- Email: [email protected]
u Xe54+-N2 collisions at HIRFL-CSR
Nuclear Instruments and Methods in Physics Research B 269 (2011) 692–694
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Short Communication
Observation of K- and L-REC in 200 MeV/u Xe54+-N2 collisions at HIRFL-CSR Deyang Yu a,⇑, Yingli Xue a, Caojie Shao a,b, Zhangyong Song a,b, Rongchun Lu a, Fangfang Ruan a, Wei Wang a, Jing Chen a,b, Bian Yang a,b, Zhihu Yang a, Jianjie Wan c, Chenzhong Dong c, Xiaohong Cai a,⇑ a
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China c College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, PR China b
a r t i c l e
i n f o
Article history: Received 30 September 2010 Received in revised form 29 January 2011 Available online 4 February 2011 Keywords: Radiative electron capture Highly charged ions Heavy ion storage ring
a b s t r a c t The 200 MeV/u Xe54+ ions were utilized to collide with N2 molecules and the K- and L-REC, as well as the Lyman lines of Xe53+ were observed. After electron cooling, the ion beam momentum spread DP/P 2.2 105 was achieved and the target thickness was stabilized at about 1013 atom/cm2. As the first atomic experiment at HIRFL-CSR with the internal target, its feasibility and stability were verified. Ó 2011 Elsevier B.V. All rights reserved.
In energetic collisions between highly charged ions (HCIs) and atoms or molecules, the target electrons may be captured by the projectile ions, either radiatively or non-radiatively. During Radiative Electron Capture (REC), a target electron is transferred to the projectile accompanying with electromagnetic field excitation, i.e., a real photon is emitted simultaneously, which carries away the excess energy and momentum. The REC was first observed in 1970s [1–3], since then it was extensively studied both experimentally and theoretically [4–6]. With the development of heavy ion cooling storage rings and internal targets [7,8], the REC research was pushed ahead to very high-Z HCIs (e.g., bare and hydrogen-like uranium ions) and wider energy ranges (from several MeV/u to several hundred MeV/u). Since relatively loosely bound electrons are more likely to be captured radiatively, in this case REC can be approximated as Radiative Recombination (RR), which in turn is the time-inverse process of the photoelectric effect [9,10]. The Xe54+ of 197 MeV/u was employed by Anholt et al. [11] to observe REC into K-shell (K-REC) in 1984. However, because a solid beryllium foil target was used, the X-ray spectrum had a high background due to the primary bremsstrahlung (PB), and especially the secondary electron bremsstrahlung (SEB). Therefore, the L-REC was not directly observed. Later, Andriamonje et al. observed REC of 25 MeV/u Xe53+ ions via channeling in a thin silicon crystal, and the orientation dependence of the positions and shapes of associated photon lines were shown [12]. Briand et al. measured Xe52+,53+ Lyman and Balmer lines by the beam-foil method, but the REC process was not observed [13]. ⇑ Corresponding authors. E-mail addresses: [email protected] (D. Yu), [email protected] (X. Cai). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.128
In this paper, the observation of K- and L-REC in Xe54+-N2 collisions is presented, which is the first atomic physics experiment carried out at HIRFL-CSR (Heavy Ion Research Facility at Lanzhou – Cooling Storage Ring) [14] with the internal target [15]. The Xe27+ ion beam produced by a superconductor ECR ion source was accelerated to 2.9 MeV/u by the SFC cyclotron (an injector of the storage rings), and then was accumulated, cooled and accelerated up to 200 MeV/u in the main ring, CSRm, which worked as a synchrotron in this experiment. Before injection into the experimental ring, CSRe, the Xe27+ ions were stripped by a 0.2 mm-thick carbon foil. The energy loss on the carbon foil was estimated to be about 4 MeV/u, and the Xe54+ yield was about 70%. After stripping, the Xe54+ ions were selected and injected into CSRe, where the beam was cooled by a hollow electron beam at the electron cooler section. The electron beam energy was set at 108.9 keV, and the intensity was about 500–600 mA with a diameter of 25 mm. During circulation, the ion velocity was dragged to the same value as the velocity of the cooling electrons, i.e., the circumrotating frequency was 1.27 MHz in CSRe. After cooling, the typical momentum spread in CSRe was DP/P 2.2 105, and the beam intensity was 30–60 lA (i.e., about 3–6 106 ions). CSRe has three typical operation modes, i.e., the internal target mode, the normal mode and the isochronous mode. In the present case, the internal target mode was imposed, in which the ion beam was most focused on the target region to minimize the scrape on the vacuum chamber. Otherwise, it would produce neutron background in the experimental area. In the internal target section, the beam crossed with a N2 cluster jet, which was produced by a four-stage internal target system with its nozzle which was cooled to about 90 K [15]. The jet had a well bounded intensity profile
D. Yu et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 692–694
with a 3–4 mm-diameter at the interaction zone. The target thickness was stabilized to be around 1013 atom/cm2 while the vacuum in the ring was better than 3 1011 mbar. To optimize the beam-target overlap, a PMT was employed behind a quartz window of the collision chamber to count the visible and ultraviolet photons, which were emitted when the ions passed through the internal target. Without the ion beam, the counting rate of the PMT was typically less than 10 Hz, while with optimized beam overlap the counting rate was at a range from 2000 to 3500 Hz. By locally shifting its orbit and monitoring the PMT counting rate, the beam was able to simultaneously overlap with the cooling electron beam and cross with the internal target jet. In addition, a neutron detector was placed closely under the collision chamber to monitor the neutron background, since most of the emitted neutrons concentrate in the forward direction for high energy nuclear collisions. It recorded about 10–30 neutrons during each beam injection (within 1 ls), while after cooling, the neutron counting rate was less than 0.1 Hz. To restrain the background, the data acquisition system would reject any X-ray data from the detectors during the injection and the cooling. According to the numerical calculations [16], the most important ion-loss mechanisms were recombination in the electron cooler and electron capture at the internal target. In the present case, the REC cross section was estimated to be about 30 barns per target electron by the Stobbe formula [17], and the NRC cross section was estimated to be about 600 barns per atom [18]. Experimentally, the storage lifetime of the Xe54+ beam in CSRe was typically 8–10 min with the electron cooling and the internal target turning on. The produced X-ray was detected by two lead-shielded HPGe detectors which were located at 90° and 120°, with the open solid angles (X/4p) of 3.0 105 (3 8 mm2 slit at 260 mm) and 2.5 105 (5 7 mm2 slit at 340 mm), respectively. The corresponding x-rays spectra are shown in Fig. 1. The K- and L-REC, as well as the Lyman lines (including the M1 transition) of Xe53+⁄ were observed. Obviously, the x-rays emitted by the moving ions were Doppler shifted, which satisfy
Elab ¼ Eion =cð1 b cos hlab Þ
ð1Þ
Here Eion and Elab denote the photon energies in the projectile and the laboratory frame, respectively. b is the ion velocity in units of the speed of light, c = (1 b2)1/2 is the Lorentz factor, and hlab represents the angle between the ion direction and the photon direction in the laboratory frame. The shifted energies and counts of K- and L-REC X-ray, as well as the corresponding intrinsic detector efficiencies are listed in Table 1. It is established that the cross section of REC into 2s is much larger than that into 2p in higher energy region [10,19], and thus the angular distribution of L-REC roughly follows the sin2hlab law, which is similar to the distribution of K-REC [10]. Then, the angular relative intensity between L- and K-REC is almost independent from the observation angle hlab, especially at larger observation angles, such as 90° and 120°. Under the present conditions, we calculate [20] the ratio of total cross sections between L- and K-REC including E1, E2, M1 and M2 transitions, which is 0.14. However, due to the large background and poor statistics, as shown in Fig. 1, it was not possible to obtain quantitative results in this first experiment. The Pb characteristic Ka1, Ka2 and Kb x-rays were also observed without any Doppler shift, as shown in Fig. 1. To clarify the origin of this radiation, the X-ray background in the HPGe detectors were measured after the experiment (with the beam off), both with and without lead shields. Another HPGe detector (more suitable for high energies) was also employed and it showed the 1.46 MeV c-ray of the 40K in the environment. With the lead shields, the low energy X-ray detectors recorded the Pb Ka1, Ka2 and Kb x-rays.
693
Fig. 1. X-ray spectra obtained by 90° and 120° detectors in 200 MeV/u Xe54+-N2 collisions. The Pb characteristic x-rays originated from the shields which were illuminated by the high energy c-ray background, while the low energy background mainly came from the primary bremsstrahlung.
On the contrary, when the lead shields were removed these Pb characteristic x-rays disappeared. As shown in Fig. 1, the background is higher in the low energy range. The low energy X-ray owing to the primary bremsstrahlung (i.e., in the projectile frame, the target electrons are decelerated by the ions and with simultaneous emission of a Bremsstrahlung photon) [21] has an energy endpoint Emax:
Emax ¼ ðc 1Þme c2 c1 ð1 b cosðhlab ÞÞ1
ð2Þ
Here mec2 is the electron rest mass in keV (i.e., 511 keV). In present conditions, at 90° and 120° the endpoint energies were 90.4 and 70.4 keV, lower than the L-REC energies, respectively. Owing to the low density of the internal target, the primary bremsstrahlung, and especially the secondary electron bremsstrahlung (i.e., the ejected electrons in the primary collisions collide on other target atoms then produce bremsstrahlung x-rays) were strongly restrained compared with those in the case of solid targets [11,22]. However, above the energy endpoints, the SEB on account of the chamber walls and the beryllium windows, as well as the scattered c-ray in the lead shields produced higher energy X-ray background yet. The measured energy spread (FWHM) of K-REC x-rays at 90° and 120° was 4.5 and 3.2 keV, respectively. It was contributed by the detector resolution, the Doppler broadening and the Compton profile of the N2 molecules, which are listed in Table 2. As
694
D. Yu et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 692–694
Table 1 K- and L-REC X-ray energy, counts and intrinsic detector efficiency. Detector
X-ray
Energy (keV)
Detector efficiency (%)
Net counts
90°
K-REC L-REC
123.6 98.2
48 ± 5 75 ± 5
3958 ± 366 348 ± 58
120°
K-REC L-REC
96.3 76.5
76 ± 5 91 ± 5
2932 ± 338 183 ± 46
In summary, the Xe54+ ions were produced, stored, cooled, and overlapped with the internal target at HIRFL-CSR. The experimental layout and feasibility, as well as the long-term stability of the internal target were verified. By using the internal gas target, the SEB background was strongly restrained, and the REC processes were clearly observed. In present conditions, the dominant REC X-ray energy spread mechanism at big observation angles was the Compton profile of N2 molecules. Acknowledgements
Table 2 Different contributions to the K-REC energy spread. Energy spread Detector resolution around 75 keV (FWHM) Doppler broadening (maximal) Transformed Compton profile of N2 (FWHM) Totally measured (FWHM)
90° Detector (keV) 0.99
120° Detector (keV) 0.57
±1.7 4.1
±0.9 3.1
4.5
3.2
mentioned above, the Pb characteristic x-rays originated from the shields which were illuminated by the high energy c-ray present in the environment. Therefore, these lines are free of the Doppler broadening. This is seen in Fig. 1 where the Pb characteristic lines are much narrower than the K-REC peaks. Via making Gaussian fits to the Pb Ka2 lines, the energy resolution of the detectors were obtained. The angular opening Dhlab of detectors caused the Doppler broadening, which was contributed by both the detection window and the target size. In the present case, the corresponding maximal Doppler broadening was estimated to be to be ±1.4% and ±0.9% of the shifted X-ray energy in the laboratory frame, for the detectors at 90° and 120°, respectively. The energy spread of K-REC induced by N2 Compton profile was theoretically estimated by fitting c-ray Compton scattering data [20,23] and converted into laboratory frame [6,17]. It shows that at larger observation angles (hlab P 90°), the broadening of the REC peaks was mainly caused by the Compton profile of the N2 molecules, rather than the detector resolution or the Doppler broadening.
The authors would like to acknowledge the HIRFL-CSR operation group for providing the ion beam and carefully adjusting the beam orbit in the internal target section. This work is supported by the National Natural Science Foundation of China under Grant No. 10874188 and the Chinese Academy of Sciences key foundation under Grant No. KJCX1-YW-N30. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
G. Raisbeck, F. Yiou, Phys. Rev. A 4 (1971) 1858. H.W. Schnopper, H.D. Betz, J.P. Delvaille, et al., Phys. Rev. Lett. 29 (1972) 898. P. Kienle, M. Kleber, B. Povh, et al., Phys. Rev. Lett. 31 (1973) 1099. J. Eichler, T. Stöhlker, Phys. Rep. 439 (2007) 1–99. D.P. Dewangan, J. Eichler, Phys. Rep. 247 (1994) 59–219. J. Eichler, Phys. Rep. 193 (1990) 165–277. T. Stöhlker, X. Ma, T. Ludziejewski, et al., Phys. Scr. T92 (2001) 432–434. T. Stöhlker, C. Kozhuharov, P.H. Mokler, et al., Phys. Rev. A 51 (1995) 2098. A. Ichihara, T. Shirai, J. Eichler, Phys. Rev. A 49 (1994) 1875. J. Eichler, A. Ichihara, T. Shirai, Phys. Rev. A 51 (1995) 3027. R. Anholt, S.A. Andriamonje, E. Morenzoni, et al., Phys. Rev. Lett. 53 (1984) 234. S. Andriamonje, M. Chevallier, C. Cohen, et al., Phys. Rev. Lett. 59 (1987) 2271. J.P. Briand, P. Indelicato, A. Simionovici, et al., Europhys. Lett. 9 (1989) 225– 230. J.W. Xia, W.L. Zhan, B.W. Wei, et al., Nucl. Instr. Meth. A 488 (2002) 11–25. X. Cai, R. Lu, C. Shao, et al., Nucl. Instr. Meth. A 555 (2005) 15–19. Y. Xue, X. Cai, D. Yu, Nucl. Phys. Rev. 25 (2008) 355 (in Chinese). J. Eicher, W. Meyerhof, Relativistic Atomic Collisions, Academic, New York, 1995. I.S. Dmitriev, V.P. Zaikov, E.A. Kral’kina, et al., Nucl. Instr. Meth. B 14 (1986) 515–526. A. Ichihara, J. Eichler, At. Data Nucl. Data Tables 74 (2000) 1–121. J.-J. Wan, C.-Z. Dong, X.-B. Ding, et al., Phys. Rev. A 79 (2009) 022707. T. Ludziejewski, T. Stöhlker, S. Keller, et al., J. Phys. B 31 (1998) 2601. R. Schule, H. Schmidt-Bocking, I. Tserruya, J. Phys. B 10 (1977) 889. P. Eisenberger, W.A. Reed, Phys. Rev. A 5 (1972) 2085.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Short Communication
Observation of K- and L-REC in 200 MeV/u Xe54+-N2 collisions at HIRFL-CSR Deyang Yu a,⇑, Yingli Xue a, Caojie Shao a,b, Zhangyong Song a,b, Rongchun Lu a, Fangfang Ruan a, Wei Wang a, Jing Chen a,b, Bian Yang a,b, Zhihu Yang a, Jianjie Wan c, Chenzhong Dong c, Xiaohong Cai a,⇑ a
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China c College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, PR China b
a r t i c l e
i n f o
Article history: Received 30 September 2010 Received in revised form 29 January 2011 Available online 4 February 2011 Keywords: Radiative electron capture Highly charged ions Heavy ion storage ring
a b s t r a c t The 200 MeV/u Xe54+ ions were utilized to collide with N2 molecules and the K- and L-REC, as well as the Lyman lines of Xe53+ were observed. After electron cooling, the ion beam momentum spread DP/P 2.2 105 was achieved and the target thickness was stabilized at about 1013 atom/cm2. As the first atomic experiment at HIRFL-CSR with the internal target, its feasibility and stability were verified. Ó 2011 Elsevier B.V. All rights reserved.
In energetic collisions between highly charged ions (HCIs) and atoms or molecules, the target electrons may be captured by the projectile ions, either radiatively or non-radiatively. During Radiative Electron Capture (REC), a target electron is transferred to the projectile accompanying with electromagnetic field excitation, i.e., a real photon is emitted simultaneously, which carries away the excess energy and momentum. The REC was first observed in 1970s [1–3], since then it was extensively studied both experimentally and theoretically [4–6]. With the development of heavy ion cooling storage rings and internal targets [7,8], the REC research was pushed ahead to very high-Z HCIs (e.g., bare and hydrogen-like uranium ions) and wider energy ranges (from several MeV/u to several hundred MeV/u). Since relatively loosely bound electrons are more likely to be captured radiatively, in this case REC can be approximated as Radiative Recombination (RR), which in turn is the time-inverse process of the photoelectric effect [9,10]. The Xe54+ of 197 MeV/u was employed by Anholt et al. [11] to observe REC into K-shell (K-REC) in 1984. However, because a solid beryllium foil target was used, the X-ray spectrum had a high background due to the primary bremsstrahlung (PB), and especially the secondary electron bremsstrahlung (SEB). Therefore, the L-REC was not directly observed. Later, Andriamonje et al. observed REC of 25 MeV/u Xe53+ ions via channeling in a thin silicon crystal, and the orientation dependence of the positions and shapes of associated photon lines were shown [12]. Briand et al. measured Xe52+,53+ Lyman and Balmer lines by the beam-foil method, but the REC process was not observed [13]. ⇑ Corresponding authors. E-mail addresses: [email protected] (D. Yu), [email protected] (X. Cai). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.128
In this paper, the observation of K- and L-REC in Xe54+-N2 collisions is presented, which is the first atomic physics experiment carried out at HIRFL-CSR (Heavy Ion Research Facility at Lanzhou – Cooling Storage Ring) [14] with the internal target [15]. The Xe27+ ion beam produced by a superconductor ECR ion source was accelerated to 2.9 MeV/u by the SFC cyclotron (an injector of the storage rings), and then was accumulated, cooled and accelerated up to 200 MeV/u in the main ring, CSRm, which worked as a synchrotron in this experiment. Before injection into the experimental ring, CSRe, the Xe27+ ions were stripped by a 0.2 mm-thick carbon foil. The energy loss on the carbon foil was estimated to be about 4 MeV/u, and the Xe54+ yield was about 70%. After stripping, the Xe54+ ions were selected and injected into CSRe, where the beam was cooled by a hollow electron beam at the electron cooler section. The electron beam energy was set at 108.9 keV, and the intensity was about 500–600 mA with a diameter of 25 mm. During circulation, the ion velocity was dragged to the same value as the velocity of the cooling electrons, i.e., the circumrotating frequency was 1.27 MHz in CSRe. After cooling, the typical momentum spread in CSRe was DP/P 2.2 105, and the beam intensity was 30–60 lA (i.e., about 3–6 106 ions). CSRe has three typical operation modes, i.e., the internal target mode, the normal mode and the isochronous mode. In the present case, the internal target mode was imposed, in which the ion beam was most focused on the target region to minimize the scrape on the vacuum chamber. Otherwise, it would produce neutron background in the experimental area. In the internal target section, the beam crossed with a N2 cluster jet, which was produced by a four-stage internal target system with its nozzle which was cooled to about 90 K [15]. The jet had a well bounded intensity profile
D. Yu et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 692–694
with a 3–4 mm-diameter at the interaction zone. The target thickness was stabilized to be around 1013 atom/cm2 while the vacuum in the ring was better than 3 1011 mbar. To optimize the beam-target overlap, a PMT was employed behind a quartz window of the collision chamber to count the visible and ultraviolet photons, which were emitted when the ions passed through the internal target. Without the ion beam, the counting rate of the PMT was typically less than 10 Hz, while with optimized beam overlap the counting rate was at a range from 2000 to 3500 Hz. By locally shifting its orbit and monitoring the PMT counting rate, the beam was able to simultaneously overlap with the cooling electron beam and cross with the internal target jet. In addition, a neutron detector was placed closely under the collision chamber to monitor the neutron background, since most of the emitted neutrons concentrate in the forward direction for high energy nuclear collisions. It recorded about 10–30 neutrons during each beam injection (within 1 ls), while after cooling, the neutron counting rate was less than 0.1 Hz. To restrain the background, the data acquisition system would reject any X-ray data from the detectors during the injection and the cooling. According to the numerical calculations [16], the most important ion-loss mechanisms were recombination in the electron cooler and electron capture at the internal target. In the present case, the REC cross section was estimated to be about 30 barns per target electron by the Stobbe formula [17], and the NRC cross section was estimated to be about 600 barns per atom [18]. Experimentally, the storage lifetime of the Xe54+ beam in CSRe was typically 8–10 min with the electron cooling and the internal target turning on. The produced X-ray was detected by two lead-shielded HPGe detectors which were located at 90° and 120°, with the open solid angles (X/4p) of 3.0 105 (3 8 mm2 slit at 260 mm) and 2.5 105 (5 7 mm2 slit at 340 mm), respectively. The corresponding x-rays spectra are shown in Fig. 1. The K- and L-REC, as well as the Lyman lines (including the M1 transition) of Xe53+⁄ were observed. Obviously, the x-rays emitted by the moving ions were Doppler shifted, which satisfy
Elab ¼ Eion =cð1 b cos hlab Þ
ð1Þ
Here Eion and Elab denote the photon energies in the projectile and the laboratory frame, respectively. b is the ion velocity in units of the speed of light, c = (1 b2)1/2 is the Lorentz factor, and hlab represents the angle between the ion direction and the photon direction in the laboratory frame. The shifted energies and counts of K- and L-REC X-ray, as well as the corresponding intrinsic detector efficiencies are listed in Table 1. It is established that the cross section of REC into 2s is much larger than that into 2p in higher energy region [10,19], and thus the angular distribution of L-REC roughly follows the sin2hlab law, which is similar to the distribution of K-REC [10]. Then, the angular relative intensity between L- and K-REC is almost independent from the observation angle hlab, especially at larger observation angles, such as 90° and 120°. Under the present conditions, we calculate [20] the ratio of total cross sections between L- and K-REC including E1, E2, M1 and M2 transitions, which is 0.14. However, due to the large background and poor statistics, as shown in Fig. 1, it was not possible to obtain quantitative results in this first experiment. The Pb characteristic Ka1, Ka2 and Kb x-rays were also observed without any Doppler shift, as shown in Fig. 1. To clarify the origin of this radiation, the X-ray background in the HPGe detectors were measured after the experiment (with the beam off), both with and without lead shields. Another HPGe detector (more suitable for high energies) was also employed and it showed the 1.46 MeV c-ray of the 40K in the environment. With the lead shields, the low energy X-ray detectors recorded the Pb Ka1, Ka2 and Kb x-rays.
693
Fig. 1. X-ray spectra obtained by 90° and 120° detectors in 200 MeV/u Xe54+-N2 collisions. The Pb characteristic x-rays originated from the shields which were illuminated by the high energy c-ray background, while the low energy background mainly came from the primary bremsstrahlung.
On the contrary, when the lead shields were removed these Pb characteristic x-rays disappeared. As shown in Fig. 1, the background is higher in the low energy range. The low energy X-ray owing to the primary bremsstrahlung (i.e., in the projectile frame, the target electrons are decelerated by the ions and with simultaneous emission of a Bremsstrahlung photon) [21] has an energy endpoint Emax:
Emax ¼ ðc 1Þme c2 c1 ð1 b cosðhlab ÞÞ1
ð2Þ
Here mec2 is the electron rest mass in keV (i.e., 511 keV). In present conditions, at 90° and 120° the endpoint energies were 90.4 and 70.4 keV, lower than the L-REC energies, respectively. Owing to the low density of the internal target, the primary bremsstrahlung, and especially the secondary electron bremsstrahlung (i.e., the ejected electrons in the primary collisions collide on other target atoms then produce bremsstrahlung x-rays) were strongly restrained compared with those in the case of solid targets [11,22]. However, above the energy endpoints, the SEB on account of the chamber walls and the beryllium windows, as well as the scattered c-ray in the lead shields produced higher energy X-ray background yet. The measured energy spread (FWHM) of K-REC x-rays at 90° and 120° was 4.5 and 3.2 keV, respectively. It was contributed by the detector resolution, the Doppler broadening and the Compton profile of the N2 molecules, which are listed in Table 2. As
694
D. Yu et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 692–694
Table 1 K- and L-REC X-ray energy, counts and intrinsic detector efficiency. Detector
X-ray
Energy (keV)
Detector efficiency (%)
Net counts
90°
K-REC L-REC
123.6 98.2
48 ± 5 75 ± 5
3958 ± 366 348 ± 58
120°
K-REC L-REC
96.3 76.5
76 ± 5 91 ± 5
2932 ± 338 183 ± 46
In summary, the Xe54+ ions were produced, stored, cooled, and overlapped with the internal target at HIRFL-CSR. The experimental layout and feasibility, as well as the long-term stability of the internal target were verified. By using the internal gas target, the SEB background was strongly restrained, and the REC processes were clearly observed. In present conditions, the dominant REC X-ray energy spread mechanism at big observation angles was the Compton profile of N2 molecules. Acknowledgements
Table 2 Different contributions to the K-REC energy spread. Energy spread Detector resolution around 75 keV (FWHM) Doppler broadening (maximal) Transformed Compton profile of N2 (FWHM) Totally measured (FWHM)
90° Detector (keV) 0.99
120° Detector (keV) 0.57
±1.7 4.1
±0.9 3.1
4.5
3.2
mentioned above, the Pb characteristic x-rays originated from the shields which were illuminated by the high energy c-ray present in the environment. Therefore, these lines are free of the Doppler broadening. This is seen in Fig. 1 where the Pb characteristic lines are much narrower than the K-REC peaks. Via making Gaussian fits to the Pb Ka2 lines, the energy resolution of the detectors were obtained. The angular opening Dhlab of detectors caused the Doppler broadening, which was contributed by both the detection window and the target size. In the present case, the corresponding maximal Doppler broadening was estimated to be to be ±1.4% and ±0.9% of the shifted X-ray energy in the laboratory frame, for the detectors at 90° and 120°, respectively. The energy spread of K-REC induced by N2 Compton profile was theoretically estimated by fitting c-ray Compton scattering data [20,23] and converted into laboratory frame [6,17]. It shows that at larger observation angles (hlab P 90°), the broadening of the REC peaks was mainly caused by the Compton profile of the N2 molecules, rather than the detector resolution or the Doppler broadening.
The authors would like to acknowledge the HIRFL-CSR operation group for providing the ion beam and carefully adjusting the beam orbit in the internal target section. This work is supported by the National Natural Science Foundation of China under Grant No. 10874188 and the Chinese Academy of Sciences key foundation under Grant No. KJCX1-YW-N30. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
G. Raisbeck, F. Yiou, Phys. Rev. A 4 (1971) 1858. H.W. Schnopper, H.D. Betz, J.P. Delvaille, et al., Phys. Rev. Lett. 29 (1972) 898. P. Kienle, M. Kleber, B. Povh, et al., Phys. Rev. Lett. 31 (1973) 1099. J. Eichler, T. Stöhlker, Phys. Rep. 439 (2007) 1–99. D.P. Dewangan, J. Eichler, Phys. Rep. 247 (1994) 59–219. J. Eichler, Phys. Rep. 193 (1990) 165–277. T. Stöhlker, X. Ma, T. Ludziejewski, et al., Phys. Scr. T92 (2001) 432–434. T. Stöhlker, C. Kozhuharov, P.H. Mokler, et al., Phys. Rev. A 51 (1995) 2098. A. Ichihara, T. Shirai, J. Eichler, Phys. Rev. A 49 (1994) 1875. J. Eichler, A. Ichihara, T. Shirai, Phys. Rev. A 51 (1995) 3027. R. Anholt, S.A. Andriamonje, E. Morenzoni, et al., Phys. Rev. Lett. 53 (1984) 234. S. Andriamonje, M. Chevallier, C. Cohen, et al., Phys. Rev. Lett. 59 (1987) 2271. J.P. Briand, P. Indelicato, A. Simionovici, et al., Europhys. Lett. 9 (1989) 225– 230. J.W. Xia, W.L. Zhan, B.W. Wei, et al., Nucl. Instr. Meth. A 488 (2002) 11–25. X. Cai, R. Lu, C. Shao, et al., Nucl. Instr. Meth. A 555 (2005) 15–19. Y. Xue, X. Cai, D. Yu, Nucl. Phys. Rev. 25 (2008) 355 (in Chinese). J. Eicher, W. Meyerhof, Relativistic Atomic Collisions, Academic, New York, 1995. I.S. Dmitriev, V.P. Zaikov, E.A. Kral’kina, et al., Nucl. Instr. Meth. B 14 (1986) 515–526. A. Ichihara, J. Eichler, At. Data Nucl. Data Tables 74 (2000) 1–121. J.-J. Wan, C.-Z. Dong, X.-B. Ding, et al., Phys. Rev. A 79 (2009) 022707. T. Ludziejewski, T. Stöhlker, S. Keller, et al., J. Phys. B 31 (1998) 2601. R. Schule, H. Schmidt-Bocking, I. Tserruya, J. Phys. B 10 (1977) 889. P. Eisenberger, W.A. Reed, Phys. Rev. A 5 (1972) 2085.