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Basis achieved at the BIPM for novel comparison measurements of beta emitter activity: the ESIR system

02.09.2020

International comparison measurements play an important role in many fields of metrology. By taking part in comparison measurements, national metrology institutes (NMIs) can demonstrate their measurement capabilities and in this way satisfy the conditions for the international mutual recognition of relevant standards [1].

In the field of radioactivity and the realization of the becquerel unit, the performance of comparison measurements is often very resource‑intensive, primarily due to the decay characteristics of the given radionuclide. One very simple and well‑established method is for a pilot lab to distribute aliquots of a stock solution to participating metrology institutes. The participants then use their various measurement techniques to determine the specific activity and return their results to the pilot lab. If the pilot lab itself is taking part in the comparison, it will have previously deposited its results at another trusted institute. Once all results have been received, a comparison and assessment of all the values can be undertaken. This procedure of comparison measurement is, however, associated with a great deal of logistical cost and effort seeing as the aliquots often need to be shipped to many countries around the world.

Another well‑established procedure is known as the International Reference System (Système International de Référence, or SIR) based at the BIPM in Paris. The SIR relies on ionization chambers in which a great number of radionuclides can be measured using suitable photon radiation. A participating institute can at practically any time send in a solution of a radionuclide in a sealed ampoule of well‑defined geometry while also reporting the activity determined at that institute. BIPM staff then determine the relationship between the measured ionization current and the reported activity, and ensure the long‑term stability of the SIR by means of a long‑lived radium‑226 reference source. If at least two institutes make submittals to the SIR, comparisons between these measurement results can be carried out as well. In particular for high‑capacity NMIs looking to demonstrate their calibration capabilities for large numbers of radionuclides, the SIR allows the required comparison measurements to be performed. Since the establishment of the SIR in 1976, over 1000 ampoules of 70 radionuclides have been submitted, underscoring the enormous significance of this system.

When it comes to comparison measurements of very short‑lived radionuclides, however, both of the comparison procedures described above have their drawbacks. Keeping in mind that the participating institutes are scattered across the globe, it is not possible, for example, to send a fluorine‑18 solution (which has a half‑life of less than 110 minutes) from Europe to other continents and expect to gain meaningful measurements. But the demand for comparison measurements of short‑lived radionuclides is considerable, given the fact that they play an important role in medical applications. To satisfy this demand, a new comparison procedure known as SIR‑TI was established a few years ago. TI here stands for "traveling instrument" and refers to a sodium iodide detector with reference sources which journeys – along with operating staff – to the various destinations around the world. Here again, the logistical resources required are enormous, but the system has done much to narrow a broad metrological gap. The SIR‑TI has made calls to institutes in the Americas, Asia, Australia, Africa and Europe.

Given the very high efficiency of the classic SIR in particular, scientists have for years been calling for the establishment of a similar instrument for beta emitters as well. This desired extension has been designated ESIR (extended SIR). In this new system, liquid scintillation counters are used due to the fact that many detectors cannot detect certain beta emitters (especially low‑energy sources such as tritium and nickel‑63), or do so very inefficiently. The establishment of an ESIR has, however, proven to be much more difficult than setting up the classic SIR. One major challenge is how to deal with parameters that can change over time (many decades). Such parameters include the composition of the liquid scintillators used as well as changes to the actual apparatus. One further aim is to keep the comparison procedure itself as free as possible of complex model calculations.

The enormous energies invested by the BIPM in the past three years combined with the support of further metrology institutes (LNE/LNHB, NPL, NIM, POLATOM and PTB) have now led to the realization of the ESIR system. This work involved not only the development and validation of the new measuring system, which represents a specially re‑engineered TDCR system [2], but also of suitable new assessment procedures. These included experiments that simulated possible changes to the apparatus caused by "disturbances" in the form of neutral density filters. Other important effects, such as possible changes to the composition of samples, were likewise examined. The new procedures and validations have now been published [3], clearly establishing the fundamental groundwork for a well‑functioning ESIR system. Moreover, a very simple method has been worked out for conducting comparison measurements on radionuclides with electron capture (e.g. iron‑55). This procedure is even suited to cases where the responses of the three photodetectors of a TDCR system are not identical and/or change over time. This is described in detail in another recently produced work [4].

Before bringing the ESIR into routine operation, though, it must first be subjected to a critical examination involving pilot comparison measurements. Here, the radionuclides used will include cobalt‑60, which is measurable in both the classic SIR and in the new ESIR system. Following the (hopefully) successful conclusion of these tests, many NMIs will be able to conduct significantly more comparison measurements at minimal cost and effort.

Literature

[1]        https://www.bipm.org/en/cipm-mra/

[2]        Broda, R., Cassette, P., Kossert, K., 2007. Radionuclide metrology using liquid scintillation counting. Metrologia 44, S36-S52.

[3]        Coulon, R., Broda, R., Cassette, P., Courte, S., Jerome, S., S., Judge, S., Kossert, K., Liu, H., Michotte, C., Nonis, M.: The international reference system for pure β-particle emitting radionuclides: an investigation of the reproducibility of the results. Metrologia 57 (2020), 035009.

[4]        Kossert, K., Sabot, B., Cassette, P., Coulon, R., Liu, H., 2020. On the photomultiplier-tube asymmetry in TDCR systems. Appl. Radiat. Isot., in press, https://doi.org/10.1016/j.apradiso.2020.109223.

Contact:

Opens local program for sending emailDr. Karsten Kossert, Department 6.1, 6.14 Fundamentals of Radioactivity