good question

 Try substituting an enclosing sphere for the enclosing box.

View your geometry to ensure there are no gaps between or overlaps of the shapes. Your geometry sounds simple, but you may have size or position errors.

If this is a class assignment, you should be seeking help at TAM.

Thank you very much. What exactly do you mean by 'ensure no gaps between the shapes' 

Thanks again

Usually a problem with more complicated geometries. Boundary placements may be incorrect.

 dear Leonard; if you trying to simulate a radiation therapy, why do you need a point isotropic source? You should define a source with pos, axs and dir parameters. On the other hand, you can define your geometry much more detiled.. Your recent scenario is a bit "general"... Do not forget that, how much you define your geometry with the real parameters, you will have more efficient results.. 

I think there may be undefined areas or overlaps between several cells. 

To my knowledge, the program SuperMC can be used to visualize the particle lose and check the geometry modelling, as is shown in the attached figure. Maybe you can try with SuperMC. 

You have an undefined region of space (volume) in your problem. MCNP will tell you where that particle gets lost. If you use that location (x,y,z) in the plotter then you can see where that location is and where your problem in your geometry definition is.

 Thank you guys for your assistance

I am actually new to MCNP but I am catching up quickly. I think.

I checked my problem on MCNP X and I found out that Cell 3 in my problem is actually not showing and my results on MCNP 6 shows that I am losing particles because it cannot detect that same cell.

Please Check the 1)code 2) the 3d diagram 3) the crossection*

*When i check all crossections: XY XZ YZ and all I dont see cell 3. 

How can I solve it?

Thank you all for your assistance. I have been able to stop losing particles and detected the overlapping cells.

Now I specified the point source and I am trying to run an F6 tally. Everything seems alright but it gives this statement: 'there are no nonzero tallies in the tally fluctuation chart bin for tally'.

It also fails the 10 statistical checks( which I feel is a variable reduction technique)

I tried increasing the nps(brute force) but didnt get any viable result.

Thanks

Thanks for the information George.

I have tried the line and disk sources(degenerate cylindrical source) but my run terminates and I only get statistical checks and tfc tables for my tally 6.

Find attached my output file

thanks