MAXIMAL BREAKUP IN HEAVY ION REACTIONS FROM 10A TO 2100A MeV

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MAXIMAL BREAKUP IN HEAVY ION REACTIONS FROM 10A TO 2100A MeV

B. Jakobsson, G. Jönsson, L. Karlsson, B. Norén, K. Söderström, E.

Monnand, F. Schussler

To cite this version:

B. Jakobsson, G. Jönsson, L. Karlsson, B. Norén, K. Söderström, et al.. MAXIMAL BREAKUP IN

HEAVY ION REACTIONS FROM 10A TO 2100A MeV. Journal de Physique Colloques, 1986, 47

(C4), pp.C4-369-C4-371. �10.1051/jphyscol:1986441�. �jpa-00225807�

JOURNAL DE PHYSIQUE

Colloque C4, supplBment au no 8, Tome 47, aoGt 1986

MAXIMAL BREAKUP IN HEAVY ION REACTIONS FROM 10A TO 2100A MeV

B. JAKOBSSON, G. J ~ N S S O N , L. KARLSSON, B. NORBN, K. S ~ D E R S T R ~ M , E. MONNAND' and F. SCHUSSLER'

Divivion _{o f} Cosmic and Subatomic Physics, University of Lund, SBlvegatan 14, S-223 62 Lund, Sweden

'centre d'atudes Nucleaires de Grenoble, 53 Avenue des Martyrs, F-38041 Grenoble Cedex, France

Abstract - Event-&-event studies of 160 + Ag and "Kr + Ag collisions w i t h maxirrdl mltiplicitieshavebeen performed i n nuclear emlsions. For 160 + Ag wi? observe a s n ~ ~ ~ t h increase in the mxirral m l t i p l i c i t y from 10A to 2100A MeV except possibly in the region 130A - 180A MeV. C c q l e t e breakup into singly charged particles is mt reachedbelar 2.1A GeV in the aqntm?tric 160 + Aq case whereas it is reached a t about 1A GeV i n the f a i r l y symetric 84Kr + 0 7 ~ g sy-•

The evolution of collisions betwen extended objects, like heavy nuclei, is strongly governed by pure g-trial. variables, such as the sizes and the impact parameter.

Therefore one needs an inpact paramter trigger to obtain real selective experinmtal information. Reactions a t very large impact paran&ers always carry the signature of weakly excited projectile-like and taxget-like nuclei whereas it is m r e difficult tn find unique parmters for the mst central mllisions. A mmpound nucleus w i t h m x i m a l mxmmtum and energy transfer is of course a relevant signature a t l o w m u g h energies whereas a t high energies sinple parmters like maximal multiplicity and absence of projectile-like and target-like fragmnts are probably mst reliable.

This report deals with an at- to investigate the mst central heavy ion collisions by mans of a Low-statistics visual conp?lete 4r detector for charged particles - ^nuclear emzlsions.

A direct horisontal exposure i n vacuum of a mdim sensitive emlsion stack was perforrred with the 94A &V 160 beam a t GANIZ. The d l total range d i s p e r s i o n ( 3 8 ~ ) and the rathex high possible integrated beam flux due to the low sensitivity allowed

us to collect collisions with energies &wn to about 10A &V. I n EEW&?C q s u r e s of electron sensitive enulsion stacks in air W have utilized 160 beams of 23aA &V and 2.1A CeV as ell as a ' ' K r beam of 1 .4A GeV. The energy regions where collisions have been c o l l e t e d are thus 10A - ^{220A MeV +} 2.1A GeV for 160 and 100A - ^{1400A MeV}

for "~r.

In the scanning(110dly along-the track) vie registrate all collisions whereas only those w i t h a total charged particle multiplicity, Q > 4 for 160 arad > 10 for

8 4 ( s t a t i s t i c s 5000 ~ and 500 collisions) are selected _for further msurements.

A f t e r target determination only high multiplicity collisions w i t h m or Ag targets rerrain for charge (2) detennination by m of range and various dE/& mzasmmmts.

Nonnally W use gap counting for a f i r s t selection of high energy Z=1 particles and for separation be- Z=1, 2 and 3 for short range particles / l / . High energy

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986441

JOURNAL DE PHYSIQUE

projectile-like fragmnts are identified by semi-autcwatic fixed slit photcmtric liqht absorption measurements whereas for stopping heavy fra~pnents W= nreasure the absorption profile with a m i n g slit photomter /2/. Both these systems have been provided w i t h IBM PC conputeps for data storing and direct charge estirmtion.

Since v e w i l l particularly discuss results on the maximdl m l t i p l i c i t y collisions we show a s an example such ones for 60 + 7 ~ g a t energies below 1% , 35A and 75A MeV in Fig. 1.

Fig. 1 - The ^{1 6 0} + l 0 ' ~ g event with the highest charged particle d t i p l i c i t y for energies klow 15A, 3% and 75A MeV. The low energy event has 9m heavy (A between 100 and 120) conpound residual track (6-0.02) .

I11 - RESULTS

The m h l charged particle nultiplicity, -%h, in 60 + Ag collisions a t various energies has been es-ted £ram the 10- level of polinomial f i t s to the exprirental distribution functions F{>,%~). The pints i n Fig.2 show these results h e r e a s the crosses give the Ikh value for the largest observed went. In general v e observe a mmth increase of lbh,, w i t h increasing energy w i t h a possible irregularity in the region 130A - ^{130A W .} In f a c t here W also observe events w i t h a larger

M& than given by the F =. 1 0 - ~ level but a verification of the significance of this

irregularity has to be perform& by a background MonteCarlo simulation.

Tne w:hl multiplicity event below 15A * V . - left) has the clear signature of a fusion reaction and the M,- level should be canfronted w i t h a ccmplete ccgllpwnd nucleus evaporation calculation. A t 3% l@V the largest event is not fusion- like but rather confirms the onset of a multifracpe.ntation process. ~ e v e r a E f these processes &ich are discussed a t this conference /3/ w i l l be confronted w i t h the f u r t l ~ e r increase of M&,=. The c q l e t e brekup into Z = 1 particles is never reached, a t l e a s t not below 2.1A GeV thich either indicates that the clean-cut participant-spectator is valid for very central collisions o r a t least t h a t the c q l e t e energy equilibration is never obtained.

To discuss t h i s point further W look a t Fig. 3 which gives the parameter

for the very symetric "Kr + l o 7 ~ g collision. Clearly we reach i n t h i s case the

q l e t e breakup into Z = 1 particles but anly a t about 1A GeV, i.e. very f a r above

the boiling l i m i t where the t o t a l excitation energy per nucleon exceeds the

bincling e n e q y per nucleon (3% t l e ~ ) .

60

50

I l l l

10 102 103

PROJECTILE ENERGY I MeVIA 1

Fig. 2 - The maximal charyed p a r t i c l e rmltiplicity versus the bakarding e n q . The p i n t s shm t h e r e s u l t s of the distribution function methafitsee text> and the crosses show the mind &served event. The pion d t i p l i c i t y is calculated from the assumption of a Poisson distribution with empirical /4/ averagenforn -

m l t i p l i c i t i e s .

REFERENCES

loo

80

X 4

5 6 0 I

/ 1 / Jakobsson ,B. , Jonsson,G. , Lindkvist ,B. , Oskaxsson,A. and Soderstrom,K. ,

2. Physik (1977) 293.

/2 / Jacobsson,L., Dissertation, Univ. of Lund (1977) .

/3 / Aichelin, J. , Bondorf , J., Campi, X., Knoll, J., Contributions t o t h i s conference, /4/ Fun9,S.Y. e t al., Phys. Rev. Lett. 40 (1976) .

- ZKr+ZAg+Nat(95%) x A---'-

x x

-

x X X

- 36Kr

C ~ A S

8 0

~ 7 ~ 9

0 0.5 10 1.5 2.0

PROJECTILE ENERGY ( GeVIA 1

Fig. 3 - ^{The xaiml charged} p a r t i c l e multiplicity in Kr+ Ag ^(X) and in 0 + Ag( m )

events.