N U C L E A R PHYSICS A

~:~st~vu.:~ Nuclear Physics A583 (1995) 461-464

Chimera: a project of a 4II detector for heavy ion reactions studies at intermediate energy

S.Aiello b , A.Anzalone ~ , M.Baldo b, G. Cardella b, S.Cavallaro ~,~, E.De Filippo b , A.Di Pietro ~'~ , S.Feminb~'Y, P.Figuera ~ , P.Guazzoni d,g, C.Iacono-Manno ~, G.Lanzanb b , U.Lombardo b'~, S.Lo Nigro b,~, A.Musumarra ~,~, A.Pagano b, M.Papa b, S.Pirrone b, G.Politib,~, F.Portoa, ~, A.Rapisarda b, F.Rizzo ~,~, S.Sambataro b,~, M.L.Sperduto ~,~, C.Sutera b, L.Zetta d,g.

INFN: ~Lab. Naz. de1 Sud, bSez. di Catania and CGruppo coll. di Messina, aSez. di Milano

Dip. di Fisica: ~Univ. di Catania,/Univ. di Messina,gUniv. di Milano

One of the most interesting goals of the intermediate energy heavy ion research is to probe the properties of the nuclei under extreme conditions of density and temperature. The hot and compressed system formed in the early stage of the collision can deexcite leading to multifragment final states. This multifragmentation is predicted to be the major decay mode for a nuclear system produced at high density and temperature [1].

An experimental investigation on the appearance of this process is of particular relevance to understand the basic properties of the equation of state (EOS) of the nucler matter , but the occurence of different reaction mechanism and the large number of nucleons involved in a reaction event largely increases the experimental work.

A complete experimental investigation, indeed, needs to detect and identify almost all the reaction products, and requires an event by event analysis of the nuclear reaction with an accurate determination of the primary properties of the system undergoing multifragmentation, of the space - time characteristics of the process [2,3] and of the multifragment correlation function [4]. Consequently the availability of experimental devices suitable to carry out exclusive experiments has become necessary in order to satisfy such requirements by means of precise measurements of the number of particles, their energy and their spatial and charge and mass distribution.

With the purpose of investigating the above mentioned topics of the heavy ion physics at intermediate energies, we designed a 47r detector for charged particles, named CHIMERA, which is a very suitable device for studying the fundamental heavy ion nuclear physics, as nuclear matter equation of state, liquid vapour phase transitions, hot nuclei instability, ion-ion collision dynamics and multifragmentation.

CHIMERA can be schematically described as a set of 1192 detection cells, arranged in cilindrical geometry around the beam axis, in 35 rings. The forward 18 rings are assembled in 9 wheels covering the polar angle between 1 ° and 30 °, and are placed at a distance from the target variable from 350 to 100 c m with increasing angle. The

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462c S. Aiello et al. / Nuclear Physics A583 (1995) 4 6 1 - 4 6 4

remaining 17 rings, covering the angular range 30 ° - 1 7 6 °, are assembled in such a way to shape a sphere 40 crn in radius. Considering the beam entrance and outgoing holes and the frame of the detectors, the overall detection solid angle is about 0.94% of 4~r. The fig.1 shows a view of the mechanical structure and in table I the geometrical features of the detector are summerized.

T a b l e I

Whe~ l D(cm~ R inq T h i n T h M relict I~hi S(cm 2) f l (ms r !

I 350 1 I.O I,O 16 ~ lli~ o.13 2 I.O ~-6 16 22.5 ~ O.3]

2 300 3 2 ~ ~ 24 ISO 12.2 02~ • 3.6 ~ 24 IILO

6 l l I I .~ l I O+d6 • 21o ? ?.o 8~ m ILO ~Ldl

O 5.5 Ilto ,Io !,0 ~ . I OJ~ 5 laO 9 10+0 II.S ~ U ~ 0,77

I0 11.5 13.0 40 ~ ~2 6 160 I I 13.0 145 411 "/.S 20.8 OJII

12 14S 16.0 40 73 331.1 ¢LgO 7 140 13 16,CI 18.0 48 "/.~ ~ I.~4

14 18.0 33.0 ~1 ?,S ZS, I 1.49 II 120 15 %100 22,O m 7,5 Z3.,II 1.54

I~ 220 24+0 4O 73 :~7 1.7e g ;oo 17 340 2?•0 40 73 2g.5 ~gS

18 27.0 310.0 411 73 33.? 327

S p h e r e ~0 is 30.0 ~ l l l 32 112t ~ IlL.t~ 20 :leo 4+.0 33 1125 ~ 1134 |1 41.0 540 ~1 11.+15 ,~ 21.00

S(O ~ 32 I I . I~ 312 63.0 70,0 33 II.2S 40.1 2S.0~i

24 ?~0 71L0 32 11.15 ~ ~L15 2S ?8.O 04.0 32 I 1~ ,Cl.4 2T.IS 215 IN,O 940 32 11,.~ ,l~.l 2?.42

~ . . . . . . . ~ " ~ ~ I10.o IIILO II~I~ ~.01

30 |IILO I~.O 33 I I ~ I ~nJ2~l

32 134.0 Id~l,O 11.2S 111.34

31 150.0 153.0 ~.7 ~ l .~ 35 163.0 176.0 II 45,0 S09 3t,79

. . . . . ~ % . .

fig.1

Each detection celt is a telescope made of a 300 # m thick silicon detector and of a CsI(T1) crystal thick enough to stop all the charged particles. Both detectors have a trapezoidal cross section.

Besides reducing multiple fire, the high granularity allows a significant total re- construction of the events, as pointed out in fig.2, where the probability of a total recostruction of the events is reported versus the number of cells, for different multi- plicities and coverings (e) of the solid angle.

,o]

0

~- 4 13_

Mutt=30 Mult=40

. . . . - " " e = 0.94 . .

x "

/M"

. / e:O.90

400 800 1200

41 . I " x

e - - 0 9 4 31 ..'""

N "

21 / "

11 -" e : 0.90

400 800 1200

n ° del. ceils

fig.2

S. Aiello et al. I Nuclear Physics A583 (1995) 461-464 463c

In fig.3 the distribution of the charged particle multiplicity for the 5SNi +03 Cu reaction at 50 M e V / A is displayed, as "measured" by CHIMERA compared to the one predicted by the code FREESCO after integrating over the whole range of the impact parameter. In the calculation, performed using the code GEANT, a threshold of 1.4 cm/nsec has been taken into account.

The shape and dimensions of CHIMERA made it suitable for TOF technique mea- surements and enables us a mass identification of the fragments stopped in the first stage of the telescope. This technique overtakes the limit, peculiar to the detectors based on charge identification, due to the Bragg peak placed at about 1 MeV/A . The way CHIMERA works in identifying the fragments detected with a 300 cm path of fligth is sketched in fig.4, where an energy and time resolution of 1% and 1 ns, respectively, has been assumed.

5BNioG3Cu 50MeV/A

101

l,q (-

o ~ lO

1

0

. . . . FREESCO

Ill.,. 10 20 30 L,0

Charged particle multiplicity 50

Mass separation vs E/A

?o

0 10 20 mass 40 50

A - E>Ethr ~ Z B -M/AM >M

C-Z&M D-M>M/AM> M/2

6O

fig.3 fig.4

In table II the maximum values of A !dent!fed with F W H M = I are reported for each part of the detector; the second value reported for E > 2 M e V / A corresponds to an hypothesis of 0.5% in energy resolution, which is more reahstic.

Table I1

M~VlA I{IMG d(m) Ot .= O~(') 0.2 0.$ 1 2 5 IO

| 3.S I "-2.G 73 64 55 47/61 3G/44 28/33

2 3.0 2.6 -{.- 4.6 70 60 52 43/$$ 32/39 25/29

3 2.5 4.6 -;- 7.0 66 56 47 39/48 28/33 22/25

4 : 2.1 7.0 -;- 10. 63 5! 43 35/42 25/29 !9/2!

5 1.8 10. + ]3. 59 45 39 31/37 22*/25 17./!8

6 16 13. + 16. 56 45 36 29*/33 20*/23 15,/16

7 1.4 16. + 20. 53 41 33 26/30 !8./20 !3./15

8 L2 20. -;- 24. 49 38 30 23/26 16/18 12./]3

g I.O 24.-;-30. 44 34 26 20*/22 14./15 IQ*/I!

> |0 0.4 >_ 31[]. 2'~ 17 12 9/ !0 6/6 4/4

464c S. Aiello et al. / Nuclear Physics A583 (1995) 461-464

Some tests, both on silicon detectors and on CsI(T1) crystals, have been carried out before defining the project. A timing resolution as good as 400 psec has been easily obtained with 25 cm ~ 300 Frn thick silicon detector . Fig.5 displays a E - T matrix for the 19F +lz C reaction at 5.6 M e V / A , obtained with such a detector placed at 200 cm from an MCP giving the start signal of the TOF.

In order to discriminate light particles detected by the CsI(T1) crystal, we performed a pulse shape analysis of the signal derived from the photodiodes, by using the two gates method. A good identification has been obtained for energies larger than 25 M e V / A as we can see in fig.&

g

O1

19F.12C E = 105MeV • 0 : 6 ° L =2m

/ ~ ' i ~:

i / " " i'

r0 ,., .[::. ~ ~ 2 8 . .!

0 25 50 75 ' 100 E (MeV)

8 ( ] ~ . . ,

,...~

0 Gate 2 °75/Gate l(arb, un.)

/

gate 2 , ~ . . , ,

g

50 100 0 5 10 15 time(ps)

fig.5 fig.6

We can conclude that all features and characteristics of CHIMERA, we briefly men- tioned above , will make it complementary and, under some aspects, competitive with the more performant detectors for heavy ion physics at intermediate energy currently in use, if we look at the present prospects of developping this wide area of the fundamental nuclear physics research.

1) D.H.E.Gross, Rep.Prog.Phys. 53(1990)605 2) E.Bauge et al., Phys.Rev.Lett. 70(1993)3705 3) D.R.Bowmann et al., Phys.Rev.Lett. 70(1993)3534 4) R.A.Lacey et al., Phys.Rev.Lett. 70(1993)1224