<html><head><style type="text/css"></style></head><body><div style="font-family:times new roman,new york,times,serif;font-size:12pt">Dear GATE users:<br><br><br>I'm trying to get the sensitivity image for a ECAT system using a series of  simulations of point sources. But I met some problems: for a point source (a sphere with 1000 Bq, 0.1~0.5 mm radius, back-to-back gammas in vacuum) at center, the coincidences increase with radius !!! This seems odd to me. Should #coincidences keep same (or almost same)? The following data is radius----#coincidences for scans with 1 second only:<br><br>            0.1---11; 0.2---106; 0.3---192; 0.4---247; 0.5---319<br><br>I copy a part of code here for your checking:<br><br>gate/source/addSource PointSource<br>/gate/source/PointSource/setActivity 1000 Bq<br><br>/gate/source/PointSource/gps/particle gamma<br>/gate/source/PointSource/setType backtoback<br>/gate/source/PointSource/setForcedUnstableFlag true<br>/gate/source/PointSource/setForcedHalfLife 6586 s<br>/gate/source/PointSource/gps/energytype Mono<br>/gate/source/PointSource/gps/monoenergy 511. keV<br><br><br>/gate/source/PointSource/gps/type Volume<br>/gate/source/PointSource/gps/shape Sphere<br>/gate/source/PointSource/gps/radius 0.4 mm<br>/gate/source/PointSource/gps/centre 0.0 0.0 0.0 cm<br><br>And #coincidences is too high (=0.319 for sensitivity) for this ECAT system when radius=0.5 mm. By some simple calculations, the sensitivity at center is about 0.18. Could anyone point me out what is wrong, or tell me how to do "point source" simulations?<br><br>Thanks a lot.<br><div> </div>Hongwei<div><br></div></div><br> </body></html>