Results of spectral monitoring of environmental gamma background in a fault zone of the Western Caucasus for seismological application

Results of continuous spectral monitoring of indoor gamma background are reported for seismological application in one fault zone of the Western Caucasus. Background count rates were recorded every 5 min since 2004. Two detectors (a scintillation crystal CsI(Tl), 200 mm × 150 mm in a low background metal screen and a NaI(Tl) detector, 90 mm × 90 mm outside of the screen) were used. Various intervals of energy of gamma rays, for example, (2.50–3.40) MeV, (1.70–2.00) MeV, (1.35–1.55) MeV and others were analyzed. Averages for all measurements as well as seasonal and hourly changes of count rate of detectors were obtained. As an example, for the interval (1.70–2.00) MeV a minimal monthly count rate of detector CsI(Tl) was observed in June, and the maximal monthly count rate of detector NaI(Tl) was observed in July. Factors of correlation between different intervals of energy of gamma rays were also investigated. Strong changes of annual, monthly and daily factors were obtained.In each daily curve of all intervals of energy of gamma rays, the first Fourier harmonic was calculated. The deviations from 3σ (“splash”), which are upward or downward from the first harmonic, were compared with the seismological data. It was possible to correlate the low background dates of “splashes” with the dates of regional earthquakes through 9 ± 1 days.Change of ratio of count rate in different intervals of energy of gamma rays can be used to study processes of preparation of earthquakes.The factor of correlations between intervals (2.50–3.40) MeV and (1.70–2.00) MeV increased to almost 1.00 close to earthquakes. This effect was obtained in low background screen.Connection of the gamma data with the meteorological data was not obtained.     

Effect of swift heavy ion irradiation on thermal,optical and structural properties of poly vinylidene chloride

Poly vinylidene chloride (PVDC) irradiated with lithium (50 MeV), carbon (85 MeV), nickel (120 MeV) and silver ions (120 MeV) having fluence range of 1 × 10¹¹ ions/cm² to 3 × 10¹² ions/cm² have been studied using different techniques i.e. XRD (X-ray diffraction), FTIR (Fourier transform infrared), UV–Visible and TGA (thermo-gravimetric analysis). In XRD analysis, the intensity of diffraction peaks of PVDC irradiated with lithium ions was enhanced at lower fluence as compared to pristine. The shift in optical absorption edge in irradiated PVDC was correlated with the decrease in optical band gap energy. The distinguishable characteristic peaks were observed due to UV–Vis analysis, in lithium irradiated samples of PVDC at higher fluences. The % age decrease in optical band gap energy for the respective ions were 30.9%, 34.16%, 81.1%, 87.02% respectively. Formation of double carbon bonds and breaking of C–O and C–Cl bonds with the release of Cl in irradiated PVDC was observed in FTIR spectra. In Thermogravimetric analysis (TGA), the % age weight loss observed for irradiated samples with increase in ion fluence was lesser than the % age weight loss observed in pristine sample.     

Physical and chemical response of polypropylene irradiated with 70 MeV carbon and 150 MeV nickel-ions

Polypropylene films were irradiated with C⁵⁺ (70 MeV) and Ni¹¹⁺ (150 MeV) ions of varying fluence. The ion induced optical, chemical and structural changes were investigated using UV-Vis and Fourier Transform Infrared (FTIR) spectroscopy and X-ray diffractometry (XRD). UV-Vis data showed an increase of optical absorbance and a shift of the absorption edge towards the red end of the spectrum when the ion fluence was increased. This shift may be attributed to the greater degree of conjugation. FTIR analysis revealed the formation of alcoholic and ketonic groups. Significant loss of crystallinity occurred with increasing ion fluence.     

Dependence of the thermoluminescent high-temperature ratio (HTR) of LiF:Mg,Ti detectors on proton energy and dose

The high-temperature ratio (HTR) is a parameter quantifying changes of the shape of the high-temperature part of the LiF:Mg,Ti glow-curve after exposure to densely ionizing radiation. It was introduced in order to estimate the ‘effective LET’ of an unknown radiation field and to correct the decreased relative TL efficiency for high-Linear Energy Transfer (LET) radiation.In the present work the dependence of HTR on proton energy (14.5–58 MeV) and dose (0.5–30 Gy) was investigated. All measured HTR values were at the level of 1.2 or higher, therefore significantly different from the respective value for gamma-rays (HTR = 1), but HTR was found to be insensitive to changes of proton energy above 20 MeV. As a result the relationship between HTR and relative TL efficiency is not unequivocal. The HTR was found to be dependent on absorbed dose even for the lowest studied doses.     

Characterisation of the high-energy neutron beam at iThemba LABS

Monoenergetic and white neutron beams with a maximum energy of 200 MeV are produced at the neutron beam facility of the iThemba Laboratory for Accelerator-Based Sciences (iTL) by (p, n) reactions on Li, Be and C targets. The iTL separated-sector cyclotron can accelerate protons from 25 MeV to 200 MeV. Time of flight (TOF) measurements can be carried out by increasing the time separation between proton bunches up to 500 ns with a beam pulse selector. In the present work, the methods of beam characterisation used at this facility are discussed. Comparisons with metrological standards are presented. Specific issues related to monitoring the neutron fields at different accelerator settings are analysed.     

Influence of Bonner sphere response functions above 20 MeV on unfolded neutron spectra and doses

Monte Carlo (MC) codes for neutron transport calculations such as MCNP, MCNPX, FLUKA, PHITS, and GEANT4, crucially rely on cross sections that describe the interaction of neutrons with nuclei. For neutron energies below 20 MeV, evaluated cross sections are available that are validated against experimental data. In contrast, for high energies (above 20 MeV) experimental data are scarce and, for this reason, every neutron transport code is based on theoretical nuclear models to describe interactions of neutrons with nuclei in matter. Here we report on the calculation of a complete set of response functions for a Bonner spheres spectrometer (BSS), by means of GEANT4 using the Bertini and Binary Intranuclear Cascade (INC) models for energies above 20 MeV. The recent results were compared with those calculated by MCNP/LAHET and MCNP/HADRON MC codes. It turns out that, whatever code used, the response functions were rather similar for neutron energies below 20 MeV, for all 16 detector/moderator combinations of the considered BSS system. For higher energies, however, differences of more than a factor of 2 were observed, depending on neutron energy, detector/moderator combination, MC code, and nuclear model used. These differences are discussed in terms of neutron fluence rates measured at the environmental research station (UFS), “Schneefernerhaus”, (Zugspitze mountain, Germany, 2650 m a.s.l.) for energies below 0.4 eV (thermal neutrons), between 0.4 eV and 100 keV (epithermal neutrons), between 100 keV and 20 MeV (evaporation neutrons), and above 20 MeV (cascade neutrons). In terms of total neutron fluence rates, relative differences of up to 4% were obtained when compared to the standard MCNP/LAHET results, while in terms of total ambient dose equivalent, relative differences of up to 8% were obtained. Both the GEANT4 Binary INC and Bertini INC gave somewhat larger fluence and dose rates in the epithermal region. Most relevant for dose, however, those response functions calculated with the GEANT4 Bertini INC model provided about 18% less neutrons in the cascade region, which led to a roughly 13% smaller contribution of these neutrons to ambient dose equivalent. It is concluded that doses from secondary neutrons from cosmic radiation as deduced from BSS measurements are uncertain by about 10%, simply because of some differences in nuclear models used by various neutron transport codes.     

Relative thermoluminescent efficiency of LiF detectors for proton radiation: Batch variability and energy dependence

The available experimental data on the relative thermoluminescent efficiency of the LiF:Mg,Ti dosimetric peaks for protons are contradictory. There are several reports showing that the efficiency exceeds unity by even more than 30%, however, many others show the efficiency close to unity or even lower. These contradictory data might be a result of the real variability of TLD properties or of not perfectly reproduced experimental conditions.In an attempt to resolve this issue, the efficiency of 16 batches of LiF:Mg,Ti (MTS) detectors for 60 MeV protons produced at the IFJ Kraków over the last 20 years was measured. All values of the relative TL efficiency were found to exceed unity significantly, with an average of 1.09. Dispersion between different batches was very low, all data were within 4% of the mean value.In second part of experiment the dependence of the relative efficiency of LiF:Mg,Ti and LiF:Mg,Cu,P detectors on proton energy was determined. The efficiency for LiF:Mg,Ti dosimetric peaks was found to have a maximum of 1.20 at about 20 MeV. For LiF:Mg,Cu,P the relative efficiency decreases systematically with decreasing proton energy, from 0.96 at 56 MeV, to 0.61 at 11 MeV.     

Neutron,photon and proton energy spectra at high altitude measured using a phoswich-type neutron detector

Neutron energy spectrum from 7 to 180 MeV, photon energy spectrum from 4 to 50 MeV and proton energy spectrum from 94 to 145 MeV were measured simultaneously using a phoswich-type neutron detector with particle discrimination methods at atmospheric depth of 249 g/cm², a vertical cut-off rigidity of 10.2 GV and at a heliocentric potential of 312 MV. We compared our results with other measured and calculated particle energy spectra. Our measured results give a large, sharp neutron peak around 70 MeV, although Bonner balls show a broad peak around 100 MeV due to low energy resolution. The measured photon and proton spectra are between the calculated energy spectra. This onboard study provides the first experimental neutron energy spectrum over 10 MeV with a high-energy resolution.     

Effective atomic number,electron density and kerma of gamma radiation for oxides of lanthanides

An attempt has been made to estimate the effective atomic number, electron density (0.001 to 10⁵ MeV) and kerma (0.001 to 20 MeV) of gamma radiation for a wide range of oxides of lanthanides using mass attenuation coefficient from WinXCom and mass energy absorption coefficient from Hubbell and Seltzer. The values of these parameters have been found to change with energy for different oxides of lanthanides. The lanthanide oxides find remarkable applications in the field of medicine, biology, nuclear engineering and space technology. Nano-oxides of lanthanide find applications in display and lighting industry.     

Radiation defects introduced by MeV electrons in argon implanted MOS structures

The influence of MeV electron irradiation on the interface states of argon implanted thin oxide MOS samples has been studied by the thermally stimulated current (TSC) method. The oxide thickness of the structures is 18 nm. Two groups of n-type MOS structures non-implanted and implanted with 20 keV Ar⁺ ions and a dose of 5×10¹² cm⁻² are examined. Both groups are simultaneously irradiated by 23 MeV electrons with doses of 1.2×10¹⁶, 2.4×10¹⁶ or 6.0×10¹⁶ el/cm². The energy position and density of the interface states (generated by electron irradiation, ion implantation or both treatments of the samples) are determined. It is shown that MeV electron irradiation decreases the concentration of interface states (like an oxygen-vacancy and di-vacancy) slightly and creates additional interface states (like an impurity-vacancy) at the Si–SiO₂ interface of argon implanted MOS structures.     

An imaging spectrometer based on high resolution microscopy of fluorescent aluminum oxide crystal detectors

Fluorescent Nuclear Track Detector (FNTD) technology was tested as an imaging, spectroscopic tool for radionuclide analysis. This investigation intended to distinguish between characteristic α-particles of ²³⁹Pu (5.2 MeV), ²³⁴U (4.8 MeV) and ²³⁸U (4.2 MeV). FNTDs are Al₂O₃:C,Mg single crystals with color centers that undergo radiochromic transformation. FNTD readout is non-destructive and is performed with fluorescence laser scanning confocal microscopy. Ionization events register in the detector as bright fluorescent features on a weak fluorescent background. Images were acquired at several incrementing depths in the detector to produce 3D datasets. Spectroscopic information was obtained by measuring α-particle range in the detector after 3D image reconstruction. The resolution of this technique is fundamentally limited by particle range straggling (about 3.8% (k = 1) at these α-particle energies). The spectroscopic line-width as full width at half maximum (FWHM) was determined to be 0.4 MeV enabling discrimination between the isotopes of interest.     

Simulation and experimental study of an indigenously designed and constructed THGEM-based microdosimeter for dose-equivalent measurement

Most of the GEM/THGEM-based microdosimetric detectors presented in the literature simulate 2 μm of tissue which results in a flat neutron dose-equivalent response in the MeV region. The objective of this work was to introduce a neutron microdosimeter with a more extended flat response. In this regard, a THGEM-based microdosimeter with plexiglas walls, simulating 1 μm of tissue was designed and constructed. Its performance was investigated by both simulation and experimentation to determine the microdosimetric quantity of “lineal energy”.In the simulation study, lineal energy distribution, mean quality factor and dose-equivalent response of the microdosimeter for eleven neutron energies from 10 keV to 14 MeV, along with the energy spectrum of ²⁴¹Am-Be neutrons, were calculated by the Geant4 simulation toolkit. Obtained lineal energy distributions were compatible with the distributions determined by a Rossi counter. Also, the mean quality factors agreed well with the values reported by the ICRU report 40 which confirmed tissue equivalent behavior of the microdosimeter. They were different from the effective quality factor values within 15% between 20 keV and 14 MeV. This led to a flat dose-equivalent response with 20% difference from a median value of 0.82 in the above energy range which was an improvement compared with other THGEM-based detectors, simulating 2 μm of tissue. In spite of the satisfactory determination of the dose-equivalent, the microdosimeter had low detection sensitivity.In the experimental study, the measured lineal energy distribution of ²⁴¹Am-Be neutrons was in agreement with the simulated distribution. Further, the measured mean quality factor and dose-equivalent differed by 1.5% and 3.5%, respectively, from the calculated values. Finally, it could be concluded that the investigated microdosimeter reliably determined the desired dose-equivalent value of each neutron field with every energy spectrum lying between 20 keV and 14 MeV.     

Radiation chemical yields for the losses of typical functional groups in PADC films for high energy protons registered as unetchable tracks

A series of FT-IR spectrometric studies has been performed to understand the latent track structure in poly(allyl diglycol carbonate), PADC, which were exposed to proton beams with energies of 20, 30 and 70 MeV. These energies are too high to register etchable tracks in PADC. Chemical damage parameters, such as damage density, effective track core radius and radiation chemical yields, for losses of ether bond, carbonate ester bond and CH groups in PADC are evaluated as a function of the stopping power, which were compared to the previous results for 5.7 MeV proton and heavy ions, between He and Xe. Graphs of the chemical damage parameters are given at the wide stopping powers ranging from 1 to 12,000 keV/μm. The decreasing behaviors of the ether and carbonate ester bonds are on the almost identical trends with those of the heavy ions. On the contrary to this, the reducing behavior of CH groups is similar to that of the gamma rays. Different dependence of the chemical damage parameters for the loss of CH groups is found on the stopping powers between the both sides of the detection threshold as an etched track detector.     

Calibration of CR-39 with atomic force microscope for the measurement of short range tracks from proton-induced target fragmentation reactions

We studied the track response of CR-39 plastic nuclear track detectors (PNTD) for low (<6 MeV/n) and high (>100 MeV/n) energy heavy ions using the atomic force microscope (AFM). CR-39 PNTD was exposed to several heavy ion beams of different energy at HIMAC (Heavy Ion Medical Accelerator in Chiba). For AFM measurement, the amount of bulk etch was controlled to be ∼2 μm in order to avoid etching away of short range tracks. The response data obtained by AFM for ∼2 μm bulk etch was in good agreement with data obtained by the conventional optical microscope analysis for larger bulk etch. The response data from low energy beams (stopping near the surface) was also consistent with the data from high energy beams (penetrating the detector) as a function of REL (restricted energy loss) with the δ-ray cut off energy of ω₀ = 200 eV. We experimentally verified that REL (ω₀ = 200 eV) gives a universal function for wide energy range in CR-39 PNTD. This work has been done as part of a basic study in the measurement of secondary short range tracks produced by target fragmentation reactions in proton cancer therapy fields.     

Study on energy dependence of PAGAT polymer gel dosimeter evaluated using X-Ray CT

The normoxic polymer gel dosimeter evaluated with X-Ray computed tomography has emerged as a promising tool for measuring the dose delivered during radiotherapy in three dimensions. This study presents the dependence of PAGAT normoxic polymer gel sensitivity to different photon and electron energies. PAGAT polymer gel was prepared under normal atmospheric condition and irradiated with different photon energies of 1.25 MeV from Co-60 and 6 MV and 15 MV from linear accelerator and electron energies of 6, 9, 12, 15, 18 and 21 MeV from linear accelerator. Evaluation of dosimeter was performed with an X-Ray CT scanner. Images were acquired with optimum scanning protocols to reduce the signal-to-noise ratio. The averaged image was subtracted from the unirradiated polymer gel image for background. Central axis depth dose (PDD) curves obtained for each energy and polymer gel dosimeter measurements were in good agreement with diode and film measurements. Hounsfield (HU) – dose response curve for each photon and electron energy were derived from the PDD curve obtained from the gel dosimeter measurements. From the study it was clear that the HU-dose response curve was linear in the region 1–10 Gy. The dosimeter sensitivity was defined as a slope of these linear HU-dose response curves and found that the sensitivity of polymer gel decreases with increase in both photon and electron energies. This trend in dependence of PAGAT gel dosimeter sensitivity to different photon and electron energies was not dosimetrically significant. However, to evaluate the test phantom exposed with one energy using the calibration curve derived at another energy can produce clinically significant error.     

The influence of field size and off-axis distance on photoneutron spectra of the 18 MV Siemens Oncor linear accelerator beam

At present, high energy electron linear accelerators (LINACs) producing photons with energies higher than 10 MeV have a wide use in radiotherapy (RT). However, in these beams fast neutrons could be generated, which results in undesired contamination of the therapeutic beams. These neutrons affect the shielding requirements in RT rooms and also increase the out-of-field radiation dose to patients. The neutron flux becomes even more important when high numbers of monitor units are used, as in the intensity modulated radiotherapy. Herein, to evaluate the exposure of patients and medical personnel, it is important to determine the full radiation field correctly. A model of the dual photon beam medical LINAC, Siemens ONCOR, used at the University Hospital Centre of Osijek was built using the MCNP611 code. We tuned the model according to measured photon percentage depth dose curves and profiles. Only 18 MV photon beams were modeled. The dependence of neutron dose equivalent and energy spectrum on field size and off-axis distance in the patient plane was analyzed. The neutron source strength (Q) defined as a number of neutrons coming from the head of the treatment unit per x-ray dose (Gy) delivered at the isocenter was calculated and found to be 1.12 × 10¹² neutrons per photon Gy at isocenter. The simulation showed that the neutron flux increases with increasing field size but field size has almost no effect on the shape of neutron dose profiles. The calculated neutron dose equivalent of different field sizes was between 1 and 3 mSv per photon Gy at isocenter. The mean energy changed from 0.21 MeV to 0.63 MeV with collimator opening from 0 × 0 cm² to 40 × 40 cm². At the 50 cm off-axis the change was less pronounced. According to the results, it is reasonable to conclude that the neutron dose equivalent to the patient is proportional to the photon beam-on time as suggested before. Since the beam-on time is much higher when advanced radiotherapy techniques are used to fulfill high conformity demands, this makes the neutron flux determination even more important. We also showed that the neutron energy in the patient plane significantly changes with field size. This can introduce significant uncertainty in dosimetry of neutrons due to strong dependence of the neutron detector response on the neutron energy in the interval 0.1–5 MeV.     

Hydrogen analysis in diamond like carbon by elastic recoil detection

Among the available surface analytic instruments, elastic recoil detection (ERD) is known as a reliable method for hydrogen analysis. Since conventional fluence determination i.e. beam current integration is incredible at a large tilt angle, ion fluence is determined by the scattering spectrum that is simultaneously measured with recoil spectrum. However scattering cross sections deviate Rutherford values in the ERD energy of 1–3 MeV. Carbon scattering cross section is different from Rutherford value for higher beam energies over 1.8 MeV. As a result hydrogen content is exaggerated when fluence is determined by carbon matrix because of fluence underestimation due to lower value of scattering cross section than Rutherford’s. Therefore in order to quantify hydrogen in diamond like carbon (DLC) incident beam energy lower than 1.6 MeV should be used where carbon scattering cross sections are well agreed with Rutherford’s.     

An analysis of 175 MeV Nickel ion irradiation and annealing effects on silicon NPN rf power transistors

In this study NPN rf power transistors were irradiated by 175 MeV Ni¹³⁺ ions in the dose range of 100 krad to 100 Mrad. Their characteristics such as excess base current (ΔI_B = I_Bpost ? I_Bpre), dc current gain (h_FE), transconductance (g_m) and collector-saturation current (I_CSat) were studied before and after irradiation. The damage factor (K) for h_FE was calculated using Messenger–Spratt relation. The base current (I_B) was found to increase significantly after irradiation and in turn decreases the h_FE. The g_m and collector current (I_C) in the saturation region (I_CSat) were found to decrease with increase in radiation dose. The results obtained here were also compared with that obtained by 140 MeV Si¹⁰⁺ ions, 100 MeV F⁸⁺ ions, 95 MeV O⁷⁺ ions and 50 MeV Li³⁺ ions irradiation studies in the same dose ranges to understand the LET effects. The recovery in the I–V characteristics of irradiated NPN transistors were studied by isothermal and isochronal annealing methods.     

Oxygen and gold ion irradiation effects on hydroxyethylammonium (l) tartrate monohydrate single crystals

Hydroxyethylammonium (l) tartrate monohydrate (HEALT) single crystals were grown by slow evaporation solution growth method and irradiated at room temperature with 100 MeV oxygen and 200 MeV gold ions. The powder X-ray diffraction study shows that the title crystal has undergone lattice disorder after irradiation. UV–Visible study reveals that there is a decrease in band gap values on irradiation. The scanning electron and atomic force micrograph discloses the defects which was formed due to irradiation. The unirradiated as well as irradiated crystals were characterized by photoluminescence. Further dielectric and Vickers microhardness measurements were studied for oxygen and gold ions irradiated crystals and compared with pristine HEALT single crystals.     

Three-dimensional simulation of laser–plasma-based electron acceleration

A sequential three-dimensional (3D) particle-in-cell simulation code PICPSI-3D with a user friendly graphical user interface (GUI) has been developed and used to study the interaction of plasma with ultrahigh intensity laser radiation. A case study of laser–plasma-based electron acceleration has been carried out to assess the performance of this code. Simulations have been performed for a Gaussian laser beam of peak intensity 5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 1 × 10¹⁹ cm^− 3, and for a Gaussian laser beam of peak intensity 1.5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 3.5 × 10¹⁹ cm^− 3. The electron energy spectrum has been evaluated at different time-steps during the propagation of the laser beam. When the plasma density is 1 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~14 MeV, with an energy spread of ±7 MeV. On the other hand, when the plasma density is 3.5 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~23 MeV, with an energy spread of ±7.5 MeV.