PHYSICAL REVIEW C VOLUME 38, NUMBER 4

Brief Reports

OCTOBER 1988

Brief Reports are short papers which report on completed research or are addenda to papers previously published in the PhysicalReview A. Brief Report may be no longer than 3g printed pages and must be accompanied by an abstract.

Fine structure in "Tm a decay

K. S. TothOak Ridge National Laboratory, Oak Ridge, Tennessee 37831

P. A. %ilmarth, J. M. Nitschke, R. B. Firestone, and K. VierinenLawrence Berkeley Laboratory, Berkeley, California 94720

M. O. Kortelahti*Louisiana State Uniuersity, Baton Rouge, Louisiana 70803

F. T. Avignone, IIIUniversity ofSouth Carolina, Columbia, South Carolina 29208

(Received 31 May 1988)

In an investigation of A = 153 isotopes fine structure was observed in ' Tm a decay. Besides thepreviously known a transitions that feed the ' Ho h»/2 (0.0 keV) and s&/2 (49.0 keV) isomers, two,very much weaker, a groups were found to populate the d3/2 (220.4 keV) and d5/2 (564.4 keV) statesin ' 9Ho. Based on the ' 9Ho level scheme and on the Q values for the two main '"Tm a transi-

tions, we determine that the h»/2 level in '"Tm is the ground state and that the s&/2 state lies 43+7keV above it. Energy systematics for the s

& /2 and h» zz proton states in even-N odd-Z nuclei nearN= 82 are discussed.

The 5.11-MeV a-particle group known' for more than20 years to follow the a decay of ' Tm has recently beenshown to encompass, besides the main group at5. 103+0.003 MeV, a less intense peak at 5.096+0.004MeV. This lower-energy peak is associated with the

Tm low-spin, s»2, isomer (whose existence was antici-pated but not verified until the work of Schardt et al. )

while the 5.103-MeV peak follows the decay of the high-spin, h»&z, state. In the present investigation by per-forming (a-particle)-(y-ray) coincidence measurements,fine structure was observed in the ' Tm a-decay spec-trum.

A 1.85-mg/cm foil of Mo (enriched to 97.37%) wasbombarded with 285-MeV Zn ions from the LawrenceBerkeley Laboratory SuperHILAc. At the center of thetarget the Zn energy was calculated to be 267 MeV.Reaction products with 3 = 153 were then mass separat-ed at the OASIS on-line facility, collected with a pro-grammable tape system, and transported to a countingstation. There, a Si particle hE-E telescope and hyper-pure Ge detector faced the radioactive layer, while a 1-mm thick plastic scintillator and an n-type Ge detector(relative efficiency of 52%) were located on the other sideof the tape. In addition, a 24% n-type Ge detector,oriented at 90 with respect to the other two Ge detec-tors, was placed -4.5 cm from the radioactive source.Coincidences between particles, y rays, x-rays, and posi-

trons were recorded in an event-by-event mode; all eventswere tagged with a time signal for half-life information.Collection and counting cycles of 1.28, 4.0, and 12.8 swere used in this investigation. Singles data were alsotaken with the 52% n type and the hyperpure Ge detec-tors in a multispectrum mode wherein cycle times weredivided into eight equal time intervals.

Figure 1(a) shows the a-particle spectrum recorded inthe Si b,E-E telescope. Besides the ' Tm a particles, oneobserves a 4.67-MeV group and a 3.91- and 4.01-MeVdoublet. They are known to belong to the a decays of

Er (Ref. 4) and of the ' Ho high- and low-spin isomers(Ref. 5), respectively. The full-width at half maximutnfor the a-particle peaks observed in our spectra was -60keV. Thus we could not resolve the two ' 3Tm a groupsreported in Ref. 2. However, we observed y ray transi-tions that follow the P decays of both the s, &2 and h»zz

Ho isomers. In our investigation these ' Ho y rayscan originate only from ' Tm a decay. Therefore, inagreement with Ref. 2, there must be two a-decaying iso-mers in ' Tm.

Low-lying levels in ' Ho have been investigated ' viathe P decay of ' Er. These studies established that fourof the five states located below —1 MeV were represented(in ascending order of excitation) by the s, &2, d3&2, d5&2,and g7/2 proton orbitals; the fifth one, represented by the

h»&2 orbital, was assumed to be close in energy to the

38 1932 1988 The American Physical Society

38 BRIEF REPORTS 1933

zZxlA

Z 060

153H

(3910,4010)

zZ

20

ZCO

~o-0

(b)

Z 15 - (C)Z io-

ZI-V)

0 5

O~ - (0)

163E

{4e7~)

. nA 1

Tm

(5096,5103)

CVOC5

+CV

+CV

0z

80 +CV +

CV

ID

11X 60 +

OLU 0CL Ll Z

40CO

ZD0

20—

60 150 250 350 460ENERGY (keV)

+CV

0Zr

FIG. 2. Portion of the y ray spectrum measured in promptcoincidence with a particles (transitions are labelled by energyand element). The 171.4- and 344.0-keV y rays that deexcite thed 3 /2 and d 5 //2 levels in ' Ho were used as gates to generate thea spectra shown in Figs. 1(b) and 1(c), respectively. The arrowmarks the energy at which the 436.9-keV y ray (g7/2~d5/&transition) should have been observed. The 91.8- and 299.3-keVpeaks are intense y rays in '"Yb and '"Tm P-decay, respective-

ly; their presence in the figure is due to random coincidences.

n n. , nn n4250 4550

ENERGY t~V)

FIG. 1. Part (a) shows the singles a-particle spectrum mea-

sured at A =153, while parts (b) and (c) show a groups ob-

served in prompt coincidence with the 171.4-, and 344.0-keV yrays in ' Ho, respectively.

n

5050

s»z level. A detailed decay scheme for the ' Er h»/2and s &/2 isomers has recently been constructed. Itverifies the sequence of the s, /p G3/2 15/2 and g7/2states and shows, for the first time, that the ground statein ' Ho is the h»/2 proton level while the s&/2 state isisomeric and is located at 49.0 keV. The resultant excita-tion energies of the d3/p (f5/2 and g7/2 states are 220.4,564.4, and 1001.3 keV, respectively.

In parts (b) and (c) of Fig. 1 we show a spectra inprompt coincidence with the 171.4- and 344.0-keV y raysthat connect the d3/2 +s]/2 and d5/2~d3/2 Ho states,149

respectively. While the two transitions were too weak tobe observed in the raw, total coincidence y-ray spectra,they were visible in the spectrum (see Fig. 2) recorded bythe 52% Ge detector in coincidence with a particles.Gates were set on the 171.4- and 344.0-keV peaks, and, aparticles that gave rise to these y-ray events were thenback projected; the coincident a spectra generated in thatmanner are shown in Figs. 1(b) and 1(c), respectively.One sees that there are peaks, 4586+10 and 4902+15keV, in Fig. 1(b) which, because of their very low intensi-ties, could not be observed in the singles spectrum; one ofthem, 4586 keV, also appears in coincidence with the344.0-keV transition [Fig. 1(c)]. Based on these coin-cidence relationships it is clear that the two new a groupsrepresent fine structure transitions from ' Trn whichproceed to the d~/2 and d3/2 levels at 564.4 and 220.4keV.

The expected energies of the fine structure transitionsfor each ' Tm isomer to the 564.4- and 220.4-keV statesare [4553 (from h»/2), 4594 (from s, /z) keV] and [4888(from h»/z), 4929 (from s, /2) keV], respectively. We,however, are unable to tell whether the 4586- and 4902-keV a groups originate from the h»/z ground state, orthe s&/2 isomer, or from both of these levels because ofthe small number of recorded events and the 60-keV en-ergy resolution of the particle telescope. (Note that thewidths of the two a groups are comparable to those ofpeaks in the singles spectrum, i.e., —100 keV at the base. )

The intensities of the 4586- and 4902-keV peaks, vis-a-Uis the main ' Tm doublet, are (4. 5+0.6)X10 and(1.8+0.4)X10, respectively. There is no indication inFig. 2 of the 436.9-keV y ray which connects the g7/2(1001.3 keV) and d5/2 (564.4 keV) levels in ' Ho; withinour detection limits the intensity of the ' Tm a transi-tion to the 1001.3-keV state is &6)&10 . In general,our fine structure results are similar to those reported byLiang et aI. in their recent investigation of ' 'Ho a de-cay to ' Tb, i.e., transitions to the g7/2 level at 719.2 keVwere not observed, and the strongest fine structure decaywas to the d5/2 level at 354.1 keV. Additionally, theycould only set intensity limits for the transitions to thed 3/2 level at 253.4 keV; as in our study these transitionsmust be substantially less intense than the ones that pop-ulate the d»z state.

Our proposed ' Trn a decay scheme is shown in Fig. 3where a transitions are indicated to the d3/2 and d5/2states from both the h»/2 and s, /2 parent levels. We alsoshow the h»/2 state as the ' Tm ground state with thes&/2 level isomeric at an excitation energy of 43+7 keV.This separation energy was deduced from the ' Ho levelscheme (Ref. 8) and Q values (Ref. 2) of the main ' Tma groups that connect the respective h»/2 and s, /z statesin the parent and daughter nuclei.

1934 BRIEF REPORTS 38

(2.5s) TABLE I. Excitation energies of h11/2 and s1/2 proton states.

0"~Tm„(kev) Nucleus [Z, N]

Energy (keV)

$1/2 h»/2 References

O

147Tb

149Tb

15]Tb'4'Ho151H

153H

'4'Tm'"Tm'"Tm

[65,82][65,84][65,86][67,82][67,84][67,86][69,78][69,84][69,86]

0.00.00.0

49.041.468'6743

-41

50.636.099.50.00.00.0'0.00.00.0b

9101189

10,1213

2,814

d&cp

S54s

21s

f49,7H.S,FIG. 3. Alpha-decay scheme of ' Tm. The ' Ho level

scheme is from Ref. 8, while the Q values shown for the '"Tmh»/2 and s1/2 isomers are from energies measured by Schardtet al. (Ref. 2). Because of the 60-keV (FWHM) resolution ofour particle telescope we could not determine from which of thetwo isomers the fine structure '"Tm a groups originate. Thus,transitions from both isomers to the 220.4- and 564.4-keV '" Holevels are indicated. The 2.5-s half-life of the '"Trn s]&2 isomeris taken from Ref. 2.

The two proton orbitals represent the lowest-lying lev-els in odd-Z even-N nuclei near N =82 after the g7/2 and

d spaz orbitals have been filled at Z =64. While these sepa-ration energies are important for determining massesfrom Qzc and Q tneasurements and for providing levelenergies to compare with shell-model calculations, theyare difficult to obtain because the two states are not con-nected by isomeric y rays. Table I summarizes excitationenergies of the s& 2 and h»/2 proton levels in ' ' ' 'Tb(Refs. 9—11), ' ' " Ho (Refs. 8, 9, and 12), and ' Tm(Refs. 2 and 8). We include energies for ' Tm (deducedfrom direct proton decay results' ) even though its neu-tron number of 78 removes it somewhat from theremainder of the nuclei listed in the table, and for ' Tmwhere the value is based on the assumption that the sin-gle a group reported' for its a decay is in fact a doubletas is the case for ' Trn. One notes that in terbium(Z =65) the s, &2 orbital is the ground state while in hol-mium (Z =67) and thulium (Z =69) the h»t2 orbital isthe ground state and the s, /2 level has become a holestate; this reversal of level sequences between ' Tb and

Ho was predicted by Hartree-Fock-Bogoliubov calcula-

'The excitation energy for the low-spin isomer in '"Ho was

originally reported (Ref. 12) to be -60 keV. Our value of 68keV is calculated by using a decay energies reported in Ref. 12and the 36.0-keV excitation energy recently determined (Ref.10) for the 11/2 isomer in ' Tb.Based on the assumption that the a group reported (Ref. 14)

for the a decay of '"Tm is in fact a doublet where the two atransitions originate from the s1/2 and h»/, isomers in '"Tmand feed the corresponding proton states in "'Ho.

tions. Based on the information in Table I the s &/2 orbit-al most probably is the first-excited level in ' 'Tm (it andthe d3/2 d5/2 and g7/p states have been observed from15

' 'Yb P decay). However, as one proceeds to higheratomic numbers this orbital's energy should increase andat some point the low-lying d3/2 state should drop belowthe s, /2 state in excitation. This change in level ordering

may provide an additional explanation as to why ' Lu(Ref. 16) and ' Lu (Ref. 17) have been observed to haveonly one a group, i.e., the low-spin isomer may berepresented by the d3/2 proton orbital. The change in theconfiguration would cause retardation in a decay andthus make it more difficult to resolve a close-lying dou-blet.

The assistance of L. F. Archambault, R. M. Chasteler,A. A. Shihab-Eldin, and A. A. Wydler during this experi-ment is gratefully acknowledged. Oak Ridge NationalLaboratory is operated by Martin Marietta Energy Sys-terns, Inc. for the U.S. Department of Energy under Con-tract No. DE-AC05-84OR21400. Work at the LawrenceBerkeley Laboratory is supported by the Director, Officeof Energy Research, Division of Nuclear Physics of theOffice of High Energy and Nuclear Physics of the U.S.Department of Energy under Contract DE-AC03-76SF00098. Support was also provided by the U.S.Department of Energy through Contract No. DE-FG05-84ER4-0159 with Louisiana State University.

Permanent address: University of Jyvaskyla, SF-40100Jyvaskyla 72, Finland.

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38 BRIEF REPORTS 1935

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