Ionizing radiation

Radioactivity

Radioactivity metrology focuses on the characterisation and quantification of the radiations produced during the decay of various radionuclides.

The diversity of sources in terms of chemical nature (metals, halogen) or physical state (solid, gaseous, liquid) combined with the variety of radiations emitted leads to a difficult use of standards. The very fact that these radioactive sources are by nature non stable generates additional difficulty : decays occur in a continuous way over time, and thus changes the original characteristics of the sources. Contrary to other fields of metrology, it is not possible to use primary standards even though this role had been played by the Radium standard in the past. Consequently, the measurements of radioactivity are made by direct measurement (i.e. without calibration) and require therefore having a combination of specific instruments and measurement methods for each radionuclide. And they thus constitute the so-called primary references.

We will more particularly mention the following direct measurement techniques :

• Concerning the sources emitting γ radiation, the use of ionisation chambers as reference that are used to compare these sources to standards with very long life (such as radium) ;
• regarding the sources emitting β radiation, the use of counting method by liquid scintillation based on the detection of luminous impulses emitted by fluorescent species contained in the scintillator interacting with ionising radiations. One application is the method of the triple to double coincidences ratio (RTDC) that is commonly used at the LNE-LNHB. Whilst the methods involving internal proportional counters or well-type 4π-detectors are used for the ß gaseous emitters ;

Installation for the measurement of emitters via RTCD method

• For the sources emitting a radiation, the use of the so-called defined solid angle method (DSA) that consists in using defined geometric conditions and calculating the solid angle of interaction between the source and the detector ;
• Concerning the sources emitting at least two radiations likely to be distinguished (α-γ,β-γ, ...), the method of coincidences is used. Two paths of detection are used, each one is equipped with the detector adapted to the radiation required. One comparative path of the quantity of synchronous detections coming from one single source allows retracing the activity of such a source.

Measurement device for -α emitters with DSA

Dosimetry

Metrology of the dose consists in measuring the quantities characterising the energy transfers and deposits in irradiated media.

Similarly to radioactivity metrology, the variety of radiations – be it their nature (electrons, photons, neutrons, ...) and/or energy – as well as the instrumental limits, imply that there is no single standard for each one of the dosimetric quantities. One is indeed led to consider as references the instrumentation chains that go from the radiation source to the measurement method.

Regarding the fields of reference radiations, and in order to cover the useful ranges of energy, sources of irradiation and accelerators should be available. To date, the laboratory has implemented and/or developed the following equipment and benches for references :

• For the photons, sources of ²⁴¹Am (60 keV), ¹³⁷Cs (660 keV), ⁶⁰Co (1,25 MeV), installations such as one linear accelerator (4 MV to 25 MV) and the installations of X-rays (up to 320 kV) ;

Linear accelerator of medical type at the LNE-LNHB

• Concerning the electrons, sources of ¹⁴⁷Pm (Emax = 225 keV), ⁸⁵Kr (Emax = 687 keV), ⁹⁰Sr + ⁹⁰Y (Emax = 2,28 MeV) and one linear accelerator (4 to 21 MeV) ;
• Regarding the neutrons, in addition to the sources of ²⁵²Cf (with or without screen (heavy water sphere) and of ²⁴¹Am-Be, the laboratory holds one beam line of one Van de Graff electrostatic accelerator of 4 MV (for the monoenergetic neutrons ranging from 120 keV to 2,8 MeV).

Concerning the measurement instrumentations, ionometry (measurements by ionisation chambers) and calorimetry are primarily used.

Ionometry, based on the measurement of the ionisation current generated b radiation in one ionisation chamber, is used more particularly for the low and medium energy photons for the ⁶⁰Co photons (cavity chambers) and for the electrons obtained through sources (extrapolation chambers).

Implementation of a reference ionisation chamber in front of a photons beam

Calorimetry allows direct access to the energy transferred to the matter through radiation, via the measurement of the quantity of heat evacuated by the system. On the one hand, one standard graphite calorimeter is used for the ⁶⁰Co, photons as well as for the high energy photons and electrons and one water calorimeter for the high energy photons and electrons used in radiotherapy, on the other hand.

Chemical dosimetry, and especially Fricke dosimetry, is also used as a transfer tool for calculating the absorbed dose in water compared with the absorbed dose in graphite.

Reading system of Fricke dosimeters (spectrophotometer)

Photos courtesy: LNE-LNHB ; Expressions