Thursday, August 20, 2015

Summer days at JLab

From June 2nd through the 5th and 17th through the 18th, and on August 3rd, Dr. Gabriel Niculescu, accompanied by the students Alex Winkley, Justin Le, and Wayne Morris, worked together on a project at Jefferson Lab in Newport News, Virginia.   Jefferson Lab, also known as JLab, is a national accelerator facility funded by the U.S. Department of Energy.  The lab seeks to research and gather information regarding the atom’s nucleus, more specifically its structure, using the lab’s particle accelerator known as the Continuous Electron Beam Accelerator Facility (CEBAF).  The lab houses scientists from around the world as well as graduate and undergraduate students from across the country.

The JMU PNP team took a total of three trips to JLab and in that time they managed to install two frames with scintillators and photomultiplier tubes (PMTs) for the Super High Momentum Spectrometer (SHMS) to be used in Hall C at JLab.  Scintillators are used to detect ionizing radiation from particles that pass through them and are wrapped with a reflective material. When these particles pass through the scintillators they produce photons that bounce around (due to the reflective material) until they reach the ends where the PMTs are located.  Once the photons reach the Photomultiplier tubes, they utilize the photoelectric effect to convert the light produced into an analog signal.

These scintillators are mounted on two separate frames and the frames are positioned roughly 5 meters away from each other. The scintillators are attached to the frame in two layers, one layer oriented horizontally and the other vertically, forming a mesh, so that no charged particles can pass through undetected.  This setup is good for getting an accurate time of flight of the particles that pass through.

Upon completion of mounting the scintillators/PMTs to the frames, the team also had to test the PMTs to see if they could produce a strong signal. In order to accomplish this, a high voltage power supply was connected to a PMT via a coaxial cable, and then the PMT was connected to an oscilloscope via a second coaxial cable.  As the PMTs came from two different sources, it was necessary to vary the supply voltage depending on the PMT’s peak efficiency, 1200 V for one source and 1500 V for the other.  A good signal is characterized by a high signal to noise ratio and a high amplitude on the oscilloscope. Pictured below is an example of an ideal signal on an oscilloscope. 

Due to concern of light leaks in some of the scintillators, the team had to head back to JLab one more time on August 3rd.  These light leaks are spots on the scintillators where photons produced from an external source are able to enter the scintillator and, as a result, fill the signal with so much noise that the data coming from the signal is useless.  A light leak is easy to fix; an application of electrical tape in the leaking area fixes the leak just fine.  The difficulty comes with finding all of the possible light leaks, of which there were numerous on the PMTs the team had to work on. The origin of the light leaks appeared to be a result of a design flaw with the way the scintillator was attached to the PMT.  However, by the end of the day, a system had been developed and the team was able to pinpoint where light leaks existed consistently from one PMT to another.