Directions of heavy ion research at low and intermediate energies

Nuclear Physics A53R (1992) 17c-36c North-Holland, Amsterdam

Directions of Heavy Ion Research at Low and Intermediate Energies P. Kienle Gesellschaft für Schwerionenforschtrrng (GSI), Postfach 11 05 52 D-6100 Darmstadt 11, Fed. Rep. Germany Abstract

Recent developments and advances in heavy ion research at low and intermediate energies are reviewed . Emphasis is layed on the progress of accelerator technology, especially in producing high phase space density heavy ion beams in cooler-storage rings. A report is presented on some selected topics of current interest in nuclear structure research such as the production of wiper-heavy e!ements, the excitation of multi-phonon vibrational states and giant-dipole resonances . Nuclear dynamics studies from the Coulomb barrier to relativistic energies will be addressed including the production of dense heated and excited nuclear matter and the study of the properties of hadrons in such a medium . Finally, some atomic physics experiments with heavy ions will be presented with emphasis on quasi-atomic e"e - pair production . 1 . Introduction The progress of heavy ion research . in particular at intermediate up to relativistic energies, is closely related to the development of accelerators and experimental facilities for heavier ions, higher intensities, and higher energies . In this respect two avenues have been persued . On the one hand existing proton accelerators were upgraded to include heavy ion acceleration capabilities e.g . Bevatron (Berkeley), AGS (Brookhaven), PS-SPS (CERN)), on the other hand many dedicated facilities have been built . With one of the latter, the UNILAC of the GSI Darmstadt, all elements rip to uranium were accelerated to energies above the Coulomb-barrier already during the second half of the seventies. It was recently converted to an injector for the newly built heavy ion synchrotron (SIS) and the storage-cooler ring (ESR). With the new facility heavy-ion and also radioactive beams with the highest possible phase space density and tap to energies of 1-2 GeV/u will be attainable . In the following we will first make some remarks on directions in heavy ion acceleration and especially in phase space cooling . Then we turn to advances in nuclear structure research, including topics like the production of neutron rich and deficient heavy fragments and the synthesis of super-heavy elements . This will be followed by recent investigations on the excitation of mialUphonon vibrational slates and giant-dipole-resonances . 0375-9474/92/$05 .00 © 1992 - Elsevier Science Publishers B .V. All rights reserved .

P. Kiewle /

IQ

Thon meVmded topics from oudomr

ir ectio n mf hea @/ On research

dyn a vnd ca Ynzrn the Coulomb barrier 8o relativistic en-

We will conclude with some interesting directions in atomic heavy ions including e'(,- pair production and various other atomic

er gi ps will be prosonted .

physics e todies using

of high Z-atoms . prop eH&s and processes in strong Coulomb-fields

celemti

SS Aa am Mo du c \ion into

the impressive sce nark of avaUab!e heavy ion beams Fig . I

shows the atomic number c h a r ade ri M !os of some typical heavy ion facilities. Linacs and whereas above this energy cyclotrons dominate the energy regime b p U ovv 100 s ync!`r Ar on facilities deliver beams up to 200 G eV9u ( SPS-CER N) .

100

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~

~

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Energy and ionspecies characteristics of sonne heavy ion accel-

GA NK.

sn

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20

40

60

z

_~-/

oo !

!

For the study of nuc h ua- nucleus collisions at energies still higher than 200 GeV/u

coMid ers are needed .

A doub>e ring A ruc tuno using ouperoonduoöng magnets (Brn =

3 .8 T) will be constructed e1 B ML in the 3 .8 l(rn circumference existing tunnel to reach

/n a xinnurn energies of 100 G eVYper beam ( R H!C) .

The upgraded AGS will be used as

injector . T he most diffic u!1 VoblennxvUhcoUiderois toechieveodequate!unninoo itieo .The luminosity design aim, for RH I C is L = 2x18"' cm -' s --' for Au'Au collision . At the pro-

P. Kienle / Directions of heavy `on research

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posed LHC collider at CERN collisions with 3.5 TeV/u on 3.5 TeV/u at luminosities of 102' cm -2 s - ' may be possible . At KEK a PS heavy ion collider with beams of 4-7 GeV/u and luminosities of 1025 cm - ' s' is discussed. GSI began a conceptual study of a collider with energies up to 50 GeV/u per beam which will take advantage of beam cooling and make use of an injector chain optimized for heavy ions (UNILAC, SIS/ESR) to achieve as high as possible luminosities .

Large progress has also been made in the field of ion sources and injector technology. At GSI this will allow to modify the UNILAC injection in such a way that we can run a truly independent low energy program with a free choice of ion species and energy in parallel to a low duty-factor (1 %) high current injection cycle into the synchrotron SIS.

Fig. 2:

General layout of the heavy-ion facility at GSI which consists of the linear accelerator UNILAC (2-20 MeV/u), the heavy ion synchrotron SIS (1-2 GeV) and the storage cooler ring ESR with half the magnetic bending power of SIS. The UNILAC can be used in parallel for a low energy program and as injector for SIS with an independent choice of energy and ion species (in fall 1991).

For the low energy program an independent injector has been constructed. This injector

consists of a 14 .5 GHz ECR-source, a RFQ linac for energies up to 300 keV/u, followed by an interdigital line structure up to the injection energies of the Alvarez section (1 .4 NIeV/u) of the UNILAC . The radio frequency structures will be operated at 108 MHz and a dutyfactor of 50 % . The U28+ -current is specified as 5 /A which is more than one order of

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P . Kienle I Directions of heavy iota research

magnitude larger than the one presently available . It also has an improved microstructure delivering a pulse every 9 ns, which is favourable- for coincidence experiments with fast detectors . In addition it can be debunched, thus achieving do beams, if desired . For SIS-injection several methods are studied (high current low charge state source, rf- or magnetically pulsed ECR-source} to provide currents which are 100-1000 times higher than available at present . Fig . 2 shows the layout of the new heavy ion acceleration facility of GSI with the UNILAC as an injector into the medium energy heavy ion synchrotron SIS (1-2 GeV/u) which is combined with the storage ring ESR equipped with a powerful electron cooling device /1/. In the ESR a "cool" high current density electron beam of well defined velocity is merged with the circulating ion beam of larger spread but the same average velocity over a distanco of two meters . Mott scaltering provides the cooling mechanism for the ions which repeatedly traverse the continuously renewed electron beam . Fig . 3 shows the result from a recent cooling experiment for Kr"' of 150 MeV/u energy using a 60 keV electron bear'" of ^- 1A current . The Schottky noiso frequency spectrum of the 40th harmonic of the revolution frequency reveals a width of 35 Hz at 56 MHz which using Afff = al fAp/p (with >> = if-1 ? - 1/7,' where y = (i - IS,) "z and -It = 2.6 is a machine parameter characteristic of the ESR) corresponds to a longitudinal momentum spread of ^- 10 --6 . This is the lowest value of Ap/p reached so far for heavy ions in the ESR cooler ring . In addition very small radial emittances of less than 0.1 7rmin mrad could be achieved for the cool teams. -SE 29Srtir x-3=, .782[7'!8 o .- . + ,,o ~A°2B .~H56p YA°2 pOVaEp oE C2 T-~-3s~A~çl OXO v1~

Fig . 3:

~®~_n

- r-oo .00Y2

Longitudinal Schottky-noise frequency . .peclra of a cooled (150 IVIeVIu) Kr °6+ coasting bear'" at 40th harmonic of the revolution frequency. The measured frectuericy width -uorresponds to a moarnenturn spread of Ap/p -- 10-6_

The Schottky noise frequency spectrum displayed in Fig_ 4 shows the simultaneous storage of two charç;e states, Kr- and Kr- , in the ESR . Because of the larger magnetic rigidity, the Kr- ions revolve on an outer orbit, which is reflected by the lower frequency_

P. Kiente / Direcriorzs of izeauy ion researeti

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The intensities of Kris" and Kr 36} are in stationary equilibrium which is the consequence of two compensating processes the radioactive electron capture (REC) producing Kr"' ions in the cooler section and the ionization of Kr" -` retie to the residcia : gas Molecules in the storage ring .

0 CS

In 4 -1

Fig . 4 .

!C P

i

RM :

Longitudinal Schottky noise frequency spectra of coasting Kr` and Kr"' beams (150 McVtu), simultaneously stored in the ESR (see text) .

Kris-'- ions In Fig. 5 the longitudinal momentum spread of PIPctron-cooled (Eß ~ = 160 McWij) is displayed versus the number of stored beam particles . For low intensities (N -~ 101 ) one expects [lie beam particles to forin a string with an average inter-particle distance d ~ I/N where I ~ 1013 rn is the circrirt~feretu:e of the storage ring . In ttie experiment shown in Fig . 3 conditions were reached in which the ptasn-ta parameter (Zze?lci)/kT (where VP'/d is equal to the Coulomb-energies of ions at tti" Wigner Seitz radius a and kT = (Apfp) z EF , is the longitudinal b¬-,arn temperature) comes out to be of the order of 1 which is consistent with the condition of string order .

r

The hiUliest please space density is reached when troll) in longitudinal and transverse direction the beam makes a phase transition to a condensed and finally crystalline please as predicted by molecular dynamics calculations, seine results of which are shown in Fig . 6. When the plasma parameter assumes values of about 140, the beam particles order on concentric shells with helically ordered particles on the shell surfaces I2i . Also cooled radioactive beams of energies up to 560 McVfu will soon be available at

GS3 . They will be produced by projectile fragmentation of SIS-beariis and separated with a special fragment separator (FRS) using magnetic deflection combined with energy loss deters-_ination, The radioactive beams will be accumulated and cooled in the ESR_

ienle / Directions of heavy ion research

10-1

-S-qu 11 Kr36-

Fig . 5: Longitudinal momentum spread of cooled Kr36+ ions (160 beam intensity.

-L25

0 z

0.25

z

eV/u) versus

Fig.6: Crystalline beam structure obtained in molecular dynamics calculations (Ref. 2).

0.5 -1 .0

-0.5 0 0.5 X("BEAM" AXIS)

1 .0

ii far off s

One goal of present day heavy ion physics is to synthesize and study nuclei far off stability with unusual proton to neutron ratios up to the limits of instability against various particle emission processes like proton-, neutron-, a-decay and fission . Besides tire 263 stable nuclei only about 2200 of the potentially existing 6000 radioactive nuclei could be synthesized until now. ®n the neutron deficient side fusion-reactions the proton dripline p -- 0) has been reached in a few cases, using heavy ion induced fusion reactions with

P. ICienle / Directions of heavy ion research

c

the observation of proton radioactivity. Future progress may be achieved using neutron deficient projectiles for the synthesis by fusion gained by projectile fragmentation at higher energies. On the neutron rich side much progress has been made recently especially for light nuclei using projectile fragmentation in the 100 MeV energy range as synthesis reaction. It is now possible to study neutron rich light nuclei at the neutron dripline. These nuclei have very loosely bound neutrons, which may form a low density halo with properties coming close to neutron matter. In the future more work will be done at GSI to study heavier neutron rich nuclei produced by fragmentation reactions using heavy beams. '79Au +j3Al at 1000 eV/u '77 r

100

'7®Pt

' ;9 Au

50 Fig . 7: Two dimensional spectrum for the projectile framents of 1 GeV/u Au bombarding an AI-target (see text) .

c

-100

'86 Ir -100

' 7éPt

`79AU

-50

0

50

100

Position at Final Plane (mm)

Fig . 7 shows results from a fragmentation experiment in which a 1 GeV/u 'e' Au beam was bombarded on an AI-target /3/ . The fragments are separated using a large separator consisting of two magnetic analysis stages with an energy loss degrader in between . In Fig . 7 a two dimensional specfru m of the Au fragments is disniaved, where the positions of the isotopes at the final focal plane of the separator are drawn versus their mid-plane positions . The mid-plane positions scale with the magnetic rigidity of the fragments beoaind the target, thus reflecting an A/, separation . The positions at the final plane are the result of the energy loss measurement followed by a second magnetic analysis which for a given A/Z allows a Z separation . A further active field of heavy ion physics is the synthesis of very heavy elements. The most heavy ones known until now, the elements 107, 108 and 109 were synthesized using cold fusion reactions bombarding "'Pb and 2 ° 9 8i-targets with beams of 10P, 54 Cr and 18Fe at bombarding energies close to the Coulomb-barrier /4/ . Fig . 8 shows the production cross sections, which drop exponentially with increasing Z. From binding energy measurements deduced from a-decay energies, fission barriers of 6-7 MeV height were discovered in these heavy elements (Fig. 9), although the liquid drop barrier has gradually

P. Kienle / Directions of heavy ion research disappeared . This is the outstanding discovery of this field, that shell effects lead to fission barriers sufficiently high to make these nuclei relatively stable against fission ; the main dti~cay mode is a-decay . With these new mass data on high Z nuclei, one can ex-

trapolate to still heavier elements, which are predicted to be even more stable .

In 6

10°~

t

2

t

1

104

106

9

109

Proton Number

90

110

Fig. 8: Cross sections for the production of the heaviest elements. Circles, cold fusion (1n channel, Dubna); asterisks: actinide based reactions (4n channels); squares: results from SI (Ref. 4) .

Fig . 9:

95

100

105

Element Number

110

Energy fission barriers for the doubly even isotopes with N-Z --- 48 (solid line) compared with shell corrected and liquid drop barriers .

ASE

S

i._

t

Target

v

Detectors

Fig . 10: Sketch of the velocity filter SHIP and the post separator NASE constructed to seperate fusion residues of element 110 and 111 (Ref. 4). At GSI an attempt is in progress to synthesize the elements Z =110 and 111 using cold fusion reactions of "Ni with ao'pb and z°9Bi targets, for which cross sections in the pbarn

range are predicted. Fig . 10 shows the experimental set-up consisting of the velocity filter

P. Kicn1e / Directions ofheavy ion research

25c

SHIP and a post-separator NASE to isolate the fusion residues which are implanted in position sensitive silicon detectors for measuring their decay chains. The a decay of the isotope (269/110) will feed a decay chain observed in earlier experiments which will allow identification by delayed coincidences to the known a-decays of the daughter nuclei . 4. Collective Excitations The investigation of collective properties of nucleons in nuclei forms another main field in nuclear research . For many years the doubly-magic nucleus 21'Pb has provided a crucial testing-ground for theories on the nuclear many body problem . In this connection the properties of the 2.615 MeV, J' = 3 - first excited state has played a vital role in particular, the static electric quadrupole moment Q(3 - ) and reduced transition probability B(E3,0+--" 3-) . From the collective enhancement of these values, the first excited state has been interpreted as one-phonon vibration of octupole character. 4*,6* - 5.684 MeV O' .2',4',6' ------------ ---- 5 .230 MeV 2'

(Et)

5- -

E2

3-

3.198 MeV

N C O u

2.615 MeV

E3 0'

8

500

0 MeV

Particle Yf coincidences obtained for the system 208 Pb + "'Pb at 6.2 MeV/u . A gate was set on the 2 .485 MeV transition (Ref. 5) .

Fig . 12:

1000

1500

2000

energy (keV)

2500

3000

Level scheme of 2°. Pb showing the dominant decay channels for the "'Pb + 208 Pb system (6.2 McV/U) . The dashed line corresponds to the harmonic vibrator limit of the two-phonon octupole states.

This interpretation of the 3 - levels as a collective state implies the existence of multiphonon states which remained unobserved so far. The search for such excitation is

therefore of great interest as its observation would provide a direct check on the degree of unharmonicity of the associated vibrational motion . Figs. 11 and 12 show results from

recent in-beam-7-spectroscopy measurements for 211APb + "'Pb collisions at 6.2 MeV/u, which reveal a new level with I 7r = 4+ or 6 + at the energy 5.683 MeV /5/ . An analysis of the Coulomb excitation cross sections resulted in a dynamical deformation parameter ß3 which agrees with the value extracted for the collective 3 - state . Thus it is very likely that a member of the long searched two-phonon octupol vibrational state has been located .

Further support for this interpretation is given by the excitation energy being roughly twice the energy of the 3 - state and by the decay path and the scattering angle depend-

P. Kiem le / Directioms of heavy ion researcit

26e

gnc e of the inelastic cross sections, which comes out to be consistent with the spin as+ signment 4" or 8 . L ARD

AADIN

Y. Fig. 13.

Experimental set-up for studying the Coulomb excitation of the giant dipole resonance in "'Xe using the mm og ne tic spectrometer ALADIN and the neutron detector LAND ( Ref .8) .

'to I~ 80 . , 40 rm

20

om

'0

kinetic energy ot neutron

so x eV

Fig. 14« 136 Excitation spectrum of Xe (bottom) constructed from an event by event analysis of the neutron energy (top) and the deexcitation -i-rays (middle) .

~

~ me so »m xm ~ ~ °

oo

20

wv

gamma sum enemy

yo Mem

4m om

0

0

nm

0

om

wm

excitation energy

*o

MeV

!n another oxpehrn ent et GS! the Coulomb excitation of giant dipole resonances via peripheral heavy ion coHisWns has been investigated /6/. This experiment was carried

out using a "'Xe beam of 700 MeV/u energy and carbon or lead targets. The experimental set-up is shown in Fig. 13 . The impact parameter was classified on the basis of charged pa oic(e nnu R ip!ici1y

The heavy residues

measured by a multiplicity detector built around the target .

analyzed with the dipol magnet ALADIN . Deexcita t iony'rayo were e observed in 4 8B aF, detectors placed close to the target . A large were

magnetically

area neutron detector (LAND) was placed 10 m downstream from the target at 0" to measure the neutron energies by time of flight techniques . Fig. 14 displays preliminary

P. Kienle / Directions of heavy ion research

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data for the neutron energy (top) and y deexcitation spectra (middle) which have been measured in coincidence with the Xe fragments /6/ . Using an event by event analysis the excitation spectrum of "6Xe has been reconstructed (bottom) which indeed seems to be dominated by the coulomb excitation of the giLnt dipole resonance at E' ^_- 15 MeV and which shows indications of further contributions at higher energies. 5. Nucleai Dynamics

Heavy ion beams with kinetic energies ranging from the Coulomb- barrier up to 200 GeV/u allow to study various dynamic processes in nucleus-nucleus collisions which occur when nuclei interact and penetrate, ranging from slow to fast collisions with the Fermi velocity of a nucleon (E F ^- 30 MeV) setting an useful scale. The dissipation and transport of energy, angular momentum and mass may be studied in various phenomena, which occur if one changes bombarding energy and impact parameter, the most relevant kinematic variables . In slow collisions the interacting nucleons have time to arrange themselves in the mean field of tire collision system, which heats up, rotates and vibrates violently. In fast collisions, nuclear matter may become compressed and heated, the nucleons may become excited and new hadrons may be created. The compressed matter expands again leading to nuclear fragmentation . The dynamics of these fast collisions are governed by the equation of state of nuclear matter at high density and temperature as well as by relaxation times and transport properties of nuclear matter under very extreme conditions . These collisions may also become a unique tool to study a fundamental property of the QCD-vacuum, namely qq condensation by restoration of the chiral symmetry at high densities and/or high temperatures . In the following we will sketch some interesting directions of nuclear dynamic studies ir. the low, medium and high energy regime . At energies around the Coulomb-barrier various effects in fusion reactions are currently studied . At bombarding energies below the Coulomb-barrier fusion cross sections which are orders of magnitude larger than expected, using a simple potential tunnelling model, were found in various systems . This enhancement is most likely caused by large amplitude shape vibrations and transfers of nucleons by quantum tunnelling in the approach phase of both nuclei . For high Z-systems a severe limitation of the fusionability of heavy nuclei has been observed . The heaviest symmetric system for which fusion is still observed is Pd-Pd, which exhibits a large fusion hindrance (Fig . 15) /7/. The low absolute cross sections measured for ""Pd + "°Pd as compared to '°°Mo +'°°Mo and '°°Mo + "°Pb result from combined effects of increased fusion hindrance and fission competition . In a series of recent experiments performed at GSI the evaporation residue distribution has been investigated and compared to predictions of the statistical model (Fig . 16) /7/. While the experimental and the calculated distributions agree well for the lightest system, there is a drastically increasing discrepancy for the heavier systems . The evaporation of

J» . Kl e m le / Directions of heavy ion research

chuged pa rUWes im

much

stronger than expected .

The results are consistent with tile

idea that light charged particles may be emitted already during the dynamic evolution

towards fusion . This change in the nuclear composition would have mn impact on the subsequent dynamic evolution . In particular, the repulsive electrostatic force would be

reduced by the emission of charged particles . As a consequence, the extra-push would also be reduced, and those reactions would lead to fusion with a higher probability . The results clearly show the !nn por\ a n ce of the interplay between dynamical evolution and particle evaporation !n the fusion of very massive systems .

1-1 -~^ -- MO m - ~" . m- - ., _ ~m -^ m "^

ju

T z~

~M

10 ~

. .

~.~

nvp d ~n : p d

1

~

j O'z L 10 -3

1

ncpm~mo M n

L

Complete fusion cross sections for the ' m' yWo, heavy syn i me t r! "'Mo f /vv 1VI o f "'Pd and "'> Pd- "'Pd . Note the strong h i drance of fusion for the ' m Pd` m pdreacbon .

Fig . 1[i

~

10 `-

-a no

~

~

/ ~

Fig . 16 :

--4-

e4

86

8e 8e 88 Proton number

90

92

:j

Experimental and theoretical distribution of the evaporation residues for the system ' «» Mo f ' m pdand mv pd + ovpd .

Another fie!d of interest is the dynarnioa) bahav i or of d/nuo)eor systems /n d/oaip a dme reactions, which have been investigated via the spectroscopy of the emitted 8-rays . It has been shown that the 6- ray spectrum is a measure of the ratio of the relative velocity R(t) of the collision partners and their relative distance R(t) which are both time dependent . At G SY 8-ray spectra have been measured in the energy range O .2 M eV 0 Q - < 3 .5 M eV in coincidence with elastic end deep inelastic collisions . Fig . 17 shows an example for the system U + Po at 6 .1 M eV/u where an unexpected high 8-electron yield for quasi elastic grazing c o M sionn has been observed /8/ .

This result can be qualitatively explained by a

distortion of the RuUhi e r Aord' tr ainr t ory resulting horn the strong interaction of tile colliding

P. Kienle / Directions of heavy ion research

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nuclei . This interpretation implies the assumption of a nuclear potential with a diffuse surface, which has also been suggested by data on the elastic scattering of "O + `® /8/ .

10 -2 10-3

Fb

U+Pd 6 .1 MeV/u~ binary collisions

1 . 10 -5

10

-a

Fit with standard Scaling-Law ' " additional dynamic

E e_

Fig . 17 :

[MêV]

8-electron spectrum in coincidence with binary scattering processes of 238U on ' °"N in the grazing region ( =4.6 fin). The calculated spectra correspond to the R/R deppndonce for a pure Coulonéb trajectory (dashed line) and a fitted trajer-tory (full line) incl~rding a distortion of the Coulomb traioutory by the strong interaction of the colliding nuclei (Ref . 8) .

Fig . 18 :

Mean intermediate mass fragment multiplicity plotted versus the calculated deposited energy/nucleon . The squares, circles and stars represent collisions on the C, AI, and Cu targets respectively . Each point within a target group corresponds to peripheral, mid-central and central collisions with the deposited energy increasing (Ref . 9) .

with

centrality

At medium energy the formation of hot nuclei has been studied intensively . It is expected that if the heat supplied to the nucleus exceeds the binding energy of the nucleons, it will go into the formation of internal surfaces and the nucleus would blow up in several intermediate fragments . There are controversial experimental results on this process, but very recent experiments at the GSI using inverse Idnernatic reactions of 600 MeV/u Au with C, AI, and Cu targets, strongly indicate a correlation between intermediate fragment multiplicity and the energy deposited in the Au remnant as deduced from transport model calculations for various targets and impact parameters (Fig . 18) . The results reveal a sharp rise of intermediate fragment formation at deposited energies around 8 MeV/u /9/ . Central heavy ion collisions studied at the Bevalac in the bornbarding energy range from 200 - 2000 MeV/u with exclusive, high statistics 47r-particle detection, have shown that thermalized, compressed and heated nuclear matter is produced . One goal is to learn about the equation of state W (n, T) and dynamical properties of nuclear matter from reaction observables like the rapidity distribution and especially from quantities which

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P. Kienle !

irections of heavy ion research

MM

Fig . 19: Schematic lay-out of the 47t detector at GSI Darmstadt . It consists of a central drift chamber (A) and a helitron drift chamber (B) housed in a large superconducting coil, a forward wall with ionisation chambers (C) and plastic scintillators (D), and a forward spectrometer (E) with position sensitive multiple sampling ionization chambers (F) and a time-of-flight wall (G) . measure the collective flow of compressed matter in the expansion phase. These experiments are taken up again at GSI with the aim to measure the complete momentum flow

of all parti-les produced in such collision . A 47r-detector shown in Fig . 19 was constructed which allows to measure the momentum flux of all identified charged particles in a large dynamical range /10/. First experiments provided a complete momentum flow measurement of charged particles and clusters (Z > 2) for Au-Au collisions at incident energies PM5, R1

015 L

0 .25

!

Z=2

U

E I--,

0 0 21 -

0

0

Z=3

Fig . 20: Distributions in the transverse momentum per nucleon versus rapidity plane for clusters with Z =1,2,3,4 as measured for the most central collisions of Au + Au at 170 MeV/u (Ref. 10).

P. Kfenle / Directions of heavy 1on research

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between 100 and 400 MeV/u . Fig . 20 shows double differential cross sections for the production of particles and clusters with Z =1-4 as function of the transverse velocities and relative rapidities (y/yp) for a measurement at 170 eV/u bombarding energy /10/. For a selection of most central collisions, events with charge particle multiplicities larger than 34 and small total transverse momenta (< 2 Gev®/c) are plotted . For these collisions there is strong indication that even heavy clusters are emitted froïn a thermalize sourc close to midrapidity . An analysis of such data at higher bombarding energies is in progress as well as comparison with quantum molecular dynamics calculations.

4®Ar+Co 1 GeV /u

C

ô 10 V

l~ 1. .l.

!I - I -.II 100 200 300 400 500 600 700 800 invariant mass m, t MeV l Fig . 21 :

Invariant mass spectru rn of photons measured in the reaction Ar + Ca at 1 GeV/u . The jy° peak are indi7r°-peak and cated (Ref. 11) .

Fig . 22:

Mass spectrum of charged particles measured for the system Au + Au at 1 GeV/u (accepted momentum band 640 MeV/c < P,,,, < 1140 MeV/c, emission angle "lab = 44±4') (Ref. 12) .

In addition a strong program has started to study the creation of neutral and charged mesons at bombarding energies from far below the N-N-threshold to 2 GeV/u . The neutral scalar mesons (n(', i1°) are detected via their two photon decays. Fig . 21 shows the result from a recent experiment with the two arm photon spectrometic TAPS at GSI, in which the

invariant mass (m ) spectrum of the photons has been measured for the reaction Ar + 77 Ca at 1 GeV /11/ . In addition to the no line at 135 MeV a weak line centered around the invariant mass of the 1-meson (550 MeV) is observed, signaling the subthreshold production of q mesons .

The production of charged mesons is studied with a large solid angle (30 msr) magnetic spectrometer (KAOS) . First data have been taken for Au + Au collisions at 1 GeV/u bombarding energy. Fig . 22 shows a preliminary mass spectrum measured within the momentum band 640 MeV/c < p < 1140 MeV/c at n,,b = 44°. The subthreshold K+ production is reflected by the peak -around mass = 500 MeV/c2 /',2/ . Further measurements

V Hante /

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ireW oo

nf heavy ion research

of the K ^/~^ production ratio as function of the bombarding energy will give information especially on the s u bt hneoho!d Ka on production mechanism and on the equation of state . Furthermore the study of s uLdh r gah ol d particle production im the hot dense collision zone should also give some insight !n a possible change of their effective mass and coupling oon sd a nt in the hot dense niediurn . In this respect the investigation of central heavy !on collisions may be a un!qu e tool to learn abouta basic feature of QCD . The question is whether some properties of hadmono Ile their effective mass and coupling constant may be changed in the high density interaction region of central heavy ion collisions, due to pa rti i ! chiral symmetric restoration.

Fig . 23 :

Iv

12

a3

14

15

16

17

0.8

Invariant mass spectrum of dileptons for the reaction "Ca + "'Ca at 1 GeV /u . The solid line gives the contribution of massive photons produced in pn bremsstrahlung, the short-dashed curve that of the ArOaU t z decay (A ---, N8' -- Ne + e - ) . The dotted curve shows the contribution from 7r + 7r - annihilation and the dash-dotted curve that of 7rN brernmo t r a h!ung( R ef . 10) .

Such effects may be studied by detection of the decay of hadrons in dense matter into

photons and leptons which interact only e!ect rorvveak with nuclear matter . One

exp er-

irnoni . which has been started at the Beva !ac !mtho e W dyoydUe pton (e + , e - production )

fron) the annihilation o[ 2 pions . By reconstruction of the invariant mass of the emitting

object one may hope to sW dy the pion mass in hod dense nuclear matter . Fig . 23 shows and cxperinmen1e l results theoretical prediction for the reac tion Wo f^'Cm at 1 GeV/u /13V. The spectrum shows a dominant contribution from r, + 7r - -anihilation at invariant masses higher than 508 MeV. Unfortunately the region near threshold seems to be masked by a dominan t 'pn'bremas t rah!u/ i g contribution, which does not allow to study a shift of the threshold to lower invariant masses . More promising would be a high statistic study in

the invariant mass region of

the // meson (7O 0 'BOO M eV) . This would open to investigate vve t her t he y- mass decreases in dense nuclear matter due

metry restoration .

an opportunity t ochina! sym-

lette

t

ic Physics

With the availability of heavy ion bearns many unique atomic physics problems have been addressed, ranging from the study of ionisation, electron capture, an transfer in ion-atom collisions to precision spectroscopy of transitions in highly stripped ions. The most interesting reaction process discovered and studied at U ILAC energies is the formation of quasi-molecules and quasi-atoms in adiabatic ioa-atom collisions /14/. During such a collision the electrons are exposed to a two center Coulomb-field determined by Z,,Z, and the internuclear distance R(t), which is time dependent corresponding to the motion of both nuclei on Rutherford trajectories . This time-dependent Coulomb-field transfers energy and momentum to the electrons, which results in both an increase of their binding energies with decreasing R as well as an ejection of bound electrons with a certain probability . For the most strongly bound electrons with velocities close to c. there is a high probability that they will adjust their charge distribution nearly adiabatically during a slow collision (v/c - 0.1) to the two center Coulomb-field . They will thus form quasi stationary states ("quasi-molecular" or "quasi-atomic" states) when R(t) becomes much smaller than the radius of Me state considered . ®f special interest are "quasiatoms" with very high Z like Z = 184 which may be formed in U-U-collisions . In such atoms the binding energy of the K shell exceeds 2mc 2, so it becomes embedded in the negative energy continuum . The formation, characteristic binding energies and wave functions of these high Z-atoms were intensively studied by the observation of inner shell ionization and 8-ray production . In further experiments the "ionization" of the QED vacuum was investigated by studying the e +e--pair production in the high time-dependent field. The surprising result was that the positron spectrum (Fig. 24) showed lines between 250 and 400 keV energy superimposed on ,a continuous distribution . The cross section for the production of the e+ lines as we .l as those for continuous e } production were found to rise with a high power of (Z, + ZJ2°, indicating a strong field effect in the production mechanism of both processes . As a further surprise the measurement of e {e- coincidence spectra revealed narrow monoenergetic lines /15/ . For some of these monoenergetic pairs a strong 180° correlation in the emission pattern was observed whereas for others all opening angles appeared. Fig . 25 displays a sum energy spectrum for the system U+Ta at 6.3 NIeV a narrow line at 635 keV . Compared to quasielastic collisions the monoenergetic pairs is enhanced by more than an order of magnitude in collisions which reflects a strong effect of the characteristic times, the fields are present, on the production mechanism .

which exhibits production of deep inelastic high Coulomb

24

We/ Di rccxünmoy heavfùom research

oo

u + Ta

. 25

6.3 Mev/u

~

f a mm

nm sj: =E, +E,

1100

mm

jk e V,'

12

C Io 00 ~X m

o

Fig . 24 :

Positron spectra for U-U

and U-Th collisions at 5 .9 MeV/u bombarding energy taken with a resolution of about 8Ohe V.The nuclear background 0 is already subtracted . Curve a represents theoretical expectations for positron creation by the strong time changing Coulomb field, with a normalisation constant [iEi

Fig. 25 :

-200

'100 0 /00 2ou E s =t--b Jpr

300

Sum (upper) and difference (lower) energy spectrum of e+ e--pains from U-Ta collisions at 6.3 K8e V/u triggered for long collision times (Re[ 15) .

In contrast to the sum energy the difference energy spectrum displayed !n Fig . 25 shows a broad distribution and a shift towards positive energies . Furthermore, for the 635 keV hne the highest intensity has been found at an opening angle L.+ -'~ 90" . These results O e are in disagreement with the assumption of a two body kinematics. Together with the absence of a noticeable Doppler broadening of the sum lines they provide evidence, that

P. Kî rnle / irections of heavy ion research

35c

a third heavy partner which is essentially at rest, is involved . This heavy partner is pr bably a target atom. Since the sum lines are observed !n coincidence with two scattered heavy ions, one has to infer that the mnonoenemge1!m pairs arm produced in a two step process. For the moment we do not have a crisp clear explanation of all observations . At present, our working scenario, which !o under test, !s about the following . >n the high Coulomb-field of the d!num!ear system formed !n close distance collisions, composite extended objects of different masses are formed which decay either unpertmrbed into e *e - pairs (180" corre!oUonm) or get dissociated by scattering on target nuclei, to which momentum is t ranofered b u t only anunmn eamureab/e amount of energy. The heavy-ion atom collisions are also o good source for the production of h' h
Summary !n conclusion ono notes 1hal heavy ion physics hon become o research field with a huge arsenal of accelerators and measuring devices to study very fundamental problems of C> CO and QED, a great variety of structure topics of nuclei under extreme conditions and finally nuclear dynamics aspects in o wide energy open . Although the problems are complex many of them contain either fundamental aspects of interactions or of the many-body behavior ofan intriguing quantum system . Thus we hope and expect that heavy !on research will have o bright future.

36C

1. 2. 3. 4. 5. 6. 7. 8. 9. 11 11 . 12. 13. 14. 15. 16.

P.

jenle / Directions

Of heavy

ion research

P. Kienle, in Nuclear utter and Heavy ion Collisions ed. by M. Soyeur, H. Flocard, Tamcin and M. Pouent, Plenum Publishing Corporation (1989) p. 429 and References therein. A. Rahmann, J .P. Schiffer, Phys. Rev. Lett . 57, 1136 (1986) . H.G . Clerc et al., Contribution to this Conference and to be published. G . Manzenbery Rep. Prog . Phys. 51, 57 (1988) . H.J . Wollersheim et al.. GSi-Report 91-1, 22 (1991) and to be published. H . Enfling, Private Communication and to be published . W . Marawek et al ., GSi- Report 901, 31 (1991), GSI-Report 89-1 (1989) . K Backe et al. . GSI-Report 91-1 . 34 (1991) and References therein. W. Trautniann, Contribution to this Conference and C.A. Ogilvie et al., submitted to Phys . Rev. LeK A. Gobbi A al., GSi Nachrichten 4-91, 3 (1991) and to be published. V. Metag, Private Communication and to be published. P. Senger et al., Contribution to this Converence and to be published . U . Mosel, Annual Review of nuclear and Particle Science, Vol . 41, in press. P. Kienle, Anti . Rev. Nucl . Part . Sci. 36, 605 (1986) . W. Konig, in Proceedings of Vacuum Structure in Intense Fields, ed. M. Fried, IVASI-Series, Carg(§se, (1990), in press. J . Gasses et al., Phys . Lett . A 147, 365 (1990) .