Volume 77B, number 2 PHYSICS LETTERS 31 July 1978

THE FISSION PROCESSES 236U(ot, a ' f ) AT E~, = 50 MeV AND 235U(d, pf) AT E d = 23 MeV

AND THE TOTAL KINETIC ENERGY RELEASE IN FISSION OF EXCITED NUCLEI

P. DAVID, J. DEBRUS, F. LOBKE, R. SCHOENMACKERS and J. SCHULZE Institut fi~r Strahlen- und Kernphysik der Universittit Bonn, D-5300 Bonn, W-Germany

Received 24 April 1978

The fissioning nucleus 236U was investigated by two different reactions. Total kinetic energies (TKE) as function of the excitation energy above the highest fission threshold are shown to have different behaviour for E x below the two quasi- particle threshold 2A and above. A strong dependence on the mass splitting is observed.

Energy dissipative processes have been studied very intensively in heavy ion reactions during the last years at relatively high excitation energies [ 1 ]. Contrary to these reactions the fission process allows to study the phenomenon of energy dissipation from the state of a cold nucleus at the barriers with mainly collective degrees of freedom being involved, over two quasi- particle breakup thresholds, up to high excitation ener- gies, where many degrees of freedom contribute. While there are many ways to get information about nuclei by exciting them at the ground state-deformation, some ways to explore the shape isomeric states, mainly the total kinetic energies (TKE) of the (binary) fission fragments give indications on what happens during the descent from the saddle configuration to the scission point in finite nuclear matter of extreme deformation. In the thermodynamical model of N~3renberg [2] the observable collective modes (e.g. mass asymmetry, stretching, distortion, asymmetry, charge-to-mass ratio) are assumed to be in thermodynamical equilibrium. But one has to consider that for excitation energies within the pairing gap above the highest fission thresh- old the collective temperature is not much higher than the characteristic frequency of the (normal) mode considered. Thus, for excitation energies of the fis- sioning system less than 2 A above the highest fission threshold, pairs may be broken only in a relatively late stage during the descent from saddle to the scission point. From this it follows that energy dissipation should be relatively small. Or stated in different terms

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of particles colliding with a moving potential wall, given by Swiatecki [3], one-body viscosity will do- minate. Features characteristic for such effects should show a change in their behaviour as function o f E x at the 2& threshold. For excitation energies above 2A the number of two-body collisions may increase and dissipation may become larger. So in terms of the hydrodynamical model with viscosity given by Nix et al. [4] an increase in excitation energy may lead to an increase in two-body viscosity.

In this letter we report on first results of the beha- viour of the TKE on the excitation energy of the fissioning system 236U as excited in the 235U(dp)236U fission reaction and by inelastic 4He-scattering 236U(ot, ot'f)236U. This continues earlier work of fissioning 238U by inelastic 4He scattering [5]. The comparison of fission processes induced by different excitation mechanisms leading to a nucleus with the same mass number seems worthwhile, since microsco- pic effects may be differently excited in different reactions.

In two triple coincidence experiments the mass and energy distributions of the fissioning systems 236U were measured. In both experiments the mass distribu- tions and the fragment TKE were obtained for E x --- 5 .5 -20 MeV excitation energy.

The measurements were carried out at the isochro- nous cyclotron of the University of Bonn. The 4He- beam specifications are E = 49.98 MeV, A E / E = 1/2000. The 236U target was prepared by electrospraying at

Volume 77B, number 2 PHYSICS LETTERS 31 July 1978

2 ~ A TKE - . 2 3 5 U ( d , p f ) (MeV)

Ed= 23 MeV

1

0

_, Bf B2n

0 1 2 3 4 5 6 7 8 9 10111213Ex~MeV)

Fig. 1. Differences of the TKE(Ex) and TKE at E x = 11.75 MeV for 235U(d, pf).

CBNM-Euratom Mol with a thickness of 95/ag/cm 2 from uranium acetate on a 45/ag/cm 2 thick vyns foil. The E = 23 MeV deuteron beam had a resolution of E/AE = 2500. The 235U target was prepared at NRE Harwell by evaporating 35/ag/cm 2 235U on a 12C- backing of 40/ag/cm 2. The particle identification sys- tem in the (d, p)-experiment consisting of a AE(500/am) , and two E (2000/am) silicon surface barrier detectors was mounted at 55 ° to the beam direction to detect the protons. In two heavy ion surface barrier detectors 60/am thick both the fission fragments were detected in coincidence with the proton. One detector was mounted at 38.5 ° , the other 180 ° with respect to this average recoil axis. The 4He-inelastic scattering on 236U was performed as described in ref. [5]. The de- tectors were calibrated following the procedure of Schmitt et al. [6]. The electronics and the calibration process are described in reL [5] and references therein.

For the 235U(d, pf) reaction fig. 1 displays the dif- ferences between the TKE(Ex) and the TKE value at

E x = 11.75 MeV. This reference point was chosen, since it is the lowest TKE value in the energy range for E x considered and close to the threshold E x = B2n for second chance fission. The TKE averaged over all frag- ment masses taken from the fission threshold up to the maximum excitat ion energy where second chance fission can be excluded have different slopes with excitation energy. But these values are weighted by the mass yield distribution thus not allowing specific conclusions.

To get more detailed information on the microscopic

dIKE dEx / i i I I I i i

L dIKE ( A ) l o ~ . IT

0,t l/ °'t 1t t 0.2 l o 0 . . . . . . . . , . . . . . . . . . . . . . . . . . . . . .

02 [ t -0.4 -0.6 -0.8 -1.0 - 1.2 v 226Ra1 He,d) 2Ac - | , i ~ • 235U ( d" P 1236U

• 236U ( a ~o~ ' ) 236U

1.6 I I I I I I

I10 t20 130 140 150 t60 170 A H

Fig. 2. Slopes of linear least square fits to the TKE(A H) for E x ranging from the fission threshold to the threshold for second chance fission. 228Ac from ref. [12].

structure the TKE were evaluated in steps of 0.5 MeV of excitation energy for single mass pairs, grouping five massos together. The resulting TKE(AH, Ex) are ana- lysed by a linear least squares fit in the region from threshold up to 11 MeV, giving the slopes dTKE(AH) / dE x for an average mass JIH of each A H-group. These slopes for the inelastic c~-scattering and for the (d, p) reaction are displayed in fig. 2 and compared with results from other experiments.

For symmetric mass break up and at masses around A H = 142 the dTKE/dE x have zero or small positive values. A minimum is formed between massA H = 140 and symmetric masses. These observations are consistent with neutron induced fission reactions from 239U to 233Th by G6nnenwein et al. [7]. Moreover they also find the posit ion of the minimum to be shifted towards symmetric mass break up. This may be explained by the changing yield of symmetric masses weighing the TKE distribution with increasing E x. The average kinetic energies o f fragments as a function of mass number from this experiment may be understood in the frame of the scission point model of Dickmann and Dietrich [8 ], which includes shell and pairing effects.

In the extended scission point model given by Wilkins et al. [9] the excitat ion energy dependence of the potential energy surface leads to changing deformations of the final fragments due to a subshell. This in turn

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Volume 77B, number 2 PHYSICS LETTERS 31 July 1978

3

2

1

0

-1

-2

-3

"-1 r 3 r T ~ r ~ - - d T K E IA d E x '~H'

~ m J . !10 120 130 I/,0

23SU(~'ct')236U • 1 23s U (d, p 12~s U •

Ex=(Be ,Bf * 1.5MeV)

J 150 160 A

Fig. 3. Slopes of linear least square fits to the TKE(A H) for Ex ranging from the fission threshold to 1.5 MeV above threshold.

results in strong negative changes of TKE around A H = 132, less strong changes fo rA a about 142 and should lead to increasing TKE for symmetric mass splits.

The TKE(AH) indicate effects near threshold, which are different from the behaviour observed for higher E x up to the threshold for second chance fis- sion. Therefore in fig. 3 dTKE/dEx(AH) is displayed as determined for E x in the range of 1.5 MeV above the highest fission threshold. The fission thresholds for 236U were determined by Back et al. [10], who assumed a double humped barrier structure. Despite of the large error bars of the values in fig. 3 the con- sistent values from the two independent reactions indicate that the slopes of the TKE(Ex) in a linear least squares fit approximation for E x within 1.5 MeV above the threshold are positive and have values be- tween zero and one for mass splittings w i t h A H above about 128. An exception seems to be present in the mass region A H = 142 to 146, where negative values appear. The large error bars still prevent us from seeing further details, e.g. possibly fine structures due to shells. But the display shows that energy dissipation in this region above the fission threshold is small. Further experiments are in progress to reduce the statistical errors. This result is in favour of the model of N6renberg [2], who assumes a small dissipation due to two-quasi-particle breaking and considers mainly collective degrees of freedom. It is also in favour of the other long mean free path model of one-body viscosity by Swiatecki [3]. Lang et al. [11 ] recently observed e v e n - o d d effects in the energy distribution

of fission fragments with Z = 33 to 42 from the reac- tion 235U(nth, f). Under plausible assumptions the authors explain these effects by a final conservation of about 23% of unbroken pairs in the descent from the saddle to the scission point. This result is consistent with the results from our experiments.

The results show the influences of shell effects ex- pressing themselves in the slopes of the TKE, which strongly depend on the mass splitting. Further, fission is a process with relatively small dissipation of energy. This is most evident for excitat ion energies above the highest barrier of the fissioning system within about 1.5 MeV, which may be identified with the pairing gap.

We thank Prof. Dr. T. Mayer-Kuckuk for his en- couraging support of the experiments. Financial sup- port by the Bundesministerium for Forschung und Technologic is gratefully acknowledged.

References

[1] W. N6renberg, Z. Phys. A274 (1975) 241; Phys. Lett. 52B (1974) 289.

[2] W. N~Srenberg, in: Proc. 2nd Intern. Atomic Energy Agency Symp. on the Physics and chemistry of fission (Wien, 1969) p. 51.

[3] W.J. Swiatecki, Lawrence Berkeley Report, No. LBL- 4296 (1975), unpublished.

[4] K.T.R. Davies, A.J. Sierk and J.R. Nix, Phys. Rev. C13 (1976) 2385; K.T.R. Davies, R.A. Managan, J.R. Nix and A.J. Sierk, Phys. Rev. C16 (1977) 1890.

[5] P. David et al., Phys. Lett. 61B (1976) 158. [6] H.W. Schmidt, J.H. Neiler and F.J. Walter, Phys. Rev.

141 (1966) 1146. [7] W. Holubarsch and F. Gt~nnenwein, Verhandlungen der

Deutschen Physikalischen Gesellschaft VI, 11 (1976) 972, Spring Meeting Baden-Wien.

[8] F. Dickmann and K. Dietrich, Nucl. Phys. A129 (1969) 241.

[9] B.D. Wilkins, E.P. Steinberg and R.R. Chasman, Phys. Rev. C14 (1976) 1832.

[10] B.B. Back, O. Hansen, H.C. Britt and J.D. Garrett, Phys. Rev.C9 (1974) 1924.

[11] W. Lang, H.-G. Clerc, H. Wohlfahrt, K.-H. Sehmidt and H. Schrader, Verhandlungen der Deutschen Physikalischen Gesellschaft, 4 (1978) 928, Spring Meeting, Heidelberg (1978).

[12] E. Konecny, H.J. Specht and J. Weber, Proc. 3rd IAEA Symp. on the Physics and chemistry of fission (Rochester, 1973) Vol. II, p. 3.

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