Setting up and carrying out a check source measurement

Transcription

1 Setting up and carrying out a check source measurement 1 Introduction Ionisation chambers are relatively fragile devices and can be faulty with little or no visible sign of damage. Because of this, most instruments used as secondary standards are supplied with a radioactive source that can be used to check the overall sensitivity of the ionisation chamber and its electrometer. The source, usually a 90 Sr foil, is incorporated in a holder designed to make it safe to handle. The ionisation chamber can be inserted in the holder in a reproducible position close to the source and the instrument then used in the normal manner to determine the time required to give a specified reading or the ionisation current. Since the decay constant for the source is known the instrument reading at any time in the future can be predicted and compared with the actual indication. Any discrepancy greater than the normal random variation indicates that a fault exists in the instrument. 2 Method of Measurement 2.1 General When an instrument is submitted for calibration at the National Physical Laboratory a set of measurements with the check source is the first operation that would be carried out following its receipt. The check-source time/current is then compared with any previous determinations, for example a previous calibration, and any discrepancy investigated. The last operation before the newly calibrated secondary standard is returned to the customer is also a determination of the check-source time/current. Again, any discrepancy between the results obtained before and after the X-ray calibrations would be investigated. 2.2 Equipment Required All test equipment used must have a valid calibration. a) Timer (if used) b) Barometer c) Thermometer d) Electrometer 2.3 Data To Be Recorded All measurements should be entered in the attached sheets given in Appendix 2; where corrections have to be applied to an instrument reading and the reading, correction and corrected value must all be recorded. The following are the minimum data that must usually be recorded. a) Type and serial number of electrometer, ionisation chamber and source. b) Type and serial number of thermometer and barometer used. c) Date of measurement. d) Source temperature at start and end of measurements. e) Atmospheric pressure during the measurements. f) Air density correction applied (calculated from d) and e) above). g) All electrometer and time readings. h) Mean readings and standard uncertainty of the mean. i) Corrected mean of readings and comparison with previously recorded measurements. Setting up and carrying out a check source measurement Page 1 of 10

2 2.4 Measurement Sequence Source, chamber and electrometer should be brought together in the measuring room at least ½ hour before the measurements are to be carried out. This will need to be longer depending on any temperature difference between the store and measurement room in order to ensure that the source and chamber are at similar temperatures. In the case of Grenz-ray, Roos and NACP chambers this time is much reduced owing to the much smaller size of the chamber and check source. Note: to ensure that the temperature of the Grenz-ray, Roos and NACP chamber systems are sufficiently stable, the chamber, source and holder should be placed in a thermally insulating box (e.g. made of expanded polystyrene). The thermometer should be rested in the box on or as close as possible to the chamber. If the electrometer is mains powered it should be switched on approximately 3 hours before measurements are started. If the electrometer is battery-powered it should be switched on about ¼ hour before measurements are started. About ¼ hour before measurements commence the chamber should be inserted in the source housing or placed on the source holder to allow the equipment to stabilise. Before starting, ensure that the source temperature is sufficiently stable. In the case of Grenz-ray, Roos and NACP chambers, measurements should be made only with the chamber fitted in its own holder. Ensure that the chamber is pushed fully into the holder and that it is not pulled out again by the weight of the cable. The measurement sequence below illustrates the general principles and may be modified for a particular system. a) Carry out and record the results of any pre-measurement checks specified by the instrument manufacturer, e.g. check and if necessary adjust instrument zero, check batteries, check saved instrument factors etc. b) Record the initial temperature of the check-source housing. c) For each measurement, record the electrometer indication (normally close to the reference setting) and the time to reach that indication. The number of readings will depend on the system and the uncertainty required; normally five will be taken. After each reading check that the zero of the instrument has not changed significantly. d) About half-way through the measurements, record the atmospheric pressure at the height of the chamber (mbar or kpa). If the atmospheric pressure is changing rapidly, i.e. more than 1 mbar between the start and end of the readings then the measurement should normally be repeated when the pressure is more stable. e) After the final reading note the check-source housing temperature. f) If the temperature of the source housing has changed rapidly, i.e. by more than 0.3 C between the start and end of the readings then the measurement should normally be repeated. However, if it is impossible to avoid a large temperature variation then the temperature should be noted about half-way through each reading. Setting up and carrying out a check source measurement Page 2 of 10

3 2.5 Calculations a) Calculate and record the average of the initial and final temperatures. b) Calculate and record the air-density correction, i.e. the correction to give the check-source reading at 20 C and mbar. The final requirement is normally the time required to give a specified instrument reading; the air density correction is thus: - p T (1) where p = atmospheric pressure in mbar T = mean source temperature in C If it is necessary, because the temperature change is large, to measure the temperature for each reading, then an air-density correction must be calculated for each reading in exactly the same way. NOTE: Equation (1) is used when applying an air-density correction to measurements of TIME but should be inverted when applying the correction to measurements of CHARGE or CURRENT. c) For each reading, if appropriate, calculate the time taken for the instrument to reach the reference setting, multiplying by the air density correction determined in b) above if the temperature change was large. d) Calculate and record the mean time taken to reach the reference setting or the mean current and the standard deviation of the mean (SDOM). You will need to repeat the check source measurement if there is a trend in the readings or if the standard deviation of the mean is too large [i.e. greater than 0.015%]. Q. What might cause a trend in the readings? e) If the air-density correction was not applied to the individual reading, multiply the mean calculated in d) by the air-density correction calculated in b) and record the results as the mean corrected to 20 C and mbar. f) The mean time should be corrected for the decay of the source using the relationship below. The reference date should be recorded and the decaycorrected value. The correction factor may be found from decay tables (see Appendix 1) or calculated using the relationship: - t t e t mean, 0 mean (2) Where tmean = Mean air-density-corrected time on the day of measurement tmean, 0 = Mean air-density-corrected time on the reference day = Decay constant (see Appendix 1) t = Time difference between measurement day and reference day in the same units as decay constant Setting up and carrying out a check source measurement Page 3 of 10

4 g) If the mean ionisation current is required then the decay corrected value is calculated using the expression:- I I e o t (3) Where I = mean air-density-corrected current on the day of measurement Io = mean air-density-corrected current on the reference day h) Record the decay-corrected mean value of the time or current, as appropriate, and compare with any previous determinations. Agreement between successive measurements within a period of 2-3 months should be better than 0.2%. Setting up and carrying out a check source measurement Page 4 of 10

5 APPENDIX 1 Decay table, strontium-90 t e t e t (days) t (weeks) t (months) (30 days) t (years) (365 days) Half-life for 90 Sr = days, = per year Setting up and carrying out a check source measurement Page 5 of 10

6 APPENDIX 2 Check Source Measurement Record I (CHARGE or CURRENT) Ionisation chamber type Check source type Electrometer type Thermometer Barometer Reference settings:- Reading a Quantity Range Electrometer charge calibration b (nc/scale rdg) Scale Linearity Correction c HT set to Volts Measurements made by Measurement date Atm pressure (mbar) Source temp, start (degc) Source temp, end (degc) Source temp, mean (degc) reading corrn corrd rdg d e Air density correction = ( e)/d x ( /293.15) f Reading Time (s) Current (pa) Mean current at reference setting Standard deviation of mean (%) Corrd to 20 degc, mbar [fxg] Reference date Decay period [-ve if in the past] Sr-90 half life Decay correction=exp((ln(2)/j)xi) Corrd to reference date [h/k] Mean current on reference date [bxcxl] pa g pa h 1-Oct-15 days i days j k pa l pa m Setting up and carrying out a check source measurement Page 6 of 10

7 Check Source Measurement Record II (TIME) Ionisation chamber type Check source type Electrometer type Thermometer Barometer Reference settings:- Reading a Quantity Range Electrometer charge calibration b (nc/scale rdg) Scale Linearity Correction c HT set to Volts Measurements made by Measurement date Atm pressure (mbar) Source temp, start (degc) Source temp, end (degc) Source temp, mean (degc) reading corrn corrd rdg d e Air density correction = (d/( e))x(293.15/ ) f Reading Time (s) Time to Ref Mean time to reference setting Standard deviation of mean (%) Corrd to 20 degc, mbar [fxg] Reference date Decay period [-ve if in the past] Sr-90 half life Decay correction=exp((ln(2)/j)xi) Mean time on reference date [hxk] Mean current on reference date [axbxc/l] s g s h 1-Oct-15 days i days j k s l pa m Setting up and carrying out a check source measurement Page 7 of 10

8 22.1 Check source measurement (Reference date: 1 October 2015) Chamber type 2611A 193 Check source type Current, pa Oct 02 Apr 03 Oct 03 Oct 04 Apr 06 Apr 09 Feb 11 Jan 12 Mar 12 Jan 13 Mar 13 Jun 14 Feb 15 Oct 15 Jan 16 Date of measurement Setting up and carrying out a check source measurement Page 8 of 10

9 20.2 Check source measurement (Reference date: 1 October 2015) Chamber type 2611B 207 Check source type Current, pa Sep 03 Oct 03 Feb 05 Apr 05 Aug 05 Oct 05 Feb 10 Mar 10 Oct 12 Feb 13 Aug 13 May 14 Aug 14 Feb 15 Oct 15 Jan 16 Date of measurement Setting up and carrying out a check source measurement Page 9 of 10

10 16.6 Check source measurement (Reference date: 1 October 2015) Chamber type TW Check source type Current, pa Mar 03 Mar 04 Apr 04 Feb 05 Apr 05 Apr 06 Mar 09 Feb 10 Mar 10 Jan 12 Mar 12 Jan 14 Jan 16 Date of measurement Setting up and carrying out a check source measurement Page 10 of 10