Characterization of a flexible a-Si:H detector for in vivo dosimetry in therapeutic x-ray beams

BACKGROUND

The increasing use of complex and high dose-rate treatments in radiation therapy necessitates advanced detectors to provide accurate dosimetry. Rather than relying on pre-treatment quality assurance (QA) measurements alone, many countries are now mandating the use of in vivo dosimetry, whereby a dosimeter is placed on the surface of the patient during treatment. Ideally, in vivo detectors should be flexible to conform to a patient's irregular surfaces.

PURPOSE

This study aims to characterize a novel hydrogenated amorphous silicon (a-Si:H) radiation detector for the dosimetry of therapeutic x-ray beams. The detectors are flexible as they are fabricated directly on a flexible polyimide (Kapton) substrate.

METHODS

The potential of this technology for application as a real-time flexible detector is investigated through a combined dosimetric and flexibility study. Measurements of fundamental dosimetric quantities were obtained including output factor (OF), dose rate dependence (DPP), energy dependence, percentage depth dose (PDD), and angular dependence. The response of the a-Si:H detectors investigated in this study are benchmarked directly against commercially available ionization chambers and solid-state diodes currently employed for QA practices.

RESULTS

The a-Si:H detectors exhibit remarkable dose linearities in the direct detection of kV and MV therapeutic x-rays, with calibrated sensitivities ranging from (0.580 ± 0.002) pC/cGy to (19.36 ± 0.10) pC/cGy as a function of detector thickness, area, and applied bias. Regarding dosimetry, the a-Si:H detectors accurately obtained OF measurements that parallel commercially available detector solutions. The PDD response closely matched the expected profile as predicted via Geant4 simulations, a PTW Farmer ionization chamber and a PTW ROOS chamber. The most significant variation in the PDD performance was 5.67%, observed at a depth of 3 mm for detectors operated unbiased. With an external bias, the discrepancy in PDD response from reference data was confined to ± 2.92% for all depths (surface to 250 mm) in water-equivalent plastic. Very little angular dependence is displayed between irradiations at angles of 0° and 180°, with the most significant variation being a 7.71% decrease in collected charge at a 110° relative angle of incidence. Energy dependence and dose per pulse dependence are also reported, with results in agreement with the literature. Most notably, the flexibility of a-Si:H detectors was quantified for sample bending up to a radius of curvature of 7.98 mm, where the recorded photosensitivity degraded by (-4.9 ± 0.6)% of the initial device response when flat. It is essential to mention that this small bending radius is unlikely during in vivo patient dosimetry. In a more realistic scenario, with a bending radius of 15-20 mm, the variation in detector response remained within ± 4%. After substantial bending, the detector's photosensitivity when returned to a flat condition was (99.1 ± 0.5)% of the original response.

CONCLUSIONS

This work successfully characterizes a flexible detector based on thin-film a-Si:H deposited on a Kapton substrate for applications in therapeutic x-ray dosimetry. The detectors exhibit dosimetric performances that parallel commercially available dosimeters, while also demonstrating excellent flexibility results.

Preparation and Properties of Low-Dielectric Polyimide Films Containing Tert-Butyl

The design of high-performance polyimide (PI) films and understanding the relationship of the structure-dielectric property are of great significance in the field of the microelectronics industry, but are challenging. Herein, we describe the first work to construct a series of novel tert-butyl PI films (denoted as PI-1, PI-2, PI-3, and PI-4) based on a low-temperature polymerization strategy, which employed tetracarboxylic dianhydride (pyromellitic anhydride, 3,3',4,4'-biphenyl tetracarboxylic anhydride, 4,4'-diphenyl ether dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic anhydride) and 4,4'-diamino-3,5-ditert butyl biphenyl ether as monomers. The results indicate that introducing tert-butyl branches in the main chain of PIs can enhance the free volume of the molecular chain and reduce the interaction between molecular chains of PI, resulting in a low dielectric constant. Particularly, the optimized PI-4 exhibits an excellent comprehensive performance with a high (5) wt% loss temperature (454 °C), tensile strength (117.40 MPa), and maximum hydrophobic angle (80.16°), and a low dielectric constant (2.90), which outperforms most of the results reported to date.

Efficient Strategy for Radiative Cooling Based on Ultra-Broad-Band Infrared Regulation of Flexible Bilayer Film

Flexible thermal radiation films with adjustable broad-band infrared radiation could maintain various heat-generating electronic devices working stably in corresponding operating temperatures, making them good candidates for radiative cooling (RC) material. However, the controllable radiation peaks of the metamaterial were narrow, and manipulation was a time-consuming and complex process. Herein, we design a simple TiN/Si bilayer film with controllable broad-band radiation peaks at a thermal radiation wavelength of 3.5-20 μm by impedance matching. Meanwhile, the different bilayer films applied to aluminum devices at different temperatures exhibit outstanding heat dissipation efficiency and maintain the corresponding equilibrium temperature to ensure that devices work stably for a long time. Moreover, the bilayer films deposited on the flexible PI substrates exhibit preferable thermostability and higher tensile strength than existing radiative cooling materials deposited on PDMS, PE, PMMA or TPX, etc. This work provides an effective strategy to realize efficient radiation cooling for flexible electronic devices and spacecraft appendages.

A Comprehensive Review on the Thermal Stability Assessment of Polymers and Composites for Aeronautics and Space Applications

This review article provides an exhaustive survey on experimental investigations regarding the thermal stability assessment of polymers and polymer-based composites intended for applications in the aeronautical and space fields. This review aims to: (1) come up with a systematic and critical overview of the state-of-the-art knowledge and research on the thermal stability of various polymers and composites, such as polyimides, epoxy composites, and carbon-filled composites; (2) identify the key factors, mechanisms, methods, and challenges that affect the thermal stability of polymers and composites, such as the temperature, radiation, oxygen, and degradation; (3) highlight the current and potential applications, benefits, limitations, and opportunities of polymers and composites with high thermal stability, such as thermal control, structural reinforcement, protection, and energy conversion; (4) give a glimpse of future research directions by providing indications for improving the thermal stability of polymers and composites, such as novel materials, hybrid composites, smart materials, and advanced processing methods. In this context, thermal analysis plays a crucial role in the development of polyimide-based materials for the radiation shielding of space solar cells or spacecraft components. The main strategies that have been explored to improve the processability, optical transparency, and radiation resistance of polyimide-based materials without compromising their thermal stability are highlighted. The combination of different types of polyimides, such as linear and hyperbranched, as well as the incorporation of bulky pendant groups, are reported as routes for improving the mechanical behavior and optical transparency while retaining the thermal stability and radiation shielding properties. Furthermore, the thermal stability of polymer/carbon nanocomposites is discussed with particular reference to the role of the filler in radiation monitoring systems and electromagnetic interference shielding in the space environment. Finally, the thermal stability of epoxy-based composites and how it is influenced by the type and content of epoxy resin, curing agent, degree of cross-linking, and the addition of fillers or modifiers are critically reviewed. Some studies have reported that incorporating mesoporous silica micro-filler or microencapsulated phase change materials (MPCM) into epoxy resin can enhance its thermal stability and mechanical properties. The mesoporous silica composite exhibited the highest glass transition temperature and activation energy for thermal degradation among all the epoxy-silica nano/micro-composites. Indeed, an average activation energy value of 148.86 kJ/mol was recorded for the thermal degradation of unfilled epoxy resin. The maximum activation energy range was instead recorded for composites loaded with mesoporous microsilica. The EMC-5p50 sample showed the highest mean value of 217.6 kJ/mol. This remarkable enhancement was ascribed to the polymer invading the silica pores and forging formidable interfacial bonds.

Synthesis and characterization of amide-bridged colorless polyimide films with low CTE and high optical performance for flexible OLED displays

Starting from three novel amide-incorporating dianhydride monomers, we synthesized a series of amide-bridged cPI films that have ultra-low CTE and high Tg due to the formation of hydrogen bonds as well as great optical performance.

Soluble and transparent polyimides with high Tgs from a new semi-aliphatic diamine with cyclohexyl and ortho-methyl groups

Herein a semi-aliphatic diamine 4,4′-(cyclohexylmethylene)bis(2-methylaniline) (CHMBMA) with a pendant cyclohexyl and two ortho-substituted methyl groups is synthesized from o-toluidine and cyclohexanecarbaldehyde by Mannich and rearrangement reactions. Then CHMBMA is polycondensed with five commercial aromatic dianhydrides by the high-temperature one-step method with the thermal imidization to produce a series of polyimides (PIs, PI-H(1–5)). The weight-averaged molecular weights ( M ws) of PI-H(1–5) are in the range from 9.08 × 104 to 27.48 × 104 g mol−1 with the polydispersity indices (PDI) from 3.29 to 6.13 by gel permeation chromatography (GPC) measurement. They are soluble in common organic solvents (such as THF, CHCl3 etc.) and can form transparent, tough films with light-color (Thickness: 20–26 μm) by the solution-casting method. The light transmittance of them is above 80% in the visible range from 400 to 760 nm. They exhibit excellent mechanical properties with tensile strength from 72.2 to 97.06 MPa and tensile modulus from 0.9 to 1.9 GPa. Furthermore, they also display low water absorption rates (<2.5%), good thermal stability (5% weight loss temperatures ( T 5%) in the range from 466 to 480°C under N2 atmosphere and high glass transition temperatures ( T gs ≥ 319°C). As comparison, we also synthesize these PIs (PI-L(1–5)) by the low-temperature two-step method with the chemical imidization. The M ws of PI-L(1–5) are lower than those of PI-H(1–5), but the film color of PI-L(1–5) is relatively lighter than the corresponding one of PI-H(1–5). In summary, the introduction of cyclohexyl and ortho-substituted methyl groups into the backbone can improve the solubility of PIs and the transparency of their corresponding films without reducing their T gs.

Novel poly(imide-ether)s based on xanthene and a corresponding composite reinforced with a GO grafted hyperbranched polymer: fabrication, characterization, and thermal, photophysical, antibacterial and chromium adsorption properties

High-performance polyimides (PIs) with ether linkages and trifluoromethyl (–CF₃) groups based on xanthene were designed and synthesized via a polycondensation reaction of novel diamine monomers with available aromatic dianhydrides.