PHYSICAL REVIEW C VOLUME 36, NUMBER 2 AUGUST 1987

Observation of ' Tm decay to ' Er levels and P-delayed proton emission

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

J. Gilat, * J. M. Nitschke, P. A. Wilmarth, and K. VierinenLawrence Berkeley Laboratory, Berkeley, California 94720

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

(Received 23 March 1987)

With the use of an on-line isotope separator, ' Tm (Tlq2 ——0.9+0.2 s) was identified in "Ni bom-

bardments of Mo. Seven y rays were observed to follow the isotope's (f3++ electron capture) decay,

including the 111.3-keV transition known from ' Er isomeric decay to be the d3/p (111.3keV)~s&z& (0.0 keV) y ray in ' Er. In addition to the previously known levels at 111.3 (d3/Q) and

741.5 (hll&2) keV in ' 'Er, our data establish three other excited states, at 907.4 ( —' ), 1066.2 (—', ),

and 1482.9 ( —, , —',' ) keV. Coincidences observed between Er K x rays and protons in the A=149mass chain show that ' 'Tm is a P-delayed proton precursor.

Nuclei with a few particles or holes outside the '64Gdq2core are of interest since their low-lying structure can pro-vide information on shell structure in a mass region locat-ed midway between the major closed proton shells ofZ =50 and Z =82. One such series of nuclei includeseven-Z isotones with cV =81; their low-energy excitationsshould be describable in terms of single-neutron holestates. Known isotopes at the higher atomic number endof the series are ' Gd, ' Dy, and ' Er. Levels in allthree have been investigated by means of their isomericdecays. ' Additional states in ' Gd and ' Dy havebeen observed, ' populated via the P decays of ' Tb and

Ho, respectively. Herein we report on the identificationof ' Tm and on its decay to levels in ' Er.

A 2.0-mg/cm thick foil of molybdenum, enriched inMo to 93.9%%uo, was bombarded with 259-MeV Ni ions

accelerated in the Lawrence Berkeley Laboratory Su-perHILAC. The beam energy at the center of the targetwas calculated to be 242 MeV. Products recoiling out ofthe target were mass separated with the OASIS facilityon-line at the SuperHILAC. Nuclei with 3 = 149 werecollected with a programmable tape system for 4 s andthen transported to a counting station. A Si particlehE-E telescope and a hyperpure Ge detector faced the ra-dioactive layer, while a 1-mm thick plastic scintillator andan n-type Ge detector with a relative efficiency of 52%were located on the other side of the collector tape. Inaddition, a 24%%uo n-type Ge detector, oriented at 90 withrespect to the other two Ge detectors, was placed —4.5cm from the radioactive source. (A schematic of this ar-rangement, with the 24%%uo and 52%%uo detectors inter-changed in position, can be seen in Ref. 8.) Coincidencesbetween particles, y rays, x rays, and positrons wererecorded in an event-by-event mode; all events weretagged with a time signal for half-life information. Singlesdata were also taken with the 52%%uo y-ray detector in amultispectrum mode in which the 4-s cycle time was di-

vided into eight 0.5-s intervals.At 242 MeV the calculated cross section for the

Mo( Ni, p2n)' Tm reaction is 1.5 mb, while cross sec-tions for the production of ' Er and ' Ho are predictedto be 40 and 60 mb, respectively. Nuclides with 3 =149Z ) 69 have negligibly small yields; mass-149 isobars withZ &67 could be produced only in interactions involvingmolybdenum isotopes with 3 ) 95, which comprised lessthan 6.0% of the target material, or by P decay.

A new activity (Ti~q ——0.9+0.2 s), with at least seven y

rays following its p decay, was observed in the 3 =149mass chain. It is assigned to the hitherto unknown iso-tope ' Trn because the y rays are in coincidence with ErK x rays and because several of them are also in coin-cidence with the 111.3-keV transition seen' in ' Erisomeric decay.

In Fig. 1 we show y-ray spectra recorded by the 52%Ge(Li) detector in coincidence with the following gates seton the intrinsic Ge detector: (a) Er Ka~ x rays, (b) the111.3-keV transition, and (c) a new intense 158.8-keV y'

ray assigned to ' Tm decay. Based on this and othercoincidence information, and on y-ray energies and inten-sities (summarized in Table I) we constructed a partial de-

cay scheme for ' Tm. The scheme, shown in Fig. 2, isbuilt on the known ' levels at 111.3 and 741.5 keV in

Er. Three additional excited states at 907.4 (=,' + ),

1066.2 (—,'), and 1482.9 ( —,', —", ) keV are proposed.Their suggested spin assignments (Fig. 2) are based ondeexcitation patterns that are analogous to those of levelslocated in ' Gd (Refs. 5, 6, and 10) and ' Dy (Ref. 6) atabout the same excitation energies. The parent level in149Tm is proposed to be the h~~~2 proton state. Our sug-gestion is that the P decay, as for the other X = 80 odd-Zisotones, proceeds primarily to —,', —", , and —' levels (one ofthese is the new state at 1482.9 keV), which then emit yrays to the lower spin levels.

We were unable to observe an initial growth period in

36 826 1987 The American Physical Society

36 BRIEF REPORTS 827

200159

I .631 I

(o) COINCIDFNCES WITHEr K X RAYS

100

200

1711091(Ho)

796

f 907 955w. :g" ~ ~+:rr..~:.' ...~. "+~'~+Am )err: % ':~ w» ~' v'

. 5« I I

(b) COINCIDENCES WITH111—keV GAMMA

, (Er) gil~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~~eo

I ~ Q ~ ~ ~ 'IM ~

OC3 159

100

0200

171(Er) g

y~e+ ~0 q ~ gP 4y ~ Jg qyQJp ~~ ~ q ~ ~ +1 +0 Q ~ ~ yyO ~ V ~ 0 ~ ~

~ ~

I I

1091X 796 (Ho)

~ ~ ~ I ~ ~ ~ ~ Q ~ ~;+'8' ~ 'o .4o o~'III" &~'i +"~ W r "WQ i~'M~~' ~ 'W' CiL -= k 94M e ~I

(c) COINCIDENCES WITH159—keV GAMMA

100111

171(Er) 417

~0

f ~ ~ J + ~ i ~ eYJ

1091I 796 907 (H )++8 +v+ + + I t + JP4+, ;'v~ Ao~d P+hP vA - NP J =-~ ' vl

50 100 150 200 250CHANNEL NUMBER

300 350 400 450

FIG. 1. Gamma-ray spectra observed in coincidence with Er Ka~ x rays [part (a)], and with the 111-keV [part (b)] and 159-keV[part (c)] y rays that follow the P decay of ' Tm.

TABLE I. Energies and y-ray intensities for transitions ob-served in the decay of ' Tm.

Er (keV

111.3(1)158.8(1)416.7(3)437.4(2)796.2(2)907.3(3)'955.2(3)'

Iy (Relative)

100'175(35)150(50)

—100"-250—120—120

'Normalization point for intensities. This y ray has an M1 mul-tipolarity (Ref. 4); the total transition intensity is therefore 316on the same relative scale.Intensity deduced from coincidence spectra. Peak in singles in-

cludes a y ray of the same energy which belongs to another3= 149 nuclide.'Gamma ray observed only in coincidence spectra.Gamma ray is not in coincidence with the 111.3-keV transition;

hence it does not proceed between the 1066.2- and 111.3-keVlevels.

the decay curve of the 630.2-keV transition that deexcitesthe 9-s —", level at 741.5 keV, due to the following: (1)the low isomeric decay branch of 2.7% (Ref. 4); (2) thelarge independent production cross section for ' Erand, (3) the presence of a y ray with the same energyfrom the decay of ' Ho. However, the relative intensityof the population to this level from ' Tm decay was es-timated as follows: by assuming an isomeric transitionbranch of 2.7% for ' Er (Ref. 4), a 2:1 preponderanceof ' Tm decays via the 111.3-keV level over directground state transitions (estimated from the 796.2- and907.3-keV y-ray intensities), a calculated productioncross section ratio (' Er /' Tm) of 25, and acorrection for the expected higher ionization efficiency of40% for erbium versus thulium, we find that about 70%%uo

of all ' Tm direct decays and indirect feedings fromhigher-lying levels lead to the —", isomer at 741.5 keV.

Figure 3 shows systematics of levels below 1.5 MeV inthe three known N =81 even-Z isotones with Z )64. Be-sides the earlier noted ' regular behavior of the s&~2, d3/2,and h ~ ~ ~2 neutron-hole states, one can also see the smooth

828 BRIEF REPORTS 36

(9/ — 11y —)

0.9 sf4969™80

Q -1& Mev

3482.9

1066.2

&- 1.0C3CLILJ

LLJ

Z.'O~~ 0.5C3&CLL)

&45Gd f47D)j &49Er

(~„,)(dp )

h

deS)~

907.4 FIG. 3. Systematics of neutron states in the à =81 nuclei' 'Gd, ' Dy, and ' Er.

741.5

I/2

PO

Og)

CU0

( ((

1(

$49esE'si

0.0(kev)

trends with proton number of the second set of states, i.e. ,between 1.0 and 1.5 MeV. In ' Gd and ' Dy these havebeen proposed ' ' to be the d5/2p f7/7 and g7/Q neutron5, 6, 10

hole states. The interested reader is referred to the paperof Pakkanen et al. ' for detailed arguments, which indi-cate that, while these single-neutron orbitals in ' Gd aresomewhat fractionated, their strengths are primarily con-centrated in the three levels shown in Fig. 3 above thehii/2 state.

The new ' Er levels at 907.4 and 1066.2 keV, whichwe suggest are the ds/7 and f7/2 states, respectively, fttwell into the energy systematics shown in Fig. 3. Wewere unable to observe, either in singles or in coincidence

FIG. 2. Proposed partial decay scheme for ' Trn. Observedcoincidences are indicated by dots. An overall ' Tm feed of—70% was estimated (see the text) for the 741-5-keV ' 'Er level.The probability for isomeric decay of this level has been estimat-ed (Ref. 4) from systematics to be 2.7%, thus, most of the decaystrength from the 741.5-keV state proceeds via P decay to levels

in '" Ho.

spectra, evidence for the g7/2 state in ' Er. Thegp/2~d3/2 transition is the one that primarily establishesthe g7/2 levels in ' Gd (Refs. 6 and 10) and in ' Dy(Ref. 6). In the /3 decays of their respective parents theg7/2 levels in ' Gd and ' Ho are less heavily populatedthan the dq/q and f7/2 states; the ' Gd g7/p level, in fact,was not reported in the original work on ' Tb /3 decay.Therefore, given our small yield for ' Tm, it is notsurprising that we do not observe deexcitation of the g7/2state in ' Er.

We conclude this paper by noting that delayed protonswere observed in ' Tm decay. Its P-decay daughter,

Er, is a proton emitter that has sharp peaks in its parti-cle spectrum (see, e.g. , Ref. 11). A gate was set on alldetected protons and a careful analysis of the coincidentK x rays was made. An Er Kn~ peak above the Ho Ka~ xrays was observed. Also, a weak 0.9-s component wasresolved in the delayed-proton decay curve dominated byprotons from ' Er. By using the total ' Tm f3 decay in-tensity derived from our data (see the discussion above)we estimate the P-delayed-proton branch of ' Tm to beabout 2& 10 with an uncertainty factor of 2.

Oak Ridge National Laboratory is operated by MartinMarietta Energy Systems, Inc. for the U.S. Department ofEnergy under Contract No. DE-AC05-84OR2 1400.Work at the Lawrence Berkeley Laboratory was support-ed by the OfTice of Energy Research, Division of NuclearPhysics of the Office of High Energy and Nuclear Physicsof the U.S. Department of Energy under Contract No.DE-AC03-76S F00098.

Permanent address: Soreq Nuclear Research Center, Yavne70600, Israel.

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