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Alpha radioactivity of unknown origin and the discovery of a new pleochroic halo
EARTH AND PLANETARY SCIENCE LETTERS 1 (1966) 453-454. NORTH-HOLLAND PUBL. COMP., AMSTERDAM
ALPHA RADIOACTIVITY OF UNKNOWN ORIGIN AND T H E D I S C O V E R Y OF A N E W P L E O C H R O I C H A L O R o b e r t V. GENTRY Institute of Planetary Science. Columbia Union College, Takorna Park, Maryland, USA Received 8 November 1966
Several pleochroic halos of anomalous radii have been found. In general it is not possible to relate these halos to the uranium or thorium decay series alpha emitters. The discovery of a new halo (typeG) corroborates previous evidence of presently unknown alpha radioactivity in crustal rocks. P l e o c h r o i c , or r a d i o a c t i v e , halos r e s u l t f r o m prolonged alpha e m i s s i o n f r o m m i c r o s c o p i c r a d i o a c t i v e i n c l u s i o n s in the host m i n e r a l . Since the alpha p a r t i c l e has a well defined r a n g e , a m i c r o s c o p i c i n c l u s i o n containing u r a n i u m or t h o r i u m will e v e n t u a l l y develop s e v e r a l s p h e r i cally c o n c e n t r i c r a d i a t i o n damage r e g i o n s about the c e n t r a l inclusion. M i c r o s c o p i c e x a m i n a t i o n of d i a m e t r a l thin s e c t i o n s of 238U and 232Th h a los r e v e a l s the r a d i o a c t i v e inclusion s u r r o u n d e d by s e v e r a l c o n c e n t r i c c o l o r e d r i n g s , whose r a d i i a c c u r a t e l y r e f l e c t the alpha p a r t i c l e e n e r g i e s of the daughter p r o d u c t s in the r e s p e c t i v e decay s e r i e s [1]. While c e r t a i n dwarf halos have been a t t r i b -
uted to one of the s a m a r i u m i s o t o p e s [2], giant halos [3] s i m i l a r to those i n v e s t i g a t e d by H i r s c h i [4] and Wiman [5] have been r e p o r t e d , and m o r e r e c e n t l y another v a r i a n t halo (designated type Y) was d i s c o v e r e d in Canadian biotite. The type Y halo is a d i s c - l i k e s t r u c t u r e of 26 ~ r a d i u s c o r r e s p o n d i n g to an alpha e m i t t e r of about 6.55 MeV [6]. The p u r p o s e of this l e t t e r i s to r e p o r t that, a s a r e s u l t of e x t e n s i v e r e s e a r c h on r a d i o a c t i v e halos in biotite f r o m s e v e r a l a r e a s of the world, still another a n o m a l o u s halo has been d i s c o v e r e d . T h i s halo (fig. 1) was found in the black Ytterby m i c a and p o s s e s s e s a v e r a g e r i n g r a d i i of 13.6 ~ and 19 ~, c o r r e s p o n d i n g to alpha p a r t i c l e e n e r g i e s of 4.5 MeV and 5.4 MeV. T h e s e r i n g s a r e not c o n s i s t e n t with the d e v e l o p m e n t of e i t h e r the u r a n i u m or t h o r i u m halos or any c o m bination t h e r e o f . F u r t h e r m o r e , f i s s i o n t r a c k a n a l y s i s of the m i c a shows no f i s s i o n t r a c k s in the v i c i n i t y of the c e n t r a l inclusion, thus e l i m i n a t i n g the p r e s ence of any spontaneous f i s s i o n a b l e n u c l i d e s such as 238U in the c e n t r a l inclusion. W h a t e v e r the i m p l i c a t i o n s , it s e e m s that these halos r e p r e s e n t alpha r a d i o a c t i v i t y of p r e s e n t l y unknown o r i g i n which at one t i m e e x i s t e d in the e a r t h ' s c r u s t a l r o c k s . J u s t how the e x i s t e n c e of t h e s e halos may be r e c o n c i l e d with c u r r e n t t h e o r i e s of n u c l e o s y n t h e s i s and c r u s t a l f o r m a t i o n is t r e a t e d e l s e w h e r e [7]. REFERENCES
Fig. 1. Type G new halo (380 x).
[1] G. HtHenderson and S. Bateson, A quantitative study of pleochroic halos, Proc. Roy. Soc. A 145 (1934} 563.
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R . V . GE NTRY
[2] E . E , Picciotto and S. Deutsch, Pleochroic haloes, Comitato Nazionale per I ' E n e r g i a Nucleare, Rome (1960). [3] R. V. Gentry, Abnormally long a l p h a - p a r t i c l e t r a c k s in biotite (mica), Appl. Phys. L e t t e r s 8 (1966) 65. [4] H . H i r s c h i , Radioactivity of a few Swiss rocks. ORNL-tr-702. [5] E.Wiman. Studies of some Archaean rocks in the
neighbourhood of Uppsala, Sweden, and of their ueological position, Bull. Geol. Inst. Uppsala 23 (1930) 1. [6] R. V. Gentry, Additional evidence of extinct r a d i o activity from pleochroic halos and the formation of the lunar crust, T r a n s . Am. Geophys. Union 47 (1966) 487. [ 7] R. V, Gentry, in preparation.
ALPHA RADIOACTIVITY OF UNKNOWN ORIGIN AND T H E D I S C O V E R Y OF A N E W P L E O C H R O I C H A L O R o b e r t V. GENTRY Institute of Planetary Science. Columbia Union College, Takorna Park, Maryland, USA Received 8 November 1966
Several pleochroic halos of anomalous radii have been found. In general it is not possible to relate these halos to the uranium or thorium decay series alpha emitters. The discovery of a new halo (typeG) corroborates previous evidence of presently unknown alpha radioactivity in crustal rocks. P l e o c h r o i c , or r a d i o a c t i v e , halos r e s u l t f r o m prolonged alpha e m i s s i o n f r o m m i c r o s c o p i c r a d i o a c t i v e i n c l u s i o n s in the host m i n e r a l . Since the alpha p a r t i c l e has a well defined r a n g e , a m i c r o s c o p i c i n c l u s i o n containing u r a n i u m or t h o r i u m will e v e n t u a l l y develop s e v e r a l s p h e r i cally c o n c e n t r i c r a d i a t i o n damage r e g i o n s about the c e n t r a l inclusion. M i c r o s c o p i c e x a m i n a t i o n of d i a m e t r a l thin s e c t i o n s of 238U and 232Th h a los r e v e a l s the r a d i o a c t i v e inclusion s u r r o u n d e d by s e v e r a l c o n c e n t r i c c o l o r e d r i n g s , whose r a d i i a c c u r a t e l y r e f l e c t the alpha p a r t i c l e e n e r g i e s of the daughter p r o d u c t s in the r e s p e c t i v e decay s e r i e s [1]. While c e r t a i n dwarf halos have been a t t r i b -
uted to one of the s a m a r i u m i s o t o p e s [2], giant halos [3] s i m i l a r to those i n v e s t i g a t e d by H i r s c h i [4] and Wiman [5] have been r e p o r t e d , and m o r e r e c e n t l y another v a r i a n t halo (designated type Y) was d i s c o v e r e d in Canadian biotite. The type Y halo is a d i s c - l i k e s t r u c t u r e of 26 ~ r a d i u s c o r r e s p o n d i n g to an alpha e m i t t e r of about 6.55 MeV [6]. The p u r p o s e of this l e t t e r i s to r e p o r t that, a s a r e s u l t of e x t e n s i v e r e s e a r c h on r a d i o a c t i v e halos in biotite f r o m s e v e r a l a r e a s of the world, still another a n o m a l o u s halo has been d i s c o v e r e d . T h i s halo (fig. 1) was found in the black Ytterby m i c a and p o s s e s s e s a v e r a g e r i n g r a d i i of 13.6 ~ and 19 ~, c o r r e s p o n d i n g to alpha p a r t i c l e e n e r g i e s of 4.5 MeV and 5.4 MeV. T h e s e r i n g s a r e not c o n s i s t e n t with the d e v e l o p m e n t of e i t h e r the u r a n i u m or t h o r i u m halos or any c o m bination t h e r e o f . F u r t h e r m o r e , f i s s i o n t r a c k a n a l y s i s of the m i c a shows no f i s s i o n t r a c k s in the v i c i n i t y of the c e n t r a l inclusion, thus e l i m i n a t i n g the p r e s ence of any spontaneous f i s s i o n a b l e n u c l i d e s such as 238U in the c e n t r a l inclusion. W h a t e v e r the i m p l i c a t i o n s , it s e e m s that these halos r e p r e s e n t alpha r a d i o a c t i v i t y of p r e s e n t l y unknown o r i g i n which at one t i m e e x i s t e d in the e a r t h ' s c r u s t a l r o c k s . J u s t how the e x i s t e n c e of t h e s e halos may be r e c o n c i l e d with c u r r e n t t h e o r i e s of n u c l e o s y n t h e s i s and c r u s t a l f o r m a t i o n is t r e a t e d e l s e w h e r e [7]. REFERENCES
Fig. 1. Type G new halo (380 x).
[1] G. HtHenderson and S. Bateson, A quantitative study of pleochroic halos, Proc. Roy. Soc. A 145 (1934} 563.
454
R . V . GE NTRY
[2] E . E , Picciotto and S. Deutsch, Pleochroic haloes, Comitato Nazionale per I ' E n e r g i a Nucleare, Rome (1960). [3] R. V. Gentry, Abnormally long a l p h a - p a r t i c l e t r a c k s in biotite (mica), Appl. Phys. L e t t e r s 8 (1966) 65. [4] H . H i r s c h i , Radioactivity of a few Swiss rocks. ORNL-tr-702. [5] E.Wiman. Studies of some Archaean rocks in the
neighbourhood of Uppsala, Sweden, and of their ueological position, Bull. Geol. Inst. Uppsala 23 (1930) 1. [6] R. V. Gentry, Additional evidence of extinct r a d i o activity from pleochroic halos and the formation of the lunar crust, T r a n s . Am. Geophys. Union 47 (1966) 487. [ 7] R. V, Gentry, in preparation.