WO2015056025A1

WO2015056025A1 - Scintillating optical fiber

Info

Publication number: WO2015056025A1
Application number: PCT/GB2014/053118
Authority: WO
Inventors: Christopher EMSLIE
Original assignee: Fibercore Limited
Priority date: 2013-10-17
Filing date: 2014-10-17
Publication date: 2015-04-23
Also published as: GB201318351D0; GB2519326A

Abstract

An optical fiber(10) for detecting ionising radiation, the optical fiber(10) comprising an inorganic glass central core (11) surrounded by a cladding layer(12), wherein the cladding layer (12) comprises an organic scintillating component and the core (11) is preferably substantially non-scintillating, and wherein the refractive index of the core is greater than the refractive index of the cladding.

Description

SCINTILLATING OPTICAL FIBER

Technical Field

The invention relates to the detection of ionising radiation, and particularly but not exclusively to a scintillating optical fiber.

Background

It is well known to use scintillating materials for the detection and measurement of ionising radiation. Example applications include neutron and high energy particle physics experiments, CT and gamma cameras in medical diagnosis, X-ray security, nuclear cameras, computed tomography and gas exploration.

Scintillating materials may be classified as either organic or inorganic. Organic scintillators fluoresce as a result of atomic transitions of valence electrons, whereas inorganic scintillators comprise a crystalline electronic band structure and fluoresce as a result of transitions between the valance and conduction bands. As a result of this difference in the way in which scintillation photons are produced, organic and inorganic scintillators having widely differing properties. For example, organic scintillators have a greater sensitivity than inorganic scintillators. An inorganic scintillator has typically around 40% of the sensitivity of an organic scintillator. This means that 2.5 times as much input radiation is required to generate scintillation photons and light with an inorganic scintillator than with an organic scintillator material.

Organic scintillators may be in the form of monocrystals, but are more commonly in the form of liquid or plastic scintillator solutions. A scintillator solution, whether liquid or plastic, comprises an aromatic base material into which small amount of fluorescing compounds, herein fluors, are dissolved or suspended. The fluors absorb the scintillation of the base and then emit secondary at larger wavelength, thereby converting the ultraviolet radiation of the base into visible light, which is the range in which photomultipliers are most effective. In liquid scintillators, the base material is an aromatic solvent such as benzene, toluene, or naphthalene. In plastic scintillators, the base material is a solid aromatic polymer such as polystyrene or polyvinyltoluene.

Inorganic scintillators are crystals formed of alkali halides or oxides, for example sodium iodide (Nal) and bismuth germanium oxide (BGO). A small amount of activator impurity is often added so that the emitted light is in the visible range or the near-UV range where photomultipliers are most effective.

Known scintillating optical fibers are formed of plastic organic scintillating materials. These materials suffer from high intrinsic optical attenuation, for example typical attenuation is in the range of between 100 dB/km and 1 ,000 dB/km. This level of attenuation severely restricts the length of fiber that may be used for radiation detection procedures. Silica-based fibers are a known alternative to plastic organic fibers. These fibers comprise a core that is sensitised to ionising radiation, for example by doping with phosphorus or alumina. One problem with this design is that the core is found to progressively darken under prolonged or repeated exposure to radiation. This darkening increases the optical attenuation coefficient and hence limits the lifetime of the fiber. Another problem with silica-based fibers is that such fibers provide limited information about the spectrum of the incident radiation.

There is thus a need for an improved optical fiber for detecting ionising radiation. Definitions

The scintillation efficiency is defined as the fraction of energy deposited by the incident radiation that is converted into photon energy.

Summary of the invention

In accordance with the present invention, as seen from a first aspect, there is provided an optical fiber for detecting ionising radiation, the optical fiber comprising an inorganic glass central core surrounded by a cladding layer, wherein the cladding layer comprises an organic scintillating component, and wherein the refractive index of the core is greater than the refractive index of the cladding.

In use, ionising radiation incident on the optical fiber triggers scintillation within the cladding layer. At least a portion of the scintillation light is coupled to the central core for transmission along the optical fiber. One advantage of the present invention is that the core may be optimally configured for the transmission of scintillation light, without the additional constraints relating to the generation of the scintillation light. Preferably the scintillation efficiency of the cladding layer is greater than that of the core.

Preferably the core is substantially nonscintillating.

The core is preferably substantially undoped and more preferably wholly undoped. Alternatively, the core may be doped with an index-modifier, for example Germania.

The refractive index of the core is greater than the refractive index of the cladding so as to facilitate total internal reflection within the core. The core is preferably substantially transparent to visible light.

The organic scintillator is preferably in the form of a plastic solution comprising a base and a primary fluor. A plastic solution can be used for ease of manufacture and durability. The organic scintillator is arranged in the cladding layer so that scintillation light is coupled to the intrinsically lower loss core for transmission along the optical fiber.

The base may be solid polymer matrix, which is preferably at least weakly scintillating.

The primary fluor preferably chosen such that, in use, the primary fluor absorbs scintillating radiation produced by the base and emits secondary scintillating radiation at a longer wavelength.

The primary fluor is preferably adapted to emit scintillating radiation in the visible or near-UV range. Alternatively, the cladding layer may comprise a secondary fluor and optionally a tertiary fluor, at least one fluor preferably being adapted to emit radiation within the visible or near-UV range when in use.

The or each fluor may be dissolved in the base prior to bulk polymerisation. Alternatively, the or each fluor may be associated with the base directly, for example by covalent bonding or through coordination. In an alternative embodiment, the cladding layer may comprise an inorganic scintillator.

The cladding layer may comprise such as a doped glass such as serium activated lithium silicate glass.

Preferably the core is substantially circular in cross-section.

Preferably the cladding layer is substantially annular in cross-section and arranged to circumferentially surround the core. The cladding layer is preferably bonded to the core at the interface therebetween.

Preferably said cladding layer constitutes the only cladding layer. In other words, the fiber is preferably a single clad fiber.

A coating may be applied to the outer surface of the cladding layer to minimise eliminate crosstalk between closely packed fibers. In an embodiment the coating is provided in addition to the cladding layer. Preferably the coating is between 10 and 15 microns thick. The coating may be black. Alternatively, the coating may be white.

In accordance with the present invention, as seen from a second aspect, there is provided an ionising radiation detection apparatus, the apparatus comprising an optical fiber as hereinbefore described and a photon detector located at a longitudinal end of the optical fiber.

Preferably the apparatus further comprises a second photon detector located at an opposing longitudinal end of the optical fiber.

The or each photon detector preferably comprises a photodiode. Alternatively, the or each photon detector may comprise a photocathode.

The apparatus preferably comprises means for measuring the height of pulses produced by the photodiode or photocathode. Advantageously, this permits measurement of the energy of the incident radiation and hence provides spectral information. It will be appreciated that any ionising radiation incident on the optical fiber will generate photons that will travel in both longitudinal directions along the core. Accordingly, in the embodiment in which the apparatus comprises detectors at each end of the optical fiber, photons will arrive at both detectors providing the attenuation within the optical fiber is sufficiently low. The apparatus preferably comprises means for measuring the time delay between a pulse of radiation being received at the first photon detector and a corresponding pulse of radiation being received at the second photon detector. The apparatus may further comprise a processor for determining a location of incidence of the ionising radiation with respect to the optical fiber.

The apparatus preferably further comprises output means, which may comprise a display screen and/or a printer.

In accordance with the present invention, as seen from a third aspect, there is provided a method for detecting emission of ionising radiation within an area, the method comprising:

providing an apparatus as hereinbefore described; and,

arranging the optical fiber on the perimeter of said area to substantially enclose said area.

Brief description of the drawings

Figure 1 is a perspective view of a portion of scintillating optical fiber in accordance with an embodiment of the present invention as seen from the first aspect;

Figure 2 is a schematic illustration of an ionising radiation detection apparatus in accordance with an embodiment of the present invention as seen from the second aspect, the apparatus incorporating the optical fiber illustrated in figure 1 ;

Figure 3 is a flow diagram of a method for detecting ionising radiation in accordance with an embodiment of the present invention as seen from the third aspect. Detailed description

With reference to figure 1 of the drawings, there is illustrated a portion of scintillating optical fiber 10 in accordance with an embodiment of the present invention. The fiber 10 is a single clad optical fiber, consisting of a central core 1 1 and a cladding layer 12. The fiber 10 may be of any length, depending on the particular application. The central core 11 is substantially circular in cross-section, comprising a diameter of 25μηι to 400μηι. The core 11 is formed of a material that is substantially transparent to visible light and is non-scintillating. The material for the core 11 is ideally chosen to provide a low attenuation coefficient, particularly in relation to radiation of the wavelength of the scintillation photons produced in the cladding layer 12. In one embodiment, the core 11 is formed of undoped inorganic glass such as silica or fused quartz. Alternatively, the core 11 may be doped with an index modifier such as germania. The core may comprise part of a relatively large core multimode fiber to facilitate capture of the cladding scintillation.

The cladding layer 12 is substantially annular in cross-section and circumferentially surrounds the core 11. The outer diameter of the cladding layer is in the range 45 μηι to 600 μηι. The radial depth of the cladding layer 12 is typically with the range 10 μηι to 100 μηι i.e. 50 to 20 % of the diameter of the core 11. Like the core 1 1 , the cladding layer 12 is formed of a material that is substantially transparent to visible light. However, unlike the core 1 1 , the cladding layer is sensitised to ionising radiation through the addition of a scintillating compound. In one embodiment, the cladding layer 12 comprises a plastic scintillator. Alternatively, it is possible to use an inorganic glass cladding doped with a suitable scintillator, for example bismuth germinate (BGO). However, but for most applications this is not preferred due to the slower photodetection provided by inorganic scintillators. The cladding layer 12 is formed of a material having a lower refractive index than the core 11 so as to facilitate total internal reflection within the core 1 1. Both the cladding layer 12 and the core 1 1 are sufficiently flexible to permit curvature of the fiber, for example to bend the fiber into a circular or elliptical shape so as to enclose a circular or elliptical area therein.

With reference to figure 2 of the drawings, there is illustrated an ionising radiation detection apparatus 20. The detection apparatus 20 includes the optical fiber 10 illustrated in figure 1 and a detection device 21.

The detection device 21 comprises photodetectors 22a, 22b provided at respective longitudinal ends of the optical fiber 10 for detecting radiation that has been transmitted through the fiber 10. The photodetectors 22a, 22b each comprise a photodiode (not shown) configured to convert scintillation light pulses incident thereupon into charge pulses. The detection device 21 further comprises processing circuitry 23 configured for measuring the height of pulses produced by the photodiodes within the photodetectors 22a, 22b. The processing circuitry 23 may comprise components known in the art for measurement of photodiode charge pulses, for example a charge sensitive preamplifier. Upon measuring the height of the charge pulses, the circuitry 23 is configured for determining the energy of the incident radiation in accordance with the pulse height, which may include making the appropriate corrections for attenuation within the optical fiber 10 and the like.

A timer 24 is provided for measuring the time delay between a pulse of radiation being received at the first photodetector 22a and a corresponding pulse of radiation being received at the second photodetector 22b. The processing circuitry 23 is configured to receive the output of the timer 24 and determine the longitudinal position at which the associated ionising radiation was incident on the optical fiber 10.

Output means in the form of a display screen 25 and a printer 26 are provided for outputting information from the detection device 21. In an alternative embodiment (not shown), the apparatus may be configured for detecting the presence of ionising radiation but not providing additional details such as spectral information and/or positional information. In this embodiment, the apparatus is similar to that described above, but the processing circuitry 23 may be simplified and the timer 24 may not be required.

With reference to figure 3 of the drawings, the above-described apparatus may be provided at step 101 for detection of ionising radiation emitted within a certain area A.

At step 102, the scintillating optical fiber 10 is positioned on the perimeter of the area A so as to substantially enclose the area A. In this way, any emission of ionising radiation within the area A will be incident on at least part of the optical fiber 10. Accordingly, the incident radiation will cause the cladding layer 12 of the optical fiber 10 to emit scintillation photons at step 103. Optical coupling between the cladding layer 12 and the core 11 permits the scintillation photons generated in the cladding layer 12 to pass into the optical core 1 1. Upon passing into the core 11 , the photons are guided, at step 104, along the longitudinal axis of the optical fiber 10 by total internal reflection at the interface between the core 11 and the cladding layer 12.

Typically, the scintillation photons will travel in both longitudinal directions along the optical core and hence will arrive at both photodetectors 22a, 22b, providing the attenuation of the optical fiber 10 is sufficiently low. Accordingly, at step 105, the photodiodes of each of the photodetectors 22a, 22b convert the scintillation light pulse incident thereupon into a corresponding charge pulse. In this step, the time delay between a scintillation light pulse being received by the first photodetector 22a and a corresponding light pulse being received by the second photodetector 22b is also recorded.

At step 106, the processing circuitry 23 analyses the charge pulses generated at the photodiodes of each of the photodetectors 22a, 22b. In particular, the processing circuitry measures the heights of the charge pulses and, applying the appropriate correction factors, calculates the energy of the incident ionising radiation.

At step 107, the time delay between a scintillation light pulse being received by the first photodetector 22a and a corresponding light pulse being received by the second photodetector 22b recoded at step 105 is analysed by the processing circuitry 23. The processing circuitry 23 calculates the longitudinal position at which the ionising radiation responsible for the scintillation light pulses was incident on the optical fiber 10 based on this time delay. The skilled person will appreciate that, in addition to the numerical value of the time delay, this calculation takes into account factors such as the speed of light within the core 11 of the optical fiber 10 and the order in which the scintillation light arrived at the detectors 22a, 22b.

Finally, the position of the incident ionising radiation calculated at step 107 and the spectral information calculated at step 106 are output at step 108 via the display screen 25 and/or printer 26. Steps 103 to 108 may be repeated upon subsequent emission of ionising radiation within area A.

Claims

1. An optical fiber for detecting ionising radiation, the optical fiber comprising an inorganic glass central core surrounded by a cladding layer, wherein the cladding layer comprises an organic scintillating component, and wherein the refractive index of the core is greater than the refractive index of the cladding.

2. An optical fiber as claimed in claim 1 , wherein the scintillation efficiency of the cladding layer is greater than that of the core.

3. An optical fiber as claimed in claim 1 or claim 2, wherein the core is substantially non-scintillating.

4. An optical fiber as claimed in any preceding claim, wherein the core is substantially undoped.

5. An optical fiber as claimed in any one of claims 1 to 3, wherein the core is doped with an index-modifier.

6. An optical fiber as claimed in any preceding claim, wherein the core is substantially transparent to visible light.

7. An optical fiber as claimed in any preceding claim, wherein the organic scintillator is in the form of a plastic solution comprising a base and at least a primary fluor.

8. An optical fiber as claimed in any preceding claim, wherein the core is substantially circular in cross-section.

9. An optical fiber as claimed in claim 8, wherein the cladding layer is annular in cross-section and arranged to circumferentially surround the core.

10. An ionising radiation detection apparatus, the apparatus comprising an optical fiber as according to any one of claims 1 to 9 and a photon detector located at a longitudinal end of the optical fiber.

1 1. An ionising radiation detection apparatus according to claim 10, further comprising a second photon detector located at an opposing longitudinal end of the optical fiber.

12. An ionising radiation detection apparatus according to claim 11 , further comprising means for measuring the time delay between a pulse of radiation being received at the first photon detector and a corresponding pulse of radiation being received at the second photon detector.

13. An ionising radiation apparatus according to claim 12, further comprising a processor for determining a longitudinal position on the optical fiber at which the ionising radiation was incident.

14. An ionising radiation detection apparatus according to any one of claims 10 to 13, wherein the or each photon detector comprises a photodiode.

15. An ionising radiation detection apparatus according to claim 14, further comprising means for measuring the height of pulses produced by the photodiode of the or each photon detector.

16. A method for detecting emission of ionising radiation within an area, the method comprising:

providing an apparatus according to any one of claims 10 to 15; and, arranging the optical fiber on the perimeter of said area to substantially enclose said area.