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Forum: Fields: Magnetic and Otherwise
Re: Question discripancy between firing a particle with a given energy and using electric field to accelerate it to the same energy (Robert Nnaemeka)
Re: Feedback Re: discripancy between firing a particle with a given energy and using electric field to accelerate it to the same ener (Gumplinger Peter)
Re: Feedback Re: discripancy between firing a particle with a given energy and using electric field to accelerate it to the same ener (Robert Nnaemeka)
Re: Feedback Re: discripancy between firing a particle with a given energy and using electric field to accelerate it to the same ener (Michael H. Kelsey)
Re: Feedback Re: discripancy between firing a particle with a given energy and using electric field to accelerate it to the same ener (Robert Nnaemeka)
Date: 30 Apr, 2015
From: Michael H. Kelsey <Michael H. Kelsey>

Robert Nnaemeka writes:
> Thanks very much for the explanations. I launched the proton with energy
> of 1 eV. They are all accelerated, although the kinetic energy they
> acquire a the opposite end of the box is less than the energy the field
> is expected to give them.

Perfect!  That's exactly what you'd expect.  As the protons gain energy from
the field, they also lose energy in collisions (ionization). 

> I guess it might be due to energy loss from collision. This quession: does
> decreasing the step length gives a better tracking? CPU power is not a
> problem as my code isn't big and my computer has a generous processing
> power.

Geant4 works out the "step length" dynamically at each step, using the
cross-sections for all available processes to compute interaction lengths,
and throwing a random distance for those ILs [exp(-x/IL)].  There's a lot of
bookkeeping involved to ensure that the various processes obey the correct
statistics (which you can read about in the Application Guide, or in our
tutorials).

Your problem, I think, is that you are looking for actual electrons :-)
Most of the energy loss involves multiple ionizations barely above
threshold, such that the produced electrons don't travel far enough to
observe.  In G4 we avoid tracking all those "invisible" particles by setting
a production threshold.  If an interaction would produce a particle below
threshold, then the energy loss is accounted for in the step, but no
secondary is tracked.

What you may want to try is to set the "production cut" for electrons to a
small value, and see how that changes your observations.  See the
Application Guide for how do to this.
      -- Michael Kelsey

> On Thu, 30 Apr 2015 16:56:59 GMT, Michael H. Kelsey wrote:
> > On Wed, 29 Apr 2015 22:57:04 GMT, Robert Nnaemeka wrote:
> > 
> > > I have a box filed with nitrogen gas at very low pressure. I Then
> > > created a uniform magnetic field and associated it with the box logical
> > > volume. The electric field strength is such that when a proton is
> > > released from one end it will accelerate to a kinetic energy of 120 keV
> > > when it reaches the opposite end. The purpose is to study electron
> > > production via ionization at low pressure. However, there is no
> > > ionization. But if I deactivate the field and fire a proton with 120 keV
> > > kinetic energy I get some electrons as should be expected.
> > 
> > I have a few observations. When you "release" the proton in your job, do
> > you give it exactly zero energy, or do you give it a small but finite
> > energy (e.g. 0.1 eV)? Geant4 does not have a mechanism to "restart" a
> > particle which is Stopped; a stopped proton will simply be killed. If
> > you give it a small energy, then G4Transportation will be able to apply
> > the E field during the step and accelerate it.
> > 
> > Second, from your description, the two tests are different. When you
> > launch a very slow proton at one end, it is probably too low energy to
> > ionize until it gets partway through the box. The 120 keV at the far end
> > is the _maximum_ energy, not the average.
> > 
> > Finally, as you know, ionization energy loss is a steeply rising
> > function as you go down to zero energy. I'm not sure that protons will
> > behave the way you think they will: the slowest protons should lose
> > their energy all in one step, with no opportunity to be accelerated very
> > much. Your tests above will tell you for sure, but you may need to be
> > launching your protons with energy above the "single step" limit, so
> > that they can both lose energy and be accelerated by the field, without
> > being stopped.
> > 
> >   -- Michael Kelsey
> > 
> 
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-- 
[ My opinions are not endorsed by SLAC, DOE or Stanford University ]
"I've seen things you people wouldn't believe.  Attack ships on fire
 off the shoulder of Orion. I've watched C-beams glitter in the dark
 near the Tannhauser Gate.   All these moments will be lost in time,
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