Patent GB-1409480-A (GB)
Title: Neutron activation analysis system
IPC: G21H5/00
Filed: 1973-08-07
Abstract: 1409480 Activation analysis UNITED STATES ATOMIC ENERGY COMMISSION 7 Aug 1973 [8 Sept 1972] 37344/73 Heading G1A In activation analysis of a fluid two samples are activated respectively by fast and thermal neutrons and the gamma radiation emitted subsequently by the samples is measured. The use of fast and thermal neutrons allows the gamma radiation from the two samples to be compared so that ambiguities are removed. The invention is described as applied to analysis of a cement slurry which is fed to inlet 31 of the apparatus shown in Fig. 1. Valves 27 and 29 allow the slurry to enter the two measurement loops 11  and 13 where pumps 19 and 21 circulate the slurry samples through activation cells 15, 25 and through gamma detectors 17 and 23. The apparatus also has a radiation source 44 which directs radiation through both measurement loops to detectors 43, 45 so that the density of the samples can be measured.  The outputs from the two gamma detectors 17, 23 are connected to a pulse spectra analyzer 57 which produces a spectrum of the gamma emission from each sample, such as that shown in Fig. 4 (not shown) which is derived from dectector 17. This figure indicates which radioisotopes formed by irradiation of the slurry sample by thermal neutrons are causing the various peaks; for example the peak at 0.84 MeV is due to <SP>27</SP>Mg and <SP>56</SP>Mn. However <SP>27</SP>Mg can be formed from <SP>27</SP>AL or <SP>26</SP>Mg, and similarly for the isotope shown at 1.78 MeV it is not possible to determine from the single spectrum which elements in the slurry give rise to these peaks. The peak at 3.09 MeV is unambiguous however, and is due to <SP>49</SP>Ca formed from <SP>48</SP>Ca. The spectrum produced by detector 23 in response to irradiation of the slurry by fast neutrons allows the uncertainty at 0.24 MeV and 1.78 MeV to be resolved, because when fast neutrons are used the peak at 0.84 MeV is due to <SP>27</SP>Mg formed from <SP>27</SP>AL, and the peak at 1.78 MeV is due mainly to <SP>28</SP>AL formed from <SP>28</SP>Si though a small error due to <SP>28</SP>Al formed from <SP>27</SP>A1 has to be allowed for using the <SP>27</SP>Al concentration determined from the 0.84 MeV measurement. As a result the two spectra allow the concentration of Ca, Al and Si oxides in the slurry to be determined once the apparatus has been calibrated by use of a standard slurry. A typical calibration result is given, Table 1 (not shown) Sources and detectors: The thermal neutron source is shown in Fig. 2 and comprises three isotopic neutron sources 61, e.g. of Cf 252, though a single source can be used. The slurry passes through a coiled pipe 63 to maximize the irradiation and is surrounded by a moderator layer 65, lead shield 69, and enclosure 71, 73. The three sources 61 are spaced by inert material 61. The fast neutron source is basically similar, Fig. 3 (not shown), but preferably uses <SP>238</SP>Pu as an alpha emitter combined with <SP>9</SP>Be to emit neutrons. This fast source also contains thermal neutron absorbers to ensure only fast neutrons reach the sample. The gamma detectors can be gas filled counters or a lithium drifted germanium semiconductor or, preferably, a thallium doped sodium iodide crystal scintillator associated with a photodetector. The density measuring apparatus uses a gamma source, such as cesium 137 or cobalt 60; the detectors may be a scintillator as above.