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Re: Question Recording (n, gamma) reactions, and misc. newbie questions (Karen Pease)
Date: 18 May, 2015
From: Michael H. Kelsey <Michael H. Kelsey>

On Mon, 18 May 2015 21:01:39 GMT, Karen Pease wrote:
> I'm a newbie to Geant4 here and have been messing around with it for two
> days or so. Unfortunately I'm getting a bit confused.

Not entirely surprising. Geant4 is a "toolkit", rather than a turnkey application. That has the strong benefit that different users (and categories of users) can build applications with all of, and just, the components they need for their job. But it has the equally strong deficit that a new user, who isn't part of a group which already has an application, can get completely lost and befuddled by all of the different choices which need to be made.

Besides the examples (, which you're already reviewing, you may also want to look at the presentations and exercises from one of our recent tutorial seminars ( We generally hold a couple of these each year, in both Europe/Asia (e.g., and North America (e.g.,

> My goal is to have neutron beam and, for each component in the setup, report the
> energy deposited and the number / type of (n, gamma) reactions. I've set up a
> scene based on the B1 example but I don't know which examples to look at
> that would be closest to what I need for analyzing transmutation. I
> assume that I have to do something with G4ParticleChange? Somehow? In
> some manner? Via grep I see G4ParticleChange used in some of the
> advanced examples but I don't understand them.

You won't be looking at G4ParticleChange directly; an instance of that object is generated for each physics process which touches the track on a given step. Rather, in your SteppingAction, you'll look at the G4Step and G4StepPoint, which integrate all of the G4ParticleChange's from all of the processes during that step.

> I see where I have a "SteppingAction" which gets the total energy
> deposit for the "step", although I'm not really positive on what a step
> is exactly (is it anything, anything at all, that happens?).

Yes, that's right. As a particle (G4Track) traverses your geometry model, each time it interacts corresponds to a "step." Steps may have different lengths, and different time durations, depending upon what process caused the step to happen (or in G4 language, "limited the step"), and steps need not represent straight-line motion of the particle. Multiple processes (see above) will have their effects applied to the track during a single step.

> I think that should let me categorize the energy on a per-object basis. But as
> for counting the (n, gamma) reactions and categorizing them by type and
> the properties of the newly generated isotopes, I really don't know
> where to start.

I think B2 and B4 may be useful to you, creating sensitive detectors and hit-data objects to record the information of interest.

>  * When I call G4PVPlacement and add a child of a mother volume, when a particle
> moves into the child volume, it stops interacting with matter made out of the
> material of the mother and starts interacting with matter made out of the material
> of the child, right?

Yes, that is exactly right. Child volumes overlay (replace) the portion of the mother volume which they occupy.

>  * When I set in the macro file "/gps/time 1ms" and then "/run/beamOn 10000", is
> that 1ms per particle emitted or 1ms for the whole beam?

Neither. That sets the initial time stamp for each particle in each event. The time clock is reset from zero for each event.

> And if it's creating radioactive particles will they decay while the beam is on, but
> then anything that's not yet decayed when the beam shuts off will not continue its
> decay processes, the simulation will just end? And there's no way to make it just
> keep going for X length of time if desired?

No, a single event will follow all of the decays in the entire chain right through until only stable isotopes remain. The time-stamp for each decay will be assigned by throwing randomly according to the half-life. The daughters of that decay will inherit that time stamp, and then have their times incremented as they are tracked through the geometry.

If you want to simulate what happens in real life, where you have some trigger gate during which you record data, and long-lived activation products sit around and may produce secondaries during a future event, then you need to do more. You'll need to set up a StackingAction, where you can check the timestamp of newly created isotopes (for example), and set flags so that they are stored, undecayed, until the next event (or event some event in the far future.

>  * Is there any way to simulate the "long term" results of a scene - for example,
> rather than firing off 10000 particles at the target and stopping, having it extrapolate
> how the target will evolve after being exposed for, say, hours, days, years, or
> whatnot? (this isn't critical for me at the moment).

Yes, there definitely is. You may want to look at examples/extended/radioactivedecay. It's probably better for you to get your basic application working, and understand examples, before jumping into this level of complication.

  -- Michael Kelsey

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1 Question: Re: Recording (n, gamma) reactions, and misc. newbie questions   (Karen Pease - 20 May, 2015)
(_ Feedback: Re: Recording (n, gamma) reactions, and misc. newbie questions   (Michael H. Kelsey - 20 May, 2015)
(_ None: Re: Recording (n, gamma) reactions, and misc. newbie questions   (Michael H. Kelsey - 21 May, 2015)
(_ Feedback: Re: Recording (n, gamma) reactions, and misc. newbie questions   (Michael H. Kelsey - 21 May, 2015)
(_ None: Re: Recording (n, gamma) reactions, and misc. newbie questions   (Karen Pease - 21 May, 2015)
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