The Influences of Inorganic Scintillator Optical Fiber Radiation Dosimeter in Some Conditions

In order to meet the increasing demands of modern radiotherapy, real time in-vivo dose measurement has recently attracted significant attention. A small, flexible optical fiber radiation dosimeter, with high signal-to-noise ratio (SNR) that employs inorganic scintillator materials is presented. In this paper, some properties are investigated under special conditions, such as saturation properties when the intensity of the X-Ray is increased and the influence of the temperature of the environment. These properties are important to practical considerations if the sensor is to be successfully deployed in-vivo.


Introduction
It is widely accepted that it is important to accurately deliver the dose during radiotherapy to adequately damage to the tumor cells whilst ideally causing no harm to the surrounding healthy tissue.The quality of the delivery of radiation therapy treatment depends on the ability to predict and measure the absorbed dose received by the whole volume being irradiated.As a "gold standard" instrument for Quality Assurance (QA purposes), Ionization chambers (ICs) are the instrument of choice for clinicians and radiotherapists.However, their relatively large size and the requirement for high voltage power supply make them unsuitable for in-vivo and internal use during patient treatment.In order to meet this requirement, a plastic optical fibre (POF) scintillation dosimeter which do not require pressure, or humidity corrections, with a simple "scintillator-fiber-Photomultiplier tube" (PMT) structure has previously been presented by Beddar et al. [1].However, this dosimeter has a low light conversion efficiency, which means that it has reduced performance e.g. in terms of its Signal to Noise Ratio (SNR) particularly in the presence of external noise sources such as Cherenkov radiation and changes in the sensor response due to external temperature fluctuations.Therefore, many methods have been investigated to subtract the Cherenkov component [1] [5].In these investigations, the contribution from Cherenkov radiation was lower than the background level of the noise.This dosimeter has exhibited good repeatability, excellent dose linearity (R 2 of 0.9999) as well as being isotropic with respect to the radial angular dependence of its signal.In this paper, the saturation characteristics when the intensity of the X-Ray are increased and the temperature response are investigated.

Method
The experimental set up and placement of the equipment used in this investigation is shown schematically in Figure 1

Saturation Property of the Inorganic Scintillator Materials
Subject to fluorescence yield restrictions of the scintillation material, when the intensity of the X-ray increases, the inorganic scintillator materials must reach a saturation status.Experiments to investigate the saturation property of different scintillation materials were designed by increasing the dose rate in steps from 100 MU/min, 200 MU/min, 300 MU/min, 400 MU/min, 500 MU/min to 600 MU/min (the maximum dose rate available from the clinical Linac of this investigation) at a submerged depth of 5 cm in water.Each exposure lasted 20 seconds.
Figure 2 shows the relationship between the intensity integrated over the full 20 seconds and the dose rate for two different scintilator materials.A linear regression analysis shows that the intensity of the dosimeter which use Gd 2 O 2 S: Tb (Figure 2(a)) as the luminescent material followed a highly linear trend (R 2 of over 0.9999).Therefore, within this range of dose rates it is clear that there exists no observable saturation.However, the results obtained using other scintillator materials e.g.Gd 2 O 2 S:Pr exhibit a weaker linear trend (Figure 2(b)) due to the fact that the intensity of the fluorescence at 600 MU/min results in a sub-linear response.According to calculation, the actual intensity of 600 MU/min is only equivalent to the value when the dose rate is 564 MU/min had a perfectly linear trend been the case.
Similar experiments have been conducted using CsI: Tl, another scintillator material which also exhibits a limited saturation phenomenon, has been surveyed under different depth exposure conditions.In this case the dosimeter was placed at the depth of 5 cm and 10 cm in the water and the results of varying the dose rate between 100 and 600 MU/min in each case are shown in Figure 3.
Figure 3(a) shows that there is a clear saturation phenomenon at the depth of 5 cm.According to calculation, the intensity of 600 MU/min (5.94E+06) is equivalent to the value at the dose rate is 579 MU/min.But the result shows perfect linear trend (R 2 of 0.9999) when tested at the depth of 10 cm.The measured intensity (5.37E+06) at 600 MU/min and a depth of 10 cm is less than the saturation point which occurs around an intensity value of 5.94E+06 in the case of the 5 cm depth measurement.

Temperature Property of the Inorganic Scintillator Materials
When the dosimeter is used in-vivo (within the human body), the temperature is about 37˚C, which is considerably higher than normal room temperature is 21˚C.It is therefore desirable to investigate the temperature response characteristics of the dosimeter.An experiment was conducted whereby the sensors was immersed in the water tank as shown in Figure 1 and the temperature was varied by regulating the temperature of the water in the tank to the desired value and recording the resulting output intensity.Four groups of experiments at different temperatures in the range 25˚C to 40˚C have been investigated using the dosimeter fabricated using the scintillator CsI:Tl.Figure 4 shows the results of varying the dose rate between 100 and 600 MU/Min and the temperature as stated above.Figure 4 clearly shows that the dosimeter is affected by the temperature, and that the intensity decreases linearly with temperature.

Conclusion
A series of experiments have been undertaken to establish the characteristics of inorganic scintillator materials when used for optical fiber radiation dosimetry under a wide range of operating conditions including dose rate, depth of immersion in water and temperature.The results indicate that inorganic scintillator materials show saturation phenomenon when the dose rate increase continuously.The saturation point of the materials that have been studied occur at a dose rate of over 550 MU/ with the exception of Gd 2 O 2 S: Tb which exhibits no saturation effect across the full dose rate range tested.However, for normal treatment the dose rate is limited to 400 MU/min or less and in these circumstances the saturation effect would not occur.The output intensity of the dosimeter was also tested at different temperatures and it was found that the intensity varies with temperature linearly.Therefore, the temperature of the testing points must be known before using the inorganic scintillator materials dosimeter.However if this use is for internal dose measurement the temperature is relatively stable being about 37˚C for humans.
which details a plastic fiber-optic dosimeter submerged in a water equivalent tank in a radiotherapy bunker room.The plastic fiber-optic dosimeter relies on the conversion of the incident radiation dose to a measurable visible optical signal by fluorescence.According to this phenomenon, the radiation dose could be tested by measuring the intensity of the fluorescence.

Figure 1 .
Figure 1.The device scheme of the experiment.

Figure 2 .
Figure 2. Intensity response at different dose rate for different Sensor materials (a) Gd 2 O 2 S: Tb.(b) Gd 2 O 2 S:Pr.

Figure 3 .
Figure 3. Intensity response at different depth in the water using CsI:Tl.

Figure 4 .
Figure 4. Intensity response at different temperature.