Details

Published: 12 May 2020

Fast neutron detection and their energy measurement is complex, because neutrons are electrically neutral particles, so they can’t be detected directly.

Mimac-FastN is a tight enclosure filled with a neutral gas at roughly the atmospheric pressure, with non-flammable and not regulated matters (so no ³He, no high pressure, no hydrogen, that limit operation in some industrial areas).

Neutrons can interact with the detector gas nucleii. This interaction results in a nuclear recoil : there is a partial energy transfer from the incident neutron to the gas nucleus.

The detector has a very fast sampling camera (40 MHz). Thanks to this camera, the detector provides 3D pictures of the nuclear recoils’ tracks in the gas.

At the same time, the energy deposited in ionization by the nuclear recoil in the gas is measured.

From these two information, tracks and ionization energy, we can calculate the energy of the incident neutron.

Hereafter a drawing of the detection principle :

Developments of Mimac-FastN result from 15 years of gaseous detectors know-how. Some specificities are listed below :

1/ A low noise and fast electronics, that opens the 3D detection field with a good resolution.

2/ The acquisition software, that controls physical events triggers.

3/ The ability of reconstructing the nuclear recoils kinetic energy from the measurement of their ionization energy. This reconstruction is specific to each gazeous mixture, and evolves with ionization energy. The higher the neutron energy is, the higher is the impact of this parameter on the kinetic energy calculation of the incident neutron.

4/ The data analysis software, that allows the selection of the events to consider for the neutron spectrum reconstruction or for the neutron source location.

Mimac-FastN differentiates from existing technologies with its performance that is not limited to neutron counting but also allows their energy measurement, with its mobility, with the 3D approach that gives access to the discrimination of all the physical contributions, and with its directionnal feature.

The proof of concept has been conducted in monoenergetic neutron fields, with a small mobile prototype, with data acquisitions of 1 hour.

Use cases are currently explored, for applications as diverse as detection of fissile matter in radioactive waste, characterization of atmospheric neutrons, or neutron dose measurements in industrial areas using neutron sources.

Reference : Article published in the NIM journal : https://doi.org/10.1016/j.nima.2020.163799

Nadine Sauzet : Scientific & technical responsible, simulations & data analysis

Olivier Guillaudin : Detector developments, micromegas & drift field cage

Marc Marton : 3D design, production & assembly

 

Details

Published: 01 October 2018

• Mentoring of early LabVIEW software developements by trainees.
• Code as well as version (GitLab) and configuration control upgrades. Refactoring to better suit experimentation needs.
• Improvement of parameters control, monitoring, data acquisition and analysis tools.

Contact : Olivier Zimmermann

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Details

Published: 01 July 2018

• Design of the 9Be neutron production target

        

A prototype target of Beryllium, of reduced size compared to a full power device, is currently under development. This target consists of a rotating graphite matrix 30 cm in diameter serving as both a structural material and a mass dissipating the heat generated by the energy deposition of the beam of deuterons or protons. On the surface of the graphite matrix, a thin layer of Beryllium 9 μm thick is deposited at the level of the annular impact zone of the beam. This deposition of Beryllium is carried out by ion beam spraying (IBS) inside the target's vacuum chamber.

This original concept has the double advantage of being able not only to produce the initial layer of Beryllium but also to regenerate it, if necessary, as the target is used. The energy transmitted to the graphite mass by the beam is evacuated by thermal radiation towards the walls of the enclosure, themselves cooled by circulation of water. The temperature reached by the Beryllium should not exceed 850 °, in order to stay in a partial vapor pressure range below 3.10-6 mbar, numerical simulations have been performed to size the cooling system and define the optimum speed of rotation . To validate the results of the numerical simulations, and also to study the stability of the beryllium thin layer, thermal tests under electron beam are in progress before proceeding to the tests in real conditions under beams of deuterons or protons.

      

 

• Design, realization and exploitation of the 3 kW thermal test bench. (18 keV-167 mA electrons beam)

        

In order to be able to test and characterize the targets 9Be and 7Li during development, a thermal test bench capable of producing an electron beam of 3 kW on 1 cm² of surface has been developed. The electrons, produced within a COMIC-type ECR source, are extracted from Argon plasma and accelerated to an energy of 18 keV for a total current of 167 mA. A removable and cooled Faraday cup measures the current extracted from the source. A beam optic consisting of a solenoid and two deflectors (steerer) makes it possible to focus and shape the electron beam. Two cameras, placed perpendicular to each other, continuously visualize the fluorescent light of the residual gas produced by the passage of the electron beam. A pixel-by-pixel image intensity analysis program extracts the position and profile of the beam. Beryllium and Lithium targets can be coupled at the end of this electron beam line to test their thermal behavior at a representative power density of 3 kW / cm2

• Neutron field characterization

 

AB-nCT requires an epithermal neutron field to treat patients cancerous tumour, so the conception of a moderator is needed to reduce the energy of fast neutrons coming from the target, as well as a neutron spectrometer to characterize the neutron field around the moderator.

Thanks to simulations performed with the Monte Carlo codes MCNP and GEANT4, the neutron field features can be optimized for the patient treatment being the most efficient with a minimized secondary dose. With these simulations, we can also explore and define the moderator structure, in order to minimize the residual fast neutron proportions in the treatment field.

Spectrometry measurements are performed with the Mimac-FastN spectrometer developed in the laboratory, filled with a gas mixture adapted to the detection of low energy neutrons.

Dosimetric measurements can also be performed with Mimac-FastN, by inserting an active target (such as a boron coating).

 

 Example of active target in B₄C, and of the interactions detection with the active target inside Mimac-FastN

 

Jean-François Muraz : Technical Responsible, 3D design & beam line optics simulation (COMSOL)
Mohammed Chala : Assembly & cabling
Olivier Guillaudin : Neutron field caracterization
Murielle Heusch : Beryllium security studies
Julien Marpaud : Remote control (LabVIEW)
Nadine Sauzet : Neutron field caracterization, moderator design & optimization(MCNP)

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Details

Published: 01 July 2018

  1. Introduction:

ITk is the future inner tracker, all silicon (pixels), of the ATLAS detector for the HL-LHC. The IN2P3 is engaged in the Outer Barrel (OB) part, namely the 3rd, 4th and 5th central pixels layers (L2, L3, L4) of this detector.

 Sectional diagram of the ITk ½ detector

 

The LPSC participates in different work packages:

• Loading of detection modules on local mechanical supports
• The design and manufacture and possibly the integration of the intermediate supports separating the pixel layers
• The design and the implementation of type 1 electrical connections services on the OB

The SDI department is strongly involved in the “loading” activities with five members, including the technical responsible of this activity at the LPSC.

2. Loading :

The loading of the modules is done in two steps:

The detection modules, which are an assembly of the silicon sensor and the associated reading electronics, are first bonded to graphite supports with high thermal conduction (TPG, Thermal Pyrolytic Graphite) called cells.

Detail of the Modules assembly

These cells are mounted on local supports, which are either “longerons” or half-rings. This assembly in addition of the installation of the electrical supply and reading connections of the modules represents the step of integration of the local supports. This phase includes a final activity before delivery, the precise geometric control of the 3D position of the integrated modules, carried out using a high-precision three-dimensional coordinate measuring machine (CMM).

An inclined half-ring and a “longeron”, equipped with Modules

The goal is to assemble approximately 750 cells and to integrate 10 “longerons” and 10 half-rings at the LPSC, all this in two years between December 2022 and December 2024.
Beforehand, a development phase is planned up to 2021, dedicated to the development of loading and integration tools, as well as the electrical test benches necessary to verify the functioning of the modules, before and after the phases of loading.

Activities work flow for loading and integration

People involved:

Patrick Stassi : Technical responsible

Murielle Heusch : Loading, developments and production

Marc Marton : Loading, developments and production

Adeline Richard : Loading, developments and production

Olivier Zimmermann : Electrical test benches and databases

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