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Question Questions about scintillation phenomenon? 

Keywords: scintillation mechanism
Forum: Physics List
Date: 25 Nov, 2007
From: Shahrokh <Shahrokh>

I have some questions regarding scintillation

I want to simulate NaI(Tl) scintillation detector. For
this purpose, I refer to an example of the
scintillation effect given in /novice/N06 example in
Geant4 toolkit. At this application, I notice that the
scintillation material is water. The optical and
scintillation properties of water, that include
refractive index, absorption length, light photon
emission spectrum, decay time and fast and slow
scintillation components, are stored in a
G4MaterialPropertiesTable pointer. About these
empirical properties, I have some questions as

1. Spectral distribution:
- What does photonEnery[32] matrix consisted of? 
- In other words, do the elements of photonEnery[32]
matrix include light photons energy emitted by water?
- Can I conclude that the water emits 32 light photons
with the energies given in this matrix?
- If it is true, I need to have the details of NaI(Tl)
emission spectrum light produced by gamma interaction
in NaI(Tl). Although I find this spectrum curve of
NaI(Tl) in the textbooks related to radiation
detection titled  “Radiation detection and
measurement” by Glenn F. Knoll, but this curve does
not include the details of each NaI(Tl) line spectrum
such as the wavelengths of light emission and its
intensities as are needed. 
Additionally, I have come the following sentence in a
across book titled “The Theory and Practice of
Scintillation Counting” by J. B. Birks, 1964:
“The scintillation emission spectrum is mainly
characteristics of the luminescence centre, i.e. Tl+
ion in the case of the thallium-activated crystal.”
Based on the above phrase, I would like to raise the
following questions:
- Is it correct to use scintillation emission spectrum
of Tl+ ion for photonEnergy matrix? 
- I get the 65 lines of Tl I from NIST atomic spectra
database site. Although I know that the wavelength of
maximum emission NaI(Tl) is 415 nm as it is mentioned
in the radiation detection textbooks, so I expect to
see this wavelength through 65 lines of Tl I, but I do
not find this wavelength mentioned anywhere through 65
lines of Tl I. Can anyone explain it to me?
In addition, I think that the intensity of the
spectral lines emitted by a crystal (e.g. NaI(Tl)) is
related to the energy of gamma ray excitation. If it
is true, then I need to have the emission spectrum as
a function of gamma ray energy. Do you agree with me?

2. Refractive index:
I need to have the refractive index of NaI(Tl)
crystal. For this purpose, refractive index data of
pure NaI crystal are available in the following
article: “Refractive Index of Alkali Halides and Its
Wavelength and Temperature Derivatives” published J.
Phys. Chem. Ref. Data, vol. 5, No. 2, 1976
p329-429.Since this data are related to pure NaI
crystal, can I use it for Thallium-activated NaI

3. Absorption length: 
For the implementation of optical photon bulk
absorption, I need to have the absorption length data
for the spectral lines emitted by NaI(Tl) crystal.
Where can I get this data?

4. Fast and slow scintillation components:
In the N06 example, I notice that the scintillation
water must consist of two components: a fast component
with decay time of 1.0 ns and a slow component with
10.0 ns as it is shown in the following lines:

- Although I see two matrixes including 32 elements
with the name of ScintilFast [32] and ScintilSlow
[32], but I do not exactly know what these matrixes
consist of. Please explain to me. 
- Are the emission spectrum lines of slow
scintillation component similar to that of the fast
- If it is the case, are the relative intensities of
the fast and slow emission spectra different from each
- Again if that is case, can I conclude that
photonEnery[32] matrix consists of the emission
spectrum lines of slow and fast scintillation
components and additionally ScintilFast [32] and
ScintilSlow [32] matrixes include the relative
intensities of the emission spectrum lines of slow and
fast scintillation components? 
- Is it correctly defined?

About NaI(Tl) crystal, I notice that this crystal
exhibits a fast component with the decay time of 250
ns and a slow component of 620 ns. Are these
parameters constant in NaI(Tl) crystal produced in the
world? Also I do not have the information about the
emission spectrum lines of slow and fast scintillation
components from NaI(Tl) and also their relative
intensities. Please guide me.  

5. Resolution scale: 
Is the resolution a function of radiation source? In
other words, is the resolution of NaI(Tl) crystal for
Co-60 gamma radiation similar to that of Cs-137? I
think that the NaI(Tl) crystal resolution is a
function of gamma ray energy. Therefore, I need to set
this parameter (RESOLUTIONSCALE) for each gamma ray
source. Is it true?

6. Reflection mechanisms: 
In the NaI(Tl)that I have used, the material of
crystal housing is Aluminum . I do not know the
constant values of the following parameters:
In addition, I do not find any explanations about the
meaning of the reflectivity and efficiency parameters.
. Where can I get this data?

Please help me for understanding these subjects.
Your sincerely

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