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Basics

The research activities of the Radiation Physics Group in the field of dosimetry are largely related to the
phenomenon of stimulated luminescence and its applications for measurements of ionizing radiation
doses. The specialty of our group is thermoluminescence, which has been studied and used in dosimetric
measurements at the Atominstitut already in the eighties of the twentieth century by Prof. N. Vana. In
recent years, more attention is also paid to optically stimulated luminescence.
(Radio)Thermoluminescence (TL) is the emission of light when some crystalline solids are heated up after
previously being irradiated with ionizing radiation. Thermoluminescent detector (TLD) crystals accumulate
energy deposited by ionizing radiation in interstitial energy levels (traps). Upon heating, the stored energy

is released as light emitted by the crystals whose intensity is proportional to the absorbed dose. Usually a
TL material has traps with different energy levels. Electrons in shallow traps will be able to leave traps at
lower temperatures than deeper traps. Therefore, a characteristic glow-curve (see Figure 1) can be
obtained when a TL material is heated with a linear temperature profile. TLD is recognized universally as
an excellent material for broad dosimetric applications as it exhibits high sensitivity, low fading, low
background and resistivity against environmental conditions. The small size of TLDs, tissue-equivalence
and lack of electronic and cabling make them favorable for use for in vivo or in phantom dosimetry,
without excessive impact on the results.
Our Group use the TL detectors for Space radiation monitoring and for dosimetry in radiotherapy beams.


Figure 1: TLD glow curve of MCP detector.

An important part of our work is currently the development of measuring techniques for microdosimetry and the use of microdosimetric detectors in dosimetry of complex radiation fields in radiotherapy. The microdosimetric distributions provide single-event spectra that relate the absorption of ionizing radiation in matter to the size of biological targets (e.g. cell). Typical microdosimetric investigations use tissue-equivalent proportional counters (TEPC) and solid-state detectors. Both measure ionisation events and derive distributions of energy deposition under the assumption that the amount of energy absorbed is proportional to the amount of ionisation recorded. Microdosimetric quantities may allow for new radiotherapy planning strategies, complementing the dose optimization, aiming to choose the radiation characteristics that guarantees a homogeneous biological response across the target volume.


Research Areas

Space dosimetry

Figure 2. MATROSHKA experiment at the ISS
Picture source: http://iss.jaxa.jp/spacerad/NI004.html

Increasing presence of humans in Space stimulated growing importance of exposure to cosmic radiation that cannot be perceived by the human senses. The astronauts are exposed to the effects of cosmic radiation on board the International Space Station (ISS) for a long time. The radiation doses are around 100 times higher than on Earth. According to radiation quality and exposure duration, health effects cannot be excluded entirely. The determination of the radiation dose, to which the astronauts are exposed, is therefore necessary to protect the health of the astronauts. Passive TL detectors offer an easy method for precise dose assessment in the complex mixed radiation environment aboard the space station.

Since 1991, the Atominstitut has been represented in space through the exposure of TDs for the detection of ionizing radiation as part of an Austrian-Russian cooperation through the project AUSTROMIR;. The acquired knowledge has been applied to numerous space missions such as ADLET, BIOPAN, LETVAR, RADO, MATROSHKA, DOSIS or DOSIS-3D in conjunction with international partners such as ESA, DLR, IMBP, etc. The experiments were designed to improve our understanding of the radiation environment onboard the ISS and to provide a set of data that can be used by the radiation research community for benchmarking of radiation transport codes, as well as for providing input to build a real 3D model of the radiation environment inside the ISS.

In the last two decades, a special evaluation technique (HTR method) for the radiation quality of an unknown field of ionizing radiation has been developed. This informative value should be challenged in the near future by the correlation with radiation biological defects.

In 2020 an exposure of TLDs to NASA's ORION EM-1 spacecraft in the context of the "MARE" project is planned.

Dosimetry for Radiotherapy

Figure 3. Human phantom for inside body measurements.

Proton radiotherapy allows reduction of the dose burden but still the existing knowledge and understanding of the out-of-field doses and associated risk of inducing secondary malignant neoplasms is not sufficiently mature to justify the use of modern techniques for treating children. Much of the concern is related to the carcinogenic risk from secondary neutrons, which are unavoidably produced by the beam modifying devices and tissues traversed. Furthermore, non-elastic nuclear reactions will also produce secondary protons, heavier ions and photons. Especially the secondary protons and heavier nuclei might have a significant detrimental effect on the surrounding healthy tissue and should be studied more carefully. Therefore, a full characterization of the secondary radiation following proton radiotherapy treatment requires special attention for radiation protection.

An important part of our research at MedAustron radiotherapy center addresses concerns in view of radiation protection in radiotherapy. Hence, a tissue equivalent human phantom equipped with different types of TL detectors is used for dose distribution measurements at the target and outside, in distant located organs being of particular risk for generating secondary cancers. Based on response of different TLD types, a method for radiation quality assessment is being recently developed.

In parallel with experimental tasks also numerical simulations are being performed. A numerical human phantom, developed on the base of CT scans, is being used for Monte Carlo transport for detailed dose distribution inside and outside the target volume. The results are going to be further compared with the measured data.

Microdosimetry

The absorbed dose as a macroscopic quantity works fine for radiation producing a fairly uniform pattern of energy deposition, such as high-energy x-rays or electrons, but fails to correlate well with the biological effects of radiation, where the energy deposition is concentrated around the particle tracks, such as hadron beams. For these beams, knowledge of the microscopic energy deposition patterns is needed to describe the physical influence of radiation on biological effects. To date, ion-beam therapy relies on correlations of biological data with the mean LET, generated by Monte Carlo simulations.


Within the recent research plan at MedAustron, our tasks address the applied microdosimetry investigations in proton and carbon radiotherapy beams with a close collaboration with MedAustron and the Radiobiology Group from the Medical University of Vienna. The research aims at finding a correlation of the physical parameters with the biological endpoints in the treatment field and outside, for further radiotherapy planning improvement. Microdosimetric quantities may allow for new radiotherapy planning strategies which complementing the dose optimization, aiming to choose the radiation characteristics that guarantees a homogeneous biological response across the target volume.

Thermoluminescence dating

Figure 4. 7th centuries BC Baku's Maiden Tower. Picture source: https://theculturetrip.com/europe/azerbaijan/articles/murky-past-bakus-maiden-tower/

Certain objects of archaeological finds, such as clay pottery or terracotta, and building structures, such as bricks or ceramic tiles, contain a small amount of quartz crystals. These tiny grains have more or less good TL properties according to their natural contamination. The arbitrariness of nature has thus created microscopic crystals that works as tiny TL dosimeters. These small radiation sensors are initialized by the burning of a ceramic object, called "the archaeological clock is zeroed". Thereafter, the crystals accumulate both a part of the everywhere and constantly present ionizing ambient radiation, as well as alpha, beta and gamma radiation of the intrinsic activity of the sample itself. Measurable samples are obtained via a special preparing process consisting of drilling or milling, fractionation of the sample granules and sedimentation of the grains of suitable size onto sample carriers. The sample plates produced in this way are measured fully automatically using the processing unit HVK, developed at the Atominstitut. The measured, resulting light intensity represents the archaeological radiation dose of the object to be dated. After the sample-specific TL light yield is obtained and the annual environmental dose including intrinsic activity is determined, then the age of the subject can be calculated. The method of TL dating can also be seen as a special form of environmental dosimetry.

In the last three decades, numerous buildings (e.g. Maiden Tower in Baku in Azerbaijan or the church in Waldhausen in Lower Austria) and countless ceramic finds have been dated at the Atominstitut. The current tasks include both real dating of objects and work to improve the evaluation technique of TL dating.

Main equipment

ATI developed reader

Figure 5. TL-DAT III reader developed at Atominstitut.

Manual TL-DAT III reader

The TL-DAT III reader was designed for TLD measurements. The maximum counting rate is up to 2 x 105 counts per time interval (without dead time correction). Different filters can be assembled to the photodetector in order to do not exceed that maximum number of counts. The reader is connected to a vacuum pump, a nitrogen feed and a Peltier cooler. The vacuum pump evacuates the chamber after positioning the chip at the heating plate. During the readout, the chamber is flooded with poor nitrogen (purity of 99,999% N2) to prevent any dust artifacts. Thus, surface effects and other emissions, e.g. chemiluminescence, are eliminated. With the increasing number of measurements, the photomultiplier tube will warm up slowly. This could cause a relatively small electric current (dark current), generated by intrinsic effects of the material. Therefore, the photomultiplier tube is cooled down with a Peltier cooler.

Riso reader

Figure 6. Risø TL/OSL reader at MedAustron.

Automated Risø-TL/OSL-DA-20 reader (access at MedAustron)


The Risø TL/OSL-DA-20 reader was manufactured by Risø DTU National Laboratory and designed for measurements of thermally and optically stimulated luminescence spectra of luminescent materials. It consists of optical and thermal stimulation systems and the light detection system with a bialkali photomultiplier (PM) tube EMI 9235QA. The spectral sensitivity of the PM is between 150 nm and 700 nm with a maximum at 400 nm. The reader allows to load up to 48 samples, which can be pre-heated or heated while reading from room temperature up to 700°C, with a heating rate from 0.1 to 10°C/s. Optical stimulation can be performed with the blue LED diodes of 470 nm (~80 mW/cm2) and the Infrared diodes of 870 nm (~145 mW/cm2). The reader is also equipped with a beta source of ionizing radiation, 90Sr/90Y (activity ~ 0.148 GBq, dose rate ~ 0.2 mGy/s).

Tissue Equivalent Proportional Counter (TEPC)

Low pressure TEPC (LET-1/2 from Far West Technology, Inc., USA) is widely used in radiation protection studies to measure and evaluate a lineal energy and a specific energy of a radiation, which are the microscopic analogs of the LET and the absorbed dose, respectively. Microdosimetric applications of the TEPC are based on simulating a microscopic tissue mass by a macroscopic TEPC cavity filled with a tissue-equivalent gas at low pressure under a Bragg-Gray’s cavity principle on the charged particle equilibrium. By using a lineal energy distribution one can easily obtain the absorbed dose by integrating over the spectrum.

The detector is a spherical cavity in tissue equivalent (TE) plastic (Shonka Type A-150) with a 1.27 cm internal diameter. An aluminum can surrounds the TE plastic that provides electrostatic shielding and serves as a vacuum tight container. A propane-based tissue-equivalent gas with a pressure of 4.4 kPa in the chamber simulates the energy imparted to a spherical tissue with a unit density of 1 μm in diameter. A small size of the TEPC makes it suitable for measurements in small cavities within a phantom.

Team

Coordinator

Dr. Monika Puchalska
e-mail: monika.puchalska@tuwien.ac.at
phone: +43 (2) 62226 100951

Staff

Ing. Fugger Manfred
e-mail: manfred.fugger@tuwien.ac.at
phone: +43 (1) 58801 141364

Assigned

DI Bergmann Robert (TL dating)
e-mail: robert.bergmann@tuwien.ac.at
phone: +43 (1) 58801 141304

Univ.Prof.i.R. DI Dr. Vana Norbert
e-mail: norbert.vana@tuwien.ac.at
phone: 43 (1) 58801 141377