Physics and Chemistry of Scintillators
Fast Growth of Large Scintillators by the EFG Method (#1870)
S. E. Swider1, G. D. Calvert2, M. Overholt1, R. S. Feigelson2
1 CapeSym, Inc., R&D, Natick, Massachusetts, United States of America
SrI2(Eu) and Cs2LiYCl6(Ce) (CLYC) scintillators offer improved nuclide detection via high-resolution gamma detection, and dual-mode gamma-neutron detection, respectively. However, the high production cost and low throughput of the incumbent Bridgman method of crystal growth has limited availability of these crystals, and impeded their adoption. To achieve uncracked 2”-inch diameter crystals, growth rates are typically limited to less than 1 mm/hr, in order to minimize the stresses that accumulate at the wall. A large boule can require over 1 month to grow. However, with the Kyropoulos, Czochralski, Stepanov, and edge-defined film-fed (EFG) methods, solidification proceeds from a free meniscus, unbound by an ampoule wall, which greatly lowers the incidence of defect formation and their negative impact upon fracture toughness. Therefore the crystal may sustain higher levels of stress without cracking, as compared to Bridgman. EFG offers the added advantage of having a die that guides the meniscus, which reduces faceting, reduces the need for rotation, and enables pre-formed round, square, plate, and even tubular shapes. The free meniscus allows for growth rates 10 times faster than Bridgman. Including turn-around, a new crystal can be made every two days. That said, apparatus design must protect these deliquescent materials from impurity incorporation. For that reason we have explored both in-glove-box and external apparatuses. We will share our latest results of EFG growth of SrI2(Eu) and CLYC. We will complement our discussion with 2D axi-symmetric, conjugate thermal models of the system.
This work was supported by US Department of Homeland Security, Domestic Nuclear Detection Office, under the competitively awarded contract HSHQDC-15-C-B0040. This support does not constitute an express or implied endorsement on the part of the Government.
Keywords: strontium iodide, clyc, EFG, Czochralski, scintillator
Photoconductivity Studies of Carrier Traps in CsPbBr3 (#1894)
S. Derenzo1, G. Bizarri1, E. Bourret-Courchesne1, A. Canning1, D. Perrodin1, F. Moretti1
1 Lawrence Berkeley National Laboratory (LBNL), Berkeley, United States of America
Semiconductor scintillators have advantages over conventional scintillators in exhibiting (1) high electron-hole yields that are proportional to the energy deposited and (2) ultra-fast decay times. Their development has been hindered by an incomplete understanding of the dopants that enhance donor-acceptor emission and of the defects that contribute to non-radiative recombination. In this work we measure the electrical conductivity of undoped and doped crystals as they are illuminated by photons from an IR monochromator. The charge trapping energy is determined as the minimum photon energy necessary to contribute to the electrical conductivity. This method can identify charge traps whether they are radiative or non-radiative and is complementary to methods that use an intermediate radiative state such as thermally stimulated luminescence. We validated the method using the EL2 level in semi-insulating GaAs and applied it to CsPbBr3 doped with 0.05% Se to make the first measurement of the Se acceptor level as being 0.44 eV above the valence band. This is in good agreement with the value of 0.3 eV determined by our first-principles calculations. We have found that some undoped CsPbBr3 samples have an as yet unidentified shallow defect that exhibits strong persistent photoconductivity.
Keywords: scintillator, photoconductivity, carrier trap, non-radiative recombination
Study of activator non-uniformities in CsI:Tl scintillators using pulse shape analysis and luminescence mapping (#2490)
D. R. Onken1, S. Gridin1, R. T. Williams1, V. Gayshan2, A. Gektin3, S. Vasyukov3
1 Wake Forest University, Department of Physics, Winston-Salem, North Carolina, United States of America
Non-uniformities in scintillation crystals may limit their energy resolution and consequently their applicability in radiation spectrometry. We have investigated both standard quality CsI:Tl crystal scintillators and some crystals that exhibit a secondary peak in the pulse height spectrum. One goal of this work is to investigate the origin of these secondary peaks. In the course of this study we have digitized and analyzed the pulse shapes forming the primary full absorption peak and the secondary peak. We also have used scanning absorption and luminescence spectroscopy to quantify the activator non-uniformities. While measuring high-resolution luminescence maps of standard CsI:Tl scintillation crystals, we observed fine-grained inhomogeneities of photoluminescence response on the scale of 10 μm measured by our two-photon confocal photoluminescence mapping experiment.
Keywords: CsI:Tl, luminescence mapping, pulse height spectrum
Strong reduction of the effective radiation length in an axially oriented PWO crystal (#3035)
L. Bandiera1, V. V. Tikhomirov2, A. Mazzolari1, A. Berra3, M. Romagnoni1, M. Prest3, D. De Salvador4, E. Bagli1, E. Vallazza5, A. Sytov2
1 INFN, Sezione di Ferrara, Ferrara, Italy
On behalf of the INFN-AXIAL experiment group
Since the invention of scintillator materials, they have found many applications in physics. Several e.m. calorimeters in HEP, e.g. ECAL CMS, Fermi and Gamma-400, use crystal scintillators with high-Z elements as CsI and PWO.
The impact of crystal orientation of scintillators is usually poorly considered, whereas in fact the influence of the crystalline structure on radiation and pair production in common crystals, such as Si and Ge, has been known since decades. Indeed, due to the interaction with the strong crystalline field for small particle incidence angles with respect to crystal planes/axes, the radiation emitted by high-energy e± and pair production by photons may strongly increase as compared to an amorphous medium. Together these two processes substantially reduce the effective radiation length of the crystal, thus modifying the shower development.
We report the experimental measurement carried on at the CERN SPS-H4 line of e.m. radiation emitted by 120 GeV/c electrons in a 4 mm thick (0.45 X0) PWO crystal. The experimental results demonstrated a strong enhancement in the bremsstrahlung radiation. Indeed, the electron energy loss distribution under axial orientation, which peaks at more than 100 GeV demonstrates a several-fold reduction of the effective radiation length, in agreement with theoretical expectations.
This effect can be exploited to reduce the shower dimension and leakage in HEP calorimeters based on crystals, for instance to measure the TeV-energy cosmic rays or to decrease the calorimeter dimension and provide high angular resolution in gamma-telescopes.
Keywords: PWO, crystal, electromagnetic shower
Optimization of Yttrium Doping for BaF2 Crystals with Suppressed Slow Scintillation Component (#3855)
C. Hu1, C. Xu2, F. Yang1, 3, L. Zhang1, Q. Zhang2, R. - Y. Zhu1
1 California Institute of Technology, 256-48, HEP, Pasadena, California, United States of America
Because of its fast scintillation with sub nanosecond decay time BaF2 crystal has attracted a broad interest in the HEP community. One of the crucial issues of BaF2 application is its slow scintillation component with 600 ns decay time, which causes pile-up. Our early work indicates that Y3+ doping is effective in suppressing the slow component. In this investigation, BaF2 crystals grown with different level of Y3+ doping were characterized. BaF2 crystals with the highest F/T ratio of 91% are observed with optimized Y3+ doping. BaF2 crystals of this type would have broad applications beyond future HEP experiments.
Keywords: crystal scintillator slow component
Garnet scintillation crystals: Y3Al5O12, Y3(Alx-Ga1-x)5O12 and Gd3(Alx-Ga1-x)5O12, doped with Ce to operate with high light yield and time resolution in a harsh irradiation environment (#2216)
V. I. Dormenev1, K. T. Brinkmann1, G. Dosovitskiy2, 3, M. Korjik4, R. W. Novotny1, O. Sidletskiy5, H. - G. Zaunick1
1 Justus-Liebig-University, II. Physical Institute, Giessen, Germany
Future detector systems, especially for accelerating facilities, based on scintillator materials for high energy physics will operate in intensive radiation fields. It requires a high resistance level of a scintillation material both to electromagnetic and hadronic parts of the radiation environment. Possible candidates for such types of detectors can be cerium doped garnet materials from the line of Y3Al5O12 (YAG) Y3(AlxGa1-x)5O12 (YAGG) and Gd3(AlxGa1-x)5O12 (GAGG) doped with Ce with an option to regulate the level of Ga concentration in mixed garnets. These materials can be produced in a fiber or bulk geometry from inexpensive raw materials. Samples of YAG, YAGG with x = 0.17, 0.35, 0.65 and Gd3Al2Ga3O12 garnets with Ce doping and different codoping were tested. For every sample we have evaluated a light yield dependence on integration time, optical transmittance, scintillation kinetics before and after the irradiation with gamma-quanta of 100 Gy dose as well as with 190 MeV protons of integral fluence of 5*1013 protons/cm2. The test results will be presented in our work.
Keywords: Scintillation detectors, Garnets, Radiation damage