The decays of ${}^{119m}$Cs and ${}^{119g}$Cs to ${}^{119}$Xe have been studied on mass separated samples, using $\gamma\rm{-ray}$ and internal conversion electron measurements. Several new low-lying levels have been established in the ${}^{119}$Xe level scheme. Half-life evaluations for ${}^{119m}$Cs and ${}^{119g}$Cs have been revisited. The results are compared with other experimental data known in light odd-mass xenon isotopes and with calculations performed in the frame of the multi-shell interacting boson-fermion model

Bucurescu, D

Genevey, J

Gizon, A

Mantica, P F

Marguier, G

Walters, W B

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN{EP/2001{02012 January 2001Low-lying levels in 119XeJ. Genevey, A. GizonInstitut des Sciences Nucl¶eaires, IN2P3 -CNRS/ Universit¶e Joseph FourierF-38026 Grenoble Cedex, FranceP. F. Mantica, W. B. WaltersDepartment of Chemistry and Biochemistry, University of Maryland,College Park, MD 20742, USAG. MarguierInstitut de Physique Nucl¶eaire IN2P3 -CNRS/ Universit¶e Claude Bernard Lyon-1F- 69622 Villeurbanne, FranceD. BucurescuHoria Hulubei National Institute of Physics and Nuclear IngineeringR- 76900 Bucharest, Romaniaand the ISOLDE collaborationCERN, CH 1211 Geneva 23, SwitzerlandABSTRACTThe decays of 119mCs and 119gCs to 119Xe have been studied on mass separatedsamples, using °¡ray and internal conversion electron measurements. Several newlow-lying levels have been established in the 119Xe level scheme. Half-life evaluationsfor 119mCs and 119gCs have been revisited. The results are compared with otherexperimental data known in light odd-mass xenon isotopes and with calculationsperformed in the frame of the multi-shell interacting boson-fermion model.(PACS numbers : 23.20.Lv; 23.20.Nx; 27.60.+j.)1 INTRODUCTIONThe present study is part of a program developed at the ISOLDE facility to inves-tigate the fl¡decay schemes of light cesium isotopes [1], [2]. The experiments have beenfocused on the 119Xe low-lying level scheme populated in the fl=EC decays of 119mCs and119gCs for which a few results have been previously reported [3]. This approach is verysimilar to the one already applied for the 121m;gCs to 121Xe beta decays [4].Spin and parity of the 119Xe ground-state are deflnitely known to be I… = 5=2+ [5].A flrst partial level scheme for the lowest excited states fed by beta decay in 119Xe hasbeen reported to stand a comparison with the 117Xe isotope [6]. From in-beam studiesof 119Xe a wealth of data is available at intermediate - and high-spin states [7], [8], [9],[10]. Negative-parity band structures having similar characteristics and based on a ”h11=2quasineutron conflguration have been reported by all works. The situation is signiflcantlymore complex for the low-spin positive-parity states. A comparison of the band structuresdeveloped in 119Xe with those recently established in the neighbouring isotope 121Xe [11],[12] shows clearly a similar behaviour for the ¢ = 1 bands built on the ”d5=2[402]5=2+ground-state of the two nuclei.Additional experiments are needed to flrmly establish the spin, parity assignmentsand deexcitation modes of the other low-lying states. Comparing the 121Xe low-lyingpositive-parity states fed in the 121m;gCs beta-decays [4] and those with spin-parity I… •11=2+ populated in the 109Ag(16O; p3n) reaction (bands 9 ¡ 10 and 11 ¡ 12 in [12]) oneobserves an interesting agreement. Keeping this similarities in mind, a special efiort hasbeen made in the present 119m;gCs beta-decay study to establish the spin and parityassignments of the low-lying states fed in 119Xe and to combine them with those proposedfrom recent in-beam °¡ray spectroscopy studies [9], [10].In section 2 of this paper the experimental procedures and the results are reported.The level scheme is established in section 3 and the fl¡feedings in section 4. The discussionof the new results is presented in the last part, including comparisons with other experi-mental data and with calculations performed in the frame of the interacting boson-fermionmodel (IBFM).2 EXPERIMENTAL PROCEDURES AND RESULTSThe present study of the fl=EC decay of 119m;gCs to levels in 119Xe was completedin part at the ISOLDE facility at CERN and at the UNISOR isotope separator at OakRidge National Laboratory (ORNL).In the studies at ISOLDE, cesium isotopes were produced in a thick molten lan-thanum target bombarbed by the 600 MeV proton beam available at CERN. The ex-perimental set-up was similar to the one described in a previous paper on the 121m;gCsdecay [4]. Separated radioactive samples were collected during various collecting times inthe 1 to 30 seconds range, and sequentially transported to the counting station by anautomatic tape driver system. The counting times were typically of 30 seconds. Singles°¡ray spectra and time-dependent multiscaling spectra were recorded with an intrinsicGe detector having a resolution of 800 eV at 122 keV for the low-energy °¡rays and withseveral other Ge(Li) coaxial detectors (15% e–ciency and R … 2:5 keV at 1 MeV).Conversion electron spectra were detected with a movable 3 mm thick Si(Li) de-tector having a resolution of 1.6 keV FWHM at 624 keV and placed in the vacuum,nearthe collected samples. Three parameter °¡°¡t and °¡e¡ t coincidence events have beenrecorded. For fl+¡° coincidence measurements, a typical arrangement including a 4…fl1plastic scintillator and a Ge(Li) detector has been used. This disposal is of the same typeas the one used at the TRISTAN mass separator [13].In the studies at UNISOR, samples of 119Cs were produced in the heavy-ion reactionbetween a 6 mg=cm2 92Mo target, which made the window of the FEBIAD-B2 ion source[14], and a 175 MeV 32S beam provided by the 25 MV Tandem accelerator at the HolifleldHeavy Ion Research Facility at ORNL. The evaporation products from the heavy-ionreaction recoiled in the center of the ion source, where they were ionized to a singly-positive charge and extracted towards the mass separator. The recoil ions were then massseparated, and the 119Cs products were deposited into a moving tape, which transportedthe activity to two counting stations with a cycle time of 40 seconds. The flrst countingstation (G1) consisted of two °¡ray detectors : a Ge detector with a full energy rangeset to 1:8 MeV at a distance of 4:4 cm from the source and a Ge(Li) detector, whichwas placed 4:7 cm from the source position and had a full energy range of 4:0 MeV: ASi(Li) electron detector was also placed at the flrst detection station (G1) at a distanceof 12 cm from the source. The e¡ detector was equipped with a mini-orange fllter, whichwas placed between the source and the detector, leaving a distance of 4:5 cm between thedetector and the fllter. This placement of the mini-orange fllter allowed for a peak in thee–ciency curve for the electron detector at an energy of 750 keV:The positions of these three detectors at the flrst counting station (G1) were suchthat the two °¡ray detectors were at 90–, and the electron detector was at 180– relativeto one of the Ge detector.A Ge detector and a Si(Li) electron detector were placed at the second countingstation (G3). The Ge detector was placed 4:6 cm from the source position and had afull energy acceptance of 6:0 MeV: The electron detector was placed at 180– to the Gedetector at a distance of 7:0 cm from the source position.This e¡ detector was also served by a mini-orange fllter, which was placed 1:5 cmfrom the face of the detector and resulted in a peak e–ciency curve for this detector atan energy of 250 keV:Singles spectra were collected for each of the flve detectors described above. Two-fold coincidences were collected between the two °¡ray detectors and the electron and Gedetectors at the flrst counting station (G1), and between the electron and Ge detectorsat the second counting station (G3).Examples of low-energy °¡ray and conversion electron spectra recorded at ISOLDEare shown in Figs. 1 and 2. Energies and intensities of the °¡rays associated to the 119Csdecay are listed in Table 2. For both experiments at ISOLDE and at UNISOR the °¡rayintensities were determined relative to the 176:5 keV gamma-ray transition.E–ciency curves were derived for each °¡ray detector using a standard referencesource containing 154;155Eu, 125Sb and 125mTe. Energy calibrations were performed bycollecting °¡ray singles spectra for sources of 152Eu and 228Th with the simultaneouscollection of the 119Cs activities.The experimental internal conversion coe–cients are listed independently for thetwo series of measurements. Table 2 concerns low-energy °¡rays and a few intense tran-sitions at intermediate energy studied at ISOLDE. The coe–cients were estimated fromsingles spectra and normalized to the 176:5 keV; E1 transition, in agreement with itsK/L experimental ratio. Table 3 concerns very low-energy transitions for which only Lconversion lines were observed. The conversion coe–cients measured at UNISOR for themore intense transitions in the decay of 119Cs are listed in Table 4.2Table 3: Same as for Table 2 but for experimental and theo-retical fiL internal conversion coe–cients.E° (keV) fiL Multipolarityexp E1 E2 M167.5 5 (2) 0.075 2.92 0.29 E268.0 0.9 (4) 0.073 2.78 0.29 M170.3 0.4 (1) 0.066 2.38 0.26 M1Figure 1: Partial conversion electron (upper panel) and gamma-ray (lower panel) spectraobserved in the 119m;gCs decay at ISOLDE (collecting time 30 s, counting time 30 s).302000040000CountsK76K86K86*K88L67.5 + L68L70K99*M70L76+ g 70M76 L86L88Xe IElectrons50100150200 250 300 350 400 450 500 550x10368.070.376.485.585.9*88.3 94.698.5*g -raysChannelFigure 2: Expanded partial low-energy spectra for °¡rays (lower panel) and electrons(upper panel), recorded at ISOLDE in the 119m;gCs decay. They show the complexity ofthe spectra in the 50 to 100 keV energy range as well as the difierences in transitionmultipolarities. Lines belonging to the decay of 119Xe are labelled with ⁄. Collecting time1 s, counting time 30 s.The electron intensities, for each transition, were determined using relative e–ciencycurves constructed using sources of 207Bi and 133Ba. The curves were normalized by con-sidering the 176:5 keV; which has an E1 multipolarity. As mentioned above,the efiect ofthe mini-orange fllter was to peak the e–ciency curve towards a speciflc electron window;the e–ciency was peaked at 750 and 250 keV for the electrons in the flrst and the secondstations, respectively.While the existence of two long-lived species in 119Cs was established and conflrmedby many experiments, half-life evaluations are still scattered. The assignment of the I… =9=2+ state to the ground-state was based on systematics of spins and magnetic momentsin the odd-A cesium isotopes [3], [5].For this state, a half-life T1=2 = 43:0§0:2 s is given in the Nuclear Data Sheets [15],in agreement with unpublished results [16]. Since the flrst evaluation T1=2 = (33 § 8) s[17], other values have been successively reported: (37:7 § 0:2) s [18], (44§ 2) s [3], 36 s[5], (44§ 3) s [6]. The situation is somewhat similar for the isomeric I… = 3=2+ state. Avery precise half-life T1=2 = (30:4§0:1) s is reported in the last compilations [15] while allthe other experimental values appear somewhat smaller: 28 s [19], [5], (29§ 2) s [3], [6].The time spectra established in the present study for a large number of °¡raysbelonging to 119Xe give a set of half-life values from 29 s to 43 s; as shown in Fig. 3.After having established the 119Xe level scheme fed simultaneously from the two long-lived 119mCs and 119gCs states (see section 3) this special situation is clearly understood.Indeed, as it will be presented later, a few high-spin (I ‚ 7=2) excited levels in 119Xe are4populated by the I… = 9=2+ 119gCs (see part (a) of Fig. 3) while the levels characterizedby spin value I = 1=2; 3=2 or 5=2 are essentially fed by the I… = 3=2+ 119mCs isomer (seepart (b) of Fig. 3). Obviously, in between these two extreme situations, as excited stateswhich are not directly fed (or partially fed) by beta decay exist, many °¡rays in 119Xewill exhibit intermediate half-lives. Two examples are shown in part (c) of Fig. 3 for theintense °¡rays at 169:8 keV and 197:4 keV which decay with a T1=2 = (38§1) s half-life.Taking into account the collecting and counting times used in the present work and thecomplexity of the 119Xe level scheme (discussed later in section 3) we estimate that it isvery di–cult to reach very precise half-life values. The following half-lives can be retained:T1=2 = (43 § 1) s for the I… = 9=2+ 119gCs ground-state and T1=2 = (29 § 1) s for theI… = 3=2+ 119mCs isomeric state. These results are in good agreement with those reportedin the Nuclear Data Sheets compilations [15], except for the error estimations.Finally the situation in 119Cs is deflnitively very similar to the one already studied in121Cs [4]. The identiflcation of two long-lived states I = 9=2 and I = 3=2 in both nuclei hasbeen flrmly established from on-line atomic beam magnetic resonance (ABMR) [19], [20]and by fl¡radiation detected optical pumping (fl¡RADOP) [21] experiments at ISOLDE.In both nuclei, the observation, by in-beam gamma spectroscopy [22] of ¢I = 1 bandstructures built on the 9=2+ states associates a 9=2+[404] …g9=2 Nilsson conflguration forthese states.T1/2 = 43– 1 s176.5 keV225.5 keV(a)T1/2 = 29– 1 sx2246.2 keV314.3 keV(b)T1/2 = 38– 1 s169.8 keV197.4 keV(c)Figure 3: Examples of time spectra measured for several 119Xe ° lines in the 119m;gCsdecays. (a) : 119gCs decay, (b) : 119mCs decay, (c) : lines having a complex feeding.5From K and L conversion electron measurements, a direct 68:5 keV; 9=2+ ! 3=2+isomeric transition, with a M3 multipolarity has been identifled in 121Cs [4]. Obviously,a highly converted isomeric transition could appear in 119Cs. However, no lines in coinci-dence with the Kfi Cs X-rays, no conversion-electron lines from about 8 keV (energy limitin the spectra) to 50 keV and no very low-energy °¡rays were observed. In conclusion, ifa highly converted M3 transition exists in 119Cs with a branching of a few %; its energywould very likely be less than 10¡ 15 keV:3 THE 119Xe LEVEL SHEMEThe 119Xe level scheme obtained in the present work is shown in Figs. 4 (Part I), 5(Part II) and 6 (Part III). It is based upon a I… = 5=2+ ground-state, previously identifledas a 5=2+[402] Nilsson neutron conflguration [5]. The strongest °¡rays populated in 119Xehave been used to establish the lowest excited states. The placement of the transitionshas been determined by their coincidence relationships, their intensities (Table 1) andmultipolarities (Table 2, 3 and 4). The (fl + EC) feedings have also added argumentsfor spin and parity assignments of several levels in 119Xe (see Table 5 and next section).Figure 7, which contains only part of the 119Xe level scheme extracted from our works, willbe used in the following, instead of Figs. 4, 5 and 6, to discuss the results and assign theflrst members of several band structures, some of them being well observed and extendedby in-beam spectroscopy studies.The negative-parity group of levels, 7=2¡; 11=2¡; 9=2¡; 13=2¡; displayed at theright hand side of Fig. 7, is strongly fed from 119gCs. It deexcites to the 5=2+ ground-state via the strongest 176:5 keV; 7=2¡ ! 5=2+, °¡ray which has an E1 multipolarityand is fed by the T1=2 = 43 s long half-life component of the119Cs fl¡decay (Fig. 3).These levels are similar to those previously identifled as the ”h11=2 yrast band from in-beam measurements [7], [8], [9], [10]. In the present fl¡decay study a °¡ray at 68 keVwas observed as a doublet which exhibits an intermediate half-life T1=2 = 39 s: However,from L-conversion electron spectra, it is possible to conflrm the E2 multipolarity of a11=2¡ ! 7=2¡; 67:5 keV transition (see Table 3) previously proposed in the flrst in-beamstudies [7], [8].

Low-lying levels in $^{119}$Xe

Matière NucléaireThe decays of ${}^{119m}$Cs and ${}^{119g}$Cs to ${}^{119}$Xe have been studied on mass separated samples, using $\gamma\rm{-ray}$ and internal conversion electron measurements. Several new low-lying levels have been established in the ${}^{119}$Xe level scheme. Half-life evaluations for ${}^{119m}$Cs and ${}^{119g}$Cs have been revisited. The results are compared with other experimental data known in light odd-mass xenon isotopes and with calculations performed in the frame of the multi-shell interacting boson-fermion model

Low-lying levels in $^{119}$ Xe

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Low-lying levels in $^{119}$ Xe