Scintillator Properties

A scintillator cocktail can be created from two or more components: the scintillator, the fluor, possibly a wavelength shifter and possibly an inert solvent. A very simple explanation:

The scintillator is a molecule that is easily excited by the passage of a charged particle, however, most scintillators on their own don't have very good quantum efficiency and are likely to relax these electronic excitations without emitting a photon. Fluors have high quantum efficiencies and can 'steal' the excitation from the scintillator (via dipole interactions - non-radiative transfer) and emit it as light. Wavelength shifters absorb photons and emit them at a different wavelength, which can be useful to stop a fluor absorbing the light it has just emitted.

The primary emission of the cocktail cannot be calculated by simply scaling the concentrations of the different components since a knowledge of the non-radiative transfer efficiencies are required. However, it can be measured. Therefore, in SNOMAN, the properties of a scintillator are separated into 2 types, bulk properties and component properties.

The bulk properties are stored in the SCBU bank. They include:

• The number of photons emitted per MeV of energy deposited (Q).
• Birks' constant in cm/MeV. This empirical constant, B, accounts for saturation of the scintillator if dE/dx (energy deposited per unit length) is too high and modifies the number of photons ( $N_{\rm photons}$) emitted according to the formula:
  

  $$N_{\rm photons} = \frac{Q.dE}{1 + B.\frac{dE}{dX}}$$ (13.12)

  
  

• The timing distribution of primary emission. This is expressed as a sum of up to 5 exponentials. The time constants (in ns) and cumulative fractions of the components are required.
• The composition of the cocktail. The cocktail can consist of up to 10 components, each with different absorption and re-emission properties (see below). The ID for each component and fractional composition are required.
• The primary emission spectrum of the cocktail. A number (currently 10) of different spectra can be given in the titles files, and the user must specify the ID of the one to be used (1-10).

Component properties are expressed in the SCCO banks, with one bank for each component. The component ID is the bank number. Each bank contains the following properties of the component:

• Absorption coefficient, A, for wavelengths 200-600nm in 1nm steps, where absorption over the range, x, changes the intensity from Io to I:
  

  $$I = I_{0}\times10^{-A.x}.$$ (13.13)

  
  

• Emission probability. This value (in the range 0.-1.) expresses the probability that a photon will be re-emitted if absorbed by this component. The probability is further modified, depending on the photons initial energy. (Emission probability is proportional to the amount of emission spectrum at lower energies than the absorbed photon).
• The Emission spectrum. The cumulative probability of emission is given for wavelengths 200-600nm in 1nm steps.
• The timing distribution of reemission. This is expressed as a sum of up to 5 exponentials. The time constants (in ns) and cumulative fractions of the components are required.

The information required for scintillation light is loaded by INPHI, and the necessary look-up tables are generated if any media have the scintillation flag set.