​​People can be exposed to neutron radiation in the nuclear industry (power plants, fuel manufacture and reprocessing plants), in research laboratories, in the medical field or in the environment. Appropriate, correctly calibrated equipment is therefore necessary to monitor them. 

IRSN’s CEZANE and AMANDE facilities at Cadarache produce all the reference neutron field types recommended by international standards (ISO 8529 and ISO 12789) for calibrating neutron measuring instruments. It is the responsibility of the Microirradiation, Neutron Metrology and Dosimetry Laboratory (LMDN) to guarantee the production of national reference standards for fluence, i.e. the number of neutrons per unit area, and the derived dosimetric values. These facilities are used to test and perform different types of calibration of neutron measuring equipment. Routine calibration is done at the CEZANE facility using the Van Gogh irradiator which has two sources: one made of americium-beryllium (²⁴¹AmBe), and the other of californium (²⁵²Cf). In the CEZANE facility it is also possible to perform calibration in a realistic field, i.e. one with similar characteristics to the neutron fields encountered in the workplace. This is done using the T400 accelerator associated with the CANEL device, which can generate a fission neutron spectrum moderated using elements representative of the shielding used in the nuclear industry. Finally, the AMANDE facility can perform a third type of calibration, which consists of determining how instrument response varies as a function of neutron energy, by producing monoenergetic neutron beams with an energy of between 2 keV and 20 MeV. 

​Together these facilities form a technical platform with few equivalents worldwide. The addition of further moderator associated with the T400 accelerator is planned in the near future, to enable a thermal neutron field to be created (energy of less than 0.5 eV).

The AMANDE and CEZANE facilities are at the center of a 300-meter radius exclusion zone. This is unique worldwide for this type of facility and means that neutrons can be generated in structurally light experiment halls (with metallic walls) limiting the proportion of scattered neutrons at the calibration point.

​The VAN GOGH irradiator consists of the two main radioactive sources recommended by the standard ISO 8529-1: ²⁴¹AmBe and ²⁵²Cf.

When they are not in use, these sources are stored in a polyethylene container. Using compressed air in a guide tube, the sources are taken to their irradiation position 3.2 m above the ground, which limits the background noise due to neutrons scattered by the ground. A motorized calibration bench is used to position the instruments at the source or at any distance between 0 and 2 m; the conventional irradiation distance is 75 cm. 

In addition to routine calibration activity, this irradiator is used to realign accredited laboratories with the national reference standards for neutron fluence and the derived dosimetric values (especially dose equivalents). Neutron flux values are determined directly from measurements of the neutron emission rate (number of neutrons emitted per second) of each source, performed in a manganese bath at the Laboratoire National Henri Becquerel (LNHB at CEA Saclay). The dosimetric values are derived from the fluence by applying the tabulated conversion coefficients given in the ISO 8529 standards [Ref.].

Calibration activities in this irradiator are accredited by the French accreditation committee (COFRAC), under the designation LA2.55 (scope available on www.cofrac.fr).

This irradiator also has a ¹³⁷Cs gamma radiation source of 11 GBq, which can be used to test the photon sensitivity of the instruments requiring calibration.

​

​Characteristics of the ‘bare’ sources

The ²⁴¹Am-Be source is covered with a 1 mm thick lead shield to limit the emission of photon radiation, whereas the ²⁵²Cf source is used without a shield.

The energy distribution of the neutron fluence associated with each of these sources is shown below. 

Graph showing the energy distributions of the neutron fluence of ²⁴¹AmBe and ²⁵²Cf sources

Moderated 252Cf source

To produce an energy distribution that is closer to those encountered in the workplace, the ²⁵²Cf source is placed in the center of a moderating heavy water sphere of radius 15 cm, with a 1.2 mm thick aluminum shell to which an 0.8 mm thick cadmium shell can be added; this configuration is the only one included in the ISO 8529-1 standard.

The two energy distributions of the fluence obtained with moderation are shown in the figure below [Ref].

Energy distributions of the neutron fluence emitted by the moderated ²⁵²Cf source [Ref]

Fluxes and dose equivalent rates

The fluxes and dose equivalent rates (individual and ambient) at a distance of 75 cm from the source are presented in the table below. 

​The T400 neutron generator is a 400 kV electrostatic accelerator bringing deuterons (deuterium nucleus) up to an energy of between 150 and 400 keV. These accelerated deuterons are sent to a deuterated titanium (TiD) target where their interactions with the deuterium atoms generate quasi-monoenergetic neutrons of around 3 MeV.

​​​

T400 accelerator for producing intense quasi-monoenergetic fields

​​​The main function of this generator is to supply high neutron fluxes of a few MeV. T400 nominally operates at an energy of 350 keV with a beam intensity of 2 mA. The target used is a thick target that completely stops the beam of charged particles. The deuteron beam is polluted by around 20% diatomic ions. This results in a neutron energy distribution consisting of two quasi-monoenergetic peaks with a full width at half maximum of several hundred keV, overlapping one another as shown in the figure below.

Energy distribution of the neutron fluence produced by the T400 accelerator (D(d,n)³He reaction) in nominal operation with 20% diatomic ions.

The neutron fluence is determined and monitored using the associated particle method: 3 silicon diodes are placed at the back of the target (175° angle) and detect the protons emitted simultaneously with the neutrons. The devices being tested can be placed at between 0 and 2 m from the target along the axis of the deuteron beam (0°). 

The T400 accelerator is used for high flux irradiation with quasi-monoenergetic beams of 3 MeV (see table below). It therefore complements the possibilities offered by the AMANDE accelerator and the Van Gogh irradiator, which have lower flux and dose equivalent rates.

​

Characteristics of the neutron fields produced by the T400 accelerator in nominal operation. The neutron energy resolution () corresponds to the full width at half maximum of the monoenergetic peak. The maximum flux and personnel dose equivalent values are given at a distance of 1 m from the target, along the axis of the deuteron beam, and correspond to 3000 counts per second in the diodes.

​CANEL is used to create realistic neutron fields. These have two major advantages. The first is that they can be used to test instruments in an environment similar to real conditions (wide energy distribution, significant contamination with photons, variable neutron incidence angles, etc.), in full knowledge of the field’s characteristics. The second is that they can be used to calibrate radiation protection equipment in a neutron field that, at least in terms of energy distribution, is representative of the workplace where it will be used, as shown by the graph below, and they therefore enable a calibration coefficient to be defined that reflects the actual use of the instrument.

Energy distribution of the neutron fluence obtained in CANEL used with the T400 accelerator and comparison with the distribution measured at workstations in PWRs.

The realistic fields are used mainly for radiation protection. They were developed by IRSN in the 1990s. In the next few years IRSN will strengthen its position as world leader for this type of field by defining new configurations for CANEL to broaden its range of realistic fields with a maximum energy of 3 MeV. 

​​​CANEL, shown in the figure below, is a modular device that is placed around the target of the T400 neutron generator to produce these realistic neutron fields. Only two examples of the device, which is mentioned as an example in the ISO 12789 standard, exist worldwide. 

The principle of CANEL is that it generates a fission neutron spectrum within a depleted uranium shell. The fission neutrons are then moderated using elements representing shielding used in the nuclear industry: an iron shield with a polyethylene plate, all placed in a polyethylene duct 70 cm in diameter and 1 m in length. This polyethylene duct confines the scattered neutrons within the containment and directs the neutron field forwards. The equipment calibration point is along the axis of the deuteron beam, 50 cm from the output from CANEL. 

CANEL device on the T400

Although only one configuration of CANEL is currently used, its modularity means that the thickness of the iron and polyethylene shields can be varied to generate realistic fields with different energy distributions. 

Fluxes and dose equivalent rates determined 50 cm from CANEL/T400 and the relative contribution of the different neutron energy domains to these values(1). These values are given for nominal operation of the T400 accelerator corresponding to a diode count of 3,000 per second.

(1)In terms of fluence, more than half of the neutrons have an energy of less than 0.5 eV, but it is the fast neutrons (energy above 100 keV) that make the main contribution (nearly 80%) to the dose equivalent.

​

​The AMANDE facility is designed to produce monoenergetic neutron fields. It consists of an accelerator of charged particles (protons and deuterons), which are accelerated then directed at a target consisting of a thin layer of scandium, lithium, tritium or deuterium deposited on a metallic surface. The interaction of the charged particles with the nuclei of these elements generates neutrons in the whole space. The corresponding nuclear reactions produce monoenergetic neutrons in a given direction for a given energy of the incident charged particle. 

Monoenergetic neutron fields are used mainly to study instrument response as a function of energy.

See details of the AMANDE facility’s characteristics

Use in radiobiology

Since 2016, a second beam line has been installed at the accelerator output to produce a microbeam, MIRCOM, capable of depositing a predefined number of ions on living biological samples. The purpose of this is to study radiation-induced effects as part of radiobiology research programs. MIRCOM has been operational for radiobiology experiments since late 2018.

See details of the MIRCOM microbeam​

​In addition to its role of associate laboratory responsible for maintaining national reference standards and transferring them to secondary calibration laboratories, LMDN conducts neutron metrology research with two main aims:

To achieve the first aim, research projects aim to:

​In terms of the second aim, the R&D projects concern:

Finally, the reference neutron fields are also used to characterize the laboratory’s spectrometry systems (Bonner spheres systems, ROSPEC multidetector) used to determine the energy distributions of the neutrons encountered at workstations in the nuclear industry, in research laboratories, in the medical field and in the environment. ​