- Email: [email protected]
Fission dynamics in the proton induced fission of heavy nuclei
ELSEVIER
Nuclear
Physics
A734 (2004)
253-256 www.elsevier.com/locate/npe
Fission dynamics in the proton induced fission of heavy nuclei V.A. Rubchenyaab*, W.H. Trzaskaac, I.M. Itkisd, M.G. Itkisd, J. Klimand, G.N. Kniajevad, N.A. Kondratievd, E.M. Kozulind, L. Krupad, I.V. Pokrovskid, V.M. Voskressenskid, F. Hanappee, T. Maternile, 0. Dorvauxf, L. Stuttgef, G. Chubariang, S.V. Khlebnikovb, D.N. Vakhtinb, V.G. LyapinC aDepartment of Physics, University of Jyvaskyll, FIN-40351, Jyviskyll,
Finland
bV G .Khlopin Radium Institute, 194021, St. Petersburg, Russia ‘Helsinki Institute of Physics, Helsinki, Finland dFlerov Laboratory of Nuclear Reaction, JINR, 141980 Dubna, Moscow region, Russia YJniversite Libre de Bruxelles, PNTPM, CP229, Bruxelles, Belgium ‘Institut Qyclotron
de Recherches Subatomiques, CNRS-IN2P3, Strasbourg, France Institute, TEXAS A&M University, College Station, 77843-3366, Texas, USA
Multi-parameter correlation study of the reaction 242Pu(p, f) at E,, = 13, 20 and 55 MeV has been carried out. Fission fragment mass and kinetic energy distributions and the double differential neutron spectra have been measured. It was observed that the two-humped shape of mass distributions prevailed up to highest proton energy. Manifestation of the nuclear shell 2 = 28 near fragment mass Af, = 70 has been detected. The experimental results were analyzed in the framework of a time-dependent statistical model with inclusion of nuclear friction effects in the fission process. The multi-parameter correlation study of the reaction During the past two decades the number and the scope of studies on fission dynamics involving heavy ion reactions have increased significantly [l]. Nevertheless, the use of light projectiles for investigation of fission dynamics of heavy compound nuclei retains at least two important advantages: (i) the contribution of the fast fission will be excluded due to small angular momenta, and (ii) it is possible to study the fission dynamics at the compound nuclei excitation energy below 50 MeV where the fusion-fission heavy ion reaction cross sections are small. The contradictions between the pre- and postscission neutron multiplicities measured in the heavy ion reactions and with light particle projectiles were already discussed [2] but data about the pre- and post-scission neutron multiplicities from light particle induced fission remain scarce and are mainly at excitation energies below 30 MeV. The other important aspects of fission dynamics are the interplay *email
[email protected]
0375-9474/$ ~ see front matter 0 2004 Elsevier doi: 10.1016/j.nuc1physa.2004.01.047
B.V.
All rights
reserved
254
FCA.Ruhchenya et al. /Nuclear Physics A734 (2004) 253-256
between various fission modes and diminishing of shell structure effects with increasing projectile energy. In particular, the search for superasymmetric fission mode connected with the influence of the nuclear shells N = 50 and 2 = 28 (78Ni fission mode) [3] is of big interest. Recently we have studied fission dynamics in the reaction 238U(p, f) at Ep = 20,35,50 and 60 MeV [4] and have demonstrated the enhancement for superasymmetric mass division. In this report the preliminary results of complex correlation measurements of fission fragment mass and kinetic energy distributions and of the double differential neutron spectra in the proton induced fission of 242Pu at Ep = 13,20 and 55 MeV are presented. The data on y-rays spectra measured in this reaction are reported in the separate contribution of this volume. The main goals of this study are: l to measure yields at extremely asymmetric mass division at Af, < 70; l to measure dependence of the post-scission neutron multiplicity on fragment mass; l to decompose pre-scission neutron multiplicity into the preequlibrium and statistical parts. The multi-parameter correlation ~(13 Me)3+Pu study of the reaction 242Pu(p,f) at Ep = 13,20 and 55 MeV has been se carried out at the Accelerator Laboratory, University of Jyvaskyla using experimental setup that included: - CORSET [5] - two-armed, timeof-flight (TOF) based fission fragment spectrometer; - 8 neutron detectors from DEMON [6]; - 3 position sensitive detector neuM 10IWIWMaIWII tron detectors from HENDES [7]; flwQmt-,m - 6 NaI(T1) scintillation detectors _. _ fission fragfor y-ray multiplicity measurements. “lgure I: Measured pre-neutron-emission ment mass distributions, the total kinetic energy and its From the CORSET data two- standard deviations in 242Pu(p, f) at Ep = 13,20 and dimensional total kinetic energy- 55 M~V, fragment mass distributions were constructed. The obtained mass distributions of the fission fragments prior to post-scission neutron emission, the total kinetic energies and its standard deviations at Ep = 13,20 and 55 MeV are shown in Fig. 1. One can see remarkable structures (local maxima at Af, M 70) connected with the nuclear shells in TKE(Af,) and cy~~(Af~) dependences. For the first time these structures are observed at Af, z 70. It can be attributed to the 2 = 28 nuclear shell. Mass distributions have the two-hump shape, which prevails up to the highest projectile energy. Pre-equilibrium and statistical pre-scission neutron emission cools substantially the compound nucleus before fission. In Fig. 2 the comparison between mass yield measured at Ep = 13 and 55 MeV, the theoretical mass distributions at Ep = 55 MeV and mass distributions of the same composite nucleus 243Am measured with LOHENGRIN mass separator in 242mAm(nth, f) reaction are presented. One can see from the Fig. 2 that fragment yields at Af, < 80 increase significantly with 6
YA. Rubchenya et al. /Nuclear Ph,vsics A734 (2004) 253-256
255
the proton energy. The yields for the very asymmetric mass division are enhanced very much in comparison with low energy fission the same composite nucleus 243Am [8]. The theoretically calculated mass curve at Ep = 55 MeV, carried out in the framework of the model with inclusion superasymmetric fission mode and dynamical effects [4], underestimates yields at Af, < 70. In Fig. 3 the neutron spectra measured with DEMON detector modules at positions with spherical coordinates relative to beam axis 02 = 18”, cp2 = 126’ and 07 = 107”, (p7 = 15” (close to direction of fission fragment detector)the direction of fission fragment detector) are presented. The calculated evaporation spectra are shown by lines. Calculations were carried in the framework of the multiple-source model, which included evaporation spectra from individual fragments and compound nucleus using experimental data on the kinetic energies and neutron multiplicities. At the low proton energy Ep = 13 MeV the contribution of the preequilibrium emission is inside of experimental errors. 50 60 70 80 90 100 110 120 At the higher proton energies the preequilibrium A’” fr
Figure 2: e Measured mass distributions
emission
in the forward
direction
gives a substantial
contribution. Preequilibrium neutron multiplicity at EP = 13 MeV (circles) and 55 MeV (triangles), and the maSs yields (stars) in can be extracted by integrating over solid angle the thermal neutron -induced fission of the difference between experimental spectra and 242mAm[a]. The theoretically calculated calculated ones within the multiple-source model. mass distribution is shown as open trian- Th e neutron statistical emission was supposed to gles. be isotropic in the source frame. The post-scission neutron multiplicity have been extracted and results are presented in Fig. 4 together with the theoretical predictions made within scission point model built in the time-dependent statistical model with inclusion of nuclear friction effects in the fission process [4]. The integral characteristics are given in the Table 1. In conclusion, the fission fragment mass and kinetic energy distributions and double differential neutron spectra have been measured in 242Pu(p, f) reaction at Ep = 13,20 and 55 MeV. An enhancement of highly asymmetric mass division (up to Af, = 60) with increasing
proton
energy
is observed.
Characteristics
of fission process did not reach the
liquid drop limit up to Ep = 55 MeV and influence of nuclear shells is still preserved. The Table 1 The measured characteristics in 242Pu(p, f) MPr=‘? TKE(MeV) Ep WV) 13 177.0 f 1.0 0.0; 0.1 o.Lo.1 20 176.7 f 1.0 0.3 f 0.1 0.3 f 0.1 55 176.2 f 1.0 0.9 & 0.1 1.1 f 0.1 MStpW
MPO"t
4.6”* 0.1 5.0 f 0.1 6.0 f 0.3
M;t 4.6 f 0.2 5.6 f 0.2 8.0 f 0.3
VA. Rubchenya et al. /Nuclear Physics A734 (2004) 253-2.56
256
L.
Em=55 MeV
* Pm fr
Figure 3. Measured neutron spectra in forward direction 02 = 18O,(pz = 126O (points) and in direction fragment detector 07 = 107°,(p7 = 15’ (triangles). Calculated evaporation spectra are shown by lines.
Figure 4. Measured post-scission neutron multiplicities (solid symbols) as function of fragment mass. Calculated values are shown by open symbols.
irregularities in the dependenciesTKE(An) and OTKE(A~~)near Af, = 70 can be attributed to the influence of nuclear shell 2 = 28. The preequlibrium emission of neutrons and protons removes substantial part of the excitation energy of the compound nucleus at Ep > 30 MeV therefore influence of the nuclear friction is small in the proton induced fission at the intermediate energies. This work has been supported by the European Union Fifth Programme “Improving Human Potential - Access to Research Infrastructure”. Contract No. HPRI-CT-199900044, and by the Finnish Academy of Finland under the Finnish Center of Excellence Programme 2000-2005 (Project No, 44875, Nuclear and Condenced Matter Physics Programme at JYFL) and the Russian Foundation for Basic Research under Grant No 0302-16779.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
D. Hilsher, H. Rossner, Ann. Phys. Fr. 17 (1992) 471. E. M. Kozulin et al., Phys. At. Nucl. 56 (1993) 166. M. Huhta et al., Phys. Lett. B 405 (1998) 230. V. A. Rubchenya et al., Nucl. Instrum. Methods A 463 (2001) 653 N. A. Kondratiev et al., In: Dynamical Aspects of Nuclear Fission, WS (2000) 431. S. Mouatassim et al., Nucl. Instrum. Methods A 359 (1995) 330; A 365 (1995) 446. W. H. Trzaska et al. In: Appl. of Accelerators in Research and Industry, AIP, N.Y. (1997) 1059. Tsekhanovich et al., Nucl. Phys. A 658 (1999) 217.
Nuclear
Physics
A734 (2004)
253-256 www.elsevier.com/locate/npe
Fission dynamics in the proton induced fission of heavy nuclei V.A. Rubchenyaab*, W.H. Trzaskaac, I.M. Itkisd, M.G. Itkisd, J. Klimand, G.N. Kniajevad, N.A. Kondratievd, E.M. Kozulind, L. Krupad, I.V. Pokrovskid, V.M. Voskressenskid, F. Hanappee, T. Maternile, 0. Dorvauxf, L. Stuttgef, G. Chubariang, S.V. Khlebnikovb, D.N. Vakhtinb, V.G. LyapinC aDepartment of Physics, University of Jyvaskyll, FIN-40351, Jyviskyll,
Finland
bV G .Khlopin Radium Institute, 194021, St. Petersburg, Russia ‘Helsinki Institute of Physics, Helsinki, Finland dFlerov Laboratory of Nuclear Reaction, JINR, 141980 Dubna, Moscow region, Russia YJniversite Libre de Bruxelles, PNTPM, CP229, Bruxelles, Belgium ‘Institut Qyclotron
de Recherches Subatomiques, CNRS-IN2P3, Strasbourg, France Institute, TEXAS A&M University, College Station, 77843-3366, Texas, USA
Multi-parameter correlation study of the reaction 242Pu(p, f) at E,, = 13, 20 and 55 MeV has been carried out. Fission fragment mass and kinetic energy distributions and the double differential neutron spectra have been measured. It was observed that the two-humped shape of mass distributions prevailed up to highest proton energy. Manifestation of the nuclear shell 2 = 28 near fragment mass Af, = 70 has been detected. The experimental results were analyzed in the framework of a time-dependent statistical model with inclusion of nuclear friction effects in the fission process. The multi-parameter correlation study of the reaction During the past two decades the number and the scope of studies on fission dynamics involving heavy ion reactions have increased significantly [l]. Nevertheless, the use of light projectiles for investigation of fission dynamics of heavy compound nuclei retains at least two important advantages: (i) the contribution of the fast fission will be excluded due to small angular momenta, and (ii) it is possible to study the fission dynamics at the compound nuclei excitation energy below 50 MeV where the fusion-fission heavy ion reaction cross sections are small. The contradictions between the pre- and postscission neutron multiplicities measured in the heavy ion reactions and with light particle projectiles were already discussed [2] but data about the pre- and post-scission neutron multiplicities from light particle induced fission remain scarce and are mainly at excitation energies below 30 MeV. The other important aspects of fission dynamics are the interplay *email
[email protected]
0375-9474/$ ~ see front matter 0 2004 Elsevier doi: 10.1016/j.nuc1physa.2004.01.047
B.V.
All rights
reserved
254
FCA.Ruhchenya et al. /Nuclear Physics A734 (2004) 253-256
between various fission modes and diminishing of shell structure effects with increasing projectile energy. In particular, the search for superasymmetric fission mode connected with the influence of the nuclear shells N = 50 and 2 = 28 (78Ni fission mode) [3] is of big interest. Recently we have studied fission dynamics in the reaction 238U(p, f) at Ep = 20,35,50 and 60 MeV [4] and have demonstrated the enhancement for superasymmetric mass division. In this report the preliminary results of complex correlation measurements of fission fragment mass and kinetic energy distributions and of the double differential neutron spectra in the proton induced fission of 242Pu at Ep = 13,20 and 55 MeV are presented. The data on y-rays spectra measured in this reaction are reported in the separate contribution of this volume. The main goals of this study are: l to measure yields at extremely asymmetric mass division at Af, < 70; l to measure dependence of the post-scission neutron multiplicity on fragment mass; l to decompose pre-scission neutron multiplicity into the preequlibrium and statistical parts. The multi-parameter correlation ~(13 Me)3+Pu study of the reaction 242Pu(p,f) at Ep = 13,20 and 55 MeV has been se carried out at the Accelerator Laboratory, University of Jyvaskyla using experimental setup that included: - CORSET [5] - two-armed, timeof-flight (TOF) based fission fragment spectrometer; - 8 neutron detectors from DEMON [6]; - 3 position sensitive detector neuM 10IWIWMaIWII tron detectors from HENDES [7]; flwQmt-,m - 6 NaI(T1) scintillation detectors _. _ fission fragfor y-ray multiplicity measurements. “lgure I: Measured pre-neutron-emission ment mass distributions, the total kinetic energy and its From the CORSET data two- standard deviations in 242Pu(p, f) at Ep = 13,20 and dimensional total kinetic energy- 55 M~V, fragment mass distributions were constructed. The obtained mass distributions of the fission fragments prior to post-scission neutron emission, the total kinetic energies and its standard deviations at Ep = 13,20 and 55 MeV are shown in Fig. 1. One can see remarkable structures (local maxima at Af, M 70) connected with the nuclear shells in TKE(Af,) and cy~~(Af~) dependences. For the first time these structures are observed at Af, z 70. It can be attributed to the 2 = 28 nuclear shell. Mass distributions have the two-hump shape, which prevails up to the highest projectile energy. Pre-equilibrium and statistical pre-scission neutron emission cools substantially the compound nucleus before fission. In Fig. 2 the comparison between mass yield measured at Ep = 13 and 55 MeV, the theoretical mass distributions at Ep = 55 MeV and mass distributions of the same composite nucleus 243Am measured with LOHENGRIN mass separator in 242mAm(nth, f) reaction are presented. One can see from the Fig. 2 that fragment yields at Af, < 80 increase significantly with 6
YA. Rubchenya et al. /Nuclear Ph,vsics A734 (2004) 253-256
255
the proton energy. The yields for the very asymmetric mass division are enhanced very much in comparison with low energy fission the same composite nucleus 243Am [8]. The theoretically calculated mass curve at Ep = 55 MeV, carried out in the framework of the model with inclusion superasymmetric fission mode and dynamical effects [4], underestimates yields at Af, < 70. In Fig. 3 the neutron spectra measured with DEMON detector modules at positions with spherical coordinates relative to beam axis 02 = 18”, cp2 = 126’ and 07 = 107”, (p7 = 15” (close to direction of fission fragment detector)the direction of fission fragment detector) are presented. The calculated evaporation spectra are shown by lines. Calculations were carried in the framework of the multiple-source model, which included evaporation spectra from individual fragments and compound nucleus using experimental data on the kinetic energies and neutron multiplicities. At the low proton energy Ep = 13 MeV the contribution of the preequilibrium emission is inside of experimental errors. 50 60 70 80 90 100 110 120 At the higher proton energies the preequilibrium A’” fr
Figure 2: e Measured mass distributions
emission
in the forward
direction
gives a substantial
contribution. Preequilibrium neutron multiplicity at EP = 13 MeV (circles) and 55 MeV (triangles), and the maSs yields (stars) in can be extracted by integrating over solid angle the thermal neutron -induced fission of the difference between experimental spectra and 242mAm[a]. The theoretically calculated calculated ones within the multiple-source model. mass distribution is shown as open trian- Th e neutron statistical emission was supposed to gles. be isotropic in the source frame. The post-scission neutron multiplicity have been extracted and results are presented in Fig. 4 together with the theoretical predictions made within scission point model built in the time-dependent statistical model with inclusion of nuclear friction effects in the fission process [4]. The integral characteristics are given in the Table 1. In conclusion, the fission fragment mass and kinetic energy distributions and double differential neutron spectra have been measured in 242Pu(p, f) reaction at Ep = 13,20 and 55 MeV. An enhancement of highly asymmetric mass division (up to Af, = 60) with increasing
proton
energy
is observed.
Characteristics
of fission process did not reach the
liquid drop limit up to Ep = 55 MeV and influence of nuclear shells is still preserved. The Table 1 The measured characteristics in 242Pu(p, f) MPr=‘? TKE(MeV) Ep WV) 13 177.0 f 1.0 0.0; 0.1 o.Lo.1 20 176.7 f 1.0 0.3 f 0.1 0.3 f 0.1 55 176.2 f 1.0 0.9 & 0.1 1.1 f 0.1 MStpW
MPO"t
4.6”* 0.1 5.0 f 0.1 6.0 f 0.3
M;t 4.6 f 0.2 5.6 f 0.2 8.0 f 0.3
VA. Rubchenya et al. /Nuclear Physics A734 (2004) 253-2.56
256
L.
Em=55 MeV
* Pm fr
Figure 3. Measured neutron spectra in forward direction 02 = 18O,(pz = 126O (points) and in direction fragment detector 07 = 107°,(p7 = 15’ (triangles). Calculated evaporation spectra are shown by lines.
Figure 4. Measured post-scission neutron multiplicities (solid symbols) as function of fragment mass. Calculated values are shown by open symbols.
irregularities in the dependenciesTKE(An) and OTKE(A~~)near Af, = 70 can be attributed to the influence of nuclear shell 2 = 28. The preequlibrium emission of neutrons and protons removes substantial part of the excitation energy of the compound nucleus at Ep > 30 MeV therefore influence of the nuclear friction is small in the proton induced fission at the intermediate energies. This work has been supported by the European Union Fifth Programme “Improving Human Potential - Access to Research Infrastructure”. Contract No. HPRI-CT-199900044, and by the Finnish Academy of Finland under the Finnish Center of Excellence Programme 2000-2005 (Project No, 44875, Nuclear and Condenced Matter Physics Programme at JYFL) and the Russian Foundation for Basic Research under Grant No 0302-16779.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
D. Hilsher, H. Rossner, Ann. Phys. Fr. 17 (1992) 471. E. M. Kozulin et al., Phys. At. Nucl. 56 (1993) 166. M. Huhta et al., Phys. Lett. B 405 (1998) 230. V. A. Rubchenya et al., Nucl. Instrum. Methods A 463 (2001) 653 N. A. Kondratiev et al., In: Dynamical Aspects of Nuclear Fission, WS (2000) 431. S. Mouatassim et al., Nucl. Instrum. Methods A 359 (1995) 330; A 365 (1995) 446. W. H. Trzaska et al. In: Appl. of Accelerators in Research and Industry, AIP, N.Y. (1997) 1059. Tsekhanovich et al., Nucl. Phys. A 658 (1999) 217.