At the heart of CosmicRay lies the neutron sensors, the focus of the research done by the group at Sheffield.

To understand these sensors and their capabilities, we first need to understand what cosmic rays are, where they originate, and what they reveal about the Greenland Ice Sheet.

From Supernova

The term, coined by Robert Millikan in 1925, is used to describe rapid particles that travel across space as a result of events such as supernovae (stars exploding) and solar flares from the sun. These particles are made up of elements such as hydrogen which means they are charged and carry energy with them through space.

These events catapult these particles through space, nearly reaching lightspeed until some inevitably hit Earth. Trillions of these particles hit our atmosphere every day, with most blocked by our magnetic field.

Despite this, many still pierce the field, making contact with other particles in our atmosphere. When this occurs, the cosmic rays shatter into a shower of secondary particles, these are neutrons are muons, which are both subatomic particles, with neutrons carrying no charge and muons carrying charge. After raining down they reach the ground and display some interesting properties.

• They create a small amount of background radiation, which everyone experiences, particularly when taking flights
• They contribute to background noise in physics experiments
• Due to the amount of cosmic rays hitting ground level, it could mean that solar flares are
  doing more damage than originally thought

The above describes their journey to Earth from space, but for us to find a use for them, we have to create equipment capable of detecting these particles.

To Snow

Other than living several kilometres underground, there is no way to avoid these particles, so CosmicRay aims to find good use for them within the ARIA program by developing sensors capable of detecting the ice sheet’s melting rates in real time, but how do they do this?

Firstly, the shower of particles from cosmic rays will land on the Greenland Ice Sheet. Once this happens, they will penetrate the ice.

After this, they will bounce back out of the ground, but when they are inside the ground the experience nature’s shield to neutrons, water, the high hydrogen content found in water has the property of absorbing the particles and lowering their energy afterwards. This means cosmic rays have a much higher chance of being absorbed when water is present.

If these particles have landed within approximately 200 metres of a CosmicRay detector, there are two possibilities:

• The particle does not get absorbed and bounces towards the sensor, interacting and converting from a neutron (no charge) to a charged particle the sensor can detect.
• The particle is absorbed and does not bounce out of the ground.

Inside the sensor

The CosmicRay group is centred on developing weather-suitable sensors, meaning each of these components must be optimised to handle the conditions. The sensor itself is an intricate piece of equipment with plenty of parts, from the power system to the smaller sensors, but to fully understand it, we can look at each part individually:

• Neutron Scintillator Material: Absorbs the neutrons which have bounced back up, producing pulses of light every time a neutron is absorbed.
• Waveguide: A piece of equipment which guides the light created by the scintillator to the photosensor.
• Photosensor: Detects the light particles and converts it to an electrical signal.
• Pulse Processor: Analyses the electrical signals, filtering out background particles to focus on the neutron signals.
• Modem: Sends data to orbiting satellites, allowing for data to leave Greenland.
• Solar Power System: A solar panel converting sunlight to power the sensor, essential for remote power in Greenland.
• Waterproof Shell: Keeps the equipment safe from potential weather conditions
• Mounting System: Holds the sensor in place when deployed into the ice, supporting all the components.
• Data Logger: Records both neutron data, but also surrounding environmental data such as temperature and humidity.

Testing the Sensor

To ensure these sensors are suitable for ice sheet deployment, they must go through rigorous testing. Firstly, to know that the equipment works in general, a physics lab just outside the department building serves as a radiation testing facility.

Radioactive materials are used as they serve as a neutron source, releasing neutrons towards a constructed sensor. This allows readings to be checked before deployment to ensure the sensor is working properly. Since radioactive particles work similarly to cosmic rays, this allows for effective and safe testing, with the testing room itself surrounded by thick concrete.

Secondly, to ensure that the sensors can withstand the cold, it is not a good idea to send them out without subjecting them to prolonged exposure to cold temperatures.

On the outskirts of Sheffield, there is the Laboratory for Verification and Validation, set up by the University of Sheffield’s Engineering department. In this building, there are “climatic test rooms” which can emulate the conditions found in Greenland, going down to –50°C and simulating harsh winds, making them perfect for testing the sensors.

What does the data mean

The sensor’s ability to detect the amount of water and snow on the ice sheet means that they are able to monitor the melting rates, but not in the traditional satellite way, whilst they often view snow thickness, the CosmicRay sensor will be able to take how compact the snow is into account.

The CosmicRay sensors will be able to capture data in real time, without human interaction after installation. Since the ARIA program will last 5 years, this will allow for several stages of melting and refreezing monitored.

The data found through CosmicRay will greatly improve the understanding of melting rates at various altitudes, meaning models will be more accurate for understanding the ice sheet’s tipping point.