The RL response shown in Fig. 4 demonstrates the characteristics of the Ge-doped optical fiber scintillator for different doses and dose rates. As shown in the figure, the RL response at high dose rates of 600 cGy/min has a slower ramp-up, akin to a plateau effect. This is likely the result of the scintillator’s reduced capacity to respond to high-dose-rate ionizing radiation, the plateau effect being less visible at lower dose rates, for example, at 100 cGy/min. A phosphorescence decay is also observed in the return to baseline of each pulse in the dose measurement, lasting for a few seconds in returning to the initial background baseline value. The phosphorescence emission is observed to be more prominent at higher dose rates, likely the result of the high number of decays due to the high dose rate of ionizing radiation. The RL response of the scintillator with varying dose for 12 MeV electron beam is illustrated in Fig. 5. The RL response is highly linear (R2 = 0.99971) for the dose rate in the range of 25 to 400 cGy. This demonstrates the suitability of this Ge-doped optical fiber scintillator to perform clinical dosimetric measurements within the intended dose range.
Figure 6 shows the accumulated photon counts with a 200 µs gating time, for the ionizing radiation from the linac irradiating only the PMMA optical fiber without any scintillator. For a fixed dose of 200 cGy, the contribution arising from the PMMA within the irradiation field (of much greater mass compared to that of the sensor itself) is of the order of 15%. The observed ‘top hat’ response for the lower dose rates (state range here) is indicative of an absence of pulse pile-up, the latter clearly only being seen at the higher rates (state range). The latter represents only a secondary effect for the given conditions. Figure 7 shows the dose rate dependency in terms of the integral dose for each of the dose rates given in Fig. 6. As seen, the integral dose at the lower dose rate is clearly greater than at the higher dose rate. This is due to the summation of the contribution from the phosphorescence decay from each pulse at the lower dose rate.
Figure 8 illustrates the high time resolution capability of the Ge-doped silica optical fiber scintillator, this enabling investigation to be made of the temporal dependency of the administered dose on a pulse-by-pulse basis, including the pulse train duty cycle (the ratio of the beam on and off time). The ability of the PMT, photon counting circuit, and scintillator fast rise and decay time to capture the single pulse profile indicates further the potential for this dosimetry be used for ultra-fast dose rate rad radiotherapy.
Single pulse time-resolved dose profiles measured using the analogue voltage output port of the MPPC detector are shown in Fig. 9, which presents clear evidence of intra-pulse sub-structure with resulting dose distribution variations. This effect results in the adjustment of dose deposition parameters in the high-resolution monitoring, allows an evaluation of the accuracy of dose delivery, potentially impacting normal tissue toxicity and tumour response. In the case of higher energies (18 MeV), the pulse amplitude is greater than that at lower energies (state value or range), reflecting greater dose deposition within the scintillator material. The scintillator produces pulses with distinct shapes, representing the real temporal profile of the emitted light within a single pulse. At 18 MeV, the scintillator pulse shape is seen to include less variability than at the lower energy values, also exhibiting faster rise and decay times and increased measured voltage amplitude (corresponding to beam intensity).
The sub-structure is suggested to originate from key components of the linac. For instance, the magnetron used for (the microwave power source of the Electa Synergy linac) accelerating the electrons. Variations in operation and interactions of the electrons potentially lead to fluctuations in the produced microwave power and hence the energy of the accelerated electrons, consequently impacting dose delivery. This possibility depends on the specific design of the linac. Figure 9 therefore indicates that the energy levels used in the linac could influence the characteristics of the radiation beam, including the sub-structure of the individual pulses. Greater energy levels generally lead to greater radiation output, potentially affecting the temporal distribution of dose delivery within each pulse. In principle, the sub-structure could change if the linac operating parameters, including the required beam energy, were to be altered. The results show the ability of silica-based scintillators to capture the distinct pulse shapes obtained at the different energies available in a standard-linac when operating in the electron mode.
Accordingly, one feature that could be investigated in electron FLASH radiotherapy dosimetry would be the observed intra-pulse variations residing within the microsecond duration pulses. To investigate this would potentially require a Ge-doped silica optical fiber scintillator coupled with a fast response MPPC capable of measuring high dose rates at high temporal resolution at high efficiency. Such facility could provide information on associated characteristics, including intensity variations, duration, and uniformity or irregularity of intra-pulse spacing. The results of Fig. 9 give a strong indication that he Ge-doped Silica optical fibre scintillator could be used in the ultra-fast FLASH linac devices which will be the focus of future work by the authors of this article.
The decay time is an important characteristic of all scintillators used in radiation detection and dosimetry. They determine the baseline value for the time resolution of the scintillator system, ultimately determining its ability to accurately measure the temporal distribution of the pulses and any intra-pulse sub-structure which is known to exist in the case of electron FLASH radiotherapy. The extended duration of light emission due to the scintillator material’s decay time limits the temporal resolution for accurately measuring the duration of one-pulse, as shown in Fig. 10. The decay time is indeed defined as the time it takes for the pulse signal to decrease from its maximum value (Vmax). Therefore, the calculation of the decay time determines the time when the pulse signal reaches 90% of its maximum value (0.9) and the time when it reaches 10% of its maximum value (0.1). The decay time is then calculated by subtracting the initial time value from the final time value. The decay time was found to be 435 ns, which makes it suitable for high-temporal resolution detection systems.
In electron FLASH radiotherapy, where high dose rates are delivered in single microsecond pulses, scintillators with fast decay times are essential. A fast decay time allows the accurate capture of rapid fluctuations and temporal characteristics that characterise the radiation pulses. Accordingly, a suitably doped optical fiber with a particularly rapid decay time (< 1 µs) may allow measurement of the sub-structure variations in electron FLASH radiotherapy.
The fast decay time (state value) of the scintillator of this investigation has indicated that it is well-suited to real-time monitoring of linac output pulses. The capability of the sensor, as currently shown, allows the temporal distribution of dose to be studied, considering the intensity variations within the pulses.
Figure 11 illustrates the rapid increase in signal output and hence confirms the ability of the scintillator material to capture the onset and initial rise of radiation pulses. Such high temporal resolution of the rapid fluctuations is vital for accurate assessment of the dose distribution and thus optimization of treatments. The fast rise time of the present scintillator (~ 140 ns), as shown in Figs. 9 and 10, would enable accurate time-resolved measurement of electrons of a typical FLASH-RT dose delivery, including capturing the sub-structure features of individual pulses. These temporal characteristics are essential for optimizing treatment planning, monitoring treatment delivery, and evaluating the effectiveness of FLASH radiotherapy in sparing healthy tissue while effectively treating tumours.