Lead Collimator Research Articles

‘Accurate proton range shift verification by using a two-layer dense-pixel LYSO compton camera prototype

ObjectiveCompton camera (CC)-based prompt gamma imaging (PGI) is promising to realize real-time in-vivo range verification during proton therapy. Lutetium–yttrium orthosilicate (LYSO)-based CC has advantages in high detection efficiency and position resolution. However, few LYSO CC-based PGI experimental evaluations have been investigated. ApproachWe built a 46 × 46 two-layer dense-pixel LYSO CC prototype for PGI. We attempt the first-ever effort to conduct the proton range shift verification experiments with the LYSO CC prototype. A 13 MeV proton beam with a cross-section of 1 mm2 produced by the Compact Pulsed Hadron Source (CPHS) and a lead collimator irradiates a block of graphite, a block of polymethacrylates (PMMA) and a block of fresh pork, respectively. A range shift varied from 1 mm to 5 mm of the proton beam is used to evaluate the performance of the prototype in PGI. The peak current intensity was about 640 nA. The pulse width and frequency were 100 μs and 1 Hz, respectively. The rate of incident protons in each measurement is about 3.6 × 108/s, and the corresponding delivered dose is about 5.8 Gy. Main resultsThe built LYSO CC prototype can distinguish the displacement of the Bragg peak in 2 mm (reaching 1 mm) at a distance of about 13 cm, and can accurately reconstruct the positions of the Bragg peak with a mean deviation of 0.2 mm in the eighteen groups of experiments. SignificationThe experiments demonstrate the feasibility of the developed two-layer dense-pixel LYSO CC in realizing accurate range shift monitoring and show its potential in range verification during clinical proton therapy and high particle rate proton therapy (e.g., FLASH proton therapy).

Comparison of measurements and simulation results of dose for the flash radiation therapy beamline at PITZ

The high-brightness electron beam at the Photo Injector Test facility at DESY in Zeuthen (PITZ) is now also used for FLASHlab@PITZ: an R.&D platform for studying radiation biology and the FLASH effect in radiation therapy. The available parameter space of the electron beam with a momentum of 22 MeV/c allows bunch charges from 1 pC up to 5 nC, bunch durations of 0.160 ps and bunch train lengths up to 1 ms. The number of bunches in the single train can currently be varied between 1 and 1000 bunches, with an upgrade to 4500 foreseen in 2023. Radiation biology studies require accurate dose predic-tion, therefore Monte Carlo simulations based on the FLUKA code were performed. According to estimations, dose delivery of 0.002 Gy (low charge case 1 pC) and 10 Gy (high charge case 5 nC) is possible, if the beam is confined to a circular area with a radius of 5 mm with a lead collimator. For the Monte Carlo simulations, the experimental setup was accurately modelled, including the exit window, lead collimator, etc. Dose measurements were used to compare simulations with experiments. Dose profiles were experimentally measured with Gafchromic films and then compared with Monte Carlo simulations. The first experiments at FLASHlab@PITZ in 2023 have demonstrated flexible dose options for studying the FLASH effect and radiation biology studies.

Влияние коллиматора и поглотителя на спектры, измеренные с помощью прототипа ксенонового гамма-спектрометра

The paper investigates the possibility in principle of determining the nuclide composition and activity of gammasources by analyzing the measured gamma-spectra provided an absorber installed between the gamma-sources and a xenon gamma-spectrometer placed in a cylindrical lead collimator.

Half-value layer measurements using solid-state detectors and single-rotation technique with lead apertures in spiral computed tomography with and without a tin filter.

Solid-state detectors (SSDs) may be used along with a lead collimator for half-value layer (HVL) measurement using computed tomography (CT) with or without a tin filter. We aimed to compare HVL measurements obtained using three SSDs (AGMS-DM+ , X2 R/F sensor, and Black Piranha) with those obtained using the single-rotation technique with lead apertures (SRTLA). HVL measurements were performed using spiral CT at tube voltages of 70-140kV without a tin filter and 100-140kV (Sn 100-140kV) with a tin filter in increments of 10kV. For SRTLA, a 0.6-cc ionization chamber was suspended at the isocenter to measure the free-in-air kerma rate ( ) values. Five apertures were made on the gantry cover using lead sheets, and four aluminum plates were placed on these apertures. HVLs in SRTLA were obtained from decline curves. Subsequently, SSDs inserted into the lead collimator were placed on the gantry cover and used to measure HVLs. Maximum HVL differences of AGMS-DM+ , X2 R/F sensor, and Black Piranha with respect to SRTLA without/with a tin filter were - 0.09/0.6 (only two Sn 100-110kV) mm, - 0.50/ - 0.6mm, and - 0.17/(no data available) mm, respectively. These values were within the specification limit. SSDs inserted into the lead collimator could be used to measure HVL using spiral CT without a tin filter. HVLs could be measured with a tin filter using only the X2 R/F sensor, and further improvement of its calibration accuracy with respect to other SSDs is warranted.

Matrix metal collimators studies for the spatially fractionated radiation therapy

Traditional radiation therapy is hindered by limitations in dose delivery and the risk of over-irradiation, which can harm healthy tissues surrounding tumors. Spatially fractionated and intensity-modulated radiation therapies have emerged as promising techniques to mitigate these issues by reducing damage to healthy tissues. This study explores the potential of enhancing radiation therapy's therapeutic efficacy through spatial fractionation of X-ray beams, a technique that can significantly increase radiation efficiency by reducing the dose load on healthy tissues. This research hypothesizes that microscopic lesions within the paths of micro/mini-beams can be repaired by minimally irradiated cells adjacent to the irradiated tissue slices, based on observations with high-energy (MV) photons. Spatial fractionation is particularly valuable for X-ray and electron radiation therapy, which is more widespread and cost-effective than proton therapy. The study introduces a new type of cost-effective metal matrix collimators designed for beam fractionation. These collimators are constructed using lead plates for thickness variation implementation and employ a 5x5 hole matrix with 1 mm diameter and 3 mm pitch, covering an area of 14x14 mm². To visualize beam distribution, a Timepix detector, capable of providing real-time 2D beam profiles, was employed. Experiments were conducted using a medical LINAC with typical therapeutic energy ranging from 6 to 18 MeV. Results indicate that spatial fractionation can be achieved for X-ray radiation, with a PVDR of approximately 3 for a 3 cm lead collimator. However, issues with hole uniformity and blurring in the peak area prompted a shift to using copper collimators due to their superior manufacturing properties. This study presents a novel matrix collimator design made from various materials for shaping mini beams, aimed at improving the efficiency of spatial dose fractionation for different ionizing radiation types. Geant4 simulations have been instrumental in optimizing collimator features. The findings suggest that high levels of dose fractionization can be achieved for X-rays, electrons, and proton beams. These results warrant further biological studies to evaluate the effects of fractionation on normal and tumoral tissues, supporting the practical implementation of collimation in radiation therapy.

Preclinical Intraoperative FLASH Radiotherapy

Intraoperative radiation therapy (IORT) is a method of delivering high radiation doses to an internal target with low dose to adjacent organs. Most organs at risk can often be moved out of the radiation field, so dose is usually limited by in-field vascular and/or nerve structures. It is known that ultra-high (FLASH) dose rate (>40 Gy/s) radiation reduces normal tissue toxicity compared to conventional, while preserving tumor control. We hypothesize that FLASH radiation can increase the dose tolerance of vascular and nerve structures, thus permitting for dose escalation in IORT. Here, we present a novel platform using orthovoltage x-rays to study FLASH effects in preclinical IORT. The experimental setup and dosimetry of the system have been established for FLASH irradiation of the abdominal aorta in rats to investigate dose tolerance. A 150 kVp rotating anode x-ray source with a 75-kW generator was used to deliver FLASH and conventional irradiation. A 3D printed immobilization platform was designed to reproducibly place a surgically exposed rat abdominal aorta at 55mm SSD. A 3-mm thick lead collimator with a 10mm x 10mm aperture was placed into a flange adapter, which docks into the immobilization platform. Phantom and in vivo dosimetry were performed using both calibrated radiographic film and thermoluminescent dosimeters (TLD). Anesthetized 2-month-old Wistar rats underwent laparotomy to expose the abdomen for irradiation. A 10x12x1 mm3 tungsten plate was placed under the aorta to reduce spinal cord dose. Animals were then docked into the irradiator using the immobilization platform. Radiation dose in the range of 20 - 50 Gy were delivered to the abdominal aorta at FLASH and conventional dose-rates. Following treatment, animals undergo MR angiography every two months for 6 months, at which a histological evaluation of aorta wall will be performed. The FLASH dose rate measured at the aorta wall in a rat carcass was 49.7 ± 1.2 Gy/s distal to the collimator. Dose measurements between the films and TLDs showed an agreement within ±3%. Positional uncertainty mainly depends on the visual localization of the aorta on the collimator window, which is verified before irradiation. Vertical uncertainty is less than 1 mm, which is important given the rapid dose fall off of the system. Eight animals went successfully through the procedure, are healthy and currently being follow up with MR angiography. More animals are being treated for meaningful statistical results. MR angiography and IORT are performed on different days to avoid longer times under anesthesia, which initially resulted on mortality. Results of the MR angiography and histological analysis will be presented. The proposed platform has been successfully utilized for precision setup for FLASH IORT in a preclinical laboratory setting. Evaluation of FLASH effects in IORT setup will be an important step in the potential translation of FLASH IORT, where only one high-dose fraction can be delivered.

Absorbed dose calculation for a realistic CT-derived mouse phantom irradiated with a standard Cs-137 cell irradiator using a Monte Carlo method.

Computed tomography (CT) derived Monte Carlo (MC) phantoms allow dose determination within small animal models that is not feasible with in-vivo dosimetry. The aim of this study was to develop a CT-derived MC phantom generated from a mouse with a xenograft tumour that could then be used to calculate both the dose heterogeneity in the tumour volume and out of field scattered dose for pre-clinical small animal irradiation experiments. A BEAMnrc Monte-Carlo model has been built of our irradiation system that comprises a lead collimator with a 1 cm diameter aperture fitted to a Cs-137 gamma irradiator. The MC model of the irradiation system was validated by comparing the calculated dose results with dosimetric film measurement in a polymethyl methacrylate (PMMA) phantom using a 1D gamma-index analysis. Dose distributions in the MC mouse phantom were calculated and visualized on the CT-image data. Dose volume histograms (DVHs) were generated for the tumour and organs at risk (OARs). The effect of the xenographic tumour volume on the scattered out of field dose was also investigated. The defined gamma index analysis criteria were met, indicating that our MC simulation is a valid model for MC mouse phantom dose calculations. MC dose calculations showed a maximum out of field dose to the mouse of 7% of Dmax. Absorbed dose to the tumour varies in the range 60%-100% of Dmax. DVH analysis demonstrated that tumour received an inhomogeneous dose of 12 Gy-20 Gy (for 20 Gy prescribed dose) while out of field doses to all OARs were minimized (1.29 Gy-1.38 Gy). Variation of the xenographic tumour volume exhibited no significant effect on the out of field scattered dose to OARs. The CT derived MC mouse model presented here is a useful tool for tumour dose verifications as well as investigating the doses to normal tissue (in out of field) for preclinical radiobiological research.

Light-transport incorporated plastic scintillator response to prompt gamma-rays for use in range verification of proton therapy

In this study, the prompt gamma-rays (PGs) produced during the interactions of proton beams of various energies have been investigated using the MCNPX2.6 Monte Carlo code. Having considered a detection setup consisting of a thick lead collimator and a commercial plastic scintillator, the event-by-event data, generated by the PTRAC card of the MCNPX code, when the scintillator is exposed to PGs have been extracted before they are used in a dedicated light transport code, PHOTRACK. The plastic scintillator response when it is moved along the phantom axis determines the flux variation of proton-induced PGs which can be further related to the longitudinal deposition energy profile of protons, i.e., the Bragg curve. The present study shows that the Bragg peak location is ∼3.7 cm after the PG peak for different proton energies of interest. The uncertainty in the Bragg peak location data is about 0.02 cm which can be improved by taking into account the precise electron-transport simulation details.

Новый алгоритм для модернизации портальных радиационных мониторов с пластиковыми детекторами. Часть 4. Использование коллиматоров для повышения эффективности работы алгоритма

The way of improving of new algorithm by using additional lead collimators is discussed in the fourth part of the article. The results of calculations and direct measurements of collimators of different sizes for two variants of algorithm with four and six energy windows are presented in the paper. It is shown that using of lead collimators increases efficiency of algorithm and provides detection of radioactive sources less activity.

Design of 4π High-Efficiency Directional Radiation Detector Based on Compton Scattering

Obtaining directional information is required in many applications such as nuclear homeland security, contamination mapping after a nuclear incident and radiological events, or during the decontamination work. However, many directional radiation detectors are based on directional shielding, made of lead or tungsten collimators, introducing two main drawbacks. The first is the size and weight, making those detectors too heavy and irrelevant for utilization in handheld devices, drone mapping, or space applications. The second drawback is the limited field of view (FOV), which requires multiple detectors to cover the whole required FOV or machinery to rotate the detector&#x2019;s narrow FOV detector. We propose a novel <inline-formula> <tex-math notation="LaTeX">$4\pi $ </tex-math></inline-formula> directional detector based on a segmented hollow cubic detector, which uses the Compton effect interactions with no heavy collimators. The symmetrical cubical design provides both higher efficiency and <inline-formula> <tex-math notation="LaTeX">$4\pi $ </tex-math></inline-formula> detection ability. Instead of the traditional two types of detectors (scatterer and absorber) structure, we use the same type of detector, based on Gd₃Al₂Ga₃O₁₂ (GAGG) (Ce) scintillator coupled to silicon photomultiplier. An additional advantage of the proposed detector is obtained by locating the photon sensors inside the detector, behind the scintillators, which improves the radiation hardness required for space applications. Furthermore, such an arrangement flattens the temperature variation across the detector, providing better gain stability. The main advantage of the proposed detector is an efficient <inline-formula> <tex-math notation="LaTeX">$4\pi $ </tex-math></inline-formula> radiation detection for high-energy gamma rays without the use of heavy collimators.

Experimental Evaluation of Scattered X-Ray Spectra due to X-Ray Therapeutic and Diagnosis Equipment for Eye Lens Dosimetry of Medical Staff

Background: For proper monitoring of the eye lens dose, an appropriate calibration factor of a dosimeter and information about the mean energies of X-rays are indispensable. The scattered X-ray energy spectra should be well characterized in medical practices where eye lenses of medical staffs might be high.Materials and Methods: Scattered X-ray energy spectra were experimentally derived for three different types of X-ray diagnostic and therapeutic equipment, i.e., the computed tomography (CT) scan, the angiography and the fluoroscopy. A commercially available CdZnTe (CZT) spectrometer with a lead collimator was employed for the measurement of scattered X-rays, which was performed in the usual manner.Results and Discussion: From the obtained energy spectra, the mean energies of the scattered X-rays lied between 40 and 60 keV. This also agreed with that obtained by the conventional half value layer method.Conclusion: The scattered X-rays to which medical workers may be exposed in the region around the eyes were characterized by means of spectrometry. The obtained mean energies of the scattered X-rays were found to match the flat region of the dosimeter response.

Development of a direction-sensitive gamma-ray monitoring system using a gamma camera with a dual-sided collimator: A Monte Carlo study

Nuclear explosions, sabotage, and dirty bomb materials are considered a security threat. This paper discusses the development of a gamma-ray monitoring system that enables the screening of nuclear materials moving simultaneously on both sides of the system at ports. This direction-sensitive gamma-ray monitoring (DSGM) system consists of a monolithic plastic scintillator surrounded by 28 photomultiplier tubes and dual-sided parallel-hole lead collimators. With Monte Carlo simulation, the monitoring performance of the DSGM system was assessed for static and moving sources. A multilayer perceptron model was employed to estimate the energy-deposited position of the gamma-rays emitted by nuclear materials in the scintillator.

Measurement of cumulative photofission yields of 235U and 238U with a 16 MeV Bremsstrahlung photon beam

In the frame of a long-term research program on the characterization of large radioactive waste packages by photofission, the Nuclear Measurement Laboratory of CEA IRESNE, France, has measured cumulative photofission yields of 235U and 238U short-lived and long-lived fission products by using a Bremsstrahlung photon beam produced by a 16 MeV electron linear accelerator (LINAC). To this aim, a characterization of the Bremsstrahlung photon beam has been carried out by photon activation analysis with different samples of gold, nickel, uranium and zirconium. The residual neutron flux exiting the LINAC head (lead collimator, borated polyethylene and cadmium shield) has also been characterized by neutron activation analysis with indium samples to estimate the contribution of photoneutron fissions in the uranium samples used to assess the photofission yields. Finally, 49 fission product yields are reported for 238U and 26 for 235U, with half-lives ranging from 64 s to more than 3 days, some of them not recorded so far in the literature Some photofission products cumulative yields show significant differences between 235U and 238U, which confirms the possibility of an isotopic discrimination method based on delayed gamma-ray ratios analysis for radioactive waste characterization.

USE OF MONTE CARLO SIMULATIONS FOR OPTIMAL GEOMETRY STUDY IN CALCULATION OF ATTENUATION COEFFICIENT FOR ELEMENT, COMPOUND AND MIXTURE

MCNP is a code extensively used to simulate experiments involving transport of radiation using the Monte Carlo method. This code allows the study of different geometries, materials, and radiation types (e.g. gamma, neutron, and electron), enabling the building of approximate models before the experimental implementation. The objective of this study is to develop an optimal geometry for the calculation of the mass attenuation coefficient for different materials using the MCNP code. Several measurement geometries were tested with different radiation energies, and the best results were obtained using lead collimators on both detector and radiation source. The considered geometries were isotropic source without any collimation, isotropic source with detector and/or source collimation, and a point source collimated into a cone of directions. The last case was proposed as a replacement for the computationally time expensive simulation of the two-collimator geometry. The energies 59.54 keV, 81 keV, 356 keV, and 662 keV were used to model 241Am, 133Ba, and 137Cs radiation sources, respectively. The materials were, NaI for the detector, aluminum, water, and sea water (3.5% NaCl) for the target sample, and lead for the collimators. The values of mass attenuation coefficient obtained from the simulations were compared with the theoretical NIST XCOM values for validation of the geometries.

Innovative experimental setup for X-ray imaging to study energetic magnetized plasmas

The research of magnetically confined plasmas with high energy content is nowadays an important branch of the plasma physics with several options considering the chosen technics. One of the ways is the detection of photons emitted by the plasma itself. A new experimental setup was built in the ECR Laboratory of Atomki (Debrecen, Hungary) to detect in 2D the dense EM-radiation emitted by the ECR-plasma. The main elements of the setup are: an ECR ion source (as plasma source, operating at 13.6–14.6 GHz RF pumping frequency) and a pinhole X-ray camera (operating in the 500 eV–20 keV energy domain). An innovative lead collimator system was designed and built between the plasma and the camera. As a result, it has been possible to acquire X-ray pictures up to 200 W total incident RF-power. This value represents the highest operative RF power for which X-ray imaging has been acquired in the field of ECR Ion Sources and ECR compact traps. A new treatment for the noise reduction was applied to study plasma morphology. The new setup gives opportunity not just to study energetic stable plasmas, but even plasma turbulences and instabilities.

Application of radiochromic gel dosimetry to commissioning of a megavoltage research linear accelerator for small-field animal irradiation studies.

To develop and implement an efficient and accurate commissioning procedure for small-field static beam animal irradiation studies on an MV research linear accelerator (Linatron-M9) using radiochromic gel dosimetry. The research linear accelerator (Linatron-M9) is a 9MV linac with a static fixed collimator opening of 5.08cm diameter. Lead collimators were manually placed to create smaller fields of 2×2cm2 , 1×1cm2 , and 0.5×0.5cm2 . Relative dosimetry measurements were performed, including profiles, percent depth dose (PDD) curves, beam divergence, and relative output factors using various dosimetry tools, including a small volume ionization chamber (A14), GAFCHROMIC™ EBT3 film, and Clearview gel dosimeters. The gel dosimeter was used to provide a 3D volumetric reference of the irradiated fields. The Linatron profiles and relative output factors were extracted at a reference depth of 2cm with the output factor measured relative to the 2×2cm2 reference field. Absolute dosimetry was performed using A14 ionization chamber measurements, which were verified using a national standards laboratory remote dosimetry service. Absolute dosimetry measurements were confirmed within 1.4% (k=2, 95% confidence=5%). The relative output factor of the small fields measured with films and gels agreed with a maximum relative percent error difference between the two methods of 1.1 % for the 1×1cm2 field and 4.3 % for the 0.5×0.5cm2 field. These relative errors were primarily due to the variability in the collimator positioning. The measured beam profiles demonstrated excellent agreement for beam size (measured as FWHM), within approximately 0.8mm (or less). Film measurements were more accurate in the penumbra region due to the film's finer resolution compared with the gel dosimeter. Following the van Dyk criteria, the PDD values of the film and gel measurements agree within 11% in the buildup region starting from 0.5cm depth and within 2.6 % beyond maximum dose and into the fall-off region for depths up to 5cm. The 2D beam profile isodose lines agree within 0.5mm in all regions for the 0.5×0.5cm2 and the 1×1cm2 fields and within 1mm for the larger field of 2×2cm2 . The 2D PDD curves agree within approximately 2% of the maximum in the typical therapy region (1-4cm) for the 1×1cm2 and 2×2cm2 and within 5% for the 0.5×0.5cm2 field. This work provides a commissioning process to measure the beam characteristics of a fixed beam MV accelerator with detailed dosimetric evaluation for its implementation in megavoltage small animal irradiation studies. Radiochromic gel dosimeters are efficient small-field relative dosimetry tools providing 3D dose measurements allowing for full representation of dose, dosimeter misalignment corrections and high reproducibility with low inter-dosimeter variability. Overall, radiochromic gels are valuable for fast, full relative dosimetry commissioning in comparison to films for application in high-energy small-field animal irradiation studies.

Design of 4π Directional Radiation Detector based on Compton Scattering Effect

Obtaining directional information is required in many applications such as nuclear homeland security, contamination mapping after a nuclear incident and radiological events, or during the decontamination work. However, many directional radiation detectors are based on directional shielding, made of lead or tungsten collimators, introducing two main drawbacks. The first is the size and weight, making those detectors too heavy and irrelevant for utilization in handheld devices, drone mapping, or space applications. The second drawback is the limited field of view, which requires multiple detectors to cover the whole required field of view or machinery to rotate the narrow field of view detector. We propose a novel 4π directional detector based on a segmented hollow cubic detector, which uses the Compton effect interactions with no heavy collimators. The symmetrical cubical design provides both higher efficiency and 4π detection ability. Instead of traditional two types of detectors (scatterer and absorber) structure, we use the same type of detector, based on GAGG(Ce) scintillator coupled to silicon photomultiplier. Additional advantage of the proposed detector obtained by locating the photon sensors inside the detector, behind the scintillators, which improves the radiation hardness required for space applications. Furthermore, such arrangement flattens the temperature variation across the detector, providing better gain stability. The main advantage of the proposed detector is the ability of 4pi radiation detection for high energy gamma-rays without the use of heavy collimators.

Study on Effect of TIR Lens and Reflector for LED Collimation

Study on Effect of TIR Lens and Reflector for LED Collimation

Selective Isotope CT Imaging Based on Nuclear Resonance Fluorescence Transmission Method

The isotope selectivity of computed tomography (CT) imaging based on nuclear resonance fluorescence (NRF) transmission method using a quasi-monochromatic laser Compton scattering (LCS) gamma-ray beam in the MeV region was demonstrated at the Ultra Violet Synchrotron Orbital Radiation-III (UVSOR-III) Synchrotron Radiation Facility (Institute of Molecular Science, National Institute of Natural Science) for two enriched lead isotope rods ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">206</sup> Pb and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">208</sup> Pb) implanted in an aluminum cylinder. Since these two rods show the same gamma-ray attenuation in atomic processes, it is impossible to differentiate between them using a standard Gamma-CT technique based on atomic attenuation of gamma rays. The LCS gamma-ray beam had a maximum energy of 5.528 MeV and an intensity of approximately 5.5 photons/s/eV at the resonance energy (J <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sup> = 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> at 5.512 MeV in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">208</sup> Pb). A lead collimator with a hole diameter of 1 mm was used to define the size of the LCS gamma-ray beam at the CT target. The CT image of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">208</sup> Pb rod was selectively obtained with a 2-mm pixel size resolution, which was determined by the horizontal step size of the CT stage.

Determination of mass attenuation coefficient of polylactic acid using gamma densitometry in 50-1000 keV energy range

Polylactic acid (PLA) is a biodegradable organic acid of polymeric chain that has as chemical structure the monomer C3H4O2 and it is one of the most used materials in rapid prototyping among 3D printers. Due to the ease of manufacturing PLA parts and because of its low atomic number, PLA can be used as holder to radiation source and detectors to establish fixed geometries in experiments using gamma radiation, hence the importance in knowing more about PLA characteristics. In this way, the novelty of this paper is to determine the mass attenuation coefficient of 3D printed PLA samples using gamma densitometry. A mathematical model was developed using MCNPX code and consisted of a PLA sample, a NaI(Tl) detector and gamma rays of 661.6 keV as radiation source (narrow beam). The mathematical model was experimentally validated using the same geometry and a lead collimator. Besides that, PLA weight fractions and density were also determined in order to use this information as input data in the MCNPX code. The obtained mass attenuation coefficients were in good agreement with the NIST theoretical reference value, presenting relative errors of 0.99% (MCNPX) and 2.00% (experimental). The proposed methodology was repeated replacing PLA for aluminum samples and comparing its results with NIST. In addition, the mass attenuation coefficient for some polymers was calculated using this model aiming to validate the geometry developed with MCNPX code.