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International comparison for the determination of the activity and half-life of protactinium-231

22.04.2020

Protactinium–231 is a long–lived radionuclide that is part of the natural actinium decay chain which starts with uranium–235 and ends with lead–207, a stable radionuclide. Protactinium–231 is important in nuclear forensics, but it is also used in geosciences for dating purposes. In both fields, reliable age dating, however, requires accurate measurement standards that are used to calibrate the respective instruments.

In order to provide the required metrological fundamentals, an international comparison was organized under the auspices of the National Physical Laboratory (NPL), to which PTB made a significant contribution. For this purpose, NPL first carried out radiochemical activities to manufacture a suitable protactinium–231 solution, hereby removing the radioactive progenies to a large extent. Aliquots of the solution were distributed to the participating institutes. Six of them determined the activity concentration. All results are in very good agreement [1].

PTB used different methods to determine activity. Among them, alpha–spectrometry with semiconductor detectors is particularly important. This technique can be used to determine activity if the solid angle of the detector/source system is known. Thanks to its energy resolution, this measurement procedure is used to determine the contributions of other alpha–emitters. Knowing these contributions is important to determine activity by means of liquid scintillation counters. Liquid scintillation counting allows a detection probability of 100 % to be achieved for alpha–emitters, but due to its rather low energy resolution, it does not allow several alpha–emitters to be differentiated. Information about other contributions from alpha–emitters obtained by alpha–spectrometry is therefore often necessary to determine activity by means of liquid scintillation counting – as is the case with protactinium–231. At PTB, both liquid scintillation counters equipped with two photomultiplier tubes and self–made TDCR detectors equipped with three photomultiplier tubes were used.

The results for the activity concentration, which are obtained from alpha–spectrometry at a defined solid angle and from the two LSC methods, are in very good agreement.

It must be pointed out that good results can often only be obtained by combining those measurement procedures. In certain cases, the activity available is very limited, and/or the uncertainties of the solid angle to be determined are very high. Correspondingly, this leads to higher uncertainties in alpha–spectrometry. In such cases, the method is used to determine the relative contributions of different alpha–emitters, so that the activity concentration can then be determined by means of liquid scintillation counting with a small uncertainty.

In addition to the previously mentioned procedures, calibrated gamma–spectrometers were also used at PTB to determine the activity concentration of the protactinium–231 solution, thereby precluding gamma–emitting impurities. This method requires photon emission probabilities as input data. Small deviations compared to the other methods seem to suggest that these input data are not accurate enough yet. In principle, gamma–spectrometry is also suitable for identifying and quantifying progenies and, if applicable, other impurities due to other radionuclides.

If the results of all participants in the comparison are combined, an activity concentration of 41.461(48) kBq/g is obtained for the solution. At the Lawrence Livermore National Laboratory (USA), the protactinium–231 atomic concentration of the solution was determined as 61.48(23) × 1015 atoms/g by means of mass spectrometry. This results in a half–life of 32 570(130) years. This value is in good agreement with values found in the literature; it contributes to reducing the uncertainty. Considering all half–life measurements carried out to date, a new recommended half–life of 32 570(98) years is obtained. The numbers indicated in brackets correspond to the standard uncertainties.

More details about the comparison, the measurements performed and the half–life determination have now been published [1].

Literature

[1]    Jerome, S., Bobin, C., Cassette, P., Dersch, R., Galea, R., Haoran, L., Honig, A., Keightley, J., Kossert, K., Liang, J., Marouli, M., Michotte, C., Pommé, S., Röttger, S., Williams, R., Zhang, M.: Half-life determination and comparison of activity standards of 231Pa. Applied Radiation and Isotopes 155 (2020) 108837.

Contact:

Opens window for sending emailK. Kossert, Department 6.1, SeSc 6.14

Opens window for sending emailS. Röttger, Department 6.1, Working Group 6.13