Source Capsule Research Articles

Calculations to support JET neutron yield calibration: Neutron scattering in source holder

After the coated CFC wall to ITER-Like Wall (Beryllium/Tungsten/Carbon) transition in 2010–11, confirmation of the neutron yield calibration will be ensured by direct measurements using a calibrated 252Cf neutron source deployed by the in-vessel remote handling boom and Mascot manipulator inside the JET vacuum vessel.The paper describes preliminary calculations and the results of numerical study of the effect of source holder on neutron detector response. The source baton was designed in such a way, that it does not significantly affect the neutron spectrum, angular neutron flux distribution or activation detector response. All effects are approximately equal to or less than 1%. The largest disturbance to the neutron flux angular distribution and to the neutron spectrum arises from the source capsule. Hence one should obtain as much information as possible about the capsule and the 252Cf source material in order to avoid additional systematic errors.

EVALUATION OF SCATTERED RADIATION IN A CALIBRATION RANGE USING EXPOSURE RATE ENERGY SPECTRA

ISO standard 4037 specifies that for calibrating protection level dosimeters, scattered radiation should contribute less than 5% of the exposure. In previous work, the authors reported the results of an MCNP analysis of the shadow shield technique that was performed for a calibration range with a Cs irradiator. This paper examines the energy distribution of the photons contributing to the exposure percent scatter (S%) and the detailed origin of the scatter that originates in the irradiator. In summary, it reports that: 1) the majority of S% is due to photons with energies that are significantly below the source energy, 2) a significant percentage of S% is due to photons that scatter within the source and source capsule walls, and 3) S% due to scatter within the irradiator is even more significant than previously reported.

Modeling a Hypothetical170Tm Source for Brachytherapy Applications

Purpose: To perform absorbed dose calculations based on Monte Carlo simulations for a hypothetical170Tm source and to investigate the influence of encapsulating material on the energy spectrum of the emitted electrons and photons. Methods: GEANT4 Monte Carlo code version 9.2 patch 2 was used to simulate the decay process of170Tm and to calculate the absorbed dose distribution using the GEANT4 Penelope physics models. A hypothetical 170Tm source based on the Flexisource brachytherapy design with the active core set as a pure thulium cylinder (length 3.5 mm and diameter 0.6 mm) and different cylindrical source encapsulations (length 5 mm and thickness 0.125 mm) constructed of titanium, stainless-steel, gold, or platinum were simulated. The radial dose function for the line source approximation was calculated following the TG-43U1 formalism for the stainless-steel encapsulation. Results: For the titanium and stainless-steel encapsulation, 94% of the total bremsstrahlung is produced inside the core, 4.8 and 5.5% in titanium and stainless-steel capsules, respectively, and less than 1% in water. For the gold capsule, 85% is produced inside the core, 14.2% inside the gold capsule, and a negligible amount (<1%) in water. Platinum encapsulation resulted in bremsstrahlung effects similar to those with the gold encapsulation. The range of the beta particles decreases by 1.1 mm with the stainless-steel encapsulation compared to the bare source but the tissue will still receive dose from the beta particles several millimeters from the source capsule. The gold and platinum capsules not only absorb most of the electrons but also attenuate low energy photons. The mean energy of the photons escaping the core and the stainless-steel capsule is 113 keV while for the gold and platinum the mean energy is 160 keV and 165 keV, respectively. Conclusions: A170Tm source is primarily a bremsstrahlung source, with the majority of bremsstrahlung photons being generated in the source core and experiencing little attenuation in the source encapsulation. Electrons are efficiently absorbed by the gold and platinum encapsulations. However, for the stainless-steel capsule (or other lower Z encapsulations) electrons will escape. The dose from these electrons is dominant over the photon dose in the first few millimeter but is not taken into account by current standard treatment planning systems. The total energy spectrum of photons emerging from the source depends on the encapsulation composition and results in mean photon energies well above 100 keV. This is higher than the main gamma-ray energy peak at 84 keV. Based on our results, the use of 170Tm as a brachytherapy source presents notable challenges.

Determination of photon fluence spectra from a 60Co therapy unit based on PENELOPE and MCNP simulations

Abstract Photon fluence spectra of the Seibersdorf Labor/BEV Picker 60Co therapy unit were calculated using two generally recognised Monte Carlo codes, PENELOPE-2006 and MCNP5. The complexity of the simulation model was increased in three steps (from a pure source capsule and a simplified model using rotational symmetry to a realistic model of the facility). Photon fluence spectra of both codes generally agree within their statistical standard uncertainties for the case of identical geometry set-up and particle transport parameter settings. Resulting total fluence values were about 0.3% higher for MCNP as compared to PENELOPE. The verification of the simulated photon fluence spectra was based upon depth–dose measurements in water performed with a PTW 31003 ionisation chamber and a thick-walled chamber type CC01. The depth–dose curve calculated with PENELOPE agreed with the curve obtained from measurements within 0.4% across the available depth region in the 30 cm × 30 cm × 30 cm water phantom. The comparison of measured and simulated beam quality indices (TPR20,10) revealed deviations of less than 0.2%.

Evaluation of Gafchromic EBT2 film for the measurement of anisotropy function for high-dose-rate 192Ir brachytherapy source with respect to thermoluminescent dosimetry

Aim The aim of this work was to assess the suitability of the use of a Gafchromic EBT2 film for the measurement of anisotropy function for microSelectron HDR 192Ir (classic) source with a comparative dosimetry method using a Gafchromic EBT2 film and thermoluminescence dosimeters (TLDs). Background Sealed linear radiation sources are commonly used for high dose rate (HDR) brachytherapy treatments. Due to self-absorption and oblique filtration of radiation in the source capsule material, an inherent anisotropy is present in the dose distribution around the source which can be described by a measurable two-dimensional anisotropy function, F( r, θ). Materials and methods Measurements were carried out in a specially designed and locally fabricated PMMA phantom with provisions to accommodate miniature LiF TLD rods and EBT2 film dosimeters at identical radial distances with respect to the 192Ir source. Results The data of anisotropy function generated by the use of the Gafchromic EBT2 film method are in agreement with their TLD measured values within 4%. The produced data are also consistent with their experimental and Monte Carlo calculated results for this source available in the literature. Conclusion Gafchromic EBT2 film was found to be a feasible dosimeter in determining anisotropy in the dose distribution of 192Ir source. It offers high resolution and is a viable alternative to TLD dosimetry at discrete points. The method described in this paper is useful for comparing the performances of detectors and can be applied for other brachytherapy sources as well.

A novel ytterbium-169 brachytherapy source and delivery system for use in conjunction with minimally invasive wedge resection of early-stage lung cancer

Purpose To describe a novel source–delivery system for intraoperative brachytherapy in patients with early-stage lung cancer that is readily adaptable to a video-assisted thoracoscopic surgery approach and can be precisely delivered to achieve optimal dose distribution. Methods and Materials Radioactive ytterbium-169 ( 169Yb) was sealed within a titanium tube 0.28 mm in diameter and then capped and resealed by titanium wires laser welded to the tube to serve as the legs of a tissue-fastening system. Dose simulations were performed using Monte Carlo computer code (Los Alamos National Laboratory, Los Alamos, NM) to mimic the geometric and elemental compositions of the source, fastening apparatus, and surroundings. Results Five test source capsules were subjected to a tensile load to failure. Failure in each capsule occurred in the wire of the fastener leg; there were no weld failures. Monte Carlo simulations and subsequent dose measurement showed the perturbation by the source legs in the deployed (bent over) position to be small (4–5%) for 169Yb and much less than that for iodine-125 (32%). Conclusion We have developed a 169Yb brachytherapy source–delivery system that can be used in conjunction with commercially available surgical stapling instruments, facilitates the precise placement of brachytherapy sources relative to the surgical margin, assures the seeds remain fixed in their precise position for the duration of the treatment, overcomes the technical difficulties of manipulating the seeds through the narrow surgical incision associated with video-assisted thoracoscopic surgery, and reduces the radiation dose to the clinicians.

SU-GG-T-92: Gamma Dose Near a New Miniature Cf-252 Brachytherapy Source

Purpose: A new generation of high‐activity miniature 252Cf sources for interstitial brachytherapy has recently been encapsulated at the Oak Ridge National Laboratory (ORNL). The neutron and gamma doses in water near the source were measured. The measured gamma doses were found to be slightly greater than the neutrondoses, contradicting the established rule of thumb that neutron‐to‐gamma dose ratio is approximately 2:1. The current study was conducted to resolve this discrepancy. Method & Materials: Previous report suggests that the source gamma spectrum used by the Monte Carlo (MCNP code) calculations may have underestimated the intensity of low‐energy photons. These photons can be direct emissions from prompt fissions or fission product decays. They can also be produced indirectly via bremsstrahlung from slowing down of the beta particles emitted from fission products. To thoroughly account for the low‐energy photons, we updated the ORIGEN‐S (an ORNL code) data library to include the experimentally measured 252Cf prompt fission gamma spectrum and explicitly simulated the production and decay of fission products using spontaneous 252Cf fission yield data. We also employed the code BETA‐S in conjunction with ORIGEN‐S to generate a beta spectrum from the fission products. This beta spectrum was then used as an input to MCNP to calculate the bremsstrahlung photons and the corresponding doses in water. Results: The inclusion of beta emissions in MCNP calculations doubles the gamma doses in water, and therefore, the MCNP‐predicted gamma doses now closely match the measured results. Conclusions: The aforementioned discrepancy is now resolved. The gamma doses near the new 252Cf source are indeed slightly greater than the neutrondoses. This is because the new source capsule is so thin that many bremsstrahlung photons are able escape the capsule.

Development of a spin-polarized positron source

We have started the development of a spin-polarized positron beam. In order to obtain a highly spin polarized positron beam, a 68Ge-68Ga source will be produced using a nuclear reaction of 69Ga(p,2n)68Ge. The optimum proton bombardment energy for 69Ga target is simulated to be 25 MeV using the Induced Radioactivity Analysis Code (IRAC). A newly designed 69Ga target holder will also be used as a 68Ge-68Ga source capsule.

EGSnrc‐based Monte Carlo dosimetry of CSA1 and CSA2 brachytherapy source models

AAPM TG-56 recommends the use of a specific dosimetric dataset for each brachytherapy source model. In this study, a full dosimetric dataset for indigenously developed 137Cs source models, namely, the CSA1 and CSA2, in accordance with the AAPM TG-43U1 formalism is presented. The study includes calculation of dose-to-kerma ratio D/K in water around these sources including stainless steel encapsulated 137Cs sources such as RTR, 3M, and selectron/LDR 137Cs. The Monte Carlo-based EGSnrcMP code system is employed for modeling the sources in vacuum and in water. Calculations of air-kerma strength, S(K) for the investigated sources and collision kerma in water along the transverse axis of the RTR source are based on the FLURZnrc code. Simulations of water-kerma and dose in water for the CSA1, CSA2, RTR, 3M, and selectron/ LDR 137Cs sources are carried out using the DOSRZnrc code. In DOSRZnrc calculations, water-kerma and dose are scored in a cylindrical water phantom having dimensions of 80 cm diameter x 80 cm height. The calculated dose-rate constants for the CSA1 and CSA2 sources are 0.945(1) and 1.023(1) cGy/(h U), respectively. The calculated value of S(K) per unit source activity, S(K)/A for the CSA1 and CSA2 sources is 7.393(7) x 10(-8) cGy cm2/(h Bq). The EGSnrcMP-based collision kerma rates for the RTR source along the transverse axis (0.25-10 cm) agree with the corresponding GEANT4-based published values within 0.5%. Anisotropy profiles of the CSA1 and CSA2 sources are significantly different from those of other sources. For the selectron/LDR single pellet 137Cs spherical source (modeled as a cylindrical pellet with dimensions similar to the seed selectron), the values of D/K at 1 and 1.25 mm from the capsule are 1.023(1) and 1.029(1), respectively. The value of D/K at 1 mm from the CSA1, CSA2, RTR, and 3M 137Cs source capsules (all sources have an external radius of 1.5 mm) is 1.017(1) and this ratio is applicable to axial positions z = 0 to z = -L/2. This is in contrast to a published GEANT4-based Monte Carlo dosimetric study on RTR and 3M 137Cs sources wherein the authors have assumed that collision kerma is approximately equal to absorbed dose at 1 mm from the source capsules. Collision kerma is approximately equal to absorbed dose for distances > or = 2 mm from source capsules as opposed to > or = 1 mm reported in published studies. A detailed electron transport is necessary up to 2 mm from source capsules. The Monte Carlo-calculated dose-rate data for the CSA1 and CSA2 sources can be used as input data for treatment planning or to verify the calculations by radiotherapy treatment planning system.

Ir-192 방사선원의 밀봉 용접부 품질에 미치는 저항용접 공정변수의 영향

Ir-192 radiation sealed sources are widely employed to the therapeutic applications as well as the non-destructive testing. Production of Ir-192 sources requires a delicate but robust welding technique because it is employed in a high radioactive working environment. A GTA(Gas Tungsten Arc) welding technique is currently well established for this purpose. However, this welding method requires a frequent replacement of the electrode, which results in the delay of the production to take a preparatory action such as to isolate the radiation sources from the working place before getting access to the welding machine. Hence, a resistance welding technique is considered as an alternative method of the GTA welding technique. The advantages of resistance welding are high welding speed and high-rate production. Also it has very long life of electrode comparing to GTA welding. In this study, the resistance welding system and proper welding conditions were established for sealing Ir-192 source capsule. As a results of various experiments, it showed that electrode displacement can be employed as a indicator to predict welding quality. We proposed two mathematical models(linear and curvilinear) to estimate electrode displacement with process parameters such as applied force, welding current and welding time by using regression analysis method. Predicting results of both linear and curvilinear model were relatively good agreement with experiment.

Teletherapy sources with imported and indigenous 60 Co activity

Board of Radiation and Isotope Technology, a unit of the Department of Atomic Energy, fabricates and supplies radioactive sources for medical, industrial, agriculture and research applications. High specific activity cobalt-60, required for teletherapy is normally imported. There was a proposal for manufacturing high specific activity sources indigenously. A study was carried out to observe the feasibility of mixing imported and indigenous cobalt-60 pellets to fabricate teletherapy source capsules. The specific activity of imported pellets is more than 300 Ci/g, whereas that of indigenous pellets obtained from Indian power reactors is 140 Ci/g. The radiation output from a capsule for different combinations of specific activity was evaluated. Losses due to self-absorption were accounted in the evaluations. In another study, the optimized lengths of the capsule for an output of 200 RMM and the additional activity to be added to compensate losses due to self-absorption were also estimated for different specific activity pellets. Sources fabricated on the basis of this study showed a good agreement with the estimations. Source capsules with a combination of different specific activities are yet to be fabricated.

Calculations of anisotropy factors for radionuclide neutron sources due to scattering from source encapsulation and support structures

A model has been developed for calculating the angular neutron fluence distributions for radionuclide neutron sources that are heavily encapsulated or surrounded by source support structures as a source holder and a source movement system. These structures may cause an anisotropic neutron fluence distribution. This should be taken into account in the neutron-measuring instruments calibration procedure. The calculations were made for two types of widely used neutron sources, (241)Am-Be and (252)Cf, by combining an in-house code simulating the (9)Be(alpha,n) reactions and the Monte Carlo code MCNP-4C. As a result, anisotropy factors in the direction perpendicular to the source capsule axis for bare neutron sources were evaluated to be 1.012, 1.030 and 1.039 for (252)Cf in a standard Amersham X1 capsule, (241)Am-Be in a X3 capsule and (241)Am-Be in a X4 capsule, respectively. These values are in reasonable agreement with the published data. If the support structures are included in the MCNP simulation, the anisotropy factors for these neutron sources increase by approximately 10%.

MCNP simulation of a Theratron 780 radiotherapy unit

A Theratron 780 (MDS Nordion) 60Co radiotherapy unit has been simulated with the Monte Carlo code MCNP. The unit has been realistically modelled: the cylindrical source capsule and its housing, the rectangular collimator system, both the primary and secondary jaws and the air gaps between the components. Different collimator openings, ranging from 5 x 5 cm2 to 20 x 20 cm2 (narrow and broad beams) at a source-surface distance equal to 80 cm have been used during the study. In the present work, we have calculated spectra as a function of field size. A study of the variation of the electron contamination of the 60Co beam has also been performed.

Monte Carlo simulation of response function for a NaI(Tl) detector for gamma rays from 241Am/Be source

The general purpose computer code MCNP4B was used to simulate the response function of a bare NaI(Tl) detector crystal for gamma rays from an 241Am/Be source capsule. The simulated spectral shape generated by the MCNP4B code was compared with the measured spectral shape obtained using a gamma-ray spectrometer with a cylindrical shape, 7.62 cm×7.62 cm, NaI(Tl). In general, the agreement between the simulation and the experimental response function was good.

Radiochromic film measurement of anisotropy function for high-dose-rate Ir-192 brachytherapy source

The dose distribution produced by the high-dose-rate (HDR) 192Ir source is inherently anisotropic due to self-absorption by the high-density source core, oblique filtration by the source capsule and the asymmetric geometry of the source capsule. To account for the dose distribution anisotropy of brachytherapy sources, AAPM Task Group No 43 has included a two-dimensional anisotropy function, F(r, θ), in the recommended dose calculation formalism. Gafchromic HS radiochromic film (RCF) was used to measure anisotropy function for microSelectron HDR 192Ir source (classic/old design). Measurements were carried out in a water phantom using specially fabricated PMMA cylinders at radial distances 1, 2, 3, 4 and 5 cm. The data so generated are comparable to both experimental and Monte Carlo calculated values for this source reported earlier by other authors. The RCF method described in this paper is comparatively high resolution, simple to use and is a general method, which can be applied for other brachytherapy sources as well.

A 22Na positron source for use in UHV

A source capsule for 22Na isotope sources for use in UHV positron beam systems is described. The capsule is suitable for a source strength up to 0.1 Ci (3.7 × 109Bq).

A 22Na positron source for use in UHV

A source capsule for 22Na isotope sources for use in UHV positron beam systems is described. The capsule is suitable for a source strength up to 0.1 Ci (3.7×109Bq).

Monte Carlo dose calculations for radiotherapy machines: Theratron 780-C teletherapy case study

The Monte Carlo transport code MCNP was used to simulate the photon beam from a Theratronics 780-C cobalt therapy unit and to calculate some dose-dependent parameters as functions of field size. The simulation process has included the source capsule, collimators (fixed and adjustable), lead in the unit head, and the field sizes as ranged from 5 × 5 to 35 × 35 cm2. Calculations have been carried out in a water phantom at a fixed source–surface distance of 80 cm. Detailed simulation of the major components of the therapy unit made it possible to calculate the effects of each unit component on the photon spectrum at the phantom surface. Percentage depth dose and peak scatter factor were evaluated for various field sizes. And tissue–air ratios were also determined for a field size of 10 ×10 cm2, as a function of depth down to 30 cm. To test the accuracy of the calculated results, they were compared with the published data of the British Journal of Radiology (BJR) suppl. 25 and good agreement between measurements and calculations has been obtained. Deviations typically were found to be of the order of 1%.

Peak scatter factors for high energy photon beams.

The peak scatter factor (PSF) for a photon beam is defined as the ratio of the total dose and the primary dose at the depth of dose maximum. The values of the PSF for photon beams ranging from 60Co to 24 MV are calculated using the EGS4 Monte Carlo technique, to avoid measurement difficulties. The calculation shows that the effect of SSD on PSF for high energy photon beams is not significant for small fields, but can be as high as 1% for large fields. For the 60Co beam, the calculation agrees with the data tabulated on BJR Supplement 25 to within 0.8%. The BJR value (1.054) of 10 x 10 cm2 for 60Co is 0.6% lower than the present value due to the underestimation of scatters from the source capsule and collimators. For a given field size, PSF is varied by up to 2% when beam quality changes from 60Co to 24 MV. For normalized PSF, the values of BJR Supplement 25 (which are assumed to be the same for beams ranging from 60Co to 50 MV) agree well with the present calculation for small field sizes, but are higher than our data by up to 2% for large field sizes. The presently calculated PSFs are related to field size(s) by an empirical expression, PSF = 1 + ms/(s + n), where m and n are the fitting parameters. This equation describes the PSFs within 0.4% (0.15% on average).

Two-dimensional scatter integration method for brachytherapy dose calculations in 3D geometry

In brachytherapy clinical practice, applicator shielding and tissue heterogeneities are usually not explicitly taken into account. None of the existing dose computational methods are able to reconcile accurate dose calculation in complex three-dimensional (3D) geometries with high efficiency and simplicity. We propose a new model that performs two-dimensional integration of the scattered dose component. The model calculates the effective primary dose at the point of interest and estimates the scatter dose as a superposition of the scatter contributions from pyramid-shaped minibeams. The approach generalizes a previous scatter subtraction model designed to calculate the dose for axial points in simple cylindrically symmetric geometry by dividing the scattering volume into spatial regions coaxial with the source-to-measurement point direction. To allow for azimuthal variation of the primary dose, these minibeams were divided into equally spaced azimuthally distributed pyramidal volumes. The model uses precalculated scatter-to-primary ratios (SPRs) for collimated isotropic sources. Effective primary dose, which includes the radiation scattered in the source capsule, is used to achieve independence from the source structure. For realistic models of the HDR and PDR sources, the algorithm agrees with Monte Carlo within 2.5% and for the type 6702 seed within 6%. The 2D scatter integration (2DSI) model has the potential to estimate the dose behind high-density heterogeneities both accurately and efficiently. The algorithm is much faster than Monte Carlo methods and predicts the dose around sources with different -ray energies and differently shaped capsules with high accuracy.