Re: Air dose to water dose conversion

From: Bertrand H. Biritz <beb2015_at_med.cornell.edu>
Date: Sat, 19 Mar 2011 00:05:55 -0400

Hello Thomas,

Thank you for the prompt reply. It answered some questions but also
raised a couple of new ones.

Regarding the secondary electron equilibrium, if I make the volume large
enough so that even the electrons with the largest range are stopped
would that be a better set-up? I tried this and now even the measured
dose in air versus calculated one using the NIST table differ by 50-60%
(with the measured being higher this time).
Perhaps I am doing the calculation incorrectly: I take the stopping
power at the energy of the incident electron and also its range to
determine the energy deposited and then just divide by the mass of the
target. I got this idea after coming across a prior post. Looking at
another thread dealing with having FLUKA simulate only CSDA for protons,
there the graph they point to shows CSDA giving a larger energy
deposited than after including various additional energy loss paths.
What sort of agreement should there be between the energy deposited
using the NIST CSDA based tables and FLUKA simulations?

A more fundamental conceptual question: if one measures a dose of 1 Gray
in air, does that mean it is also 1 Gray in any other material? For some
reason my head is spinning about this one. Since the Gray is just J/kg
and the Sievert (which is used to give the dose a quantitative
biological meaning) is just Gray times the quality factor this would
imply either that the radiation detectors are calibrated in Sv for
humans or one Sv in air, water, human is the same quantity regardless of
the material being hit and by extension the same thing holds for the
Gray. This though does not seem to be the case if I understood correctly
the NCRP paragraph I quoted earlier, as there one has a conversion
factor from dose in air to dose in tissue.
The former of "a Sv is a Sv" makes more sense as it would seem very
inconvenient to constantly have to include what material one is making
the measurement to and is not very particle when it comes to determining
the potential damage to a human.

Sorry for all these fundamental questions, I just have not found a good
text book which covers all of these basic things (any recommendations)
and there is no one else here that I can ask.

Sincerely und schoenes Wochenende,
Bertrand

On Mar 18, 2011, at 5:10 AM, Thomas Otto wrote:

> Bertrand,=20
>=20
> =46rom the description of your set-up I have doubts that you are in =
secondary electron equilibrium, which is generally a precondition to use =
conversion coefficients.=20
>=20
> To estimate the personal dose to a person standing close to the device =
you describe, I would use a USRBIN and specify with AUXSCORE to score =
the quantity DOSE-EQ. This estimator converts particle fluence in =
ambient dose equivalent H*(10), a good estimator for personal dose (it =
is the quantity that you would measure with portable radiation detector =
calibrated in Sv)
>=20
> Greetings, Thomas
>=20
> ------------------------------------------------
> Thomas Otto
> Safety Officer Technology Department
> TE-HDO
> CERN
> CH-1211 Geneve 23
>=20
> Tel (+41)(0) 22 76 73272
> GSM (+41)(0) 76 487 0648
>=20
>=20
> -----Original Message-----
> From: owner-fluka-discuss_at_mi.infn.it =
[mailto:owner-fluka-discuss_at_mi.infn.it] On Behalf Of Bertrand H. Biritz
> Sent: Friday, March 18, 2011 2:03 AM
> To: fluka-discuss
> Subject: Air dose to water dose conversion
>=20
> Dear FLUKA forum,
>=20
> I have a basic question regarding the applicability of a paragraph=20
> I read in the NCRP report 151. There they mention
> ...the result from an instrument calibrated for exposure [in roentgen =
(R)]=20
> is divided by 114 to obtain air kerma (Ka) (in gray), or by 104 to =
obtain=20
> an acceptable approximation for absorbed dose (in gray) or dose =
equivalent=20
> (in sievert) at a point in tissue.
>=20
> I combined these two conversions to go from dose-to-air to =
dose-to-water.=20
> For a simple simulation where I have a T=3D1 MeV electron beam hitting =
a=20
> 1cc of water the dose-to-air (if I replace the cube of water with air)=20=

> converted to dose-to-water is within 20-40% of what the USRBIN dose =
estimator=20
> gives (the measured value is less than the converted one).=20
> This is with PRECISIOn and using the EMFCUT option. Since I am new to=20=

> this I wasn't sure if this difference fell under the "acceptable =
approximation"=20
> mentioned in the NCRP report.
>=20
> When I run the actual simulation though the agreement between =
converted=20
> dose-to-air for water and measured dose are off by about 90%, again =
with=20
> the measured value being less. Is this because this time photon's with =
an=20
> energy spectrum are hitting the water target? The basic set-up is a=20
> mono-energetic T =3D 4 MeV electron beam hitting a tungsten target and =
then=20
> the collimated beam of photons irradiates the water phantom.
>=20
> Ultimately I was hoping to use a USRBIN EM-ENERGY estimator to =
measured=20
> the deposited energy in the air surrounding the machine and then =
convert=20
> that to dose-to-water to estimate the dose a person would receive if =
they=20
> were standing a certain distance away from the machine. Is this the =
way=20
> one would do it or am I going about this the wrong way?
>=20
> Any insights would be greatly appreciated,
> Bertrand
>=20
Received on Sat Mar 19 2011 - 09:02:51 CET

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