Ionizing radiation

Radioactivity

Development of methods used for preparation of radioactive sources

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 :

• Study of the stability of the standard radioactivity solutions; application to ¹³³Ba.
• Comparative analysis of the chemical composition, the preparation and storage conditions of the standard radioactivity solutions developed by the LNE-LNHB.
• Realisation of the sources required by the metrology traceability programme of the LNE-LNHB and used for inter-laboratories comparisons with BIPM, CCRI (CIPM’s Consultative Committee for Ionizing Radiation) and Euromet.
• Realisation of sources for the inter-laboratory tests programmes (ITP). This inter-laboratory tests programme is designed for any laboratory willing to validate their measurement methods by comparing their results to the reference value given by the LNE-LNHB.
  And over the last years, the following one have been proposed :
  - Measurements of specific activity of the components of a mixture of γ emitters fission products and activated products ;
  - Measurements of specific activity of solutions of ²³⁹Pu and of the components of one mixture of emitting radionuclides α,β et γ simulating a La Hague V effluent ;
  - une mesure de l'activité d'une solution d'un radio-nucléide utilisé en médecine nucléaire.
• Development of manufacturing processes for the radioactive sources by lyophilization.

Measurements and evaluation of the nuclear data

Dosimetry

Development of the measurement methods

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 :

a- Calorimetry

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

b- Ionometry

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)

Modelling and simulation of the radiation-matter interactions

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 :

• Calculation via the Monte Carlo code of the D_water/D_graphite transfer factor for an ionisation chamber and for a Fricke dosimeter.
• Calculations of the references for the 15 MeV and 21 MeV high energy electron beams.
• Calculation of the correction coefficients for the reference free-air chamber in radiodiagnosis.

Development of the references for radiotherapy and applications

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.

Development of curietherapy references

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.

Development of references for radioprotection and industrial irradiation

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.

Development of X-ray dosimetry references for energy below 300 keV and applications

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 :

• Characterisation of the high energy (< 320 kV) RX beam ;
• International comparison works (BIPM) mid-2007 ;
• Installation and set-up of the low energy (up to 160 kV) RX beam ;
• International and inter-laboratory comparisons anticipated on this line in 2008.

Development of references in neutron dosimetry

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.