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Particle emission time sequence in intermediate energy heavy ion reactions
Nuclear Physics A72 1 (2003) 3 13~~316~ www.elsevier.comilocateinpe
Particle emission reactions
time
sequence
in intermediate
energy
heavy ion
R. Ghetti,a J. Helgesson,b and the CHIC Collaboration. 9epartment
of Physics, Lund University, Box 118, S-221 00 Lund,
Sweden
bSchoo1of Technology and Society, Malma University, S-205 06 MalmB, Sweden The CHIC Collaboration has performed a series of simultaneous pp, np and nn fermion interferometry experiments in order to study the space-time evolution of the emission sources in intermediate energy heavy ion reactions. Furthermore, when correlations of non-identical particles are available, additional model independent information on the emission chronology of the particles is obtained. In this contribution, after reviewing the method to determine the order of emission of non-identical particles, we discuss the emission chronology of neutrons and protons from the E/A = 45 MeV 58Ni + 27A1reaction and from the E/A = 60 MeV 36Ar + 27A1reaction, shown here for the first time. 1. INTRODUCTION Two-nucleon correlation functions are normally utilized to extract information on the size of the emitting source and on the time duration of the emission. The combined information from singles n and p spectra, pp, 7272and np correlation functions is very powerful to set constraints on reaction models [l]. Furthermore, when correlations of non-identical particles are considered, additional model-independent information on the emission chronology of the particles is obtained [2]. The possibility to access information on the particle emission time sequence from the np correlation function, is connected with the fact that an average time difference in the emission times implies a difference in the average distance between the two particles (when they start to interact) for pairs E, > EP and En < EP [3]. These different pairs will in turn experience a different strength of the nuclear final state interaction. From a merely classical viewpoint, those pairs with a smaller average distance experience a stronger interaction and exhibit an enhanced correlation (or anticorrelation) strength, as compared to those pairs with a larger average particle distance. Thus, the neutron-proton emission chronology can be studied by constructing the correlation functions Cn(q), gated on pairs En > EP, and C,(q), gated on pairs E,, < Ep, and looking at their ratio C,,fCP. If the proton is emitted earlier (later) than the neutron, the ratio &/C, will show a peak (dip) in the region of q where there is a correlation, a dip (peak) where there is an anticorrelation, and will approach unity both for q + 0 (since the energy difference of the two emitted particles is negligible) and q + 00 (since modifications of the two-particle phase space density arising from final state interactions are negligible). 0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V. All rights reserved. dOi:lO.1016/So37~-~474(o3)olos8-3
R. Ghetti et al. /Nuclear Physics A721 (2003) 313c-316~
314c
2. EXPERIMENTAL
RESULTS
The experimental correlation function is constructed by dividing the coincidence yield by the yield of uncorrelated events, coiistructed from the product of the singles distributions. After introducing the relative momentum a= (pi -&)/2 and the total momentum Pyb, = pi + & of the particle pair, the correlation function can be rewritten as: W, P;t) = k . Wq’, P;,)lN&Z P-t,,t) , w h ere the normalization constant k is determined so that the correlation function goes to unity at large values of Q, where no correlations are expected. In all the correlation functions shown here, the normalization range is 80 < 4 < 120 MeV/c. For all the data shown here, the neutron and proton kinetic energies, E, and Ep, are calculated in the reference frame of a source moving with a velocity determined from a Maxwell-Boltzmann fit to the single particle kinetic energy distributions. A single normalization constant, calculated from the ungated correlation function, is utilized for both energy-gated correlation functions Cn(q) and C,(q). 2.1. Results from the LNS experiment The np correlation function for the E/A = 45 MeV 58Ni + 27A1 reaction was measured at the superconducting cyclotron of LNS, Catania. The np interferometer was made of 13 CsI detectors for protons and 12 liquid scintillators for neutrons, covering the angular range &b=39-51” and centered at 45O (Fig. 1). Two additional liquid scintillator arrays, centered at 25“ and 90°, were utilized to detect large-angle np pairs, used for normalization purposes. Details on the detector setup, particle identification, energy calibrations and background corrections can be found in Ref. [I]. 95”
83”
51”
39”
28”
16”
114” 102” 90”
78”
66”
54”
lt.i.
l ooDoooDoooDeooD l DoDoDoB*D*DoDoD l oomooomooe~eee~
Figure 1. LNS interferometer.
Figure 2. KVI interferometer.
20~.
42”
30”
Shown in Fig. 3 are the np correlation function C(q) (solid dots in panels a-e), compared to Cn(q) (open circles in panels a,d) and C,(q) (open squares in panels b,e). The ratio Cn[CP is shown in panels c,f. In order to disentangle the interplay of dynamical (preequilibrium) and statistical emission, a gate is placed on the total momentum Ptot of the particle pair, calculated in the frame of the particle emitting source moving with a source velocity of 0.16~. The results shown in Fig. 3 left panels, are obtained for low-totalmomentum pairs (P,,, < 300 MeV/c). Th ose of Fig. 3 right panels, instead, represent the complementary gate, Ptot > 300 MeV/c, that is believed to enhance preequilibrium emission. The high Ptot selection slightly enhances the correlation function strength and the Cn/CP ratio, in correspondence to the correlation region (q < 25 MeV/c). The comple-
R. Ghetti et al. /Nuclear Physics A721 (2003) 313~316~
P,<300MeV/c
2
315c
P,>JOOMeV/c
1.5
0.5 0.5 0 25 50 ? 1.5 1 wuCP(‘4 1:
: r++t+++,
75
100
0 25 50 I (c) 1 CW/CPh)
I$++bwm.
75
100
25
50
75
25
50
75
100
0
25
50
0
25
50
75
100
0
25
50
75
100
75
100
(f)
++~~-+$&+
0.5 0
-0
100
0
25
Relative Momentum,
50
75
100
q (MeV/c)
Figure 3. E/A=45 MeV 58Ni+27A1reaction, measuredat &&,=39°-510 at LNS.
Relative Momentum,
q (MeV/c)
Figure 4. Preliminary E/A=60 MeV 36Ar+27A1 reaction, measured at &b=30°-60’ at KVI.
mentary low-total-momentum gate, causes a slight suppression in the correlation function strength, a weak anticorrelation for q x 20-50 MeV/c, and a Cn/Cp ratio quite close to unity for all values of q. The small enhancement of the C,,/Cr ratio in the low-q region observed in Fig. 3f, and confirmed by the analysis of the correlation function gated on large values of the parallel velocity of the particle pairs [2], might indicate that for those np pairs, protons are on the average emitted earlier than neutrons [2]. However, the experimental effect is rather small, and further investigations are needed to unambiguously asses the neutron-proton time emission sequence. This was the main motivation for the new KVI experiment, described in the next section. 2.2. Results from the KVI experiment The E/A = 60 MeV 36Ar + 27A1 reaction was measured at the AGOR cyclotron of KVI in February 2002. The data are presented here for the first time. Protons (as well as deuterons and tritons) were measured with 16 CsI proton detectors, covering polar angles 6~,b=30°-l140. Neutrons were detected with a neutron setup of 32 liquid scintillators, placed at 8~,b=36°-126”. A schematic representation of the detector setup is shown in Fig. 2. The preliminary emission chronology analysis is performed for neutrons and protons detected with 4 CsI and 4 liquid scintillators covering the “forward” angular region (81ab=300-60’). The result for a sample of about 100.000 np pairs is shown in Fig. 4. The left panels show the results from pairs with &,t < 300 MeV/c (calculated in the frame of a source moving with velocity 0.25~). The complementary gate (P,,, > 300 MeV/c) is shown in Fig. 4, right panels. Although the statistics is low, the results are similar to those obtained for the E/A = 45 MeV 58Ni + 27A1 reaction. The high Ptot selection enhances the overall np correlation function strength and yields a Cn/Cp
316~
R. Ghetti et al. /Nuclear Physics A721 (2003) 313c-316~
ratio slightly larger than unity in the low-q region. The complementary gate (PtO, < 300 MeV/c) yields a Cn/Cp ratio consistent with unity for all values of q.
’C(q)
1.s 1.6
-
o WI)
1.4-
D Cp(q)
1.2 :;;-
jiti+$’
0.4 o.50-
0 Relative
Momentum.
q (M&‘/c)
* Cn(q)/Cp(q)
0
j~i;l,&. 1 ‘I’?++
311, “II
I I. I,, b I,, I.. c L 20 40 60 so 100 120 Relative Momentum. q (M&4/c)
Figure 5. Preliminary E/A=60 MeV 36Ar+27A1reaction, measuredat &&=84°-1200 at KVI. The higher capability of the KVI interferometer allows us to investigate the neutronproton emission chronology also for more backward emission, where the contribution from the preequilibrium and midrapidity sources should be suppressed [4]. The preliminary result from this analysis, performed with about 80.000 np pairs collected with the most backward detectors (5 CsI crystals and 14 liquid scintillators covering the angular region 8~,b=84°-120”), is shown in Fig. 5. For this data set, no further gates are applied, due to the limited statistics. The C,,/Cp ratio, shown in Fig. 5 right panel, is substantially below unity in the correlation region, indicating that for these np pairs, neutrons are on the average emitted earlier than protons. 3. CONCLUSIONS The experimental results from differently gated np correlation functions support the interpretation that for forward emitted- high-total-momentum selected events, that enhance projectile-like and/or intermediate velocity sources, the proton is on average emitted earlier than the neutron. Selection of events that suppresses preequilibrium and midrapidity source emission, instead, indicates a shorter average emission time for neutrons as compared to protons. These results are new and puzzling. They deserve to be paied attention to and ,to receive theoretical investigation. REFERENCES 1. 2. 3. 4.
R. Ghetti, et al., Nucl. Phys. A660 (1999) 20; Nucl. Phys. A674 (2000) 277. R. Ghetti, et al., Phys. Rev. Lett., 87 (2001) 102701-l. R. Lednicky, V.L. Lyuboshitz, B. Erazmus, D. Nouais, Phys. Lett. B 373 (1996) 30. Ph. Eudes, et al., Phys. Rev. C 56 (1997) 2003.
Particle emission reactions
time
sequence
in intermediate
energy
heavy ion
R. Ghetti,a J. Helgesson,b and the CHIC Collaboration. 9epartment
of Physics, Lund University, Box 118, S-221 00 Lund,
Sweden
bSchoo1of Technology and Society, Malma University, S-205 06 MalmB, Sweden The CHIC Collaboration has performed a series of simultaneous pp, np and nn fermion interferometry experiments in order to study the space-time evolution of the emission sources in intermediate energy heavy ion reactions. Furthermore, when correlations of non-identical particles are available, additional model independent information on the emission chronology of the particles is obtained. In this contribution, after reviewing the method to determine the order of emission of non-identical particles, we discuss the emission chronology of neutrons and protons from the E/A = 45 MeV 58Ni + 27A1reaction and from the E/A = 60 MeV 36Ar + 27A1reaction, shown here for the first time. 1. INTRODUCTION Two-nucleon correlation functions are normally utilized to extract information on the size of the emitting source and on the time duration of the emission. The combined information from singles n and p spectra, pp, 7272and np correlation functions is very powerful to set constraints on reaction models [l]. Furthermore, when correlations of non-identical particles are considered, additional model-independent information on the emission chronology of the particles is obtained [2]. The possibility to access information on the particle emission time sequence from the np correlation function, is connected with the fact that an average time difference in the emission times implies a difference in the average distance between the two particles (when they start to interact) for pairs E, > EP and En < EP [3]. These different pairs will in turn experience a different strength of the nuclear final state interaction. From a merely classical viewpoint, those pairs with a smaller average distance experience a stronger interaction and exhibit an enhanced correlation (or anticorrelation) strength, as compared to those pairs with a larger average particle distance. Thus, the neutron-proton emission chronology can be studied by constructing the correlation functions Cn(q), gated on pairs En > EP, and C,(q), gated on pairs E,, < Ep, and looking at their ratio C,,fCP. If the proton is emitted earlier (later) than the neutron, the ratio &/C, will show a peak (dip) in the region of q where there is a correlation, a dip (peak) where there is an anticorrelation, and will approach unity both for q + 0 (since the energy difference of the two emitted particles is negligible) and q + 00 (since modifications of the two-particle phase space density arising from final state interactions are negligible). 0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V. All rights reserved. dOi:lO.1016/So37~-~474(o3)olos8-3
R. Ghetti et al. /Nuclear Physics A721 (2003) 313c-316~
314c
2. EXPERIMENTAL
RESULTS
The experimental correlation function is constructed by dividing the coincidence yield by the yield of uncorrelated events, coiistructed from the product of the singles distributions. After introducing the relative momentum a= (pi -&)/2 and the total momentum Pyb, = pi + & of the particle pair, the correlation function can be rewritten as: W, P;t) = k . Wq’, P;,)lN&Z P-t,,t) , w h ere the normalization constant k is determined so that the correlation function goes to unity at large values of Q, where no correlations are expected. In all the correlation functions shown here, the normalization range is 80 < 4 < 120 MeV/c. For all the data shown here, the neutron and proton kinetic energies, E, and Ep, are calculated in the reference frame of a source moving with a velocity determined from a Maxwell-Boltzmann fit to the single particle kinetic energy distributions. A single normalization constant, calculated from the ungated correlation function, is utilized for both energy-gated correlation functions Cn(q) and C,(q). 2.1. Results from the LNS experiment The np correlation function for the E/A = 45 MeV 58Ni + 27A1 reaction was measured at the superconducting cyclotron of LNS, Catania. The np interferometer was made of 13 CsI detectors for protons and 12 liquid scintillators for neutrons, covering the angular range &b=39-51” and centered at 45O (Fig. 1). Two additional liquid scintillator arrays, centered at 25“ and 90°, were utilized to detect large-angle np pairs, used for normalization purposes. Details on the detector setup, particle identification, energy calibrations and background corrections can be found in Ref. [I]. 95”
83”
51”
39”
28”
16”
114” 102” 90”
78”
66”
54”
lt.i.
l ooDoooDoooDeooD l DoDoDoB*D*DoDoD l oomooomooe~eee~
Figure 1. LNS interferometer.
Figure 2. KVI interferometer.
20~.
42”
30”
Shown in Fig. 3 are the np correlation function C(q) (solid dots in panels a-e), compared to Cn(q) (open circles in panels a,d) and C,(q) (open squares in panels b,e). The ratio Cn[CP is shown in panels c,f. In order to disentangle the interplay of dynamical (preequilibrium) and statistical emission, a gate is placed on the total momentum Ptot of the particle pair, calculated in the frame of the particle emitting source moving with a source velocity of 0.16~. The results shown in Fig. 3 left panels, are obtained for low-totalmomentum pairs (P,,, < 300 MeV/c). Th ose of Fig. 3 right panels, instead, represent the complementary gate, Ptot > 300 MeV/c, that is believed to enhance preequilibrium emission. The high Ptot selection slightly enhances the correlation function strength and the Cn/CP ratio, in correspondence to the correlation region (q < 25 MeV/c). The comple-
R. Ghetti et al. /Nuclear Physics A721 (2003) 313~316~
P,<300MeV/c
2
315c
P,>JOOMeV/c
1.5
0.5 0.5 0 25 50 ? 1.5 1 wuCP(‘4 1:
: r++t+++,
75
100
0 25 50 I (c) 1 CW/CPh)
I$++bwm.
75
100
25
50
75
25
50
75
100
0
25
50
0
25
50
75
100
0
25
50
75
100
75
100
(f)
++~~-+$&+
0.5 0
-0
100
0
25
Relative Momentum,
50
75
100
q (MeV/c)
Figure 3. E/A=45 MeV 58Ni+27A1reaction, measuredat &&,=39°-510 at LNS.
Relative Momentum,
q (MeV/c)
Figure 4. Preliminary E/A=60 MeV 36Ar+27A1 reaction, measured at &b=30°-60’ at KVI.
mentary low-total-momentum gate, causes a slight suppression in the correlation function strength, a weak anticorrelation for q x 20-50 MeV/c, and a Cn/Cp ratio quite close to unity for all values of q. The small enhancement of the C,,/Cr ratio in the low-q region observed in Fig. 3f, and confirmed by the analysis of the correlation function gated on large values of the parallel velocity of the particle pairs [2], might indicate that for those np pairs, protons are on the average emitted earlier than neutrons [2]. However, the experimental effect is rather small, and further investigations are needed to unambiguously asses the neutron-proton time emission sequence. This was the main motivation for the new KVI experiment, described in the next section. 2.2. Results from the KVI experiment The E/A = 60 MeV 36Ar + 27A1 reaction was measured at the AGOR cyclotron of KVI in February 2002. The data are presented here for the first time. Protons (as well as deuterons and tritons) were measured with 16 CsI proton detectors, covering polar angles 6~,b=30°-l140. Neutrons were detected with a neutron setup of 32 liquid scintillators, placed at 8~,b=36°-126”. A schematic representation of the detector setup is shown in Fig. 2. The preliminary emission chronology analysis is performed for neutrons and protons detected with 4 CsI and 4 liquid scintillators covering the “forward” angular region (81ab=300-60’). The result for a sample of about 100.000 np pairs is shown in Fig. 4. The left panels show the results from pairs with &,t < 300 MeV/c (calculated in the frame of a source moving with velocity 0.25~). The complementary gate (P,,, > 300 MeV/c) is shown in Fig. 4, right panels. Although the statistics is low, the results are similar to those obtained for the E/A = 45 MeV 58Ni + 27A1 reaction. The high Ptot selection enhances the overall np correlation function strength and yields a Cn/Cp
316~
R. Ghetti et al. /Nuclear Physics A721 (2003) 313c-316~
ratio slightly larger than unity in the low-q region. The complementary gate (PtO, < 300 MeV/c) yields a Cn/Cp ratio consistent with unity for all values of q.
’C(q)
1.s 1.6
-
o WI)
1.4-
D Cp(q)
1.2 :;;-
jiti+$’
0.4 o.50-
0 Relative
Momentum.
q (M&‘/c)
* Cn(q)/Cp(q)
0
j~i;l,&. 1 ‘I’?++
311, “II
I I. I,, b I,, I.. c L 20 40 60 so 100 120 Relative Momentum. q (M&4/c)
Figure 5. Preliminary E/A=60 MeV 36Ar+27A1reaction, measuredat &&=84°-1200 at KVI. The higher capability of the KVI interferometer allows us to investigate the neutronproton emission chronology also for more backward emission, where the contribution from the preequilibrium and midrapidity sources should be suppressed [4]. The preliminary result from this analysis, performed with about 80.000 np pairs collected with the most backward detectors (5 CsI crystals and 14 liquid scintillators covering the angular region 8~,b=84°-120”), is shown in Fig. 5. For this data set, no further gates are applied, due to the limited statistics. The C,,/Cp ratio, shown in Fig. 5 right panel, is substantially below unity in the correlation region, indicating that for these np pairs, neutrons are on the average emitted earlier than protons. 3. CONCLUSIONS The experimental results from differently gated np correlation functions support the interpretation that for forward emitted- high-total-momentum selected events, that enhance projectile-like and/or intermediate velocity sources, the proton is on average emitted earlier than the neutron. Selection of events that suppresses preequilibrium and midrapidity source emission, instead, indicates a shorter average emission time for neutrons as compared to protons. These results are new and puzzling. They deserve to be paied attention to and ,to receive theoretical investigation. REFERENCES 1. 2. 3. 4.
R. Ghetti, et al., Nucl. Phys. A660 (1999) 20; Nucl. Phys. A674 (2000) 277. R. Ghetti, et al., Phys. Rev. Lett., 87 (2001) 102701-l. R. Lednicky, V.L. Lyuboshitz, B. Erazmus, D. Nouais, Phys. Lett. B 373 (1996) 30. Ph. Eudes, et al., Phys. Rev. C 56 (1997) 2003.