structure of unstable nuclei. We name the method OROCHI, which stands for the Optical Radioisotope Observation in Condensed Helium as Ion-catcher. In the OROCHI, He II works as a trapping medium to stop accelerated ion beams and as a host matrix for laser spectroscopy of trapped atoms. We are planning to measure Zeeman splittings and hyperfine structure splittings to determine nuclear spins and moments using laser-radio frequency/microwave (RF/MW) double resonance spectroscopy of stopped atoms in He II. The most prominent characteristic of He II as host matrix for laser spectroscopy is the large spectral shift between emission ⇑ Corresponding author. Tel.: +81 48 467 8548. E-mail address: [email protected] (K. Imamura). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.08.024
and absorption of the embedded atoms [1]. Thanks to this characteristic, we can perform highly sensitive detection of the signal form introduced atoms by making use of this characteristic. So far, Zeeman splittings and hyperfine structure splittings measurements using double resonance method have been performed for stable isotopes of Cs and Rb [2,3]. We also have successfully demonstrated that precision spectroscopy of atoms in He II is applicable to alkali and alkali-like atoms using the combination of optical pumping and double resonance method [4]. A high degree of spin polarization is achieved for stable species of Rb (50%), Cs (90%), Ag (80%), and Au (80%) atoms [5]. It is also shown that the nuclear spins and moments can be deduced from the Zeeman splitting and hyperfine splitting measurements using stable isotopes of these elements. In order to apply the OROCHI method to atoms with unstable nuclei generated in accelerator facilities, we need to construct a liquid helium cryostat with optical windows specially designed for the experiments to be installed in the beam line.
596
K. Imamura et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 595–598
Compared with the cryostat being used for the stable atoms in the laser laboratory, the design and the operation of the cryostat for beam line experiments are different in many aspects. For instance, all the liquid helium operation processes need to be remote controlled, the location of the optical detection window is restricted to the bottom of the cryostat to avoid interference with the beam-line, and so forth. It is inevitable to establish the operation procedure of such a cryostat for the success of the experiment because the cryostat is a key apparatus to realize superfluid helium condition (<2 K) that is necessary for both stable beam injection and highly sensitive laser spectroscopy. In this paper, we describe the details of the cryostat such as the design, performance, and operating method. 2. Design and characteristic 2.1. Basic structure of a helium cryostat The schematic layout of the helium cryostat with optical windows is shown in Fig. 1. Basically, the cryostat consists of four baths. The role of each part is described below. (1) The outer vacuum chamber which is to prevent thermal radiation from the outside. The pressure of the outer vacuum chamber is continuously monitored by a cold cathode pirani gauge. (2) Liquid nitrogen bath (77 K bath) which is filled with liquid nitrogen and suppresses evaporation of liquid helium. (3) Liquid helium bath (4.2 K bath) which is filled with liquid helium which is supplied to the superfluid helium bath. The 4.2 K bath has a diaphragm pressure gauge and a liquid helium level sensor. These instruments enable us to monitor the pressure variation and the amount of liquid helium remaining. (4) Superfluid helium bath (1.5 K bath) which is the most important part of our cryostat as an observation region. The 1.5 K bath has a thermometer, a diaphragm pressure gauge and a liquid helium level sensor. We can monitor the temperature, pressure and liquid helium level inside of the 1.5 K bath. To stabilize the 1.5 K bath, a needle valve and a butterfly valve, which are used to control the flow rate of liquid helium and pumping speed, respectively, are indispensable. They are controlled by LabVIEW with a GPIB interface. 2.2. Superfluid helium bath As mentioned above, the most important part of the cryostat is the superfluid helium bath (1.5 K bath). In this section, we will describe it more in detail. The 1.5 K bath plays two important roles.
Fig. 2. Schematic diagram of the 1.5 K bath.
One is to maintain the superfluid helium condition steadily. The other is to realize the environment suitable for laser spectroscopy. Fig. 2 shows the schematic diagram of the 1.5 K bath. In the 1.5 K bath, a pair of Helmholtz coils which are important to produce atomic spin polarizations and double resonance method are placed in the parallel direction along the laser path. A pair of RF coils for measuring Zeeman splittings is placed in the perpendicular direction to the laser path. A MW loop antenna that is necessary to measure hyperfine structure splittings is installed upward of the laser path. These instruments are carefully located at the position where they do not disturb the laser path. If these instruments interfere with the laser path, intense scattered light would be induced, which disturbs the observation of laser-induced fluorescence (LIF) emitted from atoms in the observation region. Special care should be taken to use quartz windows at the bottom of the 1.5 K bath to detect LIF efficiently. 3. Cooling procedure The cooling procedure is as follows: (i) Pump out the 1.5 K and 4.2 K baths (24 h). (ii) Pre-cool the 1.5 K, 4.2 K and 77 K baths using liquid nitrogen (24 h). (iii) Remove liquid nitrogen from the 1.5 K and 4.2 K baths (3 h). (iv) Pump out the 1.5 K and 4.2 K baths again (2 h). (v) Fill the 1.5 K and 4.2 K baths with helium gas (two times) (2 h). (vi) Transfer liquid helium to the 4.2 K bath (1 h). (vii) Transfer liquid helium from the 4.2 K bath to the 1.5 K bath slowly (1 h). (viii) Pump out the 1.5 K bath slowly (1 h). (ix) Finally, liquid helium in the 1.5 K bath becomes superfluid (He II). After we achieve the He II condition of the 1.5 K bath, we stabilize the height of the liquid surface and the pressure inside the 1.5 K bath using the GPIB controlled instruments such as stepping motors to move the butterfly valve and the needle valve. 4. Results
Fig. 1. Schematic layout of the helium cryostat. The helium cryostat has four baths. Each part has a different role. The outer vacuum chamber is pumped continually using a rotary pump and a turbo molecule pump. We can connect the 1.5 K bath to a rotary pump and a mechanical booster pump. After introducing liquid helium in the 1.5 K bath, we pump out the 1.5 K bath to decrease the temperature of liquid helium using vaporization heat.
In September 2012, we performed a beam-line experiment using the cryostat. Fig. 3 shows variations of parameters during the experiment. As a consequence of our development, we evaluated the continuous operation time with a single helium transfer as 19 h and we could stabilize the liquid helium level of the 1.5 K
K. Imamura et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 595–598
597
Fig. 3. Results of the cryostat monitoring. (a) The temperature and pressure variation of the superfluid helium bath (1.5 K bath) during an experiment. The dark blue line corresponds to the time variation of the temperature. The pink line corresponds to the time variation of the pressure. Upper window is the enlarged view of the temperature variation. Lower window is the enlarged view of the pressure variation. (b) Time variation of the pressure of the outer vacuum chamber. The inset figure is the enlarged view of a part of the experiment. (c) Time variation of the liquid helium level of the superfluid helium bath (1.5 K bath) and the liquid helium bath (4.2 K bath) during experiment. The black line shows the helium level of the 1.5 K bath and the red line shows the helium level of the 4.2 K bath relative to the total capacity of each bath respectively. Upper window shows the enlarged view of the the liquid helium level variation of the 1.5 K bath. A sudden increase of the helium level in the 4.2 K bath corresponds to a transfer of liquid helium. The liquid helium level of the 1.5 K bath is kept constant, because liquid helium is supplied from the 4.2 K bath through a needle valve. While liquid helium is not supplied to the 4.2 K bath continuously, the liquid helium level gradually decreases. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bath at 90% of the total capacity. In terms of continuous operation time, in this work, we successfully elongated twice as long as that in the previous work [6]. We attribute the reason of the successful extension of the continuous operation time to the establishment of the efficient cooling procedure described in Section 3. In the previous experiment, such a cooling procedure was not established yet, thus the protocol changed time by time. In this experiment, we could finally optimize the procedure after a series of cooling tests. We could also confirm that the outer vacuum chamber pressure was stably maintained at 10 7 Torr. In Fig. 3(b), a sudden increase of the outer vacuum chamber pressure was observed every 3–4 h. We can understand these peaks as the consequence of outgassing from the inner wall of the outer vacuum chamber. When low temperature liquid is introduced to the cryostat, it acts as a cryo-pump so that the pressure of the outer vacuum chamber is lowered by one or two orders of magnitude because atoms and molecules in the chamber stick to the inner wall. Then, such atoms and molecules may evaporate when the inner wall temperature of the chamber increases due to the decrease of the amount of the low temperature liquid such as liquid helium and liquid nitrogen. Because the period of the increase is limited to a few seconds, it does not affect the condition of the 1.5 K bath. We could achieve stable conditions of the temperature and the pressure as 1.79 0.02 K, 12 2 Torr of the 1.5 K bath, respec-
tively. It is clearly seen that the correlation between the temperature and the pressure of the 1.5 K bath is quite reasonable. We have also confirmed that the temperature evaluated from the vapor pressure of liquid helium is in agreement with the measured temperature within 0.1 K. In addition, although the high-energy ion beams, microwave, radio-frequency wave, and laser light were irradiated to the 1.5 K bath, no influence caused by the irradiation was observed. The result will validate the stable condition of the superfluid environment for the laser-RF/MW double resonance spectroscopy. Finally, we successfully observed a laser-RF double resonance signal for the measurement of Zeeman splittings for 85;84 Rb atoms using this cryostat [7]. 5. Conclusion We are developing the OROCHI method as a new method for nuclear laser spectroscopy of atoms with unstable nuclei that are produced at accelerator facilities. The OROCHI uses superfluid helium as a stopping medium of high energetic ion beams as well as a host matrix of laser spectroscopy. We have constructed a helium cryostat with optical windows for the experiments at the accelerator facility. As characteristics of the cryostat, it has to fulfill two demands. One is to produce the stable He II condition. The
598
K. Imamura et al. / Nuclear Instruments and Methods in Physics Research B 317 (2013) 595–598
other is to realize the environment suitable for laser spectroscopy. As a result of the development and careful operation of the cryostat, we have successfully achieved a very stable environment in the 1.5 K bath of the cryostat. The evaluated continuous operation time with a single transfer of liquid helium is 19 h. We have also successfully measured the Zeeman splittings of 85;84 Rb atoms with this cryostat. As further plans, we are going to develop an automatic control system for the cryostat and liquid helium transfer. In the present system, we can supply just 30 L liquid helium from a 100 L liquid helium tank to the 4.2 K bath by a single transfer. After we finish developing the automatic control system, we will be able to keep the liquid helium level of the 4.2 K bath and fully consume liquid in a tank with a single transfer. Calculating from liquid helium decrease time of the liquid helium level in the 4.2 K bath, continuous operation time with a single transfer is estimated to be 2 days. Thus it will contribute to a more reliable and stable system.
Acknowledgment This work was partly supported by KAKENHI (Grant-in-Aid for Scientific Research) 19204029 and 19740170. References [1] [2] [3] [4] [5] [6] [7]
Y. Takahasi et al., Phys. Rev. Let. 71 (1993) 7. Y. Takahasi et al., Phys. Rev. B49 (1993) 5. Y. Takahasi et al., Z. Phys. B98 (1995) 391. T. Furukawa et al., Hyp. Int. 196 (2010) 191. T. Furukawa et al., Physica E43 (2011) 843. T. Furukawa et al., J. Phys. Conf. Ser. 302 (2011) 012054. T. Furukawa et al., Nucl. Instrum. Method. B, 317 (2013) 590–594.