Evidence for quasi-static scission configurations obtained from long range particle accompanied fission distributions

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Evidence for quasi-static scission configurations obtained

from long range particle accompanied fission

distributions

C. Guet, C. Signarbieux, P. Perrin, H. Nifenecker, B. Leroux, M. Asghar

To cite this version:

L-213

EVIDENCE

FOR

_QUASI-STATIC

SCISSION

CONFIGURATIONS

OBTAINED

FROM LONG RANGE PARTICLE ACCOMPANIED FISSION DISTRIBUTIONS

C.

_GUET,

C. SIGNARBIEUX

_(*),

P. PERRIN

_(**),

H. NIFENECKER

_(**),

B. LEROUX

_(***)

and M. ASGHAR

Institut

_{Laue-Langevin,}

156X Centre de

Tri,

38042 Grenoble

Cedex,

France

(Re~u

le 27

avril 1978,

accepte

le 18 mai

₁₉₇₈₎

Résumé. 2014 De _nouveaux résultats _sur la fission

accompagnée

de

_particules

_légères de 236U sont

présentés. Les _largeurs de la distribution _angulaire de la

_particule

03B1 sont les mêmes dans les fissions de 236U

_(nth)

et

_{de 252Cf(s.f).}

Il

_apparait

_que cette _{largeur angulaire} est une fonction décroissante de

l’énergie

cinétique totale des

_fragments

de fission. Ces résultats _suggèrent que les

caractéristiques

de

ce mode de fission ne sont _pas

_{incompatibles}

avec un modèle de la scission _supposant

_{l’équilibre}

thermodynamique

des

_degrés

de liberté collectifs.

Abstract.

2014 New

experimental results on

_{light-particle-accompanied}

neutron-induced fission

of 236U are

_presented.

The widths of the

_{03B1-particle}

_angular distributions are the same for 236U

_(nth)

and 252Cf(s.f).

This _{angular width appears} to be a

_decreasing

function of the total fission

fragment

kinetic energy. These results suggest that the characteristics of this mode of fission are not inconsistent with a

_{semi-equilibrium}

model for scission.

LE JOURNAL DE PHYSIQUE - LETTRES _TOME 39, ler _JUILLET 1978,

Classification

Physics Abstracts 25.85

The

_knowledge

of the

_fragment

_shapes

and kinetic

energies

at scission is essential for _any

_dynamical

theory

of fission. Since

_{light charged particles}

emitted in

_ternary

fission _emerge from the neck

_joining

the I

nascent

_fragments

at a time close to

_scission,

a detailed

study

of this mode of fission should

_{provide unique}

information on the last _stage of the fission _process and

more

_generally

on the

_dynamical

_properties

of nuclear matter at low and moderate

_{temperatures. However,}

the extraction of scission _parameters from

_ternary

fission data is

unfortunately

not

_{straightforward.}

As

we are confronted with a

three-body

_problem,

the

simplest

procedure

is to _try a set of initial conditions which could be identified with the scission parameters,

to _compute the

_trajectories

_[1],

and to _compare the calculated

_quantities

at

_infinity

with the

_experimental

data.

_By

scission _parameters we mean

_essentially

the

prescission

kinetic _energy

_EKo

of the fission

_fragments,

the initial momentum of the

_a-particle

and their relative

_positions.

_{EKo is}

the kinetic _energy

_acquired

in the fission mode

_during

the descent from saddle to scission.

(*) Centre d’Etudes Nuc1éaires, DPhN/MF, BP 2, 91190 Gif

sur Yvette, France.

(**) Centre d’Etudes Nucleaires, DRF/CPN, 85X, 38041

Gre-noble, France.

(***) Universite de Bordeaux, Gradignan, France.

The results which are

_presented

below _may

_provide

new information on the

_sharing

between

_potential

and kinetic

_{energies during}

the descent from saddle to scission. These new features of

_{a-accompanied}

neutron induced fission of

236L1

have been obtained

_recently

in an

_experiment

carried out at the Grenoble

High

Flux Reactor where a beam of 5.5 x 109

nth/cm2/s

is

available. The

_angular

distribution of the

_a-particles

was measured with a

_good

_accuracy as a function of

the kinetic _energy and mass of the fission

_fragments.

A silicon

_rectangular

detector

_{( 1}

cm x 3

_cm)

fixed the a direction while 7 circular

_(yr

_{cm 2)}

fission detectors

were

_placed

at

_angles

from 68° to 98° with

_respect

to

the a direction in

_steps

of 5°. The I

cm 2

circular uranium

_target

was 150

J.1g/cm2

_UF4

(235U

enrichment

of 97

_{%) evaporated}

on to a 20

J.1g/cm2

VYNS

backing.

The

_experimental

_set-up allowed for a

_geometrical

angular

dispersion

(FWHM)

of

5°,

and for a mass

dispersion (FWHM)

of 6 amu. The mass of the fission

fragment

was determined

_by

_measuring

its kinetic

energy and

velocity,

this last

quantity being

derived

from the difference of

_{times-of-flight}

of fission

frag-ment and a

_particle

after

_appropriate

corrections.

A

_large

number of correlations have been derived and will be

_presented

in a detailed

publication.

We

present here two of these results which we believe deserve

_special

attention :

L-214 JOURNAL DE _{PHYSIQUE -} LETTRES

a)

The widths of the

_global

_{a-particle angular}

distributions are

_nearly

identical for 236U

_(nth)

and 252Cf

(st).

The

_angular

distribution is defined with _respect to the

_light

_fragment

direction. Its most

_probable

value and width

_depend

_slightly

_upon the a _energy threshold below which the events are

rejected.

The most

_probable

angle,

81.3° ± 0.4 for

_particles

with kinetic _energy

higher

than 10 MeV

₍₈₂₀

_± 0.4 for

_Ea

> 12.5

_MeV)

is lower

_by

about 20 than the

_angle

measured for

2 5 2Cf

_{(sf) [2, 3].}

This difference is

_easily

understandable in terms of the more

_asymmetric

ratio of the mean

fragments

masses for

236U

than for

_252Cf;

this leads to a

_stronger

deflection _away from the

_{perpendicular}

direction for

236 U.

After correction of the

_geometrical

dispersion,

the width

_(FWHM)

of the distribution

is 18.70 + 0.80 for

_Ea

> 10 MeV. This width is lower than those measured

_previously

_[4].

We want to underline the

_striking

_similarity

between this value and

18.50 ± 10

_(FWHM)

measured for

252Cf

_{[2, 3].}

In so far as the classical

_{three-point-charge}

model is

valid,

these _present observations disfavour the

dyna-mical

_{liquid drop}

treatment of fission

_proposed

_by

Nix et al.

_{[5, 6].}

In addition to

_{high pre-scission}

kinetic

energies

(around

25 MeV for

_236U),

the

_{liquid drop}

calculations

_predict

a marked increase of this

quantity

as a function of the

fissibility

_parameter.

For

example,

the

_pre-scission

kinetic energy for

252Cf

is

predicted

to be about 1.5 times of the value for

236U.

Since the final

_angular

distribution of the

_a-particles

is

essen-tially dependent

on their initial

positions

and the

initial

_fragment

kinetic energy

[1, 7],

one should

_expect

a wider

_angular

_spectrum

for

252Cf

than for

236U

which is

_contrary

to the

_experimental

results. In order to

_reproduce

such

_{global angular}

widths,

trajectory

calculations have shown

that

with

reasonable

dis-persions

of initial

_parameters,

one cannot rule out the

possibility

of low initial

_fragment

kinetic _energy

(E~

10

_{MeV) [7, 8].}

b)

The

_angular

distribution _gets narrower as the

kinetic energy of the fission

fragments

increases. In

_figure

1 we show the

dependence

of the _average

angle

0,, - L >

and the

_angular

width

_(FWHM)

_upon the total kinetic _energy

_EK

of the fission

_fragments

for

given

values of

_fragment

mass and for a-kinetic

energies higher

than 10 MeV. The _average

_angle

does not show _any

_significant

variation with

_EK.

_However,

there is a clear decrease of the width as

_EK

_{goes up.}

In order to

_preclude

the effect of the well-known anti-correlation between

_E«

and

_{EK [9,}

_10],

_{the’angular}

distribution was derived after

_setting

a narrow

window

_{(2 MeV)}

on the _a-energy

_spectrum.

The same

trend was observed : the

_angular

distribution is

narrower for

_{higher fragment}

_energy events. It is

generally accepted

that the final a-kinetic _energy is

strongly

determined

_by

its initial value

_2~

[1].

There-fore,

for a

_given

final a-kinetic _energy

_(hence

a small

spread

in

_E,,,.),

the

_equation

for variances derived from

FIG. 1. -

Average angle « (}rz-L » and _angular width (FWHM) as a function of total _fragment energy with the conditions that

Ea > 9.5 MeV and ML = 85-87 _{amu (0), or ML} = _{91-93 amu (~),}

or _ML = 97-99 amu _{( x ).}

the _energy conservation relation :

can be written in a reasonable

_{approximation}

as

follows :

Here C is the Coulomb

_potential

energy of the

frag-ments and is sensitive to their deformations. Let us

consider two extreme situations :

i)

First we assume that the

_spread

in

_{EKo is}

much

larger

than the

_spread

in the Coulomb energy. In this

case there is a one-to-one

_{correspondence}

between

_EK

and

_EKo.

The

_fragment

_charge

centre distance is then almost constant as is the

_dispersion

in the initial

position

of the

_a-particle.

The faster the

_fragments

separate, the less the

_a-particle

will be

_focused,

thus

leading

to a

_broadening

of the

_angular

distributions

as

_EK

increases. This is _contrary to our

_experimental

observations.

ii) Secondly,

we now _suppose

_EKo

to be a _constant,

then7(~) ~

r(C);

in this case the

_higher

values of

_EK

configu-L-215

QUASI-STATIC SCISSION CONFIGURATIONS

rations,

and

_consequently

a narrower distribution of

the initial

_positions

for the emission of

_a-particles.

As the final emission

_angle

is

_strongly

determined

_by

the initial

_{position along}

the

_fragment

_axis,

this extreme model

_predicts

the same

_qualitative

variation

as observed

_{experimentally.}

In

conclusion,

the

_experimental

results

_presented

above tend to indicate that the

_dispersion

of the total fission

_fragment

kinetic _energy results

_mainly

from the fluctuations of the scission

_{shapes (stretching}

modes)

rather than

_from

the distribution of _the pre-scission kinetic _energy which is

_certainly

rather low.

This conclusion would mean that the features of the

a-accompanied

fission are not

_{contradictory}

to the

models based on a

_{quasi-statistical equilibrium}

_among

the collective

_degrees

of freedom at scission

_{[11, 12]}

as discussed and

justified

in terms of fission bands

_by

Norenberg

[12].

Acknowledgments.

- We

are

grateful

to S.

Bjorn-holm,

M.

Brack,

K. Dietrich and P. Schuck for fruitful discussions. We are indebted to M. Brack for

reading

the

_manuscript.

References

[1] BONEH, Y., FRAENKEL, Z. and NEBENZAHL, I., Phys. Rev. 156

(1967) 1305.

[2] TSUJI, K., KATAZE, A., YOSHIDA, Y., KATAYAMA, T., TOYOFUKU, F. and YAMAMOTO, H., Proc. 3rd Symp. on

the physics and _chemistry of fission _(Rochester, 1973,

Vienna) p. 405.

[3] FLUSS, N. J., KAUFMAN, S. B., STEINBERG, E. P. and WILKINS,

B. D., Phys. Rev. C 7 (1973) 353.

[4] PIASECKI, E. and BLOCKI, J., Nucl. Phys. A 208 (1973) 381.

[5] NIX, J. R., Nucl. _Phys. A 130 ₍₁₉₆₉₎ 241.

[6] DAVIES, K. T. _R., SIERK, A. J., NIX, J. R., Phys. Rev. C 13

(1976) 2385.

[7] RAJAGOPALAN, M. and THOMAS, T. D., Phys. Rev. C 5 ₍₁₉₇₂₎ 2064.

[8] GAVRON, A., Phys. Rev. C 11 (1975) 580.

[9] FRAENKEL, Z., _Phys. Rev. 156 (1973) 1283.

[10] MEHTA, G. K., POITOU, J., RIBRAG, M. and SIGNARBIEUX, C., Phys. Rev. C 7 ₍₁₉₇₃₎ 373.

[11] WILKINS, B. D., STEINBERG, E. P. and CHASMAN, R. R., Phys.

Rev. C 14 ₍₁₉₇₆₎ 1832.

[12] NÖRENBERG, W., Proc. 2nd _Symp. on _physics and _chemistry