THE DARMSTADT-HEIDELBERG CRYSTAL BALL R.S. Simon

To cite this version: R.S. Simon. THE DARMSTADT-HEIDELBERG CRYSTAL BALL. Journal de Physique Colloques, 1980, 41 (C10), pp.C10-281-C10-293. .

HAL Id: jpa-00220650 https://hal.archives-ouvertes.fr/jpa-00220650 Submitted on 1 Jan 1980

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JOURNAL DE PHYSIQUE

THE DARMSTADT-HEIDELBERG

CoZZoque ClO, suppZ6ment a u 72'12,

Tome 41, ddcembre 1980, page C10-281

CRYSTAL BALL

R.S. Simon.

G.S.I. Darmstadt, P.O. B. 11 05 41, 6100 Darmstadt, Federal RepubZic o f Gemany.

Abstract.- 'A modularized Ma1 detector with close to 4Tgeometry can provide unique information on the decay y-radiation from highly excited nuclei. GSI Darmstadt, the Ilax-Planck-Institute (WI) for Nuclear Physics and the University of Heidelberg have joined in a collaboration to realize such a detector. The detector has the shape of a spherical shell with a free inner radius of 25 cm and a thickness of 20 cm and comprises 162 individual modules of equal solid angle. This contribution explains our special choice of configuration and gives an outline of the present status of the mechanical and electronic assembly.

1. INFORMATION CONTENT OF NUCLEAR Y-RADIATION

one preceding i t . Such a r e g u l a r spectrum i s charac-

The y - r a d i a t i o n f o l l o w i n g heavy-ion r e a c t i o n s pro-

t e r i s t i c o f quantized r o t a t i o n a l motion o f a d e f o r -

vides i n s i g h t i n t o t h e p r o p e r t i e s o f n u c l e i a t h i g h

med nucleus. The t o t a l Y-decay energy and t h e s p i n

e x c i t a t i o n energy and h i g h angular momentum. Proba-

as d e r i v e d from t h e m u l t i p l i c i t y then have t o obey

b l y the most d i r e c t information i s coming from the

the rotational I(I+l)-relationship;

i n d i v i d u a l t r a n s i t i o n energies. But a l s o the t o t a l

has t o be stretched-E2 from t h e angular d i s t r i b u t i o n ;

Y -decay energy and t h e number o f e m i t t e d y -rays a r e

and t h e cascade has t o be prompt and proceed w i t h i n

important parameters. High y - r a y mu1t i p 1 i c i t y i n

a few picoseconds from t h e e n t r y p o i n t down t o t h e

p a r t i c u l a r corresponds t o h i g h angular momentum o f

l a s t l e v e l s above t h e ground s t a t e due t o t h e en-

t h e decaying system, and t o t a l energy and m u l t i p l i -

hanced s t r e n g t h o f t h e t r a n s i t i o n p r o b a b i l i t i e s .

c i t y together may d e f i n e t h e e n t r y p o i n t o f t h e Y -

the r a d i a t i o n

Obviously one has t o study such c o r r e l a t i o n s , be-

cascade i n t h e r e s i d u a l nucleus under study. The an-

tween t h e t r a n s i t i o n energies as w e l l as between a l l

g u l a r d i s t r i b u t i o n and t h e p o l a r i z a t i o n o f t h e i n d i -

o t h e r observables, t o o b t a i n the f u l l i n f o r m a t i o n

v i d u a l y - r a y s a r e a d d i t i o n a l parameters. They a l l o w

contained i n t h e y-decay. This, however, r e q u i r e s

t o deduce t h e o r i e n t a t i o n o f t h e decaying nucleus

"complete" measurements where t h e values o f t h e r e -

and t h e mu1t i p o l a r i t y o f the y - t r a n s i t i o n s . The i n -

l e v a n t parameters a r e already d e f i n e d by a s i n g l e

s t a n t o f decay and i t s i n t e r n a l t i m e s t r u c t u r e f i -

observation. The measurement o f a t r a n s i t i o n ener-

n a l l y complete t h i s l i s t of b a s i c p r o p e r t i e s o f nu-

gy f o r example i s n o t considered complete i n t h i s

clear Y-radiation.

sense, i f Compton s c a t t e r e d quanta can leave t h e

The various observables i n t h e y-decay are n o t i n -

d e t e c t o r unobserved, s i n c e i t takes many measure-

dependent from each o t h e r . An obvious example would

ments u n t i l t h e r e s u l t i n g pulse-height d i s t r i b u t i o n

be t h e d e t e c t i o n o f a sequence o f t r a n s i t i o n s each

can be connected t o t h e f u l l t r a n s i t i o n energy. If,

d i f f e r i n g by a constant amount o f energy from t h e

on t h e o t h e r hand, t h e d e t e c t o r i s enclosed i n an

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801031

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anti-Compton s h i e l d one has a complete measurement o f t h e f u l l t r a n s i t i o n energy w i t h i n t h e r e s o l u t i o n o f t h e detector, unless t h e s h i e l d generated a veto which r e l a t e s t h e i n c i d e n t y-ray i n t e n s i t y I . t o the

signal. D e t a i l e d i n v e s t i g a t i o n s o f t h e y-continuum f o l l o w -

t r a n s m i t t e d i n t e n s i t y I. The q u a n t i t y u represents

i n g heavy-ion r e a c t i o n s a r e r a t h e r new and up t o

t h e t o t a l l i n e a r a t t e n u a t i o n c o e f f i c i e n t o f NaI. I t

now a t b e s t t h e t o t a l y-decay energy has been mea-

depends on t h e y-ray energy and i s shown i n f i g . 1.

sured completely, t h a t i s on an event-by-event bas i s . The experiments, however, c l e a r l y i n d i c a t e t h e general f e a t u r e s o f a y - d e t e c t o r which w i l l a1 low complete measurements o f a d d i t i o n a l q u a n t i t i e s and make p o s s i b l e t h e study o f t h e i r c o r r e l a t i o n s 11,2\.

2 . DESIGN OF THE DETECTOR.

- The d e t e c t o r

has the

shape o f a s p h e r i c a l s h e l l o f NaI. This s h e l l encloses t h e t a r g e t chamber completely, i t i s t h i c k enough t o f u l l y absorb t h e b u l k o f t h e y - r a d i a t i o n and i t i s subdivided i n t o many separate counters o f equal s o l i d angle t o d i s c e r n t h e i n d i v i d u a l y - t r a n s i t i o n s . With t h i s d e t e c t o r one can measure t h e t o t a l decay energy o f each y-cascade. A t t h e same time i t w i l l be p o s s i b l e t o determine t h e m u l t i p l i c i t y o f t h e y-rays and t h e i r angular d i s t r i b u t i o n and i t w i l l a l s o be p o s s i b l e t o o b t a i n several i n d i v i d u a l t r a n s i t i o n energies per cascade. Good r e s o l u t i o n i n

0

0.01 0.01 0.02

0.05 0.1 0.2

0.5

1

2

5

10

ENERGY (MeV)

these measurements depends m a i n l y on the y-ray e f f i ciency o f t h e d e t e c t o r and on t h e a b i l i t y t o i s o l a t e t h e various y - t r a n s i t i o n s .

These requirements

Fig. 1

Linear a t t e n u a t i o n c o e f f i c i e n t s f o r y r a d i a t i o n i n NaI

i n p a r t i c u l a r s e t lower l i m i t s f o r the thickness o f t h e NaI s h e l l , f o r t h e number o f elements composing

Since t h e spectrum o f y-rays f o l l o w i n g heavy-ion r e -

i t and f o r i t s r a d i u s and they a l s o determine t h e

a c t i o n s extends up t o t h e MeV-range i t r e q u i r e s about

shape o f t h e i n d i v i d u a l modules.

20 cm of NaI t o s t o p t h i s r a d i a t i o n e f f e c t i v e l y . For y-rays of 1 MeV, f o r example,

i s around 0.20 cm-I

and w i t h d = 20 cm eq. 1 g i v e s an e f f i c i e n c y

2.1

DETECTOR PARAMETERS

a)

Thickness of NaI s h e l l .

- To e s t i m a t e t h e

Q= 1-I/Io = 0.98 f o r d e t e c t i n g t h e r a d i a t i o n . The

average amount of energy absorbed i n t h e d e t e c t o r per

t h i c k n e s s o f t h e NaI s h e l l vie use t h e expression y - r a y emitted, however, i s smaller than 0.98 MeV i n

t h i s example, s i n c e n o t a l l t r i g g e r i n g events y i e l d t h e f u l l photopeak energy.

An i n n e r f r e e r a d i u s o f 25 cm i s probably t h e m i n i mum d i s t a n c e acceptable f o r t h e NaI s h e l l . Then neut r o n s from t h e t a r g e t a r r i v e about 7 ns l a t e r than

bf

Number o f d e t e c t o r elements.

-

I n order t o resol-

prompt y-rays and a t l e a s t those modules, which were

ve i n d i v i d u a l y - t r a n s i t i o n s i n the d e t e c t o r , t h e num-

t r i g g e r e d by a neutron only, can now be r e s o l v e d

ber o f elements N i n t h e NaI s h e l l has t o be l a r g e

from the r e s t i n t h e s h e l l . NaI elements which a r e

as compared t o t h e y-ray mu1 t i p 1 i c i t y M. A more accu-

t r i g g e r e d by a Y-ray and then d e t e c t a r e a c t i o n neu-

r a t e estimate f o r N comes from t h e f r a c t i o n o f mul-

t r o n w i l l y i e l d an energy s i g n a l t h a t c o n t a i n s a

t i p l e h i t s one can t o l e r a t e i n a g i v e n element. Under

neutron component. It seems d i f f i c u l t , however, t o

t h e assumption t h a t t h e y - r a d i a t i o n i s i s o t r o p i c t h e

recognize such a s i t u a t i o n by a crude p u l s e shape

p r o b a b i l i t y p t h a t an element w i l l t r i g g e r i s g i v e n

a n a l y s i s u s i n g two independent thresholds i n t h e

by

t i m e - d e f i n i n g CFT d i s c r i m i n a t o r . A t 25 cm from t h e center, on t h e o t h e r hand, t h e i n d i v i d u a l modules have q u i t e reasonable dimensions. I

c l o s e t o u n i t y and i t s energy dependence i s n e g l i g i -

I

I

I

I

CRYSTAL DIMENSIONS

Here, 8 i s t h e t o t a l e f f i c i e n c y o f the system. I t i s

1.0

DIA.

x

HGT.

b l e . The p r o b a b i l i t y f o r s i n g l e h i t s i s

and the f r a c t i o n r

o f m u l t i p l e h i t s i s f i n a l l y given

r = -P-Q w - . Q M-I

2N

P

TO have r 5 0.10 f o r a cascade o f 30 y-rays f o r i n stance r e q u i r e s N

' 150 i n the h i g h - e f f i c i e n c y

system

considered.

c)

Free i n n e r r a d i u s .

-

S c a t t e r i n g between neigh-

bouring elements and d e t e c t i o n o f neutrons which

ENERGY (MeV)

a r e f r e q u e n t l y e m i t t e d together w i t h y-rays i n heavyi o n r e a c t i o n s d i s t u r b t h e performance o f the detect o r . But w i t h i n c r e a s i n g o v e r a l l s i z e o f the system t h e p r o b a b i l i t y f o r c r o s s - t a l k decreases and a t t h e same t i m e neutrons become d i s t i n g u i s h a b l e from yrays because o f t h e i r d i f f e r e n t f l i g h t times.

Fig. 2

R a t i o o f t h e i n t e n s i t i e s observed i n t h e photopeak and i n t h e t o t a l l i n e s h a p e f o r v a r i o u s standard-size NaI c r y s t a l s - a s a f u n c t i o n o f y-ray energy. F o r a 3Itx3" c r y s t a l t h e peak-to-t0ta.l r a t i o i s around 0.33 a t 1 MeV, i . e . 67% of t h e detected y-rays do n o t d e p o s i t e t h e i r f u l l energy but are scattered out o f the crystal.

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-

With about 150 elements i n t h e s h e l l f o r i n s t a n c e

d)

t h e f r o n t face o f each module i s s t i l l b i g g e r than

of t h e i n d i v i d u a l modules have t o be polygons i n o r -

t h a t o f a standard o f 3" x 3" c r y s t a l . The s c a t t e r -

der t o p e r f e c t l y t i l e t h e d e t e c t o r she1 1. To keep

i n g o f y - r a d i a t i o n o u t o f such a c y l i n d r i c a l stan-

t h e manufacturing simple t h e r e s u l t i n g polyhedron,

dard-size c r y s t a l can be evaluated from t h e peak-to-

however, should c o n t a i n as small a number o f d i f f e -

t o t a l r a t i o s given i n f i g . 2, w h i l e f i g . 3 describes

r e n t polygons as possible. F u r t h e r r e s t r i c t i o n s a r e

t h e s i t u a t i o n i n a modularized r i n g d e t e c t o r o f

coming from t h e experimental requirements mentioned

25 cm

0

x 20 cm which had an a x i a l bore o f 6.5 cm

0

Shape o f i n d i v i d u a l modules.

The f r o n t faces

above: t h a t a l l elements o f t h e d e t e c t o r should co-

and which was separated i n t o 6 equal sectors o f 60'.

ver t h e same s o l i d angle; t h a t t h e number o f element!

From t h i s i n f o r m a t i o n we estimate t h a t t h e c r o s s t a l k

should be large; and t h a t c r o s s - t a l k between t h e mo-

among t h e elements o f a s p h e r i c a l NaI d e t e c t o r w i t h

dules should be small.

100-200 modules w i l l be around 20-40 % f o r 1 MeV r - r a y energy.

Acceptable s o l u t i o n s f o r t h i s geometrical problem a r e provided by t h e s p e c i a l c l a s s o f polyhedra t h a t c o n t a i n 12 r e g u l a r pentagons and an i n c r e a s i n g number o f hexagons which a r e i r r e g u l a r i n general. Only d i s c r e t e numbers o f hexagons can be accomodated, however. S t a r t i n g w i t h t h e basic dodecahedron o u t of 12 r e g u l a r pentagons, t h e f i r s t p o s s i b l e s o l u t i o n s have 20, 30, 60, 80, 110, 120, 150, 180, 200, and

0

1.0

2.0

3.0

Transition energy (MeV) Fig. 3

S c a t t e r i n g o f y-rays i n a NaI r i n g detect o g which i s d i v i d e d i n t o 6 segments of 60 The d e t e c t o r has an o u t e r diameter o f 25 cm and i s 20 cm long, t h e source i s l o c a t e d i n t h e c e n t e r o f t h e a x i a l bore o f 6.5 cm diameter. We have s t u d i e d ycascades w i t h 2 t r a n s i t i o n s , observing t h e f u l l energy o f one o f t h e two quanta i n an e x t e r n a l Ge counter. For each coincidence event t h e number o f a c t i v e NaI segments has been determined. The f i g u r e shows t h e p r o b a b i l i t i e s f o r double and t r i p l e t r i g g e r i n g as a f u n c t i o n o f t h e y-ray energy. S c a t t e r i n g among t h e 6 segments o f t h e NaI r i n g occurs e s s e n t i a l l y i n one dimens i o n only. For t h e proposed s p h e r i c a l det e c t o r s h e l l (two dimensions) one t h e r e f o r e expects r o u g h l y t w i c e as much scattering.

.

Fig. 4

One o f t h e 20 e q u i l a t e r a l s p h e r i c a l triangles t h a t compose t h e polyhedron w i t h 162 faces. The t r i a n g l e has a r e g u l a r pentagon i n each corner ( d e t e c t o r t y p e A) and c o n t a i n s t h r e e hexagonal shapes i n a d d i t i o n (types 6, C, and D). A l l t h r e e hexagons a r e i r r e g u l a r , b u t t h e y a r e n o t v e r y d i f f e r e n t from each o t h e r .

240 hexagons i n a d d i t i o n . These s o l u t i o n s do cover the i n t e r e s t i n g range 100

c

N < 250 adequately. Also

The average energy

5 corresponds

t o the centroid o f

the p u l s e h e i g h t d i s t r i b u t i o n , w h i l e t h e f a c t o r 2 . 3 5 ~

the number o f d i f f e r e n t types o f hexagons i s small.

2-

i s o n l y c o r r e c t f o r 1arge M when t h e d i s t r i -

The polyhedron w i t h 162 faces f o r i n s t a n c e r e q u i r e s

b u t i o n i s Gaussian. Equation ( 5 ) f o r i n s t a n c e shows

o n l y t h r e e d i f f e r e n t types o f them as schematically

t h a t ~ a has t o be b i g g e r than 0.8 i n order t o de-

shown i n f i g . 4. One corresponds t o t h e hexagons ad-

termine t h e t o t a l energy of a cascade o f 30 y - t r a n -

j a c e n t t o t h e twelve pentagons and t h e f u l l d e t e c t o r

s i t i o n s i n a s i n g l e observation t o b e t t e r than 20 I .

contains 60 modules o f t h i s kind. Another type r e -

Such ea-values seem t o be q u i t e r e a l i s t i c f o r t h e

presents t h e hexagons i n t h e c e n t r a l r e g i o n between

proposed d e t e c t o r sphere, if one e x t r a p o l a t e s from

t h r e e pentagons (60 modules i n t o t a l ) and t h e t h i r d

the experimental values i n f i g . 5 which correspond

type corresponds t o t h e hexagons h a l f way between

t o t h e NaI r i n g d e t e c t o r of 25 cm 0 mentioned above.

two neighbouring pentagons (30 modules). Only t h r e e faces meet a t any v e r t e x o f the polyhedron. This keeps t h e s c a t t e r i n g between the d i f f e r e n t modules w i t h i n reasonable l i m i t s as does t h e q u i t e favourable r a t i o o f c i r c t m f e r e n c e t o area o f pentagons and hexagons.

2.2

DETECTOR RESOLUTION.

-

I n sample c a l c u l a t i o n s

we have simulated t h e performance o f t h e d e t e c t o r under t h e assumptions t h a t o n l y one y-cascade i s present and t h a t t h e e f f i c i e n c y o f t h e d e t e c t o r i s independent o f t h e i n d i v i d u a l tran.si t i o n energies e.

a)

T o t a l y - r a y energy.

-

F o r measuring the t o t a l

y - r a y energy t h e segmentation o f t h e d e t e c t o r i s

Transition energy (MeV).

i r r e l e v a n t , s i n c e t h e r e s o l u t i o n t o be achieved depends m a i n l y on the average energy Eae absorbed per Fig. 5 y-quantum emitted. The r e d u c t i o n f a c t o r

E

takes i n t o

account t h a t n o t a l l t r i g g e r i n g y-rays d e p o s i t t h e i r f u l l photopeak energy. I n the case o f a cascade o f M

F r a c t i o n E n corresponding t o t h e energy absorbed per y-quantum emitted, as a f u n c t i o n o f t h e y-ray energy. Shown a r e r e s u l t s f o r t h e NaI r i n g d e t e c t o r described i n f i g . 3. Again t h e source was l o c a t e d i n t h e center o f t h e a x i a l bore.

t r a n s i t i o n s t h e r e s o l u t i o n o f t h e observed pulseh e i g h t d i s t r i b u t i o n i s approximately given by

b)

Multiplicity.

-

To o b t a i n t h e y - r a y m u l t i p l i c i t y

one counts t h e modules which have been t r i g g e r e d . But even f o r a p e r f e c t l y absorbing d e t e c t o r sphere t h i s would n o t y i e l d a sharp r e s u l t . S c a t t e r i n g events,where

one y-quantum f i r e s two adjacent de-

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t e c t o r elements l e a d t o double counting w h i l e mu1 ti-

dence s h i e l d and considers t h e s i g n a l o f t h e c e n t r a l

p l e h i t s i n one d e t e c t o r element would r e s u l t i n

d e t e c t o r t o represent a f u l l photopeak energy i f

t o o small a m u l t i p l i c i t y . I n o r d e r t o e s t i m a t e t h e

none o f t h e neighbours i s responding. Unfortunately,

uncertainty

t h e number n o f such i s o l a t e d s i n g l e h i t s i s small

due t o s c a t t e r i n g we assume t h a t i n -

stead o f the M o r i g i n a l quanta now ( 1

+

f ) M quanta

as compared t o t h e number of a l l s i n g l e h i t s t h a t do

a r e a v a i l a b l e per cascade, where t h e parameter f

n o t l e a d t o s c a t t e r i n g . I f one takes t h e a n t i - c o i n c i -

corresponds t o t h e f r a c t i o n o f s c a t t e r e d r a d i a t i o n .

dence requirement i n t o account, n i s approximately

The r e s o l u t i o n t o be expected i s shown i n f i g . 6,

g i v e n by

f i r s t as a f u n c t i o n o f f, then as a f u n c t i o n o f n and f i n a l l y depending on N, t h e number o f d e t e c t o r elements. Even f o r 30 % s c a t t e r i n g r e s o l u t i o n s as good as 20 % m i g h t be r e a l i z e d . From the f i g u r e i t i s c l e a r t h a t i t becomes i n c r e a s i n g l y d i f f i c u l t t o The constant a i s 5 o r 6, r e s p e c t i v e l y , and stands

improve t h e r e s o l u t i o n above N = 100.

f o r t h e number o f n e x t neighbours. I n f i g . 7 the development o f n i s given as a f u n c t i o n o f N. Only above N = 100 t h e number of i s o l a t e d h i t s becomes s i g n i f i c a n t . F o r l a r g e N s a t u r a t i o n s e t s i n , b u t much l a t e r than f o r t h e m u l t i p l i c i t y measurements.

Fig. 6

Resolution expected f o r t h e number o f f r i g g e r i n g d e t e c t o r elements, r e l a t i v e t o t h e mean value N o f t h e d i s t r i b u t i o n . The r e l e v a n t parameters a r e t h e t o t a l number o f d e t e c t o r elements N, t h e e f f i c i e n c y of t h e d e t e c t o r system 0 and t h e p r o b a b i l i t y o f s c a t t e r i n g f . The sample c a l c u l a t i o n s have been made f o r cascades o f 10, 20, 30, and 40 i d e n t i c a l y-rays, r e s p e c t i v e l y . For small N t h e y-rays begin t o exhaust t h e d e t e c t o r elements.

Number of detector elements

Fig. 7 c)

I n d i v i d u a l t r a n s i t i o n energies.

-

To i d e n t i f y

i n d i v i d u a l t r a n s i t i o n energies one uses t h e neighbours o f a given d e t e c t o r element as an a n t i - c o i n c i -

Number of i s o l a t e d h i t s expected i n a modul a r i z e d d e t e c t o r as a f u n c t i o n o f t h e number o f d e t e c t o r elements. The parameters Q and f a r e t h e e f f i c i e n c y o f t h e d e t e c t o r system and the p r o b a b i l i t y o f s c a t t e r i n g , r e s p e c t i v e l y . The curves correspond t o eq. (6) w i t h s i x neighbours f o r cascades o f 20, 30, and 40 y-rays.

2.3

THE DARMSTADT-HEIDELBERG DETECTOR CONFIGURATION

Our sample c a l c u l a t i o n s show t h a t a thickness o f

study t h e c o r r e l a t i o n between t h e i n d i v i d u a l t r a n s i t i o n energies o f a y-cascade a t NaI r e s o l u t i o n .

20 cm i s s u f f i c i e n t f o r the d e t e c t o r s h e l l . More

I n t h e second mode, on t h e o t h e r hand, one can com-

m a t e r i a l would h a r d l y improve t h e y-ray e f f i c i e n c y

b i n e t h e f i l t e r a c t i o n o f t h e spectrometer w i t h the

b u t r a t h e r increase t h e s e n s i b i l i t y o f t h e d e t e c t o r

h i g h energy r e s o l u t i o n o f Ge d e t e c t o r s ( t o do so a

a g a i n s t neutrons. I n s t e a d o f aiming f o r an u l t i m a t e

few segments have t o be removed from t h e f u l l a r r a n -

NaI e f f i c i e n c y , i t seems more important, f o r i n s t a n -

gement) o r w i t h t h e a d d i t i o n a l i n f o r m a t i o n from par-

ce, t o keep any losses i n t h e subsequent e l e c t r o n i c

t i c l e counters (which m i g h t be accomodated w i t h i n

c i r c u i t small, since they e n t e r i n t o t h e o v e r a l l

the i n n e r f r e e space o f the s h e l l ) .

e f f i c i e n c y o f t h e t o t a l system on equal f o o t i n g .

From t e s t s w i t h a pentagon p r o t o t y p e we expect t h e

The c a l c u l a t i o n s f u r t h e r show t h a t a l a r g e number o f

f o l l o w i n g s c i n t i l l a t i o n performance f o r t h e i n d i v i -

d e t e c t o r elements i s required. For mu1 t i p 1 i c i t y mea-

dual modules :

surements alone, however, i t probably would n o t be

-

f o r i r r a d i a t i o n through t h e f r o n t end;

necessary t o go beyond N=122. But i n order t o be a b l e t o i d e n t i f y several t r a n s i t i o n energies per cas-

energy r e s o l u t i o n FWHM b e t t e r than 8 % a t 662 keV

-

time r e s o l u t i o n FWHM b e t t e r than 3 ns and FW 1/10

cade one needs a f i n e r mesh o f segments. This u n f o r -

M b e t t e r than 6 ns measured f o r 6 0 ~ ocoinciden-

t u n a t e l y means r a p i d l y r i s i n g costs, because t h e r a -

ces above 200 keV t h r e s h o l d a g a i n s t a f a s t p l a -

d i u s o f t h e d e t e c t o r s h e l l has t o be increased i f

s t i c detector;

t h e c r o s s t a l k between t h e i n d i v i d u a l elements should

-

s t a b i l i t y w i t h i n + 1 % f o r t h e 662 keV photopeak p o s i t i o n of 1 3 7 ~ s under r a t e changes from 1000

s t a y constant.

t o 70000 cts/s;

As a compromise we have chosen t o d i v i d e t h e t o t a l s h e l l i n t o 162 i n d i v i d u a l modules. With a f r e e i n n e r

-

homogeneity along t h e r a d i a l dimension o f t h e

+

1.5 % o f t h e 662 keV photopeak p o s i t i o n

r a d i u s o f 25 cm and a thickness o f 20 cm we then ex-

module

p e c t the f o l l o w i n g r e s o l u t i o n s (FWHM) f o r a t y p i c a l

except f o r t h e o u t e r 4 cm c l o s e t o t h e phototube. We hope t h a t b o t h BICRON and The HARSHAW Chemical

cascade o f 30 y-rays:

-

observation o f t h e t o t a l y-ray energy t o 20 %

Company w i l l d e l i v e r a subunit o f t h e c r y s t a l b a l l . i n

o b s e r v a t i o n o f t h e y-ray m u l t i p l i c i t y t o 20 %

t h e near f u t u r e . These p i l o t assemblies, which c o n s i s t

-

spatial resolution

-

p o s s i b l e i d e n t i f i c a t i o n o f 4 t r a n s i t i o n energies

gons, w i l l a1 low us t o check whether t h e t i g h t me-

per cascade on t h e average,

chanical tolerances can be r e a l i z e d and they w i l l o f f e r

+

8

0

The proposed d e t e c t o r system w i l l a l l o w a m u l t i -

o f a c e n t r a l pentagon and t h e f i v e neighbouring hexa-

t h e p o s s i b i l i t y t o i n v e s t i g a t e c r o s s t a l k between t h e

tude o f novel i n v e s t i g a t i o n s o f h i g h l y e x c i t e d nu-

elements and t h e i r response t o neutrons i n a r e a l i -

c l e i , because i t provides an extremely e f f e c t i v e

s t i c geometry.

f i l t e r t o s e l e c t n u c l e i according t o e x c i t a t i o n energy and angular momentum. The spectrometer m i g h t MECHANICAL ASSEMBLY.

-

be used i n two modes o f operation: as a s e l f s u f f i -

3.

c i e n t d e t e c t o r o r i n coincidence w i t h a d d i t i o n a l

o f t h e c r y s t a l b a l l i s an independent y-detectar o f

counters. I n the f i r s t mode f o r i n s t a n c e one can

i t s own and i s separately canned i n a 0.5 mm t h i c k

Each o f t h e 162 modules

C10-288

JOURNAL DE PHYSIQUE

aluminium housing. The l i g h t o u t p u t i s c o l l e c t e d i n a 3 " phototube and t h e r e i s a small q u a r t z window t o connect an o p t i c a l p u l s e r . A pentagon p r o t o t y p e i s shown i n f i g . 8. For the f i n a l modules t h e a l u minium cans w i l l be e n t i r e l y hydroformed t o guarant e e the f l a t n e s s o f a l l r a d i a l faces and t o provide optimum sealing. The i n d i v i d u a l modules o f the c r y s tal

b a l l w i l l be supported by a l a r g e spherical

honeycomb s t r u c t u r e as i l l u s t r a t e d i n f i g . 9. This structure

-

and w i t h i t t h e c r y s t a l b a l l - i s d i v i d e d

i n t o two hemispheres which can be separated t o a l l o w Fig. 8

Prototype o f d e t e c t o r module.

access t o t h e c e n t r a l t a r g e t p o s i t i o n . There a r e f o u r d i f f e r e n t types o f elements i n the

/

supporting s t r u c t u r e according t o t h e f o u r types o f

Fig.10

C l u s t e r of s i x elements o f t h e supporting s t r u c t u r e b o l t e d together.

d e t e c t o r elements and t h e supporting s t r u c t u r e provides a p e r f e c t image o f the d e t e c t o r arrangement i n Fig. 9

Schematic view o f d e t e c t o r modules and supp o r t i n g s t r u c t u r e . The o u t e r r a d i u s of t h e supporting s t r u c t u r e i s 69 cm, i t s i n d i v i dual elements a r e 6.5 cm t h i c k . The f r a g i l e d e t e c t o r modules merely touch each o t h e r . They a r e k e p t i n p o s i t i o n by aluminium tubes which can be adjusted r e l a t i v e t o t h e supporting frame.

side. The

C U ~ - O U i ~n

enough

t h a t t h e corresponding d e t e c t o r module i s

SO

each supporting element i s b i g

c l e a r of t h e w a l l s and can pass f r e e l y . I n t h i s way we a r e a b l e t o remove i n d i v i d u a l segments from t h e

-

c r y s t a l b a l l . But we a l s o may remove c l u s t e r s o f seg-

4.

ments and r e p l a c e them by o t h e r d e t e c t o r s . The penta-

f o r t h e c r y s t a l b a l l has t o handle h i g h event r a t e s

gons and hexagons o f the supporting s t r u c t u r e a r e

together w i t h a l a r g e number o f parameters per event,

c a s t aluminium and a r e machined t o t h e i r f i n a l dimen-

s i n c e we o b t a i n an energy s i g n a l p l u s t h e c o r r e s -

sions on a computer-driven m i l l . F i g u r e 10 shows the

ponding time i n f o r m a t i o n from each p a r t i c i p a t i n g

c l u s t e r o f s i x elements f o r t h e p i l o t assembly, which

module, But i n a d d i t i o n t o speed we a l s o r e q u i r e

c o n s i s t s o f a c e n t r a l pentagon and i t s f i v e neighbou-

s e l e c t i v i t y , because we have t o p i c k o u t the events

r i n g hexagons. An impression o f t h e f u l l d e t e c t o r

o f i n t e r e s t from a h i g h l e v e l o f background r d d i a -

system m i g h t be provided by f i g . 11. Shown i s a sphe-

tion.

r i c a l s c a t t e r i n g chamber o f 50 cm diameter which

DATA ACQUISITIO?I.

The data a c q u i s i t i o n system

We p l a n t o reach t h i s goal i n t h r e e steps s t a r -

would f i t i n t o t h e i n n e r f r e e space o f t h e c r y s t a l

t i n g w i t h a standard m u l t i p l e ADC system. T h i s

b a l l . On t h e r i g h t t h e c l u s t e r o f s i x can be seen

system has r a t h e r moderate speed and t h e r e f o r e has

w i t h p l a s t i c dummies mounted t o i n d i c a t e t h e l o c a t i o n

t o be gated by a s e l e c t i v e t r i g g e r d e r i v e d f r o m

o f t h e d e t e c t o r s i n the NaI s h e l l .

a d d i t i o n a l e l e c t r o n i c modules i n f r o n t o f t h e ADC's. A t t h e second stage i n t h e development t h e commerc i a l ADC system w i l l be replaced by a much f a s t e r system which i s t a y l o r e d t o our s p e c i f i c r e q u i r e ments. F i n a l l y , t h i s f a s t ADC system w i l l be combined w i t h an a r r a y o f p a r a l l e l microprocessors t o p r o v i d e o n l i n e a n a l y s i s o f a l l events. From t h e hardware-oriented s i t u a t i o n of phase one we thus hope t o s h i f t t o a software-oriented s o l u t i o n which w i l l be c h a r a c t e r i z e d by b o t h f l e x i b i l i t y and h i g h speed.

4.1 HARDWARE-ORIENTED DATA SYSTEM.

-

The f u l l

c r y s t a l b a l l r e q u i r e s 2 x 162 = 324 ADC's and f o r t h e hardware-oriented s o l u t i o n of step one we have chosen t h e LE CROY 2280/2285 m u l t i p l e ADC system. Some c h a r a c t e r i s t i c p r o p e r t i e s o f t h i s CAMAC based system a r e l i s t e d i n t a b l e 1.

F i g . 11 P l a s t i c dummies o f d e t e c t o r modules mounted i n t h e c l u s t e r o f s i x , together w i t h s p h e r i c a l s c a t t e r i n g chamber.

Table 1. LE CROY 2280/2285 charge s e n s i t i v e ADC system b i t s / ADC channel

12

scale frequency

20 MHz

ADC channels / module

24

s h i f t frequency w a i t time

2 MHz

6 t 16

US

JOURNAL DE PHYSIQUE

controlled via CAMAC

threshold and walk adjust controlled via CAMAC

HV

I I1

external detector

-

1

v

- CFT .

light fibre from pulsed LASER

F i g . 12

machine trigger

-

fast

I

-Pulse

-

g. LE CROY Delay 4-1 Ei - sum E =Sum energy =discriminator fast

*

Q ADC System 2280 / 2285

-

,+ p x7 a a

Z

U U

>

Block diagram f o r hardware-oriented data system. The standard 2280/2285 LE CROY ADC's cannot handle t h e f u l l r a t e o f c r y s t a l b a l l events. Therefore a f a s t s e l e c t i v e t r i g g e r has t o be e s t a b l i s h e d by hardware i n f r o n t o f t h e ADC's.

I t i s i n s t r u c t i v e t o c a l c u l a t e t h e maximum time r e -

r a t e o f 50 kHz i n t h e c r y s t a l b a l l . T h i s r a t e has t o

q u i r e d t o nrocess one event.'Conversion o f t h e ana-

be reduced d r a s t i c a l l y t o match t h e speed o f t h e LE

l o g s i g n a l s i n t o 12 b i t takes 200 u s a t a c l o c k f r e -

CROY system. As a f a s t s e l e c t i v e t r i g g e r one may use

quency o f 20 MHz and t h e para1l e l read o u t o f t h e 24

l a r g e m u l t i p l i c i t y o r t h e s i g n a l from an e x t e r n a l

ADC's per module r e q u i r e s 12 us a t a s h i f t frequency

counter. These t r i g g e r s a r e d e r i v e d by hardware i n

o f 2 MHz. There a r e 14 ADC modules a l t o g e t h e r which

f r o n t o f t h e ADC system as schematically shown i n

are addressed s e q u e n t i a l l y . Thus t h e maximum read-

f i g . 12. To e s t a b l i s h h i g h t o t a l y-ray energy r e q u i r e s

o u t time o f 14 x 12 = 168 us becomes comparable t o

200

t h e maximum conversion time. A d d i t i o n of 22 us, which

NaI. The t o t a l - e n e r g y i n f o r m a t i o n t h e r e f o r e i s n o t

a r e necessary t o r e s e t t h e processor, f i n a l l y r e -

i n c l u d e d i n t o t h e f a s t t r i g g e r f o r t h e ADC system

s u l t s i n a maximum t o t a l time o f 390 vs per event.

i n o r d e r t o a v o i d a corresponding delay on a l l ADC

-

300 ns due t o t h e s c i n t i l l a t i o n behaviour o f

I n a g i v e n event h a r d l y more than 30 d e t e c t o r mo-

i n p u t s . We r a t h e r use t h e f a s t - c l e a r o p t i o n i f t h e

dules w i l l p a r t i c i p a t e on t h e average, b u t since t h e

t o t a l energy should d e v i a t e from t h e p r e s e t value.

LE CROY processor has zero suppression o n l y t h e r e l e 4.2

puter.

a s p e c i a l e l e c t r o n i c arrangement f o r t h e c r y s t a l b a l l

For standard experiments we a n t i c i p a t e an event

FAST ADC SYSTEM.

-

vant i n f o r m a t i o n i s t r a n s f e r r e d t o t h e o n l i n e com-

As t h e second s t e p towards

we p l a n t o develop our own m u l t i p l e ADC system. The

design i s based on commercial AOC chips as available f o r instance from DATEL of ANALOG DEVICES, who offer 12 b i t ADC's i n successive approximation technique which convert in 2.2 us.Therewil1 be 32 ADC channels per module t o d i g i t i z e the energy and time information from 16 detector elements and the total crystal ball requires 11 modules of t h i s type. Parallel read

ti

I

Ei

I

zero s u ~ ~ r e s s i o n error cdrlrection I detection bits patch data word

out of a l l modules can be achieved i n approximately 1 vs. The new system which i s roughly 100 times f a s t e r than the 2280/2285 system from LE CROY offers enough speed to handle the f u l l 'load of a l l crystal ball events directly.

4.3

SOFTWARE-ORIENTED DATA SYSTEM.

- For a f u l l

analysis q f the crystal ball experiments we have to c l a s s i f y the events with respect t o more parameters than merely mu1 tip1 i c i t y and total energy. An obvious extension i s the recognition of special pat-

4

t

4

I1 0

4

4

4

Processor

i P , , n t Host computer

I

Microprocessor Module

=--#I

Data Bus

terns on the detector sphere. Such a complete anal y s i s , however, becomes impractical in the hardwareoriented mode.

An example would be the study of the transitionenergy correlations in the y-cascades. There one r e l i e s on f u l l photopeak energies and should only combine the pulseheights from isolated h i t s as dis-

AND

i';

Fig. 14 Block diagram f o r software-oriented data system. The f a s t ADC's are able to handle the f u l l r a t e of crystal ball events and a l l events a r e analyzed online by a subsequent array of parallel microprocessors. Only events of i n t e r e s t are shifted from the microprocessor modules t o the POP 11 computer.

cussed in section 2.2. A hardware trigger f o r selecting such events i s indicated in f i g . 13. B u t since each NaI element may be once central detector and six times neighbour the number of cable connections alone i s already prohibitive. To provide the f l e x i b i l i t y required f o r any detailed online analysis we therefore will add. an array of para1 1el microprocessors to the f a s t ADC system des-

Fig. 13 Hardware trigger f o r isolated h i t in cent r a l detector.

cribed above. These microprocessors modules

-

a pos-

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JOURNAL DE PHYSIQUE

s i b l e choice f o r t h e CPU would be t h e 16 b i t MOTO-

-

ROLA MC 68000

s h a l l have 64 kbyte o f l o c a l memory

rays i n an e q u a t o r i a l band between 8 = 60'

and 120°,

which i s perpendicular t o t h e s p i n d i r e c t i o n . T h i s

and w i l l be f r e e l y programmable i n PASCAL, a language

band, however, contains o n l y 112 o f a l l d e t e c t o r mo-

which i s s i m i l a r t o ALGOL.

dules. We have chosen a v e r y f l e x i b l e mechanical as-

A schematic diagram of t h e f i n a l arrangement i s

sembly where one can withdraw i n d i v i d u a l segments o r

g i v e n i n f i g . 14. There the i n f o r m a t i o n from t h e

c l u s t e r s o f segments t o l a r g e r r a d i i o r remove them

c r y s t a l b a l l d e t e c t o r s i s c o l l e c t e d by an i n p u t /

completely from t h e f u l l sphere. I n combination w i t h

o u t p u t processor and i s t r a n s f e r r e d v i a a number o f

Ge d e t e c t o r s f o r instance, our c r y s t a l b a l l may thus

p a r a l 1e l data busses t o the microprocessors. Selec-

be used as a m u l t i p l e a n t i c o i n c i d e n c e s h i e l d . F i n a l l y

t i o n o f t h e f i r s t f r e e microprocessor i s p o s s i b l e

we p l a n t o develop our own data system f o r t h e c r y s t a l

w i t h i n 200 ns and t h e data t r a n s f e r then takes place

b a l l , which w i l l be f a s t enough t o handle t h e f u l l

a t a r a t e o f one 3 2 - b i t word per 200 ns. This means

l o a d o f d e t e c t o r events a t t h e ADC's and which w i l l

4.5 us f o r an event w i t h m u l t i p l i c i t y 30 f o r i n -

p r o v i d e a r a t h e r complete o n l i n e a n a l y s i s o f a l l

stance, i f we assume 12 b i t r e s o l u t i o n f o r each o f

events i n an a r r a y o f p a r a l 1e l microprocessors.

t h e 60 parameters. To a l l o w f o r complex a n a l y s i s

A t present we a r e several steps behind Oak Ridge.

we a n t i c i p a t e t o use 30 t o 50 microprocessors i n

We hope, however, t h a t both BICRON and The HARSHAW

p a r a l l e l . On completion o f t h e a n a l y s i s t h e events

Chemical Company w i l l d e l i v e r t h e d e t e c t o r modules

which comply w i t h t h e c r i t e r i a o f t h e experiment are

f o r a p i l o t assembly i n t h e near f u t u r e . A l l o w i n g

s h i f t e d t o the o n l i n e computer. A t t h i s l e v e l t h e

f o r a check-out p e r i o d o f several weeks we m i g h t be

data r a t e has t o be as low as a few kHz t o be han-

a b l e t o place t h e f u l l order before J u l y and should

d l ed proper1y .

r e c e i v e a l l 162 modules from t h e manufacturer b e f o r e

The o n l i n e computer has a d d i t i o n a l p e r i p h e r a l equipment l i k e teletype,magnetic tape o r d i s k . I t i s

t h e year i s out. The c r y s t a l b a l l w i l l be p u t together a t t h e post-

the h o s t f o r t h e system software and from here t h e

a c c e l e r a t o r o f t h e MP tandem i n Heidelberg (see f i g .

a n a l y s i s programs a r e t r a n s c r i b e d i n t o t h e micro-

15). A f t e r completion a whole program o f experimental

processor modules t o i n i t i a l i z e t h e data system.

s t u d i e s w i l l be p o s s i b l e a t Heidelberg, and as soon as higher energies o r heavier beams a r e re,qui red, t h e spectrometer s h a l l move t o Darmstadt.

5.

SUMMARY.

-

The Darmstadt-Heidelberg c r y s t a l b a l l

contains more than t w i c e as many d e t e c t o r modules as a s i m i l a r NaI spectrometer which has been completed

Many people have j o i n e d i n t h e c o l l a b o r a t i o n . Among

now a t Oak Ridge (11. Our sample c a l c u l a t i o n s show

those a r e R. Albrecht, H. Grosch, E. Malwitz, and

t h a t t h i s f i n e r mesh o f segments i s necessary t o safe-

D. Schwalm from G S I , V. Metag, H. Graf, E. Jaeschke,

l y d i s t i n g u i s h t h e i n d i v i d u a l y - t r a n s i t i o n s i n case

and R. Repnow from MPI, and W. Farr,,:R.

o f l o n g decay chains, e s p e c i a l l y i f we take t h e an-

P e l t e , W. Schneider, H.J. Specht and W. Weiter from

i s o t r o p y o f t h e y - r a d i a t i o n i n t o account. Under t h e

t h e U n i v e r s i t y Heidelberg. E s p e c i a l l y , however, I

assumption o f a (I-cos 48) stretched-quadrupole p a t -

would l i k e t o mention D i e t r i c h Habs, who i s t h e d r i -

t e r n f o r instance, we expect r o u g h l y 70 % o f t h e y-

v i n g f o r c e behind a l l a c t i v i t i e s i n t h e p r o j e c t .

Wanner, D.

Fig. 1 5 Experimental area f o r the crystal ball a t the postaccelerator of the MP tandem in Heidel berg. The spectrometer i s l ocated in a separate booth which i s air-conditioned to provide stable temperature and which i s shielded against RF radiation.

We have received encouragement and advice from

REFERENCES.

numerous colleagues. Speaking f o r myself I havelearnt my enthusiasm f o r the crystal ball from Gudrun Hagemann, Bent Herskind and Per Tj#m when we worked together with a N a I sum spectrometer of 2 5 cm 0 x 20 cm, which was separated into two halves.

The f i r s t results from the new instruments a t Oak Ridge and Heidelberg should certainly be interesting.

11 1 D.G. Saranti t e s , The spin spectrometer a t the Holified Heavy-Ion Research Facility and some planned experiments, contribution to t h i s conference 121 D. Habs, F.S. Stephens and R.M. Diamond, Lawrence Berkeley Laboratory PUB-5020, March 1 9 7 9