Revista Mexicana de Física 42, Suplemento 1 (1996) 144-151

Mass measurement of the doubly-magic nucleus lOOSn

A. LÉPINE-SZILy2), G. AUGER1), W. MITTIG1), M. CIJARTIER1),J .M. CASANDJIAN1), M. CIJABERT1), L.K. FIFIELD3), J. FERMÉI),

A. GILLIBERT4), M. LEWITOWICZI), M. MAC CORMICKI), M.H. MOSCATELL01),O.H. ODLAND1,5), N.A. ORR6), E. PLAGNOL7),

G. POLlTlB), C. SPITAELS1) AND A.C.C. VILLARI1)

1) GANIL, BP 5027, 14021 Caen Cedex, France2) IFUSP-Universidade de Sao Paulo, C.P. 66318, 05389-970 Sao Paulo, Brasil

3) Dep. oJ Nuclear Physics, Australian National University, GPO Box 4, Canberra,ACT 2601, Australia

4) CEA/DSM/DAPNIA/SPhN, CEN Saclay, 91191 GiJ-sur- Yvette, France5) Universitet i Bergen, Fysisk Institut, Alle9aten 55, 5007 Bergen, Norway

6) LPC-ISMRA, Bid du Maréchal Juin, 14050 Caen, Cedex, France7) IPN Orsay, BP 1, 91406 Orsay Cedex, France

8) Universitti di Catania, Dip. di Fisica, Corso Italia 57, 95125 Catania, Italy

ABSTRACT. The mass oí the doubly-magic nucleus lOOSnwas measured using the second cyc1otronof GANIL as a high resolution mass spectrometer. The fusion-evaporation reaction 50Cr+58Ni wasused to produce the A = 100 secondary ions at 5.3 MeV/nucleon incident energy. Ions of 100Ag,100Cd, 100In and 100Sn were produced and accelerated simultaneously and the masses of the threelatter isobars were measured with respect to the abundantly produced 100Ag. The total observedyield of accelerated IOOSnions was about 10 counts. The known mass of lOoCd was very wellreproduced and the masses of 100Inand IOOSnwere determined for the 6rst time with precisionsof 3 x 10-6 and 10-5 respectively. Our results do agree well with recent shell model calculationsbut disagree from extrapolations by systernatics.

RESUMEN. Se midió la masa del núcleo doblemente mágico IOOSnutilizando el segundo ciclotrónde GANIL como espectrómetro de masas ne alta resolución. Se utilizó la reacción de fusión-evaporación 50Cr + 58Ni para producir iones secundarios con A=100 y energía incidente de 5.3MeV /nucleón. Se produjeron y aceleraron simultáneamente iones de 100Ag, lOOCd, 100In y lOOSn,midiendo las masas de los últimos tres isóbaros respecto de la abundante producción de 100Ag. Laproducción total de iones acelerados de IOOSnfue de 10 cuentas. Se reprodujo muy bien la masaconocida de lOoCd y se determinaron por primera vez las masas de lOoIn y 100Sn con precisionesde 3 x 10-6 y 10-5, respectivamente. Nuestros resultados concuerdan bien con cálculos de modelode capas recientes, pero discrepan con extrapolaciones de sistemática.

PACS: 21.10.Dr; 27.60.+j

The longly searched doubly-magic nucleus lOOSnwas successfully produced in two recentand independent experiments employing the projcctile fragmcntation technique: at GSIwith a I.I GeV /nuc!con 124Xe bcam [11 and at GANIL using a 63 MeV /nuc!ean 112Sn

144

MASS MEASUREMENT OF THE DOUBLY-MAGlC NUCLEUS lOOSN 145

beam [2). Maoy attempts have beeo performed before to produce this oucleus, due to itsioterest for ouclear structure studies. The maio reasoo is its N = Z character at the doubleshell closure, with protons aod oeutroos occupyiog the same high Iyiog shell-modeí orbitsaod thus providiog ioformatioo about their ioteractioo and about the characteristics ofshell-closure oear the protoo drip-lioe. It is also the heaviest N = Z doubly-magic oucleus,stable agaiost grouod state protoo decay, since 164pb is expected to lie far beyood theprotoo drip lineo The mass of a oewly produced nucleus is the first and most fuodameotalquaotity since it provides ioformatioo 00 its ouclear binding and structure.

The high precisioo mass measuremeot of ouclei far from stability represeots ao exper-imeotal challenge. The first precision measuremeots were performed on radioactive alkaliisotopes produced by ISOL (Isotope Separation oo-lioe) techniques, with high-precisionconventional mass spectrometers mainly at ISOLDE/CERN ami at TASCC/Chalk RiverLaboratories. Receotly the use of a Penoing trap [3,4] associated with the ISOLDE/CERNmass separator allowed to increase eveo higher the precision and attaio a mass resolutiooof 10-7. In the reduced volume of the Peooiog trap, in a very homogeneous aod stablemagoetic field, the cyclotron frequeocy of the low energy exotic ouclei can be measuredwith very high precisioo, thus providing direct ioformation on their masses. However, forthe moment this technique is limited to alkali and alkali earth ioos, due to ionizatiooproperties of their atoms, and the half lives are also restricted to tens of seconds at mioi-mum d ue to time needed to cooling, bunching and confinement of the ioos io the Peooingtrap.

Masses of projectile-fragmeots produced in high energy fragmentation reactioos havebeen measured using the direct time-of-/light techniques developed [5,6) mainly with thehigh precision magoetic spectrometers SPEG [7] at GANIL and TOFI [8] at LAMPF /LosAlamos. The mass resolutioo achieved with these systems is limited by the length of the/ly path (less thao 100 m) to ~ 3 X 10-4. This resolutioo is iosufficieot to measure themass of lOOSnwith available countrates.

The path length can be substautially increased when the ions follow a spiral path aodthis consideration has 1Il0tivated us to develope a method usiug the second cyclotron ofGANIL (CSS2) as a high precision spectrometer. The mass resolution obtained with thesimultaoeous acceleration of A/q = 3 light ions (6He, 9Li) was shown 19, lO] to be less then10-6

\Ve have recently performed the ll1easurement of the lIlass of radioactive ions of A = 100,produced via the fusion-evaporatioo reaction sOCr + s8Ni, using this cyclotron technique.The A = 100 nuclei produced in this reaction were all accelerated silllultaneously sincetheir mass differeoces were smaller then the acceptance of the cyclotron (3 x 10-4). Usingthe mass of 100Ag as a reference, the masses of lOOCd, lOOInami lOOSnco,lld be determinedwith precision of 2 x 10-6, 3 x 10-6 and 10-5, respectively.

Details ofthe lIlethod applied to light ions ha,'e been published [9,10]. GANIL consists ofa system of two identical separated-sector cyclotrons (CSSl and CSS2), where the beamis injected and accelerated after delivery from an ECR ion-soun:e and preaccelerationin a small compact cyclotron CO. In standard use a stripper is located between the twocyclotrons lo iOllb~(~ami illject lile beam for further acc('lprat ion ill CSS2. In OHr mcthod tlle

stripper was sn!>stitnted by a production target, where the s('condary nudei are producedto be then iujected aud accelerated iu CSS2 (s('e Fig. 1).

146 A. LÉPIl'E-SZILY ET AL.

The mean magnetie induetion B, the radio-freqlleney applied to the eavities f (w =27rn, the harmonie h (number of radio-frequeney periods/turn) are related to the mass-eharge ratio m/q, the orbital radius P and the velocity v by

B m Bp-=1-=-w/h q v'

(1)

where 1is the relativistie factor.Considering that the radio-frequeney of the 3 GANIL eyelotrons (CO, CSSI and CSS2)

is the same, and that the harmonic of CSS1 is hl = 5, and the ratio between the injeetionradius of CSS2 and the ejeetion radius of CSSI is p21 PI = 2/5:

=VI h2 '

(2)

where h2 is the CSS2 harmonie. As harmonies are integer numbers, the ratio v2lvI =2/3,1/2, 2/5, etc, eonstitute a set of permitted solutions. The experimental set-up is shownon Fig. 1. Two ions injeeted into CSS2 with slightly dilferent masses m and m + Óm willhave dilferent time-of-f1ights during their aeeeleration inside CSS2, the heavier mass willarrive 8t la ter. To first order,

8tt

Óm

m(3)

This relation allows the determination of the unknown mass m +óm from the well knownreferenee mass m if the number of turns NT or the total time-of-f1ight t is known. If theyare not known, the ealibration can still be aehieved if more than one referenee mass issimultaneously aeeelerated with the unknown masses, or can be obtained by variation ofthe magnetie field and/or frequeney [9,101.

The first GANIL eyelotron aeeelerated the sOCr beam to an ineident energy of 5.3 Mev /nueleon and the fusion-evaporation reaetion sOCr + SBNiwas used to produce radioaetivenuelei of A = 100. The target was used simultaneously to produce the seeondary ions andto reduce their velocity ratio to 2/5. Due to eooling and rotation of the target, relativelyintense primary beam (i = 300-500 nAe) eould be used.

The seeondary ions injeeted and aeeelerated in CSS2 are also deteeted inside the ey-elotron in a silieon detector teleseope (2.E35 IIm, Exy300 ¡un) with a 2 mm radialdead zone. In preliminary tests [11] we have verified that the intense magnetie fieldsdo not alfeet the performance of the silieon deteetors. The teleseope was mounted ona radial probe whieh can be moved from the injeetion radius 1.25 m up to the extrae-tion radius 3.0 m. The time-of-flight of the detected ions was measured between thesignal given by the silieon detector and the radio-fre'lueney RF signal of the CSS2 ey-elotron.

The injeetion into CSS2 and its magnetic field B were tuned with the primary SOCrll+beam degraded to the same velocity ratio in a 22 mg/em2 Ta target. To conserve the sameseuine:s in the transnort. 1ine and in th(' CSS2. t.hf' A=10n ~f'r()nrl;Hvion~Wi1'rfl ~plprtprl in

MASSMEASUREMENTOF THE DOUBLY-MAGlCNUCLEUSlOOSN 147

CSS2cyclotron

50Crbeam(5.3 MeVlnucleon)

CSSlcyclotron

Si/iean detector te/escapemounted on a radial movableprobe

FIGURE 1. Schemalic diagram of lhe experimenlal sel-up.

neo L

CYclotronP

the 22+ charge state. After the corrections for isochronism and tuning, the phase is con-stant with radius (isochronism) and the individual orbits are perfectly separated [14,131.\Vhen the tuning was completed, the target was changed from Ta to n"Ni, in order toproduce the A = 100 ions and proceeded to their injection and acceleration in the chargestate q=22+ without any change in the transport line, in the injection, or in CSS2 set-tings. The A = 100 ions arrived in the silicon telescope when it was localized at the radiusR = 2970 mm, ehosen for data acquisition. \Ve show in Table I the relative m/ q ratios ofthe di£ferent nuelides of A = 100 and q = 22+, all calculated with respect to 100Ag22+ [seeEq. (4)]:

Ó(m/q)m/q

m/q(~xq+) - m/q(IOO Ag22+)m/q(IOOAg22+)

(4)

In Fig. 2 we present the actual total energy vs. phase spectrum. The IOOSn22+ions areexpected at the phase -10 degrees and the lighter nuelei are detected with smaller time offlight (at larger phase channels). Figure 3 shows the simulation of the total energy vs. phasespectrum of the radioactive ions of A = 100 and q = 22+, that could be obtained if theradial position of the detector would be varied over a range of several orbits. At the centralphase, due to isochronism, the phase of the partieles does not change when the radialposition and thus the energy is increased. The "banana" like figures are the interceptionsof the detection system with the increasing orbits (with their phase dispersion). Notice thatincreasing phase channels correspond to decreasing time of flight and smaller masses inFigs. 2 and 3. Thc lighter ions are not any more isochronolls, thcir time of Aight decreasesas their energy increases and several of their orbits can be simultaneously intercepted inthe detection system at a fixed radial position. On the experimental total energy vs. phase

148 A. LÉPINE-SZILYET AL.

TABLEI. Differential m/q ratios calculated with respect to 100Ag22+with the atomic mass excessesand their uncertainties, the asterisks indicate the mass values obtained from extrapolations [12J.\Ve can check that the difference betwccn the m/q is smaIl enough to allow the simultaneousacceleration of nuelei between Ag and Sn.

M.E. (MeV) [121

-78.180:!: 0.080

-74.31O:!: 0.100

-63.130 •• :!: 0.380

-56.860 •• :!: 0.430

~xq+ A

100Ag 100IOOCd 100IOOln 10010°50 100

~~....•~::::

"'"'-'~'"i:'~~-~,::

270

265

260

255

250•25.

z47484950

.15 .

q

22222222

.5. s. 15.

b(m/q)m/q

O.OOOOOOOO:!:0.00000086

0.00004148:!: 0.00000107

0.00015092:!: 0.00000408

0.00022912:!: 0.00000462

", "

;~{~'.~:'.:.

25. 35. 45.Phase (degrees)

FIGURE 2. Experimental spectrum in which the the several adjacent orbits intercepted for themost produced ions (loOAg22+and 100Cd22+)are c1early visible.

spectrum shown in Fig. 2 we can effectively see the interceptions of several orbits of the100Ag and lOoed nuclides with the detector.

The large counting rate of the 100Ag22+ ions produces a background even at the centralphase, difficulting the observation of the IOOSn22+ions. \Ve used the identification paralll'eter j(Z) presented iu Fig. 4 in order to clean the euergy spectrulll. By gating the totalenergy vs. phase spectrum by different cuts iu the identification spectrum for each uucliJe(see Fig. 4), we couid clean the background of 100Ag aud get a clear signature for thepresence of 10-12 events of 100Sn at the expected phase and Z value. Iu Fig. 5 we showthe total energy vs. phase spectrulll obtaineJ for two s!ices in the identificatiou spectrum"j(Z) vs. phase", one for the lOoln22+ ions, the other for the IOOSn22+ions. \Ve can clearlysee that in the In spectrum no background or taH of 100Ag is visible around the phase-10 degrees. The background, calculated scaliug the relative intensities in the In aud iuthe Sn spectra, was estimated to be inferior to 1 count. So the eveuts around the phase-10 degrees are due to the presence of IOOSU22+ions.

MASSMEASUREMENTOF THE nOUBLY-MAGICNUCLEUS100SN 149

4.2

o SR(; InO Cd'!:r Ag

detector

4.15-25. -15. .5. 5. 15. 25. 35. 45.

Phase (degrees)

FIGURE 3. Results oí simulation for the simultaneous acceleration oí 100Ag22+. looCd22+, lOo1n22+and 100Sn22+ The detector intercepts severa! adjacent orbits (black symbols).

15.5.-15.205

-25.

~ 265'"'-~:::

" Sn""""::::245 JnN<::::;

Cd

225Ag

25. 35. 45.Phase (degrees)

FIGURE4. Identification parameter I(Z) vs. ph¡¡.<;espectrnm. where I(Z) is proportionalto Z andw¡¡.<;calculated from (;E and E parameters. The different slices corresponding to different atomicnumbers Z are also shown.

Measuring the phase differences of the different ions with respect to the 100Ag22+ andmeasuring the number of turos in the cyclotron, we could determine the following massexcesses:

M.E.¡I°OCd) = -74.180:I: 0.200(syst.) MeV,

M.E.(1001n) = -64.650:I: 0.100(stal.):I: 0.300(syst.) MeV,

M.E.(IOOSn) = -57.770:I: 0.900(stal.):I: 0.300(syst.) MeV.

(5)

(6)

(7)

150 A. LÉI'INE-SZILY ET AL.

(e)

(d)

ill' '

I

.)5. .5. :5. 15. 25. 35. 4:5.

Phase (degrees)

1'0 ~ 270..:::: (a) .::¡~ ""16:5 "" '" 265 -~ ...,-::: '"'~'""o ~Z60'" 160 -~ "-"~ tii- 255 - 255 -~ S.~ ~•....

250 I250

.25. .l5. .5. 5. 15. 25. 35. ". .25.

Phase (degrees)

~,o' ::¡ ,o'

(110~(b) "eS ~10=

P . 10

L'10 I

.25. .1:5. .:5. S. 15. 25. JS. 45.

Phase (degrees).25. .15. .5. S. 15. 25. JS. 4:5.

Phase (degrees)

FIGURE 5. a) and e) "Total energy vs. phase" speetra gated by cut s in the identifieation funetion¡(Z) for IOOln22+and l°05n22+ respeetively. The arrow indieates the loeation of the lOo5n22+counts. b) and d) Projeetion of the speetra on the phase axis.

These masses are to be eompared with the values presented in the Audi- \Vapstra masstable 112] (Tabl~ 1).

Our mass of lOoCd agrees wen with the previous experimental result [15) and this faetgives good eonfidenee in our new results for lOoln ami lOo5n.

When eomparing these new results with reeent shen model ealculations of Johnstone and5kouras [l6], and The Audi-\Vapstra lIlass tables, a good agreement is observed betweenan masses for nuelei eloser to the stability line where previous experimental results wereavailable. The ealculations and our experimental values agree wen also for nuelei furtherfrom the stability line where no previous experimental data existed eooln and lOo5n) butthe Audi- \Vapstra systematies diverge from both, predieting lower binding energies for thenuelei approaehing the double shen elosure.

\Ve have shown that the method of using the C552 eyc1otron as a high preeision spee-trometer works wen also for heavy A = 100 seeondary ions. The results on the masses of100Cd, lOo1n lOo5n are very eneouraging, we reprodueed with a very good preeision theknown mass exeess of lOoCd, improved eonsiderably the precision on the mass of lOoln [12)and measured for the first time the lIlass of lOo5n with a preeision of 10-5. The identifi.eation of the lOo5n was aehieved for the first time by fusion-evaporation reaetion.

MASS MEASUREMENT OF TIIE DOUBLY-MAGIC NUCLEUS lOOSN 151

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