- Email: [email protected]
Heating nuclei with high-energy antiprotons
UCLEAR PHYSIC~
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 56A (1997) 114-117
Heating nuclei with high-energy antiprotons B. Lott a, W. Bohne b, J. Eades c, T.v. Egidy d, P. Figuera b, H. Fuchs b, J. Galin ~, F. Goldenbaum b, Ye. S. Golubeva e, K. Gulda f, D. Hilscher b, A.S. Iljinov e, U. Jahnke b, a. Jastrzebski f , W. Kurcewicz f , M. Morjean ~, G. Pausch g, A. P~ghaire ~, L. Pienkowski b, D. Polster b, S. Proschitzki h , B. Quednau ~, H. Rossner b, S. Schmid d, W. Schmid d, P. Ziem b ~GANIL(IN2P3-CNRS,DSM-CEA), 14021 Caen Cedex, France bHahn-Meitner-Institut, 14109 Berlin, Germany c C E R N - P P E , 1211-Geneva, Switzerland dTU-Miinchen, 85748 Garching, G e r m a n y eINR, Russian Academy of Science, 117312 Moscow, Russia fUniversity of Warsaw, 02-097 Warzawa, Poland gFZ-Rossendorf, 01314 Dresden, G e r m a n y hIPN Orsay, 91406 Orsay, France The distributions of thermal energy generated in different target nuclei by means of the annihilation of antiprotons onto their surfaces have been investigated. The results confirm that fairly hot nuclei are produced in this approach, as predicted by Intra-Nuclear Cascade calculations. Although the transition towards multifragmentation as the primary decay channel is not observed, these data should allow to pin down the decay patterns of hot nuclei with little excitation of their collective degrees of freedom. In turn this may shed new light on the mechanisms responsible for some events associated with high fragment multiplicities as observed in heavy-ion collisions.
1. I n t r o d u c t i o n The issue of the m a x i m u m excitation energy that a nucleus can sustain before it breaks apart into pieces has recently raised a large interest among nuclear physicists, both on the experimental and theoretical sides. The nucleus disassembly is predicted [1] to occur for excitation energies of about 5 MeV/nucleon, i.e. substantially lower than the 8 MeV/nucleon of nuclear binding energy. This limiting excitation energy should actually decrease with the nucleus mass and the Z/A ratio thanks to the destabilizing effect of the Coulomb repulsion. An active search was launched a few years ago, aiming at revealing a transition in the dominant decay channels from the regime of evaporation of light particles and fission towards the regime of the so-called mul0920-5632/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII: S0920-5632(97)00261-2
tifragmentation, which consists of the cracking of the nucleus into lighter fragments (usually referred to as intermediate mass-fragments, IMFs). Various groups claimed that this transition was indeed observed, in some cases for excitation energies as low as 3 MeV/nucleon and that the decaying nucleus exhibited a critical behavior which could be related to a phase transition. However, it was concurrently realized that several features of heavy ion collisions made the interpretation of the results quite ambiguous. For instance, dynamical production of IMFs was found, and the task of disentangling this contribution from the thermal one appeared intricate. Moreover, the thermalization process in the heavy-ion collisions is quite slow, and the associated time scale m a y come into conflict with the excited-nucleus decay time, which decreases sharply for increasing tern-
B. Lott et al./Nuclear Physics B (Proc. Suppl.) 56A (1997) 114-117
perature. Finally, the heating process is accompanied by a strong excitation of collective degrees of freedom, like rotation, compression or deformation, which alter the decay patterns and may even mask the thermal effects of interest. These considerations motivated an experimental program aiming at investigating alternative ways of forming hot nuclei free of the above shortcomings, with energetic protons or 3tie [2] and more recently with antiprotons at the Low-Energy Antiproton Ring (LEAR) at CERN (PS208 collaboration). Indeed, heating up a nucleus by means of an antiproton annihilating onto its surface offers in principle several advantages over heavy-ion collisions, according to theoretical calculations [3,4] . The pions produced in the annihilation (about 6 at V~= 2.5 GeV) propagate the released energy within the nuclear medium on a very short time scale, in a soft, radiation-like fashion, allowing thermalization to be achieved very rapidly. Moreover, both the mass loss due to preequilibrium emission and the excitation of collective degrees of freedom remain moderate. The reaction is usually described in three successive stages. First, the antiproton annihilates on a single nucleon located in the nucleus periphery because of the short mean free path associated with this process. Some energy is then transferred to the nuclear system by the pions passing through the nucleus, mainly via the formation of A resonances, a few energetic nucleons being expelled during this preequilibrium stage. Finally, once the residual energy is thermalized the excited nucleus starts evaporating low-energy particles and fragments. A series of experiments were carried out with beams of 200 MeV/c, 1.2 GeV/c and 2 GeV/c antiprotons. The main goals of these pioneering experiments were t9 assess the heat energy distribution deposited into the host nucleus and the amount of mass and energy losses suffered during the thermalization stage. The former issue was investigated in a first run by measuring the multiplicities of evaporated neutrons and light charged fragments, while the latter was tackled in the second run by means of inclusive d2a/dEdfl distributions measured for neutrons, (high-energy) protons, a-particles, pions and kaons via the
115
time-of-flight technique. Only results obtained in the first run with the 2 GeV/c beam will be presented in the following. A partial account of these results has already been published elsewhere [5]. 2. E x p e r i m e n t a l m e t h o d The incoming antiprotons were tagged thanks to a scintillator located 16 m upstream from the reaction chamber and focused onto 1-2 m g / c m 2 thick n~tCu, 165Ho, 197Au and 23sU targets. The experimental setup comprised mainly two highefficiency "4~r" devices devoted to detecting neutrons on the one hand and charged particles and fragments on the other hand. The Berlin Neutron Ball (BNB) with a volume of 1.5 m 3 housed the Berlin Silicon Ball (BSiB), consisting of 157 500pro-thick Si-detectors. The BNB measured the neutron multiplicity on an event-by-event basis while the BSiB provided energy and identification for particles with Z< 2 stopped inside the detectors as well as a rough identification for heavier fragments, i.e. IMFs and fission fragments. These two detectors are very sensitive to low-energy particles, i.e. those evaporated from the nuclei during the decay stage of the. reaction and exhibit a low efficiency for particles emitted during the INC stage. 3. E x p e r i m e n t a l r e s u l t s Fig. 1 top displays joint multiplicity distributions measured at 1.2 GeV for a variety of targets. The measured multiplicities increase with the target-nucleus mass because of the larger number of nucleons interacting with the pions released in the annihilation, giving rise to a larger excitation energy in the nucleus. These multiplicities are found to increase significantly with the antiproton bombarding energy (by about a factor of two for the neutron multiplicity between annihilation at rest and at E b o m b ~- 1.2 GeV). This increase results from the deeper annihilation site inside the nucleus stemming from a reduced annihilation cross section at the higher energy on the one hand, and from a larger focusing of the pions into the nucleus because of the Lorentz boost on the
B. Lott et al. /Nuclear Physics B (Proc. Suppl.) 56A (1997) 114-117
116
other hand. The patterns of the distributions of Fig. 1 top point to evaporation as the dominant emission process since for heavy targets, light charged particle emission sets in only for events associated with a large number of neutrons, because of the large Coulomb barrier experienced by the charged particles. An interesting finding concerns the light charged particle multiplicities observed for the Cu target, which reach values as high as 14 consisting mostly of H and He isotopes, which means that the system basically vaporizes into light fragments for some events. Further analysis is underway to explore whether or not these events exhibit a critical behavior. '~
20
@
10
f:
s o
q~
energy, the partial multiplicities for the different light particle species are found in good agreement with those predicted by the statistical model. Such a procedure yields the excitation energy distributions displayed in Fig. 2, in good agreement with predictions of the Intra-Nuclear Cascade model [5] (histograms). The average excitation energy from ~ annihilation in flight increases from Cu to U almost linearly with A, however when converted to excitation energy per nucleon, this tendency is inverted, the lighter nucleus receiving more energy per nucleon than the heavier one (2.5 MeV/nucleon and 1.5 MeV/nucleon for Cu and U respectively). The distributions extend with a sizeable cross-section to energies as large as 4 to 5 MeV/nucleon for the heaviest targets, Au and U, confirming the relevance of this approach to investigations devoted to hot nuclei.
.~10 3
20
10
10 2
0
10
20
30
40 0
10
20 30 40 Neutron multiplicity
10
8
_~ 4
~2
[
o -7.5-5-2...g 0 2.5 5 7.5 - 7 . 5 - 5 - 2 . 5 0 2.5 5 7.5 Z=I V parallel (cm/ns) Z=2 V parallel (cm/ns)
.] 10
Figure 1. The Galilei-invariant velocity distributions (Fig. 1 bottom) of the detected light charged particles are found essentially isotropic in the laboratory frame, and therefore fortify the above conclusion that most detected particles are evaporated. This observation enables one to assess the excitation energy from the measured total light particle multiplicity on an event-by-event basis, the connection between the two quantities being established by means of the statistical model. It is worth mentioning that for a given excitation
10
0
200
400
600
800
1000
E *(MeV )
Figure 2. The IMFs observed in this experiment were found to exhibit Poisson-like multiplicity distributions compatible with those predicted by the statistical model for a given excitation energy. For U, an average value of 1.2 IMFs was found for events associated with the highest excitation
B. Lott et aL/Nuclear Physics B (Proc. Suppl.) 5614 (1997) 114-117
energies observed, indicating that the multifragmentation threshold has not been overcome. The low IMF multiplicities observed both in p- and ~induced reactions suggest that the large IMF multiplicities found in intermediate-energy heavy-ion collisions are mostly of dynamical origin and do not manifest the failure of the conventional statistical model at high excitation energy. 4. C o n c l u s i o n The recent results obtained in the LEAR runs seem to confirm the interesting prospects put forward by earlier theoretical works. Although the total available energy in the system is tangibly lower than that involved in heavy ions collisions, this drawback is largely offset by cleaner experimental conditions making more reliable the conclusions regarding the thermal effects governing the decay of highly excited nuclei.
117
REFERENCES
1. E. Suraud, Nucl. Phys. A462 (1987) 109. 2. L. Pienkowski et al., Phys. Lett. B336 (1994) 147. 3. J. Cugnon et al., Nucl. Phys. A484 (1988) 542. 4. Ye.S. Golubeva et al., Nucl. Phys. A483 (1988) 539. 5. F. Goldenbaum et al., Phys. Rev. Lett. 77 (1996) 1230.
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 56A (1997) 114-117
Heating nuclei with high-energy antiprotons B. Lott a, W. Bohne b, J. Eades c, T.v. Egidy d, P. Figuera b, H. Fuchs b, J. Galin ~, F. Goldenbaum b, Ye. S. Golubeva e, K. Gulda f, D. Hilscher b, A.S. Iljinov e, U. Jahnke b, a. Jastrzebski f , W. Kurcewicz f , M. Morjean ~, G. Pausch g, A. P~ghaire ~, L. Pienkowski b, D. Polster b, S. Proschitzki h , B. Quednau ~, H. Rossner b, S. Schmid d, W. Schmid d, P. Ziem b ~GANIL(IN2P3-CNRS,DSM-CEA), 14021 Caen Cedex, France bHahn-Meitner-Institut, 14109 Berlin, Germany c C E R N - P P E , 1211-Geneva, Switzerland dTU-Miinchen, 85748 Garching, G e r m a n y eINR, Russian Academy of Science, 117312 Moscow, Russia fUniversity of Warsaw, 02-097 Warzawa, Poland gFZ-Rossendorf, 01314 Dresden, G e r m a n y hIPN Orsay, 91406 Orsay, France The distributions of thermal energy generated in different target nuclei by means of the annihilation of antiprotons onto their surfaces have been investigated. The results confirm that fairly hot nuclei are produced in this approach, as predicted by Intra-Nuclear Cascade calculations. Although the transition towards multifragmentation as the primary decay channel is not observed, these data should allow to pin down the decay patterns of hot nuclei with little excitation of their collective degrees of freedom. In turn this may shed new light on the mechanisms responsible for some events associated with high fragment multiplicities as observed in heavy-ion collisions.
1. I n t r o d u c t i o n The issue of the m a x i m u m excitation energy that a nucleus can sustain before it breaks apart into pieces has recently raised a large interest among nuclear physicists, both on the experimental and theoretical sides. The nucleus disassembly is predicted [1] to occur for excitation energies of about 5 MeV/nucleon, i.e. substantially lower than the 8 MeV/nucleon of nuclear binding energy. This limiting excitation energy should actually decrease with the nucleus mass and the Z/A ratio thanks to the destabilizing effect of the Coulomb repulsion. An active search was launched a few years ago, aiming at revealing a transition in the dominant decay channels from the regime of evaporation of light particles and fission towards the regime of the so-called mul0920-5632/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII: S0920-5632(97)00261-2
tifragmentation, which consists of the cracking of the nucleus into lighter fragments (usually referred to as intermediate mass-fragments, IMFs). Various groups claimed that this transition was indeed observed, in some cases for excitation energies as low as 3 MeV/nucleon and that the decaying nucleus exhibited a critical behavior which could be related to a phase transition. However, it was concurrently realized that several features of heavy ion collisions made the interpretation of the results quite ambiguous. For instance, dynamical production of IMFs was found, and the task of disentangling this contribution from the thermal one appeared intricate. Moreover, the thermalization process in the heavy-ion collisions is quite slow, and the associated time scale m a y come into conflict with the excited-nucleus decay time, which decreases sharply for increasing tern-
B. Lott et al./Nuclear Physics B (Proc. Suppl.) 56A (1997) 114-117
perature. Finally, the heating process is accompanied by a strong excitation of collective degrees of freedom, like rotation, compression or deformation, which alter the decay patterns and may even mask the thermal effects of interest. These considerations motivated an experimental program aiming at investigating alternative ways of forming hot nuclei free of the above shortcomings, with energetic protons or 3tie [2] and more recently with antiprotons at the Low-Energy Antiproton Ring (LEAR) at CERN (PS208 collaboration). Indeed, heating up a nucleus by means of an antiproton annihilating onto its surface offers in principle several advantages over heavy-ion collisions, according to theoretical calculations [3,4] . The pions produced in the annihilation (about 6 at V~= 2.5 GeV) propagate the released energy within the nuclear medium on a very short time scale, in a soft, radiation-like fashion, allowing thermalization to be achieved very rapidly. Moreover, both the mass loss due to preequilibrium emission and the excitation of collective degrees of freedom remain moderate. The reaction is usually described in three successive stages. First, the antiproton annihilates on a single nucleon located in the nucleus periphery because of the short mean free path associated with this process. Some energy is then transferred to the nuclear system by the pions passing through the nucleus, mainly via the formation of A resonances, a few energetic nucleons being expelled during this preequilibrium stage. Finally, once the residual energy is thermalized the excited nucleus starts evaporating low-energy particles and fragments. A series of experiments were carried out with beams of 200 MeV/c, 1.2 GeV/c and 2 GeV/c antiprotons. The main goals of these pioneering experiments were t9 assess the heat energy distribution deposited into the host nucleus and the amount of mass and energy losses suffered during the thermalization stage. The former issue was investigated in a first run by measuring the multiplicities of evaporated neutrons and light charged fragments, while the latter was tackled in the second run by means of inclusive d2a/dEdfl distributions measured for neutrons, (high-energy) protons, a-particles, pions and kaons via the
115
time-of-flight technique. Only results obtained in the first run with the 2 GeV/c beam will be presented in the following. A partial account of these results has already been published elsewhere [5]. 2. E x p e r i m e n t a l m e t h o d The incoming antiprotons were tagged thanks to a scintillator located 16 m upstream from the reaction chamber and focused onto 1-2 m g / c m 2 thick n~tCu, 165Ho, 197Au and 23sU targets. The experimental setup comprised mainly two highefficiency "4~r" devices devoted to detecting neutrons on the one hand and charged particles and fragments on the other hand. The Berlin Neutron Ball (BNB) with a volume of 1.5 m 3 housed the Berlin Silicon Ball (BSiB), consisting of 157 500pro-thick Si-detectors. The BNB measured the neutron multiplicity on an event-by-event basis while the BSiB provided energy and identification for particles with Z< 2 stopped inside the detectors as well as a rough identification for heavier fragments, i.e. IMFs and fission fragments. These two detectors are very sensitive to low-energy particles, i.e. those evaporated from the nuclei during the decay stage of the. reaction and exhibit a low efficiency for particles emitted during the INC stage. 3. E x p e r i m e n t a l r e s u l t s Fig. 1 top displays joint multiplicity distributions measured at 1.2 GeV for a variety of targets. The measured multiplicities increase with the target-nucleus mass because of the larger number of nucleons interacting with the pions released in the annihilation, giving rise to a larger excitation energy in the nucleus. These multiplicities are found to increase significantly with the antiproton bombarding energy (by about a factor of two for the neutron multiplicity between annihilation at rest and at E b o m b ~- 1.2 GeV). This increase results from the deeper annihilation site inside the nucleus stemming from a reduced annihilation cross section at the higher energy on the one hand, and from a larger focusing of the pions into the nucleus because of the Lorentz boost on the
B. Lott et al. /Nuclear Physics B (Proc. Suppl.) 56A (1997) 114-117
116
other hand. The patterns of the distributions of Fig. 1 top point to evaporation as the dominant emission process since for heavy targets, light charged particle emission sets in only for events associated with a large number of neutrons, because of the large Coulomb barrier experienced by the charged particles. An interesting finding concerns the light charged particle multiplicities observed for the Cu target, which reach values as high as 14 consisting mostly of H and He isotopes, which means that the system basically vaporizes into light fragments for some events. Further analysis is underway to explore whether or not these events exhibit a critical behavior. '~
20
@
10
f:
s o
q~
energy, the partial multiplicities for the different light particle species are found in good agreement with those predicted by the statistical model. Such a procedure yields the excitation energy distributions displayed in Fig. 2, in good agreement with predictions of the Intra-Nuclear Cascade model [5] (histograms). The average excitation energy from ~ annihilation in flight increases from Cu to U almost linearly with A, however when converted to excitation energy per nucleon, this tendency is inverted, the lighter nucleus receiving more energy per nucleon than the heavier one (2.5 MeV/nucleon and 1.5 MeV/nucleon for Cu and U respectively). The distributions extend with a sizeable cross-section to energies as large as 4 to 5 MeV/nucleon for the heaviest targets, Au and U, confirming the relevance of this approach to investigations devoted to hot nuclei.
.~10 3
20
10
10 2
0
10
20
30
40 0
10
20 30 40 Neutron multiplicity
10
8
_~ 4
~2
[
o -7.5-5-2...g 0 2.5 5 7.5 - 7 . 5 - 5 - 2 . 5 0 2.5 5 7.5 Z=I V parallel (cm/ns) Z=2 V parallel (cm/ns)
.] 10
Figure 1. The Galilei-invariant velocity distributions (Fig. 1 bottom) of the detected light charged particles are found essentially isotropic in the laboratory frame, and therefore fortify the above conclusion that most detected particles are evaporated. This observation enables one to assess the excitation energy from the measured total light particle multiplicity on an event-by-event basis, the connection between the two quantities being established by means of the statistical model. It is worth mentioning that for a given excitation
10
0
200
400
600
800
1000
E *(MeV )
Figure 2. The IMFs observed in this experiment were found to exhibit Poisson-like multiplicity distributions compatible with those predicted by the statistical model for a given excitation energy. For U, an average value of 1.2 IMFs was found for events associated with the highest excitation
B. Lott et aL/Nuclear Physics B (Proc. Suppl.) 5614 (1997) 114-117
energies observed, indicating that the multifragmentation threshold has not been overcome. The low IMF multiplicities observed both in p- and ~induced reactions suggest that the large IMF multiplicities found in intermediate-energy heavy-ion collisions are mostly of dynamical origin and do not manifest the failure of the conventional statistical model at high excitation energy. 4. C o n c l u s i o n The recent results obtained in the LEAR runs seem to confirm the interesting prospects put forward by earlier theoretical works. Although the total available energy in the system is tangibly lower than that involved in heavy ions collisions, this drawback is largely offset by cleaner experimental conditions making more reliable the conclusions regarding the thermal effects governing the decay of highly excited nuclei.
117
REFERENCES
1. E. Suraud, Nucl. Phys. A462 (1987) 109. 2. L. Pienkowski et al., Phys. Lett. B336 (1994) 147. 3. J. Cugnon et al., Nucl. Phys. A484 (1988) 542. 4. Ye.S. Golubeva et al., Nucl. Phys. A483 (1988) 539. 5. F. Goldenbaum et al., Phys. Rev. Lett. 77 (1996) 1230.