- Email: [email protected]
Optimization of the performance of a CsI(Tl) scintillator + Si PIN photodiode detector for medium-energy light-charged particle hybrid array
Nuclear
ELSEVIER
Physics
A719
(2003)
316c-321~ www.elsevier.com/locate/npe
Optimization photodiode array
of the performance of a CsI(T1) scintilla& + Si PIN detector for medium-energy light-charged particle hybrid
Z. Elekes”,b G. Kalinkab, Zs. Fiilijp b* , J. GBlb, J. Molnarb, J. Veghb, T. Motobayash?, A. Saito” and Y. Yanagisawa” “The Institute of Physical 351-0198, Japan
and Chemical
bInsitute of Nuclear Research Debrecen, H-4001, Hungary
Research,
of the Hungarian
Gy. Hegyesl ‘b, D. Novakb,
2-l Hirosawa,
Academy
Wako,
of Sciences,
Saitama
P.O. Box 51,
This paper gives the present status of the preparation of the CsI(Tl)+Si PIN photodiode detectors to be used in a compact. array at the RIKEN RI Beam Factory. We discuss the optimum selection of surface treatment, wrappers, adhesives for the size and geometry of our crystals and present test measurements carried out in RIKEN and ATOMKI. 1. INTRODUCTION For astrophysical reactions involving short-lived nuclei, indirect methods using fast radioactive beams are quite useful. Studies of radiative captures by the Coulomb dissociation [l] and ANC method [2] are such examples. More generally, spectroscopy of unstable nuclei in their particle-threshold region by direct processes are discussed as promising probes for future investigations. They include (d,p) type transfer reactions, proton inelastic scattering, and so on, in which detection of light charged-particle recoils is of importance. The RIKEN RI Beam Factory is under construction. The factory is aimed at providing very intense radioactive beams at energies of several hundred MeV/nucleon over the whole range of atomic masses. For this, a cascade of a K930 MeV ring cyclotron and a K2500 MeV superconducting ring cyclotron will be built [3]. It will open various opportunities of research including the indirect methods of astrophysical reaction studies mentioned above. In connection to this project, NaI(Tl), BGO and CsI(T1) crystals in compact arrays will be constructed to use them at the Factory. NaI(T1) detectors are planned to detect gamma-rays from fast moving nuclei produced in nuclear reactions with radioactive beams. However, from highly excited states, high-energy photons are also expected which can be monitored by BGO scintillators exploiting their higher efficiencies for energetic photons. For reactions with light target, in addition to the gamma-ray measurement, recoil particle *Supported
in part
by
OTK4
(T042733).
0375-9474/03/$ - see front matter doi: lO.l016/SO375-9474(03)00940-O
0 2003 Elsevier
Science
B.V
All rights
reserved.
Z. Elekes et al. /Nuclear
Physics A719 (2003) 316c-321~
317c
detection allows for determination of scattering angle which is difficult from the laboratory angle of the heavy- product due to the nature of reversed kinematics. Detection of the recoil particles is also useful in identifying the reaction channel. For this purpose; we intend to employ CsI(T1) crystals coupled to photodiodes. 2. THE
CSI(TL)
ARRAY
TO
BE
REALIZED
The array- is illustrated in Fig. 1. The complete setup ,covers 27i solid angle. The individual crvstals with size of 16x16~50 mm3, in which light particles with approximately Y 110 MeV/nucleon energy can be stopped, are optically coupled to 10x10 mm2 silicon photodiodes. They are equipped with optimized high performance light reflector of minimum Altogether 312 crystals will be installed thickness with ruggedness and compactness. into modular segments consisting of four crystals allowing to modify the geometry if it is needed. The charged particle discrimination will be realized by using charge sensitive preamplifiers together with ballistic deficit or zero crossing methods separately or combined. It is noted that the gamma-ray detection capability of CsI(T1) detectors could also be exploited.
Figure
1. Schematic
3. CONSTRUCTION
view of the CsI(T1)
OF INDIVIDUAL
ball
DETECTORS
The characteristics of the detectors (light collection efficiency, resolution, particle identification quality) are influenced by many factors, such as (1) quality of the crystal (doping level, uniformity, optical transparency), (2) crystal size, geometry and quality of surface (matt or polished, painted or natural); (flatness, deep scratches), (3) surface treatment (4) light guide between the crystal and the photodiode, (5) type and quality of adhesive to attach the optical elements to each other, (6) type and alignment of photodiode; (7) type and thickness of wrapping material.
318c
Z. Elekes et al. /Nuclear Physics A719 (2003) 31~5-321~
These facts indicates that a systematic study is required to find the optimum solution to make the best detector in every aspect. Therefore, light collection efficiency and resolution tests were carried out by 241Am alpha source, which emits 5.5 MeV particles. Accelerated aging tests in 60 “C environment were conducted measuring the light collection and resolution change in elapsed time. In order to reduce the number of measurements, OPTICS software [4] was used to simulate the behaviour of the crystals. 3.1. Particular data The glue to bond together scintillator and photodiode must possess: high transparency, matched refractive index, optimum bond strength to accomodate thermal expansion difference, chemical inertness against modestly reactive CsI, easy handling. Three preselected adhesives were considered: (1) KE-103 silicone, (2) TOSSEAL-78 silicone, (3) EPOTEK-302 two component epoxy. During the measurements it turned out that KE-103 has weak gluing property and coloration after long time, TOSSEAL-78 has long curing time and its refractive index mismatches that of the CsI(T1) crystals while EPOTEK-302 showed the best properties. However, because its two component, the mixing of EPOTEK-302 is very important to have good homogeneity and lack of air bubbles. In worst case air bubbles can decrease the light collection efficiency by 10%. One squared end of the crystals is tapered to 5 mm length to fit the 10x10 mm2 Hamamatsu S3590 Si PIN photodiode.This solution gives much better light collection efficiency than coupling the untapered end directly, or via a tapered lightguide to the photodiode. The selection of an appropriate light reflector around the optical part in order to increase the amount of detected light is crucial for the quality of the detector, but equally important is the precise geometry and accurate assembly of the whole detector, since the latter is the basis of a good performance. Numerous combinations of diffferent surface conditions (polished, lightly painted, matted) and of various wrapping materials (Teflon, Tyvek, Lumirror, 3M mirror film [5], aluminized Mylar films) for the sides and front face were tested to find best solution(s). Parallel with these experiments the Monte Carlo calculations were still run to simulate light collection, and find best set of empirical constants for coincidence of measured and calculated results. Simulations predicted and the measurements proved that two combinations are particularly good: (1) all surfaces highly polished, the sides wrapped loosely with approximately 225 turns of 60 pm soft Teflon tape, (2) front face polished, side surfaces specially depolished and covered with >2 layers of 60 pm 3M mirror film. For the front face Lumirror (180 pm), 3M foil or Aluminized Mylar foils (2-12 pm Mylar, 0.2-l pm Al)- in order of decreasing performance - can be used, depending on the type and energy of the particles to be detected. The characteristics are approximately the same (Fig. 2): for 5.5 MeV alpha particles tested the light collection efficiency is >70%, energy resolution is <2.5%, low energy background continuum is <3%, while for gammas the light yield is about 30 photon/keV with 10.3% nonuniformity along crystal length, the energy resolution for 511 keV is 110% . The accelerated (long term) stability test at 60 “C revealed the superiority of Epotek 302 among glues and the 3M film among wrapping materials (Fig. 3). This fact and space
Z. Elekes et al. /Nuclear
-
y=yo+a(,-e
Physics A719 (2003) 316c-321~
-by
Figure 2. Light collection efficiency vs. number of wrapping layers
Figure 3. Heating test of detectors glued with EPOTEK-302 and wrapped by different materials. Note the different vertical scales.
limitations were the basis of the selection of 3M foil for final production. Similar solutions in the third generation of Diamant detector system in conjunction with Euroball [6], or in the GLAST space detector system under construction [7] also supports the excellent use and justifies the choice of VM2000 3M foil, a multilayer interference film based on giant birefringent optics [8], as a promising new alternative in scintillation detection to the presently overwhelmingly applied diffuse reflectors. 4. BEAM
TESTS
Beam tests of the detectors were carried out in RIKEN with an intermediate energy beam and in ATOMKI with a low energy beam. The sides of the detectors were wrapped with VM2000 foil while the front faces of two detectors were wrapped with VM2000 and two of them with Aluminized Mylar. In RIKEN the crystals were investigated by 50 MeV/nucleon energy deuteron beam which was degraded by a 0.5 cm thick Al plate in which other light charged particles were created at maximum energy of 43.3 MeV/A which means 9.6 mm range for protons, 5.9 mm for deuterons, 4.5 mm for tritons, 1.1 mm for 3He and 0.9 mm for 4He in CsI(T1). Particle identification was carried out by using a simplified
32oc
Z. Elekes et al. /Nuclear
Physics A719 (2003) 316-321~
Figure 4. Particle identification test in R,IKEN. Spectra taken with different shaping times are plotted against each other (X: low ballistic deficit, i.e. energy (the end of spectrum represents 95 MeV for protons), Y: high ballistic deficit, i.e. particle type).
version of the ballistic deficit method [9,10]: two electronics channels for each detector with low (10 ,USshaping time) and high (2 ~LSshaping time) ballistic deficits respectively gave energy and particle information, as well. Clear separation of gamma, proton; deuteron, triton; 3He, 4He particles above 5-10 MeV was observed (Fig. 4). No significant difference in particle identification by different detectors was seen. Further improvements could be achieved with the application of a newly developed and tested dedicated electronics [ll] which uses the ballistic deficit and zero crossing methods combined. A typical result with this electronics is shown in Fig. 5 using 20 MeV alpha beam hitting a Mylar target. The separation is obvious even at very low energies. 5. CONCLUSION Performance of a CsI(Tl)+photodiode system was systematically studied conditions. Such detectors will be used in the CsI(T1) ball of a hybrid array RIKEN RI Beam Factory. Optimization was made for the glue that couples photodiode, surface treatment, wrapping material, and so on to maximize efficiency and resolution. Particle identification capability was successfully light charged particles from as low as 1 MeV/nucleon up to 50 MeV/nucleon. of the present development will be put in constructing the CsI(T1) ball; exploited in nuclear astrophysics research. REFERENCES 1.
T. Motobayashi,
Nucl. Phys. A 693 (2001) 258
under various being built for the crystal and light collection examined with The results which will be
Z. Helm
et nl./Ndenr
Physics A719 (2003) 316c--321~
Figure 5. 3-dimensional plot of the data taken in ATOMKI. In the experiment alpha beam hit the mylar target. The energy scale in the figure is uncalibrated.
2.
321~
20~ MeV
H.M. Xu, C.A. Gagliardi, R.E. Tribble, A.M. Mukhamedzhanov, N.K. Timofeyuk, Phys. Rev. Lett. 73 (1994) 2027. 3. A. Goto, Y. Kano, T. Katayama, J. Phys. G. 24 (1998) 1569. 4. E. Frlei, B.K. Wright, D. PoEaniC, Comp. Phys. Comm. 110 (2001) 134. 5. http://www,3m,com/lightmanagement 6. 6. Kalinka, J.N. Schemer, Gy. Hegyesi, B.M. Nyako, J. Gal, ATOMKI. Ann. Rep. (2002) 64. 7. http://glast.stanford.edu 8. M.F. Weber, CA. Stover, L.R. Gilbert, T.J. Nevitt, A.J. Ouderkirk, Science 287 (2000) 2451. 9. J. Gal, G. Kalinka, B.M. Nyako, G.E. Perez, Z. Mate, G. Hegyesi, T. Vass, A. Kerek, A. Johnson, Nucl. Instr. And Meth. A 366 (1995) 120. 10. J. Gal, G. Hegyesi, G. Kalinka, B.M. Nyako, G.E. Perez, A. Kerek, A. Johnson, Nucl. Instr. And Meth. A 399 (1997) 407. 11. J. Gal, J. Molnar, D. Novak, G. Kalinka, Gy. Hegyesi, J. Vegh, Zs. FiilBp, Z. Elekes, T. Motobayashi, A. Saito, RIKEN Accel. Prog. Rep. 36 (2003) in print.
ELSEVIER
Physics
A719
(2003)
316c-321~ www.elsevier.com/locate/npe
Optimization photodiode array
of the performance of a CsI(T1) scintilla& + Si PIN detector for medium-energy light-charged particle hybrid
Z. Elekes”,b G. Kalinkab, Zs. Fiilijp b* , J. GBlb, J. Molnarb, J. Veghb, T. Motobayash?, A. Saito” and Y. Yanagisawa” “The Institute of Physical 351-0198, Japan
and Chemical
bInsitute of Nuclear Research Debrecen, H-4001, Hungary
Research,
of the Hungarian
Gy. Hegyesl ‘b, D. Novakb,
2-l Hirosawa,
Academy
Wako,
of Sciences,
Saitama
P.O. Box 51,
This paper gives the present status of the preparation of the CsI(Tl)+Si PIN photodiode detectors to be used in a compact. array at the RIKEN RI Beam Factory. We discuss the optimum selection of surface treatment, wrappers, adhesives for the size and geometry of our crystals and present test measurements carried out in RIKEN and ATOMKI. 1. INTRODUCTION For astrophysical reactions involving short-lived nuclei, indirect methods using fast radioactive beams are quite useful. Studies of radiative captures by the Coulomb dissociation [l] and ANC method [2] are such examples. More generally, spectroscopy of unstable nuclei in their particle-threshold region by direct processes are discussed as promising probes for future investigations. They include (d,p) type transfer reactions, proton inelastic scattering, and so on, in which detection of light charged-particle recoils is of importance. The RIKEN RI Beam Factory is under construction. The factory is aimed at providing very intense radioactive beams at energies of several hundred MeV/nucleon over the whole range of atomic masses. For this, a cascade of a K930 MeV ring cyclotron and a K2500 MeV superconducting ring cyclotron will be built [3]. It will open various opportunities of research including the indirect methods of astrophysical reaction studies mentioned above. In connection to this project, NaI(Tl), BGO and CsI(T1) crystals in compact arrays will be constructed to use them at the Factory. NaI(T1) detectors are planned to detect gamma-rays from fast moving nuclei produced in nuclear reactions with radioactive beams. However, from highly excited states, high-energy photons are also expected which can be monitored by BGO scintillators exploiting their higher efficiencies for energetic photons. For reactions with light target, in addition to the gamma-ray measurement, recoil particle *Supported
in part
by
OTK4
(T042733).
0375-9474/03/$ - see front matter doi: lO.l016/SO375-9474(03)00940-O
0 2003 Elsevier
Science
B.V
All rights
reserved.
Z. Elekes et al. /Nuclear
Physics A719 (2003) 316c-321~
317c
detection allows for determination of scattering angle which is difficult from the laboratory angle of the heavy- product due to the nature of reversed kinematics. Detection of the recoil particles is also useful in identifying the reaction channel. For this purpose; we intend to employ CsI(T1) crystals coupled to photodiodes. 2. THE
CSI(TL)
ARRAY
TO
BE
REALIZED
The array- is illustrated in Fig. 1. The complete setup ,covers 27i solid angle. The individual crvstals with size of 16x16~50 mm3, in which light particles with approximately Y 110 MeV/nucleon energy can be stopped, are optically coupled to 10x10 mm2 silicon photodiodes. They are equipped with optimized high performance light reflector of minimum Altogether 312 crystals will be installed thickness with ruggedness and compactness. into modular segments consisting of four crystals allowing to modify the geometry if it is needed. The charged particle discrimination will be realized by using charge sensitive preamplifiers together with ballistic deficit or zero crossing methods separately or combined. It is noted that the gamma-ray detection capability of CsI(T1) detectors could also be exploited.
Figure
1. Schematic
3. CONSTRUCTION
view of the CsI(T1)
OF INDIVIDUAL
ball
DETECTORS
The characteristics of the detectors (light collection efficiency, resolution, particle identification quality) are influenced by many factors, such as (1) quality of the crystal (doping level, uniformity, optical transparency), (2) crystal size, geometry and quality of surface (matt or polished, painted or natural); (flatness, deep scratches), (3) surface treatment (4) light guide between the crystal and the photodiode, (5) type and quality of adhesive to attach the optical elements to each other, (6) type and alignment of photodiode; (7) type and thickness of wrapping material.
318c
Z. Elekes et al. /Nuclear Physics A719 (2003) 31~5-321~
These facts indicates that a systematic study is required to find the optimum solution to make the best detector in every aspect. Therefore, light collection efficiency and resolution tests were carried out by 241Am alpha source, which emits 5.5 MeV particles. Accelerated aging tests in 60 “C environment were conducted measuring the light collection and resolution change in elapsed time. In order to reduce the number of measurements, OPTICS software [4] was used to simulate the behaviour of the crystals. 3.1. Particular data The glue to bond together scintillator and photodiode must possess: high transparency, matched refractive index, optimum bond strength to accomodate thermal expansion difference, chemical inertness against modestly reactive CsI, easy handling. Three preselected adhesives were considered: (1) KE-103 silicone, (2) TOSSEAL-78 silicone, (3) EPOTEK-302 two component epoxy. During the measurements it turned out that KE-103 has weak gluing property and coloration after long time, TOSSEAL-78 has long curing time and its refractive index mismatches that of the CsI(T1) crystals while EPOTEK-302 showed the best properties. However, because its two component, the mixing of EPOTEK-302 is very important to have good homogeneity and lack of air bubbles. In worst case air bubbles can decrease the light collection efficiency by 10%. One squared end of the crystals is tapered to 5 mm length to fit the 10x10 mm2 Hamamatsu S3590 Si PIN photodiode.This solution gives much better light collection efficiency than coupling the untapered end directly, or via a tapered lightguide to the photodiode. The selection of an appropriate light reflector around the optical part in order to increase the amount of detected light is crucial for the quality of the detector, but equally important is the precise geometry and accurate assembly of the whole detector, since the latter is the basis of a good performance. Numerous combinations of diffferent surface conditions (polished, lightly painted, matted) and of various wrapping materials (Teflon, Tyvek, Lumirror, 3M mirror film [5], aluminized Mylar films) for the sides and front face were tested to find best solution(s). Parallel with these experiments the Monte Carlo calculations were still run to simulate light collection, and find best set of empirical constants for coincidence of measured and calculated results. Simulations predicted and the measurements proved that two combinations are particularly good: (1) all surfaces highly polished, the sides wrapped loosely with approximately 225 turns of 60 pm soft Teflon tape, (2) front face polished, side surfaces specially depolished and covered with >2 layers of 60 pm 3M mirror film. For the front face Lumirror (180 pm), 3M foil or Aluminized Mylar foils (2-12 pm Mylar, 0.2-l pm Al)- in order of decreasing performance - can be used, depending on the type and energy of the particles to be detected. The characteristics are approximately the same (Fig. 2): for 5.5 MeV alpha particles tested the light collection efficiency is >70%, energy resolution is <2.5%, low energy background continuum is <3%, while for gammas the light yield is about 30 photon/keV with 10.3% nonuniformity along crystal length, the energy resolution for 511 keV is 110% . The accelerated (long term) stability test at 60 “C revealed the superiority of Epotek 302 among glues and the 3M film among wrapping materials (Fig. 3). This fact and space
Z. Elekes et al. /Nuclear
-
y=yo+a(,-e
Physics A719 (2003) 316c-321~
-by
Figure 2. Light collection efficiency vs. number of wrapping layers
Figure 3. Heating test of detectors glued with EPOTEK-302 and wrapped by different materials. Note the different vertical scales.
limitations were the basis of the selection of 3M foil for final production. Similar solutions in the third generation of Diamant detector system in conjunction with Euroball [6], or in the GLAST space detector system under construction [7] also supports the excellent use and justifies the choice of VM2000 3M foil, a multilayer interference film based on giant birefringent optics [8], as a promising new alternative in scintillation detection to the presently overwhelmingly applied diffuse reflectors. 4. BEAM
TESTS
Beam tests of the detectors were carried out in RIKEN with an intermediate energy beam and in ATOMKI with a low energy beam. The sides of the detectors were wrapped with VM2000 foil while the front faces of two detectors were wrapped with VM2000 and two of them with Aluminized Mylar. In RIKEN the crystals were investigated by 50 MeV/nucleon energy deuteron beam which was degraded by a 0.5 cm thick Al plate in which other light charged particles were created at maximum energy of 43.3 MeV/A which means 9.6 mm range for protons, 5.9 mm for deuterons, 4.5 mm for tritons, 1.1 mm for 3He and 0.9 mm for 4He in CsI(T1). Particle identification was carried out by using a simplified
32oc
Z. Elekes et al. /Nuclear
Physics A719 (2003) 316-321~
Figure 4. Particle identification test in R,IKEN. Spectra taken with different shaping times are plotted against each other (X: low ballistic deficit, i.e. energy (the end of spectrum represents 95 MeV for protons), Y: high ballistic deficit, i.e. particle type).
version of the ballistic deficit method [9,10]: two electronics channels for each detector with low (10 ,USshaping time) and high (2 ~LSshaping time) ballistic deficits respectively gave energy and particle information, as well. Clear separation of gamma, proton; deuteron, triton; 3He, 4He particles above 5-10 MeV was observed (Fig. 4). No significant difference in particle identification by different detectors was seen. Further improvements could be achieved with the application of a newly developed and tested dedicated electronics [ll] which uses the ballistic deficit and zero crossing methods combined. A typical result with this electronics is shown in Fig. 5 using 20 MeV alpha beam hitting a Mylar target. The separation is obvious even at very low energies. 5. CONCLUSION Performance of a CsI(Tl)+photodiode system was systematically studied conditions. Such detectors will be used in the CsI(T1) ball of a hybrid array RIKEN RI Beam Factory. Optimization was made for the glue that couples photodiode, surface treatment, wrapping material, and so on to maximize efficiency and resolution. Particle identification capability was successfully light charged particles from as low as 1 MeV/nucleon up to 50 MeV/nucleon. of the present development will be put in constructing the CsI(T1) ball; exploited in nuclear astrophysics research. REFERENCES 1.
T. Motobayashi,
Nucl. Phys. A 693 (2001) 258
under various being built for the crystal and light collection examined with The results which will be
Z. Helm
et nl./Ndenr
Physics A719 (2003) 316c--321~
Figure 5. 3-dimensional plot of the data taken in ATOMKI. In the experiment alpha beam hit the mylar target. The energy scale in the figure is uncalibrated.
2.
321~
20~ MeV
H.M. Xu, C.A. Gagliardi, R.E. Tribble, A.M. Mukhamedzhanov, N.K. Timofeyuk, Phys. Rev. Lett. 73 (1994) 2027. 3. A. Goto, Y. Kano, T. Katayama, J. Phys. G. 24 (1998) 1569. 4. E. Frlei, B.K. Wright, D. PoEaniC, Comp. Phys. Comm. 110 (2001) 134. 5. http://www,3m,com/lightmanagement 6. 6. Kalinka, J.N. Schemer, Gy. Hegyesi, B.M. Nyako, J. Gal, ATOMKI. Ann. Rep. (2002) 64. 7. http://glast.stanford.edu 8. M.F. Weber, CA. Stover, L.R. Gilbert, T.J. Nevitt, A.J. Ouderkirk, Science 287 (2000) 2451. 9. J. Gal, G. Kalinka, B.M. Nyako, G.E. Perez, Z. Mate, G. Hegyesi, T. Vass, A. Kerek, A. Johnson, Nucl. Instr. And Meth. A 366 (1995) 120. 10. J. Gal, G. Hegyesi, G. Kalinka, B.M. Nyako, G.E. Perez, A. Kerek, A. Johnson, Nucl. Instr. And Meth. A 399 (1997) 407. 11. J. Gal, J. Molnar, D. Novak, G. Kalinka, Gy. Hegyesi, J. Vegh, Zs. FiilBp, Z. Elekes, T. Motobayashi, A. Saito, RIKEN Accel. Prog. Rep. 36 (2003) in print.