Properties of low-energy transitions in Dy161

Nuclear Physics 67 (1965) 428--432; ( ~ North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

PROPERTIES

OF

LOW-ENERGY

TRANSITIONS

IN

Dy tsz

R. T. BROCKMEIER and JOHN D. ROGERS t

California Institute of Technology, Pasadena, California tt Received 10 November 1964 Abstract: The decay ofTb xexto Dy xsx has been studied using the bent crystal spectrometer. Energies

and intensities of ten gamma transitions have been determined, and the results have been used to derive the reduced multipole transition probabilities of the various transitions. EI

I

RADIOACTIVITY TblSZ[from TbZSO(2n,7,)]; measured Ey, I~,. Dylsx deduced B(~.).

[

1

1. I n t r o d u c t i o n

W e present the results o f a study of the decay o f T b 16~ to D y 16~ using the D u M o n d 2 m bent crystal spectrometer. The results o b t a i n e d are i n good agreement with the decay scheme proposed b y H a n s e n et al. ~) a n d the recent accurate c o n v e r s i o n elect r o n data o f G r a h a m et aL 2). The existence of several very weak transitions is confirmed. 151.77

5/2-Vz

t03.05

/

7/z- 5/z

102

~

9/'2+9/2

74.58

312-312

43.84

7/Z +T/Z

25.65

512 -512

0 E

512+5/'z ,,oy~','

x ~, k

Fig. 1. Partial level scheme of Dyiez according to refs. z, 2). The energies are from this work except for the level at 102 keV. The level scheme o f D y 161 is shown in fig. 1. It was possible to observe ten t r a n sitions in the region f r o m 25 to 150 keV, varying in intensity b y a factor o f 104. T h e t Present address: Institute of Physics, University of R.G.S., Pbrto Alegre, Brasil tt This work has been supported in part by the U.S. Atomic Energy Commission 428

LOW-ENERGY TRANSITIONS

429

accurate intensity measurements which can be made under such conditions with the bent crystal spectrometer, together with the multipole mixing parameters of ref. 2) allow the determination of the reduced multiple transition probabilities for all observed transitions.

2. Experimental Details Sources of Tb ~6x were produced by double neutron capture in Tb203. About 5 mg of Tbz 03 enclosed in a 0.2 m m diam. quartz capillary was irradiated in the M T K at Arco, Idaho, and sources of ~, 0.3 Cur were obtained. The density of the source material determined by weighing the capillary before and after filling was 2g/era 3. The energies of the transitions observed were measured according to the standard techniques used in this laboratory 3). No attempt to obtain the ultimate resolution was made. The relative counting rates of each transition were also determined, and from these the intensities of the lines were determined using the techniques and spectrometer calibration of Edwards 3). The low energies of several of the transitions stu~ed in this decay posed some special problems for the measuring of intensities since the previous calibration of the l I E 2 dependence of the reflectivity did not extend below about 70 keV. To justify the extrapolation of this dependence to 48 k e y we measured the ratios of the intensities reflected in first and second order for several gamma rays ranging in energy from 48 to 300 keV. The work of Seppi and Boekm on a bent germanium crystal 4) suggested that the deviation of this ratio from a constant at low energies should be a sensitive measure of the breakdown of the l I E z dependence. The results of a study on bent quartz crystals 4) showed no evidence for a deviation from the l I E 2 dependence for energies down to 48.9 keV, the lowest energy studied. The correction for self-absorption in the source is another significant problem in these measurements. The correction due to this absorption is ~ 30 % for the worst case in this study, that of the 57.2 keV gamma ray (the 48.9 keV gamma ray is below the K absorption edge). The errors quoted on the intensities would allow a variation of the density between 1 and 3 g/cm 3. In this study special emphasis was laid on the study of some very weak transitions which could be expected from the level scheme. To this end very careful scans of some limited regions of the sprectrum were made. The entire spectrum between 25 keV and 600 keV was studied in a less accurate scan run, but the limit of intensity of unobserved lines may be somewhat higher than the intensity of some observed lines.

3. Results The energies and intensities of the observed transitions are given in the first two columns o f table 1. (No attempt to measure the intensities of the 25.65 and the 28.72 keV lines was made). In table 2 limits of intensity for some unobserved transitions are listed.

430

R.T.

BROCKMEIER A N D J. D. ROGERS

T h e t h e o r e t i c a l c o n v e r s i o n coefficients o f S l i v a n d B a n d s) h a v e b e e n u s e d to c o m p u t e t h e t o t a l t r a n s i t i o n intensities s h o w n in t a b l e 1, c o l u m n 4. T h e y c a n b e c o m p a r e d t o t h e results o f G r a h a m et aL 2) s h o w n in c o l u m n 5. T h e s o u r c e o f t h e d i s c r e p a n c i e s b e t w e e n t h e s e m e a s u r e m e n t s is n o t u n d e r s t o o d . It c a n b e n o t e d t h a t t h e r e s o l u t i o n o f t h e s p e c t r o m e t e r in this r e g i o n is h i g h e n o u g h t o e l i m i n a t e c o m p l e t e l y t h e p o s s i b i l i t y t h a t t h e s t r o n g e r lines h a v e a d o u b l e t struct u r e 6). TABLE 1

Properties of transitions in Dy TM following the decay of Tb TM 1

2

3

4

5

6

E:,

I:,

~tot

/tot

/tot

Multipolarity

3 × 10-2 7.6 2.7 < 2 × 10-8 1.65 4 × 10-2 3 x 10-2 2 × 10-t 2 x 1 0 -~

E1 M1 99.6 7o MI +0.4 ~o 1 97 y, M l + 3 y, E2 E1 E1 53 ~o M1+47 ~o E2 E1 E1 60 yo M1+40 y, E2

25.655 28.72 48.918 57.200 59.23 74.577 77.42 87.93 103.07 106.15 ~) b) e) d) e) ~)

(3) (1) (4) (5) (3) (9) (4) (5) (3) (7)

1.62 0.16 2.0x 1 5.4x 2.0 × 1.2 × 8.9×

2.5 15.8 3.6 11.5 1.1 0.65 5.6 0.44 0.29 1.8

10-8 (25 ~,) 10-a 10-2 10-0 10-s

7.5 2.0 4.4x 10-8 (25 ~o) 1.65 3.6 x 10-2 2.9 x 10-8 1.5 × 10-8 2.5 x 10-8

Results of this work. Estimated error 10 Yoexcept as noted. Results are normalized on 74 keV transition. Theoretical values of Sliv and Band 5) for ct~+ctL+u~. Values derived from columns 2 and 3. The estimated error is 10 Yo except where noted. Intensity values of ref. 8). The results are normalized to/tot 74 = 1.65. Results of ref. 2). TABLE 2 Limits of intensities of unobserved transitions E

Ir

Cqot

43.84 131.77 201

0.01 0.01 0.05

8.5 0 0

/tot 3 × 10-8 4) 0.01 0.05

Multipolarity 96 ~,M1 + 4 yoE2 4) E1

") Ref. 2).

4. Discussion T h e a v a i l a b i l i t y b o t h o f a c c u r a t e g a m m a r a y intensities a n d o f a c c u r a t e m u l t i p o l e m i x i n g r a t i o s f o r t h e v a r i o u s t r a n s i t i o n s in D y 161 a l l o w s t h e d e t e r m i n a t i o n o f r a t h e r a c c u r a t e r e d u c e d t r a n s i t i o n p r o b a b i l i t i e s . W e h a v e u s e d t h e e x p e r i m e n t a l d a t a in t w o different w a y s t o o b t a i n s u c h r e d u c e d t r a n s i t i o n p r o b a b i l i t i e s . T r a n s i t i o n p r o b a b i l i t i e s o f t r a n s i t i o n s o r i g i n a t i n g f r o m t h e 25.65 k e V a n d t h e 74.58 k e V levels w e r e o b t a i n e d f r o m t h e m e a s u r e d h a l f lives (2.8___0.2 x 10 - s sec a n d 3.0___0.3 nsec, r e s p e c t i v e l y 6 , 7 )

LOW-ENERGYTRANSI~ONS

431

using the experimental data. Transition probabilities o f transitions originating f r o m the 43.77, 103.6 and 131.77 keV levels were obtained f r o m the experimental data under the assumption that the E2 parts o f the transitions between two members o f the same b a n d were correctly given by the collective model relationship a) B(E2), Ii ~ If) =

5 Q2(IiK2OIIfK)2 '

using Qo = 7.38 e 2 determined f r o m C o u l o m b excitation o f the g r o u n d state band 9). (The reduced transition probabilities are related to the g a m m a ray transition probability T~ = 0.693/t, by the relationships s) B(M1) = 5.68 x IO-5Te(M1)/E3r, B(E2) = 0.81 x 102 T,(E2)/E¢, B(E1) = 6.29 x 10 -9 Te(EZ)/E~a, with T in sec-1, E in keV, B(E2) in units o f e2(b2) ~, B ( M 1 ) in units o f (eh/2Me) 2. The values obtained are collected in table 3. These results are in qualitative agreement with the predictions o f Bes lo) who considered the effects o f Coriolis mixing for this nucleus. However, these accurate and extensive values provide the basis for a m o r e detailed comparison o f the rotational b a n d structure o f D y 16~ to the unified model predictions. TABLE 3

Reduced gamma-ray transition probabilities of transitions in Dy~Bx

Ee

ItKt

6K t

25.65 28.72 43.84 48.92 57.20 59.23 74.577 77.42 87.93 t03.07 [06.15 [31.77

~-~ {--] ~r+~ ~-~ ~}-~r t-~ ]-] ~-~ ~-t ~}-~{-.-'] -~--~

]+t {--~ {~+~ {~-~ ~]-~ {+~ ~+~ ]-~ {+~ ~-+~ ~--j ~+~-

) ') ) ) )

By(MI)

B~(E2)

Be(E1) 2.6 × 10-e (25 %) e)

1.34× 10-' (25 %) e) 6.2× 10-3 (20 %) e) 1.9x 10-~ (15%) a) 0.137 (15 %) e)

1.94 (5 %) b) 4.6X 10-2 (50~) a) (1.86) ~) 1.6 × 10-a (20 %) e) 3.8 X 10-7 (15 %) a)

9.2x 10-3 (15 %) e)

(1.94) n) 5.4× 10-~ (25 %) e) 1.7 × 10-" (20 %) e)

7.2x 10-' (25%) e)

1.6× 10-' (25~0) e) < 10-~e)

Values assigned from collective model theory using 120 of the ground state band 9). 10 % error is assigned. Pet'. e). Based on the measured lifetime 7). Based on the measured lifetimes of the 74.7 keV level and the measured intensities. Determined from comparison to rotational E2 transitions and the measured intensities. A part o f the analysis reported here was carried out by one o f us (John Rogers) during the tenure o f a National Science F o u n d a t i o n fellowship at the Institute o f Theoretical Physics in Copenhagen, D e n m a r k . He would like to t h a n k Professor Aage Bohr for the hospitality offered during this period.

432

R. T. BROCKMEIER

AND

J. D. ROGERS

References 1) P. (3. Hanson, O. Nathan, O. B. Nielsen and R. K. Sheline, Nuclear Physics 6 (1958) 630 2) 1t. L. Graham, J. S. (3eiger and (3. T. Ewan, Bull. Am. Phys. Soc. 6 (1961) 72; quotation in P.F.M. Koehler, L. Slack and N. B. (3ore, Phys. 11ev. 130 (1963) 1503 3) W. F. Edwards and F. Boehm, Phys. Rev. 121 (1961) 1499 4) E. J. Seppi et al., Nucl. Instr. 16 (1962) 17; D. M. Baker and J. D. Rogers, Internal Report, California Institute of Technology (unpublished) 5) Gamma-rays, ed. L. A. Sliv (Academy of Sciences, USSR, Moscow, 1961) 6) Nuclear Data Sheets (National Academy of Sciences, Washington, D.C.) 7) J. Lindskog et al., in Perturbed angular correlations, ed. by E. Karlsson, E. Matthias and K. Siegbahn (North-Holland Publ. Co., Amsterdam, 1964) 8) K. Alder et al., Revs. Mod. Phys. 4 (1956) 432 9) B. Elbek, K. O. Nielsen and M. C. Oleson, Phys. Rev. 108 (1956) 406 10) D. R. B~s, Nuclear Physics 6 (1958) 645