/lcl,i"ta Mcxicana dc Física 38, Sup/cmcnto 2 (1992) 18-1-J 95

Hard Photon Intensity Interferometry in Heavy IonReaetions.',2

H. OSTENDORI"I, Y. ~CIIUTZI, H. MEItROIICII1, F. LEFE\'RE',

11. OELAGItAN(:E', \V. MITTIG" F.O. BEItG2, W. KÜIIN2,

V. METAG2, H. NOVOTNy2, M. PFF.IFI"EIt2, A.L. BOONSTRA3,

11. LOIINER3, L.B. VENEMA3, Il.\\'. WILSCIIUT" W. IlENNING4,

H. 1I0LZMANN4, n.s. I>¡AYER4, n. SIMoN4, D. ARDOUIN5,11. OAIlROWSKI5, B. EItAZMIIS5, C. Lf;HIt u N5, L. SÉZAC5,

P. LAUTItIDOII",.1. QIIF.HEItT", F. BALLESTEIt7, E. CASAI/,

J. DÍAZ7, J.L. FEItIIEIt07, M. MAItQUÉS7, G. MARTÍNEZ7,11. NIFENECI(EIt", B. FOItNAL9, L. FItEINI)IY, Z. SIIJI(OWSI(IIO,

T. MATIILEWICZ"

I (;11¡\,!L, /JI' 5027, 1-1021 Caen, Fmnce2 1/ Pltw.ikalú.ehes 11l8titllf Uuillcrsitiit Gicssc1l, /)-6;100 (,'i(',';,';C11, (;cnwuzy

3 líl'/, 97-17 AA Cl'Onillyen. The Ncthcr/an,/s4 CSI, /)-6100 D(l1'Inst(l(/t. (;(I'1/I'"'Y

5 /,/'¡\', -1-1072Nantes Cedex 03, Fmnec" CEN /JC, :1.1/75 CnJ(/iyn"" en/ex, Fnmcc

7 IF/e -16100 8ll7jassot, Valencia, SI"';"8 ISN ,"18026 Gl'clloblc, Fnl1ll'c9 INI', 31-3-12 límlww, 1'01",,,1

10 SINS, S'wicl'k, jJo/wul11 l\'al'saw l!nit'U'f.ily, PolawL

ABSTRACT. The prf'sent l'Xp«;>riUWlllal knowledge 011 hard photon produc.tion in hí'avy ion c.ol-lisiolls is summarized. An attelllpt lo IIwasure for tlle firsl time the intensity interf{'f('ncf' usingphotons in the ~tEV rang(' is dcscribcd. Tlit' t"lfect is illh'rpret.cd in lPrrns of spat.ial aTHI lt'lllporal

exlenl of thc pholon's SOllrce.

RESUMEN. Darnos UJI reSUllIf'n del conocilllient.o eXIH'rilllf'lItal actual sobre la producción defotones duros en colisiones dc iOIlI'S pesados. Descrihimos 1111intento de TlIf'dir por prilJlPra vezla intensidad de illterferencia IIsando fotones CII el int.ervalo de cnf'fgías de ~IEV. El resultado esinterpretado ('11 términos de la extensión espacial y tClllporal dI.' la fll('lItc de fotoll(,s.

PACS: 24.1O.-i; 2.1.20.-x

1 Experilllcnl. perfofnwd at tlw C;r\NIL facilily, Caen, Frallc('J'COIlt.rihll1.ion to XV Nlld('ar Physics SY"lposiulII, Oaxt.I'lwC, l\lexico, .January 1992

IIARD PilOTaN INTEN51TY INTEI<FEI<OMETI<Y IN IIEAVY ION REACTION5 185

1. INTI<ODUCTION

lIeavy iOIl collisiolls offer lhe ullique opporlullily lo form alld sludy uuclei under exlremeconditions. QlIantities likc tite spin, tite isospin, t1le tC'llIperature and lile density can bebroughl lo values eousiderahly larger lhall lhose eharaelerizing lhe nucleus al resl. Forinslallee, al illlermediale hombardiug ellergies, a domaill lhal slrelehes from 20 lo 1,000MeV /u, olle expeels lo form lIuclear matter al lemperalures and densilies lhal prevail inneulron slars or during SlIlH'rnova. explosiolls. 1'here[o[(', heavy ion rollisions also providea unique mean lo sludy sueh aslrophysieal phenomena in lhe laboralory.

1'0 observe lhese epl,emeral slales of uuclear matter lhe choice of lhe prohe is erilieal,sinee diITerenl prohes will ronvey a diITerenl huI eomplemenlary informalion. For example,flow eharaelerislies of nucleons lhal are emilled lhroughoul lhe duralion of lhe reaelionwill inform glohaly on lhe reaelioll ,Iy"amies. 011 lhe olher hand, parlicles like pholons al'light mesons, which do nol exist as real partirles insi<1e tlle lIuclcus, are created al ratherprecise instanls of Lhe rollision. ThllS, lhey provide liS with an inslanlaneous pictllrcviewing tite slale of lhe lI11ciear lIlediulII \vhpre llley clllC'rge [romo

In lhis papel', we will foeus 011 lhe productioll of phololl5 lhal have lhe parlirularad\'antage romparcd lo tll(' strongly intC'rarting Ilucleons al' 1I1(,50n5 as tltey expericllceonly lhe weak electromagnelie filial slale illle.-aelioll. Therefore lhey will lransmil anundislorled view of lhe eollisioll zone. The goal of lhe preselll work was lo measure lheexlenl of lhis zone. This is hesl done ill all illterferomelry experimelll where pholonselnitted from a chaotic SOllrcegellPrate a corn..'lation pa.ttern related to the source siz('.

Firsl, we will survey lhe experimelllal and lheorelieal kllow!e,lge of the photoll pro-duelion meclianism. \Ve will ll,en reeall lhe principies of inlensily illlerfel'Omelry and oflhe correlalion teehni'lue. Fillally, we will demonslrale lhe feasihilily of sueh a dimeullexperiment and prescnt tlle original alld promising rf'sldts \\'e havf' ohtained so faro

2. TIIE ORI(;IN or J1AIU> PIIOTONS.

Fig. I displays a lypiral speelnlm measured III a heavy ion reaelion indneed al illler-mediate energy.

Three eomponenls can 1)(' dislillguished. The lhennal pholons bui!d nI' lhe exponenlialdeereasc from lhe 10wesl measured energy nI' lo ahonl 18 MeV. They are emilled in lhelalesl phase of lhe reaelioll whell hol fragmellls eoo1 down lo lhe yrasl region. The GialltResonanre pholons raise lhe hroad l>llllll' ePlIlered al 1.5 MeV. They are emilled fromGianl ¡¡esollanee slales, mainly dipole in lIalme, excil"d in hol alld cold fragmenls. Thehard pholons make up •.,hove 25 MeV lhe high ellergy lail oflhe speelrum. As one ohservesin lhe speelrum on lhe righl halld side, pholons up to energies of 100 MeV are produeedallhongh lhe bomhardiug energy is ollly 4,1 MeV /u high.

Bremsslrahlung emitted duriug lhe wllision can be regarded as lhe mosl probahle originfor thesc high cnergy photolls. One has to cOlIsiclcrtwo kiuds of dowll. An inroherent ollercsults from the sl1pcrposition of radiation cmitted during individual nucleon~nllcl('on(NN) scatlering. Classieal elertrodynamics (see Ref. [1]) relates lhe maximnm energyradialed from a eharge dpeeleraled to resl to ils inilial veloeity. In lhe firsl case lhe

186 SCIIUTZ ET AL.

200 300E,.••• (MeV)

10030 40 50E,.""'(MeV)

2010O

~10'~

lO':J :JO O~ ~

-E 103 '"-e 103:J :Jo O() o102

Bremsstrahlung 102photons

10

101

FIGURE 1. Phot.on spertrllTll nwa ..<;ured in the rcart.ion 129Xe+I~( Al! al 44 l\leV fu. The spedrurTlon the r¡gllt hand side is a zoom juto t.lte IJigh cllergy part.

velodly is equa! lo lhe bl'am vl'loeily, vB. In lhe seeoud case it is l'qual to VH + vt.+ v~where 1J~ is the Fermi vl'locity of the two eollidillg IIueleons. Based on this argument, onecan negleet the colltribution from tllP eoherent radiation to the photon speetrum aboye 2,}MeV when the bombardillg ellergies are comparable lo the Fermi energy (Er ~ 37 !\IeV).We thell a"Sllme that all the photon, with energy larger than 2,} !\IeV origillate exclusivelyfrom lhe bremsstrahluug emitted iu illdividllal n-p eollisiolls exploiling the Fermi motiouto build up the relative vl'locily. The quadrupole radiation emitted in 1'-1' seattPfing beingsuppressed compared to the dipole radiation iu thl' n-p system can be negleetl'd loo. Themeasured properties of the l,anl photoll ('mission confírm this assumption. Thl' ve!oeityof lhe photon souree is the most couvinring olle. The radiation iu a uuclens-nueleus (AA)seatlering is emitted from a souree moving at a veloeity equal to the AA eenter of ma"svelodly. In the case of a NN scattering the source velodty is equal to the NN eenter ofmass veloeity that is also equal to half the beam vl'loeity. ExperinlPntaJly one mea"uresthe souree velorily by exploiting the DopplPf shift that affeets the photons emitted froma moving sourre. Fig. 2 displays thl' rl'sult obtainl'd fm various asymml'trie systems as afunelion of lhe beam ve!ority. lt ell'arly indirates that as eX¡lPctl'd the dala poillts followthe line Vf'; = 0.5. 1Jl,eamo

One can now construcl a model describing the I,ard pboton produetion through theNN seatteriug process. Sueh a model fírst must contain the produetion of photons inan individual NN scattcring. This c){,lIlcntaI'Y cross scetion, (T~N l should bf' taken fromexperimental va)ues obtained in frC'c p.n scattcritlg. Because of the scarcen('ss of suchmeasurements [21 theoretical vallll's ueed to be used. Tbe second ingredient of the mode!is the deseription of the beavy ion dyuamirs. This is usually donl' wilhin a frameworkbased on the Bolzmann- Ueliug- Ul,len bl'ck equatiou for one body phase spaee distribu tion,f(x,¡;,t). Referenee [3] gives a complete description of such a mode! where tbe photon

HARD PIIOTON INTENSITY INTERFEROMETRY IN IIEAVY ION REACTIONS

0.3

phOIOns

(Ey> 30 Me V)0.2

• AL/AH < 0.25~•a""-

0.1

C_-.g PIIyI, Rfrp, ,. (I"'}:N:I

00 0.1 0.2 0.3 0.4

~proj.

187

FIGURE 2. Measured photon sollrce velocity as a functioo of the beam velocity ror various asy~-metric systems ami roc piloto n energy larger lhan 30 MeV. The salid tine indicales a Bource velocltyequal to haif the beam velocity.

produetion eross section is written as follow:

d2(T' -' J. '" J dn3 ••• E~ . d

2Pr;..,(.fi),m. dE - 2". b db x ~ 4". E' dE'. dn'np eoll. "Y l' l'

x (1 - ¡(x,íiJ,t)). (1 - ¡(x,p.,t))

The produetion probability P~ee c.orresponding to the initially available energy (.fi) ina n-p collision with final momentulll P.l and i;. is sUlllmed over all pon eollisions andintegrated over the final momentum distribution. In addition, there is an integration overthe heavy ion impact parameter, b. The last factors, (1 - n, account for the final statePauli blocking. Primed quantities are ealculated in the n-p center of mass system. P~is defined as the ratio of the free production eross seetion of photons over the total n-pcross section. In the same way, one defines the in medium photon production probability.This quantity which depends only on the bombarding energy can then be compared toits experimental counterpart extracted via the following expression:

P~ -medium -

where <TR is the total reaction cross section and < N••I' >b is the number of first p.ncollisions averagecl aYer the impact paramcter alld calculated within a geometrical equalparticipant model [4]. P,7-e";••m is then defined as the probability to produce a photon ina single n.p collision takiug place in the nuclear medium during a heavy ion collision. InFig. 3 it is plotted for photons with energies larger than 30 MeV as a function of Coulomb

188 SCIIUTZ ET AL .

o:: .2Q 10.~.~::: pholonsQ .3 BUU calcula/ion" 10 (E, > 30 Me V)o:: \,l:l. Ol¡jt•.. ..•• 10 Tl:l.

.::- ---.~ ---- .5 /.~-<> 10

,.'t:l /

-<> .Q ;•.. •• ;

l:l.;

10 ¡¡ p+n ~p+n+y;

.7 e..uw"""".R.,, '.('IIrIO/JIU (E,> 30 MeV)10 O 20 40 60 80 100

(( E-V.JIA¡) [MEVIU]

FIGURE 3. Probability to emit a photon with energy larger than 30 ~leV per p-n collision as afunction of lhe bombarding energy rncaslIred in various syslems. The solid liDe is lhe result of aBUU simulation of lhe heavy ion reaction. Tite discolltinuous Jine is the probability ealculated infree n-p collisions.

corrected bombarding ellergy per nueleon. The expNimental data are rompared to theresult of the BUU ca!culation.

The data are underestimated by a factor of two, hut the caJculation reproduces suc-cessfully the overall trend. Most remarkah)e is the comparison with the free productionprobability plotted on the same figure. Although the ahsolute threshold for the productionof 30 MeY photons is 60 MeY /u, the in medium prohabiJity shows a huge enhancementbelow this energy. It is dne, as indirated carlier, to the extra energy gained' by couplingthe intrinsÍc motion of the nurleons to the relative motion of the colliding nuelei.

The mode) makes a qnalitatively satisfactory prediction so that one can make a furtherstep and question abont when and where the photons are produced. A detailed analysis ofthe caJculatioll shows that the photons originate esselltially from the initial impact areaand that only first chance collisions contrihllte significantly to their prodllction. This isdue to the Pauli hlocking that suppresses the photon production in secondary collisions.In other words, the photon production starts at the earliest stage of the collision, as soonas contact is established between the two colliding nudei, and lasts for a short period only.Its average duration is of the order of 15 fm/c, j.c., the time needed for a NN scattering.Therefore, since the source is localized accurately in space and time, photons are theperfect probe of an early phase in heavy ion rollisions, when maximum compression canbe achievcd. OUT goal is now lo mea.'iure lhe size of this SOllrre.

3. INTENSITY INTERFEROMETRYAND TAE CORRELATION TECIINIQUE.

The idea of using the correlatioll techllique to measure the size of a source emittingphotons has its antecedcllts iu a.strophysÍcs. Known as lIanbury-Brown and Twiss e!fect,

IIARD PIIOTON INTF.NSITY INTF.RFF.ROMF.TRY IN IIEAVY ION RF.ACTIONS 189

it was used in that context to measure the radius of stars [5). The method is based onthe intensity interferenee that ditrers radieally from the amplitude interferenee obscrvcdfor example with a Miehelson interfcrometcr. In the eonventional interfcrenee experimentthe souree size is dedueed from the periodicity of the fringes which result from thc super-position of the amplitude of two waves:

On the other hand, the intensity interferenee is revealed by mcasuring the correlationbetween the intensity of the two waves:

There also it can be shown that the reslllting pattern obtained by averaging e over theobservation time is characteristic of the sotITee size. 1I0wever it is neeessary that theemission is chaotie, i.e., the phase relation hetween the waves fluetuates randomly withtime. If one thinks in terms of photons, the amplitllde interferenre is the interferenee ofthe photon with itself, whereas the intensity interferenee is the interferenee between twoindependent photons. A typiral intensity interference experiment is designed as follow. Ata distant position fr01ll the souree, two detectors separated by a distance L identify thephotons and a correlator counts the events in coinciden ce. The distanee L is varied andthe eorrelation function, C, is eonstructed in the following way:

where k is the four vector (E,¡'), N12 the coincidenee rate averaged over the observationtime and N; the averaged singles rate in each detector. In the original IIanbury-Brown andTwiss experilllent photons with e'lual energy were seleeted so that C was only a funetionof L. Expressing the photon as the electrieal field of aplane wave, the correlation functioncan be developped in terllls of the Fourier transform of the normalized density distribution,p(i), of the photons source:

C(k) = I+ 1¡i(kW

¡i(k) = J d4x . e-iki . p(i)

where k = k1 - k2 is the !elative fOUT lIIomentum vector and x the fOUT vector (l,x ).The Fourier transform ¡i(k) has the followillg importallt properties. Sinee the densitydistribution p(i) is normalized to one, its Fourier transform at zero relative momentum,jitO), is e'lual to one and for non zero values of the relative momentum its absolute value

190 SCHUTZ ET AL.

'"<S 2 _ withoul pofarization

~~. with po/arizalion

1.8

1.6

11.=0.81.4 -_ --1.2 _

0.8O 2

refafive 4-momentum

FIGURE 4. Schemalic. (':orrelation fundion as a function of tite relative four momentum. Thediscontinuous Hne shows the shape when one takl'S juto account the photon polarization and amixing of incoherenl and coht"ft'ut emis.,;;ions.

is always smaller than one. Therefore the correlation funetion takes the sehematie formdisplayed in Fig. 4.

Two considerations will tend to reduce the value at the maximum of the correlationfunction. D. Neuhauser [61 has demonstrated that photons with orthogonal polarizationeannot interfere, and therefore the maximum is lowered to 1.5. Il may also be that inOUT case the photon emission is not fully ehaotie but partially eoherent. This will tend toreduce the maximum too and the A parameter is introduced to mimie this effeet. The finalform for the correlation function applieahle to our experiment can be written as follow:

- 1 - 2C(k) = 1 + 2 . A '1¡;(k)1 (1)

To extraet the souree extent it is neeessary to assume a shape for the density distribution.One usually adopts a double gaussian distribution normalized to one:

(2)

where R is the radius of the souree and 2r the duration of the photon emission. Replaeingrelation (2) in Ec¡. (1) gives the following form to the correlation function:

(3)

IIARD PHOTON INTENSITY INTERFEROMETRY IN IIEAVY ION REACTlONS 191

where p = Ipi - p"il and E = lE, - E21. If we further assume that the spatial and temporalextent are of similar size as it is expeeted in a heavy iou collision, Eq. (3) simplifies to:

C( Q) = I + ~ .A . exp -0.5 . Q2 .n2

where

n=!l=T

Relation (4) can uow be used to deduce !l amI T fl"om the experimeutal data.o

4. THE Two PHOTON INTERFF.ROMETRY EXPERIMENT.

So far we know that hard photous are the ideal probe to study the heavy ion dy-namies in its earliest phase. \Ve also kuow that the exteut of the lOne from whNe thephotons originate eau be measured usiug the conclation teehuique. 1I0wever, the rate atwhieh pbotons are produced renders this mea.surement really dillieult. ThNe are two mainreasons. First, the expeeted eross section (a, '" :¡ ¡lb) geuerates weak eouutiug rates inan enviroument where hadrous are produced abuudantly with high multiplicities. Next,witb sueh eross seetions the two photon prod uetiou will be in competition with the ".0produetion. The very short lived ".0 disintegrates inside the target and emit two correlatedphotons addiug to the hanl photon speetrum. They can be identified, but not on an eventby event basis, through their iuvariaut mass defiued as follow:

To overcome the low countiug rate aud to he able to climiuate iu the offline analysis theintense hadron background, we surrounded the target by an array of 247 llaF2 sciutilla.tors from the TAPS' multidetector and from eS! aud ORNL. The hexagonal seintillators(L=20 and 25 cm, q,= 65,52 aud 59 mm) wer" assembled iu packs of 19. The 13 paeks \Verepositioned at 42 cm from the target, aud eovered adose to continuous range in (1 between630 and 1590 and all augles iu <p. Except for ou", whirh had an active shicld, all bloeks wereproteeted against an excess;ve ha,!ron counting rate by a 2 cm thick pla.stie pIate. In arderto optimize the produetion of photons \Vescleeted the heavy system 129Xe + 197Au at 44MeY fu, as at this bombardiug energy the produetion of".O is not too abundant to hinderoue measurements. The target thickuess was 9 mg/cm2 aud the average beam iuteusitywas equal to 109 partides/s. AII events with two or more energetie partic1es d"positiug atleast 5 MeV in two distinct scilltillators w('re ac('eptcd anc1 all uet('ctors with a dcposited

'Two Arms photou Spectrometer: a GANIL-GIESSEN-GRO:,INGEN-GSI- MÜNSTf:Rcollaboration

192 SCHUTZ ET AL.

175

a 150

nO¡¡js: 125 I¡

100

\75

50

f~' ~25 \¡O

25 50 75 100 125 150 175 200

m;nv (Me Vlc)

FIGURE 5. Invarianl mass spf:'ctrum conslrllcted from 14,000 evcnts identifiecl as two photon ev('ntsand produced in the reaction 129Xe + 197Al! al 44 !\leV fu.

energy of at least 1 MeV were recorded. In the off line analysis, tbe energy calibration wasdeduced from radioactive sources and tbe energy deposiled by tbe cosmic muons (aboul40 MeV). The laller, togetber wilh a laser moniloring syslem, was used to cbeck the gainstability. Pbotons were identified in a pulse shape analysis exploiting tbe pulse sbape sen-sitivity to particles of BaF2 scinlillalors. Tbis analysis eliminates all tbe charged hadrons.The neutrons were rejected based on tbeir long time of Oight using the excellent timingproperlies of lhe BaF2 scintillalors. The cosmic muons enlering tbe detection system atthe speed c10se lo lhal of lighl deposil in adjacenl delectors an energy that depends onlyon the length of the Oigltt path inside lhe scinlillalor and mimic correlated photons wilbsmall relative momentum. Tbey were regislered in random coincidence with lhe beamcopiously enough lo genera te an important background. To reduce lhis most annoyingbackground additional condilions were re'luired in tbe idenlification of lwo photon events.To be accepled the event must contain a hadron multiplicity larger titan one, and the twophotons must be detecled in two different blocks. This treatment reduced considerably tbecosmic muons. Nevertheless tbere rema;ns approximately a 4% contamination. Tbe initia!energy and direclion of the photons were calculated by reconstructing the electromagneticsltower that develops inside the block. The invariant mass, mi" •• spectrum sbown in Fig.5 was then constructed.

Around mi"v = 125 MeV fc the spectrum exhibits a strong peak tbat is attributedto the ".0 decay. The ".0 mass (m.o=135 MeV) is nol well reprodnced because part oftbe shower induced by the pholons « E~ > = 80 MeV) is lost through both ends ofthe scintillators. This effect is c1early idenlified in Monte-CarIo simulations (GEANT) ofthe shower and bas not been taken into account for lhe calibration calculalions. To be

IIARD PilOTO N INTF.NSITY INTF.RFF.ROMF.TRY IN IIF.AVY ION RF.ACTIONS

......---------, 1'7y---oO'.,ii"".'p.,., ,-

o.••••pJ 32.00

;:. 5,.:.J 4

3

193

o 25 50 75 100 125 150 175 200

q. (M.Vle)

FIGURE 6. Correlat;on function measured in the reaction 129Xe+ 197Au at 44 MeV/u. The salidline is a gaussian fit throngh the data points ahoye 20 MeV/c.

compared to its theoretieal shape (relation 4), we have conslrueted the eorrelation funetionas a funetion of Q that is not Lorentz invariant. Therefore, the energy and momentumof the photons have heen ealenlated in the NN eenter of mass. Sinee the single rates inthe denominator of the eorrelation fnnetíon were not availahle, we used the event mixingmethod ¡nstead. It eonsists of combining photons belonging to different events, whieh aretherefore uneorrelated. The normalizatioll factor, J(, is defined so that the eorrelationfunetion is equal to one for large values of the relative momentum. In arder to ealculateJ(, one needs to remove the 11'0peak. This is done on an iterative basis by ealeulatingweíght faetors for the pairs of photons that have an invariant mass between 75 and 140MeV un ti! the correlation funetion converges to one in this region. The result is shown inFig.6.

The strong rise at very low relative momentum is essentially due to e+e- pairs produeedin the conversion of energetie photons erossing the I mm thiek iron seattering ehamberand deteeted as photons. We tentative!y attribule the smooth ganssian like deeay beyond20 MeV /e and up to 60 MeV /e to tbe expeeted interferenee effeet. The height of thecorrelation funetíon at zero relative momentum dedueed from a gaussian Jit between 20and 150 MeV /e equals 1.9:l: 0.2 amI is larger than the theoretieal value of \..5. 1I0wever itshould not be taken too strietly beeause of tbe strong e+e- contamination that masks thezero momentum value. The widlh dedueed from the same fit is equa! to (33 :l: 3) MeV /e.After unfolding tbe eorrelation funetion for the response funetion of our deteetion systemthe width is found to be consistent with the maximum overlap size of 8 fm and thetheoretiealy dedueed duration of 1:' fm/c.

Altbough we obta;n the right order of magnitude, we must question if we really have

194 SCHUTZ ET AL.

0.51¡~lljl;m

OliT I

25 50 75 100 125 150 175 200q. (M.Vle)

4~,::!,.:. 3.5~~u-

3

2.5 ¡2

1.5

1.178Pl 1.002P2 0.7.'4P3 46.J~

FIGURE i. Simulated correlation fUllrtion induding uncorre1ated photons 1('0,e+ e- pairs andcosmic muons. Tite salid line is a gallssian fit through the dat.a points aboye 20 MeV fe.

measured the photon source extent. The answer is not straightforward at all. We have notyet demonstrated that the ooserved rise in lhe correlation function is due to an interferenceeffect and therefore is sensitive to the sOllfce size. This can be checked by measuring thecorrelation function for two systems with very differeut sizes. The next open question isto know to what extents the cosmic muous ¡ufluence the correlation. It has already beenmentioned that they are detected as photon pairs with small relative momentum and thatthe analysis isunable to eliminate all of th,'m. A Monte-Cario simulation was performedto analyze the effects of this contaminations. Photons, ".0 and conversion e+e- weregenerated. The included photons were uncorrelated. The cosmic muons were entered withtheir energy spectrum and angnlar distribution. The response function of the detectionsystem was simulated using GEANT. From this set of events the correlation function wasconstructed in the same way as the experimental one. The result is shown in Fig. 7.

The simulation reproduces the strong rise helow 15 fm/c that is due to the conversione+e-. The structure ooserved hetween 20 and 80 fm/c comes entirely from the cosmicmuons. It appears in the same re¡!;ionwhere one expects lhe interference pattern. Althoughthere is no mean in this experiment to distinguish in the data the enhancement originatingfrom the cosmic muons from the one due to the photon interference, the simulationsindicate a net shape dilference. \Ve take this difference as a strong hint that the measuredcorrelation function indeed shows the intensity interfereuce elfect expected for a chaoticemission of photons. lf true, it would be the first time in nuclear physics that the so caBedIIBT effect has been observed allll it would become possihle to measure tbe spatia! andtemporal extent of the collision zone in a heavy ion reaction right after tbe impacto

IIARO PilOTO N INTENSITY INTERFEROMETRY IN I1EAVY ION REACTIONS 195

5. CONCLUSION.

lIard photous produced iu heavy ion reactions are believed to be a probe of the earlyphase of the collisiou. They are produced during a limited time aud do not suITer fromabsorptiou inside the nuclear medium. They therefore convey au undistorted picture ofthe phase iu the collision where high deusities may be reached. To measure the extentof the photon source, we used the fact that the intensity interfereuce pattern rellects thespatial and temporal dimeusions of the source. \Ve have showu the feasibility of such auexperiment owiug to the excellent performauces of the TAPS multidetector. The measuredrise in the correlatiou function towards low relative momeutum is teutatively attributedto the iuterfereuce eITect. To be conclusive, more experiments are ueeded. First we ncedto improve the reduction of the various uoises that partially mask the eITect. A new setupof TA PS is meant iu that way. Then we will have to investigate how the width of thecorre1atiou functiou depends ou the size of the system.

\Ve wish to thank our collcagues at the Oak Ridge Natioual Laboratory, .I.R. Beene,F.E. Dertrand, M.L. I1alhert, D. Horen, D. Olive and R. Varner for making their DaF2

detectors available to us and for their help iu the setup of the experimento \Ve thauk themembers of the technical staIT of the Grand Accélérateur Natioual d'lous Lourds (GANIL)for their help and efficient delivery of the high quality beam required for our mesuremeuts.

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