Investigation of ionization chamber perturbation factors using proton beam and Fano cavity test for the Monte Carlo simulation code PHITS

The reference dose for clinical proton beam therapy is based on ionization chamber dosimetry. However, data on uncertainties in proton dosimetry are lacking, and multifaceted studies are required. Monte Carlo simulations are useful tools for calculating ionization chamber dosimetry in radiation fields and are sensitive to the transport algorithm parameters when particles are transported in a heterogeneous region. We aimed to evaluate the proton transport algorithm of the Particle and Heavy Ion Transport Code System (PHITS) using the Fano test. The response of the ionization chamber f Q and beam quality correction factors k Q were calculated using the same parameters as those in the Fano test and compared with those of other Monte Carlo codes for verification. The geometry of the Fano test consisted of a cylindrical gas-filled cavity sandwiched between two cylindrical walls. f Q was calculated as the ratio of the absorbed dose in water to the dose in the cavity in the chamber. We compared the f Q calculated using PHITS with that of a previous study, which was calculated using other Monte Carlo codes (Geant4, FULKA, and PENH) under similar conditions. The flight mesh, a parameter for charged particle transport, passed the Fano test within 0.15%. This was shown to be sufficiently accurate compared with that observed in previous studies. The f Q calculated using PHITS were 1.116 ± 0.002 and 1.124 ± 0.003 for NACP-02 and PTW-30013, respectively, and the k Q were 0.981 ± 0.008 and 1.027 ± 0.008, respectively, at 150 MeV. Our results indicate that PHITS can calculate the f Q and k Q with high precision.

Monte Carlo calculated beam quality correction factors for high energy electron beams

PURPOSE

As the dosimetry protocol TRS 398 is being revised and the ICRU report 90 provides new recommendations for density correction as well as the mean ionization energies of water and graphite, updated beam quality correction factors kQ are calculated for reference dosimetry in electron beams and for independent validation of previously determined values.

METHODS

Monte Carlo simulations have been performed using EGSnrc to calculate the absorbed dose to water and the dose to the active volumes of ionization chambers SNC600c, SNC125c and SNC350p (all Sun Nuclear, A Mirion Medical Company, Melbourne, FL). Realistic clinical electron beam spectra were used to cover the entire energy range of therapeutic electron accelerators. The Monte Carlo simulations were validated by measurements on a clinical linear accelerator. With regards to the cylindrical chambers, the simulations were performed according to the setup recommendations of TRS 398 and AAPM TG 51, i.e. with and without consideration of a reference point shift by rcav/2.

RESULTS

kQ values as a function of the respective beam quality specifier R50 were fitted by recommended equations for electron beam dosimetry in the range of 5 MeV to 18 MeV. The fitting curves to the calculated values showed a root mean square deviation between 0.0016 and 0.0024.

CONCLUSION

Electron beam quality correction factors kQ were calculated by Monte Carlo simulations for the cylindrical ionization chambers SNC600c and SNC125c as well as the plane parallel ionization chamber SNC350p to provide updated data for the TRS 398 and TG 51 dosimetry protocols.

Standardization of Rn-222 concentration using the multi-electrode proportional counter

Standardization of the concentration of gaseous 222Rn based on a multi-electrode proportional counter (MEPC) is under development as a primary standard in Japan. In this study, the concept and evaluation of its performance are reported. The latter consists of a preliminary result for the uncertainty budget associated with the measurement of the MEPC and compensation of the electric field distortion in the MEPC. Moreover, an ionization-chamber-based gas circulation system was added for the calibration of radon monitors in the air.

Comparison of calibration coefficients for a vinten ionization chamber simulated using four Monte Carlo methods

The Vinten 671 ionization chamber (VIC) was modelled using Monte Carlo (MC) programs EGSnrc, Penelope, and TOPAS. Several national measurement institutes have VICs with well-characterized response relationships and have measured calibration coefficients for many radionuclides. Twelve radionuclides with various decay emissions were assessed as well as 14 monoenergetic photon sources and 10 monoenergetic electron sources. Calibration coefficients were calculated based on the energy deposited in the simulated VIC nitrogen gas volume and compared to experimental values from the literature.

Experimental Investigation of X-Ray Radiation Shielding and Radiological Properties for Various Natural Composites

BACKGROUND

Shielding from radiation and plan dose verification is vital during the potential applications in industrial and medical applications. A number of natural composites have been investigated for protecting against high-energy X-ray shielding.

OBJECTIVE

The aim is to learn about how natural composites behave under various X-ray energies at STP.

MATERIAL AND METHODS

The radiological parameters of wood samples were determined using computed tomography imaging, specifically relative electron density (RED), Hounsfield units (HUs), and mass density (MD). Percentage attenuation was measured using a semiflux ionization chamber incorporated with a brass build-up cap and an ionization chamber placed at the beam Isocenter for a different type of natural composite. Measurements are being carried out on a Linear accelerator at an SSD of 110 cm with different collimator sizes.

RESULTS

Measured values of  HUs, RED, and MD were -232 ± 40, 0.738 ± 0.039, 0.768 ± 0.024 g/cc,-368 ± 41, 0.662 ± 0.047, 0.632 ± 0.024 g/cc, -334 ± 44, 0.639 ± 0.042, 0.666 ± 0.026 g/cc, -370±61, 0.604±0.059, 0.63± 0.036 g/cc, -433±39, 0.543±0.038, 0.608 ± 0.035 g/cc, -382±54, 0.5±0.052, 0.618 ± 0.0316 g/cc, -292±68, 0.680±0.066, 0.708 ± 0.039 g/cc, -298±27, 0.680±0.0229, 0.702± 0.131 g/cc, for Acacia Nilotica, Mangifera Indica, Azadirachta Indica, Tectona Grandis L, Ficus Religiosa, Tecomella Undulata, Sesamum Indicum, Pinus respectively.

CONCLUSION

Measurements show that attenuation is affected by the energy of incident photons, collimator opening, and the type of density of the wood. Various radiological parameters were determined for wood samples that can be utilized to create inhomogeneous phantoms in dosimetry. The largest attenuation is found in Acacia Nilotica and Sesamum Indicum, while the lowest attenuation is found in Ficus religiosa.

Problems Affecting Accurate Dose Measurement in Small-Field for Linear Accelerator

BACKGROUND

The use of relatively narrow fields has become necessary with the advent of intricate and accurate radiation therapy delivery dose to patients; therefore, small-field dosimetry faces several difficulties. Both dose calculations and measurements require to be performed with extra care, due to the uncertainty that might be increased by using such small field sizes.

MATERIAL AND METHODS

In this study, we investigated the effect of detectors size on the dosimetry of small fields [starting with radiation fields from (1cm x 1cm), (2cm x 1cm), and (3cm x 1cm)...etc., up to (4cm x 5cm) and (5cm x 5cm)]. We used the linear accelerator and different types of ionization chambers i.e. [Farmer FC65-P, CC13, and CC01 (pinpoint)] an addition to semiconductors i.e. (IBA Razor diode)], and we investigated all detectors to read the absorbed dose in water under the reference conditions (field 10cm x10cm, SSD 100cm and depth 10cm).

RESULTS

While measuring the absolute dose under reference conditions, all detectors had a non-significant difference of less than ±2%, except for the Razor diode, which showed a significant difference of ± 5%. On the other hand, when small fields were measured, we discovered a significant difference of 48%, compared to the Razor diode.

CONCLUSION

The Razor diode is more stable in small-field dosimetry than other detectors. Also, the Razor Diode is intended for relative dosimetry but, it shall not be used for absolute dose measurements.

Charge collection efficiency, underlying recombination mechanisms, and the role of electrode distance of vented ionization chambers under ultra-high dose-per-pulse conditions

PURPOSE

Investigating and understanding of the underlying mechanisms affecting the charge collection efficiency (CCE) of vented ionization chambers under ultra-high dose rate pulsed electron radiation. This is an important step towards real-time dosimetry with ionization chambers in FLASH radiotherapy.

METHODS

Parallel-plate ionization chambers (PPIC) with three different electrode distances were build and investigated with electron beams with ultra-high dose-per-pulse (DPP) up to 5.4 Gy. The measurements were compared with simulations. The experimental determination of the CCE was done by comparison against the reference dose based on alanine dosimetry. The numerical solution of a system of partial differential equations taking into account charge creations by the radiation, their transport and reaction in an applied electric field was used for the simulations of the CCE and the underlying effects.

RESULTS

A good agreement between the experimental results and the simulated CCE could be achieved. The recombination losses found under ultra-high DPP could be attributed to a temporal and spatial charge carrier imbalance and the associated electric field distortion. With ultra-thin electrode distances down to 0.25 mm and a suitable chamber voltage, a CCE greater than 99 % could be achieved under the ultra-high DPP conditions investigated.

CONCLUSIONS

Well-guarded ultra-thin PPIC are suited for real-time dosimetry under ultra-high DPP conditions. This allows dosimetry also for FLASH RT according to common codes of practice, traceable to primary standards. The numerical approach used allows the determination of appropriate correction factors beyond the DPP ranges where established theories are applicable to account for remaining recombination losses.

The effect of geometry dependent Chamber Spatial Response Function in small field profile measurements

The effect of Chamber-Spatial Response Function (CSRF) on beam profile measurements has been studied for Standard Ionization Chamber-Radiation Field Analyzer setup. The Chamber-Spatial Response Function is assumed to be Gaussian and parameterized through the projected geometry of the chamber along the measurement axis. CSRF widths was measured by constructing chamber diameter-FWHM correlation for a set of ionization chambers. The results were analyzed through Geant4 simulations. The geometrical parameters for the chamber, used in Geant4 simulation, have been derived from the CT scan images of the chamber. An excellent agreement between measurement and simulation is obtained. Various factors effecting the CSRF, such as chamber width and Compton current are analyzed. The analysis shows that the Compton current has significant role in broadening the CSRF. Further, it is also observed that a spherical chamber geometry provides better and compact CSRF.

Activity measurement of mixed complex radionuclide like 152Eu with different methods

¹⁵²Eu has been standardized by three independent 4π β-γ coincidence counting systems with beta detectors as proportional counter, plastic scintillator and liquid scintillator along with the CIEMAT/NIST method. The average activity concentration by primary methods was linked to key comparison reference value (KCRV) by comparing it with that of 4π γ ionization chamber (GIC) whose calibration factor was determined from the KCRV (BIPM.RI(II)-K1.Eu-152 and CCRI(II)-K2.Eu-152) and deviates from GIC by ± 0.16% indicating good agreement within standard uncertainties.

Monte Carlo calculated beam quality correction factors for two cylindrical ionization chambers in photon beams

PURPOSE

Although several studies provide data for reference dosimetry, the SNC600c and SNC125c ionization chambers (Sun Nuclear Corporation, Melbourne, FL) are in clinical use worldwide for which no beam quality correction factors k_Q are available. The goal of this study was to calculate beam quality correction factors k_Q for these ionization chambers according to dosimetry protocols TG-51, TRS 398 and DIN 6800-2.

METHODS

Monte Carlo simulations using EGSnrc have been performed to calculate the absorbed dose to water and the dose to air within the active volume of ionization chamber models. Both spectra and simulations of beam transport through linear accelerator head models were used as radiation sources for the Monte Carlo calculations.

RESULTS

k_Q values as a function of the respective beam quality specifier Q were fitted against recommended equations for photon beam dosimetry in the range of 4 MV to 25 MV. The fitting curves through the calculated values showed a root mean square deviation between 0.0010 and 0.0017.

CONCLUSIONS

The investigated ionization chamber models (SNC600c, SNC125c) are not included in above mentioned dosimetry protocols, but are in clinical use worldwide. This study covered this knowledge gap and compared the calculated results with published k_Q values for similar ionization chambers. Agreements with published data were observed in the 95% confidence interval, confirming the use of data for similar ionization chambers, when there are no k_Q values available for a given ionization chamber.

Ytterbium-175 half-life determination

¹⁷⁵Yb is a radionuclide that can be generated by neutron capture on ¹⁷⁴Yb and whose decay properties make it useful for developing therapeutic radiopharmaceuticals. As it happens with many of the emerging radionuclides for medical uses in recent years, its nuclear data were determined decades ago and are not thoroughly documented nor accurate enough for metrological purposes. The last documented reference for the ¹⁷⁵Yb half-life value is 4.185(1) days and dates back to 1989, so a redetermination of the value was considered appropriate before standardization at the Institute of Radiation Physics (IRA, Lausanne, Switzerland) primary measurements laboratory. Three independent measurement methods were used to this purpose: reference ionization chamber (CIR, chambre d'ionization de référence), CeBr₃ γ-ray detector with digital electronics and a second CeBr₃ detector with analog electronics and single-channel analyzer (SCA) counting. The value obtained for the ¹⁷⁵Yb half-life is 4.1615(30) days which shows a 0.56% relative deviation to the last nuclear reference value (ENSDF 2004) and is supported with a detailed calculation of the associated uncertainty.

Ra-224 activity, half-life, and 241 keV gamma ray absolute emission intensity: A NIST-NPL bilateral comparison

The national metrology institutes for the United Kingdom (UK) and the United States of America (USA) have compared activity standards for 224Ra, an α-particle emitter of interest as the basis for therapeutic radiopharmaceuticals. Solutions of 224RaCl2 were assayed by absolute methods, including digital coincidence counting and triple-to-double coincidence ratio liquid scintillation counting. Ionization chamber and high-purity germanium (HPGe) γ-ray spectrometry calibrations were compared; further, a solution was shipped between laboratories for a direct comparison by HPGe spectrometry. New determinations of the absolute emission intensity for the 241 keV γ ray (Iγ = 4.011(16) per 100 disintegrations of 224Ra) and of the 224Ra half-life (T1/2 = 3.6313(14) d) are presented and discussed in the context of previous measurements and evaluations.

Correction of the measured current of a small-gap plane-parallel ionization chamber in proton beams in the presence of charge multiplication

PURPOSE

The aims of this work are to study the response of a small-gap plane-parallel ionization chamber in the presence of charge multiplication and suggest an experimental method to determine the product of the recombination correction factor (k_s) and the charge multiplication correction factor (k_CM) in order to investigate the latter.

METHODS

Experimental data were acquired in scanned proton beams and in a Cobalt-60 beam. Measurements were carried out using an IBA PPC05 chambers of which the electrode gap is 0.6mm. The study is based on the determination of Jaffé plots by operating the chambers at different voltages. Experimental results are compared to theoretical equations describing initial and volume recombination as well as charge multiplication for continuous and pulsed beams.

RESULTS

Results obtained in protons and Cobalt-60 with the same PPC05 chamber indicate that the charge multiplication effect is independent of the beam quality, while results obtained in different proton beams with two different PPC05 chambers show that the charge multiplication effect is chamber dependent.

CONCLUSIONS

The approach to be taken when using a small-gap plane-parallel ionization chamber with a high voltage (e.g. 300V or 500V) for reference dosimetry in scanned proton beams depends on which correction factors were applied to the chamber response during its calibration in terms of absorbed dose to water: In both cases, it is recommended to use the ionization chamber at the same operating voltage used during its N_D,w-calibration. Another solution consists of operating the PPC05 chamber at a lower voltage (e.g. 50V) with larger k_s and smaller k_CM and determining the product of both factors with higher accuracy using a linear extrapolation method.

Results of an independent dosimetry audit for scanned proton beam therapy facilities

PURPOSE

An independent dosimetry audit based on end-to-end testing of the entire chain of radiation therapy delivery is highly recommended to ensure consistent treatments among proton therapy centers. This study presents an auditing methodology developed by the MedAustron Ion Beam Therapy Center (Austria) in collaboration with the National Physical Laboratory (UK) and audit results for five scanned proton beam therapy facilities in Europe.

METHODS

The audit procedure used a homogeneous and an anthropomorphic head phantom. The phantoms were loaded either with an ionization chamber or with alanine pellets and radiochromic films. Homogeneously planned doses of 10Gy were delivered to a box-like target volume in the homogeneous phantom and to two clinical scenarios with increasing complexity in the head phantom.

RESULTS

For all tests the mean of the local differences of the absolute dose to water determined with the alanine pellets compared to the predicted dose from the treatment planning system installed at the audited institution was determined. The mean value taken over all tests performed was -0.1±1.0%. The measurements carried out with the ionization chamber were consistent with the dose determined by the alanine pellets with a mean deviation of -0.5±0.6%.

CONCLUSION

The developed dosimetry audit method was successfully applied at five proton centers including various "turn-key" Cyclotron solutions by IBA, Varian and Mevion. This independent audit with extension to other tumour sites and use of the correspondent anthropomorphic phantoms may be proposed as part of a credentialing procedure for future clinical trials in proton beam therapy.

Gamma spectrometry of iodine-125 produced in IEA-R1 nuclear reactor, using HPGe detector and fixation into epoxy matrix disc

Few places in the world produce iodine-125. In Brazil, the first production was achieved by using the IEA-R1 nuclear reactor located at Nuclear and Energy Research Institute - IPEN. To verify the quality of iodine-125 produced, and the amount of contaminants such as iodine-126, cesium-134 and caesium-137 among others, iodine-125 samples were immobilized into epoxy matrix disc, with the same geometry of a barium-133 reference radioactive source, used to calibrate an HPGe detector. The HPGe detector has a thin carbon composite window, which allows measure the iodine-125 photopeaks, between 27.1 and 35.4 keV. The method employed here was successful in producing and measurement of iodine-125.

Radionuclide calibrator responses for 224Ra in solution and adsorbed on calcium carbonate microparticles

A suspension of ²²⁴Ra adsorbed onto CaCO₃ microparticles shows promise for α-therapy of intracavitary micro-metastatic diseases. To facilitate accurate activity administrations, geometry-specific calibration factors for commercially available reentrant ionization chambers (ICs) have been developed for ²²⁴RaCl₂ solutions and ²²⁴Ra adsorbed onto CaCO₃ microparticles in suspension in ampoules, vials, and syringes. Ampoules and vials give IC responses consistent with each other to <1%. Microparticles attenuation leads to a ≈1% to ≈2.5% reduction in response in the geometries studied.

Calibration of sodium iodide detectors and reentrant ionization chambers for 212Pb activity in different geometries by HPGe activity determined samples

Lead-212 is a promising radionuclide for cancer therapy, but no primary ²¹²Pb activity standardization has been published. A need therefore exists for accurate estimation of injected doses of ²¹²Pb activity in equilibrium with progeny, when it comes to preclinical and clinical trials. In this study, ²¹²Pb activity was determined using a high purity germanium (HPGe) detector, which allowed the determination of geometry-specific calibration factors for commercially available reentrant ionization chambers (ICs) and sodium iodide (NaI) detectors.

Determination of 161Tb half-life by three measurement methods

The radiolanthanide ¹⁶¹Tb is being studied as an alternative to ¹⁷⁷Lu for targeted radionuclide tumor therapy. Both β^--particle emitters show similar chemical behavior and decay characteristics, but ¹⁶¹Tb delivers additional conversion and Auger electron emissions that may enhance the therapeutic efficacy. In this study, the half-life of ¹⁶¹Tb was determined by a combination of three independent measurement systems: reference ionization chamber (CIR, chambre d'ionization de référence), portable ionization chamber (TCIR) and a CeBr₃ γ-emission detector with digital electronics. The half-life determined for ¹⁶¹Tb is 6.953(2) days, showing a significant improvement in the uncertainty, which is one order of magnitude lower, with a deviation of 0.91% from the last nuclear data reference value. The previous large uncertainty of the half-life had a direct impact on activity measurements. Now it is no more an obstacle to a primary standardization.

Simulation of the response of an ionization chamber to 214Bi emission. Application to the measurement of 222Rn

PENELOPE simulations of a Vinten ionization chamber (IC) were performed to investigate the influence of the thickness of glass-ampoules used in ²²²Rn standardization. The simulation reveals a non-negligible variation of the energy deposited in the chamber gas region (which may induce a proportional variation of the measured current) when considering the β transition emissions of the daughters of ²²²Rn. This reinforces the idea of using a specialist container (made of metal to preserve the integrity of the container) that would circulate between the metrology laboratories in the context of international comparison exercises using the BIPM international reference system (SIR).

Development of a detector for the measurement of ambient dose equivalent, H∗(10) at low and medium photon energies

Ionization chambers are used as secondary standards for the dosimetric measurement of photon radiation as the current generated in it are directly proportional to the conventional true value of the monitoring quantity (i.e. air kerma, dose or ambient dose equivalent). The wall thickness of ionization chamber plays an important role in the dosimetric measurement of photon energies below 200 keV. A 10 mm thick, poly methyl methacrylate walled, cylindrical ionization chamber having 225 cm³ volume was developed to study ambient dose equivalent rate based response at photon energies below 200 keV. The developed ionization chamber shows a near flat energy response for the monitoring of ambient dose equivalent in the energy range 30-200 keV. It also fulfills the ISO 4037-4 requirements concerning the quality of a secondary standard dosimeter in the energy range ∼15-200 keV.

Standardization of 64Cu activity

The complex decay scheme that makes 64Cu promising as both an imaging and therapeutic agent in medicine also makes the absolute measurement of its activity challenging. The National Institute of Standards and Technology (NIST) has completed a primary activity standardization of a 64CuCl2 solution using the 4πβ(LS)-γ(NaI) live-timed anticoincidence (LTAC) counting method with a combined standard uncertainty of 0.51 %. Two liquid scintillation (LS) counting methods were employed for confirmatory measurements. Secondary measurements were made by high-purity germanium detectors, pressurized ionization chambers (IC), and a well-type NaI(Tl) counter. Agreement between the LTAC-based standard and standards from other laboratories was established via IC calibration factors. Poor agreement between methods and with theoretical IC responses may indicate a need for improved β+/- branching probabilities and a better treatment of β+/- spectra.

Uncertainty in positioning ion chamber at reference depth for various water phantoms

BACKGROUND

Uncertainty in the calibration of high-energy radiation sources is dependent on user and equipment type.

AIM

We evaluated the uncertainty in the positioning of a cylindrical chamber at a reference depth for reference dosimetry of high-energy photon beams and the resulting uncertainty in the chamber readings for 6- and 10-MV photon beams. The aim was to investigate major contributions to the positioning uncertainty to reduce the uncertainty in calibration for external photon beam radiotherapy.

MATERIALS AND METHODS

The following phantoms were used: DoseView 1D, WP1D, 1D SCANNER, and QWP-07 as one-dimensional (1D) phantoms for a vertical-beam geometry; GRI-7632 as a phantom for a fixed waterproofing sleeve; and PTW type 41023 and QWP-04 as 1D phantoms for a horizontal-beam geometry. The uncertainties were analyzed as per the Guide to the Expression of Uncertainty in Measurement.

RESULTS

The positioning and resultant uncertainties in chamber readings ranged from 0.22 to 0.35 mm and 0.12-0.25%, respectively, among the phantoms (using a coverage factor k = 1 in both cases). The major contributions to positioning uncertainty are: definition of the origin for phantoms among users for the 1D phantoms for a vertical-beam geometry, water level adjustment among users for the phantom for a fixed waterproofing sleeve, phantom window deformation, and non-water material of the window for the 1D phantoms for a horizontal-beam geometry.

CONCLUSION

The positioning and resultant uncertainties in chamber readings exhibited minor differences among the seven phantoms. The major components of these uncertainties differed among the phantom types investigated.

Reference conditions for ion-chamber based HDR brachytherapy dosimetry and for the calibration of high-resolution solid detectors

The aim of this study has been to develop a two-step method of in-phantom dosimetry around a brachytherapy ¹⁹²Ir photon source. The first step is to measure the absorbed dose rate to water with a calibrated ionization chamber under reference conditions, the second to cross-calibrate, under these conditions, small solid-state detectors such as silicon diodes, synthetic diamond or scintillation detectors suited for spatially resolved dose rate measurements at other, particularly at smaller source axis distances in the water phantom. This two-step approach constitutes a method for in-phantom dosimetry in brachytherapy, analogous to the "small calibration field" commonly used in teletherapy to provide the reference conditions for the cross-calibration of high-resolution detectors. Under reference conditions, all known corrections for radiation quality, volume averaging and position of the chamber's effective point of measurement (EPOM) have to be applied. The study is therefore particularly devoted to (1) the experimental determination of the position of the source axis, (2) a general formulation for the volume averaging correction factor of small ionization chambers and (3) the experimental determination of the EPOM positions for the PinPoint chamber 31014 and the 3D-PinPoint chamber PTW 31022 (both PTW Freiburg, Germany). The distance of 30mm from the source axis was chosen as the reference condition for cross calibrations. This concept is realized with the instrumentation available in a hospital, a scanning-type water phantom, a software package for small field dosimetry and detectors typically used in clinical routine dosimetry. The present development of a method of in-phantom dose measurement under ¹⁹²Ir brachytherapy conditions was performed in recognition of the primary role of dose calculations, e.g. according to the AAPM TG43 recommendations. But in addition, the methodology tested here is paving a practicable way for the experimental check of typical dose values under clinical conditions, should the need arise.

Changes in deviation of absorbed dose to water among users by chamber calibration shift

PURPOSE

The JSMP01 dosimetry protocol had adopted the provisional ⁶⁰Co calibration coefficient [Formula: see text], namely, the product of exposure calibration coefficient N _C and conversion coefficient k _D,X. After that, the absorbed dose to water D _w standard was established, and the JSMP12 protocol adopted the [Formula: see text] calibration. In this study, the influence of the calibration shift on the measurement of D _w among users was analyzed.

MATERIALS AND METHODS

The intercomparison of the D _w using an ionization chamber was annually performed by visiting related hospitals. Intercomparison results before and after the calibration shift were analyzed, the deviation of D _w among users was re-evaluated, and the cause of deviation was estimated.

RESULTS

As a result, the stability of LINAC, calibration of the thermometer and barometer, and collection method of ion recombination were confirmed. The statistical significance of standard deviation of D _w was not observed, but that of difference of D _w among users was observed between N _C and [Formula: see text] calibration.

CONCLUSION

Uncertainty due to chamber-to-chamber variation was reduced by the calibration shift, consequently reducing the uncertainty among users regarding D _w. The result also pointed out uncertainty might be reduced by accurate and detailed instructions on the setup of an ionization chamber.

Dosimetry in MARS spectral CT: TOPAS Monte Carlo simulations and ion chamber measurements

Spectral computed tomography (CT) is an up and coming imaging modality which shows great promise in revealing unique diagnostic information. Because this imaging modality is based on X-ray CT, it is of utmost importance to study the radiation dose aspects of its use. This study reports on the implementation and evaluation of a Monte Carlo simulation tool using TOPAS for estimating dose in a pre-clinical spectral CT scanner known as the MARS scanner. Simulated estimates were compared with measurements from an ionization chamber. For a typical MARS scan, TOPAS estimated for a 30 mm diameter cylindrical phantom a CT dose index (CTDI) of 29.7 mGy; CTDI was measured by ion chamber to within 3% of TOPAS estimates. Although further development is required, our investigation of TOPAS for estimating MARS scan dosimetry has shown its potential for further study of spectral scanning protocols and dose to scanned objects.

Calibration setting numbers for dose calibrators for the PET isotopes (52)Mn, (64)Cu, (76)Br, (86)Y, (89)Zr, (124)I

For PET radionuclides, the radioactivity of a sample can be conveniently measured by a dose calibrator. These devices depend on a "calibration setting number", but many recommended settings from manuals were interpolated based on standard sources of other radionuclide(s). We conducted HPGe gamma-ray spectroscopy, resulting in a reference for determining settings in two types of vessels containing one of several PET radionuclides. Our results reiterate the notion that in-house, experimental calibrations are recommended for different radionuclides and vessels.

Practical method for determination of air kerma by use of an ionization chamber toward construction of a secondary X-ray field to be used in clinical examination rooms

We propose a new practical method for the construction of an accurate secondary X-ray field using medical diagnostic X-ray equipment. For accurate measurement of the air kerma of an X-ray field, it is important to reduce and evaluate the contamination rate of scattered X-rays. To determine the rate quantitatively, we performed the following studies. First, we developed a shield box in which an ionization chamber could be set at an inner of the box to prevent detection of the X-rays scattered from the air. In addition, we made collimator plates which were placed near the X-ray source for estimation of the contamination rate by scattered X-rays from the movable diaphragm which is a component of the X-ray equipment. Then, we measured the exposure dose while changing the collimator plates, which had diameters of 25-90 mm(ϕ). The ideal value of the exposure dose was derived mathematically by extrapolation to 0 mm(ϕ). Tube voltages ranged from 40 to 130 kV. Under these irradiation conditions, we analyzed the contamination rate by the scattered X-rays. We found that the contamination rates were less than 1.7 and 2.3 %, caused by air and the movable diaphragm, respectively. The extrapolated value of the exposure dose has been determined to have an uncertainty of 0.7 %. The ionization chamber used in this study was calibrated with an accuracy of 5 %. Using this kind of ionization chamber, we can construct a secondary X-ray field with an uncertainty of 5 %.

Practical correction method for impurities on activity measurements using isotope calibrators

Radioactive impurities might cause significant error in the activity determination of a target nuclide using ionization chambers. In the present study, an impurity correction technique for (201)Tl sources was performed by applying two different responses of an IG12A20 and IG11N20 ionization chamber. This technique can be extended to another method in which an attenuation filter made of tin was used to obtain different responses of an argon filled IG12A20. The results obtained with these techniques were very consistent with each other and with the reference value within their uncertainty after making the impurity correction. Examples of (64)Cu activity determination were also shown.

Validation of the neutron and gamma fields in the JSI TRIGA reactor using in-core fission and ionization chambers

CEA developed fission chambers and ionization chambers were utilized at the JSI TRIGA reactor to measure neutron and gamma fields. The measured axial fission rate distributions in the reactor core are generally in good agreement with the calculated values using the Monte Carlo model of the reactor thus verifying both the computational model and the fission chambers. In future, multiple absolutely calibrated fission chambers could be used for more accurate online reactor thermal power monitoring.

Update of NIST half-life results corrected for ionization chamber source-holder instability

As reported at the ICRM 2011, it was discovered that the source holder used for calibrations in the NIST 4πγ ionization chamber (IC) was not stable. This has affected a large number of half-life measurement results previously reported and used in compilations of nuclear data. Corrections have been made on all of the half-life data based on the assumption that the changes to the ionization chamber response were gradual. The corrections are energy dependent and therefore radionuclide specific. This presentation will review our results and present the recommended changes in half-life values and/or uncertainties.

Response of ionization chamber based pocket dosimeter to beta radiation

Quantitative estimate of the response of ionization chamber based pocket dosimeters (DRDs) to various beta sources was performed. It has been established that the ionization chamber based pocket dosimeters do not respond to beta particles having energy (Emax)<1 MeV and same was verified using (147)Pm, (85)Kr and (204)Tl beta sources. However, for beta particles having energy >1 MeV, the DRDs exhibit measureable response and the values are ~8%, ~14% and ~27% per mSv for natural uranium, (90)Sr/(90)Y and (106)Ru/(106)Rh beta sources respectively. As the energy of the beta particles increases, the response also increases. The response of DRDs to beta particles having energy>1 MeV arises due to the fact that the thickness of the chamber walls is less than the maximum range of beta particles. This may also be one of the reasons for disparity between doses measured with passive/legal dosimeters (TLDs) and DRDs in those situations in which radiation workers are exposed to mixed field of gamma photons and beta particles especially at uranium processing plants, nuclear (power and research) reactors, waste management facilities and fuel reprocessing plants etc. The paper provides the reason (technical) for disparity between the doses recorded by TLDs and DRDs in mixed field of photons and beta particles.