Technical Fields

The invention of inhomogeneous transmutation detector and the method of its evaluation enables the measurement of dosimetric quantities in the nuclear-radiation fields, more specifically, the measurement of neutron fluences and spectra in the vicinity of the reactor core of nuclear power plant reactors and research reactors.

Background Arts

A number of dosimetric quantities are used for the characterization of the radiation field near the sources of radiation, such as the absorbed dose, dose equivalent, fluence, etc. These quantities are in a certain way related and for the purpose of simplification one can speak about fluence or fluence rate. The detectors of the invention mentioned hereinafter can be used for various types of irradiation (alpha radiation, gamma, proton radiation), but they are most important for the neutron irradiation, therefore for the purpose of simplification the term neutron radiation will be used.

A number of methods for the measurement of fluence and energy spectra in the fields of neutron radiation is used. But for the higher fluence rates of neutrons, which are inside and in the closest vicinity of nuclear reactors, a basic method for the measurement of neutron fluences and spectra is the neutron activation detector method. It is based on the irradiation of the activation detector in the neutron field. A radionuclide or radionuclides are created during the irradiation and after the end of the irradiation their activity is measured, mostly using the spectrometry of gamma radiation. The detector is typically made of a pure element, an alloy or a compound of more elements of known composition and it is in the form of a foil, a wire or a chip. From the measured activity or activities and the knowledge of the respective dosimetric constants one can determine the neutron fluence under the assumption that the temporal history of the neutron fluence rate is known. One can assume, for example, that the fluence rate was constant or proportional to the thermal power of the reactor measured during operation. In case the activities related to different reactions are measured it is possible to estimate the energy spectrum of the neutron radiation. These reactions can relate to one element or more elements (detectors) at one place can be used. In principle, more than one- half of elements of the periodic table can be used for the activation method with Ti, Mn, Fe, Co, Ni, Cu, Nb, In and Au being the most frequently used ones. The main disadvantage of this method is that the accuracy of the resulting value of fluence significantly depends on the knowledge of the time dependence of the irradiation, which is usually not known with sufficient accuracy. Thus, a large uncertainty arises in case the time of irradiation is comparable or larger than the half-life of the activated radionuclide. Particularly for the measurement of fast neutron fluence, a suitable activation detector cannot be often found. Further disadvantage is the work with an active material and in case of surveillance programs the fact that the activity of the detectors after certain period of time decreases under the measurable limit.

The disadvantages of activation detectors eliminated the invention of the transmutation detectors (TMD) described in patent„A method of measurement of dosimetric quantities CZ 304991 (application 2009-273). This method is analogical to the established one of the activation detectors, but with the difference that for the determination of the neutron fluence, it is not the activity of radioactive nuclides measured with radiometric methods that is used, but the concentration of the stable nuclides created during the irradiation and measured using analytical methods. High purity of material is a precondition, most importantly, the TMD material before the irradiation must not contain, as a higher-concentration impurity, the final nuclide that is created by transmutation and that is intended for the determination of the neutron fluence. After the exposure of the TMD in the neutron field and the decrease of the activity of induced radionuclides to a low level, the measurement of the concentration of the investigated nuclides created by transmutation is performed. The measurement is performed using one of the analytical methods for the measurement of concentration of nuclides or elements, such as the mass spectrometry, neutron activation analysis, nuclear magnetic resonance, x-ray fluorescent analysis, et al. From the measured concentrations, the probabilities of given reactions per one target nucleus are determined. From them, the neutron fluence and spectrum are determined in a similar way as in the method of activation detectors. For the final evaluation of the neutron spectra one can also combine the probabilities obtained using the method of transmutation detectors and the method of activation detectors. The advantages of the TMD method against the method of activation detectors are:

1) The accuracy of the resulting value of the fluence and the spectrum does not depend on the knowledge of the irradiation history

2) Most transmutation detectors have negligible activity even after the irradiation

3) The detectors store the information about the absorbed fluence infinitely long. In patent CZ 304991, the TMD are described as a homogeneous material formed from a single pure element or isotope or from several elements or isotopes of known concentrations in a compound or alloy. This type of transmutation detectors will be denoted further in the text as HTMD (Homogeneous transmutation detector). The disadvantages of the HTMD are: i) The limitation of the choice of nuclides for transmutation due to the requirement of high material purity that is not feasible for many materials for physical or economic reasons, which limits the choice of initial nuclides.

ii) The limitation of the choice of nuclides for transmutation due to the requirement of initial trace concentration of the final nuclide, which is created via transmutation and is intended for the determination of the neutron fluence. For HTMD, this practically disqualifies the use of nuclides that change via transmutation to a final nuclide of the same element, e.g., changes to with the natural abundance of 24.84 % in a material from Gd. Even if 157 Gd enriched by standard enrichment methods is used, the concentration of Gd is not low enough.

iii) In some cases, the activity after the exposure of HTMD in a neutron field due to activation of other isotopes of the initial element, not used for the HTMD evaluation.

Disclosure of Invention

The aforementioned disadvantages solves the inhomogeneous transmutation detector (ITMD) and the method of its evaluation according to this invention, which is designed for the measurement of dosimetric quantities in the nuclear-radiation fields, particularly for the measurement of the neutron fluences and spectra in the vicinity of the reactor core of nuclear power plants and research reactors, the principle of which is in that the detector consists of a substrate (matrix, carrier) into which or onto which a chosen initial (target) nuclide or element is inserted using a suitable chemical or physical method. The substrate and the initial nuclide before the irradiation of the detector contains as impurity maximally 0.1 % of the final nuclide.

The method of evaluation of the inhomogeneous transmutation detector consists in that the analytical methods used for its evaluation are chosen from the class of methods, which enable accurate measurement of concentration depth profiles with high sensitivity, better than atomic 0.1 %.

The invention exploits the conventional TMD method and has its advantages, but the concrete design differs from the HTMD by that the initial (target) nuclide or element (nuclide hereinafter) of high purity is implanted or deposited into (implanted hereinafter) or onto the substrate (matrix, carrier). Lor the substrate a high purity material, with respect to the impurity of the final (transmuted) nuclide, is used. The evaluation of the detector then requires measurement of the concentration depth profiles of the initial and the final nuclide with high sensitivity, for which a method of secondary ion mass spectrometry (SIMS) can be advantageously used.

The achieved elemental and isotopic purity eliminates the aforementioned disadvantages and by that better detection parameters and user specification can be obtained.

Brief Description of Drawings

The invention will be explained in the drawings, where in Lig. 1, there are sketches of two examples of the invention - (a) implanted ITMD, (b) thin film ITMD. In Lig. 2, there are profiles of B and Li providing the resulting integral Int( Li) = 2x3, 2x 10 at/cm and in Lig. 3, there are profiles of ⁹ Be and ⁶ Li providing result Int( ⁶ Li) = 2x 1, l x l 0 ⁹ at/cm ² .

Made for Carrying out the Invention

Example 1 - ITMD for thermal neutrons - ¹⁰ B implantation

The ITMD was prepared such that the matrix was made of silicon with 9N purity, doped with phosphorus and with dimensions of 10x 10x 1 mm . Then B (implantation energy 90 keV, implantation dose F( ¹⁰ B)=5 ^c 10 ¹⁵ at/cm ² ) was implanted into the matrix. This nuclide has one of the highest cross sections of all stable nuclides (3410 b) for the reaction with thermal neutrons B(n,a) Li. The ITMD prepared in this way was irradiated in a vertical channel of

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the research reactor for 5.7 days. After the irradiation the concentration of Li nuclide created by transmutation was evaluated as a ratio of area density (= integral of the concentration profile) of Li atoms and the implantation dose of B. The evaluation was performed using SIMS (Lig. 1, profiles of ¹⁰ B and ⁷ Li) with the resulting integral Int( ⁷ Li) = 2x3,2x l0 ¹⁴ at/cm ² . Using the cross section s = 3410 barn, the fluence of thermal neutrons F = Int( ⁷ Li)/ (F( ¹⁰ B) ^c s) = 3,8x 10 cm and the fluence rate 7,8x 10 cm .s under the assumption of constant irradiation were determined. Example 2 - ITMD for fast neutrons - ⁹ Be implantation

The ITMD was prepared such that the matrix was made of silicon with 9N purity, doped with boron and with dimensions of 10x 10x 1 mm . Then Be (implantation energy 60 keV, implantation dose 0( ⁹ Bc )=5xl0 ¹⁵ at/cm ² ) was implanted into the matrix. This nuclide has the cross section for the reaction with fast neutrons ⁹ Be(n,a) ⁶ Li equal to 0.036 barn. The ITMD prepared in this way was irradiated in a vertical channel of the research reactor for 5.7 days. After the irradiation the concentration of ⁶ Li nuclide created by transmutation was evaluated as a ratio of area density (= integral of the concentration profile) of ⁶ Li and ⁹ Be in the implanted part of the matrix. The evaluation was performed using SIMS (Fig. 2, profiles of ⁹ Be and ⁶ Li) with the resulting integral Int( ⁶ Li) = 2x l,l x l0 ⁹ at/cm ² . Using the cross section s = 0.036 b, the fluence of fast neutrons F = Int( ⁶ Li)/ (F( ⁹ Be) ^c s) = 1.2x l0 ¹⁹ cm ² and the fluence rate 2.4x 10 cm .s under the assumption of constant irradiation were determined.

Example 3 - ITMD for thermal neutrons - thin film of Au

For the detection of thermal neutrons an ITMD based on Aii(h,g) Hg nuclear reaction was prepared. Silicon with 9N purity doped with boron was used as a matrix. Then a thin film (500 nm) of Au was deposited onto that matrix. This ITMD was irradiated near the reactor core of the research nuclear reactor for 21.6 days. After the irradiation the concentration of Hg nuclide created by transmutation was evaluated as a ratio of Hg and Au atoms in Au thin film. The evaluation was performed using SIMS with the result 2.6% Hg. Using the cross section s = 88 barn and neglecting the influence of epithermal neutrons, the fluence of thermal neutrons 3.0x 10 cm and the fluence rate 1.6x 10 cm .s under the assumption of constant irradiation were determined.

Preparation of ITMD and its evaluation:

1) A very pure substrate from an electronic-quality material (better than 3N - 99.9%) is prepared, e.g., silicon of 9N purity, which will be additionally doped with boron or other suitable element for the increase of electrical conductivity. 2) Into the substrate or onto the surface of the substrate of 1) a chosen initial (target) nuclide is inserted using a suitable chemical or physical method, e.g. using ion implantation, namely, at concentration enabling the achievement of sufficient sensitivity (order of magnitude of atomic percents at the concentration maximum). Using a suitable technological procedure, e.g. the masking during the implantation or the deposition, one can obtain two- or threedimensional microstructures or nanostructures from the target nuclides with specific detection capabilities. Typically, the depth profile of the implanted nuclide has a shape of a Gaussian curve with full width at half maximum hundreds of nanometers. Implantation energies up to 100 keV for light elements up to 16 a.m.u., 200 keV for masses up to 35 a.m.u., 400 keV for masses up to 65 a.m.u., 800 keV for masses up to 150 a.m.u., 1 MeV for masses up to 200 a.m.u., and the doses of the order of lxlO ¹⁶ at/cm ² correspond to those parameters. In case of the reaction, where the initial isotope transmutes to the final isotope of the same element, the implanter must have sufficient mass resolving power (M/deltaM >> 200) to avoid a parasitic implantation of a neighbour- mass isotope. The suitable nuclides for the implantation for the ITMD are those, which do not generate radionuclides with a half-life of the order of days to years and which have high cross section for the related nuclear reaction. For the detection of thermal neutrons these are for example the nuclides of ¹⁰ B, ¹¹³ Cd, ¹⁴⁴ Dy, ¹⁵⁷ Gd, and ¹⁹⁷ Au and then for the fast neutrons ⁹ Be, ¹⁴ N, ¹⁶ 0 a ⁶³ Cu.

3) The ITMDs are placed in the detection position and there they are irradiated for the time period of the neutron fluence measurement.

4) After the irradiation, a measurement of the concentration depth profile of initial and transmuted nuclides is performed. Because of the isotopic purity and relatively low amount of the implanted material (compared to HTMD) the activity of the ITMD shortly after the irradiation is low and it is possible to proceed to the evaluation. The measurement is performed primarily using the secondary ion mass spectrometry (SIMS) or another method, which is capable of the measurement of concentration depth profiles with detection limits better than ppm.

From the measured concentrations, the probabilities of the given reactions per one target nucleus are determined. From them the neutron spectrum of is determined using a procedure similar to that of the activation detectors. For the final evaluation of the neutron spectra one can also combine the probabilities obtained using the ITMD, HTMD, and the method of activation detectors.

Industrial Application of the Invention

The invention is utilizable particularly for the determination of the fluence and spectrum of neutrons in the reactors of nuclear power plants and in the research reactors. More accurate characterization of the neutron field then leads to safer operation of the reactors and to the extension of the lifetime of the reactors in operation as a result of lowering the uncertainty of the neutron fluence measurements. The invention is also exploitable for the measurement of characteristics of the nuclear-radiation fields in other nuclear facilities, such as in particle accelerators, prospective in the thermonuclear reactors.