The decay properties of 19.0 min 162Yb

1.E.I: I 3.A [

Nuclear Physics A196 (1972) 362--368; ~ ) North-HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout written permissionfrom the publisher

THE D E C A Y P R O P E R T I E S OF 19.0 min 162yb P. F. A. GOUDSMIT, F. W. N. DE BOER, B. J. MEIJER and M. BOGDANOVI~ t Institute for Nuclear Physics Research, Amsterdam, the Netherlands Received 19 May 1972 (Revised 23 August 1972) Abstract: The decay properties of 162yb (19.0 min) were studied with Ge(Li) and Si(Li) spectrometers; ~-7 and e-~, coincidence measurements were performed. In contratliction to earlier results, the 44.7 keV 7-ray is found to be mainly an MI transition, indicating that its initial and final levels are the 2- and I - members of a K : 1 ground-state rotational band. Nilsson assignments to all levels involved are discussed. An upper limit of 15 ns determined for the halflife of the 163.46 keV state means that the F-selection rule in its decay does not act strongly. E

RADIOACTIVITY 162yb [from 16aEr(aHe, 5n), 166Er(aHe, 7n)]; measured T.t_, Ey, I~,, Ice, 7"7, cev'coin, TY-delay; deduced logft, 162Tm deduced levels, J, ~, cc, 7-multipolarity. Ge(Li), Si(Li) detectors.

I. Introduction T h e existence o f a 1 6 2 y b activity with 21 min < T.~ < 26 min was r e p o r t e d in 1963 by A b d u m a l i k o v et al. 1). I n their study o f the conversion-electron s p e c t r u m ( m e a s u r e d with a m a g n e t i c s p e c t r o m e t e r ) these a u t h o r s r e p o r t e d Lu, Lut a n d M u c o n v e r s i o n lines o f a 40.9 keV transition. M o r e recently Paris z) r e p o r t e d a half-life o f 14.7 min. His study o f the 7-ray s p e c t r u m (with a G e ( L i ) d e t e c t o r ) a n d the conversion-electron s p e c t r u m (with a Si(Li) s p e c t r o m e t e r ) yielded transitions at 44.7___0.1 keV (E2), 118.8___0.1 keV ( E l ) a n d 163.5+__0.1 keV ( E l ) ; the first energy value being in gross d i s a g r e e m e n t with t h a t o f ref. 1). The 118 a n d 162 keV transitions were also o b s e r v e d by C h u 3) who d e t e r m i n e d a half-life o f 18.87+0.14 m i n for 16Zyb. In the decay scheme constructed by Paris levels at 44.7 keV ( 2 - ) a n d 163.5 keV (1 ÷) decay to a 0 g r o u n d state o f 16ZTm(21.55+0.30 min [ref. 4)]). A g r o u n d - s t a t e spin I = 1 was, however, recently r e p o r t e d for the 21.5 m i n 162Tm activity by E k s t r 6 m et al. 5) as m e a s u r e d with an a t o m i c - b e a m technique. In view o f these discrepancies between the e x p e r i m e n t a l results o f several g r o u p s a n d their interpretation, it seemed worthwhile to re-investigate the decay o f this i s o t o p e **

2. Source production Sources o f 1 6 2 y b were p r o d u c e d by the (3He, x n ) reactions with x = 5 a n d 7 in the internal 3He b e a m o f the Institute's s y n c h r o c y c l o t r o n on targets o f the oxide o f * On leave from the Institute of Nuclear Sciences "Boris Kidric", Beograd, Yugoslavia. !* A preliminary report 6) on the present investigation was presented in 1971 ; the results reported then have recently been confirmed by Gromov et al. 7). 362

162yb DECAY

363

mass-enriched ~6¢Er (76~o) and ~66Er (95~o). For y-ray spectroscopy the irradiated powder was used immediately after the irradiations as a source. Sources for conversion-electron studies were prepared by evaporation of a few drops of a solution in HCI of the target material• l

I

i

'

i

,

i

d

i - -

od to

~05k

o

~

x

~

~©

~

~

~

~

200

o~

~ ~r-.,

~

o

r--

,,

c,i~

to

..:. 91 ' ~

~o~

~X

~

~. o ©

~$

~o~

"-

~

• m

~o~

~

600

400

800 1000 chonnel n u m b e r ~,

Fig. 1. The F-ray spectrum from an ~6ZYb source produced by the ~ E r ( a H e , 5n) reaction measured with a 7 mm thick Ge(Li) detector. Energy values and mass assignments for F-rays belonging to ytterbium isotopes are given above the spectrum; those for thulium isotopes are given below. For a compilation of ~6aYb F-rays, see ref. ~6).

TABLE

1

Energies, relative intensities and conversion coefficients for transitions belonging to the decay of 19.0 min ~62Yb E7

(keV)

Iy

ct (exp) E1

K X 44.66±0.06 118.77±0.04

230 ±60 6.4± 2.2 6.1± 1.6 a) 69.0± 3.5

163.47±0.04

100

L: 9.8 ~3.4 L: 9.8 i 3 . 0 a) K:0.25 ~0.18 L: 0.036±0.013

0.43 0.18 0.028

K: 0.083 ±0.012 L: 0.014±0.002

~t [theory 14)] E2 M1

73

Multipolarity

4.2

MI÷E2

0.70 0.71

1.6 0.25

El

0.076

0.29

0.66

El

0.Ol2

0.17

0.099

The y-ray intensities have been normalized to 100 units for the 163.47 keV),-ray transition. ") Obtained from the present coincidence measurements (sect. 4).

364

P.F.A. GOUDSM1T

et al.

3. Measurements of y-ray and conversion-electron spectra

The y-ray spectrum of 162yb was measured with a 7 mm thick surface-barrier Ge(Li) detector (Philips, resolution F W H M = 700 eV at 122 keV). In spite of the high resolving power of the y-ray detector (see fig. 1), the 44.7 keV transition was found to be hidden by the strong complex K X-ray peak. The line was made visible in a separate measurement with a critical absorber (europium oxide) placed between source and detector. In this manner the intensity of the K X-ray peak was decreased by a factor 20 whereas the 44.7 keV y-radiation was reduced by a factor 3 only. The intensity of the lines was followed for a period of several hours; as a result we obtained a value of 19.0 ± 0.5 rain for the half-life of *62yb" Values of 14.7, 18.87 and 14.5 min have been respectively reported previously by other authors 2, 3, 7). In order to further ascertain assignment to 162yb the spectra of sources produced at 72 MeV and 60 MeV 3He bombarding energy on enriched 166Er targets were compared. No new y-rays were observed in the decay of 162yb (table 1). A list of thus far unpublished y-ray transitions in the decay of ~63yb, a by-product of the present study, will be presented elsewhere 16). The spectrum of the conversion electrons was measured with a Si(Li) detector (Kevex, 2 mm thick, resolution F W H M = 2.5 keV for the 34 keV electron line). The y-ray spectra from the conversion sources were measured simultaneously with a 10.6 ~ Ge(Li) detector. A careful analysis of the conversion-electron spectra showed that the 34 keV electron line is a doublet (44.66 L with a contribution from the 91.39 K line in the decay of 164Tm) and the 118.77 K peak appeared to contain contributions from the Kp2(Er), Kpl(Tm), Kp2(Tm) and the L-conversion peak of the 68.9 keV E2 transition s) in the decay of 9.0 min 165yb. The results are listed in table 1. The intercalibration between the conversion-electron and the y-ray intensities was obtained, assuming ~K = 0.112 [ref. 14)] for the 227.52 keV E2 transition in the daughter activity. The intensity of the 44.66 keV y-ray is found to be 4 times higher than was reported by Paris 2). In order to ascertain that the y-ray peak at this energy is indeed the 44.66 keV transition, a y-y coincidence measurement was performed. 4. Measurement of y-? and e-y coincidences

The coincidence between the 118.78 and the 44.7 keV transitions was deduced in the present investigation from 7-7 as well as from e-7 coincidence measurements. In both cases the y-rays were detected with a 10.6 % Ge(Li) detector; the second detector being either the thin surface-barrier Ge(Li) or the Si(Li) detector. Use was made of a dual ADC (2 x 4096 channels)-PDP8 computer system. Digital windows on the output of the first detector were set on the position of the 118.77 and 163.47 keV peaks and on the backgrounds. The spectra from the second detector coincident with pulses from these four channels were stored simultaneously. The time resolution (2v) of the coincidence system was approximately 100 ns. In figs. 2 and 3, the y-ray and conver-

162yb D E C A Y

365

sion spectra measured in coincidence with the 118.77 and the 163.47 keV transitions are shown. This measurement supports the M1 assignment for the 44.66 keV transition since it could be shown (after correction for source-thickness and detectorwindow effects) that the conversion occurs mainly in the L~ shell. From a comparison of the singles and 119-? coincidence spectra (fig. 3) it is clear that no conversion peaks of the 40.9 keV transition reported by Abdumalikov 1) are visible. The numerical results on the coincidence intensities are presented in table 2. A separate measure10000

i

I

K(5 (Er), K(~2(Tm) i

Singles

T

FK~I (Tm) K~

(Er) ~[ ~

r-

u~ 5000

~1.~ I~1 ~

K~I(Tm),K/~2(Er)

"6

E ~

2oo

,~ Rh(Tm)

163

1 100

lit

,

I

ii K~I (Tm) ...... .'.,I :b. .._

................... .,-.:-<,S:-.:~:,'.:

119

K,h(Tm) -

200 K~12(Tm)

-7

Kj~I(Tin)

100

i

44.66

!

i

K,:~ (Tin) ..................

100

,,JL-::J 150

,,_,.,,,:...,

200

,. ........

250 channel number

Fig. 2. G a m m a - r a y spectra measured in coincidence with the 118.77 and 163.47 keV 7-rays with a Ge(Li)-Ge(Li) arrangement. A singles v-ray spectrum is s h o w n for comparison in the upper frame. N o correction was made for accidental coincidences.

ment with a system of high time resolution [NaI(T1)-Ge(Li)] was made to determine the half-life of the 163.46 keV level. A delayed-coincidence experiment between the K X-radiation [NaI(TI)] and the v-rays of 118.77 and 163.47 keV [Ge(Li)] gave an upper limit of 15 ns.

366

P . F . A . GOUDSMIT et al. ]

"I

t

200C

!

l

Ko/4(Er),~+z(Tm )

--

Singles

104.3Q-K( ~' Tn~/i K "+ " ~u~.t.~-r,t. ira) .. ~rA_~uge~ ! ~lKp,, (Er)

3 1000

vcc,dT~, ;466-L,91.39-K(~Tm) I

£3

.,,,,.

E

I " I ;'~_t( I: • i ", --J (Er)"

. " :."" ,.--:, .,.P

;

(rm)31877 '

j't,'\

"

-

+q;+,~,, i

50 I

(Tm) Auger

i E

i

~ ~n L "~ ~

~

. •.

. . ..

163

K(ez(Trn) ,Koq --

,+t ~

K~ (Tm) r

~ . ~.,... . . t~-....:,:.. i j . . ~._~=~__..+.,.+'/~,~,.,. "..... • . J

44.66-L

119

K~I (Tin) i~___~_44.66 -M Ko~a(Tm/Y -----q I K/3~ (Tin) . ,,Y~ ), .,~; ~4d,~ " " :.m'i~q~" ~ / * . ":-" .+ ~+,

,-

• •

50

,

100

.-.'M~q...:.---.~

150

¢......,~L..-_..~. 200

I

250

channel n u m b e r

Fig. 3. Conversion-electron spectra measured in coincidence with the 1•8.77 and 163.47 keV v-rays with a Si(Li)-Ge(Li) arrangement. A singles conversion-electron spectrum is shown for comparison in the upper frame. No correction was made for accidental coincidences. TABLE 2 Numerical results from 7 - 7 and e-7 coincidence measurements Gates

44.66- 7 44.66-L 44.66-M K~ K#

118.77 keV

163.47 keV

4.7=1= 1.0 46 ± I 0 8 ~ 3 43 ± 2 10 ± 1

< 0.8 < 4 < 3 62 :L 3 14=}=2

The normalization of the intensity values is based on a theoretical value of 76 % for the KX1637 coincidence; here the values for the fluorescence and relative K-capture yields were taken from ref. 15).

5. Discussion T h e d e c a y s c h e m e o f Paris is c o n f i r m e d e x c e p t for the spin o f the g r o u n d state (see fig. 4). T h e M1 a s s i g n m e n t d e d u c e d f r o m the p r e s e n t c o n v e r s i o n m e a s u r e m e n t f o r t h e 44.7 k e V t r a n s i t i o n , s u p p o r t s the d i r e c t m e a s u r e m e n t s) o f the 162Tm g r o u n d - s t a t e spin I = I. T h e m e a s u r e d t h u l i u m K X - r a y i n t e n s i t y (table 1) f o r t h e 1 6 2 y b d e c a y gives an u p p e r limit o f 40 % for the direct p o p u l a t i o n b y / / - t r a n s i t i o n o f t h e 1 - g r o u n d state. T h e m e a s u r e d half-life o f 1 6 2 y b t h e n results in a v a l u e l o g f t < 5.0 f o r t h e / 3 b r a n c h to the 1 + state at 163.46 k e V (the d e c a y e n e r g y Q = 2.3 M e V was t a k e n f r o m

162yb DECAY

367

Wapstra and Gove 9)). The allowed unhindered character of this fl-transition is in agreement only with an assignment p[523]T-n[523] ~ for the 163.46 keV state. Comparison with systematics for the available orbits in this region for the 69th proton and the 93rd neutron [e.g. ret, 10)] indicate that this state is indeed expected at low excitation energy in 1 6 2 T m . The systematics also indicate that the t 6ZTm ground state most likely corresponds to the p[411]~-n[521]T combination. On the basis of the

0 ÷

19.0 rn -.-r-

-2.I3 MeV ~

/'(>60"/* (:*~÷, < 5.0)

/ f

1+

1+

p [ 523] t.-n[ 523] I

21p [411] 1- n [521]!

21-

~" ,~.c~

1M1Ts°" E2 162~ 69 m93

21.5m

Fig. 4. T h e decay s c h e m e o f ~62yb d e d u c e d f r o m the present data. T h e decay energy Q = 2.3 M e V has been t a k e n f r o m ref. 9). T h e 44.66-118.77 coincidence relation is indicated with a dot. T h e total intensity o f the 44.66 keV transition is d e t e r m i n e d as 384-8 units a n d 414-19 units f r o m t h e coincidence (table 2) a n d v-ray singles m e a s u r e m e n t s (table 1 ) respectively.

experiments, the spin of the 44.7 keV state can be 2- or 1 -. According to systematics, no low-lying 1- states are expected apart from the ground state. The only possible combinations would be the p[404]J,-n[523]J, and the p[523]T-n[642]T ones, but for both possibilities the Gallagher-Moszkowski coupling rule predicts the 6- combination at lower energy. This would then result in the existence of an isomeric state for which no indication has been found thus far 4, it). The energy of the 44.66 keV state also favours its assignment as the first member of the ground-state rotational band. The transition between the 163.46 keV state p[523]]'-n[523]J, and the p[411]~n[521] T combination of the ground-state rotational band is of the F-forbidden type i.e. both particles take part in the transition. The experimental hindrance factors Fw < 7.4x 105 and Fw < 3.6x 105 found for the 163.47 and the 118.77 keV transitions with respect to the Weisskopf estimate are, however, not exceptionally large when compared to typical hindrance factors for odd-A El transitions with A K = O:

368

P . F . A . GOUDSMIT et al.

2X 102 < Fw < 2 x 10 5 [ref. 14)]. A similar " v i o l a t i o n " o f the F-selection rule was recently observed 13) in the decay o f 182Os. The au t h o rs are i n d e b t e d to Professors R. van L i e s h o u t an d A. H. W a p s t r a f o r valuable discussions. O n e o f us (M.B.) is grateful f o r the hospitality received at the Institute for N u c l e a r Physics R e s e a r c h in A m s t e r d a m during a research visit o f o n e year on an I A E A fellowship. This w o r k is p a r t o f the research p r o g r a m o f the Institute f or N u c l e a r Physics R e s e a r c h (I.K.O.), m a d e possible by financial s u p p o r t f r o m the F o u n d a t i o n f o r F u n d a m e n t a l R e s e a r c h o n M a t t e r ( F . O . M . ) and the N e t h e r l a n d s O r g a n i z a t i o n for the A d v a n c e m e n t o f Pure R e s e a r c h (Z.W.O.).

Note added in proof: A f t e r the submission o f this article, C h u : ) r e p o r t e d on 7-ray singles an d ~,-Vcoincidence m e a s u r e m e n t s on mass-separated 162yb sources. Excellent a g r e e m e n t is present in energy a n d intensity values between these two i n d e p e n d e n t studies.

References 1) A. Abdumalikov, A. ,Abdurazakov, K. Gromov, Zh. Zhelev, N. Lebedev, B. Dzhelepov and A. Kudryavtseva, Phys. Lett. 5 (1963) 359 2) P. Paris, Compt. Rend. 268 (1969) 1534 3) Y. Y. Chu, CERN yellow report 70-30 (1970) 931 4) F. W. N. de Boer, P: F. A. Goudsmit, J. Konijn and B. J. Meijer, to be published 5) C. Ekstr6m, M. Olsmats and B. Wannberg, Nucl. Phys. A170 (1971) 649, and private communication 6) P. F. A. Goudsmit, F. W. N. de Boer, R. van Lieshout and R. Beetz, IKO progress report 1970/1971 7) K. Ya. Gromov, T. A. Islamov, G. Ishakov, M. Jon, H. Tirroff, E. A. Usmanova, V. I. Fominih, E. I-I:errmann and H. Shtrusny, Progr. and Abstr. of the Soy. Conf. on nuclear spectroscopy and nuclear structure, Kiev, 1972, p. 132 8) T. Tamura, Nucl. Phys. Al15 (1968) 193 9) A. 1-L Wapstra and N. B. Gove, The 1971 atomic mass evaluation, Nucl. Data Tables 9 (1971) nos. 4, 5 10) W. Ogle, S. Wahlborn, R. Piepenbring and S. Frederiksson, Los Alamos Sci. Lab., preprint LA-DC-11253; Rev. Mod. Phys. 43 (1971) 425; M. E. Bunker and C. W. Reich, Rev. Mod. Phys. 43 (1971) 348 11) Y. Y. Chu, Phys. Rev. C4 (1971) 642 12) K. E. G. L6bner, Phys. Lett. 26B (1968) 369 13) S. B. Burson, P. J. Daly, P. F. A. Goudsmit and A. A. C. Klaasse, to be published 14) R. S. Hager and E. C. Seltzer, Internal conversion tables, (Cal. Institute of Technology, 1967) 15) A. H. Wapstra, G. J. Nijgh and R. van Lieshout, Nuclear spectroscopy tables (North-Holland, Amsterdam, 1959) 16) F. W. N. de Boer, P. F. A Goudsmit and B. J. Meijer, to be published 17) Y. Y. Chu, Phys. Rev. C6 (1972) 628