## 2 Materials and Methods  
### Apparatus and Experimental Setup  
LEDs are solid-state devices and are fabricated using a combination of typical materials/elements to produce different emission wavelengths. For instance, LEDs emitting white light are composed of blue-emitting GaN and a yellow-emitting phosphor [38]. A phosphor is a photoluminescence material that converts short-wavelength light (high energy photons) to long-wavelength light (low energy photons) [37; 39]. Table 1 shows some examples of material combinations that are generally used for manufacturing various emission wavelengths of LEDs. Our experiments were executed using LED strips (12V DC SMD 5050) of five emission colors: cold white, warm white, red, green, and blue. The emission wavelength range for both the cold white and warm white LED strips was \(\sim\)380-750 nm. Red, green, and blue LED strip emission wavelength ranges were \(\sim\)630-640, 520-530, and 430-470 nm respectively. LED strips are manufactured by soldering discrete LED chips on a conducting strip. This strip comprises other electronic components such as resistors. In our study, this strip was 1 cm wide, 0.2 cm thick, 500 cm long, and it consisted of 60 LED chips per meter, hence 5 m had 300 LED chips. On the strip, each LED chip was positioned at a 12 mm interval and its active area was of a diameter of approximately 4 mm. The strip was also cuttable at a junction of every three chips and its base was self-adhesive. As a luminary, the strip operated under a 12-volt potential difference across its terminals. Such an LED strip is called an SMD (Figure 1). In this experiment, the LED strips were exposed to x-ray beams from an x-ray generator (Optimus 80, Philips). The beams originated from a broad focal spot (1.0 mm) and were perpendicularly incident on all LED chip sensitive areas and the dosimeter probe. The x-ray generator is controlled by microprocessors and consists of a control panel, a high voltage generator, and an automatic exposure control (AEC) device. All these generator components are housed in a standard casing. The generator's \begin{table}
\begin{tabular}{c c} \hline
**Materials** & **Emission Wavelength** \\ \hline InGaN/GaN, ZnS & 450–530 nm \\ GaP: N & 565 nm \\ AlInGaP & 590–620 nm \\ GaAsP, GaAsP: N & 610–650 nm \\ InGaP & 660–680 nm \\ AlGaAs, GaAs & 680–860 nm \\ InGaAsP & 1000–1700 nm \\ \hline \end{tabular} Source: The Encyclopedia of Laser Physics and Technology [40]. Note: In = indium, Ga = gallium, N = nitride, Zn = zinc, S = sulphide, P = phosphide, Al = aluminum, As = arsenide.  
\end{table} 
Table 1: Typical materials and emission wavelengths of light-emitting diodes. Figure 1: Images of (**a**) cold white, (**b**) warm white, (**c**) red, (**d**) green, and (**e**) blue light-emitting diode (LED) strips. non-AEC setting ranges for radiography are as follows: tube voltage (kVp), 40-150 kV; tube current, 1-1100 mA; tube current-time product (mAs), 0.5-850 mAs; and exposure time range, 1-6000 ms. Exposure of the LED chips to x-ray photons instantly resulted in a radiation-induced current flow in the strips. A multimeter connected to the strips measured the potential difference, a result of the radiation-induced current flow. Therefore, in our study, the obtained signal was referred to as a current-voltage (C-V) signal. The multimeter used in this study was a handheld and multi-functional digital multimeter/oscilloscope (Model JDS2012A, Jinhan Company: Hefei, Anhui, China). This model has a single channel and functions as both a digital oscilloscope (with a 200 MSa/s maximum sampling rate and bandwidth 20 MHz) and as a multimeter (4000 counts). It can measure DC/AC voltages, resistance, and capacitance, and it can test diodes and continuity. The dose of the x-rays incident on the LED strip was recorded using an easy-to-use pocket-sized radiation dosimeter (Unfors Xi, RaySafe).