[Gate-users] back-to-back gamma source

[Eunsin Lee]

Dear Users,

I simulated a cylindrical PET system using point source (back-to-back gamma) at the center of the detector for a simple study..(with cylindrical phantom filled with just air).
If my understanding is correct, coincidence events should have back-to-back gamma's, which tell us LOR in r-z plane should be simply a line passing through the center (0,0). In other words Z1+Z2 should be very close to zero. Below shows actual z position detected for two gammas in each coincidence events in my simulation output. Looks like something else is going on...
Can anybody tell me what's missing?

	    Z1                    Z2
	-71.3593	46.7426
	-71.5756	-50.5808
	-71.5233	69.8813
	-71.004		35.5772
	-71.9038	15.7994
	-73.7647	-4.99394
	-71.5314	-11.8862
	-70.284		-12.6473
	-73.0036	63.4071	0	
	-71.3191	61.7319	0	
	-71.9782	-12.7522
	-72.9756	-68.4205
	-73.9974	39.802
	-73.1717	2.25218
	-71.4347	-44.9677
	-71.7795	-73.2273
	-70.6986	-19.3314
	-73.6817	-7.85506
	-73.5097	-1.3274
	-70.4753	54.6905

Thanks,
Eunsin