First successful low-energy test for the MIMAC-512 neutron detector

Développements techniques Nuclear physics

A team from the LPSC (1) has successfully tested a sensor capable of measuring the energy of low-energy neutrons (around the keV), which is lower than what existing detectors allow. Called MIMAC-512 and inspired by a technology developed for tracking 'hypothetical' dark matter particles, it could open up a wide range of applications.

The MIMAC-512 neutron spectrometer, developed at the Laboratoire de Physique Subatomique et Cosmologie (LPSC) in Grenoble, has taken a decisive step forward by passing its first test to measure the energy of epithermal neutrons, i.e. low-energy neutrons in the kilo-electronvolt (keV) range, which no detector has been able to characterise until now. In this energy range, the background dominates and current detectors can at most count them. MIMAC-512 therefore marks a breakthrough by making it possible to determine their energy.

Dark matter detection technique

« MIMAC-512 is based on a technique that we developed for a directional dark matter detection experiment," explains Nadine Sauzet, an engineer in the LPSC's detector and instrumentation department who was involved in developing the detector.  This technique is based on the detection of the nuclear recoil induced when a neutron collides with the nucleus of the gas. This recoil carries away a fraction of the neutron's energy. It then deposits its energy by stripping electrons from the surrounding gas, resulting in a cloud of electrons along its path. It is the characteristics of this recoil that allow us to trace the energy of the incident neutron. » This is how way it works. An electric field transfers the electrons to a pixelated plane sensor to form a 2D image of the recoil trace. To obtain the 3D shape, the device takes into account the order of arrival of the electrons and their drift time in the gas. It performs 50 million scans of its surface per second in order to precisely date the arrival of each electron. All this information is used to deduce the angle at which the recoil was emitted in relation to the direction of the incident neutron and the energy that the neutron had.

Infographie de fonctionnement de MIMAC-512
When a neutron passes through the metal chamber of the MIMAC-512 detector, as in a game of marbles, it can propel an helium nucleus (nuclear recoil) at an angle and energy that the instrument can measure. The nuclear recoil ionises the gas along its path, releasing electrons. These electrons migrate under the effect of an electric field and, after being amplified, reach a pixelated sensor that records their position and the timing of their arrival. All these observations make it possible to trace the 3D shape of the helium recoil trajectory and to deduce the energy of the incident neutron.

Tested on IRSN's AMANDE facility

MIMAC-512 is developed within a collaboration with IRSN (2). It was tested in an 8 keV mono-energetic neutron beam resulting from the 45Sc(p,n) reaction on the AMANDE facility ( Accélérateur pour la métrologie et les applications neutroniques en dosimétrie externe) of IRSN's Laboratoire de micro-irradiation, de métrologie et de dosimétrie des neutrons (LMDN). "The detector worked well and was able to recover the energy of the neutrons produced, which is very encouraging. The next step is to develop a metrological installation producing high-flux epithermal neutrons, then to characterise it with MIMAC-512", explains Nadine Sauzet.

Profils X, Y et Z de l'arrivée des électrons
Trace en 3D d’un recul d’énergie cinétique de 8 keV avec son profil temporel d’énergie déposée en ionisation dans le milieu gazeux.
Image : LPSC IN2P3
Spectre en énergie du faisceau incident de neutrons
Measured spectrum of the 8 keV single-energy neutron beam from the AMANDE facility.
Image: LPSC IN2P3

Hopefully, there is a growing interest in the design of facilities that produce epithermal neutron fields of a few keV, in a region where the cross sections vary rapidly with energy, in metrology to calibrate dosimeters, or in medical physics for the implementation of new therapies. This demonstration of the operation of MIMAC-512 opens up new perspectives for the characterisation of the reference installation that the LMDN is currently designing around its T400 accelerator in collaboration with the LPSC.

(1) Laboratoire de physique subatomique et de cosmologie (LPSC)

(2) Institut de radioprotection et de sûreté nucléaire (IRSN)


Eduardo Daniel Santos
Chercheur au LPSC
Nadine Sauzet
Ingénieure dans le service détecteur et instrumentation du LPSC
Arnaud Lucotte
DAS Accélérateurs, détecteurs et technologies
Emmanuel Jullien
Responsable du service communication de l'IN2P3