Organic photodiode detectors for heavy ion beam measurement (#1118)
F. Nishikido1, E. Takada2, M. Nogami2, G. Shikida2, M. Nitta1, 3, G. Hirumi1, 3, T. Yamaya1
1 National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Department of Radiation Measurement and Dose Assessment, Chiba, Japan
Organic photodiodes (OPDs) are thin, flexible, printable and inexpensive devices. Therefore, radiation detectors using OPDs are expected to be suitable for carbon therapy dosimeters. However, characteristics of the OPDs for high energy heavy ion measurements have not been investigated deeply. Therefore, we are investigating OPDs as radiation detectors for heavy ion beam irradiation.
The device structure of the OPD detectors used in the experiments consisted of layers of IZO (100 nm)/ PEDOT: PSS (30 nm) / PCBM: P3HT (200 nm) / Al (70 nm). The sizes of the sensing surface areas for the evaluated detectors were 1 mm × 1 mm, 2 mm × 2 mm, 3 mm × 3 mm and 8 mm × 4 mm. The OPDs were fabricated on 10 mm × 10 mm black ABS boards by spin coating; fabrication was done 3 days before an experiment. In the evaluation experiment, the OPD detectors were mounted in aluminum boxes for shielding. The Al and IZO electrodes were connected to readout wires with silver paste. Bias voltage was not applied in any measurements. The experiment was performed in the PH2 course of HIMAC at NIRS. The energy of the 12C beam was 290 MeV/u. The OPDs were irradiated by the 12C beam through an ion chamber to normalize the number of irradiated particles. The beam intensity was 106 to 108 particles per second (pps). The diameter of the 12C beam was 1 cm at the OPD.
We measured charges induced by the carbon beam irradiation with all the fabricated OPD detectors. Beam spill structure of 3.3 s cycle could be clearly observed at 108 pps with 0.1 s interval. The OPD detectors had similar beam intensity dependence as a reference ionization chamber and sufficient linearity to beam intensity under 1×108 pps. Bragg curves and Bragg peaks were clearly observed for all OPDs. However, compared with the ionization chamber, the results of the OPD detectors were saturated around the Bragg peak.
Keywords: organic photodiode, Carbon therapy, dosimeter
Solid-state avalanche amorphous selenium detector design and fabrication methods (#1403)
J. R. Scheuermann1, S. Leveille2, K. Tanioka1, M. Hansroul2, W. Zhao1
1 Stony Brook University, Radiology, Stony Brook, New York, United States of America
Avalanche multiplication in amorphous selenium (a-Se) has been used successful in optical imaging to amplify photo generated charge, and achieve high sensitivity with no additional noise. We have developed the first large area, solid-state high gain avalanche rushing photoconductor (HARP) for an indirect x-ray flat-panel imager. Avalanche gain requires high electric fields of ~100 V/µm which leads to challenges in detector fabrication. Blocking layers are required to prevent charge injection from electrodes, which lead to dark current. Since only holes avalanche, the dark current is dominated by hole injection. Therefore the hole blocking layer (HBL), deposited after the P-layer and I-layer, is the most critical. During fabrication the temperature of a-Se should be kept < 60 degree C to prevent the formation of polycrystalline aggregates in a-Se, therefore, room temperature hole blocking layers or novel fabrication methods must be developed. In this work we present two approaches to HARP development. First we consider an inorganic n-type oxide to provide a high injection barrier. The second strategy is a fabrication technique in which high temperature electron and hole blocking layers are deposited on separate substrates followed by a thin layer of a-Se. We then thermally fuse the two layers of a-Se together to form a continuous HARP structure. Time-of-flight (TOF) measurements confirm charge transport across the interface of the two layers. Dark current measurements from the new HBL is compared with that from a Se-metal interface alone. The oxide was shown to provide a larger injection barrier, however not sufficient to minimize dark current at avalanche fields. This can be mitigated by adding a thin, hole-trapping layer at the positive bias interface. Nonetheless an avalanche gain of two was achieved. Thermally fusing two layers of a-Se is shown to create a single uniform layer with both electron and hole transport across the interface.
Keywords: selenium, x-ray detectors, solid-state, avalanche gain
Highly Sensitive, Broad-band, Organic-Inorganic Hybrid Direct X-ray Detectors (#2071)
H. Thirimanne1, I. Jayawardena1, I. Bandara1, A. Nisbet2, 3, C. Mills1, R. Silva1
1 University of Surrey, Advanced Technology Institute, Guildford, Surrey, United Kingdom of Great Britain and Northern Ireland
Increasing the path lengths of photons within the active area of a device is a key factor in increasing the detection sensitivity in optoelectronic structures. For single and polycrystalline structures, this necessitates thick crystals with consequent high costs, reduced flexibility and high operational voltages. Such considerations are also relevant to current X-ray radiation detectors. As such, a low cost, low voltage X-ray detector which is highly sensitive over a broad energy range, would be of immense interest. In this work, we present a novel optimized ‘inorganics-in-organics’ hybrid semiconductor detector using a conjugated polymer blend incorporating high atomic number (Z) nanoparticles. The inclusion of the high Z nanoparticles improves X-ray attenuation compared to the polymer blend alone. High sensitivities of 22,000 e nGy-1 mm-2 for 50 kV soft X-rays and 5,400 e nGy-1 mm-2 under 15 MV hard X-rays generated by a medical linear accelerator (LINAC) have been obtained, importantly, at -10 V bias. This novel hybrid detector architecture allows for real time radiation monitoring, with the added advantage of being processed at temperatures compatible with flexible substrates.
Keywords: Direct X-ray detector, Inorganic-organic hybrid nanomaterial, photonic devices, broadband detection
TlSbS2, a Two-dimensional Structure Semiconductor for Hard Radiation Detection (#3168)
W. Lin1, H. Chen1, J. He2, Z. Liu2, S. Das2, J. - I. Kim2, K. M. McCall2, B. W. Wessels2, M. G. Kanatzidis1
1 Northwestern University, Chemistry, Evanston, Illinois, United States of America
We report a two-dimensional structure semiconductor compound TlSbS2 as a new hard radiation detection material. This compound crystallizes in the triclinic P1 space group, has a direct bandgap of 1.67 eV and high chemical stability. Electronic band structure calculations suggest that the facile charge transport is along the (0l0) cleavage planes. Thanks to the nature of congruent melting at 484 oC, large-size single crystals were grown from stoichiometric melt by Bridgman method, and then fabricated as detection device. The device exhibits a high resistivity above 1011 Ω·cm, which guarantees a low noise signal for detection. The device detects 22 keV Ag X-rays and 5.5 MeV a-particles from 241Am. The mobility-lifetime product for electrons along the perpendicular direction with respect to the cleavage planes was estimated as 2.4 × 10-6 cm2·V-1, based on spectral response against 5.5 MeV a-particles. Drift mobility measurements based on electron rise time of pulse induced by a-particles reveals an electron mobility of 3.3 ± 0.66 cm2·V-1·s-1.
Keywords: Crystal growth; G-ray detection; 2 D; Semiconductor
Charge drift in organic liquids at room temperature (#3282)
M. Bergevin1, S. Dazeley1, K. Gregory1, S. Sangiorgio1, N. Walsh1
1 Lawrence Livermore National Laboratory, PLS/NACS, Livermore, California, United States of America
Room temperature organic liquid Time Projection Chambers (TPCs) have great potential, with applications as high precision antineutrino physics detectors, neutron camera imaging devices and dark matter detectors. It is known that electronegative impurities, such as water and oxygen can affect the drift of charge through insulating liquids and, can in turn, impact the functionality of TPCs. Purification is therefore key to improving this technology. We will present progress made at LLNL towards the purification of a selection of organic liquid scintillators and present the impact of purification on electron-drift properties. We will describe the experimental apparatus used to evaluate charge drift-properties. We will also summarize our plans for a new organic-liquid noble-gas detector called DARTH TPC (Demonstration of A Room-Temperature Hydrogenous Time Projection Chamber).
This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344, release number LLNL-ABS-730822.
Keywords: organic liquid, room temperature