THE SPIN SPECTROMETER AT THE HOLIFIELD HEAVY-ION RESEARCH FACILITY AND SOME PLANNED EXPERIMENTS

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THE SPIN SPECTROMETER AT THE HOLIFIELD

HEAVY-ION RESEARCH FACILITY AND SOME

PLANNED EXPERIMENTS

D. Sarantites, M. Jääskeläinen, J. Hood, R. Woodward, J. Barker, D. Hensley,

M. Halbert, Y. Chan

To cite this version:

D. Sarantites, M. Jääskeläinen, J. Hood, R. Woodward, J. Barker, et al..

THE SPIN

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JOURNAL DE PHYSIQUE CoZZoque CIO, suppZdment au n012, Tome 41, dgcembre 1980, page C10-269

THE S P I N SPECTROMETER AT THE HOLIF-IECD HEAVY-ION RESEARCH FAClhJTY AND SOME PLANNED EXPERIMENTS

i

**

D.G. Saranti&s, M. JI?iskel?i&gen, J.T. Hood, 8 , Woodward, J.H. Barker

,

D.C. Hensley

,

M.L. Halbert and Y.D. Chan

.

.Department o f Chemistry, Washington U n i v e r s i t y , S t . Louis, MO 63130,U.S.A. ..Department o f P h y s i c s , St. L o u i s U n i v e r s i t y , S t . Louis, MO 631 03, U.S.A.

Oak Ridge NationaZ Laboratory, Oak Ridge, Tennessee 67830, U. S. A.

Abstract.- The 4n multidetector y-ray spectrometer at the Holifield Heavy-ion Research Facility (WH

IRF) is described in some detail. The following important features of this spectrometer are discussed: (a) the geometric arrangement, (b) the actual performance of the individual detector elements, (c) the associated electronics and data acquisition system

,

and (d) the response of the system to input y- cascades including the effect of crystal-to-crystal scattering and the response to neutrons. The first few experiments to be performed are briefly described.

1. INTRODUCTION

A new type of y-ray spectrometer for investi- gations of the mechanisms of heavy-ion induced reactions and of the structure of nuclei at high angular momentum was constructed at Washington University and installed at the HHIRF at the Oak Ridge National Laboratory. It consists of 72 NaI separate detector elements closely packed in a 4n

arrangement. This systemrecords on an event-by- event basis for each detector that fires its identifying-life., the associated pulse height, the time of triggering, and the pulse width. From these data the following information can be obtained on an event-by-event basis: (a) the coincidence fold k and thus the associated y-ray multiplicity

M

(b) the associated total pluse height

Y'

H and thus the total y-ray energy E*, (c) the associated pulse height spectrum {hiIitl,.

,

and thus the energy spectrum associated with a given (E*,M )

Y d) the identification of pulses due to neutrons and thus the neutron multiplicity, (e) the identifica-

tion of coincidence summing due to neutrons and y- rays, thus providing clean response to y-rays, (f) the time relationships of the various y-rays in each cascade, and (g) the angular correlations of the y-rays in each cascade. Additional valuable information can be obtained from second and higher

order correlations, e.g, E -E H-k-E H-k-E -E Y Y' Y ' Y Y' E -E -E H-k-t, H-k-t-E etc.

Y Y Y' Y'

For a given y-ray cascade from fusion-like heavy-ion reactions a linear relationship exists between M and the initial angular momentum J from

Y

where the y-cascade starts. Therefore, the event- by-event recording of the fold k and thus of M

Y pennits one to associate with each event a value of the transferred angular momentum J. This is the basis for choosing the name Spin Spectrometer for this instrument.

It should be understood that in most of the experiments with the Spin Spectrometer one or more additional selective detectors canbe used. These include Ge(Li) counters, AExE heavy-ion or light

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C10-270 JOURNAL DE PHYSIQUE

p a r t i c l e t e l e s c o p e s , n e u t r o n time-of-flight d e t e c t o r s , e t c .

The t h e o r y and g e n e r a l c h a r a c t e r i s t i c s of 4s y-ray m u l t i d e t e c t o r systems f o r i n v e s t i g a t i o n s of high-spin phenomena have been p r e s e n t e d i n consid- e r a b l e d e t a i l i n Ref. 1. Therefore only t h e char-

a c t e r i s t i c s of t h e s p e c t r o m e t e r a t t h e HHIRF w i l l

b e d i s c u s s e d h e r e .

2. GEOMETRY OF THE SPECTROmTER

The 72 NaI d e t e c t o r elements a r e c l o s e l y packed a s a hollow sphere w i t h a n i n n e r diameter of 35.6 cm and a s h e l l t h i c k n e s s of 17.8 cm. Each d e t e c t o r i s a t a p e r e d prism t h a t can b e removed

r a d i a l l y from t h e s p h e r e . The c r o s s s e c t i o n of twelve of t h e d e t e c t o r s i s a r e g u l a r pentagon.

Each pentagonal d e t e c t o r i s surrounded by f i v e hex-

d i z e d aluminum) connect each h a l f of t h e s h e l l t o each h a l f of t h e frame. I n t u r n each h a l f of t h e frame r e s t s on a 25 mm t h i c k 90 cm x 1.80 cm ano-

d i z e d p l a t e t h a t can t r a v e l on t r a c k s a d i s t a n c e of 60 cm p e r p e n d i c u l a r t o t h e beam d i r e c t i o n t o g i v e a c c e s s t o t h e enclosed s c a t t e r i n g chamber. The a n g l e between t h e axes of a d j a c e n t d e t e c t o r s i s 25O, and t h e t o t a l e f f e c t i v e s o l i d a n g l e i s 96.5% of 47~. The p r e s e n t s p h e r i c a l s c a t t e r i n g chamber has an i n n e r d i a m e t e r o f 3 2 cm and p r o v i d e s one p o r t f o r t h e t a r g e t h o l d e r , t h r e e p o r t s f o r Ge(Li) d e t e c t o r s , a n d mounts f o r p a r t i c l e d e t e c t o r s between 5' and 175O t o t h e beam d i r e c t i o n i n one o r two planes. Fig. 2 shows t h e s c a t t e r i n g chamber

a g o n a l d e t e c t o r s . A l l d e t e c t o r s subtend t h e same

s o l i d a n g l e . A c l u s t e r of 10 d e t e c t o r s i s s e e n i n

Fig. 1, Tfie shape of t h e polygons i n t h e i n n e r

Fig. 1. A c l u s t e r of 10 d e t e c t o r s showing t h e shape of t h e d e t e c t o r s and t h e i r c l o s e packing.

s p h e r e i s c l e a r l y seen. I n t h e complete polyhedron

t h e r e a r e 120 t h r e e - f o l d v e r t i c e s . The s p h e r i c a l s h e l l i s h e l d t o g e t h e r by 120 clamps each b o l t e d

t o t h r e e a d j a c e n t d e t e c t o r s . The s u p p o r t i n g Fig. 2. Closeup view of t h e s c a t t e r i n g c h ~ d e r w i t h one h a l f of t h e spectrometer i n i t s frame i s a s p l i t r e g u l a r dodecahedron w i t h edges z e a r e s t normal p o s i t i o n .

c o n s i s t i n g of 25.4 mm diameter,anodized aluminum

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surrounded by one of the halves of the spectrom-

meter. Fig. 3 shows the complete spectrometer

Fig. 3 . Complete view of the Spin Spectrometer

without the cables and the beam line.

without the cables and the beam line, and Fig. 4

chamber in mid position.

3. PREFORPfiNCE OF THE DETECTORS

Each detector consists of polyscin material [2] integrally coupled via a curved glass window to an RCA 4522 photomultiplier tube. Presently, resist- ive voltage dividers are employed with a chain cur-

rent of %1.8 mA at 2,000 V. The average operating

voltage for optimal overall performance is -1900 V.

The average timing resolution is 2.1 ns FWHM

and 4.7 ns FWTM when measured with a constant frac- tion timing discriminator (CFTD) with the threshold

set at 80 keV using a 6 0 ~ o source. The detectors

with the best timing characteristics were placed

at the forward angles. This is shown in Fig. 5

4 FWTM

e e

...*

e e *

FWHM

Fig. 5. Average timing resolution over 5 detec-

tors -that form an angle 9 with,respect.to

the beam. The detectors with the best timing resolution were placed at forward angles.

Fig. 4. Closeup view of the spectrometer open to where the average timing over five equivalknt de-

give access to the enclosed scattering

chamber. tectors is plotted vs. the angle with respect to

shows the complete spectrometer with the two halves the beam.

separated to a distance of 120 cmwith the scatter- The average energy resolution AE / E is 8.6%

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c10-272 JOURNAL DE PHYSIQUE

and 6.3% f o r 662 keV and 1332 keV, r e s p e c t i v e l y , The gain s h i f t with counting r a t e of a t y p i c a l

The average energy r e s o l u t i o n f o r each group of d e t e c t o r i s shown i n Fig. 7 f o r s e v e r a l o p e r a t i n g

f i v e e q u i v a l e n t d e t e c t o r s i n t h e spectrometer i s v o l t a g e s . For t h e lower v o l t a g e s t h e r e is l i t t l e

shown i n Fig. 6 a s a function of the angle 8 f o r

t h e 662 keV and 1332 keV l i n e s . For counting r a t e s

.g. 6. Average energy r e s o l u t i o n f o r each group of f i v e e q u i v a l e n t d e t e c t o r s forming an a n g l e 0 t o t h e beam.

up t o %50,000 c / s l e s s than 0.1% l o s s i n r e s o l u t i o n

a t 662 keV was observed. I n c r e a s i n g t h e counting

r a t e t o 75,000 c / s , however, r e s u l t e d i n d e c r e a s e

i n energy r e s o l u t i o n from 8.5% t o 9.8%.

The average p u l s e h e i g h t v a r i a t i o n ( u n i f o m i -

t y ) AE /E along t h e d e t e c t o r l e n g t h i s 3% when

Y Y

measured with a c o l l i m a t e d 1 3 7 ~ s source. The

d e t e c t o r response was found t o be l i n e a r f o r t h e

range of 0-2.5 MeV employed i n t h e t e s t s . Measure- ments of t h e g a i n s t a b i l i t y over a period of sev-

e r a l weeks d i d n o t show any s i g n i f i c a n t s h i f t .

For t h e temperature range 18O-24O C t h e g a i n a t

662 keV was found t o decrease by 0.3% per "C. This

s h i f t was due t o t h e combined e f f e c t of t h e de-

t e c t o r , phototube, and v o l t a g e supply.

H.V. 4 -

0 20 40 60 80 100

Counting Rote (thousaMs per second)

Fig. 7. P e r c e n t peak p o s i t i o n s h i f t f o r 1332 keV w i t h counting r a t e measured a t t h e i n d i - c a t e d o p e r a t i n g v o l t a g e s .

o r no s h i f t up t o %50,000 c / s and f o r h i g h e r v o l t -

ages a l i n e a r s h i f t of ' ~ 0 . 7 % p e r 10,000 c / s i s observed. With t h e p r e s e n t v o l t a g e d i v i d e r s , t h e

l i m i t of 1.0% i n gain s h i f t f o r 1332 kev i s found

t o be i n t h e range 30,000-100,000 c / s when t h e

h i g h e s t a c c e p t a b l e v o l t a g e s a r e used.

4. ELECTRONICS AND DATA ACQUISITION SYSTEM

A schematic diagram of t h e e l e c t r o n i c s arrange-

ment i s shown i n Fig. 8. The photomultiplier

anode s i g n a l i s f i r s t amplified by a f a s t x10 pre-

a m p l i f i e r . One of t h e pre-amplifier o u t p u t s i s

shaped t o a n approximately t r i a n g u l a r shape (FWHM

of %lo0 ns, FWTM of '1200 n s ) , delayed by 300 n s ,

and then fed i n t o a charge i n t e g r a t i n g ADC. The

second o u t p u t is used t o t r i g g e r a f r o n t edge CFTD and a second r e a r edge CFTD, which i s s e n s i -

t i v e t o a second p u l s e a r r i v i n g i n t h e range 48

n s t o 240 n s a f t e r t h e f i r s t p u l s e . The time

d i f f e r e n c e between t h e two d i s c r i m i n a t o r s pro-

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Fig. 8. Schematic diagram of t h e e l e c t r o n i c s ( s e e t e x t ) .

i s a measure of t h e time d i f f e r e n c e between t h e two

p u l s e s a r r i v i n g a t t h e same d e t e c t o r . This time d i f f e r e n c e i s d i g i t i z e d v i a a charge i n t e g r a t i n g

ADC. A f t e r an a d j u s t a b l e d e l a y (0-150 n s ) , t h e f r o n t edge CFTD output i s used t o s t o p a time-to-

d i g i t a l c o n v e r t e r (TDC) and s e t t h e corresponding b i t i n a s t r o b e d g a t e d l a t c h . A 72-channel m u l t i -

p l i c i t y summer, capable of p r o v i d i n g t h e c o i n c i - dence f o l d a s a f a s t analog s i g n a 1 , i s a l s o generat- ed. I n a d d i t i o n t o t h e 72 b i t s f o r NaI, 16 b i t s f o r o t h e r a s s o c i a t e d d e t e c t o r s can be s e t i n t h e g a t e d l a t c h . The two s e t s of 72 ADC's and one s e t of T D C ' s a r e coupled t o a CAMAC system.

The i n f o r m a t i o n management and d a t a a ' c q u i s i t i o n o c c u r s v i a t h e EVENT HANDLER [ 3 ] , a prograrmnable CAMAC based c o n t r o l l e r , which is a d e v i c e t o s e l e c t

e v e n t s t h a t meet t h e u s e r ' s c r i t e r i a , p l a c e them i n a s u i t a b l e format, and t r a n s m i t them t o t h e on-line computer. The computer program i n t u r n may apply f u r t h e r s e l e c t i o n , b u f f e r i n g and formating b e f o r e t h e e v e n t s a r e recorded permanently. It may a l s o

provide f o r o n - l i n e sampling of t h e d a t a . Data from o t h e r s e l e c t i v e d e t e c t o r s may be processed by a d d i t i o n a l C A W modules and i n t e r f a c e v i a t h e same EVENT HANDLER. The time r e q u i r e d f o r d i g i -

t i z i n g t h e s i g n a l s and t r a n s m i t t i n g them t o t h e computer l i m i t s t h e d a t a a c q u i s i t i o n t o ~5,000

e v e n t s / s .

The o p e r a t i n g c h a r a c t e r i s t i c s of a l l NaI de- t e c t o r s a r e s t o r e d i n a d i s k f i l e . The s e t u p and d a t a a c q u i s i t i o n programs have a c c e s s t o t h i s f i l e t o o b t a i n whatever i n f o r m a t i o n they r e q u i r e on t h e d e t e c t o r parameters. Changes may b e i n s e r t e d i n t o t h i s f i l e by t h e s e t u p programs. P r i n t o u t s o f t h e d i s k f i l e a r e made on r e q u e s t o r when changes a r e completed. Major d i s c r e p a n c i e s between t h e c u r r e n t f i l e and e a r l i e r ones a r e a u t o m a t i c a l l y brought t o t h e a t t e n t i o n of t h e u s e r a s a warning of p o s s i b l e

d e t e c t o r malfunction.

The phototube h i g h v o l t a g e s a r e f u r n i s h e d by computer c o n t r o l l e d power s u p p l i e s . The high v o l t - ages a r e i n i t i a l l y s e t by r e f e r e n c e t o t h e d i s k f i l e .

A l l d e t e c t o r s a r e o p e r a t e d w i t h t h e same pulse-

h e i g h t g a i n and t h e same time s c a l e i n o r d e r t o minimize t h e event p r o c e s s i n g time. The uniform- g a i n requirement i s met by a u t o m a t i c adjustment of

t h e phototube v o l t a g e on t h e b a s i s of s p e c t r a from a y-ray source. The g a i n - s e t t i n g program c y c l e s through a l l t h e d e t e c t o r s i n s u c c e s s i o n , accumulat- i n g d a t a f o r a few seconds p e r d e t e c t o r . A peak-

s e a r c h r o u t i n e then determines t h e l o c a t i o n s of t h e s e l e c t e d l i n e s and t h e high v o l t a g e i s a d j u s t e d , i f

r e q u i r e d , t o g i v e t h e s t a n d a r d g a i n .

The time s c a l e i s made uniform f o r each detec-

t o r by manual adjustment of each TDC urrit. The t i m e

s c a l e s and timing r e s o l u t i o n a r e checked automati- c a l l y by a n o t h e r program u s i n g d a t a from a s o u r c e w i t h c o i n c i d e n t y-rays. A t p r e s e n t t h e CFTD t h r e s h - o l d adjustments a r e manual, b u t p r o v i s i o n f o r com- p u t e r c o n t r o l has been made.

5. THE RESPONSE OF THE SYSTEM

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events consits of recording a collection of pulse- heights

.

,k when the input spectrum is

{ ~ ~ l ~ = ~ ,

.

, ,M

.

The maximum information is con-

Y

tained in the probability P (k,h l...$;E1...E _N ) that detector 1 records pulse-height hl, etc., when only k of hl,

...,

h are above threshold. Express-

N

ions for this and other related probabilities have been derived in Ref. [I]. This general probability can be constructed from calibrations. It is the reverse problem, however, that is of considerable interest. This involves the best characterization Of {Ei'i=l,.

. .

,M from a collection of measurements thro11g11 the connection given by

Restricting ourselves here to fusion-like reactions we note that nuclei can only be prepared with a population of the entry states [4] characteristic of the reaction system employed. Clearly, we would like to characterize the decay modes of each entry state (E*,J). The decay (E*,J) + (0,J ) to the

g.s

ground state results in a y-ray spectrum n(E)dE =

):

n(Ei)dE, where the sum is over all the possible i

decay paths. The important characteristic of these pathways is that they have a common total

*

energy E

,

but different decay spectra that pro- duce in turn a distribution in y-ray multiplicity.

*

Let this distribution be represented by P (E J -+

N

*

E My). This mapping is shown schematically in Figs.. 9a and b. The various lines in Fig. 9a rep- resent different pathways each with a given M

Y value and an associated decay spectrum. Upon measurement, each point in the (E*,M ) space maps

Y

into a domain in (H,k) (Fig. 9cJ represented by

*

the distribution P (E M + Hk)

,

where the total

N Y

pulse height is H = lihi. The latter distribution represents one of the possible response functions of the apparatus, and it can be well characterized by its first few moments

[I].

Calculations based

Fig. g, (a) Schematic representation of the pathways for the decay (E*J) + (O,JgsS]. (b) Representation of the decays shown in (a) into the (E*,My) space. (c) Response of the spectrometer to the distribution from (b). The projections on the My or the H

and k axes are shown schematically. on initial estimates of detection efficiencies for

*

this spectrometer show that for given E the mean <H> is a weak function of M and that for given M

Y Y

*

the mean <H> is essentially proportional to E

.

*

Similarly, for given E we find [see also Ref. 11 that <k> is essentially a linear function of M and

Y that for given M the values of <k> is insensitive

Y

*

to changes in E

.

Analytic expressions for the response

*

PN(E My + Hk) and for its moments have been given in Ref. 1 and comparisons for spectrometers with different numbers of detectors have been made. The main characteristics of the response

*

PN(E My + Hk) are shown in Fig. 10, where the re-

*

sponse was evaluated via eq. 49 of Ref. 1, for E = 30 MeV, and M = 30. In general, this bivariate

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*

Fig. 10. Density plot of the response PN(E

5

+

Hk) using the expression 49 in Ref. 1 for E* = 30 MeV and I-$ = 30. The quantities AH and Ak indicate the FWHM values of the projections on the H and k axes, respec- tively.

distribution will be narrower than that shown schematically in Fig. 9c1 because the latter incor- porates the width of the M distribution from Fig.

Y

9b. However, we note that in the approximation of eq. 49 in Ref. 1 the higher order correlation terms

2 2

such as <H k>, <Hk >, etc., have been ignored.

This may cause the width in k to vary for different values of H, although the overall widths oH and ok for the projections remain unaffected.

In many experiments the population distributions Q(H,k) will be determined. The desired population R(E*,M ) is then obtained as a solution to

Y

the equations

I

R(E*M~)

.

pN(E*MY + Hk) = Q(H,k). E*,M

We have employed iterative procedures for solving this system of equations.

The one-to-one correspondence shown in Figs.

*

9a,b,c between (E

,

J) (E*,s ) 6 (ill>, <k>) is Y

an importafit feature of this spectrometer. It per- mits for each experiment selection of a class of events associated with a pair of values (H,k) that corresponds to a region in (Ef,MY) and thus in

(E*,J) space. The selected distribution in (E*,J)

has a similar shape with that shown in Fig. 9 C . This particular type of selection, although not unique, is expected to be used most commonly due to its simplicity. The ability of the Spin Spectro- meter to select events associated with a

rather small region of the available (E*,J) space

opens many new possibilities for study of high spin phenomena both from the point of view of reaction mechanisms and of nuclear structure in a large number of experiments as outlined briefly in the introduction.

The actual response function P~(E*M -* Hk) of

Y

the spectrometer can be constructed from suitable calibr'ations with radioactive sources. There are, however, some important physical effects that determine in general the response of the spectrometer to a given input via the detection statistics. These were discussed in detail in Ref. 1. Briefly, these are: (a) Incomplete detection due to the fact that the total detection efficiency OT is less than 1. (b) Crystal-to-crystal scattering due to lack of shielding between detector elements which is required to minimize the effect of (a) above. This causes more than one detector to fire due to a single y-ray. (c) Coincidence summing that origi- nates from the finite probability that more than one y-ray from a given event may interact with the same detector and result in a single response. (d) Indistinguishable pulses that arise from the fact that neutrons of high energy protons may trigger some detector elements. This is particularly true in the forward detectors.

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JOURNAL DE PHYSIQUE

of the multiplicity to the fold k and the multiplicity resolution [l]. As part of the procedure to determine these relationships more accurately we have measured the crystal-to-crystal scattering for the entire spectrometer using monoenergetic sources of 137~s (662 keV) and 54Mn (834 keV).

Let R' be the triggering efficiency for the i th i

detector for single response and 51" the trigger- i j

ing efficiency for double response between the ij detector pair. We can define the total single and double triggering efficiencies by

a'

=

1

R; and

6111 =

1

Oij. If the ith detector is~selected

ij,ifj

to be a pentagonal one, then there are 5 equivalent j detectors that form the same angle 8 with respect to the ith dgtector. The scattering correlation function is then given by

F"(B) =

1

a;jl(n'+a"), i j

,

i+j

where for a given detector i the sum over j is limited to the five detectors with the same 8. The total double scattering is then I?" =

1

F"(8). Fig.

e

11 shows the scattering correlation function F1'(B) measured for 662 keV (open circles) and 834 keV

(closed circles). The total scattering factors F" are 0.19 and 0.24 for 662 and 834 keV, respectjve- ly. It is seen that for 6

2

60" the scattering is essentially constant.

The effect of response to neutrons on the first two moments of the response P*(E*M

*

Hk) was dis-

Y

cussed in considerable detail in Ref. 1. The large cross section for inelastic scattering of neutrons on iodine when combined with the large thickness of

NaI may cause significant interfernce due to re-

sponse to neutrons. Thus pulses due to neutrons must be identified by their time-of-flight (TOF). Fig. 12 gives the calculated TOF,Ah,as a function of neutron energy En for three distances of 18, 27 and36 cm corresponding to the front, middle, and back of each NaI detector in this spectrometer.

Fig. 11. Scattering correlation function F"(8) for groups of five equivalent detectors. Re- sults for two incident y-ray energies are given.

O O R

Neutron rejection

by time of flight

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Superimposed is shown a neutron spectrum corres- _{the y-ray and the neutron. Such responses to in-} ponding to a temperature of 3 MeV. The horizontal

-

_{dividual detectors can be identified from their} line corresponds to 5 ns. With the typical timing pulse shape information. Corrections to the pulse

resolution of 4 . 5 ns FWTM [Fig. 51 identification height can then be applied.

of all pulses from neutrons with En

5

7, 17, or 30 6. THE FIRST FEW EXPERIMENTS WITH THE SPIN SPECTROMETER

MeV for the front, middle,and back of the detectors

At present a number of necessary tests are be- can be made by their time-of-flight. This ensures

ing made to establish the proper performance of the that ~ 9 5 % of the neutrons can be thus identified.

Fig. 13 shows a density map of the TOF spectrum from a PuBe source measured with one of the detectors at the normal front distance of 17.8

Fig. 13. Density map of a TOF spectrum from a PuBe source at distance of 17.8 cm recorded with one of the NaI detectors. The start was provided by any y-ray striking an NE-213 scintillator in which neutron pulses were rejected by pulse shape discrimination.

cm from the source. The y-ray pulsesinan NE-213 scintillator were used to provide the start. The

( 4 . 4 3 MeV y-rays from 12c are clearly seen. Com-

plete identification of the Q4.7 MeV neutrons is also visible.

It should be mentioned here that significant probability exists for coincidence summing between y-rays and neutrons. In this case the TOF information will indicate that the event is a y-ray, while the associated pulse height would be the sum from

instrument. These include measurements of the background radiation on site with and without beam through the apparatus, counting rate stability with the beam on target, and response to neutrons from heavy--;.on react ions. In para.lle1, cnlibra- tions of the entire spectrometer for the determi- nation of the response to single and to multiple y-rays are being made. Below we describe the es- sential features of the first few experiments that will be carried out by several groups.

A) Population of Entry States in Fusion Reactions

At first we plan to use a Ge(Li) detector operated in coincidence with the spectrometer using previously investigated reaction systems. ana- lyzing the Ge(Li) spectra as a function of (H,k) the population of the entry states to various (H1,xn) channels will be determined. The detailed feeding patterns into the ground bands and several side bands in collective and non-collective nuclei will be examined under suitable (H,k) selection. We hope to learn to what extent one can select nuclei populated in different regions of the

*

(E ,J) space and how the intensities of the ob-

*

servable discrete transitions vary with E and J.

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selected NaI detector planes.

We plan to investigate the number of yrast and statistical y-rays as a function of selected excitation energy and angular momentum. This can be done using external large NaI detectors operated in coincidence with the Spin Spectrometer. However, use of the spectral information of the detectors 9n the spectrometer will be made to extract the same information with very good statistics. The large efficiency under -these conditions will permit us to select exit channels in addition to regions in the

*

(E ,J) space.

B) Moments-of-inertia at high spin

Knowledge ofthremQmr?nt-of-inertia is important

for the understanding of nuclear defontation, loss of pairing correlation, and the single particle contribution in creating the angular momentum states for high spins.

There are two methods that will be used to

obtain the effective moment-of-inertia as a function of spin.

The first one involves recording and unfolding the spectrum of the y-continuum with a large NaI detector external to the spectrometer as a func-

tion of (H,k). Comparisons of the structure of

the quadrupole bump from progressively higher initial spin states (suitable H,k choice) will permit one to deduce the effective moment-of-inertia as a function of spin.

The second approach involves the proper un-

folding of spectra from the detectors in the spectrometer. This method provides %500 times more statistics compared to that with an external de-

tector; This will make for selection of exit

channels with the Ge(Li) detector possible, in ad-

dition to the (H,k) selection. This method will provide data at the Ge(Li) singles rate.

In addition, the collective moment-of-inertia

associated with intra-band cascades that de-excite the same entry states can also be measured employ-

ing the E -E correlation technique 151. The sig-

Y Y

nificant advantages of carrying our such measurements with the Spin Spectrometer is to sort the data according to (H,k) and thus control the length of the y-cascades to be studied. Select- ion of the exit channel by a Ge(Li) detector in this type of measurement appears also possible. Finally, triple correlations E -E -E will be in-

Y Y Y

vestigated to learn more about the nature and number of transitions within the collective bands.

C) Particle emission as probe of fusion-like

reactions

The first experiments will employ equilibrium fusion and will involve detection of light charged particles in coincidence with evaporation residues observed with particle counters. We plan (a) to investigate the effect of angular-momentum and total y-energy selection on the energy spectra and the angular correlations of the emitted charged particles. The particle correlations can provide effective moments of inertia (spin cut-off factors) at high excitation and spin; (b) to determine the entry state population following a-particle evaporation; (c) to determine for comparison with (a) the moments-of-inertia during y-decay from unfolding of associated y-ray spectra of NaI detectors in the spectrometer.

We further plan to select from the same

measurements aligned nuclei populated in narrow J

distributions. This is accomplished by utilizing the stretched quadrupole nature of the emitted y- radiation and requiring a high order coincidence

with up to 14 Nal detectors in a plane. The nu-

clear spins are thus selected to be aligned per- pendicular to the beam-containing a ring of NaI

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be made to select only narrow ranges of 3 values. Now, when a-particles associated with evaporation residues are detected in-plane with a ring of NaI counters their angular correlation becomes iso- topic. In contrast, the out-of-plane a-correlations become more anisotropic [6].

The high efficiency of the Spin Spectrometer can provide, for example, good alignment of spins in the range 60

+

7 h. Under such conditions the angular correlations, the particle energies [7] and the particle yields

[a]

may be sensitive to the shape of the emitting system 193. This may

provide a new method for exploring the shapes of nuclei with high angular momenta. These possii bilities a 1be explored in detail.

D) Incomplete fusion reactions

Recently it has been shown that incomplete fusion 1101 of 152 MeV 160 with lS4sm is associated with peripheral collisions. These processes also appear to occur [11] only in a narrow band of

%7 5 units in the entrance channel. Such processes,highly localized in

should

lead to drasti- cally different populations of entry states following particle emission, when compared to equi- libiwn complete fusion. A careful mapping of the entry states following incomplete fusion reactions as a function of type, angle and energy of the forward emittedchargedparticles will provide the necessary data for a detailed understanding of the factors governing the onset of these processes.

In addition,using stretched-quadrupole correlation alignment techniques, we hope to determine the degree of spin alignment introduced in incomplete fusion. If significant alignment is introduced by this process itself, then combining it with the correlation alignment mentioned earlier may turn out to be a better approach in answering

the question about nuclear shapes at very high excitation and spin,

E) Non-equilibrium neutrons from fusion reactions

Various models have been proposed to account for fast forward neutrons observed [12] in heavy- ion fusion reactions. For example, incomplete fusion is highly peripheral, while PEP emission 1131 (Fermi jets) is expected to be more central and cover a broader range of lower angular momenta. Conventional multiplicity arrangments cannot easily provide the associated y-ray multiplicity [12] with the fast component of neutrons because of limited statistics. With the use of the Spin Spectrometer measurements of the y-ray multiplicity associated with emission of neutrons of various energies and angles should be easier. Such results should provide a basis for selecting among the models and should stimulate theoretical work on the angular momentum dependence.

In addition to the multiplicity information,use of the Spin Spectrometer would provide valuable data on the excitation energies of the entry states following non-equilibium emission of neutrons.

F) Role of angular momentum in deeply inelas-

tic collisions

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JOURNAL DE PHYSIQUE

(d) What are the differences or similarities of the population of entry states following DIC and fusion-like reactions? Are the products formed at all the available J-values or are only low J values populated? (e) What is the degree of spin alignment in the final fragments? (f) Is it pos- sibleto determine separately the number and characteristics of the y-rays from the target-like and projectile-like fragments?

Such questions will be addressed first with simple measurments involving a few heavy-ion AExE telescopes operated in coincidence with the Spin Spectrometer. Future experiments will incorporate simultaneous detection of emitted light charged particles and neutrons.

The excellent cooperation of the personnel at the Washington University Cyclotron Machine Shop is appreciated.

REFERENCES

*Work supported in part by the U.S. Department of Energy.under contract No. EY-76-S-02-4052.

**

Operated for the U.S. Department of Energy by Union Carbide Corporation under contract NO. W-7405-eng-26.

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