WO2015037981A2

WO2015037981A2 - Optical fiber for highly sensitive dosimeter

Info

Publication number: WO2015037981A2
Application number: PCT/MY2014/000238
Authority: WO
Inventors: Amouzad Mahdiraji GHAFOUR; Rafiq Bin Mahamd Adikan FAISAL
Original assignee: University Of Malaya
Priority date: 2013-09-12
Filing date: 2014-09-25
Publication date: 2015-03-19
Also published as: MY174287A; WO2015037981A3

Abstract

The present invention discloses a method of fiber pulling that the output optical fiber can sense radiation dose with very high sensitivity. In a specific embodiment, the method comprising steps of applying temperature with or without vacuum pressure onto a preform during optical fiber fabrication to form collapsed air holes optical fiber. The collapsed or fused area made in the invented fiber increases radiation dose sensitivity. The output fiber is not limited to any specific diameter, shape, single mode or multimode. The method can be applied for any diameter size, any shapes (flat, circle, hollow or solid), single mode, multi-mode and micro structured optical fibers. Also, the method can be used for doped or undoped or combination of them. The invented fiber can be used for detecting irradiation dose using either thermoluminescence or photo luminescence approaches.

Description

OPTICAL FIBER FOR HIGHLY SENSITIVE DOSIMETER

FIELD OF THE INVENTION

The present invention relates to an optical fiber. More specifically, the present invention relates to a method to fabricate a high sensitivity radiation dose detection optical fiber for use in dosimetry applications.

BACKGROUND OF THE INVENTION

Dosimetry is an important knowledge and technique which is used broadly in medical centers and radiation industries. Thermo luminescence detector or dosimeters (TLD) is recently attracting the interest of scientists. An irradiated TLD absorbs some amount of radiations and stimulated electrons remain in the higher level of energy for sometimes. Those electrons will come back to their initial energy levels by thermal energy. A good estimation of radiation can be measured by reading the number of photons emitted from the TLD. The most commonly used TLD is TLD phosphors (LiF:Mg,Ti and LiF:Mg,Cu,P), which is most typically used in medical applications due to their tissue equivalence characteristics.

However, these well-established materials have several notable drawbacks, including being hygroscopic and having relatively poor spatial resolution, ~ up to a few mm (McKeever and Moscovitch, 2003). With these restrictions in mind, novel TLD materials are currently identified, based on doped Si0₂ optical fibers, which offer characteristics that provide good potential for broadening the applicability of TLD. Compared with the use of TLD phosphors, the optical fibers offer not only the possibility of improved positional sensitivity (optical fiber diameters are sub- millimeter, typically ~ 200 μιη) but also, since the optical fibers are impervious to water (the silicates forming a glass in the optical fiber preforming process), this paves the way for their use in intercavitary and interstitial measurements (Abdulla et al, 2001). Such capability, in conjunction with the appreciable flexibility of the optical fibers (accommodating relatively small radii of curvature) also makes an optical fiber dosimetry system suitable for studies related to intra-coronary artery brachytherapy. The capability of optical fiber thermo luminescence (TL) systems also promises to be of considerable interest for dosimetry in a variety of vascular procedures involving high radiation doses to the skin, such as procedures being carried out under fluoroscopic guidance. In patients with severe medical problems for whom few, or no, alternative diagnostic techniques can be envisaged, doses have been delivered to an extent resulting in severe skin necroses (Aznar et al., 2002). Geise and O'Dea (1999), cite doses of several tens of Gy to the skin in several such medical investigations, also reviewing moves towards ensuring skin dose reduction in so far as this may be possible for the particular situation. At this point, it is sufficient to point out that there are many interesting dosimetric applications of optical fibers that could be imagined in such situations.

So far, numerous works have been reported on analyzing characteristics of optical fibers as TLD. The TL response of Ge-doped and Al-doped optical fiber were reported in Yaakob publication (Yaakob et al. 2011). Further analysis are also reported (Bradley et al. 2012; Benabdesselam et aL 2013) which emphasis the high potential of optical fibers as an alternative candidate to detect irradiation dose. So far, the most interesting dopant that respond good in sense of TL were Ge. One of the privileges of optical fiber rather than a commercially available thermo luminescence dosimeters (TLD 100) is that the price of one sample of optical fiber is 100 times cheaper than a TLD 100. Also very small size of the optical fiber (about 125 microns x 5mm) let it to be sued in many more case of dosimetry. These fiber can stand a high dose of radiation without any damage, but, perhaps one of the most interesting characteristics of the optical fiber TLD is that they can be applied in real time dosimetry and even remote dosimetry.

As described below, nothing else compares with the unique aspects of the present invention. European Patent No. EP2591380A2 discloses an optical fiber dosimeter probe for sensing radiation dose and the method to make the dosimeter probe. US Patent No. US6087666A discloses a fiber optic radiation dosimeter for monitoring radiation sources such as ultraviolet, x-ray, gamma radiation, beta radiation, and protons. US Patent No. US8344335B2 disclosed a dosimeter for radiation fields is described. The dosimeter includes a scintillator a light pipe having a first end in optical communication with the scintillator and a light detector. From the above mentioned prior arts, we can foresee that use of optical fiber in a field of dosimetry is getting more and more popular. Thus, it would be advantageous to have a high sensitivity radiation dose detection optical fiber that can improve the quality of the dosimeter developed in the prior arts.

In view of the foregoing, the present invention has developed a unique technique or method to pull the fiber that can result in a very sensitive optical fiber in irradiation dose sensing. In this method, regardless whether the fiber is doped or undoped, the sensitivity of both fiber can be increased. The method is mainly based on the method of fabricating process which involves drawing speed, temperature, tension, positive and negative pressure and etc. This method can strongly improve sensitivity of dosimeters.

SUMMARY

An object of the present invention is to provide a method to fabricate a high sensitivity radiation dose detection optical fiber for use in dosimetry applications. In a specific embodiment, the present invention comprises collapsing or fusing optical fibers. This can be done in different methods depending on the type of optical fiber preform An optical fiber preform can be a single glass tube or rod, or combination of several glass tubes and/or rods stacked in another tube (like microstructured optical fibers) used to draw and produce an optical fiber. The collapsing can be performed either by applying a vacuum pressure onto the optical fiber preform during optical fiber fabrication, or, applying high temperature to let the fiber holes collapsed. The optical fiber preform may also consist of several pieces of glasses with different refractive indices, to provide the core and cladding of the fiber. The method disclosed according to the present invention can be apphed in any type of optical fiber. The collapsed air holes optical fiber fabricated according to the present invention shows high sensitivity in radiation dose measurement and can be used for sensing any type of irradiation, electron, photon, gamma, radioactive, x-ray, alpha, synchrotron, nuclear or the like. Different types of collapsed air holes optical fibers have been fabricated according to the present invention and tested in various irradiation environments. All experimental test results confirm the high sensitivity of the fabricated collapsed air holes optical fibers. All collapsed air holes optical fibers show linear response in different dose measurement.

According to one embodiment of the present invention, the fabricated collapsed air holes optical fibers show about more than 20 times higher sensitivity compared to a fiber fabricated normally. This can significantly improve the current sensors and technology available in dosimetry applications.

Another object of the present invention is to provide a high sensitivity radiation dose detection optical fiber. In contrast to the conventional optical fibers, the optical fiber according to the present invention consists of collapsed air holes inside the optical fiber.

The high sensitivity radiation dose detection optical fiber according to the present invention can be used in any environment that radiation exists. Example of such applications are in radiation sources in hospitals, x-ray, electron and photon accelerators, radiotherapy, radiography, radiology, CT scan, synchrotron, nuclear environment or industry, radioactive environment and etc. This and other objects, features and advantages of the present invention will be readily apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and together with the description serve to explain the principles of the present invention. Figure 1(a) is a schematic diagram demonstrates the cross section of a conventional capillary optical fiber; 1(b) is a schematic diagram demonstrates the cross section of a capillary fiber collapsed in the form of flat fiber invented according to the present invention.

Figure 2(a) is a schematic diagram demonstrates the cross section of a conventional photonic crystal fiber (PCF); 2(b) is a schematic diagram demonstrates the cross section of a collapsed air holes PCF fiber invented according to the present invention. Figure 3 is a schematic diagram demonstrates fiber pulling process to fabricate optical fibers.

Figure 4 is a flowchart illustrating an example of various operational steps of the disclosed method in fabricating optical fibers in accordance with embodiment of the present invention.

Figure 5 shows TL response comparison between the fiber samples fabricated with our developed recipe (*), and fiber samples fabricated with normal pulling process. All are irradiated with 20 MeV electron.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, reference is made to various specific embodiments in which the present invention may be practiced. These embodiments are described with sufficient details to enable those skilled in the art to practice the present invention and it is to be understood that other embodiments may be employed and that structural and logical changes may be made without departing from the spirit or scope of the present invention.

In general, the present invention represents a high sensitivity radiation dose detection optical fiber. According to one embodiment of the present invention, the difference between the invented optical fiber according to the present invention and conventional optical fiber is the invented optical fiber consists of collapsing air hole(s) inside of the optical fiber and/or fusing multiple fibers that are stacked into a glass tube.

In an exemplary embodiment, to provide better understanding and for comparison purpose, a schematic diagram demonstrates the cross section of a conventional capillary optical fiber and a schematic diagram demonstrates the cross section of a capillary fiber collapsed in the form of flat fiber invented according to the present invention are shown in figure 1. Besides, a schematic diagram demonstrates the cross section of a conventional photonic crystal fiber (PCF) and a schematic diagram demonstrates the cross section of the collapsed air holes PCF invented according to present invention are also shown in figure 2.

Referring to figure 1(a), a conventional capillary fiber, indicated by the reference numeral (1) consists of an air hole region, indicated by the reference numeral (2); and an annular core region or capillary wall, indicated by the reference numeral (3). Referring to figure 1(b), a capillary fiber collapsed in the form of flat fiber invented according to the present invention, indicated by the reference numeral (4) consists of a collapsed air hole region, indicated by the reference numeral (5); and an annular core region, indicated by the reference numeral (3).

Comparing figure 1(a) and figure 1(b), the difference between these two capillary fibers is the air hole (2) in the conventional fiber remain the round shape while the air hole (5) in the invented fiber have been intentionally collapsed during the fiber fabrication process.

Referring to figure 2(a), a conventional PCF, indicated by the reference numeral (10) consists of a core region, indicated by the reference numeral (11); a cladding region, indicated by the reference numeral (12); and many air holes, indicated by the reference numeral (13). Referring to figure 2(b), a collapsed air holes PCF invented according to the present invention, indicated by the reference numeral (20) consists of a core region, indicated by the reference numeral (21); a cladding region, indicated by the reference numeral (22); and many collapsed air holes, indicated by the reference numeral (23).

Comparing figure 2(a) and figure 2(b), the difference between these two PCF is the air holes (13) in the cladding region of the conventional fiber remain the round shape while the air holes (23) in the cladding region of the invented PCF have been intentionally collapsed during the fiber fabrication process. Still referring to figure 2(b), the sizes and shapes of the collapsed air holes (23) can vary during the fiber pulling process. For example, some of the air holes, such as an air hole indicated by the reference numeral (23a), can have a relatively small size and some of the air holes, such as an air hole indicated by the reference numeral (23b), can have a relatively large size. The shape of air holes (23a) and (23b) are also totally different. However, all the air holes having the same characteristics that all the air holes have collapsed. The example in figure 2(b) is just to show the concept of the collapsing. However, in practice, the holes will be well collapsed or fused that there is no more hole remain to be visible. Figure 2(b) is just a schematic diagram to show how random may the holes collapsed.

The method of fabricating the collapsed air holes optical fibers is described with reference to figure 3. According to embodiment of the present invention, the air holes collapsed optical fiber is fabricated by high temperature or applying negative pressurization during the fiber pulling process. Figure 3 is a schematic diagram demonstrates fiber pulling process to fabricate the collapsed air holes optical fibers. As referring to figure 3, the air holes collapsed optical fiber, indicated by the reference numeral (40) is fabricated in a fiber pulling system (30) consists of a vacuum pressure unit, indicated by the reference numeral (34); a furnace, indicated by the reference numeral (36) to heat up and melt a optical fiber preform, indicated by the reference numeral (31); a diameter monitor, indicated by the reference numeral (37) to control the diameter of the fiber (40); a polymer coater, indicated by the reference numeral (38) to coat a layer of polymer onto the outer layer of the fiber; and a tensile strength monitor, indicated by the reference numeral (39). It should be noted that some of the components in the fiber pulling tower indicated in figure 3 may not be necessary to be used to make the present invention success. For example, polymer coater will be used only if needed, or vacuum pressure is needed if an intended fiber is large in size which cannot be collapsed with high temperature only. In this case, combination of high temperature and vacuum are used to fuse the fiber.

Referring to figure 4, it can be noticed that a method for fabricating the collapsed air holes optical fibers comprises various operational steps according to embodiment of the present invention. For optical fiber fabrication, firstly, an optical fiber preform (31) is needed, an optical fiber preform (31) is the glass tube or rod used to draw or produce an optical fiber. The preform (31) may consists a single capillary or several pieces of glasses with different refractive indices and diameters stacked in a tube, to provide the core (32) and cladding region (33) of the optical fiber. The core region (32) is the thin glass center of the optical fiber where the light travels, while the cladding region (33) is the outer optical material surrounding the core that reflects the light back into the core.

At operational step (41), the optical fiber preform (31) is arranged in the desired design. A wide variety of materials can be utilized as optical fiber preform included dope and undope silica tube or rod. Besides, any design of the optical fiber preform can be used in the present invention and there is no special design required in order to make the present invention success. For example, for single and multi-mode optical fiber, a preform is prepared by arranging the glass rod in a bigger diameter glass tube, while, in the case of PCF or micro structrued optical fiber, a preform is prepared by stacking multiple capillaries and/or rods into a glass tube and a glass rod in a desired shape.

At operational step (42), after arranging the preform in the desired design, the finished glass preform is installed at the top of a fiber pulling system (30). At operational step (43), the process begins by lowering the bottom end of the preform into an in-line furnace (36) that produces heat up to 2300 °C. The normal range of temperature usually uses in fiber pulling is from 1950 °C to 2200 °C. As the bottom end of the preform begins to melt, it forms a molten glob that is pulled downward by gravity. Trailing behind the glob is a thin strand of glass that cools and solidifies quickly. As the glob drops, it cools and forms a glass strand.

At operational step (44), after pulling of the fiber is started, according to the size of desired fiber output, if collapsing the fiber is not obtained by only using high temperature, then, vacuum pressure is used to collapse the fiber hole(s). This step can be performed by connecting the unseal upper end of the optical fiber preform to a vacuum pressure unit of the pulling system for supplying a continuous vacuum pressure into the air hole(s) of the optical fiber preform to facilitated the air hole(s) collapsing or fusing. For a larger fiber, the combination of high temperature and vacuum pressure are used to collapse the fiber.

The system (30) operator threads this glass strand through the diameter monitor (37) to control the diameter of the glass strand (40) at operational step (45). The step (45) is followed by a polymer coater (38) to coat a layer of polymer onto the outer layer of the glass strand at operational step (46). At operational step (47), a tensile strength monitor (39) is used to observe and check the tensile strength of the glass strand (47).

In a specific embodiment, the pulling speed, temperature, and tension are somehow related to each other. These parameters depend on the size of the original optical fiber preform (31) and the intended optical fiber output fiber (40). Therefore, they are not fixed. Even a specific fiber can be fabricated with many different combinations of such parameters. The relationship between speed and size of the fiber can be found using the simplified mass conservation equation:

^Al^vl ^{= A}2^V2

Where the A_\ and v\ are the area and the speed of the input optical fiber preform and the A2 and v₂ are the area and speed of the output fiber. On the other hand, temperature directly effects on the tension. The lower tension is required to pull an optical fiber at the higher temperature or vice versa. However, this relationship is not linear.

Usually, temperature and tension in different types of optical fibers are different. In some optical fibers that contain very small air holes, to keep the air holes, high temperature is not suitable but, to intentionally collapse the air holes, a higher temperature must be applied. In some fibers that has no air hole, the fiber can be pulled with higher temperature. In some specific embodiment, for a fiber with larger size or a fiber that cannot be collapsed with high temperature only, then, during the entire optical fiber drawing process, the vacuum pressure unit (34) is applying a continuous vacuum pressure to insides of the air holes (41) of the capillary tubes to ensure the air holes (41) of the capillary tubes will collapse. At this stage, a control value of vacuum pressure to be applied is determined based on the applied temperature, pulling speed, and the output fiber size.

Compare with the conventional optical fiber drawing process, the pressure is not a factor use in the conventional fibers. However, in some advanced model of optical fibers, i.e., microstructured optical fiber (MOF) including photonic crystal fiber (PCF), bandgap optical fiber, flat fiber, and etc., sometimes, the pressurization is used for the purpose of keeping the fiber structure in the desired form

In the present invention, the inventors proved that by collapsing one or multiple hollow fibers or fusing several rod fibers together (that are stacked in a tube), Thermoluminescence (TL) response of optical fiber has significantly improved. In addition, the TL response improves significantly by applying higher temperature. Also, the higher the collapsed or fused area provides the better TL response. This technique is also applicable in different model of fibers e.g. capillary, flat fiber, photonic crystal fiber, for undoped and doped fiber, or the like. To further illustrate the present invention in greater details and not by way of limitation, the following examples will be given.

EXAMPLE 1

Fiber fabrication

The pulling process initiated with a temperature of around 2100 °C to get the glass drop. In this step, the large glass tube is pulled down to a capillary cane size of around 2-3 mm diameter. In the second step, the capillary cane is pulled down into capillary fiber size of around 125 μηχ In the same process, the fiber pulling parameters include applying vacuum pressure with adjustment to provide the best sample with high sensitivity in irradiation dosimetery. The output fiber can have any different geometry or shape. All fiber samples are cut in about 5 mm length before exposing under irradiation. EXAMPLE 2

Irradiation

All samples are irradiated under different irradiation sources. For example, for clinical range, the samples are irradiated under 20 MeV electron energy using Siemens Mevatron MD2 linear accelerator (LINAC). In every energy level, five doses from 0.5 to 8 Gry are applied to the fiber samples through a field applicator size of 15 ^χ 15 cm and source to surface distance of 100 cm.

TL measurement

Thermoluminescence (TL) measurement is performed about 24 hours after the irradiation using a Harshaw 3500 TL reader. The Nitrogen gas was kept on during the TL reading to mmimize the possible spurious light signals from tritoluminescence. The sample read out is performed by setting the TL reader for a maximum temperature at 400 °C, the time temperature profile of 60 °C, acquired temperature rate of 15 °C/s, and acquisition time of 50 s. Mass normalization

For mass normalization, about ten samples from each fiber are used to measure their weight using an accurate electronic scale. For simplicity, the average mass for each fiber is used to be divided by their TL response.

Results and Discussion

Figure 5 shows TL response of different fibers. Figure 5(a) compares several pure silica based optical fibers, while Figure 5(b) compares two doped fibers both with the same doping materials, once pulled in the capillary form and next pulled as flat fiber. In Figure 5(b), TL response of optical fibers is compared with a commonly used TL detector (i.e., TLD-100) commercially available. Fibers with notation of '*' are fabricated according to the method disclosed in the present invention, while, the other fibers are fabricated with a conventional method, without consisting any coUapsing/fusing region. The results show significant improvement in dose response using the fibers that are fabricated based on the method disclosed in the present invention.

Conclusion

The fiber samples fabricated according to the present invention shows significantly higher sensitivity compared to normal silica fiber. This improvement can strongly develop the technology in optical fiber based dosimetry sensors.

References

McKeever, S.W.S and Moscovitch, M. (2003). On the advantages and disadvantages of optically stimulated luminescence dosimetry and thermoluminescence dosimetry. Rad Prot. Dos., 104, 263-270.

Abdulla, Y.A., Amin, Y.M. and Bradley, D.A. (2001). The thennoluminescence response of Ge-doped optical fibre subjected to photon irradiation. Radiation Physics and Chemistry, 61 , 409-410. Aznar, M, Polf, J., Akselrod, M., Andersen, C, Back, S., Boetter-Jensen, L., Mattsson, S., McKeever, S and Medin J. (2002). Real-time optical fibre dosimetry in radiotherapy. http://www. aapm. or g/meetings/02AM/pdf/7626-20413.pdf.

Geise, R.A. and O'Dea, T.J. (1999). Radiation dose in interventional procedures. Appl. Rad. Isot., 50, 173.

Yaakob, N.H., Wagiran, H., Hossain, I., Ramli, A.T., Bradley, D.A., Hashim, S., Ali, H. (2011). Electron irradiation response on Ge and Al-doped Si02 optical fibers. Nuclear Instruments and Methods in Physics Research A, 637, 185- 189.

D. A. Bradley, R P. Hugtenburg, A. Nisbet, A. T. Abdul Rahman, F. Issa, N. Mohd Noor, et al, (2012). Review of doped silica glass optical fibre: Their TL properties and potential applications in radiation therapy dosimetry. Applied Radiation and Isotopes, 71, 2-11.

M. Benabdesselam, F. Mady, and S. Girard (2013). Assessment of Ge-doped optical fibre as a TL-mode detector. Journal of Non-Crystalline Solids, 360, 9-12.

Claims

We claim:

1. A method to fabricate a high sensitivity radiation dose detection optical fiber for use in dosimetry applications comprising the steps of:- i) arranging an optical fiber preform in a desired design;

ii) installing the optical fiber preform at the top of a pulling system;

iii) heating the bottom end of the optical fiber preform with a furnace until a molten glob fells down by gravity. As the glob drops, it cools and forms a glass strand;

iv) collapsing or fusing the air hole(s) in the optical fiber preform by using high furnace temperature;

v) if collapsing or fusing the air hole(s) is not successful or if the optical fiber preform is large in size, connecting the unseal upper end of the optical fiber preform to a vacuum pressure unit of the pulling system for supplying a continuous vacuum pressure into the air hole(s) of the optical fiber preform to facilitate the air hole(s) collapsing or fusing; vi) threading the glass strand through a diameter monitor to monitor the diameter of the glass strand.

2. The method according to Claim 1, wherein the actual control value of temperature, drawing speed and vacuum pressure to be applied during the fabrication process is interdependence and is determined based on the output size of the fiber.

3. The method according to Claim 1, wherein the heating temperature supplied by the furnace to form the molten glob depends on the type and concentration of material doped in the preform

4. The method according to Claim 1, wherein the method can be applied in any type of optical fiber selected form the group consisting of single mode optical fiber, multi-mode optical fiber, photonic crystal fiber, and microstructured optical fibers.

5. The method according to Claim 1, wherein the method can be applied in any type of doped, undoped, and combination of doped and undoped fiber.

6. The method according to Claim 1, wherein the method can be applied in any diameter size and any shape of the optical fiber.

7. The method according to Claim 1, wherein the fabricated optical fiber shows high sensitivity in radiation dose measurement and can be used for sensing any type of irradiation, electron, photon, gamma, radioactive, nuclear, synchrotron, alpha or the like.

8. The method according to Claim 1, wherein the parameter values of the pulling speed, temperature, and tension are corresponding or interdependence to each other. These values depend on the size of the original optical fiber preform and the intended optical fiber output.

9. A high sensitivity radiation dose detection optical fiber for use in dosimetry applications consists of collapsed or fused air holes inside the area of the optical fiber.

10. The optical fiber according to Claim 9, wherein the optical fiber can be used as irradiation dosimeter sensor based on either thermo luminescence or photo luminescence.

11. The optical fiber according to Claim 9, wherein the optical fiber is fabricated according to method 1 to 8.

12. The optical fiber according to Claim 9, wherein the optical fiber can be used for sensing any type of irradiation, electron, photon, gamma, radioactive, nuclear, synchrotron, alpha or the like.