53Com: A proton-unstable isomer

Volume" 33B, number 4

P HYSIC S L E T T E R S

53

Co

m

: A PROTON-UNSTABLE

26 October 1970

ISOMERJ

K. P . J A C K S O N *, C. U. C A R D I N A L **, H . C . EVANS :~ and N. A. J E L L E Y

Nuclear Physics Laboratory, University of Oxford, England J . C E R N Y :~

Nuclear Physics Laboratory, University of Oxford, England: and Lawrence Radiation Laboratory and Department of Chemistry, University of California, Berkeley, California 94720, USA. Received 23 September 1970

A 1.53 =~0.04 MeV proton activity with a 245 :e 20 ms half-life has been observed in the reaction of 160 on 40Ca. The most plausible origin of this activity is the proton radioactivity of 53Com, although the decay of this i s o m e r by beta-delayed proton emission remains a possibility.

A r e c e n t l y r e p o r t e d e x p e r i m e n t [1] h a s d e m o n s t r a t e d the f e a s i b i l i t y of u s i n g h e a v y - i o n i n d u c e d r e a c t i o n s to p r o d u c e n e w n u c l i d e s of the A =4n + 1, TZ =½ ( N - Z ) =-~ s e r i e s of b e t a - d e l a y e d p r o t o n e m i t t e r s . P r o t o n s e m i t t e d f r o m the l o w e s t T = 3 / 2 s t a t e in 49Mn w e r e o b s e r v e d f o l l o w i n g the 4 0 C a ( 1 2 C , 3 n ) 4 9 F e r e a c t i o n . As a n a t u r a l e x t e n s i o n of t h i s p r o g r a m a s i m i l a r e x p e r i m e n t w a s d e s i g n e d to p r o d u c e 53Ni v i a the 40Ca(160,3n) reaction. This communication rep o r t s an u n f o r e s e e n r e s u l t of t h e s e e x p e r i m e n t s - t h e o b s e r v a t i o n of a p r o t o n a c t i v i t y f o r w h i c h the m o s t p l a u s i b l e e x p l a n a t i o n i s d i r e c t p r o t o n d e c a y of an unbound i s o m e r in 53Co p r o d u c e d in the 4"0Ca(160,2np) r e a c t i o n . I n the i n i t i a l e x p e r i m e n t s d e s i g n e d to p r o d u c e 53Ni, a n a t u r a l c a l c i u m t a r g e t w a s b o m b a r d e d with a p u l s e d b e a m of z 0 . 5 # A of 160(4+) i o n s at e n e r g i e s up to 81 MeV p r o d u c e d by the Harwell variable energy cyclotron. Delayed p r o t o n s w e r e d e t e c t e d and t h e i r l i f e t i m e s m e a s u r e d with the s a m e s y s t e m a s that d e s c r i b e d in r e f . [1] e x c e p t that the s e m i c o n d u c t o r t e l e s c o p e ~onsisted oLa 23~m AE counter followed by a J P a r t of this work was supported by a grant from the Science Research Council which is gratefully a c knowledged. * Present Address: Department of Physics, U n i v e r s i ty of Toronto, Toronto Ontario, Canada. ~* National Research Council of Canada Overseas P o s t doctoral Fellow. SPermanent Address: Physics Department, Queens University, Kingston Ontario, Canada. :~:1969-1970 John Simon Guggenheim Fellow.

100 ~ m E d e t e c t o r w h i c h fed a p a r t i c l e i d e n t i f i e r [2]. As shown in f i g . l ( a ) , the i d e n t i f i e d p r o ton s p e c t r a f r o m this r e a c t i o n w e r e d o m i n a t e d by a s t r o n g g r o u p n e a r the m i n i m u m e n e r g y ( ~ 1 . 3 MeV) r e q u i r e d f o r r e l i a b l e d e t e c t i o n . ( H i g h e r - e n e r g y p r o t o n s e x p e c t e d f r o m the d e c a y of 53Ni w e r e in f a c t o b s e r v e d at the h i g h e r b o m b a r d i n g e n e r g i e s but t h e i r p r o d u c t i o n c r o s s s e c tion w a s r o u g h l y an o r d e r of m a g n i t u d e l o w e r than that found f o r p r o d u c i n g 4 9 F e . ) In o r d e r to o b t a i n e n e r g y s p e c t r a u n a f f e c t e d by the t e l e s c o p e cutoff, t h i s low e n e r g y a c t i v i t y w a s a l s o m e a s u r e d with a s i n g l e 5 0 ~ m , t o t a l l y d e p l e t e d , s i l i con d e t e c t o r as shown in f i g . l ( b ) . T h e two d e t e c t i o n s y s t e m s a l w a y s g a v e i d e n t i c a l r e s u l t s f o r t h i s a c t i v i t y which can be s u m m a r i z e d a s f o l l o w s : (a) the e n e r g y of the p r o t o n s a f t e r c o r r e c t i o n f o r e n e r g y l o s s in the t a r g e t i s 1.53 =~0.04 M e V and t h e i r h a l f - l i f e is 245 +20 m s ; and (b) t h e i r e x c i t a t i o n f u n c t i o n i n d i c a t e s a t h r e s h o l d b e l o w an 160 b o m b a r d i n g e n e r g y of 39 MeV with a p e a k c r o s s s e c t i o n of ~ 8 pb o c c u r r i n g in the r e g i o n of 53 MeV ( l a b o r a t o r y ) . T h e low t h r e s h o l d e l i m i n a t e s 53Ni a s a s o u r c e of t h e s e p r o t o n s and i r r a d i a t i o n of o t h e r t a r g e t s i n d i c a t e d that the o b s e r v e d a c t i v i t y could not be a t t r i b u t e d to any l i k e l y c o n t a m i n a n t . In o r d e r to i n v e s t i g a t e f u r t h e r the r e a c t i o n s of 160 on 4 0 C a , d e l a y e d y - r a y s p e c t r a w e r e r e corded with a Ge(Li) detector followingthe bombardment of a calcium target with pulses of 160 ions at energies below 50 MeV produced by the coupled injector and EN tandem facility at Oxford. As was anticipated, prominent peaks were 281

Volume 33B, n~umber4

4O

i

PHYSICS LETTERS

i

i

26 October 1970 . I146 ~ ~ 9 / ~ "

i

~ 2 •

Telescope

a)

O÷ ~2F +

3O

~'6 +

7.52 :~Mn

(8.30}

(7/2 -)

+p

U

/

Ec.m.=l.56 MeV

3.04 2.34

~ (i9]2 -)

L33

{912 -)

O

Single 50 j~m Counter

(11/2 - }

~

7•2S3Fe 26 27

Fig.2. The proposed decay scheme for the proton unstable isomer 53com. Energies are given in MeV. The probable decay modes are given by solid arrows while the less-likely production of beta-delayed protons is indicated by dashed lines. ~9

0

0.5

h:l

t.O L5 Ep Observed {MeV)

2.0

2.5

Fig.l.(a) An identified proton spectrum arising from the bombardment of calcium by 160 at 49 MeV. Protons between 1.3 and 3.6 M e V could be linearly detected.

(b) A spectrum recorded in a single 50~m detector of the activity arising from bombardment of calcium by 49 MeV 160. The center-of-mass energy shown results from assuming 53Com to be the origin of this activity. Events below 0.6 MeV arise from the high fi-background and those below 0.4 MeV are not shown. o b s e r v e d c o r r e s p o n d i n g to the known [3] c a s c a d e s in 54Fe and 50Cr following fl+-decay of the h i g h - s p i n i s o m e r s 54Com and 50Mn m. T h e s e i s o m e r s w e r e populated v i a the (160,pn) and (160, pn) r e a c t i o n s with c r o s s s e c t i o n s of ~0.8 mb and ~ 4 mb, r e s p e c t i v e l y , at a mean bombarding e n e r g y of 47 MeV. At this e n e r g y , h o w e v e r , the o b s e r v e d i n t e n s i t i e s of y - r a y s of e n e r g i e s 0.701, 1.011, 1.328 and 2.339 M e V f n d i cared a c r o s s s e c t i o n of ~ 50 mb f o r the (160,2pn) r e a c t i o n leading to 5 3 F e m - a 2.5 min i s o m e r at an excitation of 3.04 MeV [4]. T h i s i s o m e r is p r e s u m e d to be 1 9 / 2 - , T=l/2 on the b a s i s of its o b s e r v e d p r o p e r t i e s and shell model p r e d i c t i o n s 282

for h i g h - s p i n s t a t e s with the configuration (f7/2)-3. C o n s i d e r i n g the o b s e r v e d yield of 53Fern f r o m the (160,2pn) r e a c t i o n , it is r e a s o n a b l e to expect a l o w e r but si g n i f i can t population of i t s i s o b a r i c analogue state in 53Co f r o m the (160,2np) r e a c t i o n . The m a s s e x c e s s of the 53Co ground state is p r e d i c t e d to be -42.64 MeV f r o m the known m a s s of 53Fe [5] and the Coulomb d i s p l a c e m e n t energy c a l c u l a t i o n s of H a r c h o l et al. [6]. F r o m the known m a s s of 52Fe [7], the binding e n e r g y of a p r o t o n in 53Co is then e s t i m a t e d to be 1.6 MeV and thus 5 3 c o m , the i s o b a r i c an al ogue st at e of 53Fe m, must be unbound. If the obs e r v e d p r o t o n group is attributed to the d i r e c t decay of 5 3 c o m to a 2 F e , one obtains a value of 8.42 MeV for the m a s s d i f f e r e n c e between the analogue i s o m e r s . This r e s u l t is in adequate a g r e e m e n t with the value 8.30 MeV c a l c u l a t e d in r e f . [6] for the c o r r e s p o n d i n g ground s t a t e s . * The c a l c u l a t e d t h r e s h o l d for the 4 0 C a ( 1 6 0 , 2 n p ) 5 3 c o m r e a c t i o n is 33.0 MeV, cons i s t e n t with the e x p e r i m e n t a l o b s e r v a t i o n s , and the excitation function a g r e e s well with that ex p e c t e d f r o m the s i m p l e s t a t i s t i c a l model c a l c u lations [8]. Based on 53Co m as the o r i g i n of the p r o t o n act i v i t y , the p r o b a b l e decay s c h e m e p r e s e n t e d in fig. 2 is c o n s i s t e n t with our data. The o b s e r v e d 245 ms h a l f - l i f e i m p l i e s that the dominant mode

* Some difference in Coulomb displacement energies might be expected due to the differing seniorities of the ground states and the isomers.

Volume 33B, number 4

PHYSICS LETTERS

of decay is by p o s i t r o n e m i s s i o n to 53Fe m. The p a r t i a l h a l f - l i f e f o r the F e r m i component alone of the s u p e r a l l o w e d fl+-decay can be c a l c u l a t e d with c o n s i d e r a b l e p r e c i s i o n [9] to be 0.35 s; if the G a m o w - T e l l e r m a t r i x e l e m e n t i s then in cluded ( a s s u m i n g p u r e (f7/2) -3 configurations), one obtains 0.2 s as the anticipated h a l f - l i f e [10] in r e a s o n a b l e a g r e e m e n t with o b s e r v a t i o n . P r o t o n e m i s s i o n apparently a r i s e s only as a weak b r a n c h in the decay of 53Com. A rough o r d e r of magnitude e s t i m a t e of the p a r t i a l l i f e t i m e for this branch can be obtained f r o m s t a t i s t i c a l mode l c a l c u l a t i o n s [8]; at an 160 e n e r g y of 47 MeV the p r e d i c t e d r a t i o of cr(160,2pn)/cr(160,2np) is ~ 7 0 / 1 , while the o b s e r v e d r a t i o of cr(53Fem)/ ~ p r o t o n a c t i v i t y ) is ~ 15000/1, i m p l y i n g a p a r tial half life ~ 50 s. Since p e n e t r a t i o n through the Coulomb and I =9 c e n t r i f u g a l b a r r i e r s * l e a d s to an e x p e c t e d h a l f - l i f e of ~ 0.3tzs for v 2 = l , this rough e s t i m a t i o n i m p l i e s y 2 ~ 6 × 10 -9 for this v e r y c o m p l e x decay. A 0.7 MeV (c.m.) p r o t o n group leading to the f i r s t e x c i t e d (2 +) state [11] of 52Fe is also e n e r g e t i c a l l y p o s s i b l e (with a b a r r i e r p e n e t r a b i l i t y 50 t i m e s l e s s ) but could not have been o b s e r v e d in t h e s e e x p e r i m e n t s . The best a l t e r n a t i v e explanation f o r this a c t i v ity is of c o u r s e to a t t r i b u t e it to b e t a - d e l a y e d p r o t o n e m i s s i o n . The only known nuclei expected to the m o r e than v e r y weak d e l a y e d - p r o t o n e m i t t e r s and which could p o s s i b l y be p r o d u c e d in the p r e s e n t e x p e r i m e n t s have T Z =- 3 / 2 , A =4n+ 1. The m e a s u r e d h a l f - l i f e and e n e r g y s p e c t r u m a r e quite i n c o n s i s t e n t with the known m e m b e r s of this s e r i e s (9C, 130, ... 41Ti,and49Fe) and the obs e r v e d t h r e s h o l d p r e c l u d e s p r o d u c t i o n of the unknown T Z = - 3 / 2 nuclide 45Cr. In this vein one m u s t also c o n s i d e r the ground s t a t e s and i s o m e r s of all o th e r nuclei for which the e m i s s i o n of b e t a - d e l a y e d p r o t o n s of 1.53 MeV is e n e r g e t i c a l l y p o s s i b l e . Of t h e s e the m o s t r e a sonable e x a m p l e follows the a s s u m p t i o n that the p a r e n t nuclide is s t i l l the i s o m e r 53Com, but that its d i r e c t p r o t o n decay is too weak to be obs e r v e d . One would then a t t r i b u t e the o b s e r v e d a c t i v i t y to d e l a y e d - p r o t o n e m i s s i o n following fl+-decay to a l e v e l in the r e g i o n of 9.08 MeV in 53Fe, as shown in fig. 2. In the unlikely event that such a s t at e w e r e populated by~ + - d e c a y f r o m 5 3 c o m having a l o g f t as low as 3.8, such a b r a n c h would r e p r e s e n t only 4 × 10 -4 of a l l d e c a y s of the i s o m e r . This would im p ly that the r a t i o -

E~

* An interaction radius of 6.63 fm was used in this calculation.

26 October 1970

(x(53com)/a(53Fem) was at l e a s t one o r d e r of magnitude higher than expected on the b a s i s of s t a t i s t i c a l model c a l c u l a t i o n s [8]. S i m i l a r a r g u m e n t s and the e x c i t a t i o n function data make it i m p r o b a b l e that any o t h er T Z = - 1 / 2 o r -1 nuclide below 5 6 N i - e v e n if p r o d u c e d in a f i s s i o n type r e a c t i o n - i s the o r i g i n of this activity. Howe v e r , as is obvious, our a r g u m e n t s r e l y upon the p r e s e n t l y known s y s t e m a t i c s of the decay of such nuclei. It is our opinion that the best explanation for the o r i g i n of the o b s e r v e d activity is the weak proton r a d i o a c t i v i t y of 53Co m. Its o r i g i n as a b e t a - d e l a y e d proton r a d i a t i o n f r o m 53Com can not, of c o u r s e , be r u l e d out by the p r e s e n t data nor can the a c t i v i t y without question be a t t r i buted to this nuclide. F u r t h e r e x p e r i m e n t a t i o n i s obviously needed to c l a r i f y t h ese points. At the v e r y l e a s t , h o w e v e r , it would a p p e a r that 5 3 c o m is the best p r e s e n t l y known candidate to be an o b s e r v a b l e p r o t o n - r a d i o a c t i v e n u cl eu s [12]. We wish to thank P r o f e s s o r K.W. Allen f o r his support and e n c o u r a g e m e n t of this work. F u r t h e r we should like to acknowledge the a s s i s tance given to us at the H a r w e l l Atomic E n e r g y R e s e a r c h E s t a b l i s h m e n t by Mr. R . W . McIlroy, Mr. E . J . J o n e s and Mr. B, r e n a r d .

References [1] j . Cerny et al., Phys.Rev.Lett. 24 (1970) 1128. [2] P.S. Fisher and D.K. Scott, Nucl.Instr. 49 (1967) 301. [3] C.M. Lederer, J.M. Hollander and I. Perlman, Table of isotopes (John Wiley and Sons, 1967). [4] K. Eskola, Phys.Letters 23 (1966) 471; I. Dernedde, Z.Physik 216 (1968) 103. [5] A. Tr i er et hi. NuchPhys. A l l l (1968) 241. [6] M. Harchol et hi., Nucl.Phys. A90 (1967) 459. [7] J.H.E, Mattauch, W. Thiele and A.H. Wapstra, Nucl.Phys. 67 (1965) 1. [8] M. Dlann, University of Rochester, private communication. The calculations follow the model discussed in Nucl.Phys. 80 (1966) 223. We wish to thank Professor Blann for performing these computations for us. [9] J.M. Freeman, J.G. Jenkin, G. Murray and W.E. Burcham, Phys.Rev.Lett. 16 (1966) 959. [10] A. dc-Shalit and I. Talmi, Nuclear shell theory, (Academic Press, 1963) We wish to thank Dr. LS. Towner for discussions on this calculation. [11] G. Bassini, N.M. Hintz and C.D.Kavaloski, Phys. Rev. 136 (1964) B1006. [12] V.L Goldanskii, Ann.Rev.Nucl.Sci. 16 (1966) 1; J.J~necke, Nucl.Phys. 61 (1965) 326.

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