Doubts about units in Geant4

[lmqmeiqi]

I recorded the dose deposited in the same volume using “UI-command base scoring” and the method in exampleB1, but the results confused me.This is the macro command file：

And the result_1 is(using “UI-command base scoring” ,I use “/score/quantity/get/unit” and it shows the unit is “Gy”):

The result_2 is:

The two results differ by exactly 10^12, why?

[mkelsey]

When you print out a quantity in Geant4, you need to (a) divide out the units, and (b) print the units you divded by. Geant4 internally uses a large collection of multipliers which together form a consistent set of unit. But all of the numerical quantities are simply doubles.

For a simple example, let’s store the radius of the Earth:

  G4double Rearth = 6380*km;
  G4cout << Rearth << G4endl;

The output will be

  6.38e+9

What?!? That’s not the 6380 I put in! Internally Geant4 uses units where mm = 1. Consequently, km = 1e+6, with the result you see. The correct way to print out a variable with units is

  G4cout << "R(earth) = " << Rearth/km << " km" << G4endl;

which reports

  R(earth) = 6380 km

Just as we expect.

For your problem, we can ask why it’s off by 10^12. Well, let’s find out what the unit “gray” is, from $G4INCLUDE/CLHEP/Units/SystemOfUnits.h:

  static constexpr double      gray = joule/kilogram ;

and then we find (skipping “joule” for now),

  static constexpr double  kilogram = joule*second*second/(meter*meter);   

So we have gray = metermeter / (secondsecond). Geant4 uses units, as I noted above of mm=1 and ns=1. So gray = 1e31e3 / (1e91e9) = 1e6/1e18 = 1e-12. If you print out your result as

  G4cout << result_1/gray << " Gy" << G4endl;

you’ll get 3.19e-19 / 1e-12 = 3.19e-7 Gy, just as you expect.

[lmqmeiqi]

I totally get it ！Thanks a lot for your serious and detailed answer. Best wishes！