WO2011091803A1

WO2011091803A1 - Li2B4O7 BASED TRANSPARENT TISSUE-EQUIVALENT RADIATION DETECTOR FOR THERMALLY OR OPTICALLY STIMULATED LUMINESCENCE DOSIMETRY AND FABRICATING METHOD THEREOF

Info

Publication number: WO2011091803A1
Application number: PCT/EE2010/000002
Authority: WO
Inventors: Mihkel KERIKMÄE; Mihhail Danilkin; Aime Lust
Original assignee: Tartu Ülikool (University Of Tartu)
Priority date: 2010-01-29
Filing date: 2010-01-29
Publication date: 2011-08-04
Also published as: RU2012133553A; UA102655C2; RU2516655C2

Abstract

There is provided a transparent tissue-equivalent radiation detector for thermally or optically stimulated luminescence dosimetry and a fabricating method thereof. Detector comprises a Li₂B₄O₇ base material, a dopant Mn, binding material silicon dioxide SiO₂, co-dopant which is selected from the group consisting of a divalent cation of either alkaline earth metal or transition metal with stable 2+ charge state, whereas said co-dopant does not reduce transparency of the detector in the region of wavelengths 320-750 nm. In fabricating the detectors the co-dopant is added to a pecursor together with lithium carbonate Li₂CO₃.

Description

Li₂B₄0₇ BASED TRANSPARENT TISSUE-EQUIVALENT RADIATION DETECTOR FOR THERMALLY OR OPTICALLY STIMULATED LUMINESCENCE DOSIMETRY AND FABRICATING METHOD THEREOF TECHNICAL FIELD

This invention relates to radiation dosimetry, in particular to detectors of ionizing radiation based on Li2B₄0₇ for use for thermally or optically stimulated luminescence dosimetry.

BACKGROUND OF THE INVENTION

Solid-state luminescence dosimetry is well-known for ionizing radiation dose measurements. There are two basic classes of detectors: thermoluminescent detectors (TLD) and detectors with optically-stimulated readout (OSL

Optically Stimulated Luminescence) . Both of them are used to measure integrated dose for some known period of time. Dose information is stored as trapped electrons and holes capable to be transferred to luminescence centres by stimulation, either by thermal stimulation (heating) or by optical stimulation (irradiating with light of known wavelength) .

Thermoluminescent dosimetry is known and widely used means of radiation dose measurement. The basic requirements for TLD are the following:

human tissue equivalence (should have the same radiation dose dependence on radiation energy as human tissue has) , information storage permanence (the absence of fading) , sufficient sensitivity (high efficiency of radiation energy transformation) .

The same requirements are applicable for OSL detectors, too. Nevertheless, there are some exceptions. Fading effects are not essential for some TLD-s if there are several thermoluminescence maxima at different temperatures, and those involved in fading low-temperature maxima are not used in measurements. To eliminate unstable low-temperature maxima, one uses a pre-annealing before reading out such TLD. However, a pre-annealing is not convenient for optical readout, because it makes the detector reader more complex and less portable. Hence, the absence of fading effects is yet more essential for OSL detectors compared with TLD.

There are often the same materials suitable both for thermoluminescent and optically stimulated readout of dose information. OSL readout principle opens a possibility for very compact portative solutions due to the absence of heater in readers. Also, there is a possibility of a nondestructive dose information reading in case of OSL readout .

Presently, there is a well-known commercial implementation of OSL dosimetry performed by Landauer®, Inc. They offer InLight® and LUXEL® commercial dosimetric systems. These systems are all based on corundum (Al₂0₃:C) OSL detectors. While being very transparent and possessing a high sensitivity, corundum is not human tissue equivalent. It has much higher effective atomic number than human tissue. This is not a problem in case of known radiation source with a fixed energy of X-ray quanta or known energy distribution of particles - detectors can be calibrated for each particular case. However, the strong energy dependence in the range of tens of keV can give a large uncertainty (up to 300%) in case of unknown radiation energy or in complex radiation fields. This disatvantage is absent in case of tissue-equivalent TLD detectors like Li₂B₄0₇ : Mn, Si . However, the classical Li₂B₄0₇ : Mn, Si TLD has a low- temperature thermoluminescence maximum which cannot be used in dose measurements because of essential fading.

DISCLOSURE OF THE INVENTION

Herein is disclosed Li₂B₄0₇:Mn, Si TLD convenient as well for OSL readout due to a suppressed low-temperature maximum, and which is transparent both for stimulating light and for the output luminescence.

It is an object of the present invention to provide L,i₂B₄0₇ based transparent tissue-equivalent radiation detector for thermally or optically stimulated luminescence dosimetry comprising a Li₂B₄0₇ base material, a dopant Mn, and binding material silicon dioxide Si0₂, wherein detector further comprises a co-dopant which is selected from the group consisting of a divalent cation of either alkaline earth metal or transition metal with stable 2+ charge state, and wherein said co-dopant does not reduce transparency of the detector in the region of wavelengths 320-750 nm.

It is a further object of the present invention to provide the method for fabricating abovementioned detector comprising the stages a) mixing detector precursor components, including deionized water, boric acid H₃B0₃, a dopant Mn, and a binding material silicon dioxide Si0₂; b) increasing the temperature of said mixture to 75-85C and adding to said mixture lithium carbonate Li₂C0₃ in order to obtain Li₂B₄0₇ base material; c) aging, drying and pre- annealing said precursor; d) crunching, grinding and sifting said precursor, e) molding the precursor under a pressure to form detectors; f) sintering the molded detector bodies, wherein the co-dopant, which is selected from the group consisting of a divalent cation of either alkaline earth metal or transition metal with stable 2+ charge state, and which does not reduce transparency of the detector in the region of wavelengths 320-750 nm, is added to a pecursor at the stage (b) together with lithium carbonate Li₂C0₃.

The obtained detector (s) have basic dosimetric TL peak at higher temperature compared with a „standard" Li2B₄0₇:Mn, Si where an additional co-dopant is absent. The intensity of W

4

basic TL peak becomes higher than the intensity of a low- temperature part of TL curve. This means that the stored information becomes more stable against fading effects. The detectors with a lower fraction of unstable TL peaks are better suitable for OSL readout because of a lower uncertainty of readout results.

The amount of co-dopant should be at least in equal to Mn molar fractions or exceeding that. At the same time, a co- dopant should not break down the crystal structure of Li2B₄0₇ and should not increase the effective atomic number of the material to preserve the tissue-equivalence. This makes the upper limits different for different co-dopants, depending on ionic radii, atomic masses, and their ability to change coordinations of oxygen bonds in Li₂B0₇.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 Fading effect in a „standard" Li₂B₄0₇ : Mn, Si detectors .

Fig. 2 The improvement of stable to unstable peaks ratio in the Li₂B₄0₇:Mn, Si, Be compared to a „standard" Li₂B₄0₇ : Mn, Si .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail .

The invention comprises Li₂B₄0₇ based transparent tissue- equivalent radiation detector for thermally or optically stimulated luminescence dosimetry, which

comprise Li₂B0₇ base material, a dopant Mn, and binding material silicon dioxide Si0₂ and a co-dopant which is selected from the group consisting of a divalent cation of either alkaline earth metal or transition metal with stable 2+ charge state, whereas said co-dopant does not reduce transparency of the detector in the region of wavelengths 320-750 nm. The preferred embodiment for fabricating of Li₂B₄0₇ based transparent tissue-equivalent radiation detector comprises the following stages:

a) ceramics detector precursor components are mixed together in glass vessel, including deionized water, boric acid H₃BO₃, Si0₂, Mn dopant in a form of either carbonate or other suitable for reaction with boric acid compound, with the amount of Mn taken in the range of 0.15-0.3 molar percent of Li₂B₄0_7j and the amount deionized water is taken in the range of 2000-2500 molar percent of Li₂B0₇;

b) said mixture is heated to a temperature of 75-85C with simultaneous and continuous stirring;

c) Lithium carbonate Li₂C0₃ is added to a precursor together with the co-dopant Be oxide, whereas the amount of Lithium carbonate Li₂C0₃ is taken according to reaction 4H₃B0₃ + Li₂C0₃ = Li₂B₄0₇ + 6H₂0 + C0₂ , and the amount of Be is equal to the amount of Mn in molar fractions (0.15-0.30 molar percent of Li) ; this amount of co-dopant Be is low enough to preserve the tissue-equivalence of detector material and to preserve the lattice structure and transparency of Li₂B₄0₇; both Be compound and Li₂C0₃ are added at continuous and intensive stirring during 10-15 minutes;

e) said components are further continuously heated and stirred, with holding the temperature in the range of 75- 85C until the reaction between Boric acid H₃BO₃ and Li₂C0₃ completes, and the mixture becomes paste-like and hardly stirreable ;

f)the ceramics detector precursor is aged during 40-50 minutes at the temperature about 80C and continuously rubbing the precursor to prevent it from being glued to the reaction vessel;

g)the ceramics detector precursor is transferred then to a smooth plate and dryed at 105C during 12 hours; h)the ceramics detector precursor is pre-annealed at 575C for 2 hours in isolated from air atmospheric tube under flowing protecting inert gas;

i) ceramics detector precursor is then crunched and grinded to obtain the powder suitable for pressing ceramics detector;

j) ceramics detector precursor is sifted with a close- meshed (about 0.1 mm) sieve;

k)the detector bodies are molded with a percussive press from said ceramics detector precursor under a pressure of 1000-2000 MPa, taking into account size reduction during the subsequent sintering: for example, the diameter of detectors decreases from 5.7 mm to 4.5 mm when they are sintered at 900°C;

1) the molded ceramics detectors are placed onto the smooth non-adhesive inert plate in the way preventing detectors to contact one another;

m) the ceramic detectors are sintered in the furnace or atmosferic tube isolated from air under flowing protecting inert gas at the temperature in the range of 870-915C during 40-50 minutes.

The obtained ceramic detectors are very hard semi- transparent ceramic bodies, usually molded in the form of tablets 0.8 mm thick and 4.5 mm in diameter. They are transparent both for stimulating ultraviolet light and for intrinsic luminescence. The stimulating ultraviolet radiation of 360 nm is penetrating well through the detector bodies, so the exposed to nuclear or Roentgen radiation detectors yield the OSL suitable for dose readout. The detectors can be also emptied (blanked) by ultraviolet light before the next measuring cycle. To improve the surface and decrease light scattering, the detectors may be polished similarly to standard optical parts, but this is an optional procedure.

Fig. 1 shows the stored energy losses from a low- temperature part of TL curve for ^standard" Li₂B₄0₇ :Mn, Si TLD. The low-temperature peak is yet comparable to the basic dosimetric TL peak after 100 seconds since the end of excitation, and disappears completely in several hours. Fig. 2 compares the normalized by low-temperature peak TL curves for „standard" Li₂B₄0₇ : Mn, Si and Li₂B₄0₇ : Mn, Si, Be where the amount of Be is equal to the amount of Mn (0.25 mol.%). The ratio of stable dosimetric peak to unstable low-temperature peak increases at least by factor of 2.5. At the same time, the position of basic dosimetric peak shifts towards higher temperatures from 490 K to 570 K.

The idea of present invention is based on creation of additional traps where elctrons could be stored at higher temperature without fading effects. The concentration of created traps is higher, than the concentration of low- temperature traps in a «standard» Li₂B₄0₇ : Mn, Si TLD. This causes the fraction of reliably stored energy to increase essentially over the unstable part undergoing fading effects.

The detector with increased fraction of a high-temperature dosimetric TL peak against low-temperature unstable peak gives much lower uncertainty at OSL readout without pre- annealing. This makes possible to use such detector in portable compact OSL dosimetric systems. A relatively low amount of dopant and co-dopant preserve the tissue- equivalence of Li₂B₄0 . This kind of radiation detectors is very useful for radiation dose monitoring at medical radiological treatments and Roentgen investigations. Portable and small-size readers will be applicable for measuring doses obtained both by medical personnel and patients .

The example demonstrates the properties of Li₂B₄0₇ : Mn, Si, Be radiation detector, where a co-dopant is Be, added in the amount equal to main dopant Mn (in molar fractions) . The tissue-equivalence is preserved due to both low amount of co-dopant and low atomic number of Be. The basic dosimetric peak at TL curve occurs at higher temperatures then for „standard" detectors, but it is still well-readable with common TLD techniques. The co-dopants selected among the other alkaline-earth metals make the basic peak to occur at yet higher temperatures, so the OSL readout would be more preferrable for such radiation detectors.

The embodiments described herein illustrate the principles of the invention and are not intended to be exhaustive or to limit the invention to the form disclosed; it is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

1. Li2B₄0₇ based transparent tissue-equivalent radiation detector for thermally or optically stimulated luminescence dosimetry comprising a Li₂B₄0₇ base material, a dopant Mn, and binding material silicon dioxide Si0₂,

which is characterized in that the detector further comprises

co-dopant which is selected from the group consisting of a divalent cation of either alkaline earth metal or transition metal with stable 2+ charge state, and

wherein said co-dopant does not reduce transparency of the detector in the region of wavelengths 320-750 nm.

2. A method for fabricating detectors of claim 1, comprising the stages:

a) mixing detector precursor components, including deionized water, boric acid H₃B0₃, a dopant Mn, and a binding material silicon dioxide Si0₂;

b) increasing the temperature of said mixture to 75-85C and adding to said mixture lithium carbonate Li₂C0₃ in order to obtain Li₂B₄0₇ base material;

c) aging, drying and pre-annealing said precursor;

d) crunching, grinding and sifting said precursor,

e) molding the precursor under a pressure to form detectors;

f) sintering the molded detector bodies,

which is characterized in that the co-dopant, which is selected from the group consisting of a divalent cation of either alkaline earth metal or transition metal with stable 2+ charge state, and which does not reduce transparency of the detector in the region of wavelengths 320-750 nm, is added to a pecursor at the stage (b) together with lithium carbonate Li₂C0₃.