Monte Carlo method for simulating γ-ray interaction with materials: A case study on Si

In the present work, a Monte Carlo (MC) method has been developed to simulate various quantum mechanical processes for the energy loss of photons and fast electrons. The MC model is demonstrated by application to the interaction of photons with silicon over the energy range from 50 eV to 2 MeV and the subsequent electron cascades. The electron cascade process is commonly represented by two macroscopic parameters, the mean energy required to create an electron–hole pair, W, and the Fano factor, F, describing the electron yield and its variance. At energies lower than 5 keV, W generally decreases with increasing photon energy (from 3.96 to 3.58 eV), and it exhibits a sawtooth variation, as observed previously. However, discontinuities at the shell edges follow the photoionization cross-section, in contrast to previous results. The function, F(E_p), initially increases with increasing photon energy, E_p, to a maximum value of 0.187 around 155 eV, and then decreases at higher energies. Above the K shell edge, F has a value of 0.135. These results are consistent with experimental observations. The simulated distribution indicates that the interband transition and plasmon excitation are the most important mechanisms of electron–hole pair creation, while core shell ionization appears to be significant only at high energies.     

Contribution of 210Pb bremsstrahlung to the background of lead shielded gamma spectrometers

Lead, which is often used as a shielding material, contains ²¹⁰Pb (T_1/2=22.3 y). The 46.54 keV γ-intensity of ²¹⁰Pb can be easily reduced by an inner lining, but the bremsstrahlung caused by the β-decay of its daughter, ²¹⁰Bi, with a maximal electron energy of 1.16 MeV, will contribute to the gamma detector background. The spectrum of this bremsstrahlung is calculated by numerically fitting the β-spectrum and integrating the Koch–Motz formula. The absorption of the bremsstrahlung in the lead and detection efficiencies for the HPGe detector are calculated by the effective solid angle algorithm, using corrections for the photopeak/Compton ratio of cross-sections in Ge. By comparison with the measured background spectrum, it is shown that, for the lead with 25 Bq/kg of ²¹⁰Pb up to 500 keV of gamma spectrum, the bremsstrahlung contribution to the background is about 20% for our surface-based detector system. Also, we compared our calculations with a Monte Carlo simulation of another detector system with a shield containing 1 Bq/kg of ²¹⁰Pb and found that our analytical method gives a value of roughly two times higher than the Monte Carlo one for the total bremsstrahlung contribution. The quality of the analytical semi-empirical method is proved by the reasonable agreement with the experimental results published.     

Compositional dependence of damage buildup in Ar-ion bombarded AlGaN

Results are presented for the RBS/channeling study of the structural defect behavior in ion bombarded Al_xGa_1 ? xN (for x = 0; x = 0.44; x = 1) compounds at RT. Epitaxial layers of approximately 1 μm thickness were grown by the MOVPE technique on sapphire substrates. Experiments consisted of implantation with 320 keV Ar ions to fluences ranging from 5 × 10¹² to 1 × 10¹⁷ at/cm² followed by RBS/channeling measurements using 1.7 MeV ⁴He ions. Damage distributions were evaluated using the McChasy Monte Carlo simulation code assuming that they consist of randomly displaced lattice atoms. Detailed dependence of damage buildup on the alloy composition was determined and evaluated in the frame of the Multi-step Damage Accumulation model.     

Characterisation of micro-sized and nano-sized tungsten oxide-epoxy composites for radiation shielding of diagnostic X-rays

Characteristics of X-ray transmissions were investigated for epoxy composites filled with 2–10 vol% WO₃ loadings using synchrotron X-ray absorption spectroscopy (XAS) at 10–40 keV. The results obtained were used to determine the equivalent X-ray energies for the operating X-ray tube voltages of mammography and radiology machines. The results confirmed the superior attenuation ability of nano-sized WO₃-epoxy composites in the energy range of 10–25 keV when compared to their micro-sized counterparts. However, at higher synchrotron radiation energies (i.e., 30–40 keV), the X-ray transmission characteristics were similar with no apparent size effect for both nano-sized and micro-sized WO₃-epoxy composites. The equivalent X-ray energies for the operating X-ray tube voltages of the mammography unit (25–49 kV) were in the range of 15–25 keV. Similarly, for a radiology unit operating at 40–60 kV, the equivalent energy range was 25–40 keV, and for operating voltages greater than 60 kV (i.e., 70–100 kV), the equivalent energy was in excess of 40 keV. The mechanical properties of epoxy composites increased initially with an increase in the filler loading but a further increase in the WO₃ loading resulted in deterioration of flexural strength, modulus and hardness.     

Two new cerium-doped mixed-anion elpasolite scintillators: Cs2NaYBr3I3 and Cs2NaLaBr3I3

We recently discovered that mixed-anion Br–I elpasolite scintillators, Cs₂NaYBr₃I₃: Ce and Cs₂NaLaBr₃I₃: Ce, have promising performance. Ce concentration of these compounds was optimized in terms of light yield. Cs₂NaLaBr₃I₃ with 5% Ce (by mole) has a light yield of 58,000 ph/MeV, and excellent energy resolution of 2.9% at 662 keV. It is better than both endpoint compounds of the Br–I solid solution. Cs₂NaYBr₃I₃ with 2% Ce doping shows energy resolution of 3.3% at 662 keV, despite a relatively modest light yield of 43,000 ph/MeV. Non-proportionality of the mixed Br–I compounds was measured using gamma ray sources ranging in energy from 14 keV to 835 keV.The electronic band gaps of undoped Cs₂NaLaBr₃I₃ and Cs₂NaYBr₃I₃ were determined from optical transmittance and absorbance measurements. The band gaps of the compounds are 4.4 ± 0.1 eV, and 4.3 ± 0.1 eV, respectively.The crystal structures of Cs₂NaLaBr₃I₃: Ce and Cs₂NaYBr₃I₃: Ce are tetragonal and cubic respectively. The high symmetry leads to fewer cracks during crystal growth and minimizes light scattering at grain boundaries. The ease of crystal growth is promising for the scale-up of the growth process to larger sizes.     

Crystal growth and characterization of europium doped KCaI3, a high light yield scintillator

The presented study reports on the spectroscopic characteristics of a new high performance scintillation material KCaI₃:Eu. The growth of ∅ 17 mm boules using the Bridgman–Stockbarger method in fused silica ampoules is demonstrated to produce yellow tinted, yet transparent single crystals suitable for use in spectroscopic applications due to very promising performance. Scintillation light yield of 72,000 ± 3000 ph/MeV and energy resolution of 3% (FWHM) at 662 keV and 6.1% at 122 keV was obtained from small single crystals of approximately 15 mm³. For a much larger 3.8 cm³ detector, 4.4% and 7.3% for the same energy. Proportionality of the scintillation response to the energy of ionizing radiation is within 96% of the ideal response over an energy range of 14–662 keV. The high light yield and energy resolution of KCaI₃:Eu make it suitable for potential use in domestic security applications requiring radionuclide identification.     

Response of a BaF2 scintillation detector to quasi-monoenergetic fast neutrons in the range of 45 to 198 MeV

We have studied the neutron response of a scintillation detector consisting of a 14 cm long, hexagonal-shaped BaF₂-crystal with an inner diameter of 8.75 cm coupled to an EMI9821QB photomultiplier tube. The detector was exposed to calibrated quasi-monoenergetic neutron fields obtained from ⁷Li(p,n)⁷Be reactions. The measurements were performed at neutron energies of 45, 60, 96, 147 and 198 MeV as given by the energies of the incident protons. The experimental pulse-height spectra of the BaF₂-detector are compared with Monte Carlo simulations using the FLUKA code. The detection efficiency of the BaF₂-detector in the energy range of 45–198 MeV was determined as a function of the discriminator threshold and compared to the literature data. At neutron energies above 100 MeV the detection efficiency of the BaF₂-detector was found to be a factor of two higher than that of an NE213-detector of comparable size.     

Ultralow-concentration Sm codoping in CsI:Tl scintillator: A case of little things can make a big difference

CsI:Tl scintillators were hindered from computer tomography and high-speed imaging applications by a serious afterglow problem. In this study, the effects of ultralow-concentration Sm codoping on the scintillation characteristics of CsI:Tl were investigated. Pulsed X-ray excited afterglow after 50 ms in 0.005 mol% Sm-codoped CsI:Tl was lowered by over one order of magnitude in comparison with Sm-free one. The beneficial effects of ultralow-concentration Sm codoping also appeared to be maintaining the light yield and energy resolution. The light yield and energy resolution after 0.005 mol% Sm codoping were 71,700 ± 6000 photons/MeV and 6.9% at 662 keV, respectively.     

Crystal growth and scintillation properties of potassium strontium bromide

In this work, potassium strontium bromide activated with divalent europium, (KSr₂Br₅:Eu) has been studied. It has a monoclinic crystal structure and a density of 3.98 g/cm³. Two single crystals of KSr₂Br₅ doped with 5% Eu²⁺, with diameters of 13 mm and 22 mm, were grown in a two zone transparent furnace via the Bridgman technique. The X-ray excited emission spectrum consisted of a single peak at ∼427 nm due to the 5d–4f transition in Eu²⁺. The measured light yield and energy resolution at 662 keV was 75,000 ph/MeV and 3.5%. At low energies KSr₂Br₅:Eu 5% also displays good energy resolution, 6.7% at 122 keV and 7.9% at 59.5 keV.     

Photon spectrometry in thermal neutron standard field

An NE213 liquid scintillation counter (5.08 cm in diameter and 5.08 cm long) with an LiF filter was used to measure the energy distribution of photons mixed in a thermal neutron field. The response function matrix of photons in an energy range up to 10 MeV was calculated by the EGS4/PRESTA code and properly folded with a resolution function. Pulse height spectra measured with a set of reference γ-ray sources were compared to the calculated response function and agreed very well for all reference γ-ray sources. The GRAVEL and MIEKE codes from the HEPRO program were used to unfold measured pulse height spectra. Energy distributions obtained by the unfolding were applied to evaluate the effective dose equivalent of photons mixed in a thermal neutron field.     

Improved version of the triode electron gun

Results of the triode electron gun which delivers 500 mA current pulses of 2 μs duration at an energy of 40 keV are presented. This new gun uses LaB₆ cathode as emitter indirectly heated by a tungsten filament, a focusing electrode, non-intercepting modulating anode and anode. Various improvements and additions carried out to the earlier gun which delivered only 120 mA current pulses is described.     

Characterization of Yb:YAG and Yb:YAP scintillators by means of LAAPD at temperature around 100 K

A new class of scintillators based on charge-transfer luminescence of the Yb³⁺ ion is being investigated for the last few years. The main prospect for these scintillators is in neutrino physics. The crystals manifest maximum light output at temperatures 100 K<T<150 K. In this range Large Area Avalanche Photodiodes (LAAPD) are the best photodetectors. In this work Yb(25%):YAG and Yb(5%):YAP scintillators were characterized by means of a 16 mm diameter API LAAPD at temperatures around 100 K. Light yield and energy resolution were determined. Light yield non-proportionality was detected for all the crystals comparing light output at 661.6 and 59.6 keV peaks.     

Growth and characterization of polycrystalline lanthanide halide scintillators

We are exploring a novel time- and cost-efficient approach to produce robust, large-volume polycrystalline lanthanide halide scintillators using a hot wall evaporation (HWE) technique. To date, we have fabricated LaBr₃:Ce and LaCl₃:Ce films (slabs) measuring up to 7 cm in diameter and 7+ mm in thickness (20–25 cm³ in volume) on quartz substrates. These polycrystalline scintillators exhibit very bright emissions approaching those exhibited by their melt-grown crystal counterparts. Scanning electron micrographs (SEMs) and X-ray diffraction analysis confirm polycrystalline growth with columnar structures, both of which help in light piping, thereby contributing to the observed high light yields. The new scintillators also exhibit good energy resolution for γ-rays over the tested range of 60 keV (²⁴¹Am) to 662 keV (¹³⁷Cs), although they have not yet reached that of the corresponding crystals. The measured response linearity over the same energy range is comparable for both our HWE synthesized films and melt-grown commercially-available reference crystals. Similar consistency in response is also observed in terms of their decay time and afterglow behaviors. The data collected so far demonstrate that our HWE technique permits the rapid creation of scintillators with desired structural and compositional characteristics, without the introduction of appreciable defects, and yields material performance equivalent to or approaching that of crystals. Consequently, the deposition parameters may be manipulated to tailor the physical and scintillation performance of the resulting structures, while achieving a cost per unit volume that is substantially lower than that of crystals. In turn, this promises to allow the use of these novel scintillation materials in such applications as SPECT, PET, room-temperature radioisotope identification and homeland security, where large volumes of materials in a wide variety of shapes and sizes are needed. This paper describes our growth and testing of polycrystalline LaBr₃:Ce scintillators and provides comparative characterizations of their performance with crystals.     

Expected performance of a hard X-ray polarimeter (POLAR) by Monte Carlo simulation

Polarization measurements of the prompt emission in gamma-ray bursts (GRBs) can provide diagnostic information for understanding the nature of the central engine. POLAR is a compact polarimeter dedicated to the polarization measurement of GRBs between 50 and 300 keV and is scheduled to be launched aboard the Chinese Space Laboratory around the year 2012. A preliminary Monte Carlo simulation has been accomplished to model the expected performance of POLAR while a prototype of POLAR is being constructed. The modulation factor, efficiency and effective area, background rates and minimum detectable polarization (MDP) were calculated for different detector configurations and trigger strategies. With the optimized detector configuration and trigger strategy and the total weight constraint of less than 30 kg, the primary science goal to determine whether most GRBs are strongly polarized can be achieved, and about 9 GRBs/yr can be detected with an MDP<10% for the conservative detector configuration.     

LaBr3:Ce crystal: The latest advance for scintillation cameras

Recent availability of LaBr₃:Ce crystal is attracting researchers for the development of new advanced SPECT e PET systems. The crystal shows excellent energy resolution values good radiation absorption properties and speed. At present, LaBr₃:Ce crystal is available with continuous shape covering 5×5 cm² area with a thickness up to 1 in. With the aim of analysing the imaging performances of LaBr₃:Ce for Single Photon Emission Tomography (SPET), we tested three continuous crystals with the same detection area of 5×5 cm² and various thicknesses ranging between 4 and 10 mm. Three small scintillation cameras were assembled by coupling LaBr3:Ce crystal to Hamamatsu H8500 Flat panel PMT. The results show very good imaging performances for single photon emission application with superior energy and spatial resolution up 7.5% and 0.9 mm, respectively, and a detection efficiency up to 95% at 140 keV photon energy.     

Optical functions and time-resolved luminescence of lithium hydride single crystals upon far-ultraviolet excitation

Low-temperature reflection spectra of lithium hydride (LiH) single crystals cleaved in ultrahigh vacuum (3 × 10⁻¹⁰ Torr, T = 10 K), were recorded using synchrotron radiation in vacuum ultraviolet spectral region. Based on the obtained experimental data, the optical functions of LiH in the energy range from 3.7 to 35 eV were analyzed using the Kramers–Krönig relations. Time-resolved photoluminescence excitation spectra were studied in detail for the near edge free exciton-phonon luminescence at 4.67 eV and photoluminescence at 2.4 eV due to the Bi³⁺ impurity centers. The effect of multiplication of electronic excitations due to inelastic scattering of hot photoelectrons and hot photoholes was revealed at photon energies above 15 eV (more than 3E_g). It was found that the radiative lifetime for free excitons in LiH at 4.67 eV is less than 1 ns as low temperatures as at 10 K. The interpretation of the electronic band structure of lithium hydride in the ultraviolet and vacuum ultraviolet spectral regions were carried out on the basis of the present experimental results with the involvement of the available band structure calculations.     

Carrier dynamics and intervalley scattering in InN

The carrier cooling and the carrier relaxation of an InN thin film illuminated with two excitation energies of 1.53 and 3.06 eV were studied by an ultrafast time-resolved photoluminescence upconversion apparatus. The hot phonon effect could be accounted for longer effective phonon emission times as compared to the theoretical prediction. The rise time and the LO phonon emission time for 3.06 eV excitation were much smaller than those for 1.53 eV excitation. These differences were attributed to the intervalley scattering between the Γ₁ and Γ₃ valleys in InN when carriers were excited with the energy of 3.06 eV. The intervalley scattering times of 250 fs and 2 ps were estimated for the intervalley scattering from the Γ₁ to Γ₃ valley and the reversed scattering process, respectively.     

A 20 keV electron gun system for the electron irradiation experiments

An electron gun consisting of cathode, focusing electrode, control electrode and anode has been designed and fabricated for the electron irradiation experiments. This electron gun can provide electrons of any energy over the range 1–20 keV, with current upto 50 μA. This electron gun and a Faraday cup are mounted in the cylindrical chamber. The samples are fixed on the Faraday cup and irradiated with electrons at a pressure ∼10⁻⁷ mbar. The special features of this electron gun system are that, at any electron energy above 1 keV, the electron beam diameter can be varied from 5 to 120 mm on the Faraday cup mounted at a distance of 200 mm from the anode in the chamber. The variation in the electron current over the beam spot of 120 mm diameter is less than 15% and the beam current stability is better than 5%. This system is being used for studying the irradiation effects of 1–20 keV energy electrons on the space quality materials in which the irradiation time may vary from a few tens of seconds to hours.     

A Monte Carlo method for calculating the energy response of plastic scintillators to polarized photons below 100 keV

The energy response of plastic scintillators (Eljen Technology EJ-204) to polarized soft gamma-ray photons below 100 keV has been studied, primarily for the balloon-borne polarimeter, PoGOLite. The response calculation includes quenching effects due to low-energy recoil electrons and the position dependence of the light collection efficiency in a 20 cm long scintillator rod. The broadening of the pulse-height spectrum, presumably caused by light transportation processes inside the scintillator, as well as the generation and multiplication of photoelectrons in the photomultiplier tube, were studied experimentally and have also been taken into account. A Monte Carlo simulation based on the Geant4 toolkit was used to model photon interactions in the scintillators. When using the polarized Compton/Rayleigh scattering processes previously corrected by the authors, scintillator spectra and angular distributions of scattered polarized photons could clearly be reproduced, in agreement with the results obtained at a synchrotron beam test conducted at the KEK Photon Factory. Our simulation successfully reproduces the modulation factor, defined as the ratio of the amplitude to the mean of the distribution of the azimuthal scattering angles, within 5% (relative). Although primarily developed for the PoGOLite mission, the method presented here is also relevant for other missions aiming to measure polarization from astronomical objects using plastic scintillator scatterers.     

A position-sensitive γ-ray detector for positron annihilation 2D-ACAR based on metal package photomultiplier tubes

A new position-sensitive γ-ray detector to be used in a two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) apparatus has been developed. It consists of 36 compact position-sensitive photomultiplier tubes (PS-PMT: HAMAMATSU R5900-00-C8), a light guide, and 2676 Bi₄Ge₃O₁₂ (BGO) scintillator pieces of size 2.6 mm×2.6 mm×18 mm. A high detection efficiency for 511 keV γ-ray is achieved with the length of BGO scintillators used. The detection area is about 160 mm×160 mm. The 288 anode outputs of the PS-PMTs are wired and connected to resistor chains from which 16 outputs (8 outputs each along the X and Y directions) are taken to identify the incident position of the γ-ray. The spatial resolution is about 3 mm (FWHM). The timing signal taken from the last dynodes of the PS-PMTs gives a timing resolution of 7.7 ns (FWHM) for 511 keV positron annihilation γ-rays.