Gamma detectors and flux to dose

Separate section for discussion on gamma transport
Peter Wolniewicz
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Re: Gamma detectors and flux to dose

Post by Peter Wolniewicz » Tue Jan 31, 2017 3:03 pm

Andy_Turner wrote:
Jaakko Leppänen wrote:Serpent allows using user-defined response functions with reaction identifier -100:

Code: Select all

dr -100 <int> <np> <E1> <f1> <E2> <f2> ...
where <int> is the interpolation type used between the points (1 = histogram, 2 = lin-lin, 3 = lin-log, 4 = log-lin, 5 = log-log), <np> is the number of points and <En> <fn> are the energy-value pairs.
Hi, I am having difficulties with the correct syntax here,
My detector is defined like below, but I get the following error message:

"Input error in parameter "det" on line 302 in file "FC1m":

Invalid detector parameter "25""


det 1d dc 200 dr -100 1 25
1.00E-02 7.72E-12 1.50E-02 3.08E-12 2.00E-02 1.63E-12 3.00E-02 7.11E-13 4.00E-02 4.33E-13 5.00E-02 3.33E-13
6.00E-02 3.08E-13 8.00E-02 3.33E-13 1.00E-01 4.08E-13 1.50E-01 6.61E-13 2.00E-01 9.58E-13 3.00E-01 1.54E-12
4.00E-01 2.14E-12 5.00E-01 2.53E-12 6.00E-01 3.17E-12 8.00E-01 4.08E-12 1.00E+00 4.97E-12 1.50E+00 6.78E-12
2.00E+00 8.42E-12 3.00E+00 1.11E-11 4.00E+00 1.32E-11 5.00E+00 1.54E-11 6.00E+00 1.74E-11 8.00E+00 2.14E-11
1.00E+01 2.53E-11

Also, should there be a material defined somewhere in the above detector definition?

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Jaakko Leppänen
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Re: Gamma detectors and flux to dose

Post by Jaakko Leppänen » Tue Jan 31, 2017 3:24 pm

The syntax was changed in 2.1.27:

http://serpent.vtt.fi/mediawiki/index.p ... _reactions

Reaction identifier -100 is followed by a function defined using the fun-card:

http://serpent.vtt.fi/mediawiki/index.p ... inition.29
- Jaakko

Peter Wolniewicz
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Re: Gamma detectors and flux to dose

Post by Peter Wolniewicz » Tue Jan 31, 2017 3:25 pm

Thanks! You saved my day ! :)
Jaakko Leppänen wrote:The syntax was changed in 2.1.27:

http://serpent.vtt.fi/mediawiki/index.p ... _reactions

Reaction identifier -100 is followed by a function defined using the fun-card:

http://serpent.vtt.fi/mediawiki/index.p ... inition.29

Peter Wolniewicz
Posts: 135
Joined: Mon Dec 13, 2010 5:50 pm

Re: Gamma detectors and flux to dose

Post by Peter Wolniewicz » Wed Feb 01, 2017 12:02 pm

It seems that when using dr -200 (and -201 etc...), the unit in the output becomes Gy/hour.
At:
http://serpent.vtt.fi/mediawiki/index.p ... ite_note-2

I thought seconds was the standard time unit in Serpent, so I am a bit confused if it really is Gy/seconds or Gy/hours (However the serpent 1 manual states that unit time depends on case)?

If someone could clarify I would be very happy !

Peter

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Jaakko Leppänen
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Re: Gamma detectors and flux to dose

Post by Jaakko Leppänen » Wed Feb 01, 2017 12:22 pm

It should be Gy/h. Does the order of magnitude seem reasonable?
- Jaakko

Peter Wolniewicz
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Re: Gamma detectors and flux to dose

Post by Peter Wolniewicz » Wed Feb 01, 2017 12:29 pm

Jaakko Leppänen wrote:It should be Gy/h. Does the order of magnitude seem reasonable?
Well, yes...

I took tabulated values from 1.) ICRP 21 and compared to 2.) DR -200,
when I multipled the detector value from 1.) by 3600 (i.e., seconds per hour) I got the almost same result down to 1% as in 2.) .....

I haven't compared to RL measurements yet though :) But will soon...

Calculating the dose at some distance and gamma spectrum from a fission chamber that's been in a reactor here in Sweden.... I will measure perhaps next week and see if it is matching :D

Peter Wolniewicz
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Re: Gamma detectors and flux to dose

Post by Peter Wolniewicz » Wed Feb 01, 2017 5:25 pm

Do I have to specify the detector volume (using dv entry) in order to get the correct Dose rates?'

Edit:
The reason I ask is because in a gamma field without much variation, and using a void "filled" detector, the calculated dose rate seems to change proportionally to detector volume. I was expecting the detector result to be almost independent on detector volume.

I am trying to calculate the dose rate in some position near a radioactive source...

Edit2:

det 1g dc 200 dr -200 void
cell 200 0 void -250 %Dose counter
surf 250 cylx 0 0 0.34 -0.56 -0.36 %r=0.34

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Jaakko Leppänen
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Re: Gamma detectors and flux to dose

Post by Jaakko Leppänen » Wed Feb 01, 2017 6:21 pm

Yes. All detector results are integrated over volume, and to get volume-averaged results you need to divide with the volume of the detector region. Defining the volume using the dv entry does the trick for simple detector types.
- Jaakko

Peter Wolniewicz
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Re: Gamma detectors and flux to dose

Post by Peter Wolniewicz » Thu Feb 02, 2017 10:35 am

Jaakko Leppänen wrote:Yes. All detector results are integrated over volume, and to get volume-averaged results you need to divide with the volume of the detector region. Defining the volume using the dv entry does the trick for simple detector types.
Im still a bit unsure about how to get the units right.
First, the unit Gy is J/Kg. Since I dont have any material (no mass) in my detector, is it correct that the conversion from gamma flux to Gy happens "as if" there IS material at the detector? MT -203 converts to dose in air, and I suspect it is the dose to air and not to human matter (although air and human matter both contains alot of water :) )?
Do I have to put air in that volume instead of void to get the results right or does it not matter much (since air is low density anyways)?

What if I would like to have dose rate that is typically used in industry for exposure to humans and that is used in hand-worn instruments? Do I have to create a "phantom" which is 30x30x15 cm and consisting of:
element fraction (density 1 g/cm3)
1 0.101172
6 0.111000
7 0.026000
8 0.761828

or does some MT card convert from flux to dose for such a body?

Furthermore, Is this correct:

Lets say my cell is 5 cm3

without dv entry: I get J/Kg * 5 *cm^3
with dv entry : I get J/Kg (i.e., Gy)


Thanks!

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Jaakko Leppänen
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Re: Gamma detectors and flux to dose

Post by Jaakko Leppänen » Thu Feb 02, 2017 9:27 pm

The detector calculates volume-integrated flux [cm/s]. The -2XX response functions multiply flux by energy [MeV] and the tabulated mass energy-absorption coefficient [cm2/g]. This gives for the units:

[cm/s]*[MeV]*[cm2/g] = [cm3*MeV/g/s]

The result is then multiplied by a factor that converts the energy, mass and time units into J, kg and h --> [cm3*J/kg/h]. When the results is divided by volume you get [J/kg/h], i.e. [Gy/h].

Or am I missing something?

When you use the pre-determined mass energy-absorption coefficients for mixtures and compounds (-201 to -248) the medium in which the dose rate is calculated has no direct effect on the result (there is obviously an indirect effect that comes from the attenuation of photon flux). With response -200 Serpent calculates the mass energy-absorption coefficient for the medium automatically based on tabulated elemental data.
- Jaakko

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