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Home , Isotopes of platinum

EUROPEAN 0EGAN1sAT10N EOE NUELEAE RESEARCH

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COMPLIS : QOllinear Spectroscopy Measurements using a Pulsed Laser Ion Source

Orsaym, Mainzs, Montreal4, Pariss, Lyons, GSI7 Collaboration

J. Arianerl, N. Bo0s3, F. Buchinger‘*, J. Crawford‘*, L. Davey4, I. Delonclel, H.T. Duong2·, G. Huber3, P. Juncars, P. Kilcherl, M. Krieg3, T. Kiihl7, F. LeBlanc1, J .K.P. Lee4, J. Libertl, R. Neugarts J. Obertl, J. Omsl, M. Pellerin6, J. Pinardz, J.C. Putauxl, P. Quentinl, B. Roussierel, J. Sauvagel and J. L. Vialles

Spokesman: G. Huber Contactperson: C. Serre

SUMMARY

A pulsed laser spectroscopy experiment is proposed for the study of hyperfine structure and shift of refractory and daughter elements from ISOLDE beams. It includes de celerated ion—implantation, laser desorption, element-sensitive laser ionization, magnetic and time—of-flight mass separation. The laser spectroscopy will be performed in a set-up at the PSB Isolde allowing high resolution collinear spectroscopy with the secondary pulsed ion beam. In the first step very neutron-deficient Pt will be studied to search for a second shape transition around A=18O and to investigate the parabolic baseline in the isotope shift, well documented in lighter elements only. Neighbouring refractory elements as well as isotopes and their heavy neighbours should be investigated. OCR Output

2/&»<452 Introduction

In order to test the usual nuclear models very far from beta-stability, we want to get in formation about the static behaviour of nuclear matter, especially the equilibrium shape of exotic nuclei. Shape instability and isomerism have already been found below Z=82 around the mid neutron shell in Hg [1], Tl [2], Pb [3], and more recently in Au [4], and Pt [5,6] (Fig. 1). In the case shape transition has been found which has a rather weak signature in 5 < rz > but it is strongly supported by a detailed nuclear spectroscopy ( ·y —- cy and conversion electrons ), magnetic and quadrupole moments. There are hints from nuclear spectroscopy for a second shape transition around A=180: Systematics of the energy of the rotational states built on the even- even plati num ground states show a sudden change in the moment of inertia associated with the shape transition at A=186. A similar change can be deduced from in-beam experiments [7], which suggests a second shape transition around A=180. Lattice Hartree-Fock +BCS calculations for an axially asymmetric solution [8] have been performed for even—even platinum isotopes. They reproduce the general trend of the ex perimental 5 < rf >; they predict an oblate shape for the heavier isotopes, a prolate for the lighter ones. However the shape transition is predicted 3 masses too high and the flat part of the 5 < rg > curve is not reproduced in these calculations. The potential energy surfaces are found to be very soft close to N=110. From this it is expected that dynamical effects, which have not been taken into account, play an important role. Moreover the exact locationof the shape transition strongly depends on the pairing force. In order to explain this specific behaviour of the 5 < rf > curve of the platinum isotopes, calculations are in progress which take into account dynamical effects according to the microscopic Bohr-Harnilton description of ref.

In the region Zf 50 a rather smooth change from the spherical tin isotopes to the transitio nal isotopes is seen. The isotope shift for the elements in the tin region is governed by a clear parabolic slope with a superimposed odd-even staggering [10]. A long chain from 1°4'13°In has been studied but there are incomplete or low precision data in silver and . No results are available for Z=51 (). These isotope chains could be reached by direct production and by decay from ISOLDE beams from fission targets. Similar parabolic slopes between neutron-magic numbers are known at Z=20 and Z=28. But at Z§82 the isotope shift shows an opposite bend in the region of shape instability. Pt is the element which should allow us to follow the charge radii to extremely neutron deficient isotopes since the cx decay of very light Hg isotopes is an effective production channel.

Aim of the Experiment

The existing data from experiments at ISOLDE and ISOCELE should be extended by the proposed experiments to lighter platinum isotopes with A§182. There are two special questions to be answered for these isotopes: Is there a second shape transition around OCR Output Mean square nuclear charge radii

Sn (Z=50) Pb (2:2,2) T1 (Z=a1)

In (Z=4g) Cd (Z=4B) 2 .. .. I

Hg (Z=80) Au (Z=79) Pt (Z=78) O.5 fm Ag (2:47) Ir (2:77) Os (Z=76)

50 80 70 80 90 100 110 120 130 Neutron number

Figure 1: Summary of the bf < 1*2 > results near the shell closures at Z=50 and Z=82 using an arbitrary relative position of the isotope chain in order to provide a clear figure. The straight lines belong to the predictions of the droplet model. A shape transition from oblate or more likely triaxial towards prolate has clearly been seen for Hg ( between A=l87 and A.=185 ), Au ( between A=].87 and A=186 ), Pt ( between A=187 and A=l85).

A=180 ? Do the very neutron-deficient isotopes follow the parabolic baseline of the isotope shift as seen in the lighter elements? The general question concerns data from daughter elements of ISOLDE beams.

For this purpose laser experiments with good spectroscopic resolution, especially on the odd isotopes of platinum, are needed in order to determine the static nuclear moments from the hyperfine structure. We propose to combine the pulsed laser spectroscopy of the so far tested set-up at ISOLDE III with a collinear geometry in order to obtain good efficiency and high resolution.

We have built a beam line with an implantation target at the ISOLDE III separator running at full acceleration voltage. Daughter products from the implanted ISOLDE beams are prepared as pulsed mass-separated ion beams by laser desorption and ioniza tion. Up to now this scheme has been used for Doppler-reduced pulsed laser spectroscopy (PILIS) [6,11], of Au and Pt isotopes at the ISOCELE (Orsay/France) mass-separator. In the first test runs on line at ISOLDE, the system efficiency was of the order of 10' for "°Pt obtained by the decay of 188Hg, deceleration to 600eV before implantation. A new set-up prepared for the Booster-ISOLDE will allow this pulsed secondary beam to be sent to a collinear section for high resolution laser spectroscopy experiments. The ion-optical calculations by J .Arianer have shown that the phase space requirements for OCR Output HN. oYE LASER Fox LONIZATION GRAPHITE STDPPER uv DYE LASER SELEcT1vE 1** STEP

MAGNET mu DETECTOR

CHARGE ¤EELEcT¤R- EXCHANGE

ISGLDE

u v DYE LAS ER HIGH RES. 1=TSTEP DESORPTION LASER 0YE LASERS Fun ro NI z A Tro N

Figure 2: Scheme of the experiment. The upper laser system is used for desorption and first ionization. The lower system with high resolution is used to ionize the neutral fast beam in the collinear section.

collinear experiments with this secondary ion beam are fulfilled. Such experiments are also well adapted to measure nuclear quadrupole moments in light Au isotopes which are essential for complete characterization of the nuclear shape. Furthermore, when this collinear spectroscopy set-up is combined with a dedicated pulsed ion source, either a laser ion source or a more sophisticated one as briefly described below, the overall system efficiency will be increa.sed and thus will allow us to study the isotopes further away from stability than those previously investigated. A In the new scheme of a. pulsed ion source under study, the implantation target is a solid gas film prepared in situ. The ions, once implanted, should retain their charge in the sub strate; they are then directly desorbed as ions. The losses due to conversion of implanted ions to thermal atoms and reionization will be drastically reduced. This cryogenic target is being tested in off-line experiments.

Experimental Set—up

The beam from ISOLDE is deflected and implanted after deceleration on a graphite stopper. By laser desorption and laser ionization the implants are transformed into short ion pulses with low repetition rate.

For the survey experiments laser spectroscopy can be performed in the laser ionization OCR Output 0f the desorbcd beam ( PILIS ) and the photo-ions are than detected with a, channel plate detector. The D0ppler—limited resolution may be sufHcient for extension of even Pt isotope shifts. But generally the collinear section is needed for pulsed ionization spectro scopy ( COMPLIS ) of the secondary pulsed beam prepared with laser desorption and ionization of daughter elements from implanted ISOLDE beams. In the collinear section, the short ion pulses are neutralized in the charge exchange cell, and with a second laser system reionized with high resolution. The complex laser insta.llation will be simplified by a. solid Xe target replacing the graphite stopper in Fig. 2. In this cryogenic target under development the implanted ions should remain singly charged [12] and can then be released by laser desorption.

Our sensitivity estimates for the COMPLIS set-up are better than 10"3 transmission for the laser desorption and maximum laser ionization. The ionization yield in collinear geometry can be pushed to 50 %, whereas the resonant charge exchange of Pt beams in a Cd vapour has been observed with 50 % yield, so that the total sensitivity should allow to work with production yields of 105 / s.

Technical Requirements at the PSB ISOLDE

We need space in the experimental hall as given in the plans of K.Elsner and, in ad dition, two rooms in the light buildings for the laser set-up and the electronics part of the experiment. We need energy and cooling for a 40 kW magnet used for the mass selection and a similar power connection for the cw laser system included into the pulsed laser set-up.

Beam Time Requests

We plan three runs totalling 10 shifts for Hg production to investigate light even Pt isotopes. An amount of 15 shifts is requested for and the odd platinum isotopes.

References

[1] G.U1m et al. Z. Phys. A325 (1986) 247 E.W. Otten, in: Treatise on Heavy-Ion Science (ed.: D.A. Bromley), Vol. 8, (1989) 515 [2} J.A.Bounds et al., Phys. Rev. C36 (1987) 2560 and R.Menges et al., submitted to Z. Phys. [3] M.Anse1ment et al., Nucl. Phys. A451 (1986) 471 U.Dinger et al., Z. Phys. A328 (1988) 253 [4] U.Kr6nert et al., Z. Phys. A331 (1988) 521 OCR Output [5] .I.K.P.I.ee et al., Phys. Rev. C38 (1988) 2985 H.'I`.Duong et al., Phys. Lett. B217 (1989) 401 T.Hi1berath et al., Z. Phys. A332 (1989) 107 [ 2 G.D. Dracoulis et al., J.Phys G12 (1986) L97 [8] P.Bonche et al., Nucl. Phys. A 500 (1989) 308. [9] I.De1onc1e et al., Phys. Lett 233 B (1989) 16. [10] G.Hubcr, Proc. of the 5th Int. Conf. on ’Nuclei far from Stabi1ity’, Rosseau Lake, Ontario, Canada, 1987, AIP Conf. Proc. 164, (ed.: I.S. Towner), 1988, p.105 .I.K.P.Lee et al., Nucl. Instr. Meth. B34 (1988) 252 [ S;} N.Kakahashi et al., Proc of the Int. Conf. Struct. through static and dynamic moments (ed.: H.H.Bolotin), Melbourne (1987) p.334