Basic scintillation setup

crude scintillator setup

Pictured above is my recently build scintillation setup. Scintillation detection is detection of radiation by means of a material that emits small pulses of light when struck by radiation. Most scintillation setups consist of a scintillation crystal, most often thallium doped potassium iodide, NaI(Tl) and a photomultiplier tube (PMT).

The PMT is a very sensitive sensor which converts light pulses into electrons.  These electrons are multiplied by putting a high voltage across the cathode, a number of auxiliary electrodes called dynodes and the anode. The result is a clear pulse for each detected photon emitted by the scintillation crystal.

The voltage of these pulse is directly proportional to the energy of the radiation striking the scintillator crystal. Scintillators using solid materials will only detect gamma and x-rays. Different isotopes of elements generate gamma rays with specific energies. So, basically you can identify isotopes present in samples by looking at the pulse heights and comparing them with the peaks of a calibration source.

My current setup is as follows:

  • Hamamatsu R1548 photomultiplier tube
  • 4 Bismuth Germanate scintillation crystals
  • -1250V power supply (schematic)

The pulses are displayed on my old CRT oscilloscope or on the DSO Nano v2 as shown in the picture. The scope is directly connected to the PMT’s anode, this is not ideal but it works for testing purposes. I also recorded some audio by connecting the anode to the mic input of my netbook. First you hear some background radiation, then some Cs-137 gammas and finally Ra-226 with (I guess) its decay products. The background itself is already quite noisy which is a complete different experience compared with a simple GM tube…

So, it is working but I still need to do some improvements. I want to feed the tube with a positive voltage because then output pulses will be positive instead of negative, and the supply can also be used with geiger tubes. I also need a better interface for audio because I eventually want to make some nice spectra in software. There is a free piece of software, PRA, Pulse Recording and Analysis available for this purpose.

Stay tuned!


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