1、 7009276 O009877 208 D Designation:D 3085-75(Reapproved 1983)Standard Practice for MEASUREMENT OF LOW-LEVEL ACTIVITY IN WATER This standard is issued under the fixed designation D 3085:the number immediately following the designation indicates the year of original adoption or.in the case of revision
2、.the year of last rekion.A number in parentheses indicates the year of last reapproval.A superscript epsilon(0 indicates an editorial change since the last revision or reapproval.1.Scope 1.1 This practice describes the process nec-essary to obtain meaningful data from water samples that contain low
3、levels(cl0 pCi)of radioactivity.2.Summary of Practice 2.1 The measurement of low level(up to 10 pCi)fission product or natural activities in environmental samples usually requires sepa-ration of the radioelement sought from the water sample.Normally,a measurement of the gross activity of a sample is
4、 non-specific and.therefore,of little value.Also,the sample size required to detect a given nuclide quantita-tively makes gross counting impractical.An adequate separation of the nuclide sought from the bulk matrix can usually be effected using either camer or carrier-free techniques.2.2 It is conve
5、nient although not absolutely necessary to add a radioactive tracer to the sample prior to the chemical separation.The tracer is usually another isotope of the element sought and one that has a different mode of decay.For example,strontium-85,a gamma emitter,is commonly used to trace strontium-90,a
6、beta emitter.The chemical recovery of each sample may then be easily and accurately determined by gamma counting the final prod-uct of the analysis and comparing this count with a known aliquot of the original tracer solution.2.3 If the tracer interferes with the measure-ment of the nuclide sought,t
7、hen other methods for determining recovery are necessary.One very common method is the gravimetric recov-ery of the carrier.2.4 When investigating low levels of fission products or natural activities in environmental samples.contamination of the sample may oc-cur due to the presence of the nuclide o
8、f interest in reagents used in sample processing.Careful analyses of these reagents must be performed to determine the extent of this contamination.so that it may be subtracted from the final sample value.In some instances the extent of contamination of the reagents may dictate the chemical procedur
9、e selected for a particular analysis.3.Significance and Use 3.1 This practice is suited to the gross mea-surement of alpha,beta and gamma radioactiv-ity in environmental samples with activity levels less than 10 pCi.4.Instrumentation 4.1 The counters used should be standard-ized periodically with so
10、urces prepared in a manner identical to that used for samples.but with a known amount of the nuclide present.In this way,sample count rates can be con-verted to disintegration rates.It is.however,impractical to prepare and count such sources each time samples are to be measured.A check source of a p
11、ermanent nature must be counted whenever samples are measured.and daily.if at all possible.to ensure that the instrument is functioning properly.Control charts carefully maintained on the check source and instrument This practice is under the jurisdiction of ASTM Com-mittee D-19 on Water and is the
12、direct responsibilit!of Subcommittee D 19.04 on Methods of Radiochemical Anal-Current edition approved Oct.31.1975.Published De-cember 1975.Originally published as D 308-72 T.Last previous edition D 3085-75.ysis.94 1).NOTICE:This standard has either been superseded and replaced by a new version or d
13、iscontinued.Contact ASTM International(www.astm.org)for the latest information.9009276 0009878 144 background will almost always detect serious instrument problems.From the control charts it can be determined if the instrument is behav-ing statistically and in a manner predictable by a Poisson distr
14、ibution.If no large fluctuations occur,it is always better to use long-term av-erage background and standardization data in the actual calculation of results.4.2 When counting a low-activity sample,it is always desirable to count for as long a time as possible.Sample workload is usuaiiy the determin
15、ing factor in counting times,but it should also be mentioned that the counting time should be compatible with instrument sta-bility.Overnight counts are usuaiiy considered to be a reasonable maximum counting time.5.Measurement of Alpha Emitters 5.1 The most suitable sample form for the measurement o
16、f low-level alpha activity is the electrodeposited source(l).2 Since alpha parti-cles are strongly absorbed,even in small sample masses,the counting efficiency of a precipitated source will be much less than that of an elec-trodeposited source.5.2 The alpha background of most detectors presently available is low enough so that anti-coincidence circuitry is not necessary.Com-monly used alpha counters employing a zinc sulfide(ZnS)scintillator or solid-state detector have backgrounds ranging from l
