REPORT ON THE MAY 1983 POLARIZED ELECTRON SOURCE WORKSHOP AT SLAC

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REPORT ON THE MAY 1983 POLARIZED ELECTRON SOURCE WORKSHOP AT SLAC

C. Sinclair

To cite this version:

C. Sinclair. REPORT ON THE MAY 1983 POLARIZED ELECTRON SOURCE WORKSHOP AT SLAC. Journal de Physique Colloques, 1985, 46 (C2), pp.C2-669-C2-682. �10.1051/jphyscol:1985284�.

�jpa-00224603�

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JOURNAL DE PHYSIQUE

Colloque C2, supplément au n°2, Tome *6, février 1985 page C2-669

REPORT ON THE MAY 1983 POLARIZED ELECTRON SOURCE WORKSHOP AT SLAC*

C.K. Sinclair

Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94305, U.S.A.

Résume - Les travaux ainsi que les conclusions des journées de travail sur les électrons polarisés en 1983 à SLAC sont passés en revue. Sont aussi inclus quelques progrès achevés depuis.

Abstract - The work and conclusions of the 1983 Polarized Electron Source Workshop at SLAC are reviewed. Some mention of progress since that meeting is also included.

Introduction

A workshop on polarized electron sources was sponsored by the International Commit- tee for Symposia on High Energy Spin Physics, and held at SLAC on May 16-19, 1983.

This workshop brought together 18 physicists from the fields of atomic physics, surface physics, condensed matter physics, and of course, high energy and nuclear physics. Our goals were to survey the current situation in polarized electron pro- duction and electron polarization measurement, and to study ways in which the state of the art in these fields might reasonably be advanced. Our attendees came from four western European institutions, and nine in the U.S. Of these, only eight had as their primary discipline either high energy or nuclear physics. In this brief report, I will review the activities of the workshop and also mention, to the extent I am familiar with it, work accomplished since the time of the workshop.

The workshop opened with a review of the application of polarized electron sources in the areas of high energy and nuclear physics, condensed matter and surface physics, and atomic physics. These discussions will not be reproduced here as they are either well known to readers of these proceedings, or are not germane to the topic of this conference. In a final recommendation session, we prepared a lengthy list of experiments which should be attempted and future directions for new work. Some of these will be covered in the text of this report under the particular sections to which they apply.

Although the meeting was open to, and encouraged, discussion of all types of polarized electron sources, the primary interest of a great majority of the participants was in the GaAs source and other sources of this general type, which employ optical pumping in non-magnetic solids. The GaAs source has seen very wide application to a wide variety of problems in basic and applied physics, and is by far the most commonly employed polarized electron source at this time. In fact, a fairly large number of polarized electron measurements have been made only because of the existence and particular characteristics of this source. Despite all this, the GaAs source delivers only a mediocre polarization and is a demanding source to maintain in operation with a particle accelerator. A large part of the workshop was devoted to a discussion of methods to overcome these weaknesses. No doubt the large emphasis the workshop placed on the GaAs source was due to the presence of physicists from fields other than high energy and nuclear physics, where the low polarization and relatively short operating lifetime of this source is less of a problem than for accelerator based experiments.

Other polarized electron sources which were reviewed included those based upon optical

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985284

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C2-670 JOURNAL DE PHYSIQUE

o r magnetic o r i e n t a t i o n i n f r e e a t o m i c beams, and t h o s e employing photoemission t o r e l e a s e p o l a r i z e d e l e c t r o n s from magnetized s o l i d s . None of t h e t e c h n i q u e s employing magnetized s o l i d s a p p e a r p a r t i c u l a r l y s u i t e d a s s o u r c e s f o r a c c e l e r a t o r based p h y s i c s . Of t h e a t o m i c beam s o u r c e s , one, based upon c h e m i - i o n i z a t i o n of a n o p t i c a l l y pumped h e l i u m a f t e r g l o w , a p p e a r s c a p a b l e of b e i n g developed t o t h e p o i n t where it c o u l d d e l i v e r a c o n t i n u o u s e l e c t r o n beam of about 1 mA w i t h E 90% p o l a r i z a t i o n . Such a s o u r c e would d e f i n i t e l y be u s e f u l f o r some e x p e r i m e n t s c u r r e n t l y c o n s i d e r e d . T h i s p a r t i c u l a r t y p e o f s o u r c e i s r e l a t i v e l y f r e e of many of t h e t r o u b l e s o m e f e a t u r e s of o t h e r p o l a r i z e d s o u r c e s (e.g. a l k a l i m e t a l beams, e x c e p t i o n a l v a c u a , d i f f i c u l t o p t i c a l s o u r c e s , e t c . ) .

A f a i r p e r i o d of t i m e was s p e n t d i s c u s s i n g t h e v a r i o u s t e c h n i q u e s which have been developed t o measure e l e c t r o n p o l a r i z a t i o n a t low energy. Such measurements have, i n t h e p a s t , o f t e n been c h a r a c t e r i z e d by r a t h e r l a r g e and u n c e r t a i n s y s t e m a t i c e r r o r s , even i n c i r c u m s t a n c e s where t h e r e was good s t a t i s t i c a l p r e c i s i o n . T h e r e i s a d e f i n i t e need t o have a n e l e c t r o n p o l a r i z a t i o n measurement t e c h n i q u e which o f f e r s h i g h s t a t i s - t i c a l p r e c i s i o n i n a s h o r t measuring t i m e , freedom from s y s t e m a t i c u n c e r t a i n t i e s and s p u r i o u s e f f e c t s , and a w e l l known a n a l y z i n g power, a l l c o n t a i n e d i n a s u f f i c i e n t l y s i m p l e a p p a r a t u s t o p e r m i t w i d e s p r e a d u t i l i z a t i o n . Such a method i s y e t t o b e i n v e n t e d , a l t h o u g h a c o u p l e i d e a s were p r e s e n t e d which might w e l l p r o v i d e s u c h a method i n t h e f u t u r e .

The t e c h n i q u e s t o measure e l e c t r o n p o l a r i z a t i o n a t h i g h beam energy were n o t a d d r e s s e d , a l t h o u g h t h e need f o r b e t t e r methods i n t h i s a r e a i s v e r y i m p o r t a n t , p a r t i c u l a r l y f o r t h e coming g e n e r a t i o n of h i g h e r energy machines which p l a n t o u t i l i z e p o l a r i z e d e l e c - t r o n s : SLC, LEP, and HERA.

The d e t a i l s of t h e GaAs s o u r c e were reviewed i n a s e s s i o n which covered t h e o p e r a t i o n a l o r n e a r - o p e r a t i o n a l s o u r c e s developed f o r a c c e l e r a t o r a p p l i c a t i o n s . These i n c l u d e o p e r a t i o n a l s o u r c e s a t Mainz and Chalk R i v e r , and s o u r c e s u n d e r development f o r t h e SLC and t h e MIT B a t e s l i n a c ( t h i s l a t t e r s o u r c e i s c u r r e n t l y b e i n g i n s t a l l e d on t h e a c c e l e r a t o r ) . The p h y s i c s of producing p o l a r i z e d c o n d u c t i o n band e l e c t r o n s i n GaAs, and e x t r a c t i n g them from t h e b u l k m a t e r i a l , r e c e i v e d t h e a t t e n t i o n of one s e s s i o n . A s e s s i o n c o v e r i n g t h e p r e p a r a t i o n of GaAs p h o t o c a t h o d e s r e c e i v e d a g r e a t d e a l of

i n t e r e s t . A l a r g e number of t h e workshop recommendations d e a l t w i t h producing photo- c a t h o d e s w i t h good quantum e f f i c i e n c i e s and l o n g o p e r a t i o n a l l i f e . I n c r e a s e d p o l a r - i z a t i o n from s o u r c e s o f t h e GaAs t y p e r e c e i v e d t h e a t t e n t i o n of one s e s s i o n a s w e l l . B e f o r e l e a v i n g t h i s i n t r o d u c t i o n , it i s w o r t h w h i l e t o make two remarks. The f i r s t i s t h a t , a l t h o u g h p o l a r i z e d e l e c t r o n s o u r c e s a r e u s u a l l y t h o u g h t of o n l y i n t e r m s of t h e i r u s e w i t h l i n e a r a c c e l e r a t o r s , t h e r e i s no r e a s o n i n p r i n c i p l e why t h e y cannot be employed w i t h c i r c u l a r machines, a t l e a s t up t o beam e n e r g i e s where t h e beam e n e r g y s p r e a d makes p o l a r i z a t i o n p r o b l e m a t i c . The Bonn U n i v e r s i t y group h a s s u c c e s s f u l l y a c c e l e r - a t e d p o l a r i z e d e l e c t r o n s t h r o u g h s e v e r a l r e s o n a n c e s , and e x p e c t t o b e a b l e t o p r o v i d e p o l a r i z e d e l e c t r o n beams from t h e i r n e x t machine. T h i s work i s d i s c u s s e d i n a r e c e n t p r e p r i n t by W. B r e f e l d g. &. (Bonn U n i v e r s i t y p r e p r i n t 84-23).

The second remark i s t h a t v e r y l i t t l e p o l a r i z e d e l e c t r o n s o u r c e development work i s b e i n g done i n h i g h e n e r g y and n u c l e a r p h y s i c s l a b o r a t o r i e s . S i n c e h i g h beam p o l a r - i z a t i o n and l a r g e beam i n t e n s i t i e s a r e n o t e s s e n t i a l t o s u c c e s s f u l p o l a r i z e d e l e c t r o n e x p e r i m e n t s i n o t h e r f i e l d s of p h y s i c s , one c a n n o t e x p e c t t h e development of such s o u r c e c h a r a c t e r i s t i c s from e x p e r i m e n t e r s i n t h e s e f i e l d s . To t h e e x t e n t t h a t t h e s e s o u r c e c h a r a c t e r i s t i c s a r e r e q u i r e d f o r work i n o u r f i e l d s , we w i l l have t o do t h a t development.

The GaAs S o u r c e

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To s e t t h e s t a g e f o r f u r t h e r d i s c u s s i o n , l e t u s b r i e f l y r e v i e w t h e o p e r a t i o n of t h e GaAs p o l a r i z e d e l e c t r o n s o u r c e . A s i m p l i f i e d (though r e a s o n a b l y a c c u r a t e ) view of t h e band s t r u c t u r e of GaAs n e a r t h e minimum d i r e c t bandgap, a t t h e c e n t e r of t h e B r i l l o u i n zone, i s shown i n f i g u r e 1. T r a n s i t i o n s d u e t o photon a b s o r p t i o n a r e v e r t i c a l l i n e s i n t h i s f i g u r e . When t r a n s i t i o n s from t h e P3/2 v a l e n c e band t o t h e S112 c o n d u c t i o n band a r e caused by 100% c i r c u l a r l y p o l a r i z e d p h o t o n s , t h e c o n d u c t i o n band e l e c t r o n s

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F i g . 1 - The band s t r u c t u r e of GaAs near t h e bandgap minimum i s shown on t h e l e f t . On t h e r i g h t , t h e r e l a t i v e t r a n s i t i o n r a t e s between t h e v a l e n c e and conduction bands f o r r i g h t ( l e f t ) c i r c u l a r l y p o l a r i z e d photons a r e shown a d j a c e n t t o t h e s o l i d (dashed) l i n e s . For photon e n e r g i e s between Eg and E + A , only t r a n s i t i o n s from t h e P 3 / 2 v a l e n c e band

a r e p o s s i b l e . g

have a -50% p o l a r i z a t i o n . This i s i l l u s t r a t e d i n t h e diagram of r e l a t i v e t r a n s i t i o n r a t e s on t h e r i g h t i n f i g u r e 1, and i s a consequence of angular momentum c o n s e r v a t i o n . A s t h e photon energy i n c r e a s e s t o t h e p o i n t where t r a n s i t i o n s from t h e s p i n - o r b i t s p l i t o f f band a r e allowed, t h e p o l a r i z a t i o n d e c r e a s e s t o z e r o ,

E l e c t r o n s photoexcited t o t h e conduction band a r e nominally bound t o t h e c r y s t a l by about 4 eV. However, by t h e a d d i t i o n of monolayer q u a n t i t i e s of cesium and oxygen t o t h e GaAs s u r f a c e , t h e work f u n c t i o n can be lowered t o t h e p o i n t where e l e c t r o n s a t t h e bottom of t h e conduction band can e n e r g e t i c a l l y l e a v e t h e c r y s t a l , a c o n d i t i o n known a s n e g a t i v e e l e c t r o n a f f i n i t y (NEA). This t h e n completes t h e b a s i c p i c t u r e of t h e GaAs p o l a r i z e d source: o p t i c a l pumping with a c i r c u l a r l y p o l a r i z e d photon beam t o produce conduction band e l e c t r o n p o l a r i z a t i o n , and t r e a t m e n t of t h e c r y s t a l s u r f a c e t o lower t h e work f u n c t i o n t o t h e p o i n t where conduction band e l e c t r o n s may be e m i t t e d . The measured e l e c t r o n p o l a r i z a t i o n from a t y p i c a l NEA GaAs photocathode i s shown i n f i g u r e 2 .

I 2 3 4

l a 8 4 PHOTON ENERGY (eV) ,,,,,,

Fig. 2 - Measured e l e c t r o n p o l a r i z a t i o n a s a f u n c t i o n of c i r c u l a r l y p o l a r i z e d photon energy f o r a t y p i c a l NEA GaAs photocathode.

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C2-672 JOURNAL DE PHYSIQUE

A s t h e s p i n - o r b i t s p l i t t i n g i s s i z e a b l e i n GaAs ( S 0.34 eV) t h e e l e c t r o n p o l a r i z a t i o n i s f a i r l y i n s e n s i t i v e t o t h e o p t i c a l photon energy i n t h e v i c i n i t y of t h e bandgap energy. GaAs i s a v e r y e f f i c i e n t p h o t o e m i t t e r , making it e a s y t o o b t a i n q u i t e l a r g e c u r r e n t s o f p o l a r i z e d e l e c t r o n s . It i s s t r a i g h t f o r w a r d t o r e v e t s e t h e p o l a r i z a t i o n of t h e o p t i c a l pump beam, and w i t h it t h e e l e c t r o n p o l a r i z a t i o n , r a p i d l y and w i t h o u t s i g - n i f i c a n t l y a l t e r i n g any of t h e e l e c t r o n - o p t i c a l p r o p e r t i e s of t h e p o l a r i z e d s o u r c e , t h u s making it p o s s i b l e t o a c h i e v e a h i g h l y d e s i r a b l e freedom from s y s t e m a t i c e f f e c t s upon p o l a r i z a t i o n r e v e r s a l . Each of t h e s e p r o p e r t i e s makes t h e GaAs p o l a r i z e d s o u r c e most u s e f u l .

Not a l l of t h e c h a r a c t e r i s t i c s of t h e GaAs s o u r c e a r e s o n i c e , however. The monolayer q u a n t i t i e s of cesium and oxygen which lower t h e work f u n c t i o n must b e a p p l i e d t o a c r y s t a l which h a s f a r l e s s t h a n a monolayer of c o n t a m i n a t i o n from any o t h e r a r b i t r a r y s u b s t a n c e . The a c t i v a t e d photocathode s u r f a c e i s v e r y s u s c e p t i b l e t o d e g r a d a t i o n i n t h e p r e s e n c e of r e s i d u a l g a s e s i n t h e b e s t of vacuum s y s t e m s . Cesium may d e s o r b from t h e a c t i v a t e d s u r f a c e , r e d u c i n g t h e quantum e f f i c i e n c y a s a f u n c t i o n of t i m e . T y p i c a l o p e r a t i n g l i f e t i m e s f o r p o l a r i z e d s o u r c e s on a c c e l e r a t o r s a r e on t h e o r d e r of 10 t o 40 h o u r s . Sub-monolayer q u a n t i t i e s of some c o n t a m i n a n t s a n d / o r poor c r y s t a l q u a l i t y o r

s u r f a c e morphology r e a d i l y p r e v e n t achievement of good quantum e f f i c i e n c i e s . A l l of t h e s e d i f f i c u l t i e s make t h e c l e a n i n g , h a n d l i n g , and p r e p a r a t i o n of t h e GaAs c r y s t a l s of g r e a t importance. C a r e f u l c l e a n i n g p r o c e d u r e s and g r e a t a t t e n t i o n t o d e t a i l i n vacuum p r o c e d u r e s a r e e s s e n t i a l t o a s u c c e s s f u l GaAs p o l a r i z e d s o u r c e . F i n a l l y , t h e maximum p o l a r i z a t i o n o f 50% t h e o r e t i c a l l y , and o f t e n 35 t o 45% i n p r a c t i c e i s a r e a l l i m i t a t i o n , p a r t i c u l a r l y f o r t h o s e e x p e r i m e n t s u s i n g a p o l a r i z e d t a r g e t , where r a d i a - t i o n damage a n d / o r t a r g e t h e a t i n g l i m i t t h e a c c e p t a b l e beam c u r r e n t .

T a b l e I. T y p i c a l O p e r a t i n g C h a r a c t e r i s t i c s of GaAs P o l a r i z e d E l e c t r o n Sources Beam P o l a r i z a t i o n 35% t o 45%, l o n g i t u d i n a l

Beam C u r r e n t 1 m.4 c o n t i n u o u s t o 15 A peak c u r r e n t p u l s e s

Duty F a c t o r 1-2 n s e c . p u l s e s t o c o n t i n u o u s beam Cathode Quantum E f f i c i e n c y 0.1% t o a b o u t 5%

Cathode O p e r a t i n g Temperature 77K t o 300K

Source Vacuum 1 0 - l o t o r r .

O p e r a t i n g L i f e t i m e

O p t i c a l Source

A few h o u r s t o a b o u t 100 h o u r s , b e f o r e i n s i t u r e c l e a n i n g and r e a c t i v a t i o n i s n e c e s s a r y

T y p i c a l l y a dye l a s e r , wavelength between 630 and 800 nm, 100% c i r c u l a r p o l a r i z a t i o n

Table I g i v e s a summary of some of t h e o p e r a t i n g p a r a m e t e r s of GaAs s o u r c e s used w i t h p a r t i c l e a c c e l e r a t o r s . These numbers come from e x p e r i e n c e w i t h s o u r c e s a t SLAC / I / , Mainz 121, and Chalk R i v e r / 3 / . A d d i t i o n a l s o u r c e s a r e under development f o r t h e

SLC 141, t h e MIT B a t e s l i n a c 151, and a t Nagoya, Bonn U n i v e r s i t y , and Mainz. Perhaps t h e most n o v e l of t h e s o u r c e s under development a r e t h o s e f o r t h e SLC and t h e Mainz m i c r o t r o n . The f i r s t i s n o t a b l e f o r t h e a b i l i t y t o d e l i v e r v e r y s h o r t , h i g h c u r r e n t bunches, and t h e l a t t e r i s a l o v e l y p i e c e of s o u r c e e n g i n e e r i n g . The Mainz m i c r o t r o n s o u r c e w i l l o p e r a t e a t t h e h i g h v o l t a g e t e r m i n a l of t h e i n j e c t o r Van d e G r a a f f , and t h u s be p h y s i c a l l y i n a c c e s s i b l e . I t i s b o t h s m a l l and l i g h t , and i n c o r p o r a t e s d i f f - e r e n t i a l pumping t o p e r m i t c o n n e c t i o n t o t h e poor vacuum of t h e Van de G r a a f f . The Mainz group h a s a l s o accomplished a n o t h e r i m p r e s s i v e f e a t : t h e s o u r c e t h e y u s e w i t h t h e i r l i n a c i s r e g u l a r l y removed and r e - i n s t a l l e d , i n a l t e r n a t i o n w i t h a t h e r m i o n i c gun! The s o u r c e h a s been d e t a t c h e d and t r a n s p o r t e d away from t h e l i n a c under vacuum

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a number of t i m e s .

While each p o l a r i z e d e l e c t r o n gun h a s i t s own p a r t i c u l a r f e a t u r e s , f i g u r e 3 shows a view of a t y p i c a l such gun. T h i s one was employed f o r t h e SLAC p a r i t y v i o l a t i o n work a few y e a r s ago 161. The Mainz and Chalk R i v e r guns a r e v e r y s i m i l a r t o t h i s gun.

T h i s gun is, i n f a c t , v e r y s i m i l a r t o t h e t h e r m i o n i c guns used on SLAC, w i t h t h e major d i f f e r e n c e s b e i n g t h e r e p l a c e m e n t of t h e t h e r m i o n i c e m i t t e r w i t h a GaAs w a f e r , t h e a d d i t i o n of a p p a r a t u s t o a c t i v a t e t h e GaAs s u r f a c e , and a means t o c o o l b o t h t h e GaAs c r y s t a l and t h e i n n e r w a l l s of t h e vacuum chamber.

F i g u r e 4 d i s p l a y s a complete p o l a r i z e d e l e c t r o n i n j e c t i o n system, a g a i n as used a t SLAC f o r t h e p a r i t y v i o l a t i o n experiment. Most of t h e f i r s t g e n e r a t i o n of p o l a r i z e d e l e c t r o n i n j e c t o r s were i n s t a l l a t i o n s of s i m i l a r complexity. A l l t h e s e complex i n s t a l l - a t i o n s p a l e by comparison t o t h e v e r y compact and s i m p l e s o u r c e f o r t h e Mainz m i c r o t r o n . O p e r a t i o n a l e x p e r i e n c e w i t h GaAs s o u r c e s a t a number o f l a b o r a t o r i e s h a s l e d t o t h e c o n c l u s i o n t h a t it i s n o t n e c e s s a r y t o o p e r a t e t h e GaAs a t l i q u i d n i t r o g e n ( o r c o l d e r ) t e m p e r a t u r e s t o a c h i e v e good p o l a r i z a t i o n , and t h a t it i s u n n e c e s s a r y t o i n c o r p o r a t e a Mott s c a t t e r i n g a p p a r a t u s t o measure t h e s o u r c e p o l a r i z a t i o n . I n a l l p o l a r i z e d e l e c t r o n e x p e r i m e n t s t o d a t e , p r o v i s i o n i s made t o measure t h e beam p o l a r i z a t i o n a t t h e h i g h energy end of t h e a c c e l e r a t o r . G e n e r a l l y s p e a k i n g , t h e h i g h energy beam pol- a r i z a t i o n measurements a r e a s f a s t and p r e c i s e a s Mott s c a t t e r i n g a t t h e s o u r c e energy, and a r e t h u s a s good a measure of s o u r c e performance. Of c o u r s e , s o u r c e development work r e q u i r e s some means of p o l a r i z a t i o n measurement i n a l a b o r a t o r y s e t t i n g , and f o r t h i s Mott s c a t t e r i n g i s s t i l l a p p r o p r i a t e .

L N 2 FOR CATHODE CERAMIC INSULATOR

PLATINUM COATED

Ga As GATHODE CATHODE ELECTRODE

ANODE ELECTRODE

TO 3 0 I/s ION

-L

PUMP

RETRACTABLE CESl

(IN POSITION FOR CA LNp COLD SHIELD -

ACTIVATION 1

F i g . 3 - The p o l a r i z e d e l e c t r o n gun developed f o r t h e SLAC p a r i t y v i o l a t i o n experiment.

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Fig. 4 - The complete polarized electron injection system constructed at SLAC for the parity violation experiment.

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Two v a r i a t i o n s of t h e b a s i c GaAs s o u r c e a r e worthy of n o t e . I n t h e f i r s t of t h e s e , one employs a semiconductor w i t h t h e same h a s i c band s t r u c t u r e a s GaAs, but w i t h a l a r g e r d i r e c t bandgap. T h i s moves t h e o p t i c a l wavelength r e q u i r e d t o produce t h e p o l a r i z e d beam toward s h o r t e r wavelengths, which may b e convenient f o r some a p p l i c a - t i o n s . It i s q u i t e f e a s i b l e t o produce a p o l a r i z e d e m i t t e r which o p e r a t e s a t t h e wave- l e n g t h of t h e common helium-neon l a s e r , f o r example. The Nainz s o u r c e u t i l i z e s a

6fP0 38 c a t h o d e which p e r m i t s o p e r a t i o n a t a 650 nm wavelength. T h i s i s a par- t i c u &r y i n t e r e s t i n g t e c h n i q u e , b e c a u s e i n p r i n c i p l e , l a r g e r bandgap GaAsP photocathodes s h o u l d b e f a r more s t a b l e t h a n t h o s e of GaAs. Although GaAsP i s commercially a v a i l a b l e , it i s normally n-type, w h i l e p-type m a t e r i a l i s r e q u i r e d f o r photocathodes.

Mainz u s e s n-type m a t e r i a l which i s c o n v e r t e d t o p-type a t t h e s u r f a c e by i o n implanta- t i o n . I t may b e t h a t t h e low quantum e f f i c i e n c y of t h e Mainz s o u r c e i s t h e r e s u l t of c r y s t a l damage caused by t h i s i o n i m p l a n t a t i o n . It would b e u s e f u l t o o b t a i n some p-type GaAsP m a t e r i a l p r e p a r e d by some h i g h q u a l i t y e p i t a x i a l p r o c e s s , t o s e e i f good quantum e f f i c i e n c y and good p o l a r i z a t i o n could be o b t a i n e d from t h i s m a t e r i a l . It i s n o t f e a s i b l e t o t r y t o produce a s o u r c e o p e r a t i n g a t l o n g e r wavelengths t h a n GaAs, a s t h e c a t h o d e s become l e s s s t a b l e , and it i s d i f f i c u l t t o o b t a i n n e g a t i v e e l e c t r o n a f f i n - i t y c o n d i t i o n s on s m a l l e r bandgap m a t e r i a l s .

A group a t KFA ( J i i l i c h ) h a s a l s o experimented w i t h wider bandgap m a t e r i a l s , which t h e y grew i n t h e i r l a b o r a t o r i e s by molecular beam e p i t a x y 171. They observed h i g h p o l a r - i z a t i o n from A10.3GaQ.7As, w i t h a bandgap of about 1.75 eV. While AlGaAs i s e a s i e r t o grow t h a n GaAsP, ~t s u f f e r s from h a v i n g a v e r y r e a c t i v e s u r f a c e . It i s d i f f i c u l t t o p r e p a r e good quantum e f f i c i e n c y photocathodes on t h i s m a t e r i a l i f it h a s e v e r been exposed t o atmosphere, a p p a r e n t l y due t o t h e aluminum o x i d e which r a p i d l y grows on t h e s u r f a c e . The J u l i c h group h a s s o l v e d t h i s problem by a p r o c e s s known a s a r s e n i c cap- p i n g . An a r s e n i c cap l a y e r i s grown on t h e AlGaAs s u r f a c e i n t h e growth chamber, and p r o t e c t s t h e s u r f a c e from o x i d a t i o n d u r i n g t r a n s f e r t h r o u g h t h e atmosphere. T h i s a r s e n - i c l a y e r i s r e a d i l y removed by h e a t i n g t h e sample a f t e r i n s t a l l a t i o n i n t h e p o l a r i z e d s o u r c e vacuum system. Use of t h i s p r o c e s s may make it p o s s i b l e , i n t h e f u t u r e , t o employ p o l a r i z e d p h o t o c a t h o d e s of AlGaAs. I t i s u s e f u l t o n o t e t h a t b o t h t h e Mainz and J u l i c h groups a c h i e v e good p o l a r i z a t i o n from t h e i r c a t h o d e s i n room t e m p e r a t u r e o p e r a t i o n .

The J t i l i c h group h a s a l s o r e p o r t e d t h e h i g h e s t p o l a r i z a t i o n s t o d a t e from GaAs, 49%, o r e s s e n t i a l l y t h e t h e o r e t i c a l l i m i t ^181. T h i s i s a t t r i b u t e d t o t h e r e l a t i v e l y t h i n l a y e r s of GaAs t h e y use. The p o i n t i s t h a t t h e e l e c t r o n s have t o come from a s m a l l d e p t h i n t h e c r y s t a l , and have n o t had a g r e a t d e a l of o p p o r t u n i t y t o d e p o l a r i z e . A t t h e workshop, Lampel p r e s e n t e d a s t r a i g h t f o r w a r d model f o r t h e e l e c t r o n p o l a r i z a t i o n i n t h e b u l k GaAs and of t h e e m i t t e d e l e c t r o n s . The model was checked by t h e a c t u a l measurement of b o t h of t h e s e p o l a r i z a t i o n s i n a v e r y l o v e l y experiment 191. The r e - s u l t s i n d i c a t e t h a t t h e s p i n r e l a x a t i o n c o n s t a n t i s a b o u t h a l f t h e b u l k recombination c o n s t a n t i n t h e GaAs. T h i s model, and t h e r e s u l t s o b t a i n e d a r e s u f f i c i e n t t o i n d i c a t e a h i g h e r p o l a r i z a t i o n from t h i n GaAs l a y e r s , and t o a l s o show i n d i r e c t l y t h a t t h e r e i s no d e p o l a r i z a t i o n of t h e e l e c t r o n s i n p a s s i n g through t h e cesium a c t i v a t i o n l a y e r . Improved P o l a r i z a t i o n From GaAs Type Sources

I n c r e a s e d p o l a r i z a t i o n from GaAs t y p e s o u r c e s i n v o l v e s removal of t h e degeneracy a t t h e t o p of t h e P312 v a l e n c e band. T h i s may be accomplished by s e v e r a l methods. The a p p l i c a t i o n of a u n i a x i a l s t r e s s t o t h e GaAs, f o r example, removes t h e degeneracy.

T h i s h a s been s t u d i e d t h e o r e t i c a l l y , and t h e i n c r e a s e d p o l a r i z a t i o n of t h e e l e c t r o n s i n t h e b u l k m a t e r i a l observed by measurement of t h e c i r c u l a r p o l a r i z a t i o n of t h e r e - combination r a d i a t i o n / l o / . However, t h e d e g e n e r a t e bands a r e s p l i t by only a few meV p e r k b a r of a p p l i e d s t r e s s . To o b t a i n s p l i t t i n g s l a r g e enough t o enhance t h e p o l a r i z a t i o n i n p r a c t i c a l s o u r c e s , v e r y l a r g e s t r e s s e s a r e r e q u i r e d , o f t e n r e s u l t i n g i n broken c r y s t a l s r a t h e r t h a n broken degeneracy. It i s a l s o d i f f i c u l t t o imagine a p o l a r i z e d s o u r c e which i n c o r p o r a t e s u n i a x i a l s t r e s s a l o n g w i t h a l l of t h e o t h e r r e q u i r e m e n t s i n a p r a c t i c a l arrangement. T h i s i d e a h a s y e t t o b e attempted i n a pol- a r i z e d s o u r c e .

A r t i f i c i a l semiconductor s t r u c t u r e s , such a s GaAs-GaAlAs m u l t i l a y e r s , which have a u n i a x i a l v a r i a t i o n i n t h e bandgap energy, can a l s o produce h i g h l y p o l a r i z e d e l e c t r o n s

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C2-676 JOURNAL DE PHYSIQUE

i n t h e conduction band, a s - h a s been demonstrated by luminescence measurements a t B e l l Labs /11/. However, most of t h e v a r i a t i o n . o f t h e bandgap energy o c c u r s i n t h e conduc- t i o n band, making t r a n s p o r t of t h e h i g h l y p o l a r i z e d e l e c t r o n s t o t h e s u r f a c e v e r y pro- b l e m a t i c . Experiments a t KFA f a i l e d t o show any enhancement from t h i s e f f e c t /12/.

A t SLAC, experiments on somewhat d i f f e r e n t m a t e r i a l showed a s m a l l p o l a r i z a t i o n enhancement a t t h e c o r r e c t wavelength, b u t no o n e h a s r e p o r t e d any l a r g e p o l a r i z a t i o n improvement from m u l t i l a y e r s t r u c t u r e s t o d a t e . I n p r i n c i p l e , i f one could d e s i g n a m u l t i l a y e r s t r u c t u r e i n which t h e bandgap energy v a r i a t i o n appeared i n t h e v a l e n c e band, r a t h e r t h a n t h e conduction band, h i g h l y p o l a r i z e d e l e c t r o n s should be produced and e a s i l y e x t r a c t e d . No one h a s p r e p a r e d such a s t r u c t u r e , however.

Perhaps t h e b e s t method f o r p r o v i d i n g i n c r e a s e d p o l a r i z a t i o n from o p t i c a l l y pumped non-magnetic s o l i d s i s t o employ a m a t e r i a l w i t h a band s t r u c t u r e i n which t h e undes-

i r a b l e degeneracy i s n o t p r e s e n t . I n p r i n c i p l e , band s t r u c t u r e s e x i s t which s h o u l d d e l i v e r 100% e l e c t r o n p o l a r i z a t i o n . Many members of t h e 11-IV-V2 f a m i l y of chalco- p y r i t e semiconductors have band s t r u c t u r e s which should d e l i v e r h i g h p o l a r i z a t i o n . While a l l members of t h i s f a m i l y should have a good v a l e n c e band s t r u c t u r e , i . e . no p o l a r i z a t i o n d e f e a t i n g d e g e n e r a c i e s a s i n GaAs, t h e r e a r e d i f f e r e n t s t r u c t u r e s f o r t h e conduction bands of t h e s e m a t e r i a l s , some of which a r e n o t s u i t e d f o r h i g h p o l a r - i z a t i o n . The b e s t c a n d i d a t e m a t e r i a l f o r a h i g h l y p o l a r i z e d s o u r c e from t h i s f a m i l y i s CdSiAs2. F i g u r e 5 compares a s i m p l e view of t h e band s t r u c t u r e of t h i s m a t e r i a l t o t h a t of GaAs, i n d i c a t i n g t h e removal of t h e v a l e n c e band degeneracy, and t h e t r a n s - i t i o n a t 1.74 eV which should g i v e h i g h p o l a r i z a t i o n . U n f o r t u n a t e l y , t h i s m a t e r i a l i s n o t r e a d i l y a v a i l a b l e . SLAC h a s s u p p o r t e d growth experiments f o r t h i s m a t e r i a l a t t h e Research T r i a n g l e I n s t i t u t e , and t h e f i r s t e p i t a x i a l l a y e r s of t h i s m a t e r i a l have r e c e n t l y been produced. The m a t e r i a l grows on a n InP s u b s t r a t e , i n e x a c t l y t h e c o r r e c t o r i e n t a t i o n t o produce h i g h p o l a r i z a t i o n . D e t a i l e d s t u d i e s of s u r f a c e c l e a n - i n g , c a t h o d e p r e p a r a t i o n , and p o l a r i z a t i o n measurement should b e g i n b e f o r e t o o l o n g . A t ETH ( Z u r i c h ) , o t h e r c h a l c o p y r i t e s have been measured a s c a n d i d a t e p o l a r i z e d e l e c t r o n e m i t t e r s / 1 3 / . They had samples of ZnSiAs2 and ZnGeAs . The f i r s t of t h e s e m a t e r i a l s h a s a conduction band s t r u c t u r e n o t w e l l s u i t e d t o h i g 2 p o l a r i z a t i o n , w h i l e t h e second m a t e r i a l h a s a band gap t o o s m a l l t o g i v e a NEA s u r f a c e , and t h u s t h e presumably h i g h l y p o l a r i z e d e l e c t r o n s cannot b e e m i t t e d . More r e c e n t l y , SLAC h a s o b t a i n e d some v e r y h i g h q u a l i t y samples of ZnSiAs2 and have a t t e m p t e d t o reproduce t h e ETH r e s u l t s . The ETH r e s u l t s a l o n g w i t h t h e SLAC r e s u l t s a r e p r e s e n t e d i n f i g u r e 6. The ETH samples were s m a l l and of u n c e r t a i n c r y s t a l l i n e q u a l i t y , w h i l e t h e SLAC c r y s t a l s were grown e p i t a x - i a l l y by MOCVD, and were of t h e c o r r e c t o r i e n t a t i o n t o produce h i g h p o l a r i z a t i o n . A s can b e observed i n f i g u r e 6, t h e s e c r y s t a l s produced lower p o l a r i z a t i o n t h a n t h e ETH samples. It i s worth n o t i n g t h a t t h e quantum e f f i c i e n c y from t h e SLAC samples was d r a m a t i c a l l y g r e a t e r t h a n t h a t of t h e ETH samples. T h i s may mean t h a t t h e e l e c t r o n s were a b l e t o d i f f u s e t o t h e s u r f a c e of t h e c r y s t a l from d e e p e r i n t h e m a t e r i a l , and t h u s had a g r e a t e r o p p o r t u n i t y t o d e p o l a r i z e . Continued experiments a t SLAC a r e p l a n - ned t o determine i f o p e r a t i n g w i t h a non-NEA s u r f a c e could produce h i g h e r p o l a r i z a t i o n from t h e s e samples.

Cubic Z incblende Chalcopyrite

Ga As CdSiAs

F i g . 5 - Comparison of t h e band s t r u c t u r e s of GaAs and CdSiAs2 i n t h e r e g i o n of t h e minimum d i r e c t bandgap. The v a l e n c e band degeneracy of GaAs 1s removed i n t h e c h a l - c o p y r i t e m a t e r i a l , and t h e h i g h p o l a r i z a t i o n t r a n s i t i o n i s i n d i c a t e d .

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-

i

-

1r2-r;

1 1 1 1 1 1 1 1 1 1 1 1

2.0 2.5 3.0

PHOTON ENERGY (eV) 4 9 5 7 ~ 4

F i g . 6 - Measured p o l a r i z a t i o n from ZnSiAs* a s a f u n c t i o n of photon energy. The pol- a r i z a t i o n from s e v e r a l c a t h o d e p r e p a r a t i o n s i s i n d i c a t e d . The quantum e f f i c i e n c i e s of t h e s e p r e p a r a t i o n s a r e i n d i c a t e d q u a l i t a t i v e l y a s "poor" and " b e t t e r " . The SLAC quantum e f f i c i e n c y was much h i g h e r t h a n t h a t r e p o r t e d by t h e ETH group. The l o c a t i o n of t h e t r a n s i t i o n which should produce h i g h p o l a r i z a t i o n i s i n d i c a t e d by t h e dashed l i n e . I t may a l s o b e t h a t f u r t h e r work w i t h ZnGeAs* could be s u c c e s s f u l i n producing h i g h e r p o l a r i z a t i o n . The ETH c a t h o d e s were made by a p p l y i n g cesium a l o n e t o t h e s u r f a c e . I t i s d i f f i c u l t t o produce NEA on t h i s m a t e r i a l a s i t s fundamental band gap i s q u i t e s m a l l , about 1.2 eV, and cesium a l o n e would n o t be e x p e c t e d t o produce NEA. However, t e c h n i - ques a r e known which can lower t h e work f u n c t i o n t o about 1.0 eV, through a c t i v a t i o n w i t h cesium and f l u o r i n e , r a t h e r t h a n cesium and oxygen 1141. P o l a r i z a t i o n measurements should be made on t h i s m a t e r i a l t r e a t e d w i t h cesium and f l u o r i n e .

I n any e v e n t , e m i t t e d e l e c t r o n p o l a r i z a t i o n s i n e x c e s s of 50% from o p t i c a l pumping i n non-magnetic s o l i d s h a s y e t t o be observed . ^Itdoes appear t h a t a l l t h e avenues f o r h i g h e r p o l a r i z a t i o n from s o u r c e s of t h i s t y p e have n o t been e x h a u s t e d , and s e v e r a l promising l i n e s of work a r e s t i l l underway. The d i f f i c u l t i e s a r e p r e s e n t , however, a s i n d i c a t e d by t h e o f f e r of P r o f . H. C. Siegmann of ETH. He h a s o f f e r e d a c a s e of champagne and p l a n e . t i c k e t s t o Zurich t o t h e f i r s t p e r s o n t o produce 70% p o l a r i z a t i o n by t h i s method, and t o come t o Zurich t o reproduce t h e r e s u l t s !

Techniques For P r e p a r a t i o n Of Photocathodes

I n a s e s s i o n which aroused g r e a t i n t e r e s t and provoked much d i s c u s s i o n , P r o f . W. S p i c e r of S t a n f o r d reviewed f o r s e v e r a l h o u r s t h e t e c h n i q u e s f o r photocathode p r e p a r a t i o n , and t h e e x t e n t of q u a n t i t a t i v e knowledge about a c t i v a t e d cathode s u r f a c e s . A s can be imagined, q u a n t i t a t i v e and s t r u c t u r a l knowledge about s u r f a c e s h a v i n g sub-monolayer q u a n t i t i e s of m a t e r i a l s i s d i f f i c u l t t o o b t a i n . Much of S p i c e r ' s work was done i n c o n j u n c t i o n w i t h V a r i a n , and a s such, was p r o p r i e t a r y , and t h u s was n o t d i s c u s s e d . However, p u b l i s h e d work by S p i c e r ' s group on a c t i v a t e d GaAs photocathodes was presen- t e d and d i s c u s s e d 11'51. Much of t h e i n f o r m a t i o n on t h e p r e p a r a t i o n of t h e s e s u r f a c e s i s s t i l l e m p i r i c a l . An e x c e l l e n t r e c e n t review of NEA photocathodes i s c o n t a i n e d i n t h e a r t i c l e of Escher 1161.

I t was s u g g e s t e d t h a t p r e p a r a t i o n of c a t h o d e s w i t h cesium and f l u o r i n e might g i v e b e t t e r r e s u l t s t h a n t h e more s t a n d a r d cesium and oxygen p r e p a r a t i o n . A t SLAC, we have

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C2-678 JOURNAL DE PHYSIQUE

s u b s e q u e n t l y p r e p a r e d a number of photocathode a c t i v a t i o n s on t h e same sample of GaAs, u s i n g both of t h e above t e c h n i q u e s . P r e p a r a t i o n w i t h f l u o r i n e c o n s i s t e n t l y gave a h i g h e r quantum e f f i c i e n c y t h a n p r e p a r a t i o n w i t h oxygen. I n a d d i t i o n , c a t h o d e s a c t i v a - t e d w i t h f l u o r i n e showed about one o r d e r of magnitude l e s s s e n s i t i v i t y t o e l e v a t e d p r e s s u r e s of CH4 and CO, compared t o t h e oxygen a c t i v a t i o n s . The view was e x p r e s s e d a t t h e workshop t h a t CH4 was a v e r y bad contaminant g a s f o r t h e s e photocathodes, a s it could be t h e s o u r c e of carbon c o n t a m i n a t i o n , which i s known t o be bad. The experiments t o d a t e a t SLAC do n o t b e a r t h i s o u t . Both oxygen and f l u o r i n e a c t i v a t e d c a t h o d e s showed t h e l e a s t s e n s i t i v i t y t o CH4, g r e a t e r s e n s i t i v i t y t o CO, and v e r y g r e a t s e n s i - t i v i t y t o C02. Work is underway t o reproduce t h e s e r e s u l t s on a d d i t i o n a l GaAs samples p r i o r t o p u b l i c a t i o n . Cathode a c t i v a t i o n w i t h f l u o r i n e r a t h e r t h a n oxygen may r e p r e s e n t a s i g n i f i c a n t improvement f o r p o l a r i z e d s o u r c e work, i f t h e s e r e s u l t s prove t o b e r e p r o - d u c i b l e .

Atomic Beam P o l a r i z e d Sources

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Three t y p e s of p o l a r i z e d s o u r c e s employing atomic beams were d i s c u s s e d . The f i r s t of t h e s e used a beam of o r i e n t e d ~i~ atoms which a r e p h o t o i o n i z e d by u n p o l a r i z e d l i g h t from an a r c lamp. T h i s s o u r c e h a s been employed a t SLAC f o r s e v e r a l experiments on t h e s c a t t e r i n g of p o l a r i z e d e l e c t r o n s from p o l a r i z e d p r o t o n s , and h a s been t h o r o u g h l y d e s c r i b e d i n t h e l i t e r a t u r e / 1 7 / . One development f o r t h i s s o u r c e would be t o pump t h e ~i~ atoms w i t h c i r c u l a r l y p o l a r i z e d resonance r a d i a t i o n , a g a i n followed by i o n i z a - t i o n by u n p o l a r i z e d a r c lamp l i g h t . I n p r i n c i p l e , t h i s c o u l d produce somewhat h i g h e r beam p o l a r i z a t i o n , a b o u t a f a c t o r of t e n g r e a t e r beam c u r r e n t , and t h e g r e a t advantage of o p t i c a l p o l a r i z a t i o n r e v e r s a l , r a t h e r t h a n t h e magnetic r e v e r s a l r e q u i r e d i n t h e p r e s e n t v e r s i o n of t h i s s o u r c e .

A second t y p e of atomic beam s o u r c e h a s been developed f o r a c c e l e r a t o r a p p l i c a - t i o n s by t h e Bonn U n i v e r s i t y group. It u t i l i z e s t h e p h o t o i o n i z a t i o n of a l k a l i m e t a l beams by c i r c u l a r p o l a r i z e d l i g h t of a wavelength which g i v e s h i g h e l e c t r o n p o l a r i z a - t i o n t h r o u g h t h e Fano E f f e c t . Two of t h e s e s o u r c e s have been developed a t Bonn, one u s i n g cesium, and one rubidium /18,19/. Sources of t h i s t y p e a r e c a p a b l e of t h e high- e s t beam p o l a r i z a t i o n s developed f o r a c c e l e r a t o r work.

The t h i r d s o u r c e d i s c u s s e d h a s never been used w i t h an a c c e l e r a t o r . I t i s based upon t h e c h e m i - i o n i z a t i o n of o p t i c a l l y pumped h e l i u m m e t a s t a b l e s /20/. T h i s s o u r c e i s a b l e t o d e l i v e r q u i t e a h i g h beam p o l a r i z a t i o n i f enough o p t i c a l pumping power i s provided, and should b e a b l e t o d e l i v e r c o n t i n u o u s beam c u r r e n t s of up t o 1 mA. Cur- r e n t s of t h i s s i z e a r e p o s s i b l e from t h e ~i~ s o u r c e and t h e Fano s o u r c e o n l y on a p u l - sed b a s i s , due t o t h e o p t i c a l s o u r c e s r e q u i r e d f o r t h e i o n i z a t i o n . An a d d i t i o n a l advantage of t h e helium c h e m i - i o n i z a t i o n s o u r c e i s r e l a t i v e s i m p l i c i t y . A l k a l i m e t a l beams and good vacuum systems a r e n o t r e q u i r e d . While 1 mA i s n o t a l a r g e c u r r e n t by contemporary s t a n d a r d s , t h e r e a r e p o l a r i z e d e l e c t r o n e x p e r i m e n t s under c o n s i d e r a t i o n which could p r o f i t from such a s o u r c e . _7. B a s i c a l l y , microwaves a r e used t o produce a beam of 2 S helium m e t a s t a b l e s , which T h i s s o u r c e i s s h S wn s c h e m a t i c a l l y i n f i g u r e a r e t h e n o p t i c a l l y pumped by a n e a r i n f r a r e d l a s e r . Chemi-ionization of t h e s e pumped m e t a s t a b l e s l i b e r a t e s t h e p o l a r i z e d e l e c t r o n s .

Mott Analyzer

I I I

He Gas r M~crowave

Source 4 9 5 7 * 5 ^{10 8 4}

Fig. 7 - The h e l i u r cherni-ioni7ation p o l a r i z e d e l e c t r o n s o u r c e .

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The o p e r a t i o n a l c h a r a c t e r i s t i c s of t h e s e t h r e e s o u r c e s a r e g i v e n b r i e f l y i n T a b l e 11, where t h e r e s u l t s a c t u a l l y achieved w i t h each of t h e s e s o u r c e s i s n o t e d , a l o n g w i t h t h e p a r a m e t e r s which t h e workshop concluded could be reached w i t h a maximal development of each of t h e s e s o u r c e t y p e s . I n g e n e r a l , t h e s e s o u r c e s d e l i v e r much h i g h e r beam p o l a r i z a t i o n and much lower beam c u r r e n t t h a n t h e GaAs s o u r c e s , and a r e i n genera more complex i n s t a l l a t i o n s t h a n t h e c u r r e n t g e n e r a t i o n of GaAs s o u r c e s . For t h o s e experiments where t h e beam c u r r e n t i s l i m i t e d by o t h e r c o n s i d e r a t i o n s , a s i n p o l a r i z e d t a r g e t experiments, t h e s e s o u r c e s may be v e r y u s e f u l .

T a b l e 11. C h a r a c t e r i s t i c s of Atomic Beam P o l a r i z e d E l e c t r o n Sources Source Type R e s u l t s Achieved P o s s i b l e w i t h Maximum

Development P h o t o i o n i z a i o n of i ²x 10' e l p u l s e 2 x lo1'' e l p u l s e

o r i e n t e d L i P " 85% P " 90%

180 pps 180 pps

magnetic r e v e r s a l o p t i c a l r e v e r s a l Fano E f f e c t 3 x l o 9 e l p u l s e 1.5 x 10" e l p u l s e

P s 93% (Cs); 1 pps P & 93% (Cs) P

-

67% (Rb); 50 pps 180 pps

o p t i c a l r e v e r s a l o p t i c a l r e v e r s a l

Chemi-ionization of P z 70-80%, 1 uA CW 1 mA CW,

helium m e t a s t a b l e s P

-

^40%,50 uA CW P G 90%

o p t i c a l r e v e r s a l o p t i c a l r e v e r s a l

Two f i n a l p o i n t s need t o be made r e g a r d i n g atomic beam s o u r c e s . We have n o t s o f a r mentioned t h e e m i t t a n c e of t h e s o u r c e because t h e GaAs s o u r c e s have b o t h e x c e l l e n t e m i t t a n c e and v e r y s m a l l energy s p r e a d , and have no d i f f i c u l t y i n meeting t h e a c c e p t - ance s p e c i f i c a t i o n s of a c c e l e r a t o r s . T h i s i s n o t n e c e s s a r i l y t r u e of atomic beam s o u r c e s . Those which have a magnetic f i e l d i n t h e r e g i o n from which t h e e l e c t r o n s o r i g i n a t e w i l l have an enlar.ged e m i t t a n c e . Those w i t h a s i g n i f i c a n t e l e c t r i c f i e l d o v e r t h e volume from which t h e e l e c t r o n s o r i g i n a t e w i l l have a s u b s t a n t i a l beam energy s p r e a d . These e f f e c t s can be very i m p o r t a n t . For example, a t SLAC, t h e l i n a c a c c e p t e d t h e same f r a c t i o n of t h e beam from t h e GaAs s o u r c e t h a t i s a c c e p t e d from t h e normal t h e r m i o n i c guns, w h i l e it d i d n o t a c c e p t t h i s f r a c t i o n from t h e ~i~ s o u r c e , which had both a l a r g e r e m i t t a n c e and a s u b s t a n t i a l beam energy s p r e a d . The helium a f t e r g l o w s o u r c e i s b e l i e v e d t o have an e m i t t a n c e about a s good a s t h a t of t h e G a A s s o u r c e . The second p o i n t concerns t h e importance of o p t i c a l r e v e r s a l of t h e s o u r c e p o l a r i z a t i o n . Even i n experiments which do n o t e x p l o r e s m a l l p a r i t y v i o l a t i n g asymmetries, such a s i n p o l a r i z e d e l e c t r o n - p o l a r i z e d p r o t o n s c a t t e r i n g , t h e e x p e r i m e n t a l asymmetries a r e g e n e r a l l y q u i t e s m a l l a f t e r two p o l a r i z a t i o n s , u s e f u l t a r g e t c o n t e n t , backgrounds, k i n e m a t i c d e p o l a r i z a t i o n s , and t h e r e a l p h y s i c s asymmetry a r e a l l c o n s i d e r e d . The

s y s t e m a t i c e f f e c t s a s s o c i a t e d w i t h a magnetic p o l a r i z a t i o n r e v e r s a l scheme, no m a t t e r how c a r e f u l l y c o n t r o l l e d , a r e a r e a l l i m i t a t i o n . O p t i c a l p o l a r i z a t i o n r e v e r s a l i s much t o be d e s i r e d .

P o l a r i z e d E l e c t r o n s from Magnetized M a t e r i a l s

P o l a r i z e d e l e c t r o n s may be e m i t t e d from magnetized m a t e r i a l s by e i t h e r f i e l d o r photo- e m i s s i o n . Examples i n c l u d e p o l a r i z e d e l e c t r o n photoemission from Fe, Co, N i , and EuO, and f i e l d e m i s s i o n from EuS coated t u n g s t e n n e e d l e s . To d a t e , f i e l d emission h a s r e q u i r e d o p e r a t i o n a t LHe t e m p e r a t u r e , and h a s d e l i v e r e d only v e r y s m a l l c u r r e n t s w i t h o u t d e s t r u c t i o n of t h e e m i t t e r . This t e c h n i q u e was n o t d i s c u s s e d . Photoemission from EuO h a s d e l i v e r e d modest pulsed c u r r e n t s of r e a s o n a b l e p o l a r i z a t i o n , b u t is no l o n g e r pursued a s a p o l a r i z e d s o u r c e . Photoemission from m e t a l s l i k e Fe, Co, and N i i s c h a r a c t e r i z e d by: ( 1 ) e m i t t i n g m a t e r i a l s which a r e d u r a b l e and s t a b l e ; ( 2 ) t h e need t o f l i p a magnetic f i e l d t o e f f e c t p o l a r i z a t i o n r e v e r s a l ; (3) a quantum y i e l d n e a r t h r e s h o l d , where p o l a r i z a t i o n i s t h e h i g h e s t , which i s q u i t e low ( c a . lo-' e / p h o t o n ) ; and ( 4 ) t h e need f o r q u i t e h i g h photon energy ( c a . 5 eV). The work f u n c t i o n