In this post we explore with some detail the phenomena of fluorescence. This topic has already been presented in the previous post : What is Fluorescence. As already explained, fluorescence is the property of some substances to re-emit (in most cases at longer wavelength so at lower energy) the received electromagnetic radiation, in particular they absorb the ultraviolet radiation and re-emit it in the visible.
The mechanism of fluorescence is the following : an incident radiation excites the atoms of the fluorescent substance, promoting an electron to a less bound energy level (orbital), with more energy and thus more “external”. Within few tens of nanoseconds, the excited electron returns to its previous level in two or more stages, ie through one or more intermediate-energy excited states, as shown in the figure below :
Exciting the sample with an incident radiation pulse, instead of a prolonged radiation, ie with very short duration (~ns), it is possible to study the fluorescence emitted from the sample over time. It occurs that the decay phenomenon from the excited state has an exponential trend with a specific decay time which can vary from picoseconds to nanoseconds, this constant τ is called the fluorescence lifetime :
To make an experimental measurement of the exponential decay of the fluorescence it is necessary to excite the fluorescent material with a very short pulse, preferably of sub-nanosecond duration, and perform the measurement of the intensity of the light emitted in the instants following the impulse excitation.
Make a measurement of this type is not simple at amateur level, both for the difficulty in the generation of excitation pulse and for the acquisition of the weak fluorescence signal.
The technique of scintillating materials for the measurement of gamma radiation provides us with a method for the qualitative verification of the exponential decay law and for an estimate of the lifetime.
Actually the excitation event, that is the passage into the scintillating material of an ionizing particle or a gamma photon, is virtually instantaneous. Afterwards, with a photomultiplier, it is possible to observe the decay of fluorescence. However, it is necessary to ensure that the time constant of the anode circuit of the PMT is less than the fluorescence decay time.
With a 50Ω load resistance you can get a τ value of the order of nanoseconds, and then you can appreciate the decays with decay time of the order of 100ns. Interesting materials are sodium iodide (doped with thallium) which has a decay time of 230ns and zinc sulfide (doped with silver) which has a decay of 110ns.
Acquisition carried out with NaI(Tl) crystal coupled to a PMT : it is clear the exponential trend of the PMT response signal, with a time constant of the order of 250ns, in good agreement with the correct value of 230ns.
Acquisition carried out with a ZnS(Ag) layer coupled to a SiPM : it is quite evident the exponential trend of the SiPM response signal, with a time constant of the order of 100ns, in good agreement with the correct value of 130ns.