two-nucleon emission experiments at mainz

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Hadron and Nuclear Physics with Electromagnetic ProbesK. Maruyamaand H. Okuno (Editors)2000 Elsevier Science B.V. All rights reserved. 1 8 3

Two-nucleon emission experiments at MainzP e t e r G r a b m a y r aa P h y s ik a li s c h e s I n s t i t u t d e r U n iv e r s i t s T i i b in g e n ,A u f d e r M o r g e n s t e l l e 1 4, D -7 2 0 76 T i ib in g e n , G e r m a n ye-mail address: grabmayr@ pit .physik .uni- tuebingen.de

T w o - n u c l e o n e m i s s io n e x p e r im e n t s p e r f o r m e d w i th r e a l a n d v i r t u a l p h o to n s a t t h ec w - e l e c t r o n a c c e l e r a to r M A M I a t M a in z ar e d i sc u s se d . W e ha v e s t u d i e d t h e r e a c t i o nme cha n ism s o f pho ton abso rp t io n . Ev iden ce fo r e f fec ts due to nuc leon -nuc leon co r re la t ionsa t sho r t in te rnuc leo n d is tan ces i s p resen ted . Th e need of a com bined ana lys i s o f h ighr e s ol u t io n ( y , N N ) a n d ( e , e ' N N ) e x p e r i m e n t s is e m p h a s is e d .1. I N T R O D U C T I O N

T h e m e a n f ie ld c o n c e p t t o d e s c r ib e p r o p e r t i e s o f n u c l e a r s t a t e s h a s b e e n r a th e r s u c ce s sf u lin the energy r ange a round the Fermi su r face wh ich r ep resen ts the c lass ica l low-energy ,l o w - m o m e n tu m r e g im e . O n ly m o r e r e c e n t o n e - p a r t ic l e t r a n s f e r a n d k n o c k o u t e x p e r i-m e n t s [ 1, 2] r e v e a l e d t h a t a b o u t 2 5% o f t h e s t r e n g th w a s m i s sin g. I m m e d ia t e ly t h i s w a st r a c e d b a c k t o t h e p r i n c ip a l w o r ri e s s t a t e d a l r e a d y a t t h e t im e o f b i r t h o f t h e s h e l l m o d e lm o r e t h a n 5 0 y e a r s a go , na m e l y, t h a t t h e h i gh m o m e n t u m c o m p o n e n t s a p p a r e n t a t s h o r ti n t e r n u c l e o n d i s t a n c e s s p o i l t h e m e a n - f i e ld c o n c e p t a n d t h a t t h e y s h i f t s t r e n g th h ig h u pin to t h e c o n t i n u u m w h e r e t h e t y p i c a l s p e c t r o s c o p y c a n n o t p u r s u e d a n y m o r e . T h e s t r o n grepu ls ion observed a t smal l r e la t ive d i s tances i s wel l known in f r ee nuc leon -nuc leon (NN)sca t te r ing ; to a ce r ta in deg ree th i s sho r t - r ange f ea tu re su rv ives in a tomic nuc le i desp i teth e c r e a t i o n o f a n a v e r a g e p o t e n t i a l c o n t r i b u t e d b y a ll n u c le o n s a n d t h e s u p p o r t i v e P a u l ip r inc ip le .

Thus one observes dev ia t ions f rom the mean f ie ld behav iou r wh ich a re the so -ca l lednuc leon -nuc leon co r re la t ions . Th ere a re severa l k inds o f t r iv ia l co r re la t ions as e.g. due toPau l i p r inc ip le o r conse rva t ion o f cen t r e -o f -m om en tu m which a re inc luded in the " fr ee"tw o - n u c l e o n d e n s i t y P N N :

~ [ p N ( r l ) " p n ( r 2 ) - p n ( r l [ r 2 ) . p N ( r 2 [ r l ) J (1 ). . . . .f r e e e . g . P a u l i - c o r r e l a t i o n s

T h e d y n a m i c a l c o r r e la t i on s Cdyn are in the ce n t r e o f in te res t a nd in par t icu la r tho se wh ichc a n b e c o n n e c t e d w i th s h o r t i n t e r - n u c l e o n d i s t a n c e s :C d y n ( r ] , r 2 ) = P N N - - P ~_N N ( 2 )

d y n a m ic g e n u in e f re e


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The long ranged cor re l a t ions have been incorpora t ed in to spec i f i c mean f i e ld approaches(see. e .g . r ef . [3 ] ) . However , t he t r ea tm en t o f t he shor t r an ged co r re l a t ions is more in -t r i ca t e . One of the s t rong es t a rg um ent fo r shor t r ange d ef fec ts i n the nuc lea r med iumst il l ar i se s f rom theore t i ca l i nves t iga t ions o f nuc lea r b ind ing . A recen t comp i l a t ion byH. M i i the r [4] dem ons t ra t e s the need o f NN cor re l a t ions fo r b ind ing o f nuc lea r m a t t e r a tthe empi r i ca l sa tu ra t ion po in t us ing rea l i s t i c NN po ten t i a l s . S imple Har t ree -Fock ca l cu la -t ion y i e ld b ind ing on ly wi th e f fec tive po ten t i a l s . W hi l e fo r d i f fe ren t mod ern NN po ten t i a l sa b o u t t h e s a m e b i n d i n g e n e r g y p e r n u c l e o n i s e x t r a c t e d f r o m B r u e c k n e r - Ha r t e e - Fo c k c a l -cu la t ions d i f feren t con t r ibu t ions o f t he 7r exchange have been found . Th i s c l ea r ly ind ica t e sd i f fe ren t o f f- she ll behav io ur o f t hese p o ten t i a l s which f it t he ph ase sh i ft s fo r NN sca t t e r ingwi th equ a l qua li ty . Th e t ensor p a r t o f t hese po ten t i a l s i s poor ly de f ined , however tensortype cor re l a t ions p l ay a c ruc ia l pa r t i n the ca l cu la t ions fo r t he n p channel [4] .

F igure 1 . The two-nuc leon dens i ty fo r two l p l / 2 nucleons (adapted f rom ref . [4]) .Recen t i nves t iga t ions a im towards modern t echn iques to p roduce two-ho le spec t ra l func -

t ions for rea l nucle i [4 ,5 ,9] . Here , a resul t based on the exp(S)-method for 160 i s given [4] .Th e cor re l a t ed wave func t ion ~ i s de r ived f rom the un cor re l a t ed one (I) t h roug h:9 =oxp 9

w i t h t h e o p e r a t o r1S'~ = n' -~ ~ < P l . . . P n l S n [ " l . . . " n > a ; 1 . . . a;, av,~ . . . a~,~ (4 )

uiPiwh ere the c rea t ion an d ann ih i l a t ion op e ra to rs p roduce se t s of n pa r t i c l e -n ho le s t a t e s .Limi t ing the ca l cu la t ions to n=2 the cor re l a t ed wave func t ions read

r > = 1.1 > + 11-1 >r > = A OIIU , > r > +$21"1"2 > (5)T h e t wo - b o d y d e n s i t y f o r t wo p r o t o n s i n t h e l p l / 2 orbi t of 160[ < r ' l r * e [ r - - 1 >[2 (6)is given in Fig. 1 . Th e x , z -plan e i s se lec ted for y =0 wh ere the s econd p ar t ic le i s f ixeda t ( x=0 , y= 0 , z= 2 f ro ). On the le f t s ide o f F ig . 1 the ope ra to r $2= 0 and the re fore


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t h e t w o - b o d y d e n s i t y i s j u s t t h e p r o d u c t o f t wo o n e - b o d y d e n s it ie s ( i rr e sp ec t iv e o f th eloca t ion o f t he second pa r t i c l e ) . For $2~0 how ever, a c l ear dep le t ion in the tw o-b od ydens i ty can be obse rved a t t h e pos i t i on o f t he second pa r t ic l e . S imi l a r ca l cu la t ions fo rt h e T = 0 np pa i r r evea l s no t on ly an exp ec ted inc rease o f s t ren g th by a fac to r o f ab ou t 4bu t a l so i t s concen t ra t ion a t shor t i n t e rnuc leon d i s t ances desp i t e l a rge r meson exchangec o n t r i b u t i o n s .

As one -nuc leon remova l i s no t an adequa te too l one has to inves t iga t e the two-nuc leonr e m o v a l e x p e r im e n t s . T h i s e x p e r i m e n t a l m e t h o d f or a n ac ce ss to wa r d s c o r r e la t i o n s h a dbeen sugges t ed a l ready in the 50- i e s , i n pa r t i cu la r p ion and pho ton induced reac t ions .B o t h p r o b e s a r e u n d e r s t o o d t o b e a b so r b e d d o m i n a n t l y o n n u c l e o n p a i r s a t r e l a t i v e c l o sep r o x i m i t y [ 1 0 ] . Ho we v e r , b e a m q u a l i t y , e n e r g y r e so l u t i o n a n d b a c k g r o u n d r e a c t i o n s , a se .g . t he no to r ious f ina l - s t a t e in t e rac t ion , p reven ted se r ious p rogress . However , t he newgen era t ion o f exp e r im enta l t echn iques fo r cw acce le ra to rs and l a rge a rea, h igh re so lu t iond e t e c t o r s o n o n e s i d e a n d n e w d e v e l o p m e n t s o f n u c l e ar s t r u c t u r e a n d r e a c t i o n m o d e l sp e r m i t a r e n e we d a p p r o a c h t o wa r d s NN c o r r e l a t i o n s .

Th e a im of the p resen t exp e r im enta l se ri es is a comb ined ana lys i s o f (~ ,,np), (%pp ) ,(e ,e 'np) and (e ,e 'pp) to exp lo i t t he adv an tages o f each o f t he reac t ions .2. E X P E R I M E N T S A T M A I N Z W I T H R E A L A N D V I R T U A L P H O T O N S

Figure 2 . Ex-p e r i m e n t a ls e t u p w i t h t a g -ge r, p ro ton ho- F igure 3 .d o sc o p e L a y o u tP i P [ 1 1 ] a n d o f t h en e u t r o n t i m e - t h r e eof-f light a r- spe ct ro-r a y [ 12 ] . T h r e e m e t e rk ine m at i ca l r e - f ac i li t ygion s are indi- [13] atca t ed . Mainz .

T h e e x p e r i m e n t s o f t h e P i P /T O F g r o u p a re p e r fo r m e d a t t h e cw e l e c tr o n a c ce l e r at o rMA M I [ 14 ] a t Ma i n z u s i n g t h e G l a sg o w ta g g i n g sp e c t r o m e t e r [ 1 5 ]. E l e c t r o n b e a m s o f8 5 5 Me V i m p i n g e o n 4 # m t h i n N i r a d i a t o r o r o n 1 0 0 # m t h i c k d i a m o n d c r y s t a l i n c a sepo la r i sed pho tons a re needed . The pho ton beam i s co l l ima ted in o rde r to be we l l de f ined(d iam e te r o f approx . 1 ,5 cm) on the nuc lea r t a rge t s w hich cons is t e i the r o f l iqu id he l iumwi t h i n a k a p t o n c a n o r o f sh e e ts o f g r a p h i t e o r l i th i u m . T h e p h o t o n f l u x is c a l i b r a t e dto f ree runn ing sca l e r s wi th in the t agge r foca l p l ane by so-ca l l ed ' t agg ing e f f i c i ency ' mea-su r e m e n t s . T y p i c a l v a lu e s o f ~ t =4 5 : t: 2 % h a ve b e e n o b ta i n e d . T h e t a r g e t i s su r r o u n d e dby a r ing o f I mm th ick p l a s t i c coun te r s , which ac t a s s t a r t de t ec to rs fo r cha rged pa r t i c l e sand a s ve to de tec to rs fo r neu t ron s to be sel ec t ed f rom TOF[12] . Ch arged pa r t i c l e s a re


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de te c t ed by the fivefo ld l aye red hodo scope PiP[11] . Pro tons a re d i sc r im ina ted aga ins td e u t e r o n s a n d p i o n s v i a t h e A E - E t e ch n i q u e. T h e e n e r g y o f t h e se p a r t ic l e s is e x t r a c t e dexc lus ive ly f rom the pu l se he igh t a f t e r p rope r cor rec t ion fo r quench ing and ene rgy lossesin a i r and dead l aye rs . The neu t ron ene rgy i s ob ta ined f rom the t ime-of - f l i gh t measuredb y T O F wi t h a n a v e ra g e FW H M o f a b o u t 0 .7 n s. T h e c h a r g e d p a r t ic l e s in T O F c a n bese lec t ed by appropr i a t e cu t s on pu l se he igh t vs . t ime-of - f l i gh t . An ove ra l l ene rgy re so lu-t ion (Era ) o f 6 MeV has been ob ta ined . The angu la r re so lu t ion o f 3 ~ in po la r and o f 9 ~ ina z i m u t h a l d i r e c ti o n c a n b e t r a n s l a t e d i n t o a m o m e n t u m r e so l u t i o n of 3 5 M e V/c .

T h e 1 6 0 ( e , e ' p p ) 1 4 C e x p e r i m e n t s w i t h v i r t u a l p h o t o n s e m p l o y t h e t h r e e sp e c t r o m e t e rfac il i ty [13] wi th h igh beam cur ren t s ( I ~8 0 pA ) . a wa te r fa l l t a rge t [16] was used and aso-ca ll ed supe r -pa ra l l e l k in em at i c s was se l ec t ed [17] , which supp ressed A cur ren t s .3. R E A C T I O N M E C H A N I S M S O F T H E (~/ ,N N) R E A C T I O N S

T h e t h r e e o b se r v a b l es o f i m p o r t a n c e a r e th e m i s s in g en e rg y , t h e m i s s i n g m o m e n t u m a n dt h e r e l a ti v e m o m e n t u m b e t we e n t h e two n u cl eo n s o f t h e p ai r. T h e m i s s in g m o m e n t u mp~m is de fined a s the d i ffe rence be tween incoming ph o ton m om en tum k and the ou tgo ingn u c l e o n m o m e n t a Pn a n d gp" /T in = k - /T n - g p. I n p l a n e wa v e i m p u l se a p p r o x i m a t i o n( PW I A) Pm c a n b e i n t e r p r e t e d a s t h e m o m e n t u m P o f t h e p a i r b ef o r e t h e i n t e r a ct i o n .I t gove rns the g ross fea tu res o f t he an gu la r and ene rgy depend ence o f t he c ross sec tions .T h e m i s s i n g e n e r g y E m - k - T n - T p - TR i s def ined as th e di f ference of kine t ic en ergies ofpa r t i c l e s in the in - and o u tgo in g reac t ion channe l s . In pa r t i cu la r , TR i s t he k ine t i c ene rgyof the reco i l ing ( A - 2 ) sys t em ca lcu la t ed re l a t iv is t i ca l ly f rom pro . F ina l ly , t he re l a t ive

1momentum i s de f ined a s /Tr = 7 (Pp- g~)" Note , t ha t t he re i s no mode l f ree poss ib i l i t y toge t a hand le o f t he re l a t ive m om en tum ~r in the in i t ia l s t a te .

F igu re 4 . Th e (7 ,np) reac t ion fo r t h reep h o t o n e n e r g y r a n g e s i s c o m p a r e d f o r t h et a rge t s 12C and 4He .Figu re 5 . Th e (~/,pp) reac t ion for threep h o t o n e n e r g y r a n g e s i s c o m p a r e d f o r t h et a rge t s 12C and 4He .


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An o v e ra l l p i c t u r e o f t h e c o m p e t i n g r e a c t i o n m e c h a n i sm s is o b t a i n e d b e s t b y c o m p a r i -son [18 ,19,7] o f t he da ta to the p red ic t ions o f t he V a lenc ia mo de l [20]. For th ree p ho tonene rgy b ins the miss ing ene rgy spec t ra fo r t he (7 ,np) reac t ion on 12C and 4He a re p lo t t ed .Th e 12C da ta a re we ll desc r ibed by the ca l cu la t ions . For 4He a qua l i t a t ive agree me ntcan be s t a t ed . I t is obse rved th a t even a t h ighes t pho ton ene rg ie s the y i e ld a t l ow E ,~a r i se s f rom the two-body pho ton absorp t ion p rocess . The y i e ld a t h ighe r pho ton ene rg ie s(~15 0 MeV ) fo r 4He is r educed re l a t ive to tha t fo r 12C due the reduce d in f luence o f f ina ls t a t e in t e rac t ions . Th e desc r ip t ion o f t he (~ ,pp) reac t ions is l ess sa t is fy ing . These f ind ingspe rm i t t he co nce n t ra t io n on low exc i t a t ion ene rg ie s Ex = E m - Q of the re s idua l nuc le i.

In con t ra s t t o the Va lenc ia mode l , s t a t e -o f - the -a r t ca l cu la t ions [21 ,22] pe rm i t a p rope rt rea tm en t o f t he two-nuc leon knockou t reac t ion inc lud ing the com pe t ing p rocesses . E f fect so f t h e NN c o r r e la t i o n s a r e e x p e c t e d t o b e r e c o g n ise d t h r o u g h e x ce s s o f h i g h m o m e n t u mc o m p o n e n t s i n a n g u l a r d i s t r i b u t i o n s d i f f e r e n t f r o m t h e q u a s i d e u t e r o n p r e d i c t i o n o r i ngene ra l i n unex pec ted va r i a t ion o f c ross sec tions , pa r t i cu la r whe n f ina l s t a t e s w i th spec if icqu an tum num bers can be se l ec t ed because o f a su ff ic i en t ly h igh ene rgy re so lu t ion .

F igure 6. An gula r d i s t r ibu t ion o f t he 12C(~ /,np) reac t ion a t Ez < 50 M eV and a tE ~ = ( 1 3 5 + 2 0 ) M e V .

Next , we focus on the an gula r d i s t r ibu t ion o f t he 12C(~ /,np) reac t ion a t E~


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a n d p m e so n e x c h a n g e c u r r e n t s c a n a c c o u n t f or sh a p e a n d m a g n i tu d e o f t h e d a t a .

9 9 9 i . . 9 i 9 9 9 ! 9 . 9 , 9 9

~ ' 1 . 0 0 Li (T,np)He 150


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4 . H I G H R E S O L U T I O N T W O - N U C L E O N E M I S S I O N E X P E R I M E N T SD e sp i t e t h e n e w i n s i g h t s o b ta i n e d w i th t h e e x p e r i m e n t s d e s c r i b e d a b o v e , i t t u r n e d o u t

th a t t h e a v e r a g i n g o ve r a ll s t a t e s o f a s i n g le sh e l l m o d e l o r b i t a l d o e s n o t p i n d o w n th er e a c t i o n m o d e l s su f f i ci e n t ly a s a m b i g u i t i e s o f t h e m o d e l w i th r e sp e c t t o n u c l e a r s tr u c tu r eor reac t i on a m pl i tud e m ay no t be re sol ved . There fore , we presen t l y a im for tw o-nuc l eone m i s s i o n e x p e r i m e n t s w h i c h r e sol v e f in a l s t a t e s . I n g e n e r a l o n l y a f e w p a r ti a l w a v e scontr ib u te t o a tran s i t i on to a s i ng l e f i na l s ta te . The cross s ec t i ons are pred i c ted toe x h i b it d i s t i n c t iv e d i s t ri b u t i on s d e p e n d i n g o n t h e s p i n a n d i s os p in q u a n t u m n u m b e r s o fth e f i n a l s t a t e s .

T h e n u c l e u s 1 6 0 is th e b e s t c a n d i d a t e f r om th e n u c l e a r s t ru c tu r e p o i n t o f v i e w b o th f r ome x p e r i m e n ta l a n d f r o m th e o r e t i c a l c o n s i d e ra t i o n s . H i g h r e so lu t i o n ( e , e' pp ) e x p e ri m e n t sand the search for short range corre la t i ons o f s ca l ar type are reported e l s ewh ere [17 ] . Assh o w n i n F i g . 9 t h e tw o m o d e l c a l c u l a t i o n s b y G i u s t i a n d R y c k e b u s c h a r e q u i t e g o o db u t d o n o t g i v e su f f i c i e n t l y g o o d r e su l t s w h i c h a r e n e e d e d to s e l e c t t h e b e s t p o t e n t i a l f o rthe NN corre la t i ons . The n p c h a n n e l m u s t b e i n v e s t i g a t e d w h e n s e a r c h i n g f o r t h e t e n so rtype corre l a t i ons a s they are no t presen t i n the p p c h a n n e l . D e sp i t e t h e s t r o n g c o n tr i b u -

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/ / : - , -

I I I- IN # IN 2 N 3 1 0 4Nm i s s i n g m a m c n t u m / M e V / c

' ~ 1" (T=O) ; Ex---'O.MeV04i0.2~" , i , , . | 9.v 0.0 0.2%" ~ 0 " I ~ = ~ , 0

-0.2 ., ! , i , I . , . -0.2

I*(T=0); E x=3.95 MeV [

. = . _ _ _9 ' I I , I ,~" 1 -1 00 0 100 200 300 40 0- 10 0 0 1 00 200 300 4~

~ 0* (T=I); E=2.31 MeV 0.3 " 2+(T=0);I~=7.03MeV

~ ~ 0.~ 0~i 0.1

~ o o . . . . ' ~ - - o0. 1 1/

- 0 . 1 9 , , I , I " ~ " 4 ) . I . ' . ~ 9 ' . ' . I- 1 0 0 0 1 0 0 200 300 400 -100 0 100 200 300 400

m i s s i n g m o m e n t u m ( M e V ) m i s s i n g m o m e n t u m (MeV)

F i g u r e 9 . M i s s i n g e n e r g y sp e c t r u m forthe ]60 ( e , e~pp)14C reac t i on and the m i s s -i n g m o m e n t u m d i s t ri b u t i on f or t h e g r ou n ds t a t e t r a n s i t i o n ( f r o m Ref. [17]).

F i g u r e 1 0. P r e d i c t e d m o m e n tu m d i s t ri -b u t i o n s a n d n u c l e o n p o l a r i s a t i o n s f o r t h efour l owes t s ta te s i n the 160( e , e ' np)14N re -act ion [22] .

t i o n s o f m e so n e x c h a n g e c u r re n t s i n t h e n p channe l there i s a very spec i f i c s ens i t i v i ty toNN corre l a t i ons a s pred i c te d by J . Ryck ebu sch [22 ] . The f i na l s ta te s o f the four se t s i n


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Fig . 10 cor re s pon d to the f ir s t four s t a t e s o f Tab le 1 . Th e sp ins o f t he s t a t e s a re exp la inedb y t h e d o m i n a n t c o n f i g u r a t i o n s o f p l / 2 a n d p 3 / 2 remova l ; which in tu rn f ixes the angu la rm om en tu m a nd thus the pa r t i a l waves . The d i f fe ren t con t r ib u t ions a re re f lec t ed in thesh a p e s o f t h e m i s s i n g m o m e n t u m d i s t r i b u t io n s . T h e N N c o r r el a ti o n s ( F i g. 1 0 , d a sh e dl ines) can m ake u p to 50% of the cross sec tions; they affec t a lso the sh apes. Fi r s t (e ,e 'np)expe r iment s on 3He and 160 a re in p repa ra t ion .

Tab le 1Pro pe r t i e s o f l ow- ly ing s t a t e s in 14N. Coef fi c ien ts o f f rac t iona l pa re n tag e (cfp) a re com-

pared to measured c ross sec t ions [24] .E x / M e V J ~ T , co n fi g u ra ti o n L cf p ~d2~ [sr~-~2]0.0 1+ ,0 (p l/2 ) -2 2 0 .21 1 .4 + 0 .5

2.3 0 +, 1 (Pl/2) -2 0 -0.193.95 1+, 0 ( P 1 /2 ) - l ( p 3 / 2 ) - 1 0 -0.1 9 1.7 :t: 0.57.03 2 +, 0 (P 1/2) -1(p3/2) -1 2 0.28 1.6-t- 0.4

11.05 3 +, 0 (P3/2) -2 2 0.33

Table 1 and Fig . 11 p resen t re s u l t s f rom a low ene rgy , h igh re so lu t ion l ao(7 ,np )14Ne x p e r i m e n t [ 24 ] p e r f o r m e d a t E.y=7 2 Me V a t M AX l a b in L u n d . Un f o r t u n a t e l y , n o m is s -i n g m o m e n t u m d i s t r i b u t i o n c o u l d b e e x t r a c t e d i n t h i s e x p e r i m e n t i n o r d e r t o v e r i f y t h eang ula r m om en tu m t ransfe r . However , t he ob ta in ed c ross sec t ions g ive a n i ce exam ple o ft h e s e n s i t i v i t y o f t h e p h o t o a b so r p t i o n p r o c es se s t o n u c le a r s tr u c t u r e . De sp i t e t h e s a m ecoeff ic i en t o f f rac t iona l pa re n tage p red ic t ed by s imple she ll mo de l a rg um ent s the c rosssec t ion fo r t he f ir s t exc i ted s t a t e i s l ess tha n 5% of i t s ne ighbours . Th i s s t a t e has i sosp inT = I and i s t he i soba r i c ana logue s t a t e to the g ro und s t a t e s o f 14C and 140.

In p re pa r a t io n o f t he new h igh re so lu t ion expe r im ent s a l so the 160(3 ,,pp)14C reac t ionwi ll be inves t iga t ed in o rde r to comp lemen t , t oge the r w i th the (e ,e 'pp) da t a o f ref . [17],t h e f o u r f o ld d a t a s e t of r e al a n d v i r t u a l p h o t o a b so r p t i o n e a c h wi t h p p a n d n p emiss ionf rom 160 fo r a com bined ana lys is . Em ploy ing so lid s t a t e H P-G e de tec to rs w i th la rgeso l id ang les (up to 0 .7 s r ) and o f 25 mm th ickness , a p i lo t run o f 5 hours c l ea r ly showedthe feasib il it y . In F ig . 12 the g ro und s t a t e t r ans i t i on i s we l l sep a ra t ed f rom l eve ls a tEx ~6 MeV; a s the re so lu t ion i s ma in ly governed by the t agge r re so lu t ion which wi l l beimp roved b y a " t agge r mic roscop e" , a l so a sepa r a t ion o f t he exc i t ed s t a t e s i s expe c ted infuture (~/ ,pp) exper iments .5 . S u m m a r y

The presen t s t a tu s in sea rch of NN cor re l a t ions in nuc le i has been repor t ed . The newg e n e r a t i o n o f h i g h r e so lu t i o n e x p e r i m e n t s o n 1 6 0 wi t h r ea l a n d v i r t u a l p h o t o n s w i ll p e r m i ta s tr i n g e n t s t a t e - b y - s t a t e c o m p a r i so n of m e a su r e d m o m e n t u m d i s t r i b u t i o n s w i t h v ar io u smode l ca l cu la t ions . I t i s expec ted tha t t he cor re l a t ion func t ions f rom d i f fe ren t po ten t i a l s


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20 01 7 51 501 25

5. 100" o 7 5._~> - 50

250

-2 5

A E = 1 .6 M e V ( F W H M )

_ - - - 1 lub lsr2

0.12 , .

0 . 0 4

, , , , , I , , , | . . . . . w . 0 , I .1 0 1 5 2 0 2 5 3 0 3 5 4 0 0

Missing Energy (MeV)

Il l

9 ,

2 0 4 0 6 0e x c i t a ti o n e n e r g y / M e V

Fig ure 11. Hig h reso lutio n 160(~,,np)14Nexperiment [24] .

F i g u r e 1 2 . 5 h o u r p i l o t r u n f o r t h e h i g hr e s o l u t i o n 1 6 0 ( V , p p ) 1 4 C e x p e r i m e n t [ 8 ] .

can be se lec ted and an unders tanding of the in-medium proper t ies of nuc leon pa i r s wi l lbe gained.A c k n o w l e d g e m e n t

I wo uld like to acknow ledge useful discussions wi th J . Ryck ebusch and H. Miither . Ia m inde b te d to a ll me m be r s o f the P iP /T O F group fo r the s t r ong suppor t o f ou r c ommoninterest .R E F E R E N C E S1. P. G rab m ay r, Prog. Par t . N ucl . Physics 29 (1992) 2512. P.K.A. de W itt H ube rts , J . Phys. G, Nucl. Par t . Phy s 16 (1990) 5073. P. Ring and P. Schuck, "The Nu clear M any -Bo dy Problem ", Springer , 19804. H. Mf i ther , Proc . 4 thWorkshop on "Elec t romagnet ica l ly Induced Two-H adron Emis-

sion", Granada, May 26-29,1999; eds. P. Grabmayr and A. Lallena; p. 15. W . Dickhoff, i b i d , p. 22 6; A. Fabroccini , i b i d , p. 2066. D. W atts e t a l . , i b i d , p. 182; D. W atts e t a l . , submi t t e d to Phys . R e v .7. T. Heh l, e t a l . , i b i d , p. 708. P . G rab m ayr e t a l . , i b i d , p. 3319. G. O rland ini and R. Sarra , 2nd Workshop on "Elec t romagnet ica l ly Induced Two-

Had ron E miss ion" , G ent , May 1997, p . 110. K. Gottfried, Nucl. Phys. 5 (1958) 55711. I .J .D. MacGregor e t a l . , Nucl. Ins t r. M ethods A 382 (1996) 47912. P . Grabmayr e t a l . , Nucl . Ins t r . Method s A 40 2 (1998) 8513. K.I . Blom qvist e t a l ., Nucl . Inst . an d M eth. A 4 03 (1998) 26314. H. Herminghaus e t a l . , Proc. Lin. Accelerator Conference, Albuquerque, N M 199015. I . Anthony et a l . , Nucl. Ins tr . M ethods A 301 (1991) 230;

S.J. Hall e t a l . , Nucl. Ins t r . Metho ds A 36 8 (1996) 698


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16.17.18.19.20.21.22.23.24.

N. Voegler and J. Friedrich, Nucl. Instr. and Meth. 198 (1982) 293G. Rosner e t a l . , contr ib, to Int . School on Nuclear Phy sics, Erice , 1 999, Prog. Par t .Nucl. Phys. 44 (2 000)T . L a m p a r t e r e t . a l . , Z. Phys. A355 (1996) 1P . Grabmayr , in "cor re la t ions and c lus te r ing phenomena in suba tomic physics" , ed .O.Scholten, NATO ASI B (1997) 53R.C. Car rasco e t a l . , Nucl. Phys. A570 (1994) 701C. Giusti , i b i d , p. 14 8; Nucl. Phys. A 57 1 (1994) 694J . Ryckebusch, i b i d , p. 19; Phys. Lett. B383 (1996) 1; private comm.T .T . -H . Ya u e t a l . , Eur. Phys. J. A1 (1998) 241L. Isaksson e t . a l . , Phys. Rev. Lett. 83 (1999) 3146

two-nucleon emission experiments at mainz

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