RE: Air dose to water dose conversion

Bertrand,

From 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.

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)

Greetings, 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: Friday, March 18, 2011 2:03 AM
To: fluka-discuss
Subject: Air dose to water dose conversion

Dear FLUKA forum,

I have a basic question regarding the applicability of a paragraph
I read in the NCRP report 151. There they mention
...the result from an instrument calibrated for exposure [in roentgen (R)]
is divided by 114 to obtain air kerma (Ka) (in gray), or by 104 to obtain
an acceptable approximation for absorbed dose (in gray) or dose equivalent
(in sievert) at a point in tissue.

I combined these two conversions to go from dose-to-air to dose-to-water.
For a simple simulation where I have a T=3D1 MeV electron beam hitting a
1cc of water the dose-to-air (if I replace the cube of water with air)
converted to dose-to-water is within 20-40% of what the USRBIN dose
estimator gives (the measured value is less than the converted one).
This is with PRECISIOn and using the EMFCUT option. Since I am new to
this I wasn't sure if this difference fell under the "acceptable
approximation" mentioned in the NCRP report.

When I run the actual simulation though the agreement between converted
dose-to-air for water and measured dose are off by about 90%, again with
the measured value being less. Is this because this time photon's with an
energy spectrum are hitting the water target? The basic set-up is a
mono-energetic T = 4 MeV electron beam hitting a tungsten target and then
the collimated beam of photons irradiates the water phantom.

Ultimately I was hoping to use a USRBIN EM-ENERGY estimator to measured
the deposited energy in the air surrounding the machine and then convert
that to dose-to-water to estimate the dose a person would receive if they
were standing a certain distance away from the machine. Is this the way
one would do it or am I going about this the wrong way?

Any insights would be greatly appreciated,
Bertrand
Received on Fri Mar 18 2011 - 10:10:57 CET