Srishti ’19 Spotlight series: Physics and Astronomy Club

News, 8th March 2019

As the final interview of the Srishti’19 Spotlight series, we speak to Mr Piyush Marmat, a member of the Physics and Astronomy Club, and his project on rediscovering the origin of modern particle detectors through cloud chambers.

GG: A cloud chamber sounds pretty interesting. Can you explain how it works?

Piyush: Basically, It is a particle detector that was used first used in 1932 by Carl Anderson to discover the positron. It is used to detect other charged particles. It is the simplest of all particle detectors. The device is literally what the name implies. We use a chamber made of glass or some other transparent material and fill it with supersaturated vapors of isopropyl alcohol or ethanol. A copper plate is fitted at the bottom of the chamber with a woolly cloth stuck at the top. The top side is warm and evaporates the alcohol absorbed on a sheet of material like wool or felt. The vapor generated sinks to the bottom.  At the bottom of the chamber is the cold metal plate. It can be cooled in a number of ways—using dry ice or a thermoelectric cooler, as I have in my project. The concern with using dry ice is finding a supplier, which can get tedious. We are using thermoelectric cooling, which makes use of the Peltier effect. On supplying current, a temperature difference is produced. This temperature difference is used to cool the plate.

Piyush with his project

The alcohol vapors generated need ionization centers to condense, which are initially absent. The vapors form a supersaturated cloud. Now, you place some radioactive material in the chamber which emits ionising radiation. Particles enter and ionises the gas. The alcohol vapors, on contact with the ionized particles, condense on these molecules. As particles move through the chamber, contrails are formed. The shape of the contrails can be used to identify particles.

For example alpha particles—helium nuclei—are heavy and have low range due to their low energy. Therefore, the trails formed are thick and short. They enter, ionise and get absorbed quickly because of their low energy. Whereas muons are light, and their trails are thin and long. Trails for electrons and positrons would be short and zigzag. These trails can be seen with naked eyes, i.e. specific instruments are not needed to differentiate between particles. However, the cloud chamber cannot detect neutral particles. Even advanced chambers like bubble or spark chambers cannot directly detect neutral particles.

GG: What was your motivation behind this project? Was it related to your course or purely as a hobby?

Piyush: I am having a course on nuclear physics and detectors were a part of the structure. Since I was interested in the field, I began exploring. I read about 3-4 types of detectors, including cloud chambers. I tried googling information about it and found it easy and implementable, so I started procuring construction materials and acquired most of them, and here we are!

GG: Can you tell us about the importance of cloud chambers in particle physics?

Piyush: Paul Dirac theoretically predicted the existence of anti-matter, and 4 years later Carl Anderson  discovered the positron with a cloud chamber. The principle behind cloud chambers was observed in a rather amusing way. The inventor Charles Wilson, an Scottish physicist, while observing the Brocken spectre, discovered that ionising radiation form contrails in the cloud leading to the discovery of cloud chambers. Between 1920-50, before the invention of the bubble chamber, cloud chambers were the primary means to observe particles. One could say these are the predecessors to modern particle detectors.

GG: You mentioned bubble and spark chambers. Could you elaborate?

Piyush: The only difference is that bubble chambers are huge, spherical and use liquid hydrogen. Instead of supercooling, a superheated liquid is used. It is not possible to create one on a small scale with such a limited budget. A spark chamber uses an electric spark. When a particle is detected, an electric spark is produced. These chambers are much more complicated and it is more difficult to make observations as they are opaque.

GG: On a scale of 1 to 10, how difficult was this to make?

Piyush: 7/10. It’s actually quite simple. You will find YouTube videos of second graders building them on the internet. But these are too simple and do not produce any viable result. Contrary, the cloud chamber I have built works perfectly. I’ve used one of those rechargeable electric fly swatter to generate the  electric field to increase the visibility. We will also use a magnetic field generated using small magnets. Next, I had to procure the items and assemble them. But the assembling procedure is sophisticated and any error may lead to non-workable setup.

GG: Did you require any specialised, made-to-order equipment, and did you face any hurdles while acquiring these materials?

Piyush: The glass chamber was procured by placing an order to a vendor. The copper plates I used were ordered online. Finding the other materials was not difficult, but finding dry ice was. Once I decided to discard dry ice for Peltier cells, everything was sorted out.

GG: Do you have any plans for any future projects that you would like to try?

Piyush: I will try to build a Geiger-Muller counter or a better detector. I am quite interested in nuclear physics and want to work in this field. We at Physics and Astronomy Club, always try something new in Srishti. Last year we demonstrated quantum levitation using a superconducting material. The magnet is cooled using liquid nitrogen. The superconducting material above it would levitate and get locked in position. This is called the Meissner effect. We try to create such instruments – basic principles which can be observed easily in the exhibition. The cloud chamber was one such project, hence I worked on it.

Make sure to visit the Physics and Astronomy club this Srishti, from 9th-10th March, for more info on their amazing projects.

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