1、INTERNATIONAL STANDARD IS0 10573 First edition 1995-l 2-15 Soil quality - Determination of water content in the unsaturated zone - Neutron depth probe method Qualit - the gradients in this composition that occur within the measuring volume; - the gradients in soil water content that occur within the
2、 measuring volume; - the method of access tube installation; - the characteristics of the access tubing; - the specifications of the apparatus used. The calibration curve usually differs for each soil layer. In homogeneous layers that are thicker than the measuring volume, calibration curves are gen
3、erally linear, their para- meters depending on the soil composition. In the case of thin or non-homogeneous soil layers, however, calibration curves will often be non-linear due to the different effects of gradients in soil composition and water content under wet and dry conditions. 5 Apparatus 5.1
4、Neutron depth probe, consisting of a fast neu- tron source and a thermal neutron detector combined with a read-out unit. 5.2 Thin-walled access tubing, with an inner diameter slightly larger than that of the neutron probe. The tubing shall consist of material that is very “transparent” to fast and t
5、hermal neutrons (e.g. alu- minium, aluminium alloy) and which is resistant to chemical corrosion and to deformation due to instal- lation activities. Stainless steel, galvanized iron and plastics (polyethylene) are also suitable, though less transtoarent to neutrons. 5.3 Equipment for installing acc
6、ess tubes. 5.4 Equipment for drying and cleaning the access tubes, if necessary, a dummy probe for testing the tubing performance. 5.5 Calibration curves, for conversion of count rate to water content. 5.6 Usual apparatus for taking soil samples, for carrying out a field calibration to determine the
7、 volu- metric water content 8 gravimetrically according to IS0 11461. 6 Procedure 6.1 Installation of access tubes The location shall be representative of the immediate surroundings and care shall be taken to avoid surface water from concentrating on the spot. Use a platform to prevent damage to sur
8、rounding vegetation and 2 IS0 IS0 10573:1995(E) compaction of the soil surface whilst installing a tube. each soil layer in accordance with IS0 11461, under Ensure that radial soil compaction around the tube, several different hydrological conditions, to derive a compaction below it and the creation
9、 of voids adjacent calibration curve for each layer. to it are prevented as far as possible. Install access tubes by either of the following methods. NOTE 10 The subdivision of the soil Drofile into layers is determined initially by differences in sbil compositidn, but the form of soil water content
10、 gradients that systemati- a) Push the tube into the soil using a hammer and tally recur should also be considered. Further divisions may empty it using an auger. It is recommended that be necessary to meet the objectives of the investigation. the lower end of the tube be closed with quick The hydro
11、logical conditions under which the calibra- drying cement or a stopper, to prevent infiltration tion is conducted shall differ as much as possible so of ground water. that the calibration curves are representative of the b) Push the tube into a prepared hole of the same or range of conditions which
12、occur at the site. To meet slightly smaller diameter and of the required the requirement for time invariant gradients that do depth, then seal the lower end as in 6.1.1. Alterna- not vary with time as much as possible, the calibration tively, the lower end of the tube may be sealed shall not be cond
13、ucted after heavy rain or irrigation before insertion. applications, or immediately after the sudden begin- ning of extremely warm weather. Holes can be prepared using a guide tube or an auger or by a combination of these two methods. Close the top of the tube with a tight rubber stopper to keep out
14、 rain or surface water. The tubing shall always be dry inside. Determine the calibration curves by analysing the va- rious combinations of neutron count rate and water content for each soil layer by regression analysis. The count rate is considered as the independent variable (x) and the water conte
15、nt as the dependent variable NOTES (y). Calibration curves so derived are specific to the neutron probe used. Use of reference counts to nor- 8 It is recommended that access tubes be cut to protrude above the soil surface as little as the apparatus permits, so as to minimize the radiation dose recei
16、ved by the operator when lowering the probe. 9 More specific guidelines for installation are given in 131 and 4 in annex E. After installation, take great care to minimize distur- bance of the soil and vegetation at the site whilst conducting measurements in the access tube. 6.2 Calibration malize t
17、he count rate measurements used in the re- gression allows calibrations to be used with different probes of the same geometry (see annex C). Further guidelines for carrying out a field calibration are given in 121, 131, 41 in annex E and in annex B. NOTES 11 The calibration curves may change in time
18、 due to the following processes: - changes in the chemical composition of the soil including that of the soil water, and changes in bulk density. This can be corrected for, to a certain extent, on the basis of known (chemical) properties (see 131 in annex E); In most cases, calibration curves suppli
19、ed by neutron probe manufacturers, and those published in the litera- ture, give only a rough indication of the absolute soil - decrease of the source strength of the probe due to ra- dioactive decay, and/or decrease in the sensitivity of the detector. This can be corrected for by the use of referen
20、ce counts made in a medium with invariant water content, because no or insufficient recognition can be given to the specific influences of the site mentioned in note 7 in clause 4 (see also annex A). characteristics (see annex C). 12 The guidelines given here apply to the measurement of absolute wat
21、er content. When only relative measurements The influence of chemical composition and bulk den- sity (see A.2) is accounted for in calibrations derived theoretically from the macroscopic neutron-interaction cross-section of the soil concerned (see Ill, 141, 191 in annex El. (i.e. changes of water co
22、ntent in time) are to be assessed, the requirements for calibration and demands on accuracy may be less stringent. The combined influence of gradients in water content, chemical composition and bulk density is only accounted for by a field calibration. Therefore an in situ field cali- bration is nec
23、essary for accurate measurements of absolute water content. 6.3 Measurements The neutron depth probe shall be used in accordance with the manufacturers instructions as much as poss- ible, and particularly with respect to technical handling and safety. The field calibration is based on simultaneous d
24、eter- mination of the neutron count rate and sampling for the determination of the volumetric water content of Lower the probe in the access-tube to the depth at which it is required to make the measurement. IS0 10573:1995(E) IS0 Conduct the counts according to one of the following met hods: a) with
25、 a fixed counting time; in this case the num- ber of thermal neutrons detected is recorded; ed by appropriately skilled persons. Periodic checks to test for leakage from the sealed source shall be car- ried out by a competent agency. b) with a fixed number of detected thermal neu- trons; in this cas
26、e the counting time is recorded. 7 Expression of results NOTES Calculate the count rate R, which is the number of de- 13 When changes of water content in time are to be de- termined, precise positioning of the probe at a specified tected thermal neutrons per unit of time, using the following equatio
27、n: depth is important. 14 The second method mentioned for taking the counts has the advantage that the accuracy of the measurement is rela- tively constant (i.e. precision of the count rate), whereas the ac- R=f where curacy depends on the water content in the first method R is the count rate, in co
28、unts per minute; Instead of conducting a single count for a long time, it can be advantageous to make a number of counts for a short time because this provides quantitative infor- mation about the spread of the measurements. This information allows detection of certain types of failure in the appara
29、tus. N is the number of counted thermal neutrons; t is the counting time, in minutes. Calculate the water content 0, using the equation: 0 = fCR P) It is recommended that reference counts in a medium with invariant characteristics, such as a large water barrel (see C.3.11, be made at frequent interv
30、als to check the overall performance of the instrument. For example, a reference count might be carried out before and after each series of measurements in a specific access tube. A certain amount of drift in the reference count is to be expected. However, a sudden change from the general pattern al
31、most certainly indi- cates a failure of the apparatus, which should be re- paired or replaced. where 8 is the water content, expressed as a volume fraction; f is the calibration function (calibration curve) calculated by regression analysis; R is the count rate, in counts per minute; P represents th
32、e parameters of the calibration curve. 6.4 Safety and maintenance SAFETY PRECAUTIONS - The radioactive source within a neutron depth probe is a potential hazard to the operator, the public and the environment. Most governments and organizations have legally enforceable regulations concerning the acq
33、ui- sition, operation, transport, storage and disposal of radioactive devices, which must be adhered to. In the absence of specific radiological safety regu- lations, the guidelines of the International Atomic Energy Agency U* I and of the International Commission on Radiological Protection *I shoul
34、d be consulted. The half-life (458 years) of the americium commonly used in neutron depth probes is longer than the time over which the integrity of the source container (e.g. about 30 years) can be expected to last. When a neu- tron depth probe is no longer required, the radioactive source must be
35、disposed of at a repository for radio- active waste. Neutron depth probes shall only be used by suitably trained operators. Maintenance shall only be conduct- When necessary, the count rate can be corrected for the difference between the actual reference count rate (R,) and the expected reference co
36、unt rate (R,). In most cases, a correction of the type R = R(Rse/Rs) may apply, where R is the corrected count rate. For further explanations, see annex C. 8 Accuracy 8.1 The accuracy of the water content determined with the neutron probe is influenced principally by the following error sources. a)
37、The scatter in individual counts or count times as a result of the random variation in the number of neutrons emitted by the neutron source. The magnitude of this error is usually expressed as the standard deviation of the number of neu- trons counted. As the emission process follows a Poisson distribution, the resulting standard devi- ation in the number of detected neutrons is SN = JN
