simulating low energy EM showers

Hi,

I was wondering if any expert may be able to help guide my investigation in the right direction. A quick summary of my problem: 15 MeV positrons don't seem to be fully tracked if they are ~2 meters away from my detector objects, shown by the plot
http://www.nikhef.nl/~cjreed/poslenVsRad.gif

I would appreciate very much any advice!

Details:

I am trying to simulate positrons of 10-20 MeV in water. My geometry consists of a large tank of water (cylinder dz=dr=25 meters) and 3 PMT detectors about 60 cm from the origin.

I throw the positrons evenly in a 5 meter sphere about the origin.

However, it seems that when the photon is thrown 2.5 meters or more away from the origin, the positron track length is always less than 0.3cm! (It should be ~9cm in water.)

Looking at the location of positrons which eventually produce an optical photon that hits one of my active detectors, as shown in this plot:

http://www.nikhef.nl/~cjreed/posxyWithHit.gif

There seems to be some strange cutoff at 2.5m

This is more visible if I plot the radial distance of the positron vertex from the origin:

http://www.nikhef.nl/~cjreed/posRdists.gif

(red = thrown, teal = op-photon gets to cathode, blue = photoelectron produced)

Finally, the positron track length vs the radial vertex position:

http://www.nikhef.nl/~cjreed/poslenVsRad.gif

I am using the physics lists "emStandard+optical" with:

/mcPhysics/rangeCuts 0.1 mm

/mcPhysics/setCerenkovMaxPhotons 100
/mcPhysics/setTrackSecondariesFirst True

/mcPhysics/selectOpProcess Scintillation
/mcPhysics/setOpProcessActivation True
/mcPhysics/setScintillationYieldFactor 1.0

Thanks!

• Corey