Target fragmentation in intermediate energy heavy ion collisions

Nuclear Physics A447 (1985) 101~114~ North-Holland, Amsterdam

TARGET FRAGMENTATION W. LOVELANDa,

IOlc

IN INTERMEDIATE

K. ALEKLETTb

ENERGY HEAVY ION COLLISIONS

and G. T. SEABORGc

aOregon State University, Corvallis, OR 97331, bstudsvik Science Research CLawrence Berkeley Laboratory, Laboratory, S-6118$, Nykoping, Sweden, Berkeley, CA 94720. Radiochemical measurements of tarqet fraqment isobaric yields, excitation etc. functions, fission cross sections, fragment angular distributions, are presented for the interaction of lo-100 A MeV light (C-Ne) heavy ions with heavy targets (Sm-U). The characteristics of various aspects of target fragmentation are summarized and the possibility of a new "fast fission" mechanism is suggested.

INTRODUCTION

1.

One

interesting

is that

of

aspect

slow-moving

nucleus.

This

experimental a

rich more

the

energy

production

nucleus-nucleus

collisions

of

A 5

large

(0.02 < E < 5 A MeV) fragments

phenomenon

fragmentation,

that

intermediate

target -fragmentation, -

relatively

is

of

although

is

it

difficulties

is

complementary not

studied

of detecting

and

complex

array

than

justify

the efforts

of

these

phenomena of

to as

that

of

extensively

fragments. associated

various

(20 5

ACN),

of the initial target projectile due

to

Nonetheless, with

these

experimental

the there

processes

groups

engaged

in studies of target fragmentation. In this

report,

measurements targets

of

using

requisite along

simplicity

of

variety

energies that,

light

projectiles

on single

techniques. the

aspects

characteristics

(C,O)

sensitivity with

some

The

for

use

of

detecting

ability

to

study

measurements

of

(LBL,

the

all

the correlations

accelerators region

single

of

CERN,

interest.

particle

between observables

the

of

intermediate

fragmentation

inclusive

low MSU,

in

heavy

-

105 A

simplicity

measurements,

gives target

processes.

studies this

made

techniques

The

to be performed case)

has

whose

its

beam

price

information

in

about

is lost.

*Work sponsored in part by the U.S. Dept. of Energy under Contract 76SFOOO98 and DE-AM06-76RL02227 and the Swedish Natural Sciences Council. 0375-9474/86/$03.50 OElsevier (North-Holland Physics Publishing

of

Z, slow moving

probability

allows

(8.5

survey

measurements

radiochemical the high

This

inclusive

of a set of broad

of

particle

the experimental

span

like

discuss

We shall focus

fragments a

shall

general

by

radiochemical

us the

at

the

induced

MeV) energy.

we

Science Publishers Division)

B.V.

DE-AC03Research

102c

W. Loveland et al. 1 Target fragmention

The work discussed of

a

large

working

in

California 2.

in this report

international the

group

laboratories

is primarily

of nuclear

of

Prof.

G.

the result of the efforts

scientists T.

who

Seaborg

have

at

the

been

or are

University

of

at Berkeley.

TARGET FRAGMENT YIELDS 2.1

Evolution

of

target

fragmentation

mechanisms

with

increasing

pro-

jectile energy. figure

In

teraction sionable

1, we

show

of various I54Sm.

the target

energy

heavy

fragment

ions with

isobaric

yieldsI

the n-rich,

for the

relatively

in-

non-fis-

At the lowest energy (8.5 MeV/A), the fragment yield + 85M.V/A _

-

om.v/n

‘0 f “‘Sm “0. “‘Sm

-

-

35M.V/A W.M.V/A

“C. “C.

1

“‘Se, “‘Sm

xl’~

Figure 1 Fragment isobaric yield distributions for the reaction of various heavy ions with I54Sm. The lines are to guide the eye through the data points. distribution fused

is sharply

system,

mechanism. are

due

with

to complete

the energy deposited

the

of

fragment

in the mass

isobaric

yield

fusion).

proceeding

of the fragments

As

of

isobaric

the projectile

lower

yield

transfer

distribution

by complete

35 A MeV).

reactions

and

lower

distribution

from

projectile

to be asymmetric

fusion

drops

fusion

energy

increases,

so does

to ~35%

mass

numbers.

(after

Correspond-

becomes

broader

- but

the

to target

nucleus

causes

the

with

tail towards lower masses.

It is interesting

via a complete

show > 95% of the events

leading to the production

in the target nucleus,

a long"spallation-like" proceeding

at a mass number near that of the completely

the

spectra

fragments

decrease

at

of

(The velocity

deexcitation) ingly,

peaked

most

at

a steeper

upper edge and

(The fraction of reactions 19 A MeV

and

is <5%

to note that even at a projectile

energy

W. Loveland et al. / Target fragmention

of

MeV/A,

85

although

significant

with

somewhat

formation

less

of

103c

trans-target

probability

than

species

in the

is

reaction

observed

of 85

MeV/A

I2C with 205Pb, 20gBi2. 2.2 A by

FISSION EXCITATION

complementary

12.0

- 83.8

FUNCTIONS

study

A MeV

of

the

heavy

for 154Sm with the additional from ~50% A MeV. the

the most

interaction

showed

cross section

interesting

of various

of

similar

the

more

trends

fissionable

to those

Au

observed

feature that the fission cross section decreased

of the total reaction

Perhaps

fragmentation

ions3

energy

fission

heavy

at 12 A MeV to %8% at 84 excitation

ions with

function

I65Ho as shown

comes

from

in figure

2.

I I 10' -)

h

l

a >

I I I I I

cC,OtHo

loo -. (L

2

lo-'-

-

10'

--$

loo

.@

lo l

(L

l

-2

l

lo-' b'

b' lo-*a 10-a,

’

’

’

4

’

8

’

’

12

’

’

’

16

-

10-2

?z 20

lo-so

a 20

40

60

(1)

&/VW Figure 2.

(a) Fission excitation function from the reaction of 12C and 160 with 165Ho (b) Same data as (a) except Of/OR iS now plotted vs. the average angular momentum of the fissioning system. Data from ref. 4 (solid points) and ref. 5 (open points). For

a nucleus

rapidly,

one

like

165Ho,

observes

as the projectile this observation as the

average

latter

quantity

the

energy

was

fission

will exit

only

fission

channel

to

shown

momentum

estimated

in Figure the

made in

to

importance

The physics

simple

system,

.

approximate

(ofusion)si

Eproj < 10 A MeV

lrX2 = gcrit ('II/PCN)

behind

This

semiclassical

relationships: = 2/3

rotate

2b) in which of/UP is plotted

of the fissioning using

when

decrease

is raised from 17 to 84 A MeV.

is better angular

which

Eproj L 10 A MeV (2)

(1)

104c

Values

W. Loveland et al. / Target fragmention

Of

ficrit, and

transferred

linear

dfusion momentum

were

taken

was

calculated

from

ref.

6.

using

and

the

fractional

semiempirical

the

relationship7 (pll/PCN) = -0.092 It appears of

the

fissioning

decreasing energy

system from

(which

that

+ 1.273

(3)

the data as a function of the angular momentum essential

transfer

physics

is shown,

The

singular

rare earth fissionability

the

deposit

fission

cross

excitation

i.e.,

to the target nucleus

17 to 84 A MeV.

in determining

remembering

protons

the

angular momentum

increases

momentum by

that by organizing

4:

section

energies

that

of

a

as the projectile

importance

of angular

can be further demonstrated for

of

the

several

interaction hundred

of

MeV)

GeV with

rare earth nuclei is
LIGHT FRAGMENT of

the

most

reactions

producing

At

projectile

higher

(20 5 A 5 60) EXCITATION

interesting nuclear energies

aspects

fragments (Eproj 2

of that

FUNCTIONS

target have

fragmentation Afragment

2-3 GeV),

5

are

those

l/3 Atarget.

such fragments

are known9

12 Heavy Ion t U,--> 9'

9

5 d

-

24Na /"SC /I

6

Li 3

0

s

8

16

24

Eproj (GeV)

Figure 3. (a) Excitation function for the production of 2 typical light fragments from the interaction of C and Ne with U. (b) Same data as (a) replotted to illustrate a simple functional form of the excitation function. Data from ref. 10.

W. Loveland et al. / Target fragmention

to

have fragment

to

these

the

production

(Figure

multiplicities

events

3)

in light

is called of

and

these

show

calculations

of

after

interaction

with

about

3

MeV/A,

the excitation

If into

in

at the lower energies

in intermediate

the

in a simple

production

to be Q-3

excitation

agreement

measurements

of

regime

behavior

of

energy

with

light

of

these

for

manner

fragments

of A = ZOO-240

theoretical

MeV,

nuclei

i.e.,

estimatesll

light

4),

we

excitation see

fragment

collisions,

of light fragments.

of

reactions

functions

evidence

excitation

for

a

function

indicative of a different This finding

of Lynen -et al. l2 that the production

energy

rise

Firestreak

GeV.

fragment

(Figure

the

in nucleus-nucleus

for the production

with the observation

for

giving

functions

for multifragmentation.

energy

different

Excitation

2-3 GeV C ions show Emax* s 600-700

estimate

intermediate

qualitatively

mechanism

the maximum

these

1 and the process

be parameterized

collisions

energy threshold

we extend the

an

can

threshold

nucleus

model

than

"multi-fragmentation."

fragments

the

nucleus-heavy

greater

105c

is a binary process

is consistent

of these fragments

rather than a multi-body

breakup as observed at higher energies.

1o-4

P+

lg7Au ,28M9

E proj (MeV) Figure 4 Excitation functions for the production of a typical li ht fragment, 28Mg9 The p + 9 g7Au data is from in the interaction of 12C and 1~ with lg7Au. ref. 13.

W. Loveland et al. / Target frag~e~~io~

106c

The mechanism

for the production

energy region is discussed COMPARISON

2.4

Although

WITH PHENOMENOLOGICAL

there

have

a phenomenological reactions, target

there

our

models

is, at

present,

in this

experimental

lower

energy

incomplete

fusion

localized fusion cross the

in

of

near

are

Q

also

value

generalized are

higher

projectile

critical

governed

by

of

reactions model.15

energies

(>250

successful

pheno~nological

the

firestreak

model.17

of

nuclear

is localized

matter

completely

traditional

from

inelastic

function).

After

equilibrate

two

their

the

as

the

Siemens

particularly

nuclei

binary

space

et --

chosen or

have

to

greater

reactions for

target

MeY).

the

this

model,

transfer by

implementation

of

interactions

the

region,

projectile by

tubes

collided,

they

thermal

complete given

this

assumed

assumed

If the

One is

between

co-linear

empirical

are

energies.

well

picture

interaction

where

are

an

are

picture.

employing

(governed

have

are

is given in ref. 1.)

A

model

and

the

considerations (The

shown model,

that

The

~participant-spectators

In

been

In this

nucleus.

al.16

to the overlap

and

than

momentum

target

collisions

kinetic

ion for

phenomenological

less

fusion

phase

by

nuclear

have

heavy model

just what new physical

sum-rule

angular

with

represented

colliding

the

we

different

either

described

cluster

relation

developing

energy target fragmentation.

this model for treating target fragmentation At

two

energies

incomplete

the

transferred

sections

optimum

the

reactions

l-space

the

MeY)

by

Therefore,

with

to

energy

phenomenological

in hopes of discerning

inte~diate

to be well-described

region.

projectile

10 A

beginnings14

intermediate

no appropriate

measurements

for

(<

promising

of treating

energy

used in this work)

features characterize

in the intermediate

MODELS

several

capable

(appropriate

than those

been

model

fragmentation

compare

of these light fragments

further in Section 3.

to

tubes undergo

transparency to

fuse

resulting

and

kinetic

energy of a fused tube is less than its binding energy in the target remnant, then

it

~mentum.

is

captured

(Thus

both

and

contributes

the

sum-rule

region of impact parameters In figure models

5a, we

the

of 8.5 A MeV

compare

shape

of

(apart

due to a-transfer

the

the

reactions).

broadening

1.

of

energy the

remnant's

firestreak

predictions

of

As expected,

isobaric

from a small

but as the projectile observed

and

the

energy,

models

have

mass

a

and

localized

leading to a given mass transfer).

with the data of figure

describes

to

these

distribution

peak

two

phenomenological

the sum rule model generally at

a

projectile

at A = 155 which,

The data at 19 A MeV are qualitatively increases, isobaric

the model

yield

fails

distribution.

energy

in the model,

to account This

is

described for the

failure

of

W. Loveland et al. / Target fragmention

the sum-rule model section

at

the

overestimates fusion cross

and

is due to the overestimation

higher

the

extent

predicts

section

projectile of

complete

a narrower

decreases

energies.

faster

than

of the complete fusion cross As

fusion

isobaric the

107c

a

consequence

the

underestimates

distribution.

The complete

(A)-* dependence

model

incomplete

assumed

fusion

in the

sum rule model. The

firestreak

model

suffers

the

peak

in the

predicted

A %

114

(which

is

the

quasi-compound

of the yields

not

job of predicting

(In

similar (Fig.

is the

due same

difficulty.

5, 86 A MeV to

the

Mass Number

For

l*C +

example, 154Sm) at

de-excitation

distribution,

of high A shows that this model

small mass transfers)

Product

a

distribution observed)

nucleus.

of fragments

from

the

of

overestimate

also does a poor

.

A

Figure 5 (a) Comparison of the measured isobaric yield distributions (solid line) with those predicted by the sum rule model (long-line-dash curve) and the firestreak model (dashed curve) (b) comparison of the known systematics (ref. 7) of fractional linear momentum transfer in intermediate energy heavy ion reactions with the predictions of the firestreak model.

W. Loveland et al. / Target fragmention

108c

This

overestimate

of

where the measured reactions model. in

is

shown

Among

these

transparency

3.

to

is

in

can

show

that

products) must

intermediate

concerning

interferences

time

any

that

predicted

qualitative

not

or

about

is

of

them

the

valid,

will

target

(85 A

with

1%

figure

the

in

either

of

5b

energy

firestreak

the

nuclear

model

and/or

excited, emitted

that

i.e., In

cancel.

reaction

23%

and

78 different

that

to

and

large

this

a gross

distributions mechanisms.

number so that

fundamental measure

are also With

the

assumption

is populated

addition

(or

of motion

mean

statistically

these

of

useful ideas

the angular distributions

in the interaction

fragmentsI

we

statistical

phases

distribution

the angular

of the

to the direction

the

a

long-lived,

particles

of the intermediate

"long-lived",

distributed

angular

reaction,

features

normal

term

enough

randomly

fragment

certain

the

long

densities

the

a plane

By

lives

of 48 different MeV)

in

for the deficiencies

in

highly

distribution

in mind, we wish to examine the results of measuring

1%

by

changes

reflected

moderate

angular

system.

between

scale

in defining

clearly

in intermediate

that could account of

region

for

levels with

of

than

most

transfer

emission.

the

species

relationship the

phenomena

be symmetric

level

shown

in the frame moving with the velocity

species

of overlapping

less

existence

energy

species

intermediate

is

ANGULAR DISTRIBUTIONS

of

the

fusion

linear momentum

far

pre-equilibrium

intermediate break-up

the

this

TARGET FIGMENT One

be

the physical

models

substantial

complete

fractional

of intermediate

fragments19

from

the

energy

84 A MeV

+ Ig7Au reaction. For

these

reactions,

the

isobaric

yield

distributions

have

been

measured.4*11

h

IO2

z B

F lo’ 2 (;i .a -"40 s: 0

48

56 64

72

80

88

96 104

Product Mass Number A

Product Mass Number A

Figure 6 Target fragment isobaric yield distributions for A MeV 12C with lg7Au (ref. 3) and *3BU (ref. 10).

the

interaction

of 84-85

109c

W. Loveland et al. f Target fragmention

In both with

reaction

systems,

the fission

U system.

peak

For the reaction

not part of the fission Similarly

system.

there

For

the

than

the

the than of

light

fragment

238~

frame

reaction

lower

fragment

angular

distribution

with 40 ( A 5

50 are

so for the 238U reaction

system,

momentum

the

Au

are shown in Figure

light

fragment

(46Sc)

has

backward

( 83Rb) distribution is less forward-peaked but

( I5gYb)

is

the angular

transfer

in

distributions typical

with a lesser but discernible

distribution

lg7Au reaction

observed

the

system,

distribution

reaction

for the

peaks

fission for the

with A ?r 150 appear to be part of the

The typical fission fragment

heavy

they do appear

for

broader

for the U reaction system, while they are typical

laboratory

Ig7Au

a strongly forward-peaked peak.

probability

being much

products for the Ig7Au system.3

Representative 7.

significant

Ig7Au, the fragments

with

the fragments

a

distribution

peak while

fission product distribution "spallation"

is

in the yield

system,

similar

strongly

distributions

consistent

in U target target

has a roughly very

The For

are less forward-peaked

with previous The

fragmentation.

fragment

shape.

forward-peaked.

distributions

are

observationsl* small

backward

absent

in the

U target fragment distributions. These moving

laboratory

frame

encounter

frame

of the target

(Fig. 8).

The

angular

distributions

residue

following

parameter n11

were

transformed

the initial

into

the

projectile-nucleus

= (VI l/V) used to make the transforma-

tion 102

I

I

I

1.0GA' '"C

I

t

I

'97Au

Figure 7 Representative target fra ment laboratory frame angular distributions (Ref. 18, 19). the reaction of 85 A MeV I& with I97Au and 238~.

for

1lOc

W. Loveland et al. / Target fragmention

(where

VII

is

the

is the velocity ways

(which

gave

from

moving

source

integrating

longitudinal

velocity

of the fragment

the

the

same

fits

answer to

angular

of

the

in the moving within

measured

moving

frame

frame),was

experimental

velocity

to

F

give

V in two

uncertainties):

fragment

distributions

and

determined

and

spectra

6,

the

fragments recoiling forward and backward from the target (nil

(a)

(b)

from

fraction

of

= (F-B)/(F+B)).

Figure 8. Moving frame angular distributions

The

resulting

90" in the moving

frame,

equilibrium

has

asymmetric

in the

process

without

conclusion the

moving

the

system,

fragment range

The

light

indicative

of

and

by

rather

by

were

transformation. definitive

In

these

conclusion

could

the

the be

value

frame

results

but

values

for

the

about

it For

frame in which

90°,

transformations reached

nil, values

distribution).

about

included

of

of nil

In no case was

value.

are

in a "fast" (This latter

to find a moving

velocities cases,

distributions

distribution

symmetric

about

in which statistical

equilibrium.

moving

it was not possible frame

a

symmetric

production

varying

using

fragment

distributions moving

their

than a single

light

are

fragment

of statistical

a~lbiguous, double-valued

frame

no

cause

of

as and

to

the

distributions

of a "slow" process

further

parameter,

large made

frame

investigated

to symmetrize

heaviest

cases,

indicative

the establishment

was

the Au reaction

fragment

established.

the transformation

possible

the

been

transformation

to make

fission

"normal"

for the fragments shown in figure 7.

for many

that lab

could the

were

so

to moving not

synmetry

be

W. Loveland et al. / Target fragmention

However,

of the moving frame distributions.

because cf the lower initial transferred For

the

frame

U reaction

system

distributions

the

that were

for

the

momentum,

heaviest

U

reaction

system,

no such ambiguities

fragments

asymmetric,

lllc

(A = 139-169)

indicative

exist.

had moving

of a "fast" production

mechanism. In view of the observation fragments

was

a

binary

it would

be interesting

fragment

complements

of Lynen --. et al I2 that the production

process

at

these

because

processes

which

fragments/heavy With that

the

can

light

be restricted similarity fortuitous

large

"masked" the

target, clearly

for

the

low

in an

thought

the

I46Gd

is

fragments

from

events

different.

in

The

spallation-like

spallation

which

light

frame

distribution

fragment as

target

fragments

processes

appear

to

we show (in figure 9) the general

In fact,

moving such

heavy

probability

with

complementary

experiment

of

system,

of these complementary

were produced.

situation

identified

the

of light

we

For the Au reaction

fragments.

for the existence

yield

to A > 200 (fig. 6).

between

distribution

the

fragment complements

U

be

of

energies,

to see if we could find, amongst our data, the heavy

to these

we were unable to find any evidence fragments

projectile

we

to

have

and

the

calculated

It would

46Sc.

performed

to

be

find

indeed

among

the

half dozen or so heavy fragments

, whose angular distribution we have measured,

the exact

of light

a

kinematic

single-particle

to look for the

146Gd

is quite

ccmplement inclusive

kinematic distribution

and

the

in the fission-like

as

distribution. this

calculated

is

not

Furthermore the

the similarity

"46Sc-complementO'

best

way

between

distribution

scpl'ort for this idea is seen in Figure 6 where

we show the A = 46 and A = 146 fragments positions

fragment such

Nonetheless,

complementarity.

Further

striking.

experiment

to occupy approximately

cosFlementary

portion of the mass yield curve, and the nil

values of A ~50 and A ~150 fragments which are consistent with complementarity.

0

30

60

90

120

150

180

8 MF Figure 9 A comparison of the I46Gd fragment moving frame angular distribution and that calculated for the heavy fragment complement of 46~~. The reaction is 85 A MeV l*C + 238U.

112c

W. Loveland et al. / Target fragmention

Therefore

we

conclude

A *46 and A %I46

that

shapes of the l46Gd ~ving complement A ~150

due

to:

in the mass-yield

a)

the

apparent

comple~ntarity

curve, b) the similarity

frame distribution

of

between the

and that for the heavy fra~nt

of 46Sc and c) the fact that the nil values for A %50 and

are consistent

with

complementarity,

it is plausible

(but not proven)

that these fragments are kinematic complements. Thus new

it

becomes

interesting

inte~ediate

energy

non-equilibrium, well

very

be characterized

"hidden"

under

becomes Two

the

visible fission

ion

reaction

fission.

(The

by a very

broad

heavy

abundant

processes

number

observers

with

different

fission

that

asymmetric

more

in

the

speculate

have

mass

projectile-target

"fast"

energy

a

distribution

composite

fission

At

lower

the

exceeds

have

observed a

process

"fast",

could

distribution distribution

of this

interaction 9, (

"fast fission"

have

and

been

projectile

increase

acrft where

,Q.crttis the critical

the first condition study;

the

l

second

(Zl

l

is not6

only

noted

as

in the

a

width of

liquid model

of the

limit, Current

point to a system where the total

Z2 < 2500-3000) and %8f 5 angular

momentum

Z2 < 2500) is satisfied in that

is

energies,

momentum

angular

the rotating

process

potential has a pocket (Z1

very

that

i.e., when the fission barrier becomes zero due to angular momentum. theories*0

a

tails of the distribution).

significant

when

system

mass

characteristics

regimes.

reported

may

~chanism,

symmetric

"normal"

we

actual

to us in the low and high mass

occurring of

to

L-rft

While

for fusion.

in the reaction under

= 61h while

figf = 78h.

Thus

there are no "bound" partial waves that exceed Egf. If A 'L 46 and A 'L 146 fragments

are complementary

fragments,

then

a large amount of mass has been "lost" from the system in the fission process without

disturbing

processes have

than to

observed

the

those

have

fission expected

large

in

high

Thus nucleus

we

for

unlike

processes

process is apparently potential involves

that

process,

p-nucleus

kinetic But

fission.

transverse

conclude

reaction

energy

fragment

the beam axis, a condition energy

Similar

types of "fast" fission

with large mass loss prior to fission and broad mass distributions

been

events,

the two body kinematics.

momentum

and

energies these

collisions.21p were

22

observed

fragments

were

be preferentially

In

to also

emitted

these

be

larger

observed at 90"

to

not observed in our studies. our a

results fast,

are suggestive

non-equilibrium,

seen at lower or

accompanied

higher

of a new binary

projectile

by a large mass loss.

inte~ediate

division

of

energies.

the The

Since the interaction

has pockets for partial waves up to L = 61, and the "normal" fission lower

partial

waves,

(based

on

previous

estimates

of

the mean

J

113c

W. Loveland et al. / Target fragmention

of

the fissioning

system

to be 25-35ii) one is further

tempted

to associate

target

fragmentation

this process with the higher "bound" partial waves. 4.

SUMMARY What

in

have

we

learned

intermediate

energy

about

the

(lo-100

A

the answers to this question (1)

There

is

an

increasing

Fission

this decrease

important

increases.

complete

There the

a

momentum

"high due,

energy" in part,

the projectile (4)

The

production

~200)

is

a

Both

the

firestreak

while

"low

model

reaction

in

exit

channel

the

case

of

Au

as

the

system,

fissioning

excitation

model whose predictions energy"

fail

to an overestimate

with

replaced

appears to be due to a decreasing

is not a good phenomenological

observations.

being

In the case of a Ho fissioning

systems, the dominant effect is one of a decreasing (3)

mechanisms

fusion

spallation and fragmentation.

as

in fission probability angular

fragmentation

with

Among

collisions?

statements:

target

energy

less

energy

transferred

of

of

nucleus-nucleus

fusion and eventually

becomes

projectile

MeV)

are the following

evolution

projectile

by incomplete (2)

nature

general

to

sum

rule

explain

of the number

the

model

energy. explain and

observed

of events

the data

in which

is stopped in the target. of

light

(A

"fast"

process

A

I2C

< 60) without

fragments the

from

heavy

establishment

targets

of

(A

statistical

equilibrium. (5)

For

the

85

MeV

the possible occurrence

+

238~

reaction,

evidence

is presented

for

of a new "fast fission" mechanism.

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W.D. Myers, Nucl. Phys. A296 (1978) 177; see also ref. 11.

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19.

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D.H.E.

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