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Pilis: Post-ISOCELE laser isobar separation — an apparatus for laser spectroscopic studies of laser-desorbed atoms
252
Nuclear
Instruments
and Methods
in Physics Research 834 (1988) 252-257 North-Holland, Amsterdam
PILIS: POST-ISOCELE LASER ISOBAR SEPARATION - AN APPARATUS FOR LASER SPECTROSCOPIC STUDIES OF LASER-DESORBED ATOMS J.K.P. LEE, G. SAVARD,
J.E. CRAWFORD
and G. THEKKADATH
Foster Radiation Laboratory. McGill University MontrPaI, Canada
H.T. DUONG,
J. PINARD
and S. LIBERMAN
Laboratoire AimP Cotton, CNRS II, Orsay, France
F. LE BLANC,
P. KILCHER,
J. OBERT,
J. OMS, J.C. PUTAUX,
B. ROUSSIkRE
and J. SAUVAGE
Insrilut de Physique Nucl4aire. Orsay, France
and the ISOCELE Received
29 March
Collaboration
1988 and in revised form 15 April 1988
An apparatus for laser spectroscopic studies of laser-desorbed radioactive atoms has been installed on-line at the ISGCELE isotope separator (IPN, Orsay). Mass-separated gold ions were first implanted onto a substrate, and then thermally desorbed by a Nd-YAG laser pulse. A three-step, two-resonance scheme was used to selectively ionize the desorbed gold atoms. The ions created were then mass-identified through a time-of-flight technique. The laser system used has a hnewidth of 130 MHz, and an actual experimental resolution of 170 MHz for the stable “‘Au was obtained. It has been demonstrated that with 10” total implanted ions, several laser scans can be performed, and that a 5X10-s ion conversion efficiency for the desorbed gold atoms was reached at resonance. On-line measurements of the isotope shift (IS) and hyperfine structure (HFS) for several gold isotopes were carried out. With the high-resolution capability, a negative sign for the magnetic moment of t9*Au was obtained. The HFS of ‘s’Au confirms an earlier laser spectroscopic study, and the sudden variation of IS between ‘s’Au and la6Au was reproduced.
1. Introduction The laser spectroscopic study of long chains of isotopes is a very powerful method to systematically probe the static properties of atomic nuclei. The hyperfine structure (HFS) of an atomic transition can yield information about the spins and electromagnetic moments of the nuclei while the isotope shift (IS) between a pair of isotopes gives the change of the mean square nuclear charge radius (6( r2)). These studies have contributed substantially to our understanding of properties of nuclei extending far from the beta stability valley. These results, together with the various experimental techniques used, have been summarized in a recent review article [1] and the details of these works can be found in the extensive references therein. The laser spectroscopic methods utilize the interaction of laser beams with atoms or ions of thermal or high velocities. For their applications in the studies of radioactive species, the main challenge is to achieve high sensitivity while maintaining an overall resolution adequate for the particular atomic transition being 0168-583X/88/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
studied. Generally, this means that a highly sensitive, Doppler-reduced spectroscopic technique is required. Among various methods that have been applied, the most prolific is perhaps colhnear laser spectroscopy via fluorescence scattering from a fast atomic beam [2]. In this method, the mass-separated ions from an on-line isotope separator (ISOL) are first neutralized before interacting with a collinear laser beam. This method is very versatile and applicable to any ion beam produced in an ISOL, provided that the required laser beam corresponding to the atomic transitions to be studied is available. The overall sensitivity is very good, requiring only an ion beam intensity of about lO’/s to 106/s in the laser-ion interaction region, which can be achieved in many existing ISOLs. In addition, various techniques can be adapted to improve the signal to background ratio thus increasing the overall sensitivity. Another method that has proved to be very powerful is to use the laser beam to optically pump the alkali atoms to a particular magnetic substrate followed by an efficient level-sensitive atom detection [3]. The required ion beam intensity required in this case is about 104/s, but the
J.K.P. Lee et al. / PILIS
method is only applicable to alkali elements. Other methods such as fluorescence scattering from a thermal atomic beam [4] or from atoms contained inside a closed cell [S] have also been used; however, they generally have lower sensitivities and have not widely been applied. More recently, resonant ionization spectroscopic (RIS) method [6,7] have been used to improve the overall detection sensitivity. The RIS method is known to be a highly efficient and selective process for specific atom detection. In these applications for the study of radioactive atoms, pulsed lasers are used, interacting with a steady thermal atomic beam. This approach has two major deficiencies: the inherently low temporal duty cycle and the poor spectroscopic resolution of pulsed lasers. With modem techniques, these difficulties can be compensated in several ways. For example, pulsed lasers with high repetition rates such as copper vapour lasers could alleviate the duty cycle problem. However, when a dye laser is pumped by such a laser, the tunable range of its output wavelength is limited. Therefore, the variety of elements which can be studied is restricted. As for the spectroscopic resolution of the pulsed lasers, various techniques are now available to achieve a resolution close to the Fourier-transformed limit of the pulse duration. However, for reliable operating conditions, very careful tuning and control are necessary. We would like to describe a new spectroscopic apparatus PILIS (Post-Isocele Laser Isobar Separation) installed at ISOCELE [8], an ISOL at the synchrocyclotron of Orsay, that circumvents the difficulties mentioned above. This applies the RIS technique to atoms desorbed by a heating laser. The viability of this approach was tested earlier [9,10], and an overall detection efficiency of 10d4 was achieved. Recently, PILIS was successfully used for an on-line study of the IS and HFS of radioactive gold atoms. The technical aspects of this apparatus are presented here.
2. Experimental setup The schematic diagram of PILIS is shown in fig. 1. The mass separated gold ions from ISOCELE are collected on a substrate which is mounted on the surface of a cylinder. Rotation of the cylinder brings the sample to the laser-atom interaction zone where a heating laser pulse temporarily heats up the surface and re-evaporates the implanted ions. A few microseconds later, a synchronized group of three pulsed laser beams intercepts these gold atoms and selectively ionizes them. The ions thus created are accelerated and mass identified by a time-of-flight (TOF) measurement. An electronic timing system controls the relative delays required between the different laser pulses.
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253
The 45 keV ion beam from ISOCELE was focused to a spot about 2 mm high and 10 mm wide, on a graphite substrate. The heating laser beam used was the 532 nm, 10 Hz output from a frequency-doubled Nd-YAG laser, focused to a vertical line of approximately 3 mm X 50 pm size on the implanted surface. Its output energy was 0.3 to 0.5 mJ per pulse. In order to cover the entire ion-implanted area, the sample moved at the rate of about 10 nm per laser pulse. These operating conditions were chosen to optimize the desorption efficiency and the signal to background ratio of the mass peaks to be studied. The details of the tests that led to the choice of the above parameters are described in the next section. The RIS scheme adopted was the three-step process shown in the insert of fig. 1. The first step of resonant absorption at 268 nm wavelength was provided by a high-resolution injection-locked dye laser system [ll]. This was accomplished by placing an additional thin dye cell, pumped by an excimer laser, inside a conmrercial single-mode CW dye laser cavity. When operated in this fashion, the output wavelength was determined by the CW dye laser control system while its intensity consisted of a CW component with enhanced pulses (generated by the pulsed pump laser) superimposed on it. This output was further amplified through another excimer laser-pumped amplifier and then frequency doubled to produce the continuously tunable UV light pulses. When properly tuned, the resolution of the output beam of such a system is essentially Fourier limited, with a linewidth of about 65 MHz for the visible light, or 130 MHz for the frequency-doubled UV output. Photons of the second resonant transition at 406 nm were provided by a commercial pulsed dye laser pumped by the same excimer laser. This has an output linewidth of 5 GHz. The final ionizing step was provided by the output from a broadband dye laser, operated at about 700 nm. The sizes of the first two laser beams were about 5 mm x 10 mm (the 5 mm spread being along the ion acceleration direction), while the final ionizing-step laser beam was focused to about 3 mm diameter. The ionization efficiency in the present case is determined essentially by the energy of the ionizing-step laser pulse, which is in the present case of the order of 7 ml. For the other two lasers, the system outputs are 0.1 mJ and 0.3 mJ per pulse for the first and second resonant steps, respectively. However, for actual measurements, these two lasers must operate with much reduced output power (I 1 nJ) in order to avoid power broadening effects on the transition lines. The relative timing delays among these three laser pulses were adjusted to optimize the ionization efficiency by varying the corresponding pump beam path lengths. All the laser beams were placed in the same plane parallel to the electrode plates. The first and third beams were collinear to each other, while the second beam was incident perpendicular to the other two. In this way, the
254
J.K.P. Lee et al. / PILIS
- for isobar separation
Fig. 1. Schematic layout of PILE. FP Fabry-Perot interferometer, XTL frequency doubling crystal, I, iodine cell, Ti, Tr and Ts timing inputs for synchronixed operation. Insert shows the RIS scheme. For details of operation, see text.
actual width of the second beam can be adjusted to reduce the Doppler broadening effect of the diverging motion of the desorbed atoms. The relative timing between the Nd-YAG heating pulse and the group of RIS pulses was adjusted to optimize the overall ion production of the desorbed atoms. The ions created were accelerated by the three-electrode system shown in fig. 1. The first plate was held at 1.3 kV (V,) while the second plate (V,) was at about 1.0 kV. The accelerated ions passed through a 1.3 m flight path and were mass identified by the TOF signal from a microchannel plate detector. The potential I’, was adjusted to compensate for the differences of the flight times of the ions created at different geometrical locations. The voltage on the cylinder (V,) was slightly higher than 1.3 kV, and could be adjusted to ensure a uniform field gradient in the accelerating region. An overall mass resolution (M/AM) of better than 400 for A - 200 was routinely obtained. Along the flight path, a pulsed electrostatic ion deflector was inserted to suppress the background ions created directly by the heating laser pulse. For each laser shot, the TOF spectrum was recorded by a 100 MHz digital waveform analyser. To reduce the overall fluctuation due to the shot-to-shot variations, the signals were averaged over 4 or 16 laser shots for each data point. The absolute wavelength of the CW component of the first dye laser was monitored by the
absorption of its intensity in an iodine cell, and its relative displacement was deduced from the variation of the light intensities transmitted through an external etalon with 750 MHz free spectral range. These signals were multiplexed and processed by the same digital analyser. The data were then fed to an IBM-PC com-
197
Au
192
FREQUENCY
Au
R
*
Fig. 2. Top two spectra show the variation of the corresponding mass peaks in the TOF outputs. The third curve shows the output variation from a Fabry-Perot interferometer with a free spectral range (FSR) of 748 MHz The bottom curve gives the resonant absorption in the iodine cell. All four curves were obtained during the same scan of the laser frequency.
J.K.P. Lee et al. / PILIS
puter, which also controlled the overall progress of the experiment.
3. System performance
and discussion
One of the most critical elements for the performance of PILE is the effective desorption of the implanted gold atoms. The first series of tests was carried out to search for the most suitable operating conditions, which included the power, wavelength and spot size of the heating laser beam, the motion of the implanted sample with respect to the laser spot, the substrate material and its surface conditions, and the kinetic energies of the implanted ions. For these tests, a suitable number of stable gold ions, typically about 1013, was first implanted into the substrate. Under each operating condition, the magnitude of the gold ion peak created in the TOF spectrum by the RIS process was measured together with the general background ions created by both the heating and ionizing laser beams. Several substrate materials were tested, including graphite, tantalum, platinum, tungsten and molybdenum; each measurement was done with heating laser wavelengths of 1064 nm and 532 nm. In general, the desorption of gold atoms occurred at a certain threshold heating pulse energy and became more effective with increasing heating power. For all the materials tested, adequate desorption of gold atoms could be induced. However, with higher heating power, more direct ions were created and more matter was desorbed from the substrate surface, causing a deterioration of the signal to background ratio in the TOF spectra. At the point where the deterioration became significant the heating pulse energies varied over a wide range, depending on the material and the surface condition of the substrate. These tests suggested that graphite substrates provided the best desorption, requiring much lower heating power than the metallic foils for the same desorption efficiency. Also, no noticeable difference was noted for different heating wavelengths. For most of the subsequent tests, the 532 nm beam was used for ease of beam alignment. Continued studies of the desorption characteristics of the gold atoms from graphite showed the general trend that when the implanted spot was repeatedly heated by successive laser shots, the desorption efficiency was gradually reduced. The rate of reduction varied widely according to the heating power density used. The number of laser shots at the same power level required to reduce the gold ion peak in the TOF spectrum to 10% of its original value could be as low as 3 (for high powers) to as high as over 100 (for lower powers). However, higher heating power created higher background ion intensities, and extensive tests were carried out to search for the most suitable operating
- for isobar separation
255
conditions. This resulted in the operating parameters mentioned in the previous section, namely, about 0.3 to 0.5 mJ of pulse energy over an area of about 3 mm X 50 pm, with the sample moving at the rate of 10 pm per laser shot. In this way, with the implanted ions covering a 2 mm x 10 mm area, each laser shot would cover some new sample surface, and a steady desorption of the gold atoms could be maintained. To measure the absolute desorption efficiency under these operating conditions, a sample with implanted radioactive i9’Au was used. Its activities measured before and after a complete heating laser sweep showed that about 20% of the gold atoms were removed by this heating process, giving an average desorption efficiency of 4% per laser shot for the gold atoms covered by the laser spot area. In fact, under these operating conditions it was quite feasible to repeat the laser heating process over the same surface area several times. With subsequent passes, steady desorption of gold atoms could still be maintained but with lower desorption yield for each successive pass. The desorption rate of gold atoms implanted at different energies of 30 to 45 keV did not show any noticeable difference. However, when the ions were slowed to 0.5 keV before implantation, the desorption rate was much higher and the required heating laser power was lower. Preheating the substrate surface by the laser before implantation did not seem to affect the subsequent desorption rate. To evaluate the overall sensitivity of this particular setup, a sample with about 10” implanted atoms was used. At resonance, about 100 gold ions were detected for each laser shot. Assuming a 4% desorption efficiency for each heating laser pulse as obtained above, 2 X lo6 gold atoms would then be desorbed, and the ion conversion rate for the desorbed gold atoms was therefore 5 X 10p5. The fraction of the desorbed atoms covered by the overlap of the three ionization laser beams was calculated from the position of the overlapped spot, assuming a cos 0 distribution for the desorbed atoms, and their velocity distribution, and was found to be 8 x 10e3. The remaining factor of 6 X lo3 was due to the overall ionization efficiency, losses in the ion transport and to the fraction of the desorbed atoms not in their ground state. With the operating conditions thus defined, spectroscopic measurements for the gold isotopes were then attempted. To reach a good spectroscopic resolution, the geometrical width of the 406 nm beam was narrowed. The resultant spectrum showing the RIS yield of the stable 19’Au ions vs the scanning laser frequency is shown in fig. 2, indicating an actual experimental resolution (FWHM) of 170 MHz. Another spectrum obtained for the radioactive 19*Au is also shown in the same diagram. The resolution obtained was quite adequate to resolve the hyperfine splitting of its ground state (375 MHz). The relative amplitude of the two
J.K.P. Lee et al. / PILIS
256
I
-.
197
/
/
,
/
L
I
I
I FREQUENCY
‘4
10 GHz
1
I
187 186
‘I/
j
01
Fig. 3. Summary of the RIS results for the isotopes discussed in this work. Vertical lines show the relative positions of the transitions observed while the heights are theoretical peak amplitudes expected. Open circles show the position of the centroids for the transitions. From A =197 to 187, the centroid moves smoothly towards the ‘red’ direction. The sudden change between A = 187 and 186 shows a sudden change of nuclear charge radius.
resolved components, each actually containing two transitions to the hyperfine structure of the upper level, yielded a negative sign for the magnetic moment of 19’Au. The measured isotope shift with respect to the stable 19’Au obtained was in agreement with an earlier work 1121, where the sign of the magnetic moment could not be determined. Measurements were extended to other isotopes, among them ls7Au and ‘86Au. For the “‘Au the HFS measurement is in agreement with that reporied in ref. [7], and the deduced magnetic moment of 0.531(12) n,m. is quite different from the 0.726(7) n.m. obtained by the atomic beam magnetic resonance (ABMR) method [13]. Possibly another measurement with the AMBR method will resolve this discrepancy. Our results have also reproduced the abrupt change in a sudden the IS between ‘87Au and ls6Au, indicating change in the nuclear deformation between these two nuclei. These results are summarized in fig. 3. Discussions of these and other radioactive gold isotopes were presented in an earlier conference 1141.
4. Conclusions We have demonstrate that PILIS is a viable laser spectroscopic tool for the study of radioactive species. Compared to other methods, it has several distinct advantages: 1) The overall sensitivity achievable by this approach is independent of the repetition rate of the lasers used. This allows the use of more powerful pump lasers, and the resultant wide range of accessible wavelengths will allow a more general application to a large number of elements.
- for isobar separation
2) The method is applicable to ions not directly produced in an on-line ion source. For example, radioactive gold atoms can be collected, and after a suitable delay, spectroscopic studies of platinum daughter nuclei can be carried out. In this fashion, the approach presented here will be an excellent complementary method to other well established techniques. 3) We have used UV laser light with close to Fourier-limited resolution for spectroscopic studies. The linewidth is large compared to the CW beams. However, for many applications, the IS can still be adequately determined since the centroid of the output frequency is governed by the wavelength of the CW beam; this can be monitored quite accurately. In many elements, the convenient transitions for the IS and HFS measurements often lie in the UV region, which could be difficult to produce with CW beams. In these cases, PILIS could provide a convenient alternative means for such studies. This work was funded by CNRS of France, NSERC of Canada, and FCAR of Qubbec, Canada. Two of us (J.K.P.L. and G.S.) would like to express their gratitude for the hospitality accorded them by the French laboratories during the course of this work.
References PI E.W. Otten, Treatise on Heavy-Ion Physics, vol. 8, ed. A. Bromley (Plenum Press) to be published. PI A.C. Mueller, F. Buchinger, W. Klempt, E.W. Otten, R. Neugart, C. Ekstrtim and J. Heinemeier, Nucl. Phys. A403 (1983) 234. r31 G. Huber, F. Touchard, S. Biittgenbach, C. Thibault, R. Klapisch, H.T. Duong, S. Liberman, J. Pinard, J.L. Vialle, P. Juncar and P. Jacquinot, Phys. Rev. Cl8 (1978) 2342. [41 R.C. Thompson, M. Anselment, K. Bekk, S. Giiring, A. Hanser, G. Meisel, H. Rebel, G. Schatz and B.A. Brown, J. Phys. G9 (1983) 443. 151 C. Duke, H. Fischer, H.-J. Kluge, H. Kremmling, Th. Kiihl, and E.W. Otten, Phys. Lett. 6OA (1977) 303. PI V.N. Fedoseyev, V.I. Letokhov, V.i. Mishin, G.D. Alkhazob, A.E. Barzakh, V.P. Denisov, A.G. Demyatin and V.S. Ivanov, Opt. Commun. 52 (1984) 24. 171 K. WalImeroth, G. Bollen, A. Dohn, P. Egelhof, J. Griiner, F. Lmdenlauf, U. Kr&ert, J. Campos, A. Rodriguez Yunta, M.J.G. Borge, A. Venugopal~, J.W. Wood, R.B. Moore and H.-J. Kluge, Phys. Rev. Lett. 58 (1987) 1516. PI P. Paris, C.F. Liang, D. Lecouturier, M. Avianev, J. Obert, A. Caruette, A. Ferro, J. Foumet, J.C. Putaux, J.C. Sarrouy and The ISOCELE Collaboration, Nucl. Instr. and Meth. 186 (1981) 91. [91 J.K.P. Lee, V. Raut, G. Savard, and G. Thekkadath, Proc. Third Int. Symp. on Resonance Ionization Spectroscopy, Swansea (1986) Inst. of Phys. Conf. Ser. no. 84 (1987) p. 139. WI J.K.P. Lee, J.E. Crawford, V. Raut, G. Savard, G. Thek-
J. K.P. Lee et al. / PILIS
kadath, T.H. Duong, and J. Pinard, Nucl. Instr. and Meth. B26 (1987) 444. [ll] J. Pinard and S. Liberman, Opt. Commun. 20 (1977) 344. [12] 1. Streib, H.-J. KIuge, H. Kremmhng, R.B. Moore, H.W. Schaaf and K. WaIlmeroth, Z. Phys. A321 (1985) 537. [13] C. Ekstrom, L. Robertsson, S. Ingelman, G. Wannberg, I. Ragnarsson and the ISOLDE Collaboration, Nucl. Phys. A348 (1980) 25.
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[14] J.K.P. Lee, G. Savard, J.E. Crawford, G. Thekkadath, H.T. Duong, J. Pinard, S. Liberman, F. Le Blanc. P. Kilcher, J. Obert, J. Oms, J.C. Putaux, B. Roussitre, J. Sauvage and the ISOCELE Collaboration, Proc. 5th Int. Conf. On Nuclei Far From Stability, Lake Rosseau (1987). AIP Conf. Proc. no. 164 (1988) p. 205.
Nuclear
Instruments
and Methods
in Physics Research 834 (1988) 252-257 North-Holland, Amsterdam
PILIS: POST-ISOCELE LASER ISOBAR SEPARATION - AN APPARATUS FOR LASER SPECTROSCOPIC STUDIES OF LASER-DESORBED ATOMS J.K.P. LEE, G. SAVARD,
J.E. CRAWFORD
and G. THEKKADATH
Foster Radiation Laboratory. McGill University MontrPaI, Canada
H.T. DUONG,
J. PINARD
and S. LIBERMAN
Laboratoire AimP Cotton, CNRS II, Orsay, France
F. LE BLANC,
P. KILCHER,
J. OBERT,
J. OMS, J.C. PUTAUX,
B. ROUSSIkRE
and J. SAUVAGE
Insrilut de Physique Nucl4aire. Orsay, France
and the ISOCELE Received
29 March
Collaboration
1988 and in revised form 15 April 1988
An apparatus for laser spectroscopic studies of laser-desorbed radioactive atoms has been installed on-line at the ISGCELE isotope separator (IPN, Orsay). Mass-separated gold ions were first implanted onto a substrate, and then thermally desorbed by a Nd-YAG laser pulse. A three-step, two-resonance scheme was used to selectively ionize the desorbed gold atoms. The ions created were then mass-identified through a time-of-flight technique. The laser system used has a hnewidth of 130 MHz, and an actual experimental resolution of 170 MHz for the stable “‘Au was obtained. It has been demonstrated that with 10” total implanted ions, several laser scans can be performed, and that a 5X10-s ion conversion efficiency for the desorbed gold atoms was reached at resonance. On-line measurements of the isotope shift (IS) and hyperfine structure (HFS) for several gold isotopes were carried out. With the high-resolution capability, a negative sign for the magnetic moment of t9*Au was obtained. The HFS of ‘s’Au confirms an earlier laser spectroscopic study, and the sudden variation of IS between ‘s’Au and la6Au was reproduced.
1. Introduction The laser spectroscopic study of long chains of isotopes is a very powerful method to systematically probe the static properties of atomic nuclei. The hyperfine structure (HFS) of an atomic transition can yield information about the spins and electromagnetic moments of the nuclei while the isotope shift (IS) between a pair of isotopes gives the change of the mean square nuclear charge radius (6( r2)). These studies have contributed substantially to our understanding of properties of nuclei extending far from the beta stability valley. These results, together with the various experimental techniques used, have been summarized in a recent review article [1] and the details of these works can be found in the extensive references therein. The laser spectroscopic methods utilize the interaction of laser beams with atoms or ions of thermal or high velocities. For their applications in the studies of radioactive species, the main challenge is to achieve high sensitivity while maintaining an overall resolution adequate for the particular atomic transition being 0168-583X/88/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
studied. Generally, this means that a highly sensitive, Doppler-reduced spectroscopic technique is required. Among various methods that have been applied, the most prolific is perhaps colhnear laser spectroscopy via fluorescence scattering from a fast atomic beam [2]. In this method, the mass-separated ions from an on-line isotope separator (ISOL) are first neutralized before interacting with a collinear laser beam. This method is very versatile and applicable to any ion beam produced in an ISOL, provided that the required laser beam corresponding to the atomic transitions to be studied is available. The overall sensitivity is very good, requiring only an ion beam intensity of about lO’/s to 106/s in the laser-ion interaction region, which can be achieved in many existing ISOLs. In addition, various techniques can be adapted to improve the signal to background ratio thus increasing the overall sensitivity. Another method that has proved to be very powerful is to use the laser beam to optically pump the alkali atoms to a particular magnetic substrate followed by an efficient level-sensitive atom detection [3]. The required ion beam intensity required in this case is about 104/s, but the
J.K.P. Lee et al. / PILIS
method is only applicable to alkali elements. Other methods such as fluorescence scattering from a thermal atomic beam [4] or from atoms contained inside a closed cell [S] have also been used; however, they generally have lower sensitivities and have not widely been applied. More recently, resonant ionization spectroscopic (RIS) method [6,7] have been used to improve the overall detection sensitivity. The RIS method is known to be a highly efficient and selective process for specific atom detection. In these applications for the study of radioactive atoms, pulsed lasers are used, interacting with a steady thermal atomic beam. This approach has two major deficiencies: the inherently low temporal duty cycle and the poor spectroscopic resolution of pulsed lasers. With modem techniques, these difficulties can be compensated in several ways. For example, pulsed lasers with high repetition rates such as copper vapour lasers could alleviate the duty cycle problem. However, when a dye laser is pumped by such a laser, the tunable range of its output wavelength is limited. Therefore, the variety of elements which can be studied is restricted. As for the spectroscopic resolution of the pulsed lasers, various techniques are now available to achieve a resolution close to the Fourier-transformed limit of the pulse duration. However, for reliable operating conditions, very careful tuning and control are necessary. We would like to describe a new spectroscopic apparatus PILIS (Post-Isocele Laser Isobar Separation) installed at ISOCELE [8], an ISOL at the synchrocyclotron of Orsay, that circumvents the difficulties mentioned above. This applies the RIS technique to atoms desorbed by a heating laser. The viability of this approach was tested earlier [9,10], and an overall detection efficiency of 10d4 was achieved. Recently, PILIS was successfully used for an on-line study of the IS and HFS of radioactive gold atoms. The technical aspects of this apparatus are presented here.
2. Experimental setup The schematic diagram of PILIS is shown in fig. 1. The mass separated gold ions from ISOCELE are collected on a substrate which is mounted on the surface of a cylinder. Rotation of the cylinder brings the sample to the laser-atom interaction zone where a heating laser pulse temporarily heats up the surface and re-evaporates the implanted ions. A few microseconds later, a synchronized group of three pulsed laser beams intercepts these gold atoms and selectively ionizes them. The ions thus created are accelerated and mass identified by a time-of-flight (TOF) measurement. An electronic timing system controls the relative delays required between the different laser pulses.
- for isobar separarion
253
The 45 keV ion beam from ISOCELE was focused to a spot about 2 mm high and 10 mm wide, on a graphite substrate. The heating laser beam used was the 532 nm, 10 Hz output from a frequency-doubled Nd-YAG laser, focused to a vertical line of approximately 3 mm X 50 pm size on the implanted surface. Its output energy was 0.3 to 0.5 mJ per pulse. In order to cover the entire ion-implanted area, the sample moved at the rate of about 10 nm per laser pulse. These operating conditions were chosen to optimize the desorption efficiency and the signal to background ratio of the mass peaks to be studied. The details of the tests that led to the choice of the above parameters are described in the next section. The RIS scheme adopted was the three-step process shown in the insert of fig. 1. The first step of resonant absorption at 268 nm wavelength was provided by a high-resolution injection-locked dye laser system [ll]. This was accomplished by placing an additional thin dye cell, pumped by an excimer laser, inside a conmrercial single-mode CW dye laser cavity. When operated in this fashion, the output wavelength was determined by the CW dye laser control system while its intensity consisted of a CW component with enhanced pulses (generated by the pulsed pump laser) superimposed on it. This output was further amplified through another excimer laser-pumped amplifier and then frequency doubled to produce the continuously tunable UV light pulses. When properly tuned, the resolution of the output beam of such a system is essentially Fourier limited, with a linewidth of about 65 MHz for the visible light, or 130 MHz for the frequency-doubled UV output. Photons of the second resonant transition at 406 nm were provided by a commercial pulsed dye laser pumped by the same excimer laser. This has an output linewidth of 5 GHz. The final ionizing step was provided by the output from a broadband dye laser, operated at about 700 nm. The sizes of the first two laser beams were about 5 mm x 10 mm (the 5 mm spread being along the ion acceleration direction), while the final ionizing-step laser beam was focused to about 3 mm diameter. The ionization efficiency in the present case is determined essentially by the energy of the ionizing-step laser pulse, which is in the present case of the order of 7 ml. For the other two lasers, the system outputs are 0.1 mJ and 0.3 mJ per pulse for the first and second resonant steps, respectively. However, for actual measurements, these two lasers must operate with much reduced output power (I 1 nJ) in order to avoid power broadening effects on the transition lines. The relative timing delays among these three laser pulses were adjusted to optimize the ionization efficiency by varying the corresponding pump beam path lengths. All the laser beams were placed in the same plane parallel to the electrode plates. The first and third beams were collinear to each other, while the second beam was incident perpendicular to the other two. In this way, the
254
J.K.P. Lee et al. / PILIS
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Fig. 1. Schematic layout of PILE. FP Fabry-Perot interferometer, XTL frequency doubling crystal, I, iodine cell, Ti, Tr and Ts timing inputs for synchronixed operation. Insert shows the RIS scheme. For details of operation, see text.
actual width of the second beam can be adjusted to reduce the Doppler broadening effect of the diverging motion of the desorbed atoms. The relative timing between the Nd-YAG heating pulse and the group of RIS pulses was adjusted to optimize the overall ion production of the desorbed atoms. The ions created were accelerated by the three-electrode system shown in fig. 1. The first plate was held at 1.3 kV (V,) while the second plate (V,) was at about 1.0 kV. The accelerated ions passed through a 1.3 m flight path and were mass identified by the TOF signal from a microchannel plate detector. The potential I’, was adjusted to compensate for the differences of the flight times of the ions created at different geometrical locations. The voltage on the cylinder (V,) was slightly higher than 1.3 kV, and could be adjusted to ensure a uniform field gradient in the accelerating region. An overall mass resolution (M/AM) of better than 400 for A - 200 was routinely obtained. Along the flight path, a pulsed electrostatic ion deflector was inserted to suppress the background ions created directly by the heating laser pulse. For each laser shot, the TOF spectrum was recorded by a 100 MHz digital waveform analyser. To reduce the overall fluctuation due to the shot-to-shot variations, the signals were averaged over 4 or 16 laser shots for each data point. The absolute wavelength of the CW component of the first dye laser was monitored by the
absorption of its intensity in an iodine cell, and its relative displacement was deduced from the variation of the light intensities transmitted through an external etalon with 750 MHz free spectral range. These signals were multiplexed and processed by the same digital analyser. The data were then fed to an IBM-PC com-
197
Au
192
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R
*
Fig. 2. Top two spectra show the variation of the corresponding mass peaks in the TOF outputs. The third curve shows the output variation from a Fabry-Perot interferometer with a free spectral range (FSR) of 748 MHz The bottom curve gives the resonant absorption in the iodine cell. All four curves were obtained during the same scan of the laser frequency.
J.K.P. Lee et al. / PILIS
puter, which also controlled the overall progress of the experiment.
3. System performance
and discussion
One of the most critical elements for the performance of PILE is the effective desorption of the implanted gold atoms. The first series of tests was carried out to search for the most suitable operating conditions, which included the power, wavelength and spot size of the heating laser beam, the motion of the implanted sample with respect to the laser spot, the substrate material and its surface conditions, and the kinetic energies of the implanted ions. For these tests, a suitable number of stable gold ions, typically about 1013, was first implanted into the substrate. Under each operating condition, the magnitude of the gold ion peak created in the TOF spectrum by the RIS process was measured together with the general background ions created by both the heating and ionizing laser beams. Several substrate materials were tested, including graphite, tantalum, platinum, tungsten and molybdenum; each measurement was done with heating laser wavelengths of 1064 nm and 532 nm. In general, the desorption of gold atoms occurred at a certain threshold heating pulse energy and became more effective with increasing heating power. For all the materials tested, adequate desorption of gold atoms could be induced. However, with higher heating power, more direct ions were created and more matter was desorbed from the substrate surface, causing a deterioration of the signal to background ratio in the TOF spectra. At the point where the deterioration became significant the heating pulse energies varied over a wide range, depending on the material and the surface condition of the substrate. These tests suggested that graphite substrates provided the best desorption, requiring much lower heating power than the metallic foils for the same desorption efficiency. Also, no noticeable difference was noted for different heating wavelengths. For most of the subsequent tests, the 532 nm beam was used for ease of beam alignment. Continued studies of the desorption characteristics of the gold atoms from graphite showed the general trend that when the implanted spot was repeatedly heated by successive laser shots, the desorption efficiency was gradually reduced. The rate of reduction varied widely according to the heating power density used. The number of laser shots at the same power level required to reduce the gold ion peak in the TOF spectrum to 10% of its original value could be as low as 3 (for high powers) to as high as over 100 (for lower powers). However, higher heating power created higher background ion intensities, and extensive tests were carried out to search for the most suitable operating
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conditions. This resulted in the operating parameters mentioned in the previous section, namely, about 0.3 to 0.5 mJ of pulse energy over an area of about 3 mm X 50 pm, with the sample moving at the rate of 10 pm per laser shot. In this way, with the implanted ions covering a 2 mm x 10 mm area, each laser shot would cover some new sample surface, and a steady desorption of the gold atoms could be maintained. To measure the absolute desorption efficiency under these operating conditions, a sample with implanted radioactive i9’Au was used. Its activities measured before and after a complete heating laser sweep showed that about 20% of the gold atoms were removed by this heating process, giving an average desorption efficiency of 4% per laser shot for the gold atoms covered by the laser spot area. In fact, under these operating conditions it was quite feasible to repeat the laser heating process over the same surface area several times. With subsequent passes, steady desorption of gold atoms could still be maintained but with lower desorption yield for each successive pass. The desorption rate of gold atoms implanted at different energies of 30 to 45 keV did not show any noticeable difference. However, when the ions were slowed to 0.5 keV before implantation, the desorption rate was much higher and the required heating laser power was lower. Preheating the substrate surface by the laser before implantation did not seem to affect the subsequent desorption rate. To evaluate the overall sensitivity of this particular setup, a sample with about 10” implanted atoms was used. At resonance, about 100 gold ions were detected for each laser shot. Assuming a 4% desorption efficiency for each heating laser pulse as obtained above, 2 X lo6 gold atoms would then be desorbed, and the ion conversion rate for the desorbed gold atoms was therefore 5 X 10p5. The fraction of the desorbed atoms covered by the overlap of the three ionization laser beams was calculated from the position of the overlapped spot, assuming a cos 0 distribution for the desorbed atoms, and their velocity distribution, and was found to be 8 x 10e3. The remaining factor of 6 X lo3 was due to the overall ionization efficiency, losses in the ion transport and to the fraction of the desorbed atoms not in their ground state. With the operating conditions thus defined, spectroscopic measurements for the gold isotopes were then attempted. To reach a good spectroscopic resolution, the geometrical width of the 406 nm beam was narrowed. The resultant spectrum showing the RIS yield of the stable 19’Au ions vs the scanning laser frequency is shown in fig. 2, indicating an actual experimental resolution (FWHM) of 170 MHz. Another spectrum obtained for the radioactive 19*Au is also shown in the same diagram. The resolution obtained was quite adequate to resolve the hyperfine splitting of its ground state (375 MHz). The relative amplitude of the two
J.K.P. Lee et al. / PILIS
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I
-.
197
/
/
,
/
L
I
I
I FREQUENCY
‘4
10 GHz
1
I
187 186
‘I/
j
01
Fig. 3. Summary of the RIS results for the isotopes discussed in this work. Vertical lines show the relative positions of the transitions observed while the heights are theoretical peak amplitudes expected. Open circles show the position of the centroids for the transitions. From A =197 to 187, the centroid moves smoothly towards the ‘red’ direction. The sudden change between A = 187 and 186 shows a sudden change of nuclear charge radius.
resolved components, each actually containing two transitions to the hyperfine structure of the upper level, yielded a negative sign for the magnetic moment of 19’Au. The measured isotope shift with respect to the stable 19’Au obtained was in agreement with an earlier work 1121, where the sign of the magnetic moment could not be determined. Measurements were extended to other isotopes, among them ls7Au and ‘86Au. For the “‘Au the HFS measurement is in agreement with that reporied in ref. [7], and the deduced magnetic moment of 0.531(12) n,m. is quite different from the 0.726(7) n.m. obtained by the atomic beam magnetic resonance (ABMR) method [13]. Possibly another measurement with the AMBR method will resolve this discrepancy. Our results have also reproduced the abrupt change in a sudden the IS between ‘87Au and ls6Au, indicating change in the nuclear deformation between these two nuclei. These results are summarized in fig. 3. Discussions of these and other radioactive gold isotopes were presented in an earlier conference 1141.
4. Conclusions We have demonstrate that PILIS is a viable laser spectroscopic tool for the study of radioactive species. Compared to other methods, it has several distinct advantages: 1) The overall sensitivity achievable by this approach is independent of the repetition rate of the lasers used. This allows the use of more powerful pump lasers, and the resultant wide range of accessible wavelengths will allow a more general application to a large number of elements.
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2) The method is applicable to ions not directly produced in an on-line ion source. For example, radioactive gold atoms can be collected, and after a suitable delay, spectroscopic studies of platinum daughter nuclei can be carried out. In this fashion, the approach presented here will be an excellent complementary method to other well established techniques. 3) We have used UV laser light with close to Fourier-limited resolution for spectroscopic studies. The linewidth is large compared to the CW beams. However, for many applications, the IS can still be adequately determined since the centroid of the output frequency is governed by the wavelength of the CW beam; this can be monitored quite accurately. In many elements, the convenient transitions for the IS and HFS measurements often lie in the UV region, which could be difficult to produce with CW beams. In these cases, PILIS could provide a convenient alternative means for such studies. This work was funded by CNRS of France, NSERC of Canada, and FCAR of Qubbec, Canada. Two of us (J.K.P.L. and G.S.) would like to express their gratitude for the hospitality accorded them by the French laboratories during the course of this work.
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kadath, T.H. Duong, and J. Pinard, Nucl. Instr. and Meth. B26 (1987) 444. [ll] J. Pinard and S. Liberman, Opt. Commun. 20 (1977) 344. [12] 1. Streib, H.-J. KIuge, H. Kremmhng, R.B. Moore, H.W. Schaaf and K. WaIlmeroth, Z. Phys. A321 (1985) 537. [13] C. Ekstrom, L. Robertsson, S. Ingelman, G. Wannberg, I. Ragnarsson and the ISOLDE Collaboration, Nucl. Phys. A348 (1980) 25.
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[14] J.K.P. Lee, G. Savard, J.E. Crawford, G. Thekkadath, H.T. Duong, J. Pinard, S. Liberman, F. Le Blanc. P. Kilcher, J. Obert, J. Oms, J.C. Putaux, B. Roussitre, J. Sauvage and the ISOCELE Collaboration, Proc. 5th Int. Conf. On Nuclei Far From Stability, Lake Rosseau (1987). AIP Conf. Proc. no. 164 (1988) p. 205.