Nouvelle méthode de dosage du radon dans l'eau par scintillation dans l'ortho-xylène A new method for the determination of radon in water by scintillation in ortho-xylene

Résumé

Une nouvelle méthode de dosage du radon dans l'eau a été mise au point. Elle consiste à purger le radon par l'argon, à le dissoudre dans l'o-xylène contenant les scintillateurs organiques 2,5 diphényloxazole (PPO) et 2,2' p-phénylènebis-5-phényloxazole (POPOP) et à compter les particules alpha et bêta par scintillation induite. La méthode est rapide, relativement simple, reproductible et sans interférences. Elle est très sensible; la limite de détection est de 0,01 Bq/L pour un échantillon de 2 litres et un temps de comptage de 1 heure. La méthode a été utilisée pour mesurer les concentrations du radon dans l'eau de certains puits au Québec. Celles-ci varient de 1,6 à 627 Bq/L. Ces concentrations sont similaires à celles mesurées dans d'autres pays.

Mots-clés:

• Radon,
• eau,
• dosage,
• scintillation liquide,
• ortho-xylene

Abstract

A new method for the determination of radon in ground water has been developed. It consists of purging radon with argon and dissolving it in 180 mL of ortho-xylene containing the organic scintillators 2,5 diphenyloxazole (PPO) and 2,2' p-phenylenebis-5-phenyloxazole (POPOP). The scintillation yield of radon in o-xylene, toluene, benzene and hexane has been measured at - 20, 0 and + 20 °C and the highest efficiency was obtained in o-xylene at - 20 °C.

The concentration of PPO in o-xylene was varied and the maximum yield occurs at a concentration of 4 g/L. The optimal purging volume of argon was Pound to be 3 liters. The counting efficiency increases from 2.7 to 3.9 counts per disintegration after 3 hours decay time due to the build-up of radon daughters and then Starts to decrease with the hall-life of radon (3.824 days). The detection limit at 95 % confidence level for a 2 liter sample and for 1 hour counting time is 0.01 Bq/L.

The method is virtually interference free. Due to the argon purging only radioactive gases can interfere with the measurement. Among these, thoron (Rn-220) and action (Rn-219) will decay before counting due to their short hall-lifes (56s and 4s resp.). The concentration of Kr-85 in air is low (0.0006 Be) when compared with radon, and only 0.05 % of the Kr-85 is transfered to soil and ground water. Kr-85 is also 35 times less soluble than Rn-222 in toluene. The decay products of radon will remain in the water and it is not necessary to know the degree of equilibrium between them and Rn-222. Consequently the counting can begin soon (10 minutes) after the purging of radon and it is not imperative to wait for equilibrium.

The method was used to determine the concentration of radon in ground water in Quebec. The activities Pound ranged tram 1.6 to 627 Bq/L; they are similar to these measured in other countries.

Keywords:

• Radon,
• water,
• determination,
• liquid scintillation,
• ortho-xylene