Preliminary characterization of the response of an organic field effect transistor to ionizing radiation

Design optimization of a 1-D array of stemless plastic scintillation detectors

BACKGROUND

A stemless plastic scintillation detector (SPSD) is composed of an organic plastic scintillator coupled to an organic photodiode. Previous research has shown that SPSDs are ideally suited to challenging dosimetry measurements such as output factors and profiles in small fields. Lacking from the current literature is a systematic effort to optimize the performance of the photodiode component of the detector. An optimized detector could permit a reduction in detector element size, thus improving spatial resolution without degradation of the signal to noise ratio values seen previously.

PURPOSE

SPSDs use an organic photodiode coupled to a plastic scintillator to measure ionizing radiation fields. The design retains the benefits of plastic scintillation detectors (energy and dose rate independence, no perturbation factors, etc.) but avoids the challenges of optical fiber-based systems (Cerenkov radiation). In this work, the design of a 1-dimensional array of SPSDs is optimized to maximize the measured signal.

METHODS

ITO-covered PET was etched using hydrochloric acid, and the substrate was cleaned.

PEDOT

PSS and P3HT: PCBM (different weight ratios) were then applied to the substrate using spin-coating. Finally, aluminum top electrodes were added using vacuum thermal evaporation to complete the fabrication process. The variables studied for the optimization included: spin coater's speed (i.e., film thickness), P3HT: PCBM ratio, solution concentration, and scintillator coating.

RESULTS

Increasing the film thickness from ∼80 nm to ∼138 nm increased the measured signal by a factor of approximately 7.7. Changing the ratio of P3HT to PCBM from (1:1) to (4:1) resulted in approximately 3.5 times higher signal. Additionally, increasing the total concentration of the solution from 2% to 4% by weight ratio increased the signal by roughly a factor of 2.5 for a P3HT: PCBM ratio of 2:1. However, for a P3HT: PCBM ratio of 4:1, increased solution concentration reduced measured signals to approximately 1.7 times lower than normal concentration. Covering the air gaps of the etched scintillator with white paint resulted in a signal increase of about 2.2 times higher compared to black paint.

CONCLUSION

An optimization process was conducted to improve the signal output of the radiation detector, which consisted of a 1-dimensional photodiode array combined with a scintillator. This approach has resulted in a sensitivity increase of about 24 times compared to the original sample prior to optimizing the fabrication parameters and scintillator's properties (∼0.02 nC/cGy vs. ∼0.5 nC/cGy). The most efficient device was found to have a weight ratio of (2:1) P3HT: PCBM and a total solution concentration of 4%. Additionally, using a scintillator painted white was found to produce superior outcomes compared to black paint.

Fabrication and characterization of a stemless plastic scintillation detector

PURPOSE

To fabricate a stemless plastic scintillation detector (SPSD) and characterize its linearity and reproducibility, and its dependence on energy and dose per pulse; and to apply it to clinical PDD and output factor measurements.

METHODS

An organic bulk heterojunction photodiode was fabricated by spin coating a blend of P3HT and PCBM onto an ITO-coated glass substrate and depositing aluminum top contacts. Eljen scintillators (~5 × 5 × 5 mm³ ; EJ-204, EJ-208, and EJ-260) or Saint-Gobain scintillators (~3 × 3 × 2 mm³ ; BC-400 and BC-412) were placed on the opposite side of the glass using a silicone grease (optical coupling agent) creating the SPSD. Energy dependence was measured by using 100, 180, and 300 kVp photon beams from an orthovoltage treatment unit (Xstrahl 300) and 6 and 10 MV photons from a Varian TrueBeam linear accelerator. Linearity, dose per pulse dependence, output factors, and PDDs were measured using a 6 MV photon beam. PDDs and output factors were compared to ion chamber measurements. A control device was fabricated by substituting polystyrene (PS) for the P3HT/PCBM layer. No photocurrent should be generated in the control device and so any current measured is due to Compton current in the electrodes, wires, and surroundings from the irradiation. Output factors were corrected by subtracting the signal measured using the control device from the photodiode measured signal to yield the photocurrent.

RESULTS

Each SPSD had excellent linearity with dose having an r² of 1 and sensitivities of 1.07 nC/cGy, 1.04 nC/cGy, 1.00 nC/cGy and 0.10 nC/cGy, and 0.10 nC/cGy for EJ-204, EJ-208, EJ-260 (5 × 5 × 5 mm³ volumes), BC-400, and BC-412 (3 × 3 × 2 mm³ volumes), respectively. No significant dose per pulse dependence was measured. Output factors matched within 1% for the large scintillators for field sizes of 5 × 5 cm² to 25 × 25 cm² , but there was a large under-response at field sizes below 3 × 3 cm² . After correcting the signal of the small scintillators by subtracting the current measured using the PS control, the output factors agreed with the ion chamber measurements within 1% from field sizes 1 × 1 cm² to 20 × 20 cm² . The impact of Cerenkov emissions in the scintillator was effectively corrected with a simple reflective coating on the scintillator. In comparison to a 6 MV photon beam, the large scintillator SPSDs exhibited 37%, 52%, and 73% of the response at energies 100 kVp, 180 kVp and 300 kVp, respectively.

CONCLUSION

The principle of the SPSD was demonstrated. Devices had excellent linearity, reproducibility, and no significant dose per pulse dependence, and a simple reflective coating was sufficient to correct for Cerenkov emissions from within the scintillator. The devices demonstrated similar energy dependence to other scintillator detectors used in a radiotherapy setting.

Organic Field-Effect Transistors as Flexible, Tissue-Equivalent Radiation Dosimeters in Medical Applications

Radiation therapy is one of the most prevalent procedures for cancer treatment, but the risks of malignancies induced by peripheral beam in healthy tissues surrounding the target is high. Therefore, being able to accurately measure the exposure dose is a critical aspect of patient care. Here a radiation detector based on an organic field-effect transistor (RAD-OFET) is introduced, an in vivo dosimeter that can be placed directly on a patient's skin to validate in real time the dose being delivered and ensure that for nearby regions an acceptable level of low dose is being received. This device reduces the errors faced by current technologies in approximating the dose profile in a patient's body, is sensitive for doses relevant to radiation treatment procedures, and robust when incorporated into conformal large-area electronics. A model is proposed to describe the operation of RAD-OFETs, based on the interplay between charge photogeneration and trapping.