Neutron Detectors

Due to their lack of electric charge, neutrons have some complementary properties to other constituents of matter such as electrons or protons. They can penetrate our matter much more easily and help to visualize the interior of objects. Therefore, they are used in scattering experiments or in imaging to study objects that cannot be penetrated by X-rays or gamma photons or do not provide sufficient contrast. However, detecting neutrons and determining their location is much more difficult than for other particles and requires special atoms for capture and subsequently decay of the neutrons. We are developing three different detectors for accurate measurement of the conversion locations.

Neutrons and protons are the only components of atomic nuclei and thus form the basis of our matter. However, in an unbound state neutrons decay with a lifetime of about 800 seconds into a proton, an electron and an antielectron neutrino. Furthermore, since neutrons carry no electric charge, they cannot ionize the surrounding matter and loose energy. For this reason, neutrons are particularly suitable for studying the internal structure of thicker blocks of matter. For this purpose, one usually uses beams of thermal neutrons (Ekin ~ 25 meV), which are analyzed behind the object. To detect the neutrons, nuclear reactions must be used in which a charged particle is emitted from the composite, as in 3He + n -> 3H + p. Gas detectors filled with helium-3 have been the most widely used detectors, both in neutron scattering experiments and in neutron imaging detectors. However, as an acute shortage of helium-3 has developed in recent years, new types of neutron detectors based on other nuclear reactions are being developed. Among others, the reaction 10B + n -> 7Li + α is very suitable and our group is developing three different detectors based on 10B.

Eine Wissenschaftlerin und ein Wissenschaftler arbeiten hinter einer Glasfassade und mischen Chemikalien mit Großgeräten.
© Pal / Universität Bonn

1.) Neutron Time Projection Chamber

Time Projection Chambers (TPC) record a 2 dimensional image of an event and with the help of the drift time of individual track segments a 3 dimensional image can be reconstructed. However, this requires a start signal, which neutrons cannot create because of their lack of electric charge. Our setup therefore uses both nuclei of the 10B decay. One of the two triggers the start signal in a thin scintillator layer, while the other creates a track in the gas volume. Since the tracks are very short, the depth of the TPC can be chosen narrow as shown in the figure. For a first prototype of this novel detector only the large area trigger is missing.

2.) Neutron MCP

High-resolution imaging detectors are needed to examine objects as precisely as possible. For this purpose, we use microchannel plates (MCPs), which features thin channels. In these channels esingle lectrons can create electron avalanches, which are then detected with the help of four Timepix3 ASICs. Due to the high time resolution of the Timepix3 ASIC, this detector will also be suitable for observing fast processes.

© S. Gürbüz
Eine Wissenschaftlerin und ein Wissenschaftler arbeiten hinter einer Glasfassade und mischen Chemikalien mit Großgeräten.
© J. Kaminski

3.) GEMs with boron layers

To reach a higher detection efficiency of neutrons with a single detector, we use a Gas Electron Multiplier (GEM), of which several layers can be used. The individual layers, as well as the cathode, can be covered with a 1 µm thick 10B layer, so that a high neutron capture probability is achieved due to the large number of layers. The signals are collected with a strip readout and digitized by the VMM3a-hybrid.

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