Patent Number: 048760560
Section: description

The apparatus 10 includes an irradiation chamber 16, cylindrical in shape, made of steel coated with an anticorrosion plastic coating, and with a small vent hole 18 in its top. The chamber 16 encloses a copper tube 20 wound into a number of helically wound layers; the portion of the tube 20 forming the innermost layer is of narrower diameter than the remainder of the tube 20. An end portion 22 of the tube 20 extends from the innermost layer of the winding out of the base of the chamber 16 and is connected via a constant volume, high pressure, injection pump 24 to the bore of the pipeline 12. The other end portion 26 of the tube 20 projects from the chamber 16, tapering to a narrower diameter, and being open at its end 28. Since the apparatus 10 is immersed in sea water, sea water fills the chamber 16 and the entire length of the tube 20; the volume of sea water in the tube 20 is about 50 liters. At the centre of the irradiation chamber 16 is a 100 microgram californium-252 neutron source 30 providing 2.3.times.10.sup.8 neutrons/sec. These are moderated by the sea water so the sea water in the tube 20 is subjected to irradiation by thermal neutrons. At a distance of 40 mm from the source 30 the flux is about 10.sup.6 thermal neutrons per cm.sup.2 per second. The pump 24 is connected by a power supply cable 32 to a microprocessor unit 34. The unit 34 is connected by a cable 36 to a sodium iodide scintillator 38 mounted on the wall of the pipeline 12 at a distance in the flow direction from the point at which the injection pump 24 is connected to the pipeline 12 such that the oil/water in the pipeline 12 takes about 10 seconds to travel that distance. The scintillator 38 itself is of such a length that the oil/water in the pipeline 12 flows past it in about a tenth of a second. The unit 34 is also connected by a power-supply and data transmission cable 40 to a power source (not shown) which may be remote from the apparatus 10. In operation of the apparatus 10 the sea water filling the tube 20 is constantly irradiated by thermal neutrons from the source 30, so that sodium-24 nuclides are generated from the sodium naturally present in the sea water. Once every fifteen minutes the pump 24 is energised to inject 50 milliliters of this irradiated sea water into the pipeline 12. After about ten seconds this injected sample of sea water will pass the scintillator 38 which detects the gamma radiation emitted as a result of the decay of the sodium-24 nuclides. From the time interval between energising the pump 24 and the detection of the gamma rays by the scintillator 38, the flow velocity of the contents of the pipeline 12 are calculated by the microprocessor 34, this data being transmitted along the cable 40. Since the end 28 of the tube 20 is open, the tube 20 remains full of sea water at all times, and the sea water flows along the tube 20 slowly (at a rate corresponding to 50 ml per 15 minutes) taking just over ten days to flow the length of the tube 20. The long helical winding and the narrow end portion 26 minimise diffusion of irradiated sea water back out of the open end 28. Assuming for simplicity that the neutron flux is uniform throughout the length of the tube 20, and considering a small volume of the sea water in the tube 20 which at a time t includes N.sub.S stable atoms and N.sub.R radioactive atoms (whose sum is N.sub.T). Then: EQU dN.sub.R =FN.sub.S dt-cN.sub.R dt where F is a constnt depending on the neutron flux and the cross-section for production of the radioactive nuclei, and where c is the decay constant for the radioactive nuclei. Consequently, after irradiation of the sea water for a time t.sub.r : EQU N.sub.R =(FN.sub.T /c)(1-e.sup.-ct.sbsp.r) is the number of radioactive atoms present. (This assumes that only a small proportion of the atoms present become radioactive). This small volume of seawater is then injected into the pipeline 12, and passes along the pipeline 12 past the scintillator 38. If the transit time taken for it to reach the scintillator 38 is t.sub.t, and the time during which gamma rays from the irradiated sea water are counted is t.sub.c, then the number of gamma rays detected will be: EQU n=(KFN.sub.T /c)(1-e.sup.-ct.sbsp.c)e.sup.-ct.sbsp.t (1-e.sup.-ct.sbsp.r) where K is a constant. Since in practice the counting time will be much shorter than the transit time and the irradiation time, and will be much less than the half-life of the radioactive nuclei (i.e. since ct.sub.c is much less than one) this can be written as: EQU n=KFN.sub.T t.sub.c e.sup.-ct.sbsp.t (1-e.sup.-ct.sbsp.r) It is evident that the number of gamma rays detected depends in general upon the decay constant c. For any given values of irradiation time and transit time there is an optimum value of c which gives the largest number of detected gamma rays. This corresponds to an optimum half-life T lying between the values of the transit time and the irradiation time, and given by: EQU T=t.sub.r log 2/log ((t.sub.r +t.sub.t)/t.sub.t) For example in the apparatus 10 the sample of sea water is irradiated for ten days before injection, and the transit time is about 10 seconds. For these values, the optimum half-life of the radioactive nuclei is: EQU T=52 700 sec=14.6 hours This is close to the half-life of sodium-24, which is 14.8 hours. However, as long as the irradiation time is several half-lives, then the equilibrium concentration of the radioactive nuclide will be achieved; and as long as the transit time is a small fraction of the half-life, then the activity will not significantly decrease during transit. Thus the values of the irradiation time, half-life, and transit time are not critical. Under such circumstances the decay constant c does not affect the number of gamma rays detected, which is given by: EQU n=KFN.sub.T t.sub.c and hence the factors which determine the number of gamma rays detected are the parameters F, relating to the neutron flux to which the test fluid is subjected, and to the cross-section for the activation reaction, and N.sub.T relating to the concentration in the test fluid of the nuclides which can undergo the activation reaction, and to the volume of the injected sample. In the present case, the cross-section of sodium is 0.54 barns; and as mentioned earlier the concentration of sodium ions typically 10 g/liter. If any other gamma-emitting nuclides are also generated (for example a chlorine isotope) this will increase the number of gamma rays detected. It will be appreciated that the dimensions of the tube 20 may differ from those described, as too may the volume of the injected sample. The injected volume is chosen to ensure that passage of the injected sample can be reliably sensed by the scintillator 38, i.e. that sufficient gamma counts are received; and that the gamma counts received form a sharp peak, i.e. that the injected test fluid occupies only a short length within the pipeline 12. In the apparatus 10 it will be appreciated that the detected gamma count can be increased by increasing the injected volume of test fluid; although this reduces the irradiation time, and hence the activity of the test fluid, this is more than made up for by the increased volume. However the injected volume must be small in comparison with the volume per unit length of the pipeline 12, and the time taken for the injection must be short in comparison with the transit time.