Z. Phys. A 358, 261–265 (1997) ZEITSCHRIFTFUR PHYSIK Ac© Springer-Verlag 1997

Study of nuclei far from stability with the AYE-Ball arrayM. P. Carpenter

Argonne National Laboratory, Argonne, IL 60439, USA

Received: 25 September 1996Communicated by B. Herskind

Abstract. The coupling of a Compton-suppressed Ge (CsGe)detector array to a recoil mass separator (RMS) has seen lim-ited use in the past due to the low efficiency for measuringrecoil-γ ray coincidences (< 0.1%). With the building of newgeneration recoil separators and gamma-ray arrays, a substan-tial increase in detection efficiency has been achieved. Thisallows for the opportunity to measure excited states in nucleiwith cross-sections approaching 100 nb. In this paper, resultsfrom the coupling of a modest array of CsGe detectors (AYE-Ball) with a recoil separator (FMA) will be presented.

PACS: 27.70.+q; 23.20.Lv; 23.60.+e; 25.70.Gh

1 Introduction

The wide use of Compton-Suppressed Ge (CSGe) detector ar-rays for nuclear structure studies began in the 1980’s. Thesearrays held between 10 and 20 detectors and were used initiallyto measure high-spin states in deformed nuclei. Presently, alarge fraction of the measurements performed with currentgeneration CsGe arrays are directed at the study of superde-formed (SD) nuclei. These new spectrometers (Gammasphere,Eurogam and Gasp) are ideally suited for these types of mea-surements, and there are a still number of open questions in-volving SD nuclei which require further attention. However,the large segmentation and efficiency of these new arrays alsoallow for the study of excited states in nuclei situated in re-gions of the chart of nuclides which were unaccessible usingprevious generation gamma-ray spectrometers.

Several of these areas lie on the neutron-rich side of sta-bility. While a systematic study of nuclei in this region willhave to wait for neutron-rich radioactive beams, certain massregions are accessible forγ-ray studies using a number ofdifferent techniques. For example, gamma-ray spectrometershave been used for a number of years to measureγ rays emit-ted from fission fragments [1]. These studies have provided awealth of information on the structure of neutron-rich nuclei.The contribution to this conference by P. Daly [2] illustrateshow this technique coupled with the improved sensitivity ofEurogam has allowed for the identification of excited states

in neutron-rich nuclei around doubly magic132Sn. Anotherrecent investigation has examined the feasibility of measur-ing high-spin states in neutron-rich nuclei with Gammaspherevia deep inelastic reactions [3]. Other studies have begun toCoulomb excite neutron-rich radioactive beams produced viaa multi-fragmentation reaction. Due to the Doppler broaden-ing of theγ transitions emitted from these fast moving ions(v/c∼0.3), high-resolution measurements are not possible. Asa result, these experiments have used arrays of NaI detectorsto measure the 2+ − 0+ transition in nuclei around Z=16 andN=24 [4, 5].

On the proton-rich side of stability, the large CSGe gamma-ray arrays allow for the opportunity to measure excited statesin nuclei at and beyond the proton-drip line. Since the cross-sections for producing the most proton-rich residues via fusion-evaporation reactions are small and compete with channelswith much larger cross-sections, nuclide identification be-comes essential for associatingγ rays to specific isotopes.

Two techniques to identify residues are currently in use forthese types of reactions. The first involves the measurementof evaporated particles,i.e. neutron, protons, andα particles,in coincidence withγ rays. The number and type of particlesmeasured give some degree of nuclide identification. The con-tribution by H. Grawe [6] illustrates how this method has beenused to identify for the first time states in nuclei near doublymagic100Sn.

The second technique measures the mass of the residue incoincidence with its emittedγ rays. The following sectionswill illustrate how this technique has been used to measureexcited states in nuclei near the proton-drip line by couplinga modest gamma-ray array (AYE-Ball) to a new generationrecoil separator (FMA).

2 The Aye-ball array

Recoil mass separators have a long history of use, however, thecoupling of a gamma-ray array to such device has been limited[7]. Some of the pioneering work in this area was performedat Daresbury where the CsGe detector array Tessa was placedat the target position of the Daresbury Recoil Mass Separator[8].

262

(FMA)Fig. 1.Schematic diagram of the Fragment Mass Analyzer (FMA)

In the 1990’s, several new recoil mass separators havecome on line. One such device is the Fragment Mass Ana-lyzer (FMA) which is installed on a beam-line of the ATLASaccelerator at Argonne National Laboratory [9]. The device isan 8.2-meter-long mass spectrometer which separates reactionproducts produced in a heavy-ion fusion reaction and dispersesthen by Mass/Charge (M/Q) at the focal plane. Figure 1 is aschematic diagram of the FMA and shows the configuration ofthe two electric dipoles and magnetic bending magnet whichconstitute the mass spectrometer. The FMA has an energyacceptance of±20% and a M/Q acceptance of±4%.

A number of experiments have been performed with theFMA coupled to an array of 10 Compton-suppressed Ge de-tectors (see for example [10]). This configuration correspondsto a 2 fold increase in efficiency for measuring recoil-γ coin-cidences over that of the Daresbury RMS when coupled to theTessa array.

In order to improve the recoil-γ efficiency at the FMA, alarger array of Compton-suppressed Ge detectors was placedat the FMA-target position in August of 1995. This array,named the Argonne-Yale-European (AYE) Ball, was equippedwith a support structure which could hold up to 25 CsGe detec-tors. Fifteen of the slots were configured for Eurogam phase Idetectors, and the remaining 10 slots were configured to holdGe detectors in Tessa-like suppression shields. Figure 2 showsa CAD drawing of the support frame indicating where the Eu-rogam and Tessa shields were placed relative to the beam-lineand FMA. This support frame was designed at Daresbury lab-oratory and fabricated at Edinburgh University. For the ex-periments performed with AYE-ball, nine large Ge detectors(∼ 70%) were available; seven from Eurogam, and one eachfrom Argonne and Yale. Ten small (∼ 25%) co-axial detec-tors were available for placement into Tessa-like shields, andthese detectors came from Daresbury, Yale and Manchester.When fully loaded with detectors, the array had a photo-peakefficiency of 1.1% for 1 MeVγ rays, giving a factor of eightincrease in efficiency for detecting recoil-γ coincidences overthat of the coupling of Tessa to the Daresbury RMS.

A number of other detectors were available for use withthis setup. At the target position, two LEPS counters fromYale were at times placed in the array, and evaporated neu-trons were detected by a ring of 11 NE213 liquid scintillatorcounters mounted on the quadrupole immediately in front ofthe FMA. A variety of detectors were used in conjunction withthe PPAC focal plane detector including a split anode ioniza-tion chamber, a 48x48 double-sided silicon-strip detector anda 50% Ge detector.

Thirteen experiments were performed with the array overa three month period involving forty scientists from 13 coun-tries. Nearly all the experiments were directed at nuclei at or

To FMABeam

Eurogam

Tessa/Yale

Fig. 2.Diagram of the support frame for AYE-Ball

near the proton-drip line and covered a mass range from A=24to A=226.

3 N=Z nuclei

The N=Z nuclei between Z=28 and Z=50 closely follow theproton-drip line, and offer a particularly interesting laboratoryfor nuclear structure studies. In this region, the single-particlespectrum shows large gaps as a function of deformation. TheN=Z symmetry reinforces these gaps resulting in predictionsof large shape changes and shape coexistence in these nuclei.Experiments carried out at the Daresbury RMS measured the2+−0+ transitions in the N=Z nuclei of this region up to84Mo[11], showing for example that80

40Zr is highly deformed in itsground state [8]. Other issues to address which are relatedto the N=Z symmetry include measuring and quantifying thedecline of isospin purity [12] and determining the importanceof T=0 and 1 n-p pairing [13].

Four of the experiments performed with AYE-Ball weredirected at the study of N=Z nuclei. Three were led by groupsfrom Surrey and Daresbury utilizing the reactions: (a)54Fe +36Ar at 120 MeV; (b)58Ni + 36Ar at 120 MeV; and (c)40Ca +24Mg at 65 MeV in order to identify for the first time excitedstates in the N=Z nuclei88Ru, 92Pd and62Ga, respectively.The fourth experiment was led by C.J. Lister at Argonne andattempted to confirm and extend the level structure of68Se us-ing the reaction12C + 58Ni at 200 MeV. One other experimenton nuclei in this region was led by D. Seweryniak (ANL) andattempted to identify states in103Sn.

For all these experiments both the neutron detectors andionization chamber were in place. The ionization chamberprovided Z-identification to the mass identified ions. Whilethe data from these experiments are still under analysis, anexample of the Z-selectivity is given in Fig. 3 where it is shownthat Z=30 (61Zn) lines from the 2pn channel can be cleanlyseparated from the dominating 3p lines associated with61Cuin the A=61 gated spectrum.

263

Fig. 3. Spectrum showing the Z discrimination from the ionization chamberfor A=61 recoils in the24Mg+40Ca experiment. The61

30Zn (2pn) lines can beclearly separated from the dominant61

29Cu (3p) lines

4 Recoil decay tagging experiments

Above the closed shell at Z=50, many nuclei near the proton-drip line decay by the emission of anα particle. Beyond theproton-drip line, odd-Z nuclei are observed to decay via theemission of a proton, and the identification of new proton emit-ters using the FMA has been a major focus of study at ATLASover the last several years (see for example [14]). A num-ber of experiments with AYE-Ball were performed on Z> 50nuclei which are near or at the proton-drip line. For theseexperiments, nuclide identification ofγ rays was made usingthe Recoil-Decay Tagging (RDT) method. This technique cor-relates the characteristic charged-particle radioactivity of anion implanted in a pixel of a double-sided silicon strip detec-tor (DSSD) with a previously implanted recoil allowing forisotopic identification of the mass separated residue and itsassociated promptγ rays. The method was first employed atGSI using an array of NaI detectors and the SHIP velocityfilter [15]. However, it was not until the technique was usedwith a CSGe array that the promise of the method to measureexcited states in nuclei at the proton-drip was demonstrated[16].

One of the experiments performed with AYE-ball was on156Hf [17], an N=84 nucleus which lies very near the proton-drip line. In this experiment, excited states in156Hf were pop-ulated with the102Pd(58Ni,2p2n) reaction at 270 MeV. Thecharged-particle decay spectrum was measured with a 48x48double-sided silicon strip detector placed 40 cm behind theFMA focal plane detector. Prior to this experiment, the onlyidentified excited state in156Hf was an isomeric level at 1959keV. Both the isomer and the ground state decay predomi-nantly byα emission. In addition, the excited state is knownto be populated after theβ decay of the (πh11/2 ⊗ νf7/2})9+

156Hf

(keV)

(keV)

(a)

(b)

(c)

(d)

0.1

0.01

Fig. 4.aGamma-ray spectrum in coincidence with A=156 residues.b Gamma-rays in coincidence with the ground-stateα decay of156Hf. c Gamma-raysin coincidence with theα decay of the isomer in156Hf. d Charged particledecay spectrum measured in the DSSD

state in156Ta suggesting the (νh9/2 ⊗ νf7/2)8+ configurationfor the isomeric state [19].

Figure 4a shows theγ-ray spectrum in coincidence withA=156 residues. Nearly all of the strongest transitions areassociated with the 4p channel156Yb. Figure 4d shows thecharged particle decay spectrum measured in the DSSD, andtheα energies associated with the decay of the ground state(5878 keV) and the isomeric state (7804 keV) in156Hf aremarked in the spectrum. These decay lines confirm that156Hfis produced in this reaction, however, these lines represent lessthan 3% of allα decays measured in the DSSD.

In order to determine whether any of theγ rays observedin the A=156 gated spectrum belong to156Hf, γ-ray spectratagged by the two156Hf α lines were created. These spectraare plotted in Figs. 4b and c. The spectrum correlated with theground-state decay shows only threeγ transitions of equal in-tensity. Conversely, the spectrum correlated with theα decayof the isomer shows many moreγ rays with varying intensi-ties. None of theγ transitions observed in either spectrum areresolvable in the spectrum gated by A=156 (Fig. 4a), thus illus-trating the power of the RDT technique in associatingγ rayswith weakly populated residues. The estimated cross-sectionfor producing156Hf is 100µb.

The angular distributions of the threeγ rays correlated withthe ground-stateα decay are consistent with E2 multipolarityand establishes the spin and parity of the levels connectedby these transitions. As Fig. 5 indicates, the 6+ level lies 44

264

1/2T = 0.52 ms

1/2T = 23 ms

Hf156

1587

2003

858

0

1959

2760

3681

4268

Fig. 5.Preliminary level structure for156Hf

keV higher in energy than the 8+ isomer, resulting in a spininversion of the 6+ and 8+ levels. The lowering of the 8+ state(νf7/2⊗ νh9/2) with respect to the 6+ (νf2

7/2) state arises fromthe closing of the energy gap between the f7/2 and h9/2 orbitals.In the lighter N=84 isotones, the systematic lowering of the 8+

level has been associated with the strong attractive interactionbetweenh11/2 proton and h9/2 neutrons coupled to I=1.

Many moreγ rays are observed in the spectrum correlatedwith the decay of the 8+ isomer, and these must correspondto transitions between states which lie above the isomer. Un-fortunately due to the low efficiency for detectingγ-γ-recoilcoincidences, only the strongest transitions could be placed inthe level scheme as indicated in Fig. 5.

Another region examined with AYE-Ball concentratedon Hg isotopes withN < 100. For Hg isotopes betweenN=100 and 108, shape coexistence has been established closeto the ground state where rotational bands built on weakly-deformed oblate (β2 ∼ −0.15) and moderately deformedprolate (β2 ∼ 0.25) shapes are observed. In addition, a recentpaper presented results from Nilsson-Strutinsky calculations[20] which predict the existence of strongly deformed oblatestates withβ2 ∼ −0.35 at excitation energies around 2.5 MeVfor N ≤ 104 for Pb isotopes as well as excited SD minimumwith β2 = 0.5−0.56 for Hg and Pb isotopes withN ≤ 98. TheSD minima are calculated to be between 3.5 and 5 MeV abovethe ground state which is comparable to the excitation energycalculated for the SD well in Hg-Pb nuclei withN ≥ 110.

With these past results and recent calculation in mind,a series of measurements were performed with AYE-ball toidentify for the first timeγ transitions in176−179Hg using the78Kr(103Rh,pxn) reaction at beam energies of 340, 360 and380 MeV [21]. The assignment ofγ-ray transitions to a par-ticular nuclide was made using the RDT method describedabove. As an example, Fig. 6 shows theγ-ray spectrum pro-

Fig. 6.RDT gated spectra for A=178

duced when tagging on theα decays of178Pt,178Au and178Hg.The transitions correlated with the decay of178Pt agree withthose previously measured. The four transitions associatedwith 178Hg are presumably members of the yrast rotationalband with the sequence 397, 355, 455, 559 keV defining the8+ → 6+ → 4+ → 2+ → 0+ cascade. The ordering in basedon the intensity of transitions determined from the RDT gatedspectrum. Theγ-γ-recoil data confirm that these four transi-tions are in coincidence with each other, however, the lack ofstatistics makes it difficult to confirm the proposed sequence.In 176Hg, only oneγ ray (615 keV) was observed to be corre-lated with the ground-stateα decay of176Hg, and thisγ ray isassigned to the 2+ − 0+ transition in176Hg.

Figure 7 plots the excitation energies of the 2+ and 4+

states in a series of even-even Hg isotopes centered aroundN=102. The open circles correspond to levels newly identifiedwith AYE-Ball. The minimum in excitation energy observed atN=102 is not due to an increase in deformation of the ground-state, rather it results from the crossing of the excited prolateband with the oblate ground state configuration. Two trendscan be inferred from the newest data. In178Hg, the prolate banddoes not cross the oblate band until I=6. This also implies thatthe 2+ levels in 176,178Hg are not influenced by the prolateband, and thus the continual increase in excitation energy ofthis level suggests an evolution from an oblate deformed tospherical ground state as N decreases.

Several other RDT measurements were performed withAYE-Ball. For example,γ-ray transitions correlated with theproton decay of the ground state and an isomeric state in147Tmhave been measured [17], as well asγ rays correlated with theα decay of200Ra [22].

265

174 176 178 180 182 184 186 188Mass

0

200

400

600

800

1000

Exc

itatio

n E

nerg

y (k

eV)

2+-0

+

4+-2

+

100 102 104 1069896

N

Hg80 2

4

+

+

Fig. 7. Systematics of 2+ and 4+ excitation energies in Hg nuclei aroundN=102. Theopen circlescorrespond to states measured with AYE-Ball

5 Summary

This contribution has described results from experiments usinga modest array of CsGe detectors coupled to a recoil massspectrometer. These experiments have shown the viability ofmeasuring excited states in nuclei at and beyond the proton-drip line. The minimum cross-sections measured were on theorder of 1µb. Gammasphere, a 110 CSGe detector array, willoperate at ATLAS in 1998. The coupling of this device to theFMA will increase theγ-recoil sensitivity down to 100 nb andshould allow for a mapping out of excited states in nuclei atthe proton-drip line.

Many people were involved in the planning, installation, operating and dis-mantling of AYE-Ball. Preparations for the implementation of the array atArgonne National Laboratory (ANL) were initiated by W. Gelletly (Surrey).

Thanks are extended to the EUROGAM collaboration for lending the Eu-rogam I CSGe spectrometers to the project as well as Daresbury Laboratory,Manchester University and Yale University for the use of the Tessa-like CSGedetectors. Acknowledgements are extended to D. Warner (Daresbury), S. Met-calfe (Daresbury), J. Simpson (Daresbury) and P. Woods (Edinburgh) for theirrole in the design and construction of the AYE-Ball support structure, C.J.Lister (ANL) who was the leader of the project at Argonne, and C.N. Davids(ANL) who oversees the operation of the FMA at ATLAS. Other membersof the collaboration who helped setup, maintain and disassemble the devicewere D. Blumenthal (ANL), D. Seweryniak (ANL/Maryland), S. Freeman(Manchester), P. Regan (Surrey), J. Schwartz (Yale), B. Field (Oberlin), D.Nisius (Purdue), W. Mueller (Univ. of TN), H. Amro (N.C. State), D. Acker-mann (ANL), S. Fischer (ANL), D. Henderson (ANL), R. Janssens (ANL) andT. Lauritsen (ANL). This research is supported by the U.S. Dept. of Energyunder contract No. W-31-109-ENG-38.

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