Laser-Microscope System as a Microsampling Device M. D. Adams and S . C . Tong’ Argonne National Laboratory, 9700 South Cuss Ave., Argonne, Ill. 60439
A method of obtaining microsamples of refractory materials has been developed. A laser-microscope system is used to volatilize a chosen area which may range from 5 to 200 1.1 in diameter and 5 to 100 1.1 deep. Vaporized material is then condensed on a transparent cover plate held 0.2 to 0.5 m m above the specimen surface. Samples on the order of 10-8 gram taken from a specimen of irradiated UOzhave been examined by four methods of analysis: alpha spectrometry, gamma spectrometry, neutron activation, and mass spectrometry.
To INVESTIGATE the chemical behavior of ceramic nuclear fuels during irradiation, a method of sampling such fuels on a microscale was required. This requirement arises as a result of complex changes caused by the chemical and thermal environment of the fuel during irradiation. Fission products, some 35 new elements, in varying concentrations and with varying anionic demands, are generated. These elements, ranging from alkali metals through noble gases, when driven by a temperature difference of up to 2000 “Cbetween the center of the fuel element and the cladding, volatilize, segregate, combine to form compounds and solid solutions, diffuse, and interact with each other, the fuel materials, and the cladding. A cross section of a uranium oxide fuel rod may have the general aspect of Figure 1. This illustrates the various regions in a fuel produced largely by the temperature differences between the core and cladding of a single fuel rod during its lifetime in the nuclear reactor. Just inside the cladding is a narrow but critical region in which chemical interactions between fission products, fuel constituents, and cladding materials take place. Inside this interface is uranium oxide which closely resembles the original fuel before irradiation. Closer to the center of the fuel is a band in which extensive recrystallization of the uranium oxide has taken place. Growth has been random, producing small equiaxial crystals. Closest to the center of the fuel is an area of extensive solid solution formation and recrystallization which may have been partially molten. This is commonly called the columnar growth region because of the radial orientation of the crystals. At the thermal center of the fuel rod a hole has been formed by shrinkage of the compacted uranium oxide granules as they recrystallized. Determinations of the distribution of fission products in a fuel rod cannot be significant unless the fuel is sampled on a scale which is comparable to the microstructural features produced during irradiation. Fission product distributions have in the past been determined most often by gamma spectrometry or chemical analyses of microdrilled specimens (1-4). Microdrills used in such preparations are usually 0.25 mm in diameter or larger. Thus, accurate sampling within a 25-p diameter granule or an interfacial area between fuel and clad1
Present address, Corning Glass Works, Corning, N. Y .
(1) J. A. Christensen, R. J. Allio, and A. Biancheria, U. S. At. En-
ergy Comm. Rept. WCAP-6065 (1965). (2) C. L. Boyd, ibid., HW-79122 (1963). (3) R. D. Hahn, ibid., NAA-SR-11134 (1965). (4) H. Hausner, R. C. Nelson, M. F. Lyons and B. Weidenbaum, ibid., GEAP-4667 (1965). 1762
ANALYTICAL CHEMISTRY
ding is not possible. Similar problems exist for ultrasonic coring or drilling. The virtually explosive heating effect obtained by firing a laser through a microscope is well known (5, 6). Such devices are currently being used for delicate welding, and micromachining, and for microbiological and microanalytical purposes. Two applications are of particular interest in this regard: the laser excited emission spectrograph (7, 8) and laser sources for mass spectrometry (9,10). For these studies of nuclear fuel materials, a simple method for obtaining microsamples has been developed using a pulsed ruby laser-microscope system. An area from which a sample is wanted is positioned under the cross hairs of the microscope. When the laser is fired through the same objective lens, a small crater is formed in the specimen. Material ejected from this crater is condensed on a transparent cover plate. Just as it is deposited on a standard microscope cover glass supported 0.2 to 0.5 mm above the specimen, it is suitable for alpha or gamma spectrometry. Any type of cover plate may be used which does not absorb the 6943-.& radiation from the ruby laser. The mass of material ejected from craters 10 to 100 p in diameter weighs a few nanograms to over a microgram. This represents sufficient material for gamma spectrometric analysis with high resolution, but relatively low efficiency, lithium-drifted germanium detectors (11). With a 20-cc detector volume, counting times of 10 to 100 minutes have been required for these samples of fuel approximately 18 months after discharge from the reactor. Alpha spectrometry of the actinide elements present can be carried out on the same specimens, but counting times up to 1000 minutes are required. Two other methods of analysis, neutron activation and mass spectrometry, have shown promise of yielding useful information of nonradioactive materials. Successful development of these techniques would extend the usefulness of the laser microsampling method to other fields with microstructural problems such as metallurgy, biochemistry, and geochemistry. EXPERIMENTAL
The Optical System. A laser adapted for microscope use was assembled by Raytheon, Special Microwave Devices Operation, Waltham, Mass. The laser portion consists of their LH6 laser head with a power supply capable of driving the 6.35 X 90 mm ruby cry2tal to an energy output of approximately 0.5 J at 6943-A wavelength. Pulse length is about 1 msec. (5) A. 0. Schmidt and Inyong Ham, “Laser Welding and Machining,” Proc. Engineering Seminar on New Industrial Technologies at The Pennsylvania State University (1966). (6) N. A. Peppers, Appl. Optics, 4 (5), 555-8 (1965). (7) F. Brech, Appl. Spectros., 16, 59 (1962). (8) S. D. Rasberry, B. F. Scribner, and M. Margoshes, Appl. Optics, 6 (l), 81-6 (1967). (9) F. J. Vastola and A. J. Pirone, Am. Chem. SOC.,Div. Fuel Chemistry, Symposium on Pyrolysis Reactions of Fossil Fuels, Preprints, 10 (2), C-53-C-58 (1966). (10) G . H. Megrue, Science, 157, 1555-61 (September 29, 1967). (11) M. R. Banham, A. J. Fudge, and J. H. Howes, Analyst, 91, 180 (1966).
Figure 1. Cross section of irradiated UO, fuel rod 13% enriched, 6.8% burnup 4-mm diameter inside cladding
11
Figure 2. 0 0 t i c ~ ldiaaam of laser microscope
,
A diagram of the optical system is shown in Figure 2. For coupling of the laser to a microscope the beam is first passed through a I-mm diameter aperture in a ceramic plate (2). This intercepts approximately 9 7 z of the laser energy. Next is a positive lens (3) of40-mm focal length which can be moved within its mount to position the spot precisely with respect to a cross hair reticle (12). Approximately 70 mm below this lens is a long focal length (approximately 250 mm) negative lens (4) termed a compensating lens by the manufacturer. It is possible to use this system with an objective lens (8) to form craters. However, smaller craters can be formed when a conventional eyepiece (5) is added to the system below the negative lens. Focusing and location of areas to be sampled are done by means of a side arm and mirror (6) in the microscope tube. The eyepiece (13) in the side arm was made parfocal with the eyepiece through which the laser was fired. A Leitz UB microscope body is used as the frame for the system. This unit was selected because of the long working distance and flexibility afforded by both stage and body tube having independent focusing mounts. Illumination is supplied through a Leitz vertical illuminator (7) designed for the Ortholux microscope. To fit the UB body it was modified by turning the light tube 90". Depending on the light requirements, a low voltage mil:roscope illuminator or a 40-U? zirconium arc was used. Although a number of objective lenses have been used -- __.:.L - 1 2 . - r r "A., m most of the work has been dolls wu LWV, a LTU n ~ u n j u . . w ) and a Leitz R lOXj0.18. Both are infinity corrected lenses with working distances of 8.3 and 13.8 mm, respectively. Leitz Periplan eyepieces of 10 and 25 power were used with these objectives. Cover plates on which samples yere condensed could be of any material transparent at 6943 A. In practice, standard microscope cover glasses were used for most sampling. Quartz of several thicknesses and a polycarbonate plastic, Lexan (12), in thicknesses from 0.05 to 0.1 8 mm have.also been satisfactory. Diameter and depth of craters are dependent on the choice of microscope eyepiece and objective, the energy output of
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(12) General Electric Co., Chemical Materials Department, F'itts-
field, Mass.
6Y
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the laser, _. L1 -t. *, and the position of focus of the microscope. The smallest craters achieved, approximately 5 p in diameter, have been made with a 2 5 X eyepiece and a H 2OX objective at just above the laser threshold. The largest, over 200 p in diameter, were made with a 1OX eyepiece and 1OX objective at maximum laser energy, 0.5 J. If the microscope focus is raised to a point 200 p over the surface of the specimen before firing, a wide shallow depression is formed. This can be 200 p in diameter and less than 5 p deep. In this manner the laser can be of value for surface sampling. Cross sections of craters of UOz have shown that in some circumstances exceptionally deep craters can be formed. Figure 3 shows one such crater approximately 35 p in diameter and 70 p deep. These have been formed while using long working distance objectives at maximum laser energy. Deep craters such as this have not been produced in metal specimens with this equipment. If a cover glass is placed in contact with the specimen, a crater is formed in the undersurface of the glass as well as in the specimen. Material ejected from the specimen is driven into the glass. Craters or cracks were also formed in quartz plates in contact with the specimen. When the cover glass is held above the specimen by a washer 0.2 to 0.5 mm thick, there is little or no damage to the cover glass. Material ejected from the crater is deposited as a thin film with small frozen droplets dispersed over an area three to 10 times the diameter of the crater. Several sampling operations were carried out in a vacuum chamber. With the use of long working distance lenses mentioned previously, there was no difficulty in focusing through a 2-mm-thick quartz cover plate and the cover glass. Normal craters were produced. When the cover glass was examined with a microscope after removal from the vacuum chamber, the deposit seemed to be spread over a larger area, and there appeared to be fewer of the small frozen droplets over the center of the crater than were found in vaporizations in air. Because of the inconvenience and lack of appreciable advantage, no further work has been done with vacuum sampling. VOL 40, NO. 12, OCIOBER 1968
1763
Figure 3. Laser crater in UO, Materials. Inasmuch as this program is oriented toward nuclear fuel studies, most of the work has been done with UOZ as a target material. Unirradiated test specimens were pressed and sintered pellets made for irradiation testing programs. These were sliced, mounted in plastic, and polished by usual metallurgical techniques finishing with a 0.5-/.I diamond lap. In addition, a number of metal specimensCd, Mg, U, Ni, and Mn-were nreoared fnr testine in the same manner.
Table I. Alpha Spechrometry of Laser Samples from Irradiabcd UOI Fuel Pin
-mricnrneni: . . . .-13ib
Burnup:
No. 1
-9%
Net counts/1000 min 5.14MeV 5.48MeV 6.1 MeV 778
591
313
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1
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II,.._,I_?.
