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

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

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

mcnpx 2.7

Code:
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.

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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 dont know what you want to simulate ...

berkeman

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.

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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

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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.

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