The new collinear resonant ionization spectroscopy (Cris) experiment at Isolde, Cernuses laser radiation to stepwise excite and ionize an atomic beam for the purpose of ultra-sensitive detection of rare isotopes and hyperfine structure measurements. The technique also offers the ability to purify an ion beam that is contaminated with radioactive isobars, including the ground state of an isotope from its isomer. A new program using the Cristechnique to select only nuclear isomeric states for decay spectroscopy commenced last year. The isomeric ion beam is selected using a resonance within its hyperfine structure and subsequently deflected to a decay spectroscopy station. This consists of a rotating wheel implantation system for alpha and beta decay spectroscopy, and up to three high purity germanium detectors for gamma-ray detection. This paper gives an introduction to the Cristechnique, the current status of the laser assisted decay spectroscopy set-up and recent results from the experiment in November 201

Cocolios, T.

Lynch, K.

Rajabali, M.

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Hyperfine Interact (2013) 216:95–101DOI 10.1007/s10751-013-0820-yLaser assisted decay spectroscopy at the CRIS beamline at ISOLDEK. M. Lynch · T. E. Cocolios · M. M. Rajabalion behalf of the CRIS collaborationPublished online: 7 February 2013© Springer Science+Business Media Dordrecht 2013Abstract The new collinear resonant ionization spectroscopy (Cris) experiment atIsolde, Cern uses laser radiation to stepwise excite and ionize an atomic beamfor the purpose of ultra-sensitive detection of rare isotopes and hyperfine structuremeasurements. The technique also offers the ability to purify an ion beam that iscontaminated with radioactive isobars, including the ground state of an isotope fromits isomer. A new program using the Cris technique to select only nuclear isomericstates for decay spectroscopy commenced last year. The isomeric ion beam is selectedusing a resonance within its hyperfine structure and subsequently deflected to a decayspectroscopy station. This consists of a rotating wheel implantation system for alphaand beta decay spectroscopy, and up to three high purity germanium detectors forgamma-ray detection. This paper gives an introduction to the Cris technique, thecurrent status of the laser assisted decay spectroscopy set-up and recent results fromthe experiment in November 2011.Keywords CRIS · ISOLDE · Laser spectroscopy · Decay spectroscopyProceedings of the 6th international conference on Laser Probing (LAP 2012),Paris, France, 4–8 June 2012.K. M. Lynch (B)School of Physics and Astronomy,The University of Manchester, Manchester, M13 9PL, UKe-mail: kara.marie.lynch@cern.chK. M. Lynch · T. E. CocoliosPhysics Department, CERN, 1211 Geneva 23, SwitzerlandM. M. RajabaliInstituut voor Kern- en Stralingsfysica,KU Leuven, 3001 Leuven, BelgiumPresent Address:M. M. RajabaliTRIUMF, Vancouver, BC, V6T 2A3, Canada96 K.M. Lynch et al.1 IntroductionThe Cris beam line combines two complementary spectroscopic techniques toprovide a wealth of information on the nuclear species under investigation [1–3].Laser spectroscopy allows nuclear observables to be extracted with nuclear modelindependence: the nuclear spin, moments and the change in the mean-square chargeradii (δ〈r2〉) between isotopes [4]. From the complementary decay spectroscopy, levelscheme information on the daughter nucleus can be obtained. The Cris techniqueprovides a combination of high detection efficiency, high resolution and ultra-lowbackground, allowing measurements to be performed on isotopes with yields downto 1 atom per second.2 Collinear resonant ionization spectroscopyLaser radiation is used to resonantly excite and ionize an atomic beam, probingthe atomic transitions of an isotope. These transitions are used to extract nuclearobservables, such as magnetic dipole and electric quadrupole moments, as wellas δ〈r2〉 between isotopes. A sequence of two transitions at selected frequenciesare required to bring the electron across the ionization potential. When the laserfrequencies are on resonance with the hyperfine component of the optical transition,the isotope is ionized. By using short laser pulses, loses associated with opticalpumping can be avoided. This process of resonant ionization [5–7] selects the isotopeof interest. The maximum selectivity S, of an isotope from an isomer or anotherisotope is given by the equationS =(ωAB)2=∏Sn,where ωAB is the separation between the respective hyperfine components and is the width of those transitions, Fig. 1a. Additional selectivity can be gaineddue to the kinematic shift between different masses of the isotopes caused bythe collinear geometry. The total selectivity of a resonant ionization process isgiven by the product of the individual selectivities, thus the higher the number ofatomic excitation steps, the greater the selectivity of the nuclear state, as illustratedin Fig. 1b. Indeed, the selectivity of resonant ionization can be compared to themass resolution of a state in mass spectrometry. The sensitivity of the techniquecomes from the detection of resonant ions (with the possibility of 100 % detectionefficiency), efficient ionization (between 10–30 %) and almost background freedetection.Due to the high degree of selectivity and efficiency of the resonant ionizationprocess, the Cris technique can be utilized as a purification method [8]. The differentnuclear observables (spin, moments, δ〈r2〉) of isotopes and isomers produce differenthyperfine structures, resulting in the lasers only ionizing the nuclear state of interestwhile on resonance with a characteristic transition. This results in the ability toperform decay spectroscopy on pure isomeric states with a suppression of the groundstate by a factor of at least 104 per resonant transition.Laser assisted decay spectroscopy at the CRIS beam line at ISOLDE 97Ground stateGSE1E2IPS1S2IsomerGround state Isomer(a) (b)Γ ΓΔωABFig. 1 a The difference in frequency ωAB between two absorption lines, due to the isomer shift.b The number of resonant transitions increases the selectivity of the process3 Laser assisted decay spectroscopyUsing the technique of in-source laser spectroscopy [9], the identification and useof isomeric beams has already been achieved, for example in the copper isotopes68,70Cu [10, 11]. However, a large isobaric contamination in the form of surfaceionized gallium was present, along with a ground state contamination due to theDoppler broadening of the hyperfine resonances of each state. Collinear laserspectroscopy [12] successfully identified the long-lived isomer in 80Ga that was notobserved by mass measurements [13]. Higher resolution can be provided by gammaray spectroscopy, but distinctions between ground and isomeric states that both beta-decay cannot be made. With collinear laser spectroscopy, isomeric states differing byonly 1 keV in energy can be resolved through their hyperfine structure, but withhigh-resolution gamma spectroscopy they cannot be seen as independent levels, forexample in 73Ga [14]. The alpha decay of the isomeric states of 202Fr and 204Fr couldnot initially be differentiated with decay spectroscopy [15]. Later measurementsgave tentative spin-parity assignments to these low-lying isomers, based on feedingpatterns in β+/EC decays and on systematics with the neighbouring isotopes andisotones.As a proof of principle for the Cris technique [8, 16], the first case to beinvestigated was the low lying isomerism in 204Fr [17]. Here, the ground and isomericstates are within 41 keV of each other [18–20]. This region of the nuclear chart hasalready received considerable attention, with experimental campaigns into β-delayedfission of 200,202Fr already undertaken [21]. Radioactive decay spectra from theground state, first and second isomeric states in 204Fr have been difficult to unravel;the emitted alpha particles have very similar energies and half lives. By exploitingthe different hyperfine structures and locking the laser onto resonance with acharacteristic transition, differentiation between the three states becomes possible.Thus the radioactive decays of pure beams can be measured, allowing the energiesof the alpha particles and gamma rays emitted, and the lifetime of the states, to beunambiguously determined.98 K.M. Lynch et al.4 The CRIS beam line at ISOLDEFrancium isotopes are produced by impinging 1.4 GeV protons on a thick target(UCx or ThCx). These radioactive products are then surface ionized, accelerated to30 keV and mass separated with the high resolution separator (Hrs) to select thefrancium isotope of interest [22]. The ions are then bunched using the radiofrequencyquadrupole Iscool [23]. The bunching of the ion beam is essential for removing theduty cycle losses due to the pulsed lasers. Alternative stable beam can be producedfrom the Cris off line ion source, which has the ability to produce caesium orrubidium ions. These beams allow for the testing of ionization schemes and theoptimization of transmission through the Cris beam line. The ion beam is passedthrough an alkali vapour charge exchange cell (Cec), which neutralizes the bunchin preparation for resonant ionization. Here, a Doppler tuning voltage is applied tothe ions entering the Cec, adapting the Doppler shift of the locked frequency of thelasers relative to the interacting atomic bunch.In the interaction region, the atom bunch is collinearly overlapped with two laserbeams. The collinear geometry of the set-up provides a reduction in thermal Dopplerbroadening by a factor of ≈ 103, increasing the selectivity of the state of interest. Thisis because the product of the velocity and velocity spread remains constant underacceleration. This decreases the Doppler broadened line width of the hyperfinetransition to below its natural width, which leads to a greater degree of selectivitybetween the ground and isomeric states of the isotope, and thus higher resolution andsensitivity to the nuclear observables. The first transition is from the 72S1/2 electronicorbital to the 82P3/2 state with 422 nm light. The second (non-resonant) transitionis from the 82P3/2 state into the continuum using 1064 nm light. A new injection-seeded Ti:Sa laser system was used to produce these two laser beams [24]. The atomswere ionized when the laser light is on resonance with a hyperfine transition anddeflected to the decay spectroscopy station (Dss). Here, they can be detected withan MCP, for hyperfine structure measurements, or implanted into a carbon foil fordecay measurements.The Dss consists of a rotating wheel implantation system, based on the designfrom KU Leuven [25, 26], and shown in Fig. 2 [27]. Four Canberra silicon Pips de-tectors are installed in the Dss for alpha and beta decay detection. Additionally,three high-purity germanium detectors can be placed around the implantation sitefor gamma-ray detection. A Canberra high purity germanium SEGe detector andan Eurisys thin beryllium window x-ray detector were used during the experiment.Pure beams of the desired isotope or isomer can be deflected to the decay station,where alpha and gamma decays are detected. The data was recorded event by eventusing a fully digital data acquisition system. Alpha-gamma coincidences were thenreconstructed off-line to identify a radioactive decay, and reduce the backgroundassociated with the dumped beam or the environment.5 Recent results from spectroscopy on francium isotopes at ISOLDELaser assisted decay spectroscopy was successfully performed on 207Fr, with anincrease in the number of alpha particles detected when the laser frequency wason resonance with the hyperfine transitions in the francium isotope by a factor ofLaser assisted decay spectroscopy at the CRIS beam line at ISOLDE 99Bunched ion beamRotating wheelOn-axis annular Si detectorOn-axis solid Si detectorOff-axis solid Si detectorsFig. 2 Technical drawing of the Dss wheel and silicon detectors. The wheel holds ten thin carbonfoils, with a thickness of 800 nm into which the ionized bunch is implanted. Two Canberra Pips silicondetectors for alpha decay detection are situated on either side of the implanted carbon foil. One soliddetector (Series A, 300 μm thick) sits behind the carbon foil, and an annular detector (Series AN,300 μm thick) is placed in front. The ion beam is directed through the 4 mm aperture of the annulardetector, allowing for backwards recoil particle detection. In addition to the on-axis detectors, twosolid Pips silicon detectors are placed off-axis. This allows for measurements on longer lived decayproducts to be performed by rotating the implanted foil from the on-axis to off-axis detector position(a) (b)(5/2-) 0T1/2=4.9 2 m 87Fr134221218.1085At132217(5/2-)(9/2-)E : 6126.3 keVI=15.1 %Qgs=6457.6 17 keV32.3 msE : 6341.0 keVI=83.4 %= 99%218.1 keV0.27 ns(c)Fig. 3 a Simulated single lines are the expected alpha-particle energy spectra for the ground state(dashed), first isomeric state (dotted) and second isomeric state (solid) for 204Fr when the laseris on resonance with a characteristic hyperfine transition. b Gamma-ray energy spectrum (top),alpha-particle energy spectrum (right) and alpha-gamma energy matrix showing the alpha-gammacoincidence for the radioactive decay of 221Fr. c The radioactive decay of 221Fr66 ± 11. The ratio of detected alpha particles when the laser was on resonanceto when the laser was blocked, can be increased in several ways. Any increase inthe overall efficiency of the experiment, e.g. laser ionization efficiency, will increasethe number of alpha-particle decays detected. Additionally, improving the vacuumin the interaction region will decrease the amount of non-resonant ionizations dueto collisional excitations. Decay spectroscopy was performed on 204Fr, with thealpha particles emitted from 204g,m1,m2Fr shown in Fig. 3a. This figure illustrates thenecessity of being able to distinguish the individual states of the 204Fr isotopes. Here,100 K.M. Lynch et al.the alpha-particle energies of 204gFr and 204m2Fr cannot be resolved, leading to largeuncertainties in the energies of the alpha particles and half lives of the states. By usingthe characteristic hyperfine structure of each state, a pure ground state or isomericbeam can be studied. Thus, by tuning the lasers onto resonance with a hyperfinetransition particular to 204m2Fr, the solid line spectrum shown in Fig. 3a would beobtained. Furthermore, the dashed line spectrum would be observed if the laser wastuned onto resonance with a transition in 204gFr. Alpha-gamma coincidences in 221Frwere observed, shown in Fig. 3b. These result from the 6126.3 keV alpha-particledecay of 221Fr to 217At* followed by prompt gamma-ray emission (218.1 keV) to itsground state 217At, see Fig. 3c. This demonstrates the ability of the Dss setup toobtain alpha-gamma coincidences, advantageous for the identification of radioactivedecay schemes.6 SummaryThe selectivity of collinear resonant ionization spectroscopy can be utilised toperform decay spectroscopy on pure isomeric beams. An experimental campaign isunderway at the Cris beam line to use complementary laser and decay spectroscopytechniques to study pure ground state and isomeric states of francium. This includesthe detailed study of the two isomers of 202Fr for β-delayed fission [21], along withother francium nuclei in the isotopic chain, 201−206,218,219Fr. Laser assisted decayspectroscopy was performed on 207Fr in November 2011, displaying an increase inalpha particles detected when the lasers were on resonance by a factor of 66 ± 11.Acknowledgements We thank the the ISOLDE team for providing excellent beams, the KULeuven technical staff for their support and assistance, and the GSI target laboratory for manufactur-ing the carbon foils. This work is supported by the U.K. Science and Technology Facilities Council,by FWO-Vlaanderen (Belgium), by GOA/2004/03 (BOF-KULeuven), by the IUAP -Belgian StateBelgian Science Policy (BriX network P6/23), and by the European Commission within the SeventhFramework Programme through ENSAR (Contract No. 262010).References1. Flanagan, K.T., et al.: AIP Conf. Proc. 1377, 38 (2011)2. Procter, T.J., et al.: J. Phys.: Conf. Ser. 381, 012070 (2012)3. Procter, T.J., et al.: LAP2012 Proceedings (2012)4. Cheal, B., Flanagan, K.T.: J. Phys. G 37(11), 113101 (2010)5. Hurst, G.S., et al.: Rev. Mod. Phys. 51(4), 767 (1979)6. Billowes, J., Campbell, P.: J. Phys. G: Nucl. Part. Phys. 21, 707 (1995)7. Fedosseev, V.N., Kudryavtsev, Yu., Mishin, V.I.: Phys. Scr. 85, 058104 (2012)8. Letokhov, V.: Opt. Commun. 7, 59 (1973)9. Weissman, L., et al.: Phys. Rev. C 65, 024315 (2002)10. Van Roosbroeck, J., et al.: Phys. Rev. Lett. 92, 112501 (2004)11. Stefanescu, I., et al.: Phys. Rev. Lett. 98, 122701 (2007)12. Cheal, B., et al.: Phys. Rev. C 82, 051302 (2010)13. Hakala, J., et al.: Phys. Rev. Lett. 101, 052502 (2008)14. Cheal, B., et al.: Phys. Rev. Lett. 104, 252502 (2010)15. Hornshoj, P., Hansen, P., Jonson, B.: Nucl. Phys. A 230(3), 380 (1974)16. Schulz, Ch., et al.: J. Phys. B At. Mol. Opt. Phys. 24, 4831 (1991)17. Lynch, K.M., et al.: J. Phys.: Conf. Ser. 381, 012128 (2012)18. Huyse, M., et al.: Phys. Rev. C 46, 1209 (1992)Laser assisted decay spectroscopy at the CRIS beam line at ISOLDE 10119. Chiara, C., Kondev, F.: Nucl. Data Sheets 111(1), 141 (2010)20. Uusitalo, J., et al.: Phys. Rev. C 71, 024306 (2005)21. Andreyev, A.N., et al.: CERN-INTC-2008-001, INTC-P-235. CERN Geneva (2008)22. Jonson, B., Richter, A.: Hyperfine Interact. 129, 1 (2000)23. Mané, E., et al.: Eur. Phys. J. A 42, 503 (2009)24. Hori, M., Dax, A.: Opt. Lett. 34(8), 1273 (2009)25. Dendooven, P.: Ph.D. thesis, Katholieke Universiteit Leuven (1992)26. Andreyev, A.N., et al.: Phys. Rev. Lett. 105, 252502 (2010)27. Rajabali, M.M., et al.: Nucl. Instrum. Methods Phys. Res. A 707, 35 (2013). doi:10.1016/j.nima.2012.12.090i

Laser assisted decay spectroscopy at the CRIS beam line at ISOLDE

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