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An isomer of high spin and energy in 53Fe
Volume 23, number 8
PHYSICS L E T T E R S
AN I S O M E R
OF HIGH
SPIN
AND ENERGY
21 November 1966
IN 53Fe
K. ESKOLA Department of Physics, University of Helsinki, Helsinki, Finland Received 20 October 1966
A decay scheme for a new 2.5 rain 3.04 MeV isomeric state of 53Fe is proposed. It is suggested that the isomeric state is a three-particle state, the two proton holes and the neutron hole in the f~ shell having been coupled to their maximum spin value ~-.
An i s o m e r i c state, having a half-life of 2.5 ± ± 0.2 m i n and an excitation e n e r g y of 3,04 MeV, has been o b s e r v e d in 53Fe. It has been produced by b o m b a r d i n g n a t u r a l m a n g a n e s e , i r o n , cobalt, and nickel a s well as t a r g e t s e n r i c h e d in the i s o tope 54Fe, by p r o t o n s in the energy range 20-70 MeV f r o m the s y n c h r o c y c l o t r o n of the Gustaf W e r n e r Institute, Uppsala, Sweden. Single and coincidence s p e c t r a were m e a s u r e d u s i n g a ND-160 FMR 4096-channel a n a l y z e r and 7.6 × 7.6 cm 2 c y l i n d r i c a l NaI(T1) c r y s t a l s and a 4 cm2 x 5 m m Ge(Li) detector. A total of four g a m m a r a y s with e n e r g i e s 702, 1011, 1328 and 2339 keV was observed. Chemical s e p a r a t i o n of i r o n from the r e s t of the r e a c t i o n p r o d u c t s was usually omitted because of the s h o r t n e s s of the half-life studied and because of the p r e s e n c e of r e l a t i v e l y few i n t e r f e r i n g a c t i v i t i e s . A g a m m a ray s p e c t r u m r e s u l t i n g f r o m a 38 MeV proton b o m b a r d m e n t of a m a n g a n e s e t a r g e t is p r e s e n t e d in fig. 1. The a s s i g n m e n t of the 2.5 min activity to i r o n was c o n f i r m e d by c h e m i c a l solvent e x t r a c t i o n methods (isopropyl ether, 8N HC1 [1]). F u r t h e r m o r e , proton b o m b a r d m e n t of c h r o m i u m showed no t r a c e of the 2.5 m i n activity. The i n d e n t i f i c a tion of 53Fe as the source of the new activity was e s t a b l i s h e d by the fact that in each of the r e a c tions 55Mn(p,3n)53Fe, 54Fe(p,pn)53Fe, 59Co(p,ot3n)53Fe and 58Ni(p,apn)53Fe the e x c i tation functions for the ground state of 53Fe and that for the 2.5 rain activity have the same shape with t h e i r m a x i m a o c c u r r i n g a p p r o x i m a t e l y at the same e n e r g y . In the case of the m a n g a n e s e b o m b a r d m e n t , where no i n t e r f e r i n g activity appeared, the decay of the i s o m e r to the ground state of 53Fe was i n d i r e c t l y seen by following the decay of the 377 keV g a m m a ray of 53Mn. No a n n i h i l a t i o n r a d i a t i o n a s s i g n a b l e to the 2.5 m i n activity was observed.
The proposed decay scheme of 53Fe m is p r e sented in fig. 2. It i s suggested that the 3.04 MeV i s o m e r i c state is a t h r e e - p a r t i c l e state, the two proton holes and the n e u t r o n hole in the f~ shell having been coupled to their m a x i m u m spin value of 19- • In addition to the decay of the i s o m e r to the 2ground state of 53Fe, the decay of the l a t t e r to the l e v e l s of 53Mn is also shown in fig. 2. The /3+ b r a n c h i n g to the 1290 keV level [2] has been omitted, because no g a m m a r a y s indicating the decay of this level to the 377 keV level or to the ground state were observed, and the fl+ b r a n c h ing r a t i o between the two lower levels has been modified accordingly. The 1290 keV level has been observed from the 5~'Cr(p,y)53Mn [3,4] and 56Fe(p,t~)53Mn [5,6] r e a c t i o n s , and on the b a s i s of g a m m a - r a y i n t e n s i t y c o n s i d e r a t i o n s the j~r of ~-- i s well founded. The half-life of 8.5 m i n has been m e a s u r e d by E b r e y and Grey [7]. Adjacent to the e x p e r i m e n t a l levels some t h e o r e t i c a l p r e dictions [8,9] a r e added. T h e r e is little d i r e c t evidence to favor the proposed spin a s s i g n m e n t s to the l e v e l s in the decay of the 53Fe i s o m e r , although the following a r g u m e n t s in favor of the p r e s e n t e d level scheme may be made: a. The Weisskopf s i n g l e - p a r t i c l e e s t i m a t e of the half-life of the 702 keV g a m m a t r a n s i t i o n , i n t e r p r e t e d as an E4, is 100 s [10], which a g r e e s with the m e a s u r e d half-life of 2.5 min. b. The fast coincidence and p r e c i s e g a m m a ray energy m e a s u r e m e n t s fix the scheme as given, except for the p o s s i b i l i t y of a r e v e r s e d o r d e r of the two low-lying c a s c a d i n g g a m m a t r a n s i t i o n s . The proposed o r d e r is favored by the t h e o r e t i c a l c a l c u l a t i o n s by McCullen et al. [8]. F u r t h e r m o r e , the choice of the lower energy g a m m a ray to compete with the 2339 keV c r o s s over is m o r e compatible with the o b s e r v e d g a m m a b r a n c h i n g r a t i o of 4: 1. 471
Volume 23, number 8
PHYSICS L E T T E R S
21 l~ovember 1966
511
53Fe
c c~
c
53Fem
,,
,
cz
702 ]
%-#
k r] %. /I ~P'~
::3
1011 ~ ~ %~'V~~
0
53Fern 1328 52Mnm t 14i5
'
14
53
Fe
m
\ 500
1000
1500
Gamma energy (keV)
2O00
2500
Fig. 1. Gamma-ray spectrum resutting from bombarding a manganese target by 38 MeV protons. No chemical separation has been done. The 1435 keV and 2312 keV peaks originate from the 55Mn(p,p3n)52Mnm and 160(p,p2n)140 reactions. The presence of the latter implies an oxygen impurity in the target foil. [8] 3041
2.5m
guration (lf{) "3, with two holes being identical. The possible pure configuration states in the jj j - c o u p l i n g scheme with s e n i o r i t y v = 3 a r e y , 3- ~s-~2 tz-~2 t~-~2 t~-)2 It~-~2 ~ - ~ - ~ n a [11]. d. ~The a s s i g n m e n t of a high spin value to the i s o m e r i c state i s strongly supported by the e x p e r i m e n t a l c r o s s section r a t i o for the production of the i s o m e r i c and the ground state. The e x p e r i mental high-to-low-spin isomeric ratios c o r r e s ponding to proton e n e r g i e s at the m a x i m a of the excitation functions a r e approximately 0.013, 0.1, and 0.3 for the r e a c t i o n s 54Fe(p,pn)53Fe, 55Mn(p,3n)53Fe, and 59Co(p,a3n)53Fe, r e s p e c tively. These values a r e well r e p r o d u c e d by i s o m e r i c r a t i o c a l c u l a t i o n s using a f o r m a l i s m b a s e d on the s t a t i s t i c a l model as suggested by Van den Bosch and Huizenga [12]. In t h e i r d e t a i l s the c a l culations follow those of Dudey and Sugihara [13] neglecting, however, competition between v a r i o u s evaporation paths. These c a l c u l a t i o n s r e s u l t in i s o m e r i c r a t i o s of 0.02, 0.13, and 0.26, r e spe ctively. At a f i r s t glance it i s t e m p t i n g to a s s i g n the whole set of e x p e r i m e n t a l l y found g a m m a r a y s to 53Mn, a s shown by dashed l i n e s in fig. 2. This would imply that the i s o m e r i c state would p r e dominantly decay by K - c a p t u r e , and that the i s o m e r i c t r a n s i t i o n would be too weak to be d e t e c ted with the s e n s i t i v i t y of the m e a s u r i n g a p p a r a -
2339
53Fe 8.5m
[B] %[9]
l
'%
11 -
1~-
I
I
I
//-~
2 . 3 8 M e V (40%)
Fig. 2. The proposed level scheme of 53Fem. c. The suggested spin and p a r i t y a s s i g n m e n t s a r e among those produced by the n u c l e a r confi472
Volume 23, number 8
PHYSICS
tus u s e d . T h e r e a r e s e v e r a l a r g u m e n t s w h i c h contradict this alternative: a. T h e l e v e l s c h e m e would be in p o o r a g r e e m e n t with the t h e o r e t i c a l p r e d i c t i o n s of K i s s l i n g e r and S o r e n s e n [14], D e - S h a l i t [9], and M c C u l l e n et al. [8], of which the l a s t two a r e shown in fig. 2. b. The g a m m a r a y i n t e n s i t i e s i n d i c a t e that the ~ - s t a t e would be p r e d o m i n a n t l y p o p u l a t e d in the d e c a y of the i s o m e r . A s s u m i n g that the i s o m e r i c s t a t e i s a l s o a ~ - s t a t e and that the t r a n s i t i o n b e t w e e n the two ~ - s t a t e s i s s u p e r a l l o w e d with a log f t v a l u e a s low a s 3.2, a ~ + m a x i m u m e n e r g y of 1.4 MeV i s p r e d i c t e d [10]. Such an e n e r g y would p u s h the i s o m e r i c s t a t e up to about 1.5 M e V a b o v e the g r o u n d s t a t e of 5 3 F e and c o n s e q u e n t l y l e a d to a f a s t t r a n s i t i o n c o n t r a r y to e x periments. c. The e x p e r i m e n t a l r e s u l t s f r o m 52Cr(p, ~)53Mn [3,4] and 56Fe(p,ot)53Mn [5,6] s h o w no l e v e l s that c o i n c i d e with the o n e s d r a w n by d a s h e d l i n e s in fig. 2. The l a t t e r m e a s u r e m e n t s , h o w e v e r , show two l e v e l s at 1.60 and 1.43 M e V , which m a y be the ~ - and ~ - l e v e l s p r e d i c t e d by the t h e o r y [8,9,14]. The e x i s t e n c e of a l o n g - l i v e d ~ - i s o m e r i c s t a t e in 53Fe can be u n d e r s t o o d a s a r e s u l t of the a p p e a r a n c e of a l a r g e " s p i n g a p " in the l e v e l s p e c t r u m of 5 3 F e . A u e r b a c h and T a l m i h a v e p e r f o r m e d s h e l l m o d e l c a l c u l a t i o n s on 93Mo, 211Po and 212Po [15,16], and t h e i r r e s u l t s l e a d to the r e q u i r e d spin g a p s to e x p l a i n the e x i s t e n c e of h i g h - s p i n i s o m e r i c s t a t e s in t h e s e i s o t o p e s . In a s i m i l a r way the p r e s e n c e of a spin gap in the l e v e l s p e c t r u m of 5 3 F e m a y be due to the a t t r a c tion b e t w e e n a f~ n e u t r o n and a f½ p r o t o n h o l e which i s s t r o n g e r in the h i g h e s t spin s t a t e , J = 7, than in the s t a t e s J= 6, 5, 4 and 3. T h e high spin (J= 7), 180 keV i s o m e r of 54Co [17] i n d i c a t e s s u c h a s t r o n g i n t e r a c t i o n , but due to the l a c k of e x p e r i m e n t a l d a t a no c o m p a r i s o n with o t h e r t w o - p a r t i c l e s t a t e s of 54Co can be m a d e . H o w e v e r , the a n a l o g o u s i s o t o p e 42Sc is b e t t e r known and a c c o r d i n g to the spin a s s i g n m e n t s of M c C u l len et al. [8] t h e r e i s a s t r o n g a t t r a c t i o n ( of the o r d e r of 2 MeV) b e t w e e n the n e u t r o n and the p r o -
LETTERS
21 November 1966
ton when t h e i r s p i n s a r e m u c h a l i g n e d a s p o s s i b l e , i . e . , J = 7. To e s t i m a t e the s t r e n g t h of the p r o t o n p r o t o n i n t e r a c t i o n in the (f~)-2 c o n f i g u r a t i o n one h a s to l o o k at the e n e r g y l e v e l s of 5 4 F e . The r e q u i r e d 2 +, 4 + and 6 + s t a t e s a r e known a s a r e s u l t of the d e c a y of 54Co m and t h e i r e n e r g i e s a r e 1.41, 2.55 and 2.97 MeV, r e s p e c t i v e l y [17]. Due to the p r o x i m i t y of the J= 4 and J= 6 l e v e l s in the (f~)-2 c o n f i g u r a t i o n , the p r o t o n - n e u t r o n i n t e r a c t i o n is s e e n to be d o m i n a n t , and t h u s q u a l i t a t i v e l y at l e a s t the e x i s t e n c e of a spin gap can be u n d e r stood. The a u t h o r is i n d e b t e d to the E m i l A a l t o n e n F o u n d a t i o n and the N a t i o n a l R e s e a r c h C o u n c i l f o r Sciences for financial support.
References 1. J . M . Nielsen, Natl. Acad. Sci. Nucl. Sci. Series 3017 {1960) 15. 2. J.O.Juliano, C . E . K o c h e r , T.D. N a i n a n a n d A . C . G . Mitchell, Phys. Rev. 113 (1959) 602. 3. S . E . A r n e l l and S. Sterner, Arkiv Fysik 26 (1964) 423. 4. P.H.Vuister, Nucl. Phys. 83 (1966)593. 5. E. Veje, C. Droste, O. Hansen and S. Holm, Nucl. Phys. 57 (1964) 451. 6. G . B r o w n a n d S . E . W a r r e n , Nucl. Phys. 77 (1966) 365. 7. T . G . E b r e y a n d P . R . G r a y , Nucl. Phys. 61 (1965) 479. 8. J . D . McCullen, B . F . Bay]nan and L. Zawick, Phys. Rev. 134B (1964) 515. 9. A. De-Shalit, in: Selected topics in nuclear theory, ed. F.Janouch ( I . A . E . A . , Vienna, 1963). 10. A.H.Wapstra, G . J . Nijgh and R. Van Lieshout, Nuclear spectroscopy tables (North-Holland Publishing Company. Amsterdam, 1959). 11. B . H . F l o w e r s , Proc. Roy. Soc. A212 (1952) 248. 12. R. Van den Bosch and J.R.Huizenga, Phys. Rev. 120 (1960) 1313. 13. N.D.Dudey and T.T.Sugihara, Phys. Rev. 139B (1965) 896. 14. L.S.Kisslinger and R.A.Sorensen, Mat. Fys. Medd. Dan Vid. Selsk. 32, No. 9 (1960) 47. 15. N.Auerbach and I.Talmi, Nucl. Phys. 64 (1965) 458. 16. N.Auerbach and I.Talmi, Phys. Letters 10 (1964) 297. 17. M.H.Brennan and A.M. Bernstein, Phys. Rev. 120 (1960) 927.
* * * * *
473
PHYSICS L E T T E R S
AN I S O M E R
OF HIGH
SPIN
AND ENERGY
21 November 1966
IN 53Fe
K. ESKOLA Department of Physics, University of Helsinki, Helsinki, Finland Received 20 October 1966
A decay scheme for a new 2.5 rain 3.04 MeV isomeric state of 53Fe is proposed. It is suggested that the isomeric state is a three-particle state, the two proton holes and the neutron hole in the f~ shell having been coupled to their maximum spin value ~-.
An i s o m e r i c state, having a half-life of 2.5 ± ± 0.2 m i n and an excitation e n e r g y of 3,04 MeV, has been o b s e r v e d in 53Fe. It has been produced by b o m b a r d i n g n a t u r a l m a n g a n e s e , i r o n , cobalt, and nickel a s well as t a r g e t s e n r i c h e d in the i s o tope 54Fe, by p r o t o n s in the energy range 20-70 MeV f r o m the s y n c h r o c y c l o t r o n of the Gustaf W e r n e r Institute, Uppsala, Sweden. Single and coincidence s p e c t r a were m e a s u r e d u s i n g a ND-160 FMR 4096-channel a n a l y z e r and 7.6 × 7.6 cm 2 c y l i n d r i c a l NaI(T1) c r y s t a l s and a 4 cm2 x 5 m m Ge(Li) detector. A total of four g a m m a r a y s with e n e r g i e s 702, 1011, 1328 and 2339 keV was observed. Chemical s e p a r a t i o n of i r o n from the r e s t of the r e a c t i o n p r o d u c t s was usually omitted because of the s h o r t n e s s of the half-life studied and because of the p r e s e n c e of r e l a t i v e l y few i n t e r f e r i n g a c t i v i t i e s . A g a m m a ray s p e c t r u m r e s u l t i n g f r o m a 38 MeV proton b o m b a r d m e n t of a m a n g a n e s e t a r g e t is p r e s e n t e d in fig. 1. The a s s i g n m e n t of the 2.5 min activity to i r o n was c o n f i r m e d by c h e m i c a l solvent e x t r a c t i o n methods (isopropyl ether, 8N HC1 [1]). F u r t h e r m o r e , proton b o m b a r d m e n t of c h r o m i u m showed no t r a c e of the 2.5 m i n activity. The i n d e n t i f i c a tion of 53Fe as the source of the new activity was e s t a b l i s h e d by the fact that in each of the r e a c tions 55Mn(p,3n)53Fe, 54Fe(p,pn)53Fe, 59Co(p,ot3n)53Fe and 58Ni(p,apn)53Fe the e x c i tation functions for the ground state of 53Fe and that for the 2.5 rain activity have the same shape with t h e i r m a x i m a o c c u r r i n g a p p r o x i m a t e l y at the same e n e r g y . In the case of the m a n g a n e s e b o m b a r d m e n t , where no i n t e r f e r i n g activity appeared, the decay of the i s o m e r to the ground state of 53Fe was i n d i r e c t l y seen by following the decay of the 377 keV g a m m a ray of 53Mn. No a n n i h i l a t i o n r a d i a t i o n a s s i g n a b l e to the 2.5 m i n activity was observed.
The proposed decay scheme of 53Fe m is p r e sented in fig. 2. It i s suggested that the 3.04 MeV i s o m e r i c state is a t h r e e - p a r t i c l e state, the two proton holes and the n e u t r o n hole in the f~ shell having been coupled to their m a x i m u m spin value of 19- • In addition to the decay of the i s o m e r to the 2ground state of 53Fe, the decay of the l a t t e r to the l e v e l s of 53Mn is also shown in fig. 2. The /3+ b r a n c h i n g to the 1290 keV level [2] has been omitted, because no g a m m a r a y s indicating the decay of this level to the 377 keV level or to the ground state were observed, and the fl+ b r a n c h ing r a t i o between the two lower levels has been modified accordingly. The 1290 keV level has been observed from the 5~'Cr(p,y)53Mn [3,4] and 56Fe(p,t~)53Mn [5,6] r e a c t i o n s , and on the b a s i s of g a m m a - r a y i n t e n s i t y c o n s i d e r a t i o n s the j~r of ~-- i s well founded. The half-life of 8.5 m i n has been m e a s u r e d by E b r e y and Grey [7]. Adjacent to the e x p e r i m e n t a l levels some t h e o r e t i c a l p r e dictions [8,9] a r e added. T h e r e is little d i r e c t evidence to favor the proposed spin a s s i g n m e n t s to the l e v e l s in the decay of the 53Fe i s o m e r , although the following a r g u m e n t s in favor of the p r e s e n t e d level scheme may be made: a. The Weisskopf s i n g l e - p a r t i c l e e s t i m a t e of the half-life of the 702 keV g a m m a t r a n s i t i o n , i n t e r p r e t e d as an E4, is 100 s [10], which a g r e e s with the m e a s u r e d half-life of 2.5 min. b. The fast coincidence and p r e c i s e g a m m a ray energy m e a s u r e m e n t s fix the scheme as given, except for the p o s s i b i l i t y of a r e v e r s e d o r d e r of the two low-lying c a s c a d i n g g a m m a t r a n s i t i o n s . The proposed o r d e r is favored by the t h e o r e t i c a l c a l c u l a t i o n s by McCullen et al. [8]. F u r t h e r m o r e , the choice of the lower energy g a m m a ray to compete with the 2339 keV c r o s s over is m o r e compatible with the o b s e r v e d g a m m a b r a n c h i n g r a t i o of 4: 1. 471
Volume 23, number 8
PHYSICS L E T T E R S
21 l~ovember 1966
511
53Fe
c c~
c
53Fem
,,
,
cz
702 ]
%-#
k r] %. /I ~P'~
::3
1011 ~ ~ %~'V~~
0
53Fern 1328 52Mnm t 14i5
'
14
53
Fe
m
\ 500
1000
1500
Gamma energy (keV)
2O00
2500
Fig. 1. Gamma-ray spectrum resutting from bombarding a manganese target by 38 MeV protons. No chemical separation has been done. The 1435 keV and 2312 keV peaks originate from the 55Mn(p,p3n)52Mnm and 160(p,p2n)140 reactions. The presence of the latter implies an oxygen impurity in the target foil. [8] 3041
2.5m
guration (lf{) "3, with two holes being identical. The possible pure configuration states in the jj j - c o u p l i n g scheme with s e n i o r i t y v = 3 a r e y , 3- ~s-~2 tz-~2 t~-~2 t~-)2 It~-~2 ~ - ~ - ~ n a [11]. d. ~The a s s i g n m e n t of a high spin value to the i s o m e r i c state i s strongly supported by the e x p e r i m e n t a l c r o s s section r a t i o for the production of the i s o m e r i c and the ground state. The e x p e r i mental high-to-low-spin isomeric ratios c o r r e s ponding to proton e n e r g i e s at the m a x i m a of the excitation functions a r e approximately 0.013, 0.1, and 0.3 for the r e a c t i o n s 54Fe(p,pn)53Fe, 55Mn(p,3n)53Fe, and 59Co(p,a3n)53Fe, r e s p e c tively. These values a r e well r e p r o d u c e d by i s o m e r i c r a t i o c a l c u l a t i o n s using a f o r m a l i s m b a s e d on the s t a t i s t i c a l model as suggested by Van den Bosch and Huizenga [12]. In t h e i r d e t a i l s the c a l culations follow those of Dudey and Sugihara [13] neglecting, however, competition between v a r i o u s evaporation paths. These c a l c u l a t i o n s r e s u l t in i s o m e r i c r a t i o s of 0.02, 0.13, and 0.26, r e spe ctively. At a f i r s t glance it i s t e m p t i n g to a s s i g n the whole set of e x p e r i m e n t a l l y found g a m m a r a y s to 53Mn, a s shown by dashed l i n e s in fig. 2. This would imply that the i s o m e r i c state would p r e dominantly decay by K - c a p t u r e , and that the i s o m e r i c t r a n s i t i o n would be too weak to be d e t e c ted with the s e n s i t i v i t y of the m e a s u r i n g a p p a r a -
2339
53Fe 8.5m
[B] %[9]
l
'%
11 -
1~-
I
I
I
//-~
2 . 3 8 M e V (40%)
Fig. 2. The proposed level scheme of 53Fem. c. The suggested spin and p a r i t y a s s i g n m e n t s a r e among those produced by the n u c l e a r confi472
Volume 23, number 8
PHYSICS
tus u s e d . T h e r e a r e s e v e r a l a r g u m e n t s w h i c h contradict this alternative: a. T h e l e v e l s c h e m e would be in p o o r a g r e e m e n t with the t h e o r e t i c a l p r e d i c t i o n s of K i s s l i n g e r and S o r e n s e n [14], D e - S h a l i t [9], and M c C u l l e n et al. [8], of which the l a s t two a r e shown in fig. 2. b. The g a m m a r a y i n t e n s i t i e s i n d i c a t e that the ~ - s t a t e would be p r e d o m i n a n t l y p o p u l a t e d in the d e c a y of the i s o m e r . A s s u m i n g that the i s o m e r i c s t a t e i s a l s o a ~ - s t a t e and that the t r a n s i t i o n b e t w e e n the two ~ - s t a t e s i s s u p e r a l l o w e d with a log f t v a l u e a s low a s 3.2, a ~ + m a x i m u m e n e r g y of 1.4 MeV i s p r e d i c t e d [10]. Such an e n e r g y would p u s h the i s o m e r i c s t a t e up to about 1.5 M e V a b o v e the g r o u n d s t a t e of 5 3 F e and c o n s e q u e n t l y l e a d to a f a s t t r a n s i t i o n c o n t r a r y to e x periments. c. The e x p e r i m e n t a l r e s u l t s f r o m 52Cr(p, ~)53Mn [3,4] and 56Fe(p,ot)53Mn [5,6] s h o w no l e v e l s that c o i n c i d e with the o n e s d r a w n by d a s h e d l i n e s in fig. 2. The l a t t e r m e a s u r e m e n t s , h o w e v e r , show two l e v e l s at 1.60 and 1.43 M e V , which m a y be the ~ - and ~ - l e v e l s p r e d i c t e d by the t h e o r y [8,9,14]. The e x i s t e n c e of a l o n g - l i v e d ~ - i s o m e r i c s t a t e in 53Fe can be u n d e r s t o o d a s a r e s u l t of the a p p e a r a n c e of a l a r g e " s p i n g a p " in the l e v e l s p e c t r u m of 5 3 F e . A u e r b a c h and T a l m i h a v e p e r f o r m e d s h e l l m o d e l c a l c u l a t i o n s on 93Mo, 211Po and 212Po [15,16], and t h e i r r e s u l t s l e a d to the r e q u i r e d spin g a p s to e x p l a i n the e x i s t e n c e of h i g h - s p i n i s o m e r i c s t a t e s in t h e s e i s o t o p e s . In a s i m i l a r way the p r e s e n c e of a spin gap in the l e v e l s p e c t r u m of 5 3 F e m a y be due to the a t t r a c tion b e t w e e n a f~ n e u t r o n and a f½ p r o t o n h o l e which i s s t r o n g e r in the h i g h e s t spin s t a t e , J = 7, than in the s t a t e s J= 6, 5, 4 and 3. T h e high spin (J= 7), 180 keV i s o m e r of 54Co [17] i n d i c a t e s s u c h a s t r o n g i n t e r a c t i o n , but due to the l a c k of e x p e r i m e n t a l d a t a no c o m p a r i s o n with o t h e r t w o - p a r t i c l e s t a t e s of 54Co can be m a d e . H o w e v e r , the a n a l o g o u s i s o t o p e 42Sc is b e t t e r known and a c c o r d i n g to the spin a s s i g n m e n t s of M c C u l len et al. [8] t h e r e i s a s t r o n g a t t r a c t i o n ( of the o r d e r of 2 MeV) b e t w e e n the n e u t r o n and the p r o -
LETTERS
21 November 1966
ton when t h e i r s p i n s a r e m u c h a l i g n e d a s p o s s i b l e , i . e . , J = 7. To e s t i m a t e the s t r e n g t h of the p r o t o n p r o t o n i n t e r a c t i o n in the (f~)-2 c o n f i g u r a t i o n one h a s to l o o k at the e n e r g y l e v e l s of 5 4 F e . The r e q u i r e d 2 +, 4 + and 6 + s t a t e s a r e known a s a r e s u l t of the d e c a y of 54Co m and t h e i r e n e r g i e s a r e 1.41, 2.55 and 2.97 MeV, r e s p e c t i v e l y [17]. Due to the p r o x i m i t y of the J= 4 and J= 6 l e v e l s in the (f~)-2 c o n f i g u r a t i o n , the p r o t o n - n e u t r o n i n t e r a c t i o n is s e e n to be d o m i n a n t , and t h u s q u a l i t a t i v e l y at l e a s t the e x i s t e n c e of a spin gap can be u n d e r stood. The a u t h o r is i n d e b t e d to the E m i l A a l t o n e n F o u n d a t i o n and the N a t i o n a l R e s e a r c h C o u n c i l f o r Sciences for financial support.
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