This is the first step in the radioactivity field. Radioactive sources are indeed necessary to carry out the studies in spectrometry and activity measurements.
The uncertainties obtained during these studies largely depend on the metrological quality of the sources themselves. Thus, preparation methods and technologies should also be improved. This constant effort allows the national laboratory to develop new preparation methods and thus remain upstream of the industrial requirements.
Source preparation device by electrodeposition of actinides
Main studies and research projects launched by the LNE-LNHB :
The LNE-LNHB is participating in the groupe DDEP (Decay Data Evaluation Project), in collaboration with foreign counterparts (Physikalisch Technische Bundesanstalt, Idaho National Engineering and Environmental Laboratory, Lawrence Berkeley National Laboratory, Khlopin Radium Institute...). This group is carrying out the evaluation of the nuclear data coming from the different research groups.
The decay characteristics of the radionuclides that have been listed are the following : the types of radiations emitted by the different radionuclides and the energies associated, their occurrence probabilities, their radioactive periods, etc. Once validated, this set of data is made available for the users and can be obtained at the LNE-LNHB. They are presented in various forms :
Nuclear data are crucial information for making certain computations that are used in nuclear medicine
and in fields such as the environment or the fuel cycle (storage of waste,...).
Main actions led by the LNE-LNHB :
These are sources which level of activity is close to the one found in the environment (natural radioactivity). For measuring such sources, the equipment with the lowest possible detection limits is required together with methods that allows to discriminate between the signal attributable to this low radioactivity and the signal generated by the activity around the measurement instrument.
These methods allow to directly obtain the activity of one source without having prior knowledge of the characteristics of the detection chain used (efficiency tracing,...), and do not require having one radioactivity standard.
Among the main methods, there are :
- The coincidence method that applies to the radionuclides characterised by the emission of several types of radiations (for instance: β-γ or α-γ) ;
- Liquid scintillation counting associated with the method of the triple to double coincidences ratio (RTDC) in the case of the β pure radiations ;
The defined solid angle method for the α radiation;
- The 4π well-type detector method (for ? emitters radionuclides with complex decay schematic that calls for one cascade of ? transitions) ;
- The method involving internal proportional counters for the gas ß emitters).
However, the assignements of the national laboratory are not limited to the use and the improvement of these methods; the LNE-LNHB is also in charge of the development of new methods based on different physical principles or innovative process. Thus the realization of one primary activity reference for radon 222 has achieved: condensing this gas on one cold spot using the measurement method of defined solid angle.
Device for measurement of the national activity references using the coincidence πβ-γ method
Primary set-up for radon measurement
The traditional detectors such as the liquid scintillation counters or detectors based on semiconductor materials that are currently used have now reached their limits in terms of detection threshold, resolution or response linearity based on energy. In order to meet the increasing needs from the users, the LNE-LNHB is exploring the path of the cryogenic detectors (bolometers) that can be used in the metrology of ionizing radiation.
Development of cryogenic detector
Some of these "primary" methods can be used by those working in the nuclear area; but improvements must still be brought to these methods so that they can be applied to an ever increasing number of radionuclides.
Secondary system for liquid scintillation measurement
Main studies and research project undertaken by the LNE-LNHB :
This section will focus primarily on the well-type ionisation chambers that are used in the laboratory as one transfer instrument. They play the role of methods memory in the primary measurements of γ emitter radionuclides. They also play a major role in the laboratory participation in the International Reference System (SIR) of the BIPM.
The well-type ionisation chambers are also well-adapted to measure the activity of the radionuclides used in nuclear medicine such as 125I, 201Tl, 67Ga, 111In,...
Main studies and research projects carried out by the LNE-LNHB :
The x-ray spectrometry and y-ray spectrometry an allow precise measurement of the activity of a large number of radionuclides. They also allow characterizing the emissions of sources of photons existing in different states (solid, liquid, gaseous) and in different geometries (focused or expended sources). Photons spectrometry is characterized by its universal character, its easy implementation and its separation power.
The software called ETNA Efficiency Transfer for Nuclide Activity measurements) has been developed, it allows calculating the correction factors of the efficiency track in the event of a change in the source-detector geometry and carrying out the coincidence corrections. This software is available on CD-Rom, and can be obtained from the LNE-LNHB. ETNA updates will be available later on the internet site of the LNE-LNHB.
Owing to its easy implementation, photons spectrometry is routinely used for several applications. These latter imply permanent need for fresh developments.
The LNE-LNHB has developed one tunable monochromatic source, called SOLEX, in collaboration with the Laboratoire de Chimie Physique Matière et Rayonnement. This system is made up of one X –ray tube and a removable crystal positioned on a rotating stage on the one hand, and one fixed detector on the other hand. System principle is based on the application of the Bragg law and allows selecting the photons energy over a range of 1 to 20 keV, by controlling the crystal direction in relation to the incident beam and the detector positioning. This instrument allows three types of studies in the X-ray emissions field:
- Study of the X-ray spectra with very good resolution ;
- Measurement of the attenuation coefficient ;
- Calibration X-ray efficiency tracking using one reference detector.
The LNE-LNHB, through the CEA, is also strongly involved in the ESARDA (European Safeguards Research and Development Association) works. And it has developed, among others, a library of references spectra covering uranium and plutonium.
Main studies and research projects initiated by the LNE-LNHB :
Curietherapy requires the best possible accuracy and this proves to be very close to what is achieved for the best national metrological realisations. To ensure transfer to the users with the minimum of additional uncertainties, it is advisable to have an intimate knowledge of the nuclear data of the radionuclides used as curietherapy sources and to establish the dosimetric references for the reference of high dose rate iridium 192 as well as for the new methods with iodine 125 and palladium 103.
A certain number of measurement techniques are used for establishing the national references of the highest level as well as for their transfer. As the difference between these national references and the users needs, in terms of accuracy and uncertainty, is quite thin, (a ratio of three or four), studies launched at the LNE-LNHB will allow certain improvement in the measurement methods at all levels of the metrological chain. These measurement techniques are as follows :
Such techniques allows the most direct access to energy supplied by the radiation to the matter (and therefore to the absorbed dose); there are three types of calorimeters :
- Graphite calorimeter ;
- Tissue-equivalent calorimeter ;
- Water calorimeter.
Only the first two calorimeters are currently used at the LNE-LNHB.
Development of one water calorimeter
This technique shows excellent qualities in terms of sensitivity and accuracy. It is based on the measurement of the ionizing current generated by the radiations in an ionisation chamber. According to what is to be measured, there are a large number of ionisation chambers.
Free air chamber set-up
c- Chemical dosimetry
In an irradiated medium, atomic and molecular ionizing can cause chemical changes that can be used for dosimetric measurements. Chemical dosimeters are often found in the form of aqueous solutions that are identical to the reference medium. The most used dosimeter is the one based on an aqueous solution of ferrous sulphate and named Fricke solution.
d- Electron Spin Resonance (ESR) dosimetry
This techniques which does not allow direct measurements, is especially used for the high dose rates (from several hundreds of grays to several thousands of grays). Alanine dosimeters are measured by RPE, quantifying free radicals generated by such irradiation.
RPE Reading device for alanine dosimeters
This measurement method is commonly applied for the calibration of the irradiators used in the sterilization field ( like sterilisation of objects for medical use or conditioning blood and food products). Several studies have been initiated to consider whether this technique can be used with the dose levels found in radiotherapy.
Reading and analysis of RPE signal of alanine dosimeter
e- Thermoluminescence dosimetry
This technique also cannot be used for direct measurements and requires prior calibration. It is suitable for the dose levels found in the radioprotection or radiotherapy fields. It is mainly based on lithium fluoride in different forms (powder, pellets, etc, ...).
Reading device for thermoluminescent dosimeters (LiF)
This activity turns out to be indispensable in dose metrology. The complexity of the interaction phenomena is such that it is very often difficult or even impossible to justify the assumption leading to the problems solving through simple analytical calculations. It is often very difficult to experimentally determinate one correction factor without any interpretation ambiguities. One often resorts to simulations codes such as those of the Monte Carlo type, and the LNE-LNHB is primarily using following codes EGS4, PENELOPE, MCNP and MORSE for the photons, the electrons or the neutrons according to the case.
Studies are concentrating on :
External radiotherapy requires the best possible accuracy which turns out to be close to what is obtained for the best national metrological realisations. This implies that the transfer towards the users should be made with a minimum of additional uncertainties. The national references of absorbed dose rate to water for the high energy photons and electrons must also be obtained with the best possible uncertainties.
Measurement of the absorbed dose to water with medical accelerator.
The national laboratory must also provide support to the users by transferring its knowledge and expertise, as for example for the implementation of new protocols defined at the international level.
Preparation of LiF powder samples before reading by thermoluminescence
The LNE-LNHB is continuing its works for references development covering the high energy electron beams and the high energy photon beams.
The problem discussed in the previous paragraph still holds true for the curietherapy area. And the best possible uncertainties must therefore be achieved for the national references.
The LNE-LNHB has developed the high dose rate References 192Ir in conjunction with the University of Wisconsin, and is optimizing a set-up for the measurement dose rate of 125I and of 103Pd sources.
The needs are not only restricted to the single absorbed dose to water supplied by the photons. Concerning the new operational quantities in radioprotection, references are required for radiation (absorbed dose to tissue, directional dose equivalent) and for the photons (ambient or directional dose equivalents).
Similarly, France should also have references for the high dose rates from the 60Co for the industrial irradiation and sterilization fields.
In radioprotection, new primary beta references (90Sr + 90Y, 85Kr, 147Pm) have been characterized in terms of directional dose equivalent.
LThe current national references are constituted of two installations of filtered x-ray sources of 100 kV and 320 kV, coupled with their own ionisation chamber obtaining the air kerma.
In radiotherapy, the current absorbed dose to water is obtained through the air kerma and generates high uncertainties. Today’s needs are concentrating on the references that would allow having direct calibrations in absorbed dose to water.
Likewise, in the area of radioprotection, there are requirements in terms of ambient dose equivalent.
There are numerous applications, especially in the field of radioprotection (for the people exposed to X-rays consequential to their professional activities or people than can be exposed for other reasons (patients,...) and in the area of medical diagnosis (mammography, radiodiagnosis).
Radiodiagnosis equipment at the LNE-LNHB
The main actions performed by the LNE-LNHB are :
The field of neutrons turns out to be very complex owing to the nature and the diversity of the radiation-matter interactions that may be found. Neutron radiation fields may be of either natural or industrial source.
AMANDE: DC Tandem type accelerator Tandem
This is also a field in which difficulties are faced for the realisation of metrological references or measurement means, regarding the wide energy range that is to be , extended from some 10-2 eV up to several hundreds of MeV.
Radioactive sources of 252Cf with or without heavy water sphere) and of 241Am-Be are used as wide spectrum neutron references for the calibration of dosimetric instruments utilised in the areas surveillance and individual surveillance for the sites with "neutrons" risks.
One DC Tandem accelerator, called AMANDE, has been developed. This installation will allow in the longer term to produce continuous or pulsed monoenergetic neutron beams between 2 keV and 20 MeV; and also to implement monoenergetic neutron references in this range of energy. Supplementary information is available on the site of the IRSN.
These monoenergetic neutron references have several fields of applications. In addition to the neutron references characterised in fluence rate and used as national standards, references will be applied in spectrometry to determine the response and efficiency functions of the "neutron" spectrometers efficiency, as well as for R&D in the dosimetric systems.
efficiency, as well as for R&D in the dosimetric systems. There are also realistic and wide spectra neutron references obtained through SAMES electrostatic accelerators in the 120 kV and 400 kV domain, that must be representative of the real spectral conditions likely to be found in practice, i.e. at work.
Photos courtesy: LNE-LNHB ; IRSN ; Expressions