Change in peak shape over time

[Changkun]

We temporarily replaced PETJ crystal with used one just for checking malfunction of the current PETJ crystal.
The red line in the attached figure is obtained by current PETJ crystal whereas the blue line by used PETJ crystal brought by a JEOL Korea engineer.
Since the same counter was used for PETJ crystal comparison, JEOL Korea engineer concluded that there may be a "contamination" issue on the our current PETJ crystal.
I could not disagree so the crystal will be replaced with new one next year.

Just curiosity. Is there anyone who knows how to clean PET crystals?? Is it a stupid thing?

Looks like a cracked crystal to me (as opposed to dirty).  Sometimes these cracks are barely visible to the eye.

But I do remember a service engineer (Jim Niccolino in Florida) who told me how to "dip" my TAP crystals in distilled water to remove/dissolve away the damaged surface of these crystals (which are very water soluble).

Thank you for the comment.
Our PETJ and PETL crystals have the same problem but PETH crystal is good. Because the PETH crystal is very rarely used, I could not argue the PETJ and PETL crystals were originally or recently cracked.
Also this may not be the heating issue due to malfunction of static filter.

JEOL engineer also told me Do Not Touch TAP crystal and PET crystals might be cleaned with ethyl alcohol but no one does such cleaning now.

[Changkun]

We temporarily replaced PETJ crystal with used one just for checking malfunction of the current PETJ crystal.
The red line in the attached figure is obtained by current PETJ crystal whereas the blue line by used PETJ crystal brought by a JEOL Korea engineer.
Since the same counter was used for PETJ crystal comparison, JEOL Korea engineer concluded that there may be a "contamination" issue on the our current PETJ crystal.
I could not disagree so the crystal will be replaced with new one next year.

Just curiosity. Is there anyone who knows how to clean PET crystals?? Is it a stupid thing?

Looks like a cracked crystal to me (as opposed to dirty).  Sometimes these cracks are barely visible to the eye.

But I do remember a service engineer (Jim Niccolino in Florida) who told me how to "dip" my TAP crystals in distilled water to remove/dissolve away the damaged surface of these crystals (which are very water soluble).

Thank you for the comment.
Our PETJ and PETL crystals have the same problem but PETH crystal is good. Because the PETH crystal is very rarely used, I could not argue the PETJ and PETL crystals were originally or recently cracked.
Also this may not be the heating issue due to malfunction of static filter.

JEOL engineer also told me Do Not Touch TAP crystal and PET crystals might be cleaned with ethyl alcohol but no one does such cleaning now.

Probeman may be correct.
PETL (Ch3) is always used, PETJ is less used than LiF (Ch4), LiFH is much more used than PETH (Ch5).
If something from samples maked the PETL and PETJ crystals dirty, why LiFH is not dirty.
It may be plausible PET crystals were initially cracked or more easily cracked than LiF crystals if PET crystals are weaker than LiF crystals during the integrated analysis time since installation.
Unfortunately, we did not thoroughly examine initial conditions of the PET crystals when they were installed.