METHOD FOR PRODUCING ULTRALOW-MASS FISSIONABLE DEPOSITS FOR REACTOR NEUTRON DOSIMETRY BY RECOIL ION-IMPLANTATION

Abstract: An alpha recoil ion-implantation method uses an alpha-emitting source that is a radioactive parent of the daughter isotope of interest to implant into a suitable substrate the recoil daughter ions resulting from alpha decay of the parent. For example, a ²⁴¹Am source in thin layer form is placed next to a substrate such as a solid state track recorder in a vacuum which houses an assembly for rotating opposing disks receiving the alpha-emitting source and the substrate, respectively. Each alpha decay of ²⁴¹Am results in a ²³⁷Np ion with enough recoil energy to be implanted into the substrate.

Description: This invention relates to a method and apparatus for producing fissionable deposits and, more particularly, to a method for producing fissionable deposits of ultralow-­ mass for reactor neutron dosimetry on a substrate by recoil ion-implantation, and an apparatus related thereto. Ultralow-mass fissionable deposits have proved useful as fissioning sources for solid state track recorder fission rate measurements in high intensity neutron fields. These fission rate measurements are used to derive informa­ tion for neutron dosimetry purposes. A solid track recorder placed adjacent to a thin fissionable deposit records tracks from the recoil­ ing fission fragments which result from the fission in the deposit. If the fissionable deposit is sufficiently thin, the effects of self-absorption can be ignored. The number of these tracks observed with an optical microscope after chemical etching of the solid state track recorder is proportional to the number of fissions that has occurred in the fissionable deposit. Thus, the number of fission fragment tracks per square centimeter, i.e., the track density, in the solid state track recorder can be used to calculate the fission rate per unit area in the fissionable deposit. For typical high neutron fluence applications, such as reactor core dosimetry or reactor component dosimetry, it has been found that a limitation is placed on using solid state track recorders due to the maximum track density that can be used without excessive track overlap, usually about 10⁶ tracks/­ cm². In order to avoid excessively high track densities, low-mass fissionable deposits can be used to reduce the number of fissions that will occur at a given neutron fluence. For example, in dosimetry applications for light water reactor pressure vessel surveillance, ²³⁵U deposits with masses as low as 1.5 × 10⁻¹³ grams are required to produce a usable solid state track recorder track density. Similarly, low masses of other isotopes, such as ²³⁷Np, ²³⁸U, ²³⁹Pu, are required for dosimetry in light water reactor pressure vessel surveillance. It has been found that the technical problems associated with the manufacture of such low-mass deposits can be overcome through the use of isotopic spiking/electroplating techniques to characterize the masses of these ultralow-mass fissionable deposits. For example, ultralow-mass deposits can be produced by an electroplating technique using, e.g., ²³⁷U (7 day half-life) as an isotopic spike for ²³⁵U and ²³⁸U, ²³⁹Np (2.4 day half-life) as a spike for ²³⁷Np, and ²³⁶Pu (2.85y half-life) as a spike for ²³⁹Pu. The shorter half-life isotopic spike is used as a chemical tracer to overcome the fact that the radioactivity of the principal isotope of the respective fissionable deposit renders the principal isotope undetectable when present in such low masses as can be employed according to the present invention. However, it has also been found that the amount of the isotopic spike that can be added to a fissionable deposit is limited by the nuclear properties of the isotopic spike. For example, ²³⁷U decays to ²³⁷Np, which is itself fissionable. As as result, the amount of the isotope to which the spike eventually decays must be kept small enough (by limiting the amount of spike added) to keep the fission rate of the isotope to which the spike decays small relative to the decay rate of the isotope of interest in the deposit. Also, in order to ensure that the added ²³⁷U is a valid radiochemical tracer for ²³⁵U, a series of chemical steps or chemical equilibration procedures must be carried out. After the addition of ²³⁷U to ²³⁵U, the mixture must be subjected to an alternating series of chemical oxidations and reductions to drive the ²³⁷U and ²³⁵U into an identical mixture of oxidation states. These chemical procedures typically take 1-2 days. In particular regard to ²³⁹Pu deposits, the ²³⁶Pu isotopic spike itself is fissionable and must therefore be used in limited amounts. In addition, for ²³⁹U deposits spiked with ²³⁶Pu, several experimental problems arise. For example, in the case of ²³⁹Pu fission rates in a solid state track recorder measured at the mid-plane location in the reactor cavity in the annular gap of an operating commercial power reactor during a typical operating cycle, ²³⁹Pu fissionable deposits with masses on the order of 10⁻¹³ gram are required to produce an optimum number of fission tracks. Namely, due to the previously explained limitations, the maximum allowable ²³⁶Pu/²³⁹Pu spike ratio results in a count rate of only about 0.3 disintegrations per minute (dpm) for such a ²³⁹Pu deposit of 10⁻¹³ gram. In order to desirably characterize the decay rate of this deposit to better than 2% for mass calibration purposes, a counting time of about twelve days may be required. In practice, higher masses (e.g. 6 × 10⁻¹³ gram) are produced resulting in higher count rates (e.g., 1 dpm) and shorter count times (e.g., 2 days). The resulting track densities are higher and are more difficult to count. Also, due to the low sample count rate, counters with very low background count rates of about 0.1 dpm must be used. However, the decay properties of the isotopic spike make maintenance of the low backgrounds difficult. For example, ²³⁶Pu decays as follows: Thus, many radioactive decay products accumulate from the decay of ²³⁶Pu, and these decay products must be periodi­ cally removed from the counters by cleaning to maintain low counter backgrounds. Accordingly, it is the principal object of the present invention to provide a method for producing ultralow-mass fissionable deposits for reactor neutron dosimetry by recoil ion-implantation capable of eliminating the need for isotopic spiking procedures. With this object in view, the present invention resides in a method for producing a fissionable deposit of selectively low mass for neutron dosimetry, characterized by the steps of: (a) spacing in opposing relation a substrate and an alpha-emitting parent source which decays to implant into the substrate a fissionable daughter ejected from the parent source as a result of the decay; and (b) holding the opposing relation for a period of time until the parent source decays to form a corres­ ponding mass of the fissionable daughter on the substrate. For example, a ²⁴¹Am source in thin layer form can be placed next to a substrate such as a solid state track recorder in a vacuum which houses an assembly for rotating opposing disks receiving the alpha emitting source and the solid state track recorder, respectively. Each alpha decay of ²⁴¹Am results in a ²³⁷Np ion with enough recoil energy to be implanted into the solid state track recorder. Fissionable deposits of ²³⁹Pu, ²³⁵U, and ²³⁸U can also be made by using this method and apparatus. Fissionable deposits with masses appropriate for high neutron fluence dosimetry are thusly prepared. The accompanying drawing, which is incorporated in and constitutes a part of the specification, illustrates an embodiment of the invention, and together with the description, serves to explain the principles of the invention. Figure 1 is a schematic view of the apparatus according to the present invention for producing ultralow-­ mass fissionable deposits by recoil ion-implantation. The method for producing ²³⁵U and the apparatus used therewith are described first. ²³⁵U is produced by the decay of ²³⁹Pu according to the following Equation (1): The alpha (α) particles emitted by ²³⁹Pu have an energy of 5.16 MeV. Conservation of momentum dictates that: (α Energy) (α Mass)=(Recoil Energy) (Recoil Mass) (2) Using Equation (2): Recoil Energy = (4/235) (5.16 MeV) (3) = 8.78 × 10⁻² MeV (4) = 87.8 keV (5) As a result, each ²³⁹Pu decay yields a 5.16 MeV alpha particle accompanied by an 87.8 keV ²³⁵U recoil. If a thin deposit of ²³⁹Pu is placed in a vacuum, about 50% of the alpha decays thereof will result in a ²³⁵U ion being recoiled into the vacuum. If a suitable substrate is placed in the vacuum adjacent to the ²³⁹Pu source, the recoil ²³⁵U ions will be implanted in the surface of the substrate, resulting in an ultralow-mass deposit of ²³⁵U. The present invention is best suited for alpha recoil ion implantation because beta or gamma emissions lack sufficient momentum for the respective daughter to be implanted in the substrate. By using an extrapolation of the well-known range-energy tables found in "Range and Stopping Power Tables for Heavy Ions," L.C. Northcliffe and R. F. Schilling, Nuclear Data Tables, A-7, 233-463 (1970), the recoil range of 87.8 keV ²³⁵U ions is estimated to be about 10µg/cm². Thus, a 10µg/cm² layer of ²³⁹Pu would retain about 50% of its recoils, so that an effective maximum thickness of a ²³⁹Pu layer can be about 5µg/cm². For a 10 cm diameter layer of ²³⁹Pu of a thickness greater than the range of the ²³⁵U recoils, the number of ²³⁵U recoils that would escape from the ²³⁵Pu layer is calculated as understood in the art to be: For a substrate at a distance of 5 cm from this ²³⁵Pu source, roughly 2.5 × 10⁸ ²³⁵U ions/cm²/day of ²³⁵U would be implanted. The ²³⁵U mass requirement for a mid-plane reactor cavity solid state track recorder neutron dosimeter is for instance 1.5 × 10⁻¹³ grams. This deposit is typically spread over an area with a diameter of 1/4 in. The resulting atomic density of the deposit is 1.21 × 10⁹ atoms/cm² of ²³⁵U. The time required to produce a deposit with this thickness is: Solid state track recorder neutron dosimeters typically have an overall diameter of 1.1 cm. As a result, about 25-50 solid state track recorders could be simultaneously irradiated adjacent to a 10 cm diameter ²³⁵Pu source. An apparatus such as that shown in FIG. 1 and referred to generally by reference numeral 1, which will now be described, is used. The apparatus 1 includes an assembly 10 enclosed in a vacuum chamber 12. The assembly 10 has a disk 14 opposing a disk 16, with a separation of about 5 cm therebetween. The disk 14 and the disk 16 are each about 10 cm in diameter. Preferably, high purity nickel is used as the material of the disk 14. Any other high purity metal, such as stainless steel, can be substituted as long as good adherence characteristics exist between the radioactive parent described below and the material of the disk 14. The bottom surface of the disk 14 is plated with a layer 19 of 10µg/cm² of ²³⁹Pu which is the radioactive parent source. The disk 16 includes thereon facing the layer 19, at least one substrate 15, for example, a plurality of mica, quartz or zircon solid state track recorder wafers 17, each about 1.1 cm in diameter, for receiving a fissionable daughter 21 deposited by recoil ion-implantation. The disk 14 is rotated about an axis 22 through the center of each disk 14 and 16 via, e.g., a motor 18, to ensure deposit uniformity. Each of the wafers 17 is also rotated about its respective axis relative to the disk 16. This results in a uniform deposit on each wafer 17, and a calibration function for the apparatus wherein the amount of respective uniform deposit on each wafer can depend on the radial position of each wafer from the axis of rotation of the disk 14. This rotation can be accomplished, e.g., by using a set of gears at each individual wafer 17 which is operatively connected to a motor to form a drive means 20. Disk 16 may also be rotated by the drive means 20. Generally, there should be at least two independent rotations, preferably of the source layer 19 and each wafer 17. It is preferable that the plurality of wafers 17 be loaded in a clean (laminar flow) area prior to rapid insertion into the vacuum chamber 12 which would be located in a glovebox. A glovebox is a container conventionally used to isolate from the environment radioactivity from a source in the glove box. When put into practice, this new method is simpler and more economical than previous methods. In addition, some of the fissionable deposits formed according to the present invention can be very durable, such that they are able to withstand scrubbing to clean the substrate. About 25 deposits can be made in 5 days, and deposit calibration requirements diminish as the assembly 10 is calibrated. That is, a set of trial experiments can be preformed to measure how many ions hit a wafer 17 as a function of time. For example, where a ²⁴³Am source is used to make ²³⁹Np recoils which would decay to plutonium, as described below, the assembly 10 is run under a carefully controlled set of conditions. Then it is determined radiometrically how much ²³⁹Np is present on the wafer 17. This calibration can then be used to produce other wafers 17, without requiring further re-calibration. That is, if a certain recoil thickness is made on a wafer 17 in 10 minutes, a corresponding doubled recoil thickness would be expected when the irradiation is allowed to occur for 20 minutes. Furthermore, the production of uniform ²³⁵U, ²³⁷Np, ²³⁸U, ²³⁹Pu deposits as described herein simplifies the track counting enormously. This is because quantitative track counting over the entire surface of non-uniform deposits is required. With this new method, only sampling of the uniform track density is required. An alternate embodiment directed to producing ²³⁷Np will now be described. ²³⁷Np is produced by the decay of ²⁴¹Am as follows: The alpha particles emitted by²⁴¹Am have an energy of 5.49 MeV. Using Equation (2): Recoil Energy = (4/237) (5.49 MeV) (10) =9.27 × 10⁻² MeV (11) =92.7 keV (12) As a result, each ²⁴¹Am decay yields a 5.49 MeV alpha particle accompanied by a 92.7 keV ²³⁷Np recoil. If a thin deposit of ²⁴¹Am as determined according to the preceding example is placed in a vacuum, a corresponding amount of the decays will result in a ²³⁷Np recoil into the vacuum. If a substrate is placed in the vacuum adjacent to the ²⁴¹Am source as described above, recoil ²³⁷Np ions will be implanted in the surface of the substrate to provide an ultralow-mass deposit of ²³⁷Np. By again using an extrapolation of the above-­ cited range-energy tables of Northcliffe and Schilling, the recoil range of 92.7 keV ²³⁷Np ions is estimated to be about 10µg/cm². Thus, a 10µg/cm² layer of ²⁴¹Am would retain about 50% of its recoils, or, stated otherwise, the effective maximum thickness of an ²⁴¹Am layer is about 5µg/cm². For a 10 cm diameter layer of ²⁴¹Am of thickness greater than the range of the ²³⁷Np recoils, the number of ²³⁷Np recoils that would escape from the ²⁴¹Am layer is calculated as above to be: For a substrate at a distance of 5 cm from this ²⁴¹Am source, roughly 1.3 × 10¹⁰ ²³⁷Np ions/cm²/day would be implanted. The ²³⁷Np mass requirement for a mid-plane reactor cavity solid state track recorder neutron dosimeter is for instance 6.3 × 10⁻¹¹ gram. This deposit is typically spread over an area with a diameter of 1/4 in. The resulting atomic density of the ²³⁷Np deposit is 5.05 × 10¹¹ atoms/cm². The time required to produce a deposit with this thickness is: As stated above, solid state track recorder neutron dosimeters typically have an overall diameter of 1.1 cm. About 25-50 solid state track recorders can be simultaneously irradiated adjacent to a 10 cm diameter ²⁴¹Am source. An arrangement such as that shown in Figure 1 and described above could be used. An alternate embodiment directed to producing ²³⁸U will now be described. ²³⁸U is produced by the decay of ²⁴²Pu as follows: The alpha particles emitted by ²⁴²Pu have an energy of 4.90 MeV. Using Equation (2): Recoil Energy = (4/242) (4.90 MeV) (17) = 8.10 × 10⁻² MeV (18) = 81.0 keV (19) As a result, each ²⁴²Pu decay yields a 4.90 MeV alpha particle accompanied by a 81.0 deV ²³⁸U recoil. If a thin deposit of ²⁴²Pu is placed in a vacuum, about 50% of the decays will result in a ²³⁸U recoil into the vacuum. If a substrate is placed in the vacuum adjacent to the ²⁴²Pu source as described above, recoil ²³⁸U ions will be implanted in the surface of the substrate, resulting in an ultralow-­ mass deposit of ²³⁸U. By again using an extrapolation of the above-­ cited range-energy tables of Northcliffe and Schilling, the recoil range of 81.0 keV ²³⁸U ions is estimated to be about 10µg/cm². Thus, a 10µg/cm² layer of ²⁴²Pu would retain about 50% of its recoils so that an effective maximum thickness of a ²⁴²Pu layer would be about 5µg/cm². For a 10 cm diameter layer of ²⁴²Pu with a thickness greater than the range of ²³⁸U recoils, the number of ²³⁸U recoils that would escape from the ²⁴²Pu layer is: For a substrate at a distance of 5 cm from this ²⁴²Pu source, roughly 1.6 × 10⁷ ²³⁸U ions/cm²/day would be implanted. The ²³⁸U mass requirement for a mid-plane reactor cavity solid state track recorder neutron dosimeter is 7.0 × 10⁻¹⁰ gram. This deposit is typically spread over an area with a diameter of 1/4 in. The resulting atomic density of the deposit is 5.59 × 10¹² atoms/cm² of ²³⁵U. The time required to produce a deposit with this thickness is: Although such a time requirement is larger than practical for production of reactor cavity dosi­ meters, the production of dosimeters for higher fluence application where the flux is a factor of 10³ to 10⁵ higher than in the reactor cavity is clearly feasible. As stated above, solid state track recorder neutron dosimeters typically have an overall diameter of 1.1 cm. About 25-50 solid state track recorders can be simultaneously irradiated adjacent to a 10 cm diameter ²⁴²Pu source. An arrangement such as shown in Figure 1 and described above can be used. A still further embodiment directed to producing ²³⁹Pu will now be described. ²³⁸Pu is produced by the decay of ²⁴³Am as follows: The alpha particles emitted by ²⁴³Am have an energy of 5.28 MeV. Again, using Equation (2): Recoil Energy = (4/239) (5.28 MeV) (24) = 8.84 × 10⁻² MeV (25) = 88.4 keV (26) As a result, each ²⁴³Am decay yields a 5.28 MeV alpha particle accompanied by an 88.4 keV ²³⁹Np recoil. If a thin deposit of ²⁴³Am is placed in a vacuum, a corresponding amount of the decays will result in a ²³⁹Np recoil into the vacuum. If a substrate is placed in the vacuum adjacent to the ²⁴³Am source as described above, recoil ²³⁹Np ions will be implanted in the surface of the substrate. These ²³⁹Np atoms will, over a period of weeks, all decay to ²³⁹Pu, resulting in an ultralow-mass deposit of ²³⁹Pu. By again using an extrapolation of the above-­ cited range-energy tables of Northcliffe and Schilling, the recoil range of 88.4 keV ²³⁹Np ions is estimated to be about 10µg/cm². Thus, a 10µg/cm² layer of ²⁴³Am would retain about 50% of its recoils, so that an effective maximum thickness of a ²⁴³Am layer can be about 5µg/cm². For a 10 cm diameter layer of ²⁴³Am of a thickness greater than the range of the ²³⁹Np recoils, the number of ²³⁹Np recoils that would escape from the ²⁴³Am layer is given by: For a substrate at a distance of 5 cm from this source, roughly 9.2 × 10³ ions/cm²/day of ²³⁸Np would be implanted. Assume for instance that the mass requirement for a ²³⁹Pu mid-plane reactor cavity solid state track recorder neutron dosimeter is 8.9 × 10⁻¹⁴ grams. This deposit is typically spread over an area with a diameter of 1/4 in. The resulting atomic density of the deposit is 7.08 × 10⁸ atoms/cm² of ²³⁹Pu. The time required to produce a deposit with this thickness is: As discussed above, solid state track recorder neutron dosimeters typically have an overall diameter of 1.1 cm. About 25-50 solid state track recorders could be simultaneously irradiated adjacent to a ²⁴³Am source of 10 cm diameter. An arrangement such as that shown in Figure 1 could be used. After deposition, a deposit of ²³⁹Np of for instance 8.9 × 10⁻¹⁴ gram has a beta or gamma activity of 4.59 × 10⁴ dpm, which allows easy mass calibration as it decays to ²³⁹Pu. Further, this method makes possible the prospect of applying solid state track recorder neutron dosimetry to higher neutron fluence measurements. An application of immediate relevance is the use of solid state track recorder neutron dosimeters in the pressure vessel surveillance capsules of operating commercial nuclear power reactors. Previous attempts at solid state track recorder neutron dosimetry in this location have not been fruitful because of formidable problems with high track densities resulting from inherent mass limitations in the electroplating methods used to produce fissionable deposits, dosimeter size limitations, and deposit uniformity. It is believed these problems can be overcome with the new method and apparatus described herein for producing for instance ²³⁵U, ²³⁷Np, ²³⁸U, and ²³⁹Pu fissionable deposits. In light of the above, the present invention has the following advantages over the previously known methods: (1) isotopic spiking methods are not needed; (2) electroplating methods and the resulting high purity chemical requirements are not needed; (3) uniform deposits can be made, whereas high deposit uniformity has not been achieved with electroplating methods; (4) only the apparatus need be calibrated, and, after calibration, the time of exposure will be proportional to deposit mass at a given substrate location; a variety of substrates may be used; (6) the masses of the ²³⁵U, ²³⁷Np, ²³⁸U, ²³⁹Pu deposits can be controlled precisely; (7) the resulting material in the ²³⁵U, ²³⁷Np, ²³⁸U, ²³⁹Pu deposit is isotopically pure; (8) extremely low masses are possible; (9) deposits formed by ion implantation are durable; (10) the present invention is simpler and more econo­ mical than previous methods; and (11) the present invention simplifies track counting significantly.

Claims: 1. A method for producing a fissionable deposit of selectively low mass for neutron dosimetry, character­ ized by the steps of: (a) spacing in opposing relation a substrate (15) and an alpha-emitting parent source (19) which decays to implant into the substrate (15) a fissionable daughter (21) ejected from the parent source (19) as a result of the decay; and (b) holding the opposing relation for a period of time until the parent source (19) decays to form a corresponding mass of the fissionable daughter (21) on the substrate (15). 2. A method as recited in claim 1, characterized in that the parent source (19) and at least one substrate (15) are disposed on respective, opposing holding means (14, 16), and the respective, opposing holding means (14, 16) are rotated relative to each other to promote uniformity of the implantation of the fissionable daughter (21) on the substrate (15) 3. A method as recited in claim 1, characterized in that the parent source (19) is selected from the group consisting of ²³⁹Pu, ²⁴¹Am, ²⁴²Pu and ²⁴³Am. 4. A method as recited in claim 1 or 2, charac­ terized in that the parent source (19) is selected so that the fissionable daughter (21) is ²³⁹Pu. 5. A method as recited in claim 1 or 2, charac­ terized in that the parent source (19) is selected so that the fissionable daughter (21) is ²³⁵U. 6. A method as recited in claim 1 or 2, charac­ terized in that the parent source (19) is selected so that the fissionable daughter (21) is ²³⁷Np. 7. A method as recited in claim 1 or 2, charac­ terized in that the parent source (19) is selected so that the fissionable daughter (21) is ²³⁸U. 8. A method as recited in any of claims 1 to 7, characterized in that the respective holding means (14) for the parent source (19) is a disk having its axis (22) aligned with an axis of relative rotation of the other holding means (16), and the parent source (19) is provided in the form of a layer on the disk (14). 9. A method as recited in any of claims 1 to 8, characterized in that a calibration is performed by mea­ suring the amount of the fissionable daughter (21) implanted on the substrate (15) during the period of time of the holding step (b); and subsequently a predetermined amount of the fissionable daughter (21) is implanted in a further sub­ strate (15); and the period of time being selected to form the predetermined amount of the fissionable daughter (21) on the further substrate (15). 10. A method as recited in any of claims 1 to 9, characterized in that wherein the first means (14) is of nickel, and the parent source (19) is in the form of a layer formed thereon.

Inventors: RUDDY, FRANCIS HENRY
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1987
Main classification: G01T3
IPC codes: G01T5, G01T3