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Lifetime and magnetic moment of the 36.2 keV level of 189Os
V o l u m e 28B, n u m b e r 8
LIFETIME
PHYSICS
AND
MAGNETIC
F. WAGNER,
G. K A I N D L ,
MOMENT H. B O H N ,
LETTERS
OF
3 F e b r u a r y 1969
THE
U. B I E B L ,
36.2
keV
H. S C H A L L E R
LEVEL
189
OF
Os
and P. KIENLE
Physik-Departrnent, Technische Hochschule, Miinchen, Germany R e c e i v e d 24 D e c e m b e r 1968
T h e r e c o i l l e s s n u c l e a r r e s o n a n c e a b s o r p t i o n of t h e 36.2 keV ~ r a y s of 189Os w a s o b s e r v e d . T h e l i f e t i m e ~"= 0.72 * 0.04 n s and t h e m a g n e t i c m o m e n t ~ -- + 0.226 ± 0.029 n m of t h e 36.2 k e V s t a t e (/Tr = ½-) w e r e determined.
Recently the magnetic dipole m o m e n t and the electric quadrupole m o m e n t of the 69.6 keY state of 189Os were determined [1-3] by recoilless nuclear resonance spectroscopy. The resuRs have been discussed in terms of two rotational bands coupled by Coriolis interaction [3]. According to this interpretation the 69.6 keV level (I~ = ~-) is the first excited state of the K ~ = ~ground-state rotational band, and the K ~ --½band is built on the 36.2 keV state (I~ = ~-). In the present w o r k the M~Jssbauer absorption of the 36.2 keV 7 rays w a s observed and the lifetime as well as the magnetic m o m e n t of the 36.2 keV state were determined. Conventional transmission experiments were performed at 4.2°K. Single-line absorbers of metallic Os enriched in 189Os to 87.7 % were used, and the 36.2 keV ~ rays were detected by a high-resolution Si(Li) detector operated with a colled F E T preamplifier. A source of 1891r (T½ = 11 d) was produced by the 187Re(a, 2n)1891r reaction with 30 M e V a particles. After the cyclotron irradiation the R e metal target w a s dissolved in nitric acid, s o m e Ir carrier w a s added, and the Ir was selectively reduced with formic acid [4]. The precipitate w a s annealed in hydrogen at 1000oc for several hours. With this single-line source of 1891r in cubic Ir metal and absorbers of 24.6 m g / c m 2 and 44.3 m g / c m 2 189Os metal two spectra were obtained. One of these is reproduced in fig. la. The magnetic m o m e n t of the 36.2 keV level was determined frorrt the hyperfine splitting of the recoilless ~ line emitted by dilute Os impurities in an iron matrix. A n Ir0.01Fe0.99 alloy w a s prepared from the 1891r activity by melting the constituents in an induction furnace. A thin foil was rolled from the resuRing pellet and annealed in argon at 1000oc. This source served to measure the spectra of 548
1.0
--"°° • • , o
°o
J o+
Z 0.98
0.~-
•
'YJ
o.-t
\ /
;
"
I
.-.f: .."
•o
tO oeo
e••
•
•
•
• +
0.98
0.~
Fig. i. Relative intensityN(v)/N(oo) transmitted through absorbers of (a) 24.6 m g / c m 2 and (b and c) 44.3 m g / c m 2 metallic 189Os. The solid curves are the results of the least-squares fits, the vertical lines indicate the line positions and relative intensities.
Volume28B, number 8
PHYSICS LETTERS
3 February 1969
Table 1 Summary of results obtained with absorbers of enriched 189Os metal and sources of 189Ir in Ir and Fe.
Source
Absorber (mg/em2)iSSOs
½W
"r
(ram/s)
(ns)
g(~/g(12)
189Iri!
24.6
9.0 + 0.7
0.84 ~- 0.07
./.
189IrIr
44.3
12.2 ~- 1.3
0.75 ~- 0.08
./.
189irFe Hex t -----0
44.3
13.3 ~: 1.0
0.69 ~- 0.05
1.11 :~ 0.20
1 8 9 i r Fe Hext = 29.7 kOe
44.3
13.5 ± 1.0
0.68 ~- 0.05
1.01 ± 0.14
fig. l b and c. S p e c t r u m b w a s obtained with the s o u r c e u n p o l a r i z e d ; in s p e c t r u m c the longitudinal m a g n e t i c field of a s u p e r c o n d u c t i n g solenoid, H e x t = 29.7 kOe, e l i m i n a t e d all but the Am = +1 e m i s s i o n lines. A M S s s b a u e r s p e c t r u m of the 69.6 keV ¥ r a y s m e a s u r e d with the m a g n e t i c f i e l d on a g r e e d with that of ref. 3, showing that the s o u r c e was c o m p l e t e l y p o l a r i z e d and f r e e f r o m i m p u r i t i e s of u n al l o y e d Ir. The E2 a d m i x t u r e [5] in the 36.2 keV t r a n s i tion is too s m a l l to n o t i c e a b l y influence the shape of the M 6 s s b a n e r s p e c t r a . A s u p e r p o s i t i o n of six o r four L o r e n t z i a n l i n e s having the r e l a t i v e i n t e n s i t i e s e x p e c t e d f o r a p u r e M1 t r a n s i t i o n was t h e r e f o r e fitted to the Z e e m a n p a t t e r n s . As in ref. 3 the g r o u n d - s t a t e m a g n e t i c s p l i t t i n g w a s c a l c u l a t e d f r o m the NMR v a l u e [6] f o r the m a g n e t i c h y p e r f i n e f i el d at Os in iron, H i = = 1130 + 25 kOe, and kept constant during the l e a s t - s q u a r e s fit of the data. The r e s u l t s a r e c o m p i l e d in t a b l e 1 t o g e t h e r with t h o s e obtained with the s i n g l e - l i n e 189IrIr s o u r c e . As a weighted a v e r a g e one f i n a ll y g e t s g(12)/g(~) = 1.03 + 0.13 and, u s i n g ~(~) = = 0.6565 + 0.0003 nm [7,8], the m a g n e t i c m o m e n t of the 36.2 keV state g(½) = 0.226 + 0.029 nm. T he final e r r o r s include c o n t r i b u t i o n s due to the e r r o r s of the h y p e r f i n e field and the v e l o c i t y calibration. The e x p e r i m e n t a l line widths ½W w e r e c o r r e c t e d for n o n - z e r o a b s o r b e r t h i c k n e s s a c c o r d ing to ½W = (~/T)(1+0.135 - t ) [9] (cf. ta b l e 1). T h e e f f e c t i v e t h i c k n e s s t = n . f . a o was c a l c u l a t e d with f a = 0.841, which c o r r e s p o n d s to a D e b y e - t e m p e r a t u r e 0D = 375°K [10,11] f o r Os m e t a l , and with a o = 4.25 × 104 b. The weighted a v e r a g e of all r e s u l t s is T = 0 . 7 2 + 0 . 0 4 n s . A 1 0 ~ o u n c e r tainty in the c o r r e c t i o n f o r n o n - z e r o a b s o r b e r
t h i c k n e s s is included in the e r r o r . F r o m the known e l e c t r i c field g r a d i e n t in hexagonal Os m e t a l [11] and the n u c l e a r quadrupole m o m e n t [12] the splitting ~ e Q V z z = 0.66 m m / s of the 189Os ground state in the a b s o r b e r s could be c a l c u l a t e d . It is s m a l l c o m p a r e d to the e x p e r i m e n t a l l i n e width, and a d e t a i l e d c a l c u l a t i o n showed that i t s influence on the value obtained f o r T is n e g l i g i b l y small. The 36.2 keV state of 189Os has b e e n a s s u m e d to be the ½-[510] N i l s s o n l e v e l [3,13]. Due to the spin I ~ = ½- i t s wavefunction cannot contain ad m i x t u r e s f r o m the K ~ = ~- g r o u n d - s t a t e r o t a t i o n a l band and the l e v e l may be r e g a r d e d as a f a i r l y p u r e i n t r i n s i c state. F o r n e g a t i v e - p a r i t y s i n g l e n e u t r o n s t a t e s with spin ½ the N i l s s o n m o d el p r e dicts. - zgs]
,
g R being the r o t a t i o n a l g - f a c t o r , which f o r 189Os is given in ref. 14 as gR = 0.32, g s the spin g - f a c t o r of the odd n eu t r o n and a the e n e r g y d e coupling p a r a m e t e r . F o r all r e a s o n a b l e v a l u e s of a the main contribution to g a r i s e s f r o m the g s t e r m and a r a t h e r s m a l l e f f e c t i v e spin g - f a c t o r [15], g s , e f f ~ 0.3 g s , f r e e , has to be u s e d to explain the s m a l l m a g n e t i c m o m e n t of the 36.2 keV l e v e l . Such s m a l l m a g n e t i c m o m e n t s s e e m to be t y p i c a l f o r ½-[510] s t a t e s ; they a r e a l s o found f o r ½-[510] l e v e l s in neighbouring n u cl ei , such a s the g r o u n d - s t a t e s of 187Os (~ = 0.067 nm) and of 183W (~ = 0.117 nm).
The authors wish to thank Dr. G. Schatz for the irradiations in the cyclotron of the Gesellschaft ff/r Kernforschung, Karlsruhe, and Dr. Ursel Zahn for her help in preparing the sources.
549
Volume 28B, number 8
PHYSICS
References 1. B. P e r s s o n , H. Blumberg and M. Bent, Phys. Rev., to be published. 2. M. G. Gregory, B. L. Robinson and S. Iha, to be published. 3. D. Kucheida, F. Wagner. G. Kaindl and P. Kienle, Z. Physik 216 (1968) 346. 4. G.W. Leddieotte, The r a d i o c h e m i s t r y of iridium, NAS-NS-3045 (Office of Technical S e r v i c e s , U.S. Department of C o m m e r c e , Washington, D . C . , 1961). 5. B. H a r m a t z , T. H. Handley and J . W . Mihelich, Phys. Rev. 128 (1962) 1186. 6. M. Kontani and I. Itoh, J. Phys. Soe. Japan 22 (1967) 345.
550
LETTERS
3 F e b r u a r y 1969
7. H.R. Loeliger and L. R. S a r l e s , Phys. Rev. 95 (1954) 291. 8. A. Sehwenk and G. Z i m m e r m a n n , Phys. L e t t e r s 26A (1968) 258. 9. D.A. O'Connor, Nuel. Instr. and Meth. 21 (1963) 318. 10. R . J . M o r r i s o n , A. Atac, P. Debrunner and H. F r a u e n f e l d e r , Phys. L e t t e r s 12 (1964) 35. 11. L. Grodzins and Y. W. Chow, Phys. Rev. 142 (1966) 86. 12. G. Himmel, Z. Physik 211 (1968) 68. 13. Nuclear Data, See. B, 1, No.2 (1966). 14. O. P r i o r , F. Boehm and S. G. Nilsson, Nuel. Phys. Al10 (1968) 257. 15. Z. Bochnacki and S. Ogaza, Nucl. Phys. 69 (1965) 186.
LIFETIME
PHYSICS
AND
MAGNETIC
F. WAGNER,
G. K A I N D L ,
MOMENT H. B O H N ,
LETTERS
OF
3 F e b r u a r y 1969
THE
U. B I E B L ,
36.2
keV
H. S C H A L L E R
LEVEL
189
OF
Os
and P. KIENLE
Physik-Departrnent, Technische Hochschule, Miinchen, Germany R e c e i v e d 24 D e c e m b e r 1968
T h e r e c o i l l e s s n u c l e a r r e s o n a n c e a b s o r p t i o n of t h e 36.2 keV ~ r a y s of 189Os w a s o b s e r v e d . T h e l i f e t i m e ~"= 0.72 * 0.04 n s and t h e m a g n e t i c m o m e n t ~ -- + 0.226 ± 0.029 n m of t h e 36.2 k e V s t a t e (/Tr = ½-) w e r e determined.
Recently the magnetic dipole m o m e n t and the electric quadrupole m o m e n t of the 69.6 keY state of 189Os were determined [1-3] by recoilless nuclear resonance spectroscopy. The resuRs have been discussed in terms of two rotational bands coupled by Coriolis interaction [3]. According to this interpretation the 69.6 keV level (I~ = ~-) is the first excited state of the K ~ = ~ground-state rotational band, and the K ~ --½band is built on the 36.2 keV state (I~ = ~-). In the present w o r k the M~Jssbauer absorption of the 36.2 keV 7 rays w a s observed and the lifetime as well as the magnetic m o m e n t of the 36.2 keV state were determined. Conventional transmission experiments were performed at 4.2°K. Single-line absorbers of metallic Os enriched in 189Os to 87.7 % were used, and the 36.2 keV ~ rays were detected by a high-resolution Si(Li) detector operated with a colled F E T preamplifier. A source of 1891r (T½ = 11 d) was produced by the 187Re(a, 2n)1891r reaction with 30 M e V a particles. After the cyclotron irradiation the R e metal target w a s dissolved in nitric acid, s o m e Ir carrier w a s added, and the Ir was selectively reduced with formic acid [4]. The precipitate w a s annealed in hydrogen at 1000oc for several hours. With this single-line source of 1891r in cubic Ir metal and absorbers of 24.6 m g / c m 2 and 44.3 m g / c m 2 189Os metal two spectra were obtained. One of these is reproduced in fig. la. The magnetic m o m e n t of the 36.2 keV level was determined frorrt the hyperfine splitting of the recoilless ~ line emitted by dilute Os impurities in an iron matrix. A n Ir0.01Fe0.99 alloy w a s prepared from the 1891r activity by melting the constituents in an induction furnace. A thin foil was rolled from the resuRing pellet and annealed in argon at 1000oc. This source served to measure the spectra of 548
1.0
--"°° • • , o
°o
J o+
Z 0.98
0.~-
•
'YJ
o.-t
\ /
;
"
I
.-.f: .."
•o
tO oeo
e••
•
•
•
• +
0.98
0.~
Fig. i. Relative intensityN(v)/N(oo) transmitted through absorbers of (a) 24.6 m g / c m 2 and (b and c) 44.3 m g / c m 2 metallic 189Os. The solid curves are the results of the least-squares fits, the vertical lines indicate the line positions and relative intensities.
Volume28B, number 8
PHYSICS LETTERS
3 February 1969
Table 1 Summary of results obtained with absorbers of enriched 189Os metal and sources of 189Ir in Ir and Fe.
Source
Absorber (mg/em2)iSSOs
½W
"r
(ram/s)
(ns)
g(~/g(12)
189Iri!
24.6
9.0 + 0.7
0.84 ~- 0.07
./.
189IrIr
44.3
12.2 ~- 1.3
0.75 ~- 0.08
./.
189irFe Hex t -----0
44.3
13.3 ~: 1.0
0.69 ~- 0.05
1.11 :~ 0.20
1 8 9 i r Fe Hext = 29.7 kOe
44.3
13.5 ± 1.0
0.68 ~- 0.05
1.01 ± 0.14
fig. l b and c. S p e c t r u m b w a s obtained with the s o u r c e u n p o l a r i z e d ; in s p e c t r u m c the longitudinal m a g n e t i c field of a s u p e r c o n d u c t i n g solenoid, H e x t = 29.7 kOe, e l i m i n a t e d all but the Am = +1 e m i s s i o n lines. A M S s s b a u e r s p e c t r u m of the 69.6 keV ¥ r a y s m e a s u r e d with the m a g n e t i c f i e l d on a g r e e d with that of ref. 3, showing that the s o u r c e was c o m p l e t e l y p o l a r i z e d and f r e e f r o m i m p u r i t i e s of u n al l o y e d Ir. The E2 a d m i x t u r e [5] in the 36.2 keV t r a n s i tion is too s m a l l to n o t i c e a b l y influence the shape of the M 6 s s b a n e r s p e c t r a . A s u p e r p o s i t i o n of six o r four L o r e n t z i a n l i n e s having the r e l a t i v e i n t e n s i t i e s e x p e c t e d f o r a p u r e M1 t r a n s i t i o n was t h e r e f o r e fitted to the Z e e m a n p a t t e r n s . As in ref. 3 the g r o u n d - s t a t e m a g n e t i c s p l i t t i n g w a s c a l c u l a t e d f r o m the NMR v a l u e [6] f o r the m a g n e t i c h y p e r f i n e f i el d at Os in iron, H i = = 1130 + 25 kOe, and kept constant during the l e a s t - s q u a r e s fit of the data. The r e s u l t s a r e c o m p i l e d in t a b l e 1 t o g e t h e r with t h o s e obtained with the s i n g l e - l i n e 189IrIr s o u r c e . As a weighted a v e r a g e one f i n a ll y g e t s g(12)/g(~) = 1.03 + 0.13 and, u s i n g ~(~) = = 0.6565 + 0.0003 nm [7,8], the m a g n e t i c m o m e n t of the 36.2 keV state g(½) = 0.226 + 0.029 nm. T he final e r r o r s include c o n t r i b u t i o n s due to the e r r o r s of the h y p e r f i n e field and the v e l o c i t y calibration. The e x p e r i m e n t a l line widths ½W w e r e c o r r e c t e d for n o n - z e r o a b s o r b e r t h i c k n e s s a c c o r d ing to ½W = (~/T)(1+0.135 - t ) [9] (cf. ta b l e 1). T h e e f f e c t i v e t h i c k n e s s t = n . f . a o was c a l c u l a t e d with f a = 0.841, which c o r r e s p o n d s to a D e b y e - t e m p e r a t u r e 0D = 375°K [10,11] f o r Os m e t a l , and with a o = 4.25 × 104 b. The weighted a v e r a g e of all r e s u l t s is T = 0 . 7 2 + 0 . 0 4 n s . A 1 0 ~ o u n c e r tainty in the c o r r e c t i o n f o r n o n - z e r o a b s o r b e r
t h i c k n e s s is included in the e r r o r . F r o m the known e l e c t r i c field g r a d i e n t in hexagonal Os m e t a l [11] and the n u c l e a r quadrupole m o m e n t [12] the splitting ~ e Q V z z = 0.66 m m / s of the 189Os ground state in the a b s o r b e r s could be c a l c u l a t e d . It is s m a l l c o m p a r e d to the e x p e r i m e n t a l l i n e width, and a d e t a i l e d c a l c u l a t i o n showed that i t s influence on the value obtained f o r T is n e g l i g i b l y small. The 36.2 keV state of 189Os has b e e n a s s u m e d to be the ½-[510] N i l s s o n l e v e l [3,13]. Due to the spin I ~ = ½- i t s wavefunction cannot contain ad m i x t u r e s f r o m the K ~ = ~- g r o u n d - s t a t e r o t a t i o n a l band and the l e v e l may be r e g a r d e d as a f a i r l y p u r e i n t r i n s i c state. F o r n e g a t i v e - p a r i t y s i n g l e n e u t r o n s t a t e s with spin ½ the N i l s s o n m o d el p r e dicts. - zgs]
,
g R being the r o t a t i o n a l g - f a c t o r , which f o r 189Os is given in ref. 14 as gR = 0.32, g s the spin g - f a c t o r of the odd n eu t r o n and a the e n e r g y d e coupling p a r a m e t e r . F o r all r e a s o n a b l e v a l u e s of a the main contribution to g a r i s e s f r o m the g s t e r m and a r a t h e r s m a l l e f f e c t i v e spin g - f a c t o r [15], g s , e f f ~ 0.3 g s , f r e e , has to be u s e d to explain the s m a l l m a g n e t i c m o m e n t of the 36.2 keV l e v e l . Such s m a l l m a g n e t i c m o m e n t s s e e m to be t y p i c a l f o r ½-[510] s t a t e s ; they a r e a l s o found f o r ½-[510] l e v e l s in neighbouring n u cl ei , such a s the g r o u n d - s t a t e s of 187Os (~ = 0.067 nm) and of 183W (~ = 0.117 nm).
The authors wish to thank Dr. G. Schatz for the irradiations in the cyclotron of the Gesellschaft ff/r Kernforschung, Karlsruhe, and Dr. Ursel Zahn for her help in preparing the sources.
549
Volume 28B, number 8
PHYSICS
References 1. B. P e r s s o n , H. Blumberg and M. Bent, Phys. Rev., to be published. 2. M. G. Gregory, B. L. Robinson and S. Iha, to be published. 3. D. Kucheida, F. Wagner. G. Kaindl and P. Kienle, Z. Physik 216 (1968) 346. 4. G.W. Leddieotte, The r a d i o c h e m i s t r y of iridium, NAS-NS-3045 (Office of Technical S e r v i c e s , U.S. Department of C o m m e r c e , Washington, D . C . , 1961). 5. B. H a r m a t z , T. H. Handley and J . W . Mihelich, Phys. Rev. 128 (1962) 1186. 6. M. Kontani and I. Itoh, J. Phys. Soe. Japan 22 (1967) 345.
550
LETTERS
3 F e b r u a r y 1969
7. H.R. Loeliger and L. R. S a r l e s , Phys. Rev. 95 (1954) 291. 8. A. Sehwenk and G. Z i m m e r m a n n , Phys. L e t t e r s 26A (1968) 258. 9. D.A. O'Connor, Nuel. Instr. and Meth. 21 (1963) 318. 10. R . J . M o r r i s o n , A. Atac, P. Debrunner and H. F r a u e n f e l d e r , Phys. L e t t e r s 12 (1964) 35. 11. L. Grodzins and Y. W. Chow, Phys. Rev. 142 (1966) 86. 12. G. Himmel, Z. Physik 211 (1968) 68. 13. Nuclear Data, See. B, 1, No.2 (1966). 14. O. P r i o r , F. Boehm and S. G. Nilsson, Nuel. Phys. Al10 (1968) 257. 15. Z. Bochnacki and S. Ogaza, Nucl. Phys. 69 (1965) 186.