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An isomeric state in 24Al
Volume 21, number3
PHYSICS LETTERS
AN ISOMERIC
STATE
15 May 1966
IN 24A1 t
A. J. ARMINI, J . W . SUNIER, R . M . POLICHAR and J. R. RICHARDSON University of California, Los Angeles Received 29 April 1966
The existence of a 1+ isomeric state is reported in 24A1, with a half-life of 129~5 ms. The energies and intensities of the isomeric gsmm~t transition and of three beta transitions have been measured.
I s o m e r i c s t a t e s m a y be expected to e x i s t in o d d - o d d o u c l e i d e s [1] when the two odd nucleons belong to the s a m e Schmidt group and have l a r g e a n g u l a r m o m e n t a . The ch2 nucleons involved in the s t r u c t u r e of the i s o b a r i c spin m u l t i p l e t s of m a s s 22, 24, 26 f a l l in this c a t e g o r y , and i s o m e r i c t r a n s i t i o n s have been r e p o r t e d in the d e cay of 26A1 and 24Na [2]. P a r t i c u l a r l y , the a n a logue p r o p e r t i e s of the s e l f - c o n j u g a t e n u c l e i d e s 24Na and 24A1 s t r o n g l y s u g g e s t the e x i s t e n c e of an i s o m e r i c s t a t e in 24A1, with an e x c i t a t i o n e n e r g y b e t w e e n 400 and 500 keV. We have b o m b a r d e d n a t u r a l m a g n e s i u m with an e x t e r n a l b e a m of 25 MeV p r o t o n s f r o m the UCLA S e c t o r F o c u s e d C y c l o t r o n , and o b s e r v e d a p r o m i n e n t g a m m a t r a n s i t i o n of 439±2 keV, which d e c a y s with a h a l f - l i f e of 126~10 m s . The p r o duction t h r e s h o l d , m e a s u r e d by d e g r a d i n g the 25 MeV b e a m e n e r g y with A1 a b s o r b e r s , i s 15.5¢-0.8 MeV and c o r r e s p o n d s to the r e a c t i o n 24Mg(p, n)24A1 [2]. S i m u l t a n e o u s l y , the production of 23Mg by m e a n s of the 24Mg(p,pn) r e a c t i o n , with a t h r e s h old of 17.2 MeV, h a s b e e n identified by the 449 keV g a m m a t r a n s i t i o n between the f i r s t e x c i t e d s t a t e and the ground s t a t e of 23Mg. This p r o v i d e s a convenient e n e r g y c a l i b r a t i o n of the g a m m a d e t e c t o r ( L i - d r i f t e d g e r m a n i u m of 0.4 c m 3) a s well a s a c h e c k of the techniques u s e d to m e a s u r e the y i e l d c u r v e p r e s e n t e d in fig. 1. In fig. 2 we show a) the p r o m p t g a m m a s p e c t r u m t a k e n at a p r o t o n e n e r g y of 24.3 MeV, b ) the s a m e s p e c t r u m at a p r o t o n e n e r g y Df 18.3 MeV and c) the d e l a y e d g a m m a s p e c t r u m r e c o r d e d f o r 80 m s with a s t a r t i n g t i m e of 50 m s a f t e r the end of a s e r i e s of 150 m s b o m b a r d m e n t s .
Supported in part by the U.S.Office of Naval Research, contract Nonr. 233-(44).
30
o) 439
keY
A124m
b) 4 4 9
keY
M g 23
IZ :3
f~--
a)
/
20 ,,¢ rr" p,,,n rr
/ /
.J
/
uJ >-
/@
I0
u.I
>_
jbl
_1 bJ (E
15
2o Ep ( MeV)-.--~
25
Fig. 1. Yield curve for the production of the 439 keV line of 24Alm and the 449 keV line of 23Mg* as a function of the proton energy Ep.
The i n t e n s i t y r a t i o of the 439 keV and 449 keV g a m m a r a y s in the p r o m p t s p e c t r u m , and the h a l f - l i f e of the 439 keV t r a n s i t i o n , exclude a s i g nificant amount of the ~+ (439 keV) ~ ~+ 23Na g a m m a r a y in the d e l a y e d g a m m a s p e c t r u m . . The 1.37, 2.73 and 2.86 MeV g a m m a t r a n s i t i o n s of 24Mg a r e o b s e r v e d to d e c a y with a h a l f life of 130+10 m s r i d i n g on the l o n g e r l i v e d c o m ponent (2.08~-0.05 s) of the d e c a y of the 24A1 ground s t a t e (4+). The annihilation r a d i a t i o n , m a l ~ due to the p o s i t r o n d e c a y of 24A1, 25A1 and Mg i n d i c a t e s a s h o r t - l i v e d component c o m p a t i b l e with a 140~60 m s h a l f - l i f e . 335
Volume 21. number 3
PHYSICS LETTERS
15 May 1966 I
104
I
I
]
I
I
"
i
i
>>~ 0'~
0~.39
0'~
4.23 I1.37
T,o 2 W Z Z
--
-r 0
--
I looo
o° °
7
1 + ~ 0 1 3
S
T 2 0 8
S
i
~, •
M 24 12 gi2
n,I--
% Z ° ' ~ , ~-~oo ~ ~ O % O o %
n" W a.
!
/'¢7~ 9.0 (0.7 %) 2 + - ~ ' ) ~ / - ~ 1 , .9 ( 1.9%) 2 + ~ -..... 13.3 (4.4%) 0+
>-
24
13All~
I
1
*o
%0°
b}
I0
t5
20
W(mc2 )
25
Fig. 3. Kurie plot of the 24Alm beta spectrum and decay scheme of 24A1m.
'02
,I0
ENERGY (CHANNEL)" ~ "
Fig. 2. Low energy gamma spectrum for the Mg target. a) Prompt spectrum with a beam energy Ep = 24.3 MeV. Prompt spectrum with a beam energy Ep = 18.3 MeV. bl Delayed spectrum 50 to 130 ms after bombardment at Ep = 25 MeV.
Table 1 Decay properties of 24A1m. Half-life
129 ±5 ms
Spin-parity 24A1-24Mg mass difference (ground states)
1+ 13.88 :~0.05 MeV
Branching of 24Alto Transition
24Alto{y) 24A1(0)
Measured energy of endpoint (MeV)
Relative intensity
log ft
(%)
0.439:E0.00293 .+2
24Alm(B+) 24Mg(4.23) 9.00 :E0.15
0.7:E0.2 6.17~0.15
2 4 A l m ~ 24Mg(1.37) 11.90 ~0.10
1.9:k0.5 6.24:E0.15
24Alm(~ +) 24Mg(0)
4.44-1.2 6.13:~0.15
336
13.30 :~0.05
A c o m p o s i t e beta s p e c t r u m with a h a l f - l i f e of 129±5 m s has b e e n m e a s u r e d s i m u l t a n e o u s l y with the beta t r a n s i t i o n s f r o m the 24A1 ground state. A wedge gap m a g n e t i c s p e c t r o m e t e r , with f o u r d e t e c t o r s having a m o m e n t u m r e s o l u t i o n of 1% and c o v e r i n g a r e l a t i v e m o m e n t u m r a n g e of 12%, has been u s e d f o r that m e a s u r e m e n t [3] in c o n nection with a p n e u m a t i c i r r a d i a t i o n s y s t e m using the i n t e r n a l 25 MeV p r o t o n b e a m of the c y c l o t r o n . The data has been m u l t i s c a l e d and analyzed with an SDS 925 o n - l i n e c o m p u t e r , and the F e r m i . K u r i e p l o t s w e r e computed s e p a r a t e l y f o r each component h a l f - l i f e . The r e s u l t s a r e g i v en in fig. 3 f o r the s h o r t - l i v e d b e t a s p e c t r u m alone. T h r e e b r a n c h e s of an allowed c h a r a c t e r a r e obs e r v e d , and the d i f f e r e n c e s of t h e i r e n d - p o i n t s a g r e e v e r y well with the l e v e l sp aci n g of the f i r s t t h r e e s t a t e s of 24Mg. F u r t h e r m o r e , the endpoint of the 24A1 4 + (/3+) 4 + 24Mg (4.12 MeV) t r a n s i t i o n has been m e a s u r e d a s E o = 8 .7 4 ± 0 .0 5 MeV. A c o m p a r i s o n with the h i g h e s t endpoint of the 130 m s component of the b e t a s p e c t r u m l e a d s to an e x c i t a t i o n e n e r g y of 440 ± 70 k e y above the 24A144 state, in e x c e l l e n t a g r e e m e n t with our value of 439 + 2 k e y f o r the g a m m a r a y en er g y . We i n t e r p r e t the o b s e r v e d 130 m s a c t i v i t y as definite e v i d e n c e f o r a 439 k e y i s o m e r i c st at e in
Volume 21, number3
PHYSICS LETTERS
24A1. The b r a n c h i n g r a t i o of the i s o m e r i c g a m m a t r a n s i t i o n h a s b e e n m e a s u r e d to be Ny/(N~+N~+) = 0.93 ± 0.02. The a l l o w e d shape of the b e t a s p e c t r a and t h e i r c o m p a r a t i v e h a l f - l i v e s allow us to a s s i g n to 24A~m a s p i n and p a r i t y of 1+, in a g r e e m e n t with the analog 1+ i s o m e r i c s t a t e of ~24Na. The p a r t i a l h a l f - l i f e of the M3 i s o m e r i c t r a n s i t i o n i s c o n s i s t e n t with the Moszkowski e s t i m a t e of 40 m s [5]. The d e c a y of 24A1 m by c o n v e r s i o n e l e c t r o n e m i s s i o n was not o b s e r v e d . The s m a l l t h e o r e t i c a l c o n v e r s i o n c o e f f i c i e n t [6] ~ ~ 5 × 10-4 and the e l e c t r o n b a c k g r o u n d on the t a r g e t (24Na) m a d e the s e a r c h f o r this mode of d e c a y u n s u c c e s s f u l . A s i m i l a r h a l f - l i f e has been p r e v i o u s l y r e p o r t e d f r o m a c t i v a t i o n of n a t u r a l M g with 23 MeV p r o t o n s [4] and a t t r i b u t e d l a t e r [2] to the 24Mg(p' 2n)23A12 r e a c t i o n . The t h e o r e t i c a l e s t i m a t e of the 23A1- 3Mg m a s s d i f f e r e n c e i s 12.2~0.3 MeV [7]. This g i v e s a (p, 2n) r e a c t i o n t h r e s h o l d of E p -- 3 0 . 8 ± 0 . 3 MeV. The m e a s u r e d m a s s d i f f e r e n c e , c o m p u t e d f r o m our h i g h e s t e n d point e n e r g y , would be 14.32~-0,05 MeV. We c o n -
15 May 1966
clude t h e r e f o r e that that e a r l y m e a s u r e m e n t [~t] c a m e in f a c t f r o m 24A1 m and that 23A1 h a s y e t to be d i s c o v e r e d . The a u t h o r s wish to thank Solid State R a d i a tion, Inc., in Santa Monica, C a l i f o r n i a , f o r making a Li-drifted germanium detector available.
References 1. A.De-Shalit, Phys. Rev. 91 (1953) 1479. 2. P.M.Endt and C.Van der Leun, Nuclear Phys.34 (1962) 1-324. 3. A . J . Armini, J.W. Sunier and R. M. Polichar (to be published in Nucl. Instr. and Methods). 4. H.Tyr~n and P.A.Tove, Phys.Rev. 96 (1954) 773. 5. 8.A.Moszkowski, in: Beta and gamma ray spectroscopy, ed.: K.Siegbahn (North-I-Iolland Publishing Co., 1955). 6. M . E . Rose, in: Internal conversion coefficients (North-Holland Publishing Co., 1958). 7. J.J~necke, Nuclear Phys. 61 (1965) 326.
****
337
PHYSICS LETTERS
AN ISOMERIC
STATE
15 May 1966
IN 24A1 t
A. J. ARMINI, J . W . SUNIER, R . M . POLICHAR and J. R. RICHARDSON University of California, Los Angeles Received 29 April 1966
The existence of a 1+ isomeric state is reported in 24A1, with a half-life of 129~5 ms. The energies and intensities of the isomeric gsmm~t transition and of three beta transitions have been measured.
I s o m e r i c s t a t e s m a y be expected to e x i s t in o d d - o d d o u c l e i d e s [1] when the two odd nucleons belong to the s a m e Schmidt group and have l a r g e a n g u l a r m o m e n t a . The ch2 nucleons involved in the s t r u c t u r e of the i s o b a r i c spin m u l t i p l e t s of m a s s 22, 24, 26 f a l l in this c a t e g o r y , and i s o m e r i c t r a n s i t i o n s have been r e p o r t e d in the d e cay of 26A1 and 24Na [2]. P a r t i c u l a r l y , the a n a logue p r o p e r t i e s of the s e l f - c o n j u g a t e n u c l e i d e s 24Na and 24A1 s t r o n g l y s u g g e s t the e x i s t e n c e of an i s o m e r i c s t a t e in 24A1, with an e x c i t a t i o n e n e r g y b e t w e e n 400 and 500 keV. We have b o m b a r d e d n a t u r a l m a g n e s i u m with an e x t e r n a l b e a m of 25 MeV p r o t o n s f r o m the UCLA S e c t o r F o c u s e d C y c l o t r o n , and o b s e r v e d a p r o m i n e n t g a m m a t r a n s i t i o n of 439±2 keV, which d e c a y s with a h a l f - l i f e of 126~10 m s . The p r o duction t h r e s h o l d , m e a s u r e d by d e g r a d i n g the 25 MeV b e a m e n e r g y with A1 a b s o r b e r s , i s 15.5¢-0.8 MeV and c o r r e s p o n d s to the r e a c t i o n 24Mg(p, n)24A1 [2]. S i m u l t a n e o u s l y , the production of 23Mg by m e a n s of the 24Mg(p,pn) r e a c t i o n , with a t h r e s h old of 17.2 MeV, h a s b e e n identified by the 449 keV g a m m a t r a n s i t i o n between the f i r s t e x c i t e d s t a t e and the ground s t a t e of 23Mg. This p r o v i d e s a convenient e n e r g y c a l i b r a t i o n of the g a m m a d e t e c t o r ( L i - d r i f t e d g e r m a n i u m of 0.4 c m 3) a s well a s a c h e c k of the techniques u s e d to m e a s u r e the y i e l d c u r v e p r e s e n t e d in fig. 1. In fig. 2 we show a) the p r o m p t g a m m a s p e c t r u m t a k e n at a p r o t o n e n e r g y of 24.3 MeV, b ) the s a m e s p e c t r u m at a p r o t o n e n e r g y Df 18.3 MeV and c) the d e l a y e d g a m m a s p e c t r u m r e c o r d e d f o r 80 m s with a s t a r t i n g t i m e of 50 m s a f t e r the end of a s e r i e s of 150 m s b o m b a r d m e n t s .
Supported in part by the U.S.Office of Naval Research, contract Nonr. 233-(44).
30
o) 439
keY
A124m
b) 4 4 9
keY
M g 23
IZ :3
f~--
a)
/
20 ,,¢ rr" p,,,n rr
/ /
.J
/
uJ >-
/@
I0
u.I
>_
jbl
_1 bJ (E
15
2o Ep ( MeV)-.--~
25
Fig. 1. Yield curve for the production of the 439 keV line of 24Alm and the 449 keV line of 23Mg* as a function of the proton energy Ep.
The i n t e n s i t y r a t i o of the 439 keV and 449 keV g a m m a r a y s in the p r o m p t s p e c t r u m , and the h a l f - l i f e of the 439 keV t r a n s i t i o n , exclude a s i g nificant amount of the ~+ (439 keV) ~ ~+ 23Na g a m m a r a y in the d e l a y e d g a m m a s p e c t r u m . . The 1.37, 2.73 and 2.86 MeV g a m m a t r a n s i t i o n s of 24Mg a r e o b s e r v e d to d e c a y with a h a l f life of 130+10 m s r i d i n g on the l o n g e r l i v e d c o m ponent (2.08~-0.05 s) of the d e c a y of the 24A1 ground s t a t e (4+). The annihilation r a d i a t i o n , m a l ~ due to the p o s i t r o n d e c a y of 24A1, 25A1 and Mg i n d i c a t e s a s h o r t - l i v e d component c o m p a t i b l e with a 140~60 m s h a l f - l i f e . 335
Volume 21. number 3
PHYSICS LETTERS
15 May 1966 I
104
I
I
]
I
I
"
i
i
>>~ 0'~
0~.39
0'~
4.23 I1.37
T,o 2 W Z Z
--
-r 0
--
I looo
o° °
7
1 + ~ 0 1 3
S
T 2 0 8
S
i
~, •
M 24 12 gi2
n,I--
% Z ° ' ~ , ~-~oo ~ ~ O % O o %
n" W a.
!
/'¢7~ 9.0 (0.7 %) 2 + - ~ ' ) ~ / - ~ 1 , .9 ( 1.9%) 2 + ~ -..... 13.3 (4.4%) 0+
>-
24
13All~
I
1
*o
%0°
b}
I0
t5
20
W(mc2 )
25
Fig. 3. Kurie plot of the 24Alm beta spectrum and decay scheme of 24A1m.
'02
,I0
ENERGY (CHANNEL)" ~ "
Fig. 2. Low energy gamma spectrum for the Mg target. a) Prompt spectrum with a beam energy Ep = 24.3 MeV. Prompt spectrum with a beam energy Ep = 18.3 MeV. bl Delayed spectrum 50 to 130 ms after bombardment at Ep = 25 MeV.
Table 1 Decay properties of 24A1m. Half-life
129 ±5 ms
Spin-parity 24A1-24Mg mass difference (ground states)
1+ 13.88 :~0.05 MeV
Branching of 24Alto Transition
24Alto{y) 24A1(0)
Measured energy of endpoint (MeV)
Relative intensity
log ft
(%)
0.439:E0.00293 .+2
24Alm(B+) 24Mg(4.23) 9.00 :E0.15
0.7:E0.2 6.17~0.15
2 4 A l m ~ 24Mg(1.37) 11.90 ~0.10
1.9:k0.5 6.24:E0.15
24Alm(~ +) 24Mg(0)
4.44-1.2 6.13:~0.15
336
13.30 :~0.05
A c o m p o s i t e beta s p e c t r u m with a h a l f - l i f e of 129±5 m s has b e e n m e a s u r e d s i m u l t a n e o u s l y with the beta t r a n s i t i o n s f r o m the 24A1 ground state. A wedge gap m a g n e t i c s p e c t r o m e t e r , with f o u r d e t e c t o r s having a m o m e n t u m r e s o l u t i o n of 1% and c o v e r i n g a r e l a t i v e m o m e n t u m r a n g e of 12%, has been u s e d f o r that m e a s u r e m e n t [3] in c o n nection with a p n e u m a t i c i r r a d i a t i o n s y s t e m using the i n t e r n a l 25 MeV p r o t o n b e a m of the c y c l o t r o n . The data has been m u l t i s c a l e d and analyzed with an SDS 925 o n - l i n e c o m p u t e r , and the F e r m i . K u r i e p l o t s w e r e computed s e p a r a t e l y f o r each component h a l f - l i f e . The r e s u l t s a r e g i v en in fig. 3 f o r the s h o r t - l i v e d b e t a s p e c t r u m alone. T h r e e b r a n c h e s of an allowed c h a r a c t e r a r e obs e r v e d , and the d i f f e r e n c e s of t h e i r e n d - p o i n t s a g r e e v e r y well with the l e v e l sp aci n g of the f i r s t t h r e e s t a t e s of 24Mg. F u r t h e r m o r e , the endpoint of the 24A1 4 + (/3+) 4 + 24Mg (4.12 MeV) t r a n s i t i o n has been m e a s u r e d a s E o = 8 .7 4 ± 0 .0 5 MeV. A c o m p a r i s o n with the h i g h e s t endpoint of the 130 m s component of the b e t a s p e c t r u m l e a d s to an e x c i t a t i o n e n e r g y of 440 ± 70 k e y above the 24A144 state, in e x c e l l e n t a g r e e m e n t with our value of 439 + 2 k e y f o r the g a m m a r a y en er g y . We i n t e r p r e t the o b s e r v e d 130 m s a c t i v i t y as definite e v i d e n c e f o r a 439 k e y i s o m e r i c st at e in
Volume 21, number3
PHYSICS LETTERS
24A1. The b r a n c h i n g r a t i o of the i s o m e r i c g a m m a t r a n s i t i o n h a s b e e n m e a s u r e d to be Ny/(N~+N~+) = 0.93 ± 0.02. The a l l o w e d shape of the b e t a s p e c t r a and t h e i r c o m p a r a t i v e h a l f - l i v e s allow us to a s s i g n to 24A~m a s p i n and p a r i t y of 1+, in a g r e e m e n t with the analog 1+ i s o m e r i c s t a t e of ~24Na. The p a r t i a l h a l f - l i f e of the M3 i s o m e r i c t r a n s i t i o n i s c o n s i s t e n t with the Moszkowski e s t i m a t e of 40 m s [5]. The d e c a y of 24A1 m by c o n v e r s i o n e l e c t r o n e m i s s i o n was not o b s e r v e d . The s m a l l t h e o r e t i c a l c o n v e r s i o n c o e f f i c i e n t [6] ~ ~ 5 × 10-4 and the e l e c t r o n b a c k g r o u n d on the t a r g e t (24Na) m a d e the s e a r c h f o r this mode of d e c a y u n s u c c e s s f u l . A s i m i l a r h a l f - l i f e has been p r e v i o u s l y r e p o r t e d f r o m a c t i v a t i o n of n a t u r a l M g with 23 MeV p r o t o n s [4] and a t t r i b u t e d l a t e r [2] to the 24Mg(p' 2n)23A12 r e a c t i o n . The t h e o r e t i c a l e s t i m a t e of the 23A1- 3Mg m a s s d i f f e r e n c e i s 12.2~0.3 MeV [7]. This g i v e s a (p, 2n) r e a c t i o n t h r e s h o l d of E p -- 3 0 . 8 ± 0 . 3 MeV. The m e a s u r e d m a s s d i f f e r e n c e , c o m p u t e d f r o m our h i g h e s t e n d point e n e r g y , would be 14.32~-0,05 MeV. We c o n -
15 May 1966
clude t h e r e f o r e that that e a r l y m e a s u r e m e n t [~t] c a m e in f a c t f r o m 24A1 m and that 23A1 h a s y e t to be d i s c o v e r e d . The a u t h o r s wish to thank Solid State R a d i a tion, Inc., in Santa Monica, C a l i f o r n i a , f o r making a Li-drifted germanium detector available.
References 1. A.De-Shalit, Phys. Rev. 91 (1953) 1479. 2. P.M.Endt and C.Van der Leun, Nuclear Phys.34 (1962) 1-324. 3. A . J . Armini, J.W. Sunier and R. M. Polichar (to be published in Nucl. Instr. and Methods). 4. H.Tyr~n and P.A.Tove, Phys.Rev. 96 (1954) 773. 5. 8.A.Moszkowski, in: Beta and gamma ray spectroscopy, ed.: K.Siegbahn (North-I-Iolland Publishing Co., 1955). 6. M . E . Rose, in: Internal conversion coefficients (North-Holland Publishing Co., 1958). 7. J.J~necke, Nuclear Phys. 61 (1965) 326.
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337