Lucas Sifoni

35c3 Leipzig notes - Deep underground stars


I had the pleasure to go to the 35th edition of the Chaos Communication Congress. Here are some notes of some talks.

Deep underground stars

Radioactive beta decay, observed > 100 yrs ago, Observed emitted energy was lower than expected -> something according to energy conservation should have this energy : neutrinos ?? Neutrinos = Ghosts

If neutrinos are emitted by the sun, they reach us => estimated 60MM through my thumb. In 1934 truth was established as “neutrinos are impossible to observe”, until recently. In the 1970s unified theories predicted that protons should decay as well. 1983: Kamioka Nucleon Decay Experiment -> nothing was observed, the detector was adjusted to maybe observe neutrinos. Koshiba received the nobel prize for those works in 2002. In 1996, Super Kamiokande was built to be 20x “bigger”. Discovered that neutrinos are of different types, and can switch state as they travel. We’re still attaining limits from this detector.

In early 2020s, we’ll build a “Hyper Kamioka” to get more information. Through SuperKa, 10^22 neutrinos pass every day, between 10-15 get detected.

An electron “floats” in water, and neutrinos sometimes collide with it. They move it at a speed higher than the speed of light in water, that emits Cherenkov light. This light gets detected by photosensors around the walls. We can deduce from this the energy of the neutrino, and its provenance. Hyper Kamiokande will be an underground cylinder 74m radius * 78m height. Located 650m underground. Rock and mountain above acts as a shield for other particles.

Physicists need to collaborate with geologists and conduct “boring surveys” to find a location. Local infra also comes into play (roads, water, electricity, where do we store the rocks ?) (+- 500000 m^3 of rocks). Then it should be filled with water. It’s cheap and efficient to detect neutrinos. (For HyperK, +- 5000 people of annual consumption of water). How to get this water in japanese mountains ? Rivers, Spring, melting snow ? Needs a water purification system, to avoid dust, radioactivity, […]. (Pure water sucked nutriments out of the lecturer’s supervisor hair).

Pixels = Sensors = Photomultiplier tubes.

We had 1 cable / PMT before. Now, PMTs are used in arrays with 1 cable per array, but electronics need to be put into the water. Should be low-power, and avoid heating the water, or there will be signal loss. Redundancy is also needed. 1 PMT is an half-sphere of 50cm, that has a vacuum inside. Glass should be free of structural weaknesses. Super Kamiokande had sensors implosions, that killed half of the sensors.

Why does the sun shine ?

Imagery of the industrial revolution suggested it was a giant ball of burning coal. Lifetime => 0-1000 We then suggested it was shrinking, but lifetime would be 1M-100M We now know nuclear fusion is the answer. We can confirm this by detecting neutrinos. Quantity of neutrinos emitted = f(temperature), so we know the temperature into the core of the sun. Since we have the direction of neutrinos, we can take a picture of neutrinos emission from the sun.

Exploding stars A supernova can shine as bright as a star cluster or galaxy. Supernova observed from the distance sun-earth > hydrogen bomb against your eye. Produce a neutron star or a black hole. Produce Calcium, Silicium, Oxygen

Life couldn’t exist without those explosions. We need to know what’s “inside” a supernova, but we need to observe inside => neutrinos detection. Neutrinos compose up to 99% of the energy, they arrive before the light. This generated more than 1600 papers on this matter.

Hypothesis : The 4 forces play a role Hydrodynamics are nonlinear 0.1c infall velocity Extreme temperature is needed

Stars often do not explode in those computer simulations, there are code complexity and verification issues. Any simulation result has to be taken with distance.

Starting with a massive star 8 times the size of our sun, H becombes HE, C, N, […] until Fe. At Fe, fusion stops in the core. At a certain size, it collapses under its own weight.It continues to shrink, and density is high enough to make neutrinos “captive”. A shock wave forms, neutrinos are emitted, encourage the reaction, and the star explodes. This whole proccess spans +- 1 second.

With good enough detectors, we could have a timeline with enough precision of an exploding star. Since neutrinos arrive earlier, this can trigger an optical observation.

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