The Mainz microtron facility MAMI

The Mainz Microtron Facility MAMI

TH. WALCHER

lnstitut fiir Kernphysik, University of Mainz, 6500 Mainz, F.R.G.

ABSTRACT

At the Univers i ty of Malnz a novel e lec t ron acce le ra to r with a max imum energy of 8S5 MeV and 100 ~ duty cycle is being buil t . Some cha rac t e r i s t i c s of th is machine and an ou t l ine of i ts exper i - men ta l p r o g r a m m e are p resen ted .

KEYWORDS

In t e rmed ia t e Energy Physics. E l ec t romagne t i c In te rac t ions , Coincidence Expe r imen t s with E lec t rons . Pho tonuc lea r React ions, Polar ized Elec t rons , Formfac to r of the Nucleon.

INTRODHCTION

At the l . lniversity of Mainz a novel e lec t ron acce le ra to r with a maximum ene rg ) of 855 MeV and I00~. duty cycle is being buil t . The hear t of th is acce le ra to r is a big race t r ack mic ro t ron which has given the name MAMI for Mainz Microtron to the whole project , This a r t i c l e will p r e sen t a t f i r s t a s h o r t desc r ip t ion of the acce lera tor . In the fo l lowing three pa ragraphs th ree expe r i men t a l fac i l i t i es which are rea l ized by three in t e rna t iona l co l l abo ra t i ons are descr ibed. These th ree c o l l a b o - r a t ions a t MAMI are

AI: "Coincidence e x p e r i m e n t s with e l ec t rons" A2: "Real pho tons" A3: "Polar ized e l e c t r o n s and e lec t r i c fo rmfac to r of the neutron"

The mot iva t ion and ac tua l rea l iza t ion of the fac i l i t i es are developed in some detai l . The expe r imen- ta l p r o g r a m m e in tended for the f i r s t round of expe r imen t s with t he se fac i l i t i es is out l ined .

The three c o l l a b o r a t i o n s have s t a r t e d in 1985 and many ideas, much mone,~ and hard work have been c o n t r i b u t e d s ince then. No a t t e m p t wil l be made to t race back all c o n t r i b u t i o n s in a s y s t e - mat ic way.

THE MICROTRON MAMi

The MAMI acce l e r a to r c o n s i s t s of a l inear acce le ra to r as injector and a cascade of th ree micro- t rons us ing normal conduc t ing RF s t ruc t t t r e s (Herminghaus et al., 1976, Herminghaus and Euteneuer . 1979) (see Fig. 1). In th is way a con t inuous wave opera t ion becomes poss ib le for the f i r s t t ime at medium high energ ies (Emax~SSSMeV) and high beana cu r ren t s ( i~lOOgA). The energy may be varied f rom 180 MeV to 8,55 MeV in s t eps of IS MeV b,~ kicking the beam off the s epa ra t ed t r acks into the e x t r a c t i o n channel . The beam will be d i s t ingu i shed by an e x c e l l e n t qual i ty both in t r ansve r se as in long i tud ina l phase space .

189

190 Th. Walcher

d•l•lO0 keY 0 . . . . Sm

°

3 . 5 MeV 0

,-0 ~ 0 A O Tuhl

180 ~ V J

Ta99~,

H2

Ai

- 2 0 m

-IOr~

Fig. 1. The cascade of three race Fig. 2. track microtrons with the injector linac.

The layout of the buildings of the MAMI faci l i t 3. A new building houses the injector linac and the three race track microtrons RTM I. II. Il l. The old halls HI, H2. H3. H4 and the beam switch- )ard hall (now tagger hall) were used for the old LINAC. The tagger hall wi l l be used b) the A2 collaboration and the old halls H3 and H4 by the A3 collaboration for the measurement of the electric formfactor of the neutron Fn(q2). The 3-spectrometer set up of the A1 collaboration wil l be accomodated in a new large spectrometer hall.

A f i rst two stage version of the machine, known as MAMI A, operated successfully from 1983 until 1'}87 with a maximum energy of 187MeV and a beam current of bS~A. A report on the results with this machine has been collected recently (Drechsel and Walcher, 1988). This operation demonstrated the working of the principle and its technical realization.

Since the end of 1987 the full machine, s o m e t i m e s ca l led MAMI B, is being a s s e m b l e d in a new bui ld ing (see Fig. 2). The e s sen t i a l pa rame te r s of the machine are summar ized in Table I. The s t a - tus (August 1989) of the a s s e m b l y of the d i f fe ren t s t a g e s in the new bu i ld ings is the fo l lowing:

The injector linac and microtron I have delivered a beam exceeding the specifications of Table 1 in all parameters. The surface correction coils for homogenizing the large 180 ° magnets of microtron [1 and ][] are in place and have been verified. The assembl) of the vacuum systenl and the RF cavi- ties is in progress and the f i rs t beam at 85,S MeV is expected to be produced in earl) summer 1990. The dipoles, a contr ibution of the CEN. Saclav. and the quadrupoles of the transferl ines are in house and are being installed together with the pertaining vacuum system. First experiments are planned for summer 1990.

T h e M a i n z M i c r o t r o n F a c i l i t y M A M I

Table 1. The e s s e n t i a l pa r ame te r s of MAM!

191

s t a g e 1 I1 I l l

General data:

input energy [MeV] 3 . 5 14 180 o u t p u t energy [MeV] 14 180 855 l inac t r a v e r s a l s 18 51 90 t o t a l power [kW] 280 900

Magnet s ) s t e m :

d i s t ance of m a g n e t s [m] 1. (>7 5 . 5 9 12 .86 f lux dens i ty [T] 0 . 1 0 0 . 5 6 I .28 max. o rb i t d i ame te r [m] 0 . 9 7 2 . 1 7 4 . 4 3 we igh t of one magne t [ t] 1.3 43 450 gap wid th [cm] 6 7 10

R.F. sys tem:

number of k l y s t r o n s I 2 5 l inac l eng th {el.) [m] 0 . 8 0 3 . 5 5 8 . 8 7 to ta l r.f. power [kW] 9 65 l b8 beam power [kW] 1.1 17 66 energy gain per pass [MeV] 0.bO 3 . 2 4 7 . 5

Bee m:

energy wid th [keV] *9 * 18 - 6 0 e m i t t a n c e vert. [n .mm.mrad] 0. l 7 O. 014 0 . 0 4

hor. [~ .mm.mrad] O. 17 0 . 0 1 4 O. 14

ln iec tor : 100 keV gun, in jector linac

Klys t rons : Thom s on-CSF TH 2075. 50 kW c.w., ~I=60Z

Frequency: 2449.3 MHz

THE A1 COLLABORATION: "COINCIDENCE EXPERIMENTS WITH ELECTRONS"

The AI c o l l a b o r a t i o n has des igned and is bui ld ing a s e t - u p of three large high r e so lu t i on magne t ic s p e c t r o m e t e r s . These s p e c t r o m e t e r s will s i t on a large t u r n t a b l e and are r o t a t a b l e around a common pivot. This a r r a n g e m e n t can be comple t ed by fur ther magne t ic and non -magne t i c de t ec to r s , as e.g. neu t ron t i m e - o f - f l i g h t . It will be the cent ra l faci l i ty for coincidence e x p e r i m e n t s wi th e lec- t rons , nuc leons and mesons .

I ts op t imiza t ion has been made with cer ta in typical expe r i men t s in mind which sha l l be s h o r t l ) ou t l ined in the fo l lowing . This will serve to d e m o n s t r a t e the mot iva t ion and special rea l iza t ion of the Mainz 3 - s p e c t r o m e t e r sys tem. The s t r u c t u r e of t he formulae of coincidence e x p e r i m e n t s with e l ec t rons wil l not be repea ted here. A concise r ep resen ta t ion can be found in Drechsel and Giannini, 1989 whose n o m e n c l a t u r e is used t h r o u g h o u t this ar t icle .

Quasi Free Nucleon Knock Out

A(e,e'N)A' : N ={p,n}

In this reaction a precision separation of the longitudinal and transverse cross sections 0T/O L shall be performed. This investigation will serve to reproduce the existing results (Drechsel and Giannini,

1989, and references therein) and try to extend them to a broader kinematical range and different nuclei. The investigation of neutrons is practicall) missing so far. However, neutrons are difficult

to measure in the environment of an electron beam with high y background. Therefore, a special neutron cave has been planned to measure neutrons in the most difficult situations (see Fig. 2, left hand side of spectrometer hall).

192

A Resonance Product ion in Nuclei

A ( e , e ' A ) A ' [ '- '~N } A ( e , e ' ~ N ) A '

Th. Walcher

In this react ion the exc i t a t ion of a A resonance in a nucleus and i ts p ropaga t ion th rough nuclear s t a t e s shal l be inves t iga ted . Lip to now only the quas i f ree product ion b~ s ing le arm e lec t ron exper - iments or coincidences of the e l ec t ron with one nucleon have been s tud ied (see ref. Drechsel and Giannini, 1989, and re fe rences therein). Such i ncomple te m e a s u r e m e n t s leave a large room for in te r - p re ta t ion of the reac t ion mechanism and prohibi t conc lus ions on the fundamenta l A-nucleus in te r - action.

The s tudy of t r ip le coincidences would a l low a complete r econs t ruc t ion of the A electroproduction in the quas i f ree regime. If the r e so lu t ion for all three pa r t i c l e s e ' ~ N will be good enough even nuclear s t a t e s in the nuc leus A' may be d i s t inguished . The exc i t a t ion probabi l i ty of these s t a t e s will s t r o n g l y depend on the A propaga t ion th rough the nuclear s t a t e s of the i n t e rmed ia t e s3s tem. A compar i son of the A-nucleus and N-nuc leus in te rac t ion will, therefore , teach s o m e t h i n g about the sp in - i so sp in dependence of the ba r~on-nuc leus in terac t ion .

7t Product ion f rom the Nucleon

p(e,e '~ t+)n

In this react ion a precise separat ion o f the G T and °L over an as large as possib le k inemat ica l range is aimed for . Wi th this data one mas address three di f ferent physics issues:

(i.) The p h o t o p i o n - p r o d u c t i o n at M A M I A has shown exc i t ing deviat ions f rom the low energs theorems (Schoch, 1989). I t w i l l be ver) interesting to s tud) this effect in (e,e'~).

(ii.) In the long i tud ina l cross sect ion the pion pole term wi l l largel~ dominate. Since the pion f o r m f a c t o r is wel l known f rom a CERN experiment in the k inemat ica l range in quest ion i t w i l l be possib le to ex t rac t the probabi l i t~ o f p re fo rmed pions in the nucleon (" the pion con- ten t o f the nucleon") .

(iii.) Given the pion f o r m f a c t o r the axial vec tor fo rmfac tor GA(Q 2) o f the nucleon can be de te r - mined f rom the t ransverse cross sect ion. Taken toge ther w i th the de te rmina t ion o f GA(Q 2) f rom a recent neut r ino sca t te r ing exper iment (Ahrens L A . et al., 1988) Jow energy theorems l ike the PCAC assumpt ion or the Goldberger-Treiman re lat ion can be checked.

E 2 / M I ra t io in the A exc i ta t ion

p l e . e ' ) A +

A possible E2 adm ix tu re in the vers dominant M1 exc i ta t ion o f the A resonance wou ld be a very impor tan t cons t ra in t on models o f the nucleon. However, th is adm ix tu re is so smal l tha t a normal long i tud ina l (E2) f r om t ransverse (M1) separat ion o f the cross sect ions does not suffice, A wa~ to render th is measurement possib le is the use o f polar ized e lect rons or o u t - o f - p l a n e measurements (see ref. Drechsel and Giannini. 1989)

n ° and .//o th resho ld p roduc t ion

p ( e . e ' p ) T: ° or

An in t r igu ing poss ib i l i t ) to s tud) the s t ruc tu re o f the nucleon is the compar ison o f n ° and qo elec- t rop roduc t i on . Since the qo conta ins a s,s quark component whereas the ~t ° = 1 7-~ (uS + c la ) such a

compar ison ma~ a l low conc lus ions about the strange quark con ten t o f the nucleon.

The Mainz Microtron Facility MAMI 193

T h i s m e a s u r e m e n t w h i c h d e t e r m i n e s t h e m e s o n as a m i s s i n g m a s s p o s e s , h o w e v e r , t h e p r o b l e m , t h a t t h e p r o t o n s p e c t r o m e t e r h a s t o b e t u r n e d t o very f o r w a , - d a n g l e s . F o r i n s t a n c e , a t a p r i m a r y e n e r g y o f 1E o = 8 5 5 M e V t h e m o m e n t u m a n d s c a t t e r i n g a n g l e o f t h e o u t g o i n g e l e c t r o n a r e k e , = 1 0 9 M e V / c a n d ® e . = 4 0 ° a n d t h e m o m e n t u m a n d s c a t t e r i n g a n g l e o f t h e o u t g o i n g p r o t o n a r e kp=484MeV/c a n d E)p=5.7 ° .

F o r e a c h o f t h e s e e x p e r i m e n t s a n o p t i m a l s o l u t i o n t o t h e e x p e r i m e n t a l s e t - u p c o u l d b e d e v i s e d . H o w e v e r , t h e m o s t e f f e c t i v e a p p r o a c h is t h e r e a l i z a t i o n o f a m u l t i p u r p o s e s e t - u p i f it is p o s s i b l e t o c o m p r o m i s e n o t t o o m u c h . I t is b e l i e v e d t h a t a 3 - s p e c t r o m e t e r - s e t - u p is a c l o s e t o o p t i m u m s o l u t i o n f o r t h e k i n d o f p h s s i c s d e s c r i b e d . Fig. 3, Fig. 4, a n d Fig. 5 s h o w t h e d e s i g n o f t h e t h r e e s p e c t r o m e t e r s a t M A M I . T h e m o s t s i g n i f i c a n t p a r a m e t e r s o f t h e s e t - u p b e i n g r e a l i z e d a r e s u m m a - r i zed in T a b l e 2. W h e r e a s t h e s p e c t r o m e t e r s A a n d C a r e o f QSDD t y p e a n d a l l o w f o r a l a r g e s o l i d a n g l e , t h e c h o i c e f o r t h e s p e c t r o m e t e r B is o f " c l a m p - s h e l l " t y p e in o r d e r t o r e a c h ve ry s m a l l s c a t t e r i n g a n g l e s . Fig. 6 d e p i c t s s o m e d e t a i l s o f t h e a r r a n g e m e n t o f t h e s p e c t r o m e t e r s in t h e i r new e x p e r i m e n t a l ha l l .

T a b l e 2. P a r a m e t e r s o f t h e MAMI 3 - s p e c t r o m e t e r - s e t - u p

A B C

M a x i m u m m o m e n t u m [ M e V / c ] 73S 8 7 0 SSO P r o t o n k i n e t i c ene rg> [ M e V ] 2 5 0 3 4 0 150 Pion k i n e t i c e n e r g } [ M e V ] 610 7 4 0 4 3 0 S o l i d a n g l e [ m s r ] 28 5 . 6 28 M o m e n t u m a c c e p t a n c e [%] 20 1S 25 L o n g t a r g e t a c c e p t a n c e [ m m ] SO SO SO A n g u l a r r a n g e I0 ° - l b O ° 5 ° - 5 0 ° 4 0 ° - 1 6 0 ° M o m e n t u m r e s o l u t i o n ~ p / p < 10 - 4 < 10 - 4 < 10 - 4 A n g u l a r r e s o l u t i o n a t t a r g e t [ m r a d ] ~ 3 ~ 3 ~ 3 S p a t i a l r e s o l u t i o n a t t a r g e t [ m m ] 4 s 1 4

IMAGE SPECTROMETER A PLANE /

QSDD " PMAX = 735 MzV/c c '.. I"' ~",

AP/P = 20 ~ i. ~, '. ",

~ = 28.se ' t - " ~ "\ "

1 7

SPECTROMETER C

QS00

PMAX = 550 MtV/c

Ae/e= 25 %

f~ = 2B MSR

IMAGE PLANE ,

TARGET ~ ~

Fig. 3. S p e c t r o m e t e r A Fig. 4. S p e c t r o m e t e r C

194 Th. Walcher

Fig. 5. Spectrometer B

Fig. 0. Spectrometrers A, B. C in the new experimental hall


Three impor tant fea tures shall be especially mentioned:

195

Coverage of f o rward scat ter ing angles

As i t is wel l known f rom previous set-ups at e.g. DESY. Hamburg or SLAC, Stanford. i t is manda- tory to reach very smal l scat ter ing angles at e lectron scat ter ing faci l i t ies. This was alread~ demon- s t ra ted in the example o f the ~1 ° e lec t roproduct ion p (e ,e 'p )~ l ° ment ioned above. More general ly i t is needed fo r a precise separat ion o f °L and °T" This separat ion needs an as wide var iat ion o f the t ransverse po lar izat ion ~ at constant Q2 as possible. Fig. 7a demonst ra tes this f o r the e lec t rop ion product ion p ( e . e ' T t ) N at E0=8~t0MeV. Fig. 7b shows the angle o f the v i r tual photon w i th respect to the beam di rect ion as a funct ion of the e lectron scat ter ing angle e e. One realizes that the co in- cident haclron is emi t ted in a ra ther fo rward direct ion. Finally Fig. 7c depicts the f l u x o f the v i r tu - al photons as a funct ion o f 8 e. I t demonst ra tes that the count ing rates drop dramat ica l l ) w i th O e and that there fo re angles ee~ 40 ° w i l l need very long measuring t imes.

900 12..I I 80 °

30 o

o

0 0.2 0.4 0.6 Q8 1.0 E

28° b)

24 o

2C~ 400 60 ~ 80 ~ 100" e e

MeV..~

10

lo 20 ° 4.0 ° 60* 800 100 ° 8 e

Fig. 7. 7a: The four momentum t ransfer Q2 as a funct ion o f the t ransverse polar izat ion fo r p(e.e'x)N at an invar iant mass o f W= I I7S MeV. Time elect ron scat ter ing angle (~e is parameter.

7b: The angle of the momentum t ransfer w i th respect to time inci- dent part ic le Oyv as a funct ion o f e e fo r the same react ion.

7c: The vir tual photon f lux F- iv as a funct ion o f e e for the same react ion.

Versa t i l i t y o f three spect rometers

For the indicated ph)sics one needs d i f fe ren t spect rometers fo r d i f fe ren t k inematical ranges. The small sol id angle spec t rometer B wi l l normall.~ serve as e lect ron spect rometer ill f o rward d i rect ion where a large sol id angle is less desirable because o f high count ing rates. This is also valid i f i t is used as a pro ton spect rometer . The large sol id angle spectrometers A and C ma~ he used at larger scat ter ing angles where count ing rates are low for e lec t ron-hadron coincidences. Al l three together are also wel l sui ted fo r the t r ip le coincidence needed for the & resonance invest igat ions ("zl-spec- t rometer " ) .

196 Th. Walcher

O u t - o f - p l a n e m e a s u r e m e n t s

A particularl-s impor tan t opt ion in coincidence exper iments wi th e lect rons is the measurement o u t - o f -p lane in order to determine 0TL and 0TT (Drechsel and Giannini, Iq89). These cont r ibu t ions o f the cross sect ion stem f rom the inter ference o f the longi tudinal w i th the transverse and the t ransverse w i th the t ransverse photon ampl i tudes. The) o f f e r the possibilit-s to invest igate ver-s small L or T admixtures in dominant t rans i t ions o f the opposi te character.

A detai led stud.~ o f the range of the ou t - o f - p l ane angle, which has to be accessed. -sields 0 °~ ~e ~45 ° w i th a par t i cu la r need fo r the small angles. In the M A M I set -up this range is covered b-s combining two methods:

(i.) For O°<( I )e< lO ° the spec t rometer B (weight 2SOt) is moved by means o f a h3draul ic system o u t - o f plane.

(ii.) The range 15°~ (I>e~ 45 ° ma) be reached b-s means of a special beamhandl ing system as indicated in Fig. 8 B v means of this s-sstem the beam is incl ined b-s an angle y wi th r e spec t to the hor izonta l plane. The angle" is varied by sh i f t ing the whole sy s t em horizontall-s and a d - jus t ing the magne t exc i t a t i ons onl) .

beam ~ " . • e~ ~ ' ¢ ~ t ~ =2o°

Fig. 8. Beam handl ing s.~stem for o u t - o f - p l a n e m e a s u r e m e n t s

The whole of the 3 - s p e c t r o m e t e r s-sstem is being bui l t and shal l be read-s in au tomn 1990. The tu - ning will s t a r t la te in 1990 and it is an t ic ipa ted tha t f i rs t expe r imen t s can begin in spr ing Iq91.

THE A2-COLLABORATION: "REAL PHOTONS"

The centra l facilit-s fo r the A2-co l labora t ion is a large, broad band photon tagger. This tagger wi l l al low to cover an energ-s range of 50=E,~,~80OMeV. It is a special con t r ibu t ion of the Liniversities of Glasgow and Edinburgh and f inanced b s the British Engineer ing and Research Council ,

This faci l i t ) with i ts per ta in ing e xpe r im e n t s will be housed in the t agge r hall (see Fig. 2). Some de ta i l s are depic ted in Fig. 9. It is wor th ment ioning tha t the hall provides a reasonab le space for expe r imen t s and tha t the beam dump can be placed in a separa te hall in order to op t imize background condi t ions .

A cont inuous photon beam o f this energ3 range wi l l a l low for a great number o f a new class of exper iments. For the f i r s t phase the fo l l ow ing exper iments are planned:

Photoproduct ion o f n ° and rj f rom the proton, f rom li~:ht and complex nuclei

('f.n °) • (~(,zl)

The n° -p roduc t ion is wel l su i t ed for a stud-s of the A(1232) (Pl1(1232)) resonance. This resonance is due to a M1 t r ans i t ion in the pho toabso rp t i on c ross sec t ion and domina t e s the to ta l c ro s s sect ion. One d i s t i n g u i s h e s two final s t a t e s . In ti le "coherent product ion" the final nucleus is in the ground

The Mainz Microtron Facility MAMI 197

s t a t e and the recoil is taken by the whole nucleus. For " incoherent product ion" the final nucleus s t ays in an exc i ted s ta te . In order to make sens i t ive compar i sons with theore t i ca l mode l s a separa t ion of the two s i t u a t i o n s is needed. With the good re so lu t ion expec ted f rom the d e t e c t o r s which will be ment ioned be low a separa t ion may be poss ib le in favourable nuclei.

The ~O-production shou ld serve in an ana log manner to stud), the N(1535) (S1~(1535)) resonance. However. due to spin and par i ty conserva t ion th is resonance cannot con t r i bu t e to coheren t ~l-product ion if for the init ial and final nuclear spin If--I i =0 holds. In th is case of coheren t p roduc t ion one may, therefore , s u p p r e s s the con t r ibu t ion of the SH(1S3S) and will be sens i t ive to the Roper Pit(1440) resonance . For the pho top roduc t ion of ~°s very few da ta exis t .

Compton scatter ing o f f the proton and l ight nuclei

A(y,y)A , p(y,y)p , d(y,y'n)p

From the measurement o f angular d ist r ibut ions of this reaction at d i f ferent energies strength dis- t r ibut ions o f the d i f fe rent contr ibut ing mult ipoles can be derived. From these the po lar izab i l i t ) o f the proton or heavier nuclei can be derived (see Ahrens e t a ] . in Drechsel and WaJcher, 1988).

A part icu lar ly interest ing in format ion for tests o f nucleon models would be a comparison o f the proton and neutron polar izabi l i t ) . In a measurement on the deuteron d (T , ' f ' n ) p in which the recoi l ing neutron is suf f ic ient ly well measured to establish a quas i - f ree reaction mechanism, this polar izabi- l i t ) o f the neutron could be determined. The precision o f such investigations wi l l be great ly im- proved with polar ized photons since with them the E1 and M1 ampli tudes can be separated. Polar- ized photons wi l l be available f rom the product ion o f coherent bremsstrahlung f rom a diamond crys- tal. Polarized electrons fo r the product ion o f c i rcu lar l ) polarized photons wil l be provided b) the A3 col laborat ion as out l ined below.

Photo induced b reak-up o f d and 3He

d i%pp~-) ; 3He(%pp)n

The pho to d i s i n t e g r a t i o n of the d and 3He offer a sens i t ive d i s t inc t ion of d i f f e ren t reac t ion mecha- nisms. Of pecul ia r i n t e r e s t are the NN. the ~N and AN in te rac t ions in the final s t a t e which modify the q u a s i - f r e e descr ip t ion .

A part icu lar ly intr iguing problem is that o f three bod) forces which can be st,Idled in the photoabsorpt ion on 3He. In a recent experiment at Saclay N. d'Hose (d'Hose et al . , 1989) measured the channels 3 H e ( % p ) X and 3He(y . x± )X . They could explain these inclusive data on l ) with the as- sumption o f three bod~ forces. Exclusive studies as e.g. 3He(y ,pp )n which wil l become possible at MAMI ma~ al low to fu r ther elucidate this ver) interest ing question.

Tota] p h o t o a b s o r p t i o n c ross sec t ion 6to t

During the last years the photoabsorpt ion cross sections in the exci tat ion range o f the L~ resonance have been determined fo r a great number o f nuclei (Ahrens e t al . . 1985). These invest igat ions showed that shape and cross section per nucleon were almost equal fo r nuclei ranging f rom 6Li to 23813. It seems that binding o f the nucleons plays l i t t le role and that a "universal" curve fo r all nuclei exists. A comparison of the universal curve with the cross section fo r the free proton shows that the exci tat ion energy o f the A resonance sta)s practical ly the same whereas the width becomes large," by a seemingl5 constant factor. This broadening may be part ia l ly a t t r ibuted to the Fermi- motion o f the 5, during i ts decay. In order to conf i rm this almost too simple picture more precise data are needed. Addi t ional in format ion which may help to check these ideas wi l l be the measurement o f the tota l cross section between S00MeV and 8SOMeV, the region o f some higher exci ta- t ion nucleon resonances fo r which only few data exist.

198 Th. Walcher

Photoproduc t ion of the A resonance in nuclei

A(.y,nTt°) A"

The photon is evident ly verb' well su i ted to exc i te the A(1232) resonance in nuclei s ince th is r e so - nance domina te s l a rge ly the pho toa bs o r p t i on c ross sect ion. With the duty cycle of 1 the recon- s t ruc t i on of the decay h ~ n ° n or r~-p is poss ib le . Since the nucleon can have ra ther smal l k ine t ic energies (Eml n =28 MeV) the m e a s u r e m e n t of the 7z°n channel wil l be advan t ageous in some cases . De tec tors su i t ed for th i s channel are ment ioned below. As with e l ec t rons the aim will be to com- p le te ly r e c o n s t r u c t the A and in th is way d i s t ingu i sh exci ted s t a t e s in the final s t a t e nucleus A'.

A case of specia l i n t e r e s t is the " reco i l l ess" p roduc t ion of a A in a nucleus, i.e. the A s t ays in the nuclear shel l o rb i t of the nucleon on which it was produced. Such a s t a t e will be only popu la t ed if the recoil m o m e n t u m of the A is taken by the nucleus as a whole. An a l t e rna t ive mechanism is the t r ans fe r of the A recoi l m o m e n t u m in a second react ion to a nucleon which is then ejected in the d i rec t ion of the photon. Whereas the f i r s t s i tua t ion may be d i f f icu l t to d i s t i ngu i sh expe r imen ta l l3 the second may be feasible .

Two nucleon knock -ou t on l ight and heav~, nuclei

A(~',pn)A'

The t w o nucleon k n o c k - o u t on nuclei w i l l a l low conclusions on two nucleon corre la t ions. These cor re la t ions are directJy re lated to the long-s tanding quest ion o f shor t range in teract ions o f nucleons in nuclei. This in fo rmat ion is not only in terest ing fo r modern mean f ie ld models o f the nucleus but may also te l l something about s t ruc ture o f nucleons at shor t distances. The invest igat ions o f these react ions have suf fered so far f rom the pool" duty fac tor o f ex is t ing photon faci l i t ies. There- fore, at M A M I a drast ic improvement can be expected.

• ' ~ ' I ",, ", Llin~ofcons=~tresoution ,' ,'"

..... T"- . ......................... 7 . . ' _

Fig. q. The exper imental area in the tagger hall

For these invest igat ions a large variety o f de tec tors has been designed and bui l t . The most impor - tant are:

1.) CATS (Col laborat ion A2 Spectrometer) This de tec to r arrangement Is, Fig. lO) consists o f two parts:

- a segmented NaJ crysta l (48cm diameter, b3 cm length) w i th a reso lu t ion A E / E ~ 3 Z (FWHM) at 300 MeV and

- a ha l f sphere app rox ima ted by 70 hexagonal BaF 2 c r y s t a l s of 25cm length.

2.)

The Mainz Microt ron Facility MAMI / /

/

- - < .,~7 Z ~ U~ - ~ - "~:" N~I : , ," , . , 3* ?:i3!_

~ \ ~ \ \ \ ~ \ \ \ \ \ \ \ \ \ \ \ \ N \ ~ L\\ 1 6Li neutron ahsorb/er

lm

199

Fig. 10. CATS: a s e t - u p o f t w o l a rg e y d e t e c t o r s f o r t h e measurement of ~ ° ' S

This detector is particularl) suited for the measurement of r~ o at large production angles.

This arrangement has been conceived and realized b) ph)sicists from the universities of Mainz and Bonn.

DAPHNE (D6tecteur~grande Acceptance pour la Ph)sique Nucl6aire Electromagn@tique).

This is a c) l indrical d e t e c t o r (s. Fig. II) which c o n s i s t s of a s a n d w i c h o f wi re c h a m b e r s .

s c i n t i l l a t o r s a nd a Pb c o n v e r t e r . It is p a r t i c u l a r l ) ~,ell s u i t e d fo r t h e r e a c t i o n d(y.ppTt-) and 3He(y .pp 1.

Th is d e t e c t o r has been d e s i g n e d and bu i l t b) ALS ICEN) Sacla3 and t h e INFN. Pavia.

C I

Pbconverter

%;° 0h.mUe,,

-I- r- ~ ~_~ Iszintillators

DAPHNE D~tecteur k grande Acceptance pour la Physique Nudda~re Electromagndtique

Fig. 11. DAPHNE: a c)l indrical close to 4x detector for the detection of x - , p and ~('s

3.) TAPS {Two Arm Photon Spectrometer)

This spectrometer consists of four identical blocks of 88 BaF 2 cr)stal each. Two of these blocks are placed together on one turnable arm (s. Fig. 12). This arrangement has been designed for investigation of n ° and q production in heavy ion and photon induced reac- tions.

I t is an instrument realized b) the Universit) of Giel3en, GSI Darmstadt. GANIL Caen and KV! Groningen.

200 Th. Walcher

Fig. 12. TAPS: a t w o a r m p h o t o n s p e c t r o m e t e r f o r t h e m e a s u r e m e n t o f n ° a n d

F u r t h e r d e t e c t o r s i n c l u d e a p l a s t i c wa l l o f IOO s c i n t i l l a t o r s (300 c m h e i g h t . 20 c m w i d t h . 5 c m d e p t h ) , l i qu id s c i n t i l l a t o r f o r s l o w n e u t r o n s (2m 2 a r ea ) , s c i n t i l l a t o r s a n d w i c h e s f o r c h a r g e d p a r t i c l e s a n d a 4 n s e t u p o f p o s i t i o n s e n s i t i v e p a r a l l e l p l a t e d e t e c t o r s (PPAC's ) f o r t h e d e t e c t i o n o f h e a r ) f r a g m e n t s f r o m p h o t o i n d u c e d f i s s i o n .

M o s t o f t h e s e d e t e c t o r s wi l l b e r e a d ) b) e n d o f 1989 s o t h a t f i r s t e x p e r i m e n t s wi l l s t a r t in s u m - mer 1~]90.

T H E A3 C O L L A B O R A T I O N : " P O L A R I Z E D E L E C T R O N S AND E L E C T R I C F O R M F A C - T O R OF T H E N E U T R O N "

The A3 C o l l a b o r a t i o n p r o v i d e s a s o u r c e o f p o l a r i z e d e l e c t r o n s t o t h e MAMI f a c i l i t ) , In a f i r s t a p - p l i c a t i o n it wi l l u s e t h e a d d i t i o n a l a c c u r a c 3 w h i c h is p o s s i b l e w i t h p o l a r i z e d e l e c t r o n s f o r t h e m e a s - u r e m e n t o f t h e e l e c t r i c f o r m f a c t o r o f t h e n e u t r o n G~(Q2) . T h e r e is s p e c i a l r e p o r t o n t h i s s u b j e c t a t t h i s c o n f e r e n c e ( O t t e n . 1989) w h i c h d e a l s in p a r t i c u l a r w i t h p o l a r i z e d e l e c t r o n s a n d p o l a r i z e d t a r g e t s . T h e r e f o r e , o n b a f e w s a l i e n t f e a t u r e s o f t h e e x p e r i m e n t a l s e t u p f o r time f o r m f a c t o r m e a s - u r e m e n t wi l l be d e s c r i b e d .

Three measure l 'nents are envisaged:

(i.) 3~e(e~,e n ) p p

(ii,) d ( ~ e ~ ) p

l i i i . ) d ( e . e n ) p

R e a c t i o n (i.) n e e d s a p o l a r i z e d t a r g e t a n d p o l a r i z e d e l e c t r o n s in t h e e n t r a n c e c h a n n e l . In i t s ex i t c h a n n e l t h e d e t e c t i o n o f u n p o l a r i z e d e l e c t r o n s a n d n e u t r o n s s u f f i c e s . R e a c t i o n (ii.) u s e s p o l a r i z e d e l e c t r o n s a n d a n u n p o l a r i z e d d e u t e r o n t a r g e t and , t h e r e f o r e , a m e a s u r e m e n t o f p o l a r i z a t i o n t r a n s f e r o f t h e e l e c t r o n t o t h e n e u t r o n is r e q u i r e d . R e a c t i o n (iii.) n e e d s no p o l a r i z a t i o n b u t w a n t s t o d e t e r -

m i n e G ~ ( Q Z ) by an o u t - o f - p l a n e ItI~: O) m e a s u r e m e n t .

F o r t h e s e e x p e r i m e n t s l a r g e s o l i d a n g l e s f o r an e l e c t r o n s p e c t r o m e t e r a n d f o r a p o l a r i m e t e r f o r t h e n e u t r o n is n e e d e d . Fig. 13 s h o w s a s c h e m a t i c lab o u t o f t h e p l a n n e d a r r a n g e m e n t . All i m p o r t a n t p a - r a m e t e r s a r e g i v e n in t h e f i g u r e . H o w e v e r . t w o f e a t u r e s a r e w o r t h m e n t i o n i n g :

1.)

2.l

The Mainz Microtron Facility M A M I 201

In f ron t of the l eadg las s ma t r ix a large focuss ing Cerenkov d e t e c t o r wil l a l low an e f fec - t ive supp re s s ion of the cumber some background of so f t photons . In th is way it is hoped to avoid a magne t i c s p e c t r o m e t e r and work wi th a ra ther large sol id angle of 5tq=10Omsr.

The neu t ron polarimeter is of c lass ica l and economical design. It c o n s i s t s of two layers of p l a s t i c s c i n t i l l a t o r with veto de tec to r s . The f i r s t one serves as ana lyse r b) means of the ana lys ing power of the reac t ion p i n ~ , n ) p and the second as a neu t ron hodoscope in order to measure the a s3mmet r ) . An unavoidable background may come from the 12C(~ ' .n)X ine las t i c b r eak -up react ion. However, in the p ( ~ , n ) p reac t ion ex i s t s a s t r ong cor re la t ion be tween sca t t e r ing angle and ampl i tude in the ana lyse r layer which is not p re sen t for l a C ( ~ , n ) X . This a l l ows for an ef fec t ive cut. For the m e a s u r e m e n t s (i.) and (iii.) the=)polar imeter will just serve as a qui te ef f ic ient neu t ron de tec to r . For the reac- t ions 3 H e ( ~ . e n ) p p and d ( ~ . e ~ ) p a precis ion of 5 G n / G e n = ±10% in a bin of AQ2 = ±l fm -2 is a imed for. At Q2 = l O f m - 2 th is s t a t i s t i c a l accurac) will be ob ta ined in abou t IO0 h of measur ing time. The f i r s t m e a s u r e m e n t s are planned for au tomn Iqqo.

16,16 le~d - glass anay Cerenkov-/ ~ ~ ~.40.40-290 ...... 3

~0 = ~ -~ +0.2 °

~ / / ~ / l ~-- 1,90 m

~H-or ~ . . ;I/fe - target Jleutluzl - pohu imet, er

~ Aq_~ 25u msr &~ _ ±17 °

~ /_4.0m ~ / ~ -~ :t: 13°

I \ \ ~ ~, 5.20.300 CIII 3

Fig. 13. The set Lip for the measurenlent of the electric formfactor of the neutron. The netltron pola,-imeter consists of two plastic scint i l lator hodoscopes of 2 and 3 layers of scint i l lators each. It is surrounded b) a shielding against low energ 3 y's. The electron detector is a Ibx l6 lead glass matrix,

202 Th. Walcher

CONCLUSIONS

The Mainz Microtron MAMI is approaching completion. The first cont inuous beam of 85,SMeV is expected earl) sum m er 1990. The f i rs t photon exper iments (A2 collaboration) will then s ta r t imme- diately. The f i rs t exper iment with polarized electrons, the electric fo rmfac tor of the neutron, (A3 collaboration) is planned to begin in automn 1990. The 3 -spec t romete r set up (AI collaboration) will be mechanically installed in automn lq90. After a tuning period of 6 months the first experiments wil l s tar t in spring 1991.

The realization of three rathe=" big facilities has been possible because the interested phss ic is ts or- ganized themselves in three col laborat ions s tar t ing about 4 years ago. There are three evident ad- vantages of this organizational scheme:

1.) Concentrat ion of resources (money, technical support , man power) to a few ambit ious facilities.

2.) Common responsibi l i t ) of all col laborators for the realization of these facilities.

3.) Common interest of all col laborators in the ph.~sics to be persued with these facilities.

Therefore the MAMI project could be realized despite its size in the f ramework of a university inst i tute. Nevertheless. it is open to col laborators from outside to the extent posed b.s the limited resources and infras t ructure .

This is i l lus t ra ted b) the fo l low ing l ist o f col laborators:

AI collaboration: I. Blomqvist. Lund: J. Calarco, Durham: H. Emrich, Mainz; G. Fricke, Mainz: J. Friedrich, Mainz: G. K/Sbschall. Mainz: M. Korn. Mainz: K.-W. Krygier. Mainz: V. Kunde. Mainz: A. Liesenfeld. Mainz; G. LiJhrs. Mainz: K. Merle, Mainz: R. Neuhausen, Mainz; N. Potocnik, Mainz: M. Potokar, Ljubljana; A. Richter, Darmstadt; A, Richter. Mainz: G. Rosner, Mainz: P. Sauer. Mainz: S. Schardt, Mainz; B. Schoch, Bonn; Th. Walcher: Mainz.

A2 collaboration: J. Ahrens. Mainz: G. Audit. Saclay: A. Baumann. Mainz: R. Beck. Mainz: D. Brandford. Edinburgh: j. Capitani, Frascati: j. Crawford. Glasgow: N. D'Hose. Sacla): H. Dittmayer. Mainz: R. Gothe, Mainz; P. Grabmayr, TEbingen: S. Hall, Glasgow: S. Herdade, Sao Paulo: F. Kalleicher, Mainz: J. Kellie. Glasgow: S. Klein, TUbingen: U. Knell31. Giel3en: K.-H. Krause. Mainz: W. Lang, Mainz; M. Ludwig, GiSttingen: G. LUhrs, Mainz: D. MacGregor, Glasgow; C. McGeorge. Glasgow: G. Mertens. TUbingen: V. Metag, Gieflen: R. Owens. Glasgow: P. Pedroni. Pavia: J. Peise. Mainz; M. Pfeiffer. Giel3en; T. Pinelli. Pavia; P. Rullhusen, GiSttingen; M. Sanzone, Genua: H. Schmieden, Mainz; M. Schneider, Mainz: B. Schoch, Bonn: M. Schumacher. GiJttingen; J. Sobolewski. Mainz: F. Steiper. Giel3en; G. Tamas, Saclay; G. Wagner. TLibingen: Th. Walcher, Mainz; T. Weber, Gie{3en: F. Wissmann. Mainz; B. Ziegler, Mainz:

A3 collaboration: H. Andresen, Mainz: J. Annand. Glasgow; K. Aulenbacher, Mainz; D. E)I. Mainz; A. Frey, Mainz; S. Hall, Glasgow; W. Hell, Mainz; F. Klein, Mainz; M. Leduc, Paris; G. LiJhrs, Mainz; G. Miller, Glasgow; P.J. Nacher. Paris; R. Owens, Glasgow: E. Otten, Mainz: E. Reichert. Mainz: L. Schearer, Rolla Missouri; K.-H. Steffens, Mainz; Th. Walcher. Mainz:

The project is receiving support by the fo l low ing inst i tut ions:

- Land Rheinland-Pfalz - Deutsche Forschungsgemeinschaf t . Bonn-Bad Godesberg - British Science and Engineering Research Council, U.K. - Bundesministerium fur Forschung und Technologie, Bonn - CEN. CEA, Sacla)


REFERENCES

203

Ahrens, J. (1985). The to ta l abso rp t ion of p h o t o n s by nuclei. NucI.Ph)'s. A446. 229-240c Ahrens, L.A.. K. Abe. K. Amako, S.H. Aronson , E.W. Beier, J.L. Cal las , D. Cu t t s , M. Diwan, L.S. Durkin, B.G. Gibbard, S.M. Heagy, H. Hedin, J.S. Hof tun , S. Kabe, Y. Kurihara, R.E. Lanou, A.K. Mann, M.D. Marx, M.J. Mur tagh , Y. Nagashima, T. Shinkawa, E. Stern, Y. Suzuki, S. Terada. D.H. White . Y. Yamaguchi , (1988). A stud) ' of the ax ia l -vec tor fo rm fac tor and s e c o n d - c l a s s cur - r en t s in an t ineu t r i no quas ie la s t i c sca t t e r ing . Physics Letters B, 202, 284-288 Drechsel . D. and Th. Walcher (1988). Physics with MAMI A. Univers i ty of Mainz. Drechsel , D and M.M. Giannini (1989). Elect ron sca t t e r ing of f nuclei. Reports on Progress in Physics. 5_22, 1083-1163 d 'Hose, N., P. Argan, G. Audit , A. Bloch, N. de Bot tom L. Ghedira, L. J a m m e s , J.M. Lager, J. Martin, E. Mazzucato . P. Pedroni, M. Rodgers , A. Sasaki, C. Schuhl , P. Stoler , G. Tamas , (1989). T w o - N u - c leon Photon Abso rp t i on on 3He in the A(1232) Region. Physical Review Letters, 63, No. 8, 856-859 Herminghaus , H., A. Feder, K.H. Kaiser, W. Manz, H. yon der Schmit t . (1976). The des ign of a cas - cade 800 MeV normal conduc t ing c.w. race t rack microt ron. Nucl. Instr. and Mech. 138, 1-12 Herminghaus , H. (1988). The Mainz Microtron, Opera t ion exper ience and upgrade p rogress . Procee- d ings of the t988 Linear Acc. Conf. , Wi l l iamsburg , VA. Ot ten , E. (1989). Elect , 'on s ca t t e r ing with polarized beams and 3He- t a rge t . Con t r ibu t ion to th is con- ference Schoch, B. (1988). Recent Results with MAMI A. In Workshop on "Electron-nucleus scattering" E]PC, Marciana Marina, I ta ly; (A. Fabrocini, S. Fantoni. S. Rosati. M. Viviani, ed.) World Scientif ic.

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The Mainz microtron facility MAMI