The ISOLDE resonance ionization laser ion source (RILIS) allows to ionize efficiently and selectively many metallic elements. In recent yield surveys and on-line experiments with the ISOLDE RILIS we observed $^{23-34}$Mg , $^{26-34}\!$Al , $^{98-132}$Cd , $^{149}$Tb , $^{155-177}\!$Yb , $^{179-200}$Tl , $^{183-215}$Pb and $^{188-218}$Bi. The obtained yields are presented together with measured release parameters which allow to extrapolate the release efficiency towards more exotic (short-lived) nuclides of the same elements

Andreyev, A N

Bergmann, U C

Catherall, R

Cederkäll, J

De Witte, H

Dietrich, M

Fedorov, D V

Fedosseev, V

Fraile-Prieto, L M

Franchoo, S

Fynbo, H O U

Georg, U

Giles, T

Górska, M

Hannawald, M

Huyse, M

Joinet, A

Jonsson, O C

Kratz, K L

Kruglov, K

Köster, U

Lau, C

Lettry, Jacques

Mishin, V I

Oinonen, M

Partes, K

Peräjärvi, K

Pfeiffer, B

Ravn, H L

Seliverstov, D M

Thirolf, P G

Van de Vel, K

Van Duppen, P

Van Roosbroeck, J

Weissman, L

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN-EP/2002-0493 July 2002ON-LINE YIELDS OBTAINED WITH THE ISOLDE RILISU. Ko¨ster1) V.N. Fedoseyev1)2) A.N. Andreyev3) U.C. Bergmann1) R. Catherall1)J. Cederka¨ll1) M. Dietrich1) H. De Witte4) D.V. Fedorov5) L. Fraile1) S. Franchoo1)H. Fynbo1) U. Georg1) T. Giles1) M. Gorska4) M. Hannawald6) M. Huyse1)4) A. Joinet1)O.C. Jonsson1) K.L. Kratz6) K. Kruglov4) Ch. Lau1) J. Lettry1) V.I. Mishin2)M. Oinonen1) K. Partes1) K. Pera¨ja¨rvi1) B. Pfeiffer6) H.L. Ravn1) M.D. Seliverstov5)P. Thirolf7) K. Van de Vel4) P. Van Duppen4) J. Van Roosbroeck4) L. Weissman1)for the IS365, IS387, IS393 and ISOLDE Collaborations1)AbstractThe ISOLDE resonance ionization laser ion source (RILIS) allows to ionize effi-ciently and selectively many metallic elements. In recent yield surveys and on-line experiments with the ISOLDE RILIS we observed 23−34Mg, 26−34Al, 98−132Cd,149Tb, 155−177Yb, 179−200Tl, 183−215Pb and 188−218Bi. The obtained yields are pre-sented together with measured release parameters which allow to extrapolate therelease efficiency towards more exotic (short-lived) nuclides of the same elements.(Submitted to Nuclear Instruments and Methods in Physics Research B)1 IntroductionThe resonance ionization laser ion source (RILIS) [1] provides an efficient, fast andselective way to ionize many metallic elements. The elements ionized with the ISOLDERILIS were reviewed recently [2, 3, 4]. In this article we will discuss on-line results, i.e.release curves and yields measured in recent target tests, nuclear spectroscopy experimentsand on-line collections.The measured release curves were fitted with the empiric 4-parameter-formula de-scribed in [5, 6]. Table 1 shows a summary of the release parameters: tr the rise time,tf the fall time of the fast component, ts the fall time of the slow component and α therelative intensity of the fast component. Parameters which had to be fixed to obtain astable fit are marked in parentheses.The release parameters were used to determine for each radio-isotope the fractionwhich is released before it decays. The on-line efficiency of the RILIS cannot be easilymonitored as in the case of a plasma ion source where the support gas is injected at awell-defined flowrate. Still a rough estimate can be made by scaling the off-line measuredefficiency with the laser power available in the on-line run. Also the ion yield of a long-lived isotope with known production cross-section gives an indication of the ionizationefficiency. Correcting the ion yields for the released fraction and the ionization efficiencyallows to deduce the in-target production yield. In some of the following graphs bothvalues are plotted: the ion yield, showing the presently available radioactive ion beamintensity for the user, and the in-target production yield showing the theoretical limitthat could be obtained with an ideal target (no decay losses) of the same thickness anddimensions coupled to an ideal ion source (100 % efficiency). The ratio between bothcurves shows the overall efficiency of the present target and ion source unit.2 MagnesiumNeutron-rich Mg beams have been produced formerly at ISOLDE SC [7] from aUCx/graphite target with a chemically unselective “hot plasma” ion source (MK5) [8].Although this ion source is optimized for the production of singly charged ions, it alsoproduces with less efficiency multiply charged ions. Hence, at a mass separator settingA/q = 31 in addition to most A = 31 isobars also doubly charged ions of mass 62 (e.g.62Mn2+), triply charged ions of mass 93 (e.g. 93Kr3+), quadruply charged ions of mass 124(e.g. 124Sb4+) and molecules (e.g. 12C19O+) may appear. In particular the contaminationfrom abundantly produced fission products may be disturbing. Special ion source tuningcan reduce this kind of background, but it cannot suppress it completely.With the RILIS there is much less background of multiply charged ions. The mainlyoccuring beam contaminants are the surface ionized Na and Al isobars, the intensity ofthe former being about one order of magnitude lower compared to Mg. The measured Mgion yields and the deduced in-target production rates are shown in Fig. 1.The pure RILIS Mg beams were used for precision mass measurements with the RFspectrometer MISTRAL [9]. In a former beamtime with a hot plasma ion source 32Mg1) ISOLDE, CERN, 1211 Gene`ve 23, Switzerland2) Institute of Spectroscopy, Russian Academy of Sciences, 142190 Troitsk, Russia3) Oliver Lodge Laboratory, University of Liverpool, Liverpool, L69 3BK, UK4) Instituut voor Kern- en Stralingsfysica, K.U. Leuven, 3001 Leuven, Belgium5) Petersburg Nuclear Physics Institute, 188350 Gatchina, Russia6) Institut fu¨r Kernchemie, Universita¨t Mainz, 55128 Mainz, Germany7) Sektion Physik, Ludwig-Maximilians-Universita¨t Mu¨nchen, 85748 Garching, Bavaria1could already be measured, but the strong background had to be reduced with narrow slitsettings which compromised the transmission and hence the statistics. With the RILISbeams the slits could be opened wider and the statistics was significantly improved. Thusalso the mass of the N = 21 isotope 33Mg could be measured for the first time [10].3 Aluminum11010 210 310 410 510 610 710 822 23 24 25 26 27 28 29 30 31 32 33 34 35MassYield or in-target prod. (atoms/µC)Mg yields RILISMg in-target prod. RILISMg yields MK5 SCAl yields surf. ioniz. 1.0 GeVAl yields RILIS 1.4 GeVAl yields MK5 SCFigure 1: Ion yields and in-target production rates of magnesium and aluminum isotopes.For 26Al only the yield of the 6.3 s isomeric state was measured.Al is moderately well surface ionized. Neutron-rich Al beams up to 35Al [11] werealready studied at ISOLDE from a UCx/graphite target with W surface ionizer. Theseyields obtained with a 1.0 GeV proton beam are shown in Fig. 1 compared to yields froma MK5 ion source at ISOLDE SC.Off-line tests had shown that the RILIS can increase the Al ionization efficiency byup to a factor 70 compared to surface ionization in a 2200 ◦C hot W cavity [3]. This wasobtained by simultaneous excitation of the two fine structure levels of the ground statewith two frequency doubled dye laser beams (308.2 and 309.3 nm respectively). For theon-line tests (dedicated for another purpose) only the 308.2 nm beam was used to excitethe less populated fine structure level. Moreover the second step laser power focused tothe HRS frontend was lower, giving a “laser on”/”laser off” ratio of only about three.There is no doubt that the shown yields will be tripled when using both laser beamssimultaneously. Even more could be gained by increasing the power focused to the target.Fig. 1 shows the yields measured with the 308.2 nm beam only.Due to long desorption times, elemental Al is released very slowly1). Adding CF4 willconsiderably speed up the release of Al, then as AlF molecule [12]. To allow for ionizationwith the RILIS (tuned to excite atomic Al) the AlF molecule needs to be dissociated inthe ionizer cavity. A successful combination of fluorination and RILIS would increase thebeam intensity of short-lived Al isotopes enormously and would allow to produce beamswell beyond 36Al.1) With the available tracer isotopes no indication for a fast release component was observed, henceα = 0 was assumed for the fit.24 CadmiumNeutron-deficient Cd nuclides were one of the first beams separated at ISOLDE[13]. Concerning beam intensity and purity, these beams from a molten tin target are stillshowpieces at ISOLDE. A drawback of the molten targets is their rather slow release timeof the order of tens of seconds to minutes [14]. Thus, considerable decay losses occur forthe isotopes below 102Cd.A LaCx/graphite target (molar ratio La:C≈1:4) combined with the RILIS wastested2). Due to a reduced target density (≈0.22 mol/cm2 La versus 1 mol/cm2 Sn) andsmaller cross-sections the yields of 100+xCd are lower than from molten Sn targets, seeFig. 2. For 98,99Cd they are comparable, but the most short-lived isotope 97Cd, where thefaster release should give the biggest advantage, was not observed in this test [15]. Withan efficient and selective identification method (e.g. detection of βp) the RILIS could beused to perform atomic spectroscopy directly in-source [4], i.e. to measure magnetic mo-ments and nuclear charge radii of the very neutron-deficient Cd isotopes 97+xCd aroundthe N = 50 shell closure.Neutron-rich Cd isotopes up to 130Cd had already been separated at the ISOLDESC with a UCx/graphite cloth target and a plasma ion source [16]. Some chemical selec-tivity was provided by a quartz transfer line at 600 ◦C, suppressing part of the indiumand cesium isobars but leaving (on mass 130) still a molecular beam of 40Ca90Br+ asbackground. To detect the heavier Cd isotopes with still shorter lifetimes the decay losseshave to be minimized by keeping the full target and ion source unit at a sufficiently hightemperature. Using the chemical selectivity of the RILIS, the short-lived neutron activitiesof the isotopes 130−132Cd could thus be identified [17].Fig. 2 shows the yields of Cd isotopes obtained with the RILIS at ISOLDE PSB.These yields were measured with four different targets, fluctuating3) by up to an order ofmagnitude, probably due to a varying on-line ionization efficiency. Using a “neutron con-verter” [18], the yields are lower than in direct bombardment, but the isobaric backgroundof neutron-deficient Cs is suppressed considerably.The RILIS provides now beams free of molecular contaminations but leaves mostof the In and Cs isobars in the beam. The ultimate beam purity could be obtained bycombining a chemically selective transfer line (suppressing Cs and In) with the hot cavityRILIS (suppressing most molecules).5 TerbiumNeutron-deficient Tb isotopes are abundantly produced by spallation of a Ta tar-get and moderately well surface-ionized. The 4.1 h isomer (1/2+) of 149Tb has a 17 %branching ratio for alpha decay [19] and is therefore considered for radiotherapeutic ap-plications [20]. A recent in vivo experiment at the University Hospital of Geneva using149Tb marked antibodies gave very promising results for the treatment of lymphoma [21].To obtain sufficient quantities of 149Tb for therapeutic purposes any possible increase inyield is important. Therefore we tested on-line the prospects of the RILIS for Tb ion-ization. We used an ionization scheme which had been developed in Troitsk [22]: threevisible transitions (579.6 nm, 551.7 nm and 618.3 nm) leading to an autoionizing state.Due to the presence of many surface ionized isobars and oxides in the lanthanide region2) This run was performed with 1.0 GeV protons, all other yields shown in this paper were measuredwith 1.4 GeV protons.3) Also the fitted release times of Cd fluctuate by up to a factor two when comparing the different targetsat different “age”. Table 1 shows an average.31010 210 310 410 510 610 710 810 9102 104 106 108 110 112 114 116 118 120 122 124 126 128 130 132MassYield or in-target prod. (atoms/µC)Cd yieldsCd in-target productionFigure 2: Ion yields of cadmium isotopes. For 115,117Cd the 11/2− isomers are more abun-dantly produced than the 1/2+ ground state.it is difficult to obtain a clean Tb signal by simple counting of atoms or beta-decays. Twocollections at mass 149, one with surface ionization (“laser off”) and one with additionaluse of the RILIS (“laser on”) were performed and analysed by γ-spectroscopy. The latterincreased the 149Tb yield from 1.2 · 107/µC to 2.1 · 107/µC. This 70 % increase is muchsmaller than expected4), indicating that a dominant part of the off-line measured Tb isindeed produced indirectly as decay daughter of 149Dy+ and 149Ho+. Hence, using theRILIS to ionize rather Dy than Tb could give a much higher cumulative 149Tb yield. Asuitable RILIS scheme for dysprosium still needs to be developed.6 YtterbiumYb was the first element tested off- and on-line at the ISOLDE RILIS [1]. Laterneutron-deficient Yb isotopes down to 153Yb were ionized and used for in-source atomicspectroscopy with the RILIS of the IRIS facility in Gatchina [23]. Here we report theyields obtained at ISOLDE with a standard Ta foil target and W ionizer5). With theRILIS the Yb yields were enhanced by a factor of about 20 against surface ionization.The now measured on-line efficiency was probably below the off-line measured 15 %[1]. Note that the W ionizer is “optimized” to create isobaric background from otherlanthanides and lanthanide oxides. Hence only the most abundant isotopes (down to155Yb) could be clearly identified by measurements using a 4piβ detector, Faraday cup,“laser on/laser off” and half-life checks, respectively, see Fig. 3. Before irradiation of thetarget no contamination with stable Yb was observed, i.e. all shown yields are radiogenic.168Yb shows a significantly higher yield than the neighboring isotopes6). This shows that 6minute 168Lu which accumulates from the decay of the refractory (and thus not released)higher-Z isobars is not significantly released before decaying towards 168Yb. Thus therelease time of lutetium is under these target conditions well above 1000 s! From ourmeasurement we cannot exclude that also Yb has an additional very slow (>100 s) releasecomponent.4) Also the absolute yields of Tb were very low since target and ion source were rather cold during thisexperiment.5) This particular target had generally low yields. Thus the given yields can be regarded as a lower limitfor standard conditions.6) The dip at 164,165Yb may just be due to a fluctuation of the laser power during the measurement.410 410 510 610 710 810 9156 158 160 162 164 166 168 170 172 174 176MassYield (atoms/µC)Yb yieldsFigure 3: Ion yields of ytterbium isotopes from a Ta foil target.7 ThalliumTl is moderately well surface ionized and clean Tl beams were already available atISOLDE SC. Off-line it was shown that the RILIS allows to increase the Tl ionizationefficiency by about a factor 20 compared to surface ionization in a 2200 ◦C hot W cavity.In the on-line experiment the laser power was lower, but the surface ionization efficiencyin the 2100 ◦C hot Nb cavity was also reduced, providing again a selectivity of about 20.Fig. 4 shows the measured yields of neutron-deficient Tl isotopes from a W surfaceionizer and the RILIS respectively. In cases of several detected isomers the sum yield isshown. The yields of 179−181Tl were estimated from the observed alpha emission rate andassumed branching ratios for α decay of 80 % (179gTl), 10 % (180Tl) and 20 % (181gTl).For comparison are shown the yields of surface ionized Tl isotopes which were obtained atthe IRIS facility by 1.0 GeV proton induced spallation of a special 10 g/cm2 ThCx target(produced by pyrolysis of a thorium diphtalocyanine precursor) coupled to a W/Re surfaceionizer [24, 25].8 Lead and BismuthPb and Bi beams, ionized with an unselective hot plasma ion source, were availablebefore at ISOLDE [7]. However, the admixture of many isobars (Hg, Tl, Po, At, etc.)made them difficult to use for nuclear spectroscopy and other applications. The use ofthe RILIS allows now to provide rather pure beams of Pb and Bi, the remaining isobariccontaminations being the surface ionized Tl and Fr. The former is mostly on an acceptablelevel, but the latter is intense enough to be disturbing for most applications at A > 206.Only for masses 215 to 218 the Fr isotopes are so short-lived that this background can besuppressed completely by a delayed opening of the electrostatic beamgate [26].In cases of several detected7) isomers the sum yield is shown, see Fig. 4. In mostmeasured cases the yield of the high-spin isomer was higher. The yields of 188−192Bi weredetermined from the alpha decay rates, assuming ≈100 % branching ratios for α decay of188g,188m,189g,189mBi. Note that the laser power in the Bi run was lower than in the formeroff-line experiment.The RILIS can also be used to produce isomerically pure beams by tuning the7) Not always could all isomers be detected. This causes the fluctuations between neighboring isotopes,as apparent in the plotted yield curve.5wavelength of the resonantly exciting laser to the hyperfine structure of one or anotherisomer [27]. For the heavier elements Tl, Pb and Bi the Doppler broadening is rathersmall compared to the hyperfine splitting and this technique works particularly well. Itwas recently used to perform nuclear and atomic spectroscopy on two isomers in 185Pb[28].11010 210 310 410 510 610 710 810 9178 180 182 184 186 188 190 192 194 196 198 200 202 204MassYield or in-target prod. (atoms/µC)Tl yieldsTl in-target prod.Tl yields IRISPb yieldsPb in-target prod.Bi yieldsBi in-target prod.Figure 4: Ion yields and in-target production rates of neutron-deficient thallium, lead andbismuth isotopes. The yields of 215Pb, 215mBi and 218Bi (not plotted) were 0.3, 1400 and44 per µC respectively.9 SummaryThis article summarizes release functions of eight different elements and the yieldsof over hundred isotopes which were measured with the ISOLDE RILIS in the last years.Detailed data of the individual experiments are shown in table 1.Table 1: Overview of target characteristics, operation conditions, release parameters andestimated ionization efficiencies during the different on-line runs. Note the uncertaintyin the determination of the ionization efficiency: about a factor two. The UCx/graphitetargets contain about 10 g/cm2 of carbon in addition to the ≈50 g/cm2 238U.Ele- Ion. Target Ionizer Ion. Releasement pot. eff. α tr tf tseV g/cm2 ◦C ◦C % ms ms msMg 7.65 UCx/gr. 50 2020 W 2200 10 0.98 66 190 860Al 5.99 UCx/gr. 49 2070 W 2250 1–10 (0) (100) – 22000Cd 8.99 UCx/gr. 50 ≈2000 Nb ≈2000 10 0.98 ≈60 ≈300 ≈3000Tb 5.86 Ta foil 42 2200 W 1900 ? – – – –Yb 6.25 Ta foil 42 2150 W 2000 ≤15 0.91 (300) 100 35000Tl 6.11 UCx/gr. 50 2000 Nb 2100 5 0.98 45 600 5700Pb 7.42 UCx/gr. 50 2050 Nb 2100 3 0.96 70 580 4700Bi 7.29 UCx/gr. 50 2050 Nb 2100 1 0.86 45 540 80006AcknowledgementsThese yield measurements profited from support by the EU-RTD projects EURISOL(HPRI-CT-1999-50001) and TARGISOL (HPRI-CT-2001-50033) and by the BMBF undergrant 05 KK1TSA/7.References[1] V.I. Mishin et al., Nucl. Instr. Meth. B 73 (1993) 550–560.[2] V.N. Fedoseyev et al., Hyp. Int. 127 (2000) 409–416.[3] U. Ko¨ster, Nucl. Phys. A 701 (2002) 441–451.[4] V.N. Fedoseyev et al., Nucl. Instr. Meth. B, these proceedings.[5] J. Lettry et al., Nucl. Instr. Meth. B 126 (1997) 130–134.[6] U.C. Bergmann, Nucl. Instr. Meth. B, these proceedings.[7] E. Hagebø et al., Nucl. Instr. Meth. B 70 (1992) 165–174.[8] S. Sundell et al., Nucl. Instr. Meth. B 70 (1992) 160–164.[9] D. Lunney et al., Phys. Rev. C 64 (2001) 054311.[10] D. Lunney, CSNSM Orsay, priv. comm.[11] S. Nummela et al., Phys. Rev. C 63 (2001) 044316.[12] R. Kirchner, Nucl. Instr. Meth. B 126 (1997) 135–140.[13] P.G. Hansen et al., Phys. Lett. 28B (1969) 415–419.[14] J. Lettry et al., Nucl. Instr. Meth. B 126 (1997) 170–175.[15] U. Ko¨ster, Ph.D. thesis, TU Mu¨nchen (2000).[16] K.L. Kratz et al., Z. Phys. A 325 (1986) 489–490.[17] M. Hannawald et al., Phys. Rev. C 62 (2000) 054301.[18] R. Catherall et al., Nucl. Instr. Meth. B, these proceedings.[19] G. Audi et al., Nucl. Phys. A 624 (1997) 1–124.[20] G.J. Beyer, Hyp. Int. 129 (2000) 529–553.[21] G.J. Beyer et al., to be published.[22] V.N. Fedoseyev et al., J. Phys. B 24 (1991) 1575–1583.[23] A.E. Barzakh et al., Proc. of ENAM-01 (2002) in press.[24] V.A. Bolshakov et al., Nucl. Instr. Meth. B 70 (1992) 69–74.[25] A.E. Barzakh et al., Nucl. Instr. Meth. B 126 (1997) 150–153.[26] P. Van Duppen et al., Nucl. Instr. Meth. B 134 (1998) 267–270.[27] U. Ko¨ster et al., Nucl. Instr. Meth. B 160 (2000) 528–535.[28] A.N. Andreyev et al., Eur. Phys. J. A 14 (2002) 63–75.7

On-line yields obtained with the ISOLDE RILIS

On-line yields obtained with the ISOLDE RILIS

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