Applied Radiation and Isotopes 87 (2014) 38–43

Contents lists available at ScienceDirect

Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso

“Realisation of the becquerel”—reducing the impact of equipment failure G. Suliman 1, J. Paepen n, U. Wätjen European Commission, Joint Research Centre, Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium

H I G H L I G H T S

   

The goals of the “Realisation of the becquerel” project of CCRI(II) are presented. Reproducible source holder designed and tested. New ionisation chamber design with better definition of sensitive volume proposed. The advantages of the new design are discussed.

art ic l e i nf o

a b s t r a c t

Available online 10 December 2013

The goal of the CCRI(II) “Realisation of the becquerel” project is to design a reproducible radioactivity standard which will increase the robustness of the current international reference system for radioactivity measurements. Tests performed with a first prototype ionisation chamber of this project, built in 2005, are presented. Based on experience with the first prototype, a new design was proposed in 2010 aiming at achieving the very ambitious project goals. The article discusses the status of the project and the advantages of the new design. & 2013 Elsevier Ltd. All rights reserved.

Keywords: International reference system (SIR) Ionisation chamber prototype Reproducibility Monte Carlo simulations

1. Introduction: the SIR One of the most extensively calibrated ionisation chamber systems to date is the SIR (Système International de Référence) which relies on two commercial ionisation chambers. These chambers have been calibrated for γ-ray emitting radionuclides over a long period of time (about 40 years) with a significant effort. Ratel (2007) provided a description of the practical realisation of this system and of its pivotal role in the hierarchy of radioactivity reference standards. The most important function of the SIR is that it is the basis of the traceability chain used in radionuclide metrology. Should any of the critical components of the SIR fail (be it a gas-leak of the chambers themselves, radon leaking from the 226Ra reference sources or malfunctioning of the current measurement system) a significant amount of effort would be in peril. To mitigate the risks, over 15 years ago the BIPM Comité Consultatif des Rayonnements Ionisants, Section II (CCRI(II)) established the

n

Corresponding author. Tel.: þ 32 14 571 329; fax: þ 32 14 584 273. E-mail address: [email protected] (J. Paepen). 1 Present address: Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, IFIN-HH, 30 Reactorului Str., Magurele, Romania. 0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.130

Working Group on the Realisation of the becquerel, with the task to investigate how the continuity of such a system can be assured. This article describes the goals of the project and gives a summary of the work carried out so far.

2. Goal of the project The broad goal of the project is to replace the dependence on a unique system based in one location, like the SIR, with a system that can be rebuilt identically by any skilled laboratory, anywhere and at any time in the future. The best approach was found to be the realisation of a new measurement system, based on an ionisation chamber, with characteristics similar to the SIR, but fully reproducible. The deliverables of the project are a complete set of specifications concerning all the parameters of the system that have an impact on the measurement result. This includes the materials and their composition (including the gas), the dimensions of all the parts of the chamber, and the properties of the current measurement system. To ensure reproducibility, the values and tolerances of all the parameters must be the first unequivocally specified in SI units wherever possible. The parameters shall also be specified in a way which allows SI-traceable measurement of the actual values of the realised parameters in order to verify that the

G. Suliman et al. / Applied Radiation and Isotopes 87 (2014) 38–43

reproducibility requirements are met. Also the ionisation current generated by the chamber should be measured in a manner which ensures full traceability to the SI, over the full range required, from a few tens of fA (for background measurements) up to hundreds of nA. The objective in this project concerning reproducibility is that the response, of any ionisation chamber, for any photon in the energy range between 30 keV and 2.6 MeV, shall not differ by more than 0.2 % from that for any other ionisation chamber. On a more fundamental level, the ultimate goal of the project is to eliminate the dependency on the set of 226Ra reference sources. Once the first chamber has been built and calibrated using radionuclide sources that have been standardised by primary methods, the calibration should be valid for all other chambers within the 0.2 % criteria. The calibration is then expressed in electrical current per unit of activity of the radionuclide (e.g. pA/MBq). The use of long-lived radionuclides such as 226Ra (with sufficient activity to reduce the effect of counting statistics) will still be useful as a mean of checking the long-term stability of the system. The current measurement system can be verified regularly for low currents or recalibrated using an accurate sub-picoampere current generator, as described by van den Brom et al. (2005).

39

Other design parameters result from the requirement of the envisaged very long operation time of the chamber, e.g. the selection and availability of the materials, and the possibility to

k

biased electrode

collecting electrode

biased electrode

j

guard ring

i

Fig. 1. Schematic drawing of the first ionisation chamber prototype.

Fig. 2. Assembly of the second prototype holder, consisting of a tube (i) with the ampoule, a ruler stick (j) and a cone (k) that fits on a conical ring fixed to the re-entrant tube of the ionisation chamber.

40

G. Suliman et al. / Applied Radiation and Isotopes 87 (2014) 38–43

weld the chamber once it leaves the research phase and enters the operation phase.

3. The first prototype Investigations into materials, designs and construction methods began in 1997 and simulations were carried out (Johansson, 2001) to assess the feasibility and the ease of construction. The first prototype was built in 2005 (Švec, 2005) and extensively tested for many of the operational and design parameters under study, such as the dependence of the response on the gas pressure; the applied high voltage and the reproducibility of source positioning (Camps and Paepen, 2006). A second prototype was proposed in 2010 and built upon the lessons learned from the work with the first prototype. During this time it became clear that the success of the project does not only depend on the machining specifications of the ionisation chamber and on its current measurement system but also on details like the source holder and the quality of the ampoules containing the source.

A few clear drawbacks of the first prototype were identified. The first was that the use of proprietary materials should be avoided as their composition might change in time at the discretion of the manufacturer. This difficulty and the concerns regarding the ageing and radiation damage of plastics led to the decision that the inner electrode and the re-entrant tube should be manufactured from metal with a known composition. The chamber also proved difficult to open and repair. The outside of the chamber being at high voltage posed a safety hazard and increased the susceptibility for pickup of ambient electrical signals, for which extra insulation and shielding was needed. The guard ring configuration was effective in minimising leakage currents flowing from the biased parts to the current collecting electrode but did not allow a clear definition of the sensitive collection volume, which is very important to assure the reproducibility. The use of glued home-made sub-assemblies for the electrical feedthrough and internal standoffs turned out to be a major issue since the feedthrough started leaking gas and one of the standoffs broke for an unknown reason. 3.2. Reproducibility of the source holder

3.1. Principles of operation The prototype has a cylindrical geometry, with a single collecting electrode (Fig. 1). The high voltage is applied to the outside of the pressure vessel, so that the current can be read from the virtually grounded electrode in the centre. The material used for the re-entrant tube was Vespels, a polyimide. The current measurement is performed by a commercially available electrometer (Keithley, 6517B) operating as current integrator. To improve its stability and to reduce the measurement uncertainty for low currents, a calibrated high quality capacitor (IET Labs, 1404-9701) is placed in the feed-back loop. This capacitor has Invar plates and is sealed in a dry nitrogen environment. The use of an external feedback capacitor does however introduce a stray capacity internal to the electrometer that adds to the feedback capacity, thereby directly affecting the measurement uncertainty. The stray effect is estimated to be (0.25 70.02) pF for a similar electrometer (Giblin et al., 2009), and can be reduced by using a relatively large feedback capacitor, e.g. 1 nF. This capacitor value also accommodates the range of currents envisaged to be measured. In any case, it is feasible to calibrate the complete current measurement system as mentioned earlier (van den Brom et al., 2005), thereby evading the issue of the stray capacity.

Reproducibility also depends on the source holder and the ampoule or any other container for the radioactive solution. Camps and Paepen (2006) tested the source holder of the first prototype and found that there was an issue with the reproducibility of the source positioning, justifying the design of a new source holder. The newly designed holder has three parts (Fig. 2). Part i is a tube that holds the ampoule, k is a ruler stick connected to the tube and j is a cone holding the ruler stick and matching with a conical ring, rigidly connected to the re-entrant tube. The conical connexion provides very precise repositioning of the source. The ruler stick and tube can move inside the cone to raise or lower the source inside the re-entrant tube. It is noted that for the new design, the re-entrant tube preferably should have a coned end to further reduce build-up of tolerances between different parts Of the three parts, several items were built: 5 tubes identified with i¼1 to 5; 4 cones identified with j¼1, 2, 3 or 5; and 4 ruler sticks identified with k¼ 1, 2, 3 or 5. The complete holder assembly is identified as (i, j, k). The assembly (5, 5, 5) was produced by a different machine shop, taking another step in the direction of testing the reproducibility. To limit the number of tests from 80 to 20, it was decided not to separate the combination of ruler

238.6

Combinations of stick and cone (i,1,1) (i,2,2) (i,3,3) (i,5,5) 238.4

Current (pA)

238.2

238.0

237.8

237.6

237.4

237.2 i=1

i=2

i=3

i=4

i=5

Tube that holds ampoule (i) Fig. 3. Reproducibility of source placement using the new source holder. The error bars only represent the uncertainty due to the statistical nature of radioactive decay.

G. Suliman et al. / Applied Radiation and Isotopes 87 (2014) 38–43

effect of the electrons. Looking carefully at the data within every group i, one might observe more or less the same sub-structures for every i, but the spread of the data is too large to draw a meaningful conclusion. The major lesson to learn from this experiment is that it is feasible to make a reproducible source holder, provided that the dimensions of the critical part (the thickness of the tube) can be verified in practice, which is difficult for this design. More research needs to be done to assess the feasibility of using less dense material around the ampoule, reducing the tolerance on the thickness.

4. The second prototype The design of the second prototype (IC2010 design) is more complicated than the first one; a schematic drawing is presented in Fig. 4. The cylindrical chamber contains three electrodes and

first collecting electrode

guard rings

biased electrode

second collecting electrode

guard rings

stick and cone and accordingly keeping j¼k, as the major contribution to a change in response was expected to come from the tube (part i). Experiments were carried out using one 109Cd source, which is very sensitive to variations in position and thickness due to its low-energy photons (around 25 keV). After the measurement (which lasts around 300 s), the tube was removed from the stick and cone and another combination was assembled from the available parts. The results are shown in Fig. 3. The graph shows that the measured current is about 0.2 % smaller for the tube i¼5 than for the other tubes, indicating a slight difference in thickness of the plastic (PMMA) tube surrounding the ampoule. Measurements with a magnetic thickness gauge reveal that the tubes with i¼ 1–4 are indeed about 10 μm thinner than the tube made by the other machine shop. The difference in transmission of photons from 109Cd through 1.00 mm and 0.99 mm of PMMA only accounts for 0.05 %. Detailed simulations are required to include the

41

Fig. 4. Schematic drawing of the second prototype of the ionisation chamber.

42

G. Suliman et al. / Applied Radiation and Isotopes 87 (2014) 38–43

four guard rings. The first and the third electrode are collecting electrodes (virtually connected to ground potential by the current measuring system), while the second, central electrode is the biased electrode. Each collecting electrode is shielded by two guard rings, one on the upper side and one on the lower side, separated from the collecting electrode by thin ceramic insulators. It still needs investigating as to whether the reverse configuration (where the first and third electrodes are biased, and the second electrode is collecting) would be a better choice. The major expected advantage of this design with respect to the first prototype is the precisely defined collection volume: a separate volume inside the chamber defined by the electrodes which do not have the structural task of holding the gas pressure. In the following a few arguments supporting this claim will be presented, and the issue of the re-entrant tube is discussed.

(ELMER) with Geant4 (Allison et al., 2006) simulations of the energy deposition in the chamber, using a simple charge transport model (Suliman, 2013). It was assumed that all the energy deposited at each interaction point is transformed into electrical charge, neglecting recombination. Using the previously calculated electric field, the movement of the charge was traced until it reached the collecting electrode (where it contributes to the measured current) or the insulators between the collecting electrode and the guard rings (where its destiny is uncertain). The simulations show that for photon energies above 100 keV, about 0.7 % of the total charge deposited between the electrodes is collected on the insulator. To reduce this remaining fringe effect, both ends of the collecting electrode can be machined with a small overlap over the guard ring, as in (2) in Fig. 5. This also makes the definition of the sensitive volume independent of small variations of the insulator length.

4.1. The clear definition of the collection volume The guard rings at both ends of the collecting electrodes remove most of the fringe effect of the electric field which is present at the end of the collecting electrode in the first prototype due to the influence of the outer chamber potential. The proposed electrode configuration with guards creates an electrical field in the collection volume similar to that of a perfect cylindrical capacitor. Nevertheless, the existence of the insulators between the electrodes and the guard rings creates a small perturbation in the field structure. It can be argued that some of the charge deposited in this small volume (uncertainty volume) between the insulators and the high voltage electrode is collected by the guard ring or ends up on the collecting electrode, hence contributing to the measured signal. The magnitude of this contribution was assessed by coupling finite element modelling of the electric field

4.2. Clear separation between the mechanical properties and the charge collection In the first prototype, the pressure vessel itself had the important task to contain the gas, but also played a role in the electrical field configuration. Deformation of the ionisation chamber under influence of the pressure could lead to a change of the distance between the electrodes, which could affect the electrical field and the charge collection. These risks are mitigated in the newly proposed design: the expansion of the chamber will not affect the position of the electrodes with respect to each other, and therefore will not affect the charge collection. Even in case the bottom of the chamber would bend under the pressure, leading to a relative vertical displacement between the electrodes, the collecting volume will not be affected as long as the electrodes move only vertically.

Fig. 5. Precision of the definition of the collecting volume. When the collecting electrodes extend over the guard ring as in (2), the volume with uncertain collection is very much reduced, as compared with (1).

G. Suliman et al. / Applied Radiation and Isotopes 87 (2014) 38–43

4.3. Re-entrant tube One of the most difficult aspects of the project is to ensure the reproducibility at low energy. This is due to the fact that on its way from the source vial to the sensitive volume, the radiation needs to pass through several structural parts. Using well known absorption coefficients the effect of small thickness variations of these structural parts on the response can be calculated (to a first approximation using I ¼ I 0 e  μd , or to obtain a better precision by using Monte-Carlo simulations). Such studies have shown that (Camps and Paepen, 2006), using the geometry of the first prototype, the tolerance on the thickness of the inner tube needs to be smaller than 2 μm for aluminium to achieve 0.1% reproducibility of the transmission of 30 keV photons. This strict requirement has proven impossible to achieve in practice, neither by machining nor by tube drawing. The only solution to this problem is using lighter materials. Magnesium only relaxes the tolerance by a factor of two compared to aluminium and the use of plastics is excluded. Beryllium could be a viable option, relaxing the tolerance to about 0.1 mm. However, the dual use (the possible military use of civilian nuclear power technology) of that metal could be an issue for some NMIs, as some countries have strict import/export control on beryllium. 5. Conclusions and outlook The next step towards realisation of this project is to develop the blueprints of the chamber, thereby using as much as possible components that are proven reliable. Deformation under influence of the pressure should be assessed to further assure that it does not affect reproducibility, but also for safety aspects. In particular, the re-entrant tube requires special attention as this is mechanically the weakest part of the chamber. Other parts that need attention are the guarded electrical feed-through, capable of withstanding the pressure difference, and the electrical insulators used inside the chamber. When blueprints are finished, the reproducibility could be experimentally assessed by building more than one chamber, preferably by different workshops.

43

Another issue is the calibration of the current measurement system in the pA and fA range. For this calibration, a standard capacitor (which typically has air as dielectric) is charged over time while the voltage over its plates is being measured, resulting in the accurate knowledge of the charging current. Rietveld and van den Brom (2009) have shown that the DC capacitance of such capacitor shows strong dependence on relative humidity and is not always equal to the AC value, which is the one being standardised. The knowledge of the DC capacitance is however required for low current measurements and calibrations. A possible solution would be to use vacuum or inert gas capacitors with monitoring of the hygrometry, but this certainly needs careful study. The most prominent decision to be made is the choice of the material for the re-entrant well. Further studies about the dual use of beryllium and stress calculations to verify that beryllium could withstand the pressure difference are ongoing. References Allison, J., Amako, K., Apostolakis, J., et al., 2006. GEANT4 developments and applications. IEEE Trans. Nucl. Sci. 53, 270–278. Camps, J., Paepen, J., 2006. Development of an ionisation chamber for the establishment of the SI unit becquerel. Report EUR 22609 EN, ISBN: 92-7904588-1, ISSN: 1018-5593, 〈http://bookshop.europa.eu〉. ELMER, Open Source Finite Element Software for Multiphysical Problems. 〈http: //www.csc.fi/elmer〉. Giblin, S.P., Bakshandier, E., Fletcher, N.E., Lines, K.J., Sephton, J.P., 2009. An SI traceable electrometer system for radionuclide metrology. Nucl. Instrum. Methods Phys. Res. A 606, 824–828. Johansson, L., 2001. A new design of the SIR ionization chamber – Monte-Carlo simulations. IRMM Internal Report no. GE/R/RN/04/01. Ratel, G., 2007. The Système International de Référence and its application in key comparisons. Metrologia 44, S7–S16. Rietveld, G., van den Brom, H., 2009. DC and low-frequency humidity dependence of a 20 pF air-gap capacitor. IEEE Trans. Instrum. Meas. 58 (4), 967–972. Suliman, G., 2013. Report on the Geant4 simulations performed for the “Realisation of the Bq” project. Report EUR 25676 EN, ISBN: 978-92-79-28074-0, ISSN: 18319424, http://dx.doi.org/10.2787/70585, 〈http://bookshop.europa.eu〉. Švec, A., 2005. Reference ionisation chamber for radioactivity measurements. IRMM Internal Report no. GE/R/IM/RN/10/05. van den Brom, H., de la Court, P., Rietveld, G., 2005. Accurate subpicoampere current source based on a differentiating capacitor with software-controlled nonlinearity compensation. IEEE Trans. Instrum. Meas. 54 (2), 554–558.

"Realisation of the becquerel"--reducing the impact of equipment failure.

The goal of the CCRI(II) "Realisation of the becquerel" project is to design a reproducible radioactivity standard which will increase the robustness ...
504KB Sizes 0 Downloads 0 Views