MCNPX - How calculate Kerma (kinetic energy released per unit mass) in Air?

[Alexander Camargo]

Hi, my name is alexander, i am student from Institute of radioprotection and dosimetry (IRD). My project is calculate MGD (mean glandular dose) from womans with augmented breast. i am having dificulties to calculate Kerma in air with mcnpx. I drew a block of air above the breast, i am using the "tally" f6 (energy deposited in cell) for obtain the kerma in MeV/g. However, I'm not obtained good results, i obtain very low values.

I ask for help for ideas or suggestions.

thanks.

 

[Astronuc]

What is the type of radiation source, and what is its distribution?

 

[PSRB191921]

Hi,
Can you put your file (in ascii) ?

 

[Alexander Camargo]

mcnpx 2.7

C Alexander Camargo - 23/10/2018
C *******************Celulas********************************
10 2 -1.090 (-1 2 -3 4) #20 imp:p=1 VOL=149.2256511   $ espessura / thickness                     
20 1 -0.9850 (-5 6 -7 8) imp:p=1 VOL=353.4291735           $ mama / breast 
30 3 -0.001205 (-10 -11 12 13) #10 #20 imp:p=1 VOL=10.3059947 $ ar / air
C ----- Acompanhamento -----
80 3 -0.001205 (-80) #10 #20 #30 imp:p=1                   $ Mundo-Interno / universe
90 0 (80) imp:p=0                                          $ Fim do Acompanhamento

C -------------Geometrias / geometries ----------------
C ------------ espessura / thickness ----------
1 cz 8
2 pz -2.5
3 pz 2.5
4 px 0
C ------------ mama / breast -------------
5 cz 7.5
6 pz -2
7 pz 2
8 px 0.5
C ------------ AR / air -------------
10 cz 8.1
11 pz 2.6
12 pz 2.5
13 px 0
C --- Fim de Acompanhamento / universe ---
80 SO 225 $ Mundo

C area 0 0 0 0 0 0 0 0 0 0 100.5312 0 0 0
mode P $ fotons / photons
C -------------- fonte e energia (monoenergetico) ----------------
sc1  raio x monoenergetico para mamografia                               
SDEF X=d1 Y=d2 Z=60 PAR=2 ERG=0.018 VEC=0 0 1 DIR=-1
SI1 -1 9 $ sampling range Xmin to Xmax
SP1 0 1 $ weighting for x sampling: here constant
SI2 -10 10 $ sampling range Ymin to Ymax
SP2 0 1 $ weighting for y sampling: here constant
C                                                           
C **************************** Materiais *********************************                               
C ***************************** mama padrao 50/50 **********************************   
m1    1000          -0.1140  $ H
      6000          -0.5250  $ C
      7000          -0.0060 $ N
      8000          -0.3535 $ O
      11000         -0.0005  $ Na
      16000         -0.0005 $ S
      17000         -0.0005  $ Cl
C                                                                           
C ------------------------ Pele -------------------------------
m2    1000          -0.100  $ H
      6000          -0.199  $ C
      7000          -0.042 $ N
      8000          -0.650 $ O
      11000         -0.002  $ Na
      15000         -0.001  $ P
      16000         -0.002 $ S
      17000         -0.003  $ Cl
      19000         -0.001 $ K
C
C ****************************** AR ***************************************     
m3    6000           -0.000124   $ C
      7000           -0.755268   $ N
      8000           -0.231781   $ O
      18000          -0.012827   $ Ar
C
C ************************** Tungstenio *********************************
C m4    74000          -1.000
C
C *************************************************************************
C
C                               
C ***************************** Tallies ***********************************
f4:p 30
f6:p 30
c
C
C
C
C *************************** tempo de Input **********************************     
NPS   16000000   $ Stop after XX source particles have been run.               
C CTME 120          $ Stop run after XX minutes.

 

[PSRB191921]

I don't see a trivial error.
I suppose you have multiplied F6 by 1.602e-10 to convert in Gy. Remember that to compare results, you have to normalize to the fluence (K/phi) to obtain Gy.cm².

Note that for me the "SDEF" is Strange. If you want a rectangular parallel beam you must write something like that "SDEF X=d1 Y=d2 Z=0 pos 0 0 60 …" but in fact I don't know what you want to simulate ...

 

[Alexander Camargo]

i need to kerma in air, but, i found very low results. I need multiplicate my results per 16000000 to found the valor?

I want simulate a mamography machine with focal point the 60cm the breast. My card "SDEF" is configured for a one-dimensional collimated beam in direction the -z axis.

sorry for my lack of knowledge, I'm new to the mcnpx area and I'm in the third year of graduation.

 

[PSRB191921]

Hi,
No ! A F6 result is in MeV/g for one photon emitted
If you want in Gy you have (F6)*1E6 (eV/g)*1E3 (eV/kg)*1.602E-19 (J/kg=Gy)=F6*1.602e-10 in Gy
But you must remember that MCNP normalizes the results to one photon. So you must multiplied by the photon flux (photon/s) to have a result in Gy/s.

For geometry I am not sure to understand. Do you want to simulate a beam like this :

and

Furthermore a mamography device not emitted a monocinetic energy of photon. For example :

In first approximation a photon of 18 keV like in your simulation could be ok.
Hope it's help

 

[Alexander Camargo]

Thank you so much, you helped me a lot.

I understood everything your said, but, when I multiply my results in mev / g by 1.602E-10, I get very low results for what I want.

In cell 30 (semi-circular cylinder of air. to obtain kerma in air) i obtained = 6.01751E-05 (mev/g per history). multiplyed by 1,602E-10 i obtain 9,64E-15 (Gy por history).

my fluence obtained in cell 30 is 4.41740E-03 (1/cm**2)

I simulate one million histories. 4.867E+06 histories/minute.

please, help me in this question.

 

[PSRB191921]

Your result is 9.64E-15 Gy for one Xrays emitted by your device.
You must known the flux of your device in Xrays/s and multiplied the flux by your results and you have Gy/s