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Decay of hot nuclei formed with energetic antiprotons (PS208)
N
NUCLEAR PHYSICS A
ELSEVIER
Nuclear Physics A655 (1999) 269e-274e
www.elsevier.nl/locate/npe
Decay of Hot NucleiFormedwith EnergeticAntiprotons (PS208) L. Pienkowski a'f, W. Bohne a, J. Eades b, T.v.Egidy c, P. Figuera ~, H. Fuchs a, J. Galin a, F. Goldenbaum a, Ye.S. Golubeva e, K. Gulda f, D. Hilscher a, A.S. Iljinove, U. Jahnke a, J. Jastrzebski f, W. Kurcewicz f, B. Lott a, M. Morjean d, G. Pausch g, A. P6ghaire d, D. Polster ~, S. Prosehitzki h, B. Quednau a, H. Rossner a, S. Sehmid c, W. Schmid c, P. Ziem a ~Hahn-Meitner-Institut Berlin, D-14091 Berlin, Germany bCERN-PPE, CH-1211-Geneva 21, Switzerland cTU-Miinehen, D-85748 Garching, Germany dGANIL (IN2P3-CNRS, DSM-CEA), F-014076 Caen-Cedex 05, France eINR, Rassian Academy of Science, 117312 Moscow, Russia fWarsaw University, ul. Pasteura 5a, 02-093 Warszawa, Poland gFZ-Rossendorf, D-01314 Dresden, Germany hIPN Otsay, F-91406 Orsay-Cedex, France Energetic antiproton-nucleus interactions allow to build up thermal excitation energies exceeding 800 MeV in nuclei of masses close to 200. We found that for these high excitation energies the fission process still remains a very significant exit channel. At excitation energies close to 800 MeV the fission process approaches 20-50% of the cross section for Ho, Au ~nd U-like nuclei depending on the fission events selection criteria. The average intermediate mass fragment multiplicity at such high excitation energies is equal to about 1 and alpha particle multiplicity is equal to about 5 for all three reactions independently of the exit channel, including fission. A transition towards the multifragmentation is not observed.. 1. I N T R O D U C T I O N In nuclear physics the formation and decay of hot nuclei is a subject of great theoretical and experimental interest since many years. This widespread research is based on the interest in the basic properties of hot nuclear matter. Due to the unique superposition of the short range nuclear force and the long range Coulomb repulsion in nuclei one expects some kind of a multifragmentation when at high excitation energies the surface tension is thought to vanish. This subject was extensively studied with heavy ion induced reactions where the purely statistical properties are superimposed on collective effects like deformation, compression and high angular momentum. Such collective effects are 0375-947Z./99/$ see front matter © 1999 ElsevierScience B.V. All fights reserved. PII S0375-9474(99)00212-2
270c
L. Pienkowsl~" et al./Nuclear Physics A655 (1999) 269c-274c
somewhat reduced in light ion induced reactions. The situation is even better in reactions induced by energetic antiprotons where the nuclear excitation is mediated by pions in a "radiation-like heating"[1]. In addition to the softer way of heating with antiprotons one can produce hotter nuclei than in light-ion induced reactions. PS208 was designed to detect all nuclear particles including charged mesons emitted into 4~r with high efficiency: prompt if-rays, neutrons, light charged particles, fission fragments and to some extent also heavy residues. Since the detector system is sensitive to the total reaction cross section it is possible to deduce relative probabilities for various exit channels. 2. E X P E R I M E N T
The experiment PS208 was performed at L E A R / C E R N with stopped and energetic antiprotons of 1.22 GeV, we will report here on recent progress in the data analysis of 1.22 GeV antiproton induced reactions on Ho, Au and U targets of thickness 1-2 m g / c m 2. The multiplicity of neutrons evaporated from the hot nuclei was measured with high efficiency (about 85%) by the 4~r Berlin Neutron Ball (BNB), a spherical shell of 1500 1 gadolinium loaded liquid scintillator. Inside BNB, in the center of a 40 cm diameter reaction chamber the Berlin Silicon Ball (BSiB) was installed. BSiB is also a 4~r detector, a sphere of 20 cm diameter, composed of 158 independent silicon detectors of 500 #m thickness which are able to detect protons and all heavier charged fragments with high geometrical efficiency of about 85% and low energy thresholds (1-2 MeV). The experimental methods and previous results can be tbund in ref. [2-6]. 3. E X P E R I M E N T A L
RESULTS AND DISCUSSION
One of the first results from PS208 experiment was a combined information from BNB and BSiB detectors: the measured correlation between neutron multiplicity and light charged particle (LCP, particles not heavier than alpha particles) multiplicity. It was found that LCP are emitted isotropieally strongly indicating emission from a thermally equilibrated source. This information was sufficient to deduce event-by-event the excitation energy of the produced nucleus. The procedure to extract the excitation energy is presented in ref. [2]. For heavy targets such as Ho, Au and U the thermal excitation energies exceed 800 MeV and the further data analysis was focused on the decay mechanism of these hot nuclei. Fig. 1 presents as an example for the reaction 1.22 GeV ~+U the deduced fragment mass distributions for various excitation energies. The fragments were identified by means of the measured energy and associated time of flight over a path length of 10 cm. Mass resolution (RMS) for fragment mass A=20 is about 3 mass units and for A=100 is about 10 mass units. At relatively low excitation energy, below 350 MeV, the picture seems to be clear. The fission fragments are in the mass regime around A = l l 0 and are well separated from the intermediate-mass fragments (IMF) and LCP. The expected mass peak for heavy residues (HR), fragments with mass around A=200, cannot be found in Fig. 1 at the lowest E* bin. The HR formed at low excitation energy are too slow to escape the uranium target of 2 m g / e m 2 thickness with enough energy of at least 1-2 MeV in order to be detected. At larger excitation energies the picture becomes more and more complicated and the question is whether any new decay mechanisms can be identified.
L. Pienkowski et al./Nuclear Physics A655 (1999) 269c-274c
8
250
l0 4 ~ 0
<
10 4 ~
< + < v
1
0
350
~
271c
[-
200 150
.
100 10 4. 102
700
~,, L
,3 50 100 150 200 Fragment mass Figure 1. Fragment mass distribution at three excitation energy (E*) bins for ~-induced reaction on uranium target.
50
O+U
0 0 ":/6b"£b
~+Au o" :6b
~+Ho . . .o.
8oo E* (MeV)
Figure 2. Mean value (.) of the sum A1 and A2 as a function of excitation energy for p + U, Au and Ho. A1,2 are the masses of the two heaviest detected fragments. The error bars correspond to RMS of the distribution. The dotted lines are the result of the Inter Nuclear Cascade (INC) model followed by the statistical model calculations (GEMINI). The solid (dashed) lines correspond to the total detected mass, ATOT, (not) including IMFs, not corrected for detection efficiency.
The events with at least two detected heavy fragments were selected for further data analysis. Figure 2 shows the sum of the masses of the two heaviest detected fragments as a function of excitation energy. This plot was performed for a selected subset of data with the condition that both fragment masses A1 and A2 are larger than 20 mass units. If we maintain this condition but include into the sum of A1 + A2 also the masses of all other detected particles (n, p, (~, IMFs) then we get the total detected mass ATOT indicated by the the solid line which actually agrees very well with the calculated mass after the prompt iatra-nuclear cascade (INC) cascade (the fast light ejectiles being detected with a low efficiency). The contribution from IMFs is demonstrated by comparing the solid and dashed lines, with the latter not including IMFs contribution in the total detected mass. Since the, mean IMF multiplicity is about one at high excitation energy we see that the mean mass carried off by an IMF is about 10 mass units. The sum of the fragment masses A1 and A2 was compared with a theoretical prediction. The mocel calculations were performed in two steps. The fast part of the reaction was simulated by INC model [7,8] and the decay of the hot thermalized nucleus was simulated by the standard statistical code GEMINI [9,10] with a constant level density parameter a=A/10, without any correction for dynamical effects. The experimental results presented in Fig. 2 are in good quantitative agreement with the theoretical prediction in which the two heaviest fragments are fission fragments. This indicates that it is interesting to look closer at other properties of events with two heavy fragments detected. The fluctuations in the statistical decay chain can result in considerable recoil energies.
272c
L. Pienkowski et al./Nuclear Physics A655 (1999) 269c-274c
4 ~ ~*U
p+U
0
I 2
.....
! 4o 20
>
0
;>
20
350
i 4
.... :;~ . . . . . .
2 0
°
• ....
;i
0 120
~ 80
11130
~ 40 ..... 0 700
120
20
~°
~ 10 -4
-2
0
2
4
~0
Vii (cm/ns) Figure 3. Invariant cross section (in relative units) of alpha-particles plotted versus vii and v± in lab system with respect to the recoil direction of the fissioning nucleus for p-induced reactions on U at three bins of excitation energies. The plot was performed for events selected with the condition that both fission fragments are heavier than 20 mass units.
U° -4
-2
0
2 4 Vii (cm/ns)
Figure 4. As in Fig. 3, but velocities are plotted with respect to the fission axis in the lab system. The included histograms correspond to the velocity distributions of the fission fragments A1 and A2 at positive and negative vii, respectively, and A1 >_ A2.
This is very nicely demonstrated in Fig. 3 showing the Galilean invariant cross section of a-particles with respect to the recoil momentum of the fissioning nucleus deduced from the velocity vectors of the two fission fragments. We observe that the alpha particles are mostly emitted in a direction opposite to the momentum of the fissioning nucleus. This correlation explains the relatively large velocity of the fissioning nuclei and large energy of detected heavy residues at high excitation energies, what cannot be explained only by the momentum transfer from the incident energetic antiproton. One of the conclusions from the nuclear fission studies is that this process is slow compared to the evaporation of neutrons, LCPs and IMFs. For review see for example ref. [11]. This conclusion was obtained from the angular correlations of light particles with respect to the fission axis. Figure 4 displays the invariant cross section of alpha particles versus vii and v± with respect to the fission axis. The important point is that the tips of the velocity vectors are almost evenly located on a circle at vii -- 0, indicating that the alphas are essentially emitted isotropically prior to acceleration of the fission fragments in their mutual Coulomb field. If the a-particles were evaporated from the accelerated fission fragments we would expect circles around the centers of the respective fragment velocities• The observations presented in Figures 2, 4 are consistent with the expectations of binary and slow fission, which can be observed up to very high excitation energies. It
L. Pienkowski et al./Nuclear Physics A655 (1999) 269c-274c
0.6
_
O+U %.° ~
p+Au.~TT~fl
273c
~+Ho
"..,i~I
~,0.4
-~ 0.2
"". . . . . . . . . . . . . . . . . .
""
~,:; ............. IL.~.-.~--.-~-, 0 0 ' 2 5 0 500 750 " 0
250500750
0
250500750
E (MeV) Figure 5. Fission probability for the reaction 1.22 GeV p + U, Au and Ho as a function of excitation energy. The open (filled) points represent the results for events selected with a condition that both fission fragments are heavier than 20 (40) mass units. The solid (dashed) lines are the result of the INC model followed by the evaporation calculations (GEMINI) and fission events selected with a condition that both fission fragments are heavier ;han 20 (40) mass units.
should be added that even at the highest excitation energy the relative velocity of the two heaviest fragments, is very close to the Coulomb velocity due to the repulsion of two sticking spherical fragments, what also fits in the binary fission scenario [12]. Fig. 5 shows the fission probability as function of excitation energy for ~-induced reactions on three heavy targets: U, Au and Ho. The measured probability to detect fission fragments was corrected for detection efficiency according to a simulation program. It is observed that at excitation energies up to 800 MeV the fission process approaches 20-50% of the cross section for U, Au and Ho-like nuclei, depending on the fission events selection criteria. The integrated fission probabilities, Pf, for the 1.22 GeV p induced reaction on U, Au and Ho are: Pf = 49 :t: 5, 16 4- 3 and 14 + 3 %, respectively. These results are for fission events selected with a condition that both fragments are heavier than 20 mass units. In the case of the data set selected with a condition that both fragments are heavier than 40 mass units the fission probabilities for U, Au and Ho are: Pf = 44 + 5, 9 + 3 and 6 + 2 %, respectively and these values are in good agreement with previously published results i:a ref. [6] which were obtained with a somewhat different data analysis method. The fission probability at high excitation energy for events selected with a condition that both fragments are heavier than 40 mass units is much smaller than in the case when the fission fragments mass cut is defined as A1,2 > 20 mass units (see Fig. 5). This is related to the observation that the mass of the fissioning nucleus decreases with increasing excitation energy as indicated in Fig. 2. At high excitation energy up to 30-40% of the available mass is carried off by neutrons, LCP and IMF emission. Additionally at high excitation energy we observe that the mass asymmetry distribution of the two heaviest fragmenLs is much broader than at low excitation energy. Fissioa probabilities for p induced reactions on Au and Ho predicted by the INC model followed by the standard statistical code GEMINI are also shown in Fig. 5. They differ sig-
274c
L. Pienkowsla" et al. /Nuclear Physics .4655 (1999) 269c-274c
nificantly from the experimental data at both low and high excitation energies. It should be stressed that the calculations were performed with a very simple set of parameters available in the GEMINI code. The level density parameter was taken to be a=A/10, no difference between the level density parameter at the equilibrium deformation and at the saddle point deformation was assumed nor any correction for dynamical effects was done. This simplification can explain the differences only at low excitation energy, whereas at high excitation energy the statistical model might be questionable. It is also of particular interest to deduce the probability that the three heaviest fragments are larger than 20 or 40. We find that for the highest excitation energies achieved in the three reactions about 10-15% or less than 1-3% of the fission events contain three fragments with masses larger than 20 or 40, respectively. In other words the probability for a mass split into three fragments heavier than 20 or 40 is about 5-8% or less than 0.2-0.8%, respectively. The possibility of the decay mode with only IMFs emission was also studied. It was found that among all events with total detected mass greater than 70% of the available mass the heaviest fragment is always heavier than 20-30 mass units (heavier than IMF) and the measured average IMF multiplicity, not corrected for detection efficiency (about 85~) at excitation energy above 700 MeV is about 0.7-0.8. 4. C O N C L U S I O N S Antiproton-nucleus interactions at 1.22 GeV allow to dissipate thermal excitation energies exceeding 800 MeV in nuclei of masses close to 200. We have shown that for these high excitation energies the fission process still remains a very significant exit channel. The average intermediate mass fragment (IMF) multiplicity at such high excitation energies is about one and the average mass of IMFs is about 10 mass units. The important role of the fluctuations in the statistical emission of light particles on the recoil kinetic energy was presented. At the highest excitation energies the probability to observe a mass split with three fragments heavier than 20 or 40 is very small. A transition towards the multifragmentation is not observed.
REFERENCES 1. D. Polster et al., Phys. Lett. B30O (1993) 317. 2. F. Goldenbaum et al., Phys. Rev. Lett. 77 (1996) 1230. 3. D. Hilscher et al., Proc. LEAP'94, edited by G. Kernel, P. Kri~an, M. Miku2, (World Scientific 1995) p. 381. 4. U. Jahnke et al., Proc. NAN'95, Yadernaya Fizika 59 (1996) 1625. 5. B. Lott et al., Proc. LEAP'96, Nucl. Phys. B (Proc. Suppl.) 56 A (1997) 114. 6. S. Schmid et al., Z. Phys. A 359 (1997) 27. 7. A.S. Iljinov et al., CRC press, (1994). 8. Ye.S. Golubeva et al., Nucl. Phys. A 483 (1988) 539; A.S. Iljinov et al., Nucl. Phys.
9. i0. II. 12.
A 382 (1982) 378. R.J. Charity et al., Nucl. Phys. A 483 (1988) 371. R.J. Charity, Phys. Rev. C 51 (1995) 217. D. Hilscher and H. Rossner, Ann. Phys. Fr. 17 (1992) 471. V.E. Viola et al., Phys. Rev. C 31 (1985) 1550.
NUCLEAR PHYSICS A
ELSEVIER
Nuclear Physics A655 (1999) 269e-274e
www.elsevier.nl/locate/npe
Decay of Hot NucleiFormedwith EnergeticAntiprotons (PS208) L. Pienkowski a'f, W. Bohne a, J. Eades b, T.v.Egidy c, P. Figuera ~, H. Fuchs a, J. Galin a, F. Goldenbaum a, Ye.S. Golubeva e, K. Gulda f, D. Hilscher a, A.S. Iljinove, U. Jahnke a, J. Jastrzebski f, W. Kurcewicz f, B. Lott a, M. Morjean d, G. Pausch g, A. P6ghaire d, D. Polster ~, S. Prosehitzki h, B. Quednau a, H. Rossner a, S. Sehmid c, W. Schmid c, P. Ziem a ~Hahn-Meitner-Institut Berlin, D-14091 Berlin, Germany bCERN-PPE, CH-1211-Geneva 21, Switzerland cTU-Miinehen, D-85748 Garching, Germany dGANIL (IN2P3-CNRS, DSM-CEA), F-014076 Caen-Cedex 05, France eINR, Rassian Academy of Science, 117312 Moscow, Russia fWarsaw University, ul. Pasteura 5a, 02-093 Warszawa, Poland gFZ-Rossendorf, D-01314 Dresden, Germany hIPN Otsay, F-91406 Orsay-Cedex, France Energetic antiproton-nucleus interactions allow to build up thermal excitation energies exceeding 800 MeV in nuclei of masses close to 200. We found that for these high excitation energies the fission process still remains a very significant exit channel. At excitation energies close to 800 MeV the fission process approaches 20-50% of the cross section for Ho, Au ~nd U-like nuclei depending on the fission events selection criteria. The average intermediate mass fragment multiplicity at such high excitation energies is equal to about 1 and alpha particle multiplicity is equal to about 5 for all three reactions independently of the exit channel, including fission. A transition towards the multifragmentation is not observed.. 1. I N T R O D U C T I O N In nuclear physics the formation and decay of hot nuclei is a subject of great theoretical and experimental interest since many years. This widespread research is based on the interest in the basic properties of hot nuclear matter. Due to the unique superposition of the short range nuclear force and the long range Coulomb repulsion in nuclei one expects some kind of a multifragmentation when at high excitation energies the surface tension is thought to vanish. This subject was extensively studied with heavy ion induced reactions where the purely statistical properties are superimposed on collective effects like deformation, compression and high angular momentum. Such collective effects are 0375-947Z./99/$ see front matter © 1999 ElsevierScience B.V. All fights reserved. PII S0375-9474(99)00212-2
270c
L. Pienkowsl~" et al./Nuclear Physics A655 (1999) 269c-274c
somewhat reduced in light ion induced reactions. The situation is even better in reactions induced by energetic antiprotons where the nuclear excitation is mediated by pions in a "radiation-like heating"[1]. In addition to the softer way of heating with antiprotons one can produce hotter nuclei than in light-ion induced reactions. PS208 was designed to detect all nuclear particles including charged mesons emitted into 4~r with high efficiency: prompt if-rays, neutrons, light charged particles, fission fragments and to some extent also heavy residues. Since the detector system is sensitive to the total reaction cross section it is possible to deduce relative probabilities for various exit channels. 2. E X P E R I M E N T
The experiment PS208 was performed at L E A R / C E R N with stopped and energetic antiprotons of 1.22 GeV, we will report here on recent progress in the data analysis of 1.22 GeV antiproton induced reactions on Ho, Au and U targets of thickness 1-2 m g / c m 2. The multiplicity of neutrons evaporated from the hot nuclei was measured with high efficiency (about 85%) by the 4~r Berlin Neutron Ball (BNB), a spherical shell of 1500 1 gadolinium loaded liquid scintillator. Inside BNB, in the center of a 40 cm diameter reaction chamber the Berlin Silicon Ball (BSiB) was installed. BSiB is also a 4~r detector, a sphere of 20 cm diameter, composed of 158 independent silicon detectors of 500 #m thickness which are able to detect protons and all heavier charged fragments with high geometrical efficiency of about 85% and low energy thresholds (1-2 MeV). The experimental methods and previous results can be tbund in ref. [2-6]. 3. E X P E R I M E N T A L
RESULTS AND DISCUSSION
One of the first results from PS208 experiment was a combined information from BNB and BSiB detectors: the measured correlation between neutron multiplicity and light charged particle (LCP, particles not heavier than alpha particles) multiplicity. It was found that LCP are emitted isotropieally strongly indicating emission from a thermally equilibrated source. This information was sufficient to deduce event-by-event the excitation energy of the produced nucleus. The procedure to extract the excitation energy is presented in ref. [2]. For heavy targets such as Ho, Au and U the thermal excitation energies exceed 800 MeV and the further data analysis was focused on the decay mechanism of these hot nuclei. Fig. 1 presents as an example for the reaction 1.22 GeV ~+U the deduced fragment mass distributions for various excitation energies. The fragments were identified by means of the measured energy and associated time of flight over a path length of 10 cm. Mass resolution (RMS) for fragment mass A=20 is about 3 mass units and for A=100 is about 10 mass units. At relatively low excitation energy, below 350 MeV, the picture seems to be clear. The fission fragments are in the mass regime around A = l l 0 and are well separated from the intermediate-mass fragments (IMF) and LCP. The expected mass peak for heavy residues (HR), fragments with mass around A=200, cannot be found in Fig. 1 at the lowest E* bin. The HR formed at low excitation energy are too slow to escape the uranium target of 2 m g / e m 2 thickness with enough energy of at least 1-2 MeV in order to be detected. At larger excitation energies the picture becomes more and more complicated and the question is whether any new decay mechanisms can be identified.
L. Pienkowski et al./Nuclear Physics A655 (1999) 269c-274c
8
250
l0 4 ~ 0
<
10 4 ~
< + < v
1
0
350
~
271c
[-
200 150
.
100 10 4. 102
700
~,, L
,3 50 100 150 200 Fragment mass Figure 1. Fragment mass distribution at three excitation energy (E*) bins for ~-induced reaction on uranium target.
50
O+U
0 0 ":/6b"£b
~+Au o" :6b
~+Ho . . .o.
8oo E* (MeV)
Figure 2. Mean value (.) of the sum A1 and A2 as a function of excitation energy for p + U, Au and Ho. A1,2 are the masses of the two heaviest detected fragments. The error bars correspond to RMS of the distribution. The dotted lines are the result of the Inter Nuclear Cascade (INC) model followed by the statistical model calculations (GEMINI). The solid (dashed) lines correspond to the total detected mass, ATOT, (not) including IMFs, not corrected for detection efficiency.
The events with at least two detected heavy fragments were selected for further data analysis. Figure 2 shows the sum of the masses of the two heaviest detected fragments as a function of excitation energy. This plot was performed for a selected subset of data with the condition that both fragment masses A1 and A2 are larger than 20 mass units. If we maintain this condition but include into the sum of A1 + A2 also the masses of all other detected particles (n, p, (~, IMFs) then we get the total detected mass ATOT indicated by the the solid line which actually agrees very well with the calculated mass after the prompt iatra-nuclear cascade (INC) cascade (the fast light ejectiles being detected with a low efficiency). The contribution from IMFs is demonstrated by comparing the solid and dashed lines, with the latter not including IMFs contribution in the total detected mass. Since the, mean IMF multiplicity is about one at high excitation energy we see that the mean mass carried off by an IMF is about 10 mass units. The sum of the fragment masses A1 and A2 was compared with a theoretical prediction. The mocel calculations were performed in two steps. The fast part of the reaction was simulated by INC model [7,8] and the decay of the hot thermalized nucleus was simulated by the standard statistical code GEMINI [9,10] with a constant level density parameter a=A/10, without any correction for dynamical effects. The experimental results presented in Fig. 2 are in good quantitative agreement with the theoretical prediction in which the two heaviest fragments are fission fragments. This indicates that it is interesting to look closer at other properties of events with two heavy fragments detected. The fluctuations in the statistical decay chain can result in considerable recoil energies.
272c
L. Pienkowski et al./Nuclear Physics A655 (1999) 269c-274c
4 ~ ~*U
p+U
0
I 2
.....
! 4o 20
>
0
;>
20
350
i 4
.... :;~ . . . . . .
2 0
°
• ....
;i
0 120
~ 80
11130
~ 40 ..... 0 700
120
20
~°
~ 10 -4
-2
0
2
4
~0
Vii (cm/ns) Figure 3. Invariant cross section (in relative units) of alpha-particles plotted versus vii and v± in lab system with respect to the recoil direction of the fissioning nucleus for p-induced reactions on U at three bins of excitation energies. The plot was performed for events selected with the condition that both fission fragments are heavier than 20 mass units.
U° -4
-2
0
2 4 Vii (cm/ns)
Figure 4. As in Fig. 3, but velocities are plotted with respect to the fission axis in the lab system. The included histograms correspond to the velocity distributions of the fission fragments A1 and A2 at positive and negative vii, respectively, and A1 >_ A2.
This is very nicely demonstrated in Fig. 3 showing the Galilean invariant cross section of a-particles with respect to the recoil momentum of the fissioning nucleus deduced from the velocity vectors of the two fission fragments. We observe that the alpha particles are mostly emitted in a direction opposite to the momentum of the fissioning nucleus. This correlation explains the relatively large velocity of the fissioning nuclei and large energy of detected heavy residues at high excitation energies, what cannot be explained only by the momentum transfer from the incident energetic antiproton. One of the conclusions from the nuclear fission studies is that this process is slow compared to the evaporation of neutrons, LCPs and IMFs. For review see for example ref. [11]. This conclusion was obtained from the angular correlations of light particles with respect to the fission axis. Figure 4 displays the invariant cross section of alpha particles versus vii and v± with respect to the fission axis. The important point is that the tips of the velocity vectors are almost evenly located on a circle at vii -- 0, indicating that the alphas are essentially emitted isotropically prior to acceleration of the fission fragments in their mutual Coulomb field. If the a-particles were evaporated from the accelerated fission fragments we would expect circles around the centers of the respective fragment velocities• The observations presented in Figures 2, 4 are consistent with the expectations of binary and slow fission, which can be observed up to very high excitation energies. It
L. Pienkowski et al./Nuclear Physics A655 (1999) 269c-274c
0.6
_
O+U %.° ~
p+Au.~TT~fl
273c
~+Ho
"..,i~I
~,0.4
-~ 0.2
"". . . . . . . . . . . . . . . . . .
""
~,:; ............. IL.~.-.~--.-~-, 0 0 ' 2 5 0 500 750 " 0
250500750
0
250500750
E (MeV) Figure 5. Fission probability for the reaction 1.22 GeV p + U, Au and Ho as a function of excitation energy. The open (filled) points represent the results for events selected with a condition that both fission fragments are heavier than 20 (40) mass units. The solid (dashed) lines are the result of the INC model followed by the evaporation calculations (GEMINI) and fission events selected with a condition that both fission fragments are heavier ;han 20 (40) mass units.
should be added that even at the highest excitation energy the relative velocity of the two heaviest fragments, is very close to the Coulomb velocity due to the repulsion of two sticking spherical fragments, what also fits in the binary fission scenario [12]. Fig. 5 shows the fission probability as function of excitation energy for ~-induced reactions on three heavy targets: U, Au and Ho. The measured probability to detect fission fragments was corrected for detection efficiency according to a simulation program. It is observed that at excitation energies up to 800 MeV the fission process approaches 20-50% of the cross section for U, Au and Ho-like nuclei, depending on the fission events selection criteria. The integrated fission probabilities, Pf, for the 1.22 GeV p induced reaction on U, Au and Ho are: Pf = 49 :t: 5, 16 4- 3 and 14 + 3 %, respectively. These results are for fission events selected with a condition that both fragments are heavier than 20 mass units. In the case of the data set selected with a condition that both fragments are heavier than 40 mass units the fission probabilities for U, Au and Ho are: Pf = 44 + 5, 9 + 3 and 6 + 2 %, respectively and these values are in good agreement with previously published results i:a ref. [6] which were obtained with a somewhat different data analysis method. The fission probability at high excitation energy for events selected with a condition that both fragments are heavier than 40 mass units is much smaller than in the case when the fission fragments mass cut is defined as A1,2 > 20 mass units (see Fig. 5). This is related to the observation that the mass of the fissioning nucleus decreases with increasing excitation energy as indicated in Fig. 2. At high excitation energy up to 30-40% of the available mass is carried off by neutrons, LCP and IMF emission. Additionally at high excitation energy we observe that the mass asymmetry distribution of the two heaviest fragmenLs is much broader than at low excitation energy. Fissioa probabilities for p induced reactions on Au and Ho predicted by the INC model followed by the standard statistical code GEMINI are also shown in Fig. 5. They differ sig-
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L. Pienkowsla" et al. /Nuclear Physics .4655 (1999) 269c-274c
nificantly from the experimental data at both low and high excitation energies. It should be stressed that the calculations were performed with a very simple set of parameters available in the GEMINI code. The level density parameter was taken to be a=A/10, no difference between the level density parameter at the equilibrium deformation and at the saddle point deformation was assumed nor any correction for dynamical effects was done. This simplification can explain the differences only at low excitation energy, whereas at high excitation energy the statistical model might be questionable. It is also of particular interest to deduce the probability that the three heaviest fragments are larger than 20 or 40. We find that for the highest excitation energies achieved in the three reactions about 10-15% or less than 1-3% of the fission events contain three fragments with masses larger than 20 or 40, respectively. In other words the probability for a mass split into three fragments heavier than 20 or 40 is about 5-8% or less than 0.2-0.8%, respectively. The possibility of the decay mode with only IMFs emission was also studied. It was found that among all events with total detected mass greater than 70% of the available mass the heaviest fragment is always heavier than 20-30 mass units (heavier than IMF) and the measured average IMF multiplicity, not corrected for detection efficiency (about 85~) at excitation energy above 700 MeV is about 0.7-0.8. 4. C O N C L U S I O N S Antiproton-nucleus interactions at 1.22 GeV allow to dissipate thermal excitation energies exceeding 800 MeV in nuclei of masses close to 200. We have shown that for these high excitation energies the fission process still remains a very significant exit channel. The average intermediate mass fragment (IMF) multiplicity at such high excitation energies is about one and the average mass of IMFs is about 10 mass units. The important role of the fluctuations in the statistical emission of light particles on the recoil kinetic energy was presented. At the highest excitation energies the probability to observe a mass split with three fragments heavier than 20 or 40 is very small. A transition towards the multifragmentation is not observed.
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