Studies in superdeformation at Chalk River

Nuclear Physics A520 (1990) 139c- 152c North-Holland

139c

STUDIES IN SUPERDEFORMATION AT CHALK RIVER

David WARD

Atomic Energy o f Canada L i m i t e d , R i v e r , O n t a r i o , Canada, KOJ 1JO

Chalk R i v e r N u c l e a r

Laboratories,

Chalk

Recent work w i t h t h e 8n s p e c t r o m e t e r l o c a t e d a t t h e TASCC f a c i l i t y i n Chalk R i v e r has c o n c e n t r a t e d on two a s p e c t s o f s u p e r d e f o r m a t i o n . Firstly, the s t u d y o f n u c l e i n e a r X52Dy t o s e e k and to s t u d y new c a s e s . T h i s program has l e d to t h e d i s c o v e r y o f m u l t i p l e s u p e r d e f o r m e d bands i n XS3Dy, and more r e c e n t l y t o t h e d i s c o v e r y o f two e x c i t e d s u p e r d e f o r m e d bands i n 149Gd. The s e c o n d program u t i l i z e s an a r r a y o f c h a r g e d p a r t i c l e d e t e c t o r s i n s i d e the spectrometer, and e x p l o r e s t h e p o s s i b i l i t y that following a HI-fuslon r e a c t i o n , c h a r g e d p a r t i c l e e v a p o r a t i o n c o u l d depend on d e f o r m a t i o n i n the final nucleus.

The discovery and present exploitation of superdeformed rotational bands is very much a child of the technology that made large-scale v-ray instrumentation possible.

At the TASCC facility in Chalk River our program is built around the

8~ spectrometer, a view of which is shown in Figure I.

The instrument was built

and is Jointly operated by Canadian universities and CRNL.

It is a typical

second generation instrument comprising an inner ball of 72 Bismuth germanate (BGO)

detectors

shields.

and

an outer

array

Generally we use an event

of

detectors

with

BGO

trigger requiring a suppressed

coincidence in the BPGe array together ball with K=10.

20 HPGe

Compton two-fold

with a K-fold coincidence in the BGO

The data rate to tape is typically 2500 events per second.

The nucleus 149Gd provides a good example of a superdeformed band.

Although

I might hesitate to call this a strong band (it is approximately 1.5% of the 5n channel in this reaction),

there are a number of clean gates such as the one

shown

can be identified as separate peaks

in Figure

projection

2, which

(ef inset).

in

the

total

In this data there is no difficulty in showing

every member of the band is coincident with every other member.

that

The problems

begin when we try to push to the limiting sensitivity of the instrument, example

in searching

for other

cases of superdeformation near

searching for excited superdeformed bands. much more on systematics

152Dy,

or

for in

In these situations we have to rely

and be content with summing spectra

gates.

0375-9474/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)

for individual

140c

.

D. Ward / Superdeformation at Chalk River

Figure I. A view of one-half of the 8~ spectrometer. The inner ball is made up of 72 BGO scintillation detectors, (one removed to allow the beam to enter). The hexagonal shaped holes in the inner ball, visible in this picture, allow the 20 HPGe detectors an unobstructed view of the target located at the centre. Each HPGe detector has BGO Compton shields.

D. Ward

/ Superdeformation

at Chalk River

14lc

TOTAL PROJECTION

~gGd

SUPEROEFORHEO

i

BANO

~.oooo

z zoooo S

100C

u~

>

,,-," ,=

"/SO

800

Ey (keY)

r---

D 0

N

50q t

I'I" I"1

r''

'1" '

700

500

I'

'q

1100

900

1500

1300

EV (keV) Figure 2. Example of a V-spectrum characteristic of a superdeformed band. The gating transition is visible in the total projection spectrum, and is chosen for this illustration because it is a "clean" gate bringing back in coincidence only the superdeformed band and Its decay through "normal" states in 149Gd. From reference I.

A major component give

a vlew

momentum

of

not

of the fascination

the nucleus

previously

of superdeformed

in a region of extreme

accessible

to detailed

bands

is that

deformation

study.

But

and

they

angular

to date,

no

superdeformed band near ZS2Dy has been attached to the rest of the level scheme, and none of them have firm spin assignments.

In these circumstances one might

well ask is it possible to do nuclear structure physics? one

thing we do

inertia,

j(2),

know

about

are sensitive

these

bands,

indicators

namely

It turns out that the

their dynamical

of nuclear structure.

the rate at which j(2) changes with rotational frequency ( ~ )

moments

of

In particular,

is an indicator of

the occupancy of the intruder orbitals. A summary of where we stand in the Z52Dy region is shown in Figure 3.

The

first case of multiple superdeformed bands was discovered in IS3Dy by Johansson et al.2). only

one

comparing

These three hands were populated weakly, fifth

of

the

the observed

intensity j(2)

typically

values with

found

the strongest of them being in

the ZS2Dy

those calculated

region.

for various

By

assumed

142c

D. Ward / Superdeformation at Chalk River

•

67 152Dy ~s3D'~y

151 Dy

66

1 1/,9 Tb

Z 65

•

0 •

I

144Gd I 14sGd l~6Gd %7Gd

6~

•

ol

,

l~SGd

tsl Tb 1

2

%gGd

1so Gd

3

2

1 •

2

1 L___LI

150 Tb

63

80

81

82

83

86

85

86

N

Figure 3. A summary of the present status in the Z52Dy region. The bottom right of each panel refers to the number of superdeformed bands known in that nucleus, and a zero denotes an unsuccessful search. The filling at the bottom left means that an 8~ spectrometer group have studied or searched for superdeformation in that nucleus.

structures

(cf

Figure

Nilsson-Strutinski reference

2,

some s i x

t h r e e have p o s i t i v e

rotational This

is

involving

to l i e

then j(2)

such p a r t i c l e s . remaining

two a r e

by Aberg

Figure

4).

are

j(2)

the

al.

configuration The t h r e e

cranked

given In

in

Z53Dy;

~63 and t h r e e positive

are not

parity the ones

v a l u e s drop s h a r p l y w i t h i n c r e a s i n g

observed first

In et

predicted

l o w e s t f o r s p i n s above 50~,

an example o f a g u i d e l i n e two o r

assignments.

the i n t r u d e r

(cf

experiment because t h e i r the

cut

pairing

s u p e r d e f o r m e d bands

i n v o l v i n g ~64

f r e q u e n c y whereas

when t h e r e a r e e i t h e r orbital

without

low-lying

although predicted

seen in t h i s

we can make c l e a r

parity

have n e g a t i v e p a r i t y bands,

4)

calculations

bands have

a very

flat

n o t i c e d by B e n g t s s o n e t

three particles

in a particular

behaviour. al.3),

that

v or ~ intruder

t e n d s to d e c r e a s e more s h a r p l y than when t h e r e a r e one o r f o u r Of t h e

three

signature

predicted

partners

bands w i t h ~64,

h a v i n g v72 .

and

the

Band 2 and band 3 i n

one has v73

the

e x p e r i m e n t a r e s i g n a t u r e p a r t n e r s and can t h e r e f o r e be a s s i g n e d ~64v72,

leaving

D. Ward

A

/

Superdeformation at

100

9.0

95

8.6

=, ~e

143c

'

90

>

River

Chalk

(b)..~:_

8.2

85

?,

80

I

7.8

75 0.3

0.4

0.5

0,6

0,7

7.4 30

4O

50

60

70

SPIN, I (~)

ROTATIONALFREQUENCY(MeV/~)

Figure 4. Calculated dynamical moments of inertia and excitation energies of the six lowest superdeformed bands in *S3Dy. The intruder configurations, parity and signature (m, ~) are: curve (a) = ~64v73 (-, -I/2), curves (b) = ~64v72 (-, ±I/2), curve (c) = ~63v73 (+, -I/2) and curves (d) = ~63v72 (+, ±1/2). From reference 2.

band

1

for

the

configurations, than

the ~63

calculated Woods-Saxon

~64v73

assignment.

In

the

experiment

it

is

the

~64

which are observed, and hence they must lie at lower excitation configurations.

It

seems

clear

therefore

with standard Nilsson model parameters calculations

by Nazarewicz

et

al. 4)

that

the

is too small. do

Z=66

gap

More recent

reproduce

the correct

ordering of these levels in ZS3Dy. This is not the end of the Z53Dy story.

What the original authors failed to

notice was that the average 7-ray transition energies of the signature partners (band 2 and band 3) lie within energies

(cf Figure 5).

the data and interpret with v[51419/2.

I-2 keV of the corresponding

Z52Dy

transition

Nazarewicz et al. 5) have picked up on this aspect of the signature partner bands as the ZS2Dy core coupled

They point out that I) the decoupling parameter a-O and 2) the

addition of a neutron causes no change in the kinematic moment of inertia of the core AJ(Z)/J (I) at the level of I part per I000. The discovery of so called "twinned" bands has stimulated a great deal of experimental and theoretical excitement. technically the first example.

I suppose the Z53Dy case mentioned is

However, what we generally mean by twinned bands

are cases where the same sequence of 7-ray energies (to within I-2 keV) is found over a span of ten or more transitions in two different nuclei.

The first such

examples being the pairs (ZSZTb, *S°Gd*) and (IS2Dy, ZSZTb*), where one asterisk denotes the first excited superdeformed bandS). Recently we have performed

two new experiments

vlew to look for excited superdeformed handsT).

on Z49Gd and ZS°Tb with a

In the ZS°Th case we have not

found anything new beyond the original Berkeley studyS). found two excited bands.

However,

in ZagGd we

The reactions employed were 124Sn(3°Si,

5n)149Gd at

144c

D. Ward / Superdeformation at Chalk River

,

b)

a)

+i

-i 93/;~_

91/.2.~

,6= t

1075

1064 44+@

1052 8 9 / ~ 8712.~ 1029 1006 8 5 / ~ 83/~ 980 959 81/2-~, 79/2-

h~ =0.6 MeV

1017

42+@ 969 40 + ~'

CORE + [ 5 1 4 ] ~

EFFECTIVE

"152Dy,,

153Dy 0.55

0.60

0.65

CORE

fl=

NEUTRONS

AE 7 2 (keV) 1

,, • '

-.-."

0

.

-1

10'

12'

1

14

E y(MeV)

Figure 5. (a) Calculated single neutron routhians at a rotational frequency 0.6 MeV versus B2(b) The effective Z52Dy core transitions have been derived from the ZS3Dy data for the [51419/2 signature partners, and are compared with the actual ZS2Dy transitions as shown inset. (After Reference 5) 155 HeV and z24Sn(3ZP, 5n)ZS°Tb at 156 HeV.

In both cases we recorded about 500

x I06 coincidence events above a K=IO trigger in the BGO ball. examples

of

?-spectra

In

these

experiments.

These

Figure 6 shows

spectra

are

sums

of

reasonably clean gates taken from the efflclency-corrected background-subtracted coincidence matrices.

To get an idea of

the statistical accuracy it can be

noted that the net intensity of any particular photopeak-photopeak coincidence on the plateau region of the intensity distribution in the z49gd yrast data is approximately 2000 counts. The dynamical moments of inertia j(2) shown in Figure 7.

in z49Gd have very different slopes orbltals

are

and relative ?-ray

intensities are

In the left most panel it is apparent that the three bands

available

to their j(2)

in the calculations

values.

and how

Considering which

their j(2)

values

would

behave, it Is clear that to make z49Gd* from z49Gd we need a neutron particle hole excitation placing a neutron in an intruder orbital and leaving a hole in the 64 or [642]5/2 orbital, i.e. from ~62v7 z to ~62v72(64) "I.

The excited band

19. Ward / Superdeforrnation at Chalk River

145c

149Gd** (SECOND

EXCITED

BAND)

1400

6O0

0 I,I Z Z

< 7-

' ~i'll ~II N_ooN~_i''_' ,~iI_ ~=rll ~'""I I",li'e. II 1II'I'~'"""" 'N l~i"""'ll" =

2200

r i

,

m Lo

i

I

~

I I o S ~I ,~ ~ ~

F{

(.D

_ _

,

(YRAST)

1400 131 LIJ O3 I--

z

600 0

3000

0

u.I _J <

~

co~s ;~ ~ _0 ~ _ ~Z --

149Gd*

~

(FIRST

--

EXCITED

BAND)

1500

m

o

i

~

T

r-.

~

O0

~

I

I1000

'

'il' tO

I

'U i"

I

I I' I"

1 4 9 , . , .4

u,

LO

"~

I~

~

m 0

0

---- ,,o ~

(D

C'J

F-- ~

co c

(YRAST)

1

7000 3000 O 800

V'u,f~ vu'~C~'..-~,'u'u~-~." : " I L

1000

1200

GAMMA-RAY

• " '

' i

. . . . . . . . . . . . . I

i400 ENERGY

i600

I

i800

(kev)

Figure 6. Sample spectra of superdeformed bands observed in Z49Gd and *5°Tb with the 8~ spectrometer. Gamma-ray transition energies are shown in keV rounded to the nearest integer. The lower three panels are taken from the background-subtracted efficiency-corrected coincidence matrix and were obtained by summing coincidence spectra derived from clean gates. The upper panel is also efficiency corrected but was derived from the raw coincidence matrix by summing gates on 71269, 71323, 71377 and 71431 keV with an appropriate background subtracted. From Reference 7.

146c

,

Ward / Superdeformation at Chalk River

D.

then has positive parity and signature ~ = +1/2.

With this assignment,

149Gd*

has the same intruder configuration as the yrast superdeformed band in 15°Gd9) whose j(2) behavlour it clearly mimics (of centre panel in Figure 7). The interpretation of 149Gd** is very simple since it is a twin to the yrast superdeformed

band

configuration.

in

15°Tb

and

must

therefore

have

the

same

intruder

In this case we have a proton particle-hole configuration,

the situation

is surely parallel ---T--

T

-

P

~ --T

to the cases observed ----

--

i

~

and

by Byrski 6) where

1

149Gd I00

OGd

>- 80~ ~- 6O

~

4o

tlA

~z

2O

>

8

<_J

4

oc

2 i 06 IO0

i¢r hi Z

! 08

j(2)r

i I0 ~

~___i_. E ~ I t----J OrB 2 14 16 1808 I0 12 14 16 06 G A M M A - R A Y ENERGY (MeV)

,

,

~

T - -

--~

r

T

I --

I

",

"2"eV") ,',9 ,

90

n I0

T

" T

I 12

t 14

"

Z

16

I ,,e',,,,

I Gd

t- 2 F ¢¢~'{ ¢'¢¢¢'¢o? ~l

U_ 0 I-Z UJ

,50Gd

',

0~8~-~

.

/

~

149Gd**

E~, 4

0

o_ d


14

d**

4I

<~ z >cl

J i I 60

L

03

I 04

05

_

I

06

I

07

018

L

~

09 04 05

ROTATIONAL

06

J

~

'

:

~

-

07 08 03 0"4 0"5 0 6 07 08

FREQUENCY

~,.,.,

(MeV)

Figure 7. Relative intensities and dynamical moments of inertia, j ( 2 ) observed in reference 7, (ref. 9 for the iS°Gd case). Solid lines are drawn to link the data points and are not fits. The j(2) plot for iS°Gd is irregular beyond the uncertainty quoted in Ref. 9 and the inflections may have significance considering that 149Gd* shows a similar pattern. The J(~) for 149Gd is very regular except for the last two transitions, an effect which can be seen by eye in Figure 8. The bottom right panel contains an inset detailing the transition-energy differences between the twinned bands 149Gd**/*S0Tb. From Reference 7.

the

D. Ward / Superdeformation at Chalk River proton hole is in the 3x0 or [30111/2 orbital.

147c

The configuration of 149Gd** is

then ~63(310)'xv7 I, having positive parity and signature ~ = +1/2. In 149Gd we find both a neutron and a proton excited superdeformed band; the neutron excitation does not produce a twin whereas the proton excitation does. Can we understand this?

In part, it can be understood as a deformation driving

effect of the hole state. Nilsson

diagram,

and

In X49Gd** the [30111/2

a hole

in

that

state

orbital is up-sloping in the

increases

the deformation

thus

cancelling to some extent the effect of the change in nuclear size which scales as A s/3.

On the other hand,

the neutron hole in 149Gd* is [642]s/2 , which is

strongly down-sloping in the Nilsson diagram. a decrease

in deformation

thus reinforcing

A hole in this orbital will cause the change in nuclear size,

and we

should expect that the corresponding decrease in the moment of inertia will be larger than the average scaling, calculations by Ragnarsson,

and indeed

this is what we observe.

presented at this conference1°),

Recent

substantiate

these

simple arguments. Since

Gd**

is

excitation energy

more

weakly

populated

than

Gd*

it

must

in the feeding region around spin 55 ~.

lie

at

higher

But Gd** has

the

smaller j¢2} values and therefore a crossing of the bands might occur at lower spin.

Such a crossing should exhibit an interaction since these bands have the

same parity and signature if our assignments are correct.

We can speculate that

the sharp deviation in the energy of the lowest transitions from the systematic trends,

namely 7877 keV in Gd*

(higher

than expected),

and 7896 keV in Gd**

(lower than expected) is an indication of the crossing.

That is, the level fed

by 7877 keV is pushed down by the interaction and the level fed by 7896 keV is pushed up.

The assumption

that these levels are essentially degenerate

fixes

the relative excitation energy of Gd* and Gd**, and we find that in the feeding region, the bands would be approximately 700 keV apart.

This conclusion is very

tentative since on the basis of this experiment we cannot be sure that the key transitions

7877

Nevertheless,

and 7896

keV are

truly members

of

their

respective

bands.

we present the argument since it is the first clue pertaining

to

the dependence of population intensity on excitation energy in the second well. Since transition

in

this

known

experiment so

far

we

have

observed

(71673 keV I = 135/2 ~

the 131/2,

highest

spin

cf Figure

discrete 8),

it is

worthwhile to extrapolate the measured feeding intensities for 149Gd to estimate how the population of very high spin states might look. to a decrease in population of approximately is of interest

in connection with

The slope corresponds

1.8 per transition.

the detection of hyperdeformed

This estimate nuclei

(3:1

axis ratio) which are predicted 11) to lie near the yrast line at spins in excess of 70 ~.

For example, according to our extrapolation, a discrete transition of,

148c

D. Ward / Superdeformation at Chalk River

d W

YRAST

SUPERDEFORMED

z z

BAND

149Gd

t 'o

7000

oJ

uJ 13..

--

N

_

(./3 I.z z) o .o 3 0 0 0 ¢-., w J

"~-

--

.~.

i~ -

oJ

>¢

OrO

_

--

to

6.0 Ul ,"

1200

U ""

I

1360

1280

~'Lgup

UU

1440

GAMMA-RAY

bt.~ •

1520

,--,

,

uu~

1600

1680

(kev)

ENERGY

F i g u r e 8. Spectrum showing t h e h i g h e s t s p i n d i s c r e t e l i n e s t h e y r a s t s u p e r d e f o r m e d band i n 149Gd. T h i s s p e c t r u m was summing a p p r o p r i a t e gates in the background-subtracted corrected matrix. The a s s i g n m e n t s g i v e n i n Ref. 1 l e a d to 131/2 f o r t h e 1673 keV ? - r a y . From R e f e r e n c e 7.

say,

spin

transition populated

74~72

would

assigned bands

be approximately

in

this

should

be

ten

experiment. possible

times

The

with

weaker

observed in o b t a i n e d by efficiencya s p i n 135/2

than

spectroscopy

the

next

of

the weakest such

generation

weakly

of

v-ray

instrumentation.

2.

STUDIES OF CHARGED PARTICLE EVAPORATION SPECTRA

of

the

For n u c l e i

with large

Coulomb b a r r i e r

prolate at

evaporation of charged particles in a spherical searching

deformation

the

nucleusZ2).

tips,

and

there

it

should therefore

More r e c e n t l y

for hyperdeformed nuclei

it

has

is a significant long

h a s been s u g g e s t e d

program to i n v e s t i g a t e

possible

by v - r a y

spectroscopy

states

specific

deformation

(particularly

charged particle

to e x p e r i m e n t a l i s t s

effects. the

spectrum.

the

t h a t one way of

might be t o t a g t h e v - r a y s p e c t r o s c o p y

This sounds eminently reasonable

associated

that

be e n h a n c e d o v e r t h a t e x p e c t e d

t h e low e n e r g y component o f t h e e v a p o r a t e d c h a r g e d p a r t i c l e

in

reduction

been a r g u e d

with

spectrumZZ).

and we have i n i t i a t e d

a

We t u r n t h e c o n c e p t a r o u n d and s e l e c t

final

nucleus

which a r e

superdeformation)

and

known to have a then

examine

the

D. Ward / Superdeformation at Chalk River

149c

The apparatus is an array of charged particle detectors that fits inside the BGO

ball

crystals angle

of

(cf

appreciably were

the

coupled Figure less

8~

9).

The

This which

space

driven

ALF,

comprises

inside

the

35Z of

8~

to photodiodes.

Their

16 CsI

the

solid

spectrometer

is

at ~RNL and we

performance

has

been very

The CsI crystals are 2 cm x 2 cm x 0.5 cm thick and the photodiodes

to detect

every

fusion

The BGO ball of the 8~ spectrometer is

event

with

therefore we can use it as a start timer. outputs referenced rays,

named

approximately

in the Spin Spectrometer

2 cm x 2 cm with a rise time of 25 ns. guaranteed

array,

cover

available

than that available

to some extent

gratifying.

spectrometer.

to photodiodes,

to the start

p, d, and ~ particles.

(alpha energy).

Typical

a

timing

of

-2

ns~

Simple cross-over timing on the CsI

time then provides good The operational

energy resolutions

accuracy

threshold

identification is generally

for 72.5 MeV

are 5Z for 5 MeV ~-particles

and

1.3X for 15 MeV protons. To

build

superdeformed MeV.

up

to

hyperdeformatlon

in

easy

stages

we

first

studied

the

band in i]3Ndi~) by the reaction 1°Spd (32S, 2p, 2n)133Nd at 155

This band is remarkable in that it is populated with an intensity about

Figure 9. View of the plastic vacuum shell and mounting system for the 16 CsI - photodiode detector array, (ALF), mounted inside the BGO ball of the 8~ spectrometer.

150c

•

20%

of

19.

the reaction

Furthermore,

Z33Nd

Ward / Superdeformation at Chalk River

channel

Is Just

(compared far enough

wlth

I-2% in the rare earth

off

stability

residue in reactions with outgoing charged particles.

that

region).

It Is a prominent

Thls Is not the case for

known superdeformed bands in the Z52Dy region. Now

our

main

objective

in

these

experiments

is

to be certain

that

by

discrete llne gating techniques on the BPGe detectors we have truly isolated the associated particle spectrum. chosen gate included only from background

underlying

would be very difficult

It means we have to be very careful that I) the

the v-ray peak of interest the v-ray

10000[

F ~J 1400~ z z 12003: 0 1000.

1000

0

0

100[

I1.

°t 10

,11.

400,

-200

I

zl-" ~

1000

°

100

0 0

/

~

--

It

~

I_

~,~,. 132Nd

C)

What we can do is to

133Nd

SOb

800~ 6001

ill 8 2oo~ ~

-400 10000

subtracted.

to attain this level of quality assurance were it not

for the fact we are dealing with HPGe coincidence data.

~ z

and 2) the contribution

peak has been properly

400

0

0

500 1000 1500 GAMMA-RAY ENERGY

(keV)

2000

Figure II. The V-spectrum showing the superdeformed band of Z33Nd selected with gates and background corrections identical to those used In generating the p r o t o n c o i n c i d e n c e spectrum shown in Figure 10, Panel a.

t

1o.

I I

r 10

20

30

Figure 10. Panel (a) - the spectrum of protons at 90 ° in the laboratory leading to normal and superdeformed states in Z33Nd, (2p, 2n) channel, overlaid. Panel (b) - the difference spectrum normalized to zero net counts over the full spectrum. Panel (c) - proton spectra leading to low spin yrast states of Z32Nd (2p, 3n) channel and Z34Nd (2p, n) channel.

D. Ward / Superdeformation at Chalk River

set

gates

spectrum;

and

backgrounds

on HPGe 1 and project

151 c

out an associated

particle

the quality of this result is tested by projecting out under exactly

the same conditions

the associated V-spectrum In HPGe 2.

If we claim to have

isolated the particles leading to superdeformed states then we should see in the corresponding

coincidence V-spectrum

the lines of the superdeformed

its decay through the low spln yrast transitions. belonging to any other nucleus, intensity exceeding

band,

and

Ne do not expect to see lines

or to see low spin transitions of iS3Nd at an

those of the superdeformed

band measured below the gating

trahsition. Results for *S3Nd are shown In Figure 10. judgements spectrum

and

including

leading

to the superdeformed

band

leading to normal states in the same nucleus. at the I-2% level.

slopes

interesting proton

in

the charged

is virtually

particle

identical

to that

If there are differences they are

These certainly show large centroid shifts and have

the exponential

experimental

spectra

that

As a check we also show proton spectra associated with other

(2p, xn) reaction channels. different

After making the most stringent

only clean gates we find

is

observation

strongly

region as might that

correlated

be expected.

the degree to

the

of

total

"hardness"

number

of

It is an in

these

evaporated

particles (i.e. n + p) and not to just the number of protons involved.

3.

SUMMARY Results of recent experiments with the 8n spectrometer have been presented.

Our

understanding

superdeformed good shape.

of

the

configurations

involved

in

the

three

With regard to the charged particle work, we have obviously made a

good start with 133Nd; *S2Dy

intruder

bands now known in each of XSSDy and 149Gd seems to be in very

region

where

however,

I think we need a definitive experiment in the

the deformations

are higher

in order

to pln

down

this

question of whether shape in the final nucleus matters or not.

ACKNOWLEDGEMENTS The 8~ spectrometer is a National Facility, jointly operated by AECL and by Canadian universities

through NSERC funding.

The work I have reported on has

been performed by various 8~ spectrometer groups and collaborators. The iS3Dy study was principally J.K. Johansson, Universit~ A. Tehami.

J.A. Kuehner,

de Montreal

group

the work of the McMaster group

D. Prevost

and J.C. Waddington

including

S. Monaro,

together

A. Djaafri,

The new study of 149Gd was inspired by B. Haas,

including with

N. Nadon,

J.P. Vivien

a

and (CRN

Strasbourg) and H. Kluge (HHI Berlin) and was brought to fruition by them and a Universit4 de Montreal group (F. Taras and S. Flibotte).

The construction and

.

152c

D. Ward / Superdeformation at Chalk River

operation of the particle detection system is the main responsibility of the Toronto

group

G.C. Ball,

(A. Galindo-Urlbarri

J.S. Forster

and T. Drake)

and T.K. Alexander

experiments with evaporated charged particles.

who

in conjunction

(CRNL) were

with

the principals

in

Virtually all experiments with

the spectrometer have a major participation from my colleagues at CRNL, namely, H.R. Andrews and D.C. Radford and S. Pilotte (now at University of Tennessee) and V. Janzen.

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B. Haas et al., Phys. Rev. Lett. 60 (1988) 503.

2)

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3)

T. Bengtsson et al., Phys. Lett. B 20___88(1988) 39.

4)

W. Nazarewlcz et al., Nucl. Phys. A503 (1989) 285.

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