On the relationship between dose-, energy- and LET-response of thermoluminescent detectors

Measurements of the response of thermoluminescent (TL) detectors after gamma ray doses high enough to observe signal saturation provide input to microdosimetric models which relate this gamma-ray response with the energy response after low doses of photons (gamma rays and low-energy X rays) and after high-LET irradiation. To measure their gamma ray response up to saturation, LiF:Mg,Ti (MTS-7 and MTT), LiF:Mg,Cu,P (MCP-7), CaSO4:Dy (KCD) and Al2O3:C detectors were irradiated with 60Co gamma rays over the range 1-5000 Gy. The X-ray photon energy response and TL efficiency (relative to gamma rays) after doses of beta rays and alpha particles, were also measured, for CaSO4:Dy and for Al2O3:C. Microdosimetric and track structure modelling was then applied to the experimental data. In a manner similar to LiF:Mg,Cu,P, the experimentally observed under response of alpha-Al2O3:C to X rays <100 keV, compared with cross-section calculations, is explained as a microdosimetric effect caused by the saturation of response of this detector without prior supralinearity (saturation of traps along the tracks). The enhanced X-ray photon energy response of CaSO4:Dy is related to the supralinearity observed in this material after high gamma ray doses, similarly to that in LiF:Mg,Ti. The discussed model approaches support the general rule relating dose-, energy- and ionisation density-responses in TL detectors: if their gamma ray response is sublinear prior to saturation, the measured photon energy response is lower, and if it is supralinear, it may be higher than that expected from the calculation of the interaction cross sections alone. Since similar rules have been found to apply to other solid-state detector systems, microdosimetry may offer a valuable contribution to solid-state dosimetry even prior to mechanistic explanations of physical phenomena in different TL detectors.     

Determination of LiF:Mg,Ti and LiF:Mg,Cu,P TL efficiency for X-rays and their application to Monte Carlo simulations of dosemeter response

The simulation of response of a new passive area dosemeter for measuring ambient dose equivalent H*(10) for photons has been performed using the Monte Carlo code MCNP and experimentally determined responses of LiF:Mg,Ti and LiF:Mg,Cu,P thermoluminescent (TL) detectors for hard-filtered X-ray spectra from 20 to 300 keV and for 137Cs and 60Co gamma radiation. Relative TL efficiency for both types of detectors, determined in experiments with bare detectors and similar Monte Carlo simulations, compared favourably with prediction of microdosimetric models for proposed microdosimetric target sizes in the range of 20-40 nm. The concluding verification experiment showed small deviations between measured and simulated dosemeter energy response values in the range of a few percent.     

Microdosimetric modelling of the response of thermoluminescence detectors to low- and high-LET ionising radiation

The microdosimetric one hit detector model was applied to calculate the dose response, energy response and relative thermoluminescence (TL) efficiency, eta, of high sensitive LiF:Mg,Cu,P and Al(2)O(3):C detectors after their irradiations by X rays, gamma rays, beta electrons and heavy charged particles (HCP). Microdosimetric distributions in 60 nm targets for photons and beta rays were calculated using the TRION MC track structure code, for HCP using the analytical model of Xapsos with modified transport of secondary electrons and the model of Olko & Booz. The calculated values of eta compare favourably with a broad spectrum of experimental data, including ICHIBAN experiments with HCP. The model offers a method for calculating the thermoluminescence response of TL foils applied to 2-D dosimetry of radiotherapeutic proton beams.     

Development of cryogenic alpha spectrometers using metallic magnetic calorimeters

Cryogenic particle detectors have recently been adopted in radiation detection and measurement because of their high energy resolution. Many of these detectors have demonstrated energy resolutions better than the theoretical limit of semiconductor detectors. We report the development of a micro-fabricated magnetic calorimeter coupled to a large-area particle absorber. It is based on a planar, 1 mm² large paramagnetic temperature sensor made of sputtered Au:Er, which covers a superconducting meander-shaped pickup coil coupled to a low-noise dc-SQUID to monitor the magnetization of the sensor. A piece of gold foil of 2.5×2.5×0.07 mm³ was glued to the Au:Er film to serve as an absorber for incident alpha particles. The detector performance was investigated with an ²⁴¹Am source. The signal size comparison for alpha and gamma peaks with a large difference in energy demonstrated that the detector had good linear behavior. An energy resolution of 2.83±0.05 keV in FWHM was obtained for 5.5 MeV alpha particles.     

Internal microdosimetry of inhaled radon progeny in bronchial airways: advantages and limitations

The objective of the present study was to identify advantages and limitations of the application of microdosimetric concepts for inhaled radon progeny activities in the lungs. The methods employed for this analysis were a recently developed Monte-Carlo microdosimetry code for the calculation of energy deposition in bronchial target cells and the Probability Per Unit Track Length (PPUTL) model, which relates these microdosimetric parameters to cellular radiation effects. The major advantages of internal microdosimetry of radon progeny in bronchial airways are: (i) quantitative characterisation of non-uniform dose distributions and identification of target sites with enhanced carcinogenic potential, (ii) quantification of low doses of alpha particles by the number of cells hit and the dose received by those cells, (iii) illustration of the random variations of cellular doses by specific energy distributions and (iv) establishment of a direct link to cellular radiobiological effects. At present, a major limitation of microdosimetry is the extrapolation of the response of individual cells to the resulting tissue response, which is still not fully explored.     

Measurement and simulation of lineal energy distribution at the CERN high energy facility with a tissue equivalent proportional counter

The response of a tissue equivalent proportional counter (TEPC) in a mixed radiation field with a neutron energy distribution similar to the radiation field at commercial flight altitudes has been studied. The measurements have been done at the CERN-EU High-Energy Reference Field (CERF) facility where a well-characterised radiation field is available for intercomparison. The TEPC instrument used by the ARC Seibersdorf Research is filled with pure propane gas at low pressure and can be used to determine the lineal energy distribution of the energy deposition in a mass of gas equivalent to a 2 mum diameter volume of unit density tissue, of similar size to the nuclei of biological cells. The linearity of the detector response was checked both in term of dose and dose rate. The effect of dead-time has been corrected. The influence of the detector exposure location and orientation in the radiation field on the dose distribution was also studied as a function of the total dose. The microdosimetric distribution of the absorbed dose as a function of the lineal energy has been obtained and compared with the same distribution simulated with the FLUKA Monte Carlo transport code. The dose equivalent was calculated by folding this distribution with the quality factor as a function of linear energy transfer. The comparison between the measured and simulated distributions show that they are in good agreement. As a result of this study the detector is well characterised, thanks also to the numerical simulations the instrument response is well understood, and it's currently being used onboard the aircrafts to evaluate the dose to aircraft crew caused by cosmic radiation.     

Use of various microdosimetric models for the prediction of radon induced damage in human lungs

Exposure to radon and radon decay products in some residential areas and at workplaces constitutes one of the greatest risks from natural sources of ionising radiation. Recently, increasing attention has been paid to the precise estimations of this health risk by numerous models. The compartmental model published in ICRP Publication 66 (HRTM) has been used for calculating alpha activity concentration in human lung. Energy deposition in the tissue was calculated by the Bethe-Bloch equation. The aim of this study was to check the performance and to compare the reliability of the microdosimetric models. In this work different thicknesses of mucus in the cases of non-smokers and smokers has been considered. Transformed cells were considered as the radiation risk parameters. The radiation risk evaluation for different exposure levels was based on homogeneous and heterogeneous distributions of target cells. The results of application of these procedures were compared with the epidemiological study of Czechoslovakian uranium miners.     

Detected contrast and dynamic range measurements of CdZnTesemiconductors for flat-panel digital radiography

The detected contrast and dynamic ranges of Cd_1-xZn_xTe semiconductor detectors have been measured, within the X-ray diagnostic energy range, using a contrast sensitivity phantom. The aim of this study is to optimize the image quality parameters of these solid state ionization devices for flat panel digital radiographic applications. The experimental results of this study indicate that Cd_1-xZn_xTe detectors have excellent detected contrast response and large dynamic range     

Dose rate calculations for a reconnaissance vehicle

A Chemical Nuclear Reconnaissance System (CNRS) has been developed by the British Ministry of Defence to make chemical and radiation measurements on contaminated terrain using appropriate sensors and recording equipment installed in a land rover. A research programme is under way to develop and validate a predictive capability to calculate the build-up of contamination on the vehicle, radiation detector performance and dose rates to the occupants of the vehicle. This paper describes the geometric model of the vehicle and the methodology used for calculations of detector response. Calculated dose rates obtained using the MCBEND Monte Carlo radiation transport computer code in adjoint mode are presented. These address the transient response of the detectors as the vehicle passes through a contaminated area. Calculated dose rates were found to agree with the measured data to be within the experimental uncertainties, thus giving confidence in the shielding model of the vehicle and its application to other scenarios.     

Predicting the response of a submillimeter bolometer to cosmic rays

Bolometers designed to detect submillimeter radiation also respond to cosmic, gamma, and x rays. Because detectors cannot be fully shielded from such energy sources, it is necessary to understand the effect of a photon or cosmic-ray particle being absorbed. The resulting signal (known as a glitch) can then be removed from raw data. We present measurements using an Americium-241 gamma radiation source to irradiate a prototype bolometer for the High Frequency Instrument in the Planck Surveyor satellite. Our measurements showed no variation in response depending on where the radiation was absorbed, demonstrating that the bolometer absorber and thermistor thermalize quickly. The bolometer has previously been fully characterized both electrically and optically. We find that using optically measured time constants underestimates the time taken for the detector to recover from a radiation absorption event. However, a full thermal model for the bolometer, with parameters taken from electrical and optical measurements, provides accurate time constants. Slight deviations from the model were seen at high energies; these can be accounted for by use of an extended model.     

Microdosimetry of epithermal neutron field at the Kyoto University reactor

Microdosimetric spectra were measured in order to gain the microdosimetric parameters of some epithermal neutron fields. Changes in dose mean lineal energy YD as a function of depth of heavy water showed a trend of softening with heavy water of the beam. The neutron absorbed dose was obtained by using the frequency mean lineal energy. Results show good agreement with measurements with the activation method using gold foil. This study demonstrated how microdosimetric parameters change in radiation quality as a function of heavy water depth.     

Simulated and measured Hp(10) response of the personal dosemeter Seibersdorf

The Hp(10) energy response of the personal dosemeter Seibersdorf and its two different filtered LiF:Mg,Ti (TLD-100) thermoluminescence (TL) detectors are investigated. A close-to-reality simulation model of the personal dosemeter badge including the wrapped detector card was implemented with the MCNP Monte Carlo N-particle transport code. The comparison of measured and computationally calculated response using a semi-empirical TL efficiency function is carried out to provide information about the quality of the results of both methods, experiment and simulation. Similar to the experimental calibration conditions, the irradiation of dosemeters centred on the front surface of the International Organization for Standardization (ISO) water slab phantom is simulated using ISO-4037 reference photon radiation qualities with mean energies between 24 keV and 1.25 MeV and corresponding ISO conversion coefficients. The comparison of the simulated and measured relative Hp(10) energy responses resulted in good agreement within some percent except for the filtered TL element at lower photon energies.     

Microdosimetric characteristics of carbon track-segments

Carbon ions were introduced to radiation therapy due to their special physical and radiobiological properties. In heavy ion therapy, specification of radiation quality is an important issue and rapid calculations are often required for treatment planning. Because radiation qualities are closely related to microdosimetric spectra and parameters, this study is intended to provide microdosimetric parameters of carbon track-segments in the energy range from 50 MeV to 5 GeV. Monte-Carlo techniques are used to simulate track-segments of carbon ions in water. Microdosimetric quantities (the dose mean lineal energies) are calculated for the two components of the track-segment: the track-core formed by energy deposition of carbon ions and the track-penumbra (an extended region around the track-core produced by energy deposition of secondary electrons).     

Cellular signal transduction events as a function of linear energy transfer (LET)

In order to obtain a deeper insight into the molecular mechanism controlling the cellular response to high-linear energy transfer (LET) radiation, the number and size of pATM (S1981) and gamma-H2AX foci were compared in cultures of diploid human fibroblasts after exposure to charged particles of varying species, energy and LET at the NIRS-HIMAC-facility (Chiba, Japan). Particle LET ranged from 2.2 to 300 keV/mum, and a low fluence of 7.3 x 10(4) cm(-2) was chosen. Therefore, about 1 out of 7 nuclei was traversed by a particle. Doses and LET were verified with thermoluminescence detectors (LiF:Mg, Ti) evaluated according to the high temperature ratio method. Two hours after irradiation, fibroblasts were fixed and the subcellular distribution of pATM (S1981) and gamma-H2AX was visualised by immunofluorescence or histochemical staining using phosphorylation-specific antibodies. It was found that the number of pATM (S1981) foci per nucleus was higher after exposure to higher-LET particles. Irradiation with the two highest LET beams (Fe-ions, 197 and 300 keV/mum) gave a significant increase in the number of pATM foci, whereas ions with an LET lower than 30 keV/mum yielded similar numbers of pATM foci compared with unirradiated control samples. These data show that the early cellular response to high-LET radiation is modulated by the energy deposition of the particle. Therefore, the correlation between the microdosimetric aspect of energy deposition and biologic consequences at low radiation doses deserves further study.     

Microdosimetry of inhomogeneous radon progeny distributions in bronchial airways

A Monte Carlo code, initially developed for the calculation of microdosimetric spectra for alpha particles in cylindrical airways, has been extended to allow the computation (i) of additional microdosimetric parameters and (ii) for realistic exposure conditions in human bronchial airways with respect to surface activity distribution and airway geometry. The objective of the present study was to investigate the effects of non-uniform distributions of radon progeny activities in bronchial airways on cellular energy deposition parameters. Significant variations of hit frequencies, doses and microscopic energy deposition patterns were observed for epithelial cell nuclei, depending strongly on the assumed activity distributions. Thus, epithelial cells located at different positions in a given bronchial airway may experience a wide range of biological responses. The results obtained suggest that the hit frequency may be the primary physical parameter for alpha particles, supplemented by microdosimetric single event spectra, to be related to biological effects for chronic low level exposures.     

Construction issues surrounding a portable four-band neutron detection system

Solid state-based surface barrier detectors (SBDs) have been employed in neutron radiation dosimetry for over half a century, each combination of detector featuring positives and pitfalls. Here the implications of constructing a portable neutron spectrometer device, based upon moderating high-density polyethylene substrates and utilising different types of large-area solid state SBDs, are discussed.     

A Monte Carlo study of metastable superconducting detectors based on distributions of large volume grains

A Monte Carlo study of the behavior of a metastable superconducting detector under irradiation by beta decay electrons is described. The detector consists of a suspension of large volume tin grains well-distributed in size. Comparison with experiment suggests an encouraging agreement at current levels of experimental uncertainty, propounding the predictability of the response of such detectors and the possibility of successfully interpreting their behavior in the form of deposited energy spectra.     

Beta Spectrometry with Magnetic Calorimeters

Absolute activity measurements of alpha, beta and gamma emitting radioactive sources are important in numerous fields such as therapeutic radiology and the characterization of nuclear waste. Conventional ionization and liquid scintillation detectors, which are commonly used for these applications, have an energy dependent quantum efficiency and severe limitations in energy resolution. As a novel alternative we have developed a detector based on a metallic magnetic calorimeter (MMC) with a gold absorber that covers the full solid angle of 4π around the radioactive source. Deposition of energy in the absorber causes a temperature rise and results in a change of magnetization of a parametric Au:Er sensor, which can be measured by a low-noise high-bandwidth dc-SQUID. The detector has equal sensitivity for beta and gamma radiation. In this paper we describe a detector which has a deviation from linear behavior for energies up to 700 keV of smaller than 0.5% and an overall quantum efficiency for beta particles in this energy range close to unity. We show the data of our experiments measuring the decay of ³⁶Cl and compare the results to the theoretically expected spectrum for this second order forbidden non-unique β-decay. We discuss the observed contributions to noise, the quantum efficiency and the achieved energy resolution.     

Characterization of bismuth tri-iodide single crystals for wide band-gap semiconductor radiation detectors

Bismuth tri-iodide is a wide band-gap semiconductor material that may be able to operate as a radiation detector without any cooling mechanism. This material has a higher effective atomic number than germanium and CdZnTe, and thus should have a higher gamma-ray detection efficiency, particularly for moderate and high energy gamma-rays. Unfortunately, not much is known about bismuth tri-iodide, and the general properties of the material need to be investigated. Bismuth tri-iodide does not suffer from some of the material issues, such as a solid state phase transition and dissociation in air, that mercuric iodide (another high-Z, wide band-gap semiconductor) does. Thus, bismuth tri-iodide is both easier to grow and handle than mercuric iodide. A modified vertical Bridgman growth technique is being used to grow large, single bismuth tri-iodide crystals. Zone refining is being performed to purify the starting material and increase the resistivity of the crystals. The single crystals being grown are typically several hundred mm³. The larger crystals grown are approximately 2 cm³. Initial detectors are being fabricated using both gold and palladium electrodes and palladium wire. The electron mobility measured using an alpha source was determined to be 260±50 cm²/Vs. An alpha spectrum was recorded with one of the devices; however the detector appears to suffer from polarization.     

Measurement of the spectral fluence rate of reference neutron sources with a liquid scintillation detector

Reference neutron sources such as (241)AmBe(alpha,n) and (252)Cf are commonly used to calibrate neutron detectors for radiation protection purposes. The calibration factors of these detectors depend on the spectral distribution of the neutron fluence from the source. Differences between the spectral fluence of the neutron source and the ISO-recommended reference spectra might be caused by the properties of the individual source. The spectral neutron fluence rates of different reference neutron sources used at PTB were measured with a liquid scintillation detector (NE213), using maximum entropy unfolding and a new, experimentally determined detector response matrix. The detector response matrix was determined by means of the time-of-flight technique at a pulsed neutron source with a broad energy distribution realised at the PTB accelerator facility. The results of the measurements of the reference sources are compared with the ISO-recommended reference spectra. For the PTB (241)AmBe(alpha,n) reference source, the spectral neutron fluence was determined by means of a high-resolution (3)He semiconductor sandwich spectrometer in 1982. These measurements were the basis for the ISO recommendations. The current measurements confirm the high-energy part (E(n) > 2 MeV) of this spectrum and demonstrate the suitability of this new method for high-resolution spectrometry of broad neutron spectra.