Conversion-electron-gamma and gamma-gamma directional angular correlations in 160Dy

I

~

Nuclear Physics A211 (1973) 581 --588; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout written permission from the publisher

CONVERSION-ELECTRON-GAMMA

AND GAMMA-GAMMA

DIRECTIONAL ANGULAR CORRELATIONS

I N 16°Dy

F. C. ZAWISLAK, J. D. ROGERS and E. A. MENI~SES t Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brasil tt Received 12 June 1973 Abstract: Gamma-gamma and conversion-electron-gamma angular correlations in 16°Dy have been measured for the 298 keV-966 keV and 298 keV-879 keV cascades. Particle parameters of the 966 keV E2 transition were determined to be b2(E2; e/~)= +1.234-0.08 and b2(E2; etL+tM) = +1.274-0.23. The multipole mixing ratio for the 298 keV radiation was determined to be 6(M2/EI) = +0.04-4-0.01 and we have confirmed the value ~(E2/M1) = -- 13 4-2 for the 879 keV. The E0-E2 mixture parameter for the 879 keV transition was determined as qK = --0.03 4-0.09 assuming penetration effects to be negligible.

E/ t

RADIOACTIVITY

16°Tb [from ISgTb(n,y)]; measured 7y(0),

ce7(0 ). 16°Dy

transitions deduced 6, 2, qK.

I

I

1. Introduction T h e nucleus t 6 ° D y (fig. 1) has been studied very intensively. The e x p e r i m e n t a l d a t a before 1964 m a y be f o u n d in ref. 1); m o r e recent results are f o u n d in refs. 2 - 6 ) . T h e decay scheme is well k n o w n a n d spins a n d parities are well established for levels below 1.0 MeV. The nucleus 16°Dy is a d o u b l y even d e f o r m e d nucleus having at 0, 87 a n d 283 keV the first three levels o f the g r o u n d state r o t a t i o n a l b a n d ; the levels at 966 a n d 1049 keV are the 2 + a n d 3 + m e m b e r s o f the y - v i b r a t i o n a l b a n d with K = 2. E x p e r i m e n t a l evidence f r o m ~-~ a n g u l a r c o r r e l a t i o n a n d conversion electron m e a s u r e ments indicate t h a t the k n o w n states l o c a t e d a b o v e 1050 keV have o d d p a r i t y , being d e p o p u l a t e d m a i n l y by E l t r a n s i t i o n s with small M 2 a d m i x t u r e s to the levels o f even parity. I n the present investigation we study the n a t u r e o f some t r a n s i t i o n s in 16°Dy by using s i m u l t a n e o u s l y the techniques o f conversion-electron-~ (e-~) a n d 7-~ directional a n g u l a r correlations. E l e c t r o n conversion particle p a r a m e t e r s for p u r e E2, mixed M1 + E2 a n d E1 + M 2 transitions are m e a s u r e d a n d analyzed with the i n t e n t i o n o f d e t e r m i n i n g accurate m u l t i p o l e mixing ratios o f the transitions involved. t Present address: Instituto de Fisica Gleb W a t a g h i n , Universidade Estadual de C a m p i n a s , S~o

Paulo, Brasil. tt Work partially supported by Conselho Nacional de Pesquisas (Brasil), and The International Atomic Energy Agency (Vienna). 581

582

F.C.

ZAWISLAK

et al.

2. Experimental procedures Radioactive t6°Tb was obtained by neutron irradiation of pure natural terbium oxide. The e-7 angular correlations were measured with thin sources of the radioactive material deposited on a 200/~g/cm 2 Mylar foil supported by a plastic ring. A diluted drop of the radioactivity, TbCI 3 dissolved in HC1, with known mass was deposited on the Mylar together with diluted insuline (insuline having a small surface tension allows a more uniform mass distribution of the liquid drop). The thickness of the source was 200/~g/cm 2 and the uncertainty in thickness was estimated to be 30 %. The source was a circular spot 5 mm in diameter. Gamma-gamma angular correlations were measured with the same source and also with a liquid source of TbCI 3 in 3M HC1. The 7-Y spectrometer consisted of a planar Ge(Li) detector and a NaI(TI) scintillator with a conventional cross-over coincidence system and a multichannel analyzer. The e-7 coincidences were measured with the same electronic system but using as electron detector a circular Si(Li) counter with 80 mm 2 of area and 2 mm deep. The Si(Li) detector was cooled to liquid-nitrogen temperature by a cold-finger and placed in a cylindrical thin wall aluminum vacuum chamber having 14 cm in diameter. In measurements using a Si(Li) electro~ spectrometer a serious problem is the efficiency of the detector for y-rays. For high-energy transitions the Si(Li) detector has a low photo-efficiency for y-rays, but the conversion coefficients of these transitions are also generally small and as a result detection of photons may compete with electrons. For angular correlation experiments the corrections due to scattered photons are very important because they show angular dependence. In all the measurements reported in this paper we determined, independently for all the angles, the contribution of the y-spectrum using an electron absorber in front of the Si(Li) detector. The scattering of electrons in the source is also a severe problem, especially for thick sources and low-energy electrons. In our measurements relatively high-energy electrons are involved and this problem is less severe. The data were analyzed using the well-known expression of the integral directional angular correlation function:

w(o) = y~ 4,,,~,,(cos o), k even

where Akk = A k ( ~ ' l ) A k ( ~ 2 ) and Akk Ak(ex)Ak(~,2) for Y-7 and ex-~ correlations respectively. Expressions for Ak(~2) and Ak(ex) are given in ref. 7). For the particular case of a 2 + --* 2 + transition where both E0 conversion and penetration effects are non-negligible the expression for A2(eK) of interest to this experiment is =

Ak(eK) = b2(M1)F2(l122)Q2(2)+26,b2(M1, E2)F2(1, 2, 2, 2)Q~(2) +26e~eb2(EO, E2)+f~b2(E2)F2(2, 2, 2, 2)] x [Qo(2)+5~+ 3e]2 where

[:(L')I+

6o = 6 L ~ J

E0 K-shell electron intensity '

S# =

M1 K-shell electron intensity '

-J. ,

D I R E C T I O N A L CORRELATIONS

583

and where b k are the internal conversion particle parameters, ~(L) is the internal conversion coefficient of the transition with multipolarity L and Qo(2), Q2(2) and Q~(A) are polynomials in the penetration parameter 2. These coefficients and parameters are tabulated in refs. 8-11). The phases of the multipole mixing coefficients and fi~ follow the convention of ref. 7). For mixed transitions the data have been analyzed in terms of the coefficient 1;2 = A22(e'y)/A22(Y-Y). For a pure multipole transition/;2 reduces to the particle parameter b2.

3. Experimental results 3.1. P E R F O R M A N C E C H E C K OF THE Si(Li) SPECTROMETER

In order to check the performance of the Si(Li) electron spectrometer, we measured initially e-y correlations in 2°Tpb, comparing our results with the measurements performed by Kleinheinz et aL 12) in the same nucleus using a magnetic spectrometer. The y-y angular correlation coefficients for the ~+(1064keV)½-(570keV)½sequence in 2°7pb are very well known 13); a mean value of all measurements gives A22 = +0.231__+0.003 and A44 =-0.023--+0.003, associating multipolarities M4 (with 0.1% admixture of E5) for the 1064 keV transition and E2 for the 570 keV transition. TABLE 1 e-7 angular correlation coefficients for the 1064 keV-570 keV cascade in "°TPb and particle parameters for the transition of 1064 keV A22

A44

b, particle parameters present work

7_~,a) eg-), ezL-~ eZM-)'

+0.2314-0.003 +0.2304-0.005 +0.2424-0.010 -t-0.2204-0.020

--0.023±0.003 --0.0174-0.008 --0.0304-0.015 --0.0154-0.030

+l.00:k0.03 -t-1,054-0.05 +0.954-0.10

Kleinheinz

Bosch

et al. 12)

et al. 14)

+0.994-0.02 +1.024-0.04 b)

+1.054-0.05 +1.004-0.12 +1.164-0.15

theory

+1.047 +1.038 -t-1.030

a) Ref. 13). b) This value corresponds to ~ L + ~ M shells.

With a 2°7Bi source prepared in the same way as the Tb film source, we measured simultaneously the angular correlations between the 570 keV y-ray and K, ~ L (z~L means the unresolved Ll + L n + . . . lines) and ~ M conversion electrons of the 1064 keV transition. Table 1 shows the measured Akk(e-y ) coefficients corrected for accidentals, solid angle and scattering in the source. The obtained particle parameters are in column 4. The data of Kleinheinz et aL 12) obtained with a magnetic spectrometer are in column 5. More recent measurements of the same parameters by Bosch et al. ~4), also using a Si(Li) spectrometer are in column 6, and the last column show3 the

584

F.C. ZAWISLAK

et aL

theoretical calculation for finite size nuclei 1o). The agreement between all the experimental results as well as with the theory is very good. 3.2. THE

7"7

AND e-7 DIRECTIONAL CORRELATIONS IN l~°Dy

In this subsection we report our measurements of 7-7 and e-7 angular correlations for various sequences in 160Dy (fig. 1). The experimental ~4kk coefficients are shown in table 2; all measurements have been performed twice and the results are the mean values of both independent runs. The coefficients have been corrected for accidentals (5 ~ ) , decentering (1 ~), for solid angle and for Compton admixtures due to photons o f higher energy. This last correction was made in the displayed energy coincidence spectra. For the electron spectra, in addition, the continuous fl- spectra lying under the converted electron peaks were subtracted. 3-

7

'~4 o

1555

1408 ,599

(5-) - - ~ 4-

2-

5(I-) 24+

5÷ 2*

6+

4+

0*

leODy Fig. 1. Partial decay scheme of 16°Dy.

E(KeV)

585

D I R E C T I O N A L CORRELATIONS TABLE 2 Akk coefficients for 2'-2' and e-7, angular correlations in 16°Dy Cascade 2982'-9667 2982'-879,/ 2987(8797)872' b) 1977,.877 c) 2987,-966K 2982'-966 ( E L + Z M ) 2982'-879K 298K-9667,

A 2z

.+.0.2204-4-0.010 --0.081 4-0.003 a) --0.0474-0.006 +0.0704-0.007 q- 0.270 4-0.012 +0.2804-0.050 --0.0874-0.012 --0.420±0.025

A4.~

4-0.010±0.015 -]-0.001 -t-0.003 ") 4-0.0104-0.010 4-0.010+0.010 -+-0.020 4-0.020 4-0.0304-0.070 -+-0.0204-0.020 -+-0.0304-0.040

.) Weighted average of measurements in refs. 2.6). b) Triple angular correlation with the 879 keV transition not observed. Correction has been made for attenuation due to the 87 keV transition obtained for the 1977'-877' cascade. ~) Used to determine Gz = 0.704-0.07 for the 87 keV state. TABLE 3 Electron conversion particle parameters bz for transitions in 16°Dy Conversion electron 298K 966K 966 ( E L + E M ) 879K

Transition 2 - ( E l + M 2 ) ~ 2 +" 2+'(E2) ~ 0 + 2+'(E2) -+ 0 + 2+'(M1 + E 2 ) -+ 2 +

bz (exp)

--1.90t0.15 -+-1.23±0.08 .+.1.27±0.23 .+.1.07±0.15

The correlations involved pass through the 2 +' state at 966 keV which has a lifetime of 3 psec or through the first 2 + state at 87 keV with a lifetime of 2 nsec. To check for possible attenuation due to internal fields in the solid source the ~-~ angular correlation of the 298 keV-966 keV cascade was measured both for the solid and the liquid source. Since the results were identical it was assumed that attenuation of correlations passing this state was negligible. The attenuation factor for the first 2 + state in the liquid source was determined from a measurement of the 197 keV-87 keV, 4÷(E2)2+(E2)0 + cascade to be Gz2(OO) = 0.70___0.07. The electron conversion particle parameters shown in table 3 are obtained from the experimental angular correlation coefficients quoted in table 2. 4. Discussion of the results

The experimental 966 keV transition K-particle parameter, ~2(966 keV; %:)-= +1.23-t-0.08, agrees with the calculated value lo) for a pure E2 transition, b2(966 keV, E2; eK) = + 1.20. There are no available theoretical calculations of b2(966 keV, E2; %M) for this energy, but a reasonable extrapolation indicates that b2(966 keV, E2; %L+~M) = + 1.15 which is also in agreement with the experimental value quoted in table 3.

586

F . C . Z A W I S L A K et aL

The results of the 298 keV-966 keV ~-y angular correlation assign spin 2 for the 1265 keV level and the conversion coefficient of the 298 keV transition confirms the odd parity of the state, suggesting a small M2 admixture in the predominantly E1 transition between this 2 - state and 2 + state of the y-band. Earlier measurements of?-y angular correlations ascribe different values and signs to the multipolarity admixture parameter 6(M2/E1) for this transition. Giinther et al. 2) find 6 = +0.029_+0.005, Jaklevic et aL 4) 6 = -0.02___0.02 and Krane and Steffen 6) determine 6 = + 0.005___0.010. It should be noted however that the theoretical values of A22 for 6 = - 0 . 0 2 and +0.03 are +0.27 and +0.24 respectively. Thus the measurement of 6 from this coefficient is highly sensitive to eventual small systematic errors in the experiment. From our complementary measurement of the 298K-966y angular correlation, which is more sensitive to the multipolarity admixture in this transition, we obtain /~2(298 keV; e K ) = - 1 . 9 0 + 0 . 1 5 (table 3). Using theoretical conversion coefficients ~K(L) and K-particle parameters from refs. 9, ~o), the experimental results for both V-y and e~:-7 correlations give a common solution for the multipolarity admixture of the 298 keV transition: 6(298 keV) = + 0.040_ 0.010. A similar analysis assuming 6 < 0 reveals that with 6(298 keV) = - 0 . 0 2 the particle parameter should be b2 = - 1 . 3 in complete disagreement with the experimental value, ~2 = - 1.90-t-0.15. From previous measurements 2, 4) of angular correlations involving the 879 keV transition the weighted mean value of the M1-E2 mixing ratio in this transition is 6(879 keY) = - 13_+2. We analyze our result for the 2 - (298)2 + (879 K)2 + angular correlation (table 2) to study the existence of the E0 de-excitation mode in competition 200

,~

REGIONCOMPATIBLEWl7

I00

0

REGION COMPATIBLEWITH~'N eK- ~" CORRELATION N -0.2 0 0.2 qk

-I00-,,

~

~

'

,

,

Fig. 2. Values ofqK a n d 2 consistent with c~Ka n d A2 (eK) for the 2 +" (879 keV) 2 + transition plotted in the q~:-2 plane.

DIRECTIONAL CORRELATIONS

587

with M I + E 2 for this 2 +' ~ 2 + transition in 16°Dy. In fig. 2 we show the common values of 2 and q~ = 3e[6e, measuring respectively penetration and E0 conversion effects, which are compatible with the experimental K-conversion coefficient 15-17) ~ = (3.38 + 0 . 1 0 ) x 10-3 and the eK-v angular correlation coefficient for this transition. From the figure, upper limits of qK = 0.0-+0.17 and ;L = 20T-70 can be found. Since one expects that penetration effects should in fact be negligible in a transition between the v-vibrational and ground state band of a rotational nucleus, it is also of interest to note the lower limits of error on the E0 contribution for this case. We obtain with 2 = 0, qK = - 0.03 _+0.09. Fig. 2 has been obtained with 6 (879) = - 13 + 2 and the theoretical values of particle parameters and conversion coefficients from refs. 9-xl). Our value of qK = -0.03_+0.09 obtained for 2 = 0 agrees with a recent measurement of qK = -0.03_+0.10 by Zupancic et al. is) for the same transition. We report also a measurement of the triple V-V-V angular correlation 2-(298)2+(879)2+(87)0 + with the 879 keV transition not observed. The ARk coefficients shown in table 2 for this cascade were determined with a liquid source and are corrected for the attenuation in the 87 keV level with Tqr = 2 ns. Using the known values of the multipolarity admixture parameters for the 298 keV and 879 keV transitions (the 87 keV transition is pure E2) we calculate for this triple angular correlation A22 = - 0 . 0 4 5 + 0 . 0 0 3 and A4 g 5 x 10 -3 in excellent agreement with the experimental values quoted in table 2, confirming in this way 6(298 keV) = = +0.040+0.010 and 6(879 keV) = - 13-+2.

5. Summary and conclusions The experimental electron conversion particle parameter of the 2 + (966 keV) ~ 0 + transition, ~2 = +1.23_+0.08 is in good agreement with the calculated value b 2 = + 1.20 for a pure E2 transition. Since this is a collective transition whose transition rate is enhanced relative to the single particle estimate, no effects of penetration are expected, in agreement with our results. The spin and parity of the 2 - state at 1265 keV was confirmed by our e-~, directional correlation measurement and the magnitude and phase of the M2/E1 mixing ratio of the 2-(298 k e V ) ~ 2 + transition depopulating this level was determined to be 6(298) = +0.040+0.010. Our result also shows that the sign of 6(298) is definitely positive, because the calculated particle parameter for a small and negative 6(298) is in complete disagreement with the experimental value ~2(298 keV, e s : ) = = -1.90_+0.15. The M2/E1 mixing ratio for the 215 keV transition from this 2 - state to the 3 + state of the v-band has also been measured 4, 6). The result of 6(215) = - 0 . 1 9 +_0.05 indicates that the B(M2) for the 2 - ( M 2 ) --, 3 + transition is about 20 times that of the 2 - ( M 2 ) --, 2 + transition whereas the Alaga rules would predict factors of 1.75, 0, and 1.75 assuming that the K o f t h e 2 - state is 0, 1, or 2 respectively. The anomalous experimental value provides further evidence of the large admixtures in the wave

588

F.C. ZAWISLAK et aL

f u n c t i o n o f this state from different K-values as proposed in ref. s), a n d c o n s e q u e n t cancellations between the matrix elements involved i n the transition. I n this respect it is interesting to note that since the M 2 c o m p o n e n t s o f these transitions are o f the order o f the single-particle strength, these b r a n c h i n g ratios furnish a m u c h more reliable test o f the theoretical wave functions t h a n do the highly hindered E1 transitions, as the latter m a y be highly influenced by very small admixtures in the wave functions. O u r e-3, m e a s u r e m e n t also confirmed previous determinations of the large E2/M1 ratio in the 2 + (879 keV)2 ÷ t r a n s i t i o n between the y-vibrational b a n d a n d the g r o u n d state band. A value o f 8 ,,~ - 10 has been predicted by T a m u r a a n d Yoshida 19) in good agreement with the experimental value o f -13-4-2. The negative phase is consistent with systematics in other deformed nuclei. Our analysis o f the 2-(298~,)2+(879K)2 + a n g u l a r correlation indicates that the E0 admixture in the 879 keV t r a n s i t i o n is negligibly small, with qK = -0.03-t-0.09. This result is consistent with the fact that the E0 t r a n s i t i o n between K = 2 + a n d K = 0 + is K - f o r b i d d e n whereas the E2 c o m p o n e n t o f this transition is strongly enhanced. This value o f qK, together with the k n o w n lifetime o f 3___1 psec for the 966 keV level 20) can be used in the formulas o f ref. 2~) to derive a value for the reduced nuclear matrix element for E0 conversion, p. A value of p = ( - 3 _ 9 ) x 10 - 3 was obtained, which is, within the errors, consistent with the theoretical predictions for deformed nuclei as reported in ref. ~s).

References 1) Nucl. Data Sheets, 1964 2) C. Giinther, G. Strub~, U. Wehmann, W. Engels, H. Blumberg, H. Luig, R. M. Lieder, E. Bodenstedt and I4. J. K6rner, Z. Phys. 183 (1965) 472 3) S. L. Gupta and N. K. Saha, Nucl. Phys. 70 (1965) 203 4) J. M. Jaklevic, E. G. Funk and J. M. Mihelich, Nucl. Phys. A99 (1967) 83 5) C. Gtinther, H. Ryde and K. Krien, Nucl. Phys. A122 (1968) 401 6) K. S. Krane and R. M. Steffen, Nucl. Phys. A164 (1971) 439 7) A. J. Becker and R. M. Steffen, Phys. Rev. 180 (1969) 1043 8) L. C. Biedenharn and M. E. Rose, Rev. Mod. Phys. 25 (1953) 729 9) R. S. Hager and E. C. Seltzer, Nucl. Data A4 (1968) 1 10) R. S. Hager and E. C. Seltzer, Nucl. Data A4 (1968) 397 11) R. S. Hager and E. C. Seltzer, Nucl. Data A6 (1969) 1 12) P. Kleinheinz, R. Vukanovic, L. Samuelsson, D. Krmpotic, H. Lindstr6m and K. Siegbahn, Nucl. Phys. A93 (1967) 63 13) Nucl. Data 135 (1971) 226 14) H. E. Bosch, L. F. Gatto, M. Behar and G. J. Garcia, Phys. Rev. CI (1970) 242 15) G. E. Keller and E. F. Zgarkiar, Nucl. Phys. A147 (1970) 527 16) J. F. W. Jansen, J. 1-L Hamilton and E. F. Zganjar, Internal conversion processes, ed. J. H. Hamilton (Academic Press, N.Y., 1966) 17) M. A. Ludington, J. J. Reidy, M. L. Wiedenbeck, D. J. McMillan, J. /-I. 14amilton and J. J. Pinajian, Nucl. Phys. All9 (1968) 398 18) M. Zupancic, I. Bikit, D. Cvjeticanin and L. Marinkov, Z. Phys. 252 (1972) 237 19) T. Tamura and H. Yoshida, Nucl. Phys. 30 (1962) 579 20) Y. Yoshizawa, B. Elbek, B. Herskind and M. C. Olesen, Nucl. Phys. 73 (1965) 273 21) A. V. Aldushchenkov and N. A. Voinova, Nucl. Data A l l (1972) 299