Evidence of a new type of fission within a solid state nuclear detector

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Submitted on 1 Jan 1982

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Evidence of a new type of fission within a solid state

nuclear detector

M. Debeauvais, J. Ralarosy, J. Tripier, S. Jokic

To cite this version:

L-521

Evidence of

a new

_type

of fission within

a

solid

state

nuclear

detector

_(*)

M.

_Debeauvais,

J.

_Ralarosy,

J.

_Tripier

and S. Jokic

Centre de Recherches _Nucléaires, 67037 Strasbourg Cedex, France

(Reçu le 13 mai 1982, accepte le _{16 juin 1982)}

Résumé. 2014 Lors de l’étude des réactions de l’U de 10 MeV/n sur des cibles d’U, Au et Pb à l’aide d’un détecteur visuel solide de traces, l’on a mis en évidence une

_{configuration}

nouvelle

_{qui pourrait}

correspondre à de la fission retardée. Abstract 2014 An SSNTD

study of reactions induced _by 10 MeV/n U ions on _U, Au and Pb targets

led to the observation of a new _type of fission event which can be

_explained

as a

_delayed

fission.

LE

JOURNAL DE

_{PHYSIQUE - LETTRES}

J.

_Physique

- LETTRES 43 ( 1982) L-521 - L-524 Classification Physics Abstracts 25.85 - 27.90 1 er AouT 1982,

1. Introduction. - In the _{present paper} we deal with a new

phenomenon

observed within solid

state nuclear track detectors

_(SSNTD).

_Elastically

or

_{quasi-elastically}

uranium ions from _very

heavy

targets

(U, Pb,

Au)

were seen to

_decay

into two

_{heavy fragments.}

Three such events _among 6 100 scattered ions were found.

The detector

used,

namely

MAKROFOL,

is a threshold detector. That

_is,

_{only heavy}

ion

tracks whose energy loss rates are

_larger

than a critical value can be recorded

_[1].

_Targets

were

irradiated

_by

10

_MeV/n

uranium at the G.S.I. in Darmstadt

_(West

_Germany).

After irradiation and

development,

different

_types

of events were observed :

-

Two-pronged

events

_{corresponding}

to elastic

_(or

_{quasi-elastic) scattering}

of uranium on

the

_target

nuclei

_{(U, Pb, Au);}

- Three- and

even

_{four-, five-,}

and

_six-pronged

events in correlation and

_{corresponding}

to

sequential

fissions

_[2-4]

of the scattered

and/or

recoil ions.

In

addition,

reactions of uranium on the carbon

_and/or

_{oxygen. nuclei of the detector} were

recorded. These interactions

_generally

occurred between 1 and 40 _~m

_depth

within the detector bulk

_along

the incident uranium

path,

which was

_{perpendicular}

to the

_plane

of the detector. These

_{interactions,}

which lead to

_in-flight

fission of uranium induced

_by

carbon

_and/or

_oxygen have been

_{previously investigated}

in our

laboratory [5].

2. Characteristics of the new events. - A schematic

_diagram

of the new events is

_presented

in

figure

1. In

_figure

2 is

_given

a

_photograph

of the tracks of these events

_projected

on the

_plane

of

(*) La version française de cet article a ete _{proposee pour}

_publication

aux _Comptes Rendus de 1’Academie

des Sciences.

L-522 JOURNAL DE _{PHYSIQUE -} LETTRES

Fig. 1. - Schematic

diagram of a _typical new event.

the detector. Tracks are identified as follows : number « 1 » _represents the recoil

_{nucleus ;}

number

« 2 » is the scattered ion before its

_decay

and numbers « 3 » and « 4 » are the

_{fragments issuing}

from the

_break-up.

The characteristics of these correlated tracks are

_given

in table I.

Table I. -

Characteristics

_of

the 3 correlated tracks

_of

new events.

In all cases, a

geometrical

calculation

permitted

us to

_verify

that the tracks numbered

2,

3 and 4

are

_coplanar.

The lifetime values are deduced from the _{measured ranges of the scattered nuclei.} 3. Discussion. -

Several tentative

_suggestions

can be advanced to

_explain

these observations :

- That these

events be elastic collisions of uranium upon nuclei of the detector.

However,

this

_hypothesis

is _very

_improbable

because MAKROFOL is a

polycarbonate

whose formula is

(C16H1403)n

and thus collisions would occur with

light

nuclei

only.

This threshold detector on

the one hand would not record a _proton recoil track and on the other hand records

only

_oxygen

and _{carbon recoil ranges of ~ 12 ~m} in the standard chemical

_etching

used.

_Moreover,

the

_angle

Fig. 2. -

Photograph of the three tracks of new events

_projected

on the detector _plane.

- That these

events be nuclear reactions of the scattered uranium with a carbon or an _oxygen

nucleus of the

_detector,

since the cross section is not

_{completely negligible.}

_Indeed,

in the 36

cm2

scanned area, we found 6 100 elastic

_scatterings

of uranium for a flux of 1.6 x

106/cm2

incident

particles.

The reaction cross section

_{of U + C (or U + 0)}

is known as well as is the total number

of C and 0 nuclei. From this

information,

the number of nuclear interactions of the scattered uranium with a nucleus of the detector is calculated to be 0.18. In

_fact,

we

_{experimentally}

found 3 events, or 16 times more. In

addition,

the

_decay

of the scattered uranium nuclei occurred after

path

lengths

of

_12,

32 and 45 _~m inside the detector so that the residual

energies

of the three

scattered nuclei were

_{8.5, 6.9}

and 5.4

_MeV/n,

_{respectively.}

As for the latter two events, their

_energies

are close to the

_potential

_barrier,

so that the

probability

of

_forming

a

_compound

nucleus is _very

small.

Since the above

_hypotheses

are not

_totally

_adequate,

we _propose that the events can be some

L-524 JOURNAL DE PHYSIQUE - LETTRES

If the

_branching

ratios of these three events relative to that of the _prompt fissions

₍₃

+ 4 + 5

pronged

events)

is

calculated,

a value of 3 x

10-3

is found. At first

_glance,

this value seems too

high

in

_comparison

with that

_{usually given}

in the

literature,

which is of the order of

10-’.

Never-theless,

when reference is made to the data of P. Limkilde et al.

_{[6] concerning}

two isomers of

238Pu,

it can be

_pointed

out that the

_branching

ratio increases

_noticeably

as the isomer lifetime

decreases. The maximum of this ratio is ~ 2 x

10-5

_{for the}

238pU

_{isomer whose lifetime is}

0.5 x

10-’ s

and -

10 - 6

for that whose lifetime is 5 x

10-’

s. Provided that

_{extrapolation}

of

these results is

valid,

the

_branching

ratio for an isomer whose lifetime is ~

10-12

_s would be ~

10- 3.

This would _agree with the values _we found and thus is in favour of _our

proposition.

4. Conclusion. - Solid state nuclear track detectors would be

particularly

suited to the

measurement of lifetimes of the order of

_picoseconds.

The three events that have been described could be fission isomer ones,

issuing

from reactions of 10

MeV/n

uranium on

heavy

_targets

in

which the scattered uranium has become unstable in

_picking

_up nucleons from the

_target

nuclei.

Nevertheless,

a

_deep

inelastic nuclear reaction of uranium on a carbon or _{oxygen nucleus} cannot

be

_completely

excluded.

Acknowledgments.

- We thank the G.S.I. in Darmstadt for the irradiations and

we are

_grateful

to R.

_{Antanasijevic}

and Z. Todorovic for their effective collaboration.

References

[1] DEBEAUVAIS, M., MONNIN, M., C.R. Hebd. Séan. Acad. Sci. Paris 260 ₍₁₉₆₅₎ 4728-4730.

[2] DEBEAUVAIS, M., JOKIC, S., TRIPIER, J., Solid State Nuclear Track _Detectors,

_{Proceedings of}

the 10th Int.

_Conf.

_{Lyon, July 1979, François} et al. _{(Pergamon Press)} p. 927-937.

[3] BRANDT, R., GOTTSCHALK, P. _{A., VATER, P.,} Nucl. Instrum. Methods 173 ₍₁₉₈₀₎ 111-119.

[4] FIEDLER, G., HAAG, R., CROMBACH, P., BREITBACH, G., GOTTSCHALK, P. A., VATER, P., KHAN, E. V., BECKMANN, R. and _{BRANDT, R.,} Solid State Nuclear Track Detectors, Proceedings

of

the 11th Int. _{Conf. Bristol, Sept. 1981,} P. H. Fowler and V. M.

_Clapham

_{(Pergamon Press)} p. 779-782.

[5] DEBEAUVAIS, M., RALAROSY, J., TRIPIER, J. and _{JOKIC, S.,} Solid State Nuclear Track _Detectors,

Pro-ceedings of the 11th Int.

_Conf.

Bristol, Sept. 1981, P. H. Fowler and V. M. _{Clapham (Pergamon}

Press) p. 763-768.