MeV Gamma Photons Research Articles

Reversible Total Ionizing Dose Effects in NiO/Ga2O3 Heterojunction Rectifiers

NiO/Ga2O3 heterojunction rectifiers were exposed to 1Mrad fluences of Co-60 g-rays either with or without reverse biases. While there is small component of Compton electrons (600 keV), generated via the interaction of 1.17 and 1.33 MeV gamma-photons with the semiconductor, which in turn can lead to displacement damage, most of the energy is lost to ionization. The effect of the exposure to radiation is a 1000x reduction in forward current and 100x increase in reverse current in the rectifiers, which is independent of whether the devices were biased during this step. The on-off ratio is also reduced by almost 5 orders of magnitude. There is a slight reduction in carrier concentration in the Ga2O3 drift region, with an effective carrier removal rate of < 4 cm-1. The changes in electrical characteristics are reversible by application of short forward current pulses during repeated measurement of the current-voltage characteristics at room temperature. There are no permanent total ionizing dose effects present in the rectifiers to 1 Mad fluences, which along with their resistance to displacement damage effects indicate these devices may be well-suited to harsh terrestrial and space radiation applications if appropriate bias sequences are implemented to reverse the radiation-induced changes. Figure 1

Reversible Total Ionizing Dose Effects in NiO/Ga2O3 Heterojunction Rectifiers

NiO/Ga2O3 heterojunction rectifiers were exposed to 1Mrad fluences of Co-60 g-rays either with or without reverse biases. While there is small component of Compton electrons (600 keV), generated via the interaction of 1.17 and 1.33 MeV gamma-photons with the semiconductor, which in turn can lead to displacement damage, most of the energy is lost to ionization. The effect of the exposure to radiation is a 1000x reduction in forward current and 100x increase in reverse current in the rectifiers, which is independent of whether the devices were biased during this step. The on-off ratio is also reduced by almost 5 orders of magnitude. There is a slight reduction in carrier concentration in the Ga2O3 drift region, with an effective carrier removal rate of < 4 cm-1. The changes in electrical characteristics are reversible by application of short forward current pulses during repeated measurement of the current-voltage characteristics at room temperature. There are no permanent total ionizing dose effects present in the rectifiers to 1 Mad fluences, which along with their resistance to displacement damage effects indicate these devices may be well-suited to harsh terrestrial and space radiation applications if appropriate bias sequences are implemented to reverse the radiation-induced changes.

Reversible total ionizing dose effects in NiO/Ga2O3 heterojunction rectifiers

NiO/Ga2O3 heterojunction rectifiers were exposed to 1 Mrad fluences of Co-60 γ-rays either with or without reverse biases. While there is a small component of Compton electrons (600 keV), generated via the interaction of 1.17 and 1.33 MeV gamma photons with the semiconductor, which in turn can lead to displacement damage, most of the energy is lost to ionization. The effect of the exposure to radiation is a 1000× reduction in forward current and a 100× increase in reverse current in the rectifiers, which is independent of whether the devices were biased during this step. The on–off ratio is also reduced by almost five orders of magnitude. There is a slight reduction in carrier concentration in the Ga2O3 drift region, with an effective carrier removal rate of &amp;lt;4 cm−1. The changes in electrical characteristics are reversible by application of short forward current pulses during repeated measurement of the current–voltage characteristics at room temperature. There are no permanent total ionizing dose effects present in the rectifiers to 1 Mad fluences, which along with their resistance to displacement damage effects indicate that these devices may be well-suited to harsh terrestrial and space radiation applications if appropriate bias sequences are implemented to reverse the radiation-induced changes.

Influence of MeV Gamma Photons on Thermally Stimulated Exoelectron Emission from MgO Films

The effect of 6 MeV gamma photons on thermally stimulated exoelectron emission (TSEE) spectra of MgO films was studied. The films were fabricated on Si/SiO2 substrates using the extraction-pyrolytic method. The crystalline structure and surface morphology of the films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). TSEE spectra of MgO films had emission peaks at about 450 oC and 525 oC. The area under the TSEE peaks increased after repeated TSEE measurements. In the case of gamma-irradiated films, the percentage increase in the area depended on the radiation dose, decreasing linearly with an increase in the radiation dose from 0 to 80 Gy. The results suggest that gamma radiation reduced the density of trapped electrons present in the as-grown MgO films or created competing hole traps that inhibited TSEE from the films.

Effect of gamma radiation on thermostimulated exoelectron emission from Gd2O3 films

The effect of gamma irradiation on Gd2O3 films was studied using the thermostimulated exoelectron emission (TSEE) technique. The films were deposited on a glass and Si/SiO2 substrates using an extraction-pyrolytic method. Crystalline structure, chemical composition, film thickness and surface morphology were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The films were irradiated by 10 MeV gamma photons and TSEE was measured from the irradiated films. It was found that gamma irradiation decreases TSEE intensity and the area below TSEE spectral curves. A linear correlation between the relative decrease in the area and the delivered dose was observed in a dose range of 0–10 Gy. These findings suggest that gamma radiation might decrease the density of trapped electrons present in the as-grown Gd2O3 films or create competing electron traps that inhibit TSEE from the films.

All-optical generation of petawatt gamma radiation via inverse Compton scattering from laser interaction with tube target

A scheme for generating gamma photons from laser interaction with a hollow-tube target filled with near-critical-density plasma is investigated using quantum electrodynamics particle-in-cell simulation. The solid-density tube wall stabilizes the laser propagation in the tube, guides the laser-accelerated relativistic electrons, and its endplate reflects the laser light. As the electrons collide head-on with the reflected laser, a large number of MeV gamma photons are generated through inverse Compton scattering. It is found that bright gamma radiation with power up to 1.4 PW and 37% laser-to-gamma ray energy conversion efficiency can be obtained with 5 × 1022 W cm–2 laser pulse.

Calculations of the transmitted gamma photons through infinite slabs

The intensity and number of transmitted multiple scattered photons are calculated for 0.123, 0.320, 0.511, 0.662, and 1.115 MeV gamma photons normally incident on slabs of carbon, aluminum, iron, copper, water, muscle, bone, and concrete with thicknesses varying from 1 to 10 mean free paths. The dependence of the transmission probability and energy distribution on the incident energy and material are examined. In general, the obtained results show good agreement with the other values calculated by the Monte Carlo method.

Thermoluminescence Properties of Yb–Tb-Doped SiO2 Optical Fiber Subjected to 1.25 MeV Gamma Radiation

A thermoluminescence (TL) analysis was carried out to study the sensitivity and dose responses of Yb–Tb-doped SiO2 optical fiber subjected to 1.25 MeV gamma radiation. The results are compared with the commercially available standard LiF:Mg, Ti (TLD-100) chip. The Yb–Tb-doped SiO2 optical fiber and TLD-100 chips were placed inside Perspex and irradiated with 1.25 MeV gamma photons with doses ranging from 1.0 to 10.00 Gy. The results clearly show the superiority of TLD-100 chips in terms of response and sensitivity. The sensitivity of Yb–Tb-doped optical fiber and TLD-100 chips is 45.25 and 768.85 nc/(mg × Gy) respectively.

Gadolinium nanoparticles and contrast agent as radiation sensitizers

The goal of the present study was to evaluate and compare the radiosensitizing properties of gadolinium nanoparticles (NPs) with the gadolinium contrast agent (GdCA) Magnevist® in order to better understand the mechanisms by which they act as radiation sensitizers. This was determined following either low energy synchrotron irradiation or high energy gamma irradiation of F98 rat glioma cells exposed to ultrasmall gadolinium NPs (GdNPs, hydrodynamic diameter of 3 nm) or GdCA. Clonogenic assays were used to quantify cell survival after irradiation in the presence of Gd using monochromatic x-rays with energies in the 25 keV–80 keV range from a synchrotron and 1.25 MeV gamma photons from a cobalt-60 source. Radiosensitization was demonstrated with both agents in combination with X-irradiation. At the same concentration (2.1 mg mL−1), GdNPS had a greater effect than GdCA. The maximum sensitization-enhancement ratio at 4 Gy (SER4Gy) was observed at an energy of 65 keV for both the nanoparticles and the contrast agent (2.44 ± 0.33 and 1.50 ± 0.20, for GdNPs and GdCA, respectively). At a higher energy (1.25 MeV), radiosensitization only was observed with GdNPs (1.66 ± 0.17 and 1.01 ± 0.11, for GdNPs and GdCA, respectively). The radiation dose enhancements were highly ‘energy dependent’ for both agents. Secondary-electron-emission generated after photoelectric events appeared to be the primary mechanism by which Gd contrast agents functioned as radiosensitizers. On the other hand, other biological mechanisms, such as alterations in the cell cycle may explain the enhanced radiosensitizing properties of GdNPs.

Evaluation of the dose enhancement of iodinated compounds by polyacrylamide gel dosimetry

In this study, polyacrylamide gel (PAG) dosimetry is used to quantitatively assess the efficiency of radiation sensitizers. The local dose enhancement caused by the K-edge absorption of certain atoms such as bromine and iodine can be employed to increase the damage to neighboring molecules and cells. Clonogenic assays can assess the radiation survival of cells to evaluate the efficiency of radiation sensitizers, but this technique requires reliable cell growth in culture media and is time consuming. Our purpose is to use PAG dosimetry to investigate the sensitizing potential of radiation sensitizers such as iodinated compounds. Incorporation of iodinated radiation sensitizers such as NaI and an iodinated contrast agent leads to a quantifiable dose enhancement ratio. When irradiated at low energy (∼40 keV), the dose enhancement ratio of the iodinated contrast agent at concentrations of 0.01 (3.5 mg ml−1), 0.05 (6 mg ml−1) and 0.1 (12 mg ml−1) M are 1.16 ± 0.02, 1.39 ± 0.03 and 1.82 ± 0.04, respectively. No dose enhancement was observed when the samples were irradiated with 1.25 MeV gamma photons.

Investigations of multiple backscattering and albedos of 1.12 MeV gamma photons in elements and alloys

The energy and intensity distributions of multiple backscattering of 1.12 MeV gamma photons emerging from targets of elements and alloys are observed as a function of thickness and atomic number ( Z) of the target. The numbers of these multiply backscattered events show an increase with increase in target thickness, and then saturate for a particular thickness of the target called saturation thickness (depth). The saturation thickness decreases with increasing atomic number and varies as e − Z . The multiple backscattering, an interfering background noise in Compton profile, has been successfully used to assign the ‘‘effective atomic number’’ to alloys. Monte Carlo calculations also support the present experimental results. The number, energy and dose albedos are also found to be saturating for the same thickness where the numbers of multiply backscattered events saturate.

Dark matter with invisible light from heavy double charged leptons of almost-commutative geometry?

A new candidate for cold dark matter arises from a novel elementary particle model: the almost-commutative AC-geometrical framework. Two heavy leptons are added to the standard model, each one sharing a double opposite electric charge and an own lepton flavour number. The novel mathematical theory of almost-commutative geometry (Connes 1994 Noncommutative Geometry (London: Academic)) wishes to unify gauge models with gravity. In this scenario two new heavy (mL ⩾ 100 GeV), oppositely double charged leptons (A,C), (A with charge −2 and C with charge +2), are born with no twin quark companions. The model naturally involves a new U(1) gauge interaction, possessed only by the AC-leptons and providing a Coulomb-like attraction between them. AC-leptons posses electro-magnetic as well as Z-boson interaction and, according to the charge chosen for the new U(1) gauge interaction, a new ‘invisible light’ interaction. Their final cosmic relics are bounded into ‘neutral’ stable atoms (AC) forming the mysterious cold dark matter, in the spirit of the Glashow's Sinister model. An (AC) state is reached in the early Universe along a tail of a few secondary frozen exotic components. They should be here now somehow hidden in the surrounding matter. The two main secondary manifest relics are C2+ (mostly hidden in a neutral (C2+e−e−) ‘anomalous helium’ atom, at a 10−8 ratio) and a corresponding ‘ion’ A2− bounded with an ordinary helium ion (4He)2+; indeed the positive helium ions are able to attract and capture the free A2− fixing them into a neutral relic cage that has nuclear interaction (4He2+A2−). The cage preserves the leptons to later recombine with neutral (C2+e−e−) into (AC) evanescent states. In early and late cosmic stages (AC) gas is leading to cold dark matter gravity seeds. It can form dense cores inside dense matter bodies (stars and planets). Binding (C2+e−e−) + (4He2+A2−) into (AC) atoms results in a steady decrease of the anomalous isotopes and a growing concentration of AC-gas. However the (AHe) influence on big bang nucleo-synthesis and catalysis of nuclear transformations in terrestrial matter appears to be a serious problem for the model. Moreover the zero lepton OLe-helium (AHe), (C2+e−e−) pollution and its on-going (AHe) catalysis in terrestrial waters may release frequent tens MeV gamma photons whose pair production leads to nearly aligned electron pairs; their consequent expected presence by twin Cherenkov rings poses a crucial test to the model. Their total absence in Super-Kamiokande or SNO records might point to the failure of the model, while their eventual discovery (above the background) may hint to the double charge AC-model to solve the cold dark matter puzzle. The new invisible light attraction allows us to stimulate the effective A–C recombination into (AC) atoms inside dense matter bodies (stars and planets), resulting in a decrease of anomalous isotopes below the experimental upper limits. OLe-helium pollution of terrestrial matter and (OHe) catalysis of nuclear reactions in it is one of the dramatic problems (or exciting advantages?) of the present model.

The response of pristine and doped poly(p-phenylene sulfide) towards MeV gamma photons

The effect of γ -photons on the potential engineering thermoplastic poly( p -phenylene sulfide) (PPS) has been investigated. The structural modifications are studied by elemental analysis, Rutherford back Scattering, Fourier transform infrared spectroscopy and differential scanning calorimetry. The polymer is resistant to irradiation up to a maximum dose of 540 kGy. Introduction of KBr that undergoes photo-decomposition resulted in disruption of the aromaticity of the polymer moiety with possible introduction of extensive cross-linking. Evidence for this structural modification was obtained from a decrease in the melting point from the differential scanning calorimetry profile and from FTIR spectral features with complete disappearance of the characteristic aromatic skeletal vibration bands at 1467 and 1379 cm −1 and with reduced band intensity for the in-plane and out-of-plane C–H bending modes and aromatic thio-ether linkage at 1008 and 1088 cm −1 respectively.

Neutron dosimetry in French nuclear power plants. Problems and their solutions in 1995

Personal neutron dosimetry in nuclear power plants is one of the hardest to achieve because there is no nuclear reaction that is both efficient and which ensures a suitable discrimination between 50 keV to 2 MeV fast neutrons and gamma photons. Neutron doses being low, the method used until now has been based on ambient dosimetry and the spent time in exposed areas. The advent of bubble detectors meets our immediate requirements until the direct reading electronic neutron dosimeter, which is still at the feasability research stage, can be utilized.

Assessment of natural radioactivity levels in building materials and evaluation of indoor radiation exposure

Abstract Building materials used is some cement houses in Greater Bombay and nearby areas were analysed gamma speotrometrically using a bulk material gamma counter, having 20 cm (dia) x 10 cm (thickness) NaI (T1) detector. Radium‐226 was determined through Bi‐214 (1.76 MeV) and Thorium‐232 through T1–208 (2.61 MeV) while Potassium‐40 by its 1.46 MeV gamma photons. Average Ra‐226 content in cement was found to be the highest (60.4 Bq, Kg‐1) and that in granite the lowest (10.7 Bq. Kg‐1). Ra‐226 gives ‘Q’ the Radon‐222 emanation rate from which Rn‐222 concentration is calculated. From Rn‐222, indoor internal exposure in mWLM Y‐1 and from Bi, T1 and K indoor air dose in urad h‐1 inside a model room that can be built from such materials has been computed. It is 3.9 urad h‐1(0.04 uGy h‐1) and is comparable with values obtained by actual measurements in other countries (1).

Energy and angular flux density spectra of 6 MeV gamma-rays penetrating shielding materials

Double differential flux spectra are presented for 6 MeV gamma photons penetrating aluminium, steel, lead and lead-aluminium shielding slabs. A large diameter disc radiator filled with activated primary cooling water from the Universities Research Reactor provides the source of high energy photons. Experimental pulse height distributions from a NaI spectrometer have been unfolded with the code RADAK. Comparisons are made with theoretical values using the code PIPE. Good agreement is established for most of the energy range, but experimental results show the need for the inclusion of secondary sources of photons in theoretical calculations.

Scalar leakage flux spectra for 6 MeV gamma photon penetrations through PbAl and FeAl laminated shields

Leakage scalar flux spectra are established for 6 MeV gamma photons penetrating laminated shielding assemblies of lead-aluminium and steel-aluminium. A large diameter disc radiator filled with activated water provides the source of high energy photons. Scalar leakage spectra are presented for penetration through 5, 10, 20 and 30 cm aluminium following 10 cm steel and 7.5 cm lead. Results show that leakage spectra in laminated shields are characterized by the material properties of the outermost layer.

Intercomparison of unfolding codes for benchmark gamma spectrometry

The unfolding codes FERDOR and RADAK were selected to compare gamma photon spectra from experimental measurements. The input data were obtained from penetration measurements of 6 MeV gamma photons from a 0.86 m diameter uniform isotropic disc source through 10 cm of steel and 5 cm and 10 cm of lead. Both codes produced similar spectra but RADAK allows adjustment to the response matrix elements within real constraints and has a realistic approach to error analysis. RADAK is recommended as a standard unfolding code to provide experimental benchmark data.

Benchmark data for the penetration of 6MeV γ photons through steel and lead

Benchmark data is presented for the penetration of 6·13 MeV gamma photons through various thicknesses up to 35 cm of steel and 15 cm of lead. Experimental data has been obtained using an isotropic uniform disc source, a NaI(Tl) spectrometer and the unfolding code GAMAK. Calculated dose has been determined using the Monte-Carlo code McGID. Good agreement has been established between experimental and calculated values. The Bremsstrahlung contribution is significant for 6 MeV source photons penetrating lead.

The nitrogen-16 radiation shielding unit at the Universities Research Reactor Centre

A source of 6·13 MeV gamma photons has been designed in the form of a disc having a diameter of 80 cm and a surface activity uniform to ±10%. The photons arise from 16N in the cooling water, activated in the core of the Liverpool and Manchester Universities Research Reactor. The surface exposure rate is in excess of 2 R/hr and a programme of shielding and dosimetry studies is outlined.