Isotropic vector generation

[nkx111]

restG4/src/PrimaryGeneratorAction.cc

Line 671 in 8e6c7ef

G4ThreeVector PrimaryGeneratorAction::GetIsotropicVector() {

Generating vector in this way may not be isotropic. Shouldn't we use θ and Φ to randomize direction?

[jgalan]

I believe the present implementation should be isotropic. Using angles would be more complex implementation

Here is a good example
http://corysimon.github.io/articles/uniformdistn-on-sphere/

Do you have any clues pointing to a problem on isotropy with present vector?

[jgalan]

Our present implementation is closer to the "Alternative method 1", however, we do not protect normalization by very small vectors, as they do, this could cause problems for us:

They require that the norm is larger than 0.001, we could improve that in our method.

[lobis]

I think we should really use the internal features of geant4 for this, using the G4GeneralParticleSource. it will also work with the random seed internally. I will update this function as an example to see the difference it makes, I think it will greatly simplify our code.

We can only use the angular distribution from the generator, we do not need to change from G4ParticleGun.

I will do this later this morning.

[nkx111]

The currecnt method is by throwing points randomly in a cube, and calculate their directions. I guess there will be more events near the 8 corners of the cube

[nkx111]

OK, I see we rejected the events with n >1. Then this is right. Isotropic indeed. Just be less elegant.

[jgalan]

Yes, you always get a uniform distribution within a sphere of radius 1. Indeed, efficiency goes with 4/3 * PI * (D/2)^3/D^3. Volume of the sphere versus volume of the cube.

However, the calculation of cosines and other mathematical functions required in other methods believe is more expensive computationally.

[jgalan]

Ok, finally I believe there is a bug there, because the random number is running between -0.5 and 0.5, so the radius is 0.5 and not 1!

[jgalan]

No no, it is ok, it is ok, it is divided by 0.5 so it is runnning between -1 and 1. :)