Re: Air dose to water dose conversion

Hello Thomas,

Thank you very much for the detailed explanation and also the reference
material.

I would have hopefully one last question: what USRBIN bin size should
one use when measuring DOSE or DOSE-EQ?
For the test simulations of an electron beam hitting a 1 cm^3 water
target, I can either have one bin covering the entire target volume or
have many bins. If I find the average value for the multi-bin measurement
then I get the same value as measured by the single bin covering the same
target volume, makes sense. When looking at the individual bin values
though, the bins in the direct path of the beam naturally measure a higher
value than the ones further away =96 which averages out in the end.

Now this raises the conceptual question of over how many bins to average
(if at all) when using USRBIN to measure the DOSE or DOSE-EQ in the volume
surrounding the accelerator. Right now I have 1 cm^3 bins but perhaps that
is to fine a mesh.

Apologies if this is covered in one of the texts you mentioned, but I
thought I would be better to ask right now rather than after I have
gotten the texts and read through them and found out it was not covered
in it.

Sincerely,
Bertrand

On Mar 21, 2011, at 8:02 AM, Thomas Otto wrote:

> Hello Betrand,
>
> I have little experience with secondary electron equilibrium in FLUKA,
there may be others who have an authoritative opinion on the concept.
In FLUKA, as in any other simulation code, the simplifications of
"nature", such as cut-off energies, will have an influence on the
outcome of calculations.
>
> In measurement, however, an "equilibrium ionization chamber" must be
as large as the range of secondary electrons of the radiation it is
considered to measure. This is technically feasible for photon energies
energies up to approx 100 or 200 keV, afterwards the chambers would
become too large to operate. This only as a caveat about the size of the
geometry you may need to establish equilibrium !
>
> Under secondary electron equilibrium, absorbed dose from photon
radiation at energy E in material N can be converted to absorbed dose in
material M by the ratio of mass-energy-transfer coefficients
(mu_en(E,M)/rho):
> D(E,M) =3D D(E,N) * (mu_en(E,M)/rho) / (mu_en(E,N)/rho)
> The coefficients are tabulated by Hubbell and Berger and are
reproduced for a mall range of materials in many textbooks. For
electrons (charged particles in general), a similar relation with the
mass-stopping coeffients S is holding.
>
> As for useful literature on fundamental concepts of dosimetry, try to
find a copy of Vol 1 of Attix, Roesch and Tochilin (eds.) "Radiation
Dosimetry", even if the quantities and units have other names today, the
discussion is chapter 1 by Roesch and Attix is very good. Another good
fundamental text is from Gudrun Alm Carlsson in Kase, Bjaerngard, Attix
(eds.) "The Dosimetry of ionizing radiation 1".
>
> Best regards, Thomas
>
> ------------------------------------------------
> Thomas Otto
> Safety Officer Technology Department
> TE-HDO
> CERN
> CH-1211 Geneve 23
>
> Tel (+41)(0) 22 76 73272
> GSM (+41)(0) 76 487 0648
Received on Mon Mar 21 2011 - 16:24:57 CET