DETECTION AND IDENTIFICATION OF HEAVY ION REACTION PRODUCTS

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DETECTION AND IDENTIFICATION OF HEAVY

ION REACTION PRODUCTS

P. Armbruster

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C5, supplément au n° 11, Tome 21, Novembre 1976, page C5-161

DETECTION AND IDENTIFICATION OF HEAVY ION REACTION PRODUCTS P . A r m b r u s t e r

G e s e l l s c h a f t f u r S c h w e r i o n e n f o r s c h u n g mbH, 61 D a r m s t a d t , W.Germany

Résumé. Les différentes réactions entre ions lourds nécessitent des méthodes différentes pour la détection et 1' identification. Une revue générale des méthodes nécessaires est présentée. Des expériences réalisées montrent ce que nous avons atteint. Les améliorations instrumentales futures sont proposées.

Abstract. There are different types of heavy ion reactions, which ask for different de-tection and identification methods. An outline of methods adequate for the different reactions is given. What has been achieved is demonstrated following different experi-ments actually performed. Some instrumental improveexperi-ments still to be realised are pro-posed.

The subject wide and open, as announced in the title, needs restriction in several aspects. Heavy ion reaction products will be understood as being produced in a nuclear reaction of two heavy nuclei (A>10). Reaction products are to be detected and identified as fast (g * 15%) energetic residues of the nuclear reaction. The detection time, governed by the flight time to the detection systems, is

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assumed to be larger than 10 s, but wxll hardly become larger than 10 s . Prompt emission of y-rays, as well as radioactive decay processes, are not discussed as means of detection or identifica-tion. Still they are one of the most sensitive tools of detection. Chemistry and mass spectroscopy are not covered, although they contribute to the field considerably.

Following Webster's Dictionary, to detect means "to discover the hidden subject, which itself is known", whereas to identify is explained as "to es-tablish the subject to be of a particular kind". There is only one detection problem in heavy ion reactions, that is fusion. Here we actually know the reaction product, its mass, nuclear charge, e-nergy and direction of flight, but it is hidden in the primary beam. The discovery of the fusion pro-duct hidden in the projectile beam is the only de-tection problem, I foresee in heavy ion reaction physics. How it may be solved, will be discussed in the first section. All other reactions pose identi-fication problems. Mass, charge, momentum, and ener-gy of the reaction products are themselves subject of the experiment. Identification techniques to es-tablish the individuals as complete as necessary

will be covered in the second section. In the third section I try to demonstrate the power of the iden-tification techniques achieved in our times follo-wing examples obtained during the last two years. In the last section a few conclusions concerning the question what is possible in the future, and is the possible needed will be drawn.

1. Fusion, a detection problem.

Being precise, there is only the radiative cap-ture process of two nuclei which leads to a fully determined fusion product. However, until now this process has not yet been established, and it will have to be established by an identification of the fused product. Only, if we are not too strict in our definitions, we arrive at a pure detection pro-blem at all. Within the accuracy of the number of evaporated nucleons, the mass of the fusion products and its momentum is known. The evaporation residue is emitted into a small cone around the direction of the projectiles. Its velocity v = m.vi/nii+mo is given by the target (nO and projectile mass ( n O , and the velocity of the projectile (v,). Out of all properties of the fusion product which are open to a measurement, its velocity differs most from the corresponding property of the projectile. Elec-tric and magnetic stiffnesses both depend on the accidental ionic charge, a quantity which is of help to the experimentalist, but of no interest to nu-clear physics. A selection of fusion products accor-ding to their velocity allows to separate them from the primary beam. A number of velocity selectors have been discussed [Y] . The use of detector arran-gements, as time of flight (TOF) or energy

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P. ARMBRUSTER

TARGET DIPOLEMAGNETS EXIT-

I

OUADRUPOLE- OUADRUPOLE - 1 TRIPLET TRIPLET I VELOCITY SLIT

\

l

PRIMARY BEAM l

I EL FIELD I1 EL. FIELD

l

109 MeV Kr 288 MeV Xe

EV RESIDUES LOOMeV EVRESIDUES

1

6 i 7 MeV Xe

i

t l- I 2 ENERGY LO Ar 1 2 3 6 ~ e ~ ) - ' ~ ~ ~ r n 8 L ~ r ( L ~ ~ M ~ V I

-

93 Nb 13'xe 16i7 M ~ V I - - r S 6 ~ e

F i g . l ( a ) . P r i n c i p a l set-up of SHIP ( S e p a r a t o r f o r Heavy I o n r e a c t i o n Pro- d u c t s ) . The double W i e n - f i l t e r i s i o n o p t i c a l l y s y m e t r i c . I n t h e symmetry p l a n e a v e l o c i t y d e f i n i n g s l i t i s p o s i t i o n e d , which a l l o w s t o v a r y t h e a c c e p t e d energy range between 2% and 20%. The condensers a r e s u p p l i e d by two 400 kV DC-generators. (b-d). TOF-energy a n a l y s i s of t h e beam l e a v i n g SHIP. A r , K r and Xe a r e used t o produce masses A Q 180. With A r t h e eva- p o r a t i o n r e s i d u e s have been found w i t h a r e d u c t i o n of t h e primary beam by a f a c t o r of 1012. With K r and Xe beams t h e r i d g e of primary p a r t i c l e s

i s seen. Reductions of t h e primary beam of 1010 and 108 have been found. The main background i s primary p a r t i c l e s having t h e v e l o c i t y s e t by t h e window. They a r e e i t h e r coming from t h e a c c e l e r a t o r o r a r e produced by s c a t t e r i n g somewhere i n t h e t a r g e t r e g i o n .

ments i s n o t p o s s i b l e i n beam d i r e c t i o n . The number of p r o j e c t i l e s i s much too h i g h f o r a l l e l e c t r o n i c d e t e c t i o n systems. Combinations of s t a t i c e l e c t r i c and magnetic f i e l d s 121, o r v e l o c i t y d e f i n i n g RF-

choppers o r d e f l e c t o r s

[d

may s e r v e a s v e l o c i t y se- l e c t o r s . Two systems have been b u i l t i n t h e l a s t y e a r s . RF-chopping and magnetic d e f l e c t i o n i s used

i n a system i n s t a l l e d a t t h e Munique Tandem Lab. 141. The S e p a r a t o r f o r _Heavy Son r e a c t i o n _Products (SHIP) i s o p e r a t i n g s i n c e 1975 a t UNILAC [5] . F i g . l a shows t h e p r i n c i p a l set-up. Quadrupole f o c u s s i n g t o g e t h e r w i t h d e f l e c t i o n i n e l e c t r i c and magnetic f i e l d s a l l o w s t o f o c u s s a n energy and i o n i c c h a r g e spectrum of 20% w i d t h each, on a p o s i t i o n 1 1 m be- yond t h e t a r g e t p o s i t i o n [6]. SHIP s e p a r a t e s t h e r e a c t i o n p r o d u c t s s p a t i a l l y . Thus i t i s n o t o n l y

s e d r e c e n t l y [:7]. F i g s . lb-Id demonstrate t h e sup- p r e s s i o n of t h e primary beam f o r d i f f e r e n t r e a c t i o n s

l e a d i n g t o f u s i o n i n t h e A Q 180 mass range. Sup- p r e s s i o n depends on t h e v e l o c i t y d i f f e r e n c e between p r o j e c t i l e s and e v a p o r a t i o n r e s i d u e s . Ar, K r , Xe beams have been suppressed by f a c t o r s 1012, 101°,

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in the nb-region can be measured. Such a combined _{sec after the reaction. If evaporation processes} system will be used to look for very rare events _{occur they are much faster than the experimental i-} produced in fusion reactions, a modified technique _{dentification techniques. ~h~ quantities actually} as applied by a French Group in 1972

181 .

_{measured are not the primary ones, which define the} 2. Identification of reaction products from N-body reaction, but secondary ones.

break-ups. _{2.1. Unperturbed kinematics. Let us first assume,}

The properties of reaction products which are of interest for the nuclear reaction are not identical with those which are open to a measurement. The nuclear reaction is characterized by the masses A,

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nuclear charges 2 , and the momenta p of the reaction products, and the energy release of the reaction. The quantities open to our experiments are angles

(g), energies (E), flight-times (t), energy losses (AE), ionic charges

(q),

magnetic stiffnesses (Bp)

and electric stiffnesses (FP) at a time (10-~-10-~)

there is no evaporation. The questions to be answe- re are : how many pieces have to be measured to re- construct the nuclear event, and which pieces are experimentally the most appropriate? The kinematics of a reaction are governed by equations bedween the kinematic variables, masses, momenta, and the reaction Q-value. These equations connect the experimental observables, angles, flight-times, and energies with the kinematic variables.

Table 1

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N-body kinematics (An,pnQ) with mass- and energy transfer in exit channels

Prototype reaction Radiative capture tranefer ?

Number of kinematic variables Conservation laws 3 4 5 Minimum number of observables Observable angles 1 2 2N Number of additional, necessary observables Redundance of (B, t)- experiment Redundance of (8, t,E)- experiment 3 1 for N=3 0 for .N=4

Typical instruments 0'-detector (Fig. 1 ) a) 83, t3,E3,AE3 (Fig. 2)

covering complete ki- b) e3,t3,Bo3.AE3(Fig.3)

nematics and nuclear C) ~ ~ , F P ~ , B P ~ , A E ~ (Fig.5)

charge derermination d) 83,04,At34,AE3 (Fig.4)

Table 1 gives a comparison of the number of kinematic variables defining the event completely and the experimental observables for reactions with N bodies in the exit channel. For example, in case of a 2-body reaction, it is swfficient to measure three quantities. If all angles, flight-times and energies are measured the reaction kinematics are determined redundantly. More pieces are measured than needed. The number of redundant observables is given in the table. TOF and angle measurements define the kinematics fully up to 4-body reactions. For more than 5 bodies in the exit channel (N-4) energy measurements become unevitable. If complete sets

of kinematic observables ( B , (P

,

t ,E) are observed, the conservation laws allow to reduce in a N-body reaction the number of identified reaction products to (N-l). This general statement applied to a two- body reaction leads to the possibility to learn all, what we need in order to describe the reaction from one reaction product, the angle, the energy and the time of flight of which is measured. This method has lead to the development of (TOF,E)-telescopes

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b o t h r e a c t i o n p r o d u c t s a r e d e t e c t e d a t d e f i n e d an- g l e s , o n l y one f u r t h e r q u a n t i t y i s needed t o com- p l e t e t h e k i n e m a t i c s . S p e c t r o m e t e r s u s i n g t h e k i - nematic r e l a t i o n s h i p between t h e p a r t n e r s of a two-body r e a c t i o n have f i r s t been a p p l i e d i n f i s - s i o n P5]. I n heavy-ion r e a c t i o n s t h e s i m p l e s t ver- s i o n of a k i n e m a t i c s p e c t r o m e t e r i s a t i m e - o f - f l i g h t d i f f e r e n c e measurement of t h e two c o i n c i d e n t f r a g - ment s

.

The advantages of t h i s method a r e t h e r e d u c t i o n t o one TOF-meas.urement which c a n b e done w i t h b e t - t e r a c c u r a c y t h a n energy measurements. Optimum mass and energy r e s o l u t i o n a r e o b t a i n e d f o r a b o u t equal masses of t h e r e a c t i o n p r o d u c t s 1 1 6 , 1 7 , 1 g ( s e e F i g . 4) The kinematic r e l a t i o n s hold between t h e primary r e a c t i o n p r o d u c t s . Primary r e a c t i o n products' a r e observed i n t r a n s f e r r e a c t i o n s and i n fu- s i o n by r a d i a t i v e c a p t u r e . 2.2. Nuclear c h a r g e i d e n t i f i c a t i o n . B e s i d e s t h e k i - nematic v a r i a b l e s t h e r e a c t i o n p r o d u c t s a r e charac- t e r i z e d by t h e i r n u c l e a r c h a r g e number. The n u c l e a r c h a r g e d e t e r m i n e s a t a g i v e n v e l o c i t y t h e a v e r a g e i o n i c c h a r g e of t h e r e a c t i o n p r o d u c t . The avera- g e i o n i c charge governs t h e d e f l e c t i o n i n g a s f i l l e d magnetic f i e l d s f i g , 2 4

,

where m u l t i p l e c h a r g e chan- g i n g c o l l i s i o n s i n t h e g a s p r o v i d e an a v e r a g i n g of t h e a c c i d e n t a l i o n i c c h a r g e s t a t e s , a s w e l l a s i t

governs t h e energy l o s s of heavy i o n s i n m a t t e r E 1 , 2 2 , 3 , where t h e a v e r a g e i o n i c charge determi- n e s t h e s t o p p i n g power. X-ray t r a n s i t i o n s which a r e induced d u r i n g t h e s l o w h g down p r o c e s s o f f e r an a d d i t i o n a l way t o i d e n t i f y t h e n u c l e a r charge of a r e a c t i o n p r o d u c t [ I 2 3 , 2 3 . Only t h e energy l o s s measurement h a s found a wide a p p l i c a t i o n f o r n u c l e a r c h a r g e d e t e r m i n a t i o n , a s i s demonstrated by t h e f a c t t h a t n e a r l y a l l experiments p r e s e n t e d i n sec- t i o n 3 make u s e of t h i s method.

Energy l o s s measurements demand i n o r d e r t o se- p a r a t e h i g h n u c l e a r c h a r g e s of heavy i o n r e a c t i o n p r o d u c t s energy d e g r a d i n g a b s o r b e r s which a r e t h i n

( < l 0 pm) and homogeneous i n t h i c k n e s s (< 1%). Gas l a y e r s e a s i l y a l l o w t o produce a b s o r b e r s of s u i t a - b l e v a r i a b l e t h i c k n e s s and homogeneity. The r e v i v a l of i n t e r e s t i n h i g h r e s o l u t i o n i o n i s a t i o n chambers 1151 i s observed a l l over t h e heavy i o n l a b o r a t o r i e s p 5 - 2 g

.

Optimum r e s o l u t i o n i s o b t a i n e d , i f about 50% of t h e energy of t h e r e a c t i o n p r o d u c t i s l o s t i n t h e a b s o r b e r . The s t r a g g l i n g of t h e energy l o s - s e s l i m i t s t h e charge r e s o l v i n g power of AE-detec- t o r s . The main c o n t r i b u t i o n t o t h e energy s t r a g g l i n g

stems from t h e f l u c t u a t i o n s of t h e a v e r a g e i o n i c c h a r g e v a l u e s , averaged o v e r a l l i o n i c c h a r g e va- l u e s d u r i n g t h e slowing down p r o c e s s @ 9 , 2 7 , 3 g . A comparison of energy s t r a g g l i n g i n s o l i d s and g a s e s shows s m a l l e r v a l u e s f o r s o l i d s , and h i g h e r c h a r g e r e s o l v i n g powers P o r degrading i n s o l i d s , respec- t i v e l y

PfJ.

The charge r e s o l v i n g power o b t a i n e d by degrading i n 1 mg/cm2 s o l i d carbon o r s i l i c o n was found t o be a f a c t o r 1.5 h i g h e r t h a n i n e q u a l amounts of A r gas. I n s t e a d of measuring t h e energy l o s s i n an i o n i s a t i o n chamber o r a S i - d e t e c t o r , t h e r e s t energy o r v e l o c i t y a f t e r p a s s a g e through a n homogeneous s o l i d a b s o r b e r may b e measured D 2 , 3 g

.

T h i s method y i e l d s t h e b e s t charge s e p a r a t i o n f o r f i s s i o n p r o d u c t s ( s e e Fig.5) and may be a p p l i e d t o - g e t h e r w i t h TOF-measurements f o r heavy i o n r e a c t i o n p r o d u c t s .

2.3. P e r t u r b e d k i n e m a t i c s . I n most heavy i o n reac- t i o n s , e s p e c i a l l y i f we go t o t h e h e a v i e s t - p r o j e c - t i l e s , t h e c o l l i s i o n par.tners l e a v e t h e e n c o u n t e r h i g h l y e x c i t e d . A c a s c a d e of e v a p o r a t i o n p r o d u c t s changes t h e mass and charge d i s t r i b u t i o n s of t h e r e a c t i o n p r o d u c t s . The w e l l d e f i n e d k i n e m a t i c s of t h e primary r e a c t i o n p r o d u c t s a r e d i s t u r b e d by t h e l o s s of nucleons. Assuming i s o t r o p i c e v a p o r a t i o n i n t h e center-of-mass system t h e k i n e m a t i c s of t h e secondary p r o d u c t s a r e modified i n f i r s t approximation a s f o l l o w s :

a ) A l l d e f l e c t i o n a n g l e s a r e o n l y d e f i n e d a s ave- r a g e v a l u e s w i t h v a r i a n c e s d e f i n e d by t h e number of evaporated p a r t i c l e s . The average v a l u e

g

e- q u a l s t h e corresponding primary a n g l e 8

.

P

b) I n two-body r e a c t i o n s t h e p r i m a r i l y w e l l d e f i - ned r e a c t i o n p l a n e i s widened by t h e v a r i a n c e of t h e secondary d e f l e c t i o n a n g l e s .

c ) A l l secondary v e l o c i t i e s have t h e same average value-:as t h e primary v e l o c i t i e s . The v a r i a n c e of t h e v e l o c i t y d i s t r i b u t i o n a g a i n i s a consequence of t h e e v a p o r a t i o n p r o c e s s .

d) A l l momenta a r e changed p r o p o r t i o n a l l y t o t h e mass.

The k i n e m a t i c o b s e r v a b l e s , a n g l e s and v e l o c i t i e s which a r e unchanged on t h e average a l l o w t o recons- t r u c t t h e primary k i n e m a t i c s from measurements of t h e secondary p r o d u c t s . T h i s r e c o n s t r u c t i o n i s n o t a one t o one c o r r e l a t i o n of secondary masses and primary masses, b u t a c o r r e l a t i o n of a v e r a g e v a l u e s

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HEAVY I O N REACTION PRODUCTS C5-165

The n u c l e a r c h a r g e i s modified by t h e e v a p o r a t i o n of charged p a r t i c l e s . As t h e e v a p o r a t i o n of charged p a r t i c l e s s e t s i n a t h i g h e r e x c i t a t i o n energy, i n many c a s e s t h e n u c l e a r c h a r g e g i v e s a b e t t e r ima- g e of t h e primary d i s t r i b u t i o n t h a n a mass measurement. P r i n c i p a l l y t h e r e i s no way t o r e c o n s t r u c t t h e primary c h a r g e d i s t r i b u t i o n , i f i t h a s been a l t e r e d by e v a p o r a t i o n , b u t t o d e t e c t d i r e c t l y t h e l o s t charged p a r t i c l e s .

The mass l o s s by e v a p o r a t i o n i s s m a l l i n reac- t i o n s l i k e f i s s i o n (AA/A%I%) where only n e u t r o n s a r e l o s t . A d e t a i l e d s t u d y of t h e secondary mass d i s t r i b u t i o n i s j u s t i f i e d a s c o r r e c t i o n s i n t h e 1%- range s t i l l a l l o w a n a c c u r a t e r e c o n s t r u c t i o n es- p e c i a l l y i n a c a s e where t h e n u c l e a r c h a r g e s + a r e unchanged D 4 , 3 3 , 3 g ( s e e F i g . 5 )

.

I n heavy i o n re- a c t i o n s - t h e mass l o s s r e a c h e s 10%, e.g. i n f u s i o n and deep i n e l a s t i c r e a c t i o n s . I n f u s i o n r e a c t i o n s a measurement of t h e i s o t o p i c d i s t r i b u t i o n of a l l e v a p o r a t i o n r e s i d u e s a l l o w s a d e t a i l e d s t u d y of t h e e v a p o r a t i o n p r o c e s s , and assuming t h e l a t t e r i s understood, t h e d i s t r i b u t i o n of e x c i t a t i o n energy and s p i n i n t h e primary f u s e d compound system may be r e c o n s t r u c t e d E a , 3 7 3 ( s e e F i g . G ) . I s o t o p i c a n a l y s e s of heavy e v a p o r a t i o n r e s i - dues seem t o be a most c h a l l e n g i n g a p p l i c a t i o n of h i g h r e s o l u t i o n mass and charge i d e n t i f i c a t i o n methods.

I n deep i n e l a s t i c two-body break-ups primary d i s - t r i b u t i o n s s t i l l may be o b t a i n e d from measurements of t h e k i n e m a t i c o b s e r v a b l e s t and 8 . The mass l o s s governs t h e v a r i a n c e of t h e d i s t r i b u t i o n s of observed t- and 8-values. These v a r i a n c e s t r a n s f o r m in- t o t h e accuracy of t h e r e c o n s t r u c t i o n of t h e p r i - mary mass v a l u e s . The primary mass v a l u e s o b t a i n e d have an accuracy of about 30% of t h e r e l a t i v e mass l o s s by e v a p o r a t i o n . A s o n l y t h e primary d i s t r i b u - t i o n i s of i n t e r e s t i n a s t u d y of t h e r e a c t i o n me- chanism a l l measurements of secondary d i s t r i b u t i o n s w i t h a n accuracy b e t t e r t h a n 30% of t h e r e l a t i v e mass l o s s become d o u b t f u l , a s long a s t h e primary masses a r e n o t known more a c c u r a t e . A second way

t o r e c o n s t r u c t t h e primary d i s t r i b u t i o n i s t o d e t e r - mine t h e secondary d i s t r i b u t i o n and t o measure i n a d d i t i o n t h e mass l o s s . Measurements of t h e energy of a fragment t o g e t h e r w i t h TOF g i v e t h e secondary masses. To t h e k i n e m a t i c o b s e r v a b l e s N a d d i t i o n a l

E , B p , o r Fp ,--measurements kcome n e c e s s a r y t o determine t h e secondary masses i n a N-body r e a c t i o n . A d i r e c t measurement of t h e e v a p o r a t e d p a r t i c l e s

from t h e i n d i v i d u a l fragments, a s i n t h e c a s e of charge d i s t r i b u t i o n , i s one p o s s i b i l i t y t o determine t h e m i s s i n g mass. On t h e o t h e r hand i n a two- body r e a c t i o n t h e m i s s i n g mass is o b t a i n e d from t h e s p r e a d of t h e a n g u l a r d i s t r i b u t i o n . The spread of

9 and i s p r o p o r t i o n a l 'to t h e m i s s i n g mass. The a v e r a g e number of e v a p o r a t e d p a r t i c l e s p e r fragment o r t h e average a n g u l a r widening g i v e t o g e t h e r w i t h average secondary masses an average v a l u e of t h e primary masses. The v a r i a n c e of t h e d i s t r i b u t i o n s of t h e m i s s i n g masses o b t a i n e d depends on t h e de- t a i l s of t h e e v a p o r a t i o n cascade, which i t s e l f i s governed by t h e s p i n and e x c i t a t i o n energy d i s t r i - b u t i o n s of t h e s i n g l e primary fragments. Again, an i s o t o p i c d i s t r i b u t i o n of t h e secondary r e a c t i o n pro- d u c t s would n o t a l l o w t o r e c o n s t r u c t t h e primary i- s o t o p i c d i s t r i b u t i o n , a s t h e mass l o s s e s depend on t h e i n t e r n a l d e g r e e s of freedom of t h e primary break-up, which p r i n c i p a l l y i s n o t f i x e d by mass,

charge, and momentum a l o n e . Angular momenta and ex- c i t a t i o n e n e r g i e s of i n d i v i d u a l fragments have t o be measured i n a d d i t i o n . I t seems t o me s t i l l a n o- pen q u e s t i o n t o what a c c u r a c y m i s s i n g mass measurements s h o u l d b e c a r r i e d . However, i t i s e v i d e n t ,

i n e l a s t i c r e a c t i o n s w i t h mass l o s s e s of 10% a r e n o t t h e f i e l d of high r e s o l u t i o n mass and charge measurements.

Table 2 shows f o r t h e c a s e of p e r t u r b e d kinema- t i c s t h e number of o b s e r v a b l e s which have t o be measured under d i f f e r e n t l i m i t a t i o n s . These a r e j u s - t i f i e d e i t h e r by t h e type of r e a c t i o n o r t h e a t t i - tude of t h e experiment,whether a n e x p l o r a t i v e s u r - vey o r d e t a i l e d i n f o r m a t i o n i s wanted.

1) N e g l e c t i n g e v a p o r a t i o n of charged p a r t i c l e s and assuming an unique r e l a t i o n s h i p between primary mass and charge of a fragment, l e a d s i n c a s e of a two-body r e a c t i o n t o a (AE-E)-telescope a t f i x e d b u t v a r i a - b l e a n g l e B7-407 o r t o a p o s i t i o n s e n s i t i v e l a r g e a r e a d e t e c t o r measuring a l l a n g l e s s i m u l t a n e o u s l y

.

The above n e g l e c t i o n s govern t h e u s e f u l accu- r a c y of t h e measurement. I n deep i n e l a s t i c r e a c t i o n s t u d i e s t h i s t e c h n i q u e h a s become s t a n d a r d ( s e e F i g . 7 ) . Two c o i n c i d e n t s i n g l e E - d e t e c t o r s a t f i x e d

(7)

P. ARMBRUSTER Table 2

-+

Perturbed Kinematics

(5

< IOZ,

l

vs/ =

I

:p\,

Es=ep)

*n

Prototype reaction

l-body 2-body 3-body 4-body

Evaporation Deep inelastic collision Deep inelastic collision Deep inelastic

residues with subsequent fission collision with

subsequent fission

I) Minimum number of observa- 0 bles neglecting evaporation, if assumed Z -A .Z /A

P P 0 0

Typical instruments ,0°

-

detector R) (AE-E)-telescope et 03 3 (AB-E)-teleacopes 4(AE-E) telescopes

(Fig. 1 ) (Fig. 7)

b) (AE-E-0) detector

(Fig.9)

-

C) 2~-detectors at 03

and 04 CRef.421 2 ) Minimum number of observe- Kinematic observables and I missing mass bles, if assumed temperature

of all fragments constant (T-const.) , and Z -A

.

Z /A

P P 0 0

Typical instruments 0'

-

detector + a) (AE,E,t)-telescope at O3 Large area detector (8.+.t) + energy of

energy (Fig.8) one fragment

b) (Bp-E-AE) magnetic spectr. at 03 [Ref.45.46]

3) Minimum number of observa- Kinematic observables and 1 missing mass and N nuclear charges bles, if assumed T-const.

Typical instruments (AE,E.t)-telescope (AE,E, t,B)-detector + Large area detectors (~,+,~;AE,E)

at small angles (AE,t,0)-detector

(Fig. 6 ) (Fig.9)

4) Minimum number of o b s e ~ a - Kinematic observab1es.N missing masses ans N nuclear charges bles with complete kinematics

and (Z,A)- analysis

Typical instrumente N (AE,E,t)-telescopes or large area detectors (B.@,~,AE,E)

2) If the nuclear system which breaks up, has a constant nuclear temperature the level density parameters of the fragments determine how the excitation energy is shared between the reaction products. The missing mass measured for one fragment allows within this approximation to calculate the missing masses of all other fragments. In a two- body reaction 4 parameters measured for one fragment are sufficient to determine the kinematic variables and the mass loss due to evaporation (743,43 (see Fig. 8). The Dubna-group applied a system u- sing magnetic deflection together with a (AE,E)- detector to the deep inelastic reaction between Ar on Th

[ 4 q .

A similar system is used by the Orsay- group C4g.

3) The assumption made in the last section together with a kinematic mass measurement and a nuclear charge determination allows to determine primary mass and charge distributions and missing masses. Two detectors, one of them a (AE,E,t,9)-detector, the other a (AE,t,0)-detector, are a system tested

C411 and to be applied for deep inelastic reaction

adequate to the problem (see Fig. 9).

4 ) The most complete detection system giving pri-

mary masses, nuclear charges, secondary masses, e- nergies and momenta for all fragments technically seems feasible. Large area detectors measuring (0 ,$

,

t,E ,AE) for each of the fragments are developed by a Heidelberg-GSI collaboration C487

.

In a 4-body break-up an event will be characterized by 20 parameters. The data evaluation pvblems arising in the analysis may be solved, but go to the limit of the possibilities of 1976-computer systems. 3. Direct Identification methods, what has been achieved 1976.

(8)

Table 3.

Specifications obtained for different Identification Methods applied to Heavy Ion Reactions.

S ~ccics power eft icicnt for

Ilef. Encr- I'rojoc- Target Ohscrv. Pcrlur- (A,%\- A isotropic emission

gy/AMU tile Angle bation )$:(AU E E AE

iMeV) ("1 AA/A(X) Fission Fusion Deep ine Z a s t i c r e a c t i o n (E,AE) (E,AE) Table 4

Rates of detected reaction products.

Type of reaction Reaction do/dn Target at ms ₉ Beam intensity Reaction products Time for a 3%

-,

b r ) pd(mg/cm ) (part.s-l) detected ($-l) o = 1 0 - ~ o _max

-

experiment (days) Transfer < I O - ~ 3 10~~t0.1) 1o12 3 Ar + Au 4 df? = I msr Kr + Au < IO-' 1.5 X 10~~(0.05) 3 X 10" 0.45 27 E/AE = lo3 Xe + Au < 10-2 6 X 10~~(0.02) 1011 6 X 10-2 200

E/A = 5 ~eV/amu U + AU < 10-2 3 x 1016(0.01) 5 x to1O 1.5 X I O - ~ 800

Pusion aF = 0.2 b A r + S m + H g < 4 4 X (1) 4 X 10l2 1.2 X 10' 1 0 - ~ dn = 2msr Kr + Nb -t Ir 15 6 X 10" (1) 8 X 1011 1.3 X 105 I O - ~ G I A = 5 MeV/amu Xe + Ni + Pb 70 l0l9 (1) 2 X 1011 2.8 X lo5 5 I O - ~ Zeep irie Z u s t i o do = 5 msr AT + Ca 0.1 1 . 5 1019 (1) 4 x 10l2 3 I O ~ 4 10-~

E/AE = 100 Kr + sr < 0.5 3 X 1o18 (0.5) 1oI2 7.5 lo3 2 I O - ~

E/A = 7 MeV/amu Xe + Sn < 1 1018 (0.2) 5 X 1o1] 2.5 X 103 I O - ~

(9)

C5- 168 P. ARMBRUSTER

Table 3 gives typical experiments from different fields which have to identify heavy ion reaction products. Only measurements where at least either the mass or the charge number have been identified

0 1

by a direct method have been included. The techniques renouncing high resolution but aiming at a

&E- ,OF - E

F:,*EL%+p

p

complete survey of the most important parameters

- m c m i

of the reaction are not discussed. However, their 1o3-

bE E 5, SURFACE BARRIER DETECTORS

merits in fission [l51 and deep inelastic reaction work

1

39,42

1

cannot be emphasised enough.

Table 4 gives estimates of maximum detected ra-

. -. tes which may be obtained by suitable instruments

already running or planned. A range of many orders

cl d l

of magnitude spans between fusion and transfer re- *U h' GZ

105 actions from heavy projectiles. Measuring times are given in the last column assuming 1 0 - ~ of the dif-

2

m'

ferent maximum cross sections should still be mea- surable and a 3% statistical accuracy should be

102

reached.

3.1 Transfer. Transfer reactions have unperturbed

kinematics. High resolution in energy with an uni- CHANNEL NUMBER CWNNEL NUNBEE

que isotope identification is wanted. The relative energy resolution of the detection system has to be increased proportionally to the mass of the projectile, if equal absolute accuracy is demanded. This trend together with the increasing kinematic aberrations, the increasing energy and angular strag- 'gling per unit target thickness for heavier ions, restrict e.g. the energy resolution for U-beams to

2 about l MeV at a target thickness of 10 ~.lg/cm

.

A

10 pb/sr transfer cross section leads in the case of U as projectile to measuring times of 800 days as may be seen from table 4. The small counting rates to be expected in transfer reaction studies for very heavy projectiles together with the unsol- ved identification of heavy masses and charges make transfer studies the domain of medium mass and light projectiles (A 6 40).

16 120Sn, 1 18Sn) IS0 Fig. 2. gives the reaction O(

investigated by the (E,t ,AE)-technique [l01

.

Two Si-detec-tors at a distance of 20 cm have been used to measure 2 energies and TOF. Isotopes have b~:en separated: An energy resolution of 300 keV has been achieved. Reactions with S-ions have been reported to be investigatedwith the same techniques. A first

experiment done with Xe on Pb at UNILAC showed mass and charge resolution to be insufficient for

separation of heavy isotopes [49]. The small thickness of the AE-detectors, their insufficient

Fig.2. a) Specifications reached with (AE-E)-telescopes using Si-surface barrier detectors in experiments with 160- and 3 2 ~ - beams [10]. b) Mass spectrum of the reaction 74 MeV 160 on 122~n. ,

c) Element spectrum.f_or the same reaction. d) Ener,

18

gy spectrum of the 0 reaction products. Energy resolution 300 keV.

homogeneity, the lack of knowledge of pulse height defects spoiling the energy resolution, and radia- tion damage of the detectors ape the reasons why for very heavy ions the application of the technique is restricted. However, AE-detectors have been made homogeneous enough in areas of a few nun2 to separate light fission fragments C351

.

Principally energy- and time- resolution should not be much worse than the values obtained with S-ions. The low detection efficiency of the telescope is a drawback,

I

which hardly will be overcome. Still the simplicity of the set-up often compensates for the larger measuring times.

Magneticlspectrometers have to be equipped to i- dentify isotopes from heavy ion reactions with

(10)

HEAVY ION REACTION PRODUCTS

lenghts in the spectrometer are suitable conditions for an optimum TOF-measurement. A system operating at the 88'-cyclotron at Berkeley zs \sed fdr traris- fer reaction studies

(

1 1

[

.

High energy resolution and high acceptance of magnetic systems suggest in case the isotope identification may be improved an extension of the technique to heavier projectiles 1471

-

GSI MAGNETIC SPECTROMETER L

E CTOR TYPE QODQ DEFL ANGLE 45' 3 Trn ?&AL LENGTH 750m SOLID ANGLE l rnsr I s 3 rnsrl

$$&%EN

1 5 6crnl%

FOCAL PLANE 50crn.l PARTICLES ENERGY

ACCEPTANCE ' 3 3 8 5 %

ENERGY 5 10.' FWHM k=O Du- c.U-

-

RESOLUTION 10.' -,,- k=l I

FLIGHT PATH 6 . 0 X A€, AE, T, .E

DIFFERENCES l'' DETECTOR SYSTEM SCATTERING t o +l350

ANGLE

Fig. 3. Magnetic spectrometer to be installed at

GSI [SO]. The detector system will register TOF, en-

trance angle, radial position in the focal plane, AE, and E.

Fig. 3 gives the specifications of a spectrometer planned at UNILAC [12,50]

.

The time resolution of the TOF-measurement is hoped to be improved considerably compared to the Berkeley system in order to achieve separation of isotopes up to

A

Q 100 with

unchanged acceptance of 1 msr.

Kinematic coincidences together with a time-difference measurement allow to obtain high mass re- solutions. Fig. 4 demonstrates the state of the art 1511. Together with high resolution nuclear charge

1

detectors this method will combine unique isotope identification, fair energy resolution and good detection efficiency. It avoids the energy detectors, the weak point of (AE,E, t) telescopes, and, it is much simpler than a (Bp,E,t,AE)-system. It seems to be an adequate compromise between Si-detector techniques and magnetic spectrometer systems.

I Counts

Fig. 4 TOF-difference method [51] demonstrated with the elastic scattering of 310 MeV 8 6 ~ r on "sr. 83, 84, and At34 have been registred for each scattering event. Elastic scattering events are seen as two groups in the (03,04) projection (upper left), as well as in the (83, At34) and (83,m3)-projection, respectively (upper right). In the lower right a mass spectrum is presented demonstrating the mass resolution reached.

3.2 Fission. Fig. 5 shows the isotopic separation of fission fragments. "Lohengrin", a system installed at the HFR-Grenoble allowed for the first time a direct separation of single fission fragments 114,33,35]

.

The production cross sections for all secondary light fission fragments as a function

235* of the total kinetic energy are measured in

thermal fission. "Lohengrin" demonstrates how principally it is possible to separate reaction products in a heavy ion reaction. However, the low efficiency of 2 ~ 1 0 - ~ tolerable at a HFR does not recommend' an application at accelerators-with primary reaction rates'which are a factor 10" smaller than at a reactor, unless favourable kinematics, as in fusion reactions, help to compensate for the difference in reaction rates. Spatially separated rates

3

of less than 10 evaporation residues per sec would be obtained at UNILAC.

(11)

Source - -- '\ AE

E

8

urn

S i

-

detectors U

carbon zero - time stop absorber detector detector

channel number T O F l arbitrary

scale

(12)

HEAVY ION REACTION PRODUCTS mass resolut on A I 6A 70

-

I I 132 MeV 3 2on ~ 2 7 ~ ~ , 8 = L 0 S<! . . . 0 5 0 100 M eV ENERGY CHANNEL

Pig. 6. a)-Mass energy projection of evaporation residues from the 132 MeV 3 2 ~ on 2 7 ~ 1 fusion reaction. Electrons knocked but by the heavy-ion passing a thin C-foil produce a fast signal from a channel plate detector. This start _signal togethet with the stop signal taken from a surface barrier energy detector defines TOF [ 3 6 ] . b) Elements produced as evaporation residues in the 495 MeV '8'4~r on 2 7 ~ 1 fusion reaction are separated by a (AE,E)-ionisation chamber l301

.

present. state of the art L321

.

,Behind SHIP (see Fig. l), an isotopic analysis of heavy evaporation residues using (AE,E, t)-telescopes is planned. 3.4. Deep inelastic reactions. Ionisation chambers, :

S high resolution charge detectors, have been u- sed in (AE,E)-telescopes in different laboratories to investigate deep inelastic reactions [38,40]. The motivation to use this technique has been explained in section 2. Fig. 7 gives a (AE-E)-plot

for the reaction Xe on Fe [40]. The resolving of single atomic numbers was possible up to Z=46.

- '00

1

.C.* A.'' % d..'.C ... L ..*.... in ,..C.'* .,C' 8 -1 300 - > 0 W W zoo -

~ E V I N 132XE O N 56FE 16 DEGR

I

-

350 LOO

An analysis of the light and fast component of the ENERGY IMeVI

secondary isotope distribution has been done using _Pig._7. _{(AE,E)-plot of the reaction products}

magnetic spectrometers with (AE-E) detector systems a MeV/AMU. 132xe On 5 6 ~ e 1401

-

The same chamber as Fig. 6b has been used. The in- [45,46]. An alternative giving comparable resolution _{sert shows the element}

is a (AE,E,t) analysis of the reaction products

Fig. 9 shows a two-detector system (AE,t,E,O)- [43,44. Fig. 8 shows a system using a plastic scin-

detector and (AE,t,E,B)-detector, respectively. The tillator foil as start detector, an ionisation cham-

example shows an application to the A; on Ni reac- ber as 2-detector and a Si-diode as energy detector.

tion at 7 MeV/AMU [41j. The use of large area detec- Reaction products up to 60~i, have been separated,

tors gives the angular coordinate as one of the pa- a result nearly as good as the best measurements with

rameters of the measurement. The (E ,9 )-plot for

(13)

P. ARMBRUSTER

vy ions themselves cannot be overcome by technical means. The limits which are set depend on the reac-

tion we are investigating. Thus the improvements a-

channel

Fig. 8. Isotopic distribution of the reaction products from 240 MeV Ar on 5 9 i . a) Principal set- up of a (E,t,AE)-telescope l441. Light produced in a thin scintillator, resistered in a multiplier gives the start signal for a TOF-measurement stop- ped by a signal from a Si-surface barrier detector. AE is measured in an ionisation chamber [26]. b)

(~t~,~)-plot of the reaction products. Masses are separated up to A=60. c) (AE,E)-plot of the reaction products. Elements are separated up to 2=28. d) Isotopes produced in the reaction. About 50 species are identified up to 6 ~ i .

the Ar-reaction products immediately shows how the deep inelastic component is spread towards large angles, which physically are negative angles. The position sensitive counters give in addition the angular spread in the reaction plane and out of the reaction plane. The spread itself is a measure of the missing mass, which on the other hand may be obtained from the differclice of primary and secondary mass. Both masses will be obtained with the set-up. The ratio of the angular spread in and out of the reaction plane gives first information of the number of evaporated particles in these two di- rections and the alignment of the reaction products with respect to the beam direction. A prototype counter of the large area detectors planned for the many-body break-up experiments has been successfully tested recently in this set-up [51].

4. Improvements needed in direct detection techni-

W.

gain should be discussed in connection with the different types of reactions.

4.1. Transfer reactions. Transfer studies are rate-

limited for heavier projectiles. Spectroscopic studies will hardly be done wit-h heavy projectiles. The information gained per time unit and the energy resolution are less compared to work with ligh- ter ions.

Existing, detection systems allow to separate medium mass isotopes. But the upper limit d£ solid angle set by kinematic aberrations by far has not been reached by (E, t ,AE)-telescopes. Larger solid angles become accessible with gas-counter systems. Parallel plate counters in front of (AE,E) ionisa-

tion chambers allow to reach I msr solid angles. TOF-paths of sufficient lenght are possible even at

these apertures if quadrupole focussing along the flight path is provided. [52].

Magnetic spectrometers which do have advantages concerning the background of particles from elastic reaction channels need development of large area (B,E,t,AE)-detector systems. The long term reliabi- lity of these detectors is of special importance.

4.2. Fusion-evaporation residues. No intensity problems in fusion reaction studies are foreseen. The strong kinematic forward peaking allows to detect 10 nb cross section under the constraints of Table 4. The stcdy of very rare events with cross sections of 0.1 nb still are possible at rates of a few e- vents per day.if a sufficient reduction of the primary beam has been achieved. Up to A-200 yields of

5

10 /sec may be ohtained but a direct isotopic separation until now is not possible.

(E,t,AE)-telescopes need better energy resolution. Thin, homogeneous, Si-surface barrier detectors of better quality are needed to improve the Z-resolution of Si surface barrier telescopes. Whether ionisation chambers actually are giving better energy resolution urgently should be settled. The double TOF-technique should be developed further. A separation up to A8200 is within the scope of the possible improvements of detector telescopes.

(14)

HEAVY ION REACTION PRODUCTS C5-173

Lab J I

-

o3

Fig. '9. a) Two-detector set up to study two-body break-up reactions[41]. Parallel plate aetectors are used as TOF-detectors. Position detectors, AE-and

E

detectors are gas proportional counters and ionisation chambers, respectively. The detector 1 (right in the figure) measures O,$,E,t, and AE, the detector 2 (left in the figure) measures B,@,AE and the time difference with respect to detector 1 . b) (AE,E)-plot for the reaction 4 0 ~ r on 58~i. c) (Z3,83)-plot for the same reaction. d) Ar-reaction products in a

(15)

C5- 174 P. ARMBRUSTER

promise at 2 MeV/AMU a charge separation up to 2-92. 3

Production rates of 10 /sec spatially separated isotopes are possible. They allow simple decay sche- me studies. Identification of isotopes, kinetic e- nergies of evaporation residues and excitation func- tions may still be measured with rates of ]/sec. Studies of the evaporation process demands to measure angular distributions. The angle between beam direction and spectrometer has to be varied within at least 10'.

4.3. Deep inelastic reactions. There are no serious restrictions in rates. With large area detectors e- ven in experiments with U-beams cross sections of 50 pb/sr may still he measured

.

The mass, charge and energy resolution of existing systems is within the accuracy demanded by the physics of the deep inelastic reaction process and the secondary evaporation processes. The isotopes produced in the reaction are especially interesting in the light

mass region, as here new neutron-rich species may be found. Telescopes or existing magnetic spectrometers allow an identification. Spatially separating electric or magnetic combinations suffer from the low intensity of (l-l~-~)/sec. They are not recom- mended.

4.4. Many-body break-ups. The existence of true break-ups in 3 or more pieces would be an interesting new type of reaction. The separation from abundant sequential processes needs an identification method sensitive to detect the rare true many-body.channe1. The large-area-detectors which have been developed will be useful tools in these experiments. The data evaluation may pose the most serious problem of the experiments.

Stimulating discussionbwith many scientists at GSI are acknowledged.Especially, I am grateful to those who made available their first unpublished experimental data.

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,

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HEAVY ION REACTION PRODUCTS C5- 175

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