From: Stan Seibert (volsung@physics.utexas.edu)
Date: Thu Jan 19 2006 - 22:07:35 CST
Hi all,
I just wanted to let people know that the most recent CVS commit has
fixed the neutron capture to use a "real" simulation of the neutron
diffusion and capture, rather than a parameterized model (with a
fixed capture time not applicable to our scintillator) that was
inadvertently used before.
The energy distribution of the neutron captures is not changed, but
the capture times are now ~ 15 microseconds for 0.002 Gd fraction
rather than > 150 microsec. Moreover (as Matt will show in the
slides tomorrow) the capture time naturally tracks the Gd fraction of
the scintillator stored in the material database, as you would expect
if GEANT4 is simulating the all the interactions for us.
This has no measurable effect on the runtime of inverse beta-decay
events, or probably any other neutron study that also tracks optical
photons. Photon propagation is almost all of our run time,
especially since the Gd capture energy is > 8 MeV. Theoretically,
neutron simulations that dropped all the photons would run much
slower now, but I don't know that anyone is needing to do that yet.
(And I don't even know how to turn off the photons at the moment,
even if I wanted to.)
--- Stan Seibert
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