NZ213777A - Quantitative analysis of a mixture from gamma and neutron source irradiation - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £13777 213777 ^214666 Patents form No 5 Number 213777/214666 PATENTS ACT 1953 Dated io October 1985 COMPLETE SPECIFICATION NUCLEAR MEASUREMENT METHOD AND APPARATUS FOR MIXTURE COMPONENT ANALYSIS X5/WE HER MAJESTY THE QUEEN in right of New Zealand do hereby declare the invention for which i/we pray that a Patent may be granted to i33§/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - 1 - (followed by page la) 213777^ /^14666 This invention relates to the non-destructive and non-contact determination of proportions of two or more components of differing chemical compositions in homogeneous, non-homogeneous or layered mixtures.

The object of the invention is to provide a method and apparatus for the rapid and regular or continuous measurement of, for example, fat content in meat, moisture content in wood or cream content in milk, where measurements may be made through the 10 thickness of the material being examined but must be independent of its thickness.

Previous methods for readily measuring fat content in meat have been limited to taking measurements at specific points on a 15 carcass and sensing a boundary between two layers, for exampl-e by ultrasonic measurement or by inserting a colour-sensitive probe through the meat thickness. Such methods have not been available for inspecting meat packed into boxes for shipment. This production-line situation has required the destructive cutting-up 20 and weighing of the separate fat and lean components.

Nuclear measurement techniques for non-destructively determining the proportions of two-component systems have previously been limited to situations where - 1. The object or object-stream being sampled is of uniform or fixed thickness or diameter. 2.

The neutron mass attenuation coefficients of the components being sampled differ typically by a factor of 2 or more. 2 2t3777_ ^214666 3. The mixture of the components is relatively homogeneous.

The method and apparatus of this invention goes at least some way towards overcoming these limitations and provides a useful, non-destructive production line measuring technique. It may be used for other non-uniform-distribution situations, for example for determining the width and density of the middle layer in a three layer sandwich. This technique has medical applications such as in the study of osteoporosis where the middle layer is bone and the outer layers tissue. It can be extended to measuring more than two components.

The method may also be used to measure the position of the steel belt in belted tires to ensure there is no separation between the belt and the rubber.

According to the invention, therefore, there is provided a substantially non-destructive and non-contact method for quantitative analysis of a mixture of two or more known components, which comprises irradiating the mixture with simultaneous fast neutron and gamma source radiation, measuring the extent to which each species of the source radiation is transmitted through the mixture, and determining the analysis by reference to the known mass attenuation coefficient of each component for each species of the source radiation.

The neutron and gamma source radiation may comprise radiation species of different energies so that there will be always at least two distinguishable species of radiation. To obtain 3 214666 quantitative analysis of an n-component system, at least n distinguishable species of radiation are required, the analysis being effected, in mathematical terms, by solution of the n simultaneous equations describing the extent of transmission of 5 each species of radiation, as illustrated more particularly below.

Simple solution of the n simultaneous equations provides a quantitative determination of the weight fraction of each 10 component. The radiation is passed through the object being measured, and the transmitted radiation intensities are measured simultaneously by a suitable detector or array of detectors connected to electronic measuring and computing apparatus.In a layered system, a more complex analysis, using also the known 15 length of material through which the radiation passes and the densities of the layered components, permits the depths of each layer to be measured.

Each component of the object mixture will have a different known 20 mass attenuation coefficient for each species of source radiation. Measurements will have been made of the unattenuated intensities (countrates) of neutron and gamma radiation when there is no object in the beam.

During object measurement, the intensities of neutron and gamma radiation received by the detector(s) will be given (for example in a two-component system) as follows: (2) (1) 213777^- ^214666 wherein In and I-y are the countrates of neutron and gamma rays after passing through the object, and I are their unattenuated countrates, no yo u , u ., u and u , are the neutron and gamma ray mass wna ^nb Yb attenuation coefficients of components a and b, for the source radiation employed and Ma, Mfc are the mass thicknesses (mass per unit area in the beam) of the components a and b.

Solving these two simultaneous equations will give the values of Ma and Mfc. The weight fraction, for example of component a, will be given by Ma/(Ma + M^). •] 5 It will be noted that this simple calculation has not required the object thickness to be measured or known. Integrating the measurement over the object volume sensed by the detector or detector array will give the overall weight fraction for that volume.

By computing with data obtained from equipment calibrations using standard samples of different known component fractions and known constant thickness, the object thickness over the measuring volume may be automatically calculated.

The above mathematical determination may for example be employed in analysis of fat levels in meat, moisture levels in partially-dry timber, water and soil in a soil/water mixture, or cream content in milk. 213777 ^ -214666 In certain situations, such as partially dry timber, identifiable layers may be present and in an extension of the above mathematical analysis the nature of the layers may be investigated in a substantially non-destructive and non-contact manner.

For example, a mathematical 'model' of a partially-dry wood block can be as shown in Fig. 5 of the drawings accompanying the provisional specification of Application No. 214666. 3y including the densities of the wood and of water, and the length L of the 'block', in the analysis, the moisture levels in the interior (moist) part of the wood and the effective width of this part can be determined, as follows: In general situation 1, as depicted in Figure 5, a beam passes through three layers, of thicknesses respectively x, y and x, the densities of the material, considered uniform within each layer, being f]_ s.nd ^3 and their respective attenuation coefficients Pi' f>2 an<z f3 - T-e is to determine the thickness and the density (y and central layer using neutron/gamma ray- transmission.

The solution to general situation 1 is: yi(Pi+c 3} L+ 2 lcgQ ( -r ) io y = — + Ps ) " (P,+P=)T050{(3 • ion j.y and p= = — 2l°se {(^n)!:=-'(i2a'-"H(0i+03)L(P^=r,J._nvJ 6 213777^ -214666 The neutron and gamma radiation may be provided from any suitable source, for example a radioactive isotope source such as 252q£ or Am-Be or a nuclear particle accelerator target system.

According to a further aspect, the present invention provides an apparatus for substantially non-destructive and non-contact quantitative analysis of a mixture of two or more components, which comprises means for irradiating a sample of the mixture under analysis with simultaneous fast neutron and gamma source radiation, means for detecting each species of said radiation after transmission thereof through the sample, means for measuring the extent to which each said species is transmitted through the sample, and means of determining the analysis according to the method of the invention by reference to the mass attenuation coefficient of each component for each species of- the source radiation.

It has been found that the energies of the neutrons from an Am-Be neutron source are approximately optimum for distinguishing between fat and lean meat packed into an export box of standard thickness (16 cm). This allows the use .of a relatively inexpensive source, the radioactivity of which can be kept well ■within the limits required for operation in a public work-place. It" also provides sufficient precision when measuring lean and fat meat, the neutron absorption coefficients of which vary by only approximately 20%.

The detector system may comprise typically four detectors placed underneath a meat box. Each detector may conveniently be, for * 213777 ;214666 ;5 example, a Nuclear Enterprises NE213 liquid scintillator (an organic scintillator utilizing proton recoil and pulse shape discrimination), in a cell 15 cm diameter and 15 cm thick, ;coupled to a Philips XP2041 or RCA 8854 photomultiplier tube. ;Standard electronic measuring equipment is used. such a system, 10 used in conjunction with a 1 Curie Am-Be source for checking standard export meat boxes has been found to provide a precision of 2 percent after a measuring interval of one minute. ;An alternative radiation source is a nuclear accelerator used 15 with a target system having adjustable parameters. Since the attenuation coefficients of different materials vary with incident neutron and gamma ray energies in ways that are characteristic of those particular materials, this allows variation in incident neutron and gamma ray energies to optimise 20 the measuring system for the particular mixture components to be determined. ;Accelerator targets may be solid, liquid or gaseous. Gaseous targets allow the elemental composition to be easily changed to 25 yield the required neutron and gamma ray energies. Further control may also be provided by adjustment of the incident beam energy and the neutron detection angle relative to the incident beam direction The relative weight ratio of the mixture of gases can be adjusted for an optimum ratio of neutron to gamma 30 ray yield. ;Using a radioactive neutron source in a gauge system suffers from the problem that detected scattered flux from the surroundings is difficult to eliminate and source collimation is desirable. By ;213777 ;-Iff. ;contrast no collimation is required with a nuclear accelerator target system where the time-of-flight technique eliminates scattered flux. Background components may also be eliminated. Accelerators can furthermore be scaled to an industrial 5 application in which environment they offer the advcintage that the radiation source can be switched off. ;A practical gaseous accelerator target for use as a radiation source consists of a cell of deuterium, this being bombarded with 10 approximately 1.5 MeV aeuterons. The entrance window to the gas cell comprises 2.5 micrometre thick nickel. In practice the gamma rays produced by ^®Ni (d,d') and ^Ni (d,d') reactions are suitable as the gamma component of the measuring beam, while suitable neutrons are produced by the (d,n), (d,n) and ;15 6^Ni (d,n) reactions, primarily the former. The deuteron beam is stopped at the end of the gas cell on a heavy metal plate (made of tantalum for example) from which the neutron and gamma yield is relatively low. Time variation in the beam conditions can cause the relative proportions of the (d,n) and the Ni (d,n) 20 reaction contributions to vary as more or less beam strikes the cell window and support. This can cause-a time variation in the mass attenuation coefficient. However, this variation can be accounted for in a single measurement. ;25 To explain this, the means of normalising the runs will now be described, with reference to Figure 1 of the drawings accompanying the provisional specification of Application No. 214666, in which 1 represents the gas target, 2 is the sample, 3 is the sample detector and 4 is a monitor detector. ;The time of flight spectra in the monitor and sample detectors are represented in Figure 2 of the drawings accompanying the provisional specification of Application No. 214666. ;The magnitude of the gamma ray peak in the monitor detector is a measure of the incident gamma ray flux on the sample. The magnitude of this peak is also a measure of the Ni(d,n) contribution to the total incident reaction flux because the gamma rays are being produced by Ni(d,d') reactions, the yield from the (d,d') reactions being proportional to the yield from (d,n) reactions. Consequently, in measuring the transmission through the standard materials (e.g. soil and water for soil/water mixtures) a plot of the form of Figure 3 of the drawings accompanying the provisional specification of Application No. 214666 can be produced. The horizontal scale, of this plot can be converted to units of mass attenuation coefficient using the calculation procedures outlined above. ;Consequently, in any particular subsequent run on a soil/water sample (for example), the correct attenuation coefficients for that run can be obtained from the 1-^/ % - ratio. In practice this correction can desirably be minimised by maintaining stable beam conditions. ;An alternative practical target is a variation on that just described where gas is added to the deuterium gas to enhance the yield of gamma rays with Ni (d,d') reactions. If the gas cell end plate material is now graphite a low energy neutron group (3 MeV) distinguishable from the neutron group previously described is produced. Such multiple neutron groups may be utilised to improve ;213777> ;10 -^14666 ;measurement precision or to analyse the object in terms of more than two components (e.g. three components by solving three simultaneous equations). The electronic procedure of v time-of-flight can be used to separate the neutron and gamma ray 5 groups after transmission through the sample. ;A further alternative accelerator target is natural Lithium. The neutron source reaction is then ^Li(p,n)^Be, and the gamma ray source reaction is ^Li(p,p*)^Li. A feature of the ^Li(p,n) 10 reaction is its negative Q-value, with 2MeV incident protons the reactions produced in the forward direction have maximum energies of about 300KeV and are readily detected with a ^Lil(Eu) scintillator mounted on an RCA 8575 pnotomultiplier. The detection reaction is the ®Li(n,a)t reaction, which has a cross 15 section resonance at 25 0KeV (3 barns). These low energy neutrons are suitable for transmission measurements through thin layers.

The following non-limiting examples are included as further illustration of the present invention. " 213777 .

^ZU666 Example 1 Analysis of a layered mixture Measurements were made on a layered structure comprising water layers 5mm thick and slices of wholemeal bread 10mm thick. The time-of-flight spectrum observed was as shown in Figure 4 of the drawings accompanying the provisional specification of Application No. 214666. The gamma rays formed a sharp peak and the neutron energies formed a continuous distribution from the level of the random background to a maximum energy at point "A" on the cycle. The two components were extracted by subtraction techniques to enable mathematical analysis of the layered 15 structure as previously described.

Examole 2 1 0 Analysis of soil/water mixtures Tests were performed with soil/water mixtures. The mean energies, attenuation coefficients and optimum material thicknesses for experiments with Am-Be and accelerator-based sources are compared in Table 1. For this particular combination 25 of coefficients, using a 0.15 m thick sample a given precision can be achieved more readily using the accelerator. The factor" relating, for example, the uncertainty in uJ to the statistical error in I^/I^ or In/Ino (equations 1 and 2 above) for the accelerator source is about half that for the radioactive source. 3 0 in Table 2 are shown the values of u and p measured by the accelerator method for soil samoles. The ratios Za, /I= 0.990 w R and = 0.992i 12 213777^ 214666 Table 1: Comparison of the parameters for neutron-gamma experiments with water/soil mixtures. The value of tQp is the optimum thickness for a water/soil mixture with &»=0.I.

- Attenuation coefficients Radiation Am-Be Accelerat or (Ejj=l. 5 MeV) E water soil E water soil neutrons 4.5 8.7 2.9 4.3 12.8 4.4 y-rays 4.4 2.2 1.8 1.0 3.4 3.1 MeV xlO-3 MeV xlO-3 m2/kg tOD metres neutrons y-rays neu trons y-rays 0 .15 0.41 0 .10 0.25 13 213777 x /214666 Table 2: The » (mass faction) and © (density) values for wet soil measured by the radiation and weighing methods.

Radiation method Weighing Method w(% water) density w(% water) density 1.25 1.52 1.01 1.50 1-95 1.43 1.92 1.47 2-90 1.3 6 2.92 1.38 .10 1.41 4.08 1.40 4.40 1.44 .04 1.41 6.10 1.42 .98 1.38 7.40 1.47 6.96 1.46 9.30 1.47 8.06 1.48 8.60 1.50 9.06 1.48 .10 1.55 .14 1.54 13.70 1.63 14.92 1.61 3:0. 01 £0.01 40.5% £0.01 xlO3 kg/m3. xiO3 kg/m3 f R "w ^w Example 3 213777/ ^2liG66 Analysis of meat containing lean and fat components To achieve similar precision to the soil/water studies of Example 2 using an Am-3e source, fat/lean meat measurements need accumulated counts 10 times higher. Typical attenuation coefficients for neutrons and gamma rays in fat are 9.6 and 2.4 (xlO-3 m2/kg) and for lean meat 8.1 and 2.3 (xlO-3 m2/kg). It 10 can be seen that the attenuation coefficients for neutrons differ by about 18% (referred to the lean meat value) and it is this small difference which necessitates the higher counts. Fortuitously the thickness of a standard New Zealand box of export meat has been found to be about optimum for best precision 15 in O and p for given errors in the transmission ratios (equations 1 and 2 from above). In a typical result for commercial minced meat the measured values of u and p are (12.8£2.3)% and (1.032± 0.00 7) x 103 kg/m3 compared with (12tl)% chemical determination and (1.040±0.004) x 103 kg/m3 gravimetric determination.

Claims (2)

WHAT WE CLAIM IS: 15 213772-^314666

1- A method for quantitative analysis of a mixture of two or more components which comprises irradiating the mixture with simultaneous fast neutron and gamma source radiation, measuring the extent to which each species of the source radiation is transmitted through the mixture, and determining the analysis by reference to the mass attenuation coefficient of each component for each species of the source radiation.

2. A method as claimed in claim 1 wherein more than one 10 distinguishable species of neutron and/or gamma source radiation is present and each species of said neutron and gamma radiation is characterised by a different energy, by which means each species may be distinguishably detected.. 15 3. a method according to claim 1 or claim 2 wherein the source radiation is obtained from a radioactive isotopic source. 4. A method according to claim 3 wherein the source is 252Cf or Am-Be. 20 5. A method according to claim 1 or claim 2 wherein the source radiation is obtained by means of a particle accelerator/target system. 25 6. A method according to any one of the preceding claims wherein the mixture is or comprises partially-dry timber having in cross section dry outer portions sandwiching a moist inner portion and the source radiation is passed through the lb kv -j / - 1 ^ / (y.i 14 b' t> G mixture such that it passes through each said portion. A method according to any one of claims 1 to 5 wherein the mixture is or comprises a soil/water mixture. A method according to any one of claims 1 to 5 wherein the mixture is or comprises meat containing lean and fat components. A method according to any one of claims 1 to 5 wherein the mixture is or comprises animal bone and tissue. A method according to any one of claims 1 to 5 wherein the mixture is or comprises steel belted rubber tyres. A method according to any one of claims 1 to 5 wherein the mixture is or comprises milk containing milk and cream components. A method for quantitative analysis of a mixture of two or more components, substantially as herein described with reference to the accompanying Examples and to the drawings accompanying the provisional specification. An apparatus for quantitative analysis of a mixture of two or more components, which comprises means of irradiating a sample of the mixture under analysis with simultaneous fast neutron and gamma source radiation, means for detecting each species of said radiation after transmission thereof through the sample, means for measuring the extent to which each said species is transmitted through the sample, and means for ZSZllI^ 7 / 214666 determining the analysis according to the method of claim 1 by reference to the mass attenuation coefficient of each component for each species of the source radiation. \ 5 14. An apparatus according to claim 13, wherein the means for irradiating the sample comprises a radioactive isotopic source. 15. An apparatus according to claim 13, wherein the means for 10 irradiating the sample comprises a particle accelerator/target system. 16. An apparatus according to any one of claims 13 to 15, wherein the means for irradiating the sample comprises means for 15 irradiating with more than one distinguishable species of- neutron and/or gamma source radiation, each species of said neutron and gamma radiation being characterised by a different energy, by which means each species may be distinguishably detected. 20 17. An analytical apparatus for quantitative detection of the fat content of meat, comprising means for irradiating a sample of the meat with simultaneous fast neutron and gamma source • radiation, detector means for scintillation detection of each 25 species of said radiation after transmission through the sample, electronic means for measuring the extent to which each said species is transmitted through the sample, and means for determining the analysis according to the method of claim 8 by reference to the mass attenuation coefficient of 30 each component for each species of the source radiation. 213777/-'8 /314f An analytical apparatus for quantitative detection of the water content of soil, comprising means for irradiating a sample of the soil with simultaneous fast neutron and gamma source radiation, detector means for scintillation detection of each species of said radiation after transmission through the sample, electronic means for measuring the extent to which each said species is transmitted through the sample, and means for determing the analysis according to the method of claim 7 by reference to the mass attenuation coefficient of each component for each species of the source radiation. An analytical apparatus for quantitative detection of the water content of partially-dry timber, comprising means for irradiating a sample of the timber with simultaneous fast neutron and gamma source radiation, detector means for scintillation detection of each species of said radiation after transmission through the sample, electronic means for measuring the extent to which each said species is transmitted through the sample, and means for determining the analysis according to the method of claim 6 by reference to the mass attenuation coefficient of each component for each species of the source radiation. An analytical apparatus for quantitative detection of bone strength in a human or animal body, comprising means for irradiating a sample of said bone and tissue with simultaneous fast neutron and gamma source radiation, detector means for scintillation detection of each species of said radiation after transmission through the sample, electronic means for measuring the extent to which each said _ ,5 213777 214666 species is transmitted through the sample, and means for determining the analysis according to the method of claim 9 by reference to the mass attenuation coefficient of each component for each species of the source radiation. '6 21. An analytical apparatus for quantitative detection of steel in a steel belted rubber tyre, comprising means for ^ irradiating a sample of said steel belted rubber with simultaneous fast neutron and gamma source radiation, 10 detector means for scintillation detection of each species of said radiation after transmission through the sample, electronic means for measuring the extent to which each said species is transmitted through the sample, and means for determining the analysis according to the method of claim 10 15 by reference to the mass attenuation coefficient of each • component for each species of the source radiation. 22. An analytical apparatus for quantitative detection of cream in milk, comprising means for irradiating a sample of said 20 milk with simultaneous fast neutron and gamma source radiation, detector means for scintillation detection of each species of said radiation after transmission through the sample, electronic means for measuring the extent to which ■ each said species is transmitted through the sample, and 25 means for determining the analysis according to the method of claim 11 by reference to the mass attenuation coefficient of each component for each species of the source radiation. An apparatus for quantitative analysis of a mixture of two or more components, substantially as herein described with reference to the Examples and drawings. WEST-WALKER. McCABE pen ATTORNEYS^FOR >HE APPLICANT/