Personal Dosimetry
Following the work by Farrington Daniels and colleagues, the application of TL to the world of dosimetry developed at a rapid pace. TL dosimetry (TLD), using a variety of TLD materials, became the foremost method to be used in the measurement of dose to people. The use of RPL in dosimetry developed slowly in parallel with the dominant application of TLD, while OSL did not become a popular dosimetry tool until the 1990s. Nevertheless, OSL dosimetry (OSLD) has since grown to become perhaps the dominant personal dosimetry method throughout the world, even though the availability of OSLD materials is rather limited. Today, in the world of luminescence personal dosimetry, TLD, OSLD, and radiophotoluminescence dosimetry (RPLD) each have their niches and one or other is the preferred dosimetry method in many institutions around the world.
TLD personal dosimeter badges have been designed by a wide range of companies and institutions, and a myriad of badge designs are available. Figure 1.5 displays several of them. (OSLD and RPLD badges are also included in this figure.) Each TLD badge contains a suitable TL material (the most popular of which is the first TLD material to be developed, namely LiF:Mg,Ti), along with an array of radiation filters (different metals or plastics of different thicknesses) to enable the analysis of the TLD badges to reveal not only dose, but also to provide some information about the radiation type and quality (i.e., energy). This is important to allow the dosimetry service provider to present information concerning how penetrative was the radiation to which the badge wearer was exposed.
Figure 1.5 Examples of personal dosimeters, including TLD, OSLD and RPLD badges. (Also included are some examples of electronic dosimeters.) Source: Dr. Hannes Stadtmann, © Hannes Stadtmann, European Radiation Dosimetry Group (EURADOS).
TLD badges are used everywhere that requires the monitoring of radiation workers at radiation facilities (nuclear power stations, hospitals, industrial complexes, and many others), including guest visitors to those facilities. Although TLD badges are slowly being phased out and replaced by other technologies (such as electronic dosimeters, some of which are also shown in Figure 1.5), they remain a powerful presence in the world of personal radiation dosimetry.
Also shown in Figure 1.5 are some of the OSLD and RPLD badges that are commercially available. While OSLD has become an extremely popular luminescence method for personal dosimetry, RPLD occupies a smaller portion of the luminescence dosimetry market share.
Commercial TLD materials include those based on LiF (e.g. LiF:Mg,Ti and LiF:Mg,Cu,P), CaF2 (e.g. CaF2:Mn, CaF2:Dy, and CaF2:Tm), Li2B4O7 (e.g. Li2B4O7:Cu), MgB4O7 (e.g. MgB4O7:Mn, MgB4O7:Dy, and MgB4O7:Cu), Al2O3 (e.g. Al2O3:C; Al2O3:Si,Ti, and Al2O3:Mg,Y) and CaSO4 (e.g. CaSO4:Tm and CaSO4:Dy). Other, less well-known and utilized materials have often been described in the published literature, including natural minerals (e.g. calcite, fluorite, quartz). OSLD is dominated by Al2O3:C and BeO, while RPLD is dominated by Ag-doped alkali phosphate glass, and Al2O3:C,Mg.
The material properties that are important in personal dosimetry are a high sensitivity (large signal for a given dose), a linear dose-response relationship, angle independence, and energy independence. The latter is difficult to achieve at low radiation photon energies. However, the important property in this regard is that the material responds to low-energy photons in the same way as does human tissue – a property known as tissue-equivalence. The most important characteristic for tissue equivalence is the effective atomic number Zeff of the material. Here, materials such as LiF (Zeff = 8.14), BeO (Zeff = 7.4) and Li2B4O7 (Zeff = 7.13) have advantages over other materials since these values are close to that of tissue (Zeff = 7.4). However, tissue equivalence is not essential as long as the response at low energies is well defined such that suitable corrections can be made (for example Al2O3:C with Zeff = 10.2).
1.3.2 Medical Dosimetry
Tissue equivalence is also a desirable characteristic of dosimeters used in medical dosimetry, but not an essential property. The high sensitivity of the TL, OSL, and RPL processes can be exploited in medical dosimetry applications since the dosimeters can be made small, which in turn gives them the property of high spatial resolution. This, in turn, means that they have the potential for dose measurement in regions of severe dose-gradients.
Modern advances in radiation medicine – in radiodiagnosis, radiotherapy, and interventional radiography – are now presenting dosimetry challenges that did not exist previously. For example, the movement toward the use of charged particles (protons and carbon ions) in radiotherapy has resulted in new requirements for luminescence dosimeters compared to the dosimetry of high-energy photons. Known responses to charged particles means careful calibration of the dosimeters in charged-particle fields. This consideration becomes especially significant beyond the Bragg peak for energy deposition by charged particles. Here, the linear energy transfer (LET) of the particles increases rapidly as the particles lose energy and slow down. Knowledge of the behavior of dosimeters in such regions, where the physical dose is rapidly decreasing but the relative efficiency of biological damage (compared to high-energy photons) is rapidly increasing, is a research area of growing importance. Secondary neutron production is also a feature of charged-particle irradiation and the response of the dosimeter in neutron fields also needs to be established.
Sophisticated intensity-modulation techniques in radiotherapy, whether using photons or particles, also create new challenges for dosimetry. This is due to the production of sharp dose gradients where the therapists try to protect healthy tissue surrounding the tumor as much as possible. It is also necessary to understand the response of the dosimeter in either steady or rapidly pulsed radiation fields.
The luminescence dosimeters may be used in vivo or ex vivo. Their small size enables their innovative use internally as the patient is treated (for example, in brachytherapy). The dosimeters may also be used externally to measure entrance and exit doses on the surface of the patient without disturbance of the radiation field.
Luminescence (particularly OSL) has also been used as an imaging technique in radiodiagnosis (where it has long been known by its alternate name of photostimulated luminescence, PSL). The sensitivity and speed of readout of the stimulated luminescence signal has given radiologists the ability to reduce radiation doses to patients and yet provide high-resolution images to aid diagnosis. However, the use of OSL in this way is not dosimetry. The actual dose to the patient is still determined by other methods.
In interventional radiography the real-time dose to patients (and surgeons) during surgical procedures can be significant. Adaptation of OSL techniques using fiber optics can provide dose measurements in real time, without disturbing the x-ray image.
In all of the above applications, the preferred properties of the needed dosimeters include tissue equivalence, high sensitivity, small size, rapid response, and known responses as functions of dose, angle, photon energy, particle LET, and neutrons.
1.3.3 Space Dosimetry
Dosimetry (for astronauts and equipment) in space environments is one of the most challenging applications for luminescence dosimetry. The space environment consists of an extremely wide range of highly energetic charged particles originating from cosmic rays (CR), solar particle events (SPE), and the radiation belts trapped by the Earth’s magnetic field (Earth’s radiation belts, ERB). Additionally,
