DE3628511A1 - Method of determining the dose of and instant of exposure to radioactive radiation, material for carrying out the method and a method of preparing such a material - Google Patents

Abstract

The invention relates to a method of determining the dose of and instant of exposure to radioactive and/or ionising radiation by means of thermoluminescence. A thermoluminescent material of which the glow curve has at least two thermoluminescence peaks is used. For determining the overall dose, the entire glow curve or the peak at the higher temperature is evaluated; for determining the instant of exposure, the relationship between the thermoluminescent intensities of the two peaks is found. The particularly preferred material for carrying out this method contains alpha -tridymite doped with metal ions, in particular copper ions. Furthermore, a method of preparing such an alpha -tridymite is described. <IMAGE>

Description

Various materials are known in the art, which are based on thermoluminescence for determination the radiation dose of radioactive or ionizing Radiation are suitable. The corresponding measurement technology is also known as thermoluminescence dosimetry. An example of a thermoluminescent material is Lithium fluoride. When used, this material becomes too exposed to measuring radiation; the radiation causes in the material an occupation of imperfections or imprisonment through load carriers. The lifespan of this electronic Occupation conditions depend on the material and temperature and can last up to 100,000 years.

If the material is heated, the recombination the load carrier favors, and on the return of the Charge carriers in their initial states are created characteristic light radiation - thermoluminescence.

If one carries the emitted light radiation in a diagram depending on the temperature during the Heating process, you get the so-called glow curve. As a measure of the thermoluminescent material fallen radiation can be the height of the maximum of Glow curve or the total emitted radiation energy, i.e. use the integral over the glow curve.

With the previously known evaluation methods of Thermoluminescence dosimetry can only be relative Dose values and relative spatial dose distributions determine; it has not been possible with this method so far been able to determine at what time the thermoluminescent Material exposed to radiation.

The object of the invention is accordingly a method to determine the dose of radioactive or to design ionizing radiation by means of thermoluminescence, that the time of irradiation is also determined can be.  

It is a further object of the invention to implement one to specify particularly suitable material in this method. Such a material is said to be largely chemically in particular inert, non-toxic, reusable and in the personal and Environmental dosimetry may be applicable.

After all, it's supposed to be a simple and inexpensive Manufacturing process specified for such a material will.

This task is done with a method according to the generic term of claim 1 solved the invention according to the specified in the characterizing part of this claim Is designed.

Further refinements are in the subclaims of the method according to the invention and preferred materials for use in this process and a manufacturing process specified for a particularly preferred material.

The invention takes advantage of the effect of "fading". A Part of the excitation states caused by the radiation in the thermoluminescent Material caused are already sweeping return to their rest states at room temperature, whereby this transition while emitting phosphorescent light or can also be done without radiation. This effect of "fading" has the disadvantage for measurement purposes and undesirable consequence that the glow curve and in particular decrease the height of their maxima. The effect of "fading" is at maxima in the lower temperature range of the glow curve larger than at maxima in the higher temperature range. The requires that with increasing distance from the Irradiation time those at low temperature Maxima of the glow curve decrease more than those at higher temperature maxima. According to the invention this fact is exploited by making a comparison between the peak heights or between the integrals these peaks were used to draw conclusions about the time span between measuring the glow curve and the Irradiation time has passed.  

As "material" in the sense of the present invention are not only to be understood as pure chemical compounds, but also mixtures of two different thermoluminescent centers Fabrics, where one fabric has a glow Curve peak at a first, low temperature and the other substance has a glow curve peak on a second one, has a higher temperature. In principle there is also one Design possible in which two bodies from different composed of thermoluminescent substances or are arranged in close spatial proximity. "Material" i.S. the invention is then the arrangement of these two Body.

In the following, the invention is based on exemplary embodiments and described and explained in more detail in the figures.

Fig. 1 shows a Thermolumineszenzkurve (glow curve) for explaining the invention;

Fig. 2 is a diagram showing the relationship between the measured intensity of Thermolumineszenzstrahlung and voraufgegangenen irradiation dose.

A first material that can be used according to the invention for thermoluminescence dosimetry with simultaneous determination of the time of irradiation is a mixture of dysprosium-doped calcium sulfate (CaSO₄: D _y ) and manganese-doped calcium sulfate (CaSO₄: Mn). These two materials have the following properties:

The sensitivity values are in each case on lithium fluoride (LiF TLD-100). In this example, the dysprosium is doped Calcium sulfate the material with the second,  peak at higher temperature, the manganese-doped Calcium sulfate is the material with the lower temperature lying "first" peak. Although these materials show different despite their same host lattice (CaSO₄) Sensitivities to different energies of the incident Radiation so that they have different thermoluminescent properties to have; however, this can be done by an appropriate Calibration and preparation of calibration tables can be balanced.

This thermoluminescent material can be used as a mixture loose crystal powder are used, being used for wrapping Capsules made of plastic, glass or aluminum are used. It is also possible, the powdery material in a carrier such as e.g. Embed Teflon or another plastic. In the end this material can also be used as a sintered or pressed body be used.

As already mentioned, the two substances with different thermoluminescent properties, in the given example CaSO₄: Dy and CaSO₄: Mn not necessarily to be mixed as a thermoluminescent material to be able to serve, it is also possible to serve each one tablet of CaSO₄: Dy and another tablet CaSO₄: Mn to squeeze and both tablets side by side to arrange. From the fading determined by calibration measurement Both tablets can then be used for dosimetric measurements a comparison of the glow curve peaks of these two materials a conclusion about the time of irradiation will.

Another procedure that is suitable for implementation of the method according to the invention is terbium doped magnesium orthosilicate (Mg₂SiO₄: Tb). The glow Curve of this material shows different peaks at 50, 90, 170, 300 and 420 ° C, with the largest peak being at 300 ° C.

Investigations by the inventor have shown that a material which contains α- tridymite doped with metal ions is well suited for use in the process according to the invention. Doping with Fe or Cu ions provides a material in which the glow curve has two peaks, the fading of the low-temperature peak being brief and following first-order kinetics. Compared to a material that is composed of two different thermoluminescent substances, there is the further advantage that the same absorption and weakening coefficients are present for the formation of the defects, so that the calibration of the material is particularly simple. Doping with Cu ions provides a material which has good sensitivity and in which the position of the glow curve peaks and the peak height ratio are particularly favorable.

The glow curve of a thermoluminescent material of this type is shown in FIG. 1, the dependence between the irradiated dose and the intensity of the thermoluminescent radiation in FIG. 2. It can be seen from the illustration that the first maximum of the glow curve is around 400 K. , the second maximum is around 520 K. The heights and positions of the glow curve peaks of this material are particularly favorable for determining the time of irradiation. The glow curve peak, which is at a low temperature, is particularly high, so that its decrease due to the fading effect can be determined particularly precisely in the period between irradiation and measurement. The fading of the first peak obeys first-order kinetics and thus allows the time of irradiation to be determined between one day and several months.

Another advantage of this material is its particularly good reusability because apparently the Radioactive radiation in this material does not cause permanent damage to the thermoluminescent properties worsen. Furthermore, this is quartz-like material chemically inert and can be used in many different ways Use ambient atmospheres. The sensitivity of this  Materials as well as the peak position and the peak height ratio can by adjusting the copper concentration and the annealing times are determined.

A particularly preferred process for the production of a copper-doped α- tridymite-containing material proceeds as follows:

An aqueous sodium silicate solution (water glass), for example 10 ml, is mixed with stirring with an equal amount (10 ml) of a 0.02 molar copper (II) chloride dihydrate solution and a drop of concentrated hydrochloric acid. The reaction mixture is dried in a drying cabinet at 400 K and aged for about 12 hours. The reaction product is then comminuted and purified, the cleaning being carried out, for example, with dilute hydrochloric acid. After cleaning and fractionation of the ground material with a test sieve, a batch with a particle size of 100 to 250 μm was annealed in an aluminum oxide crucible, in the example 1 hour at 1280 K with a subsequent brief heating to 1420 K for 5 minutes. With repeated sieving, a particle size fraction of 160 to 250 μm was separated off, washed with hydrochloric acid and rinsed several times with distilled water and isopropanol and then dried in a drying cabinet at 350 K. The material thus obtained consists of metastable α- tridymite, which is doped with copper, and also contains amorphous silicate glass and traces of copper and sodium chloride. This material is then placed in a quartz tube, which is then evacuated and flushed several times with nitrogen and then with oxygen-free helium. After setting an atmospheric pressure of 500 torr helium, the material was annealed at about 670 K in this atmosphere for 30 minutes. With this tempering, the defects caused during the production of the α- tridymite are emptied and in this way the occurrence of trace peaks is avoided. This process prepares the material for use as a dosimeter and compensates for any previous radiation.

The material obtained by this method was irradiated with a test dose of 5 mGy in a Cs-137 source with an activity of 9 µm Ci. A 25 mg sample was then heated in a thermoluminescent apparatus at a heating rate of 5 K / s and a working pressure of 5 torr of helium. The spectral transmission of the filters of the arrangement was 340 to 550 nm. The thermoluminescent radiation was measured with a photon counting unit at a photomultiplier voltage of 1000 V. The glow curve shows two peak maxima at approximately 400 K and 520 K. The first glow curve peak measured directly after the irradiation has a sensitivity of V = 1.45 Gy / cm or 0.09 µ Gy / count and the second peak has a sensitivity of H = 2.28 m Gy / cm or 0.14 µ Gy / count. Under these measurement conditions, the lowest sensitivity limit was about 0.2 m Gy.

With this thermoluminescent material, the fading of the first glow curve peak obeys first-order kinetics. The rate constant k was found to be 0.059 1 / d, which means a half-life of 11.75 days. The second peak is stable over at least 10 half-lives of the first peak and is a direct measure of the irradiated dose. The ratio of the heights or counts of the two peaks to one another, after calibration, gives the time of irradiation:

t = 1 / k · ln [ F · L _x (H) / L _x (V) ]

in which

k = 5.9 x 10 ^-2 / d

and
F = H / V = 1.56-1.57, depending on the evaluation method

( H / V means the quotient of the light intensity of the rear second peak to that of the first, front peak, both being measured immediately after a defined irradiation)
and L _x (H) the light intensity of the second peak after the storage time t _x after the irradiation,
and L _x (V) are the light intensity of the first front peak after the storage period t _x .

A dosimeter material based on α- tridymite, produced with the method described, was used in a personal dosimeter. The glow curve was measured after the dosimeter was exposed to radiation at an unknown time. The front peak of the glow curve gave a height of 16.9 mm or 26 400 counts, the rear peak a height of 20.6 mm or 32 200 counts. With the values of the parameters k and F given above, a time period of t = approx. 263 hours was calculated, which had passed since the irradiation. The total dose taken was calculated to be 4.5 m Gy.

Claims (9)

1. A method for determining the dose of radioactive and / or ionizing radiation by means of thermoluminescence, in which a material possessing thermoluminescent properties is irradiated by the radiation to be measured, is heated after the irradiation and in which the light radiation emerging from the material upon heating is measured,
characterized,
that the radiation is exposed to a material in which the glow curve has at least two thermoluminescent peaks, and that
the intensities of the two peaks and / or the peak heights are related. 2. The method according to claim 1, characterized in that a material is used in which the first maximum the glow curve short-lived and the second maximum long-lived is with no noticeable fading within the Dose measurement period. 3. The method according to claim 1 or 2, characterized in that a material is used at which the first maximum in the temperature range between about 300 K and 550 K, especially at about 400 K, and the second maximum above 400 K and in particular is 520 K. 4. Use a mixture of Dy-doped CaSO₄ and Mn-doped CaSO₄ in a method according to a of claims 1 to 3. 5. Use of Tb-doped Mg₂SiO₄ in a process according to one of claims 1 to 3. 6. Use of a material containing α- tridymite doped with metal ions in a process according to claims 1 to 3. 7. Use of a material containing α- tridymite doped with Cu ions in a method according to one of claims 1 to 3. 8. A process for producing a material which contains α- tridymite doped with metal ions, characterized by the following process steps:

• (1) preparing a mixture of water glass and an acidic metal salt solution;
• (2) aging the mixture by heating;
• (3) grinding the aged material;
• (4) annealing the ground material at a temperature of about 500 K to 1500 K for a period between one and fourteen hours;
• (5) annealing again in an inert gas, especially in helium.

9. The method according to claim 5, characterized in that a mixture of sodium silicate and copper (II) chloride solution is used to produce a material which contains Cu-doped a tridymite.