when creating particle, each particle have a unique number (ID).
start from 0,1,2 .. etc.
when some particle start to die, the number is shifted.
i mean if the particle number 0 died, then particle no.1 will be assigned as no.0, 2 as 1, 3 as 2… etc.
the problem arise when i use copy SOP and using stamp function, coz my stamp function use $PT variable as input, so as the particle id($PT) is changed/shifted, the stamp will then immediatelly give different result in the middle of simulation.

anyone know how to deal with this problem?

thx.

From what I understand, the variable $PT references the particle number as you said, but the variable $ID references the absolute number of the particle as it was birthed. Try plugging in $ID instead of $PT.