The AMANDE facility (AMANDE = "Accelerator for metrology and neutron applications for external dosimetry"), commissioned in 2005, produces monoenergetic neutron reference fields, with two objectives:
- metrology related to neutron fluence and dose equivalents quantities
- testing and calibrating neutron sensitive devices at multiple specific energies over a broad range (between 2 keV and 20 MeV).
Background and objective
AMANDE is one of the facilities of IRSN's Neutron Metrology and Dosimetry Laboratory (LMDN). Through LMDN, IRSN is Designated Laboratory of the LNE (French National Metrology Institute), and in that context develops and operates facilities that produce neutron reference fields. These fields are used to:
- define national fluence references (i.e. in terms of the number of neutrons per surface area unit) or dose equivalents references (ambient or personal) related to neutrons,
- perform various types of calibration on radiation protection instruments (routine calibration, calibration in "realistic" fields, determination of the energy variation with neutron energy).
AMANDE is beneficial in that it provides monoenergetic neutron fields, i.e. those that have a single energy (within a degree of uncertainty); this makes it easier to conduct tests and interpret them, and makes it possible to study the variation of an instrument's response with neutron energy.
Principle and description of the facility
Monoenergetic neutrons are obtained using ion beams (protons or deuterons). These particles are accelerated to a given energy and directed onto a target formed of a few microns thick reactive layer deposited on a metallic backing. The accelerated ions interact with the target reactive layer nuclei, thereby producing neutrons. The deposit is made of scandium, lithium, or titanium, with tritium or deuterium trapped inside.
The ion beam hitting the target is achieved in two steps. First, negatively-charged hydrogen and deuterium ions (H- and D-) are accelerated by a voltage until they reach the centre of the accelerator tube, where their charge is reversed by passing through a nitrogen flow, which strips two electrons from them. They are thereby transformed onto protons or deuterons (i.e. stable nucleus formed of one proton and one neutron) and these now-positive ions are accelerated again by the same voltage.
The energy of the neutrons emitted from the target depends not only on the type of particles used (protons or deuterons) and its energy, but also on the reaction used. The resolution of the monoenergetic peak depends mainly on the type and thickness of the deposit chosen for the target. Multiple target thicknesses are therefore used.
To generate neutrons with a specific energy, the energy of the ion beam is adjusted: a 15° bending magnet located after the source selects the type of ion - proton or deuteron - to be accelerated, and a 90° analysis magnet located at the accelerator's output defines the desired ion beam energy. The neutrons' energy also varies with their emission angle in relation to the direction of the ion beam. This property is used to extend the energy domain of the available monoenergetic neutron fields: it is sufficient to place the detector to be calibrated at a given emission angle. However, the 0°angle (i.e. in the same axis as the beam) is preferred whenever possible, as the characteristics are best (maximum energy and flux, minimal contribution of scattered neutrons, optimal homogeneity of the field on the detector's surface). The energy range of neutrons produced at 0° is specified in the table below. By taking into account the emission angle up to +/- 150°, the neutron energy domain covered by the facility extends from 2 keV to 7.3 MeV and from 12 MeV to 20.5 MeV.
Table of nuclear reactions for achieving monoenergetic fields (energy domain)
© IRSN (Thesis of Amokrane Alloua)
The AMANDE ion accelerator, a 2 MV TandetronTM, was designed so as to have a very low ion beam energy dispersion (about 500 eV), a very high energy stability (about 100 eV) and makes it possible to set the ion beam energy (owing to its high accelerator voltage control system using the 90° bending magnet) with a relative error less than 6×10^-4. A high positive voltage (up to 2 MV) is applied to the center of the accelerator using a rectifier system, without any mechanical load transport, resulting in the excellent energy stability of the ion beam. AMANDE makes it possible to accelerate protons and deuterons continuously or in pulse mode, with energies between 100 keV and 4 MeV (i.e. twice the maximum high voltage). Pulse mode is used to perform neutron time-of-flight measurements: their speed - and therefore their energy - is calculated by measuring the time they take to travel a known distance.
View of the AMANDE accelerator © IRSN
Particular care has been taken to reduce neutron scattered background in the experiment area, which might have complicated the accurate characterization of the radiation fields. The target is located 7.2 meters from the ground, in the center of a pit 6 meters in radius surrounded by a metal grating. The 400 m² experiment hall is surrounded by metal walls that limit the scattered neutrons contribution to measurement. Furthermore, an automated system (moving arms) makes it possible to accurately and reproducibly position the detectors to a distance between 50 cm and 6 meters from the target, and within an angular range of –150° to +150° in relation to the axis of the ion beam. The resulting uncertainty on the neutron flux is less than 0.1%. Finally, the experiment hall is equipped with a temperature and hygrometry control system using an air curtain around the calibration zone, so that at the time and location of the measurement, these conditions are as close as possible to the environmental conditions specified in international standards.
View of the experimental hall © IRSN
Determining reference levels of flux and energy
In a facility producing neutrons with an accelerator, the neutron emission rate cannot be accurately determined. This is because neutron emission depends on reaction cross-sections that are rather poorly known, as well as on target characteristics with high levels of uncertainty (thickness, density of the deposit, etc.) that may change over time. Thus, in order to define the reference quantities (fluence, dose equivalents, energy), the use of measurement reference standards (detectors or reference methods) is necessary.
The measurement reference standard for fluence on AMANDE is a long counter developed by IRSN: a low-energy neutron detector (cylindrical tube filled with 3He gas) inside a moderator whose geometry has been optimized in order to achieve a high and constant response between a few eV and several MeV energy range. This detector had previously been calibrated to the 252Cf source of IRSN's Van Gogh irradiator. This long counter makes it possible to determine the neutron fluence with a relative accuracy on the order of 3%. This reference fluence is measured (or adjusted after correction for distance) at the calibration point and normalized by the counting in the monitors (neutron detectors) placed at fixed points of the AMANDE experiment hall. The ambient and personal dose equivalents are obtained from the fluence by applying a conversion factor tabulated in international standards.
The energy reference standard, above 1 MeV, is the time-of-flight method performed with a liquid scintillator. The relative uncertainty associated with the energy is 1.5%. This standard makes possible to confirm the calculation of the neutron field's energy, by the kinematics of nuclear reactions, based on accurate knowledge of the incident beam's energy and the target's characteristics.
The monoenergetic neutron fields produced by AMANDE are used for:
- characterizing "reference or standard" detectors by determining their response functions,
- the development, qualification, characterization, and calibration of neutron sensitive devices, particularly for radiation protection,
- the development and characterization of neutron spectrometry and dosimetry systems, used for inspecting or/and determining neutron energy distribution around nuclear facilities or at workplaces.
With respect to radiation protection instruments, particular attention is paid to dose-equivalent calibration. The energy range of the neutrons encountered by exposed workers is very broad, and requires special monitoring for neutrons combined with gamma ray monitoring. These workers are active in the nuclear, medical, and aerospace industries.
From mid-2011 to mid-2012, some neutron fields of the AMANDE facility are undergoing an international comparison exercise in neutron metrology, wherein several National Metrology Institutes are harmonizing their national measurements standards for neutron fluence in monoenergetic fields. The results are expected from mid-2013 and will make it possible to support the accreditation request of the AMANDE facility's calibration activities, presented end of 2012 and effective for 2014.
Two systems, aimed to become measurement reference standards, based on the principle of proton recoil telescopes are being developed (TPR-CMOS and µTPC). Measurements with these systems, which are still in the prototype stage, will be compared in the years to come (2015-2016) with the current references. These detectors will ultimately complete the potential of energy and fluence measurements on the AMANDE facility.
The AMANDE facility will get a new beam line in 2013 (developed in collaboration with CENGB) capable of targeting, with micrometer-precision, cellular or subcellular elements with a defined number of charged particles (ranging from protons to heavy ions). This new beam line will be used for radiobiology experiments conducted by IRSN, the first of which are expected for mid-2014.