The response of a large CsI (TI) detector to light particles and heavy ions in the intermediate energy range

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1 Nuclear Instruments and Methods in Physics Research A344 (1994) North-Holland NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A The response of a large CsI (TI) detector to light particles and heavy ions in the intermediate energy range A.S. Fomichev a, I. David a, S.M. Lukyanov a, Yu.E. Penionzhkevich a, N.K. Skobelev a, O.B. Tarasova, A. Matthiesb 1, H.-G. Ortleppb,1, W. Wagner b,1, M. Lewitowicz 1, M.G. Saint-Laurent, J.M. Corre, Z. Dlouhy d, I. Pecina d, C. Borcea e a FLNR, JINR, PB 79, Moscow, Russian Federation b RCR Inc., PB , D-01314, Dresden, Germany I GANIL, BP 5027, F Caen Cedex, France d NPI, CS-25068, Aei, Czech Republic IAP, PB MG-6, RO Bucharest, Romania (Received 29 September 1993 ; revised form received 20 December 1993) Particle dependent response of a CsI(TI) scintillation detector with an entrance surface of 314 cmz has been investigated using a secondary beam facility at the energy range from 2 to 77 MeV/A. The mass and charge identification of particles with Z = 1-18 has been performed by means of time-of-flight and energy measurements. The dependence of light output on E, Z and A has been studied. Using a pulse shape analysis possibility to identify the masses of momentum separated particles with charges of Z = 1-7 is presented. 1. Introduction Inorganic CsI (Tl) crystals possess many features [ 1-31 which are particularly useful for multi-module detection systems having a large, nearly 47t sr, geometrical efficiency [4-6]. The most important features are: i) light output and energy resolution, which are comparable with Nal(TI) ; ii) strong dependence of light output as well as of pulse shape on the type of ionizing radiation ; iii) good plasticity, low hygroscopicity and other features which make it possible to build a detector having a large effective surface. However, not many of the experimental data obtained up to now allow a demonstration of these advantages. Recently, in refs. [7,8 ], the authors have analyzed the dependence of the scintillation properties of a CsI (TI) detector on mass and charge of light and heavy ions with energies up to 40 MeV/A. Similar measurements have been done with relatively small CsI (Tl) crystals mostly combined with highly effective photodiodes [2,9-11 ]. The aim ofthe present paper is to study the response of a CsI (TI) crystal with a large entrance surface viewed by the photomultiplier to the broad spectrum oflight and intermediate mass fragments obtained at the output of a magnetic spec- Present address: FLNR, JINR, PB 79, Moscow, Russian Federation /94/$ Elsevier Science B.V. All rights reserved SSDI (94)00004-Q trometer. We have also investigated the pulse shape analysis (PSA) in CsI(TI) scintillator for different ions. 2. Description of the CSI(TI) detector The CsI (TI) detector, shown in Fig. 1 a, represents an element of the scintillator shell of the multidetector array FOBOS [6] built at FLNR of JINR in Dubna. A total number of 210 detectors are placed behind the gas detector ball of FOBOS at a distance of about 80 cm from the target. They have been manufactured by the Monocrystalreactiv Company in Kharkov, Ukraine, applying Kyropoulos' method of crystal growing and contain f 0.003% of thallium. The investigated CsI (TI) crystal has a cylindrical shape with diameter of 200 mm and thickness of 15 mm. Both the front and rear surfaces of the crystal have been polished in a glycerol-alcohol mixture in order to prevent the activator to be washed out from the surface layer. At a distance of 3 mm in front ofthe crystal, a 2.5,um thick aluminized Mylar reflector has been placed in order to increase the light collection efficiency. The construction ofthe light guide was based on the idea to use ahollow cone with a diffuse reflecting inner surface. Therefore, the side surfaces of the crystal and the inner surface of the cone to which the detector is fasten have been covered with an organic enamel

2 A.S. Fomichev et al. / Nucl. Instr. and Meth. in Phys. Res. A 344 (1994) polished surface 5 10cm IIIIIIIIIIIIIIIII aluminized mylar reflector 1r- CSI (TI) diffuse reflector r7 ph o tom ul tipper TYPE 173 R -100 k 0 Fig. 1. The scintillation Csl(TI) detector: (a) The detector setup. (b) The progressive voltage divider for FEU-173 photomultiplier. based on titanium dioxide. This setup ensures uniform light collection from the large crystal to the photomultiplier and, in summary, the position dependent detector response is smaller than 6%. The photocathode of the FEU-173 (Novosibirsk,Russia) with diameter of 150 mm and the spectral range nm) is very suitable for the CsI(TI) (~max = 550 nm). The photomultiplier works with progressive inter-dynode voltage, see Fig. lb, which guarantees the nonlinearity coefficient to be less than 0.5% for the anode signal within a dynamical range of mv at an output impedance of 50 S2. Recently, an investigation of this detector [12,13] has been performed to prove its ability to separate light charged particles with Z = 1-3 and y-rays using the pulse shape analysis. It has been shown that the energy resolution is about 7.3% for 5.5 MeV a-particles from a 238Pu source. 3. Experimental setup Our investigation of the response of the scintillation detector to light and intermediate mass ions has been performed by means of secondary beams produced in the reaction (20 MeV/A) +18'Ta at FLNR, JINR [14] (Fig. 5) and in the reactions 36Ar (44 MeV/A) + 5sNi and 180 (63 MeV/A) e at GANIL using the magnetic spectrometer LISE [ 15,16 ]. In both cases the detector was placed in the focal plane of the spectrometers perpendicularly to the beam direction. The energy calibration of the detector has been done by measuring at several values of the magnetic rigidity Bp. In order to check the particle identification a 300,um thick Si(Li) detector has been temporally located in front of Csl (Tl) and for each of the detectors the time-of-flight and energy has been measured on the 18 m TOF base using a HFreference signal. The anode pulse of the photomultiplier was integrated in the time interval, denoted as (0 ; 4.0,us), that started well before the pulse rise and had a duration of 4.0,us. In Fig. 2, the two matrices (AEs,,TOF) and (Ecs1,TOF), obtained in the measurement of the 36Ar (44 MeV/A) + 58 Ni reaction at a magnetic rigidity Bp = 1.78 Tin are shown. The measurement indicates that the CsI (Tl) detector ensures the mass and charge identification of particles with Z = Comparing the two matrices one can observe that the slope of the isotopic lines is changed and the energy resolution becomes worse by a factor of 2.5 reaching a value of about 2.1% for CsI (Tl ). Despite the

3 380 A.S. Fomichev et al. / Nucl. Instr. and Meth. to Phys. Res. A 344 (1994) Fig. 2. The identification matrices (DES,, TOF) and (Ecst, TOF) in arbitrary units obtained in the measurements of secondary particles from the reaction 36Ar (44 MeV/A) + S 8Ni. The time-of-flight parameter is derived from the timing signal of the corresponding detector and the cyclotron high frequency. large surface of our detector the resolution achieved is comparable with that obtained for substantially smaller crystals [ 10,111. 4, Results and discussion The energy dependence of the CsI (TI) light output for different isotopes with Z from 2 to 13 has been measured for several values ofspectrometer magnetic rigidity Bp = 1.78, 2.05 and 2.20 Tm using the 36Ar (44 MeV/A) + 58 Ni reaction and of Bp = 2.58, 2.84 Tm using the ' 80 (63 MeV/A) + 9Be reaction. The energy E of a given particle has been determined by the relativistic formula : E=931.5A(~ ( 2-1), (1) B AZ ) where A is the rest mass in amu and Bp is the magnetic rigidity in T m. The relative energy resolution AE/E of the CSI(TI) crystal slightly depends on the particle energy, changing from 2.1 to 0.7% over the whole energy range (22-77 MeV/A) and does practically not depend on the particle type. Fig. 3 shows the measured light output, in arbitrary units, as a function of the energy for different isotopes from 'He up to 30A1. The symbols represent the experimental data, while the curves are the results of a common least squares fit with the function : L(E,A,Z) = ao + a, {E - a 3AZ 2 In [(E + a2az 2 ) /a 2AZ 2 ] }, (2) where the ao, and a l coefficients denote the intercept and the slope respectively and the a2, a3 parameters are related to quenching factor (Birks' constant), which was described in ref. [71 by only one parameter a2. The first two parameters are determined by the experiment's electronics and the scintillation efficiency, while the next a2, a3 coefficients take into acount the nonlinear CSI (TI) light output defined as a function of the energy for the entire energy- and particle range. In our case the best results of fits were obtained with a2 = MeV and a3 = MeV, different to ref. [7], where a2 = a 3 = MeV. It can be explained by a nonlinear relationship between the total quenching of light output and AZ 2 [9,17] which is a consequence of the simple approximation del dx - const x AZ 2/E for energies above a few MeV per nucleon [ 7 ]. Moreover, the differential scintillation efficiency dl/ de as a function of the specific energy loss de/ dx has no linear dependence for the different ions and has been discribed by only available models [ 1,18 ]. A detailed analysis of this processes is beyond the scope of the present work, while the two empirical coefficients a2 and a3 help to reproduce the studied calibration curves. The fitted data with a standart error of 5.1 channels or 0.3 MeV were obtained with values of coefficients ao = , a l = 1.765, a2 = and a 3 =

4 A.S. Fomichev et al. / Nucl. Instr. and Meth. m Phys. Res. A 344 (1994) i. ci i C S Q- 0,a0 a L=a~ +a,(e-a,az + Lr,(l *E )) a,az' He6, Bel2 Cl6, 019, Ne24, Mg28 He-I, Bell, C15,OI8,Ne2? NIg Energy, MeV o b) Li Na 600 w; Fa L19, BI.:, N17, F22, Na26, A130 " L8, 617, N16, F21, Na :5, A129 b7, B12, N15. F20, Na2a, AI28 L,6, BI 1, NIJ. F19. Na25, AL7 Y'S BIO FIB Na Energy, Me V Fig. 3. The energy dependence of the CsI(Tl) light output for isotopes produced in different reactions at high energies 36Ar (44 MeV/A) + 5sNi and 180 (63 MeV/A) + 913e. The photomultiplier anode signal was integrated in the interval from 0 to 4 jus. Uncertainties in the observed channel number are typically smaller than the size of the plotted points.

5 382 A.S. Fomichev et al. / NucL Instr. and Meth. in Phys. Res. A 344 (1994) Fig. 4. The identification of secondary particles emitted in the 180 (63 MeV/A) + 9Be reaction using (left) the time-of-flight method ; (right) the PSA method. The a-peak is out ofz = 2 systematics because its energy (77.4 MeV/A) exceeds the limit (61 MeV/A) needed for a-particles to be stopped in the 15 mm thick Csl(TI) detector. We have also investigated, for some elements, the response of the CsI (Tl) scintillator to different isotopes. For example, in the 20 MeV per nucleon range 9Be, "Be and 160, 180 have pulse heights differing by about 11% and 6% respectively, somewhat distinguishing from the data of refs. [ 7,8 ]. Our data also pointed out that at energies higher than 25 MeV/A there exists a rather strong mass dependence of the light output for nuclei with atomic number up to 13. Therefore, it was of interest to determine limits of particle identification by pulse shape analysis in the CsI (TI) crystal instead oftof-e analysis for the high energy range. In Fig. 4 the identification matrices obtained for the reaction 180 (63 MeV/A) + 9 Be are shown. On the left the matrix for TOF and integration time interval of T2 (0.0; 4.0 Its) is presented, on the right a time interval of T1 (0.0 ;0.08,us) has been used instead of the TOF parameter. As one can see, the PSA applied to our large CsI (TI) detector allows to identify all particles up to Z = 7. The selected integration gates T,, T2 can provide both the suitable energy and particle resolution opposite to the results of ref. [2,171 where two intervals T, (0.0 ; 0.4,us) and T2 (1.6 ; 1.2 us) give only the best particle identification. A choice of optimum combination of T,, T2 for a good PSA is essential for low particle energies. To compare the pulse shape analysis for the different time interval combinations the matrices (T1, T2) and (T1, T2) have been built, as shown in Fig. 5, when Tz has been delayed by 1.5 us. For the given value of magnetic rigidity Bp = 1.13 T m the energies of the secondary ions ( 1 H, He, 6He, 7 Li, 9Li) emitted from 11 B (20 MeV/A) + 181Ta reaction amount to 59.38, 58.41, 35.96, 70.98, and MeV, respectively. By decreasing the energies of the secondary particles by magnetic rigidity of 0.81 T m the lower energy limit for a suitable identification of 6He and 9Li isotopes has been found for both time combinations. The lower level of energy is equal to 1 MeV/A for the (T,, Tz) matrix while for the second plot (TI, T2 ) this value is increased by a factor of Conclusion The investigation of the particle identification properties of a large CsI(TI) crystal viewed by a FEU-173 photomultiplier has been performed for light and heavy ions (Z < 18) in an energy range of 2-77 MeV/A. Summarizing we can conclude that our detector with an effective surface of 314 cm 2 has been proved to be a suitable tool for measuring wide and divergent particle beams and its characteristics are comparable due to the uniform light collection with those of small crystals. The good fits of expression (2) to the measured points have been achieved introducing two coefficients a2, a3 that do not depend on Z, A, E in the whole energy range. The dependence of the light output on the particle energy is practically linear for energies higher than a value of a 3 AZ2 MeV. It has

6 A.S. Fomichev et al. / Nucl. Instr. and Meth. in Phys. Res. A 344 (1994) L(TI ) L(TI) Fig. 5. The identification of secondary particles emitted in the IIB (20 MeV/A) + 1s1Ta reaction using the PSA method with various time integration intervals. (a) The identification matrix using the T, (0.0 ;0.4 ps) and TZ (1.5 ;2.0 us) windows. (b) The matrix build-up using the T, (0.0 ;0.4,s) and TZ(0.0 ;2.0,us) windows. been shown that in the proper description of the light output the mass dependence plays a very significant role and begins to manifest itself at about these energies. With increasing energy its contribution to the light output increases. The measurement has shown that using a TOF-E measurement our detector could ensure the mass and charge identification of particles with Z = 1-18 with an energy resolution better then 2% for 20 MeV/A particles. The pulse shape analysis has been applied to CsI(TI). To optimize the PSA condition special attention has been paid to the choice of integration time intervals. It has been shown that this choice depends on whether one prefers an improved resolution in energy or in particle identification. The pulse shape analysis applied for suitable integration time intervals allowed to identify particles with Z = 1-7, provided that the energy is determined by some other means, for example, at a magnetic spectrometer. The energy resolution together with the presented features makes such a CsI (TI) detector very suitable for use in many physical tasks. It can be used as a large solid angle detector in the experimental study of elastic scattering [ 19 ] and other reactions with heavy ions. References [ 1 ] J.B. Birks, The Theory and Practice of Scintillation Counting (Pergamon, Oxford, 1964). [2] J. Alarja et al., Nucl. Instr. and Meth.A 242 (1986) 352. [3] P. Kreutz et al., Nucl. Instr. and Meth. A 260 (1987) 120. [4] D. Drain et al., Nucl. Instr. andmeth. A 281 (1989) 528. [5] D.W. Stracener et al., Nucl. Instr. and Meth. A 294 (1990) 485. [ 6 ] H.-G. Ortlepp et al., Proc. Int. Conf. on New Nucl. Phys. with Advanced Techniques, Ierapetra, Crete, Creece, 1991, eds. F.A. Beck, S. Kossionides and C.A. Kalfas (World Scientific, Singapore, 1992) p [7] D. Horn et al., Nucl. Instr. and Meth.A 320 (1992) 273. [8] N. Colonna et al., Preprint LBL-32313, Berkeley (1992). [9] C.J.W. Twenh6fel et al., Nucl. Instr. and Meth. B 51 (1990) 58. [ 10 ] E. Valtonen, J. Peltonen and J. Torsti, Nucl. Instr. and Meth. A 286 (1990) 169. [ I 1 ] G. Viesti et al., Nucl. Instr. and Meth. A 252 (1986) 75. [ 12 ] A.S. Fomichev et al., Preprint JINR, PI Dubna (1992). [ 131 W. Wagner et al., JINR Rapid Communications, no. 4 (61) Dubna (1993) 49, submitted to Nucl. Instr. and Meth. [ 14 ] S.M. Lukyanov et al., Preprint JINR, P Dubna (1991). [ 15 ] R. Anne et al., Nucl. Instr. and Meth. A 257 (1987) 215. [ 161 A.C. Mueller and R. Anne, Nucl. Instr. and Meth. B 56/57 (1991) 559. [ 17 ] F. Benrachi et al., Nucl.Instr.and Meth. A 281 (1989) 137. [ 18 ] A. Meyer and R.B. Murray, Phys. Rev. 128 (1962) 98. [ 19 ] M. Lewitovicz et al., Preprint GANIL, P92-20 (1992).