Plastic Scintillation and dryness evaporation: a new procedure for Alpha/Beta determination in water samples

Transcription

1 a new procedure for Alpha/Beta determination in water samples Bouchra Aboudou, Hector Bagán, Alex Tarancón Sanz, Gemma Rauret, Jose Francisco García Departmento de Química Analitica de la Universidad de Barcelona, Spain LSC 2010 Advances in Liquid Scintillation Spectrometry Paris, France, 6-10 September 2010

2 Introduction - Gross Alpha/Beta are an important parameters to monitorize the radioactivity in water Depending on the level, specific analysis are required. - Procedures to determine Gross Alpha/Beta activity.. Proportional counters: sample evaporated to dryness and deposited on a planchet. - Well established - Low detection efficiency. Liquid Scintillation Counting: sample mixed with scintillation cocktail (PSA). - High detection efficiency - Amount of sample? Mixed waste production.

3 Introduction - Plastic Scintillation is an alternative method to liquid scintillation.. In general, sample solution is added to a vial filled up with plastic microspheres (PSm) - High detection efficiency (except for low beta emitters) No limited amount of sample No mixed waste production. - New procedure for Gross Alpha/Beta determination by Plastic Scintillation:. Water sample evaporated to dryness on the PSm inside the counting vial. Measurement in a Scintillation counter (PSA).

4 Objective To evaluate the capability of the new procedure based on Plastic Scintillation and dryness evaporation to the determination of gross Alpha/Beta parameters in aqueous samples. (Partially achieved)

5 Experimental Apparatus and reagents. - Detector Quantulus liquid scintillation (EG&G Wallac) - Poliethylene vials (20 ml) from Packard Instruments Co. - Plastic Scintillation microspheres (Detec-Rad) diameters ( , and µm) - Active solutions: 90 Sr/ 90 Y, 14 C and 241 Am Sample preparation procedure: 20 ml scintillation vial 2 g of PSm 1-20 ml of solution (1 ml) Evaporate to dryness at 40ºC Measurement 40ºC

6 Optimum conditions determination. - Liquid Scintillation vs Plastic Scintillation vs Plastic Scintillation+ dryness evaporation - Infuence of the PSm diameter. - Influence of the beta emitter energy. - Influence of sample volume

7 Liquid Scintillation vs Plastic Scintillation vs Plastic Scintillation+ dryness evaporation. Conditions: PSm ( µm), 90 Sr/ 90 Y 241 Am 241 Am LS -> PS: higher distance lower energy shorter pulse PSA shifts to lower values PS -> PS+dryness : intermediate.

8 Liquid Scintillation vs Plastic Scintillation vs Plastic Scintillation+ dryness evaporation. Conditions: PSm ( µm), 90 Sr/ 90 Y 241 Am 241 Am CL CP CP evap 1 Eff (%) channels Detection Efficiency: PS 27.6 PS+dryness evaporation 99.3 % LS -> PS: higher distance lower energy shorter pulse PSA shifts to lower values PS -> PS+dryness : intermediate.

9 Liquid Scintillation vs Plastic Scintillation vs Plastic Scintillation+ dryness evaporation. Conditions: PSm ( µm), 90 Sr/ 90 Y 241 Am 90 Sr/ 90 Y LS -> PS: higher distance lower energy shorter pulse PSA shifts to lower values PS -> PS+dryness : intermediate.

10 Liquid Scintillation vs Plastic Scintillation vs Plastic Scintillation+ dryness evaporation. Conditions: PSm ( µm), 90 Sr/ 90 Y 241 Am 90 Sr/ 90 Y CL CP CP evap 0.5 Eff (%) channels Detection Efficiency: PS PS+dryness evaporation % LS -> PS: higher distance lower energy shorter pulse PSA shifts to lower values PS -> PS+dryness : intermediate.

11 Liquid Scintillation vs Plastic Scintillation vs Plastic Scintillation+ dryness evaporation. Conditions: PSm ( µm), 90 Sr/ 90 Y 241 Am LS -> PS: higher distance lower energy shorter pulse PSA shifts to lower values PS -> PS+dryness : intermediate.

12 PS+ dryness evaporation. Influence of the PSm diameter Conditions: PSm ( , , µm), 90 Sr/ 90 Y 241 Am 241 Am PSm diameter increase: total surface decreases available surface increases? thinner recovery layer - higher energy longer pulses - PSA shifts to higher values

13 PS+ dryness evaporation. Influence of the PSm diameter Conditions: PSm ( , , µm), 90 Sr/ 90 Y 241 Am 241 Am With solution Evaporated Eff (%) Diameter Eff±STD Eff±STD D1: 59,6 ± 0,9 D1: 101,5 ± 1, D3: 41,1 ± 0,7 D3: 101,7 ± 1, D4: 27,6 ± 0,8 D4: 99,3 ± 1, channels PSm diameter increase: total surface decreases available surface increases? thinner recovery layer - higher energy longer pulses - PSA shifts to higher values

14 PS+ dryness evaporation. Influence of the PSm diameter Conditions: PSm ( , , µm), 90 Sr/ 90 Y 241 Am 90 Sr/ 90 Y PSm diameter increase: total surface decreases available surface increases? thinner recovery layer - higher energy longer pulses - PSA shifts to higher values

15 PS+ dryness evaporation. Influence of the PSm diameter Conditions: PSm ( , , µm), 90 Sr/ 90 Y 241 Am 90 Sr/ 90 Y Eff (%) With solution Evaporated Diameter Eff±STD Eff±STD D1: 178,2 ± 3,4 D1: 191 ± 6, D3: 173,5 ± 2,6 D3: 192,7 ± 2, D4: 163,2 ± 2,3 D4: 190,5 ± 2, channels PSm diameter increase: total surface decreases available surface increases? thinner recovery layer - higher energy longer pulses - PSA shifts to higher values

16 PS+ dryness evaporation. Influence of the PSm diameter Conditions: PSm ( , , µm), 90 Sr/ 90 Y 241 Am Proposed diameter: higher ( µm)

17 PS+ dryness evaporation. Influence of beta emitter energy Conditions: PSm ( , , µm), 14 C (156 kev) - 90 Sr/ 90 Y ( 2.27 MeV) 14 C 90 Sr/ 90 Y PSm diameter increase: total surface decreases available surface increases? thinner recovery layer - higher energy longer pulses - PSA shifts to higher values

18 PS+ dryness evaporation. Influence of beta emitter energy Conditions: PSm ( , , µm), 14 C (156 kev) - 90 Sr/ 90 Y (2.27 MeV) C 90 Sr/ 90 Y Eff (%) Eff (%) channels With solution channels Evaporated Diameter Eff±STD Eff±STD D1: 39,7 ± 0,8 D1: 80,3 ± 1, D3: 25,4 ± 0,6 D3: 78,5 ± 0, D4: 18,3 ± 0,6 D4: 78,3 ± 1,1 Low-Medium energy beta emitters can be detected with high efficiency

19 PS+ dryness evaporation. Influence of the sample volume Conditions: PSm ( µm) Solution volumes: 1, 5, 20 ml. Calibration procedures: Comprimise PSA: 90 Double PSA: no 90 Sr/ 90 Y at α spectrum (quantification 241 Am) 60 - no 241 Am at β spectrum region (quantification 90 Sr/ 90 Y) Compromise PSA 90Sr/ 90 Y 241Am Double PSA 90Sr/ 90 Y 241Am Volume Eff β Eff α 1 ml ± ± ml ± ± ml Volume Eff β Eff α 1 ml ± ± ml ± ± ml Detection efficiency is not constant with sample volume change (material deposited) -> calibration in progress.

20 Results and discussion sample quantification (exploratory results) PS+ dryness evaporation. Conditions: PSm ( µm) Calibration: Constant Efficiency Volumes: 5 ml. Drinking water (Fontvella) (3 Replicates). Act Sr Act Am Prop compromise Double PSA Err Sr Err Am Err Sr Err Am : ± ± ± ± : ± ± ± ± : ± ± ± ± :1-9.66± ± ± ± : ± ± ± ±3.90 Exploratory Relative Errors in Gross Alpha/Beta Compromise PSA > Double PSA Exploratory Relative Errors - Double PSA < 12% (except proportion 1:4)

21 Conclusions - A new procedure for Alpha/Beta determination in water samples is defined based on the use of plastic scintillator microsphere and dryness evaporation. - Evaporated water samples on PSm produce:. pulses of intermediate energy between LS and PS. pulses of intermediate duration between LS and PS. spillover vs PSA distribution shifted to lower values - Changes in the PSm diameter produce slight changes in spillover vs PSA distribution. - Low energy beta emitters are detected with high efficiency. - Detection efficiency depends on the material deposited (calibration is required) - Exploratory results using using constant efficiency and double PSA allow Gross Alpha / Beta activity determination with relative errors < 12 %

22 Acknowlegments. Ministerio de Ciencia e Innovación (Spain) for Financial support (CTM ). Plastic Scintillation and dryness evaporation: a new procedure for Alpha/Beta determination in water samples Bouchra Aboudou, Hector Bagán, Alex Tarancón Sanz, Gemma Rauret, Jose Francisco García Departmento de Química Analitica de la Universidad de Barcelona, Spain