The calibration of monitors of radon decay products is a difficult task, and even more so in the case of monitors detecting the products of thoron decay. An important issue here is the lack of primary standards, both in relation to the products of radon as well as thoron decay. In Poland, for many years, measurements of potential alpha energy concentration (PAEC) were carried out with the application of special devices, called ALFA probes. These in turn featured thermo-luminescent detectors (TLD), the readout of which provided information about the potential alpha energy, with no dependence on equilibrium or other factors alike. In this paper, we propose modifying this technique, which had been used only for measurements of products of radon decay. We specifically examine how it can be used to allow simultaneous measurements of radon and thoron PAECs. In employing the method, the idea is to use two sets of TL detectors in the same device during and after sampling. The first set of detectors is used to store the energy of alpha particles during the course and for a short while after the sampling, up to 5 h after its end. Using this method, the first set of detectors holds respectively the full and partial PAECs of products of radon and thoron decay. After that, the first set of TLDs is replaced with a second one. These are then left in the device for another 55-60 h. This set contains only the PAEC from thoron decay products. Afterwards, it is necessary to subtract thoron PAEC from the first set result and add it to the result provided by the second set in order to estimate the PAEC for the products of thoron decay. On a practical level, using such a procedure can potentially difficult during underground or field monitoring nevertheless. This is why another approach can be adopted as well. This is specifically to leave the device for 60 h after sampling without any changes, the original set of TLDs providing a “total” PAEC readout – this being a sum of radon and thoron PAECs. In this paper, we will explore the value of using such a readout for the measurement that is carried out.
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