Temporal variation of Radon measurements in three wells in the Phlegraean Fields, using LR-115 Type II Nuclear Track Detector

Radon-222 (222Rn), formed within the 238U chain by decay of 226Ra, is the main radioactivity source of groundwater. Radon has three radioactive isotopes but only 222Rn is of interest having a half-life of 3.8 days, since 219Rn and 220Rn are short-lived isotopes (3.8 s and 55.6 s, respectively).

The values of radon and temperature measured in three wells in the Phlegraean Fields in the 1980s, 1990s and early 2000s are reported.

The wells are: Damiani hot well, Costagliola cold well, Tortorelli hot well.

Measur at Tortorelli were interrupted in 1993, because of technical problems, while for Damiani and Costagliola were interrupted in 2006

To measure radon content we used the track-etch method.

This technique uses a particle sensitive cellulose nitrate film (Kodak LR-115 type II) as the detector. The measured track density (Track/cm2 /day).

A 2 x 4 cm strip of alpha-particle sensitive film is fixed into the internal top of a bell-shaped brass cylinder, long 0.2m, opened at its lower end, into which gas is permitted to flow.

The cylinder was positioned about 30 cm above the water, so that the film strip can record the traces made by the radon expelled from the water.

The method is commonly used in wells because water collects radon as it migrates through the rocks not only about the measurement point but for a wide area.

The time interval between measurements was about 10-15 days for the cold well (Costagliola well) and about 7 days for the hot wells (Damiani and Tortorelli).

The sensitive films were treated with 10% NaOH at 60°C for 90 minutes in the laboratory to enlarge the recorded alpha-tracks. The track shape depends on the energy and angle of incidence of alpha tracks

Tracks were counted using a microscope with a magnification of 160X.

Low radon content is detected in the cold well (Costagliola; temperature ranges between 20-27°C) , whilst radon contents in the hot wells (Damiani and Tortorelli; temperature 44°-52°C and 60-64°C, respectively).

The well temperature was measured when the films were changed using a K-type thermocouple