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Hyperfine structure measurements in metastable atomic states — nuclear spins of 201Pb and 203Pb
Volume 72B, number 2
HYPERFINE
PItYSICS LETTERS
STRUCTURE MEASUREMENTS IN METASTABLE N U C L E A R S P I N S O F 201 Pb A N D 2 ° 3 p b
19 December 1977
ATOMIC STATES -
M. GUSTAFSSON, I. LINDGREN, J. LINDGREN, A. ROSEN and H. RUBINSZTEIN Department of Physics, Chalmers University of Technology and the University of Gdteborg, G6teborg, Sweden Received 7 October 1977 A new atomic-beam source has been used to populate metastable states within the atomic ground-state configuration for lead. With this source and the atomic-beam magnetic resonance method, hyperfine structure measurements have been performed in metastable states to determine the nuclear ground-state spins of two lead isotopes as 2°3pb(I = 5/2), 2m Pb(I = 5/2).
During recent years there has been increased interest in determining the hyperfine structure (hfs) in several states within one atomic configuration. This interest originates from the work of Harvey [ 1], who introduced a parametrized operator in analysing the hfs of 170 and 19F. He found that in order to explain the experimental data, a non-zero contact-interaction and two different (r -3) values must be used for the orbital and spin-dipole parts of the hyperfine Hamiltonian. It has been shown by Judd and others [2] that such an operator can describe most of the polarization and correlation effects, and Kelly [3] was, at an early date, able to reproduce the parameters of 170 using manybody perturbation techniques. By Sandars and Beck [4] it has been shown that for the magnetic dipole interaction the same effective operator includes also relativistic effects, which will be important for heavy elements. A review of the effective operator technique, with particular emphasis on the relativistic effects, has recently been given by two of the present authors [5]. From that compilation it is evident that the hfs is still unknown (or only known with a large uncertainty) for many states in the ground-state configuration of many elements. For some elements, like lead, the atomic ground state has the electronic angular momentum J equal to zero. Measurements of nuclear spins and moments from the hfs for such elements must be performed in excited states. The lack of accurate hfs data for metastable states is mainly due to the fact that these states are too high in energy to be thermally populated in conventional atomic-beam magnetic resonance (ABMR) experiments. 166
For example, the Boltzmann factor for the first excited state 3P 1 in lead is of the order 10 -5 . Optical spectroscopy has therefore been used for hfs measurements in such states. In order to perform resonance experiments on such states Lurio et al. [6] developed a technique to populate the metastable states by electron excitation. The detection was performed by means of a cesium-coated surface. In connection with optical double-resonance and level-crossing experiments, another type of metastable atomic-beam source, based on a plasma discharge, has been developed [7]. We have now applied a similar technique to populate metastable atomic states for ABMR measurements on radioactive isotopes. The sample is heated in a boron-nitride oven to produce an atomic beam. The front part of the oven consists of a channel where a discharge can be initiated by a voltage of 1 0 - 1 0 0 V, when the atomic density is high enough. In this discharge a high percentage of the atoms are brought to metastable states, from which they cannot decay by optical electric-dipole transitions. Hfs measurements in these excited states can then be performed in the conventional way [8]. In these experiments the atomic-beam apparatus at G6teborg has been used. In spin measurements the nuclear spin I is determined from the transition frequency between magnetic substates belonging to the same hyperfine level F. At low magnetic fields B the transition frequency is approximately given by u ~ gF IJBB/h, where gF = Igj[F(F+ 1) + J ( J + 1 ) - I ( I + 1 ) ] / 2 F ( F + 1)l and where/1B is the Bohr magneton. The electronic splitting factors g j for lead have earlier been measured for stable isotopes with the result [6] gj(3P2) =
Volume 72B, n u m b e r 2
PHYSICS L E T T E R S i
2O3pb 11
I
loo1
1D2 ~r
p
I
11
t
t
zo3 aePb 1=5/2 (53.7H1 6p 2 1D2
~
I i
3P 2
]
gj=1.2745 1 •
11
3P 1
.
1"5I I
0.5
5
6
Fig. 2. Decay curve of the spin I = 5/2 (1DZ, F = 9/2, gF = 0.5450) resonance signal. Sample obtained from a 28 MeV proton irradiation of thallium.
gj =1.5001
[
*
DAYS AFTER EXPOSURE
, ,1,,T
1,0 0
19 December 1977
i
1.0 iF
Fig. 1. Spectrum from a frequency scan at an external magnetic field of 1.5 gauss. Above the spectrum the gF values corresponding to different nuclear spins o f the 3 p 1 , 3 p 2 , and 1 D2 states o f lead are given. The lengths o f the arrows indicate the nuclear spin.
= - 1.2745(13), gj(3 P1 ) = - 1.S001 (15), gj(1 D2 ) = -1.2263(1). For a certain magnetic field B the resonance frequency is a discrete function of the nuclear spin I. In the first measurements some neutron-deficient lead isotopes 203pb and 201pb have been investigated. The isotopes were produced by Tl(p, xn) reactions in the synchro-cyclotron at the Gustaf Werner Institute at Uppsala. After the irradiation a chemical separation was performed to separate out the lead isotopes. The sample was then placed in the oven and heated. For detection, the atoms were collected on carbon-coated aluminium foils and the activity of 7-rays or X-rays, which follow the electron capture decay, was measured with scintillation counters. Resonances corresponding to the nuclear spin I = 5/2 in the 3P1,3P 2 and 1D 2 states have been observed. The experimental result from a frequency scan
at an external magnetic field of 1.5 gauss is shown in fig. 1. The upper part of the figure gives the gF values corresponding to different nuclear spins for the 3P1, 3P 2 and 1D 2 atomic states. The lengths of the arrows indicate the nuclear spin values. The results from the spin search are shown in the bottom of the figure. Nuclear spins ranging f r o m / = 1/2 to I = 11/2 were tried, but as can be seen only a spin 5/2 is consistent with the observed signals. The components of the resonance signals were identified using the known halflives [9] and a least-squares fit. Fig. 2 shows the decay curve of the spin 5/2 resonance signal of 203pb. Another method to excite the atoms in an atomic beam to metastable states using resonance radiation from a lamp has been suggested by Redi et al. [12]. However, to our knowledge, no results have appeared. Lead isotopes close to the double magic isotope 2°Spb are expected to be well described by the nuclear shell model [10]. The nuclear spins I = 5/2 for 2°3pb and 201pb correspond to the nuclear configurations (2f5/2) 3 and (2f5/2) 1 , respectively. These results are also in agreement with data obtained from nuclear spectroscopy [9]. The method described in this paper has now been further applied to stable isotopes using laser detection [11]. The authors gratefully acknowledge the fruitful discussions with Dr. Sune Svanberg and the technical assistance of H. Riedl and L. Lindstr6m. This work has been supported financially by the Swedish Council for Atomic Research.
167
Volume 72B, number 2
PHYSICS LETTERS
References [1] J.S,M, Harvey, Proc. Roy. Soc. A285 (1965) 581. [2] B.R. Judd, Proc, Phys. Soc. 82 (1963) 874; J. Bauche and B.R. Judd, Proc. Phys. Soc. 83 (1964) 145; B.R. Judd, Hyperfine structure in the atomic 2p shell, in: La structure hyperfine magndtique des atomes et des molecules, eds. R. Lefebvre and C. Moser, Colloques Internationaux du Centre National de la Recherche Scientifique, Paris (1967) No. 164; P.G.H. Sandars, Adv. Chem. Phys. 14 (1969) 365. [3] H.P. Kelly, Phys. Rev. 173 (1968) 142, 180 (1969) 55, A2 (1970) 1261. [4] P.G.H. Sandars and J, Beck, Proc. Roy. Soc. A289 (1965) 97. [5] 1. Lindgren and A. Ros6u, Case Studies in Atomic Physics 4 (1974) 97. [6] A.G. Blachman, D.A. Landman and A. Lurio, Phys. Rev. 181 (1969) 70; A. Lurio, Phys. Rev. 126 (1962) 1768;
168
19 December 1977
A.G. Blachman, D.A. Landman and A. Lurio, Phys. Rev. 150 (1966) 59; D.A. Landman and A. Lurio, Phys. Rev. A1 (1970) 1330; A. Lurio and D.A. Landman, J. Opt. Soc. Am. 60 (1970) 759. [7] U. Brinkmann, J. Goschler, A. Steudel and H. Walther, Z. Physik 228 (1969) 427; U. Brinkmann, Z. Physik 228 (1969) 440; S. Garpman, G. LidiS, S. Rydberg and S. Svanberg, Z. Physik 241 (1971) 217. [8] A. Ros4n, C. EkstriSm, H. Nyqvist and K.E./~.delroth, Nucl. Phys. A154 (1970) 283. [9] C.M. Lederer, J.M. Hollander and I. Perlrnan, Table of isotopes (Wiley, New York, 1967). [10] M. Goeppert Mayer and J.H.D. Jensen, Elementary theory of nuclear shell structure (Wiley, New York; Chapman and Hall, London, 1955). [11] M. Gustavsson et al., Phys. Lett. 62A (1977) 250. [12] O, Redi, J. Milch and E. Wang, Abstract of the Third Internat. Conf. on Atomic physics, Boulder (1972) p. 234.
HYPERFINE
PItYSICS LETTERS
STRUCTURE MEASUREMENTS IN METASTABLE N U C L E A R S P I N S O F 201 Pb A N D 2 ° 3 p b
19 December 1977
ATOMIC STATES -
M. GUSTAFSSON, I. LINDGREN, J. LINDGREN, A. ROSEN and H. RUBINSZTEIN Department of Physics, Chalmers University of Technology and the University of Gdteborg, G6teborg, Sweden Received 7 October 1977 A new atomic-beam source has been used to populate metastable states within the atomic ground-state configuration for lead. With this source and the atomic-beam magnetic resonance method, hyperfine structure measurements have been performed in metastable states to determine the nuclear ground-state spins of two lead isotopes as 2°3pb(I = 5/2), 2m Pb(I = 5/2).
During recent years there has been increased interest in determining the hyperfine structure (hfs) in several states within one atomic configuration. This interest originates from the work of Harvey [ 1], who introduced a parametrized operator in analysing the hfs of 170 and 19F. He found that in order to explain the experimental data, a non-zero contact-interaction and two different (r -3) values must be used for the orbital and spin-dipole parts of the hyperfine Hamiltonian. It has been shown by Judd and others [2] that such an operator can describe most of the polarization and correlation effects, and Kelly [3] was, at an early date, able to reproduce the parameters of 170 using manybody perturbation techniques. By Sandars and Beck [4] it has been shown that for the magnetic dipole interaction the same effective operator includes also relativistic effects, which will be important for heavy elements. A review of the effective operator technique, with particular emphasis on the relativistic effects, has recently been given by two of the present authors [5]. From that compilation it is evident that the hfs is still unknown (or only known with a large uncertainty) for many states in the ground-state configuration of many elements. For some elements, like lead, the atomic ground state has the electronic angular momentum J equal to zero. Measurements of nuclear spins and moments from the hfs for such elements must be performed in excited states. The lack of accurate hfs data for metastable states is mainly due to the fact that these states are too high in energy to be thermally populated in conventional atomic-beam magnetic resonance (ABMR) experiments. 166
For example, the Boltzmann factor for the first excited state 3P 1 in lead is of the order 10 -5 . Optical spectroscopy has therefore been used for hfs measurements in such states. In order to perform resonance experiments on such states Lurio et al. [6] developed a technique to populate the metastable states by electron excitation. The detection was performed by means of a cesium-coated surface. In connection with optical double-resonance and level-crossing experiments, another type of metastable atomic-beam source, based on a plasma discharge, has been developed [7]. We have now applied a similar technique to populate metastable atomic states for ABMR measurements on radioactive isotopes. The sample is heated in a boron-nitride oven to produce an atomic beam. The front part of the oven consists of a channel where a discharge can be initiated by a voltage of 1 0 - 1 0 0 V, when the atomic density is high enough. In this discharge a high percentage of the atoms are brought to metastable states, from which they cannot decay by optical electric-dipole transitions. Hfs measurements in these excited states can then be performed in the conventional way [8]. In these experiments the atomic-beam apparatus at G6teborg has been used. In spin measurements the nuclear spin I is determined from the transition frequency between magnetic substates belonging to the same hyperfine level F. At low magnetic fields B the transition frequency is approximately given by u ~ gF IJBB/h, where gF = Igj[F(F+ 1) + J ( J + 1 ) - I ( I + 1 ) ] / 2 F ( F + 1)l and where/1B is the Bohr magneton. The electronic splitting factors g j for lead have earlier been measured for stable isotopes with the result [6] gj(3P2) =
Volume 72B, n u m b e r 2
PHYSICS L E T T E R S i
2O3pb 11
I
loo1
1D2 ~r
p
I
11
t
t
zo3 aePb 1=5/2 (53.7H1 6p 2 1D2
~
I i
3P 2
]
gj=1.2745 1 •
11
3P 1
.
1"5I I
0.5
5
6
Fig. 2. Decay curve of the spin I = 5/2 (1DZ, F = 9/2, gF = 0.5450) resonance signal. Sample obtained from a 28 MeV proton irradiation of thallium.
gj =1.5001
[
*
DAYS AFTER EXPOSURE
, ,1,,T
1,0 0
19 December 1977
i
1.0 iF
Fig. 1. Spectrum from a frequency scan at an external magnetic field of 1.5 gauss. Above the spectrum the gF values corresponding to different nuclear spins o f the 3 p 1 , 3 p 2 , and 1 D2 states o f lead are given. The lengths o f the arrows indicate the nuclear spin.
= - 1.2745(13), gj(3 P1 ) = - 1.S001 (15), gj(1 D2 ) = -1.2263(1). For a certain magnetic field B the resonance frequency is a discrete function of the nuclear spin I. In the first measurements some neutron-deficient lead isotopes 203pb and 201pb have been investigated. The isotopes were produced by Tl(p, xn) reactions in the synchro-cyclotron at the Gustaf Werner Institute at Uppsala. After the irradiation a chemical separation was performed to separate out the lead isotopes. The sample was then placed in the oven and heated. For detection, the atoms were collected on carbon-coated aluminium foils and the activity of 7-rays or X-rays, which follow the electron capture decay, was measured with scintillation counters. Resonances corresponding to the nuclear spin I = 5/2 in the 3P1,3P 2 and 1D 2 states have been observed. The experimental result from a frequency scan
at an external magnetic field of 1.5 gauss is shown in fig. 1. The upper part of the figure gives the gF values corresponding to different nuclear spins for the 3P1, 3P 2 and 1D 2 atomic states. The lengths of the arrows indicate the nuclear spin values. The results from the spin search are shown in the bottom of the figure. Nuclear spins ranging f r o m / = 1/2 to I = 11/2 were tried, but as can be seen only a spin 5/2 is consistent with the observed signals. The components of the resonance signals were identified using the known halflives [9] and a least-squares fit. Fig. 2 shows the decay curve of the spin 5/2 resonance signal of 203pb. Another method to excite the atoms in an atomic beam to metastable states using resonance radiation from a lamp has been suggested by Redi et al. [12]. However, to our knowledge, no results have appeared. Lead isotopes close to the double magic isotope 2°Spb are expected to be well described by the nuclear shell model [10]. The nuclear spins I = 5/2 for 2°3pb and 201pb correspond to the nuclear configurations (2f5/2) 3 and (2f5/2) 1 , respectively. These results are also in agreement with data obtained from nuclear spectroscopy [9]. The method described in this paper has now been further applied to stable isotopes using laser detection [11]. The authors gratefully acknowledge the fruitful discussions with Dr. Sune Svanberg and the technical assistance of H. Riedl and L. Lindstr6m. This work has been supported financially by the Swedish Council for Atomic Research.
167
Volume 72B, number 2
PHYSICS LETTERS
References [1] J.S,M, Harvey, Proc. Roy. Soc. A285 (1965) 581. [2] B.R. Judd, Proc, Phys. Soc. 82 (1963) 874; J. Bauche and B.R. Judd, Proc. Phys. Soc. 83 (1964) 145; B.R. Judd, Hyperfine structure in the atomic 2p shell, in: La structure hyperfine magndtique des atomes et des molecules, eds. R. Lefebvre and C. Moser, Colloques Internationaux du Centre National de la Recherche Scientifique, Paris (1967) No. 164; P.G.H. Sandars, Adv. Chem. Phys. 14 (1969) 365. [3] H.P. Kelly, Phys. Rev. 173 (1968) 142, 180 (1969) 55, A2 (1970) 1261. [4] P.G.H. Sandars and J, Beck, Proc. Roy. Soc. A289 (1965) 97. [5] 1. Lindgren and A. Ros6u, Case Studies in Atomic Physics 4 (1974) 97. [6] A.G. Blachman, D.A. Landman and A. Lurio, Phys. Rev. 181 (1969) 70; A. Lurio, Phys. Rev. 126 (1962) 1768;
168
19 December 1977
A.G. Blachman, D.A. Landman and A. Lurio, Phys. Rev. 150 (1966) 59; D.A. Landman and A. Lurio, Phys. Rev. A1 (1970) 1330; A. Lurio and D.A. Landman, J. Opt. Soc. Am. 60 (1970) 759. [7] U. Brinkmann, J. Goschler, A. Steudel and H. Walther, Z. Physik 228 (1969) 427; U. Brinkmann, Z. Physik 228 (1969) 440; S. Garpman, G. LidiS, S. Rydberg and S. Svanberg, Z. Physik 241 (1971) 217. [8] A. Ros4n, C. EkstriSm, H. Nyqvist and K.E./~.delroth, Nucl. Phys. A154 (1970) 283. [9] C.M. Lederer, J.M. Hollander and I. Perlrnan, Table of isotopes (Wiley, New York, 1967). [10] M. Goeppert Mayer and J.H.D. Jensen, Elementary theory of nuclear shell structure (Wiley, New York; Chapman and Hall, London, 1955). [11] M. Gustavsson et al., Phys. Lett. 62A (1977) 250. [12] O, Redi, J. Milch and E. Wang, Abstract of the Third Internat. Conf. on Atomic physics, Boulder (1972) p. 234.