Behr, J

Billowes, J

Campbell, P

CERN Document Server

Qféigndirectly with a bismuth beam. OCR Output2mSmaller samples of 211,mBi can be made in the same way and Bi can be collectedfrom a thermal oven in an off-line gas cell for a laser resonance fluorescence measurement.bismuth isotopes by sequential 2-cy-emission. The bismuth will subsequently be releasedcatching intense beams of 22°Fr and 2"Fr on thin lead foils. The francium decays to thethe small samples available. The bismuth samples of 212Bi and 213Bi will be prepared bySUNY, Stony Brook to the neutron deficient bismuth isotopes and it makes efficient use ofreleased into a gas cell trap. This is a method that has been applied very successfully atLaser resonance fluorescence will be used on the isotopic bismuth samples as they arecore.the first measurement of the odd proton hg/2 moment in the region above the closed 2°8Pbcharge radii change with neutron number seen in the Pb isotopic chain, and will representthe shell closure. This will provide new information on the systematics of the kink in thebe deduced. These will be the first isotones of lead to be measured immediately aboveshell closure. The change in nuclear mean square charge radii and static moments canhyperfine structures of bismuth isotopes in the region 21°'2“Bi which lie above the N=126The aim of the proposed experiment is to measure the optical isotope shifts andSummaryContactman: G. HuberSpokesman: J. BillowesP.D. Wheelera, F. Xu‘* and the ISOLDE Collaboration]G. Huber], M. Keimz, J. Kilgallon3, A. Kleinz, R. Neugart2, M. Neurothz, G.D. Sprouse‘*,J. Behr4, J. Billowes3, P. Campbell3, T.G. Cooperg, U. Georg2, I.S. Grant3, G. Gwinner4,CERN1 — Mainz? — Manchester3 — Stony Brook4 CollaborationLaser Spectroscopy of Neutron Rich Bismuth Isotopes9 Le-?GEM} »- emit 1=·1>.o1=·osAL TO THE rsotos COMMITTEEscPP60Q/pm lK, O CERN/ISC 94-7Q DCD [L]SC00000267new lllllllllllIllllllllllllllllllllllllllllllllCERN LIBRARIES, GENEVA2HBi isotope. OCR Outputposition during the end of the experiment, we will have the ability to measure the 2.17 minof 21°Bi is longer than the beam time requested) and, by moving the gas cell to an on-linelarge sample of 21°Bi using a bismuth beam with extended collection times (the half-lifeHeavier francium isotopes do not 04-decay. It should be possible to acquire a sufficientlydepend critically on the oe—decay branching ratio of 222Fr which lies between 1% and 0.01%.are shown in figure 1. We will attempt to extend the measurement to 214Bi but it will2]3the proposal. The expected positions of 2"Bi and Bi in the charge radii systematicsThe measurement of these two isotopes is straightforward and is the primary goal ofto establish the change in gradient at the kink and an odd-even staggering parameter.The laser spectroscopy of these isotopes in an off-line measurement will be sufficienteasily and quickly at ISOLDE. These are products in the decay chains of 22°Fr and 2Fr.21Relatively large samples of two isotopes of bismuth, 212Bi and 213'Bi, can be preparedeffects and hence the re-normalization of spin and orbital g factors.hg/2 moments in the same region. This provides information on first order core polarizationThe proposed experiment will also represent the first measurement of the odd protonhave been measured (above N=132) are prolate deformed.isotopes are near-spherical throughout this region, whereas the Rn and Fr isotopes thatbetween Pb and Bi below the shell is repeated above the shell. Both the Pb and BiFurthermore, it will be interesting to determine whether the apparently exact agreementthe region above N=126, measurements are presently only feasible on the bismuth chain.ing the nature of the kink but, because of the short half-lives of most of the isotones inComparisons between the lead and other isotopic chains are important in understandfor predictions of nuclear properties further from the line of stability.it becomes important to understand why the differences arise since this has implicationsthis, the apparent failure of the Skyrme mean field approach is not fully understood butthe relativistic mean field theory can reproduce the trend very successfully. In the light ofdescription of this behaviour has been elusive but a recent publication[2] has now shownfamiliar feature in isotope chains at all other neutron magic numbers. A good theoreticalclosure than below and this produces the kink in the lead chain at N=l26 and it is aThe increase in charge radius as neutrons are added is more rapid above the shell»— to develop modest oblate deformations[l].start to deviate from these systematics below N:121 and this is where they are expectedthe case for the Tl isotones which have a single proton hole in the Pb core). Rn and Frbehaviour in nuclear size and the odd-even staggering is identical (intriguingly this is notBi and Po isotopes are expected to to be almost spherical and, within the errors, theirhave been internally normalized to 6(r2}m·12° = -1. Below the N=l26 shell the Pb,Z2 82 in the region of the N=l26 shell are shown in figure 1 where the isotope chainsgenerally hindered by the very short half-lives in this region. The available data fortron shell are known for the lead isotopes alone. Measurements of other isotones areChanges in the nuclear mean square charge radii immediately above the N=l26 neuThe entire sample is evaporated from the oven in 5-10 minutes. This time is so short that OCR Outputa lens and filter system and detected by a single-photon counting photomultiplier tube.some milliseconds actually in the laser beam. The fluorescent light is collected throughtake typically 1 second to diffuse from the oven to the walls of the cell and could spendrepeatedly scanned across the 306.7 nm resonance region. A single bismuth atom wouldby passing an electric current through it. As the oven is heated the laser frequency isoven inside the cell. The oven is made of a tantalum tube and is heated to about 400°C5 mBar. A 1 mW beam of narrow bandwidth UV laser light passes close to the end of theactivity is not removed from its container. The cell is evacuated and filled with argon toremoved from the ISOLDE beam line and inserted into the off-line gas cell, so the collectedtube mounted on a flange that becomes the oven of an off-line gas cell. The entire unit iswould take a maximum of one half-life. The catcher foil is contained within a tantalumto the ground states of 2"Bi (T1/2 :1 hour) or 213Bi (45.6 mins). Each implantationure 2. Within a few minutes most of the francium has decayed by sequential 2-04-emissioncaught on thin self-supporting foils of Pb. The decay schemes of 22°·221Fr are shown in figwould proceed in the following way. Beams of francium from the separator would bedirectly into the laser interaction volume of the gas cell. The experiment at ISOLDEpressure at modest temperatures (400°C) and can be evaporated from a lead-catcher foiltained on the nuclear parameters of size and static moments. Bismuth has a good vapourDoppler and pressure broadenings inherent in the technique and high precision is obThe bismuth isotope shifts and hyperfine structures are large compared with theexpect from a vapour sample (with no repeated excitation).of the hyperfine structure are present with the same relative intensities that one wouldquickly in the buffer gas. There is little evidence of optical pumping and all componentsunderstand) is that the metastable state populated by the 472 nm transition relaxes verybuffer gas can be filtered out. A very beneficial property of the system (that we do not yetfeature of the system is that the huge background of Rayleigh scattered UV light from theproduced by intra-cavity frequency doubling with a lithium iodate crystal. An importantas the laser is tuned across the 306.7 nm (483/2——"P1/2) transition, the UV light beingThe fluorescence is observed on the 472 nm line (a 6% branch from the excited state)measure extremely small samples. The atomic levels of bismuth are shown in figure 3.then a high fluorescence rate per atom can be achieved, providing sufficient signal toatom can be repeatedly cycled by resonance—excitation and spontaneous photon emissionlaser interaction time compared to collinear-beams or crossed-beams techniques. If ancell filled with an inert buffer gas acts as an atom trap which greatly extends the atomfrom the daughter products of intensely produced elements such as francium. Here aideally suited to the gas cell method[3] which can be used even when the sample comesany radioisotope measurement. However, it was discovered that the bismuth atom wasof laser spectroscopy and, even at ISOLDE, the ion beam fluxes would be too low forshown that the bismuth atomic system is poorly suited to the collinear-beams methodOff-line tests on the isotope separator at the Daresbury Nuclear Structure Facility havefor laser beam delivery. We require a beam line on ISOLDE for repeated foil implantations OCR OutputThe gas cell will be brought to ISOLDE and mounted off-line in a position convenient5. Apparatus to be usedNone.4. Technical, financial and manpower requestsisotopes with suflicient yield in any heavy-ion reaction on a tandem accelerator.the neutron—rich side of the line of stability. It is simply not possible to produce thesein a reaction with a tandem accelerator beam and, critically (iii) we will have access to(ii) the samples will be at least two orders of magnitude larger than could be producedThe ISOLDE facility offers tremendous advantages: the samples will be mono-isotopicshown in figure 4. The work is being extended to even lighter isotopes at Stony Brook.in a gas cell. Sample sizes were typically 109*10 atoms and examples of the spectra areallowing the vapour to condense inside a tantalum oven which was subsequently insertedbeams of 75-87 MeV HB. The bismuth was released from the gold by baking at 1000°C andshown in figure l. The samples were produced by the bombardment of gold foils withstructures of the neutron deficient 2°2·2°3*2°4Bi at SUNY, Stony Brook[4]. The results areWe have recently used the same gas cell method to measure isotope shifts and hyperfine3. Status of researchin the oven. This is destroyed by the evaporation.evaporation procedure. Before each sample is collected, a new lead catcher foil is insertedwe will be using or is trapped on the walls of the gas cell which is sealed during theThroughout the experiment all the activity accumulates in one of the several ovensto become an on-line cell, reducing the time between collection and evaporation.For the short-lived 2HBi the gas cell will simply connect to the ISOLDE beam line flangesamples can be collected while the laser spectroscopy is carried out on earlier samples.cover the whole hyperfine structure. The advantage of using the gas cell off-line is thatthese isotopes. A number of samples must be successfully measured for each isotope tosizes of z 109 atoms could be obtained in about 1 second from the isotope separator forpossible since the lineshapes are understood and can be fitted very well. Minimum sampleif components were to overlap with the stable hyperiine structure then deconvolution iswill actually have more radioactive 2"Bi or 213Bi material than the stable 2°9Bi. Evennents will probably be fully separated from the natural signal. The ISOLDE samplesprovides the perfect local frequency calibration. The 2"Bi and 213Bi hyperfine compoknow hyperfine structure of stable 2°°Bi (which is inevitably present in the catcher foil)shown in figure 4) so for most of the time the laser is actually on resonance. The preciselyit is best to restrict the laser scan to cover only one set of the hyperfine components (asManchester University Nuclear Physics Report 1994. OCR Output[4}[3} G.D. Sprouse et al., Phys. Rev. Lett. 63 (1989) 1463.[2] M.M. Sharma, G.A. Lalazissis and P. Ring, Phys. Lett. B317 (1993) 9.and W.J. Swiatecki, (1993)Nuclear Ground—State Masses and Deformations P. Moller, J .R. Nix, W.D. MyerslllReferencesapparatus and laser beam delivery.shifts for 21°·2“Bi which includes the mounting of the gas cell on-line and re-arranging the3Bi since that will not require continuous production of samples. So we estimate 5212·21but we may be able to collect this sample during the earlier laser spectroscopy work onbeam flux of 105 s’1 it would take 14 hours to produce a useful sample of 5 >< 109 atoms2 shifts. The 2l°Bi measurement is merely a problem of collection time. For an ISOLDEone hour whether it is worthwhile to pursue a full 214Bi measurement which may then takealso includes the preparation work with the ISOLDE beam. We will be able to assess incell. Thus we estimate that 212·2l3Bi measurements would be completed in 4-5 shifts whichdelay measurements for some hours to reduce the background due to radioactivity in theand laser conditions right simultaneously. At some point it might also be necessary tocell and doing the laser spectroscopy. It may take several attempts to get the evaporationbeams of 220*221Fr are so intense that we are limited by the speed of preparing the 0H-lineWe estimate that 12 shifts will be sufficient for the complete set of measurements. The6. Beam time requestIS 304.in the laser room, and counting electronics and computers will also be taken mainly fromto the existing beam line. Additional equipment for calibrating laser scans can be placedfrequency-doubled version. Therefore it would be convenient to put the apparatus closeThe cw lasers of the collinear laser spectroscopy experiment IS 304 can be used in thethen accept either the oven and catcher foil unit or the gas cell itself for on-line running.and removals. We will fit an isolation valve and end-flange on the beam line which willFigure 4: Fits for 209Bi (upper plot) and the residual 2°3Bi (lower)Frequency (GHz from 209 centroid)-5o 00 5p“ *“‘;_%O m I . . gn; " • .·“ . ' I O.! n | || •._•»,•·| /1 000200030004000500060007000Frequency (GHz from 209 centrold)10 2015e ...e e..•*"`°·;·..4"*'L’.10*10°Figure 3: Atomic level scheme and hyperiine structure of 306.7 nm line.shown belowScans of this regionole4 S Q; guiggssdxxmuum luserpumplng \ V ’_.»‘ U};Nbjmn202472.2nml=5203F-1+*;I: 9, 2MJ ZM-9;l=6F-ry 2Rh}; OCR OutputOCR Output.. 9 I- 2

Laser spectroscopy of neutron rich bismuth isotopes

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