New physics research!

I’ve been back at school for a couple weeks now, and have successfully arranged to do research in the physics department this year!  I will be working with a professor to continue his work studying the lifetimes of positronium in various materials.  I hope to post here every once in a while with understandable explanations of what I’ve been doing and anything cool I find out.  I haven’t done much real research so far, but I have been doing some basic experiments to get used to the equipment I will be working with.

Gamma Ray Detector

Gamma ray detector consists of two main parts – the scintillator and the photomultiplier tube (PMT).  Gamma rays enter a slit on the front of the scintillator and a sodium iodide crystal changes the rays into a lower wavelength (about 420 nm) photon.  This can happen in 2 ways:  Compton scattering or the photelectric effect. In Compton scattering, the photon hits the electron at an angle, only imparting some of its energy and bouncing off at an angle.  With the photoelectric effect, the photon gives up all of its energy as it is completely absorbed by the electron.  The electron later re-radiates this energy at a lower wavelength.

The photons generated in the scintillator are then sent to the PMT, where they hit the photocathode.  Here more electrons are knocked off the plate via the photoelectric effect.  They are then sent through a series of charged plates, called dynodes, which double the number of electrons on each plate.  By the end of the PMT, a large collection of electrons hits a final plate with an energy related to the energy of the initial gamma ray that hit the scintillator.

This energy is then amplified several times and fed into a computer where we can analyze the data.  We get plots like this:

The spectrum of radioactive Cobalt 60.

The spectrum of radioactive Cobalt 60.

which show the counts (number of times) for a certain bin #.  The bin #s actually correspond to energy, and by taking data on radioactive sources with know gamma ray energies we can calibrate the x axis to tell us the energy of gamma rays we detect.  In this plot you can see the detection of two gamma rays on the right hand side of the plot.  These peaks are higher, sharper, and have more energy than the broad peak on the left.  This is because the higher energy peaks are from photoelectric effect electrons in the scintillator (higher energy peak because the gamma rays gave the electrons all of their energy), while the broad peak on the left is the lower energy Compton radiation!

Eventually I will use this type of detector to calculate the time between the birth and death of positronium.  Both the creation and annihilation of positronium emits a gamma ray, so I will be able to time how long positronium lives with these detectors.


September 6, 2009. Physics, Positronium Research.

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