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Fast Neutron Spectrometer

As a part of the BEXUS (Ballon EXperiments for Students) we, a group of physics students from CAU Kiel, are building a Fast Neutron Spectrometer.

Exploring new measurement principles

While measuring charged particles is easy, detecting neutrals is a lot harder. Different measuring principles have been tested by previous BEXUS teams from Kiel (PING, ADAM, THANOS), which helps us as we have a large heritage to build up on. So what is new about this project?
The idea is to measure fast neutrons. Doing this is additionally hard as the neutrons have to be slowed done before being captured. Our boron-doped scintillator performs both tasks of moderating and capturing the neutrons.

What is a neutron?

Neutrons are subatomic particles, meaning they are smaller than atoms and actually part of most atoms. Contrary to protons and electrons, which are the other constituents of all usual matter, neutrons are electrically neutral.
As every measurement requires interactions between a detector and the particle to be measured, their lack of electro-magnetic interactions requires special measurement principles to measure neutrons.
Neutrons outside of atomic nuclei are unstable and decay into a proton, an electron and a neutrino with a half life time of about 10 minutes. Thus all neutrons measured in the atmosphere have to be produced inside the atmosphere.

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What makes a neutron fast?

Fast in the sense of particles means they have energies excessing those expected in particles at the temperature of their surroundings.
Our specific instrument is designed to measure energies from around 0.5 MeV up to some 10 MeV.

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What is an MeV?

MeV means Mega Electron Volt. The electron Volt is a unit that describes an energy just like Joule, but is much smaller  (19 orders of magnitude). It is used in particle physics as particles masses are many times smaller than those of everyday objects, making their kinetic engeries also far smaller.

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What is a scintillator?

A scintillator is a material that emits light when charged particles travel through it. A special kind of scintillator used in FaNS is the so called plastic scintillator. As the name says it is made of plastic, which is a hydrocarbon, meaning it contains large amounts of hydrogen.
As only charged particles lead to light emissions in scintillators, they cannot directly measure neutrons.

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How to measure neutrons with a scintillator?

The amount of ligth emited by the scintillator is a direct measure of the charged particles energy passing through the scintillator. Still, neutrons itself are not charged. FaNS uses a combination of multiple effects to be able to measure them. Upon entering the scintillator, the neutrons may collide with the hydrogen of the plastic, losing significant parts of its energy by knocking the proton out of the molecule. This proton now carries part of the neutrons initial energy. Since protons are charged, the scintillator starts emitting light which can be measured after amplification by a photomultiplier tube (PMT).
This alone still is not sufficient to identify neutrons, as a free charged particle inside the atmosphere could do the very same. This is where the Boron-doping comes into play. Some isotopes of the Boron easily capture the now low-energy neutrons, leading to emission of additional charged particles with a specific energy.
Measuring a first light-emission, followed by a second emission with this specific energy makes sure a neutron was measured.

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Why even bother about neutrons in 20 km of altitude?

Neutrons, espescially fast neutrons are capable of heavily damaging human cells and genes. While there are not many humans in the given altitude, there are many fast neutrons, making it an ideal environment to test the new principle implemented in FaNS.
Once its funcionallity is proven, FaNS-like instruments may be used in more human-friendly environments.

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We are fans of FaNS, so I created a FanBlog of FaNS called FaNSBlog.

I don't really know if this is necessary, but it might be cool.

Our History

Things that definitely happened


FaNS' BEXUS campaign


Tanos' BEXUS campaign

Our predecessor, also built in Kiel to measure neutrons, but using another measuring principle.


Carstens' Master

Basic concepts of FaNS developed.

Fast Neutron Spectrometer

About us

We are a small team of 5 physics students, ranging from bachelor to PhD. We participate in the BEXUS program and are planning to fly a Fast Neutron Spectrometer.

The REXUS/BEXUS programme is realised under a bilateral Agency Agreement between the German Aerospace Center (DLR) and the Swedish National Space Agency (SNSA). The Swedish share of the payload has been made available to students from other European countries through the collaboration with the European Space Agency (ESA). Experts from DLR, SSC, ZARM and ESA provide technical support to the student teams throughout the project. EuroLaunch, the cooperation between the Esrange Space Center of SSC and the Mobile Rocket Base (MORABA) of DLR, is responsible for the campaign management and operations of the launch vehicles.


Leibnitzstraße 11
Kiel, S-H 24118, Germany

© 2020 Team FaNS