Interferometry measurements selected by neutron calorimetry

NUCLEAR PHYSICS A Nuclear Physics A583 (1995) 401-406

t-1 .~;EV II'IP,

INTERFEROMETRY MEASUREMENTS SELECTED BY NEUTRON CALORIMETRY C. Ghisalberti a, C. Lebrun a, L. Sezac a, D. Ardouin a, A. Chbihi b, H. Dabrowski c, B. Erazmus a, PbEudes a, J. Galin b, D. Guerreau b, F. Guilbault a, P. Lautridou a, R. Lednicky d, M. Morjean , A. Peghaire b, J. Pluta e. J. Quebert ~, A. Rahmani a, T. Reposeur a and R. Siemssen g. a Laboratoire de Physique Nucl6aire (CNRS/IN2P3 et Universit6 de Nantes) 2, rue de la Houssini~re, 44072 NANTES, cedex 03 (France) b Laboratoire GANIL, BP 5027, 14021 CAEN (France) c Institute of Nuclear Physics, im. H. Niewodciczanskiego, Zaklad I - Radzikowskiego 152, 31342 CRACOW (Poland) d Institute of Physics, Academia of Sciences of the Czech Republic, CZ-18040 Prague, Czech Republic e Institute of Physics, Warsaw Technical University, ul. Koszykoma 75, 00-662 WARSAW (Poland) f Centre d'Etudes Nucl6aires de Bordeaux-Gradignan, le Haut Vigneau, 33170 GRADIGNAN (France) g K. V. I. Zemikelaan 25-9747 AA GRONINGEN (The Netherlands).

Abstract

: Two-oroton correlation functions have been measured in the reaction 208pb+93Nb at 29 MeV per nucleon at GANIL in the scattering chamber of ORION used as a neutron calorimeter. In the same experiment preequilibrium and equilibrium emissions are observed. Very strong effects are seen after centrality selection and directional cuts.

1. I n t r o d u c t i o n Information on space-time characteristics of an emitting source can be deduced from two-proton correlations at small relative momenta [i-3]. Some experimental correlation functions exhibit a peak near 20 MeV/c and others are relatively flat depending of the studied system and of the location of the detectors. The first case corresponds to a small emission area and a short time scale while the second one clearly indicates a longer time between the emission of two particles for a fixed source radius. These two extreme situations have been reported for different experiments [3-6] and generally without centrality selection (with some exceptions [7,8] ). So it is important to test the feasibility of measuring in the same experiment different space-time values by using some suitable cuts in the data and to deduce possible dynamic properties of the reaction. In the following we report such experimental results from an experiment performed in the reaction chamber of ORION at GANIL. In the fwst part we will present theoretical predictions of the behaviour of correlation functions depending of different parameters. In the second part the experimental set-up will be described and finally the proton-proton correlation results will be analysed. 0375-9474/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0375-9474(94)00694-6

402c

C. Ghisalberti et al. / Nuclear Physics A583 (1995) 401-406

2. Theoretical calculations To study the theoretical evolution of the p-p correlation function R depending of the relative momentum q we used the model of Lednicky and Lyuboshitz [9] taking into account Coulomb and nuclear interactions between the two protons and antisymetrization effects. The analytic calculation deduces the value of R(q) from a set of given values for V and 0 (respectively velocity and polar angle of the proton pair) and for q~ and ~t which define ¢, the direction of the relative momentum in the pair frame (Fig. 1). The proton source is sup- F i g u r e 1 - Illustration o f variables used in posed to have a radial gaussian shape with a the theoretical calculation to describe two mean radius r and a characteristic emission protons. time "~. Figure 2 shows various shapes of correlation functions obtained for fLxed mean values of V, 0 and {p. Other variables ('~, r and ~ ) are considered as parameters. A resonant effect can be observed at 20 MeV/c due to the attractive nuclear interaction between the two protons and the repulsive Coulomb interaction leads to cancel out the function at low relative momentum values. The correlation effect disappears above 50 MeV/c. The main information is contained on the behaviour of the function near 20 MeV/c. To analyse the influence of the time parameter we studied the evolution obtained for a source radius equal to 2.9 fm and for ~ equal 0 (fig. 2-a). To obtain a maximum greater than 1.2, the value has to be lower than 50 fm/c. On figure 2-b are presented the results corresponding to various source radius when the x parameter is set to 20 fm/c. A maximum greater than 1.2 implies a r value lower than 4 fro. On the opposite, for a r parameter equal to 2.9 fm, the positive correlation at 20 MeV/c disappears if the x value becomes greater than about one hundred frn/c. Figure 2-c shows the influence of the tp value on the shape of the correlation function. If the relative momentum vector is perpendicular to the emission direction a clear decrease of the correlation can be observed. In the model this result is induced by the antisymetrization effect. These plots give a clear indication of the possibility to deduce the space-time parameters from the shape of the p-p correlation function when the proton pairs are supposed to be emitted by an unique and spherical source. However, before using these theoretical predictions to analyse experimental results, we must keep in mind the complexity of the reaction mechanism. Two main effects occur. The first one corresponds to the mixing of impact parameters and the second one to the dynamical time evolution of the system during the collision. This time can be simply shared in two parts. At the begining of the reaction some nucleons can be emitted without collision or after few collisions. The corresponding proton pairs are called "preequilibdum" pairs. The short time scale will induce a strong correlation near 20 MeV/c. During the second part of the time evolution the system thermalizes and proton pairs are essentially emitted by equilibrated sources. Then the time delay between the emission of the two protons will depend on the involved excitation energy but will induce a flat shape near 20 MeV/c.

C. Ghisalberti et al. / Nuclear Physics A583 (1995) 401-406

1.8

.

1,

1.4 1.2 o.

1.o

t'(frn/cl

'

a)

'

t

i ---i

1.2

I

f(~(.) I

' b)

'

-

j,

400 I~U=o

t ~

,-,.,

1.4

/~", r ( f m )

403c

,°

I

:

,.,o

,°,o.

~=0° I

,

t,

I

c)

O ~

1.0 0.8 0.6

="

r',2.9 fm

-

U,J..O"

"r.- 50 f m / ¢

0"40

10 210 3'0 4fO 0

1;)

210'''

3LO

4'0

50

q(MeV/c)

F i g u r e 2 - Theoretical p-p correlation functions obtained with different sets of variables. In d) $1 corresponds to ~=20 fm/c and $2 to x=4OOfm/c. The result obtained with mixed sources (30%, 70%) is labelled $1+$2.

To illustrate the influence of these dynamical effects the figure 2-d gives a shape ($1) corresponding to a "preequilibrium" source and another one ($2) corresponding to an "equilibrated " source. On the same figure is plotted the function obtained when the p-p pairs are emitted by the two mixed sources (S1+$2). In such case the x and r parameters deduced from the global shape will represent average values weighted by the number of pairs emitted by each source. Therefore it is useful to investigate in the same experiment the conditions allowing to observe separately the preequilibrium emission and the thermal o n e .

3. Purposesandset-upof the experiment In order to measure the c e n t r ~ t y of each event we chose the n ~ o n calorimetry with the ORION detector at GANIL. A "" Nb target was bombarded by a " Pb beam at 29 MeV per nucleon. This system favours the neutron emission and the reverse kinematic allows to observe in the forward direction the particles emitted by the heavy partner. The beam energy was set to reach high excitation energy with a relatively small amount of preequilibriurn par-

404c

C. Ghisalberti et al. / Nuclear Physics A583 (1995) 401-406

ticles. Charged particles were detected by 48 CsI detectors (1.27 cm diameter). Among them 4 detectors (10 cm long) were put at 5.5, 10, 15 and 20 degrees in the forward direction. At backward angles, 4 other detectors (4.5 cm long) were set at 100, 120, 140 and 160 degrees. The correlator itself was formed with scintillators (4.5 cm long) left in a compact geometry covering radial angles between 25 and 80 degrees and spanning 50 degrees in azimuth. This system allowed to measure the proton-proton correlation function with a relative momentum threshold of about 10 MeV/c. For each coincidence event a centrality parameter called QP was recorded. It corresponds to the total charge of prompt light pulses induced by the neutron detection in the liquid scintillator loaded with Gadolinium surrounding the reaction chamber. In a ftrst step we demonstrated the selectivity of this parameter which is well correlated to the neutron multiplicity (measured at low reaction rate) and roughly estimated the relation between QP and the projectile-like velocity [10]. Counts 1

1 i

10e ,

\

\ Peripheral Intermedi [ ate] I,\q]

J

103

tL.

Va 10 Target

C.M.

Projectile Like

Figure 3 - Veloci~ diagram illustrating limits in polar angle and energy of the protons detected in the correlator.

I

Ice'trail

i

i ,, I ! 10 12 QP (a.u.)

,

2

4

:"

Figure 4 - Distribution of the coincident events against the centralit3, parameter called QP (see the text).

Figure 3 shows a velocity diagram describing the acceptance of the correlator. On this figure the length Vr of the relative velocity vector approximately corresponds to 20 MeV/c relative momentum value. In the following we will classify events according to the direction of this vector (defined by the ~ value only) with the assumption of a projectile-like emission and also according to the parallel component of the pair velocity as it is shown on figure 3. Figure 4 presents a QP spectrum obtained for all coincidents events. We defined in this spectrum seven classes of events according to seven QP intervals covering the total studied range. Single events used for the normalisation procedure of correlation functions were recorded for each separated class. To increase the statistics we will group some centrality classes together to define only three parts called "peripheral", "intermediate" and "central" respectively (fig. 4).

C. Ghisalberti et al. / Nuclear Physics A583 (1995) 401-406 4. p-p experimental correlation functions

..-,1.5

~ '"1'

"cY --'1.4

+ 1.3 -

As we already stressed the forward mean position of the proton detectors favoured the detection of the particles emitted by a fast source, especially the heavy projectile-like partner. On this assumption the expected correlation function obtained without any selection will be of the "thermal" type. That is confirmed by the experimental result shown on figure 5. However we should analyse the influence of the various cuts presented in the previous paragraph and deduce possible concealed contributions to the correlation.

o) ~2.5 cY +

,

~

,

,~

i~"1 ....

j

-~ i

v///v~<

0.6

I"~'l""l~'"i~'"i

''''

No selection

-

L

1.2

L

1.1

1 _ - - - - ~ - - i - ~ ~ ~ - % 0.9 0.8 0.7

b

_[

L

_]

0.6 0.5 0 20.30

40

50

60

70 80 90100 q (MaY/c)

Figure 5 Experimental p-p correlation function obtained without any selection.

b) i

405c

i

c) k

I

'

_

ii I ~,~.~. Per,, pneral

i

P ,~

'

:

i,',,:

o°-~,<

4o o

2

:v,

1.5

I:ll I I e I,.L

±*r,$

r

: 0.5

.T

$'

:

~ ' + ~"-{, I

I

'

i

i

I

......

', ~ ' , , -~,,'f4

i

_1

O. 6 ~_V///V~_~ 0.8 -

L~

Intermediate

-O ,

1.5

+

~ ,

.,~ ,~

1 0.5

40°~-~1'<

~, -, ' ~,!,i,

80 o -

i',

•

_1~I

i

:

:-

~u_ rr

e' i~

:

:,i.L

Ill I~T £' I I-I ~1,£~TI-

i

I

,

!

-i

1.5 _

1.2~V///V~

_

P

-~

Central

80°~

-i

-ii

0.5

i

~,~

1

'~ ~'

]1"

I

~o

~ r :

,

3'

,K,,,

,oo 1"1"

i

I

so

,

, , :

.

+:

go o _

÷ ,~g ;+r , % ~ : ' ,?, ,~ ~ .

leo/r

.

.

.

5o

~oo q(MeV/c)

F i g u r e 6 - Experimental p-p correlation functions obtained after some selections according to the parallel projected velocity of the pair (a), to the centrality parameter (b)(with only "aligned" events), and to the aligment (~! value) (c). Arrows indicate the 20 MeV/c q value.

406c

C. Ghisalberti et al. / Nuclear Physics A583 (1995) 401-406

A first cut on the parallel pair velocity can favour more or less the involved emitting sources. As shown on the figure 6-a the correlation function changes from a "preequilibrium" shape with a very strong correlation near 20 MeV/c (more than 2) when the more backward events are selected, to a "thermal" shape for the more forward events. We can now select events corresponding to the so-called "preequilibrium" part of the previous cut and analyse the centrality effect. Again very big changes can be observed in figure 6-b. More peripheral events give a correlation function strongly peaked at 20 MeV/c while more central ones produce a fiat shape. This behaviour confn'ms the efficiency of the centrality parameter. The decrease of the correlation at 20 MeV/c with the centrality might be interpreted by an increase of the relative weight of "thermal" protons compared with "preequilibrinm" ones. Finally, it is interesting to observe the directional effect corresponding to ~ values. Figure 6-c presents the correlation function in the more extreme "preequibrinm" situation obtained with the more aligned events (~ less than 40 degrees), compared with two non-aligned cases. In spite of the lack of statistics induced by cumulated cuts a very nice increase of the correlation with the alignment is measured. It seems that the range of variation is incompatible with the theoretical prediction using a spherical emission source assumption (fig.2-c). We suggest that, at this energy and with the used system, our measurement is able to reveal an azimuth asymmetry of the preequilibrium source of proton pairs.

5. Conclusion

Two-proton correlation measurements reported in this paper show, for the first time in the same experiment, the contribution of both preequilibrium and equilibrium protons. Strong directional effects have also been measured for the preequilibrium events. These results exhibits the importance of the centrality measurement and the requirement to get sufficient statistics allowing many selections to give information on the dynamics of the reaction.

REFERENCES [ 1] J. Quebert, Ann. Phys. Fr. 17 (1992) 99 [ 2] W.G. Lynch et al. Phys. Rev. Lett. 51 (1983) 1850 [ 3] J. PochodzaUa et al. Phys.Rev. C35 (1987) 1695 [ 4] D. Ardouin et al. Nucl. Phys. A495 (1989) 57c [ 5] D. Goujdami et al. Z. Phys. A339 (1991) 293 [ 6] P. Lautridou, Thesis University of Bordeaux I, France (1989) [ 7] A. Ferragut, Thesis University of Caen, France (1990) [ 8] M. A. Lisa et al. Phys. Rev. Lett. 70 (1993) 3709 [ 9] R. Lednicky et al. Sov. J. Nucl. Phys. 35 (1982) 770 and Proc. of CORINNE 90 Nantes, France (1990) 42 [10] L. S6zac, Thesis University of Grenoble 1, France (1993) C. Ghisalberti et al. Proc. of the XXXI Intern. Winter Meeting on Nucl. Phys. Bormio, Italy (Jan. 1993) 293.