Define Cs137 by GPS

Dear Experts,
I am using GPS to define Cs137 in my .mac like this:

/gps/particle ion
/gps/ion 55 137 0 0.
/gps/pos/type Point
/gps/pos/centre -4625. 0. 200. mm
/gps/ang/type iso
/gps/energy 0 MeV
/run/bemOn 1

and i use
/tracking/verbose 2
to check soure information.

But i only see Ba137 ,not Ba137[661.2keV]

    :----- List of secondaries ----------------
G4WT0 >            Ba137:  energy = 4.459 eV   time = 24.61 y  
G4WT1 >   Number of events processed : 0
G4WT0 >        anti_nu_e:  energy = 616.4 keV  time = 24.61 y  
G4WT0 >               e-:  energy = 559.3 keV  time = 24.61 y  
G4WT0 >     :------------------------------------------

So,i can’t get 662 keV gamma.

If there is any small example to just check this then it will be great help.


I used the macro you provided and i get Ba137[661] in my simulation

 List of generated particles:
  Ba137	         10000	  Emean = 2.13187 eV 	( 0.0360596 eV  --> 9.87414 eV )	stable	stable
  Ba137[661.659]  9436	  Emean = 1.37854 eV 	( 0.00110595 eV  --> 3.0409 eV )	half life = 	153.088 s 
  anti_nu_e	     10000	  Emean = 364.586 keV	( 14.2558 keV --> 1.17516 MeV)	stable	stable
  e-	        230086	  Emean = 15.1808 keV	( 1.6151e-05 eV  --> 1.11606 MeV)	stable	stable
  gamma	          9601	  Emean = 588.218 keV	( 266.508 eV  --> 661.657 keV)	stable	stable

My code to count secondaries comes from extended example Hadronic/hadr03. I guess you could just use this to test.


Thank you, I find the cause devote it is because i just beam On 1 event. I can see Ba137(661.6) when beam On is 2.
And I have some other questions need you help.

  1. If i set up the beamOn to 100, dose it mean the source activity is 100Bq, or 10Bq source decay in 10sec? As i want to calculate dose equivalent for radiation protection.
  2. What means of “stable stable “in the fifth column?


  1. Geant4 simulates particles not activities. beamOn 100 means 100 particles are simulated.
    We had a similiar question here:
    Statistical error / uncertainty - #11 by BenjaminW

  2. “stable” means this isotope is stable and does not decay. i need two columns for my analysis so i decided to use print “stable” twice.


You can run example rdecay01 with following macro :
zhenyu.mac.txt (149 Bytes)
zhenyu.out.txt (2.2 KB)

Hello maire,
Thanks for your reply. I saw the output file you uploaded, i want to make clear some questions which one of it is why every events can get Ba137(662keV) in yours and another is what relation of the sentence “Activity of Cs137 = 4.671e+12 Bq/g (126.2 Ci/g) “ with beamOn(10 events in your input).
Hope you reply , thanks again.

1- Not all; only 94.66 % ! This comes from data set RadioactiveDecay5.6, file z55.a137
2- activity of primary ion (eg. Cs137) is calculated in, lines 342-355

zhenyu.mac.txt (156 Bytes)

Hi maire,
i check the code of at lines 342-355, but i have some comfuse about it:

  1. i simulate it under beamOn 1 and beamOn 10, but the result of Activity of Cs137 is no relation with event Number. i don’t clear relation between activity with event number.
    2.the code at lines 342-355 introduce into track time, i don’t clear relation between activity with track time,so could you please tell me it?

What is computed is the mean activity of 1g of pur Cs137
To do that, you need only the mean time life of Cs137. You can read it from table or compute it from the simulated events. This is what is done at line 344. Of course, this computation has no meaning with only few events; the statistic must be big enough …

Thank you maire,
i clear the mean of this code that it purpose to caculate activity of all radioactive ion per gram. By the way, if i want to set source activity (suppose 10^6 Bq), what should i do ?

What will be the time of exposure to the radioactive source ?

I want caculate ambient doserate to Cs137, so the time of exposure may 1 hour is siutable.

GEANT4 simulates a given number of gammas, N. In real life the activity is decay rate – a number of gammas in a given amount of time. So you decide what length of time, t, is represented in your simulation, then the rate is N/t.

One caution: You are using an isotropic emission. In a future simulation, if you decide to restrict the direction of the gammas that you generate to be less than the full 4pi solid angle, then the same number of simulated gammas corresponds to a higher activity of the source. The proportionality is the ratio of total solid angle (4pi) to simulated solid angle.


1 Like

Hi Brian,
Thank you answer. Decay rate, for your reply, is equal to Number of events divided by time(seconds). So, if i want to simulates 1000 Bq Co60 and 1000 Bq Cs137 in one hour, i should set up events of number N to 3.610^6 (10003600)?

Correct. One Becquerel (Bq) is identical to 1 decay per second. So 1000 Bq * 3600 sec/hour = 3.6e6 gammas simulated.

Thank you careful response.
i have make clear that the relation between N with activity after your reply.
Thank you again.

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