WO2003021234A1

WO2003021234A1 - Density/level gauge having ultra-low activity gamma-ray source

Info

Publication number: WO2003021234A1
Application number: PCT/AU2002/001218
Authority: WO
Inventors: Brian David Sowerby
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2001-09-06
Filing date: 2002-09-05
Publication date: 2003-03-13
Also published as: AUPR751101A0

Abstract

Density and level gauges can be used for on-line measurement of mass per unit area (MPUA) (bulk density multiplied by thickness) of either (a) solids such as material (11) on a conveyor belt or (b) slurries such as material (11) flowing in a tank or through a pipeline. The gauge makes use of a substantially un-collimated ultra-low activity source (10) of gamma-rays and a high efficiency scintillation detector (12). The ultra-low activity gamma-ray sources are selected to be exempt from licensing requirements.

Description

DENSITY/LEVEL GAUGE HAVING ULTRA-LOW ACTIVITY GAMMA-RAY SOURCE

Technical Field

This invention relates to density and level gauges. These gauges can be used for on-line measurement of mass per unit area (MPUA) (bulk density multiplied by thickness) of either (a) solids such as material on a conveyor belt or (b) slurries such as material flowing in a tank or through a pipeline. In a further aspect the invention concerns a method of measuring density or level of materials.

Background Art

Conventional nuclear density gauges have found widespread use in industry, as they allow rapid, non-invasive and reliable on-line measurement of bulk density (ref. 1 ). When the carrier fluid density remains constant, nuclear density gauges provide highly accurate measurement of percent solids for most slurries.

Nuclear density gauges generally comprise a sealed gamma-ray source in a source holder with a shutter and collimator and a collimated scintillation detector mounted on the opposite side of a pipe or vessel to the source. A collimated beam of radiation is transmitted from the source through the pipe or vessel and material to the detector. As the density of the material in the pipe or vessel changes, the amount of radiation reaching the detector changes. The greater the density of the material, the lower the radiation field at the detector; the lower the density of the material, the higher the radiation field at the detector. When the radiation strikes the scintillation crystal, pulses of light are emitted which are detected by the photomultiplier tube. This amount of light is proportional to the amount of radiation striking the detector. The photomultiplier tube then creates a signal that is proportional to the amount of light received which is sent to the electronics for conversion to a useable process signal.

Most commercial nuclear density gauges use either Cs-137 or Co-60 sources of activity between 185,000 and 1 ,850,000 kilobequerels (kBq) (5 and 50 millicurie (mCi)). The safety of such a device is improved if the radiation beam from the source is confined to a narrow beam by strong collimation. The use of strong collimation also defines a "narrow beam" or "good geometry" in which only transmitted gamma rays that do not interact with the material are counted by the detector. In a "narrow beam" geometry the response of the gauge closely follows the absorption law

l/lo = exp (-μpx) where I = intensity of transmitted gamma ray with material present, lo = intensity of transmitted gamma ray with material absent; μ = gamma ray mass absorption coefficient; p = density of material; x = material thickness.

In an uncollimated geometry the detector will respond to both gamma rays direct from the source and to gamma rays scattered from the sample and surrounds. In this case the equation above is replaced by:

l/lo = B(x,Eγ) exp (-μpx) where the factor B(x,Eγ) is the buildup factor and Eγ the gamma-ray energy

(ref. 2).

A number of patents refer to the use of "low activity" sources in nuclear moisture and density gauges. One example is US 4766319 (ref. 3) that describes a portable nuclear moisture-density gauge in which the preferred low activity gamma-ray source is 60 microcuries (2220 kBq) cobalt-60, well above the exempt quantity of 100 kBq (see Table 1 ). A second example is US 4614870 (ref. 4) that describes a miniature isotopic soil moisture gauge using low activity naturally occurring radiation sources. However the activity of these sources is at least 100 microcuries (3700 GBq) that is well above exempt levels.

Commercial nuclear density gauges utilising low activity sources are those manufactured by Berthold Systems Inc (BSI). The so-called BSI "Photon Gauges" (US patent pending) use an uncollimated Cs-137 source and a collimated Nal(TI) scintillation detector. The nominal source strength used is 370 kBq (10 microcurie) of Cs-137 that is about 37 times the exempt level of Cs-137. With these low source strengths a general license is required in the USA that tracks use, ownership and location.

The main disadvantage of conventional nuclear density gauges is the radiation hazard associated with the use of medium-level radioactive sources. To minimise these hazards there are increasingly stringent regulatory requirements placed upon possession and use of radioisotope sources. These regulations require full safety approvals from the relevant authorities regarding source transportation, use and disposal for all applications, including wipe testing, leak testing, shutter testing and radiation surveys.

It should be noted that a gamma-ray transmission gauge is used to directly measure the mass per unit area (bulk density multiplied by thickness) of material in the radiation beam. In slurry applications, the measured bulk density can be used to determine the solids weight fraction (SWF) provided the specific gravity of the solid material is constant and provided significant air bubbles are not present (ref. 1 , 8).

Summary of the invention

The invention is a gauge for measuring the density or level of materials, the gauge including:

• a substantially un-collimated ultra-low activity source of gamma-rays that is below exemption levels (such as one of those shown in Table 1 ), and that, in use, is positioned with respect to a material to be examined to radiate gamma rays to impinge on the material; and,

• a high efficiency scintillation detector (such as bismuth germanate (BGO) or sodium iodide (Nal(TI)) capable of measuring the energy of gamma-rays and that, in use, is positioned with respect to the material to receive at least some of the gamma rays that have impinged on the material, have been transmitted through the material, and have exited the material.

An optional shield may be associated with the detector in such a way as to reduce natural background radiation, and direct transmission from the source, but to not shield source photons from reaching the detector from the material. The pulse height spectrum measured by the detector may be stored on a computer or other suitable device. A readout device may provide information about the density or level of the material being examined based on the number of detected photons within a specified energy range that is computed.

The improvements inherent in the current invention are associated with the benefits of using ultra-low activity (exempt) radioactive sources. The ultra-low activity gamma-ray sources are selected to be exempt from licencing requirements in the same way that exempt Am-241 radioisotopes are used in common household smoke detectors. When implemented in industrial density and level gauges, safety is significantly improved and the sources are exempt from the regulatory requirements placed upon possession and use of radioisotope sources. No special handling or shipping is required.

The invention can be used to measure the MPUA of either solids or slurries. An example of the former is material on a conveyor belt and examples of the latter include material flowing in a tank or through a pipeline. The invention is especially applicable to measure the solids weight fraction of the slurry in combination with ultrasonic transmission to analyse the particle size distribution of slurries. A second particular application is for the determination of belt loading in an on belt microwave moisture monitor (ref. 11). The invention can also be used to determine the level of material in containers. It can also be used in a backscatter geometry.

Ultra-low activity sources (below the exemption levels) are free from the regulations surrounding higher activity sources. Table 1 below gives a listing of the exemption levels of some commonly used gamma-ray sources. Probably the best everyday example of the widespread use of exempt radioisotopes is the use of Am-241 in household fire alarms. It is clear from Table 1 that the conventional commercial nuclear density gauges use at least 18,000 times the exempt level of Cs-137.

In a further aspect, the invention is a method for measuring the density or level of materials, including the steps of: • positioning a substantially un-collimated ultra-low activity source of gamma-rays such that, in use, it radiates gamma rays to impinge upon a material to be examined; and

• positioning a high efficiency scintillation detector capable of measuring the energy of the gamma rays such that, in use, it receives at least some of the gamma rays that have impinged on the material, after they have been transmitted through the material, and have exited the material.

Table 1. Exemption levels of radioactive sources (refs. 9, 10)

Brief description of drawings

Examples of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic drawing of an embodiment of the invention operating in transmission mode;

Figure 2 is a schematic drawing of another embodiment of the invention operating in backscatter mode;

Figure 3 is a graph showing the measured values of -In (l/IO) for the 1.274 MeV

Na22 gamma ray as a function of solids weight fraction (SWF) for silica slurry;

Figure 4 is a graph of the calculated counting statistical error in wt.% SWF versus counting time for a slurry density gauge; Figure 5 is a graph showing the measured values of -In (l/IO) for the 1.274 MeV

Na22 gamma ray as a function of sample thickness for a water sample of thickness 36 cm; and

Figure 6 is a graph of the calculated relative counting statistical error in thickness determination versus counting time for a gamma ray transmission gauge. Best Modes for carrying out the invention

In Figure 1 , photons from source 10 are directed through the material 11 towards detector 12. A fraction of these photons will be transmitted directly to the detector and a fraction will be scattered towards a detector 12. The detector 12 has a high efficiency. The spectrum of measured energies is read out and stored on a computer or other suitable device.

The source 10 is a sub-licensable radioisotope producing gamma rays with discrete energies. The sources Na-22, Bi-207 and Mn-54 are particularly suitable as activities up to 1000 kBq are exempt, but alternative lower activity sources such as Co-60 and Cs-137 could be used for particular applications.

Other geometric arrangements can be imagined, within the scope of the current invention, that allow the detector to measure photons back scattered from the target without registering unscattered photons direct from the source. An alternative embodiment is shown in Figure 2. The shield 20 is made of a suitable material strongly absorbing of the gamma rays produced by the source. Normally, a high atomic number material such as lead or tungsten would be used.

Experimental Demonstrations

Laboratory measurements were carried out to demonstrate the applicability of the technique in two applications, namely the determination of solids weight fraction (SWF) in slurries and the determination of the mass loading of material on a conveyor belt.

(a) Solids Loadings in Slurries

A prototype gamma ray transmission gauge was constructed that could be immersed in a slurry. The design is similar to that shown in Figure 1. The gauge was tested with a 1000 kBq Na-22 source and both 51x51 mm BGO and 51x51 mm Nal(TI) detectors. The source to detector separation was 110 mm and count rates were measured with no sample and with water-based slurries of both silica and iron oxide with solids loadings from 0 to about 50 wt.%. The results are summarised in Table 2.

Table 2. Summary of results for a gamma ray density gauge using a 1000 kBq Na-22 source and either a 51x51 mm BGO or Nal(TI) detector.

The measured values of In (l/IO) for the photopeak of the 1.274 Na-22 gamma ray are shown in Figure 3. The value of the build-up factor B(x,Eγ) for 1.274 MeV gamma rays was about 1.2. Figure 4 shows the calculated error in measured SWF due to counting statistics for a 100 sec counting time using a 1000 kBq Na-22 source and 51x51 mm Nal(TI) detector. The count rates of transmitted 1.274 MeV gamma rays were determined using the energy window 1145 - 1410 keV. Reduced counting statistical errors are obtained by using broader energy windows around both the 0.511 and 1.274 MeV gamma rays.

(b) Conveyor Belt Loadings

A prototype gamma ray transmission gauge was constructed with a source to detector spacing of 600 mm and tested with various thicknesses of water between the source and detector. The gauge simulates a gamma ray transmission gauge for on-conveyor belt use. The gauge was tested with a 1000 kBq Na-22 source together with a 51x51 mm BGO and a 76x76 mm Nal(TI) detector. Water thicknesses from 0 to 360 mm were used. The results are summarised in Table 3. Table 3. Summary of results for a gamma ray density gauge using a 1000 kBq Na-22 source and either a 51x51 mm BGO or a 76x76 mm Nal(TI) detector.

The measured values of In (1/10) for the photopeak of the 1.274 MeV Na-22 gamma ray are shown in Figure 5. Figure 6 shows the calculated error in measured water thickness due to counting statistics for a 100 sec counting time using a 1000 kBq Na-22 source and 51x51 mm Nal(TI) detector. The count rates of transmitted 1.274 MeV gamma rays were determined using the energy window 1145 - 1410 keV. Reduced counting statistical errors are obtained by using broader energy windows around both the 0.511 and 1.274 MeV gamma rays.

Industrial Applicability

The invention has wide applicability to the measurement of density and level in a range of industries. Examples include tank level control, mass loadings on conveyor belts and the measurement of percent solids in a range of slurries. In a number of applications the low activity density gauge can be used in conjunction with other measurements to accurately determine other parameters. One example is in determining solids loading in a slurry particle size analyser (refs. 5, 6 and 7). A second particular application is for the determination of belt loading in an on-belt microwave moisture monitor (ref. 11 ).

References

1. Carr-Brion, K.G., Trans. Inst. Min. Metall. (Section C) 76_(1967) C94-C100 2. Knoll, G.F., Radiation Detection and Measurement (John Wiley & Sons, New York, 1989)

3. Regimand, A., Portable Nuclear Moisture-Density Gauge with Low Activity Nuclear Sources, US Patent No. 4766319 (Feb. 1987)

4. Morrison, R.G., Miniature Isotopic Soil Moisture Gauge, US Patent No. 4614870 (Dec, 1983)

5. Coghill, P.J., Millen, P.J. and Sowerby, B.D., 2002. "On-line measurement of particle size in mineral slurries", Minerals Engineering 15, 1-2, pp. 83-90

6. Coghill, P.J. and Sowerby, B.D., Determining the size distribution particles in a fluid, Australian Patent No. 695306, Priority Date 19 June 1995 and 18 July 1995

7. Sowerby, B.D., A method and apparatus for determining the particle size distribution, the solids content and the solute concentration of a suspension of solids in a solution bearing a solute, Australian Patent No. 676,846, 17 August 1992.

8. Watt, J.S., Determination of solids weight fraction and mineral matter or ash concentration of coal in slurries, Int. J. Applied Radiation and Isotopes, 34 (1983) 55-62

9. Australian Radiation Protection and Nuclear Safety Regulations 1999; Schedule 2

10. International Basic Safety Standards for Protection Against Ionizing Radiation and for the Safety of Radiation Sources, International Atomic Energy Agency Safety Series No. 115, 1996, p. 84

1 1 . N.G. Cutmore, D.G. Miljak, T.G. Rowlands, D. Crnokrak and A.J. McEwan, On-Conveyor Measurement of Moisture in Coal Using Low Frequency Microwaves, Eighteenth Annual International Pittsburgh Coal Conference, Newcastle, 4-7 Dec 2001 (ISBN 1 890977 18-7)

Claims

1. A gauge for measuring the density or level of materials, the gauge including: a substantially un-collimated ultra-low activity source of gamma-rays that, in use, is positioned with respect to a material to be examined to radiate gamma rays to impinge on the material; and a high efficiency scintillation detector capable of measuring the energy of the gamma-rays and that, in use, is positioned with respect to the material to receive at least some of the gamma rays that have impinged on the material, have been transmitted through the material, and have exited the material.

2. A gauge according to claim 1 , where the source of gamma rays is below exemption levels.

3. A gauge according to claim 2, where the source of gamma rays is as one of Cs-137, Co-60, Na-22, Bi-207 or Mn-54.

4. A gauge according to claim 1 , 2 or 3, where the detector is bismuth germanate (BGO) or sodium iodide (Nal(TI)).

5. A gauge according to any preceding claim, where a shield is associated with the detector to reduce natural background radiation, and direct transmission from the source, but to not shield source photons from reaching the detector from the material.

6. A gauge according to any preceding claim, further comprising a computer connected to the detector to store the pulse height spectrum of measured energies.

7. A gauge according to claim 6, further comprising a readout device to provide information about the density or level of the material being examined based on the number of detected photons within a specified energy range that is computed.

8. A gauge according to any preceding claim, used to measure the MPUA of either solids or slurries.

9. A gauge according to claim 8, used to measure the solids weight fraction of the slurry in combination with ultrasonic transmission to analyse the particle size distribution of slurries.

10. A gauge according to claim 8, used to measure the MPUA of solids on a conveyor belt in combination with microwave transmission to analyse the moisture content of the solids.

1 1. A gauge according to any preceding claim, where the source and detector are positioned relative to a material in a backscatter geometry.

12. A method for measuring the density or level of materials, including the steps of: positioning a substantially un-collimated ultra-low activity source of gamma-rays such that, in use, it radiates gamma rays to impinge upon a material to be examined; and positioning a high efficiency scintillation detector capable of measuring the energy of the gamma-rays such that, in use, it receives at least some of the gamma rays that have impinged on the material, after they have been transmitted through the material, and have exited the material.

13. A method according to claim 12, where the source of gamma rays is below exemption levels.

14. A method according to claim 13, where the source of gamma rays is as one of Cs-137, Co-60, Na-22, Bi-207 or Mn-54.

15. A method according to claim , 12, 13 or 14 where the detector is bismuth germanate (BGO) or sodium iodide (Nal(TI)).

16. A method according to any one of claims 12 to 16, comprising the further step of shielding the detector to reduce natural background radiation, and direct transmission from the source, but to not shield source photons from reaching the detector from the material

17. A method according to any one of claims 12 to 16, comprising the further step of using a computer connected to the detector to store the spectrum of measured energies.

18. A method according to claim 17, further comprising the step of using a readout device to provide information about the density or level of the material being examined based on the number of detected photons within a specified energy range that is computed.

19. A method according to any one of claims 12 to 18, used to measure the MPUA of either solids or slurries.

20. A method according to any one of claims 12 to 18, used to measure the solids weight fraction of the slurry in combination with ultrasonic transmission to analyse the particle size distribution of slurries.

21. A method according to claims 12 to 18, used to measure the MPUA of solids on a conveyor belt in combination with microwave transmission to analyse the moisture content of the solids.

22. A method according to any one of claims 12 to 18, where the source and detector are positioned relative to a material in a backscatter geometry.