S. Gridin

Channels of Energy Losses and Relaxation in CsI:A Scintillators (

Radiative relaxation channels and energy losses in In and Tl doped CsI scintillation crystals have been investigated as a function of temperature and excitation conditions to evaluate scintillation efficiency of the activator channel. Two activator concentration series of crystals were grown by the Bridgman method. Temperature dependence of excitation and luminescence spectra were measured under VUV and X-ray excitation; thermostimulated luminescence was also studied. The observed drop of radioluminescence yield of doped CsI crystals at room temperature relative to the pure crystal is explained by the migration losses caused by charge carrier trapping on the activator centers. The energy losses in CsI:A at low temperatures are due to the trapping of charge carriers on different centers: self-trapping of holes and capture of electrons by the activator centers. We suppose that migration energy losses are the main reason for significantly lower luminescence yield of CsI:A at room temperature than that of self-trapped excitons in pure CsI crystal.

{\rm A}={\rm Tl}

The measurement in 2014 of the nonproportionality of each decay component in CsI:Tl found opposite slopes of the fast and tail nonproportionality curves above about 10u2009keV. Somewhat earlier experiments on nonproportionality and resolution versus shaping time in NaI:Tl and CsI:Tl showed that proportionality and intrinsic resolution could be improved by including the slower “tail” component of the scintillation pulse. The observed opposing nonproportionality trends of fast and tail components constitute a basis for improvement of scintillator nonproportionality if they are added in a suitable linear combination. We examine whether combining the rise and decay components pulse by pulse with an algorithm of optimized proportions may also improve energy resolution. The premise is that a scintillation pulse carries more information about the particle stopping event than is conveyed in a simple measurement of the pulse height. In this work, we measured pulse shapes of individual gamma events in CsI:Tl and other scintillators using an eMorpho multichannel analyzer as a digital oscilloscope. Decomposition of every scintillation pulse into a rise and three exponential decay components allowed us to represent the pulse height spectrum as a linear combination of them. We found that energy resolution of CsI:Tl can be altered and improved through changing the weight of decay components in the linear combination.

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Thallium bromide is a promising semiconductor for room temperature radiation detection. A primary remaining challenge of this soft material is defects associated with impurities, strain, reactivity, or electromigration. Defect states near mid-gap promote rapid loss of carriers at high densities by Shockley-Read-Hall bimolecular recombination. We describe measurement and 3-D mapping of nonlinear photoconductive response using femtosecond pulse trains at 836 nm (1.48 eV), somewhat above mid-gap of TlBr. A confocal two-photon microscopy setup with photocurrent detection affords spatial resolution perpendicular to the surface. We report analysis of the irradiance dependence of the induced current as a function of depth in the sample. There are three separate contributions to the second-order current response. There is intrinsic two-photon absorption of non-defective TlBr. A finding of this work is that at relatively low irradiance, there is additional significantly enhanced two-photon absorption concentrated within about 1 mm of the surface and attributed to resonant enhancement by real intermediate states near mid-gap. Those same mid-gap states potentially contribute an electron-hole recombination term proportional to the square of carrier density (for intrinsic production as in a radiation detector), and this is in fact confirmed by analysis of the measurements. The spatial distribution of the defects can be displayed and the cross section for bimolecular carrier quenching can be extracted.

Pulse shape analysis of individual gamma events—Correlation to energy resolution and the possibility of its improvement

A custom designed cryostat was constructed to measure the response of a CsI:Tl scintillator at temperatures close to the boiling point of liquid nitrogen (LN2). The scintillation light was collected by an HUV-HD SiPM from FBK with 6×6 mm2 area and 25×25 μm2 cell pitch. The crystal size was 5×6×7 mm3. All surfaces except the one facing the SiPM were covered with Teflon tape to enhance light collection by the photodetector. The performance of the experimental setup was verified at room temperature using analog electronics for signal processing. The crystal was mounted on a copper frame placed inside the LN2 cryostat. Since our goal was to measure the scintillation decay profiles, and the SiPM response at low temperatures becomes substantially slower than that observed at room temperature, the SiPM was mounted on a separate copper frame connected with the outer housing to keep it close to room temperature. The separation between the crystal surface and the SiPM was about 1.5 mm at room temperature, and it became smaller once the setup was cooled down to LN2 temperature, but even so the crystal and the photodetector were still separated. This approach allowed us to analyze scintillation pulse shapes of CsI:Tl at LN2 temperatures. An energy spectrum of 662 keV γ-rays from a 137Cs source was also recorded. The light yield of the CsI:Tl sample at LN2 temperature stands at about 6 % ÷ 8 % of the value observed at room temperature.

Three-dimensional mapping and analysis of mid-gap defect distributions in TlBr by a two-photon photocurrent microscope

Laser shock peening (LSP) is applied to potassium iodide (KI) crystals. A selective etching technique is used to reveal the dislocation structure induced by the LSP process, confirming that LSP multiplies and moves dislocations in an alkali halide. Increasing the crystal temperature during LSP is shown to eliminate cracking along cleavage-planes caused by the LSP process and increases the range of dislocation motion in the crystal. LSP has long been used to increase surface toughness and fatigue life in metals. Having shown that LSP affects the mechanical properties in the ionic crystal KI, we propose studying the possible benefit of LSP in other halide crystals, including scintillators. One potential application might be reducing crack initiation and propagation at surfaces during Bridgman growth.