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Forum: Event and Track Management
Re: Question Energy loss of Gamma (Rohin)
Re: Feedback Re: Energy loss of Gamma (Gumplinger Peter)
Re: Question Re: Energy loss of Gamma (Rohin)
Re: Feedback Re: Energy loss of Gamma (Gumplinger Peter)
Date: 13 Nov, 2009
From: Robert Penny <Robert Penny>

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.

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,


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 message (to reply or unsubscribe) at: 
Robert D. Penny <>

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1 None: Re: Energy loss of Gamma   (michel maire - 14 Nov, 2009)
(_ Ok: Re: Energy loss of Gamma   (Rohin - 16 Nov, 2009)
(_ Ok: Re: Energy loss of Gamma   (michel maire - 16 Nov, 2009)
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