Beta-gamma directional correlation in the decay of 148Pm

I

~

NuclearPhysics A l l 8

(1968) 3 3 - - 4 0 ; ~ )

North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

BETA-GAMMA DIRECTIONAL CORRELATION IN THE DECAY OF 14Spm R. A. AMADORI, N. K. GUPTA and K. S. R. SASTRY

Department of Physics and Astronomy, University of Massachusetts, Amherst, Massachusetts t Received 28 June 1968 Abstract: The directional correlation involving the 1930 keV fl-group in the decay of the 5.4 d 14spin and the 551 keV cascade y-ray in lasSm has been measured at nine fl-ray energies in the range 1050-1700 keV. The correlation coefficient As is found to be small and positive (As ~ +0.02) in fair agreement with the expectations in the ~-approximation and the work of Baba et al. The present results do not support the rather large negative values reported by Nainan and Moore. An attempt is made to interpret the data in terms of the fl-decay matrix elements. An essentially isotropic correlation is measured for the 1-(fl)l-(y)0 + cascade. E

RADIOACTIVITY 14spm[from 147pm(n,y) ]; measured fly(O).

]

1. Introduction The decays of the 5.4 d g r o u n d state and the 43 d isomeric level of 148pm to levels in 148Sm have been investigated by Schwerdtfeger et al. 1), Baba et al. 2) and others 3). The intense ground-state to ground-state ( 1 - ~ 0 + ) fl-transition (see decay scheme 2) in fig. 1) has been f o u n d to display a strong deviation from the statistical shape 1,4). I n their study o f the first-forbidden rank-one matrix elements 5) (I r, I i~, I ia x r) governing this transition, Baba et al. 4) have shown that the experimental shape factor can be understood only in terms of the relationship between S i~ and S r predicted by Fujita 6) on the basis of the C V C hypothesis 7). The deviation o f the fl-spectrum f r o m the allowed shape is ascribed to art accidental cancellation of the matrix elements, while the retardation of the decay is presumed to be due to the smallness of the matrix elements themselves. In view o f the above findings, one m a y expect to obtain interesting information by an investigation o f the characteristics of the 1 - ~ 2 + inner fl-transition. The rather large l o g f t value ( ~ 9.4) suggests that this fl-group m a y show deviations f r o m n o r m a l behaviour (i.e. large fl-7-anisotropies, energy dependent spectrum shapes etc.), which would facilitate investigation of the fl-decay matrix elements (the three rank-one matrix elements mentioned above and .[ Bq of rank two) contributing to this transition. A n u n a m b i g u o u s determination o f these parameters requires accurate and independent information on at least three fl-decay observables besides the f t value. A t W o r k s u p p o r t e d by the U.S. A r m y R e s e a r c h Office, D u r h a m , N o r t h Carolina. 33

34

R.A. AMADORIet al.

series of experiments have been undertaken in this laboratory to obtain such information. In this paper, experimental results on the directional correlation involving the 1-(fl)2+(7)0 + cascade will be presented. According to Baba et al. 2), the A 2 coefficient in the distribution function N(O) oc [1 +A2P2(O)] is < +0.04 for 1500 keV < Ep < 1800 keV, whereas Nainan and Moore 8) have recently reported rather large negative A 2 values ranging from - 0 . 0 7 to - 0 . 1 6 in the energy interval 800-1600 keV. Such a strong discrepancy between the results of both the groups provided additional motivation for these measurements. 5D 146pm 0%(7"7) OkeV keV 1465 (9.4) 0

%O9"1

551

z+

o

o÷

148Sm

Fig. 1. Decay scheme of 5.4 d l=Pm.

2. Experimental details and resnlts The 5.4 d 14Spm activity was produced by the l*TPm(n, 7)14Spm reaction. The 7-spectrum was measured using a 7 mm deep Ge(Li) planar detector with a resolution of 5 keV for the 661 keV 137Ba ~-ray. All the observed 7-rays could be explained on the basis of the decay schemes proposed by Baba et aL 2) for the 148pm isomeric pair. No new transitions were seen, and there was no evidence of other source impurities (apart from ~47pm). The 43 d ~4aPm activity did not interfere with the measurements, since the maximum energy of the intense t-group in this decay is only 690 keV, which is well below the energy range (1050 keV to 1700 keV) of interest in this work. It was necessary to confine the measurements to t-energies above 1000 keV to prevent interference from the 1- ~ 1- t-transition. Sources were prepared by evaporating a drop of the active PmCI3 solution onto a 6.2/~m mylar foil supported on an aluminium ring. The insulin method was used to obtain sources of fairly uniform thickness ( ~ 0.1 mg/cm2). The source thickness was too small to cause attenuation of the directional correlation involving energetic t particles. The lifetime of the intermediate state (551 keV) is too small 9) (7 ps) to affect the correlation.

148Prn DECAY

35

The directional correlation measurements were performed using a conventional scintillation/?-V coincidence spectrometer with five slow-fast type coincidence channels which permitted simultaneous measurements at five different/?-ray energies. An NE 560 plastic scintillator (3.81 cm diam. x 1.27 cm thick) served as the B-detector and a 3.81 cm diam. x 3.81 cm thick NaI(TI) crystal was used for detection of v-rays. Coincidence resolving times (2z) of about 40 nsec were used. The source and the fixed /?-detector were kept in vacuum. Disturbing effects of scattered v-rays were strongly suppressed by means of a 3.81 cm thick lateral lead shield around the N a I crystal. Only the photopeak of the 551 keV v-ray was accepted in the gamma single-channel analyser. The source centering was accurate to the extent that the v-singles rates at 90 ° and 180 ° positions were equal to within a few tenths of a percent. The/?-detector was calibrated using the K-conversion electrons from 2°Tpb. The 14Spm directional correlation data were obtained at nine/?-ray energies in the interval 1050-1700 keV. Window widths of 50 keV were used in the beta single-channel analysers. The intense 1 - ~ 0 +/?-transition rendered the ratio of true coincidences to accidentals rather unfavourable. At lower energies, this ratio was about 4 and at higher energies about 2. N o evidence for V-V coincidence background was observed. The data were corrected for chance coincidences, source decay, finite solid-angle effects and finite resolution of the /?-detector and backscattering effects in the usual manner. Considerable interference was expected from the contribution of/?-bremsstrahlung coincidences due to the strong 2480 keV/?-transition. An attempt was made to evaluate this effect by shifting the v-lower level discriminator beyond the photopeak of the 551 keV v-ray keeping the window the same. The coincidence counting rate at 90 ° was found to be about 0.5-0.75 % of the rate measured on the 551 keV peak. While there was evidence for a large negative anisotropy, the beta-bremsstrahlung coincidences were too few to permit an accurate measurement of the anisotropy. If the theoretical anisotropy 10) of - 1 were assumed, our results would be slightly enhanced. TAm~ 1 Summary of l~sPm directional correlation data Present work E# (keY)

1070 1140 1200 1270 1330 1400 1470 1610 1650

Results of others A2

+0.0244-0.003 +0.020+0.003 +0.0364-0.004 +0.0194-0.004 +0.023+0.007 +0.0274-0.004 +0.027 4-0.003 +0.041 4-0.008 +0.019±0.010

E# (keY)

A~

Nainan and Moore s) 800 --0.0664-0.009 1000 --0.095 4-0.010 1100 --0.1034-0.013 1300 --0.1374-0.016 1500 --0.1474-0.013 1650 --0.156 4-0.015 Baba et aL a) 1500-1800 < +0.04

36

g.A. AMADORIet al.

The experimental results are given in table 1 as well as the results of Nainan and Moore s) and Baba et al. 2) for comparison. Our data consistently show that the A 2 coefficient is small and positive ( ~ +0.02) over the energy range 1000-1700 keV in agreement with the result of Baba et al. In view of the strong discrepancy between the present results and the work of Nainan and Moore, several tests have been performed to ascertain the reliability of our instrument. (i) The fl-y anisotropy for the 1 - ( f l - ) l - ( y ) 0 + cascade in l~Spm decay was measured without disturbing the source position and was found to be - 0 . 0 0 5 __+ 0.003. The true anisotropy, however, would be considerably smaller when corrected for fl-bremsstrahlung coincidence contribution ( ~ 0.4 ~ ) . The very small anisotropy obtained is consistent with expectations for an allowed fl-transition and is in general agreement with similar results obtained for a large number of allowed fl-transitions in this laboratory 11) and elsewhere 12). (ii) The source was deliberately off-centred so that the y-singles ratio Nr(90°)/Nr(180 °) was 1.02 and 0.98, respectively. The 1930 keV fl-551 keV y-anisotropies in both cases were consistent with a small correlation. (iii)The anisotropy for the well-known ~a_+ ( e - ) ~ - - ( y ) l - cascade in 2 o7pb was measured to be + 0.315 + 0.005 in fair agreement with the theoretically expected value of +0.33 for this cascade; due account was taken of the smearing out of the angular distribution by the finite size of the detectors. (iv) Finally, the fl-~ correlation involving the 3-(fl)2+(~)0 + cascade (W o = 1850 keV, Er = 123 keV) in 154Eu was measured using a solid chloride source. This cascade was chosen mainly because of the similarity of A 2 values in this case to those reported by Nainan and Moore for 148pm. Our values for A 2 ( W ) , namely, - 0 . 1 1 5 + 0 . 0 0 6 (1050), - 0 . 1 2 8 + 0 . 0 0 6 (1200), - 0 . 1 4 3 ___0.008 (1350), --0.154-t-0.010 (1500) and -0.166_+0.012 (1600), are in good agreement with the results published in the literature 13). The numbers in the parentheses represent the respective fl-ray energies in keV. The above considerations show that our ~48pm measurements are dependable. Very recent results of two independent groups 14) are essentially in agreement with this work. 3. Discussion According to Kotani

15), the

directional correlation coefficient t A2 is given by

Az = ( p 2 / W ) ( R 3 k + e k W ) / C ( W ) , where W is the total energy of the electron in mc z units, p the momentum, R3 k, ek and the shape factor C (W) are functions of the parameters x, u, y and z, which represent the relevant matrix elements as follows: ~/x = - Cv lr,

~/u = CA Iia x r,

n y = - C v Ii~,

n z = c A I B , s,

t This corresponds to the coefficient e in Kotani's notation.

37

148pm DECAY

q being the standard matrix element to be determined by t h e f t value. Two convenient linear combinations o f these matrix elements are also defined r

= y-~(u+x),

~ = Y+(u-x)~Wo, where the C o u l o m b energy factor ~ = ~Z/2R. In this section, an attempt is made to interpret the experimental results on t4apm in terms of the above parameters. On the basis of our experimental data, the following qualitative statements m a y be made. First of all, it m a y be noted that the measured A 2 values (plotted in fig. 2 as a

0.25

-

-

Pure

Bij

0.20

0.15

0.10

0.05

,C I 3.0

3.5

W (mc 2 )

4.0

4.5

Wo 4.7

Fig. 2. Plot o f As as a function o f fl-energy W. T h e expectations for a 1-(fl)2+(y)O + cascade (with

only the B~ matrix element contributing) are shown by the curve designated pure B~j. The solid curves A-C are generated by the respective sets of the Kotani parameters given in the text. function of W) are very different f r o m theoretical expectations for a pure B u. transition (x -- u = Y = 0, z = 1), which indicates that the rank-one matrix elements play a significant role in understanding the 1930 keV fl-transition in ~48Pm. Secondly, the reduced correlation coefficient, (W/p2)A2 is very small ( ~ + 0.007)and is essentially independent o f energy within experimental limits (see fig. 3). In the i - a p p r o x i m a t i o n [ref. 16)], in which normal first-forbidden fl-transitions are adequately described, one would expect (W/p2)A2 to be energy independent with a magnitude 15) of about 1/(64). F o r 148pm, ~ is 13.8, and l(W/p2)Az] is a b o u t 0.01. Even t h o u g h these considerations show that the fl-transition m a y be understood as a C o u l o m b transition, information on the shape factor and fl-7(CP) correlation should substantiate such a conclusion.

38

R . A . AMADORI et al.

I n o r d e r t o see w h e t h e r a s e l e c t i o n r u l e o r c a n c e l l a t i o n effect is c o n s i s t e n t w i t h o u r data, a simple analysis has been carried out using Kotani's theoretical expressions f o r z42 w i t h t h e f o l l o w i n g c o n s t r a i n t s : (i)

0.03

(W/p2)A2

=

+ 0 . 0 0 7 a t * W = 3.2, (ii) t h e

(V~2) A 2

0.02

T I

O.Ol

--T ....

3.0

--~

............

B,C

4.0 W(mc2)

4,5

5.0

Fig. 3. Energy dependence of the reduced correlation coefficient (W/p~)Av The solid curves are obtained from Kotani's theoretical expressions for the sets A-C given in the text.

2,0

/C

C (W)

j.

/./'/

/./"B

1.5

1.0

3.0

-

4.0

-

A

4~

5.0

W(mcz) Fig. 4. Predicted behaviour of the fl-spectrum shape factor C(W) for the sets of parameters A-C. The calculated shape factors are normalized at W = 3. t This value of the reduced correlation coefficient may be larger up to about 20 % depending on the extent of corrections for fl-bremsstrahlung interference. The conclusions drawn from this analysis are not significantly affected by this uncertainty.

l~Pm D E C A Y

39

solutions generated for the nuclear parameters should satisfactorily explain the experimental data over the energy range of measurement. It was found convenient to choose the matrix element combination ~i as the standard ((1 = 1) to be determined by t h e f t value. The number of unknowns was reduced to two (x and z) by using the relation, y --- A~x, A being a constant whose value for the decay under investigation is 1.2 according to Ahrens and Feenberg 17) and 2.6 according to Fujita 4). The analysis essentially consisted of finding the values of x and z consistent with the constraints and was carried out for A = 1.2 and 2.6. The results may be summarized as follows: (i) The experimental data cart be explained by several sets of the parameters x and z for both values of A. (ii) For A = 1.2, no combination of parameters that would generate a statistical shape could be found. (iii) The solutions obtained for A = 2.6 fall into three categories. For small z, almost statistical shapes are predicted; for moderate ( ~ 1) as well as for large values of z, the predicted shape-factors are energy dependent. As art illustration we have chosen the following three sets: (A)

x=0.06,

u=

0.02,

z=

(B)

x=

7.41,

u=

12.35,

z=

(C)

x=

5.84,

u=

9.72,

z=

0.11, 100

,

1,

Y=

1.07,

~t=

1,

Y = -6.84,

~1 = 1,

Y=-5.17,

(1 = 1.

Each of the above sets satisfactorily explains our data (see figs. 2 and 3). The parameters in sets B and C are characteristic of selection-rule effect and cancellation effect, respectively, and predict measurable deviations from statistical shape (see fig. 4). It may be noted, however, that the usual model-dependent selection rule effects ~s) (K-forbiddenness and j-forbiddenness) are not applicable in this decay, the former because the parent and daughter nuclei are presumably spherical, and the latter because the transforming nucleons belong to different major shells. The relative magnitudes of the parameters in set A are in agreement with the expectations in the Capproximation, in as much as a statistical shape is predicted by this set. One may tend to favour this set somewhat in view of the experimental observation of an essentially linear Fermi plot for this E-transition by Baba et aL 4). More definite conclusions should await the availability of further information from shape-factor and ~-y(CP) correlation experiments. We take this opportunity to express our gratefulness to Professor H. J. Fishbeck for informing us of their results prior to publication and for drawing our attention to the work of Wyly et al. The assistance of J. R. Horgan is deeply appreciated.

References 1) 2) 3) 4)

C. F. C. V. S. K. C. V.

Schwerdtfeger, E. G. Funk, Jr. and J. W. Mihelich, Phys. Rev. 125 (1962) 1641 K. Baba, G. T. Ewan and J. F. Suarez, Nucl. Phys. 43 (1963) 264 Bhattacherjee, B. Sahai and C. V. K. Baba, Nucl. Phys. 12 (1959) 356 K. Baba, G. T. Ewan and J. F. Suarez, Phys. Lett. 3 (1963) 232

40 5) 6) 7) 8) 9) 10) 11) 12) 13)

14) 15) 16) 17) 18)

R.A. AMADORIet aL E. J, Konopinski and G. E. Uhlenbeck, Phys. Rev. 60 (1941) 308 Jun-Ichi Fujita, Prog. Theor. Phys. 28 (1962) 338 R. P. Feynman and M. Gell-Mann, Phys. Rev. 109 (1958) 193 T. D. Nainan and S. E. Moore, Nucl. Phys. A94 (1967) 257 M. J. Martin, Nuclear Data Sheets for A = 148; Nucl. Data B2-4-96 (1967) J. K. Knipp and G. E. Uhlenbeck, Physica 3 (1936) 425; D. Pfeifer and K. Runge, Z. Phys. 183 (1965) 195 K. S. R. Sastry, R. J. Ouellette, Y. Sharma and R. Strange, Phys. Lett. 26B (1968) 207 S. Cipolla, Z. W. Grabowski, H. M. Naser and R. M. Steffen, Phys. Rev. 146 (1966) 877 K. S. R. Sastry, R. F. Perry and R. G. Wilkinson, Phys. Rev. 123 (1961) 615; L. D. Wyly et al., Phys. Rev. 124 (1961) 841; J. W. Sunier, Helv. Phys. Acta 36 (1963) 429 H. J. Fishbeck, private communication; L. D. Wyly et al., Bull. Am. Phys. Soc. 13 (1968) 248 T. Kotani, Phys. Rev. 114 (1959) 795 T. Kotani and M. H. Ross, Phys. Rev. Lett. 1 (1958) 140 T. Ahrens and E. Feenberg, Phys. Rev. 86 (1952) 64 G. Alaga, K. Alder, A. Bohr and B. R. Mottelson, Mat. Fys. Medd. Dan. Vid. Selsk. 29, No. 9 (1955); R. W. King and D. C. Peaslee, Phys. Rev. 94 (1954) 1284