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Discription of measuring methods for Radon Decay products with Tracerlab Radon/Thoron-Progeny-Monitors:

The Tracerlab device for measurements of radon decay products is usable to measure potential alpha energy concentration (Cp) as well as activity concentration of single nuclides (Ci; Po-218, Pb-214, Bi/Po-214). Measurements are founded in sampling of decay products on filters using air flow through the filters driven by a pump and measurement of alpha radiation of sampled nuclides.

Three kinds of methods for different uses are implemented in Tracerlab device. The choice of the method can be carried out on the display of the device. Fundamentally all data are saved in ASCII-files. These are the measured spectra as well as the calculated concentrations. Structure of concentration files depends on used methods.

1. Short time measurement

Short time measurement is used for radiation protection measurements in working areas especially like mines and caves. The advantage is the short time of about 15 minutes for a result of measurement for a quick characterisation of the situation at working places. For this, a number of methods were developed in the past, cited in literature.
The principle of measurement is based on short sampling of radon decay products (typical 5 minutes) and counting alpha decays on filter in time intervals specific for the different methods after sampling or while sampling. With this method by using the measured counts and by a special algorithm the potential alpha energy concentration or concentration of nuclides can be calculated. In the Tracerlab device the method of Markov (/1/) will be used. This method has a good performance for use in mines.
Result is available after 15 minutes and systematic errors are smaller than 20 %.

How does it works, the time-schedule of a measurement:
- sampling: 5 minutes
- break: 1 minute
- counting: 3 minutes (N1)
- break: 3 minutes
- counting: 3 minutes (N2)
Using the values N1 and N2 the potential alpha energy concentration and nuclide concentrations will be calculated.
For each measurement a new filter (without activity) must be used.

2. Continuously Measurement

This method is used when the time dependence of the potential alpha energy concentration or equilibrium factor (between Radon and decay products) is needed, especially for radiation protection measurements on working places for better assessment of the situation.
Hereby air is pumped continuously through a filter. The count rate n(t) of alpha radiation is measured to calculate the potential alpha energy concentration. In practice, the measurement is carried out by counting pulse sum N in a time interval Tint.

1. Slow measurements
   Herby the potential alpha energy concentration (Cp) is calculated using the simple equation
   
   with k= calibration factor.
   The advantage of this method is a small statistical uncertainty at small concentrations. The disadvantage is the large time constant of measured values for changes in concentrations. Time constant is about 2 hours for 80 %.
2. Fast measurements
   To see quick changes in potential alpha energy concentration it is possible to use a special algorithm. Therefore 2 measurements of count sums N will be used for calculation. The disadvantage of this method is a higher statistical uncertainty compared with slow method (about factor 10).

3. Quasi continuously measurements

This method can be used to estimate the concentration of single nuclides without systematic errors. This is necessary in the case of scientific investigations for instance to estimate time constants of physical processes like wall deposition of decay products or ventilation in rooms.
Hereby a cycle of sampling and decay is realised. The decrease function of filter activity in decay time span is used to calculate concentrations of the nuclides in air with a higher precision than in short time measurement. A change of filter is not required.
The calculation of nuclide concentration is based on the method of least squares for an over estimated system of equations.
In the Tracerlab device a sampling time of 20 minutes and a decay time of 40 minutes is used. This time spans are chosen to optimize between small statistical uncertainty and good time resolution. In practice, no better time resolution is necessary. At very small concentrations in the air, the user can calculate adapted mean values of single measurements.

Reference:

/1/ Markov, K.R.; Ryabov, N.V.; Stas, K.N.
Ehkspress-metod otsenki radiatsionnoj opasnosti, swyazannoj s nalichiem w wozdukhe dochernkh productov radona.
At. Ehnerg. 12 (1962) 315