Decay of the long-lived (T1/2=160.44 d) Kπ=23/2- isomer in 177Lu was investigated using a chemically purified source and the Gammasphere array. New, high-multipolarity M3 and E4 deexcitation branches to the known Iπ=17/2- and 15/2- members of the π9/2-[514] band were discovered. The reduced hindrance factors per degree of K forbiddenness deduced for these two transitions are found to be relatively large when compared to similar decays from the Kπ=37/2- (T1/2=51.4 min) and Kπ=16+ (T1/2=31 yr) isomers in 177Hf and 178Hf, respectively. This is attributed to significant configuration changes that occur in the decay of this 177Lu isomer

Ahmad, I.

Carpenter, M.P.

Chiara, C.J.

Chowdhury, P.

Greene, J.P.

Janssens, R.V.F.

Kondev, F.G.

Lalkovski, S.

Lauritsen, T.

Seweryniak, D.

Zhu, S.

Carolina Digital Repository

PHYSICAL REVIEW C 85, 027304 (2012)M3 and E4 K -forbidden decays of the Kπ = 23/2− isomer in 177LuF. G. Kondev,1 S. Zhu,1 M. P. Carpenter,1 R. V. F. Janssens,1 I. Ahmad,1 C. J. Chiara,1,2 J. P. Greene,1 T. Lauritsen,1D. Seweryniak,1 S. Lalkovski,3 and P. Chowdhury41Argonne National Laboratory, Argonne, Illinois 60439, USA2Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA3Faculty of Physics, University of Sofia, BG-1164 Sofia, Bulgaria4Department of Physics, University of Massachusetts Lowell, Lowell, Massachusetts 01854, USA(Received 20 December 2011; published 14 February 2012)Decay of the long-lived (T1/2 = 160.44 d) Kπ = 23/2− isomer in 177Lu was investigated using a chemicallypurified source and the Gammasphere array. New, high-multipolarity M3 and E4 deexcitation branches to theknown Iπ = 17/2− and 15/2− members of the π9/2−[514] band were discovered. The reduced hindrance factorsper degree of K forbiddenness deduced for these two transitions are found to be relatively large when comparedto similar decays from the Kπ = 37/2− (T1/2 = 51.4 min) and Kπ = 16+ (T1/2 = 31 yr) isomers in 177Hf and178Hf, respectively. This is attributed to significant configuration changes that occur in the decay of this 177Luisomer.DOI: 10.1103/PhysRevC.85.027304 PACS number(s): 21.10.−k, 23.20.−g, 23.35.+g, 27.70.+qThe properties of K isomers play an important role inunderstanding the structure and collective modes of deformed,axially symmetric nuclei. While considerable progress hasbeen made in the past 30 years in identifying and charac-terizing many K isomers in different areas of the nuclearchart, predictions of their lifetimes and associated transitionstrengths still remain a challenge for theory. The lifetimecan be particularly long, especially when the isomer is beingclassified as both a “K trap” (with a large difference in the Kquantum numbers of the isomer and the final state to whichit decays) and a “spin-trap” (with a large change in nuclearspin enabling the decay to proceed only via high-multipolaritytransitions) [1]. In the rare-earth region, high-multipolarityM4 and E5 transitions were established in the decay of theKπ = 16+ isomer (T1/2 = 31 y) in 178Hf [2–5]. Searches forsuch exotic decay modes have also been performed in otherregions of the nuclear chart. The most notable example isthe E6 decay of the high-spin isomer in 53Fe [6], the highestmultipolarity transition known in any nucleus [7], whosestrength was recently calculated in a shell-model approach [8].High-multipolarity transitions can also be expected to occurin the decay of the Kπ = 23/2− isomer (T1/2 = 160.44 d) in177Lu, denoted here as 177mLu. This isomer was discoveredby Jørgensen et al. [9] and assigned the three-quasiparticleπ7/2+[404] ⊗ ν(7/2−[514], 9/2+[624]) configuration. Thelatter was confirmed by the measured magnetic momentof the isomer (see Refs. [10,11] and references therein)and by the properties of the rotational band built on thisstate [12]. Previous studies (see for example Ref. [13] andreferences therein) found that the isomer partially decays byβ− emission to a similar three-quasiparticle, Kπ = 23/2+π (7/2+[404], 9/2−[514]) ⊗ ν7/2−[514] isomer in 177Hf andby means of an E3, K-forbidden γ -ray transition to the Iπ =17/2+ member of the π7/2+[404] ground-state band of 177Lu.Here, we report on the discovery of high-multipolarity M3and E4 γ -ray decay branches from the isomer to members ofthe previously known one-quasiparticle π9/2−[514] band in177Lu. These transitions are found to be unusually hindered,and this is attributed to the significant configuration changesthat occur in the decay between the isomer and the final levels.The 177mLu isomer was produced in a neutron-capturereaction on 176Lu at the University of Massachusetts Lowellresearch reactor facility. A natural lutetium metallic samplewas irradiated for about three years. This was followed byan approximately 3-yr radiation cooling period that allowedall short-lived nuclides to decay away. The 177mLu source(∼0.5 μCi) was prepared at Argonne National Laboratoryfollowing radiochemical separation of lutetium and 182Taradionuclides. The latter was produced from natural tantalumimpurities in the target material via the 181Ta(n, γ ) reaction.The 182Ta radionuclide has a half-life of T1/2 = 114.74 d [14],comparable to that of 177mLu, and its decay generates substan-tial Compton-scattered background in the γ -ray spectra owingto the presence of several high-energy transitions of relativelyhigh intensity. Since the tantalum material was insoluble inmost dilute acids, the irradiated lutetium sample was dissolvedin a 0.2-mol HNO3 solution, leaving behind the tantalum metalpieces. The solution was separated from the insoluble solid,placed on a quartz plate, and dried.Several γ -ray measurements were carried out with thelutetium source including: (i) γ -ray singles measurementsusing 2-cm2 × 1-cm planar Low Energy Photon Spectrometer(LEPS) and 25% coaxial Ge detectors; (ii) γ -ray coincidencemeasurements using two LaBr3(Ce) scintillation detectorsand the Gammasphere array of 100 Compton-suppressed,large-volume Ge detectors [15]; and (iii) γ -ray coincidencemeasurements using the Gammasphere alone. The results fromthe first two series of measurements have been presentedelsewhere [16,17].In the Gammasphere standalone experiment, the source wasplaced at the center of the array and data were collected withthe requirement that at least one γ ray was detected. In thesubsequent analysis, the collected data were sorted offlineinto γ -γ and γ -γ -γ histograms with required coincidence027304-10556-2813/2012/85(2)/027304(5) ©2012 American Physical SocietyBRIEF REPORTS PHYSICAL REVIEW C 85, 027304 (2012)FIG. 1. 177mLu decay level scheme deduced in the present work. The quoted lifetimes are from Ref. [13].windows of ±25 ns and ±170 ns. The latter condition wasused to minimize the distortion of γ -ray intensities due to timewalk by the timing discriminators for transitions with energiesbelow 200 keV. Since single- and double-gated spectra wereused to deduce the isomer decay branches, the Gammasphereγ -ray efficiency was carefully determined using a calibrated,multinuclide source containing 57,60Co, 85Sr, 88Y, 109Cd, 113Sn,137Cs, 139Ce, 203Hg, and 241Am and monoisotopic sources of152Eu, 182Ta, and 243Am. These measurements were carriedout in the same geometry as adopted for the 177mLu source.The existence of a number of cascade and crossover transitionsin the 177mLu decay scheme itself allowed the correspondingγ -ray efficiencies to be checked internally using intensitybalances, and these were found to be in good agreement withvalues deduced using the source measurements.The decay scheme deduced in the present work for theKπ = 23/2− isomer is presented in Fig. 1. The main decaypaths via the three-quasiparticle Kπ = 23/2+ isomer in 177Hfand the Iπ = 17/2+ member of the π7/2+[404] ground-stateband of 177Lu, established previously [13], were confirmed inthe present studies. Figure 2(a) provides a γ -ray coincidencespectrum that isolates the decay via the ν9/2+[624] bandin 177Hf. The 14-keV transition that feeds the Iπ = 21/2+level was not observed directly, since it is highly converted.However, its existence is confirmed by the observation of the214- and 418-keV transitions depopulating this Iπ = 21/2+level. The coincidence spectrum of Fig. 3, produced bygating on γ rays in the ν7/2−[514] band, shows many ofthe connecting E1 transitions between the ν9/2+[624] andν7/2−[514] bands in 177Hf. The 55-keV transition that feedsdirectly the 21/2− level was not observed, because of theoverlap with the Hf Kα1 x ray. Figure 2(b) provides acoincidence spectrum where the decay via the π7/2+[404]ground-state band of 177Lu is isolated. A thorough search forthe 334-keV M4 transition that would connect the isomer withthe Iπ = 15/2+ level was carried out, but this γ ray was notobserved and only a limit for this branch was deduced, asindicated in Table I.The Kπ = 9/2− band head at 150 keV, associated withthe π9/2−[514] orbital, and the corresponding rotationalband were known from previous studies [13]. The sum ofcoincidence spectra in Fig. 4 was produced with double gateson the 150-, 139- and 163-keV transitions with the 186-keVγ ray. It is apparent that the Iπ = 15/2− and 17/2− members027304-2BRIEF REPORTS PHYSICAL REVIEW C 85, 027304 (2012)02x1054x105Hf KX129 153174204214229234282 327378418105113100 200 300 40004x1048x104116Lu KX(b)(a)122Energy (keV)Counts\147172196 218269319367413FIG. 2. (a) Background-subtracted γ -ray coincidencespectrum produced by summing γ1{105, 113}γ2{208} doublegates, i.e., γ1{105}γ2{208} + γ1{113}γ2{208}, showing theisomer decay path via the ν9/2+[624] band in 177Hf;(b) Gamma-ray coincidence spectrum produced by summingγ1{122, 147, 172, 196, 218}γ2{122, 147, 172, 196, 218} doublegates (the diagonal contributions were excluded) associated with theπ7/2+[404] ground-state band in 177Lu.of the π9/2−[514] band are fed by the Kπ = 23/2− isomer viathe 333- (E4) and 125-keV (M3) transitions. While the formerγ ray is clearly seen in the coincidence spectrum of Fig. 4, thelatter was not directly observed, owing to the large conversionHf KX50 70 90 110 130 150 170 1905x1031x1041.5x1042x104Energy (keV)Counts72 88| | |105 113117129137146153160174177/182Hf KXEnergy (keV)Counts203/204214200 250 300 350 400 45001x1042x1043x104 229|242 282/284\292/293296305 327341378\385427418466/299203/204214FIG. 3. Background-subtracted γ -ray coincidence spectrum pro-duced by summing γ1{113}γ2{296, 385, 466}, γ1{137}γ2{341, 427},and γ1{113}γ2{137} double gates associated with the ν7/2−[514]band in 177Hf.coefficient for such an M3 transition (αT (M3) = 94.3 [18]).With the relative γ -ray intensities deduced from the spectrumof Fig. 4 and the intensity balance at the Iπ = 17/2− level, therelative intensity of the 125-keV γ ray is expected to be ∼1.5%of the 208-keV yield. Hence, the former is not observableTABLE I. Reduced hindrances per degree of K forbiddenness for high-multipolarity transitions in 177Lu, 177Hf, and 178Hf. The transitionsshown in parenthesis were not observed directly.Eγ Mult. Iγ αT [18] FW ν fν(keV) (rel) (W.u.)177Lu; Kπ = 23/2−; T1/2 = 160.44 (6) d; IT = 22.8 (7)%115.9 E3 100 (2) 30.7 8.3 (4) × 108 5 60.8 (5)(334) M4  0.28 5.58  5.3 × 106 4  48125.3 M3 0.032 (8) 94.3 4.4 (12) × 1010 4 458 (30)333.1 E4 0.26 (6) 1.007 5.7 (14) × 108 3 829 (70)177Hf; Kπ = 37/2−; T1/2 = 51.4 (5) min; IT = 100%(185.3) M3 0.16 (3) a 18.6 5.5 (11) × 105 3 82 (6)214.0 E3 100.00 (3) 1.512 2.47 (6) × 105 4 22.29 (13)178Hf; Kπ = 16+; T1/2 = 31 (1) yr; IT = 100%(12.76) E3 0.050 (8) b 1.43×107 1.2 (3) × 109 5 65 (4)309.5 M4 100 (7) c 8.44 2.7 (5) × 107 4 72 (4)587.0 E5 41 (4) c 0.284 5.3 (11) × 106 3 175 (11)aThe existence of this branch was not known previously. It is inferred from the observation of the 120.5-keV γ ray in the decay of theKπ = 37/2− isomer in 177Hf [22], which was subsequently assigned to depopulate the Kπ = 25/2− bandhead in 177Hf [13].bFrom the intensity balance at the Iπ = 13− level and the relative intensities quoted in Ref. [3].cFrom the relative intensities given in Ref. [3].027304-3BRIEF REPORTS PHYSICAL REVIEW C 85, 027304 (2012)50 100 150 200 250 300 3500200400600Energy (keV)Counts150139 163208333Lu KXFIG. 4. Background-subtracted γ -ray coincidence spectrum pro-duced by summing γ1{150, 139, 163}γ2{186} double gates associatedwith the π9/2−[514] band in 177Lu.as it is buried under the background in the spectrum ofFig. 4.The very long half-life of the Kπ = 23/2− isomer arisesbecause of its low excitation energy, which results in decaypaths associated with high-multipolarity and low-energy γ -ray transitions, which are in addition K forbidden. Fora transition of multipole order λ, the reduced hindrancefactor per degree of K forbiddenness, fν (ν = K − λ), isdefined as fν = F 1/νW , where FW = T γ1/2/T W1/2 is the Weisskopfhindrance factor with T γ1/2 and TW1/2 being the partial γ -ray andthe Weisskopf estimate half-lives, respectively. The deducedhindrances are presented in Table I, together with values forother high-multipolarity, K-forbidden transitions (with ν  3)measured in neighboring 177Hf and 178Hf nuclei. In general,the magnitude of fν depends on the degree of K mixing inthe wave functions of the isomer itself and the final states towhich the decay proceeds. A common feature of the isomersselected in Table I is that all of them are yrast and, therefore, nosignificant K mixing would be expected to affect their decay.Reduced hindrances per degree of K forbiddenness arealso known to be sensitive to associated changes in theconfiguration of the states involved [19–21]. Considering thedifferences between the configurations of the Kπ = 23/2−isomer and the Iπ = 17/2+ member of the π7/2+[404]ground-state band, the E3 decay involves theπ7/2+[404] ⊗ ν(7/2−[514], 9/2+[624]) → π7/2+[404]change, which is equivalent to a ν2(7/2−[514],9/2+[624])[8−] → 0+ transition. It is worth noting thatthe E3 decay from the Kπ = 16+ isomer in 178Hf to the 13−member of the Kπ = 8− band (a mixture between ν[8−] andπ [8−] configurations) involves a similar change:π (7/2+[404], 9/2−[514]) ⊗ ν(7/2−[514], 9/2+[624])→ ν(7/2−[514], 9/2+[624])orπ (7/2+[404], 9/2−[514]) ⊗ ν(7/2−[514], 9/2+[624])→ π (7/2+[404], 9/2−[514]).Hence, the fν hindrances for these two E3 decays should beexpected to be nearly identical. This is verified experimentally,as seen in Table I (fν ∼ 60). In contrast, the E3 decay fromthe Kπ = 37/2− isomer (π (7/2+[404], 9/2−[514]) ⊗ν(5/2−[512], 7/2−[514], 9/2+[624])) in 177Hf tothe Iπ = 31/2+ member of the Kπ = 23/2+(π (7/2+[404], 9/2−[514]) ⊗ ν7/2−[514]) band has arelatively low value of fν = 22.29. In this case, thechange in configurations between the isomer and the finallevel corresponds to a ν2(5/2−[512], 9/2+[624])[7−] → 0+transition, i.e., an operation involving the f7/2 and i13/2orbitals which differ by l = 3 and j = 3, a change knownto lead to enhanced E3 transitions.A notable feature of the 177mLu decay is the significantlylarger hindrances of fν = 458 and 829 measured for the M3and E4 decays to the Iπ = 17/2− and 15/2− members of theπ9/2−[514] band, respectively. As both transitions involverearrangements of all the orbitals between the correspondingconfigurations,π7/2+[404] ⊗ ν(7/2−[514], 9/2+[624]) → π9/2−[514]one should expect larger retardation, as observed. In con-trast, the M3 transition in the decay of the Kπ = 37/2−isomer in 177Hf to the 31/2− member of the Kπ = 25/2−band (π (7/2+[404], 9/2−[514]) ⊗ ν9/2+[624]) has fν = 82,and involves a simple configuration change equivalent to aν2(5/2−[512], 7/2−[514])[6+] → 0+ transition.In summary, the decay of the long-lived, Kπ = 23/2−isomer in 177Lu was studied using a chemically-purified sourcein conjunction with the Gammasphere and other detectionsystems. High-multipolarity M3 and E4 decay branches wereobserved for the first time. These transitions were found to bemore hindered when compared to similar exotic decays knownin neighboring nuclei. This feature is proposed to arise fromthe significant rearrangement in configurations between theisomer and the final levels.The authors are indebted to the staff of the researchreactor facility at the University of Massachusetts Lowellfor irradiating the lutetium sample. This work was sup-ported by the U.S. Department of Energy, Office of NuclearPhysics, under Contract No. DE-AC02-06CH11357 and GrantNo. DE-FG02-94ER40848.[1] P. M. Walker and G. D. Dracoulis, Nature (London) 399, 35(1999).[2] J. Van Klinken, W. Z. Venema, R. V. F. Janssens, and G. T.Emery, Nucl. Phys. A 339, 189 (1980).[3] M. B. Smith, P. M. Walker, G. C. Ball, J. J. Carroll, P. E. Garrett,G. Hackman, R. Propri, F. Sarazin, and H. C. Scraggs, Phys.Rev. C 68, 031302(R) (2003).[4] R. G. Helmer and C. W. Reich, Nucl. Phys. A 114, 649 (1968).027304-4BRIEF REPORTS PHYSICAL REVIEW C 85, 027304 (2012)[5] R. G. Helmer and C. W. Reich, Nucl. Phys. A 211, 1(1973).[6] J. N. Black, Wm. C. McHarris, W. H. Kelly, and B. H.Wildenthal, Phys. Rev. C 11, 939 (1975).[7] ENSDF database [www.nndc.bnl.gov/ensdf].[8] B. Alex Brown and W. D. M. Rae, AIP Conf. Proc. 1355, 145(2011).[9] M. Jørgensen, O. B. Nielsen, and G. Sidenius, Phys. Lett. 1, 321(1962).[10] C. König, B. Hinfurtner, E. Hagn, E. Zech, and R. Eder, Phys.Rev. C 54, 1027 (1996).[11] U. Georg et al., Eur. Phys. J. A 3, 225 (1998).[12] G. D. Dracoulis, F. G. Kondev, G. J. Lane, A. P. Byrne,T. Kibédi, I. Ahmad, M. P. Carpenter, S. J. Freeman,R. V. F. Janssens, N. J. Hammond, T. Lauritsen, C. J. Lister,G. Mukherjee, D. Seweryniak, P. Chowdhury, S. K. Tandel, andR. Gramer, Phys. Lett. B 584, 22 (2004).[13] F. G. Kondev, Nucl. Data Sheets 98, 801 (2003).[14] B. Singh and J. C. Roediger, Nucl. Data Sheets 111, 2081 (2010).[15] R. V. F. Janssens and F. S. Stephens, Nucl. Phys. News 6, 9(1996).[16] S. Zhu, F. G. Kondev, M. P. Carpenter, I. Ahmad, C. J. Chiara,J. P. Greene, G. Gurdal, R. V. F. Janssens, S. Lalkovski,T. Lauritsen, and D. Seweryniak, Nucl. Instrum. Methods Phys.Res., Sect. A 652, 231 (2011).[17] F. G. Kondev et al., Appl. Radiat. Isot. (to be published).[18] T. Kibédi, T. W. Burrows, M. B. Trzhaskovskaya, P. M.Davidson, and C. W. Nestor Jr., Nucl. Instrum. Methods Phys.Res., Sect. A 589, 202 (2008).[19] G. D. Dracoulis, F. G. Kondev, A. P. Byrne, T. Kibédi, S. Bayer,P. M. Davidson, P. M. Walker, C. Purry, and C. J. Pearson, Phys.Rev. C 53, 1205 (1996).[20] F. G. Kondev, G. D. Dracoulis, A. P. Byrne, T. Kibédi, S. Bayer,and G. J. Lane, Phys. Rev. C 54, R459 (1996).[21] F. G. Kondev, G. D. Dracoulis, A. P. Byrne, and T. Kibédi, Nucl.Phys. A 632, 473 (1998).[22] Y. Y. Chu, P. E. Haustein, and T. E. Ward, Phys. Rev. C 6, 2259(1972).027304-5

M3 and E4 K-forbidden decays of the Kπ=23/2- isomer in 177Lu

https://cdr.lib.unc.edu/downloads/2514nw19f

M3 and E4 K-forbidden decays of the Kπ=23/2- isomer in 177Lu

Abstract

Similar works

Full text

Available Versions

Carolina Digital Repository