Mass, charge, and energy distributions in very asymmetric thermal fission of 235U

233~

Nuclear Physics A502 (1989) 233c-242~ North Holland, Amsterdam

MASS, METRIC

CHARGE, AND ENERGY DISTRIBUTIONS THERMAL FISSION OF 235U

J.L. SIDA BRISSOT (1) I.P.N., (4) I.S.N.,

(l), P. ARMBRUSTER (4) and H.R. FAUST (3) ORSAY,

FRANCE.

GRENOBLE,

(2) G.S.I.,

(2),

M. BERNAS

DARMSTADT,

IN

J.P.

(l),

F.R.G.

(S) I.L.L.,

VERY

ASYM-

BOCQUET

(3),

GRENOBLE,

FRANCE.

FRANCE.

Abstract: The fission fragment separator “Lohengrin” of the Institut Laue-Langevin in Grenoble was used to determine the yields of the very asymmetric light fission products (A= 84-69) as a function of A, Z, and the kinetic energy E. The proton pairing effect causes fine structures in the mass distribution, in the mean nuclear charge 2 and its variance crz, and in the mean kinetic energies of the elements. The neutron pairing effect in the production yields is found for the first time of the same order of magnitude than the proton pairing effect. In the mass region investigated both are the largest observed in fission of 2351J. A decrease in the mean kinetic energy for the isotopes of Ni and Cu was observed. It points to a large deformation at, scission. Our results support the view that very asymmetric low-energy fission is a weakly dissipative process. The highly deformed transient system breaks by a slow necking-in process. 1. INTRODUCTION Since reveal

fission was discovered,

its secrets.

became

Step

available,

gave valuable

the isotopic

informations

the extreme asymmetry. and charge

by step,

50 years ago, many experiments as increasingly

and isobaric

of production:

The present

investigation

distributions

yield distributions

on the process.

conditions

sophisticated

kinetic

on 235U (nth,f)

in the mass range

provides

from A=

kinetic

interesting

energy,

tried to

and methods

a.t different

It is especially

extreme

and theories

instruments

and/or an insight

energies

to study

now,

extreme

mass

into the mass

84 down to 69.

2. EXPERIMENT The mass and charge measured’

down to AL/AH

The

sources

yields

were

99.5010, on a Titanium source,

the

UOs

product

In order

for thermal to study

fission of 235U had been previously

the process

for mass-asymmetry

= 69/167 far away from the most probable

very low independent

fission

distributions

down to mass 80.

for the light fragment

layers backing.

layer intensity

of 40 pgjcm2 In order

was covered with

time

with

of lJOz,

250

values

(AI,/AH 2 !X/l,iO).

were measured.

to reduce

with

an isotopic

the sput.tering of Nickel.

pg/cm2

(“burn-up”)

ones

was measured

enrichment

of Uranium The

of

from the

decrease

to be less than

in the 5% per

day. A AE-E the

HFR

charge

state

0375-9474/X9/$03.50

(North-Holland

ionization

of ILL,

chamber’

Grenoble,

(A/q)

allowed

and energy

@ Elsevier

Physics

Publishing

installed

behind

to identify

over charge

slate

Science Publishers Division)

the

single (E/q)

B.V.

“Lohengrin” isotopes

selected

spectrometer

of a given

R.

at

mass

over

by the spectrometer.

A

234~

J.L. Sida et al. / Asymmetric

sufficient

AE resolution

of the chamber by about

unambiguously; first

was achieved

for element

to separate

neighbouring

allowed

5 MeV,

Fig.

Mass

1.

the entrance

experiment3

thermaljission

allowed

and atomic

identification’.

direct

The energy

A/q-values,

number

slit of the chamber

the

of “‘U

of the

selected

identification

which isotopes

a 1 MeV

resolution

differ

in energy

were determined energy

of nine new neutron

window.

rich

A

isotopes,

I”

70 >I %! w

300

Q 65

200

100 60 90

05

Fig. 1 a. Bidimensional 78/19

at 89 MeV.

spends

to hewier

spectrum

elements

for A/q

AE-E

The compolletlt

ries in the spectrometer, to A/q=82/20

AEtMeV)

hwing

spurious

the total ellergy

the compolwlt

trajecto-

onto the AE

sxis of the bidimen-

plot in Fig. 1 a, after selection of f2

Mev.

of a window

The contribution

(33) is not shown.

011 the right

measurements

were performed

97 MeV for one charge the relevant

charge

Conversion

processes state

to obtain

followed

shell closure

The data were corrected

5 MeV).

in the errors

the energy The

energies

in “Lohengrin”

a reasonable

interpolation

energies

85,

of 89 and

and to integrate

over

by Auger

cascades

for Neon-like

were not observed

configurations

in this mass

was shown

to modify

distributions.

sion of the spectrometer. into account

kinetic

(q), and at intermediate

states.

but the atomic

the ion charge

state,

at three

states

for the “burn-up”

The kinetic losses

energy

in the source

uncertainties

of the target

and for the energy

of the light fragment (a

0.7

MeV)

due to the corrections

and

was obtained

in the

NGlayer

and summations

dispertaking covering

were included

given for the yields.

3. RESULTS 3.1 Isotopic

distributions

The

integrated

yields,

of the mass

of the fragment,

and mass distribution over the kinetic

energies

and for each

element.

are given The

in Fig.

mass

yields

for

of AB

at EN94 MeV.

The

it (z

Fig. 1 b. Projection sionsl

93, and 101 MeV for 4 or 5 ion charge

region,

=

01, the top corre-

2, as a function are decreasing

J. L. Sida et al. / A.ysmmetric thermal jfission oJ‘ z.icU

extremely

quickly,

fragments/fission) extrapolation

dropping

by five orders of magnitude

following

approximately

is only

as compared neutrons).

due to the

to the pairing Our result

as observed5

in 252C

s u aJ .>

A=80

the Wahl prediction4.

larger

pairing

effects

used in the predictions

excludes

between

any peak

in this mass

and A=69

The difference

in this

(~20%

235~

very

light

per protons

region

mass

(lo-”

to this region

and ~5%

due to closed

shell

per Z=28

f.

1

1 0-l 10-2

1o-3

10-4

10-5

lo~

IT

I1

1

70

I,

I

I

I,,

75

I,,

80

,

I

A 85

Fig. 2. Mass distribution of the yields integrated over kinetic energy (crosses joint by line). The circles (squares) joint by dashes (dots) are the contribution of even (odd) elements to the mws yields. The contributions from elements se (34) and fir are not shown. The

neutron

below

As (Z=33).

paired

fragments

odd elements

pairing

effect

It is responsible are also produced

is clearly

seen

on the production

for the oscillations with larger

to be lower in yield than

yields

even elements.

around than

yields

the main

unpaired

ones.

for elements trend.

Proton

Fig. 2 shows

236c

J.L. Sida et al. / Asymmetric thermal.fission of‘ z’cV

3.2 Mean

isobaric

charge

and its variance

In Fig. 3a, the deviation Density” v(A)

is plotted

taken

of the average

as a function

from the Wahl prescription

modulated

by the proton

are pointing

odd-even

to the increased

nuclear

charge from the “Unchanged

of the pre-neutron

emission

4. The average

effect.

odd-even

A + y(A),

charge

predominantly

nuclear

The large oscillations

effect on the proton

Charge

mass A’= occuring

with is

at low masses

number.

0.0 ‘“_ -0.2 'N -0.4 s ';1-0.6 Y -0.8

Fig.

3 a.

changed tion

deviation Charge

of the

Another

primary

uz

where

the production

as a function

lower than

becoming finally

its variance

slower scission

larger

effects

plotted

proton

pairing.

isotope

a freeze-out.

than

70%

fission

(oz

adjusts

located

mass

of the oz

masses

mass

yield

:

is notice-

is associated

the motion

adiabatically,

A’L.

of the

at

(UZ N 0.4)

c- 0.63).

lower variance

as a fuac-

fragment

modulation

are

oz for As75 At scission

The

primary

in uz

of the isoba-

distribution

is the strong

is more

no longer

deviation

cbsrge

of the

minima

thermal

distribution

on the production

(“third

of necking-in it widens,

observed

stems

and

from

a

for higher

Z ‘, Fig.

4 b, is found

Such

a large odd-even

reported

method”)7. 4 a.

in Fig. The

For the first

to be of the same difference

It gives first evidence

to this mass region.

yields

effects,

difference

in Fig.

evaporation.

be restricted

effect

The

of freedom’.

of the pairing

yields than

effect, neutron

pairing

3b.

235V

nuclear

tion

A“.

in our measurement.

degree

undergoes

magnitudes

four consecutive is much

Fig.

3 b. Standard

ric

motion.

3.3 Odd-even The

mass

for the standard the charge

Fig.

Un-

of an even-even

equilibration

rapid,

the

as a func-

of the large of A’,

Ge and 84Se

” Ni,74,7G Zn,” to the charge

from value

fragment

consequence

width

ably

of z

Density

time,

order

in the yields

of a primary

4, were evaluated

proton

effect

pairing

the neutron of magnitude

using

found

here,

odd-even than

the

can not be produced

by

for neutrons

which could

J.L. Sida ef al. / Asymmetric thermal,$.wion of '"U

.

EO-

lntegrnted

0 c, x

x

,SO-

over

237c

E

Lang a cl, Er~9lMsV E=107hleV

:

60

I

: -

x

. htegrated over E 0 Lang (L 0, o E-31 MsV x E=107M*V

x

20

0

1’~“‘1’1’1” 30 32

Fig.

34

4 a. Proton

emental

yields

energies

(line)

odd-even summed

33 the values

40

the

Ref.

Neutron

55

odd-even

isotonic

yields

summed

energies

(line)

and for

than

gies as a function

1.

49 the vslues

pairing

to increase energy

be anticorrelated

to the intrinsic

dissipation

4 b.

ener-

and excitation must

50

60

N Fig.

kinetic

and neutron energy

45

in the el-

selected

from

the pairing

and/or

Only small

two

are taken

We find the proton

distance)

effect

Z

of Z. For Z higher

sum of the kinetic Q-value,

38

over

and for

gies as a function

total

36

energy

and slow scission

kinetic

selected

energy,

from

Ref.

Fig.

from the fragments

ener-

the strong

than 1.

4. AS the equals

energy

of the scission

yields

the

in the

of N. For N higher

to the deformation

(temperature) motion

over two

are taken

with kinetic

released

effect

the

(breaking

configurations.

odd-even

effects

ob-

served. Mean

3.4

kinetic

energy

of the isotopes

70

Fig.

5. Average

The

energy

elements

kinetic

scale

are giveu

used

energy is shown

by the

of the

isotopes

in the

horizontal

upper

arrows.

75

of different left

corner,

80

A

85

element. 4 as n function the

absolute

energy

of their values

mass for

the

~~umbt~~s. diffrren(.

238~

J.L. Sida et al. / Asymmetric thermalfission

The average through

kinetic

measured.

process

As for thermal to increase behaviour

fission

by neutron

for a given element

isotopes fragments

number

there

below

kinetic

the

which

of the different

Z=31

near closed shell configurations,

for Z>31,

depends

was found Fig.

involved.

5.

The

This

heaviest

evaporation.

energy,

high kinetic might

the

Ref.1.

on the temperature

less neutron

which

But

of the iisotopes*

on kinetic

show relatively neutrons

energies

distribution.

nuclides

which suffered

a Gaussian

as can be seen from

number

is no dependance

of evaporated

energy

energies

neutron

by fitting

of the five kinetic

significantly

evaporation*,

are those

elements

of the elements

a small

with

and on the Q-values

to the higher

cates

our result

and regularly

was calculated

as a function

the pre-neutron

of 23QPu the average

strongly

of the fragments

isotope

yield

will not change

was explained

isotopes

of each

evaporation

We have not yet determined

evaporation

trast

energy

the post-neutron

of “‘U

In con-

and the lightest

energies.

be expected

as those near the doubly

This

indi-

for those

light

closed shell nucleus

‘s2S7Z. 3.5 Mean

kinetic

energy

We have calculated kinetic

energy,

of the element

the average

E(A,Z),

kinetic

of the isotopes,

E(Z)

=

c

energy

of the elements

1

I

,

I5

,

. 98

2

.

-

.

I

Y(A,Z)z(A,Z)

c I

o

Oo

.

I,,

stant

O0.

higher elements,

I

occurs

; I

most

92

1 1

f

.

Thtsexp Lang801

0

1

1 30

1

’ 35

1

the

larger

ZLZH

pulsion.

2

enlellts as 3 function of 2. For 2 higher than 34 the vslues are taken from Ref. 1.

the kinetic

Another finding

that

configurations explain

characteristic

splits

even-even

mass

or higher

appears

split

higher have

prescission

clearly

than higher

kinetic

mean

energy

a decrease than

expected

dependence must

kinetic

energies

Coulomb

more elongated

of re-

scis-

to reproduce

drop.

in Fig. 6. The mean

odd ones,

by al-

is signifi-

on the basis

of the

be involved

energy

change to higher

drops

which

con-

for the

a consequence

energy. for paired

More

kinetic

energy

of the general compact

scission

fragmentations

might

this finding.

3.6 Isobaric The summed

of E(Z)

are systematically

stays

Compared

Consequently

sion shapes

E(Z)

as observed

Fig. 6. A significant

their

4 MeV,

cantly

_ 1 1 40'

(Z=30)

for Cu and Ni.

Fig. 6. Average kinetic energy of the el-

of even-Z

to Zn

l 0.5)MeV,

(98

Z values, IJg6 94

Y(A,Z)

A270

Down

,

0

from the mean

using the relation:

A270

100-m

E(Z)

average

total

kinetic

kinetic

over Z. The

total

energies

energy kinetic

I

h as been

energy

is then

calculated

calculated

using applying

equation

(1)

but

the conservation

J.L. Sida et al. / Asymmetric thermalfission

of linear

momentum

two curves

and compared

exhibit

similar

is nearly

TKE(AL/AH)

slopes

to the thus

constant,

E*

Our values

fission and lead to about

2 or 3 evaporated

3.4, that

the neutron

most of the excitation

are quite

emission

energy

E*(AL/AH)

over

the

comparable

neutrons. must

=

whole

be carried

7.

The

Q(AL/AH)

mass

-

region

to the ones from

On the other

from the light fragment

of 27 MeV

Fig.

g(AL/AH)-value’,

difference

27 MeV,

x

ered by the experiment. section

average

the

23%

of “‘U

cov-

“standard”

hand we concluded

in

was very low. Therefore

away by the heavy

partner.

,,F

,,,,,(,,,,,,,,,

75

70

Fig.

7 a.

Average

responding light

fragment

turns

total

Q-value,

80

kinetic

from

Ref.

out that

the most

perimental

kinetic

deformability

sion factor

supported” spending

probable

the

cm-

Fig.

ternary

mode could

Mean

excitation

JS a function

energy

of the

light

E'= frsg-

configuration,

low lying

which

fragments

might

in F8Ni explain

It is open to reconcile

point

reproduces

being

by microscopic

Of-state

This

a scission

It

also the ex-

deformed.

theories”,

allows

model.

A high

and exper-

a large

deformation

the weak neutron

evaporation

the calculations

with a very low

neutrons.

fission

to solve

as a source 500

times

be compensated

(y(A+l)/y(A)>4). This

and decrease hypothesis

fragments

this puzzle for these

with

by invoking

region

than

could

kinetic

be verified

the

of cy-accom-

fragmentations. binary

by the rapid

of an alpha particle the total

the possibility

very asymmetric

less probable

in this mass

The emission

energy heavy

using

to both

as predicted

energy.

light fragment.

ance problems.

calculations,

scission

fragment

by the

is typically

of excitation

7 b.

-Q-TKE

of the

corresponds

too much

of emitted

plementary

and

as a function

energies,

We have speculated panied

85 AL

preliminary

of the light

of the deformed number

80

75

111ent 1115ss.

performed

imentally

70

mass.

We have

without

energy 9,

85

AL

]

process,

This but

thus solving

by measuring

a

drop of the mass yields

would remove a significant energy,

fis-

such amount

the energy

the yields

bal-

of the com-

Z=61-64.

4. CONCLUSIONS The present

measurements

for masses

from 84 down to 69, extend

previous

mea-

240~

J.L. Sida et al. / Asymmetric

surements

described

in Ref.

in the independant

yields

as well as the modulation is a consequence on the mean primary energy

of the width

oz of the isobaric

odd-even

of the fragments,

neutron

from scission. of Nickel,

of evaporated for Z=28,

neutron

from

oc-accompanied

yields. energy,

cannot

these

effect

be its cause.

is also observed

decrease

time

a large

pairing

kinetic

in the mean shapes

effect

energy

for a very small

nuclear

effect

with excitation

It is a primary

of the mean

of more elongated

AZ,

distributions,

As the neutron

indications

The

isotopes.

the occurence

can be seen

for the first

i.e it decreases

the lack of variation

and Zinc are strong

charge

pairing

We observed

effect

of the deviation

nuclear

A proton

with kinetic

evaporation

Further,

Copper

29 indicates

The modulation

effect on the production

and regularly

the isotopes

effect.

of the element.

significantly

originating

of the odd-even

energies.

energies

odd-even

influence

at all kinetic

of the proton

kinetic

neutron

increases

1. The strong

qf “‘iJ

thermaljssion

for

number

kinetic

energy

at scission

or of

fission.

We summarize: 1) The tribution,

constant

apart

“standard”

excitation

thermal

3) The

large

to a very low excitation

with

required

fission

dis-

is still

the elongation adiabatic

and the necking-in,

process.

and the weak neutron at scission

evaporation

from

for the light fragment

and

partner.

calculation,

the mean

a very low neutron

to the Discovers 0.

Hahn,

excitation

emission

of Fission

L. Meitner

The yields of the new isotopes

from

energy the light

of 27 MeV fragment.

is difFurther

emitted

discovered

life of this isotope

from

0.5 )s. The process

in its subsequent a total

discovered

we were not yet able to produce

of about

and many

Fission

Products

and F. Strassmann

1.5 10e5 %. Using a time correlation

and the electron i

in the mass

are necessary.

Reverence

obtained

during

deformation

our first

to reconcile

of structure

this very asymmetric

cold, nearly

point

of the heavy

4) Following

pairing

rather

total

a large deformation

calculations

shows that

of neutron

for an intrinsically

the light fragment

ficult

and the absence

effect,

fission.

2) The conservation is a signature

energy

from the pairing

recently technique

@--decay, a hundred

are very low”, e. g. for 72 Ni we between

we succeeded decays

to determine

observed.

50 years ago still gives an access by any other

the fragment The

identified t.hr lralf-

half-life

to new isotopes

is (2 which

reaction.

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