**1. Introduction**

552 Advances in Cancer Therapy

Van Dyk, J. (2000). Magna-field irradiation: physical considerations. *International Journal of* 

Radiation is a type of the energy transport. It produces ionization, scintillation, and luminescence when radiation interacts with matter. By detecting these phenomena from the response of the dosimeter after exposure, one can acquire an understanding on the types and the intensity of the radiation.

Solid state dosimeters can be divided into two categories, active dosimeters and passive dosimeters. When radiation interacts with medium inside the dosimeter, the active dosimeter transfers radiation intensity into the pulse of electric signals. Based on those signals, users can determine the types and the intensity of the radiation. As for passive dosimeters, radiation interaction is detected through certain physical processes after radiation interacts with medium in the dosimeter. From the physical processes, users can also determine the types and the intensity of the radiation. Active dosimeters are used for dose measurements in areas with unknown radiation level to gather the radiation information immediately. Therefore, the proper radiation protection actions can be initialized. The common active dosimeters in the market are gas-filled counters, scintillation counters, and semi-conductor detectors…etc. On the other hands, passive dosimeters are often used as periodic radiation monitor for people work in the radiation environment to monitor the cumulated dose and the types of radiation. They can be used as personal dose measurement, long-term environmental radiation dose monitor…etc. The film badge, Thermoluminescence Dosimeter (TLD), Optically Stimulated Luminescence Dosimeter (OSLD), and Radio-photoluminescence Glass Dosimeter (RPLGD) are commonly used passive dosimeters. In clinics, many different kinds of dosimeters are applied in the procedures to verify dose delivery accuracy, to obtain dose to critical areas or organs, and to verify machine output for QA purposes.

For TLD, OSLD, and RPLGD, when radiation interacts with the medium in the dosimeters, part of the absorbed energy are first stored in a metastable energy state of the medium. Then some of this energy can be recovered later as visible light after proper physical process, such as heating.

#### **2. Radio-Photoluminescence Glass Dosimeter (RPLGD)**

OSLD is made of the same luminescent material as one used in TLD. The only differences are different excitation source and different readout technique used. However, RPLGD uses

Radio-Photoluminescence Glass Dosimeter (RPLGD) 555

FD-1 3.7 - 33.4 53.7 4.6 3.7 - 0.9

FD-3 3.6 - 34.5 53.5 5.1 3.3 - -

FD-4 3.5 - 34.0 52.7 5.0 4.8 - -

FD-5 - 9.0 33.1 51.3 6.1 0.5 - -

FD-6 - 6.6 33.2 51.4 5.5 1.4 1.9 -

FD-7 - 11.0 31.5 51.2 6.1 0.2 - -

Figure 1 shows the old RPLGD readout technique (Piesch). The pre-dose M0 (M0 (t0) = I2 x t) was obtained with photomultiplier tube (PMT) first. After RPLGD irradiated by the radiation, the total light intensity is M1 (M1 = I1 x t). The "actual" light intensity from the irradiation, M, is M1 - M0 = (I1 x t) – (I2 x t). The radiation dose can then be estimated from M. The traditional way to calculate the light intensity is to subtract the pre-dose reading (M0) from the total reading (M). With the traditional readout technique, if the glass surface is covered with dust or other material the pre-dose reading (M0) and the total dose reading (M) are both affected and results in a large error for dose estimation. Therefore the old

In 1990, a new RPLGD readout system was developed by the cooperation of ATGC (Japan) and KNRC (Germany). The major modification in this new system is to use pulse ultraviolet laser as excitation source, instead of ultra-violet light. The intensity, the excitation time and position of the pulse ultra-violet laser can be accurately controlled. Traditionally, it takes seconds for the unit to count the excitation time; however, it has changed to micro second (s) for the new system. The readout time is decreased rapidly with the new system. Furthermore, with a collimated laser beam, the laser can be delivered to the exact position in the glass. The radiation energy can also be estimated accurately with the energy

With the pulse ultra-violet laser excitation system, decay curve of fluorescence can be divided into three portions according to the fluorescence decay time of RPLGD. They are (1) pre-dose or the light signal emitted from the impurity covering the glass surface, (2) the light signal from color centers formed by radiation, and (3) the light signal emitted from pre-

Any signal detected within the fluorescence decay time between 0 to 1 s, the readout system mark it as the light signal from pre-dose or from the impurity on the glass surface.

Table 1. The types and compositions of silver activated phosphate glass

RPLGD readout technique will not measure the dose accurately.

composition ratios (mol%)

Li Na P O Al Ag Mg Ba

glass series

compensator filter.

dose after long time decay.

glass compound as the luminescent material and applies different excitation method along with different readout technique. In 1949, Wely, Schulman, Ginther, and Evans manufactured the first RPLGD system (Yokota). Schulman applied this system in radiation dose measurement in 1951 (Yasuda, Troncalli). The luminescent material used by Schulman was a compound glass of 25% of KPO3, 25% of Ba (PO3)2 and 50% of Al (PO3)2, with proper amount of AgPO3 to form silver activated phosphate glass. It is very difficult to measure dose under 1 mGy with Schulman's RPLGD system, because it has a high pre-dose (residual dose). Pre-dose is the phosphorescence light emitted from RPLGD without any irradiation and excitation process. It is the minimum radiation can be measured with RPLGD. Besides, because of the pre-mature luminescence measurement technique and the poor quality of excitation source for color centers, the measurement accuracy with Schulman's RPLGD is very poor. Therefore, RPLGD is not a popular dosimeter in day to day applications in those days.

However, there are many researchers continue to devote in the developments of RPLGD and its readout system; including people at Asahi Techno Glass Corporation (ATGC) in Japan, at Toshiba Corporation in Japan, and at Karlsruhe Nuclear Research Center (KNRC) in Germany. The developments of new generation RPLGD and readout system were completed in 1990 (Piesch). Table 1 shows the types and compositions of the glass luminescent material developed by ATGC and Toshiba. The excitation source was changed from ultra-violet into pulse ultraviolet laser. The improvements in the glass material and in readout system make the RPLGD capable for lower dose (10 Gy) measurement with excellent accuracy (A. T. G., Corporation Chiyoda Technol).

TLD is still the major dosimeter used for personal dose monitor and for dose verification in diagnostic radiology and in radiotherapy in nowadays. The major problem with TLD is its non-repeatable readout for the measurements. Based on the preliminary report by Hsu et al on the study of the characteristics of RPLGD in radiation measurement, it proves that the radiation detection characteristics of RPLGD are superior to that of TLD (Hsu). Therefore, in the near future, RPGLD will become one of the important dosimeters for dose measurement and radiation detection in the field.

The work on the radiation measurement with self-manufactured RPLGD by Schulman in 1951 opened the history of RPLGD applications in dose measurement (Yasuda, Troncalli). After exposed to radiation, stable color centers are formed in the glass and more color centers are formed with increasing radiation intensity. After irradiated by ultraviolet light, color centers are excited and emit 600 nm to 700 nm visible orange light (Burgkhardt). It is called radio-photoluminescence phenomenon. The amount of orange light emitted from RPLGD is linearly proportional to the radiation received; therefore, it is suitable for long term personal dose monitor or environmental radiation monitor. RPLGD is used in Japan for over 80% of radiation workers as an external dosimeter (Corporation Chiyoda Technol).

#### **3. Principle of RPLGD and its readout methods**

The basic principle of RPLGD is that the color centers are formed when the luminescent material inside the glass compound exposed to radiation and fluorescence are emitted from the color centers after irradiated with ultra-violet light. The excited electrons generated from the color centers return to the original color centers after emitting the fluorescence. This process is called radio-photoluminescence phenomena. Because the electrons in the color centers return to the electron traps after emitting the fluorescence, it can be re-readout for a single irradiation.

glass compound as the luminescent material and applies different excitation method along with different readout technique. In 1949, Wely, Schulman, Ginther, and Evans manufactured the first RPLGD system (Yokota). Schulman applied this system in radiation dose measurement in 1951 (Yasuda, Troncalli). The luminescent material used by Schulman was a compound glass of 25% of KPO3, 25% of Ba (PO3)2 and 50% of Al (PO3)2, with proper amount of AgPO3 to form silver activated phosphate glass. It is very difficult to measure dose under 1 mGy with Schulman's RPLGD system, because it has a high pre-dose (residual dose). Pre-dose is the phosphorescence light emitted from RPLGD without any irradiation and excitation process. It is the minimum radiation can be measured with RPLGD. Besides, because of the pre-mature luminescence measurement technique and the poor quality of excitation source for color centers, the measurement accuracy with Schulman's RPLGD is very poor. Therefore, RPLGD is not a popular dosimeter in day to day applications in those days. However, there are many researchers continue to devote in the developments of RPLGD and its readout system; including people at Asahi Techno Glass Corporation (ATGC) in Japan, at Toshiba Corporation in Japan, and at Karlsruhe Nuclear Research Center (KNRC) in Germany. The developments of new generation RPLGD and readout system were completed in 1990 (Piesch). Table 1 shows the types and compositions of the glass luminescent material developed by ATGC and Toshiba. The excitation source was changed from ultra-violet into pulse ultraviolet laser. The improvements in the glass material and in readout system make the RPLGD capable for lower dose (10 Gy) measurement with

TLD is still the major dosimeter used for personal dose monitor and for dose verification in diagnostic radiology and in radiotherapy in nowadays. The major problem with TLD is its non-repeatable readout for the measurements. Based on the preliminary report by Hsu et al on the study of the characteristics of RPLGD in radiation measurement, it proves that the radiation detection characteristics of RPLGD are superior to that of TLD (Hsu). Therefore, in the near future, RPGLD will become one of the important dosimeters for dose measurement

The work on the radiation measurement with self-manufactured RPLGD by Schulman in 1951 opened the history of RPLGD applications in dose measurement (Yasuda, Troncalli). After exposed to radiation, stable color centers are formed in the glass and more color centers are formed with increasing radiation intensity. After irradiated by ultraviolet light, color centers are excited and emit 600 nm to 700 nm visible orange light (Burgkhardt). It is called radio-photoluminescence phenomenon. The amount of orange light emitted from RPLGD is linearly proportional to the radiation received; therefore, it is suitable for long term personal dose monitor or environmental radiation monitor. RPLGD is used in Japan for over 80% of radiation workers as an external dosimeter (Corporation Chiyoda Technol).

The basic principle of RPLGD is that the color centers are formed when the luminescent material inside the glass compound exposed to radiation and fluorescence are emitted from the color centers after irradiated with ultra-violet light. The excited electrons generated from the color centers return to the original color centers after emitting the fluorescence. This process is called radio-photoluminescence phenomena. Because the electrons in the color centers return to the electron traps after emitting the fluorescence, it can be re-readout

excellent accuracy (A. T. G., Corporation Chiyoda Technol).

**3. Principle of RPLGD and its readout methods** 

and radiation detection in the field.

for a single irradiation.


Table 1. The types and compositions of silver activated phosphate glass

Figure 1 shows the old RPLGD readout technique (Piesch). The pre-dose M0 (M0 (t0) = I2 x t) was obtained with photomultiplier tube (PMT) first. After RPLGD irradiated by the radiation, the total light intensity is M1 (M1 = I1 x t). The "actual" light intensity from the irradiation, M, is M1 - M0 = (I1 x t) – (I2 x t). The radiation dose can then be estimated from M. The traditional way to calculate the light intensity is to subtract the pre-dose reading (M0) from the total reading (M). With the traditional readout technique, if the glass surface is covered with dust or other material the pre-dose reading (M0) and the total dose reading (M) are both affected and results in a large error for dose estimation. Therefore the old RPLGD readout technique will not measure the dose accurately.

In 1990, a new RPLGD readout system was developed by the cooperation of ATGC (Japan) and KNRC (Germany). The major modification in this new system is to use pulse ultraviolet laser as excitation source, instead of ultra-violet light. The intensity, the excitation time and position of the pulse ultra-violet laser can be accurately controlled. Traditionally, it takes seconds for the unit to count the excitation time; however, it has changed to micro second (s) for the new system. The readout time is decreased rapidly with the new system. Furthermore, with a collimated laser beam, the laser can be delivered to the exact position in the glass. The radiation energy can also be estimated accurately with the energy compensator filter.

With the pulse ultra-violet laser excitation system, decay curve of fluorescence can be divided into three portions according to the fluorescence decay time of RPLGD. They are (1) pre-dose or the light signal emitted from the impurity covering the glass surface, (2) the light signal from color centers formed by radiation, and (3) the light signal emitted from predose after long time decay.

Any signal detected within the fluorescence decay time between 0 to 1 s, the readout system mark it as the light signal from pre-dose or from the impurity on the glass surface.

Radio-Photoluminescence Glass Dosimeter (RPLGD) 557

Fig. 2. The luminescence decay curve of RPLGD (A. T. G.)

Fig. 3. The readout technique with pulse ultra-violet laser(A. T. G.).

The readout system takes light signal emitted in the fluorescence decay time between 1 to 40 s as the signal from radiation exposure. For light signal emitted in the fluorescence decay time up to 1 ms, the readout system takes it as the signal produced by pre-dose with long decay time characteristics. The characteristics of the fluorescence decay curve are illustrated in figure 2**.** 

Fig. 1. Old readout technique for RPLGD (Piesch)

In figure 3**,** the area of F1 is the integral of fluorescence decay curve between t1 (1 s) and t2 (40 s) and it is the luminescence signal produced by radiation. However, there are pre-dose signals included in the lower half part of F1, therefore, one should subtract this portion from F1 to obtain the "actual" luminescence signal emitted by exposure. The way to subtract the pre-dose signal is to find F2 from the longer fluorescence decay curve of pre- dose. The area of F2 is the integral between t3 and t4 where time between t3 and t4 and t1 and t2 is the same, 39 s. From the proportional relationship of trapezium area, it shows the area of pre-dose in F1 is F2 x fps (fps is the conversion factor for trapezium area). Therefore, the actual luminescence signal from the color centers is F1 - F2 x fps. The exposure received by RPLGD can be obtained from the luminescence signal emitted.

The readout system takes light signal emitted in the fluorescence decay time between 1 to 40 s as the signal from radiation exposure. For light signal emitted in the fluorescence decay time up to 1 ms, the readout system takes it as the signal produced by pre-dose with long decay time characteristics. The characteristics of the fluorescence decay curve are illustrated

In figure 3**,** the area of F1 is the integral of fluorescence decay curve between t1 (1 s) and t2 (40 s) and it is the luminescence signal produced by radiation. However, there are pre-dose signals included in the lower half part of F1, therefore, one should subtract this portion from F1 to obtain the "actual" luminescence signal emitted by exposure. The way to subtract the pre-dose signal is to find F2 from the longer fluorescence decay curve of pre- dose. The area of F2 is the integral between t3 and t4 where time between t3 and t4 and t1 and t2 is the same, 39 s. From the proportional relationship of trapezium area, it shows the area of pre-dose in F1 is F2 x fps (fps is the conversion factor for trapezium area). Therefore, the actual luminescence signal from the color centers is F1 - F2 x fps. The exposure received by RPLGD

in figure 2**.** 

Fig. 1. Old readout technique for RPLGD (Piesch)

can be obtained from the luminescence signal emitted.

Fig. 2. The luminescence decay curve of RPLGD (A. T. G.)

Fig. 3. The readout technique with pulse ultra-violet laser(A. T. G.).

Radio-Photoluminescence Glass Dosimeter (RPLGD) 559

Fig. 5. The energy levels of RPLGD. (1) After RPLGD being exposed, Ag+ at the valence band combines with electron released from PO4- and hPO4 formed by PO4- to become a color center. (2) After electron at color center excited by 337.1 nm pulse ultra-violet laser, it will be excited and emits 600 – 700 nm visible orange light, then return to the original color centers (3)After annealing at 4000C for one hour, the electron at color centers returns to the valence

The luminescence materials used in either TLD or OSLD have an ordered crystal structure with lattice defects. From the glow curve, which is generated after annealing, one has the information on the electron distribution functions at different energy trap(s). The luminescence models for TLD and OSLD are developed based on this information. However, RPLGD is a mixture of inorganic amorphous solid and does not have lattice structure and lattice luminescence centers. Therefore we cannot get the information on electron trip(s) distribution function to establish the luminescence model for RPLGD. We can only establish the radio-photoluminescence model based on the energy of the excitation

After excited with 337.1 nm pulse ultra-violet laser, RPLGD emits 600 – 700 nm visible lights. From the emitted lights we know the energy gap between the excited energy levels which electrons jump to and the energy levels at color centers is between 1.78 and 2.07 eV. Becker assumed there are many continuous energy levels at the color centers of the RPLGD (Becker), as shown in Figure 6. It shows the electrons in the valence band are excited to the conduction band after irradiation. When electrons return to the valence band, portions of electrons are captured by the electron trap(s) located at P shell and Q shell, and then form color centers. After excitation, the electrons in color centers jump to higher energy level, emit fluoresce, then return to the original color centers. RPLGD is manufactured via the process of melting various compounds under high temperature, different from the manufacture process of TLD or OSLD which is via process of long-crystal formation. Hence, the color centers of PRLGD are not built at the lattice. There are no formal reports on the

band of luminescence material (Hsu).

**5. The radio-photoluminescence model** 

source and the energy of the released visible light.

### **4. Chemical characteristics of the silver ions**

The color centers were structured at the silver activated phosphate glass. The numbers of ionic silver relate to energy levels in color centers and the numbers of electron trap(s). The numbers of electron trap(s) increase with increasing numbers of ionic silvers. However, excessive numbers of ionic silver decrease the penetration efficiency of the pulse ultra-violet laser and increases energy dependence. Therefore, a proper ratio of ionic silver is required for the best luminescence and excitation efficiency (Yokota).

At present, the most common type of glass in RPLGD for radiation dose measurement is FD-7. The AgPO4 in silver activated phosphate glass of FD-7 can be viewed as Ag+ and PO4 - . When the tetrahedron of PO4 - is exposed to the radiation, it loses one electron and forms a "positron hole". The electron released from the PO4 - will combine with Ag+ to form an Ag0. Similarly, hPO4 ("hole" formed after PO4- loses one electron) will combine with Ag+, and then gains a "positron hole" to become an Ag2+. Both Ag0 and Ag2+ can form color centers as shown in Figure 4.

$$\begin{array}{rcl} \text{Ag}^+ + \text{e} \xrightarrow{\cdot \text{} \longrightarrow} \text{Ag}^\circ \text{ } (\text{electron trap})\\ \text{Ag}^+ + \text{hPO}\_4 \xrightarrow{\cdot \text{} \longrightarrow} \text{PO}\_4 + \text{Ag}^{2+} \text{ } (\text{hole trap}) \end{array}$$

Fig. 4. The color centers formation mechanism of FD-7 (A. T. G.)

After exposure, the Ag+ at valence band of silver activated phosphate glass combines with electron released from both PO4- and hPO4 (formed by PO4-) to become color centers (Ag0 and Ag2+). When these color centers excited by 337.1 nm pulse ultra-violet laser, the electrons in Ag0 and Ag2+ excited to higher energy levels and emit 600 – 700 nm visible orange light, then return to the original color centers. Energy gained by electrons from the pulse ultra-violet laser is not high enough to let electron escape from color centers. Therefore these electrons will not return to the valence band of the glass material directly. For electrons to gain enough energy to return to the valence band, we need to anneal RPLGD at 4000C for one hour. The color centers won't disappear after readout; hence, RPLGD can be read repeatedly. Figure 5 shows the energy levels of RPLGD.

The color centers were structured at the silver activated phosphate glass. The numbers of ionic silver relate to energy levels in color centers and the numbers of electron trap(s). The numbers of electron trap(s) increase with increasing numbers of ionic silvers. However, excessive numbers of ionic silver decrease the penetration efficiency of the pulse ultra-violet laser and increases energy dependence. Therefore, a proper ratio of ionic silver is required

At present, the most common type of glass in RPLGD for radiation dose measurement is FD-7. The AgPO4 in silver activated phosphate glass of FD-7 can be viewed as Ag+ and PO4

"positron hole". The electron released from the PO4- will combine with Ag+ to form an Ag0. Similarly, hPO4 ("hole" formed after PO4- loses one electron) will combine with Ag+, and then gains a "positron hole" to become an Ag2+. Both Ag0 and Ag2+ can form color centers as

Ag+ +e- Ago(electron trap)

Ag+ <sup>+</sup> hPO4 PO4 +Ag2+(hole trap)

After exposure, the Ag+ at valence band of silver activated phosphate glass combines with electron released from both PO4- and hPO4 (formed by PO4-) to become color centers (Ag0 and Ag2+). When these color centers excited by 337.1 nm pulse ultra-violet laser, the electrons in Ag0 and Ag2+ excited to higher energy levels and emit 600 – 700 nm visible orange light, then return to the original color centers. Energy gained by electrons from the pulse ultra-violet laser is not high enough to let electron escape from color centers. Therefore these electrons will not return to the valence band of the glass material directly. For electrons to gain enough energy to return to the valence band, we need to anneal RPLGD at 4000C for one hour. The color centers won't disappear after readout; hence,

Fig. 4. The color centers formation mechanism of FD-7 (A. T. G.)

RPLGD can be read repeatedly. Figure 5 shows the energy levels of RPLGD.



**4. Chemical characteristics of the silver ions** 

for the best luminescence and excitation efficiency (Yokota).

When the tetrahedron of PO4

shown in Figure 4.

Fig. 5. The energy levels of RPLGD. (1) After RPLGD being exposed, Ag+ at the valence band combines with electron released from PO4- and hPO4 formed by PO4 - to become a color center. (2) After electron at color center excited by 337.1 nm pulse ultra-violet laser, it will be excited and emits 600 – 700 nm visible orange light, then return to the original color centers (3)After annealing at 4000C for one hour, the electron at color centers returns to the valence band of luminescence material (Hsu).
