- Email: [email protected]
Laser spectroscopy with cooled beams
Nuclear Physics A 752 (2005) 309c–316c
Laser spectroscopy with cooled beams J. Billowesa a
Schuster Laboratory, University of Manchester, Manchester M13 9PL, UK
This contribution describes the programme being carried out by a collaboration involving the Universities of Birmingham, Jyv¨askyl¨a and Manchester, using collinear laser spectroscopy to measure isotope shifts and hyperfine structures of radioisotopes. The work benefits from the unique combination of an ion guide isotope separator (IGISOL) and an RFQ ion cooler-buncher, allowing measurements on ion fluxes as low as 150 s −1 . Recent studies include Zr and Ce isotopes produced by proton-induced fission of uranium, neutron deficient isotopes of Ti, and K = 8 isomers in 130 Ba and 176 Yb. Development work on a more sensitive technique of Collinear Resonance Ionization Spectroscopy (CRIS) with bunched ion beams is also described. 1. Introduction The laser spectroscopy programme at the Cyclotron Laboratory, Jyv¨ askyl¨a (JYFL) is run by a collaboration involving the Universities of Birmingham, Jyv¨askyl¨a and Manchester. The aim of the programme has been to measure nuclear moments and charge radii of radioactive nuclei. The nuclear magnetic dipole and electric quadrupole moments are obtained directly from the hyperfine structures observed on optical transitions. The difference in nuclear mean square charge radius between two isotopes is deduced from the frequency difference (isotope shift) in the optical transition. These are part per million perturbations of the atomic energy levels that are usually masked by Doppler broadening of the transition. The standard ‘collinear beams’ method of overlapping the laser and ion beam eliminates Doppler broadening and provides the high sensitivity required for on-line measurements of short-lived isotopes. In this way many isotopes chains have been measured at on-line isotope separators [1–3]. The present status of these measurements is indicated in Figure 1. 2. The laser/IGISOL facility The IGISOL facility offers two significant advantages over conventional on-line mass separators: (i) the ion guide method can provide isotope beams of any element, regardless of chemistry and with fast extraction times in the region of one millisecond [4]; (ii) the RFQ cooler-buncher [5] installed in the mass-separated line reduces the ion beam energy spread to a level where it makes a negligible contribution to the experimental resolution of the laser spectroscopy. The device can accumulate and bunch the ions before they are presented to the laser beam. This feature allows a reduction in the laser-scattered 0375-9474/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2005.02.045
310c 2
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
152 82 126
Z 50 82
28 50 20 28
8 20
2 2
8
N
Figure 1. Present status of optical measurements (black – stable; dark shade – measured)
light in the laser fluorescence signal. This is done by gating the photomultiplier signal to accept counts only during the 15 µs when the ion bunch passes through the laser interaction region. The laser-related background is reduced by the ratio of bunch width to accumulation time (typically 300 ms) which is thus at least four orders of magnitude [6]. In this way measurements have been made with ion beam fluxes as low as 200 s −1 . 3. Fission fragment spectroscopy Proton-induced fission of uranium produces a broad range of medium-mass isotopes on the neutron-rich side of stability. Using the IGISOL fission ion guide, laser measurements have been made on the zirconium isotopes out to 102 Zr [6]. This covers the shape transition at N = 60. The apparent inconsistency between the dynamic quadrupole deformations deduced from the rms charge radii and those from B(E2; 2 + → 0+ ) values for isotopes in the range 50 < N < 60 is still not adequately explained. A similar situation occurs in the strontium chain. Measurements of the quadrupole moments of 93,95 Zr would be useful in understanding the deformation properties here, but these isotopes are difficult to produce in a nuclear reaction. An alternative approach is to look at the odd-proton yttrium isotopes (Z = 39) which lie between 38 Sr and 40 Zr. The yttrium chain is rich in isomers and it should be possible to correlate the charge radii changes with static deformation and also see orbital-dependent effects on the core polarization.
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
311c
0.45
0.35
Z=60
Z=58
2
(fm )
Z=62
samarium neodymium cerium barium xenon
0.25
2 N
-
2 N-2
Z=56 Z=54
0.15
0.05
-0.05
78
80
82
84 86 88 Neutron Number, N
90
92
Figure 2. A Brix-Kopfermann plot showing the differences in mean square charge radii for even isotopes in the region of the N=82 shape transition.
Preparations for measuring the yttrium isotopes are well advanced. Off-line tests have been made on three ionic transitions from the (s 2 )1 S0 ground state. The obvious choice was the (sp) 1 P1 (224 nm) transition but only low laser powers are presently available and the hyperfine structure for stable 89 Y is unresolved. A weaker intercombination line, (dp) 3 P1 (311 nm) was actually more promising because of the higher laser power available. On-line data for 87−89 Y were collected showing some of the hyperfine structures, albeit on a high background. The strongest ionic transition is the (dp) 1 P1 (363 nm) because of (p2 ) and (d2 ) admixtures in the ground state and an (sp) admixture in the 1 P1 state. New laser optics and intra-cavity doubling crystal have been installed and the good efficiency using this transition has been experimentally confirmed. On-line running on both the neutron-deficient and neutron-rich isotopes will begin later in the year on this transition. In the heavier mass region, the neutron-rich 144,146,148 Ce isotopes have been measured to complete the systematics near the N = 88 shape transition. The Brix-Kopfermann plot in Figure 2 shows the difference in mean square charge radii between adjacent even isotopes for elements in this region. The generally higher values above N = 82 is a common feature above all neutron magic numbers and indicates a greater increase in rms radius with neutron number than before the shell closure. The highest differences are seen 150 at 152 62 Sm and 60 Nd where there is a sharper transition to well deformed prolate shapes (β2 ∼ 0.27 − 0.30). It is these two nuclei that have been established as having the X(5) symmetry at the critical point of the SU(3) − U(5) phase transition [7]. The new data for Ce places it almost exactly midway between Sm, Nd and the more usual behaviour of Ba, Xe. Further work on the odd Ce isotopes is planned. 4. The neutron-deficient Ti isotopes The isotopes shifts of the stable titanium isotopes 46−50 22 Ti and the neutron-deficient Ti produced via 45 Sc(p,xn) reactions have been measured in collaboration with the laser spectroscopy group from JINR, Dubna [8]. The isotope shifts, measured on the
44,45
312c
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
4 F3/2 (0 cm−1 )→ 4 F3/2 (30,837 cm−1 ) transition in the Ti + ion, are shown in table 1. The even-N nuclear radii show a generally increasing tendency with decreasing neutron number in contrast to the symmetric parabolic behaviour of the 40−48 20 Ca chain. The trend is consistent with the predictions of the relativistic mean-field (RMF) theory [9,10] and is due, in part, to the decreasing proton separation energies for the neutron-deficient isotopes. Interestingly, the RMF does not predict a ‘kink’ at the N = 20 shell which is in agreement with all experimental data (for Ar, K and Ca) but it fails to reproduce the parabolic behaviour of the Ca chain. It is possible that the larger rms charge radius of 44 Ti compared with 46 Ti is due to its 40 Ca+α cluster structure. Theoretical calculations lack the necessary precision but suggest the cluster structure may only have a minor net effect on the charge radius.
Table 1 Isotope shifts measured on the 324.2 nm ionic transition in Ti + . Statistical errors are given in parentheses and systematic uncertainties are given in brackets (see ref. [8] for details). A δν 48,A (MHz) δr 2 (fm2 ) r 21/2 (fm) 44 –1558(17) +0.143(37)[25] 3.6185(38) 45 –1100(8) +0.013(17)[18] 3.6005(27) 46 –765(3) +0.110(7)[12] 3.6139(24) 47 –363(2) +0.030(4)[6] 3.6029(24) 48 3.5987(23) 49 +400(4) –0.139(9)[6] 3.5793(25) 50 +731(3) –0.160(7)[11] 3.5764(24)
5. The K = 8 isomers in 178
+
130
Ba and
176
Yb
The celebrated Hf(16 ) 4 quasi-particle isomer has the surprising property of a smaller rms charge radius than the ground state despite being no less deformed [11]. At JYFL it has been possible to measure two 8 − isomers in 176 Yb and 130 Ba which are related to the 2-neutron and 2-proton configurations which form the 16 + state. There are a number of experimental problems in these isomer shift measurements. Both K = 8 isomers are two-neutron configurations with small magnetic moments. Consequently, the hyperfine structure is bunched up around the much more intense nuclear ground state resonance peak. Furthermore, the quadrupole interaction can change the order of the hyperfine components and assignment of the peaks is difficult when some components are lost under the ground state peak. Nevertheless, an unambiguous analysis for 130 Ba(8− ) was possible [12] and, like the 178 Hf isomer, the state was found to have a smaller rms charge radius despite a similar deformation to the ground state. The 176 Yb(8− ) isomer was populated in the 176 Yb(d,pn) reaction at 13 MeV with a deuteron beam current of 5.5 µA. The isomer component of the A=176 beam was determined by gamma-ray spectroscopy to be 200 s−1 out of a total ion flux of 8,400 s−1 . The analysis has now been completed [13]. The results are compared with the measured
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
Yb Lu Hf
1
0.5 δ < r 2 > 99,N δ < r 2 > 99,106
313c
1 0.95 0.9
0
0.85 0.8
-0.5
-1
98
100
102
0.75 105.5
106
104
106
106.5
108
N
Figure 3. Relative changes in mean square charge radii for isotopes of Yb [13], Lu [14] and Hf [5,11,15]. The N = 106 isomers are shown in the panel as open symbols. 178 Hf(16+ ): diamond; 177 Lu(23/2− ): square; 176 Yb(8− ): circle.
N = 106 isomers in the neighbouring chains of Lu and Hf in Figure 3. For display purposes the data have been normalised to the ground states at N = 99 and 106 (although this is not quite consistent with the atomic factor evaluations used for the extraction of the change in mean square charge radii). It is evident from the figure that all isomers are smaller than their nuclear ground state. A comparison of the β 2 deformation parameters derived for the ground states and isomers indicate that none of the isomer shifts can be attributed to a reduction in deformation of the isomer. The reduction in rms charge radius is greatest for the 4 quasi-particle 16 + state. The 176 Yb(8− ) and 177 Lu(23/2− ) states are both 2 quasi-particle effects compared to their respective ground states, and are about twice the size of the normal odd-even staggering of isotope shifts which might be thought of as a 1 quasi-particle effect. The most probable explanation for the isomers’ smaller radii is in the pairing reduction caused by the blocking of the additional quasi-particles but theoretical work is required to test this suggestion quantitatively. 6. Development of collinear resonance ionization spectroscopy (CRIS) Laser resonance ionization of atoms using powerful pulsed laser systems has important applications in nuclear physics, for example in realizing element-selective ion sources and providing high-sensitivity methods of optical spectroscopy. The ability to bunch the ion beam at JYFL has rejuvenated interest in the collinear RIS technique. This was first demonstrated by Schulz et al [16] but it did not produce a significant improvement in
314c
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
6000
Photon counts
5000
4000
3000
2000
1000 2000 3000 De-tuning frequency relative to an arbitary origin (MHz)
4000
Figure 4. Laser fluorescence spectrum (308.2 nm) for a 60 pA
27
Al beam.
sensitivity due to the duty cycle loss with the continuous ion beam. A simple two-step CRIS scheme was demonstrated at JYFL on a stable beam of 27 Al. A sodium-vapour charge exchange cell was installed in the ion beam line just before the laser interaction region. The fluorescence spectrum on the first selected transition (308.2 nm) from the 2 P1/2 atomic ground state to the 2 D3/2 (32,435 cm−1 ) state is shown in Figure 4 for a 60 pA beam. The poor fluorescence efficiency (1 photon detected per 50,000 atoms in the beam) is partly a consequence of the optical pumping of the 27 Al atoms while on resonance in the laser beam between the charge exchange cell and the light collection region. The sub-structure in each of the six hyperfine components is caused by charge-exchanging into several states in the Al atom which feed into the ground state. The ion beam was considerably weakened to avoid space-charge problems when bunching in the RFQ cooler. The ion bunches were synchronised to arrive in the interaction region at the same time as the laser pulse. The pulsed 308.2 nm light was provided by amplifying 616.4 nm CW light from a Spectra-Physics 380 dye laser with a pulsed dye amplifier pumped by a 50 Hz Nd:YAG (532 nm) followed by frequency-doubling in a lithium iodate crystal. The second step to the continuum was made with 532 nm light from the same Nd:YAG laser. The atom–laser interaction region was 1.5 m long (equivalent to a 2.8 µs bite of the atom beam). Ions produced in the interaction region were deflected to a micro-channel plate detector and counted. The ion yield spectrum, shown in Figure 5, was taken with an optimal 1 µs gate on the ion detector signal. The measured efficiency was equivalent to one ion detected per 30 atoms within the 1 µs gate. The CRIS method does not suffer from the optical pumping problem that caused the low fluorescence efficiency, but this still represents a substantial gain in sensitivity. The resolution is dominated by power broadening but it is still far less than normal Doppler broadening. It is ultimately limited by the Fourier broadening caused by the pulsed dye amplifier. There are a number of simple improvements that will raise the overall efficiency (shorter
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
315c
60
Ion Counts
50
40
30
20
1000
2000
3000 Frequency(MHz}
4000
5000
Figure 5. Ion yield spectrum for resonance ionization (308.2 nm + 532 nm) of a weak 27 Al beam (200 atoms per bunch). The two group of hyperfine components are separated by approximately 2 GHz
bunching of the RFQ cooler, optimal RIS scheme, faster ion counting detector and electronics, improved vacuum in the interaction region) and there is great potential for the CRIS method for medium and heavy elements. An obvious application of CRIS would be in the provision of pure isomer beams for nuclear spectroscopy. In this case the ions are simply deflected down a beam line to a measurement station rather than to the microchannel plate 7. Acknowledgements The on-going work described in this paper is being carried out by a collaboration involving the Universities of Birmingham (G. Tungate, D.H. Forest, B. Cheal, M.D. Gardener, ¨ o, H. Penttil¨a, A. Jokinen, I.D. Moore, J. Huikari, S. RintaM. Bissel), Jyv¨askyl¨a (J. Ayst¨ Antila) and Manchester (J. Billowes, P. Campbell, A. Nieminen, K. Flanagan, A. Ezwam, M. Avgoulea, B.A. Marsh and B.W. Tordoff). The 130m Ba measurement was done in collaboration with Dr A. Bruce (University of Brighton), and the 176m Yb work was in collaboration with Professor G. Dracoulis (Australian National University). REFERENCES 1. 2. 3. 4. 5. 6. 7.
E.W. Otten Treatise on Heavy-Ion Science Vol. 8 (Plenum, New York 1989) p.515. J. Billowes and P. Campbell, J. Phys. G 21 (1995) 707. H-J. Kluge and W. N¨ortersh¨auser, Spectrochemica Acta B58 (2003) 1031. ¨ o, Nucl. Phys. A 693 (2001) 477. J. Ayst¨ A. Nieminen et al., Phys. Rev. Lett. 88 (2002) 094801. P. Campbell et al., Phys. Rev. Lett. 89 (2002) 082501. A. Dewald et al., Eur. Phys. J. A 20 (2004) 173.
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8. 9. 10. 11. 12. 13. 14. 15. 16.
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
Yu.P. Gangrsky et al., J. Phys. G 30 (2004) 1089. G.A. Lalazissis et al., At. Data Nucl. Data Tables 71 (1999) 1. G.A. Lalazissis et al., Nucl. Phys. A 628 (1998) 221. N. Boos et al., Phys. Rev. Lett. 72 (1994) 2689. R. Moore et al., Phys. Lett. B 547 (2002) 200–204. K.T. Flanagan, PhD thesis, University of Manchester (2004). U. Georg et al., Eur. Phys. J. A 3 (1998) 225. J.M.G. Levins et al., Phys. Rev. Lett. 82 (1999) 2476. Ch. Schulz et al., J. Phys. B 24 (1991) 4831.
Laser spectroscopy with cooled beams J. Billowesa a
Schuster Laboratory, University of Manchester, Manchester M13 9PL, UK
This contribution describes the programme being carried out by a collaboration involving the Universities of Birmingham, Jyv¨askyl¨a and Manchester, using collinear laser spectroscopy to measure isotope shifts and hyperfine structures of radioisotopes. The work benefits from the unique combination of an ion guide isotope separator (IGISOL) and an RFQ ion cooler-buncher, allowing measurements on ion fluxes as low as 150 s −1 . Recent studies include Zr and Ce isotopes produced by proton-induced fission of uranium, neutron deficient isotopes of Ti, and K = 8 isomers in 130 Ba and 176 Yb. Development work on a more sensitive technique of Collinear Resonance Ionization Spectroscopy (CRIS) with bunched ion beams is also described. 1. Introduction The laser spectroscopy programme at the Cyclotron Laboratory, Jyv¨ askyl¨a (JYFL) is run by a collaboration involving the Universities of Birmingham, Jyv¨askyl¨a and Manchester. The aim of the programme has been to measure nuclear moments and charge radii of radioactive nuclei. The nuclear magnetic dipole and electric quadrupole moments are obtained directly from the hyperfine structures observed on optical transitions. The difference in nuclear mean square charge radius between two isotopes is deduced from the frequency difference (isotope shift) in the optical transition. These are part per million perturbations of the atomic energy levels that are usually masked by Doppler broadening of the transition. The standard ‘collinear beams’ method of overlapping the laser and ion beam eliminates Doppler broadening and provides the high sensitivity required for on-line measurements of short-lived isotopes. In this way many isotopes chains have been measured at on-line isotope separators [1–3]. The present status of these measurements is indicated in Figure 1. 2. The laser/IGISOL facility The IGISOL facility offers two significant advantages over conventional on-line mass separators: (i) the ion guide method can provide isotope beams of any element, regardless of chemistry and with fast extraction times in the region of one millisecond [4]; (ii) the RFQ cooler-buncher [5] installed in the mass-separated line reduces the ion beam energy spread to a level where it makes a negligible contribution to the experimental resolution of the laser spectroscopy. The device can accumulate and bunch the ions before they are presented to the laser beam. This feature allows a reduction in the laser-scattered 0375-9474/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2005.02.045
310c 2
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
152 82 126
Z 50 82
28 50 20 28
8 20
2 2
8
N
Figure 1. Present status of optical measurements (black – stable; dark shade – measured)
light in the laser fluorescence signal. This is done by gating the photomultiplier signal to accept counts only during the 15 µs when the ion bunch passes through the laser interaction region. The laser-related background is reduced by the ratio of bunch width to accumulation time (typically 300 ms) which is thus at least four orders of magnitude [6]. In this way measurements have been made with ion beam fluxes as low as 200 s −1 . 3. Fission fragment spectroscopy Proton-induced fission of uranium produces a broad range of medium-mass isotopes on the neutron-rich side of stability. Using the IGISOL fission ion guide, laser measurements have been made on the zirconium isotopes out to 102 Zr [6]. This covers the shape transition at N = 60. The apparent inconsistency between the dynamic quadrupole deformations deduced from the rms charge radii and those from B(E2; 2 + → 0+ ) values for isotopes in the range 50 < N < 60 is still not adequately explained. A similar situation occurs in the strontium chain. Measurements of the quadrupole moments of 93,95 Zr would be useful in understanding the deformation properties here, but these isotopes are difficult to produce in a nuclear reaction. An alternative approach is to look at the odd-proton yttrium isotopes (Z = 39) which lie between 38 Sr and 40 Zr. The yttrium chain is rich in isomers and it should be possible to correlate the charge radii changes with static deformation and also see orbital-dependent effects on the core polarization.
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
311c
0.45
0.35
Z=60
Z=58
2
(fm )
Z=62
samarium neodymium cerium barium xenon
0.25
2 N
2 N-2
Z=56 Z=54
0.15
0.05
-0.05
78
80
82
84 86 88 Neutron Number, N
90
92
Figure 2. A Brix-Kopfermann plot showing the differences in mean square charge radii for even isotopes in the region of the N=82 shape transition.
Preparations for measuring the yttrium isotopes are well advanced. Off-line tests have been made on three ionic transitions from the (s 2 )1 S0 ground state. The obvious choice was the (sp) 1 P1 (224 nm) transition but only low laser powers are presently available and the hyperfine structure for stable 89 Y is unresolved. A weaker intercombination line, (dp) 3 P1 (311 nm) was actually more promising because of the higher laser power available. On-line data for 87−89 Y were collected showing some of the hyperfine structures, albeit on a high background. The strongest ionic transition is the (dp) 1 P1 (363 nm) because of (p2 ) and (d2 ) admixtures in the ground state and an (sp) admixture in the 1 P1 state. New laser optics and intra-cavity doubling crystal have been installed and the good efficiency using this transition has been experimentally confirmed. On-line running on both the neutron-deficient and neutron-rich isotopes will begin later in the year on this transition. In the heavier mass region, the neutron-rich 144,146,148 Ce isotopes have been measured to complete the systematics near the N = 88 shape transition. The Brix-Kopfermann plot in Figure 2 shows the difference in mean square charge radii between adjacent even isotopes for elements in this region. The generally higher values above N = 82 is a common feature above all neutron magic numbers and indicates a greater increase in rms radius with neutron number than before the shell closure. The highest differences are seen 150 at 152 62 Sm and 60 Nd where there is a sharper transition to well deformed prolate shapes (β2 ∼ 0.27 − 0.30). It is these two nuclei that have been established as having the X(5) symmetry at the critical point of the SU(3) − U(5) phase transition [7]. The new data for Ce places it almost exactly midway between Sm, Nd and the more usual behaviour of Ba, Xe. Further work on the odd Ce isotopes is planned. 4. The neutron-deficient Ti isotopes The isotopes shifts of the stable titanium isotopes 46−50 22 Ti and the neutron-deficient Ti produced via 45 Sc(p,xn) reactions have been measured in collaboration with the laser spectroscopy group from JINR, Dubna [8]. The isotope shifts, measured on the
44,45
312c
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
4 F3/2 (0 cm−1 )→ 4 F3/2 (30,837 cm−1 ) transition in the Ti + ion, are shown in table 1. The even-N nuclear radii show a generally increasing tendency with decreasing neutron number in contrast to the symmetric parabolic behaviour of the 40−48 20 Ca chain. The trend is consistent with the predictions of the relativistic mean-field (RMF) theory [9,10] and is due, in part, to the decreasing proton separation energies for the neutron-deficient isotopes. Interestingly, the RMF does not predict a ‘kink’ at the N = 20 shell which is in agreement with all experimental data (for Ar, K and Ca) but it fails to reproduce the parabolic behaviour of the Ca chain. It is possible that the larger rms charge radius of 44 Ti compared with 46 Ti is due to its 40 Ca+α cluster structure. Theoretical calculations lack the necessary precision but suggest the cluster structure may only have a minor net effect on the charge radius.
Table 1 Isotope shifts measured on the 324.2 nm ionic transition in Ti + . Statistical errors are given in parentheses and systematic uncertainties are given in brackets (see ref. [8] for details). A δν 48,A (MHz) δr 2 (fm2 ) r 21/2 (fm) 44 –1558(17) +0.143(37)[25] 3.6185(38) 45 –1100(8) +0.013(17)[18] 3.6005(27) 46 –765(3) +0.110(7)[12] 3.6139(24) 47 –363(2) +0.030(4)[6] 3.6029(24) 48 3.5987(23) 49 +400(4) –0.139(9)[6] 3.5793(25) 50 +731(3) –0.160(7)[11] 3.5764(24)
5. The K = 8 isomers in 178
+
130
Ba and
176
Yb
The celebrated Hf(16 ) 4 quasi-particle isomer has the surprising property of a smaller rms charge radius than the ground state despite being no less deformed [11]. At JYFL it has been possible to measure two 8 − isomers in 176 Yb and 130 Ba which are related to the 2-neutron and 2-proton configurations which form the 16 + state. There are a number of experimental problems in these isomer shift measurements. Both K = 8 isomers are two-neutron configurations with small magnetic moments. Consequently, the hyperfine structure is bunched up around the much more intense nuclear ground state resonance peak. Furthermore, the quadrupole interaction can change the order of the hyperfine components and assignment of the peaks is difficult when some components are lost under the ground state peak. Nevertheless, an unambiguous analysis for 130 Ba(8− ) was possible [12] and, like the 178 Hf isomer, the state was found to have a smaller rms charge radius despite a similar deformation to the ground state. The 176 Yb(8− ) isomer was populated in the 176 Yb(d,pn) reaction at 13 MeV with a deuteron beam current of 5.5 µA. The isomer component of the A=176 beam was determined by gamma-ray spectroscopy to be 200 s−1 out of a total ion flux of 8,400 s−1 . The analysis has now been completed [13]. The results are compared with the measured
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
Yb Lu Hf
1
0.5 δ < r 2 > 99,N δ < r 2 > 99,106
313c
1 0.95 0.9
0
0.85 0.8
-0.5
-1
98
100
102
0.75 105.5
106
104
106
106.5
108
N
Figure 3. Relative changes in mean square charge radii for isotopes of Yb [13], Lu [14] and Hf [5,11,15]. The N = 106 isomers are shown in the panel as open symbols. 178 Hf(16+ ): diamond; 177 Lu(23/2− ): square; 176 Yb(8− ): circle.
N = 106 isomers in the neighbouring chains of Lu and Hf in Figure 3. For display purposes the data have been normalised to the ground states at N = 99 and 106 (although this is not quite consistent with the atomic factor evaluations used for the extraction of the change in mean square charge radii). It is evident from the figure that all isomers are smaller than their nuclear ground state. A comparison of the β 2 deformation parameters derived for the ground states and isomers indicate that none of the isomer shifts can be attributed to a reduction in deformation of the isomer. The reduction in rms charge radius is greatest for the 4 quasi-particle 16 + state. The 176 Yb(8− ) and 177 Lu(23/2− ) states are both 2 quasi-particle effects compared to their respective ground states, and are about twice the size of the normal odd-even staggering of isotope shifts which might be thought of as a 1 quasi-particle effect. The most probable explanation for the isomers’ smaller radii is in the pairing reduction caused by the blocking of the additional quasi-particles but theoretical work is required to test this suggestion quantitatively. 6. Development of collinear resonance ionization spectroscopy (CRIS) Laser resonance ionization of atoms using powerful pulsed laser systems has important applications in nuclear physics, for example in realizing element-selective ion sources and providing high-sensitivity methods of optical spectroscopy. The ability to bunch the ion beam at JYFL has rejuvenated interest in the collinear RIS technique. This was first demonstrated by Schulz et al [16] but it did not produce a significant improvement in
314c
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
6000
Photon counts
5000
4000
3000
2000
1000 2000 3000 De-tuning frequency relative to an arbitary origin (MHz)
4000
Figure 4. Laser fluorescence spectrum (308.2 nm) for a 60 pA
27
Al beam.
sensitivity due to the duty cycle loss with the continuous ion beam. A simple two-step CRIS scheme was demonstrated at JYFL on a stable beam of 27 Al. A sodium-vapour charge exchange cell was installed in the ion beam line just before the laser interaction region. The fluorescence spectrum on the first selected transition (308.2 nm) from the 2 P1/2 atomic ground state to the 2 D3/2 (32,435 cm−1 ) state is shown in Figure 4 for a 60 pA beam. The poor fluorescence efficiency (1 photon detected per 50,000 atoms in the beam) is partly a consequence of the optical pumping of the 27 Al atoms while on resonance in the laser beam between the charge exchange cell and the light collection region. The sub-structure in each of the six hyperfine components is caused by charge-exchanging into several states in the Al atom which feed into the ground state. The ion beam was considerably weakened to avoid space-charge problems when bunching in the RFQ cooler. The ion bunches were synchronised to arrive in the interaction region at the same time as the laser pulse. The pulsed 308.2 nm light was provided by amplifying 616.4 nm CW light from a Spectra-Physics 380 dye laser with a pulsed dye amplifier pumped by a 50 Hz Nd:YAG (532 nm) followed by frequency-doubling in a lithium iodate crystal. The second step to the continuum was made with 532 nm light from the same Nd:YAG laser. The atom–laser interaction region was 1.5 m long (equivalent to a 2.8 µs bite of the atom beam). Ions produced in the interaction region were deflected to a micro-channel plate detector and counted. The ion yield spectrum, shown in Figure 5, was taken with an optimal 1 µs gate on the ion detector signal. The measured efficiency was equivalent to one ion detected per 30 atoms within the 1 µs gate. The CRIS method does not suffer from the optical pumping problem that caused the low fluorescence efficiency, but this still represents a substantial gain in sensitivity. The resolution is dominated by power broadening but it is still far less than normal Doppler broadening. It is ultimately limited by the Fourier broadening caused by the pulsed dye amplifier. There are a number of simple improvements that will raise the overall efficiency (shorter
J. Billowes / Nuclear Physics A 752 (2005) 309c–316c
315c
60
Ion Counts
50
40
30
20
1000
2000
3000 Frequency(MHz}
4000
5000
Figure 5. Ion yield spectrum for resonance ionization (308.2 nm + 532 nm) of a weak 27 Al beam (200 atoms per bunch). The two group of hyperfine components are separated by approximately 2 GHz
bunching of the RFQ cooler, optimal RIS scheme, faster ion counting detector and electronics, improved vacuum in the interaction region) and there is great potential for the CRIS method for medium and heavy elements. An obvious application of CRIS would be in the provision of pure isomer beams for nuclear spectroscopy. In this case the ions are simply deflected down a beam line to a measurement station rather than to the microchannel plate 7. Acknowledgements The on-going work described in this paper is being carried out by a collaboration involving the Universities of Birmingham (G. Tungate, D.H. Forest, B. Cheal, M.D. Gardener, ¨ o, H. Penttil¨a, A. Jokinen, I.D. Moore, J. Huikari, S. RintaM. Bissel), Jyv¨askyl¨a (J. Ayst¨ Antila) and Manchester (J. Billowes, P. Campbell, A. Nieminen, K. Flanagan, A. Ezwam, M. Avgoulea, B.A. Marsh and B.W. Tordoff). The 130m Ba measurement was done in collaboration with Dr A. Bruce (University of Brighton), and the 176m Yb work was in collaboration with Professor G. Dracoulis (Australian National University). REFERENCES 1. 2. 3. 4. 5. 6. 7.
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