A way to overcome the influence of the 360° distrubuted light quants is to guide the light inside structure, e.g. chanels where the light quants are reflected at the walls.
The classical approach is CsI (Caesium iodide), doped with sodium (CsI (Na)), or doped with thallium (CsI(Tl)) for higher light output in the green region (550nm). CsI can be grown as crystals and then have a needle structure which forces the light inside the stucture.
Due to the better efficiency CsI is taken often in medical applications. But there are two issues with CsI with higher energies (>60keV):
a) CsI tends to get
Burn In with creates ghosting (sometime you can still see the object from last session)
b) Compon Scatter is the dominating effect at energies above 150keV and the needle structure is obsolete
Here is a strange example for
Burn In with a brand new detector with CsI. A 6mm steel plate with a on-top 5mm steel plate with test-welds and a Wire IQI is exposed with 160kV at 3mA for 300s. The contrast from the plate with the welds to the base plate is 12,500 grey values (from 65,535).
Then X-ray is shut down and 180s later this image is taken (0 kV, 0mA):
You can still see all what was in the image before (and even some more), but to be fair: The contrast is 43 grey values only.
Now all objects are removed out of the beam.
After further 120s X-Ray is turned on with same dose as above and now you can see the objects from before with a good contrast (620 grey values):
There is still 5% of contrast from the image before
; if you have to meet 2-2T you have to detect flaws with 2% contrast
.
