The ionizing particle environment near earth

The r a d i a t i o n environment i n space i s d e s c r i b e d w i t h p a r t i c u l a r emphasis on t h e i n t e n s e l y i o n i z i n g p a r t i c l e s t h a t a r e capable o f p roduc ing s i n g l e event e f f e c t s i n t h e m i c r o e l e c t r o n i c s used on spacecra f t . A method i s presented f o r u s i n g models of t h e r a d i a t i o n environment t o c o n s t r u c t 1 i n e a r energy t r a n s f e r (LET) s p e c t r a i n s i d e spacecraf t . The v a r i a t i o n o f these LET s p e c t r a w i t h i n t e r p l a n e t a r y weather c o n d i t i o n s , s h i e l d i n g , and o r b i t a l parameters i s shown and t h e u n c e r t a i n t i e s i n these s p e c t r a a r e discussed. I n t r o d u c t i o n I t has been shown t h a t s i n o l e i n t e n s e l v i o n i i i n g p a r t i c l e s can p r o d G e s o f t u p i e t s l and l a t c h u p 3 i n s a t e l l i t e b o r n e microe l e c t r o n i c s 2 . 3 . The f requencies of such events must be a n t i c i p a t e d i n t h e des ign of e l e c t r o n i c c i r c u i t r y f o r use i n space. t h e r e f o r e , a means o f e s t i m a t i n g these f r e q u e n c i e s i s needed. These s i n g l e event e f f e c t s can be t h e r e s u l t o f t h e d i r e c t i o n i z a t i o n f rom p a r t i c l e s o r i g i n a t i n g o u t s i d e t h e spacecraf t3 . or t h e y canbe caused b y t h e i n t e n s e l y i o n i z i n g f ragments o f a n u c l e a r r e a c t i o n caused b y a p a r t i c l e o r i g i n a t i n g o u t s i d e t h e spacecra f tq . In t h e l a t t e r case. t h e n u c l e a r r e a c t i o n must occur i n or near t h e s e n s i t i v e p a r t o f an i n d i v i d u a l memory c e l l on t h e c h i p and t h e n u c l e a r r e a c t i o n must be o f a t y p e t h a t produces i n t e n s e l y i o n i z i n g fragments5. These f a c t o r s combine t o make t h i s mechanism v e r y i n e f f i c i e n t . Nuclear r e a c t i o n s a r e p r o b a b l y an impor tan t cause of s i n g l e event e f f e c t s o n l y i n t h e i n n e r r a d i a t i o n b e l t where i n t e n s e f l u x e s o f p r o t o n s a r e t rapped6. Because of t h e b a s i c d i f f e r e n c e i n t h e mechanism, t h e e s t i m a t i o n o f s i n g l e event e f f e c t r a t e s has t o be t r e a t e d d i f f e r e n t l y i n these two cases. Petersen5 and Bradford7 have d iscussed methods f o r e s t i m a t i n g s o f t upsets due t o n u c l e a r r e a c t i o n s caused by pro tons . The o p e r a t i o n a l upset c ross s e c t i o n s f o r p r o t o n s can a l s o be measured4. The t rapped p r o t o n environment, t h a t i s r e s p o n s i b l e f o r t h e n u c l e a r r e a c t i o n s which cause s i n g l e event e f f e c t s has been w e l l modeled6 and w i l l n o t be d iscussed here. Ins tead, t h i s paper w i l l c o n c e n t r a t e on t h e *This research was suppor ted b y NAVELEX under t h e Nuclear S u r v i v a b i l i t y 1 V u l n e r a b i l i t y Program. +Research P h y s i c i s t , Gama and Cosmic Ray A s t r o p h y s i c s Branch T h i * p s p ~ r i r d p d s r ~ d s r o r k o l l h ~ U . S . Covermmenl and Iherrrore is in lhe public domain more genera l problem o f s i n g l e event e f f e c t s caused b y t h e d i r e c t i o n i z a t i o n o f p a r t i c l e s o r i g i n a t i n g o u t s i d e t h e s p a c e c r a f t . E s t i m a t i o n of t h e S o f t Upset Rate When upsets a r e caused by i n t e n s e l y i o n i z i n g p a r t i c l e s o r i g i n a t i n g o u t s i d e t h e spacecraf t , these p a r t i c l e s g e n e r a l l y pass th rough t h e s e n s i t i v e volume i f t h e ~ memory c e l l a t a h i g h v e l o c i t y , so t h a t t h e i r r a t e of i o n i z a t i o n o r l i n e a r enerov t r a n s f e r (LET\ , ~ ~ . , ~~. does n o t change over thei i m e n s i o n s of t h a t s e n s i t i v e volume. T h i s means t h a t t h e LET s p e c t r a 8 , l E ~ ~ c a n be used t o e s t i m a t e .upset ra tes.g. lo .11 The upset r a t e i s s i m p l y , l l