Total kinetic energies and nuclear-charge yields in the fission of relativistic 233U secondary projectiles

17 April 1997

PHYSICS

Physics Letters B 398 (1997)

ELSEWER

LETTERS B

259-263

Total kinetic energies and nuclear-charge yields in the fission of relativistic 233U secondary projectiles * C. Biickstiegel a, S. Steinhguser a, J. Benlliure b, H.-G. Clerc a, A. Grewe a, A. Heinz b, M. de Jong a, A.R. Junghans a, J. Mi.iller a, K.-H. Schmidt b a Institut fir Kernphysik, Technische Hochschule, Schlossgartenstr. 9, D-64289 Darmstadt, Germany b Gesellschaftfiir Schwerionenforschmg, Received

3 December

Planckstr. I, D-64291 Darmstadt, Germany

1996; revised manuscript

received 4 February 1997

Editor: R.H. Siemssen

Abstract For isotopically separated secondary beams of 233U delivered by electromagnetic excitations in a lead target and by nuclear 420. A MeV was observed. The nuclear-charge yields and the a function of the nuclear charge of the fission fragments were thermalneutron-induced fission. @ 1997 Elsevier Science B.V. PACS: 24.75+i; 25.85.-w; 25.85.Jg; 27.90.+b Keywords: Nuclear reaction; Radioactive beams; Fission of 233U(y correlation

by the SIS-FRS facility at the GSI, fission-in-flight induced interactions in a plastic target at beam energies of about total kinetic energies released during the fission process as measured. The results are compared to data measured by

, f) ; Total kinetic energy; Nuclear-charge

The use of secondary beams has been introduced as a new tool for fission studies on short-lived radioactive nuclei. It was shown that neutron-deficient isotopes of elements up to uranium can be produced with sufficient intensities [ 1] to study fission induced in a secondary target [ 21. In a recent work, fission cross sections after electromagnetic excitation of 420 . A MeV secondary beams in a lead target have been determined, and fission barriers have been deduced for a number of neutron-deficient isotopes [ 31. In addition, the nuclear-charge distribution of fission fragments produced from 234U in electromagnetic excitations has proven to show characteristics, like mass asymmetry and a strong proton odd-even effect, very similar *This

work forms part of the PhD thesis of C. Bockstiegel.

0370-2693/97/$17.00 @ 1997 Elsevier Science B.V. All rights reserved, PlIS0370-2693(97)00214-l

yields; Nuclear charge-energy

to those observed in thermal-neutron-induced fission. These findings are in qualitative agreement with results obtained for the electromagnetic-induced fission of 238U in inverse kinematics [4,5]. Besides the mass and the nuclear charge of the fission fragments, the kinetic energies of the fission products have proven to be an efficient tool for deducing the influence of nuclear structure on the fission process. In particular it is related to the elongation of the scission-point configuration, and it is an important signature for specific fission modes. In the present work we will report on the first results on total kinetic energies, determined for the fission of secondary beams. In our secondary-beam experiments, fission is observed in inverse kinematics. Since the fission fragments are detected with kinetic energies around

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C. B&kstiegel/Physics

1

x 1 m2

Fig. 1. Experimental setup at the exit of the fragment separator.

50 GeV, a very high precision is needed to determine the total kinetic energy of the fission products in the center-of-mass system of the fissioning nucleus with the desirable accuracy of about 1 MeV. Additional difficulties arose from the thick targets which had to be used in order to efficiently use the low secondarybeam intensities. The experiment was performed at the secondarybeam facility of the GSI, which consists of the heavyion synchrotron SIS and the Fragment Separator FRS [ 61. Radioactive beams were produced by the fragmentation of a 238U primary beam of 1 . A GeV impinging on a 0.680 g/cm2 beryllium target. A niobium stripper foil of 0.212 g/cm2 served to maximize the yield of bare ions. The fragments were separated by the FRS which was operated as a momentum-loss achromat [ 71 using an intermediate degrader of about 3.8 g/cm2. For the isotopical identification, scintillation detectors [ 81 were used to measure the positions of the fragments at the center and at the exit of the fragment separator as well as their time-of-flight. The experimental setup behind the FRS is sketched in Fig. 1. As a secondary target we used a stack of lead foils with a total thickness of 3.03 g/cm2 mounted in a gas-filled chamber which acts as a subdivided ionization chamber (active target). With this device it is possible to determine the lead foil in which fission took place and thus also to discriminate fission induced

in the scintillator

in front

of the active

target.

In the lead target, the secondary beams were slowed down from 520 to 320 . A MeV. The energy-loss signals of the two fission fragments and their positions in the horizontal and the vertical direction were measured independently in a subdivided double ionization chamber (Twin MUSIC) with a position resolution of about 6 mm (FWHM) in horizontal and 0.5 mm (FWHM) in vertical direction. The time-of-flight of

Letters B 398 (1997) 2.59-263

the fission fragments was measured with a time resolution of about 170 ps (FWHM) by using a 1 m x 1 m scintillator TOF wall as a stop detector. The detector was dimensioned to cover the whole range of emission angles of the fission fragments up to 70 mrad on a flight path of about 5 m. The geometry was given by the relativistic focussing of the beam. This TOF measurement allowed to correct the measured energy-loss signals for their velocity dependence and to obtain the nuclear charge of both fission fragments with a resolution of Z/AZ N 150. From the measured fission-fragment time-of-flight values and the emission angles it was possible to determine the velocity vectors of the fission fragments. For this purpose, the deceleration of each secondarybeam particle in the layers of matter in front of the target foil in which fission took place had to be reconstructed first. The velocities of the fission fragments parallel to the beam right after the fission process were deduced from the measured time-of-flight values by taking into account the deceleration of the fission fragments down to the TOF wall. Momentum conservation was used to determine the depth of the fission reaction inside the target foil in order to minimize the target-location straggling. The two transverse components of the velocities of the fission fragments were determined by their positions measured with the Twin MUSIC (see Fig. 1). The velocity vectors of the fission fragments in the laboratory system immediately after fission were then transformed into the center-ofmass system of the secondary beam. The average preneutron-emission mass was estimated from l-he nuclear charge of a fragment by assuming the mass-to-charge ratio of the fissioning nucleus to be preserved. The total kinetic energy TKE of the two fragments was finally calculated from the fragment center-of-mass velocities and the pre-neutron-emission masses. The resulting mean total kinetic energies TKE of the fission fragments from the electromagnetic excitation in the lead target and from nuclear excitation in the scintillator target are shown in Figs. 2 and 3, respectively. In addition, the nuclear-charge yields for the two excitation modes are given, too. The spectra were accumulated under the condition that the sum of the charges of the two fission fragments was equal to the nuclear charge of the secondary beam (see Ref. [ 31) . By this condition, most events with additional nuclear or electromagnetic interactions in the different layers

C. BiickstiegeUPhysics Letters B 398 (1997) 259-263

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Nuclear charge Fig. 2. Upper part: Full line: Mean total kinetic energies TKE as a function of the fission-fragment charge for electromagnetic-induced fission of 233U in a lead target. Dashed-dotted line: Data from Asghar et al. [9] for TKE as a function of fragment mass for ‘s3U(ntt,, f). The scale of the mass range is adapted to the charge range. Lower part: Nuclear-charge yields for electromagnetic fission of 233U in a lead target (full line) and from Djebara et al. [lo] for 232U(n~. f). For the present data only statistical error bars are indicated.

of matter prior to fission were suppressed. The remaining contxibution of such events in Figs. 2 and 3 was estimated to amount to the insignificant level of about 1%. In the lead target, the projectiles may fission either after electromagnetic excitation or following nuclear collisions. Fission induced in the scintillator target is almost exclusively caused by nuclear collisions. Electromagnetic excitation which predominantly populates the giant dipole resonance, leads to an excitationenergy distribution with a mean value of 11 MeV and a FWHM of about 5 MeV as has been calculated in [ 31 by using y-absorption cross sections and the equivalent photon spectrum [ 111. Quadrupole resonances and two-phonon excitations of the GDR were

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Nuclear charge Fig. 3. Upper part: Full line: Mean total kinetic energies TKE as a function of fission-fragment charge resulting from nuclear-induced fission in the scintillator. Dashed line: Liquid-Drop-Model calculation of the post-scission kinetic energy after Wilkins et al. [ 201. Lower part: Nuclear-charge yields after fission in the scintillator. Error bars include statistical errors only.

included in this calculation which is analogous to that performed for 238U in Ref. [ 51. It is important to note that neither the equivalent photon spectra nor the mean energies of the giant resonances depend on the nuclear structure and both vary only little with mass or charge number in the range of heavy fissile nuclei. Therefore, the excitation-energy distribution populated in electromagnetic excitation does not introduce any additional structural effects. For 233U, fission after electromagnetic excitation consists of first-chance fission with a probability of about 80%. In contrast to electromagnetic excitations, nuclear reactions leave the residual nucleus with a high excitation energy of about 27 MeV per abraded nucleon on the average [ 121, which allows not only direct but also multi-chance fission. Even under the restriction to reactions which preserve the number of protons, nu-

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C. Biickstiegel/Physics Letters B 398 (1997) 259-263

clei fissioning after nuclear excitation generally have lost several neutrons and they have a broad range of excitation energies. In the case of 233U, model calculations [ 13,141 show that on the average nuclei of the isotope 230U with a mean excitation energy of 37 MeV undergo fission. The mean total kinetic energies of the fission fragments resulting from electromagnetic-induced fission were deduced by disentangling the two possible processes. For this purpose the nuclear-induced fission events produced in the plastic target were subtracted from the events produced in the lead target with an appropriate weight. This procedure is analogous to that applied in [ 31 where electromagnetic fission cross sections have been determined. The correction has only little influence on the TKE values as well as on the statistical-error bars, since only about 20% of the fission events induced in the lead target and in which the number of protons of the secondary projectiles is preserved, originate from nuclear excitation. Since the TKE values are deduced from time-offlight and position measurements, they are determined on an absolute scale without any adjustment. From the precision of the calibrations we estimate for TKE an additional systematic error of 2%. Further considerations for the precision of the TKE values obtained with the experimental setup described above include the influence of the number of neutrons lost before and after the fission process. The first case leads to a change in the mass-to-charge ratio of the compound nucleus which shifts the deduced TKE values by about 0.7 MeV per neutron removed. The latter case is used for the calculations of the mean energy loss of the fission fragments in the matter layers between the two time-of-flight detectors (see Fig. 1). The number of emitted neutrons & was estimated from experiments on thermal-neutron-induced fission of different uranium isotopes where the average number of emitted neutrons &i, was found to amount to about 2.5, almost independent of the mass number of the uranium isotope considered [ 151. Since higher excitation energies tend to increase the number of emitted neutrons [ 161, the different excitation energies of electromagnetic-induced fission and nuclear-induced fission were taken into account by adding 0.5 neutrons to a total of i& = 3 and 4 neutrons to a total of vtot = 6.5, respectively. Z&t was attributed to both fission fragments in equal parts. These considerations

have little influence on the total kinetic energies determined by this method: A variation of the average number of emitted neutrons Ptot = 6.5 by one changes the TKE values by 0.1 MeV only. In addition, it is known that the deformation of the fragments in the scission configuration modulates the number of emitted neutrons for specific fragments by about f1.5, which results in an uncertainty of the TKE values of about fl MeV. Even this effect is small compared to the variation of the Coulomb energy due to the influence of nuclear structure which amounts to more than 10 MeV. Thus, we conclude that the neutron number of the fissioning nucleus and those of the fission products which cannot be determined in our experiment do not crucially enter into the analysis. This is of particular importance, when the method is to be applied to nuclei which have not been investigated previously. The TKE values for electromagnetic fission are compared in Fig. 2 with those from thermal-neutron fission [ 91. For asymmetric mass splits the agreement is good, provided a general shift of about 4 MeV which is within the systematic uncertainties of the data is disregarded. This new experimental technique shows the influence of the nuclear structure on the total kinetic energies as clearly as it has been found in previous experiments [ 91. The symmetric component, however, shows higher total kinetic energies if compared to thermal-neutron fission. In the reactions 233U(n, f) [ 171 and 235U( n, f) [ 181 with neutron energies up to 6 MeV, the mean total kinetic energies near symmetry were found to rise with increasing neutron energy. Therefore, it is concluded that the mass-symmetric electromagnetic-induced fission events originate from relatively high excitation energies. This can also be seen in the lower part of Fig. 2 which shows the yields in near-symmetry charge splits to be appreciably larger than those observed in thermal-neutron fission. It is interesting to note that the nuclear-charge yields show a distinct odd-even effect even near symmetry. From a combination of experimental data for 238U(y, f) and 235U( n, f), a fast washing out of the proton-odd-even effect was derived, if the excitation energy exceeded the fission barrier by more than the pairing gap [ 19 I . The present fission-yield data for electromagnetic-induced fission of 233U near symmetry indicate that this conclusion may not be valid in general. In nuclear-induced fission in the plastic target, the

C. Bdckstiegel/Plzysics Letters B 398 (1997) 259-263

enhancement in TKE near 2 = 52 (corresponding to N N 82), as well as the steep decrease towards charge asymmetry observed in electromagnetic fission have almost disappeared, see upper part of Fig. 3. As was already observed in [ 17,181, the influence of the nuclear shell effects on the TKE decreases with higher excitation energy. At the same time, the nuclear-charge yield near symmetry has increased considerably. The data may be explained by the high average excitation energy associated with nuclear-induced fission. The similarity of the data to model calculations of total kinetic energies without shell and pairing corrections after Ref. [20], as shown in Fig. 3, confirm this interpretation. For these calculations a tip distance of 2.0 fm was assumed, which is proposed by the authors of Ref. [ 201. The radius of the nucleon was assumed to be 1.16 fm, the deformations of the two fission fragments were set to /I = 0.625 [20], and the volume conservation factor as well as the shape factor for the Coulomb potential were left to 1. Electromagnetic excitation of secondary beams in a lead target has proven to be an efficient tool to study low-energy fission properties of short-lived radioactive nuclei. In addition to the nuclear charges of the fission fragments, also the total kinetic energies can be determined with satisfactory precision, in spite of the difficult experimental conditions in secondary-beam experiments. Therefore, fission experiments with radioactive secondary beams provide the possibility to investigate nuclear-structure effects in fission with regard to charge distributions and total kinetic energies for many short-lived nuclei which have not been accessible before. We wish to thank K.-H. Behr, A. Brtinle, K. Burkhard and their team for the technical support of

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the experiment, H. Folger and his colleagues for the fabrication of the targets. B. Voss made an essential contribution to the experiment by designing the TwinMUSIC and by participating in the construction of the TOF wall. Supported by GSI Hochschulprogramm and Bundesministerium fur Bildung und Forschung (BMBF) under the contract number 06 DA473.

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