The adoption of vertically aligned carbon nanotubes (VACNTs) as electron emitters in x-ray generation has opened a new path for medical imaging technology advancement. With their outstanding electron emission capabilities, VACNTs provide a distinct advantage in miniaturizing and improving the performance of x-ray devices. This research focuses on the effect of electrical aging on x-ray imaging quality and the dose rate while using VACNTs as the electron source. The study includes a thorough examination of the electrical aging effects on VACNT-based x-ray systems, with an emphasis on changes in emission characteristics, beam stability, and the resulting variations in x-ray output. Experiment results show that electrical aging has a considerable impact on the performance of VACNT-based x-ray sources, with visible changes in electron emission parameters and subsequent consequences on x-ray imaging quality. Furthermore, the study investigates the relationship between electrical aging and the x-ray dose rate, providing vital insights into radiation exposure optimization in medical diagnostics.

Carbon nanotubes (CNTs) are an innovative kind of carbon that has received much attention due to their excellent electron field emission capabilities.1 They exhibit a one-of-a-kind structure, with a hollow cylindrical shape composed of carbon atoms organized in a hexagonal lattice. Iijima's discovery of carbon nanotubes in 1991 resulted in important improvements in the science of electron emission. Carbon nanotubes have been investigated as potential electron sources in a variety of applications, including x-ray tubes, due to their excellent properties such as high electrical conductivity, high aspect ratio, and low power consumption, making them ideal candidates for electron beam sources for x-ray tubes. Carbon nanotubes have also been demonstrated to have lower emission threshold fields than other emitter materials, resulting in greater x-ray tube brightness and intensity2–5 

Furthermore, when compared to metallic emitters, carbon nanotubes have shown outstanding field emission stability, ensuring continuous and dependable electron emission over long periods of time.6 However, understanding the long-term performance and reliability of these carbon nanotube-based electron sources is critical. An important topic of investigation is the influence of electrical aging on the x-ray dose rate and focal spot size (FSS) when employing a carbon nanotube-based electron source. The x-ray dose and focal spot size are critical characteristics that define x-ray imaging quality and accuracy.

The focal spot size of an x-ray tube corresponds to the size of the electron beam that impacts the target. The better the spatial resolution of the x-ray image, the smaller the focal point size. The x-ray dose rate, on the other hand, refers to the amount of radiation delivered to the target during x-ray operation.7 Understanding the effects of electrical aging on these features is critical for assessing the long-term stability and performance of carbon nanotube-based electron sources. Researchers can get insights into how carbon nanotube electron sources degrade with time and how this affects the quality and accuracy of x-ray imaging by examining the electrical aging effects on these sources.

Electrical aging is the gradual deterioration or change in the performance of an electron source caused by extended usage or exposure to electrical current.8 An intentional and regulated process known as “electrical aging” involves subjecting a vertically aligned carbon nanotube (VACNT) to prolonged electrical voltage in order to modify or enhance certain structural, electronic, or functional properties. We used electrical aging for a period of 1 h to strengthen the VACNT structure. The goal was to improve the quality and stability of emission current under DC operated high bias conditions by addressing the vulnerability of weak carbon nanotube tips within the (VACNT) structure. The results indicated that the maximum anode emission current was 0.68 mA post-x-ray imaging at gate voltage 2200 V, confirming the effectiveness of the electrical aging process in strengthening the structural robustness of the VACNT. The aging operation operates at an anode bias of 5 kV, while the x-ray imaging operation operates at 65 kV. The focal spot size of electron sources in carbon nanotube-based x-ray tubes may change due to electrical aging. The focal spot size is an area of the x-ray beam that is concentrated on the target. Electrical aging can cause changes in carbon nanotube emission parameters, such as a drop in the emission current or a rise in the beam divergence. These modifications may eventually result in an increase in the focal spot size, lowering the spatial resolution of the x-ray images. Additionally, electrical aging can have an effect on the x-ray radiation delivered to the target.9–12 As a result, as the electron source ages, the x-ray dose may become less consistent and predictable, thereby impacting diagnostic imaging accuracy and treatment planning capabilities. Thorough research and experiments are necessary to fully grasp how electrical aging affects the focal spot size and the x-ray dose in electron sources based on carbon nanotubes. In earlier research, electrical aging was applied to remove the catalyst, leading to the opening of tips and an increase in the emission current.13 Moreover, when exposed to high currents during electrical aging, resistive heating removes less robust carbon nanotube emitters. The heat produced by the CNT emitter's resistance during electron emission may alter the structure of the emitters. The joule heat enhances the crystallinity of the CNT emitters, leading to consistent electron emission.14–17 Understanding the intricate relationship between electrical aging and VACNT-based x-ray system performance is critical for improving the reliability and safety of medical imaging devices. The findings of this study contribute to current efforts to optimize carbon nanotube technology for medical applications, addressing difficulties relating to electron source lifespan and stability for enhanced x-ray imaging in medical applications.

The present study investigates the electrical aging effects on VACNT-based x-ray systems in a comprehensive manner, with a focus on changes in emission characteristics, beam stability, and subsequent variations in the x-ray dose rate.

The design in Fig. 1(a) illustrates the C-beam module, we fabricated for analyzing electron field emission to get high current. In the meantime, the schematic of the C-beam that makes use of VACNTs is depicted in Fig. 1(b). This C-beam consists of the 75 × 75 μm2 subisland arranged in a 1 × 10 array consisting of 36 VACNT emitters in each subisland.18 As seen in Fig. 2(a), this C-beam is loaded into the home-based x-ray system. Figure 2(b) is the SEM micrograph of the VACNT grown on the Si wafer, whereas Fig. 2(c) shows the aligned C-beam with gate mesh. The schematic of a dosimeter and x-ray detector for a home x-ray system using a reflection-type setup is shown in Fig. 3. Regarding SEM micrographs, please see the VACNT fabrication in Sec. II B.

FIG. 1.

(a) and (b) Schematic of the C-beam module and design, respectively.

FIG. 1.

(a) and (b) Schematic of the C-beam module and design, respectively.

Close modal
FIG. 2.

(a) Actual photo of the C-beam loaded into the x-ray system. (b) SEM micrographs of 1 × 10 emitter array. (c) Actual photo of the aligned C-beam into the gate mesh.

FIG. 2.

(a) Actual photo of the C-beam loaded into the x-ray system. (b) SEM micrographs of 1 × 10 emitter array. (c) Actual photo of the aligned C-beam into the gate mesh.

Close modal
FIG. 3.

Reflection-type x-ray system schematic.

FIG. 3.

Reflection-type x-ray system schematic.

Close modal

The multiarray VACNT emitter was grown on a silicon (Si) wafer substrate using direct current plasma-enhanced chemical vapor deposition (DC-PECVD). The resulting emitter of CNTs has unique structural properties. A radiofrequency magnetron was used to sputter a Ni catalytic layer on a silicon (Si) substrate. A photoresist layer containing micrometer-scale CNT seeds was used, and a conventional photolithography technique was developed.19,20 A DC-PECVD setup with a substrate bias of −600 V and a gate bias of +300 V was used to synthesize CNTs on the seed layer. For the purpose of carbon nanotube growth, ammonia (NH3) and acetylene (C2H2) gas with a ratio of 200 and 18 SCCM was maintained for 2 h. The seed layer temperature was kept at 850 °C while the DC-PECVD chamber's pressure was kept at 2 Torr.2,17 Figure 2(b) shows the SEM micrographs of vertically aligned CNTs that were examined with a HITACHI FESEM model (S-4700). CNTs showed an overall height of 40 μm, a pitch (distance between CNT dots) of 15 μm, and an average CNT dot size of 3 μm. The Si substrate, on which carbon nanotubes were produced, stayed remarkably clean and devoid of any impurities or debris.

The C-beam module was incorporated into a home-made reflection-type system in order to evaluate x-ray imaging, shown in the Fig. 3 schematic design. The anode target center, which had a tungsten (W) target covered in high thermal conductivity copper, was positioned 15 mm from the gate electrode and had a 12° angle. The low pressure of 3.0 × 10−7 Torr in the chamber was maintained by the employment of a Materion (PS-200) turbo-molecular pump (TMH 261P). The chamber window was fabricated with a 127 μm thick layer of 98.5% pure beryllium (Be). X-rays were produced utilizing a consistent positive bias of 65 kVp with a pulse-supplied gate voltage.

Spellman ST 120P2 was used to power the anode, and Keithley 2290 E-5 power supply was used to power the gate. Using an Agilent 34401A digital multimeter, the cathode current was measured. A tungsten (W) crossed-wire 1 mm-thick was placed in between a flat panel detector (Vieworks, Vivix-S1012N) and an x-ray source in order to test the resolution. The setup is shown schematically in Fig. 3, where the dose rate monitoring and image acquisition are carried out at a fixed magnification of 3.

The x-ray measurement was performed primarily to evaluate the C-beam resolution. We measured x-ray resolution using the EN-12543-5 standard method, which makes use of a W wire gray level line profile. In this method, we measure the intensity line profile of the generated x-ray image by inserting a W wire with 1 mm diameter on the x-ray chamber window. The focal spot characteristics, which are defined by a two-dimensional distribution of intensity in the object plane, have a substantial influence on the resolution and quality of x-ray pictures. Depending on projected magnification, the x-ray system resolution was calculated using the intensity profile. By assessing resolution from multiple angles, this technique offers a thorough evaluation of an x-ray system's performance in different imaging fields.

Figure 1(a) shows a C-beam module with a cathode, gate mesh, CNT electron source, and gate electrode, whereas Fig. 1(b) is the schematic of the C-beam design. Figure 2(a) is the actual photographs of the C-beam module (depicted in the red box) loaded in the home-based x-ray chamber, and Fig. 2(b) is the SEM micrographs of the 1 × 10 C-beam module grown on the Si wafer. Figure 2(c) depicts the VACNT aligned with a gate mesh, while Fig. 3 depicts the x-ray system schematic of the x-ray detector and dosimeter at 340 mm distance away from the source.

Before taking x-ray images, we looked into the C-beam's I-V characteristics both without and with aging. The gate voltage was methodically adjusted in 50 V increments from 400 to 2200 V while maintaining a uniform anode voltage of 5 kV. After various x-ray imaging, Figs. 4(a) and 4(b) convey the I-V characteristics. The current-voltage (I-V) measurement at a gate voltage of 1550 V revealed the reduction in the anode current. The anode current reduced from 1.1 to 0.10 mA after multiple x-ray imaging. During high bias (i.e., post-x-ray acquisition), the emission current not only decreased by 90%, but the threshold voltage also shifted by nearly 200 V. Following x-ray imaging, we examined the C-beam and VACNT structures. We observed that the VACNT structure had some deterioration, as seen in Fig. 5(a). Hence, we implemented a structural hardening process through electrical aging with the C-beam. The structure of the VACNT exhibited a significant enhancement following the aging process, as depicted in Fig. 5(b). We adapted the electrical aging process after C-beam fabrication and followed the same process of I-V and x-ray imaging. After 1 h electrical aging of the C-beam, the anode emission current even after multiple x-ray imaging displayed no significant reduction, and the threshold remained unchanged, as illustrated in Figs. 4(a) and 4(b). We obtained an anode current of 0.68 mA at a gate voltage of 2200 V, as shown in Fig. 4(b).

FIG. 4.

(a) Anode current I-V curves at the triode configuration with respect to the varying gate voltages and the fixed anode bias at 5 kV without electrical aging for before and after high bias, (b) anode current I-V curves at the triode configuration with respect to the varying gate voltages and the fixed anode bias at 5 kV with electrical aging for before and after high bias.

FIG. 4.

(a) Anode current I-V curves at the triode configuration with respect to the varying gate voltages and the fixed anode bias at 5 kV without electrical aging for before and after high bias, (b) anode current I-V curves at the triode configuration with respect to the varying gate voltages and the fixed anode bias at 5 kV with electrical aging for before and after high bias.

Close modal
FIG. 5.

(a) SEM micrographs of the VACNT without aging pre-x-ray imaging with inset magnified VACNT tip, (b) SEM micrographs of the VACNT without aging post-x-ray imaging, (c) SEM micrographs of the VACNT with aging pre-x-ray imaging with inset magnified VACNT tip, (d) SEM micrographs of VACNT without aging post-x-ray imaging.

FIG. 5.

(a) SEM micrographs of the VACNT without aging pre-x-ray imaging with inset magnified VACNT tip, (b) SEM micrographs of the VACNT without aging post-x-ray imaging, (c) SEM micrographs of the VACNT with aging pre-x-ray imaging with inset magnified VACNT tip, (d) SEM micrographs of VACNT without aging post-x-ray imaging.

Close modal

The structural hardness of VACNT was proved as shown in the SEM micrographs of Fig. 5(d); it is evident that without electrical aging, the anode current exhibited favorable emission properties before high bias (i.e., pre-x-ray acquisition). The decline in the anode emission current for the without aging C-beam was attributed to the vulnerability of the weak CNT tips, impacting the emission quality. As a result, the height of the VACNT decreases from 35 to 30 μm after the aging process. This assertion was corroborated by SEM micrographs shown in Figs. 5(a) and 5(b).

Along with the I-V characteristics, we evaluated the stability test of the C-beam after electrical aging for 1 h, as shown in Fig. 6. The results demonstrate that with electrical aging, the C-beam showed not only a greater current but also a stable emission current. In our previous work, we showed a stability test for nearly 20 h.20 So, we could enhance the structural hardness of the VACNT and sharpness. By plotting the current-voltage (I-V) curves, utilizing the Fowler–Nordheim equation helped us to clarify the features of field emission resulted in current.21 

FIG. 6.

Stability of the emission current over a 60 min period.

FIG. 6.

Stability of the emission current over a 60 min period.

Close modal

The effect of without and with electrical aging on the FSS was investigated. The FSS is divided into two parts: horizontal FSS (HFSS) for the beam's short axis and vertical FSS (VFSS) for the beam's long axis. Using the EN-12543-5 standard, we measured the blurriness of tungsten (W) wire,22–25 and we achieved an FSS HFSS of 0.58 and VFSS of 0.76 mm, respectively, as shown in Figs. 7(a) and 7(b). With aged C-beam, we conducted the evaluation again and were able to get focal spot sizes (FSS) of HFSS 0.34 and VFSS 0.55 mm, respectively. As we can see that without aging, C-beam x-ray image resolution is unsharp in both horizontal and vertical directions as compared to the electrically aged C-beam, we attribute this to the sharping of the CNT tip during the aging process as depicted in the SEM micrographs, shown in the Fig. 5(d). During high-current electrical aging, resistive heating removed very weak CNT emitters. In addition, the heat generated by the CNT emitter's resistance during electron emission might have caused structural changes in the CNT emitters along with the joule heating improving the overall crystallinity of the CNT emitters, which resulted in a steady electron flow and, hence, high resolution. The commercially available free ImageJ tool was used to calculate the gray level of the x-ray image intensity. We assessed the FSS calculation through the x-ray image line profile, considering two components: horizontal focal spot size HFSS and vertical focal spot size VFSS, as illustrated in Figs. 7(b) and 7(d). The higher values of HFSS and VFSS in the without aging C-beam suggest poor resolution, in contrast to the with aging C-beam, which exhibits lower HFSS and VFSS values, indicating superior resolution.

FIG. 7.

(a) W cross-wire FSS x-ray images without aging, (b) W cross-wires FSS x-ray images with aging, (c) and (d) without and with aging x-ray gray level, respectively.

FIG. 7.

(a) W cross-wire FSS x-ray images without aging, (b) W cross-wires FSS x-ray images with aging, (c) and (d) without and with aging x-ray gray level, respectively.

Close modal

We measured the x-ray dose using commercially available dosimeter, make IBA model MagicMaX (Application CRC: 3412FF2F), as shown in Fig. 8. The dosimeter was placed 340 mm away from the source. The dose rate without aging was 0.55 mGy/s and was measured at anode bias Va = 65 kV, gate bias Vg = 1700 V, and mAs = 0.24. With aging, we were able to obtain a dose rate of 1.1 mGy/s at mAs = 0.45, anode bias Va = 65 kV, and gate bias Vg = 2200 V. The aging impact on the C-beam enabled us to achieve a uniform and consistent dose rate even at high gate bias. It is possible to achieve a high emission current by ensuring a high electric field through high gate bias. Hence, aging plays a significant role in acquiring a high dose through high bias. Moreover, the calculated area under the curve for the aged C-beam was twice as large as that for the without aging C-beam. In practical terms, we observed a 1.87-fold increase in irradiation mAs. This outcome unequivocally illustrates that the x-ray efficiency (dose/mAs) of the without aging C-beam is lower than that of the aged C-beam. Thus, we achieved a nearly twofold improvement in x-ray efficiency through the aging process.

FIG. 8.

X-ray dose for both the cases without and with electrical aging.

FIG. 8.

X-ray dose for both the cases without and with electrical aging.

Close modal

In conclusion, an in-depth study of the electrical aging effects on VACNT-based x-ray systems indicates substantial implications for their performance, particularly in medical applications. The study focused on changes in emission characteristics, beam stability, and x-ray production variations during long operational durations, imitating real-world conditions. Without aging, the I-V measurement after high bias (i.e., after multiple x-ray measurement at 65 kV anode bias), we observed a reduction in the anode current and obtained the lowest anode current of 0.1 mA, whereas with aging, we achieved an improved anode current ranging from 1.0 to 0.68 mA. Similarly, the FSS was substantially blurred without aging, with HFSS = 0.58 mm and VFSS = 0.76 mm, but improved with aging with HFSS = 0.34 mm and VFSS = 0.55 mm. Furthermore, we achieved a higher dose rate, i.e., nearly 1.1 mGy/s with the aged C-beam. The experimental investigations provided notable results, demonstrating that electrical aging with C-beam has a significant impact on the electron emission parameters of VACNT-based x-ray sources.

This work was supported by the Technology Innovation Program (No. 20013595, Extreme ultraviolet light source using nano electron beam) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

The authors have no conflicts to disclose.

Ketan Bhotkar: Data curation (equal); Investigation (equal); Methodology (equal); Resources (equal); Writing – original draft (equal). Yi Yin Yu: Investigation (equal); Resources (equal). Jaydip Sawant: Investigation (equal); Resources (equal). Kyu Chang Park: Conceptualization (equal); Investigation (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.

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