Patent Number: 062597574
Section: description

The apparatus 30 according to the invention as illustrated in FIG. 1 has an Nd:YAG laser (Spectron SL 401), a spectrograph 4 with detector unit 23, an analyzing unit 6 and a delay unit 7 arranged between the analyzing unit 6 and the laser 1. A computer (not shown) is connected to the detector unit 23. The analyzing unit 6 is the four-channel analyzer system OMAIII from EG & G. The delay unit 7 is connected to the laser 1 and the analyzing unit 6 by way of suitable signal lines 24. To produce vaporization of the sample 3 the laser 1 is operated with its fundamental wavelength of 1064 nm and a pulse length of 5 ns. The pulse energy is preferably 20 mJ. The measuring head 14 which as shown in FIG. 2 is of a square configuration in plan can be fitted to the sample 3. Arranged thereon are a total of five tubular connecting portions 16, 17, of which only three can be seen in the side view in FIG. 1. The focussing unit 2 comprising the light guide 10 and the lenses 8 and 9 is arranged in the perpendicularly upwardly projecting portion 16 of the measuring head 14, in which respect it will be appreciated that the light guide 10 extends only over a comparatively short length within the connecting portion 16. The other four connecting portions 17 which project laterally away from the measuring head accommodate the four imaging units 5 each comprising the light guide 13 and the lenses 11, 12 and 11', 12' respectively. Because of the four imaging units 5 provided here, reference is also made to a four-armed light guide 13. The two connecting portions 17 which cannot be seen in FIG. 1 are disposed one behind the other in the direction of viewing onto FIG. 1 and symmetrically with respect to the plane of the drawing in FIG. 1. That arrangement ensures that an image of the maximum possible part of the light which is emitted in all directions in space is formed in the spectrograph 4. The light guides 10 and 13 are held in the connecting portions 16 and 17 by guide portions 25 and 26 respectively. The light guides used are preferably glass fibers or glass fiber cables. The technical data of the focussing and fiber optics of the focussing unit 2 can be seen in the table illustrated in FIG. 3. The technical data of the focussing and fiber optics of the four imaging units 5 are contained in the table shown in FIG. 4. As can best be seen from FIG. 2 the measuring head 14 has at its underside four feed conduits 22 through which a flushing or scavenging fluid flow can be fed as indicated by the arrows A into the chamber 15 of the measuring head 14. In that situation the flushing or scavenging fluid flow is intended to flow along the surface of the sample 3 and is intended to be of such a vertical extent that it flows as completely as possible around the laser-generated plasma 18. The fact that a flushing or scavenging fluid flows around the plasma means that disturbances to the plasma such as for example a reduction in plasma temperature due to air are avoided. The use of the inert gas argon as the flushing or scavenging fluid has proven to be particularly desirable. Preferably an argon flow of about 40 l/h is selected. FIG. 5 shows the structure of the optical systems used for coupling into the four-armed light guide 13. It is possible in particular to see the working spacing E of the 2-lens systems 11, 12 of the imaging units 5 in which the light-emitting plasma 18 must occur. The imaging units 5 used make it possible also to form the image of and analyze radiation from layers of the plasma 18, which are near to the surface. The table of FIG. 6 shows the pixel width and the simultaneously observable spectral window in various spectral regions of the four-channel analyzer system used. FIG. 7, 8 and 9 show the light guide coupling-in unit 19. It serves for optimum coupling of the laser beam coming from the laser 1, into the light guide 10. It has a laser beam introduction opening 21 and a light guide connection 20. The entire light guide coupling-in unit 19 is mechanically releasably fixed to the housing of the laser 1 so as to avoid disadjustment of the light guide fibers relative to the laser beam and thus possible destruction of the light guide fiber end surfaces in the event of disadjustment. To provide protection from impurities of the light guide fiber end surfaces, the fiber ends are flushed with dust-free air. A quartz lens with a focal length of 55.8 mm at the laser wavelength of 1064 nm is used for coupling the laser beam into the light guide fiber. The light guide fiber with a core diameter of 600 .mu.m is positioned about 14 mm behind the focal point of the lens. For optimum adjustment of the light guide fiber end relative to the laser beam, the light guide is allowed a total of five degrees of freedom. As shown in FIG. 8 at its end towards the laser 1, the light guide 10 which is connected to the light guide connection 20 is pivotable about its two transverse axes y and z and is linearly displaceable in the direction of those two transverse axes and its longitudinal axis x. To prevent the fiber end surface being destroyed by excessively high levels of intensity, during the adjustment operation an attenuation filter is disposed between the laser 1 and the fiber of the light guide 10 so that, with a pulse energy of about 10 mJ, only about 0.01 mJ/pulse is coupled into the light guide fiber. FIG. 10 shows the result of an investigation of the beam profile of the light issuing from the fiber of the light guide 10, which was obtained by means of a photodiode cell. This is the beam profile of the laser beam downstream of the light guide fiber with optimum coupling-in. It can be seen that the profile is symmetrical. After careful adjustment of the light guide fiber pulse energies of up to 30 mJ are coupled into the fiber without any problem. Those pulse energies are still about an order of magnitude beneath the specified destruction threshold of the light guide 10. As shown in FIG. 11, it was not possible to detect any transmission losses within the limits of measuring accuracy, in the light guide 10. The measurement procedures involved operating with a pulse energy of 20 mJ, while the radiation density (about 0.8 GW/cm.sup.2) of the light guide fiber end surfaces was about an order of magnitude below the destruction threshold. The measurements with the embodiment described herein, that is to say with the light guides 10 and 13 and the measuring head 14, were compared with measurements which were implemented with another embodiment according to the invention which does not have a measuring head 14 and light guides 10 and 13, but in which the laser beam could strike the sample 3 directly from the laser 1 through free space and the light emitted by the plasma 18 could pass directly through free space into the spectrograph 4. The measurements associated with those two embodiments are referred to hereinafter as "measurements with light guide" and "measurements without light guide". In the case of the measurements with light guide, a focus radius of about 320 .mu.m was produced on the sample 3. In the measurements without light guide a maximum signal/noise ratio was found to occur with a laser energy of 20 mJ and a focus radius on the sample 3 of 90 .mu.m, which corresponds to a radiation strength of about 6.3.times.1010 W/cm.sup.2. In the measurements with light guide the radiation density at the same pulse energy of the laser 1 was only about 5.times.109 W/cm.sup.2, because of the larger focus diameter. That resulted in a sample removal which was about an order of magnitude less. FIG. 12 shows the relative transmission of the four-armed light guide 13 for plasma radiation in the wavelength range between 200 and 600 .mu.m. Transmission measurement was effected by means of a mercury vapor lamp. The transmission curve shows the relative configuration with all optical components in the event of measurements in air. The intensity ratio of measurements with and without light guide is plotted in relation to wavelength. It can be clearly seen that there is a falling transmission configuration below a wavelength of about .lambda.=280 nm. FIG. 13 shows the absolute transmission, as specified by the manufacturer, of the light guide 13. The maximum absolute transmission is about 64%. FIG. 14 shows a calibration measurement in respect of chromium as against iron with three different matrices (steel: NBS-sample, glass: sample from Hoesch, aluminum: samples from Pechiney and Alusuisse). Atom emission lines at 425.435 nm (Cr(I)) and 302.639 nm (Fe(I)) were used. The intensity ratio is plotted in relation to the concentration ratio. Measurement was also effected with a time delay of 6 .mu.s after the laser pulse (pulse energy 20 mJ). The integration time was 40 .mu.s. 50 spectra were added up. The gradient of the evening-out straight line is one in a double-logarithmic representation. All measurement data lie on the straight line, within 5%. This shows that matrix-independent measurement is possible. This information is important for determining uranium and plutonium in glasses of different compositions. Slight changes in the composition of the fused-in sample material can alter absorption and reflection of the laser radiation, the amount removed and the plasma parameters. If however lines with similar excitation energy are selected, then upon complete atomization of the material removed and upon the assumption of a thermodynamic equilibrium the intensity ratio is proportional to the concentration ratio and thus substantially independent of the sample composition and the plasma temperatures. Samples 3 in respect of which different amounts are removed and in respect of which different temperatures occur in the laser-generated plasma 18 can thus be compared together. FIG. 15 shows spectra of the glass samples VG 98/12, GP 98/12 and GP 98/12+0.98%UO2. 50 spectra were integrated in each case, with a delay of 6 .mu.s and an integration time of 20 .mu.s. The sample removal was about 0.15 ng/shot with a radiation density of 5.times.109 W/cm.sup.2. Besides the uranium atom emission line at 591.539 nm, it can be seen that there are also two emission lines of the matrix constituent titanium (Ti(I) 591.855 nm and Ti(I) 592.212 nm), and an atom lines in respect of the protective gas argon (591.208 nm). The measurement operation shown in FIG. 16 was implemented under the same measuring conditions in relation to homogeneity of the distribution of uranium in the sample GP 98/12+UO2. Each measurement point represents the mean value from five measurements. It exhibits relatively homogenous uranium distribution, relative to the diameter of the focal spot of 320 .mu.m. The standard deviation of the individual measurements is between 6 and 12% while that of the mean value is 6%. FIG. 17 shows the signal/noise ratio of the uranium atom line 591.539 nm in dependence on time. Both in measurement with a light guide and also in measurement without a light guide, a maximum ratio can be seen at a time shift of 6 .mu.s. Admittedly, the intensity of the analysis lines is higher with shorter delay times, but they are on a very high foundation (recombination continuum of the plasma). In addition there is a certain matrix dependency with short time delays because the material removed is not yet completely atomized. The level of noise can be reduced by cooling the detector 23. That gives an improvement in the signal/noise ratio and thus also the detection limit. When using higher laser pulse energies, greater levels of line intensity are to be expected, due to the greater degree of removal. However the time window then has to be re-established for an optimum signal/noise ratio. Because of the lower level of intensity in the laser focus, the degree of sample removal in the case of measurements with a light guide is less than in the case of measurements without a light guide. However the measured intensity of the analysis lines is about 66% greater than in the case of measurements without a light guide if the degree of sample removal is comparable, due to a reduction in laser intensity, in the measurements without a light guide. FIG. 18 shows the time dependency of the signal/noise ratio with different integration times (10 .mu.s, 20 .mu.s and 30 .mu.s). An optimum ratio is obtained with an integration time of 20 .mu.s and a delay of 6 .mu.s. The table illustrated in FIG. 19 compares the measurements according to the invention without light guide 10, 13 on the one hand and with a light guide 10, 13 on the other hand. The comparison of the results shows that the absolute detection limits for uranium are lower in the case of measurements with a light guide than in the case of measurements without a light guide. The detection limit could be reduced approximately by the factor of 2-3 if the detector 23 were to be cooled, in which case flushing of the detector 23 with a dry gas would of course be required. In addition higher levels of pulse energy result in a greater degree of removal and, with a suitable choice in regard to delay of detection, lower relative detection limits. It would also be advantageous to use a spectrograph 4 of the highest possible resolution so that stronger detection lines which previously could not be adopted by virtue of line interference phenomena can be utilised for determining concentration. The apparatus according to the invention permits rapid analysis, of a simple structure, of highly radioactive samples 3. The apparatus required for analysis of the light emitted is transportable, in which respect during the measurement operation the measuring head 14 and the light guides 10, 13 represent stationary units while the laser 1, the spectrograph 4 and the detector 23 with connected computer can be installed in a mobile unit. The advantage of the measuring head 14 is in particular that it can be positioned as desired, while in addition it is possible to use flexible bendable light guides 10, 13 of very great length. Furthermore the measurements take place under an argon flow at atmospheric pressure so that there is no need to use closed sample chambers with expensive mechanisms for sample interchange. The measuring head 14 can be positioned at the sample 3 by means of a manipulator or a robot arm so that the experiment can be conducted under remote control and without direct contact with radioactive material.