AU636316B2 - Measurement of natural gamma-ray activity - Google Patents

Description

636316 COMMONWEALTH OF AUSTRALIA The Patents Act 1952 Name of Applicant(s): Address of Applicant(s):

S

Priority Details: Actual Inventor(s): Address for Service:

S

Address for Service: COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION LIMESTONE AVENUE, CANBERRA

A.C.T.

Application No. PJ1665 Lodged on 28 November 1988 Plackotto Joseph MATHEW SIROTECH LIMITED 580 Church Street RICHMOND VIC 3121 *so COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED MEASUREMENT OF NATURAL GAMMA-RAY ACTIVITY The following statement is a full description of the invention, including the best method of performing it known to us: *Mo Ct MEASUREMENT OF NATURAL GAMMA-RAY ACTIVITY ee..

*This invention relates to the measurement of natural gamma-ray activity of bulk material. One very useful application of the invention is to the determination of the ash content of coal in bulk, for 5 example in stockpiles and in wagons.

An accurate knowledge of the amount of ash present in stockpiled coal is extremely important in many aspects of coal preparation, blending and utilisation. Also, the ash content of coal loaded in wagons has to be measured prior to transport to the consumer to determine whether it is within contract specification. In many instances, a coal producer is heavily penalised by the buyer if the ash content of coal in a consignment is not within specification. coal in a consignment is not within specification.

The ash content of coal in a stockpile is usually determined by appropriate sampling and.sample analysis by conventional methods. This approach is labour-intensive and time-consuming. The availability of the ash analysis result is often delayed if the facility for ash analysis is not locally available. A gauge for the rapid measurement of ash content in stockpiles, therefore, would be of considerable benefit to the coal mining industry.

The determination of the ash content of coal in wagons is also presently carried out by conventional sampling and sample analysis. Wormald et al. Wormald, C.G. Clayton, I.S. Boyce and D.

Mortimer, Int. J. Radiat. Isot., 30 (1979) 297] 15 developed a neutron activation method for the determination of ash content in wagons. The e* technique consisted of determining the aluminium content in coal using the thermal neutron activation method: 27 28 28 S* Al(n,y) Al 28Si 28 The beta decay of 2Al(t /2 2.3 min) is 25 followed by the emission of a 1.78 MeV gamma ray.

The 1.78 MeV gamma rays are counted using a scintillation detector. The ash content of coal is 0 determined from the correlation between ash and aluminium content in coal.

This approach for the determination of ash content of coal in wagons has several disadvantages.

Firstly, the weak correlation between ash and aluminium content of coals makes this method inherently inaccurate. Secondly, the gauge is heavy and cumbersome, due to the requirement of radiation shielding, and thirdly, personnel with specialised knowledge in radiation safety are required to supervise the use and handling of the gauge. A gauge for determining the ash content of coal in stockpiles and in wagons will be most advantageous if it is portable and contains no radiation sources.

Natural radioactivity in geological materials is mainly due to the presence of potassium, uranium and thorium in trace quantitites. Coal also S. contains these three radioelements. Potassium and decay products of thorium and uranium emit deeply penetrating gamma rays. The presence and 15 concentration of these radioelements can, therefore, be determined with the aid of an efficient gamma-ray detector.

A recent CSIRO study has shown that the ash content of coal is strongly correlated with gamma-ray activity and that the ash content can be predicted from natural gamma-ray activity with a high degree of accuracy. The study also showed that the specific gamma-ray activity of the ash content of coals may t: change from one location to another. Consequently, 25 different coals require different calibration equations relating the ash content and gamma-ray .activity. The results of the CSIRO study were o. presented at the Coal Research Conference in Wellington, New Zealand, by P.J. Mathew in 1985.

It has now been appreciated, in accordance with the present invention, that the natural gamma-ray activity of coal provides an opportunity to overcome or at least substantially alleviate the disadvantages inherent in existing techniques for determining the ash content of coal in stockpiles and wagons.

The invention accordingly provides apparatus for monitoring the natural gamma-ray activity of bulk material comprising a probe and spectral analysis means, the probe comprising a tubular body closed at at least one end, a gamma-ray detector mounted within the tubular body and means coupled to the detector for outputting an electrical signal in response to receipt of a gamma-ray photon by the detector, and including means in the probe providing shock protection for the gamma-ray detector, the detector being shielded to prevent gamma-rays and x-rays of below about 100 keV from reaching it; the spectral analysis means being connected to receive the outputted electrical signals and comprising spectral energy windows for accepting signals originating from the presence of, respectively, naturally occurring potassium, uranium and thorium, or their daughter products, in the bulk material.

the invention also provides apparatus for S. monitoring the gamma-ray activity of bulk -Material, for example coal in a stockpile, wagon or other bulk mass, comprising apparatus as just described and an elongate tubular housing for the probe, which housing is closed Sat one end, carries external means to facilitate penetration of a mass of said material by the housing, and is internally dimensioned to receive and guide the probe to a measurement position within said mass of *material The apparatus may be employed with materials other than coal, for example materials consisting of a mixture of radioactive and non-radioactive components such as iron ores, oil shales, uranium ores, and potassium bearing materials.

The gamma-ray detector is shielded to prevent gamma-rays and x-rays below a threshold energy, eg 100keV, from reaching the detector and the shield preferably comprises successive sheets of aluminium, copper and S, cadmium in that order 6 outwardly from the detector.

The means for outputting an electrical signal may typically comprise a photomultiplier.

Also preferably housed within the probe body is a preamplifier coupled to receive said signal, and an associated high-voltage power supply.

The means to facilitate penetration of a mass of material preferably includes a tapered tip at the closed end of the tubular housing, and a spiral fin extending about said tubular housing. This housing may be, eg, a metre or more in length.

The invention still further provides a *se method of measuring the ash content of coal in a stockpile, wagon or other bulk mass, comprising penetrating the mass with said tubular housing to *bring the closed end of the housing to a sufficient depth therein, positioning the probe within the housing at or adjacent that depth and utilising the probe to measure the gamma-ray activity of the coal, which activity is a measure of the ash content of the coal in accordance with a calibration predetermined for the coal according to its type and/or source.

The invention will be further described, by :way of example only, with reference to the 25 accompanying drawings, in which:

S

Figure 1 is a somewhat schematic representation of a probe according to the invention, showing the probe in axial cross-section and associated electronics in block form; Figure 2 is a side elevation of a tubular housing for the probe; Figure 3 is a diagram showing the determination of the ash content of a coal stockpile with apparatus comprising the probe of Figure 1 and the tubular housing of Figure 2; Figure 4 is a schematic indication of the shielded application of the probe to surface measurement; and Figure 5 shows a means for making regular on-site recalibration.

The probe 10 depicted in Figure 1 includes a tubular body 12 closed at one end 13, a gamma-ray detector 14 mounted within the tubular body, shock-absorbing material 16 surrounding the detector, and encased electronic components 18. The electronics comprises a photomultiplier 19 and a high-voltage power supply with preamplifier Detector 14 may be, a NaI (Tl) scintillation crystal. Body 12 is typically anodised 15 aluminium, stainless steel or similar material. The shock-absorbing material 16 may be, silicone rubber.

The detector 14 is shielded by thin sheets 21 of cadmium, copper and aluminium (in that order) 20 to minimise the detection of gamma-rays and X-rays of energy below about 100keV. The aluminium sheet is nearest to the detector, followed by the copper and then the cadmium sheets.

All materials used in the manufacture of the sensor are selected for their low gamma-ray activity levels.

The gamma-ray signals from the sensor are fed into a main amplifier 22 through a well shielded armoured multi-core cable 24. The cable also carries power to the probe. The output of amplifier 22 is fed into a group of single-channel analysers 26 and counters 28 controlled by a timer 30. The windows of the single-channel analysers 26 (SCAs) are adjusted so that the first SCA accepts the gamma-ray signals in the range 80 to 2800 keV, the second from 1350 to 1550 keV, the third from 1660 to 1860 keV, and the fourth from 2500 to 2800 keV. These spectral windows are termed total, potassium, uranium, and thorium windows respectively. The total window accepts the total gamma-ray counts received by the detector. The potassium window accepts signals from the 1460 keV 40 gamma-rays originating from the isotope K, the uranium window accepts signals from the 1760 keV 214 gamma rays from Bi (a daughter product of 238U), and the thorium window accepts signals from S"208 the 2610 keV gamma rays from Tl (a daughter product of 232 Th).

In order to facilitate subsurface 15 measurement in coal stockpiles and wagons, a steel o"e •tubular housing or pipe 40 (Figure 2) with a pointed end 41 and a spiral external fin 42 is used. The tube is provided with a handle 44 for turning so that it will move into the coal in the manner of a drill.

The inner diameter of housing 40 should be slightly greater than the outer diameter of the probe so that 0.0 the probe can be received by the housing and thereby introduced into the coal for subsurface measurement.

S" This device thus acts as a form of access pipe for 25 the probe. The length of the pipe depends on the size of the coal stockpile. For example, a high coal stockpile requires a long pipe in order to gain access to its deeper regions for measurements.

However, the minimum length of the pipe or housing should be at least Im so that the measurement of the gamma-ray activity of coal can be made at least im in from the surface to prevent background gamma-rays from the environment reaching the detector. The background radiation from the environment is due to the presence of potassium and the daughter products of uranium and thorium in the soil and other materials around and below the stockpile, and from cosmic radiation.

In operation, the access housing or pipe is inserted into the coal stockpile 45 to the depth at which the measurement has to be taken, and the probe 10 is introduced into the pipe and placed at the desired position for the measurement of gamma-ray activity (Figure 3).

The gamma-rays from the coal are counted for 0 Ca time long enough to accumulate statistically significant counts in the four analyser windows. If T, K, U and Th are the gamma-ray countrates received by the total, potassium, uranium, and thorium windows *respectively, the ash content of coal around the access pipe in the vicinity of the gamma-ray detector can be determined from the calibration equation ash aT bK cU dTh e where a, b, c, d and e are constants to be determined 25 experimentally for a coal of given type and/or source.

The same probe 10 can be used for the surface measurement of coal ash in stockpiles and S. wagons. For this purpose, the probe is placed near the surface of coal for radioactivity measurement.

However, in this position the probe can receive background radiation of terrestrial and cosmic origin. A radiation shield, preferably of lead, is therefore used to shield the detector from such background radiation. Figure 4 illustrates one t method of using such a radiation shield,. The radiation shield 50 is in the form of a disc and of sufficient thickness to shield the background radiation; it is provided with a hole 52 in the centre to introduce the probe 10 into the coal. In operation, the radiation shield is placed on the surface of coal in the rrgion where the radiation measurement is to be made and the probe is introduced int, the coal, as shown in Figure 4. The gamma-ray activity of the coal is measured in the same manner as described in the case of subsurface measurement and the determination of the ash content in coal is made using a similar calibration equation. The constants a, b, c, d and e of this calibration equation should also be determined experimentally for S.o a given coal.

A major potential problem of gamma-ray-activity measurement in the field is associated with lack of stability of electronics, due 20 to change in gain. Use of. an electronic spectrum stabiliser is one way of overcoming this problem, but the use of such devices for a portable instrument is cumbersome and adds to the cost of the device. An 00@e** alternative method is the use of a calibration pad 25 for frequent checking of the calibration. The calibration pad consists of a block 60 (Figure made of concrete or similar material doped with naturally radioactive materials, such as monazite sand and feldspar containing thorium, uranium and potassium.

The block 60 is provided with a hole 63 in the centre to introduce the probe into the block for counting in a fixed geometry. The gamma-ray activity of the calibration pad should be sufficient to 11 provide adequate counts in the four spectral windows without overloading the counting system. It should be noted that the calibration pad, while providing a constant number of gamma-ray photons of different energies to the probe, acts as a shield against gamma-ray background from the surroundings. The various spectral windows should, therefore, record consistent gamma-ray countrates if the probe is placed in the calibration pad and counted in the same geometry. If the gain, and consequently the calibration, changes, the gamma-ray count in various windows would also change. Thus, the change in the calibration can easily be detected from the countrate in various windows and the gain of the counting 15 system can be adjusted to regain the original calibration.

The described arrangement has been advanced merely by way of explanation and many modifications may be made thereto without departing from the spirit 20 and scope of the invention which includes every novel feature and combination of novel features herein disclosed.

C.

S

S* C C

C

S S oO o oooa o* S oo oil oO oo o oooo 0

S

C

S C

S

S..

SO S

Claims (14)

1. Apparatus for monitoring the natural gamma-ray activity of bulk material comprising a probe and spectral analysis means, the probe comprising a tubular body closed at at least one end, a gamma-ray detector mounted within the tubular body and means coupled to the detector for outputting an electrical signal in response to receipt of a gamma-ray photon by the detector, and including means in the probe providing shock protection for the gamma-ray detector, the detector being shielded to prevent gamma-rays and x-rays of below about 100 keV from reaching it; the spectral analysis means being connected to receive the outputted electrical signals and comprising spectral energy windows for accepting signals originating from the presence of, respectively, naturally occurring potassium, uranium and thorium, or their daughter products, in the bulk material.

2. Apparatus according to claim 1 wherein the shield is ::comprised of successive layers of aluminium, copper and cadmium in that order outwardly from the detector.

3. Apparatus according to any one of the preceding claims wherein the means for outputting an electrical signal comprises a photomultiplier.

4. Apparatus according to any one of the preceding claims S: :wherein the means for outputting an electrical signal includes a pre-amplifier. Apparatus according to any one of the preceding claims further including means for the probe to facilitate penetration of a bulk material. 7 8AU/PROVNLS p, -13-

6. Apparatus according to claim 5 wherein the penetration means comprises a tubular housing closed at one end for containing the probe, the housing being adapted to penetrate a mass of material.

7. Apparatus according to claim 6 wherein the adaptation of the housing is a helical fin providing a screw.

8. Apparatus according to any one of the preceding claims wherein the spectral analysis means includes a main amplifier and a group of single channel analysers (SCA's) and counters.

9. Apparatus according to claim 8 wherein the windows of the SCA's are adjusted so that a first SCA accepts gamma-ray signals in the range 80-2800 keV, a second from 1350-1550 keV, a third from 1660-1860 keV and a fourth from 2500-2800 keV. Apparatus according to any one of the preceding claims further including a calibration means for the probe.

11. Apparatus as claimed in claim 10 wherein the calibration means is a block of material doped with naturally radioactive materials, the block including a hole allowing introduction of the probe for the counting of gamma-rays in a fixed geometry.

12. A method of measuring the ash content of coal in a stockpile, wagon or other bulk mass using apparatus as defined ir. any one of claims 5-7, comprising the steps of penetrating the mass with the tubular housing to bring the closed end of housing to a depth sufficient to exclude extraneous i gamma-rays, the probe being positioned or positioning the probe within the housing at or adjacent that depth, and measuring the gamma-ray activity of the coal, which activity is a measure of the ash content of the coal in accordance with a calibration predetermined for the coal according to its type and/or source. TW3078AU/PROVNLS -14-

13. A method of measuring the ash content of coal in a stockpile, wagon or other bulk mass using apparatus as defined in any one of claims 1-9 excluding claims 5-7, comprising the steps of placing the probe on or within and near the surface of the coal, shielding the detector from extraneous background radiation and measuring the gamma-ray activity of the coal, which activity is a measure of the ash content of the coal in accordance with a calibration predetermined for the coal according to its type and/or source.

14. A method of measuring the ash content of coal according to claim 12 or claim 13 wherein the ash content is determined from the calibration equation: ash aT bK cU dTh e where a, b, c, d and e are constants determined experimentally for a coal of given type and/or source; K, U and Th are gamma-ray count-rates due to Potassium, Uranium and Thorium respectively, and T is the total gamma-ray count-rate. Apparatus for monitoring the natural gamma-ray activity :of coal substantially as hereinbefore described with reference to the drawings.

16. A method of measuring the ash content of coal substantially as hereinbefore described with reference to the drawings.

17. A method, or apparatus according to claim 10 or 11, for .calibrating a probe substantially as hereinbefore described with reference to the drawings. Dated this 6th day of January 1993. COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION TW3078AU/PROVNLS