GENESIS OF THE FUNDAMENTAL TOOLS FOR POLARIZATION EXPERIMENTS

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GENESIS OF THE FUNDAMENTAL TOOLS FOR POLARIZATION EXPERIMENTS

J. Thirion

To cite this version:

J. Thirion. GENESIS OF THE FUNDAMENTAL TOOLS FOR POLARIZATION EXPERIMENTS.

Journal de Physique Colloques, 1990, 51 (C6), pp.C6-333-C6-346. �10.1051/jphyscol:1990626�. �jpa-

00230892�

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COLLOQUE DE PHYSIQUE

Colloque C6, supplement au n022, Tome 51, 1 5 novembre 1990

GENESIS OF THE FUNDAMENTAL TOOLS FOR POLARIZATION EXPERIMENTS

J. THIRION

ZNS, CEN Saclay, F-91191 Gif sur Yvette Cedex, France

Polarization phenomena i n nuclear physics have always bad a specific personality as illustrated by t b i s conference, tbe seventb o f tbem.

I t i s partly due to the need of new s p e c i f i c developments i n experimental equipments. Bow tbose new objects were designed and progressively made availa- ble for every day use i s tbe largest part of t b i s "historical" talk. I apologize t o a l l , physicists or engineers, I w i l l not be able t o expose their important contributions to the subject, but I w i l l l i m i t myself to the parts I have had a chance t o know directly.

1

-

P o l a r i z e d b e a m d e v e l o p m e n t s

la

-

Polarized ion sources. T y p e I

The need f o r suck s g e c i f i c s o u r c e s h a s ~ o t i s a t e d t h e work of :;any l a b o r a t o r i e s arou;;d i 3 5 S . I: was a t a meecing cf t h e German Phys:::cil S c c i e t y i n i i a r l s r u n a riiar I had t h e o p p o r t u n i t y t c meet Dr. Scliopper s n a Cr. C l a ~ i s n i t z e r who were a i r s 3 6 y engaged i n t h e f i e l d ; t h e y zon:iinced s.6 :ha:

p r o m i s i n g i n t e n s i r i f s of p o l a z i z a d p r 2 t o n s o r ? e u t e r o i i s would be o b t a i n e d i n t h e f u t u r e by d e s x g n i n g a s o r t of S i e r n and G e r l a c h d e v i c e . There :~.s:e a i r e a d y r e s u i t s o b t a i n e d by Becker f c i p r a c t i c a l s u p e r s o n i c g a s je:s (pcr;;aas n o t e x a c t l y "Lav?;" flow: $23 a l s o f o r hydrogen d i s s o c i a t i o n ( K z l l e r :,L C e r n , C l a u s n i t z a r a t E r l a n g = c ) ; t h e s i p a r a ~ i o n cf atoms a c c c r d i n g t o t h e i r e l e c t c o n s s i n by s e x t u p o l e x a g c s c i c f i e l d s was c l e a r a n d , whar was n o t s o r r i v i a l ' , ch,2 auinpinp d e s i s n .

~ h ~ - ~ - o r e t i c a l ^C^a= ,,L,c..., c - : . . - . i s sho?in i n t h e B a b i d i a g r a e : ( f i g . l ) which e v s r y b a d y kco+is a s it t a s b e s n p r o j e c i e d aii tnormcas n u a b e r of t i r e s f o r e a c n

-,i ;, cn t h e s u b j e c : ;f p o l a r i z a d s o u r c e s .

Roots pu::ps wsre a v a i l a b l e b u t o i l d i f f u s i o r i ~ u : ? p s were d i f f i c u l t t o r a i n - : a i n b e c l u s e of r h e d e s t r u c t i - e a c c i c n of a t o x i c hydrogen ; Z e r c u r y pu::igs c c u l d is u s e d , 5 ~ f o r e t h e a r r i - a i of s i i i x n o i l s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990626

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,# Z=~il..aii"

,.

^..^~, hjrperfine s t r : i c t u r i o f a t o ~ : i c h y d r o g e i ~ . I t i r ~ c l l ! d e s th e ?S 2nd ,'F s t a t e s v i t h t h e "Lamb s h i f t " v f 1947. I f or?e ilses t h e IS part o f t h e $iagi-.i,:

t h e s t l t e s l i and 2: remain a f t e r t h e s e x t d p o l e , b g t Botli proton orieo:atic:.os .ire p r e s e n t : t r a n s i t i o n s ~ u l l c o r r e c t t h i s .

The a i s s i n g i t e z u a s t h e RF t r a n s i t i o n s and t h e i o n i s , r .

XF t r a n s i t i o s s (is iC0E.S: : T h e r e were a t t h e beginning p r o g c s a i s ( f o r e x i n - p l e I i e l l e r e t a l . , CERii+j xo u s e f o r t h e s e p a r a t i o l l of s t a t e s 1:

and 21 t h e s m a i l m a g n e t i c i i e l d p a r t of t h e Xabi d i a g r l n , t h a t is * ' L ~ Ccliff*-- r e n c s s i n m a g n e t i c n c l e n t s . Bat i t i s c c t c o n ~ ? c n i e n z and one loos-2s i c t h e 3er;si:y of ? h e a t o m i c b e a n . The f i r s t t y p s of i d i a b a t i c : r a n s i t i o i i kiss p:sposed by Abragam and Wii:ter2' a s soon a s t h e y h e a r d aijout t h i s t y p e of p c l a r i z e d s o u r c e s . I t s me can is:^ i s e a s i l y u n d e r s t o c 3 by a c l a s s i c a l > r z c e s s i o n o f t h e c a g n e t i c c o n e n t i n a a a g n e t i c f i e l d (re:r.i.riber t h e u s t of t h e r o t a t i a ~ r e f e r e s c e frarae.. . j b u t e x a c t r e q u i r e m e n t s net:< a r-'-: = ~ ~ n e d d e s c r i g t i o n . 2 3 c r d e r t o t e s t i t we x a d t a con~::lete S t a r n G e r l a c h e x p e r i a e n t ( f i g . 2;" !::-L

- . a n a l y s i n g p a r t b e i n g t 5 e si:r.ple--t in>c:"ogenous :ras:ie:ic f i e i d r:: :oul-; b u i l ? r a g i d l y .

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TJi6 ar;e:ys;.r. a f t e r t h e s o k r c ? i s a ;;agfi=:ic f i , - i d gradiarlt proyidz.d by cbc pol? g i s c e s A c r i z o n t a J y i n c l i n z d by L' s!nal2 i n g l e oil t h e irooi? Seam ;:<is

. .

The f i r s t r e s u l t s we:? s e e n ~ y 7 ~ ; s u a i i s i n g t h e impact of t h e atc:cic bea:c c;n a Pklybdene oxyda s c r e e n (?!o ^{0 3}+ Ii -+ 304 C ; g (0H;2), v h i c h shsws b a a u t i f u l . . o i u e s p o t s a i t h i n a s h o r t t i z e ( r e t k o d used a: E r l a n g e n and CERN) ; a s soon a s we s b s e r a e d :ha t u c spc:s ( f i g . 3 ) we phcned t o Abragam t h a t t h e t r a n s i t i s n ( a t c x i c h y d r c g f n j worksd w s l l ; h i s cornmen: was t h a t obi7iously t h e t h = o r y was e x a c t . B u t , t o u s , i c was n o t s o e a s y a s we were n o t e x p e r t s of n i c r o w a v s t e c h n i q u e s i2853 MC,'sec a t :he magnetiz f i a l d of 884 Gauss) an2 a l s o b e c a u s e i h e a d i a b a t i c i t y : o n d i t i o n s were a c r c o m p l z t e l y s a t i s f i e d a t t h a t t i n e , h e t o :he l i i r i t c d power and poor Q c a ~ i t y v a l u e ; t h i n g s were r a p i d l y imprsved and a l m o s t f u l l e f f i c i e n c y was obtaineci by I960 ( B a s 5 l C o n f e r e n c e ) . But we were in*-.,..= C S - S - t e d a l s o i n t h e d e u t s r i u m c a s e and t h a r e t h e a d i a b a t i c i t y require:?-.'- ... ^=I,^L i s more d i f f i c u l t ; f u i i e f f i c i e n c y was o b t a i n s d o n l y l a t e r by a s i n g a 350 r s p e c i a l r a g n e t r o n c a l l s d r a r c i n o t r o n (CSF C 4 ) . But t h i s can be a v o i d e d f o r deuterini:. a: lower f i e l d s and f r e q u e n c i e s .

F i g . 3

two s p o t s 011 !~olySdenu;.: oxyde duo t o t h e h i g h frequcacq- t r i n s i t i o i ~ ; ti52:~f a s t b 6 c ! ; th,ir siispe d i f f e r because tllc ::ragkktic g r a d i e n t o f t h r ei:alyser. is / o c u s l n g for, one s t a t e :+l/'; f o r t h a e l e c t r o r ; s p i n o r l e a t a -

t i o n j and d i ? i ~ i ^ k s i i j g f o r t h e otlier

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The circumstances of adiabatic transitions of the second kind (in low aagnetic field) are interesting : I had read some day in 1962 a paper on

z h e flipping of polarized slow neutrons by rapidly changing the sign 2f rhe

sagnetic field, this in fact is similar to "Sona" transitions (1967) quoted (1968) by Clegg'). The 3pp:ication of that :c hydrogen or deuttriun atoms seemed t u me so exci:ing that I we;;: to see Abragai: who iminediately answered : Adiabatic c.ultiphoton transitions" are exeli known and very easy ; how can ue ha:;e missed that at t h e beginning ! The adiabatic rzquirergents are extreaely

"cheap" : ~agnetic field of :Le order of 8 G an2 an ozcillator 2f 7.5 113~ at a few watts power. Especially in the deuterium case the existence of suc!i low field transitions were impcrtant as, combined wi:h the first type, one cin efficiently producs beams in all various states of vector and tensor polarization needed (3 transitions).

But I like also to !n.ention that combinations of the two types enable to change at will within a nicrosecond and progranmed by a clock6' the sign of the polarization with identical conditions of beam. The experimental errors are then pcreiy sta:isrical, which is not at ail guaranteed otherwise.

Ionization is done in a "high field" ionizer of the type aade first by Gla-~lsh' in New Zeeland.

Later developments (Argonne) included the zooling to 70°K atoms before t h a injection inro :he seztupole ; This is important because the accep- tance of ;he sextupole car] be increased and also the ionization becomes more efficient.

lb - Polarized ion sources. m e I1 : negative ions

It is called now "Laab shift" type sourcs but in 1957 Zavoiskiia) proposed such a source, refering to the hyperfine structure of atomic hydrogen af:er the Laixb shift discovery of 1947.

More precisely, the sources use an incoming bfam of excited (metastable 2S state) atoms. If the atoms are in a aagnetic field of 574 Gauss (hydrogen) the % J =

-

1/2 2 5 1 , ~ and mi = + 1/2 2Pii2 hyperfine states mix (mr concerns the electron spin) and -with the aid of some extra alectric field- transitions to the unstable P state occur wnich decay to the 1s state. The

3, = +1/2 2S1. 2 states reinain unaffected. Various ways to select one proton spin state are then operated. One (MC Kibben ?968)9' uses HF transitions between one of the remaining 2S1.2 in^ = + 1/2 states with the sane mJ the other m, = 1 / 2 2SA. One advantage is the possible choice of various HF :ransitions to proyride various polarization states (hydrogen and deuteriun).

All that would have remained an intellectual game without the selective resonant groZuction of ne~ative ions from 2.5 atoms which I heard Donally (Donally and Sawyer) propose in 1964 at a conference in Germany (by electron exchange with soxe gas (CS, Rr for exanple).

* EJ refers to the elecrron spin J and mr to the nucleon spin . The effect of the RF field, in the presence of the aagnetic field and of a DC electric field is to preserve thz initial state to decay by coupling to a P state.

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Such sources have Seen in use at tandse accelerators (nega:ive ions acceleration). The >Ic Kibbea source illustrated hers (fig. 4 ) .

'To spin- 'nrecessor

Ion deilectson ~Nddubtrn

Sol. 6-0 m

Polarized H- 8 D- Source

F i g . 4

The McKibben source. For the details of the spin f i l t e r sea 9

2

-

C o u p l i n g of t h e p o l a r i z e d s o u r c e t o a n a c c e l e r a t o r

Contrary to "outside" predictions, the first acczlerat~d poiarized Seam was obtai~ied at the old Saclay cyclotron in 1962. It concarned 22 ?leV deuterons because poiarized stomc were sent to the canter of the aachine where ionization in flight was done by a "xagnetron" type ionizer and residuai -dacuum contani3ation was a~oidzd. Final intensity on e x t e r ~ a l targzts was a few ?08/sec and experiments used mu1tidetec:ors which in so:ne casts could be rotated around the bea!?. axis for separating tensor polarization effects.

The first experimen; is illustrazed in fig. 5, 6, 71°'. The preci- sion obtained iz in contrast with early "double scattering" results which often differed considerably from one laboratory to another. First tensor effects wers also neasured and soon after explained by Raynalio) using a central plus s2in orbit potential.

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Fig. 5

Targzt chamber. I t i s can be rotated around the beam a x i s f o r obserrring the dependence o f the Tarious tznsor terms

Fig. 7 F i r s t sxperiment :

Fig. G E l a s t i c s c a t t e r i n g o f vector polarized deuterons by 40Ca. The curve i s Detector assembly showing the 8 d e t e c t o r s ; eye guide

each one i s composed o f one dE/d>: and one E d e t s c t o r . A c y l i n d r i c a l absorber o f variable

thickness i s inserted between target and d e t e c t o r s , so that the deuterons end t h e i r

range i n t h e E d e t e c t o r s

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Beautiful results wre obtained at Birmingham, Erlangen and also at Hadison. In 1967 :he Visconsin group had aade a large colleztion of dp r?sults illustrating many j dspendences. A little later there were also results on pd reactions from the new Saclay cyclotron. At the 1970 Madison Symposium, Haeberli sade an impressive report. The theory made aiso considerable progress in the same years with Goidfarb, Johnson, Raynal, Satchler, Robson, and others. DWBA programs gave aany successful interpretations. The 3 state effects were cvaliiated (Johnson, Robson, Santos). Tensor polarizations were icciuda6 (elastic deutzron scattering, qBe, pd...).

For golarized protons we had to wait for our variable energy cyclotron. There the ionization was made externally in a well controled vacuum box ; but, having no axial hole for injection we had to design a special

"trochr;idalW'l injection (rig. 5). The ions were brought to the center in crossed electric and magnetic fields, a device which >.?v2 nice proper:ies noted for :>ass spectrographs, except tha: our system had his :?oments of independence and alsc tarred to be oaposed to improvemen?s.

Fig. S

Trocfioi'dal ~ n j e c t i o i ? . Tfie various eli?ctrodes needed t o

"matcl!" the 1;:sgne:ic f i e l d itake d p r e t t y pictttre

Nowaday injec:ion into cyclotrocs are ?ad? by standard nethods like at Berkeley or Grenoble (axial hole in the cbnter of t'rie nagnet yr;ke and sonetice with bunching included).

A large numbzr of inelastic proton scattoring we:e rapidly available 2nd detailed interpretations were :nade (Satchler, Raynal and others). I like to recognize the prozinen: role of d . Raynal in this regard and slso in

ail +L.... "gciarized" progra:ns at Saclay since the beginning. In :he pp' doaain,

his coupled equations codes which included the full Thanas tsrr. (Schiifer and alair;"'! are world-wide appraciated. k o n g :;any results on cacroscopic and z;croscopic analyses by Raynal T show arbitr~rkly (fig. 9 1 : " ) the case of

' 4 "

sc

^'^{. i 2}S-, '?he agreerne:lt betw:-.>, data and :heory S ~ O H S :h2 deeailed uu-

8::rscandincj :hat has seen reached.

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Fig. 9

E l a s t i c and i n e l a s t i c scat t e r i n y o f 2 4 , 5 l!eV polarized protons.

The agreements are i!.?pressive

3

-

A c c e l e r a t i o n p r o b l e m s

In 1960 Froissart and Stora14) studied theoretically the various depolarization possibilities in circular acceierators and eventual corrections :c avoid them. This is ussd for high energy machines : a "recent" exaxple wiil be gi7en later. What happens is very sixilar to thz classical description of adiabatic transitions. BistoricaIly the first predictions for cyciotrons {AVF typ2) were nade by Teng at Gatlinburg and were somewhat alarming. Of course

the +L.. b,,Goretical part was exact but after having put in reaiistic nimbers i i r i

particalar for the oscillation amplitudes of our new strong focusing cyclo- troc) I felt reassured. In fact there were no depolarization effects in the new cyclotrons.

Thz case of synchrotron is ?ore serious and, f c r the proton acceleration in strong focusing ::achit;=s, rspuires careful study for each

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c a s e . Around 52-83 a c c e l e r a t i o n was s u c c e s s f u l l y o p e r a t e d a t ZGS, AGS f o r ? a r t of t h e e n e r g y r a n g e and a t S a t u r i i e f o r a l l e n e r g i e s . I o n l y want t a show one example ( f i g . 1 0 ) o i s t r o n g r e s o n a n c e : r a j e r s a l where a d i a b a t i c r e v e r s a l i s

- < .

-

a u a ~ e ~ . The s y n c h r o t r o n o s c i l ? a t i o n s i J 1 i n t h e beam c o m p l i c a t e t h e c r o s s i c g and t h e f i g u r e shows t h e c o n s e q u e c c e s . T h i s was z new e f f e c t noc i n c l u d e d i n F r o s s a r d - S t o r a a n a l y s i s . For d s u t e r o n s t h e r e a r e no r e s o n a n c e s w i t h t h e p a r a i t e t e r s u s e d i n t h e d e s i g n of S a t u r n e 2 .

A

^(POLBRIS._SANS

A

Fig. l:!

Crossing o f the

'bS

= 2 reso!iance a t S a t k r o u 2. In t h i s casc "syncA5rotrotl"

osci.Zlations sake the adiabatic r?!rsrsal d i f f i c i i l t and exact f i e l d corrections haye ti: be used

1 6 ' - - - C

,, ^.;I;-. p o i n t I l i k e t s ~ i ' . , ' V S t h e c h a r a c t e r i s t i c s of t h e p c l a r i z e c i beat:! a: S a t u r e n 2 , i r i . : c t o r '2 !:isas i n c l u d e d . S c u r c ~ : 5 0 0 p A ( p u l s e d ) . Z x t e r n a l : > e a r : 2 1 O L : , ' p ~ : l s e . P o l a r i z a t i o n 80% up t o nraxixu:: e n e r g y .

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4

-

Polarized t a r g e t s

T h e firs: proton polarized target (Abragam and Jeffries)'6) (fig.

li) w a s made by Abragam's group and ussd at the old Saclay cyclotron for a C Z R expeiinent in 1 9 6 2 . There the polarized proton beam w a s obtained by the rscoil protons from a hydrogen carget boxbarded with Helium of 44 Key.

Fig. 11

F i r s t polarized target and C X N experiment. The target i s a nono c r y s t a l Zx2x0,l ^mm3 o f La2 Mg3 ,(?I 0 3 ) l n 24 H 2 0 v i t h , 1% Nd (replacing L a ) properly oriented i n a magnetic f i e l d o f 18,5 KG. The c a v i t y f o r m i l l i m e t r i c waaes gives an o s c i l l a t i n g f i e l d p e r p e n d i c ~ l a r t o the nairi f i e l d . I t i s cooled a t 1,5OK. Th? t r a n s i t i o n s induce s i t h e r " f l i p f l i p " cr " f l i p flop" f o r the s p i n s o f electron-proton pairs according t o the frequency choosen. The target l a t t i c e has t o provide the slow relaxation time for electron and the long relaxation time for protons. Also spin d i f f u s i o n f o r protons i s important.

There are about 700 t i n e s more protons ( i n H z O ) than " f r e e " electrons {magnetic center : ~Vddl. E:<perinental resul t s l 7 ' for AS.S and Ays correspond t o

"horizontal " and " v e r t i c a l " counters (4S0 laboratory angle). Note that the i n t e r p r e t a t i o n i n F phase s h i f t val ues i s transparent :

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Already in 1 9 5 2 the beai: golsrizacion was very precisely knoun (from p oL scattering]. The target polarization was known fros nuclear magnetic resonance :aeasurements. At the energies used, only S wave scaitering is ixportant, so &a: C A $ * - L. This firs: expeiinent showed in fact ths agree- men? between the NMR xeasurernent (19%). The sign of the target polarization was obtained by chan~inj thz RF frequency and the valze of target polarization obtained irom nuclear scattering. Later golarizations were about 70%. The aiz was to see the deviations froa -1 for C%& as energy increases due to the contribu:ion of Pwaves, as suggested by iioyes who was interested in pp scattering. That was done later at the new Saclay cyclotron with accelerated polarized proton bean up to 26 MeV. Although a first experiment it was a very precise one. The sign of the target polarization was done by changing the RF frequency. Altholigh small the target was a real piece of jewelry and the systems of cooling tc 1.5OK (Roubeaudj (fig. 12) are of the inost elaborated c:yogenics. One difficulty in :he measurement was to take into account the dspolarization due to the bea::.

\ l SERPEHRI DE POIIPZIE DU

SEPIRATWR ... . . . . _ . 2 9

--

POMPAGE PRNCIPAI

CRYOSTAT HORIZONTAL A TRANSFERT CONTINU D'HELIUM

The cooliilg syste~;: : Roubeaud invented t h i s extremely elaborated apparatus which worked p e r f e c t l y

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5

-

P o l a r i z e d = H e t a r g e t

A target xade at H o i ~ s t o n * ~ ~ j arrived at Saclzy with S. BakerLa) and was used to measure C S & P p A r s A y y at Ep = 19.5 He?. The target (fig. 13) is 3Ae gas ( N 2 torr pressure) 'in a pyrex bulb 5 cm in diameter. The polarization is obtained 5 y optical pumping, a method due to A. Kastler in 135:.

Very roughly : ", metastable states are produced ; a circularly polarized light (bright 4He lamp) induce, m =

+

1 transitions to 2 3 P states which decay to "S, sublevels (with m = l) ; the net effect is to pump the states with negative mF out of the 3 levels, leading to polarization. The ground state polarization is obtai~id by transfer froai the metastables and reaches 20%.

nolium L i n r o r kR wove lamp p o l o r ~ z e r plole

Fig. 2 3

Polarized "He t a r g e t : o p t i c a l puz?ing n o t s h ~ s i n : H s l a o l t z c o i l s to provide a few Gauss magnetic f i e l d . The pun~piag l i g h t i s 10.8 A O vaIre l e n g t h . The

Aid p l a t e has a x i s orie.rited a t 3 5 O t o t h e axis o f t h e l i n e a r p o l a r i z e r

Calibrated polarineters are important and I lika to rene?b+r so22 of them for protons.

1) 4He. Since 1955 as deduced from phase shift analysis by G a m e 1 and Thzler and :zany experixents, for esa?:.p:e by. Brockinznn. q ,

2j 1 2 C for 15,7 MeY protons as defined by A. Craigzol et al. at Harwell in 1964. Saclay used this polari:>eter to measure the proton polarizations.

* First experiments including spin correlation were mads in 1963 (Rice - liisconsln coliaborationj

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3) P ~ r n i e ~ ~ ' for protons of energy> 100 XeV ; made and calibrated at Saclay.

In fact the beam polarization values are used for intermediate energies. Thoss are abtaingd in the proton case by pp scattering on a polarized target of known polarization. For deuterons it is the polarization vaiues 3.easur@d at small energies which are used as there is no depolarization by acceleration.

Finally one calibrated tensor polarimeter (deuterons). It is AHEAD as deflned by the Alberta team and calibratzd end of 1986 at Saclay, using elastic dp scattering (deuteron energies 100 - 200 M e V ) (fig. 14).

-.3 -.4

-.5 I I I , . , , I

0 20 40 60 80 100 120 1<0 i6G i 8 C

522 A\LfLE (CM)

0 5 - 1 I I , , ' , , , ,

0.4 -

0.3 - ^I

02 -

1

0.1 ^- i

0 0 , a . . .

I

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(15)

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