Source Holder Research Articles

INNOVATIVE HOSE GAMMA FLAW DETECTOR “RID 50W”

An innovative model of a portable class hose gamma flaw detector “RID 50W” is presented, equipped with environmentally friendly tungsten components: a biological protection unit, a shutter assembly and a multi-link source holder for working with emitters based on 75Se and 192Ir. The design features of the radiation head model, the principles of release and shutdown of the radiation beam in the “RID 50W” apparatus, issues of optimization, synthesis, and computer engineering of critical components are considered, and a quantitative assessment of the technical parameters and characteristics of the equipment is presented. The exposure dose rate of the diffusion flux of penetrating gamma radiation from a working emitter based on 192Ir was calculated in the Geant4 software package, followed by practical verification using standard dosimetric monitoring tools. Conclusions based on the results of verification of the obtained data are presented. The design and operating principle of a patented laser sight designed to expand the functionality of a hose gamma flaw detector during operation, including in critical conditions of structural constraint, is also considered.

Dosimetry studies for an optimized reloading of a panoramic Cobalt-60 source using FLUKA Monte Carlo code

The Cobalt-60 source used at the Tunisian Gamma Irradiation Facility is nearing the end of its useful life. Due to the current low dose rates, the irradiation process takes a long time. To address exposure time and cost issues, source racks need to be reloaded to improve the quality of the exposure process. This work aims to find the optimal new configuration for future reloading of the Cobalt-60 source rack and to evaluate the dose rate distribution in the vicinity of the Irradiator and the Dose Uniformity Ratio (DUR) in a dummy product. When keeping old pencils, the optimal configuration will need to be adjusted to accommodate the new Cobalt-60 pencils. The replenishment method developed using FLUKA-based Monte Carlo code involved the addition of six new pencils: three in the upper top (outer) holder of the source rack and three in the lower bottom (inner) source holder. The spatial distribution of dose rates in the vicinity of the source rack before and after reloading was evaluated. For the radiation processing, the maximum and minimum absorbed dose in the dummy product were estimated to calculate the DUR value. The DUR value for a single-use medical product is about 2.5. The results obtained before and after reloading show that the spatial distribution of the dose rate is uniform for the proposed configurations, and has a value of approximately 0.4 kGy/h after reloading at the conveyor belt location.

Development of Gamma Flaw Detectors for Non-Destructive Testing

The radiographic method of control is implemented by a set of control tools in the form of gamma flaw detectors, which are distinguished by: mobility, reliability, radiation safety, adaptability to a wide climatic range, ease of operation, and the ability to control products with small and large radiation thicknesses. The paper describes portable gamma flaw detectors manufactured by JSC NIITFA: Gammarid 2010R, UNIGAM R, Stapel-5 Se75 Ir192, Stapel-5 Se75W, gamma flaw detector γ-RID-75/80R and stationary gamma flaw detectors RID K-100 and RID K-400. Also paper describes a brief technology for the production of ionizing radiation sources Ir-192 with activity of 120 Curie, a technology for rolling the source holder with Ir-192 source and completing the Ir-192 source with a portable gamma flaw detector Gammarid-192/120M in the INP AS.

Conceptual Design of a Compact Divertor Heat Load Simulation Device: HIT-PSI

Linear plasma devices have been increasingly applied in investigating plasma–surface interaction (PSI) processes and divertor/scraped-off-layer (D/SOL) physics because of their economy, flexibility, and expandability. However, only a few existing linear plasma devices are able to obtain high heat and particle fluxes. In this work, we report a compact superconducting linear device, with its scientific goals and specific design methods, at Harbin Institute of Technology (HIT), HIT-PSI, capable of implementing an extreme plasma environment with beams of a long discharge pulse, as well as high heat and particle fluxes in the future fusion reactor regime of ITER/CFETR-like parameters. A five-coil integrated superconducting magnet is designed to generate a >2.0 Tesla steady-state magnetic field for confining a long pulse plasma beam with a density of >1020 m−3 produced by a cascaded arc plasma source. With a pump set of 2500 L/s and a water-cooled target system with bias voltage, it is expected to obtain high-density and low-temperature plasma beams with a heat flux of over 10 MW/m2. Subsystems of the platform, including the plasma source, superconducting magnets, vacuum system, and target holder system, are described in detail. In addition, the function and performance of the platform are numerically simulated and represented by SOLPS-ITER code to predict the laboratory simulation results.

MEASUREMENT AND SIMULATION OF THE RADIATION DOSES AROUND THE 'MINI LABYRINTH' EXPERIMENTAL WORKPLACE AT STU.

One of the most effective ways of gaining user experience and minimizing user effects on modelling and simulation is benchmarking. Currently (2022) a new experimental workplace, the so called 'Mini Labyrinth' is being developed at Slovak University of Technology in Bratislava (STU). It is a simple neutron and gamma shielding benchmark, inspired by the ALARM-CF-AIR-LAB-001 ICSBEP experiment. The latest V2 setup consists of a PuBe neutron source, several NEUTRONSTOP C5 shielding blocks, H2O filled polylactic acid (PLA) tank, plastic source holder and detectors placed inside and outside the Mini Labyrinth. This paper is focused on the monitoring of the radiation doses around the workplace using the NuDET neutron detector and on the comparison of the measured quantities with ones simulated by SCALE 6 MONACO. The influence of the size of the model and the definition of the detector is studied in this analysis. The achieved results are presented and some discussions on further needed development are also included.

SIR-Spheres Activity Measurements Reveal Systematic Miscalibration.

The purpose of this work was to perform an independent and National Institute of Standards and Technology-traceable activity measurement of 90Y SIR-Spheres (Sirtex). γ-spectroscopic measurements of the 90Y internal pair production decay mode were made using a high-purity germanium detector. Methods: Measured annihilation radiation detection rates were corrected for radioactive decay during acquisition, dead time, source attenuation, and source geometry effects. Detection efficiency was determined by 2 independent and National Institute of Standards and Technology-traceable methods. Results: Measured SIR-Spheres vials (n = 5) contained more activity than specified by the manufacturer calibration; on average, the ratio of measured activity to calibrated was 1.233 ± 0.030. Activity measurements made using 2 distinct efficiency calibration methods agreed within 1%. Conclusion: The primary SIR-Spheres activity calibration appears to be a significant underestimate of true activity.

Micro-spacing in-air sublimation of submillimeter-scaled rubrene nanoribbons and nanosheets for efficient optical waveguides

Convenient and low-cost fabrication of large-scaled 1-dimensional (1D) and 2-dimensional (2D) organic crystals is vital for potential optoelectronic applications such as light-emitting diodes, transistors, lasers and optical waveguides. In this work, we report a feasible approach to fabricate submillimeter-scaled rubrene nano-ribbon (1D) and nano-sheet (2D), conducted in a micrometer-confined space and in air condition. It was found that the 1D nano-ribbons and 2D nano sheets can be selectively fabricated by tuning the heating rate of the source holder on which rubrene powder was placed. A vapor-to-solid mechanism is established for the growth of rubrene crystals. It was revealed that the growth of 1D ribbons was determined by the diffusion of rubrene on substrate surface while the growth of 2D sheets was governed by the homogeneous nucleation. Selected area electron diffraction and x-ray diffraction analysis revealed that the rubrene nano-ribbons and nano-sheets share the same single-crystalline structure with an orthorhombic cell of a = 14.49 Å, b = 7.21 Å and c = 26.92 Å. Luminescence and optical transportation measurement revealed the quite low loss optical-loss coefficient at ~590 nm for both rubrene nano-ribbons and nano-sheets, indicating their promising applications as efficient optical waveguides.

On the stability of well-type ionization chamber source strength calibration coefficients.

To investigate the variability and stability of brachytherapy source strength calibration coefficients for air-communicating and pressurized well-type ionization chambers among high-dose rate (HDR), low-dose rate (LDR), and electronic brachytherapy (EBT) source qualities. These qualities of an ionization chamber are important features to assess given their role in maintaining traceability to a primary national standards laboratory and facilitating efficacious patient care in brachytherapy. The calibration records from the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) customer well-type ionization chamber database were retrospectively analyzed for calibrations performed between 1996 and 2019. A statistical analysis was performed and differentiated among calibration type to quantify the distribution of chamber calibration coefficients among several chamber models. Distributions were quantified based on their moments and by quantile analysis. For LDR calibrations, chamber response was further differentiated by seed type to study the variability in seed dependence within and across chamber models. In addition to these metrics, the calibration lineage at the UWADCL was used to assess the stability of these calibration coefficients among chamber model and source type based on the ratio of subsequent calibration coefficients. The distribution of brachytherapy source strength calibration coefficients for a particular chamber model is not necessarily normally distributed and is sensitive to changes in the machining tolerances or design of the chamber model. Calibration source quality also influenced the distributions of calibration coefficients for a chamber model; the air-kerma rate calibration coefficients for EBT sources were the most variable followed by LDR and then HDR source types. The stability of a chamber's source strength calibration coefficient exhibited a similar dependence on the source quality. Air-communicating and pressurized chambers exhibited an average stability between subsequent calibrations of 0.2% and 3.0%, respectively, for HDR calibrations, but could exhibit more than double this variability characteristic of their leptokurtic distributions. For LDR calibrations, the spread and stability in a model's calibration ratio toward another seed type is extensive and notable. For some seed types and chamber models exhibit variations of <0.5% while others exceeded 2.0%. Furthermore, the magnitude of this dependence is outside the variability of the source's source strength due to manufacturing, which was determined from the manufacturer and NIST source strength intercomparison records at the UWADCL. As a result, establishing and disseminating calibration conversion factors among different source and chamber models is not advised as it would substantially increase the uncertainty in a clinical user's determination of source strength. The calibration of a well-type ionization chamber is unique to the chamber, source, and source holder. Attempting to generalize source strength calibration coefficients among chambers of the same model or source type is impractical given the variation in response observed across the calibration history of the UWADCL. Attempting to quantitate and transfer calibration coefficients in such a relative sense may significantly degrade the uncertainty relative to specifically calibrating a chamber for an individual source.

Demonstration of o-Ps detection with a cylindrical array of NaI detectors

Ortho-positronium (o-Ps), the triplet bound state of an electron and positron, is a promising system in which to search for new physics. O-Ps production and detection can be achieved with a tabletop setup, involving a 22Na source, aerogel and a detector. We present our approach to o-Ps detection using the APEX array, which consists of 24 NaI(Tl) bars, arranged cylindrically. Our approach involves tagging on the 1.27 MeV gamma ray, a technique which is used in positron annihilation spectroscopy (PALS) Gidley et al. (2006). We demonstrate the ability to tag with any one of the bars in the array. Using a NaI(Tl) array of high angular coverage (75%) with this technique provides many benefits. This method provides some advantages over tagging on the positron directly insofar as it minimizes the amount of material inside the source holder and simplifies the design of the DAQ. This has potential applications to CP- and CPT-violation searches in o-Ps.

Dosimetric characterization of a novel 90Y source for use in the conformal superficial brachytherapy device

PurposeTo characterize the dose distribution in water of a novel beta-emitting brachytherapy source for use in a Conformal Superficial Brachytherapy (CSBT) device. Methods and materialsYttrium-90 (90Y) sources were designed for use with a uniquely designed CSBT device. Depth dose and planar dose measurements were performed for bare sources and sources housed within a 3D printed source holder. Monte Carlo simulated dose rate distributions were compared to film-based measurements. Gamma analysis was performed to compare simulated and measured dose rates from seven 90Y sources placed simultaneously using the CSBT device. ResultsThe film-based maximum measured surface dose rate for a bare source in contact with the surface was 3.35 × 10–7 cGy s−1 Bq−1. When placed in the source holder, the maximum measured dose rate was 1.41 × 10–7 cGy s−1 Bq−1. The Monte Carlo simulated depth dose rates were within 10% or 0.02 cm of the measured dose rates for each depth of measurement. The maximum film surface dose rate measured using a seven-source configuration within the CSBT device was 1.78 × 10−7 cGy s−1 Bq−1. Measured and simulated dose rate distribution of the seven-source configuration were compared by gamma analysis and yielded a passing rate of 94.08%. The gamma criteria were 3% for dose-difference and 0.07056 cm for distance-to-agreement. The estimated measured dose rate uncertainty was 5.34%. Conclusions90Y is a unique source that can be optimally designed for a customized CSBT device. The rapid dose falloff provided a high dose gradient, ideal for treatment of superficial lesions. The dose rate uncertainty of the 90Y-based CSBT device was within acceptable brachytherapy standards and warrants further investigation.

Development and performance test of an in-situ soil total nitrogen-soil moisture detector based on near-infrared spectroscopy

In order to accurately acquire soil total nitrogen (STN) content and soil moisture (SM) content, an in-situ STN-SM content detector was developed, and the stability and accuracy of the detector were tested. The in-situ detector consisted of an optical unit, a control unit and a mechanical unit. The optical unit comprised eight single band Near-infrared (NIR) laser sources at 1260, 1330, 1360, 1430, 1530, 1580, 1660 and 1450 nm, collimator and an InGaAs photoelectric sensor, a laser source holder with “Eight faces” umbrella structure and sapphire glass. The control unit was composed of hardware part and software part. The hardware part included I-U conversion circuit, amplifier and filter circuit, and JN5139 module responsible for A/D conversion and data transmission. The software part of the program was written in the JN5139 microcontroller unit. The mechanical unit comprised a cylindrical detection darkroom, an electrical component mounting tray and a cylindrical shading and dust cover. The stability test results showed that the repeatability error at eight sensitive bands were 0.7758%, 0.8091%, 0.7958%, 0.5189%, 0.6405%, 0.8556%, 0.7391% and 0.5294%, respectively. The correlation coefficients between the absorbance of in-situ detector and MATRIX-I Fourier transform near-infrared spectrophotometer were all above 0.88. A portable LED STN content detector was taken as the reference in the performance test. The results showed that the calibration coefficient Rc2 and the validation coefficient Rv2of in-situ detector model were 0.8322 and 0.7832, respectively. While those of the portable LED STN content detector model were 0.7205 and 0.6851, respectively. Two regression models were established for the SM content by respectively using the portable LED STN content detector and the in-situ detector, and the results showed that both detectors had good detection effects in SM. The experimental results showed that compared with the portable LED STN content detector, in-situ detector greatly improved the STN content detection precision, and could realize STN and SM content field detection in real-time.

Do it yourself: optical spectrometer for physics undergraduate instruction in nanomaterial characterization

In this paper, we report on a homemade optical spectrometer using diffraction grating and image processing techniques. This device was designed to produce spectral images that could then be processed by measuring signal strength (pixel intensity) to obtain the light source, transmittance, and absorbance spectra of the liquid sample. The homemade optical spectrometer consisted of: (i) a white LED as a light source, (ii) a cuvette or sample holder, (iii) a slit, (iv) a diffraction grating, and (v) a CMOS camera (webcam). In this study, various concentrations of a carbon nanoparticle (CNP) colloid were used in the particle size sample test. Additionally, a commercial optical spectrometer and tunneling electron microscope (TEM) were used to characterize the optical properties and morphology of the CNPs, respectively. The data obtained using the homemade optical spectrometer, commercial optical spectrometer, and TEM showed similar results and trends. Lastly, the calculation and measurement of CNP size were performed using the effective mass approximation (EMA) and TEM. These data showed that the average nanoparticle sizes were approximately 2.4 nm and 2.5 ± 0.3 nm, respectively. This research provides new insights into the development of a portable, simple, and low-cost optical spectrometer that can be used in nanomaterial characterization for physics undergraduate instruction.

Characterization of prompt neutron spectrum of the Missouri University of Science and Technology Reactor

An activation-foil method was used to obtain the energy spectrum of the prompt-neutron flux at the Missouri University of Science and Technology Reactor (MSTR). The foils were irradiated at the center of the reactor core (120W configuration). The neutron spectrum was determined using the unfolding method implemented in the SAND-II code. Multiple foils of dysprosium, vanadium, indium, gold, aluminum, copper, cobalt, silver, nickel, and iron were placed at the source holder position and irradiated for 3min each at 100kW. The foil set cover energies from 0.025eV to 7.2MeV for a broad spectrum analysis. After irradiation, the sample was cooled using water until the dose rate reached a level that allowed safe handling. For each material, both bare and cadmium covers were used to obtain the thermal and epithermal neutron flux. The activity of each foil was determined by counting for 3min with a high-purity germanium detector, and the results were used in SAND-II to obtain the neutron flux spectrum for the MSTR. Monte Carlo N-particle (MCNP) model was used to obtain the initial guess of the spectrum at the source holder position, which was used as input for SAND-II. The spectrum is unfolded using two different energy binning structures in the initial guess for SAND-II. As expected, the results provided highly thermal neutron spectra in source-holder environment, where water thermalizes most of the neutrons; 93% of the total flux at the source-holder position is no greater than 0.55eV for the 120W reactor-core configuration. The epithermal and fast neutrons were 6% and 1%, respectively at fast/epithermal energy boundary of 100keV. A similar experiment and analysis in the past determined that 7%, 38% and 55% of the total flux at the source-holder position were respectively thermal, epithermal and fast for the 101W configuration. Notwithstanding the difference between the two configurations, the current determination is an improvement to the characterization of the MSTR flux. This is because 7% thermal flux is unlikely in an environment primarily surrounded by water. The total neutron flux calculated by MCNP is 4.98×1011±1.72×1010n/cm2s. The flux determined through SAND-II is 4.79×1011±6.10×1010n/cm2s; a difference of 4%.

Air-kerma strength determination of an HDR 192 Ir source including a geometric sensitivity study of the seven-distance method.

To perform an in-air air-kerma strength (SK) calibration of the Bebig model Ir2.A85-2 192Ir high-dose rate (HDR) brachytherapy source manufactured by Mallinckrodt Medical (Westerduinweg, Germany) with the NIST-traceable seven-distance technique established by the University of Wisconsin. A comparison was made between the reference air-kerma rate (RAKR) reported on a certificate from the Physikalisch-Technische Bundesanstalt (Berlin, Germany) (PTB) primary laboratory and the SK determined at the University of Wisconsin Madison Radiation Research Center (UWMRRC). A theoretical sensitivity study was performed to investigate the impact that variations in the experimental setup have on the computed SK from the seven-distance algorithm in order to determine if the uncertainty budget for the seven-distance method should be expanded. The manufacturer-reported SK for the source was compared to the SK determined from the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) transfer standard well chambers and the seven-distance technique. Monte Carlo techniques (MCNP6) were employed to compare the theoretical SK calibration coefficients of a Standard Imaging (Middleton, WI) HDR1000 Plus well chamber using Standard Imaging model 70010 and model 70110 source holders to determine if a holder dependence was present. Radiochromic film (EBT3, Ashland) exposures were performed to assess the dose distribution of the source in phantom. The seven-distance algorithm was coded in MATLAB®(R2013b) and benchmarked with MCNP6 with the capacity to model distance offset behaviors among nominal positions. This offset model was used in a Monte Carlo simulation coded in MATLAB to determine the average uncertainty in the SK calculations from the seven-distance algorithm. The measured SK using the seven-distance technique at the UWMRRC agreed with the RAKR reported on the PTB source certificate and the SK on the Mallinckrodt source certificate to within 0.28% and -0.79%, respectively. It was found that the difference between the SK measured from the transfer standard well chambers at the UWADCL and the seven-distance method was between 0.13% and 0.30% at the 95% confidence level. Monte Carlo results showed negligible differences between the simulated SK calibration coefficient for an HDR1000 Plus well chamber using the model 70010 or model 70110 source holder. The autoradiographs from the source in Virtual WaterTM showed that the dose distribution is symmetric. Additionally, the sensitivity study performed in MATLAB showed that the SK calculated with the seven-distance algorithm could deviate by 0.24% from randomly generated distance offsets within 1mm in magnitude. The differences between the SK measurements determined from the seven-distance technique and the accredited UWADCL measurement results were within the k=2 uncertainty reported for an accredited calibration. Excellent agreement was found between the measured SK and RAKR methods used at the UWMRRC and PTB, respectively. Additionally, the sensitivity study has shown that the seven-distance algorithm accurately determines the SK of a source while having a variable chamber offset among nominal positions; the uncertainty budget for the seven-distance method does not need to be expanded at this time. It has been determined that the current standard used by the UWADCL for well chamber calibrations is valid for the Bebig model Ir2.A85-2 192Ir brachytherapy source.

Development of Remote Sampling ESI Mass Spectrometry for the Rapid and Automatic Analysis of Multiple Samples.

We report on combining a self-aspirated sampling probe and an ESI source using a single metal capillary which is electrically grounded and safe for use by the operator. To generate an electrospray, a negative H.V. is applied to the counter electrode of the ESI emitter to operate in positive ion mode. The sampling/ESI capillary is enclosed within another concentric capillary similar to the arrangement for a standard pneumatically assisted ESI source. The suction of the liquid sample is due to the Venturi effect created by the high-velocity gas flow near the ESI tip. In addition to serving as the mechanism for suction, the high-velocity gas flow also assists in the nebulization of charged droplets, thus producing a stable ion signal. Even though the potential of the ion source counter electrode is more negative than the mass spectrometer in the positive ion mode, the electric field effect is not significant if the ion source and the mass spectrometer are separated by a sufficient distance. Ion transmission is achieved by the viscous flow of the carrier gas. Using the present arrangement, the user can hold the ion source in a bare hand and the ion signal appears almost immediately when the sampling capillary is brought into contact with the liquid sample. The automated analysis of multiple samples can also be achieved by using motorized sample stage and an automated ion source holder.

Development of a 170Tm source for mercury monitoring studies in humans using XRF

The goals of the present study were to develop a 170Tm radioisotope and generate a K XRF spectrum of mercury. Thulium foil and thulium oxide powder were both tested for impurities and the latter was found to be a better prospect for further studies. The 170Tm radioisotope was developed from thulium oxide powder following the method of disolution and absorption. A suitable source holder and collimator were also designed based on Monte Carlo simulations. Using the radioisotope thus developed, a mercury XRF spectrum was successfully generated.

Conversion electron spectrometry of Pu isotopes with a silicon drift detector

An electron spectrometry set-up was built at IRMM consisting of a vacuum chamber with a moveable source holder and windowless Peltier-cooled silicon drift detector (SDD). The SDD is well suited for measuring low-energy x rays and electrons emitted from thin radioactive sources with low self-absorption. The attainable energy resolution is better than 0.5keV for electrons of 30keV. It has been used to measure the conversion electron spectra of three plutonium isotopes, i.e. 238Pu, 239Pu, 240Pu, as well as 241Am (being a decay product of 241Pu). The obtained mixed x-ray and electron spectra are compared with spectra obtained with a close-geometry set-up using another SDD in STUK and spectra measured with a Si(Li) detector at IRMM. The potential of conversion electron spectrometry for isotopic analysis of mixed plutonium samples is investigated. With respect to the 240Pu/239Pu isotopic ratio, the conversion electron peaks of both isotopes are more clearly separated than their largely overlapping peaks in alpha spectra.

Source geometry factors for HDR 192Ir brachytherapy secondary standard well-type ionization chamber calibrations

Well-type ionization chambers are used for measuring the source strength of radioactive brachytherapy sources before clinical use. Initially, the well chambers are calibrated against a suitable national standard. For high dose rate (HDR) 192Ir, this calibration is usually a two-step process. Firstly, the calibration source is traceably calibrated against an air kerma primary standard in terms of either reference air kerma rate or air kerma strength. The calibrated 192Ir source is then used to calibrate the secondary standard well-type ionization chamber.Calibration laboratories are usually only equipped with one type of HDR 192Ir source. If the clinical source type is different from that used for the calibration of the well chamber at the standards laboratory, a source geometry factor, ksg, is required to correct the calibration coefficient for any change of the well chamber response due to geometric differences between the sources.In this work we present source geometry factors for six different HDR 192Ir brachytherapy sources which have been determined using Monte Carlo techniques for a specific ionization chamber, the Standard Imaging HDR 1000 Plus well chamber with a type 70010 HDR iridium source holder. The calculated correction factors were normalized to the old and new type of calibration source used at the National Physical Laboratory. With the old Nucletron microSelectron-v1 (classic) HDR 192Ir calibration source, ksg was found to be in the range 0.983 to 0.999 and with the new Isodose Control HDR 192Ir Flexisource ksg was found to be in the range 0.987 to 1.004 with a relative uncertainty of 0.4% (k = 2). Source geometry factors for different combinations of calibration sources, clinical sources, well chambers and associated source holders, can be calculated with the formalism discussed in this paper.

Method for verifying the air kerma strength of I-125 plaques for the treatment of ocular melanoma.

The purpose of this work was to develop a method for easily verifying that the activity or air kerma strength of pre‐assembled eye plaques, used in the treatment of ocular melanomas, agrees with the activity or air kerma strength called for in the treatment plan. A Capintec CRC‐7 Dose Calibrator with its standard vial/syringe sample holder was used to measure the activity of pre‐assembled COMS and Eye Physics EP917 eye plaques using IsoAid Advantage I‐125 seeds. Plaque activity measurements were made by placing the plaque face up in the center of a 5 cm tall Styrofoam insert in the source holder. Activity measurements were made with the source holder rotated to four angles (0°, 90°, 180°, and 270°). The average of these four values was converted to air kerma strength and divided by the assay air kerma strength, from the NIST traceable source calibration, and decayed to the plaque measurement date, to determine a plaque calibration factor. The average of the calibration factors for each plaque type was used to establish a calibration factor for each plaque type. Several partially loaded plaque configurations were included in this study and different methods were used to determine the effects of partial loading. This verification method is easy to implement with commonly available equipment and is effective in identifying possible errors. During this two‐year study, the air kerma strength of 115 eye plaques was checked and 11 possible errors were identified.PACS number: 87.55.Qr

A simple method to prolong the service life of radioactive sources for external radiotherapy.

A radioactive source is usually replaced and disposed after being used for a certain amount of time (usually a half‐life). In this study, a simple method is proposed to prolong its service life. Instead of replacing the used source with a new source of full activity, a new source of less activity is added in the source holder in front of the used one, so that the total activity of two sources is equal to the initial activity of the used source or even higher. Similarly, more sources can be added to the previous ones. Attenuation of front source(s) to the back source(s) was evaluated with exponential attenuation equation, and variation of source‐focus distance (SFD) with inverse square law for Leksell 4C Gamma Knife, which served as an example of external radiotherapy units. When the number of front sources increased from 1 to 3, the relative air kerma decreased from 36.5% to 5.0%. Both the attenuation effect and SFD variation contributed to the decrease in air kerma, with the former being the major factor. If the height of the source can be decreased in some way, such as increasing the specific activity of sources, the sources can be used more efficiently. The method prolongs the service life of sources by several factors, and reduces the expense of source exchange and reclamation.PACS number: 87.56.bg