Principle of the Momentum Distribution Techniques The momentum components py perpendicular to the propagation direction lead to angular deviations of the collinearity of the annihilation y-rays according to moc These equations hold for small angles.Ox,y can be egistered simultaneously in both x and y directions by a coincidence measurement using position-sensitive detection of the y-quanta
The momentum components px,y perpendicular to the propagation direction lead to angular deviations Θx,y of the collinearity of the annihilation γ-rays according to These equations hold for small angles. Θx,y can be egistered simultaneously in both x and y directions by a coincidence measurement using position-sensitive detection of the γ-quanta. Principle of the Momentum Distribution Techniques
Due to the limited energy resolution of Doppler- broadening spectroscopy,electron structure investigations are carried out mainly by angular correlation of annihilation radiation. Because the momentum of valence electrons is significantly lower,the momentum distribution of annihilating electrons shifts to smaller values.This means a smaller angular deviation for ACAR and a smaller Doppler broadening for DOBS. The curve of defect-rich material is thus higher and narrower than that of defect-free reference material,when both curves are normalized to equal area
D u e t o t h e l i m i t e d e n e r g y r e s o l u t i o n o f D o p p l e r - b r o a d e n i n g s p e c t r o s c o p y, e l e c t r o n s t r u c t u r e i n v e s t i g a t i o n s a r e c a r r i e d o u t m a i n l y b y a n g u l a r c o r r e l a t i o n o f a n n i h i l a t i o n r a d i a t i o n. B e c a u s e t h e m o m e n t u m o f v a l e n c e e l e c t r o n s i s s i g n i fi c a n t l y l o w e r, t h e m o m e n t u m d i s t r i b u t i o n o f a n n i h i l a t i n g e l e c t r o n s s h i ft s t o s m a l l e r v a l u e s. T h i s m e a n s a s m a l l e r a n g u l a r d e v i a t i o n fo r A C A R a n d a s m a l l e r D o p p l e r b r o a d e n i n g fo r D O B S. T h e c u r v e o f d e fe c t - r i c h m a t e r i a l i s t h u s h i g h e r a n d n a r r o w e r t h a n t h a t o f d e fe c t - fr e e r e fe r e n c e m a t e r i a l, w h e n b o t h c u r v e s a r e n o r m a l i z e d t o e q u a l a r e a
ID-ACAR The first ACAR measurements in one dimension were realized with Geiger counters by Behringer and Montgomery (1942).A position-sensitive detection can be realized in the simplest way in one dimension (1D-ACAR)by the mechanical movement of a long scintillation detector (Hautojarvi and Vehanen 1979;Mijnarends 1979). The integration in one more dimension compared with (6)results in a counting rate of N(⊙)=A∫∫σ（⊙mc,P,P:)p,dp
1D-ACAR The first ACAR measurements in one dimension were realized with Geiger counters by Behringer and Montgomery (1942). A position-sensitive detection can be realized in the simplest way in one dimension (1D-ACAR) by the mechanical movement of a long scintillation detector (Hautojärvi and Vehanen 1979; Mijnarends 1979). The integration in one more dimension compared with (6) results in a counting rate of
Magnet Sample ⊙ Slit ●e+ Slit S1 Magnet S2 Coincidence Memory The angular resolution is realized by lead slits.It can be adjusted in the range 0.2 to 5 mrad.The energy resolution of a corresponding Doppler broadening experiment would range from 0.05 to 1.3 keV.Thus,ACAR has a much better momentum resolution than the Doppler-broadening technique
The angular resolution is realized by lead slits . It can be adjusted in the range 0.2 to 5 mrad. The energy resolution of a corresponding Doppler broadening experiment would range from 0.05 to 1.3 keV. Thus, ACAR has a much better momentum resolution than the Doppler-broadening technique
2D-ACAR & 2D-detector array 2D-detector array Sample p. Coincidence 2D-Memory Coincidence counting rate N: N.(⊙，日，)=A∫o(⊙，mc,O,mc,p,)p ⑧x,y-px,y/moc Resolution:0.2~5mrad (0.05~1.2keV)
2D-ACAR Θ ✂✁ ✄ ✂✁ ✄☎