The Darmstadt-heidelberg Crystal Ball

'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 The y r a d i a t i o n f o l l o w i n g heavy-ion react ions provides i n s i g h t i n t o the p roper t ies o f nuc le i a t h igh e x c i t a t i o n energy and h igh angular momentum. Probab l y the most d i r e c t information i s coming from the i n d i v i d u a l t r a n s i t i o n energies. But a lso the t o t a l Y -decay energy and t h e number o f emi t ted y -rays are important parameters. High y r a y mu1 t i p 1 i c i t y i n p a r t i c u l a r corresponds t o h igh angular momentum o f the decaying system, and t o t a l energy and m u l t i p l i c i t y together may de f ine the e n t r y p o i n t o f the Y cascade i n the res idua l nucleus under study. The angu la r d i s t r i b u t i o n and the p o l a r i z a t i o n o f the i n d i v idual yrays are add i t i ona l parameters. They a l low t o deduce the o r i e n t a t i o n o f the decaying nucleus and the mu1 t i p o l a r i t y o f the y t r a n s i t ions. The i n s t a n t o f decay and i t s i n t e r n a l t ime s t r u c t u r e f i n a l l y complete t h i s l i s t of bas ic p roper t ies o f nuc l e a r Yrad ia t ion . The various observables i n the y-decay are n o t i n dependent from each other . An obvious example would be the de tec t ion o f a sequence o f t r a n s i t i o n s each d i f f e r i n g by a constant amount o f energy from the one preceding i t. Such a regu la r spectrum i s charact e r i s t i c o f quantized r o t a t i o n a l motion o f a deformed nucleus. The t o t a l Y-decay energy and the s p i n as der ived from the m u l t i p l i c i t y then have t o obey the r o t a t i o n a l I ( I + l ) r e l a t i o n s h i p ; the r a d i a t i o n has t o be stretched-E2 from the angular d i s t r i b u t i o n ; and the cascade has t o be prompt and proceed w i t h i n a few picoseconds from the e n t r y p o i n t down t o the l a s t l e v e l s above the ground s t a t e due t o the enhanced st rength 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 . Obviously one has t o study such cor re la t ions , between the t r a n s i t i o n energies as we l l as between a l l o ther observables, t o ob ta in the f u l l i n fo rmat ion contained i n the y-decay. This, however, requ i res "complete" measurements where t h e values o f t h e re levan t parameters are already def ined by a s i n g l e observat ion. The measurement o f a t r a n s i t i o n energy f o r example i s n o t considered complete i n t h i s sense, i f Compton scat tered quanta can leave the de tec to r unobserved, s ince i t takes many measurements u n t i l t he r e s u l t i n g pulse-height d i s t r i b u t i o n can be connected t o the f u l l t r a n s i t i o n energy. If, on the o ther hand, the de tec to r i s enclosed i n an Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801031 C10-282 JOURNAL DE PHYSIQUE 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 the r e s o l u t i o n o f the detector, unless the s h i e l d generated a veto s ignal . De ta i led inves t iga t ions o f the y-continuum fo l lowi n g heavy-ion reac t ions are r a t h e r new and up t o now a t bes t the t o t a l y-decay energy has been measured 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 the general features o f a y-detector 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 poss ib le 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 de tec to r has the shape o f a spher ica l she l l o f NaI. This s h e l l encloses the 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 the bu lk o f the y rad ia t ion 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 iscern the i n d i v i d u a l yt rans i t i o n s . With t h i s de tec to r one can measure the to t a l decay energy o f each y-cascade. At the same t ime i t w i l l be poss ib le t o determine the m u l t i p l i c i t y o f the 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 so be poss ib le t o ob ta in 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 these measurements depends main ly on the y-ray e f f i c iency o f the de tec to r and on the a b i l i t y t o i s o l a t e the various y t rans i t i ons . These requirements 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 the NaI s h e l l , f o r the number o f elements composing i t and f o r i t s r a d i u s and they a lso determine the shape o f the i n d i v i d u a l modules. 2.1 DETECTOR PARAMETERS a) Thickness of NaI she l l . To est imate the th ickness o f the NaI s h e l l vie use the expression which r e l a t e s the i n c i d e n t y-ray i n t e n s i t y I. t o t h e t ransmi t ted i n t e n s i t y I. The q u a n t i t y u represents the t o t a l l i n e a r a t tenua t ion c o e f f i c i e n t o f NaI. I t depends on the y-ray energy and i s shown i n f i g . 1. 0.01 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10 ENERGY (MeV) Fig. 1 Linear a t tenua t ion c o e f f i c i e n t s f o r yr a d i a t i o n i n NaI Since the spectrum o f y-rays f o l l o w i n g heavy-ion re act ions extends up t o the MeV-range i t requ i res about 20 cm of NaI t o s top 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, for example, i s around 0.20 cm-I and w i t h d = 20 cm eq. 1 gives an e f f i c i e n c y Q= 1-I/Io = 0.98 f o r de tec t ing the r a d i a t i o n . The average amount of energy absorbed i n the detector per yray emitted, however, i s smaller than 0.98 MeV i n t h i s example, s ince n o t a l l t r i g g e r i n g events y i e l d the f u l l photopeak energy. b f Number o f de tec to r elements. I n order t o reso l 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 detector , the number o f elements N i n the NaI s h e l l has t o be l a r g e as compared t o the y-ray mu1 t i p 1 i c i t y M. A more accur a t e est imate f o r N comes from the f r a c t i o n o f mult i p l e h i t s one can t o l e r a t e i n a g iven element. Under the assumption t h a t the y rad ia t ion i s i s o t r o p i c the 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 iven by An inner f r e e rad ius o f 25 cm i s probably the m in i mum dis tance acceptable f o r the NaI s h e l l . Then neut rons from the t a r g e t a r r i v e about 7 ns l a t e r than prompt y-rays and a t l e a s t those modules, which were t r iggered by a neutron only, can now be resolved from the r e s t i n the s h e l l . NaI elements which are t r i ggered by a Y-ray and then de tec t a r e a c t i o n neut r o n w i l l y i e l d an energy s ignal t h a t conta ins a neutron component. It seems d i f f i c u l t , however, t o recognize such a s i t u a t i o n by a crude pulse shape ana lys is us ing two independent thresholds i n the t ime-def in ing CFT d isc r im ina to r . A t 25 cm from the center, on the o ther hand, the i n d i v i d u a l modules have q u i t e reasonable dimensions. Here, 8 i s the t o t a l e f f i c i e n c y o f the system. I t i s c lose t o u n i t y and i t s energy dependence i s n e g l i g i b le . 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 I I I I I CRYSTAL DIMENSIONS 1.0 DIA. x HGT. 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 g iven P-Q M-I r = w . Q P 2 N TO have r 5 0.10 f o r a cascade o f 30 y-rays f o r i n stance requ i res N ' 150 i n the h igh-e f f i c iency system considered. c ) Free inner rad ius. Sca t te r ing between neighbouring elements and de tec t ion o f neutrons which a re f requen t l y emi t ted together w i t h y-rays i n heavyi o n reac t ions d i s t u r b the performance o f the detect o r . But w i t h increasing o v e r a l l s i z e o f the system the p r o b a b i l i t y f o r c ross ta lk decreases and a t the same t ime neutrons become d is t ingu ishab le 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. ENERGY (MeV) F ig . 2 Rat io o f the i n t e n s i t i e s observed i n the photopeak and i n the t o t a l l ineshape f o r var ious 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. For a 3Itx3" c r y s t a l the peak-to-t0ta.l r a t i o i s around 0.33 a t 1 MeV, i . e . 67% of the detected y-rays do no t deposi te t h e i r f u l l energy b u t a r e scat tered ou t o f the c r y s t a l . C10-284 JOURNAL DE PHYSIQUE With about 150 elements i n the s h e l l f o r instance the f r o n t face o f each module i s s t i l l b igger than t h a t o f a standard o f 3" x 3" c r y s t a l . The sca t te r i n g o f y rad ia t ion ou t o f such a c y l i n d r i c a l standard-size c r y s t a l can be evaluated from the peak-tot o t a l r a t i o s given i n f i g . 2, wh i le f i g . 3 describes the s i t u a t i o n i n a modularized r i n g detector o f 25 cm 0 x 20 cm which had an a x i a l bore o f 6.5 cm 0 and which was separated i n t o 6 equal sectors o f 60'. From t h i s in format ion we est imate t h a t the c ross ta lk among the elements o f a spher ica l NaI detector w i t h 100-200 modules w i l l be around 20-40 % f o r 1 MeV