In this post, I will discuss a gamma spectroscopy experiment conducted on a faintly radioactive clock apparently decorated with a glow-in-the-dark radium-containing paint. I will discuss the idea at the foundation of gamma spectroscopy and briefly present the collected data and results.

Introduction

The purpose of a gamma spectroscopy experiment is to collect and determine the energy of the gamma rays emitted by a given radioactive source. Given that each radionuclide has characteristic gamma energy emissions, a gamma spectrum allows the identification of the gamma-active isotopes allowing the screening of the type of contamination of an object or confirming the composition of a given sample.

In its simplest form, gamma spectroscopy experiments make use of a scintillating material that, when stroked by a gamma photon, gets excited and converts the incoming gamma ray energy into a burst of photons in the visible or ultraviolet region resulting in a scintillation light pulse. The intensity of the scintillation pulse is proportional to the energy deposited by the gamma ray into the scintillating crystal and, as such, can be correlated, through calibration, to the energy of the incoming gamma ray.

In simple terms, three main processes are involved in the working of a scintillation detector:

• Photoelectric effect: The incoming gamma ray transfers all its energy to an electron part of the material ejecting it from the original bounded state and exciting the material by direct ionization.
• Compton effect: The gamma-ray photon interacts with an electron in the material changing direction and transferring part of its energy, in the form of recoil, to the material.
• Pair production: For energies greater than $1.022\mathrm{MeV}$ (two times the $511\mathrm{keV}$ rest mass of the electron) the gamma-ray converts, by interaction with the atomic nucleus of an atom, in an electron-positron pair.

In practical applications, the scintillating material is optically coupled to a photo-multiplier tube (PMT) that amplifies the weak light signal to a stronger electrical pulse that is then fed to an electronic system for processing. The basic idea behind the processing of the signal is the measurement of the height of the electrical pulse that, being proportional to the light pulse intensity, is in turn correlated to the energy of the gamma interaction event. By collecting a large number of events and performing a binning operation over the pulse height, a count versus height diagram can be constructed. This, once calibrated using a known energy gamma source, represents the collected gamma spectrum.

Experimental part

The subject of this experiment was an old clock with glow-in-the-dark paint showing a very faint radioactive signature. The activity of the sample was quite low; measuring the activity in direct contact with the glass cover of the clock I managed to obtain 4 times background radiation ($86.6\mathrm{cpm}$ vs $19.6\mathrm{cpm}$ of background) with an SBM-20 tube and 6 times background radiation ($687.0\mathrm{cpm}$ vs $110.8\mathrm{cpm}$ of background) with a Si-8B mica-window pancake probe. 

To obtain the gamma spectrum of the sample, I used a 1-inch NaI(Tl) thallium-doped sodium iodide scintillation detector probe coupled to a computer using a Theremino PMT adapter and a modified USB sound card. I collected, without shielding, a background and a sample spectrum accumulating counts for around 7 hours. For the collection of the data and their binning I used the PRA software. The average count rate for the background was $37.48\mathrm{cps}$ while for the sample an activity of $38.68\mathrm{cps}$ was measured. In essence, an average activity of $1.2\mathrm{cps}$ was associated with the radioactive material present in the paint. By subtracting the background from the sample data I obtained the spectrum of the gamma emissions for the sample that, after applying an average smoothing to the data, resulted in the following gamma spectrum.