F. Quarati

CeBr

CeBr3 crystals have been studied to assess their utility as potential gamma ray spectrometers for future ESA planetary missions. Pulse height spectra, scintillation time profiles, X-ray excited emission spectra, and photoluminescence spectra have been recorded as a function of temperature between 78 and 600 K. In addition, the influence of exposing CeBr3 to various doses of gamma rays from a strong 60Co source on its scintillation performance has been investigated.

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A simple kinetic model concerning photocurrent in epitaxially grown GaN is presented. Utilizing a minimal set of rate equations and kinetic parameters, it is shown that in the presence of hole centers with small probabilities of electron-hole recombinations, the time dependence of photocurrent is ruled by competition between capture of conduction bandelectrons by deep electron traps and electron-hole recombinations. If the probability of electron capture exceeds that of recombination, the decay of current after excitation is turned off shows the usual persistent photocurrent trend. If, on the contrary, the probability of recombination is larger than that of electron capture, a slow photocurrent quenching, past a maximum, can be observed. In some circumstances, after excitation is turned off, the current drops below the steady dark current, at which point the negative persistent photoconductivity effect comes into play.

Scintillator Development for Possible Use in Space Missions

Future planetary missions such as BepiColombo are resource limited in both mass and power. Due to the proximity of the spacecraft to the Sun, the instrumentation will encounter harsh environments as far as radiation levels and thermal loads are concerned. Only radiation hard detectors that need little or no cooling will be able to successfully operate after long cruise times and over the expected mission lifetimes. The next generation of lanthanum halide scintillators promises to provide sufficient resolution in the spectral range between 1 and 10 MeV where most of the elemental gamma-ray emission lines can be detected. In order to be suitable for planetary gamma-ray spectrometers with sufficient sensitivity it had to be proven that larger crystals of size 3 can be produced and that they maintain their resolution of 3% at 662 keV. For that purpose we have produced and characterized several larger crystals and assessed their radiation hardness by exposing the crystals to radiation doses that are representative of the expected conditions in the space environment. Systematic measurements on several crystals allowed the determination of the activation potential and the performance verification from which the consequences for instrument flight performance can be derived. From these investigations we conclude that these scintillators are well suited for planetary missions, with excellent and stable performance.

Photocurrent in epitaxial GaN

We present the results of a detailed experimental study of polarization effects in thallium bromide planar x-ray detectors. Measurements were carried out in the range 10–100 keV by scanning a highly focused x-ray beam, 50 μm in diameter, from a synchrotron source across the detector. Above a certain radiation threshold the detector response showed a systematic degradation of its spectroscopic characteristics, peak channel position, peak height, and energy resolution. Using a pump-and-probe technique, we studied the dynamics of spectral degradation, the spatial extent and relaxation of the polarized region, and the dependence of the detector response on bias voltage and temperature. For comparison, we modeled polarization effects induced by the charging of traps by both electrons and holes using a model based on recent theoretical work of Bale and Szeles. We calculated the charge collection efficiency and spectral line shapes as functions of exposure time, beam position, count rate, and photon energy, and ...

Development and Characterization of Large La-Halide Gamma-Ray Scintillators for Future Planetary Missions

We present the results of an experimental study of a special type of CdZnTe detector of hard x and γ rays—a ring-drift detector. The device consists of a double ring electrode structure surrounding a central point anode with a guard plane surrounding the outer anode ring. The detector can be operated in two distinctively different modes of charge collection—pseudohemispherical and pseudodrift. We study the detector response profiles obtained by scanning the focused x-ray beam over the whole detector area, specifically the variations in count rate, peak position, and energy resolution for x rays from 10 to 100 keV. In addition, at 662 keV the energy resolution was shown to be 4.8 keV, more than a factor of 2 better than for CdZnTe coplanar grid detectors. To interpret the experimental data, we derive an analytical expression for the spatial distribution of the electric field inside the detector and neglecting carrier diffusion, and identify carrier collection patterns for both modes of operation within the...

Polarization effects in thallium bromide x-ray detectors

The scintillation response of LaBr3:Ce scintillation crystals was studied as function of temperature and Ce concentration with synchrotron X-rays between 9?keV and 100?keV. The results were analyzed using the theory of carrier transport in wide band gap semiconductors to gain new insights into charge carrier generation, diffusion, and capture mechanisms. Their influence on the efficiency of energy transfer and conversion from X-ray or ?-ray photon to optical photons and therefore on the energy resolution of lanthanum halide scintillators was studied. From this, we will propose that scattering of carriers by both the lattice phonons and by ionized impurities are key processes determining the temperature dependence of carrier mobility and ultimately the scintillation efficiency and energy resolution. When assuming about 100?ppm ionized impurity concentration in 0.2% Ce3+ doped LaBr3, mobilities are such that we can reproduce the observed temperature dependence of the energy resolution, and in particular, the minimum in resolution near room temperature is reproduced.

Hard x-ray response of a CdZnTe ring-drift detector

Abstract We report preliminary results from a synchrotron radiation study of Te inclusions in a large volume single crystal CdZnTe (CZT) coplanar-grid detector. The experiment was carried out by probing individual inclusions with highly collimated monochromatic X- and γ-ray beams. It was found that for shallow X-ray interaction depths, the effect of an inclusion on measured energy loss spectra is to introduce a ∼10% shift in the peak centroid energy towards lower channel numbers. The total efficiency is however not affected, showing that the net result of inclusions is a reduction in the Charge Collection Efficiency (CCE). For deeper interaction depths, the energy-loss spectra shows the emergence of two distinct peaks, both downshifted in channel number. We note that the observed spectral behaviour shows strong similarities with that reported in semiconductors, which exhibit polarization effects, suggesting that the underlying mechanism is common.

Energy resolution and related charge carrier mobility in LaBr3:Ce scintillators

The effect of high dose 60Co gamma irradiation on photoelectron yield, energy resolution, optical transmission, scintillation time profiles and thermoluminescence glow curves of LaBr3:5%Ce and LaCl3:10%Ce crystals has been investigated. Both materials show lower yields and deteriorated resolutions after exposure to a strong 60Co source, however their transmission, yield proportionality, scintillation decay, and thermoluminescence remain unaffected. Initial scintillation characteristics can be retrieved neither by spontaneous recovery nor upon heating in vacuum.

Synchrotron radiation studies of spectral response features caused by Te inclusions in a large volume coplanar grid CdZnTe detector

We calculate the phase space volume Ω occupied by a nonextensive system of N classical particles described by an equilibrium (or steady-state, or long-term stationary state of a nonequilibrium system) distribution function, which slightly deviates from Maxwell-Boltzmann (MB) distribution in the high energy tail. We explicitly require that the number of accessible microstates does not change respect to the extensive MB case. We also derive, within a classical scheme, an analytical expression of the elementary cell that can be seen as a macrocell, different from the third power of Planck constant. Thermodynamic quantities like entropy, chemical potential and free energy of a classical ideal gas, depending on elementary cell, are evaluated. Considering the fractional deviation from MB distribution we can deduce a physical meaning of the nonextensive parameter q of the Tsallis nonextensive thermostatistics in terms of particle correlation functions (valid at least in the case, discussed in this work, of small deviations from MB standard case).

Gamma-Ray Induced Radiation Damage in

Calibration source with monoenergetic gamma-ray lines in wide energy range designed for gamma-ray detector energetic calibration and testing has been built. Gamma-rays are obtained from thermal neutron capture, which is a suitable and cost efficient way how to provide discrete gamma-ray lines with energies above 3 MeV with reasonable intensity. With appropriate and interchangeable targets the source can generate different gamma-ray spectra with energy up to 10 MeV. We present the data obtained with neutron capture on chlorine, but other elements with high thermal neutron capture cross-section such as chrome, iron, nickel and titanium can be used as well. As neutron source we employ radionuclide sources (252Cf or 241Am-Be) with emission rate about 106 neutrons/s. The emitted fast neutrons are moderated by a moderator made of light materials such as graphite, standard water or heavy water. Performance of the source is demonstrated by calibration spectra measured by HPGe and scintillation detectors (LaBr3, NaI(Tl)).