METHOD FOR PERMANENTLY RECORDING HIGH NEUTRON FLUENCE

Abstract: A method for producing a high fluence neutron dosimeter which serves as a permanent record for fission rate measurement wherein an alpha-emitting source which is a radioactive parent of the daughter isotope of interest is used to implant recoil ions resulting from alpha decay into a substrate such as a mica solid state track recorder which is then exposed to neutrons, etched, and both the induced fission tracks and recoil ion tracks are counted. As a result, the fission rate in fissions per atom can be calculated.

Description: The invention relates to a method and apparatus for producing fissionable deposits for reactor dosimetry and, more particularly, to a method and apparatus for producing fissionable deposits of ultralow-mass on a substrate to serve as a dosimeter with a permanent record of high neutron fluence. 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 information for neutron dosimetry purposes. A solid state track recorder placed adjacent to a fissionable deposit record track from the recoiling fission fragments which result from the fissions 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, which is proportional to the fission rate per atom of the fissionable deposit and the fluence of the neutron field, given by the integration of the neutron flux over the period of exposure. 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, usually about 10⁶ tracks/cm², without excessive track overlap. 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 gram are required to produce a usable track density in a solid state track recorder. Similarly, low masses of other isotopes, such as ²³⁷Np, ²³⁸U and ²³⁹Pu, are required for dosimetry in light water reactor pressure vessel surveillance. It has been found that the technical problems associated with the production of such low-mass deposits can be overcome by using radioisotopic 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 (1.85y half-life) as a spike for ²³⁹Pu. These electroplating procedures have inherent limitations, however, due to radioisotopic spike limitations, such as chemical impurities, etc. which lead to minimum masses of each isotope that can be produced and still give a meaningful signal above background in a neutron field. Extension of deposit fabrication techniques to lower masses is described in co-owned and co-pending U.S. Application Serial No. 897,466 (W.E. 53,266), entitled "Method And Apparatus For Producing Ultralow-Mass Fission able Deposits For Reactor Neutron Dosimetry By Recoil Ion-Implantation". The recoil ion-implantation techniques described therein use an alpha emitting source which is a radioactive parent of the isotope of interest to implant recoil ions resulting from alpha decay into a suitable substrate. For example, an ²⁴¹Am source in thin layer form is placed next to a substrate layer in a vacuum. Each alpha decay of ²⁴¹Am results in a ²³⁷Np ion with enough recoil ene rgy to be implanted in the substrate. As described therein, fission deposits with masses appropriate for high neutron fluence dosimetry are thus prepared. In order to accurately determine the fission rate of a solid state track recorder neutron dosimeter produced according to the recoil ion-implantation techniques described in the referenced application, both the number of fissions that has occurred and the mass of the fissionable deposit must be determined to high accuracy. The former is simply determined from the number of fission tracks formed in the solid state track recorder. The latter requires careful and time-consuming mass calibration measurements on the fissionable deposit. Once calibrated, detailed records of the mass calibration must be maintained to ensure that the fission rate of the solid state track recorder neutron dosimeter can be calculated after exposure. These mass calibrations are usually done radiometrically, requiring accurate determinations of the uncertainties due to isotope half life, radiation branching ratios, detector efficiency, etc. It is the principal object of the present invention to provide a method and apparatus for producing a high fluence neutron dosimeter wherein the masses of the fissionable deposits produced can be controlled easily and precisely so that permanent accurate records of data of radiation exposure can be achieved. With this object in view, the present invention resides in a method for producing a permanent record for neutron dosimetry with a fissionable deposit of low mass, characterized in that ions from a parent source are implanted into a substrate to form a low mass of a fissionable daughter on the substrate; and the substrate is etched after exposure thereof to neutrons. For example, a ²⁴¹Am mica source in thin layer form can be placed next to a mica solid state substrate in a vacuum housing the assembly. Each alpha decay of ²⁴¹Am results in a ²³⁷Np ion with enough recoil energy to be implanted in the substrate recorder. If an ion-implanted mica solid state track recorder produced by this method is then exposed to neutrons, etched, and both the induced fission tracks and recoil ion tracks are counted, the fission rate in fissions per atom can be calculated. The accompanying drawing illustrates an embodiment of the invention, and together with the description, serves to explain the principles of the invention. The sole Figure is a schematic view of the apparatus according to the present invention for producing ultralow-mass fissionable deposits by recoil ion-implantation. The present invention is a new and simpler mass calibration method which also provides a permanent record of the deposit mass as well as of the number of fissions which has occurred in the deposit. Although the following description discusses in detail the implantation of ions into a solid state track recorder via alpha recoil ion implantation, other methods for implanting ions are available, e.g., using a particle accelerator to accelerate an ion and drive it into a substrate. These alternate methods are also contemplated by the present invention. .The preferred embodiments according to the present invention will now be described. 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: (αEnergy) (αMass) = (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 dep osit of ²³⁹Pu is placed in a vacuum, about 50% of the decays thereof will result in a ²³⁵U alpha recoil into the vacuum. If a suitable substrate is placed in the vacuum adjacent to the ²³⁹Pu source, recoil ²³⁵U ions will be implanted in the surface of the substrate resulting in an ultralow-mass deposit of ²³⁵U. By using an extrapolation of the range-energy tables found in "Range and Stopping Power Tables for Heavy Ions", L. C. Northcliffe and R. F. Schilling, Nuclear Data Tables, A7, 233463 (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. Accordingly, for a 10 cm diameter layer of ²³⁹Pu with a thickness greater than the range of ²³⁵U recoils, the number of ²³⁵U recoils that would escape from the ²³⁵Pu layer is: For a substrate at a distance of 5 cm from this ²³⁵Pu source, roughly 2.5 × 10⁸ ²³⁵U ions/cm²/day would be implanted. The mass requirement for a ²³⁵U mid-plane reactor cavity solid state track recorder neutron dosimeter is 1.5 × 10¹³ gram. 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 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 deposit by recoil ion-implantation. Mica is preferred due to its fine etching properties. 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 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 rotate via drive means 20. Generally, there should be at least two independent rotations, preferably the source layer 19 and the wafers 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 radioactivity emanating from a source therein from the environment. .When put into practice, this new method is simpler and more economical than previous methods. In addition, it has been found that the implanted substrates formed using mica according to the present invention are extremely durable. About 25 deposits can be made in 5 days, and deposit calibrat ion requirements diminish as the assembly 10 is calibrated. That is, a set of trial experiments can be performed to measure how many ions would hit a particular wafer 17 as a function of time. For example, where a ²⁴³Am source is used to make ²³⁹Np recoils which 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 there is on the wafer 17. This calibration can then be used to produce other wafers 17 without resorting to recalibration. 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 occurs for 20 minutes. Furthermore, the production of uniform ²³⁵U, ²³⁷Np, ²³⁸U and ²³⁹Pu deposits as described herein simplifies the track counting enormously. At present, quantitative track counting over 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 by 241Am 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 is placed in a vacuum, about 50% 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 resulting in an ultralow-mass deposit of ²³⁷Np. By again using an extrapolation of the range-energy tables cited above, 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, so that an effective maximum thickness of an ²⁴¹Am layer can be about 5µg/cm². For a 10 cm diameter layer of ²⁴¹Am with a thickness greater than the range of the ²³⁷Np recoils, the number of ²³⁷Np recoils that would escape from the ²⁴¹Am layer is: 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 mass requirement for a ²³⁷Np mid-plane reactor cavity solid state track recorder neutron dosimeter is 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 deposit is 5.05 × 10¹¹ atoms/cm² of ²³⁷Np. 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 ²⁴¹Am source with a 10 cm diameter. An arrangement such as that shown in Figure 1 and described above is 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 an 81.0 keV 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. I f 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 range-energy tables cited above, 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 such 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 mass requirement for a ²³⁸U 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 dosimeters, the production of dosimeters for higher fluence applications 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 could be simultaneously irradiated adjacent to a 10 cm diameter ²⁴²Pu source. An arrangement such as that shown in Figure 1 and described above could be used. A still further embodiment directed to producing ²³⁹Pu will now be described. ²³⁹Pu 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, about 50% 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 range-energy tables cited above, 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 or the effective maximum thickness of a ²⁴³Am layer is about 5µg/cm². For a 10 cm diameter layer of ²⁴³Am with a thickness greater than the range of the ²³⁹Np recoils, the number of ²³⁹Np recoils that would escape from the ²⁴³Am layer is: For a substrate at a distance of 5 cm from this source, roughly 9.2 × 10³ ²³⁹Np ions/cm²/day would be implanted. As stated previously, the mass requirement for a ²³⁹Pu mid-plane reactor cavity solid state track recorder neutron dosimeter is 8.9 × 10⁻¹⁴ gram. 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 recor der 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 10 cm diameter ²⁴³Am source. An arrangement such as that shown in Figure 1 can be used. After deposition, a deposit of for instance 8.9 × 10⁻¹⁴ gram of ²³⁹Np has a beta or gamma activity of 4.59 × 10⁴ dpm, allowing easy mass calibration as it decays to ²³⁹Pu. When a solid state track recorder fissionable deposit is prepared by the method described above using a mica substrate, the recoiling ions are implanted to a depth of about 7 × 10⁻³µm. Since the alpha recoils are highly ionizing heavy ions, latent tracks are produced in the mica solid state track recorder. The bulk etch rate for mica in 49% hydrogen fluoride at room temperature is about 4.4µm/hr. In an 8 second etch, tracks with breadths about equal to the ranges of the alpha recoils are produced. Although these tracks are not visible with an ordinary optical microscope, they can be observed with an electron microscope. Since the optical efficiency for perpendicularly incident heavy ions in mica is 100%, the number of recoil tracks observed is exactly equal to the number of recoil ions. If an ion implanted mica solid state track recorder is exposed to neutrons, etched, and both the induced fission tracks and recoil ion tracks are counted, the fission rate in fissions per atom can be calculated from the following relationship: The quantity ε is the optical efficiency for mica (ε = 0.9875 fission tracks/fission). In practice, one might etch the fission tracks and recoil tracks under different conditions. For example, a very small area of the solid state track recorder might be removed with a punch and etched for eight seconds for scanning with an electron microscope. The remainder of the solid state track recorder would be etched for about ninety minutes to optimize the viewing of fission tracks. Recoil tracks would have very little contrast after a ninety minute etch and would be difficult to observe. Alternatively, a set of etching conditions can be derived where the fission tracks and alpha recoil tracks are both developed for viewing after a single etch. The advantage of this approach is that each fission track could be correlated with a corresponding alpha recoil track to ensure that the fission has not arisen from contamination of the deposits. This requirement effectively lowers the background for the solid state track recorder substantially, since background tracks can be discriminated out of the total count. The use of this method and apparatus for solid state track recorder neutron dosimetry has all of the advantages inherent in the alpha recoil ion implantation techniques discussed above. In addition, a single solid state track recorder dosimeter can be used as a permanent record of all of the information needed to calculate the neutron-induced fission rate. Both the number of fissions and number of atoms of the fissioning species are permanently stored as a track record in the solid state track recorder. In addition, the correlations of fission and recoil atom tracks can be used to discriminate against background events. The permanent storage of all of the fission rate data as a track record in the solid state track recorder makes this invention particularly attractive for commercial nuclear power reactor surveillance measurements, where dosimetry records that span the reactor lifetime, about 40 years, are required by Nuclear Regulatory Commission regulations. Data storage and archiving are simplified enormously with this invention. In addition, as newer and more sophisticated methods of track analysis become available, the solid state track recorders prepared by this invention are available for a more detailed re-analysis, including analysis to higher statistical accuracy, if required. This invention also makes possible the prospect of applying solid state track recorder neutron dosimetry to higher neutron fluence measurements. An application of immediate relevance would be 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 invention described herein. In light of the above, the present invention has the following advantages over other methods 1. isotopic spiking methods are not needed; 2. electrodeposition 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 exposure time is proportional to deposit mass at a given substrate location; 5. a solid state track recorder can be used to produce a permanent record; 6. the masses of the ²³⁵U, ²³⁷Np, ²³⁸U and ²³⁹Pu deposits can be controlled precisely; 7. the resulting material in the ²³⁵U, ²³⁷Np, ²³⁸U and ²³⁹Pu deposit is isotopically pure; 8. extremely low masses are possible; 9. the present invention is simpler and more economical than previous methods; and 10. the present invention simplifies track counting significantly. The foregoing is considered illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. Accordingly, resort may be had to all suitable modifications and equivalents falling within the scope of the invention and the appended claims.

Claims: 1. A method for producing a permanent record for neutron dosimetry with a fissionable deposit of low mass, characterized in that ions from a parent source are implanted into a substrate to form a low mass of a fissionable daughter on the substrate; and the substrate is etched after exposure thereof to neutrons. 2. A method as recited in claim 1, characterized in that for implanting the ions the substrate and the parent source are spaced in opposing relation and the parent source which decays into the fissionable daughter, is maintained in position until the parent source decays to form the low-mass of the fissionable daughter on the substrate. 3. A method as recited in claim 2, characterized in that the parent source is rotated relative to the substrate to uniformly deposit the fissionable daughter on the substrate. 4. A method as recited in claim 1, 2 or 3, characterized in that the parent source is selected from the group consisting of ²³⁹Pu, ²⁴¹Am, ²⁴²Pu and ²⁴³Am. 5. A method as recited in claim 1, 2 or 3, characterized in that the parent source is selected so that the fissionable daughter is ²³⁹Pu, ²³⁵U, ²³⁷Np, or ²³⁸U. 6. A method as recited in any of claims 1 to 5, characterized in that at least two parts of said substrate are etched separately and the tracks of the implanted ion of the fissionable isotope are counted in at least a portion of a first of the parts, and, in at least a portion of the second part, the track of each fission of the fissionable isotope caused by the neutron fluence. 7. A method as recited in cla 6, characterized in that a value corresponding to the neutron fluence is determined from the counted numbers of tracks of the implanted ions and the fissions. 8. A method according to claim 6 or 7, characterized in that any tracks in said implanted, exposed solid state substrate due to background are discriminated against and excluded from the counting on the basis of there being no correlation between a track due to implantation and a track due to fission, and the value of the neutron flux is accordingly determined without error due to such background. 9. A method according to any of claims 1 to 8, characterized in that the substrate is a solid state track recorder consisting of mica, quartz or zircon. 10. The method according to claim 9, characterized in that for the ion implantation step, the substrate is disposed in a vacuum opposite a parent source that alpha-decays to eject as a daughter each ion of the fissionable isotope implanted in the solid state track recorder.

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