ARMUG New CAM Developments. Arran Morgan MSc Physicist

Transcription

1 New CAM Developments Arran Morgan MSc Physicist

2 Topics Particulate sampling considerations Alpha spectral analysis Concentration calculation Spectrum stabilisation Beta measurement Loose filter Bi detection - Decay - Testing in the presence of Radon - Test results - Initial feedback Questions

3 Particulate sampling considerations Air monitor positioning - Accurate representation of air ingested by workers - Accurate representation of air passing through a duct Dust loading - Loss of spectral resolution (peak spreading) - Peak movement (half life dependant) - Increased pressure drop/ loss of flow - Regular filter changes help to overcome these issues Filter considerations - Pressure drop - Strength of material - Efficiency to different AMAD

4 Particulate sampling considerations Interference from natural background radiation - Affects beta and alpha measurement - Some form of compensation required Air density changes between the filter and detector - Alterations to peak shape and position from reference - Compensation or added tolerance required

5 Particulate sampling considerations Radon interference Rn Radon (3.82d) Po-218 (3.05 min) Pb-214 (26.8 min) Bi-214 (19.7 min) Po-214 (164 usec) Pb-210 (22 yrs) Pb MeV alpha 6.0 MeV alpha 0.67 MeV beta 3.26 MeV beta 7.68 MeV alpha 160 KeV beta Radon and Thoron Decay Series 36% 6.05 MeV alpha Tl-208 (3.1 min) Rn Thoron (54.5 sec) Po-216 (0.16 sec) Pb-212 (10.6 hours) Bi-212 (60.5 min) 6.29 MeV alpha 6.78 MeV alpha 0.58 MeV beta Po-212 (0.3 msec) Pb-208 (Stable) 64% 2.25 MeV beta 1.8 MeV beta 8.78 MeV alpha

6 Particulate sampling considerations Estimation of Radon levels determined through fitting of the 218 Po peak Must assume equilibrium (the ratio of Rn-222 and Po-218 is stable) Equilibrium count rate of Po-218 on filter medium 1

7 Spectrum analysis Peak fitting to the Polonium isotopes in both peak fit and RoI versions. Po-212

8 Spectrum analysis Po-214

9 Spectrum analysis Po-218

10 Spectrum analysis Pu -239 Am-241

11 Concentration calculation Once interfering background has been subtracted from the count rate due to a radiation of interest, concentration may be calculated. Look at activity now compared with activity a time period ago Bq Concentrat ion = Vol Bq / m 3 Adjustment made for activity that had decayed away during the last measurement period important for short lived radioisotopes

12 Spectrum stabilisation Changes in ambient temperature and pressure may cause spectral shift from reference position at calibration Due to differences in air density of the air gap in between the filter medium and detector Temperature and pressure is measured and a shift factor is applied to the spectrum to compensate for this movement Cumulative counts kpa (reference) 70 kpa no shift factor 70 kpa shift factor applied Channel number

13 Beta measurement Compensation for gamma background via secondary detector Compensation for beta interactions due to Radon presence - Individual beta factors given for Radon progeny - Beta factors multiplied by count rates obtained through peak fitting Good efficiency to low energy beta emitters - Approximately (6-8% for 14 C, beta end point energy kev) Beta CPS Comp Off Beta CPS Comp On BG Comp off BG Comp on Dose rate Cs-137 (µsv/h)

14 47 mm Loose Filter Compatibility Provision for sample counting with 47 mm filters Re-usable filter holder Collection on 1 diameter efficiency is the same as the standard 1 card mount filter

15 Bi Detection Why measure? - Medical applications - Bi falls within the 223 Ra decay chain Ra is the active component in Radium dichloride - Used in the treatment of metastatic castration resistant prostate cancer Facilities producing Radium Dichloride may want to monitor effluent/ airborne activity in worker environments Mainly from 219 Rn migrating through filters trapping out airborne 223 Ra Bi is the first alpha emitter that can be realistically measured

16 223 Ra and Bi decay scheme 223 Ra Bi Decay 219 Rn 215 Po 11.4d 3.96s Alpha Beta % probability of alpha decay % MeV Pb 1.78ms 36.1m Bi Po 99.7% 207 Tl 0.3% 2.14m 207 Pb 4.77m 0.516s % MeV - Alpha peaks located in between 218 Po and 214 Po 2

17 Bi testing in Radon background Aim: measure simulated Bismuth count rate to a high degree of accuracy in a Radon environment of similar count rate Pulser used to simulate a range of count rates from 6.62 MeV alpha decay in the presence of Radon Count rates of 1.25 cps to 5 cps used as this is similar in magnitude to Polonium 214 count rates in testing area Spectra acquired at each count rate to determine whether peak fit was viable in Radon presence Measurement % could be determined based on simulated Bismuth count rate and count rate beneath peak This would ensure measurement of real releases during instrument application

18 Bi testing in Radon background Further testing performed by merging Radon spectra, simulated Bi 6.6 MeV emissions and simulated Bi 6.3 MeV emissions Determine what effect the lower energy, low probability Bi alpha emission has on peak fitting to the spectrum Count rate of approximately 0.5 CPS for the 6.6 MeV emission and 0.1 CPS for the 6.3 MeV emission to agree with different emission probabilities with associated energies Such a low count rate would present a challenge for the peak fit routine

19 Bi testing in Radon background 218 Po Bi 214 Po 212 Po Count Data Fitted Channel

20 Bi testing results Peak fitting with a simulated Bi input of 5 counts per second 218 Po peak observed at the edge of the Bi peak 93% of simulated count rate correctly attributed to Bi Count Channel Number Bismuth 200 ms Interval Fitted

21 Bi testing results Peak fitting with a simulated Bi input of 1.25 counts per second 218 Po peak observed at the edge of the Bi peak 95% of simulated count rate correctly attributed to Bi Count Bismuth 800ms Interval Fitted Channel Number

22 Bi testing results Difficulty to accurately measure activities of both 218 Po and Bi when both isotopes have similar count rates Due to the small separation of the alpha peaks of interest A relationship between measured activities 1.315E E-05 Bi 6.6 MeV activity (µci) 1.310E E E E E E E E E E E E Po activity (µci) Bi- 6.6 MeV Po E E Time period (minutes)

23 Bi testing results Even at low count rates and in the presence of Radon, fluctuations of less than 3% This effect becomes less important with increasing Bi activity Bi 6.6 MeV % of max activity Po % of max activity Bi- 6.6 MeV Po Time period (minutes) 70

24 Bi testing results Bi activity remains within 3% of maximum with fluctuating 214 Po activity Indication that 214 Po has minimal effect on measured result Bi % max activity Po % max activity Bi- Po Time period (minutes)

25 Bi MDL Average MDL of 0.1 Bq in the presence of approximately 16 Bq/m 3 radon maximum ime window of 60 minutes 95% confidence limit 3.50E E E-01 MDL (Bq) 2.00E E-01 MDL (Bq) Bismuth chronic 1.00E E E Time (minutes)

26 Initial feedback Correspondence with the customer has confirmed that small releases of Bi are detected Confirmation of what was observed in testing

27 References 1. D J Wagennar. Radiation physics principles. Accessed 20/01/ /2_2.3.html 2. Data from the Henri Bequerel National laboratory website. Accessed 20/01/2014.

28 Thank you Questions?