COMPOSITE MATERIAL FOR DETECTING FREE NEUTRONS WITH AN EFFECTIVE ATOMIC NUMBER SIMILAR TO BODY TISSUE BY USING BERYLLIUM OXIDE AND/OR LITHIUM TETRABORATE, DOSIMETER, AND A METHOD FOR CAPTURING OR DETECTING FREE NEUTRONS

Abstract

A method as well as a composite material for detecting free neutrons are disclosed that include a converter material, which is configured to generate in response to a capture of neutrons a secondary radiation, and a detector material, which is configured to store an information relating to the secondary radiation and to release it again in a later evaluation by optically stimulated luminance. The converter material and the detector material each are present in a plurality of particles, which are jointly present in the composite material as material mixture. In order to improve the detection of neutrons with regard to a person dosimetry, that is the estimation of a dose absorbed by a human, it is envisaged that the detector material is formed from beryllium oxide and/or the converter material is formed from lithium tetraborate.

Claims

1.-15. (canceled)

16. A composite material for detecting free neutrons, comprising: a converter material that is configured as a consequence of a neutron capture to generate a secondary radiation; and a detector material that is configured to store an information relating to a quantity of the secondary radiation and to release it again in a later evaluation by optically stimulated luminance, wherein the converter material and the detector material each are present in a plurality of particles, which jointly are present in the composite material as material mixture, and wherein the detector material is formed from beryllium oxide.

17. A composite material for detecting free neutrons, comprising: a converter material that is configured as a consequence of a neutron capture to generate a secondary radiation; and a detector material that is configured to store an information relating to a quantity of the secondary radiation and to release it again in a later evaluation by optically stimulated luminescence, wherein the converter material and the detector material each are present in a plurality of particles, which are jointly present in the composite material as material mixture, and wherein the converter material is formed from lithium tetraborate.

18. The composite material according to claim 17, wherein in the lithium tetraborate the isotopes 6Li and/or 10B compared to their natural frequency are enriched.

19. The composite material according to claim 17, wherein the detector material is formed from a different material than lithium tetraborate.

20. The composite material according to claim 19, wherein the detector material is formed from beryllium oxide.

21. The composite material according to claim 17, wherein the detector material is formed from lithium tetraborate.

22. The composite material according to claim 17, wherein shares of the converter material and the detector material in the composite material are chosen in such a way that an effective atomic number of between 6.1 and 8.1, or between 6.7 and 7.5 is rendered.

23. The composite material according to claim 17, wherein the particles of the converter material and/or the detector material have a grain size of less than 30 micrometers, or of less than 10 micrometers.

24. The composite material according to claim 17, wherein the composite material has a flat surface as well as an expansion of between 0.2 millimeter and 0.5 millimeter, or an expansion of 0.3 millimeter, perpendicular to the flat surface.

25. The composite material according to claim 17, wherein the converter material and the detector material are joined by burning or sintering into the composite material.

26. A dosimeter comprising: a composite material according to claim 17.

27. A method for capturing free neutrons, comprising: at least partially absorbing the neutrons by a composite material having a converter material and a detector material, wherein the converter material and the detector material each are present in a plurality of particles in a material mixture, and the detector material is formed from beryllium oxide; generating a secondary radiation by the converter material as a consequence of a capture of the neutrons; and storing an information relating to a quantity of the secondary radiation by the detector material of beryllium oxide, which is configured to release the information again in a later evaluation by optically stimulated luminescence.

28. A method for detecting free neutrons with the aid of a composite material comprising the steps of the method according to claim 27, and further comprising evaluating the information by: illuminating the composite material with light of a stimulation spectrum, wherein the stimulation spectrum is specific for at least one of beryllium oxide or lithium tetraborate, and detecting the neutrons based on an emission spectrum, which is emitted by the composite material in response to the illumination with the stimulation spectrum, corresponding to a predetermined provision.

29. The method according to claim 28, wherein the composite material contains lithium tetraborate as the converter material, and the illuminating of the composite material is affected with two different stimulation spectra, wherein one of the two stimulation spectra is specific for beryllium oxide and the other of the two stimulation spectra for lithium tetraborate.

30. A method for capturing free neutrons, comprising: at least partially absorbing the neutrons by a composite material having a converter material and a detector material, wherein the converter material and the detector material each are present in a plurality of particles in a material mixture, and the converter material is formed from lithium tetraborate; generating a secondary radiation by the converter material from lithium tetraborate in response to a presence of neutrons; and storing an information relating to a quantity of the secondary radiation by the detector material, which is configured to release the information again in a later evaluation by optically stimulated luminescence.

31. A method for detecting free neutrons with the aid of a composite material comprising the steps of the method according to claim 30, and further comprising evaluating the information by: illuminating the composite material with light of a stimulation spectrum, wherein the stimulation spectrum is specific for at least one of beryllium oxide or lithium tetraborate, and detecting the neutrons based on an emission spectrum, which is emitted by the composite material in response to the illumination with the stimulation spectrum, corresponding to a predetermined provision.

32. The method according to claim 31, wherein the composite material contains beryllium oxide as the detector, and the illuminating of the composite material is affected with two different stimulation spectra, wherein one of the two stimulation spectra is specific for beryllium oxide and the other of the two stimulation spectra for lithium tetraborate.

Description

[0053] FIG. 1 depicts a dosimeter containing two detector units, in a schematic front view.

[0054] FIG. 2 depicts a composite material for a detector unit in a schematic perspective view.

[0055] FIG. 3 depicts a flow diagram of an exemplary method for evaluating a neutron dose.

[0056] FIG. 1 shows a dosimeter 10, which comprises a housing 12. Within the housing 12 two detector units are arranged. A first one of the two detector units is provided by a composite material 1. The other one of the two detector units is referred to as further detector unit 11. Therein the dosimeter 10 is configured to capture neutron radiation, in particular so-called free neutrons and/or thermal neutrons. For capturing the free neutrons in particular the composite material 1 is configured. The further detector unit 11, in contrast, is configured to capture a photon radiation (gamma radiation, cosmic radiation, x-ray radiation, etc.) In the course of a later evaluation a reference value with regard to the captured photon radiation can be determined. By this reference value photon radiation captured by the composite material 1 can be subtracted so that as evaluation result solely the neutron dose detected by the composite material 1 remains. This later evaluation, however, in the following is yet to be discussed in more detail.

[0057] FIG. 2 shows the composite material 1 in a schematic perspective view. The composite material 1 in the present case exemplarily has a shape design similar to a tablet. In other words, the composite material 1 in the present case is merely exemplarily shaped in the form of a cylinder. The composite material 1 in the present case has two flat surfaces 5. In the present example the flat surfaces 5 moreover are parallel to each other. In the present example of a cylindrical shape design flat surfaces 5 are provided by the bottom and the top surface of the cylinder. Between the flat surfaces 5 in the present example extends the cylinder lateral surface 6. Perpendicular to one or both of the surfaces 5 the composite material has a thickness D. In the present example the flat surfaces 5 each are shaped to be circular, wherein a respective circle at the basis of the surfaces 5 has a diameter R.

[0058] The composite material 1 comprises a converter material 2, which is configured to generate a secondary radiation in response to a capture of free neutrons. A suitable converter material represents in particular chemical compounds containing the isotope .sup.6Li. .sup.6Li responds timely to the capture of free neutrons by a radioactive decay, in which short-range alpha radiation as well as a tritium particle are released. In the converter material accordingly, it is advantageously envisaged that the isotope .sup.6Li compared to its natural frequency is enriched. Of course, any random materials may be considered as converter material 2 if they have significant capture cross sections for neutrons. Different materials in this connection can also generate different secondary radiation. However, frequently the source of the secondary radiation is a nuclear reaction caused by the neutron capture. In other words, the converter material 2 is advantageously characterized in that it contains atoms, which in response to a neutron capture radioactively decay whilst emitting the secondary radiation. In this connection it is to be ensured that the secondary radiation is generated in a period of time that is appropriate for the respective purpose of application. Advantageously, the converter material 2 or isotopes contained in the converter material 2, which are configured for capturing the neutrons and for generating the secondary radiation, have an as large as possible capture cross section for neutrons. The isotope .sup.6Li for instance has a sufficiently large capture cross section for neutrons.

[0059] The composite material 1 further comprises a detector material 3, which is configured to store the quantity of the secondary radiation and have it determined in a later evaluation by optically stimulated luminescence. The detector material is in particular a material, which preserves the dose information by storing free charge carriers in stable energy levels. For instance, the traps which are capable of absorbing free charge carriers are energy levels which lie between valence band and conduction band of the detector material 3. A returning into the valence band or a raising into the conduction band starting from this energy level are not readily possible. This is the underlying principle to the fact that the electron is trapped on the corresponding energy level and can only be freed by further supply of energy. In the course of the later evaluation by optically stimulated luminescence by a corresponding energy supply the electron can be raised to an even higher energy level. When returning from this further raised energy level to the basic state or a different lower energy level, then a characteristic emission of light is generated, the wavelength of which corresponds to the released energy. This is explained in further detail in the following.

[0060] For application as part of the person dosimetry in the present case it is envisaged that the composite material 1 has an effective atomic number, which is very similar to the effective atomic number of human tissue. In this way measurement results, which are obtained by the composite material 1, to a considerable degree can be transferred to the human body or to a person wearing the dosimeter 10 on the body for monitoring of the exposition to radiation. In other words, by such a composite material 1 results relating to person dosimetry can be obtained, which in comparison with the prior art are improved. For instance, an effective atomic number in the composite material 1 of between 6.1 and 8.1, preferably of between 6.7 and 7.5 may be envisaged.

[0061] In order to obtain an effective atomic number, which complies with the above-named requirements, it may for instance be envisaged that the detector material 3 is formed from beryllium oxide. The effective atomic number of beryllium oxide (BeO) in good approximation (effective atomic number is 7.1) is equivalent to the effective atomic number of body tissue. Thus, radiation transport in the beryllium oxide takes place under similar conditions as in the human body. The composite material 1 thus can be used without additional filter in order to capture a dose over a wider energy range.

[0062] Beryllium oxide moreover is characterized in that a so-called fading, that is the loss of dose information over time, can be virtually neglected. Moreover, a typical detector sensitivity of beryllium oxide is high enough for reproducible measurements to be possible up into the dose range of few microsievert. Beryllium oxide in significant amounts are employed for applications as good thermally conductive insulator for example in ignition plugs and therefore are readily available as starting material also for an application in the dosimetry. As ceramic material beryllium oxide is chemically and mechanically very stable and not hygroscopic. Beryllium oxide has a sensitivity to incident photon radiation (x-ray, gamma) as well as electrons (beta radiation) and helium nuclei (alpha radiation) as far as these particles can enter the beryllium oxide, that is in the present case the detector material 3. In the pure form the detector material 3, that is in the present case the beryllium oxide, however, has no or only a minor sensitivity to the radiation with neutrons (thermal or high energy). For this reason, the admixing of the converter material 2 is envisaged in order to generate the secondary radiation, which then in turn is detectable with the aid of the detector material 3.

[0063] Another possibility to approximate the effective atomic number to the effective atomic number of human tissue consists in forming the converter material 2 from lithium tetraborate. Lithium tetraborate (Li.sub.2B.sub.4O.sub.7) may even contain two possible isotopes with a high capture cross section for neutrons, namely .sup.6Li and .sup.10B. Lithium tetraborate with regard to its effective atomic number is equivalent in terms of tissue to human body tissue. In order to improve the efficiency as converter material 2, the isotope .sup.6Li may be enriched compared to other lithium isotopes and/or the isotope .sup.10B compared to other boron isotopes.

[0064] This means that according to a first embodiment it may be envisaged to combine in the composite material 1 lithium tetraborate as converter material 2 with any random detector material 3, which facilitates optically stimulated luminescence. Alternatively, according to a second embodiment it is possible to combine beryllium oxide as detector material 3 with any random converter material 2 which is configured to generate a secondary radiation in response to the incidence of free neutrons. Therein, in each case it is to be seen to it that the shares of the beryllium oxide or the lithium tetraborate in the composite material are sufficiently large to shift the mean effective atomic number of the entire composite material 1 to a value deviating from the effective atomic numbers of human tissue to maximally a predetermined extent. For instance, the share in beryllium oxide or the share in lithium tetraborate in the composite material 1 is to be chosen high enough for an effective atomic number for the entire composite material 1 of between 6.1 and 8.1, preferably of between 6.7 and 7.5, to be rendered.

[0065] Generally, it is envisaged that the composite material 1 in each case comprises at least 10 percent of the converter material 2 and of the detector material 3. Advantageously, the detector material 3 comprises a share of more than 10 percent, for instance at least 20 percent, at least 30 percent, at least 50 percent, or at least 70 percent, in order to sustain in the later evaluation a sufficient intensity of the luminescence. In this way, on the one hand, a sufficient conversion of the neutrons and, on the other and, a sufficient storage of the secondary radiation is ensured.

[0066] An effective atomic number having a particularly high tissue equivalence then invariably is rendered if as converter material 2 lithium tetraborate and as detector material 3 beryllium oxide is used. According to a third embodiment it may thus be envisaged that both the converter material 2 as well as the detector material 3 have tissue equivalence with regard to the respective effective atomic number. In this case it is in particular possible to combine lithium tetraborate as converter material 2 with beryllium oxide as detector material 3 in the composite material 2.

[0067] Due to the fact that the composite material comprises beryllium oxide as detector material 3 and lithium tetraborate as converter material 2, the effective atomic number irrespectively of the respective weight portions is equivalent to the effective atomic number of human tissue and the volume share of the converter material freely selectable. As a matter of course, these advantages are also entailed if a different converter material 2 than lithium tetraborate and/or a different detector material 3 than beryllium oxide with a comparable effective atomic number are chosen.

[0068] According to a fourth embodiment it may be envisaged that the lithium tetraborate is employed both as converter material 2 and as detector material 3. This is due to the fact that lithium tetraborate equally facilitates the storing of information with regard to the secondary radiation as well as a later release of this information by optically stimulated luminescence. In other words, the lithium tetraborate in an application as converter material 2 and detector material 3, on the one hand, can generate the secondary radiation in response to the capturing of the neutrons and equally store an information with regard to the secondary radiation itself. In this connection the capturing of the neutrons as well as the generating of the secondary radiation is affected in particular by the atoms contained in the lithium tetraborate .sup.6Li and/or .sup.10B. The storing of information with regard to the secondary radiation, in contrast, is affected substantially by the chemical compound of the lithium tetraborate.

[0069] Due to the in parts low range of the secondary radiation and/or in order to avoid an attenuation of the secondary radiation on its path from the converter material 2 to the detector material 3, in the present case it is envisaged that the converter material 2 and the detector material 3 each are present in a plurality of particles, which are jointly present in the composite material 1 as material mixture. This is schematically shown in FIG. 2. In other words, the converter material and/or the detector material 3 each are present in a plurality of particles. The respective particles of the converter material 2 and the detector material 3 are mixed in with each other in the material mixture 1. In this way the distance to be covered by the secondary radiation from the converter material 2 to the detector material 3 can be minimized. This applies in particular if the respective particles in which the converter material 2 and/or the detector material 3 is present have a grain size of less than 30 micrometer, in particular less than 10 micrometer.

[0070] The composite material 1 can for instance be produced by pressing, burning, and/or sintering. In the present embodiment the composite material 1 is produced by hot isostatic compressing. The starting material for this are the converter material 2 as well as the detector material 3 each in powder form. As described in the above, the respective grain sizes of the particles are a possible degree of freedom in manufacture. The composite material 1 moreover optionally may contain a binding agent to improve the cohesion of the individual particles. After the hot isostatic compressing by burning at a high temperature a stable ceramic can be produced. Degrees of freedom in order to optimize the manufacture therein consist in temperatures, temperature profiles, and the burning time. Burning temperatures therein may for instance be at about 1500 degree Celsius. Binding agents possibly employed in the pressing may decompose at least partially during burning at such temperatures. By the compressing and the subsequent burning a stable ceramic is produced. The composite material 1 is mechanically stable and inert. In particular, the composite material 1 is very stable against abrasion. Moreover, a composite material 1 is produced that is chemically very stable. Also, the composite material 1 after corresponding treatment is not hygroscopic, that is it does not attract water.

[0071] Finally, FIG. 3 shows a method for detecting free neutrons. The method for detecting free neutrons comprising the steps S1 to S5 in this connection contains a method for capturing free neutrons comprising the steps S1 to S3. In a first step S1 the composite material 1 is exposed to free neutrons. Therein at least part of the free neutrons is absorbed by the composite material 1.

[0072] In a step S2 by the converter material 2 a secondary radiation is generated in response to the presence of neutrons. In particular the neutrons are captured by the converter material 2 at least partially whilst generating the secondary radiation. In particular the neutrons are captured by the converter material 2 at least partially whilst generating the secondary radiation. In a further step S3 an information with regard to the secondary radiation is stored by the detector material 3. The detector material 3 further is configured to release the information with regard to the secondary radiation again at a later evaluation by optically stimulated luminescence.

[0073] It is to be noticed that the steps S1, S2, and S3 in reality commonly are executed to be temporally overlapping or even simultaneously.

[0074] In the performance of the method, it may be envisaged that either the converter material is formed from lithium tetraborate or the detector material is formed from beryllium oxide. Alternatively, it may be envisaged that the converter material 2 is formed from lithium tetraborate and at the same time the detector material 3 is formed from beryllium oxide. According to a further alternative it may be envisaged that both the converter material 2 and the detector material 3 are formed from lithium tetraborate.

[0075] The later evaluation may substantially be given by the further steps S4 and S5. In a step S4 the composite material 1 is illuminated with light of a stimulation spectrum. Therein the stimulation spectrum for the detector material 3, that is beryllium oxide or lithium tetraborate, is specifically suited for stimulation. In particular the stimulation spectrum is at least substantially monochromatic light, wherein a wavelength of the at least substantially monochromatic light is specific for the detector material 3, that is in particular beryllium oxide or lithium tetraborate. Specific means in particular that an energy of photons of the stimulation spectrum is sufficient to free electrons from the traps. In another step S5 in particular simultaneously with step S4 an emission spectrum is detected, which is emitted by the composite material 1, in particular the detector material 3, in response to the illumination with the stimulation spectrum. Therein according to a predetermined provision an intensity of the neutrons can be derived from the intensity of the emission spectrum. In particular a neutron dose is determined from the number of photons of the emission spectrum. For instance, the determined neutron dose according to the predetermined provision may be proportional to the number of detected photons of the emission spectrum. The photons of the emission spectrum are in particular monochromatic light of a second wavelength. The second wavelength is in particular specific for the detector material 3, in particular beryllium oxide or lithium tetraborate.

[0076] The steps S4 and S5 are in particular performed simultaneously. This may be due to the fact that the detector material reacts at least nearly instantaneously with the emission of the emission spectrum in response to the stimulation with the stimulation spectrum. In order not to lose any dose information, however, it is also necessary to perform the detecting according to step S5 during the entire duration of the emission of the emission spectrum.

[0077] Finally, as part of the present method a loop 9 may be performed so that the steps of illuminating the composite material and the detecting of the neutrons, that is the steps S4 and S5, are multiply performed. Therein it is in particular envisaged that the illuminating of the composite material is affected consecutively or simultaneously with the two different stimulation spectra. This is reasonable in particular if the converter material 2 is formed from lithium tetraborate and the detector material 3 from beryllium oxide. This is because, as already described in the above, in this case two different materials, which facilitate an evaluation by optically stimulated luminescence, are present in the composite material 1. Accordingly, it may be envisaged that in the step S4 the composite material 1 is consecutively or simultaneously illuminated with the two different stimulation spectra, wherein a first one of the two different stimulation spectra is specific for beryllium oxide and the other one of the two stimulation spectra is specific for lithium tetraborate. Analogously, then two different emission spectra are detected, wherein a first one of the emission spectra may be specific for beryllium oxide and the other one of the two emission spectra for lithium tetraborate. Since both the stimulation spectra and the emission spectra each may differ from each other, it is possible to perform the illuminating with the two stimulation spectra as well as the detecting of the two emission spectra simultaneously. By the respectively different wavelengths a mutual influencing can possibly be excluded. Alternatively, it is possible that the illuminating of the composite material 1 with the two different stimulation spectra is carried out consecutively. Accordingly, in this case also the detecting of the two stimulation spectra is executed consecutively. The illuminating with the first stimulation spectrum and the detecting of the first emission spectrum is affected simultaneously. Analogously, the illuminating with the second stimulation spectrum and the detecting of the second emission spectrum is affected simultaneously.

[0078] As part of the evaluation also the reference value with regard to the captured photon radiation may be determined. The reference value is determined by the further detector unit 11. The further detector unit 11 may be modeled on the composite material 1, however, the further detector unit 11 does not comprise any converter material 2. Thus, the further detector unit 11 has no significant or only a very low sensitivity to neutron radiation. For instance, the sensitivity to neutron radiation of the further detector unit 11 compared with the composite material 1 is lower at least by the factor 10 or 100. For instance, the further detector unit 11 captures exclusively ionizing radiation, in particular the photon radiation. By the reference value then photon radiation captured by the composite material 1 can be subtracted so that as evaluation result solely the neutrons detected by the composite material 1 are obtained.

LIST OF REFERENCE SIGNS

[0079] 1 composite material [0080] 2 converter material [0081] 3 detector material [0082] 5 surfaces [0083] 6 cylinder lateral surface [0084] 9 loop [0085] 10 dosimeter [0086] 11 composite material [0087] 12 housing [0088] D thickness [0089] R diameter [0090] S1 method step [0091] S2 method step [0092] S3 method step [0093] S4 method step [0094] S5 method step