WP 6: Portable primary TDCR measuring instrument adapted to in-situ activity measurements (TDCR)
Background
The TDCR method (Triple to Double Coincidence Ratio) is a method that is relatively well established in European NMIs and was primarily developed as a primary method for the activity measurement of beta emitters. However, the method also has potential to be extended to other nuclides like alpha emitters (without any adjustments to the system), to electron-capture decaying nuclides and to nuclides with complex decay schemes including many gamma-rays (the later two cases with significant updates in the theoretical model that is used for the activity calculation). It is based on liquid scintillation, i.e. the emission of light resulting from the transfer of energy from ionizing radiation emitted by an aliquot of radioactive solution to a solvent and then to fluorescent molecules (scintillator). The light is collected by three photomultipliers (PM) and the detection efficiency is evaluated by using a model which uses the ratio of triple-to-double coincidences between the PM tubes. The counting and signal treatment is carried out using dedicated analogue electronic modules which are not commercially available. The source preparation is simple as it consists in adding a weighted drop of radioactive solution in the flask filled with the scintillating cocktail.
State of the art
Current TDCR systems are home-made metrology instruments neither aimed at nor suitable for in-situ measurements. They are generally of large size (larger than 1,5 m x 1,5 m x 1 m), of heavy weight (about 100 kg) and not transportable. The activity determination results from a calculation relying on a model specific to the decay scheme of each measured nuclide. The new scientific areas that will be pursued in this work package will be the realisation of a miniature self-calibrated primary TDCR system, which is state of the-art, for use on-site. The challenge is then to develop a versatile portable, table-top designed instrument, from this metrology device without losing its characteristics. This implies three major improvements:
Miniaturisation of the detection chamber by studying new types of photomultipliers, efficient and smaller,
Miniaturisation of electronic modules by exploring the possibilities of digital treatment (see WP7),
Validation of models and extension of them to nuclides with special beta spectrum shapes, to nuclides with complex decay schemes including many gamma- rays and to nuclides with higher atomic number decaying by electron capture.
It is a great opportunity to have the TDCR experience of scientists in NMIs coming together and producing a useful application of the TDCR which has up until now mostly been used for International Comparisons and the realisation of the Becquerel. For further details on the TDCR method please see references below.
Only one commercial company has up to now started the production of TDCR devices.
Aims of the work package
The overall aim of this workpackage is to enhance operation of new generation nuclear power plants (NNPP) by enabling on-site determination of low-energy beta-emitters created in the fuel cycle (e.g. Pu-241) and/or as activation products in the reactor and its enclosure (e.g. S-35, Ni-63, Ca-41, H-3 etc.). It is normally a greater scientific challenge to measure pure beta emitters but it should be mentioned that the same system could also be applied to alpha emitters, i.e. most of the minor actinides (which in the NNPP will be part of the fuel cycle and the reprocessing of the fuel).
The work aims more specifically at providing a world leading method for activity determination of pure beta emitters by developing a miniature size TDCR system for on-site measurements. Previous experience working with nuclear power plants has shown that there is a demand for on-site measurements as this significantly improves the efficiency and reduces the costs of the plant operation. The detector that will be developed is a state-of-the-art system, which is based on the only available method for measuring beta emitters without calibration, Triple-to-Double-Coincidence-Ratio (TDCR). This is what is normally referred to as a primary method. A primary method can provide very accurate (<1 % at k=1) activity determination without cumbersome calibrations and can measure the activity in a range of matrices. Furthermore, this method has been applied in several Key Comparisons for the International Reference System and would provide a "short" route for traceability.
In order to move towards a holistic activity measurements system with TDCR, the model for the efficiency calculation of nuclides decaying by electron capture mode and nuclides with complex decay schemes including many gamma-rays will also be investigated by two JRP-Partners with previous experience in this area.
Description of work
The work will address all the improvement needs described above. This will involve development of a portable TDCR scintillation measuring system with joint design features drawn from European NMIs with experience in TDCR development. That task will include the study and choice between different types of photomultipliers (channeltrons, silicon photomultiplier tubes, multi-anode PM tubes, etc), the study of the implementation of digital signal treatment in connection to WP7 and the study of theoretical models (validation, extension). The design protocol will also consider aspects such as facilitating a self-calibration by adding a source of Compton electrons of defined energy (Cassette et al., 2008). The implementation of the system will be tested including its portability.
The JRP will also provide the opportunity to test the measurement capabilities of a currently available commercial TDCR system (Hidex), with associated validation measurements The study will comprise measurements with several relevant radionuclides by means of the commercial system. The results will be compared with those of well-established TDCR system at NMIs. Thus, the study will yield valuable information about potential usage of the commercial system for radioactivity measurements. This comprises potential measurements in nuclear power plants as well as application in radionuclide metrology. Thus, the information will be important for potential users and for all JRP-Partners.
Due to the potential interest of the work carried out in this WP for instrument manufacturers, a dedicated chapter dealing with these specific aspects of intellectual property will be included in the JRP-Consortium Agreement.
Scientific tasks
6.1 Development of a protocol for the design of the transportable TDCR system for in-situ measurements (CEA, NPL, PTB, ENEA)
6.2 Construction of a joint prototype of a transportable TDCR system for in-situ measurements (CEA, NPL, PTB, ENEA)
6.3 Update of TDCR model in existing NMI software for relevant beta spectrum shapes (CEA, NPL, PTB, CMI)
6.4 Inter-laboratory comparison of two nuclides of relevance to new generation nuclear power plants, Pu-241 and H-3, with traceability to the International Reference System (NPL, CEA, PTB, CMI)
6.5 Report on in-situ TDCR instrument (CEA, NPL, PTB, ENEA, CMI)
6.6 Evaluation of the only available commercial TDCR system (Hidex) with associated validation for radioactivity measurements (PTB, ENEA)
6.7 Extension of the theoretical TDCR model to include nuclides with higher atomic number decaying by electron capture and nuclides with complex decay schemes including many gamma-rays (PTB, CEA)
Selected referenced
[1] R. Broda, P. Cassette and K. Kossert, "Radionuclide metrology using liquid scintillation counting", Metrologia 44 (2007) S36
[2] P. Cassette, P Do, "The Compton source efficiency tracing method in liquid scintillation counting: A new standardization method using a TDCR counter with a Compton spectrometer", Applied Radiation and Isotopes 66 (2008) 1026-1032.
[3] Kossert, K., Grau Carles, A.: Study of a Monte-Carlo rearrangement model for the activity determination of electron-capture nuclides by means of liquid scintillation counting. In: Applied Radiation and Isotopes 66 (2008) 998-1005.