Operational checks

2.4.1: CHECK OF REPRODUCIBILITY Purpose of test

To check the day-to-day reproducibility of performance of a radionuclide calibrator in measurements on commonly used radionuclides.

Materials

Long-lived sealed medium-energy gamma-radiation source, activity about 3.7 MBq (100 >uCi). A r?3Ba, 137Cs or 226Ra source is suitable. A certified source may be used, through the manner of its use does not require that its activity be known.

Source holder

Remote handling device for source Linear graph paper

Procedure

1. Select the operating conditions appropriate to the radionuclide in most common use (e.g. ^^Tc"1).

2. Note the background reading to be subtracted from subsequently measured activities. Alternatively, if an adjustable zero control is provided, adjust this for zero reading.

3. Insert the gamma-radiation source into the source holder by means of the remote handling device and introduce the source holder into

the instrument.

4. Allow sufficient time for the reading to stabilize.

5. Measure and record the apparent activity, subtracting the background reading if necessary.

6. Remove the source holder from the instrument and extract the source by means of the remote handling device.

7. Repeat steps 1-6 under operating conditions appropriate to each other radionuclide in common use.

Data analysis

Record the results on a control chart showing apparent activity plotted against date on linear graph paper (Fig. 2-4). Results on successive days should be closely distributed about a straight line corresponding to the radioactive decay of the source. An initial point on this line may be established as the mean of ten replicate measurements on the day concerned. The negative slope is determined by the physical half-life of the radionuclide constituting the source. For the purpose of the test, decay may be considered linear over a period short compared with the half-life (e.g. 1 year). Limits of acceptability may be

indicated by two other straight lines parallel to the first, but respectively above and below it at a distance determined by the precision of the instrument as specified by the manufacturer (e.g. +_ 5% of expected activity). If an individual result lies outside these limits, this may be taken to indicate faulty performance. The procedure should then be carried out a second time, but with step 5 repeated to a total of 10 successive measurements, and the individual measured activities examined for evidence of imprecision (random errors), bias (systematic errors), or both.

Observations

It should be appreciated that since the operational conditions selected for the test are not, in general, those appropriate to the

130 r

Jut. Aug.

Day of test

Sep Oct Nov.

2-4. Test 2.4.1: Check of Reproducibility. Part of control The 137Cs source used had a mean measured activity of 4.55 MBq

on 1 April. The limits of acceptability indicated correspond Fig

chart.

(123

to 4^ 5% of the expected activity. The initial discrepant result on 17 May arose from failure to allow sufficient time for the reading to stabilize.

radionuclide constituting the source, the apparent activity recorded may differ greatly from the true activity. This is unimportant in testing simple reproducibility of performance. It should also be appreciated, however, that the procedure described checks the reproducibility of performance of the instrument under the operational conditions selected, but not necessarily under those appropriate to other radionuclides. This is a significant limitation, especially with instruments having selector switches, selector push-buttons or plug-in modules for different radionuclides. With such instruments, the procedure should periodically be extended to cover all radionuclides for which specific provision is made, the apparent activity being recorded for each set of conditions in turn. Again, if the operating conditions for the radionuclide concerned can be selected either by means of a selector switch, selector push button or plug-in module or by means of a potentiometer with a dial, the relevant procedure should be carried out twice, first with the one, then with the other enabled. The results should agree.

Some manufacturers of radionuclide calibrators will supply a long-lived radiation source with the instrument at the time of purchase.

It is then possible to request that the apparent activity of this source under the operating conditions for each radionuclide for which a selector switch position, push button or plug-in module is provided or for which a potentiometer dial reading is specified be determined in the course of factory calibration, and that the values thus found be quoted to the purchaser. The values may then be confirmed with the same source as part of acceptance testing and re-determined quarterly thereafter in routine quality control, serving to indicate whether changes in the accuracy of the instrument have occurred.

Interpretation of results

Discrepant results may imply imprecision, bias, or both, as indicated by the results of replicate measurements. Whatever the nature

of such discrepancy, the defective instrument should be withdrawn from operational use pending corrective action.

Limits of acceptability

The limits of acceptability for the results of the test are determined in part by the precision of the instrument specified by the manufacturer. In general, however, for measurements on sources such as

that specified, the reproducibility of performance should be such that all individual measured activities are within +_ 5% of the mean measured activity, provided that radioactive decay has a negligible effect over the measurement period.

Conclusion

Record whether or not the results confirm acceptable performance.

If not, indicate follow-up action taken.

2.4.2: CHECK OF BACKGROUND RESPONSE Purpose of test

To check the background response of a radionuclide calibrator under the operational conditions appropriate to a particular radionuclide.

Procedure

1. Select the operational conditions appropriate to the radionuclide concerned.

2. Record the background reading to be subtracted from subsequently measured activities. Alternatively, if an adjustable zero control is provided, adjust this for zero reading and record its setting.

Interpretation of results

A significant increase in background response may indicate either radioactive contamination of the instrument or increased environmental radiation from local sources. If such an increase is observed, the liner of the instrument well should be removed and the test procedure repeated. A return to the previous response would indicate contamination of the liner, which should then be replaced. The contaminated liner may be retained for re-use, if desired, after appropriate cleaning and/or storage. Persistently high response would suggest other contamination of the instrument or increased environmental radiation from local sources.

These possibilities should then be explored.

Limits of acceptability

While specific limits of acceptability cannot be laid down for the results of the test, an increase in background response of 20% or greater would call for further investigation.

Conclusion

Record whether or not the results confirm acceptable performance.

If not, indicate follow-up action taken.

3. MANUAL AND AUTOMATIC COUNTING SYSTEMS