SETH

What is SETH?

SETH, which stands for Scintillation Event Triggering Hodoscope, is a detector developed by a student group from Kiel University. We will participate in the BEXUS (Balloon EXperiments for University Students) Cycle 16 in the autumn of 2025. We have been selected for the BX36 mission.

The acronym effectively describes how the device operates. We will utilise two sets of photodiodes and two large BGO scintillation crystals to measure the angle of incidence of Galactic Cosmic Rays (GCRs) as they enter our atmosphere at altitudes exceeding 20 km. By analysing which detectors registered the particle, we can then calculate its angle of incidence. We can also separate the incoming particle species by detecting the energy loss of a single particle across six different detector stages. In this context, “triggering” means that only particles fulfilling certain trigger conditions are recorded.

Background

In 1912, Victor Hess conducted a series of balloon flights with electroscopes and discovered that ionising radiation intensity increased with altitude. This groundbreaking discovery marked the beginning of cosmic ray research and earned him the Nobel Prize in Physics in 1936.

Later, in the 1930s, Erich Regener and his student Georg Pfotzer further investigated this phenomenon. In one pivotal experiment, they launched a balloon carrying three Geiger-Müller counters stacked vertically. This setup was designed to measure the vertical component of cosmic radiation. Their measurements revealed that radiation intensity increased with altitude up to a certain point – now known as the Regener-Pfotzer maximum – which occurs approximately 15-20 km above Earth’s surface, depending on atmospheric conditions. Beyond this point, the intensity significantly decreases due to the thinning of the atmosphere, which provides less target material for particle interactions (Regener 1935).

Today, we know that this radiation originates primarily from Galactic Cosmic Rays (GCRs) – high-energy charged particles, predominantly protons and fully ionised atomic nuclei, that originate outside our solar system, primarily from sources such as supernovae, neutron stars, and high-energy astrophysical events. These primary cosmic rays collide with nuclei of atmospheric atoms and molecules when they enter Earth’s atmosphere, producing a cascade of secondary particles – including muons, electrons, positrons and other subatomic particles. These secondary particles make up what is observed as the secondary GCRs.

In 2015, the ADAM project was conducted by a student team from Kiel University as part of the Swedish-German BEXUS balloon program. Their experiment aimed to measure the angular dependence of atmospheric particles. During the balloon flight, ADAM collected data at high altitudes and confirmed that charged particle intensity is not uniformly distributed but decreases with increasing zenith angle. This confirmed theoretical predictions and provided valuable insight into the angular attenuation of secondary cosmic radiation in the atmosphere.

Our goals

Building on the success of the ADAM experiment, our experiment aims to refine and expand the measurement of atmospheric charged particles. One of our primary objectives is to repeat the angular dependence measurements with improved accuracy by incorporating an attitude sensor, allowing for real-time correction of the zenith angle despite the gondola’s pitching during flight.

Additionally, our experiment will include measurements of the azimuth angle to investigate the east-west effect, a phenomenon related to the deflection of charged cosmic ray particles by Earth’s magnetic field. The flight will also serve as a test platform for new electronics for future balloon missions, including a heading and attitude determination system.

Furthermore, the experiment will explore particle separation capabilities using two Bismuth Germanium Oxide (BGO) detectors, enhancing the ability to distinguish between different types of cosmic ray particles.

Components

To give an overview of how all of our components assembled will look, Jasper and Milan worked on the CAD design.

The instrument consists of several core components working together to detect and characterise charged particles during flight.

The trigger diodes generate a voltage signal proportional to the energy lost as a particle passes through them. They are located at the BGO’s (green squares at the purple rectangles in the CAD design)

To complement the instrument, two Bismuth Germanate (BGO) crystals are used. Particles that deposit a high amount of energy in the BGO produce distinct signal patterns that are later used for particle identification during data analysis.

The instrument also includes a magnetometer and accelerometer, which act as a compass and provide information about the roll and pitch angles of the gondola.

The electronics unit is responsible for signal amplification and shaping, followed by analogue-to-digital conversion.

A custom FPGA-based logic system processes these signals, allowing for real-time event detection and data handling during the mission.

Together, these components form a compact and robust system.

Team SETH

Our team consists of 9 members:

Nicolas is our team lead and dedicates himself to attitude determination.

Milan took over Jasper’s responsibility for mechanics, testing, and data evaluation and is assisted by Pauline.

Leon is in charge of the electronics, calibration, and testing and is assisted by Moritz and Niklas.

Leonie and Emilia built the outreach team.

Find out more about us on Instagram @seth.bexus

Outlook

Looking ahead, we will continue to share updates and insights from our mission on our blog.

Beyond the immediate goals of the experiment, the data collected at high altitudes contributes to a broader understanding of radiation exposure in the stratosphere—a topic of growing relevance with the rising interest in commercial stratospheric flights.

While it is well known that pilots, flight attendants, and frequent flyers are exposed to elevated levels of radiation during conventional air travel, the conditions at even higher altitudes raise important questions: How much greater is the radiation exposure for passengers and crew aboard future high-altitude aircraft? Our findings aim to support ongoing research in this field.