COSINE-100's New Moves in Dark Matter Research

In the bustling world of science, there's a hot topic: Dark Matter. It's a mysterious stuff that makes up a big part of the universe, yet we can’t see it. Scientists are racing to find it, and that includes our friends at the COSINE-100 experiment. They’re upgrading their equipment to look for it even more closely. Picture it like upgrading your old phone to find better connection in a crowded area.

#The Liquid Scintillator

For this upgrade, they’ve cooked up a new batch of liquid scintillator (let’s call it LS for short). This is not your average juice; it’s a special liquid made from linear-alkyl-benzene, which sounds fancy, but it helps in catching signs of dark matter. The plan is to use 2,400 liters of this LS in a new underground lab at Yemilab. This LS is like the security guard at a party, helping to spot any unwanted guests-aka, background noise that could mess with their dark matter hunt.

#How They Measured Radiopurity

Before they could start using this LS, our scientists needed to make sure it was clean enough for the job. Imagine using a dirty sponge to clean your dishes; not ideal! They took a sample of 445 mL of the LS and placed it in a custom-made container. Two big light-catching tubes were stuck onto the container to see how much Background Radiation it’s got. They measured the levels of uranium (U) and thorium (Th), two suspects that could ruin their party if found in high amounts.

#The Dark Matter Hunt

So, what’s the deal with dark matter? It’s like that friend who keeps talking about a mysterious treasure, and you’re not sure if it exists. The DAMA experiment claimed to have found signs of dark matter through mysterious signals that change throughout the year. To check these claims, the COSINE experiment was launched, hoping to confirm or deny the findings.

After 6.5 years of hard work, COSINE-100 came back with results that raised some eyebrows, challenging the DAMA claims. Now they’re looking to step things up with the COSINE-100 Upgrade, located in their new underground facility.

#The LS System in Action

Now, about that LS system: it plays a vital role in figuring out what’s what in the crystals used for detection. The NaI(Tl) crystal targets are located in the middle of an acrylic box, surrounded by the 2,400 liters of LS. This ensures the crystals are well-protected by at least 40 cm of LS all around. It’s like wrapping your most prized possession in bubble wrap.

Eighteen light-capturing tubes attached to the box help record the light signals produced in the LS. This way, any light from outside or from the crystals themselves is noted. The LS acts as a clever system to know when a gamma ray (another tiny troublemaker) comes around. So far, it’s shown pretty good results, catching up to 75% of the signals it’s supposed to.

#Preparing for the Next Step

A little bit of chemistry was involved in making the LS. They added a substance to help it glow better, ensuring the light isn’t wasted. However, because the glow wasn’t quite right for the light tubes, they used another handy ingredient to shift the wavelength and make it work just right.

Before adding the LS to the detectors, they took the time to check it for unwanted radioactivity. They collected data for a month using a small detector at a ground lab, hoping to catch any sneaky background radiation.

#Building the Detector

To measure how clean the LS was, the team created a special detector to hold that 445 mL sample. They rigged it up with two highly efficient light tubes sitting in custom containers, ensuring everything was nice and snug. They even created some holes to let the good vibes (or light) in while keeping everything else out.

Once built, they shielded the whole thing with lead bricks to keep out stray radiation, kind of like wearing sunscreen to avoid sunburn. The setup was complete with additional layers of protective material to handle whatever the environment could throw at them.

#How They Figured Out What They Were Seeing

To know what particles were showing up, they used a method known as Pulse Shape Discrimination (PSD). This fancy term means they figured out different types of particles by examining the light they produced. By looking at the timing of the light signals, they could tell if it was uranium or thorium causing the ruckus.

They created a system to measure how good their particle identification was, which meant looking through a lot of data to develop criteria to separate the good signals from the bad. They were like detectives solving a case-excluding the bad guys to get to the good stuff.

#The Energy Calibration

During their investigation, they noticed distinct peaks in the data, which indicated the presence of uranium and thorium. Each peak gave them valuable information, helping to calibrate their energy measurements. They had to fit these peaks into models that could tell them what was really going on.

#Digging Into Time Coincidence

Our clever scientists didn’t stop there. They also looked into how different particles decay over time, especially with radioactive elements like uranium and thorium. They found that when a particle decays, it can lead to another particle’s decay, showing up in the data as a time coincidence.

By keeping track of how often these decays happened and fitting them into equations, they could measure how much of each radioactive element was in the LS. They discovered that some of the events they were counting had to do with radon contamination. This is like finding out that the friend who promised to help you is actually a no-show due to some other issue.

#Checking the Right Amounts

The collected data was divided up to analyze the contamination levels further. They realized there was a decrease in activity over time, which they could trace back to radon contamination from the LS being made. They fitted this data into their equations and determined that the contamination levels were quite low, which was good news.

#Keeping an Eye on Thorium Too

They didn’t forget about thorium; that one can be sneaky as well. By applying similar methods, they checked for thorium activity. They spotted decay events through time coincidence and quantified how much thorium was present.

#All About Backgrounds

Now, the whole reason for checking these contaminations is to ensure that the dark matter hunt isn’t spoiled by background noise. The scientists simulated what a “bad” background would look like, comparing it to the real-world background model they had from their experiment. They found the contamination levels from their LS were negligible when it came to their dark matter search, ensuring they were good to go.

#Final Checks

To further validate their results, the team turned to a different method-using High Purity Germanium (HPGe) detectors. This method checked for contaminants as well, and they found that their LS was clean enough for the purpose they needed.

#Conclusion

In summary, the scientists behind the COSINE-100 upgrade have taken all the right steps to ensure their new liquid scintillator is up to snuff. The work they did to assess the purity levels shows that they are ready to push forward with their search for dark matter. The combination of clever setups, smart analysis techniques, and a bit of patience has set them on the right path.

Who knows, with all this work, they might just find that elusive dark matter or, at the very least, make a great party story about how they dealt with the sneaky radioactivity trying to crash their party!