Proceedings of the 5th International Conference on

Dynamical Aspects Nllf:lear Fission

Casta- Papiernicka, Slovak Republic 23-27 October 2001

Editors

J.Kliman Joint Institute for Nuclear Research, Russia and Slovak Academy of Science, Slovakia

M.G. Itkis Joint Institute for Nuclear Research, Russia

v

S. Gmuca Slovak Academy of Science, Slovakia

"" World Scientific • NewJersey•London •Singapore•Hong Kong

ON FISSION FRAGMENT DE-EXCITATION AT SCISSION POINT

V. A. KALININ, V. N. DUSHIN, B. F. PETROV, V. A. JAKOVLEV

V.G. Khlopin Radium Institute, 2-nd Murinski Ave. 28, 194021, St. Petersburg, Russia

A. S. VOROBYEV, I. S. KRAEV, A. B. LAPTEV, G. A. PETROV, Y. S. PLEVA,O. A.SHCHERBAKOV,V. E. SOKOLOV

Petersburg Nuclear Physics Institute, 188350, Gatchina, Leningrad district, Russia

F.-J. HAMBSCH

EC-JRC-Institute for Reference Materials and Measurements, Retieseweg B-2440, Geel, Belgium

The number of prompt neutrons emitted in the fission event have been measured separately for each complementary fragment in coincidence with fragment mass and kinetic energies in spontaneous fission of 252 Cf, 244 Cm and 248 Cm. Two high efficient Gd-loaded liquid scintillator tanks were used for the neutron registration . Approximately 3 · 106 fission events coincident with prompt neutron emission have been accumulated for each isotope. The mean neutron multiplicity, the dispersion and the covariance of the multiplicity distributions have been obtained as a func­tion of fission fragment mass and kinetic energy. Dependencies of the moments of the multiplicity distributions on the fragment mass and total kinetic energy for different mass bins, as well as mass and total kinetic energy distributions of the fission fragments are presented, discussed and compared for the different iso­topes investigated. The results showed a different behavior of the moments of the multiplicity distribution depending on the fragment mass asymmetry that reflects changes in the dynamical effects for different fission modes. Possible reasons for the decrease of the neutron yield at low TKE of fission fragments are discussed .

1 Introduction

Neutron multiplicity is one of the important parts of the experimental data needed for better understanding of the fission process. They represent the information on the deformation of both fragments at the vicinity of the scis­sion point, on the energy partition released in the fission process between different degrees of freedom, the distribution of the excitation energy between the fragments, and on the neutron-')' competition at the fission fragment de­excitation. Generally, these detailed data can help to investigate the dynamics of the strongly deformed fissioning system at the descent from saddle to scis­sion.

The first multiparameter measurements of neutron mult iplicity combined

252

253

with the direction-selected spectroscopy of fission fragments for spontaneous fission of 252 Cf have been done at KRI (Russia) 1 more than a decade ago, followed by the analogous measurements carried out by the TUD-HMI col­laboration (Germany) 2 . These experiments revealed some unusual effects, such as deformed cold fission. Recently, a new series of such experiments was started by the KRI-PNPI-IRMM collaboration within the framework of an ISTC Project. In the present report, some results of the measurements carried out for spontaneous fission of 252 Cf, 244 Cm and 248 Cm are presented.

2 Experiment

A thin fission source was placed between two large 200 I Gd-loaded liquid scintillator tanks which were used for neutron detection in a 2x2?T-geometry - to separate contributions from complementary fragments . The efficiency of the neutron registration was about 55% for each detector. A 16 em thick iron shielding inserted between the tanks was used to decrease both fission neutrons and capture ')'-rays scattering from one tank into the other (cross talk). The same set-up after necessary changes was used for the 4?T calibration measurements. The fission fragments were collimated towards the neutron de­tectors by means of a pin-hole collimator combined with a common cathode of the twin parallel plate gas-flow ionization chamber with Frisch grids. At the counting rate of "useful" fission events (fragments in coincidence with comple­mentary collimated fission fragments) of 1-3 s- 1

, in total about 3 · 106 events for each spontaneous fissioning nuclide investigated have been accumulated.

3 Results and Discussion

To deduce mass and kinetic energy distributions of fission fragments , the pulse height data have been corrected for pulse height defect for the counting gas mixture, energy losses in the sample backing and pin-holes. Mass and kinetic energy distributions of fission fragments have been obtained on the basis of reference values of the most probable mass and energies of fission fragments. The data were accumulated in a four-dimensional array (A1 , TKE, n1, n2] , where A1 - mass of one of the fragments, TKE - total kinetic energy of the fragments , n1 , n2 - simultaneously registered number of neutrons emitted from complementary fragments. The first moment (mean multiplicity) and second moments (variance and co-variance) of the one-dimensional multiplic­ity distributions for both neutron detectors have been calculated for each mass and energy value of the four-dimensional array. These data have been cor­rected for the neutron pile-up, background and detector efficiency, including

254

3 .5

3.0

2.5

2 .0

1.5

3 .5

3.0

2.5

2.0

1 5

1.0

0 .5

2.0

1.5

1.0

0 .5

. ,..

<v>"' var.

--Yield(E)

TKE, MeV

<v>

var <V>t

--mass yield

Mass, a.m.u .

Figure 1. Left: Energy dependence of the average neutron multiplicity < llt,h >, variance vart,h of the neutron multiplicity distribution for light and heavy fragments . Full line: the energy distribution in arbitrary units. Right: Mass dependence of < 11 >, total average multiplicity < lit > , variance var . Full line: mass distribution in arbitrary units. Upper part - 252Cf, middle part - 248Cm and lower part- 244Cm

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cross talk effects. Iterative procedure was applied for introducing of the correc­tion for neutron emission based on the measured mass and TKE dependence of mean neutron multiplicity.

The average multiplicity< v > and dispersion (variance) of the multiplic­ity distribution of the neutrons ernitted from light and heavy fission fragments as a function of the TKE are shown in the left part of Fig.l together with the fragment energy distribution. The average neutron multiplicity < v >, total neutron multiplicity < lit > measured in the 471' experiment and the dispersion of the multiplicity distribution as a function of fragment mass are shown in the right part of Fig.l together with the fragment mass distribution. The minimum in < v > (A), as well as in the variance is observed around A=l28-130 for all nuclides due to the presence of the Z, N magic shells. The average level of mass dependence of the variance var(A) for 252Cf is in good agreement with results of During et al 2 and Signarbieux et al 3

The energy dependence of the mean neutron multiplicity in the case of 248Cm and 252Cf shows a significantly different slope for the light and heavy fragments unlike to 244 Cm. From the rather different behavior of < 111 > (T K E) and < vh > (T K E) in case of 248Cm and 252 Cf it may be concluded that the deformation of the heavy fragment is proportional to the deformation of the whole fissioning system at scission. And, since the dependencies for light fragments are not completely linear, the part of the deformation energy taken by the light fragment is not constant. Also, at the highest TKE close to cold fission the heavy fragments are nearly compact, but the light ones are still significantly deformed. Shape and average value of energy dependencies of the variance var of the multiplicity distribution correlates with the corresponding dependencies of< v > for all nuclides. The TKE distributions exhibit resolved structures that can mean some coincidence of TKE structures for fixed mass splits.

A comparison of different < v > (A) data for 252Cf taken from litera­ture 3,4 with the result of the present work is shown in Fig.2. The agreement is rather good in a wide range of masses especially with the results of Sig­narbieux et al 3 • Also, at fragment mass higher than 165 a.m.u. the present results do not demonstrate a significant discrepancy with the work of Budtz­J0rgensen and Knitter 4 , in which a triple 'saw-tooth' curve for 252 Cf has been observed.

The mean neutron multiplicities < v1 > (T K E) and < vh > (T K E) for the light and heavy groups of fragments as a function of TKE are shown in Fig.3. The values of Vt,h were averaged over two mass bins: first (Sl) -over the mass range corresponding (in terms of the light fragment mass) to 108 < Az < Asym, where Asym is the symmetrical mass for each nuclide and

256

1\ ;>

4

3

v 2

80 100

A Signarbieux [4] --Budtz-J0rgensen [5]

• Present work

120 140 160

Mass, a.m.u.

180

Figure 2. The mean neutron multiplicity < v > averaged over the total kinetic energy as a function of fission fragment mass.

second (S2) - over At < 108. The first bin, where the so-called Standard I mode has a considerable

contribution, was roughly denoted as Sl and the second one, corresponding to more asymmetrical fission and suppressed Standard I mode, as S2. The description of the fission process as superposition of fission modes was intro­duced in the frame of the multi-modal random neck-rupture model 5 . In this model different modes for the descent of the fissioning system from saddle to scission along the potential energy surface are obtained by means of min­imization of the potential energy of the fissioning nucleus in a given nuclear shape parameter space. The fragment mass asymmetry is a main characteris­tic of the fission modes. Figure 3 demonstrates a quite different behavior of the mean neutron multiplicity for Sl and S2 bins. For the Sl mode the light fragment is always more deformed than the heavy one, even at the highest TKE values, where the heavy fragment is rather stiff. The situation for S2 is opposite - the deformation of the heavy fragment exceeds the deformation of the light one, especially at the lowest TKE. Except in the case of 244 Cm, where the deformation of both light and heavy fragments is quite similar. The latter fact can explain the similar behavior of < Vt > and < vh > as seen in the left part of Fig.l for 244 Cm compared to 248Cm, 252Cf. It is also in

257

agreement with the known fact 6 that 244 Cm has a much stronger Standard I contribution than 248Cm.

Another remarkable effect presented in Fig. 3 is the decrease of < v1 > at TKE below 160-170 MeV. The neutron multiplicity is a measure of the fragment total excitation energy (TXE). A higher multiplicity is reached at

2s2Cf

4 4 ~ 52 ° v,

3 3 ' • vh

~ 2 _,r~

2 ~~~ v \It,

\, 0 .__~_._ __ ...._~_._~~~ 0 L....L. __ _.___.._..._~_.-" ......

140 160 180 200 220 140 160 180 200

248Cm TKE, MeV. TKE, MeV.

4~-~~~~-~--~~

~: t ~~:~ vl~

2

1\ > v

Ql...J....--'--~-'-~---''--.....;;....-' 0'--~-.L.-~-..L-~--'-..&....J 140 1 60 180 200 220 140 160 180 200

TKE, MeV. TKE, MeV.

a~~~~~~-~~-~

140 160 180 200 160 180

TKE, MeV. TKE, MeV.

F igure 3 . Mean neutron multiplicities < 111 >, < llh > as a function of TKE averaged over two mass bins: 8 1-left and 82-right (see text ) . E rrors are st atistical only.

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low TKE, corresponding to relatively weak Coulomb repulsion of strongly de­formed fragments. Observed reduction of the neutron emission for the light fragment may be explained in the assumption of emission of a neutron before fragment acceleration. Such explanation is based on the fact, that efficiency of the neutron detector used is rather .direction sensitive. Fission fragments are collimated in a narrow cone toward the centers of the neutron detectors in order to provide maximal efficiency of neutron detection and to separate registration of neutrons from complementary fragments due to fragment trans­fer momentum. Decreasing of the number of registered neutrons may means isotropic in lab. system emission of neutrons from 'pre-acceleration' frag­ments. The detection efficiency for these neutrons is considerably reduced.

This explanation may also be illustrated using TKE dependence of mean total number of emitted neutrons Vt. In Fig.4 a comparison of the present data for Vt (T K E) with result of ref. 4 is presented. Deviation from straight line at low TKE values presents in both cases, but in data of work 4 it is much smaller. It may be explained by the fact, that in this work fission fragments were not collimated and all relative angles between fragment direction and emitted neutrons were taken into consideration. A very strong deviation of Vt(TKE) for 235U(n,f) from straight line at low TKE have been observed

8

• Budtz-J0rgensen and Knitter 7

0 this work

6

5

"- 4 :> v

3

2

140 160 180 200 220

TKE, MeV

Figure 4. Mean total neutron multiplicity < lit > as a function of TKE.

259

also in work of Nishio et al 1 . The experimental method used in this work is also based on the assumption of the neutron emission from fully accelerated fragments. Authors of the both works 4•7 gave no explanation for decreasing of Vt at low TKE of fission fragments.

Such neutrons with isotropic angular distribution may be emitted due to single particle excitation of the fragment, caused by energy dissipation in a non-adiabatic transition from saddle to scission. Existence and features of these so-called scission neutrons are still discussed 8 . Emission of scission neutrons (most probably from light fragment) may cause the decrease of frag­ment TXE in the vicinity of scission point. Another indication that at the lowest TKE the fissioning system may be cold comes from the analysis of the covariance cov(vt, vh) of the neutron multiplicity distributions for light and heavy fragments.

C A1<108(S2) • 108<A1<Asym(S1) 0. 0 .---.~ • ..-. --r--.-..,.....--.---.--..---.--r--.LT"""".---...--,

-0.1

-0.2 -~ :>~ -0.3

..::... > 8 -0.4

-0.5

-0.1

-0.2

"' :>:. -0.3 ..::... i; -0.4 u

-0 .5

Cf-252

160 170 180 190 200 210 220

Cm-248

150 160 170 180 190 200 210

TKE,MeV

Figure 5. 4 Energy dependence of the cov(vhvh) averaged over Sl , S2 mass bins; upper part - 252 Cf, lower part - 248Cm.

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The possibility of strongly deformed cold fission observed by Alkhazov et al 1 for 252Cf was first described by Nifenecker et al 9 based on the fact that the cov(v1, vh) for a fixed mass is reduced to zero both at minimal and maximal values of TKE. The zero value of the cov(v1 , vh) for fixed mass means that the intrinsic excitation energy of the complementary fragments and the pre­scission kinetic energy at the scission point are vanished. Thus, a zero value of the cov(v1, vh) at highest TKE corresponds to cold compact fission and at the lowest TKE- to cold deformed fission. In Fig.5 the energy dependence of the cov(v1, vh) averaged separately over S1 and S2 fragment mass bins is presented. There is a strong cut of the covariance at lowest TKE values for more asymmetric fission (Al < 108) where the Standard I fission mode is suppressed and a statistically verified slope to zero at 108 < Al < Asym where the Standard I mode has a considerable contribution. An increase (in absolute value) of the cov(vl , vh) from highest to lower TKE means an increase of the energy dissipation. The decrease of the cov(v1, vh) at lower TKE for S1 may mean either a different energy dissipation for different fission modes, or a possible partial de-excitation of system in the scission point. The amount of considered events for the S1 bin with TKE less than 160-170 MeV is approximately 1% of the total statistics.

Hardly understandable deficit of the total excitation energy at lowest val­ues of fragment TKE has been explained above by means of contribution of isotropic distributed (scission) neutrons. In the absence of other explanations this approach seems not to be inconsistent and may be admitted as a hy­pothesis. Besides appearance of this effect for more symmetrical fission, two main features of emission of such neutrons must be noted based on Fig.3. First, isotropic distributed neutrons are emitted most probably from light fragments . Second, the effect is observed only at lowest TKE corresponding to highest deformation of fissioning system. Such features are specific for the cluster phenomena in nuclear fission and may validate indirectly formation of the light cluster in third minimum at large elongation of the shape of fissioning nucleus.

Manifestation of cluster phenomena in low energy was discussed in details earlier by Pyatkov et al. 10•11 . Revealing of cluster effects is based on finding of fine structures in TKE-M fission fragment distribution other than usual Z odd-even effect with approximately 5 mass period. Since TKE dependence of mean mass for given fragment charge is rather weak (see ref 11 ) , undisturbed by Z odd-even effect smooth TKE distributions are expected for fixed fragment mass. But in partial TKE distributions the clear structures are observed. In the present work these structures are even sharper than in refs. 10

•11

, possibly due to better correction for neutron emission. In Fig.6 example of TKE

261

8000 Cm-248 M=108 -corrected

7000 -o-- not corrected

6000

5000

"' t: 4000 :> 0 () 3000

2000

1000

0 160 180 200

TKE,MeV

1000 Cm-248 M=128 -corrected

800 -o-- not corrected

"' 600 t: :> 0 ()

400

200

160 180 200 220

TKE, MeV

Figure 6. TKE distribution for 248 Cm for masses A= 108 (upper) and A= 128 before and after correction for neutron emission.

distribution for 248 Cm for masses A= 108 (maximum yield in fragment mass distribution) and A = 128 (close to symmetrical mass split) are presented before and after correction for neutron emission.

Presence of structures in TKE may indicate clusterization of the neck most probably with a-particles as clusters and this effect should be as better pronounced as more elongated is fissioning nucleus. In Fig. 7 mass distribution without symmetrization for 248Cm for 160 MeV TKE is presented before and after correct ion for neutron emission. Very clear 4 mass structures are

262

900

800

700

600

"' 500

c :I 400 0 (.)

300

200

100

0 80 100

-<>- not corrected

120

Mass

140

Cm-248

TKE=160 MeV

160

Figure 7. Mass distribution for 248Cm at 160 MeV TKE before and after correction for neutron emission. Figures denote mass values in the peaks

observed, which are vanished with increasing of TKE. Based on this short discussion it can be assumed that cluster phenomena may play important role in some aspects of fission process. The systematic analysis of fine structures of fission fragment distribution on the basis of present results is not completed yet.

4 Conclusion

Different behavior of < v > (T K E) in spontaneous fission of 248 Cm and 252Cf for light and heavy fragments demonstrates the fact that the deformation of the light fragments is not fully proportional to the deformation of the fissioning nucleus. The energy dependence of the cov( v1, vh) averaged over different mass intervals showed that deformed cold fission may be most probably observed for the Standard I fission mode and that the energy dissipation along the descent from the saddle to the scission point may be different for different fission modes. A strong decrease of < v > (T K E) for the light fragments may indicate emission of scission neutrons. This effect may be connected to formation of cluster configuration in the final stages of the fission process. But these assumptions have to verified in the course of further data analysis and experiments.

263

Acknowledgments

The authors are grateful to Prof. F. Gonnenwein, Drs. J. Kliman and M. Jandel for interest to this work and fruitful discussions.

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