A RANGE-FINDING METHOD AND DEVICE

Abstract: The invention relates to a method for determining the distance of a target in difficult industrial conditions, method in which (i) one of the gamma quanta produced from a radioactive source in annihilation and traveling in opposite directions is aimed at the target and the other one at a detector, which detects it, and time measuring starts; (ii) the quantum Compton-scattered from the target is detected by the same or a separate detector, and time measuring stops; (iii) the distance of the target is calculated from the time difference between (i) and (ii). The invention also relates to a device for carrying out the method, the device comprising a radioactive source, a scintillation crystal counter/counters, a time-amplitude converter (TAC) and a pulse height analyser (PHA).

Description: A range-finding method and device The present invention relates to a method and device for deter mining a distance on the basis of measuring the time of travel of gamma radiation. In the metal industry there are problems of range-finding re¬ lated to the surface level in ore reduction furnaces; these problems are complicated by the corroding and hot flue gases present in the range-finding space. The object of the present invention is to eliminate these problems. Among the range-finding methods currently used in industry, th method according to the present invention is in principle simi lar to pulsed time-of-flight laser rangefinding, in which the time of travel of light is used for range-finding. Owing to th limitations associated with the use of the laser, it cannot be used in all range-finding applications. The most significant o the limitations is perhaps the requirement of visual contact with the target to be measured by the laser beam. For this reason, for example, smoke, gases, dust, etc., prevent range- finding or hamper it. Another difficulty in the use of the laser appears in the measuring of targets having a high temper ature, which increases noise in the measuring receiver and thereby increases imprecision in the measurement. On the other hand, imprecision increases because the propagation speed of light changes when the temperature of the medium changes. How¬ ever, in industry there are numerous range-finding targets of the type mentioned above, and therefore it is necessary to de¬ velop other possible range-finding methods. It has now been observed surprisingly that gamma radiation can be exploited in range-finding. Gamma quanta travel in air at the vacuum speed of light, regardless of the pressure, humidit or temperature of the air. Furthermore, owing to their high energy they will penetrate media (1 - 10 g/cm ) , and so visual contact with the target is not needed. The method and device according to the invention are thus suitable for the determina tion of the surface level in the metal industry, for example i converters in the steel industry and in flash smelting furnace in the copper industry. Furthermore, the invention also has uses in other metal industry processes, in which light arc furnaces are used in which the melting and reduction of the batch cannot be monitored using present-day surface level meters. The surface to be measured may be metal, mineral, or metal covered by slag. The method and device according to the invention can also be used for determining the thickness of a layer of slag or ceramics on top of metal. It is a further advantage of the method and device according to the invention that they save energy by improving process control. The source used may be a positron-emitting radio isotope, for example an Na-22 isotope. In the method according to the invention, collimated gamma radiation is emitted to the target to be measured, and the scattered gamma radiation returning from the target is detecte using a detector. The invention is based on β + annihilation. When the source of radiation used is, for example, an N-22 radio isotope, a 1.28 eV gamma quantum and a positron are produced as a result of radioactive decay. The life of the positron produced is short, after which it is annihilated with an electron. In the annihilation the masses of the positron an the electron are converted into electromagnetic radiation ener gy. Thus two 0.5 MeV gamma quanta, departing in precisely op¬ posite directions and usable in the measuring, are produced. One quantum being emitted from the source of radiation is de¬ tected by the START detector, and time measuring starts. The other quantum travels in the opposite direction on the straig line which connects the source and the detector. The gamma be thus formed is aimed at the target the distance of which is t be determined. When the gamma radiation impinges upon the meta surface, Compton scatter occurs. A quantum scattered from the target will depart with a probability corresponding to the solid angle towards the END detector and, when impinging upon it, will stop the measuring of the time. It is also possible t use the START detector as the END detector if the measured distance is so long that the START and END pulses can be dis¬ tinguished from each other by the same detector. Since it is known that gamma radiation travels at the vacuum speed of light, the desired distance between the detectors and the meta surface can be calculated from the travel time of each gamma quantum. The gamma quanta are detected by using scintillation crystals; when impinging upon them they produce a pulse of light. The intensity of the pulse of light depends on the scin- tillator and on the energy of the quantum impinging upon the crystal. The produced pulse of light is amplified and converte into electric form by using a photo-multiplier tube (PMT). The pulses received from the tubes are applied to a time-amplitude converter, which converts into an output pulse amplitude the time difference between the gamma quanta which have impinged upon the START and END detectors. The measuring result obtained is stored in a pulse height analyzer. A diagram of the prin¬ ciple of the measuring system is depicted in Figure 1. The detectors used in the device should be maximally rapid; at present, scintillation crystal detectors are such. The scin- tillator used may be, for example, barium fluoride (BaF 2 ) or a scintillation plastic (e.g. NE 104). The electronics of the device according to the invention in¬ cludes the actual time-interval measuring electronics, which is optimized for the BaF scintillation crystal detectors used as gamma detectors, and a PC-based result collection device. BaF detectors have the advantage of good detection effective¬ ness. However, the pulses of light received from the detector have a component having a long time constant (approx. 600 ns), which may at high frequencies weaken the performance of the measuring electronics, unless the electronics is designed spe¬ cifically for the pulses of BaF detectors. The output of the time-interval measuring electronics is an analog pulse, the height of which is proportional to the time interval being measured. The pulse is AD-converted and fed to the PC for the collection and analysis of the results. The apparatus is in¬ stalled, for example, above a converter so that it can be used for determining the surface level of molten steel in the con¬ verter. The steel is at a temperature of approx. 1500 °C, and so the apparatus must be capable of withstanding considerable radiation of heat. This is achieved by using thermal shielding and cooling. The essential characteristics of the invention are disclosed i the accompanying claims. There is a random error, corresponding to approx. ±5 cm of dis tance, in the travel time of each quantum pair. The error de¬ creases on the basis of the law of the square root of the num¬ ber of observations, i.e. when the number of observations in¬ creases one-hundred-fold, the error decreases to one-tenth. If the number of observations is N, the random error has been decreased to 1/-/Nth. The number of gamma pairs detected is restricted by the fact that the Compton-scattered quanta depar almost uniformly in all directions, so that the return quanta detected by the END detector are reduced in an inverse propor¬ tion to the square of the distance. The intensity of the sourc cannot be increased indefinitely, since the START detector wil be saturated. On the other- hand, if the activity of the source is increased, the probability that the quanta causing the START signal and the END signal are derived from different annihilations in¬ creases. However, this probability is low, since the dead time of the TAC is shorter than the average interval of a new quan¬ tum arriving in the beam of the START detector. Besides, rando measuring results will not accumulate within a delay corre¬ sponding to the distance of the target of the range-finding. Factors affecting the measuring rate include the geometry of the system, the activity of the source, the degrees of effec¬ tiveness of the scintillation crystals, and the material of t target. The Compton scatter taking place at the target also h an effect on the measuring rate, but this factor cannot be af fected. Figure 2 depicts the geometry of the measuring. It ca be seen in Figure 2 that the solid angle of the START detecto determines the direction of the gamma quanta used in the meas¬ uring. For this reason the average pulse rate T start can be calculated as a proportion, determined by the solid angle, of the activity: start = where e ι = effectiveness of START r 2 = radius of START Q = activity of the source [Bq] l χ = distance between START and the source The activity is taken in double, since two annihilation gamma quanta are produced per each radioactive decay. At the target the scattering is from electrons the number of which N e is obtained on the basis of the properties of the material, the area A 2 , and the penetration depth s of the scat¬ tered quanta: p«Z N β = -N A «A 2 »s (2) M where P = density of the target Z = ordinal number of the atoms of the target M = mass number of the target [g/mol] N A = Avogadro constant 6.202 E23 [1/mol] s = mean penetration depth of gamma quanta A 2 = area in the object, determined by the geometry The ratio of the quanta scattered into a certain solid angle t the quanta arriving within the area A 2 is (3) dβCTα? where = area of electron action in return Compton dΩ scatter The probability of quanta departing from the target towards th END detector is determined on the basis of the solid angle of the detector: %nd N.« (4) where Ω 2 = solid angle of END at the target r 3 = radius of END 1 3 = distance between END and the target When the effectiveness e 3 of the END detector is further taken into account, the formula obtained for the measuring rate is: = T start' P end " e 3 - • * • ra. 2p»Z dσ_. T = eι-< r 3 • •N_v»S» » Q 3 m τi 2 lx (5) M dΩ The method according to the invention is applicable to uses i which a high measuring rate is not required. The rate can be increased by increasing the activity of the source and by usi more effective scintillation crystals. The limit for rate in¬ crease is at present the highest possible rate of START pulse which is in the order of 100,000 - 200,000 1/s. When scintill tion crystals having a diameter of approx. 5 cm are used, thi limit has the effect that results will accumulate at a rate o approx. 10 per minute from a distance of six meters. When ave aging of 100 results is used, results are obtained at approx. six-minute intervals and the imprecision of the distance is i this case ±0.5 cm. The resolution is dependent not only on the resolution of the A/D converter of the pulse height analyzer but also on the resolution of the TAC, on the various delays of light produc¬ tion by scintillation crystals, on the size of the source, and on the travel time differences in the photo-multiplier tubes a the different points of the tube lens. Of these, the source worsens the resolution of a single measurement directly accord ing to its size. The geometry of the measuring also has its effect on the resolution, owing to the distance difference caused by scatter from different points of the target. Typical ly the half-value width of the result distribution, due to the measuring apparatus, is approx. 0.3 ns, which corresponds to 4.5 cm in the determination of distance. The resolution of a single measurement obtained according to the half-value width of the method and device according to the present invention is 10 cm. The resolution can be- improved by averaging the measure ment results. Depending on the number N of the results, the resolution will improve by the coefficient 1/./N. As a result of double coincidences caused by background radia¬ tion, by temporally successive annihilations, and by 1.28 MeV gamma quanta produced in the source, random measuring results are produced which are distributed rather evenly over the en¬ tire range (0 - 60 ns, i.e. 0 - 9 m). The distribution of the results from double coincidences is dependent on the locations of the START and END detectors. In addition, results are ob¬ tained from random scatterings from around the system. In order to reduce extra measuring results, the source and the detectors should be shielded as well as possible by enveloping them with, for example, lead in all directions except in the direction of the target. The invention is elucidated below with the help of examples. In the measurements, the geometry according to Figures 1 and 2 was used, in which the following dimensions were maintained constant: The properties of the materials used were as follows: Target: iron p = 7.83 g/cm 3 Z = 26 M = 55.8 g/mol s -= 2.3 cm (experimentally obtained thinnest iron thickness with a scatter of 100 %) Scintillation crystals: NE 104 e x = e 3 = 0.2 r x = r 3 = 2.5 cm Source: Na-22 Q = 0.3 mCi = 11.1 E6 Bq Example 1 The effect of the background was investigated using the above- mentioned values and geometry. The random results were dis¬ tributed over 3200 discrete distance values so that the average rate per channel was 0.4 1/h. Reducing the background by means of lead shields reduced the background to 0.1 1/h. Thus the proportion of the results which had accumulated within the range of the half-value width, of all of the measuring result increased from 15 % to 32 %, the distance being 120 cm. Figure 3 depicts an example of the distance spectrum. With th ADC settings used, channel 2803 corresponded to a distance of and channel 2000 to a distance of 226.9 cm. The figure shows that the peak of the result distribution is on channel 2307, corresponding to a distance of 140 cm. The extra accumulation between channels 2500 and 2800, visible in the figure, is due to double coincidences and to random scatterings from the lea shields. Example 2 The effect of a layer of a medium in front of the target was measured. A chipboard 2 cm thick, sand layers of different thicknesses, bricks, and slag tiles were used between the source and the target. The chipboard was used for checking whether range-finding can be carried out through an obstacle. In the measurement, a chi board 20 mm thick was placed between the source and the target at a distance of 75 cm from the source, the distance of the steel target being 140 cm. The number of measuring results obtained from the steel was reduced by 30 %, and the chipboard caused its own, slightly smaller accumulation. Nevertheless, the accumulation of results caused " by the steel was clearly observable. The distance spectrum is shown in Figure 4. A layer of sand in front of the target caused, on the basis of the measurements, an increase in the half-value width of the result distribution, depending on the thickness of the sand layer. Figure 5 shows the effect of sand layers of 2.2 cm and 6.2 cm on the distance spectrum. In the measurements the sand was placed in a box made of plexiglass, for which reason the thickness of the medium layer was 0.6 cm greater than that of the sand layer. The results showed that the half-value width increased by approx. 1.7 mm per one cm of the sand layer. In Figure 5, channel 2626 corresponds to a distance of 50 cm, and the channel interval is 2.83 mm. With slag and bricks the re¬ sults were similar to those obtained with sand. It was noted that, if precise results are desired, the shape of the distance spectrum should be analyzed more precisely from the layer thicknesses, and attention should be paid to the width of the spectrum. This, of course, requires considerably longer measur¬ ing times than used in mere range-finding.

Claims: Claims 1. A method for determining the distance of a target, char acterized in that (i) one of the gamma quanta, traveling in opposite directions, created from a radioactive source in β + annihilation, travels to the target and the other one to the detector, which detects it, and time measuring starts; (ii) a quantum Compton-scattered from the target is detected by the same or a separate detector, and time measuring stops; (iii) the distance of the target is calculated from the time differ ence between (i) and (ii) . 2. A method according to Claim 1, characterized in that the target is steel. 3. A method according to Claim 1, characterized in that the radioactive source used is an Na-22 radio isotope or some othe positron emitter. 4. A method according to Claim 1, characterized in that there is a layer of a medium between the target and the source 5. A method according to Claim 1, characterized in that there is a layer of medium on the target. 6. A method according to Claim 5, characterized in that the layer of medium on the target is slag. 7. A method according to Claim 5 or 6, characterized in that the thickness of the medium layer on the target is meas¬ ured on the basis of the width of the distance spectrum. 8. A device for determining the distance of a target, char¬ acterized in that it comprises: (a) a radioactive source in which two gamma quanta, traveling in opposite directions, are produced; (b) a first scintillation crystal detector, which detects the first gamma quantum and is located on the opposite side of the radioactive source in relation to the target; (c) a second scintillation crystal detector, which detects the second gamma quantum, traveling to the target and being Compto scattered from it, which detector may be the same as the detec tor detecting the first gamma quantum or another, separate detector; (d) a time-amplitude converter, to which the pulses from the first and the second photo-multiplier tubes are taken and whic converts the time difference between the observed first and second gamma quanta into an output pulse amplitude; (e) a pulse height analyzer, in which the measuring result is stored and printed out. 9. A device according to Claim 8, characterized in that the target is metal, preferably steel. 10. A device according to Claim 8, characterized in that the radioactive source is an Na-22 radio isotope or some other positron emitter.

Inventors: KARRAS MATTI, KOSTAMOVAARA JUHA, OIKARINEN JORMA, KARRAS, MATTI, KOSTAMOVAARA, JUHA, OIKARINEN, JORMA
Applicants: RAUTARUUKKI OY
Application year: 1990
Main classification: G01S17
IPC codes: G01S17