# Need Help with Sensitive Detector for Neutron Flux Calculation

Hi everyone,

I’m facing an issue with a sensitive detector in my Geant4 simulation. I’m trying to calculate the neutron flux. Here’s the setup:

1. World Volume (W): Contains all other components (filled with vacuum).
2. Container: Filled with water.
3. Rod: Also filled with water, placed inside the Container.
4. Detector: Placed inside the rod.

When I set the world volume (W) as the mother volume for the detector, I’m unable to detect anything. However, when I set either the container or the rod as the mother volume, I can detect neutrons passing through the detector.

What could be causing this discrepancy? Any insights or suggestions would be greatly appreciated.

If you set the detector as a daughter of the world volume, then you are explicitly creating overlapping volumes, which breaks the tracking system.

Thank you for your response and clarification.

In my simulation, I have a complex geometry setup where I have a mother volume (W) representing a cylindrical shape with a radius of 1000 cm and a length of 500 cm. Inside this volume, I have a container volume, also cylindrical, with a radius of 50 cm and a length of 100 cm, filled with water. Within this container, there are several components:

``````1. A neutron source placed at the boundary of the container.
2. Several cylindrical shapes resembling a target, filled with enriched uranium.
3. A hexagonal shape with a length of 50 cm, filled with water, serving as a central structure.
4. My detector placed inside this central structure.
``````

Now, my questions are:

``````1. If I place a volume (let's call it B) inside another volume (let's call it A), which is not the original mother volume, do I need to explicitly mention that volume A is the mother volume of B to avoid overlapping volume issues?
2. When defining my physics list, is the physics list applied to the entire mother volume, even in volumes where I didn't explicitly define (W) as the mother volume?
``````

Additionally, I have attached a picture of my geometry for reference.

It may be useful to you to read the Geant4 documentation on how the geometry works (Geometry — Book For Application Developers 11.2 documentation). You define your geometry hierarchically – the innermost volume(s) are placed as daughters of their nearest mother. That volume in turn is placed as a daughter of its mother, and so on. You don’t specify the full hierarchy explicitly, just each daughter-in-mother placement.

The physics list applies to the entire job, not to specific volumes. You can configure specific parameters or physics-list configuration by way of `G4Region`, but if you’re not yet comfortable with the simplest examples, you probably don’t want to go down that road yet.

Thanks for your help and the Geant4 documentation tip! Placing volumes as daughters of their nearest mother did the trick.