# RE: Air dose to water dose conversion

From: Thomas Otto <Thomas.Otto_at_cern.ch>
Date: Tue, 22 Mar 2011 13:11:39 +0100

Hello Bertrand,

No problem, this question is not covered in any of the texts I quoted, which
were written when Monte-Carlo methods were available to only a handful of
specialists on the whole globe.=20

Within the target I would only score DOSE, i.e. the quantity of physical
energy deposition. There you may define a mesh as fine as you deem necessary,
probably commensurate with beam size and a reasonably small fraction of
target size.

In the vicinity of the target, you want to score DOSE-EQ, which yields the
quantity H*(10), ambient dose equivalent. It is a conservative estimator
for effective dose E. Effective dose is defined as the sum of weighted organ
doses in a human. Conversion factors from fluence to E are calculated with
anthropomorphic, numerical phantoms. While H*(10) can be calculated by
scoring on an arbitrarily fine mesh, this does not seem to make sense to me,
becaue the relation to the size of a human is lost. I usually score H*(10)
on a mesh of at least 10-cm-edge cubes. Sometimes I put boxes of 40*20*80 cm^3
(width * depth * height) in the geometry where the torso of an exposed
person may be placed. This would average H*(10) over the region of the body
where most of the critical organs are located.

One caveat: Fluka as much as any other MC programme will output and plot
H*(10) in all regions of the geometry where the user specifies it to be
calculated. Bear in mind that H*(10) is defined in vacuum (and the definition
holds to a good approximation in air). H*(10) within arbitrary materials is
not properly defined, although it may be useful to roughly judge the
attenuation of dose equivalent in the material.
Reversely, when you score H*(10) in a certain volume in air, then a human
who occupies that volume will be exposed to the value of H*(10) you previously
scored.

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

-----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: Monday, March 21, 2011 2:53 PM
To: Thomas Otto
Cc: fluka-discuss
Subject: 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 =3D96 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
Received on Tue Mar 22 2011 - 13:50:02 CET

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