Complex fragment emission sources characterized by linear momentum transfer measurements

NucIearPhysics A471 (1987) Ulc-288~

271c

No~h-Ho~~d,~ter~

COMPLEX FRAGMENT EMISSION SOURCES CHARACTERIZED BY LINEAR MOMENTUM TRANSFER MEASURE41ENTS"

K. ~IATKOWSKI Department of Chemistry and Indiana University Cyclotron Facility, Bloomington, IN 47405

Triple coincidences between intermediate mass fragments (fms) and anglecorrelated fission fragments were measured in E/A = 35 MeV 14N + 232Th and E/A * 90 MeV 3He + 232Th reactions. These measurements indicate that fragments emitted at backward angles are associated with more central In the collisions than those detected in the forward hemisphere. 3He-induced reaction the longitudinal orientation of the missing momentum is indicative of non-equilibrium emission prior to IMF emission; the average number of IMFs per fragmentation event was deduced to be close to unity.

1.

INTRODUCTION The emission of intermediate mass fragments in nuclear reactions well above

the interaction barrier is believed to be a signature of highly excited nuclear matter.

Several recent experimentsI** have demonstrated that at

relatively low bombarding energies the emission of IMFs originates from completely equilibrated compound nuclei.

For nuclear reactions at

intermediate and high energies , particles are emitted prior to the attainment oE full statistical equilibrium, rendering the concept of compound nucleus formation and decay inadequateq3

At these higher energies IMF production has

been associated with more complex phenomena such as emission from nuclear

subsystems in local equillbrium,4 or from highly excited reaction residues, 596 or the coalescence of nucleon67 as well as the development of mechanical instabilities

in nuclei* and the liquid-gas phase transitions. 9*10

Many of

these models do not include the dynamics of the fast source-forming stage of the reaction.

Instead they rely on ad hoc assumptions concerning the amount

of excftation energy available to the source and the degree of equilibration of the composite system.

*This work was done in collaboration with M. Fatgga, V.E. Viola, R. Byrd, W. Skulski, W.G. Wilson, and L.W. Woo, of IUCF; 8. Karwowski, University of North Carolina; M.B. Tsang, W.G. Lynch, J. Pochodzalla, C.K. Gelbke, D.J. Fields and C.B. Chitwood of NSCL, Michigan State University. Research supported by the U.S. Department of Energy and the National Science Foundation.

0375~9474/87/$03.500 Elsevier SciencePublishersB.V. (North-Holland PhysicsPublishing Division)

K. Kwiatkowski/ Complex fragment emission sources

272c For

reactions

non-equilibrium case

of

emission

for

energy.

non-equilibrium energy with

silver

mechanisms

for

target

peaked

shown in

fig.

sources) of

forward

lo30I

1.

I

20 I

I,

where roughly in

Solid

the

,I1

60

both

projectile

reaction

apparent

I,

00

I

and

probability,ll

100 ’

of

represent

source

the

velocity

equilibrium

equal

to

The relative

fit

center-of-mass

exhibit

40

on the

the

analogues

l4

lines

moving

IMFs in

angles

and

co-exist,ll-l3 emission.

produced

velocity

distributions and at

equilibrium

must depend

with

fragments

to

nucleon

a system

contribute

carbon are

of

both

appear

two mechanisms

As an example

and intermediate

The angular forward

these

energies,

and compound

sources

spectra

(slow

intermediate

pre-equilibrium

probability and/or

at

to

the 200 MeV 3He

a two-component the

system

temperatures

data. are

strongly

exceeding

, 120

E LAB (MeV)

Energy spectra for carbon fragments produced in the 200 MeV FIGURE 1. Angles for each spectrum are indicated on figure. Data at 311e + Ag reaction. Solid lines represent a two-component moving source 10’ are multipled by 10. fit; at 10’ the dashed line shows the non-equilibrium component and the dotted line the equilibrium component.11

K. Kwiatkowski/ Complex fragment emission sources

that

expected

in

the

of

non-equilibrium

fragments

are

and the

isotropic

indicate with

that

angular

those

a velocity

In

order

source

2 the

of

two-component energy

velocity

of

the

nucleus,

@EQ

=

of

moving

equilibrium 0.75

source

cross

source

at

the

that

angles moving

compound nucleus.

statistical

both

analysis

backward

nature using

the

of

slow

As shown in

Z-dependence which

%Q,

the

the

by Gomez de1 Campo.15 well

these

rapidity

from a source

sections

were

and the extracted

fits.

jHe beam is becomes

indicating

@cn,

of

cross-sections,

incident

hand,

IMFs detected

on the

as implemented very

suggests

other

isotropically

that

check

equilibrium

the

to

elemental

reproduce

the

of

emitted

close

the

calculations

from

slow

very

formalism

value

the

is

are

This

On the

distributions

calculated

absolute

formation.

origin.

to make a consistency

we have

As the

nucleus

fragments

which

Hauser-Feshbach fig.

compound

213~

increased

lower

an incomplete

to E/A = 90 MeV, the

than

that

of

the

compound

momentum transfer

to

the

for

the

source.

The apparent

temperatures,

non-equilibrium values observed

for

source

the

200-

are

and 270-MeV

weak dependence

projectile-target

TRE, extracted nearly

of

in

2 independent ‘He data.

TRR on the

combinationl2,16,

This

projectile

possibly

the

same reactions

and yield

identical

confirms

the

energy

a non-thermal

200 MOV‘He + “a’~~ -c

-

.

lo- E

generally

and on

reflecting

EQUlLlBRlUM

average

nature

IMF + X

COMPONENT CALC.

\

2

-1 ;3

e

b IO2

IO’

2

I

I

I

I

I

I

I

I

3

4

5

6

7

8

9

IO

I II

2 IMF FIGURE 2. The equilibrium cross-sections for the ‘He + Ag reaction. solid line shows the calculated values for the statistical emisson fragments in the Hauser-Feshbach formalism.15

of

The complex

274~

K. Kwiatkowski/ Complex fragment emission sources

of the source. In contrast to IMFs, the apparent temperatures obtained for proton ejectiles in the intermediate energy region show a strong increase with the incident energy.17 We have investigated the emission of intermediatemass fragments in the 3He- and 14N-induced reactions on 232Th in order to characterize the sources from which these fragments are emitted and to investigate whether equilibrium and non-equilibrium components could be associated with different classes of collisions. In an effort to study the evolution of the reaction mechanism with projectile velocity, the measurements were performed with E/A = 35 MeV 14N ions, for which the inclusive cross section for complete fusion is still sizable ("4OOmb or 0.12 oR),18 and with 3He projectiles at E/A = 90 MeV where the probability for complete momentum transfer is zero.lg In the experiments reported here, triple coincidences between intermediate mass fragments and angle-correlated binary fission events were measured, in order to determine the linear momentum transfer to the heavy reaction residues.

2.

14N EXPERIMENT AND RESULTS The experiment was performed at the National SuperconductingCyclotron

Laboratory of Michigan State University.2o Angle-correlated fission fragments were detected with two x-y position sensitive parallel-plate avalanche counters. Intermediate-massfragment telescopes consisted of standard AE-BE-E silicon detectors and were placed at two angle settings: one at back angle, OIMF = +126J0,which should emphasize ejectiles emitted from an equilibrated target-like source, and one at forward angle setting, 01~~ = -51", which would favor non-equilibrium processes but avoid complications due to projectile fragmentation. The folding angle distribution between coincident fission fragments can be related to the longitudinal momentum component, pi, of the heavy reaction residue through a simple kinematic transformation.21 We can, therefore, determine the missing momentum, pm = Po - PR - PIMF'CO~(CIMF) where p. and ~IMF denote the beam momentum and the momentum of the detected fragment. Missing momenta different from zero result either from preequilibrium particle emission or from the sequential decay of highly excited primary fragments. This experiment cannot distinguish these two mechanisms.

K. Kwiatkowski / Complex fragment emission sources

21%

Figure 3 shows the measured dependence of the average folding angle, , on the atomic number of the coincident complex fragment (solid points). For the back angle data the values of were derived from Gaussian fits to the OAF distributions. Typical errors given in the figure include both statistical and systematic uncertainties. Due to counting statistics, the errors become larger for heavier fragments. The open points in the figure depict most probable folding angles calculated for various values of the missing momentum, pm.

For these calculations the direction of the missing

momentum was assumed to be parallel to the beam axis;

the fragment mass was

taken as A = 22; fission mass and energy distributions were determined according to ref. 22. At OIMF = Sl", the missing momentum is of the order of 25-30X. Apart from the trivial effects due to momentum conservation, no systematic trend with fragment charge can be established. The average missing momentum corresponds to 28 f 3% of the projectile momentum, compared to 42% for the total fission cross section.'* For incomplete fusion reactions, the linear momentum transfer to the heavy residue, PII,is closely related to the deposition of where p. is the beam momentum and Exmax excitation energy Ex = Exmax*pll/po,23 the maximum possible excitation energy. Intranuclear cascade calculations for

e*IMF=-510

e IMF = 126O

170

140 t

/

Pm q 0.3

PO---L_

o.~_p,=o.l -i

“0.. 9 *.l O‘.B 0

-+a

o..O--o’-

o__o_'O--O'~-Pm=0.2 Po 150. -

po

2 $

'Q,*__

“0. Pm=O----‘~o.~

130;;:

0

: z

0 ..i 0 I

- 120

FIGURE 3. The average value of the fission fragment folding angle, , as a function of coincident ejectile atomic number for two measured IMF angles. Open points are calculated based on various values of missing momentum, pm; solid points are experimental values.

276~

K. Kwiatkowski / Complex fragment emission sources

light projectiles (e.g. alpha particles) yield the same relationship for the average values of the excitation energy and linear momentum transfer to within f5%.24 We estimate the average excitation energies to be about 320 MeV for the sources emitting intermediate mass fragments at 01~~ = 51". The observation of non-zero missing momentum is consistent with recent measurements13 of coincidences between target-like residues and intermediate mass fragments for the 32S+Ag reaction at E/A = 22.5 MeV. The missing momenta are considerably smaller when intermediate mass fragments are emitted at backward angles, 01~~ - 126". Again, the extracted values are fairly independent of fragment mass. The average value of pm * (0.05 + O.OS)*pO indicates that these fragments are emitted in highly inelastic collisions in which nearly the entire momentum of the projectile is transferred to the heavy reaction residue. Within the experimental uncertainties, the emission of intermediate mass fragments at backward angles is consistent with the occurrence of complete fusion of target and projectile, corresponding to an excitation energy deposition of about 420 MeV. Figure 4 shows the energy spectra of beryllium and carbon fragments detected at l3Tt.g~ = 51" and 126". (Since these spectra were measured in coincidence with two fission fragments, ContrLbutions from light target contaminants are nonexistent.) At 01~

= 126', the energy spectra can be

parametrized in terms of the statistical model of ref. 25, assuming emission from an equilibrated compound nucleus formed by the complete fusion of projectile and target. The calculation shown by the dashed line was performed for a temperature of T = 4 MeV, corresponding to a level density parameter of a = A/8 MeV-l. At 01~~ = Sl", on the other hand, emission from a fully equilibrated compound nucleus (dashed line) is of minor importance. The solid curve shovs a two-source fit with contributions from the fully equilibrated compound nucleus and an additional thermal source of

velocity

v

z

4vCN,

apparent temperature T s 13 MeV, and Coulomb energy of EC = 0.8VC, where VC denotes the Coulomb barrier for two spherical nuclet.l,ll The shapes of the energy spectra measured at 01~~ - 51" are consistent with previous measurements4,13*14which established the emission of intermediate mass fragments prior to the attainment of full statistical equilibrium of the composite system. Several systematic uncertainties exist for the quantitative determination of the excitation energy deposition and the degree of equilibrium in reactions leading to the emission of intermediate mass fragments. The largest quantitative uncertainty has to be attributed to the unknown effects of sequential decay of highly excited primary reaction products. Since the

K. Kwiatkowski / Complex fragment emission sources

I

20

I

! 60

I

I 100

I

I 140

I

217c

I _ 180

EIMF IMeW FIGURE 4. Energy spectra of beryllium and carbon fragments in coincidence with angle-correlated fission fragments for OIMF = -51" and OIMF = +126'. Dashed lines give the results of parametrizationassuming emission from a fully equilibrated compound nucleus. Solid line is based on a two-source fit with parameters described in text.

momenta of undetected decay products are included in our definition of the missing momentum, the occurence of sequential decay will lead to larger missing momenta when the primary reactton products are emitted at forward angles and to smaller missing momenta when they are emitted at backward angles. As a consequence, we may have underestimated the excitation energy deposition for our measurements at 81~

= 51' and overestimated it at

OTMF = 126'. If, for example, all 12C fragments resulted from the decay of 160 primary fragments, the missing momenta carried away by the undetected alpha particle would be of the order oE pm(a)=+(O.OB-0.1)~~at 01~~ = 51' and pm(a)=-(0.05-0.06)poat 01~

= 126".

Our results demonstrate that the emission of intermediate mass fragments at backward angles must be attributed to near-equilibriumemission in fusion-like processes in which the missing momentum is surprisingly small when compared to inclusive fusion-like reactions or reactions in which the fragments are emitted at forward angles. These findings suggest that complex fragment emission at backward angles could serve as a reaction filter for the selection of highly excited nuclear systems close to thermal equilibrium.

278~

K. Kwiatkowski/ Complex fragment emission sources

3. THE 3 He EXPERIMENT 3.1. Experimental Setup The measurements with 270 MeV 3He ions were carried out at the Indiana University Cyclotron Facility.26 Relative to heavy ions 3He projectiles have the advantage of unambiguous identificationof IMFs at very forward angles, while at the same time bringing in substantial excitation energy at a moderate angular momentum. The cross-sections for IMF production are, however, an order of magnitude smaller than in 12C-induced reactions at the same total incident energy, reflecting the smaller projectile size and lower angular momentum of the emitting system. This limited the coincidence counting statistics to oxygen and lighter fragments. As in the 14N experiments triple coincidences were detected to evaluate the momentum balance in the reaction. In an attempt to determine more precisely the origin of the missing momentum the fission fragment angular-correlation technique was modified to allow identification of not only the magnitude of the recoil momentum but also of its direction.26 In order to estimate the excitation energy available to the system for IMF emission one has to know, in addition to the magnitude of the missing momentum, the time sequence of light particle emission relative to that of IMFS.

Measurements of the direction of the missing momentum should help in

establishing the time sequence, at least when the emission events are well separated in time. For instance, the direction of missing momentum created by non-equilibrium light particles emitted prior to or simultaneouslywith IMF emission should be strongly correlated with the beam direction. If, on the other hand, light particles are evaporated from the residue equilibrated after IMF emission, a correlation between the missing momentum and recoil direction should be expected. Similarly, sequential break-up of the intermediate mass fragment and/or simultaneous emission of other light particles in the average direction of the IMF will lead to correlations between the direction of missing momentum and that of the detected IMF. The experimental arrangement for these studies was modified to allow for identification of the average direction and velocity of the recoil residues. The fission fragments were detected in two 14cm x 14cm x-y position-sensitive wire chambers covering angular intervals (+25" to +65') and (-105O to -145') in the laboratory system. Six triple-elementAE - AE - E silicon telescopes for IMF identificationwere positioned symmetrically at three laboratory angles on the opposite sides of the beam axis: +15O , +75O, +160'. For each of these three angles, we derived the average velocity vector of the fissioning residue by comparing angular correlations of the fission fragments coincident with IMFs detected in either telescope of the pair.

K. Kwiatkowski / Complex fragment emission sources

279~

The fission fragment folding angle, GAB, is a known function of the fissioning residue recoil velocity, vk, and the angle, CL,between the recoil direction and the axis of the defining fission detector OAB a f(vk, a) In fig. 5 the folding angle CAB is plotted as a function of the relative recoil angle a in 'He-induced reaction, at three different projectile energies, in which complete momentum transfer to the target was assumed (i.e. OR = 0 and a = CA). The angles CR and CA are defined in fig. 6, which explains the principle of the modified fission fragment correlation technique used in this experiment. When fragments of the same species and momenta are detected in either of the two IMF telescopes, positioned on opposite sides of the beam axis, the resulting fissioning residues will on average have equal recoil velocities vk, but usually different relative recoil angles a.

This in

3He+*'*lh---cf,+fe

FIGURE 5. Calculated dependence of the folding angle between two binar fission fragments following complete fusion of the 3He projectile with T32Th target, for three values of the incident beam energy. In this reaction the fissioning residue recoil angle, CR, is zero and a = CA. Note that at OA40" the folding angle is primarily dependent on the total momentum transfer, whereas at the extreme values GAB is mostly sensitive to the recoil direction.

28Oc

k Kwiatkow~kt/ Complex fr~~e~t erni~~t~nsources

turn of

results

the

a set

of

and from 3.2. Figure of

the

the to

values

of

the

coincidence

them,

SR and vR can

Results

and Discussion

7

shows

the

folding

with

angles

either

of

@AB*

the

Thus measurements

IMF telescopes

yield

8 shows

be determined.

measured

plots

as a function

beam direction

of

of 2 of

(pmf)

is

(pmi).

folding

angles of

fissioning

beam direction

average

emission

predictions

equilibrated

the

in

two equations.

kinematic

Figure data

angles

IMF at different

indicate fully

in different

folding

(solid

OAB, assuming

@AB as a function

angles symbols).

Of Z

Open symbols

a 2-body

final

state

(IMF plus

residue). missing the

momentum components

ejectile.

consistently In addition,

The missing much larger pm8 is

determined

from

the

momentum component than

found

to

that

along

perpendicular

decrease

with

IMF

\

IMF TELESCOPE

FiSSlON DETECTOR A

Schematic outline of the experimental arrangement for the triple FIGURE 6. Intermediate mass fragments are detected in one of the coincidence systems. A fragment silicon telescopes placed on either side of the beam axis. will produce fissioning residue pointing towards the detected at OIm = -15’ fragment registered at OIL = +15” fission fragment detector A, an identical Both folding angles results in a residue moving at an angle a close to 90”. and fission-fragment velocities in laboratory system are affected by the relative recoil angle a.

281c

K. Kwiatkowski/ Complex fragment emission sources

eIYF

BJYF= 160°

= 75”

la

;880

VV.. VV

*a

V VVV

UO. VV”8 VV v:

MOL 3

5

I

7

3

I

I

5

I

I

I

7

3

I

I

5

1

I

7

of IMF atomic FIGURE 7. Fission-fragment folding angle, SAg, as a function number. Circles indicate coincidence with IMFs detected at negative angles and triangles show folding angles in coincidence with IMFs detected at positive angles. Solid symbols correspond to measured values. Open symbols show results of calculations assuming zero missing momentum. For each ZIm, it was assumed that only the most abundant stable isotope was detected. Only for 2=4 was mass separation accomplished and results for the A=9 isotope are plotted. up to 2-6, error bars are comparable to the size of the symbols.

increasing IMF charge at 15' and 160°, but appears to be constant at 75' (where our experimental setup is most sensitive). Two energy gates are shown for the IMF at 15", one for Coulomb-like energies (55-75 MeV) and one for the high energy tails of the spectra (75-200 MeV).

Both yield similar results.

Figure 9 shows a comparison of the recoil angle and the missing momentum direction for IMF emission angles -15", -75" and -160". For SIltF= 75', the missing momentum direction appears to be primarily along the beam axis, although a bias is noted for the heavy recoil side of the beam. For OIMF = 160", the recoil direction is very close to the beam axis, which limits the correlation test. Nonetheless, for all three 01~

angles, the missing

momentum points in the beam direction. No strong evidence has been found for the sequential IMF decay as a dominant process in creating missing momentum. For the same reason it is difficult to determine if sequential IMF decay contributes to missing momentum detected in coincidence with forward-emitted fragments. The lack of apparent correlation between missing momentum

282c

-0.2

f 34

-0 I

I

I

I

5

6

7

3

45

6

7

Z

FIGURE 8. Two components of the missing momentum (longitudinal,pml[,and transverse, pml) plotted as a function of the IMF atomic number. The same assumptions about the mass of the IMF as in the calculation shown in Fig. 7 were made. Low E refers to a gate on IMF energy, EIMF = 55 - 75 MeV, while high E to EIm = 75 - 200 MeV.

direction and recoil or IMF directions indicates that, on the average, up to 25% of beam momentum is lost prior to or simultaneouslywith the IMF emission. This result implies that the average excitation energy of the IMF source is E* s 150-200 MeV, rather than the 275 MeV expected for the fully equilibrated composite system. The probability of fission following IMF emission (defined here as the IMF-fission probability) has also been measured in the present experiment. The IMF-fission probability is defined as the ratio of the yield of fission-fission-IMFcoincidences (normalized with the efficiency for fission fragment detection) to the inclusive yield of intermediatemass fragments. Figure 10 shows that, for small angle IMF emission, the IMF-fission

K. Kwiatkowski / Complex fragment emission sources

16012C-

8

MeV rmr'-l5* EIMF'c55-751 - 160

iii

-120,

BO-" 40- 1 3 3 1 6z-! C_____-_------______160cn

283~

-80 z I_-_;i

8IMF=-15*Er~~=[75-200]MeV- 160% f w T T T

3456

3456 ZIMF

FIGURE 9. Recoil direction (left column) and missing momentum angle (right column) plotted as a function of IMF atomic number. The same assumptions about IMF mass as in the calculation shown in Fig. 7 were made.

probability is close to unity; whereas at larger angles, it decreases as a function of the Z of the IMF. In order to evaluate the extent to which multifragmentation of a sequential nature may be present in this system, statistical calculations of the fission probability following IMF emission have been carried out with the statistical code MREGATez7 The calculations have spanned a range of Z-values for the fissioning nuclei (Z 3 80-89), excitation energies (E* = 100-200 MeV), and angular momenta (J - 0 and 20 h). Results of the calculation indicate that the IMF-fission probability falls significantly below unity if one assumes that more than one heavy fragment is emitted prior to fission in a given event. For example, emission of 2-3 charge units in addition to a "C

or 14N ejectile is sufficient to decrease

the IMF-fission probability by 25-50% (for excitation energy E* * 150-190 MeV).

Taken together with the large experimental values of the

probability at 15" (fig. lo), the results of these calculations suggest that

284~

K. Kwiatkowski/ Complex fragment emission sources

Y 1.0 ti \

y:0.6

t 2

0.6

8EJECTILE

1 = 750

t

!

5 0.6-

b” = 0.4t

.w

b’ 0.2-

8 EXCTILE

O

I

s 160” I

3

4 2

i

91

I 5

I 6

I 7

EJECTILE

FIGURE 10. IMF-fission probability number. The IMF-fission probability coincidences to inclusive rates of

plotted as a function of is defined as the ratio IMF production.

the

per

average

i.e.

number of

no evidence As the OIm

especially

for

backward emitted the

at

of

light-charged Since angle,

increases,

the this

preferentially

are

forward is the

IMFs emitted

lower

the

angles; for

fissioning

This

requirement transverse

with

i.e. these

the

nucleus

to prior

can only to

charge

the

and/or

be met if beam axis,

these

light

but prior

angular

at

those momentum of

for

the

average

multiplicity is

independent

particles to

decrease,

than

and fission

nearly

to unity;

IMFs emitted

In order

an increased

pm11is

to

that source

IMF emission of

close system.

begins

events.

be lower,

is

this

implies

fissionable

average

component

for

probability behavior

a less

to

event

observed

backward-angle

emitted

longitudinal

is

IMF-fission

ejectiles.

associated

particles average

fragmentation

multifragmentation

heavier

angles

source

charge

for

the IMF atomic of IMF-fission

are

IMF emission

for required. of emitted (thereby

K. Kwiatkowski1 Complex fragment emission sources

28%

insuring that = 0, as shown in fig. 8). Alternatively, the observed decrease in the IMF-fission probability can be accounted for if the average angular momentum of the backward-angle IMF source is lower than that for forward angle-emission. Both interpretationsare consistent with a picture in which IMFs detected at backward angles come from more central collisions, with relatively isotropic angular distributions of non-equilibriumlight particles, while

IMFs

detected at forward angles originate in collisions with larger

impact parameters, with a lower multiplicity of accompanying non-equilibrium light particles.

4.

CONCLUSIONS In 3He-induced reactions at E/A = 90 MeV, large missing momenta are

observed, amounting to 20-25% of the beam momentum, irrespective of the angle of the coincident IMF. The direction of the missing momentum is indicative of the non-equilibrium light particle emission which precedes or accompanies IMF emission. Similarly, large missing momentum was measured in the 14N + 232Th reaction for fragments detected at forward angles. As a consequence, the average excitation energy of the source of the IMFs

is

substantially lower

than that calculated under the assumption of complete energy deposition in the composite system. Nevertheless, the average excitation energies for IMF events are well in excess of the average values deduced from the inclusive measurements of momentum transfer, which (at these projectile velocities) begin to be dominated by peripheral collisions. In deriving the excitation energies from the missing momentum data one is hindered by the lack of a reliable prescription for performing these calculations, especially in the domain of central collisions.23*28 Complex fragments detected at backward angles in the 35 MeV/u 14N-induced reactions are associated with full momentum transfer events. Furthermore, their energy spectra are consistent with the emission from a fully equilibrated source possessing (very nearly) the full beam momentum. The comparison of the results of the two experiments suggests that the probability of energy deposition and equilibration in central collisions depends primarily on the incident energy per nucleon (velocity) and on the projectile mass, and less on the total center-of-mass energy. The relative velocity between the two colliding nuclei

is

also a decisive factor for the systematics of the

linear momentum transfer in fusion-like collisions, as established by inclusive measurements.18 Hence, as long as the incident energy per nucleon remains below E/A a 50 MeV for heavy ion beams (and below E/A = 70 MeV for

K. Kwiarkowski / Complex fragment emission sources

286~

complex light ion projectiles), complete fusion in central collisions could be a viable concept, even at very high excitation energies. By using heavier projectiles, one could possibly produce long-lived composite nuclei with excitation energies close to the total binding energy of the system.

I must mention that this talk is a presentation of the hard work of all the collaborators. In particular, M. Fatyga had been primarily responsible for the 3?ieexperiment and its interpretation. We would like to express our gratitude to the staffs of the NSCL K500 Cyclotron and IU Cyclotron Facility for providing excellent beams. The assistance of K. Solberg, J. Yurkon, J. Dorsett and J. Regal in the development and construction of the fission detectors, and of B. Lozowski in the preparation of the targets are highly appreciated. It is a pleasure to thank Laurie Hicks for her excellent job of typing this manuscript.

REFERENCES 1)

L.G. Sobotka et al., Phys. Rev. Lett. 51 (1983) 2187.

2)

L.G. Sobotka et al., Phys. Rev. Lett 53 (1984) 2004, and this volume.

3)

C.K. Gelbke and D.H. Boal, Prog. Part. Nucl. Phys. (to be published).

4)

D.J. Fields et al., Phys. Rev. C30 (1984) 1912.

5)

W.A. Friedman and W.G. Lynch, Phys. Rev. C28 (1983) 16; ibid. 950.

6)

G. Fai et al., ?hys. Lett. 8164 (85) 265; Nucl. Phys. A404 (1983) 551. D.H.E. Gross et al., Phys. Rev. Lett. 56 (1986) 1544; Nucl. Phys. A461 (1987) 641.

7)

B.V. Jacak et al., Phys. Rev. C31 (1985) 704.

8)

G. Bertsch and P.J. Siemens, Phys. Lett. 8126 (1983) 9; J. Aichelin et al., Phys. Rev. C30 (1984) 107.

9)

A.S. Hirsh et al., Phys. Rev. C29 (1984) 508.

10) J-P. Siemens, Nature 305 (1983) 410; Nucl. Phys. A428 (1984) 189~. 11) K. Kwiatkowski et al., Phys. Lett. B171 (1986) 41. 12) R. Trockel et al., GSI preprint 85-45 (1985). 13) D,J. Fields et al., Phys. Rev. C34 (1986) 536. 14) M. Blann, Phys. Rev. C31 (1985) 1245.

28%

K. Kwiatkowski 1 Complex fragment emission sources

15)

J. Gomez de1 Campo, A326 (1987) 335.

Bull. Am. Phys.

de Physique,

Sot. 31 (1986) 770; Zeit. fur Phys.

16) B. Borderie,

Journal

Colloque,

17) C.K. Gelbke,

Nucl. Phys. A400 (1983) 473~.

47 (1986) C&-251.

18) M.B. Tsang et al., Phys. Lett 8134 (1984) 169; M. Fatyga Rev. Lett. 55 (1985) 1376. 19) Y. Cassagnou 20) M. Fatyga

et al., Phys.

et al., Jour. Phys. Sot. Jap. 54 (1985) 422.

et al., to be published.

21) B.B. Back et al., Phys. Rev. C22 (1980) 1927. 22) M. Fatyga

et al., Phys. Rev. C32 (1985) 1496.

23) D. Jacquet

et al., Nucl Phys. A445

(1985) 140.

24) L.W. Woo, private communications. Intranuclear cascade calculations carried out with the CLIJST code (G.J. Mathews et al., Phys. Rev. C25 (1982) 2181), see also ref. 22. 25) L.G. Moretto,

Nucl.

Phys. A247

(1975) 211.

26) M. Fatyga et al., Phys. Lett. 8185 (1987) 321; M. Fatyga, Indiana University, 1986. 27) S.E. Vigdor

and H.J. Karwowski,

28) D. Hilscher, Production emission, this volume.

Ph.D. thesis,

Phys. Rev. C26 (1982) 1068.

and decay of hot nuclei

studied

by neutron