Patent Number: 048572594
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a neutron dosimeter 10 has a first layer 20 and a second layer 30. The first layer is made out of a fissile material. As used herein, "fissile" is intended to define a group of materials which produce energetic ions via fission interactions with fast or thermal neutrons. Such materials include but are not limited to .sup.235 U, .sup.239 Pu, .sup.238 U, and .sup.237 Np. A nonconducting form of the fissile material, such as a ceramic, can most easily be used. Indeed, it would be apparent to one of ordinary skill in the art that many materials would serve well as or possibly in place of the fissile material, provided they generate sufficient numbers of ions of energy sufficient to implant themselves in a second layer as will be described below. Sandwiched together with the first layer is a second layer 30. This second layer is made of a material which changes its conductivity in proportion to a concentration of implanted ions. For this reason, such material will be referred to as a CVI (Conductivity Varying with Ion implantation) material. Such materials include: poly (phenylene sulfide) (PPS); 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA); 1, 4, 5, 8-naphthalenetetracarboxylic dianhydride (NTCDA); Ni phthalocyanine (NiPc); and poly [N, N'- (p, p'-oxydiphenylene) pyromellitimide] (Kapton H). Of course, it will be apparent to one of ordinary skill in the art that other materials may suffice. These materials change thier conductivity on the basis of impurities, introduced either through doping techiques or through ion implantation. These materials are described, for example, in U.S. Pat. No. 4,491,605 to Mazurek et al issued Jan. 1, 1985 and in articles by S. R. Forrest, et al., Large Conductivity Changes in Ion Beam Iradiated Organic Thin Films, APP. PHYS. LETT., Volume 41, No. 8, p. 708 (Oct. 1982); H. Mazurek, et al., Electrical Properties of Ion-Implanted Poly (p-phenylene sulfide), J. OF POLYMER SCIENCE, Polymer Physics Edition, Volume 21, p. 537 (1983); and T. Hioki, et al., Electrical and Optical Properties of Ion-Irradiated Organic Polymer Kapton H, APPL. PHYS. LETT., Volume 43, No. 1, p. 30 (July 1983). In practice, neutrons, indicated by numeral 40, impinge on and enter the fissile material, where they induce the production of ions 50. A determinable fraction of the ions propagate to the second layer 30 of CVI material, where an again determinable fraction implant themselves. This implantion induces a change in conductivity (specifically an increase in conductivity) in the second layer 30 of CVI material. This conductivity is then measured by a conductivity measuring unit 60. Conductivity measuring unit 60 may comprise any suitable device, such as a 4-point probe for measuring sheet resistivity. The physics underlying the production of ions in the fissile layer is relatively well understood. Many types of neutron dosimeters now used commonly employ a layer of fissile material. The neutron capture results in fission fragment ions with energies on the order of 100 million electron volts (Mev). These energetic ions produce tracks in some dosimeters and in others they are used to ionize a gas. Various fissile isotopes have different neutron energy thersholds for fission. For example, .sup.235 U and .sup.239 Pu can be made to fission by neutrons having energies from thermal (on the order of 0.01 ev) through fast (on the order of 1 Mev), while the fission cross sections of .sup.238 U and .sup.237 Np show distinct fission thresholds versus neutron energy (on the order of 0.08 Mev for .sup.237 Np and on order of 1.5 Mev for .sup.238 U). Thus, proper selection of the fissile material makes it possible to obtain neutron spectral information, i.e., the energy distribution of impinging neutrons. Also, the use of cadmium coatings, which show near complete neutron absorption below 0.41 eV, can supply information sufficient to form an inference of thermal flux by comparison of data from coated versus uncoated samples. Such a method is currently known in connection with .sup.59 Co neutron activation in vessel dosimetry for inference of damage. In sharp contrast to the understanding of the physics underlying production of ions in the fissile material, the mechanism by which certain polymers can become conducting depending on the presence of impurities is still imperfectly understood. It is known, however, that the ions in their interactions with the polymer atoms break bonds thus forming free radicals. The radicals then combine and form small conducting "islands". As the dose level increases the islands grow in size and number and their average distance of separation decreases. At some point the sizes of, numbers of, and distances between the islands reach values which permit the conductivity to increase. In one proposed model, the electrons "hop" from one island to another. The temperature dependence of the induced conductivity would tend to substantiate this model. During the course of radiation, gases tend to escape the polymer, leading to denser and darker polymer surface. The rate of change of conductivity of the polymer is dependent on several factors. The first and most obvious factor is the identity of the polymer itself. Various polymers have been investigated, and each displays a different threshold dose at which conductivity begins to change. This is presumably dependent upon the ease or difficulty of forming free radicals in the material. Again, this varation of threshold dose creates the possibility that different polymers having different sensitivies can be used for different neutron fluxes. The second factor which apears to be of importance is the mass of the implanted ion. Ions with larger masses have a higher maximum ionization rate, i.e., a higher maximum energy deposited per unit volume. Thus, more free radicals are created per unit volume, thus enhancing the probability of forming conducting islands. For some applications, the rate of change of conductivity of unimplanted polymer may not be sufficiently great to provide sufficiently sensitive measurements. At least some CVI materials, however, tend to change conductivity more rapidly as ion implantation increases. Thus, it is possible to "precondition" the CVI material by pre-implanting ions in it to obtain a rate of change of conductivity with additional ion implantation which is sufficiently great to provide sufficiently sensitive measurements. If a conducting form of the fissile material is used, then it is also desirable to interpose between the first layer and second layer a third layer 65 of an electrically insulating material so that the conductivity of the first layer does not interfere with the measurement of the conductivity of the second layer. Of course, if an electrically nonconducting first layer is used, such as ceramic material, then it is not necessary to provide an intervening electrically insulating layer 65. FIG. 2 shows one potential application of a dosimeter according to the present invention. The numeral 70 designates generally a reactor core, comprising a matrix of fuel rods 80. The reactor core 70 is surrounded sequentially by a baffle 72, a core barrel 74, and a thermal shield 90. Positioned within the rector vessel, but outside the thermal shield, is a neutron dosimeter 10, electrically connected to a conductivity measuring unit 60. As an estimate of design parameters in this application, the fast neutron flux at a location such as that depicted in FIG. 2 might be as low as on the order 10.sup.10 n/cm.sup.2 /sec. The reaction rate at such location thus would be 3.times.10.sup.-15 reactions/atoms/sec for .sup.238 U. The thicker that a one square centimeter layer of this material is, the more reactions will be produced. This provides yet another mechanism for controlling the sensitivity of the detector up to some maximum useful thickness. For .sup.238 U, the ranges of 100 Mev fission fragments lie between 5 and 9 microns. Thus, a maximum useful thickness would be approximately 5 microns. A 1 cm.sup.2 layer of .sup.238 U, 5 microns thick, contains on the order of 2.4.times.10.sup.19 atoms. After a day at the aforementioned reaction rate, it would be expected that on the order of 6.times.10.sup.9 reactions would occur. After a month and a year, 2.times.10.sup.11 and 2.3.times.10.sup.12 reactions/cm.sup.2 would have accrued. As mentioned above, sensitivity will also depend on the selection of the CVI material, and whether the CVI material is perconditioned. The flexibility in design makes it possible to produce variation in conductivity as a function of neutron dose per unit area. Measurement could be automated with an instrument constructed to measure conductivity and provide a dose readout. For on-line applications, it would be necessary to shield gamma radiation from affecting the readout. The foregoing description has been in terms of a preferred embodiment merely for the purposes of illustrating the underlying principles of the invention. Nothing in the foregoing should be construed as limiting the invention to the specific embodiments discussed. Instead, it will be apparent to one of ordinary skill in the art that the concepts underlying the particular embodiment discussed herein have extremely broad application. Therefore, the invention should not be regarded as being limited to these specific embodiments, but instead should be regarded as commensurate in scope with the underlying concept, as reflected in the following claims.