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Development of a laser-enhanced ion-guide ion source
Nuclear Instruments and Methods in Physics Research B70 (1992) 241-244 North-Holland
Beam Interactions with Materials"Atoms
Development of a laser-enhanced ion-guide ion source M . Oshima, T. Sekine, S . Ichikawa, Y . Hatsukawa, I. Nishinaka 1 , T. Morikawa 2 and H. Iimura
Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-11, Japan
For the purpose of achieving a good efficiency and pulsed output of short-lived reaction products, we propose a laser-enhanced ion-guide ion source for an on-line isotope separator . Two-step resonant, three-color resonance ionization of Ba atoms was established in an off-line experiment and the ionization efficiency per pulse was measured to be 1.9%. In an on-line ion-guide experiment, it was found that the use of heavy gas was effective to stop the recoil atoms and that most of them remain in the gas until evacuation . While enhancement of the ionization efficiency by introduction of a laser beam to the ion-guide chamberhas not been achieved so far, possible improvements are discussed . 1. Introduction
2. Off-line resonance ionization experiment
For the study of short-lived nuclei which lie far from the ß-stability line, a fast extraction of reaction products is required for an ion source of an isotope separator on-line. The ion-guide ion source is most suited for this purpose [1] : reaction products recoiling out from a target are stopped in a buffer gas and extracted immediately by an electrostatic field, allowing extraction of the ions as fast as = 1 ms. However, its application is rather limited because of its low extraction efficiency (typically 10 -3 or less) . This low efficiency is partly due to neutralization of the ions in the buffer gas. An attractive method to increase the efficiency is to re-ionize the neutral atoms by resonant photo-ionization with lasers, as used in resonance ionization spectroscopy (RIS) [2] : twoor three lasers tuned in resonance with the relevant atomic transitions are used to excite and ionize the atoms. Further advantage of this technique is the Z-selectivity, which can be achieved by the element-dependent resonance scheme . Forthis purposewe propose a laser-enhanced ion-guide ion source for the isotope separator at Japan Atomic Energy Research Institute. A similar approach is proposed by Qamhieh et al. [3]. In this report, we present results of off-line resonance ionization experiments and of a first on-line experiment of the ion-guide method in combination with resonance ionization .
We performed an off-line experiment by use of two tunable dye lasers pumped by a copper-vapor laser with a repetition rate as high as 6-9 kHz and an average output power of 30 W. The band width of the dye lasers is 0.2 cm - ', which corresponds to 6 GHz. The Doppler broadening of atoms wit)-, mass number A at a temperature T is given by
On leave from Tokyo Metropolitan University, Hachioojishi, Tokyo, Japan. 2 On leave from Hiroshima University, Kagamiyama, Higashihiroshima-shi, Hiroshima 724, Japan.
Av/v = 7.16 x 10 -6 T/A .
For a wavelength of 600 nm and at room temperature, the Doppler broadening for mass number A =130 is w = 0.52 GHz. Thus the laser bandwidth well covers the Doppler broadening of the atoms. An experiment on Ba atoms was performed in an off-line chamber by three-step resonance ionization . The experimental setup is shown in fig. 1. An atomic beam of Ba was produced by an oven at a temperature of 750°C. Dyes of Rhodamine 110 and Rhodamine 613 were used for RIS of Ba as already reported in ref. [4]. The dye laser beams were collimated to a diameter of 1 mm in the interaction region of the atomic beam, in front of which a Ceratron device was placed for detection of the ions produced . The resonance-ionization spectra were measured in multichannel scaling mode with one laser scanning at a constant speed while the other was fixed at the wavelength of an atomic transition . Clean spectra were obtained as shown in fig. 2, proving the three-step (two-step resonant), two-color resonance ionization for Ba atoms. The spectra were recorded with the laser operating at a repetition rate of 6 kHz and a dwell time in the multiscaling mode of 0.1 s. It was proved that both resonant transitions at A, = 553.548 nm as well as at A z = 578.410 nm could be saturated with the available laser powers .
0168-583X/92/$05 .00 © 1992 - Elsevier Science Publishers B.V . All rights reserved
IV . ION GUIDES/HE JETS
242
M. Oshima et aL /Laser-enhanced ion-guideion source 511 nm 15 W
8.5 w
6.5 W
pico Ammeter
Fig.
Fig. 1. Experimental setup for off-line resonance ionization spectroscopy . TFA: timing filter amplifier; CFD- constant fraction discriminator; MCS: multichannel scaling . The ionization efficiency was measured by threecolor, two-step resonant RIS under the following conditions: The wavelength of the dye lasers pumped by the green output (A = 510 nm) of the copper-vapor laser, were set to resonance wavelengths of 553.548
3.
Experimental setup for off-line measurement of ion intensity.
and 578.410 nm . Enhancement of the last step was accomplished by adding the yellow pump laser beam (A = 578 nm, average power 15 W) not used for pumping the dye lasers. The schematic setup is shown in fig. 3. The atomic beam was collected on a Mylar foil placed just above the interaction region of the atomic beam and lasers in order to determine the density of the atomic Ba beam. The quantity of Ba atoms on the fo.1 was determined by neutron activation analysis . The samples were irradiated together with a standard sample of Ba under a thermal neutron flux of 4 x 10 13
A2 = 578.41 nm
I'--
oüi .
i IL I. u I
II"~1" LI- L IJIin
lnd .ii.ili . L,1dli( .
Fig. 2. Resonance ionization spectra for Ba atoms. (a) A t is scanned while A z is fixed to 578.410 nm . (b) A Z is scanned while fixed to 553.548 nm .
At
is
243
M. Oshima er aL / Laser-enhanced ion-guide ion source
n/cm 2 - s in the JRR-4 reactor at JAERI. The 139Ba radioactivity in each sample was measured by y-ray spectroscopy. The result gave pv =2.9 X 1012 atoms/ cm2. s, p and v being the density of Ba atoms in the interaction region and the mean velocity of the atoms. Since the mean velocity v is expressed as v = 3kT/m =4.3 X 104
H .I.
beam
CM/S
at T = 1020 K, the Ba-atom density at the interaction region is estimated to be 6.9 X 10 7 atoms/cm 3. Since the diameter of the lasers was typically d, = 1 mm, the number of atoms in the interaction region was determined to be 5.4 X 10 5 atoms. An ion current of I = 10-tt A was measured in the setup shown in fig. 3. Hence, the ionization efficiency perpulse is 1.9%. This can be compared with the estimated value of 35% obtained by using the empirical formula given in ref. [5]. The lower experimental value might be due to a smaller effective interaction volume of the atoms and the laser than expected . 3. On-line experiment In order to derive what fraction of atoms interacts with the laser and its dependence on different buffer gases in an on-line ion-guide chamber, we measured the distribution inside the chamber of the radioactivity produced . A schematic layout of the chamber used is shown in fig . 4. The buffer gas of 0.2 atm He was introduced in the chamber and evacuated through a skimmer of a diameter of 0 = 0.8 mm by a turbomolecular pump with a pumping speed of 1800 1/s. The reaction na1 Sn( 12 C, xn) was used to produce 125-127Ba activities. At first the reaction products recoiling out of the target were accumulated on an Al foil placed just behind the target (not shown in the figure). It was found that 18% of the reaction products recoiled out of the target owing to its finite thickness . Secondly, Al foils were placed on the inner surface of
135% 4535% 54±5% (Inside) (0°) (ges)
the chamber, as shown in fig. 5. After irradiation by the carbon beam, the 127Ba radioactivity collected on the foils was measured. The yield of Ba atoms which were evacuated through the skimmer was derived by subtracting the radioactivity remaining inside the chamber from the total radioactivity recoiling out of the target . The result is shown in fig. 5a . The fraction of the radioactivity is normalized to the total number of Ba atoms recoiling out of the target . As mentioned above, the recoil energy of the reaction products should show a distribution from zero to the energy corresponding to the reaction occurring at the rear surface of the target. From a range calculation the fraction of atoms stopped in the gas was calculated to be 25%. This can be compared with the experimental value of 1 t 5% . This discrepancy is considered to be due to the fact that the atoms are trapped on the inner surface of the chamber during thermal motion . In a third experiment, the He buffer gas was replaced by N2 of a pressure of 0.2 atm. The radioactivity distribution is shown in fig. 5b measured in the same way as for He . The fraction of atoms evacuated from the ion-guide chamber amounts to 72%, which is comparable with the estimated value of 100% obtained in the same way as for He. Hence, the use of N2 gas has
7235% 1935% (gas) (Inside)
833% (0°)
(a)
Fig. 5. Distribution of 125-127Ba activity for buffer gases (a) He and (b) N2 after irradiation of a Sn target by a carbon beam . IV . ION GUIDES/HE JETS
244
M. Oshima et al. / Laser-enhanced ion-guide ionsource
turned out to be effective for stopping the recoils and keeping a considerable amount of the atoms in the gas phase without sticking to the inner surface, until they are evacuated through the skimmer. Ions extracted from the ion-guide chamber were mass-separated and the A = 127 radioactivity was measured on-line. From the intensity of annihilation -y-rays, the fraction of extracted ions was determined to be 1 x 10 -4 relative to the number of atoms recoiling out of the target with laser shut off. In the last step laser light, a composite of two dye-laser beams and the 578 nm copper-vapor laser beam, was introduced into the ion-guide chamber, as shown in fig . 4. Taking into account the faster flow in the ion-guide chamber as compared to the thermal atomic beam, one expects the ion-extraction efficiency to be > 0.3% rather than 1.9% obtained in the off-line experiment. Any enhancement of the extraction efficiency, however, was not observed. This indicates the photo-ionization efficiency is less than 10 -4. A probable reason is a plasma generated by the primary heavy-ion beam which produces electrons and leads to recombination of the ions . This process may explain the low ionization efficiency of the ion-guide method itself. The recombination process may not be negligible, since the laser beam interacts with atoms in almost the same region where the plasma is created in the present setup. In order to avoid recombination, the interaction region should be separated from the plasma region . Hence, a modification of the ion-guide chamber will be necessary. The primary heavy-ion beam may cause another effect: The beam decomposes impurities present in the buffer gasor a gas molecule itself (N Z), and the decomposed fragments react chemically with the reaction-
produced atoms, forming chemical compounds. Such a process disturbs the formation of atomic ions and causes reduction of the extraction efficiency. Use of a pure inert gaswill be necessary to avoid such reactions . 4. Summary Resonance ionization of Ba atoms was established in an off-line experiment and the ionization efficiency per pulse was measured to be 1.9%. In an on-line ion-guide experiment, the radioactivity distribution after the carbon-beam irradiation was measured for two different buffer gases (He and NZ). Use of the heavier NZ gas was found effective to stop the recoil atoms. The atoms which were not lost by sticking to the inner surface of the chamber, however, could not be extracted as ions with the efficiency expected from the off-line value . Modification of the chamber and/or use of a still heavier inert gas will be required to stop the recoil atoms and to increase the extraction efficiency. References [1[ J. Ärie, J. Ayströ, J. Honkanen, K. Valli and A. Hautojarvi, Nucl. Instr . and Meth . 186 (1981) 149. [2] G.S. Hurst and M.G. Payne, Principles and Applications of Resonance Ionization Spectroscopy (I .O .P . Publishing Ltd, 1988). [3) Z.N. Qamhieh, R.E . SiNerans, E. Vandeweert, P. Van Duppen, M. Huyse and L. Vermeeren,these Proceedings (EMIS-12) Nucl. Instr. and Meth . B70 (1992) 131. [4J L.W. Green, R.G . Macdonald, F.C. Sopchyshyn and L.J. Bunnell, Inst. Phys . Set. no 84, section 3 (I .O.P . Publishing, 1986) p. 133. [5) H. Kramers, Philos. Mag. 44 (1927) 836.
Beam Interactions with Materials"Atoms
Development of a laser-enhanced ion-guide ion source M . Oshima, T. Sekine, S . Ichikawa, Y . Hatsukawa, I. Nishinaka 1 , T. Morikawa 2 and H. Iimura
Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-11, Japan
For the purpose of achieving a good efficiency and pulsed output of short-lived reaction products, we propose a laser-enhanced ion-guide ion source for an on-line isotope separator . Two-step resonant, three-color resonance ionization of Ba atoms was established in an off-line experiment and the ionization efficiency per pulse was measured to be 1.9%. In an on-line ion-guide experiment, it was found that the use of heavy gas was effective to stop the recoil atoms and that most of them remain in the gas until evacuation . While enhancement of the ionization efficiency by introduction of a laser beam to the ion-guide chamberhas not been achieved so far, possible improvements are discussed . 1. Introduction
2. Off-line resonance ionization experiment
For the study of short-lived nuclei which lie far from the ß-stability line, a fast extraction of reaction products is required for an ion source of an isotope separator on-line. The ion-guide ion source is most suited for this purpose [1] : reaction products recoiling out from a target are stopped in a buffer gas and extracted immediately by an electrostatic field, allowing extraction of the ions as fast as = 1 ms. However, its application is rather limited because of its low extraction efficiency (typically 10 -3 or less) . This low efficiency is partly due to neutralization of the ions in the buffer gas. An attractive method to increase the efficiency is to re-ionize the neutral atoms by resonant photo-ionization with lasers, as used in resonance ionization spectroscopy (RIS) [2] : twoor three lasers tuned in resonance with the relevant atomic transitions are used to excite and ionize the atoms. Further advantage of this technique is the Z-selectivity, which can be achieved by the element-dependent resonance scheme . Forthis purposewe propose a laser-enhanced ion-guide ion source for the isotope separator at Japan Atomic Energy Research Institute. A similar approach is proposed by Qamhieh et al. [3]. In this report, we present results of off-line resonance ionization experiments and of a first on-line experiment of the ion-guide method in combination with resonance ionization .
We performed an off-line experiment by use of two tunable dye lasers pumped by a copper-vapor laser with a repetition rate as high as 6-9 kHz and an average output power of 30 W. The band width of the dye lasers is 0.2 cm - ', which corresponds to 6 GHz. The Doppler broadening of atoms wit)-, mass number A at a temperature T is given by
On leave from Tokyo Metropolitan University, Hachioojishi, Tokyo, Japan. 2 On leave from Hiroshima University, Kagamiyama, Higashihiroshima-shi, Hiroshima 724, Japan.
Av/v = 7.16 x 10 -6 T/A .
For a wavelength of 600 nm and at room temperature, the Doppler broadening for mass number A =130 is w = 0.52 GHz. Thus the laser bandwidth well covers the Doppler broadening of the atoms. An experiment on Ba atoms was performed in an off-line chamber by three-step resonance ionization . The experimental setup is shown in fig. 1. An atomic beam of Ba was produced by an oven at a temperature of 750°C. Dyes of Rhodamine 110 and Rhodamine 613 were used for RIS of Ba as already reported in ref. [4]. The dye laser beams were collimated to a diameter of 1 mm in the interaction region of the atomic beam, in front of which a Ceratron device was placed for detection of the ions produced . The resonance-ionization spectra were measured in multichannel scaling mode with one laser scanning at a constant speed while the other was fixed at the wavelength of an atomic transition . Clean spectra were obtained as shown in fig. 2, proving the three-step (two-step resonant), two-color resonance ionization for Ba atoms. The spectra were recorded with the laser operating at a repetition rate of 6 kHz and a dwell time in the multiscaling mode of 0.1 s. It was proved that both resonant transitions at A, = 553.548 nm as well as at A z = 578.410 nm could be saturated with the available laser powers .
0168-583X/92/$05 .00 © 1992 - Elsevier Science Publishers B.V . All rights reserved
IV . ION GUIDES/HE JETS
242
M. Oshima et aL /Laser-enhanced ion-guideion source 511 nm 15 W
8.5 w
6.5 W
pico Ammeter
Fig.
Fig. 1. Experimental setup for off-line resonance ionization spectroscopy . TFA: timing filter amplifier; CFD- constant fraction discriminator; MCS: multichannel scaling . The ionization efficiency was measured by threecolor, two-step resonant RIS under the following conditions: The wavelength of the dye lasers pumped by the green output (A = 510 nm) of the copper-vapor laser, were set to resonance wavelengths of 553.548
3.
Experimental setup for off-line measurement of ion intensity.
and 578.410 nm . Enhancement of the last step was accomplished by adding the yellow pump laser beam (A = 578 nm, average power 15 W) not used for pumping the dye lasers. The schematic setup is shown in fig. 3. The atomic beam was collected on a Mylar foil placed just above the interaction region of the atomic beam and lasers in order to determine the density of the atomic Ba beam. The quantity of Ba atoms on the fo.1 was determined by neutron activation analysis . The samples were irradiated together with a standard sample of Ba under a thermal neutron flux of 4 x 10 13
A2 = 578.41 nm
I'--
oüi .
i IL I. u I
II"~1" LI- L IJIin
lnd .ii.ili . L,1dli( .
Fig. 2. Resonance ionization spectra for Ba atoms. (a) A t is scanned while A z is fixed to 578.410 nm . (b) A Z is scanned while fixed to 553.548 nm .
At
is
243
M. Oshima er aL / Laser-enhanced ion-guide ion source
n/cm 2 - s in the JRR-4 reactor at JAERI. The 139Ba radioactivity in each sample was measured by y-ray spectroscopy. The result gave pv =2.9 X 1012 atoms/ cm2. s, p and v being the density of Ba atoms in the interaction region and the mean velocity of the atoms. Since the mean velocity v is expressed as v = 3kT/m =4.3 X 104
H .I.
beam
CM/S
at T = 1020 K, the Ba-atom density at the interaction region is estimated to be 6.9 X 10 7 atoms/cm 3. Since the diameter of the lasers was typically d, = 1 mm, the number of atoms in the interaction region was determined to be 5.4 X 10 5 atoms. An ion current of I = 10-tt A was measured in the setup shown in fig. 3. Hence, the ionization efficiency perpulse is 1.9%. This can be compared with the estimated value of 35% obtained by using the empirical formula given in ref. [5]. The lower experimental value might be due to a smaller effective interaction volume of the atoms and the laser than expected . 3. On-line experiment In order to derive what fraction of atoms interacts with the laser and its dependence on different buffer gases in an on-line ion-guide chamber, we measured the distribution inside the chamber of the radioactivity produced . A schematic layout of the chamber used is shown in fig . 4. The buffer gas of 0.2 atm He was introduced in the chamber and evacuated through a skimmer of a diameter of 0 = 0.8 mm by a turbomolecular pump with a pumping speed of 1800 1/s. The reaction na1 Sn( 12 C, xn) was used to produce 125-127Ba activities. At first the reaction products recoiling out of the target were accumulated on an Al foil placed just behind the target (not shown in the figure). It was found that 18% of the reaction products recoiled out of the target owing to its finite thickness . Secondly, Al foils were placed on the inner surface of
135% 4535% 54±5% (Inside) (0°) (ges)
the chamber, as shown in fig. 5. After irradiation by the carbon beam, the 127Ba radioactivity collected on the foils was measured. The yield of Ba atoms which were evacuated through the skimmer was derived by subtracting the radioactivity remaining inside the chamber from the total radioactivity recoiling out of the target . The result is shown in fig. 5a . The fraction of the radioactivity is normalized to the total number of Ba atoms recoiling out of the target . As mentioned above, the recoil energy of the reaction products should show a distribution from zero to the energy corresponding to the reaction occurring at the rear surface of the target. From a range calculation the fraction of atoms stopped in the gas was calculated to be 25%. This can be compared with the experimental value of 1 t 5% . This discrepancy is considered to be due to the fact that the atoms are trapped on the inner surface of the chamber during thermal motion . In a third experiment, the He buffer gas was replaced by N2 of a pressure of 0.2 atm. The radioactivity distribution is shown in fig. 5b measured in the same way as for He . The fraction of atoms evacuated from the ion-guide chamber amounts to 72%, which is comparable with the estimated value of 100% obtained in the same way as for He. Hence, the use of N2 gas has
7235% 1935% (gas) (Inside)
833% (0°)
(a)
Fig. 5. Distribution of 125-127Ba activity for buffer gases (a) He and (b) N2 after irradiation of a Sn target by a carbon beam . IV . ION GUIDES/HE JETS
244
M. Oshima et al. / Laser-enhanced ion-guide ionsource
turned out to be effective for stopping the recoils and keeping a considerable amount of the atoms in the gas phase without sticking to the inner surface, until they are evacuated through the skimmer. Ions extracted from the ion-guide chamber were mass-separated and the A = 127 radioactivity was measured on-line. From the intensity of annihilation -y-rays, the fraction of extracted ions was determined to be 1 x 10 -4 relative to the number of atoms recoiling out of the target with laser shut off. In the last step laser light, a composite of two dye-laser beams and the 578 nm copper-vapor laser beam, was introduced into the ion-guide chamber, as shown in fig . 4. Taking into account the faster flow in the ion-guide chamber as compared to the thermal atomic beam, one expects the ion-extraction efficiency to be > 0.3% rather than 1.9% obtained in the off-line experiment. Any enhancement of the extraction efficiency, however, was not observed. This indicates the photo-ionization efficiency is less than 10 -4. A probable reason is a plasma generated by the primary heavy-ion beam which produces electrons and leads to recombination of the ions . This process may explain the low ionization efficiency of the ion-guide method itself. The recombination process may not be negligible, since the laser beam interacts with atoms in almost the same region where the plasma is created in the present setup. In order to avoid recombination, the interaction region should be separated from the plasma region . Hence, a modification of the ion-guide chamber will be necessary. The primary heavy-ion beam may cause another effect: The beam decomposes impurities present in the buffer gasor a gas molecule itself (N Z), and the decomposed fragments react chemically with the reaction-
produced atoms, forming chemical compounds. Such a process disturbs the formation of atomic ions and causes reduction of the extraction efficiency. Use of a pure inert gaswill be necessary to avoid such reactions . 4. Summary Resonance ionization of Ba atoms was established in an off-line experiment and the ionization efficiency per pulse was measured to be 1.9%. In an on-line ion-guide experiment, the radioactivity distribution after the carbon-beam irradiation was measured for two different buffer gases (He and NZ). Use of the heavier NZ gas was found effective to stop the recoil atoms. The atoms which were not lost by sticking to the inner surface of the chamber, however, could not be extracted as ions with the efficiency expected from the off-line value . Modification of the chamber and/or use of a still heavier inert gas will be required to stop the recoil atoms and to increase the extraction efficiency. References [1[ J. Ärie, J. Ayströ, J. Honkanen, K. Valli and A. Hautojarvi, Nucl. Instr . and Meth . 186 (1981) 149. [2] G.S. Hurst and M.G. Payne, Principles and Applications of Resonance Ionization Spectroscopy (I .O .P . Publishing Ltd, 1988). [3) Z.N. Qamhieh, R.E . SiNerans, E. Vandeweert, P. Van Duppen, M. Huyse and L. Vermeeren,these Proceedings (EMIS-12) Nucl. Instr. and Meth . B70 (1992) 131. [4J L.W. Green, R.G . Macdonald, F.C. Sopchyshyn and L.J. Bunnell, Inst. Phys . Set. no 84, section 3 (I .O.P . Publishing, 1986) p. 133. [5) H. Kramers, Philos. Mag. 44 (1927) 836.