Characterization of miniature RAD-HARD silicon diodes as dosimeters for small fields of photon beams used in radiotherapy

The dosimetric response characterization and beam data acquisition performance of a miniature Float Zone (FZ) silicon diode for photon beams was investigated using Novalis TX linear accelerator (Varian Medical Systems^®). In all measurements the unbiased diode operated in a short-circuit mode, connected to the input of a Keithley 6517B electrometer using a water phantom. For photon beams of the 6 and 15 MV the results presented good repeatability (coefficient of variation ≤1.6%), measured through switching on/off the photon beams. Moreover, the diode showed a quite linear response, given by the charge versus absorbed dose, with charge sensitivities higher than 6.9 nC/Gy. The output factor, percentage depth dose profile (PDD) and transversal dose profile (TDP) were also measured in a water phantom. For small field sizes, the output factor values using the FZ diode were compared with measurements obtained with a SFD (Stereotactic Field Diode) commercial diode and the differences were 5.4%, 2.5% and 1.3% for the field sizes of 1 × 1, 2 × 2 and 3 × 3 cm². For larger field sizes (≥4 × 4 cm²), the maximum difference found was 0.7% in comparison with values obtained with a CC13 ionization chamber. Thus, the result demonstrates that the unshielded FZ diode has the potential to be used for measuring of, as it performed acceptably well for both small and large field sizes. The TDP experimental results obtained with the FZ diode for field sizes of: 1 × 1 cm², 2 × 2 cm² and 4 × 4 cm² are in agreement with experimental results acquired with several commercial detectors. Through the TDP study, the comparison of the field penumbra size confirmed the excellent spatial resolution of the miniature diode. However, the PDD study, requires further investigation.     

Characterization of a scintillating fiber-optic dosimeter for photon beam therapy

Abstact  A scintillating fiber-optic dosimeter has many advantages such as real-time readout, high-resolution measurement, water-equivalence and no corrections for temperature, pressure and humidity. Organic scintillator which has water or tissue equivalent characteristics is very important to measure absorbed dose, dose rate and dose distributions exactly without any corrections and due to its small size, the sensitive volume enables accurate dose measurements in regions of high dose gradients with high spatial resolution. In this study, a scintillating fiber-optic dosimeter with an organic scintillator is fabricated to measure high-energy photon beam from a clinical linear accelerator. And we have measured linear responses of a fiber-optic dosimeter according to dose rates and monitor units of a clinical linear accelerator. Also, a percent depth dose curve for 6MV photon beam with different field sizes are obtained.     

Small field output factors: Comparison of measurements with various detectors and effects of detector orientation with primary jaw setting

Accurate dosimetry in small photon fields used in modern radiotherapy is a challenging task due to electronic disequilibrium, steep dose gradients, source occlusion and size of the sensitive volume of the detector. These challenging effects and the lack of metrological dosimetric reference instigated an investigation on the acquisition of output factor with various detectors in parallel and perpendicular orientations. Small field output factor measurements of tertiary collimators such as BrainLab circular cones, BrainLab mMLC and Millennium MLC were carried out in this study. The data acquired show the differences between output factor results with different detectors for all collimating systems. Good agreement in output values was observed in field sizes greater than ∼2 × 2 cm² for all detectors and all tertiary collimators. For smaller fields when compared to electron field diode (EFD), 0.125 cm³ ion chamber underestimates the output by up to −11.1% and −20.4% and pinpoint ion chamber underestimates the output by up to −1.5% and −6.1% in their parallel and perpendicular orientation, respectively. In contrast, PTW SRS diode and photon field diode (PFD) overestimate the output factor by up to 2.5% and 6.9% respectively in its parallel orientation. The investigated data for the effect of jaw position (0.25 × 0.25 cm², 0.5 × 0.5 cm² and 1 × 1 cm²) away from the field edge generated by different tertiary collimating systems inferred that the opening of X–Y jaw highly influences the small field output factors. The orientation of the detectors and the position of the jaws could influence the output factors considerably in small fields.     

Studying the potential of point detectors in time-resolved dose verification of dynamic radiotherapy

Modern megavoltage x-ray radiotherapy with high spatial and temporal dose gradients puts high demands on the entire delivery system, including not just the linear accelerator and the multi-leaf collimator, but also algorithms used for optimization and dose calculations, and detectors used for quality assurance and dose verification. In this context, traceable in-phantom dosimetry using a well-characterized point detector is often an important supplement to 2D-based quality assurance methods based on radiochromic film or detector arrays. In this study, an in-house developed dosimetry system based on fiber-coupled plastic scintillator detectors was evaluated and compared with a Farmer-type ionization chamber and a small-volume ionization chamber. An important feature of scintillator detectors is that the sensitive volume of the detector can easily be scaled, and five scintillator detectors of different scintillator length were thus employed to quantify volume averaging effects by direct measurement. The dosimetric evaluation comprised several complex-shape static fields as well as simplified dynamic deliveries using RapidArc, a volumetric-modulated arc therapy modality often used at the participating clinic. The static field experiments showed that the smallest scintillator detectors were in the best agreement with dose calculations, while needing the smallest volume averaging corrections. Concerning total dose measured during RapidArc, all detectors agreed with dose calculations within 1.1 ± 0.7% when positioned in regions of high homogenous dose. Larger differences were observed for high dose gradient and organ at risk locations, were differences between measured and calculated dose were as large as 8.0 ± 5.5%. The smallest differences were generally seen for the small-volume ionization chamber and the smallest scintillators. The time-resolved RapidArc dose profiles revealed volume-dependent discrepancies between scintillator and ionization chamber response, which confirmed that correction factors for ionization chambers in high temporal and spatial dose gradients are dominated by the volume averaging effect. The unique scaling of the scintillator volumes indicated how such time-dependent volume averaging corrections could be quantified. The time-resolved measurements further supported the claim that small-volume water equivalent detectors are most likely to accurately detect changes in dose delivery, although exact positioning of detectors remains critical.     

Method of H1(10) evaluation using a spherical-type energy-independent neutron dose monitor

A spherical-type neutron dose monitor has been developed for the use of area monitoring around the neutron producing fields. Considering the practical fields that neutrons coming from multiple directions, the study has been performed using MCNP simulation, in order to estimate the resultant direction of the sources and to find a suitable method of H¹(10) evaluation. This was done by irradiating the monitor using two neutron sources of different energies and intensities; placed at an angle between them. A parallel neutron beam of 50 cm radius with wide energy was used for the irradiation. The direction of the neutron sources was determined comparing the responses of 12 thermo-luminescent detectors (TLDs) of first layer between single and two directional sources. It was found that in most of the cases, the direction of the source was estimated to be the average angle or near the average angle of single sources. Using a linear combinational equation the method has been developed for evaluating the dose from the mean values of selected detectors of two consecutive depths of TLDs and the TLD of core depth. It was found that the ratio of the calculated to the expected dose was almost close to unity which indicates that the method can be used for evaluating the dose with good accuracy.     

Design of a graphite-moderated 241Am–Li neutron field to simulate reactor spectra

A neutron calibration field using ²⁴¹Am–Li sources and a moderator was designed to simulate the neutron fields found outside a reactor. The moderating assembly selected for the design calculation consists of a cube of graphite blocks with dimensions of 50 cm by 50 cm by 50 cm, in which the ²⁴¹Am–Li sources are placed. Monte Carlo calculations revealed the optimal depth of the source to be 15 cm. This moderated neutron source can be used to provide a test field that has a large number of intermediate energy neutrons with a small portion of MeV component.     

Skin dose assessment in unmodulated and intensity-modulated radiation fields with film dosimetry

Superficial dose from 6- and 18-MV photon beams has been studied by measuring surface dose and shallow build-up dose using radiographic film EDR2, radiochromic film EBT2 and plane-parallel chamber. Measurements have been made for intensity- and non-intensity-modulated beams.The results show that the surface dose was found to be 19.8% and 10% of maximum dose in unmodulated fields for 6 and 18 MV photon beams, respectively. The study further showed that intensity modulation decreased surface dose by 1.1% and 0.7% for the same field size at 6 and 18 MV, respectively, and surface dose was dropped by magnetically sweeping contaminating electrons. EDR2 and EBT2 films show in good agreement in shallow build-up region.This study demonstrated the capability of EDR2 film, in addition to radiochromic film, to measure surface and build-up dose in case of treatment planning system uncertainties with regard to skin toxicity or shallow target coverage.     

Measurement of peak fluence of neutron beams using Bi-fission detectors

Fission fragments and other charged particles leave tracks of permanent damage in most of the insulating solids. Damage track detectors are useful for personal dosimeters and for flux/dose determination of high-energy particles from accelerators or cosmic rays. A detector that has its principal response at nucleon energy above 50 MeV is provided by the fission of Bi-209. Neutrons produce the largest percentage of hadron dose in most high-energy radiation fields. In these fields, the neutron spectrum is typically formed by low-energy neutrons (evaporation spectrum) and high-energy neutrons (knock-on spectrum). We used Bi-fission detectors to measure neutron peak fluence and compared the result with the calculated value of neutron peak fluence. For the exposure to 100 MeV we have used the iThemba Facility in South Africa.     

Visualization of high radiation field by radiophotoluminescence photography

We have proposed a simple technique for the visualization of high radiation fields by radiophotoluminescence (RPL) photography. Pulverized RPL glass particles were encapsulated into hundreds of polystyrene balls of accumulation-type RPL detectors. The RPL detectors were placed near an intense gamma-ray source. After irradiation, the RPL detectors were uniformly brightened with a UV illuminator. Orange RPL could be observed by the naked eye at doses above 5 Gy. For a dose above 0.5 Gy, a clear RPL photograph was taken with a digital camera. The spatial dose distribution was obtained through digital image processing of the RPL photograph. Therefore, this simple RPL photographing technique using RPL detectors is useful for detecting high levels of radioactivity.     

Characterisation of a real-time fibre-coupled beryllium oxide (BeO) luminescence dosimeter in X-ray beams

For the first time the feasibility of using beryllium oxide (BeO) ceramics as a fibre-coupled radioluminescent dosimeter is investigated. BeO ceramic exhibits both radioluminescence (RL) and optically stimulated luminescence (OSL), and has the potential to be a near tissue equivalent alternative to Al₂O₃:C. A BeO fibre-coupled radioluminescence dosimeter is demonstrated and characterised for 6 MV X-rays and superficial X-ray energies, 150 kVp and 120 kVp. Based on the results, we demonstrate the capability of the RL BeO FOD for accurate and reproducible dose measurements with a linear dose rate and dose response. It has also been found that the percentage depth dose curves for 6 MV agreed with ion chamber measurements to within 2%, except in the build up region. For the 150 kVp and 120 kVp photon beams, the depth dose measurements agreed with ion chamber measurements to within 2.5% and 4%, respectively.     

A new neutron monitor with silver activation

A moderator-type neutron monitor has been developed, which registers delayed beta rays from neutron-induced silver activation and which is able to measure dose equivalent in pulsed fields with peak dose rates of several thousand Sv h^?1. The monitor uses four silicon diodes in the centre of a polyethylene moderator, 30 cm in diameter. Two of the diodes are covered by natural silver foils and two of them by tin foils. The latter are used to subtract photon-induced pulses. For registering signals, a pulse height threshold is set at 662 keV, which minimizes the effect of ¹³⁷Cs and lower energy radiation and – in addition – enhances the detection of beta rays from the shorter half-life silver isotope ¹¹⁰Ag (25 s) as compared to the longer half-life isotope ¹⁰⁸Ag (144 s). The results of measurements in neutron and photon calibration fields, of MCNPX neutron response calculations and of first measurements in a high-intensity pulsed field at the PSI accelerator are shown.     

Monte-Carlo calculations of the activation of the accelerator target holder and shadow cone during the calibration of the ITER diagnostic devices with monoenergetic neutrons

The ITER International Fusion Energy Organization has solicited IRSN Laboratory for Neutron Metrology and Dosimetry to study the possibility to calibrate, in monoenergetic neutron fields at 14 and 2.45 MeV, the neutron detectors to be placed inside the future fusion reactor. In addition to the estimate of the necessary irradiation times, the dose equivalent rates from some of the neutron activated beam line elements had been calculated to consider the cooling time mandatory before access. Neutron activation calculations have been performed with the Fluka Monte-Carlo code. The resulting dose equivalent rates depend strongly of the neutron beam intensity as well as the neutron energy. In the worst case, for 14 MeV neutrons at an emission rate of 10¹² s⁻¹, a cooling time of 24 h would be needed for a close access to the shadow cone. Several days would be mandatory in the case of the target holder.     

Radio-photoluminescence of highly irradiated Lif:Mg,Ti and Lif:Mg,Cu,P detectors

The radio-photoluminescent (RPL) characteristics of LiF:Mg,Ti (MTS) and LiF:Mg,Cu,P (MCP) thermoluminescent detectors, routinely used in radiation protection dosimetry, were investigated after irradiation with ultra-high electron doses ranging up to 1 MGy. The photoluminescence of both types of LiF detectors was stimulated by a blue light (460 nm) and measured within a spectral window around 530 nm. The RPL dose response was found to be linear up to 50 kGy and sublinear in the range of 50 kGy to 1 MGy for MCP detectors and linear up to 3 kGy and next sublinear in the range from 5 kGy to 1 MGy for MTS detectors. For both type of LiF detectors RPL signal is saturated for doses higher than 100 kGy. The observed differences between MCP and MTS may suggest, that the RPL effect in LiF is not entirely governed by intrinsic defects (F₂ and F₃⁺ centers), but dopants may also have a significant influence. Due to the non-destructive character of the RPL measurement, it is suggested to apply combined RPL/TL readouts, what should improve accuracy of high-dose dosimetry.     

Testing of a scintillator and fibre optic based radiation sensor

We describe the optimisation of RadLine^®; a small, real time, remotely operated radiation detector, which consists of an inorganic scintillation crystal coupled to a fibre optic cable transporting produced photons to a CCD camera some distance away. RadLine^® is tested in a beta and gamma narrow radiation field of 2.4 GBq, from a Caesium-137 (662 KeV) source, at doses rates between 0.125 mSvhr⁻¹ and 10 mSvhr⁻¹. Our results establish that the lower limit of the device corresponds to a dose rate of 0.2 mSvhr⁻¹, constrained by the signal to noise ratio of the instrument. We also demonstrate the process of characterising the RadLine^® for utilisation underwater due to its partial electrical inactiveness; and to consider how the instrument might perform in aquatic environments and ultimately in a First Generation Magnox Storage Ponds (FGMSP). The RadLine^® brings a marked difference to actual underwater radiation monitoring devices such as; HPGe, CZT and GM detectors, which not only incorporate the whole electronics within and are more bulky, only perform over a short range. The RadLine^®’s design offers signification value for intermediate (>100 m) and long range detection.     

Application of phototransferred thermoluminescence (PTTL) for dose re-assessment in routine dosimetry using MTS-N (LiF:Mg,Ti) thermoluminescent detectors

Erasure of the thermoluminescence (TL) signal on detector readout is considered to be a disadvantage of TL dosimetry, as post-readout dose reassessment is then impossible in principle. A method of dose reassessment based on phototransferred thermoluminescence (PTTL) has been developed at the Institute of Nuclear Physics, Polish Academy of Sciences (IFJ PAN) and applied to MTS-N (LiF:Mg,Ti) detectors. We demonstrate the possibility of applying PTTL for dose reassessment in MTS-N TL detectors routinely applied in the dosimetric service at IFJ PAN. Readings of TL detectors exposed to relatively high doses by the customers of our dosimetry service can now be reassessed using our automatic readers. A major obstacle in applying the PTTL method at lower exposures is the presence of residual dose accumulated in LiF:Mg,Ti detectors after many field exposure and readout cycles. Since most of the TL detectors in our service have been already used for a long time (e.g. for over 10 years in the case of some detector batches), we find that our PTTL method of dose reassessment is possible only in detectors which had received doses exceeding 5 mSv.     

Negative magnetoresistance in the regime of hopping conduction through <Emphasis Type="Italic">p</Emphasis> states at quantum dots

The magnetoresistance in the system of quantum dots with hopping conduction and filling factor 2 < ν < 3 in the limit of small quantum dots has been considered. In this case, hopping conduction is determined by p states. It has been shown that the system exhibits negative magnetoresistance associated with a change in the wavefunctions of p states in a magnetic field. This mechanism of magnetoresistance is linear in magnetic field in a certain range of fields and can compete with the known interference mechanism of magnetoresistance. The magnitude of this magnetoresistance is independent of the temperature at fairly low temperatures and increases with a decrease in the size of a quantum dot.     

High-dose characterization of different LiF phosphors

The dose response of three LiF TLDs: standard LiF:Mg,Ti (denoted MTS), high-sensitive LiF:Mg,Cu,P (MCP) and a recently developed in Kraków version of LiF:Mg,Ti with modified activator composition (MTT) and increased high-LET response was measured. The TLDs have been exposed to ⁶⁰Co gamma-rays, up to dose of 10 000 Gy, i.e. beyond saturation dose of the main dosimetric peaks, which corresponds to ca. 1000 Gy. The measured glow-curves were deconvolved into separate peaks with first order kinetic function (using self-developed GlowFit software). The dose response of the main peaks was found to be supralinear for MTS and sublinear for MCP detectors, as expected. The dose response of MTT was found to be even more supralinear than that of MTS. An interesting effect has been observed with regard to glow-curve shape of MCP detectors. Up to a dose of 1 kGy it remains practically unchanged, while for higher doses a strong growth of high-temperature peaks is observed. In the same dose region a decrease of the main peak of MCP with increasing dose is observed, unlike LiF:Mg,Ti detectors.     

Real time dose rate measurements with fiber optic probes based on the RL and OSL of beryllium oxide

This work covers the examination of fiber optical probes based on the radioluminescence and real time optically stimulated luminescence of beryllium oxide. Experiments are carried out to determine the fundamental dosimetric and temporal properties of the system and evaluate its suitability for dose rate measurements in brachytherapy and other applications using non-pulsed radiation fields. For this purpose the responses of the radioluminescence and optically stimulated luminescence signal have been investigated in the dose rate range of 20 mGy/h to 3.6 Gy/h and for doses of 1 mGy up to 6 Gy. Furthermore, a new, efficient analysis procedure, the double phase reference summing, is introduced, leading to a real time optically stimulated luminescence signal. This method allows a complete compensation of the stem effect during the measurement. In contrast to previous works, the stimulation of the 1 mm cylindrical beryllium oxide detectors is performed with a symmetric function during irradiation. The investigated dose rates range from 0.3 to 3.6 Gy/h. The real time optically stimulated luminescence signal of beryllium oxide shows a dependency on both the dose rate and the applied dose. To overcome the problem of dose dependency, further experiments using higher stimulation intensities have to follow.     

Dosimetric characterization of the Exradin W1 plastic scintillator detector through comparison with an in-house developed scintillator system

New commercial dosimetry systems need careful characterization and can benefit from the comparison with similar, in-house developed solutions. A comparison between such two dosimetry systems, both based on fibre-coupled organic plastic scintillator detectors, is presented. One system is the Exradin W1, fully commercialized by Standard Imaging, while the other system is the non-commercial ME40 system, developed by DTU Nutech with the aim of fundamental dosimetric research. Both systems employ plastic scintillator detectors that can be considered similar in design, calibrated using the same method, but differing primarily in the signal detection hardware. The two systems were compared with respect to essential dosimetric properties, with the purpose of testing their performance under conditions less well discussed in the literature. A Farmer ionization chamber was used as the primary reference of the comparison. The study demonstrated that the Cerenkov light ratio calibration coefficient of both systems was not constant, but changed systematically with photon beam quality to a maximum difference of 1.1%. Calibration with respect to stem effect correction should therefore be performed for every investigated beam quality when using plastic scintillator detectors. Both systems were found to be dose rate independent, even for the highest instantaneous dose rate evaluated (1.5 mGy per pulse). Low-dose measurements revealed large uncertainties for both systems, although the ME40 system handled short beam deliveries under reference conditions with accuracy and precision within 0.4%. Changes in response due to field size dependence were investigated and found to be as large as 3.3% for the W1 and 5.4% for the ME40, biasing output factor measurements in large fields. Great caution is therefore advised if using either system for measurements in large fields or under circumstances where the fibre irradiation geometry is unfavourable. Measurements of reference dose to water yielded differences up to 1.5% when compared with the Farmer ionization chamber for all investigated beam qualities.     

Low dose TL characteristics of Nigerian fluorite

Nigerian fluorite has been characterized by β-irradiation for thermoluminescence in the low dose range (40 μGy–72 mGy). The glow curves exhibit 3 peaks recorded at 111 ± 11 °C, 196 ± 2 °C and 282 ± 4 °C at the heating rate of 5 °C s^?1. The two high temperature peaks exhibit a linear response over the range of study. The minimum detectable dose for each of the observed peaks has been determined and the lowest detection limit of fluorite was also determined. A complex fading pattern was observed for the phosphor and the possible source of the TL buildup has been discussed.