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Hadron and Nuclear P hysics w ith Electromagnet ic ProbesK. M amyam a and H. O kuno (Edi tors)

2000 Elsevier Science B .V. All rights reserved. 265

Nuclea r phys ics exper iments wi th 1 .2 -GeV STB r ing a t LNS-TohokuJ . Kasa gi ~~L abo ra tory of Nu clear Science , Toh oku U niversi ty , M ikamine , Sendal 982-0826, . Japan

The Labora to ry o f Nuc lea r Sc ience a t Tohoku Unive rs i ty has recen t ly comple t ed con-s t ruc t ion o f t he s t r e t che r -boo s te r r ing (STB r ing) . The ST B r ing is now produc ing acon t inuous e l ec t ron beam wi th ene rg ie s up to 250 MeV for ex te rna l t a rge t s t a t ions , andi s abou t r eady to de l ive r a t agged pho ton beam wi th ene rg ie s up to 1 .1 GeV. We desc r ibethe p re sen t expe r imenta l p rograms fo r nuc lea r phys ic s wi th the STB r ing .1 . I N T R O D U C T I O N

The L abo ra to ry o f Nuc lea r Sc ience (LNS) a t Tohoku Un ive rs i ty has con t r ib u ted tonuc lea r sci ence re sea rch wi th e l ec t ron beam s in the l a st t h ree decades . I t has had a300-MeV e lec t ron li nea r acce le ra to r s ince 1967 . A 150-MeV pu l se -beam s t re t che r wascons t ruc ted in 1981 a s a p ro to typ e r ing for a GeV s t re t che r r ing p roposed in l a t e 70 ' s.However , an o r ig ina l p roposa l o f t he GeV e lec t ron acce le ra to r complex fo r nuc lea r phys ic swas no t accep ted , and , t hen , t he p ro jec t was fo rced to sca l e down; the 1 .2 -GeV s t re t che r -boos te r r ing (S TB r ing) was cons t ruc ted . The STB r ing is expe c ted to work no t on ly fo rnuclear physics research but a lso as an in jec tor for a 1 .8 GeV SR r ing, which i s p lannedfor a future projec t .

The STB r ing has two func t ions ; t he pu l se -beam s t re t che r r ing and the boos t a .nds to rage r ing . In the s t r e t che r mod e ope ra t ion , t he r ing accep ts a pu lsed bea m f rom thea00-MeV Linac (300 pps) and p roduces a con t inuous b eam (abou t 8O % of du ty f ac to r ) .The ex t rac t ed beam i s de l ive red to t a rge t s t a t ions fo r l ow ene rgy nuc lea r expe r iment s .In the boos t and s to rage mode ope ra t ion , e l ec t rons a re acce le ra t ed up to 1 .2 GeV to bes to red in the r ing . Th ey can be used for i n t e rna l t a rge t expe r im ent s inc lud ing h igh-en e rgyt a g g e d p h o t o n p r o d u c t i o n .

The comiss ion ing o f t he S TB r ing was s t a r t ed in Novem ber , 1997, and th e acce le ra t ionup to 1 .0 GeV was succeeded in May , 1998 . Af t e r t he inspec t ion on rad ia t ion sa fe ty thes t re t che r mod e ope ra t ion ha d been deve loped . A t p re sen t a 200-MeV conr inuous b eamin the s t r e t che r mode ope ra t ion i s i n use fo r va r ious expe r iment s and a 1 .2 -GeV t aggedphoton beam i s be ing deve loped .2 . E X P E R I M E N T S W I T H L O W E N E R G Y B E A M S

The e xpe r ime nta l a rea o f LNS is shown in F ig . 1. T he low ene rgy pu l sed beam Dora the300-MeV l inac is used for the p rod uct i on of var ious radioisotope s a t th e s ta t ion 1 , wh eree lec t rons wi th energies up to 60 MeV are avai lab le . As i s shown in Fig . 1 , the STB r ingoccup ies more tha n ha l f a rea of t he expe r im enta l room no . 2 . Two t a rge t s t a t ions a re

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1)repared around the r ing for 1 .2-GeV exper iments; 1 .2-GeV tagged photon and internalta rge t e xper imen t . In the s t r e tcher mode opera t ion , the low energy con tinuous bea m (150_< E_~ < 2.50 M eV) is ex tra cte d from the r ing and is tra ns po rte d to the LD M stat ion , wh erea la rge d ipo le magnet ic spect rograph can be used to analyze the sca t te red e lec t rons . Theother s ta t ion fo r low energy exper iments i s the low-energy tagged photon s ta t ion loca tedin t i le exper i lnental room no. 1 .

F igure 1. 1 .2 -GeV St re tcher -Boos ter r ing and exper imenta l s ta t ions a t Labora tory o fNuclear Science.

The fol lowing is a l is t of the approved nuclear physics exper iments with low energystretched beams. Exper iments (1) and (3) have been already carr ied out .

(1) Study of quasifree elastic process in the 12C(e,e'p) reaction.(2) Study of IAS with (e,e 'p) react ions on heavy nuclei .(3) Study of IVMR and IVQR in the 28Si(e,e 'n) react ion.(4) Stu dy of the l~ react ion.(.5) Collective motion in heavy nuclei via (e,e'fission) reactions.(6) M echa nism of sub- thre shold pion produc t ion In (e,~ ~ react ions.(7) Search for exci ta t ion of two-phonon GR in the 4~ react ion.(8) S tudy of pho to-absorp t ion mec hanism in the 12C("f,n) an d 12C(7,np) react ion.Here, I d iscuss the possib i l i ty to exci te the two-phonon giant resonance in a photo-abso rpt ion process; Ex per im ent (7) proposed by Terasawa[1]. Eviden ce for the two-

phonon GDR has been found f i r s t ly in p ion doub le charge exchange reac t ion a t LAMPF

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(Los Alam os) [2] , and in heav y ion react ions a t SIS[3] and GAN IL[4]. How ever, up tot.he present ., no a t t em pts have been repor ted to observe the two -phono n GR s ta tes v iapho t o -a b s o rp t i on .

T w o p h o n o nG D R G D R

q h ~ o p h o n o n P , l s ~ P I P T w o p h o n o nC D R G D R n p

nsmD..p~. . P l Jn ( n )9 - - - ~- (n-+)

;z~if.--'- ~o A"'"/ (El)

0 + ' + D G D R D G D R 0 +, 2 + i - ( I - @ 2 + ) I -/ !

/ G Q R o r O D R/ I /t ~ 0+T o T o - 1 T o -2 T o T o T o - I

( a ) ( ~ :+ , 7 r - ) r e ac t i on ( b ) H e a v y i o n r e a c t i o n ( c) P h o t o - a b s o r p t i o n

Figu re 2 . Tw o-p ho non GR s ta tes exci ted in various reactions" (a) pion doub le chargeexchange reac t ion , (b ) heavy ion reac t ion , and (c ) photon absorpt ion reac t ion .

In the p ion double charge exchange reac t ion , the two-phonon s ta tes can be exc i t edthrou gh two-s tep par t i c le -hole exc i t a tions . For examp le , the (re + , re - ) reac t ion can ber ~ g ~ r d e d ~ s t h ~ two-s tep zTo(rc+,rc~176 react ion. As is shown inFig . 2 (a ) , the f ir st s tep reac t ion exc i t es the in teme dia te s t a tes of the A T - 1 par t i c le -hole(pro ton-n eut ro n) exc i t a t ion cor responding to the G DR of the T0-1 nuc leus. Th e seconds tep is the succes ive exc i t a t ion of anoth er p ar t i c le -hole (pro ton-ne ute ron ) exc i t a t ion , and ,t hus, t he t w o-phonon G D R ( G D R Q G D R ) c a n b e exc i te d in t he T 0 -2 nuc le us. I n he a v y ionreac t ions , the s t rong Coulomb f i e ld be tween the pro jec t i l e and the t a rge t nuc le i prov idesman y v i r tua l photons . Th en the t a rge t nuc leus can absorb , for exam ple , two photonss imul taneously, and a two-pnonon GR s ta te is exci ted, as shown in Fig. 3(b) .

In the photo-absorpt ion process , however , only one photon mus t t ake par t in exc i t inga two-phon on s ta te . Thus , the in te res ting ques t ion is "can such an exc i t a t ion occur?" A poss ib le exc i t a t ion m echan ism of the two-phono n s ta tes m ay be a coupl ing of apho t on w i t h t he me s on e xcha nge c u r r e n t ( ME C ) . T he M E C i s know n t o p la y a n i mpo r t a n tro le in photo-absorpt ion process , e spec ia l ly for exc i t a t ion energies h igher than the GDR

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e x c i t a t i o n . I n F i g . 3 ( c) t h e e x c i t a t i o n m e c h a n i s m is s h e m a t i c a l l y s h o w n . T h e i n c i d e n tp h o t o n i s c o u p l e d w i t h M E C w h i c h c o n n e c t s t h e d o u b l e p a r t i c l e - h o l e e x c i t a t i o n s . A s t h ee l e c t r i c d i p o l e a b s o r p t i o n s h o u l d b e t h e m a i n p r o c e s s , t h e t w o - p h o n o n s t a t e s s h o u l d b e1 - . A T = I s t at e : a 1 - p h o n o n c o u p le d w i th a 2 + p h on o n . T h e 1 - p h o n o n c o r r e s p o n d s t ot h e i s o v e c t o r g ia n t d i p o l e r e so n a n c e ( I V G D R ) , a n d t h e 2 + p h o n o n t o t h e i s o v e c t o r g ia n tq u a d r u p o l e r e s o n a n c e ( I V Q G R ) . A n i s o s c a l e r m o d e i s n o t e x c i t e d s i n c e t h e M E C a c t s a sa r _ r + ( o r r + r _ ) o p e r a t o r . T h u s , o n e o f t h e t w o p h o n o n s o f t h i s I V G D R c O I V G Q R 1 -s t a t e i s t h e p r o t o n - p a r t i c l e a n d n e u t r o n - h o l e t y p e w h i l e t h e o t h e r n e u t r o n - p a . r t i c l e a . n dp r o t o n - h o l e t y p e . T h e 1 - t w o - p h o n o n G R ex c i t e d b y p h o t o - a b s o r p t i o n c a n b e r e g a r d e da s t h e p h o n o n e x c i t a t i o n c o u p l e d w i t h t h e G R o f t h e n e x t n u c l eu s .

T . S u z u k i[ 5 ] p r e d i c t e d t h e p h o t o a b s o r p t i o n c r os s se c t io n t o e x c i te t h e t w o - p h o n o n G Rto be a s l a rge a s 25~ , o f tha t o f the s ing le GDR exc i t a t ion in 4~ O n e m a y e x p e c tt o o b s e r \ e t h e t w o - p h o n o n G R in t h e 4~ r ea .c ti on b y m e a s u r i n g n e u t r o n s w h i c he s c a p e f r o m t h e t w o - p h o n o n s t a t e t o o n e - p h o n o n s t a t e .3 . E X P E R I M E N T S W I T H 1 .2 G E V C I R C U L A T I N G B E A M

F o r e x p e r i m e n t s w i t h G e V e l e c t r o n s , o n e of t h e m a i n i n t e r e s t s is th e b e h a v i o r o f t h enuc leon exc i t a t ion in nuc le i. As p re sen ted by Y am azak i in th i s sympos ium [6] , we haves tud ied $11 re sonance in nuc lea r med ium a t , the E S fac i l i ty o f the fo rmer INS. We a rep l a n n i n g t o e x t e n d t h e m e a s u r e m e n t s o f (7 ,7 1 ) a n d ( 7, p7 1 ) r e a c t i o n s o n v a r i o u s n u c l e a rt a r g e t s w i t h m u c h b e t t e r s t a t i s t i c s , i n o r d e r t o s e e w h e t h e r t h e r e s o n a n c e p r o p e r t y d o e sc h a n g e i n n u c l e a r m e d i u m o r n o t .

"xXA\

T A G X S C I S S O R S

F i g u r e 3 . 1 . 2 - G e V t a g g e d p h o t o n b e a m c o u r s e a n d t a r g e t s t a t i o n .

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In Fig. 3 , we show the exper im ental a rea for 1 .2-GeV tagged photon expe r imen ts .The ph oton tagging sy stem is now being instal led . The sy stem consists of a rad iato rp laced a t the en t rance o f the bend ing mag net (BM4) of the STB r ing and 100 p las t icscint il lat , ors placed in a small room in the ben ding ma gan et . The rad iato r ma de of athin carbon foi l can be moved and set at the beam posi t ion very quickly: the radiatori s r emoved f rom the beam pos i t ion when the e lec t ron beam is in jec ted and accelera ted .The electrons em it t ing ph otons are detected with one of the plast ic scinti l lators (5 x 5 20 ram) . Each scint i l lator is conne cted w ith a special l ight guide which couples to 3-mlong l ight f iber. The n, pho tom ult ipl iers can be placed at a d is tance from the ben dingmag net . The ta rge t pos i t ion o f the tagged photon i s about 7 .5 m f rom the r ad ia to r .The re, two large detecto r system s wil l be placed: one is the TA GX sytem , which hasbeen w orking very "well in the ES facility at INS for these 15 years. Th e oth er is calledSCISSORS (Sendai CsI Scint i l lat ion System On Radiat ion Search): a large scint i l lat iondetector system, consis t ing of 200 pure CsI crystals . The former system wil l be used mailyfor charged p ar t icle detect ion , while the la t ter for -y-ray detect ion. As shown in Fig. 3 ,they can be p laced a t the ta rge t pos i t ion o f the tagged photon beam.

The 1 .2 -GeV tagged photon sys tem i s now being examined together wi th the improve-!nent, of the qual i ty of the 1 .2-GeV electron bea m c irculat ing the ST B r ing. Th e tagg edphoton exper iment is expected to s tar t in May, 2000, and, then, LNS wil l serve not onlyfor low-energy nuclear physics but also for GeV energy nuclear physics .R E F E R E N C E S1. T. Terasa.wa, pr ivate comm unicat ion.2. S. M orde cha i et al. , Phy s. Rev. Lett. 60 (1988) 408.3. J . Ri tm an et al. , Phy s. Rev. Lett. 70 (1993) 533; R. schmit et al. , Phy s. Rev . Left. 70

(1993) 1767.4. N. Fra scari a, Nucl. Phy s. A569 (1994) 111c.5 . T. Suzuki , pr ivate com mu nicat ion.6 . H. Ya ma zaki , pap er in this proceedings; T. Y or i ta et al ., Phys. Let t . B in press.