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REPORT ON THE WORKSHOP ON POLARISED TARGET MATERIALS AND TECHNIQUES BAD
HONNEF 1984 - PART II PROTONS
G. Court
To cite this version:
G. Court. REPORT ON THE WORKSHOP ON POLARISED TARGET MATERIALS AND TECH-
NIQUES BAD HONNEF 1984 - PART II PROTONS. Journal de Physique Colloques, 1985, 46 (C2),
pp.C2-695-C2-697. �10.1051/jphyscol:1985287�. �jpa-00224606�
JOURNAL DE PHYSIQUE
Colloque C2, suppl6rnent au n02, Tome 46, f k v r i e r 1985 page C2-695
REPORT ON THE WORKSHOP ON P O L A R I S E D TARGET M A T E R I A L S AND TECHNIQUES B A D HONNEF 1 9 8 4
-
PART I 1 PROTONSG.R. Court
Department of Physics, LiverpooZ University, P.O. Box 147, Liverpool, L69 3BX, U.K.
I n t h e i n t r o d u c t o r y t a l k on t h i s workshop i t was s t a t e d t h a t over t h e l a s t few years t h e r e has been a s t r o n g i n t e r e s t i n t h e development o f improved
m a t e r i a l s f o r p o l a r i s e d t a r g e t s . I n t h e case o f p r o t o n t a r g e t s t h i s i n t e r e s t has come m a i n l y from those working a t t h e h i g h energy end o f t h e s p i n physics
spectrum because t h e dynamically pol a r i s e d sol i d s t a t e t a r g e t s which a r e a t present u n i v e r s a l l y used, have two important 1 im i t a t i o n s , t h e i r re1 a t i v e l y 1 ow f r e e hydrogen content, and t h e d e t e r i o r a t i o n i n performance which occurs when t h e y are used i n h i g h i n t e n s i t y beams. A new m a t e r i a l has now been developed which has had a very s i g n i f i c a n t e f f e c t i n b o t h o f t h e s e artds. T h i s i s
dynamicall y pol a r i sed ammonia and a 1 arge p r o p o r t i o n o f t h e p r e s e n t a t i o n s made a t t h i s meeting were concerned i n some way w i t h i t s p r e p a r a t i o n , p r o p e r t i e s o r 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 , Results from t e s t s on some o t h e r new d y n a m i c a l l y p o l a r i s e d m a t e r i a l s were a l s o given, t o g e t h e r w i t h d a t a and reviews on o t h e r p o s s i b l e techniques i n c l u d i n g t h e use o f p o l a r i s e d atomic hydrogen i n t h e form o f a beam o r a s t a b i l i s e d gas.
The f i r s t advantage o f ammonia as a t a r g e t m a t e r i a l i s i t s increased hydrogen c o n t e n t compared w i t h t h e t e s t m a t e r i a l s a v a i l a b l e p r e v i o u s l y (17% as compared w i t h a maximum o f 14%). The second i s i t s increased r e s i s t a n c e t o beam r a d i a t i o n damage which i s t h e major cause of l o s s o f performance when h i g h i n t e n s i t y beams a r e used. The l a t t e r p r o p e r t y a r i s e s , a t l e a s t i n p a r t , because t h e paramagnetic centres which need t o be present i n t h e m a t e r i a l f o r t h e dynamic pol a r i s a t i o n process t o work a r e themsel ves i n t r o d u c e d by a r a d i a t i o n damage process r a t h e r t h a n by t h e chemical doping methods used p r e v i o u s l y .
D e t a i l e d experimental s t u d i e s on t h e dynamic p o l a r i s a t i o n process (Meyer) and t h e e f f e c t s o f r a d i a t i o n damage ( H a r t f i e l ) i n ammonia were presented a t t h e meeting. A t h e o r e t i c a l model was a l s o proposed, based on t h e a s s m p t i o n t h a t t h e
paramagnetic centres produced d u r i n g t h e p r e p a r a t i o n o f t h e m a t e r i a1 a r e d i f f e r e n t from some o f t h o s e generated d u r i n g o p e r a t i o n i n a beam, which s a t i s f a c t o r i l y e x p l a i n s t h e observed r a d i a t i o n damage and o t h e r p r o p e r t i e s ( N i i n i k o s k i ) .
Data from t h e Brookhaven-Michigan experiment a t t h e AGS (High PT p-p s c a t t e r i n g ) , where ammonia has been i n use on a r o u t i n e b a s i s f o r some time.
confirms t h a t t h e p r a c t i c a l r a d i a t i o n damage l i f e t i m e i s o f t h e order o f a f a c t o r o f t h r e e b e t t e r than t h e best previous m a t e r i a l s (Crabb). It a l s o demonstrates t h a t t h i s m a t e r i a l can be r a p i d l y annealed t o r e s t o r e t h e t a r g e t performance a f t e r r a d i a t i o n damage has occurred and t h a t it has a very l o n g l i f e t i m e , t h u s e l i m i n a t i n g t h e need f o r regul a r t a r g e t m a t e r i a l changes. This p a r t i c u l a r experiment uses a r e l a t i v e l y small s i z e o f t a r g e t i n an i n t e n s e beam and t h e heat generated i n t h e t a r g e t m a t e r i a l i s a l s o a s e r i o u s problem, r e d u c i n g t h e
p o l a r i s a t i o n from 70% t o 50% i n c e r t a i n circumstances. It was r e p o r t e d t h a t t h i s s i t u a t i o n can be improved by a f a c t o r o f t h r e e o r more, i n terms o f usable beam i n t e n s i t y , by u s i n g a m i x t u r e o f 3He and 4He i n s t e a d o f p u r e 3He i n t h e
e v a p o r a t i o n r e f r i g e r a t o r used t o cool t h e t a r g e t m a t e r i a l (Cameron). T h i s e f f e c t was confirmed by r e s u l t s from Bonn (Hewel) and CERN (Rieubl and).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985287
C2-696 JOURNAL DE PHYSIQUE
A completely d i f f e r e n t s e t o f problems need t o be solved f o r t h e t a r g e t used by t h e European Muon C o l l a b o r a t i o n a t t h e CERN SPS, (deep i n e l a s t i c p-p
s c a t t e r i n g ) which was r e p o r t e d on i n some d e t a i l (Court). I n t h i s case, t h e beam pot s i z e i s l a r g e and t h e beam i n t e n s i t y i s low, and so a very l a r g e amount o f t a r g e t m a t e r i a l i s r e q u i r e d (2 l i t r e s ) t o o b t a i n an adequate experimental event r a t e . Ammonia i s again being used but i n t h i s case i t i s t h e h i g h f r e e proton t o bound nucleon which i s t h e c r i t i c a l f a c t o r , as t h e experiment i s i n c l u s i v e so no k i n e m a t i c s e p a r a t i o n o f hydrogen and background events i s possible. This t a r g e t i s cooled w i t h a very 1 arge c a p a c i t y d i l u t i o n r e f r i g e r a t o r ( 2 w a t t ) as t h e h i g h e s t possi b l e pol a r i s a t i o n i s required. A maximum pol a r i s a t i o n o f over 80%
was reported, although, as t h e growth r a t e i s r a t h e r slow, t h i s t a k e s several days t o achieve. 70% p o l a r i s a t i o n can be reached i n about 12 hours, however.
This slow growth, which i s p a r t i a l l y a consequence o f t h e very 1 arge s i z e o f t h e t a r g e t and p a r t i a l l y a p r o p e r t y o f t h e m a t e r i a l i t s e l f , i s n o t a s e r i o u s problem i n t h i s p a r t i c u l a r case. T h i s i s so because t h e t a r g e t i s s p l i t i n t o two s e c t i o n s arranged i n tandem and pol a r i s e d i n o p p o s i t e d i r e c t i o n s . The primary purpose o f t h i s arrangement i s t o reduce t h e systematic u n c e r t a i n t i e s i n t h e measurement o f t h e experimental assymmetries b u t i t a l s o removes t h e n e c e s s i t y f o r r a p i d and f r e q u e n t r e v e r s a l s o f t h e pol a r i s a t i o n d i r e c t i o n .
It i s now c l e a r t h a t t h e development o f ammonia as a p r a c t i c a l t a r g e t m a t e r i a l has l e d t o a b i g improvement on t h e q u a l i t y o f d a t a o b t a i n a b l e from some c u r r e n t h i g h energy physics experiments. It has a l s o been shown t h a t t h e
m a t e r i a l can be produced i n l a r g e q u a n t i t i e s on a r o u t i n e b a s i s and t h a t t h e p r o d u c t i o n processes a r e w e l l understood (Brown). However, does i t s use r a i s e any problems? One has a1 ready been mentioned, t h e re1 a t i v e l y slow r a t e o f p o l a r i s a t i o n growth. Another p o s s i b l e source o f d i f f i c u l t y i s t h e p o l a r i s a t i o n o f t h e n i t r o g e n n u c l e i (14N) which have s p i n 1. Data was presented which confirms t h a t t h e n i t r o g e n n u c l e i a r e indeed p o l a r i s e d and a r e a l s o i n s p i n temperature e q u i l i b r i m w i t h t h e protons (Heyes). I n these circumstances, t h e n i t r o g e n p o l a r i s a t i o n i s c a l c u l a b l e when t h e p r o t o n p o l a r i s a t i o n i s known, t h e value being 11% a t 80% proton p o l a r i s a t i o n . The same s i t u a t i o n was observed when t h e 14N n u c l e i were rep1 aced by ( s p i n 4).
Some p o s s i b l e new m a t e r i a l s w i t h h i g h hydrogen c o n t e n t are under i n v e s t i g a t i o n a t a number o f l a b o r a t o r i e s . Those r e p o r t e d on were t h e borohydrides NH BH (19.5% hydrogen) and NH BH (24.5% hydrogen) t o g e t h e r w i t h 1 i t h i u n h y d r i d e (7LiH). The l a t t e r is4ma?nly o f i n t e r e s t f o r t h e
development o f 6 ~ i D. Pol a r i s a t i o n t e s t s , u s i n g c e n t r e s produced by i r r a d i a t i o n techniques, have been made on t h e two borohydrides, so f a r w i t h r a t h e r l i m i t e d success (Heyes). However, t h e r e s u l t s o b t a i n e d w i t h NH BH, suggest t h a t f u r t h e r development c o u l d be worthwhile. I n t h e case o f 7LiH, k i g h pol a r i s a t i o n s have been p r e v i o u s l y r e p o r t e d u s i n g v e r y small samples and w i t h a h i g h magnetic f i e l d (6.5 T). Devel opnent work i s now being c a r r i e d out w i t h sample s i z e s s u i t a b l e f o r use i n a t a r g e t and a t l o w e r f i e l d (2.5T). The very p r e l i m i n a r y r e s u l t s so f a r o b t a i n e d suggest t h a t t h e i r r a d i a t i o n technique i s very c r i t i c a l and much development i s s t i l l r e q u i r e d (Durand).
At present, chemical doping o f t h e pure borohydrides does n o t seem t o be possible. However, v e r y good r e s u l t s ("- 80% pol a r i s a t i o n ) have been o b t a i n e d w i t h c h e m i c a l l y doped m i x t u r e s o f borohydrides and o r g a n i c amines (Hi1 1
,
Krumpolc). These m i x t u r e s have a hydrogen content v e r y s i m i l a r t o ammonia, and t h e y c o u l d be b e t t e r m a t e r i a l s t o use i n c e r t a i n circumstances, p a r t i c u l a r l y if
f a s t r e v e r s a l o f t h e p o l a r i s a t i o n i s a c r i t i c a l experimental f a c t o r . It should be noted however, t h a t t h e boron (llB) n u c l e i a r e a1 so pol a r i s e d i n an analogous way t o t h e n i t r o g e n n u c l e i i n ammonia.
Many developments and improvements i n t h e techno1 ogy o f p o l a r i s e d t a r g e t s were covered a t t h i s meeting. Among t h o s e n o t a1 ready mentioned were a microprocessor based NMR system f o r p o l a r i s a t i o n measurement i n use a t CERN
( R i j l l a r t ) and improvements i n t h e use o f t h e f r o z e n s p i n techniques a t Saclay ( B a l l ) and LAMPF (Jarmer). The frozen s p i n technique has become p a r t i c u l a r l y
important i n t h e i n t e r m e d i a t e energy r e g i o n because i t a l l o w s a l l d i r e c t i o n s o f p o l a r i s a t i o n t o be o b t a i n e d i n combination w i t h good experimental access and a l o w magnetic f i e l d a t t h e t a r g e t . The present technology i s t o t a l l y dependent on t h e use o f h i g h power d i l u t i o n r e f r i g e r a t o r s t o o b t a i n t h e necessary very low temperatures and these have now become standard equipment i n many l a b o r a t o r i e s . The i n t e r e s t i n g suggestion was made t h a t i t might be p o s s i b l e t o produce a t a r g e t m a t e r i a l which c o u l d be used i n f r o z e n s p i n mode w i t h o u t t h e need f o r a very 1 ow temperature (Wenckebach). The dominant proton r e l a x a t i o n process i n normal m a t e r i a l s i s v i a t h e paramagnetic c e n t r e s used f o r t h e p o l a r i s a t i o n process. So i f t h e y can be removed a f t e r t h e system has been p o l a r i s e d , a h i g h temperature f r o z e n s p i n s t a t e can, i n p r i n c i p l e , be obtained. T h i s d e s i r a b l e s i t u a t i o n has been shown t o e x i s t f o r c e r t a i n t y p e s o f c e n t r e generated by U.V. r a d i a t i o n which decay r a p i d l y once t h e source o f U.V. has been switched o f f . The technique works, 12% p o l a r i s a t i o n having been obtained i n small samples o f a m a t e r i a l which c o n t a i n s 8.3% hydrogen, and i t appears t h a t f u r t h e r development i s possible.
A " b r u t e f o r c e " p o l a r i s e d t a r g e t was described which uses t i t a n i u m h y d r i d e as t h e t a r g e t m a t e r i a l (Herringa). The main advantage i s t h e h i g h f r e e p r o t o n d e n s i t y ("- x4 as compared w i t h ammonia). 60% p o l a r i s a t i o n has been obtained a t a temperature o f 1 2 mK i n a f i e l d o f 9.0T. This f i e l d and temperature c l e a r l y l i m i t t h e use o f t h i s technique t o r a t h e r special s i t u a t i o n s , i n t h i s case f a s t neutron s c a t t e r i n g experiments.
Discussions about f u t u r e developments i n t h e f i e 1 d, a t 1 e a s t as f a r ss h i g h energy physics i s concerned, were mai n l y concerned w i t h atomic hydrogen. Reviews were given on t h e c u r r e n t s i t u a t i o n w i t h p o l a r i s e d atomic beams (Gruebler) and appl i c a t i o n s o f s t a b i l i s e d atomic hydrogen (Ni i n i k o s k i ) t o g e t h e r w i t h t h e Brookhaven-Mi chigan groups' p l ans f o r an u l t r a c o l d pol a r i s e d j e t (Raymond).
Data from CERN was a1 so presented which c o n f i r m s t h a t nuclear pol a r i s a t i o n does occur i n s t a b i l i s e d atomic hydrogen ( P e n t t i l a ) .
There was a general consensus t h a t t h e most s i g n i f i c a n t f u t u r e advances i n s p i n physics a r e 1 i k e l y t o develop from t h i s research area because these
techniques w i l l f a c i l i t a t e t h e p r o d u c t i o n o f "clean" pol a r i sed t a r g e t s (and beams) w i t h very h i g h pol a r i s a t i o n . It must be p o i n t e d out, however, t h a t t h e p r e d i c t e d d e n s i t i e s o f 1013 cm-3 f o r atomic beams and up t o 1017 cm3 f o r t h e s t o r e d s t a b i l i s e d s i t u a t i o n a r e very low compared w i t h t h e f r e e hydrogen d e n s i t y i n conventional s o l i d s t a t e t a r g e t s (5 x
l o z 2
cm-3). Therefore v e r y major changes i n experimental phi1 osophy and techniques w i l l be necessary b e f o r e t h e i r p a r t i c u l a r p r o p e r t i e s can be f u l l y e x p l o i t e d .No r e f e r e n c e s have been given as t h e f u l l proceeding o f t h i s meeting w i l l be pub1 i s h e d by t h e Physi kal isches I n s t i t u t , U n i v e r s i t i i t Bonn.
