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I've only been half watching this thread, so my apologies if I'm covering ground already covered. I have found that a really good sanity check on photon interactions is the NIST XCOM photon database. http://physics.nist.gov/PhysRefData/Xcom/html/xcom1.html Requesting cross sections on element "Si", units in cm^2/gm, specifying the energy range as 10 MeV to 100 MeV, and entering an additional energy point of "20" gives me cross sections in cm^2/gm of: Energy Coherent Incoherent Photoelectric 2.000E+01 4.21E-07 9.12E-03 6.22E-07 Pair production nuclear field electron field 1.35E-02 7.05E-04 Total xsect with coherent 2.34E-02 Total xsect without coherent 2.34E-02 ("Incoherent" is what they call "Compton", by the way.) Density of Si is 2.33 gm/cm^3. Therefore we'd expect the 1/e length for any type of Compton event in silicon at 20 MeV to be: length = 1/((9.12 E-3 cm^2/gm) * (2.33 gm/cm^3)) = 47 cm The probability of a 20 MeV Compton interaction in 1cm of silicon is 1 - exp( -1cm/47cm ) = 2.1% / photon The probability of a 20 MeV Compton interaction in 50 um of silicon is 1 - exp( -(50E-6)cm/47cm ) = 1.1e-6 / photon Remember also that a photon interaction either happens or it doesn't. Unlike a charged particle, a photon has a finite probability of traversing some distance through a material without depositing any energy at all. Hope this helps, -Rob. On Fri, 2009-11-13 at 10:24 -0800, Gumplinger Peter wrote: > *** Discussion title: Event and Track Management > > > But i dont see any delta electron production. > > You mean Compton electrons? > > > One more thing, when the > > > Si thickness was set to 50 micrometers and gamma beam of 20 MeV were > > > incident on this target, there were no photoelectric effects observed > at > > > least for 10,000 runs. My physics list is adapted from exampleN03. Am > i > > > doing any mistake here. > > Well, you need to do a back of the envelope calculation to estimate how > many Compton scatters and Photoelectric absorptions you expect in your > thin layer at 20MeV. > > Explicit production of secondaries happens only for those above your > "production range cut", though any electron will be produced if it has a > chance to leave your thin layer regardless of the production cut. I > presume you tally whether or not secondaries are produced in > UserStackingAction. > > Whenever GetDeltaEnergy returns non-zero and GetEnergyDeposit returns > zero, the simulation generated a discrete secondary. > > ------------------------------------------------------------- > Visit this GEANT4 at hypernews.slac.stanford.edu message (to reply or unsubscribe) at: > http://hypernews.slac.stanford.edu/HyperNews/geant4/get/eventtrackmanage/897/1/1/1.html -- Robert D. Penny <email@example.com>
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