New insights into nuclear structure from nuclei far from stability

Nuclear Physics A520 (1990) 377c-399c North-Holland

377c

NEW INSIGHTS INTO NUCLEAR STRUCTURE FROM NUCLEI FAR FROM STABILITY

J. H. HAMILTON Department of Physics, Vanderbilt University, Nashville, TN 37235; Institute for Theoretical Physics, University of Frankfurt, Frankfurt, W. Germany

I. INTRODUCTION Across

the periodic

table,

nuclei

far from stability

us with new insights into nuclear structure. of proton to neutron composition off

stability

advances

probe

beyond

new results

nuclear

the latest

to surprise

found in proton- and neutron-rich nuclei far

matter

in new ways.

reviews 1'2 are

in light nuclei

continue

Nuclei under the extreme conditions

Some

presented

highlights

of

in this report.

and the A = 190 region are

included

recent Other,

in the next

two conference papers. One of the principal motivations to climb out of the valley of beta stability and view the nuclear scenery from the mountainsides the spherical sometimes they

are

been the

the

spherical

not,

and

proposed. single

shell

New

numbers

"deformed" spectrum

for both protons

deformation. higher Z.

magic

new spherical

particle

gaps

was

to see how magic are

shell model magic numbers far from stability.

magic magic

at

large

are

still

numbers numbers

magic

not seen associated

deformation

and neutrons

reinforce

are

As we shall see,

there

and

near with

shell

established

each

sometimes

stability

other

for

have

gaps when the

in the

same

The predicted proton and neutron drip lines are being probed to Beyond the stability

or instability

of a particular

collection

of

nucleons, measurements of the decay properties out to the drip lines are being carried out to test theoretical models in the universe.

and our

understanding

nucleosynthesis

Recent data have revealed new complexities in the coexistence

of different

nuclear

190 regions.

Clear advances are being made in both microscopic and generalized

shapes at low and higher

spins

in the mass 70,

100 and

collective model approaches to nuclear structure. Finally, new experimental facilities are opening up previously inaccessible or very difficult to reach areas of nuclei far from stability. accelerators greater

beam

with

energies

intensities

nuclear fragmentation. up to several

up are

to

100 MeV/u

probing

new

and

several

regions

of

New heavy-ion

orders

light

of magnitude

nuclei

through

The new SIS at GSI will give beams of heavy ions with

GeV/u with higher

intensities

to yield greater enhancements

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

of

J.H. Hamilton / New insights into nuclear structure

378c

the production of exotic modes.

Studies of the products of spontaneous fission

and heavy-ion and proton-induced fission are being used to extend our knowledge of neutron-rich

nuclei.

new spectroscopic tests

of

On-line

measurements

the fundamental

sources and ion-guides)

nuclear

orientation

of the properties

interactions.

Other

facilities

of nuclei

and important

new developments

and the new YASNAPP II facility

new

(bunched

ion

in Dubna presented at

this conference are further expanding research opportunities. generation recoil mass spectrometer

open up many

Finally,

a new

(RMS) being built at HHIRF for inverse and

deep inelastic reactions, when coupled with a large Ge detector array, including GAM~ASPHERE, off stability

will be a powerful new facility for nuclear structure studies far with broad applications

in many other areas.

in this field, from ground states to high spins, I-3

Further progress

can be found in other recent

reviews.

2. LIGHT EXOTIC NUCLEI Detraz and Vieira 2 have covered in considerable detail the impressive extension of the proton drip line up to cobalt (Z = 27) with gaps only at iron (Z = 26) and

chromium

(Z = 24) and the neutron drip line

stability) only up to neon (Z = 10). knowledge

of neutron-rich nuclei

GANIL and the NSCL with powerful (see ref. LAMPF.

2) and

the

(which extends farther

from

Much of the impressive expansion of our

is through fragmentation reaction studies at spectrometers

time-of-flight

such as SPE and LISE at GANIL

isochronous

recoil

spectrometer

TOFI

at

The LAMPF and GANIL groups have determined the masses of 22 previously

unmeasured neutron-rich nuclei from ~ B Recent GANIL work 4'5 has shown lines are bound,

but

260 as well

to 3ap.

29F and as

32Ne at the predicted neutron drip

250 are unbound.

More recently,

the

B

half-lives and B-delayed neutron emission probabilities have been measured for 2~,22N and 23,2~0 which are close to or at the drip line to provide unique tests of theory at the limits of particle-bound nuclei. 6

A large scale quasi-particle

random-phase approximation (QRPA) calculation of the beta half-lives of neutronrich

nuclei

out

to

the

drip

line

has

been

made

recently. 7

between the experimental values and the QRPA predictions, taken from mass formulae,

is relatively poor. 6

enhanced

QB values

if experimental

are

used.

The

agreement

if based on Q values

The predictive power of QRPA is A large

and difficult

effort

lies before us to extend the half-life and mass measurements to known neutronrich

isotopes

as well as to currently

unknown heavier

neutron-rich

isotopes.

These data not only are important to test the nuclear calculations but also to test

astrophysical

calculations

of

critically on beta decay properties.

nucleosynthesis

processes

which

depend

J.H. Hamilton / New insights into nuclear structure

379c

3. MAGIC NUMBERS FOR SPHERICAL AND DEFORMED SHAPES FAR OFF STABILITY The level structures

of ~32s0Snsz and 6828Ni~o. are characteristic

of spherical

double magic nuclei to confirm the magic character of these numbers for spherical shapes (see ref. I).

Now 38 and 60 are found to be magic numbers for deformed

shapes when both the proton and neutron numbers are a combination reinforce there like

each

should

other. I

be magic

the spherical

deformed

magic

This

confirmed

numbers

ones

numbers

for

in stabilizing only

become

the

deformed

long-standing shapes

a nucleus

important

that

prediction 8

should

at a given

when

of these to

there

that

be

important

shape.

The new

is reinforcement

of

the proton and neutron shell gaps for the same deformation. 9

This reinforcement

effect

and neutron

occurs

only over a narrow

gaps and so leads

region

around

the proton

shell

to the two new islands of super deformed ground states (B -

0.4) centered around Z = 38, N = 38 and Z = 38, N = 60,62 (refs. 1,9). The Fig.

two

I.

neutron

separation

As emphasized

and S2n yield A=130-170

important

region,

smooth trends decreasing

he

energies

(S2n)

by Audi I0 recently, information

notes,

about

demonstrates

in S2n (smooth decreases

Z for a given N):

for

light

systematic changes two

nuclei

are shown

in

tends in nuclear masses

in nuclear

departures

structure.

from

the

The

expected

with increasing N for given Z and with

A) a sharp decrease

in S2n when passing

the 82

closed shell and, B) a sudden increase with N between 88 and 90 in Pm, Sm and Eu where

there

is a sudden

deformed shapes. is interpreted

2,1

in structure

from near-spherlcal

to well-

In Fig. I, one sees a similar bump beginning at 31Na2o which

as the sudden onset of large deformation. 11

23_

~

Fig. I.

change

3,p

i

2

Two neutron separation

. . . .

N

e

energies,

S2n (refs.

10,12).

This discovery was

i

v

380c

ZH. Hamilton

/

New insights into nuclear structure

a complete surprise because one would have expected a spherical ground state for the N = 20 spherical magic number.

Only a slight bump is seen in a2Mg2o

and none in 3aA12o. The interpretation (see ref. 2) of the pronounced bump in S2n in 3~,32Na2o,2~ can be understood

by looking at

Flg.

2

(refs.

10,12).

The

odd proton

in

sodium is in the d5/2-3/21211] Nilsson level which is independent of deformation. The f7/2-I/21330] d3/2-3/21202]

neutron-orbital strongly down-slopes to "intrude" below the

level

at

large

prolate

deformation.

The

binding

energy

is

increased as a result of the deformed intruder state dropping below the energy of the normal spherical state.

In 32,33Mg the bump is only half that seen in

3~,a2Na and good agreement with shell model calculations is seen for a3-3SA120_22. In aluminum, the odd proton is in the up-sloplng d5/2-5/21202]

Nilsson level

whose energy increases rapidly with increasing prolate deformation.

The gain

in energy for the down-sloping f7/2-I/21330] orbital in sodium is countered in 33-3SA1 by the increased energy required to put the proton in the d5/2-5/21202] orbital

at large deformation.

With

this interpretation,

Detraz and Vieira 2

suggest 29F may show an even more pronounced S2n deformation effect. Recent studies 13 of the beta decay of a~A12o to a~Si20 revealed a quite new surprise for N = 20.

The level scheme of the N - 20 isotones from

~°Ca are shown in Fig. 3. its deformed nature.

The level scheme for a~Si established in on-line separator

work at ISOLDE 13 clears up earlier reaction work discrepancies. is over three times higher than in a2Mg. remarkably

like

a2Mg to

The low energy of the first 2 + state in 32Mg indicates

those

of

spherical

Indeed,

double magic

The 2 + energy

the levels of ~ S i 2 0 ~Ca20.

look

Calculations 13'14

show the (sd)-2(fp) 2 intruder states, which give a2Mg a deformed ground state, are pushed up in a~Si whose ground state is the more spherical sd configuration. Fig. 2. Single particle l e v e l s around the s p h e r i c a l magic number 20 as a f u n c t i o n of

deformation.

................ ................---- ::::~ .... :: .......... ___ii--~''! ~::::Z~I~;I"................ 3 ~ , } ~

35 ~ -

~

I

.........../

3~[2~]

I

/

v2{33o}

°

I

I

I

-0.3

-0.2

-0.1

O.O

I

I

I

0.1

0.2

0.3

/

J.H. Hamilton

(¢,5)-

4.38 4.26

3-

(3)-

3.04

(3}-

2.86

2+

2+

4.48

3-

3.81

2+

3.29

0+

3.38

2+

2.17

3.90 3.74 3.36

2+

304-

0.88

0+

0÷

, 0

32Mg

Fig. 3.

0+

0

34Si

0

0+

36S

two-hole

intruder

range with the 2 + level lowest, MeV dominated

states

are

predicted

by the sd 2 + excited state.

coexistence

intruder was

states --

invoked 15

13).

in the

observed

On the other hand, the

indeed

energy

after

36S levels

The low-lying levels of W°Ca

~°Ca was the second case where shape

Moronaga 16

explain the first few levels of double magic ~wSi and

~°Ca

and the first non-intruder state is about 5.5

are well reproduced in just the sd model space. involve

0

0÷

0

38Ar

Lowest e x c i t e d s t a t e s o f N = 20 isotones ( r e f .

Two-particle

also

~4.19

3-

3.33

381c

New insights into nuclear structure

introduced *60.

shape

coexistence

to

The strong similarities of

~°Ca and the fact the mass formula based on e energies

indicate Z =

14, N = 20 has a stronger shell closure than Z = N = 20 lead Baumann et al. 13 to suggest that ,~Siao3W is also a spherical double magic nucleus -- with Z = 14 a new spherical magic number. As a final point bearing on this question,

Audi discussed the fact that S2n

in Fig. I, based on the new GANIL and TOFI mass measurements around N = 15 at lower

Z,

added.

show

a sudden

drop

for Ne,

F and 0 when

the sixteenth

that 15 is a spherical

magic

neutron

is

number

here.

This would be very interesting since it would be the first odd-integer,

magic

number.

Such a drop could signify

How is this related to Z = 14 as a magic number and is N = 14 magic

are intriguing questions.

4. NEW INSIGHTS INTO SHAPE COEXISTENCE IN THE MASS 70 AND 100 REGIONS AS recently documented I, the mass 70 and 100 regions are important testing grounds for sophisticated nuclear models because of the wide variety of shapes and structures observed and of the rapidity with which they change as a function of both Z and N. 0.4,

The islands of super strong ground state deformations,

8 -

discovered around Z = N = 38 and Z = 38, N = 60,62 disappear rapidly as

either N or Z change by more than two particles.

These new "deformed" magic

ZH. Hamilton / New insights into nuclear structure

382c

numbers

are important

only when Z and N reinforce

at the same deformation,

in this case B - 0.4

each other with shell gaps

(see refSo

1,9) as seen in Fig.

4. Strongly deformed 0 + state

at only

The experimental

9aSr (Fig. 5, from ref. 18) has an excited near-spherical

215

keV,

deformed O + state is at 331 = 2.67.

As recently

l°°jl°2Zr60~62, measurements

the lowest

E4/E 2 is 2.99.

io~-~

of the

excited

In strongly

keV,

0 + state

deformed

known

in all nuclei.

1°°Zr the excited,

less

the second lowest excited 0 + known and E4/E 2

discussed 3, states to 8 + to 10 + have been identified

°6M062,6~and spontaneous

in

1°6-|l°Rusa_66 (ref. 19) from Y-~ coincidence

fission

decay

of

2S2cf

in the HHIRF

close-

packed Ge ball. Ey1) band,

Peker and Hamilton 20 analyzed those data in terms of AEy (=Ey2 21 spin, which is very sensitive to shape changes For a single

vs.

AEy is a smooth decreasing

when bands and

10 +

cross.

data

However,

in

presumably

l°2Zr,

decrease

1°°Zr

function

smoothly

there

of spin with clear breaks occurring

l°~,I°6Mo, to

If one assumes

1°6-11°Ru the plots with the new 8 +

indicate

is a clear

from the interference

deformations. cases,

For

break

no

band

at the

AEy line for

its 0 + state is pushed down by about gr 20 up a similar amount by the interaction.

98Sr Fig.

6 suggests

spin,

below see

10 + .

Fig.

a similar

interaction

and

~°°Zr as for the other

40 keV and the excited The same

increase

AEy analysis

of about

20 keV

the 2+-0 + energy. With these shifts for 98Sr and 1°°Zr, their E4+/E2+ I gr 3.31, 3.07, respectively, which are comparable to the rigid rotor value. Recently,

Mach et al. 18 analyzed

the 21, + 02+ and 4 I+ levels conclusions band

and

more They

state

of 23.3

remarkably

weakly

keV

close

in

unperturbed

down

and

band

in each nucleus

and up for

46.9

keV

in

to the 20 and 40 keV shifts

This close agreement

confirms

20-00+ + energies

the detailed

in the deformed

121.3 and 165.6 keV, respectively.

the ground

~°°Zr,

both

9aSr and

(3.5)

could

E4+/E2+

~°°Zr.

suggest

= 3.23)

respectively.

bands

Mach that

while

However,

are only

are

extracted

ground bands

+

02 are

from the AEy plots 3. The

in 9aSr and ~°°Zr are

These energies are very close to the 2 I+-01+

and

~°°,~°2Zr

which

of the AEy curves.

They extracted deformations

IB21 - 0.2 for the less deformed

et al. 18 noted that the unperturbed 9SSr

weakly

O + and excited

respectively,

usefulness

experimental energies in 1°°Sr and ~°2Zr as in Fig. 5. of 0.4 for the deformed

in

~°°Zr shown in Fig. 5 with the following

excited

shifts 9SSr

of

The shape coexisting strongly deformed ground

deformed

extracted

+

02

their and other recent lifetime data 22 for

in 9aSr and

for 98Sr and t°°Zr.

mixed.

6,

of the two low-lying 0 + states with different a similar

then

pushed

crossings

lowest

is a have

better very

rotor similar

than

tO°Sr

ratios

it should be noted that interactions

bands

E4+/E2+

in

ratio

(experimental

of 3.17

and 3.15,

with higher

J.H. Hamilton / New insights into nuclear structure

2;~

383c

871 o -

~.

867

~,.; ,, I

i ;

434

0 2 N --

417

21,5

~°, l- ~ .........

z,

I

, ~

144

J__o

129

2"

5O" '~

I[--O0

I00_ 38 ~ r62

98 38 Sr 60

6, 2Z

4*

SOps

~

,o63

878

°=1°11

i.,..1

-lq

331

s

--~;

0 212 ....._t...r

-0.4

-0.3

-0.2

-OJ

0.0 42

0.1

0.2

4oZr~

0.3

15__1

102 _ 40 Z r62

Fig. 5. Lowest excited states of 9s,*°°Sr and ~°°,*°2Zr (ref. 18).

Fig. 4. Single particle levels as a function of deformation. 200

180

-o~ ~ --o-

~ ~

~

102 Zr 104 Mo lOOZr 106M0+10 ke~ 8

160

140

120 100 Spin

80

I

J

l

,

I

i

I

,

0 2 4 6 8 0 Fig. 6. AEy as function of spin for heavy, Sr, Zr, and Mo nuclei. data from ref. 19, analysis from ref. 20.)

(High spin

J.H. Hamilton / New insights into nuclear structure

384c

+

lying,

presently unobserved 02+ states in 1°°Sr and 1°2Zr need to push their 01

states

down only

value of 3.3.

5-10

keV for their E4+/E2+

Such a shift would

ratios

be undetected

to equal

in present

the rigid rotor data.

The RMS

radii derived from laser studies 22 indicate that 9aSr and ~°°Sr have essentially identical As

deformations

discussed

2.86,

respectively.

like

it should

3,

76Sr

and

8°Zr have

Lister et al. 22 suggested

for the ~ softness

near-spherical

of 82 - 0.37-0.38.

in ref.

of

be.

a°Zr,

but that

However,

if one

0 + states which

~6Sr

E4+/E2+

= 2.83,

that these ratios are evidence

is not a good

assumes

interact

experimental

that

these

rotational nuclei

with their ground states

nucleus

have

excited

to push down

the 0 + states only 50 keV, then these ratios are 3.25 and 3.27, which are in gr agreement with 3.3 within the errors. Thus, in regions where nuclear shape coexistence ratios.

may be present,

it is dangerous

to draw conclusions

from E4+/E2+

In ref. 3 is discussed the very intriguing puzzle of why the unperturbed

01-02++ states

in

~,~6Kr,

which

are

170-400

keV apart,

interact

strongly

to

give shifts of 190-250 keV in both 0 + states while in 9'Sr,

*°°Zr where the 0 +

states

much,

This

are much closer,

is

especially

these two regions

in terms

should be very similar, The high recently

at Daresbury

with similar

interact because

of

and shift the

intruder

as discussed

spin levels

and 23aU(~Li,f)

they

puzzling

origin orbitals

in energy of

the shape

and

shell

in greater detail

up to 10 + in 9aSr and

much

less.

coexistence

in

gap reinforcement

in ref. 3.

*°2Zr

also

have been studied

in the heavy ion induced fission reactions

232Th(~SO,f)

(ref. 23) with the gamma-ray detector arrays ESSA30 and TESSA3

results

for

*°2Zr,

in the Oak Ridge-Vanderbilt

while

work 19.

9SSr was extended

The spontaneous

to higher

spins

than

fission of 2~'Cm also has

been studied with levels again seen to 10 + in l°o,l°2Zr

and (19/2-)

in l°IZr,

and states to 8 + in ~°4Zr and (15/2) in ~°'Zr seen for the first time. 24 The (2~) is somewhat lower, it has somewhat a 5 to

10 keV

deformations

140.3 keV, in ~°4Zr compared to ~°°Zr to suggest

larger deformation. shift

could

However,

in the 0 + energies

be essentially

equal.

as noted

and E(2~)

above,

energy

The moment

in

there could be 1°2Zr so their

of inertia

and AEy are

essentially linear for 1°~Zr to indicate there is no mixing of bands. Hotchkis 24 et al. propose that the side band in ~°~Zr and the ground band in ~°2Zr have 5/2- band heads associated with the 5/2-[532]

intruder configuration

(~h11/2).

These results are consistent with the predictions of mean-field calculations 25-27 and correspond 82 = 0.4. in ~°~Zr,

with predicted

The near degeneracy they propose,

shows

minima

in the potential

of the 5/2-[532] substantial

occupancy

and 3/2 Nilsson orbitals of the h11/2 intruder.

energy surfaces 26'27 at

orbital with the ground state in

~°°Zr of the ~ = I/2

So, in ~°2-~°~Zr they suggest

J.H. Hamilton / New insights into nuclear structure

385c

there should be significant

occupancy of the ~ = I/2-, 3/2- and 5/2- orbitals

to

deformed

support

predictions

of

mean

field

calculations

that

the

h11/2

orbit near the Fermi surface provides the driving force toward large deformation. This is in contrast to shell model calculations which attribute the deformation to the g7/2 orbit with only small

occupancy of the h11/2 orbit.

Earlier,

I

pointed out3 that it is the strong down-sloping,

~h11/2 orbit and to a lesser

extent

be

ng7/2

orbital

(see

Fig.

4) which

should

important

in driving

the

deformation and that it is not clear what this does to the p-n coupling scheme 28. Earlier

Varley

et al. 29 identified

Y rays

in 72Kr with the Daresbury RMS.

They placed the first 21+ energy at 709.1 keV based on its intensity and proposed a level

structure

near-spherical

based

ground

on

state

from these singles data. the levels with

,6Kr36

and

(ref.

well-deformed

Ge

31).

is

Ball

4) manifest

it still

excited

coexistence

band

was

with

a

interpreted

at Rochester and Y-Y coincidence studies

at HHIRF 30,

we extracted

An important question is where,

itself?

prolate?

deformation,

Y-Y coincidence

data

for

if at all, does the shell

82 ~ -0.4,

for N and Z of 36

The work of Varley et ai.29 indicates the ~2Kr

ground state is not large oblate, or

Shape

In the course of our identifying for the first time

gap predicted 17'32 at large oblate (see Fig.

intensities.

in 73Kr from RMS studies

the Compact

72

singles

but is the excited well-deformed band oblate

In ~ K r 3 ~

the spins have not been established,

but

5 the pattern of the energies of the levels are very similar to 7a6Krag, which is

prolate being dominated by the N = 38 shell gap at 82 - 0.4. Our Y-Y coincidence

data show the level scheme of Varley et al.

singles intensities is wrong. data 31

is shown

deviation of

in Fig.

7.

at low spin than

extrapolating

back

the

energy was used for ~2Kr. levels

is much

larger

in

ground state energy.

with

based on

The level scheme of 72Kr based on our coincidence The moment of inertia plot shows an even greater ~Kr

shows.

higher

The same procedure we used earlier 33

spin

data

to establish

the unperturbed

+

01

The deviation between the extrapolated and measured ~2Kr

between the two shapes in ~2Kr. 6E = E(O~) ° - E(O~)

29

than

7~Kr

to

indicate

a larger

interaction

The energy shifts are given in Table I where

E(O~) ° the extrapolated

and E(O~)

the experimental

For a two-state interaction the difference between AEo =

E(O~) ° - E(O~) ° and the experimental

levels AE = E(O~) - E(O~)

26E and the interband interaction V=[AE 2 - AEo211/2/2. from the data for ~6,~'Kr as given in Table I.

is AEo - AE =

These can be calculated

The interband interaction, V,

depends largely on the single particle structure of the two 0 + states, and it

J.H. Hamilton / New insights into nuclear structure

386c

(8+ )

3036.8

(6+ )

2111.8

Table 1. Band mixing for Kr isotopes.

Nucleus (4+ )

1321.6

(2 +)

709.1

0+

0.0

72 Fig. 7.

very

little

V

~2Kr

0.525

0.732 a

-0.318 a

0.330 a

~Kr b

0.256

0.681 a

0.169 a

0.330 a

~6Krb

0.187

0.770

0.396

0.330

~SKrb

O.102

1.017

0.813

0.305

This negative

between

and

~SKr.

for

~ZKr,

the perturbed

It was

E(O~)

AE 0 implies the unperturbed

compared

to 7~-TeKr.

oblate shape for the ~ K r in V for

~aKr

assumed

not

to change

for 7~Kr (ref. 33).

If V = 0.33

change

AE °

aE(O~) and AE o calculated with the value V. bValues for these nuclei are from ref. 2.

Kr

significantly

~Kr.

E(O~)

in MeV

~ZKr levels 31.

changes

7ZKr

6E

parameters

~2Kr would

= 732 keV and AE0 = -318

The new data are consistent ground state.

be required

keV.

0 + states have changed their role in

Otherwise,

to keep

AE0

with a well-deformed

a factor

positive

of two increase

at a value similar

to

Thus either way, viewed as a sudden change in the sign of AEo or an abrupt in the

size

of

the

interband

interaction,

appears to have changed markedly relative is now possible

the

ground

state

of

to the heavier krypton isotopes.

that the ground state of ~ZKr has a large oblate

~2Kr It

(B 2 ~ -0.4)

deformation and a low-lying excited band with large prolate (82 - 0.4) deformation as predicted by theory. 17'32

5. SHAPE COEXISTENCE

AND NEW INSIGHTS IN OTHER REGIONS

Zganjar et al.34 describe, shape coexisting

structures

in the Z = 78-82 odd-A

and even-A

structures

are

region

for

which are being discovered

in studies

nuclei.

seen

in a following paper, the wide variety of different

with on-line

Summarizing, both

even-

and

overlapping odd-A

near the ground states

isotope

in

separators

nuclear la°-~9°Hg,

shape

in both

coexisting

l'~-l'aAu,

and

*'~-~'aPt near the ground states with transitions

between the different shapes

dominated

while

by

superdeformed proceedings.

EO

radiation

(see

bands are observed

refs.

I,

34),

in ~9°-~9~Hg

above

as reported

about

elsewhere

spin

10 +

in these

ZH. Hamilton / New insights into nuclear structure

The lifetimes by a-e-t shape

of the 0 + states in '9°,'92,~'~Pb

coincidences 35.

coexisting

deformed

states,

excited

20.02 for

For

states

a deformation

the mixing was found

of

very small mixings are too small

0.3%

deviation

spherical

to be quite where

for the 02+ excited

ground

small,

states

b 2 s 0.003,

and

the

0.005

and

IO+>D = blsph> - aldef>.

to explain the variations

charge radii which have been measured measured

have recently been measured

of 0.17-0.18

the

~'~Pb to *'°Pb, respectively,

387c

down to *'2Pb (ref.

of from the spherical

droplet

in the mean square 36).

model

For *'2Pb the

is the order

of

(ref. 36) while the value based on the mixing of the deformed state into

the spherical

ground state is only the order of (3-6) x 10-'% (ref. 35).

The development of the IGISOL (Ion Guide Isotope Separator On-Line) in Finland has opened up studies of very short-lived 37).

For example,

n-rich

nuclei

proton

induced

and

they have employed the technique short-lived

fission

of

isomers

2"U.

from

These

**2To

new data

New 1.8 and 1.5 ms isomers were discovered concluded

that

these

isomers

are

facility

decays down to Ims (ref.

to identify numbers of new to

**gPd populated

provide

important

model calculations of half lives which are used in nueleosynthesis

They

These

in the

tests

of

calculations•

in *l~Pd and **~Ag, respectively. 37

most

probably

related

to

changes

in

nuclear shapes. The

first

results

from

the

new YASNAPP-2

facility,

separator

on-line with the new high intensity

in Dubna,

were presented

different

experimental

separator. of

at this conference

facilities

*SSTm was

shown

to be a doublet

I/2 + isomeric state 38.

by Gromov 38.

on separate

In one of the first studies, from

which

has an isotope

660 MeV proton synchrocyelotron

beam lines

It includes several out

of the isotope

the 4462 MeV a group from the decay the 23s 11/2-

A full range of investigations

ground

state and 47s

are now underway there.

6. NEW VISTAS FROM ON-LINE NUCLEAR ORIENTATION There are now four operational On-Line Nuclear Orientation in which

a low

temperature

He refrigerator

is coupled

(OLNO) facilities

on llne to an isotope

separator which in turn is on line to an accelerator at Leuven (LISOL), Daresbury (DOLIS), On-Line

Oak Ridge Nuclear

(UNISOR),

Orientation

the rapidly

developing

many

previously

cases,

and CERN

(ISOLDE).

and Hyperfine

importance

of

inaccessible

this

The Proceedings

Interactions technique

information

on

of the 1988

Conference 39 documents to provide

the

spins,

new

and,

parities

in and

magnetic moments of nuclear levels, multipole mixing ratios of their connecting transitions,

a and 8 correlations

and other tests of fundamental

as well as solid-state properties of matter. new

insights

into

the shapes

and

interactions

Such data are providing significant

structures

of nuclei

far from stability

to

J.H. Hamilton / New insights into nuclear structure

388c

test and extend current nuclear models.

Some recent results and new approaches

that open up new opportunities in on-line nuclear orientation are described here. A

FEBIAD

applied

pulsed

ion

to chemically

line isotope separation the ion source. released

at

intensity achieved

source

developed

at GSI

been

very

successfully

separate different elements in the ion source for on40 . The reaction products are stored on a cold trap in

When the cold trap is heated,

different

bunching

has

times.

of

the

Both

radioactlvitles

for each mass-separated

different

selectivity

chemical

species are

of a particular

emitted

from

beam by selecting

the

element

and

source

are

ion

the times of the storage-

release cycle. A significant

new application

of the pulsed

been developed at Daresbury 41 and ISOLDE 42. in studies

initially

host

technique

Recent OLNO work at NICOLE (ISOLDE)

nuclei

determines

reach

which

thermal

nuclei

equilibrium

are

accessible

When the ratio TI/2/T I falls below about 5, computational to extract nuclear is needed 43.

the power of

The spin-lattice relaxation time, TI, with which the implanted

unpolarized

environment

information

so that correction

The new technique

and radioactive implanted

for OLNO has

of the ~84Hg ÷ ~84Au ÷ ~8~Pt decay chain 42 illustrate

the application. but

source

decay 42

compared

their

OLNO

cold

studies.

methods must be used

for incomplete

cleaner

separation

With a pulsed source,

in a time short

source continuously.

provides

with for

polarization

of the relaxation

the mass-separated

to T I and TI/2 rather

nuclei are

than producing

the

Effects of incomplete relaxation are included by calculating

the time averaged polarization

attenuation

factors.

The time evolution of the

activity, that includes the decays of the initial implant, daughter and subsequent product

growth

polarization

and

decay,

time for the initial is the disadvantage The

decay

of

be

separated

which

can

be

experimentally

present

and

from

which

implant and other decay chain members.

~8~Hg and the growth and decay of

with feeding

from

TI/2

about

m 53s,

15s later

a short-lived

isomer.

The

cold

(12s TI/2) data

for

in ~8~Au.

However,

with such feeding.

Eventually

its

both decayed with

level of the 362 keV peak was higher at the end

compared to its maximum than for the 162 keV peak. as

by the

grew more rapidly but reached

than the 162 keV peak.

but the activity

there

time.

Data for the 2+÷0 +

data were not consistent

The intensity of the 362 keV peak initially maximum

Of course,

with

~8~Hg with TI/2 = 30.6s

via a 53s state (previously thought to be the ground state) the (6 + ) ÷ 4 + , 362 keV transition

nuclear

change

~8~Au were studied

implantation of ~8~Hg (ref. 42).

in :a~Pt are consistent

the

can

of using only a fraction of the experimental

above technique following transition

can

time evolution

high the

spin 362

ground

keV

peak

These data were interpreted

state

for

~e~Au fed by a 53s

shows

the

presence

of

a very

J.H. Hamilton / New insights into nuclear structure

389c

short-lived weakly-polarized component not seen in the 162 keV peak. all

the

from

data

a

suggest

high

spin

an additional

isomer

with

5s

shorter

lived

half-life.

feeding

of the

Knowledge

of

Together (6 + ) level

this

unexpected

complexity in the isomeric structure of ~e~Au revealed by these data is essential in the interpretation ~8~Au and its decay.

of all spectroscopic

and hyperfine

interaction data on

For example, the laser g-factor measurement 44 of the 53s

state should be re-evaluated.

Such unknown isomers must be investigated in the

neighboring gold isotopes to understand those decays, too. Recent OLNO studies were the chains

starting

with

carried

out at the UNISOR/NOF

implanting

~8~Au

(rather

base temperature of about 9 mK was obtained for

than

OLNO

facility

~8~Hg) and

~a6Au.

for A

~a6Au ~ ~a6pt with the baffle

open and subsequently 16h ~e6Ir to ~a60s was studied with the baffle closed at a base temperature of 4 mK (ref. 45). source

at angles

separator unique

is

bent

UNISOR

90 ° and

geometry.

implanted Both

extract unique multipole multiple mixing

Six detectors were positioned around the

of 0 °, 90 o , 135 ° , 180 °, 270 ° , and 315 ° .

A2

vertically

and

mixing ratios.

ratios are needed

A4

into

the

coefficients

For example,

The beam from the iron

must

in

foil

be

in

measured

the to

~a6Pt precise E2-MI

to extract the EO admixtures

in the AI = 0

transitions between the bands built on the different shapes and to search for mixed-symmetry UNISOR

states

facility

off-line

of

there

compared

~2As

to

and

in

to all

~2Ge

and

~a6Os,

others on-line

too.

was

This

unique

demonstrated

~°As

to

~°Ge,

feature

in recent

where

both

of the studies

A2

and

A4

coefficients must be measured to obtain unique 6 values which are essential in identifying

mixed-symmetry

states

in these nuclei 46.

study of the light As to Ge decays done

elsewhere,

In another recent OLNO

unique

6 values could not

be extracted because only one coefficient could be measured in their geometry. las,la~,lagPt

decays on- and off-line

have been the focus of the first OLNO work at ISOLDE 47.

Studies

of the

la2-1S6Au decays and

Both nuclear orientation

(NO) and nuclear magnetic carried out.

resonance

on oriented nuclei

(NMR/ON)

studies were

The g-factors and magnetic moments of L6~,~aTpg were measured by

NMR/ON.

Strong electric quadrupole alignments were observed for ~85,La~,~"gPt

in

The

Zn.

agree

with

ratios

of

the

quadrupole

moments

the laser work 48 both in magnitude

for

~agPt/19LPt,

~B~Pt/~BgPt

and in the sign change of the

quadrupole moment in ~aSPt. Another recent example of the power of OLNO is the study of the deformation of

the

light

Br

isotopes

in

an Oxford-Daresbury-Bonn-ISOLDE

collaboration

where on-line and off-line studies were carried out in the first three laboratories with

sources

strong

prepared

competition

off-line

among

the

at

oblate

ISOLDE shell

and gaps

on-line

at

Daresbury 49.

at N=Z=34,36,

the

The

spherical

390c

'

J.H. Hamilton / New insights into nuclear structure

gaps at 38, 40, and prolate gaps at N=Z=38 leads to shape coexisting structures (see ref. I).

The OLNO measurements of magnetic moments of ~ , ~ s , ~ m B r

with earlier

values

for

~9,~6Br

orbitals and deformations in ground

state

yield unambiguous

involved

deformation

assignments

in these nuclei.

between

~Br~

and

There

~,~Br~2

combined

of the Nilsson

is a similar

shift

,~o just as seen

in

the Kr isotopes 33, however, ~ m B r unexpectedly has only a small oblate deformation. There may be shape coexistence large prolate deformation.

in 7~Br with the low spin ground

This must be checked.

that OLNO studies will be essential far off stability to establish by

in-beam

~s,~SBr

work

(ref.

where

the

Earlier

in measuring

state having

it was emphasized 50

ground state spins of nuclei

the spins of extensive band structures

ground

state

51) were such cases,

spin

was

unknown.

Our

obtained

studies

and this new work now provides

of

the ground

state spin of ~SBr. Finally, and

briefly

off-line

nuclear

interactions. the

B-decay

measurements nuclei.

to probe

of mirror with

of

the such

longitudinal

the

~SO and

and

the

l~F with

of

presence

of using

electromagnetic

the

study

Leuven

of

KOOL

enlarge

the number

of

important,

they open

up

betas

of

both on-

the first OLNO

even more

polarization the

weak

significantly

Perhaps

measurements,

examples

reported

nuclei B-rays

now can be studied.

In

exciting

et al. 52 recently

OLNO measurements

which

two recent

orientation

Severljn anisotroples

facility. nuclei

consider

emitted

by polarized

right-handed

currents

in

nuclear beta decay can be tested, in some cases, with up to an-order-of-magnitude enhancement nuclei 53.

in sensitivity

compared

to current

Such studies of 150 and ~ F

measurements

with unpolarized

are being pursued at Leuven 54.

Rikovska,

Griffith and Stone 55 at Oxford are investigating Time reversal symmetry violation which

is expected

particle neutral

physics kaons,

at a level of 10 -3 . where

it was

predicted

it is of great interest

Their first studies

Since no T violation has been found in

in 56Co involving

from

CP

violation

in the

decay

the detection

of linear

the Y-rays emitted by oriented nuclei were imprecise.

However,

polarization

7.

in 292Ir

improve the reported value 56 of 4(5) x 10 -3 in ~9~Ir.

SEARCH FOR SUPERDEFORMATION IN 186Hg

The discovery has

in

they estimated

they can reach an accuracy of 10 -~ in the 605 keV, MI + E2 transition decay to significantly

of

to search for it in nuclear processes.

sparked

of a band with superdeformation,

much activity

in the A = 190-194

several papers at this conference in these proceedings).

B - 0.6,

mercury

in ~9~Hg (ref. 57)

nuclei

(see papers by Janssens,

as reported

Stephens,

in

and others

J.H. Hamilton / New insights into nuclear structure

391c

We have searched for super deformation in *~6Hg where the calculations of Dudek (see ref.

58)

indicate

such at lower

was studied with 20 Compton at HHIRF 59.

A preliminary

spin.

partial

presented

there.

*S~Gd(36S,4n)*'6Hg

in the close packed ball

level scheme is shown in Fig. 8 (ref. 59)

where the spins are based on DCO ratios. in the data

The reaction

suppressed Ge detectors

Two new striking

A new side

band

featurem are seen

beginning at a 1908.O keV 6 +

level is seen to feed into the 4 + levels of both the known near-spherical ground state

band

0.24)

[see ref.

(6 - 0.15) I].

and well-deformed The moment

of

constant and much larger than that

band with 522.0,

inertia

of

this

new

0 + band band

head

the well-deformed band with AEy

of

(8 -

is remarkably ffi

ET(I

+

2) - E7(I) much smaller and nearly constant, varying between 56-39 keV, compared with

65-68

keV

for

the

well-deformed

ground

band.

The

intensities

depopulating Y-rays are constant (-5%) for the lowest four members. 73300

Fig. 8. Levels in *'6Hg (ref. 59).

(35.1)

7357.4

(26 ° ) ~

6556. I

r-

of

(51.0)

28

r-° 6635.1

24 +

the

This band

-

26'

-

(50.3) (37.5) 5963.8

P"

5817.2

_

22 +

24 ~

_

o (53.4)

(34.6)

~

5115.8 (30.2)

5342.8 20*

~

4775.2 _ _ 2 0 "

,o

4448.9

+

4445.0

(65.9)

I8+

18 o '° 'u~

(50) 39o9,o

(42)

(25.6) 16 °

o ,.D

, ~

3423.0

o

(39)

3812.3 (29.4)

14

'~

2979.0

,2-

(44)

,050,0.

2574.0

,.o ~,o

16"

26,g7

-~

_6. 5g0,

j,

~,080.5

,2.

4"

I~

....

807.9

~ 3s6.7 T

• 4"

6752 ~%>~,~ 620~ v °~° 2: 405.3

'~

12"

J/Z-/---

(67.5)

T

0.0

16 °

~2555 I 0 ÷

1908.0

\%

3827.3

3088.8

2213.0

io\.'676.6

18"

o '~

(126.3)

14 +

(56)

I oo -

~o ""

3470.6

---"

,D

4268,3 (84.3)

o

3201.3

22"

(60.7)

2 0÷

186

8oHglo6

J./-/. Hamilton / New insights into nuclear structure

392c

has

the properties

mercury nuclei.

expected

for

in the calculations of Dudek. 58 Hg nuclei,

but

a superdeformed

band as seen in the heavier

Here, it is much lower in energy than in 19°-~9~Hg as predicted

this

The bEy are not as regular as in the heavier

can be expected

because

in this lower

more competing crossing bands to perturb the system.

regime

there

are

This would be the first

time discrete transitions de-excitlng a superdeformed band have been observed. Three new transitions are established high

spin.

level

has

rotation

The

rotation

a backbend

aligned

above

band

in the two bands which were known to that

begins

at

14 + with the lower member

the

10 + , 2833.3

keV

interpreted as a ~h9/2

aligned

aligned band and the 16 + and above states as a ~(i13/2 )2 rotational 60 band. These bands have a larger and smoothly decreasing bEy.

However,

at 12 + there is a sharp break in bEy in the earlier proposed prolate

band where bEy ~ 65-68 constant,

25-35

keV at the lower spins but is much smaller and nearly

keV from 12 + to (26+).

Such a distinct

change about

12 + in

~e6Hg is not seen in the yrast bands that feed the well-deformed excited bands in la2,1e~Hg (ref. 60). at high spins. multiple

Again, such bEy may indicate a superdeformed structure

Clearly,

structures

these data offer an unusual opportunity

which

are predicted

to occur

to probe the

in this nucleus.

Lifetime

studies of these levels are planned.

8. A NEW GENERATION RECOIL MASS SPECTROMETER FOR HHIRF The first reported in-beam gamma-ray studies in which a recoil mass spectrometer was

used

to mass gate the gamma spectra were carried out only six years ago

at the University of Rochester RMS in a Vanderbilt-Rochester The Daresbury facility began operation soon after. of such devices

to observe new,

in an earlier section.

previously

It is absolutely

collaboration 61.

Some examples of the power

inaccessible nuclei were discussed

clear from studies

at Rochester

and

Daresbury that to identify many of the unknown nuclei still farther off stability it is essential identification

that the gamma spectra be gated by both Z and mass. requires

the RMS

to be capable

of handling

To do Z

the high rigidity

recoil products produced when using heavy ion beams on light targets. Based on our work at Rochester and the work at Daresbury which have dramatically demonstrated unknown

the vastly superior

nuclei

far off

power of using

stability,

inverse reaction

a new generation

has been designed 62 for use with the accelerators Research are

very

kinematic the RMS.

Facility

at Oak

important focussing This

Ridge National

when greatly

extra

studying

increases

intensity

the

low

Inverse

cross-section

intensity

is essential

mass

to identify spectrometer

at the Holifield Heavy Ion

Laboratory.

very

recoil

of

the

reactions

also

products

because

products

through

in recoil-mass-Y-Y

coincidence

J.H. Hamilton / New insights into nuclear structure

experiments

and RM-Z-Y-~

coincidence

experiments.

393c

The key design features

of

our RMS for HHIRF are: a)

to match as well as possible the RMS rigidity to the tandem and cyclotron

beams at HHIRF for all (not just some) inverse reactions; b)

to have a large solid angle to study weak reaction channels;

c)

to be able to position nearly all of the detectors

in the GAMMASPHERE

facility around either the target or the focal plane of the RMS; to have high quality beam rejection

d) from

the

target

or

focal

plane

areas

(1013 ) and to have it well removed

to significantly

lower

the

background when using large Ge detector arrays like GAMMASPHERE, e)

to make

different The

the

system

flexible

so

that

it can

cover

gamma-ray

and;

a broad

range

of

research areas both now and in the future.

improved

design

of

the RMS

for HHIRF

achieves

all

these

goals 62.

Some

additional design features and comparisons with existing and under-construction RMS's are given elsewhere 62. A

completely

When

new

consideration

the GAMMASPHERE

detectors

design

on a frame

with

for

our

RMS

was made with 75 cm radius,

was

the GAMMASPHERE

the order the

of 110

problem

arose

large that

project. volume

Ge

none of the

existing or proposed RMS's could operate at the large image or object distances required

by GAMMASPHERE

significant With led

The

these

to a new

large

without

significant

loss

of KMS

performance

or very

loss in the number of detectors even on building a second frame. new

conditions

generation

in mind,

RMS

that

allows

image and object distances,

primary

modification

front of the original the achromat electrostatic

there

deflector

the

where

spot

momentum

use

of

to our first RMS design

all

inverse

and meets the high rigidity a momentum

RMS design.

is a focus

image of the target quadrupole,

is that

modifications

to

the In

the

between

primary

is reformed. achromat

the

beam

been

quadrupoles

is completely

With a focus

with

has

has

This is shown in the layout in Fig. 9.

formed

achromat

reactions,

needs of HHIRF.

a rigidity

added

and

the first

removed

at 4.5 meters

of 25 MeV/nucleon

solid angle acceptance of 25 msr can be used by itself,

and an

after

the

and

a

too.

The advantage and power of the addition of the achromat are seen by considering a general

example

of

inverse

study the product ~s~Yb. beam and the reaction

products

unscattered

at

totally

so

overlaps

the achromat

that

reactions

at 5 MeV/u:

~SSGd

beam

on

~2C

to

In this case for a Rochester-type RMS, both the primary

the

the products

pass through the first electrostatic focal which

plane are

the then

primary too weak

beam

deflector

significantly

to be studied.

or With

the beam particles are easily stopped while the reaction products

are passed with only a small loss.

J.H. Hamilton / New insights into nuclear structure

394c

J

i/i

RMS

UNISOR

GAMMASPHERE Layout for RMS and GAMMASPHERE at HHIRF.

Fig. 9.

The

physical

spectrometer used with neutron,

distance

element

all but

was

five

of

75

chosen

cm

from

the

to allow

detectors.

This

target

the normal distance

position

to

GAMMASPHERE

also

gives

charged particle, and other Y-ray (BaF 2) detectors,

the

first

frame

to be

good

space

as well.

75 cm with quadrupoles that have an aperture with a diameter of 20cm,

for

Even at the RMS

still has a large solid angle of 15 msr. The major features of the total RMS are: of up to 15 msr;

a) a large acceptance solid angle

b) an excellent primary beam rejection (>10 ~3 in most cases)

at 0°; and c) with software correction excellent mass resolution (m/Am)-1200 (FWHM) at 3 msr and 1000 at 12 msr (vastly superior to the 300 of current RMS's). In

Fig.

10

is

of

range

10 to 15 MeV/q

of

reactions.

target

shown

function

mass

A comparison

a plot for

of

electrical

different

is needed

rigidity

projectiles.

to reach

of the intensities

most

of

(in units

Clearly

an

E/q) E/q

the interesting

in

as

a

the

inverse

for recoil-~-Y-coincidences

in a

J.H. Hamilton / New insights into nuclear structure

normal

and

inverse

(heavy

projectile)

ratios of the recoil-X-Y-coincidences and

39

show

in

the

the

two

power

cases

shown

of the HHIRF

for

reaction

is

395c

given

in Table

2.

The

for inverse to normal reactions are 6.2 20 Compton

facility

compared

suppressed to

Ge detectors.

the Daresbury

and

To

Argonne

RMS's for normal reactions with the space each has available that al}ows 20 Ge detectors Argonne

at 12 cm at ORNL,

(however,

their space), coincidences days,

of

the

14 available

cm at Daresbury,

in a normal

reaction

and

14

at

16

cm at

probably no more than half can fit in

the recoil-Y-Y-coincidence

58 days and 31 days,

our RMS's,

30 at 19

counting

4~Ca beam on

respectively.

time to collect '22Sn to produce

By using an inverse

I0 s triple *62Yb

is 7

reaction

with

the 7 days would be a reduced factor of six in this case and more

in other cases! Another

unique

area opened

up by our high rigidity RMS is deep inelastic

collisions to produce neutron-rich nuclei far off stability. As documented in 63 the GAMMASPHERE proposal , such reactions can populate neutron-rich nuclei with spins up to 7 0 ~ . region

with

These neutron-rich nuclei comprise an essentially unexplored

quite

different

shell

structure

and

shell

gaps

and

neutron-to-

proton ratios that have long tantalized both theorists and experimentalists. Finally, sufficient

by using inverse reactions one can produce radioactive beams with

energy

to do Coulomb

excitation.

The

E/A

of

the

secondary

beam

should be about 2-5 MeV/A to produce compound nuclear reactions for structure studies.

For the normal reactions discussed in Table 2, the E/A of the resulting

compound nuclei are <0.5 MeV/A, well below that required for nuclear reactions

Table 2. A comparison of normal and inverse kinematics for the reactions of 122Sn + "~Ca and *S~Gd + 32S at projectile energies of 5-5.2 MeV/u are given. Conditions assumed for these calculations were: beam current = 2 pnA; target thickness = 500 wg/cm2; a = Ib; Y-ray multiplicity = 20; and RMS solid angle of 8 msr. '22Sn + ~ C a Normal Inverse

*5~Gd + 32S Normal Inverse

E/Q a (MeV/q)

2.5

10.2

1.7

12.8

E/A a (MeV/nucleon)

0.4

2.7

0.2

3.6

Recoil-Y/sec b

5,600

32,400

1,300

51,000

Recoil-Y-Y/sec

Ir700

I0r500

440

17~000

a)The values are for the compound nucleus. b)The Y-ray detection efficiency is calculated for 20 Compton suppressed Ge detectors located 12cm from the target and Ey = I MeV.

.

396c

J.H. Hamilton / New insights into nuclear structure

~ : 7 MeV/u 16

.\ o "-

bJ

_

8

Ato~ 0 Fig.

10.

while

the

I

I

I

40

80

t20

~60

Electrical rigidity vs. target A for different projectiles.

inverse

reactions

have

E/A's

of

2.7

and

3.6 MeV/A.

The

results

shown below demonstrate the many radioactive beam experiments that will become possible and that could not be done without the larger E/q of our RMS and the use of inverse reactions. In the first B(E2)s

example,

of low-lying

bombarding

~4Ca with

consider Coulomb

states

of nuclei

5.0 MeV/A

~22Sn

far

excitation from

the

as a means to determine valley

of stability.

(Table I), the secondary

beam of

By ~6°Yb

with E/A = 2.7 MeV/A on 160 would give 200 and 18 c/hr in the 2 ~ 0 and 4 ÷ 2 full energy peaks, respectively,

to establish in a few hours their B(E2)s.

A second example illustrates how a secondary radioactive can be used to produce and to study high-spin

further from beta stability than presently possible. with 700-MeV

~6Sn.

For a calculated

12~Ba of 220 mb, a secondary

2p2n

detected

in

coincidence

rate is 0.3/sec.

with

section

For a secondary

cross-section

gamma-ray energy of 1.0 MeV,

cross

For example, for

bombard

the 4n channel

~2C to

124Ba beam is produced with 1.2 x 10 -4 pnA at 5.0

MeV/A, and E/Q of 14.7 MeV/q. calculated

beam from our RMS

states or the decay of nuclei

leading

to

target of 1.0 mg/cm 2 ~2C, the

~a2Nd

is

130 mb.

For

the rate of gamma rays in the full recoils

is 1.7/sec

This rate could establish

and

an

average

energy peaks

the

recoil-gamma-gamma

the placement

of many gamma-rays

into a level scheme in a few days. The calculations presented for our RMS were for the currently available 20 Ge detectors

in the close packed array at HHIRF.

The use of GAMMASPHERE

in

conjunction with our RMS will give major improvements for all research, especially for recoil-Y-Y and even higher fold studies.

Indeed GAMMASPHERE and our RMS,

ZH. Hamilton / New insights into nuclear structure

397c

will provide the most powerful facility in the world for studying nuclei under extreme conditions such as high spin, high temperature, and far from stability.

ACKNOWLEDGEMENTS I would like to thank my colleagues for permission to use our data prior to publication and for their help in preparing this paper.

This work is supported

by the U.S. Dept. of Energy through a grant to Vanderbilt, DE-FGO5-88ER40407, and UNISOR DE-ACO5-76OROO033.

I would like to thank Prof. W. Grelner for his

hospitality while at the University of Frankfurt.

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