Magnetron Sputtered CsPbCl3 Perovskite Detectors as Real-Time Dosimeters for Clinical Radiotherapy

Abstract

The aim of this study is to investigate the feasibility of manufacturing thin real-time relative dosimeters for clinical radiotherapy (RT) with potential applications for transmission monitoring in vivo dosimetry and pre-treatment dose verifications. Thin (≈1μm) layers of a high sensitivity, wide bandgap semiconductor, the inorganic perovskite CsPbCl₃, have been grown for the first time by magnetron sputtering on plastic substrates equipped with electrode arrays. Prototype devices have been tested in real-time configuration to evaluate the dose delivered by a 6MV photon beam from a linear accelerator. Linearity of the charge with the dose has been verified over three order of magnitudes, linearity of the current signal with the dose rate has been also successfully tested in the range 0.5-4.3Gy/min. The combination of high sensitivity per unit volume and wide bandgap provides high signal-to-noise ratios, up to 70, even at moderate applied voltages. The Schottky diode configuration allows the detector to operate without bias voltage (null bias).The blocking-barrier structure allows to confine the active volume within sub-millimetric sizes, a quite attractive feature in view to increase granularity and achieve the high spatial resolutions required in modern RT techniques. All the above-mentioned features indeed pave the way to a novel generation of flexible, transmission, real time dosimeters for clinical radiotherapy.

Keywords: Dosimeters; Metal halide perovskite; Radiation detectors; Radiotherapy; Thin films.

Figure 1

(a) 1.3 μm thick CsPbCl₃ film deposited by magnetron sputtering on a plastic substrate equipped with a linear array of 5 Cu electrodes, (b) the set-up for dosimetric characterization of the CsPbCl₃ device at the Radiotherapy Unit of the University of Florence with the device placed in the PMMA phantom.

Figure 2

Optical and electrical characterization of the CsPbCl₃ film grown by magnetron sputtering. (a) Transmittance spectrum at room temperature; (b) Current – voltage characteristics measured at room temperature in dark (electrodes on contact pads 2 and 4).

Figure 3

CsPbCl₃ device measured under a 6 MV photon beam from a linac when a 10 V external voltage is applied between 2 and 4 electrodes. (a) Current signal for two consecutive irradiations with different doses (2 Gy and 1 Gy) but equal dose rate of 4.26 Gy/min. (b) Same data shown in a logarithmic scale to visualize both the signal and the background current measured when the beam is off.

Figure 4

(a) I-V characteristics of the CsPbCl₃ device measured under a 6 MV photon beam from linac with 4.26 Gy/min dose rate (blue) and in dark (red) (b) Current signal measured with the CsPbCl₃ device under the 6 MV photon beam with three different dose rates, null bias applied. (c) Current signal as a function of the dose-rate and best-fit showing a linear behavior.

Figure 5

Signal-to-noise ratio measured with the CsPbCl₃ device under the same 6 MV photon beam and dose-rate as a function of the external voltage.

Figure 6

(a) Current signal of the CsPbCl₃ device with 10 V applied voltage, measured under a 6 MV photon beam with 4.26 Gy/min dose rate, at increasing doses from 0.02 Gy to 10 Gy. (b) Charge collected as a function of the dose, obtained integrating signals of Fig. (a). Log-log plot in inset evidences that linearity holds on over three orders of magnitude.

Figure 7

Current signal measured by the CsPbCl₃ device, 10 V applied voltage under a 6 MV photon beam from linac with different dose-rates. Inset: Mean current at each dose-rate plotted as a function of the dose-rate and linear best-fit.

Figure 8

(a) Energy diagram of isolated Cu/CsPbCl₃/Cu materials in case of p-type conductivity of the semiconductor. (b) Sketch of the equivalent circuit of our Cu/CsPbCl₃/Cu two-terminal device, made of a resistence in series with two back-to-back Schottky barriers, band diagram evidences the bending due to the Schottky barriers at equilibrium.

Figure 9

Current -voltage characteristics of the CsPbCl₃ device measured under 6 MV X-photons with a dose rate 4.26 Gy/min. Best-fit (black solid line) is obtained as a square-root function of the voltage within $\pm 30V$. For higher applied voltages, the I-V characteristics is flattening, green and red lines evidence plateau values.

Figure 10

Mean ionization energy of various semiconductors plotted as a function of the energy gap. Region within the two black lines indicates the range covered by the Klein's rule in eq. (2). Those semiconductors obeying the Klein's rule are shown in blue. Other data concern materials not obeying the rule. Linear best-fit for halide semiconductors is also given as a guide-line. Mean ionization energy measured in this work for magnetron sputtered CsPbCl₃ film is shown as a black square, in green the value for crystalline CsPbCl₃ samples .