HARD PHOTONS AND NEUTRAL PIONS RADIATED FROM COLLIDING NUCLEI

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HARD PHOTONS AND NEUTRAL PIONS RADIATED FROM COLLIDING NUCLEI

E. Grosse

To cite this version:

E. Grosse. HARD PHOTONS AND NEUTRAL PIONS RADIATED FROM COLLIDING NUCLEI.

Journal de Physique Colloques, 1986, 47 (C4), pp.C4-197-C4-200. �10.1051/jphyscol:1986423�. �jpa-

00225789�

JOURNAL DE PHYSIQUE

Colloque C4, supplément au n' 8, Tome 47, août 1986 CU-197

H A R D P H O T O N S AND NEUTRAL PIONS RADIATED FROM C O L L I D I N G N U C L E I

E . G R O S S E

GSI, Postfach 110541, D-6100 Darmstadt, F.R.G.

Résumé- Les collisions nucléaires dans le domaine de l'énergie de Fermi

peuvent être étudiées par comparer l'émission directe des photons à la production des pions. On gagne de l'information très intéressante par ajuster la formule du rayonnement d'un corps noir aux données et par les comparer à la théorie classique du rayonnement de freinage. Une pro- duction dans la première phase de la collision est manifestée.

Abstract

- A comparison of direct photon emission to pion production observed from nucleus-nucleus collisions in the Fermi energy domain shows some very interesting features. Valuable information is gained through the fit of a standard black body radiation formula to the data as well as by a comparison to classical bremsstrahlung theory. A production

in the early phase of the collision is indicated.

Radiative processes play an important role in atomic as well as in elementary particle physics whereas in nuclear physics the investigation of bremsstrahlung has been limited to a few cases, like beta-decay

, proton-nucleus scattering

, and only very recently also nucleus-nucleus collisions. In these - especially at incident energies high above the Coulomb barrier - the participating protons experience large velocity changes and are thus possible sources of electromagnetic bremsstrahlung

'

. Also for intermediate beam energies (B = 0.3) directly produced photons have been observed

as well as elec- trons and positrons

produced from them via external conversion. Theoretically the relevance of bremsstrahlung as a probe for the dynamics of a nuclear collision has recently been pointed out

"

. For sufficiently hard photons, the energy for their production is available only in the reaction zone and only in the early stage of the collision. In that respect they have similar properties as pions which have been con- sidered a particle probing the dynamics of nuclear collisions at intermediate energies.

Respective experiments

'

have supported that idea, which has been investigated the- oretically on the basis of Fermi boosted NN-collisions

with the inclusion of mean field effects

* , Pauli blocking

, or clustering

, in a statistical model

and by a semiclassical bremsstrahlung ansatz* . The interesting features resulting from this pion production work triggered our attempt to also study the photons directly produced with medium-energy heavy ions and their analysis as a radiative process accompanying the nuclear collision. Because of their negligable interaction with the surrounding nuclear matter photons seem to have some advantage as compared to pions.

To study the features of the neutral pion and the direct photon emission from nuclear collisions we performed a series of experiments using

C ions from the CERN synchrocy- clotron of 84, 74, 60 and 48 MeV/u, impinging on targets from

C to

U . To further- more study the mass dependence we investigated the (nearly) symmetric systems

0+

Mg at 48 MeV/u and compared them to data from *'Ar+*°Ca as well as "*Kr+*Zr, taken

at the 44 MeV/u beam of GANIL. The directly produced photons were registered with the same detector

as the decay

from simultaneously

'

produced ir

's. The inclusive photon production cross section integrated within different energy bins increases char-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986423

JOURNAL DE PHYSIQUE

acteristicly with the projectile energy, as can be seen from Fig. 1: For higher photon energies the excitation function for photon production becomes steeper. It is remark- able, how the excitation function for 100 MeV 5 Ell 5 150 MeV resembles in absolute mag- nitude and slope the one for ~ ~ - ~ r o d u c t i o n (E, > m w c Z

135 MeV). Also the high energy slopes 'of the photon and pion spectra are very similar. The slope parameter E o as obtained from a fit of an exponential exp(-E/Ee) to the spectra increases gradually with beam energy as displayed in the lower part of Fig. 1, but it does not change when the atomic numbers of the colliding nuclei are varied (at a given energy per nucleon).

This indicates the source of the radiation to be composed of equally many nucleons from target and projectile.

The exponential shape of the hard photon as well as the neutral pion spectra suggests an interpretation in a thermal model (with a statistically equilibrated fireball or hot spot). We have attempted to fit the data using a simple black-body radiation expression and obtain a reasonable overall fit to the spectral slopes observed6 at different ~4~

after adjusting T = E:~/~*A~ + AT; AT =

0.5) MeV. The resulting slope parameters E,are shown as lines in the lower part of Fig. 1. (Note, that

is different fromT due to the term E .pc in the expression for the energy spectrum: dZa/dE dQ = E. pc d'a/dp3; E=pc for photons).

The dashed lines in the upper part of the Figure result from the thermal model after adjusting a "strengthu parameter S. The calculation of absolute cross sections in such a model suffers from their exponential dependence on the temperature Twhich is diffi- cult to be fixed with high enough accuracy. This is also true for our approach of read- ing T from the spectral slopes which is accurate within +- 1.5 MeV only; from the fit to the photon data it appears that S ~ i s about one order of magnitude smaller than a first guess from:

. ^A direct*comparison between pion and photon data can be per- formed with much less ambiguity: clearly Sr has to be adjusted to be at least a factor of 6 larger than SK. With these strength parameters inserted into the black body radi- ation formula one 'bbtains a rather good overall fit to the photon or pion data obtained at the different projectile energies between

and

MeV/u. But because of the need of the different ad hoc assumptions this is not a proof of the validity of the statis- tical model for these intermediate energy collisions. Neither the introduction of an additional temperature AT =

MeV nor the equal participant hypothesis can be justified from first principles. Besides that the rather small strength parameter Sr and its large difference from the respective pion value ST indicate an emission of these hard photons prior to the statistical equilibration of the energy of relative motion; such a dynamical emission is of course governed by the rather weak coupling of photons to nuclear matter (as compared to the much stronger pion-nucleus coupling). The photon emission should thus not resemble thermal radiation but rather be compared to the clas- sical dynamic radiation, namely bremsstrahlung.

The intensity of bremsstrahlung depends on the change of the charges velocity in the collision; with increasing E y it decreases as

up to some critical energy. This energy and the subsequent exponential fall off in the photon spectra are related to the cut off of the high Fourier components in the space-time development of the nuclear motions19 , i.e. to the most violent part of the collisional deceleration. For one-di- mensional (linear) motion bremsstrahlung has a dipole pattern19 perdendicular to the direction of motion (in the comoving frame and tilted forward in the laboratory). In a collision there is interference between the radiation from the decelerated projectile and the accelerated target charge, which is especially strong in the case of equal charge to mass ratio; in that case the dipole component of the radiation is extinct (completely in nonrelativistic approximation) and a quadrupole distribution

evolve^^"^. In our experiment a pure quadrupole pattern was not expected to show up since the impact parameter and the nuclear radii are comparable or even larger than the wavelength of the detected photons (1 fm < ^%c/E,.s 4 fm) leading to a filling of the minim~m'~ at 90'. Dipole components also appear due to elementary p-n collisions9~

visible due to incoherent nucleon motion and an incomplete averaging over the reaction

zone by the emitted short wavelength photons. Experimentally we do not observe6

strongly structured photon angular distributions, but a clear 90' minimum in the pion

case. The high energy fall off in the spectra is more pronounced than the one expected from total energy conservation2' and is apparently due to the finite extension of the collision process in space and time1' , which are the quantities we wish to learn about. For the quadrupole case the relation between the stopping time and the slope parameter has been studied8,' by carrying out the Fourier integrals. This numerical problem becomes trivial when assuming the deceleration of a point charge Z along a straight line caused by a Lorenz shaped slowing down function with width

(FWHM). In that case the observed values E, = 20 MeV correspond tofi/~

30 MeV and indicate colli- sion times

10'~~sec which obviously decrease with increasing beam energy (cf.

Fig. 3b). Similar to what was found1' in pion production the observed decrease is in agreement with the assumption of a beam-energy independent deceleration length

2

1.2 fm, over which the relative velocity of the collision partners is slowed down.

In conclusion it can be said that in nucleus-nucleus-collisions at 44 MeV/u to 84 MeV/u neutral pion as well as direct photon production are sufficiently strong to be used as a probe for the collision dynamics. The data6 indicate that the hard photons observed originate from a source composed from equally many participant nucleons out of target and projectile - also if these have different mass. Such a source probably is the result of the first collision between nucleons from both nuclei prior to complete thermaliza- tion. The rather high temperature - in comparison to a classical gas of the same energy content - necessary in a description as black-body-radiation is another indication of such a preequilibrium situation. The enhancement of pion emission versus photon emis- sion in that picture also points in that direction: the larger coupling of pions to nuclear matter enlarges their emission rate and thus their preequilibrium yield.

Bremsstrahlung is the prototype of a nonequilibrium dynamic radiation process; a respective analysis of our data seems feasible, but a definite proof of such an inter- pretation is hampered by the insufficient wavelength and thus coherence length of the radiation observed. Whereas coherent bremsstrahlung calculation^^^^^^^^^^ fall sig- nificantly below the experimental cross sections, data are mostly in accordance to an expression for incoherent bremsstrahlung'' , reasonably normalized and modified to fit the steep slopes in the experimental spectra. These high energy slopes are interpreted as reflecting the high Fourier components in the relative motion, which then turns out to be slowed down considerably within 1.2 fm. The first nucleon-nucleon collision dur- ing the nuclear encounter is likely to cause such a strong deceleration and the emitted pions and photons are thus a sensitive probe to this early stage of the reaction.

Recent calculation^^^ indicate, for example, that the photon and pion emission yields in such collisions may be influenced by shock compression effects.

C " " " " ' 4

F i g . 1 :

Projectile ener y dependence of the direct

photon production in l8C+l2C collisions (trans- formed to the c.m. system)

Top

Cross section for 50 MeV

Ey

100 MeV ( O ) , 100 MeV < Ey

150MeV ( a ) , and Ey

150 MeV (A),

(angle integrated after a linear extrapolation

from observation angles)

Bottom

Slope parameters Eo determined from the spectra for Ey 3 E,.

-

For comparison

production data (integrated over E) are also given

the slopes are for

30

- E,

m,

3 E,. The dotted line represents the respective thermal calculation with T as for photons and S,

6 S .

-

The dashed and full lines are for black body . radiation and bremsstrahlung, respectively.

10

- Details are explained in the text.

40 50 60 70 80 90

E ,

,

C4-200 JOURNAL DE PHYSIQUE

This work has benefitted from an inspiring collaboration with P. Grimm, H. Heckwolf, J.L. Laville, A. Mueller, W.F.J. Killer, H. Noll, A. Oskarsson, H. Stelzer and W. Rosch. The intensive support by B. Pllardyce and his collaborators from the CERN-SC and by C. Brendel, H. Dabrowski, N. Hermann, and 0. Klepper is greatfully acknowledged.

Thanks are due to J. Aichelin, W. Cassing, W. Greiner, J. Knoll, U. Mosel, B. Killer, H. Stocker and - last but not least - D. Vasak for fruitful discussions.

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