Dear Mina,
the idea that the physical meaning of fluence has anything to do with
"particles incident on a surface" is a long-standing misunderstanding
that is deeply rooted in most minds.
Fluence is a volume density of particle tracks and is a point quantity
like temperature. Fluence at a point is the limit of the track density
in an infinitesimal volume around that point, for the volume tending to zero.
By integration, you can calculate an average fluence in a macroscopic volume
or an average fluence on a surface, but only integrating over all the
volume points or the surface points.
The fact that you can score fluence based on particles crossing a boundary
derives from the fact that the surface of the boundary is a "volume"
of infinitesimal thickness. Please look at the lecture on scoring
of the last FLUKA course:
https://www.fluka.org/free_download/course/heidelberg2011/Lectures/Scoring0311.pdf
Read the first 6 slides, and especially Slide 6. You will see that the
fluence contribution of a single particle is the limit for dt --> 0 of
(dt/cos(theta)) / (S dt), where dt is the infinitesimal thickness of the
surface, S the surface area and cos(theta) the angle of incdence with respect to
the normal to the surface. The thickness dt cancels out above and below;
dt/cos(theta) is the length of particle track inside the volume, and Sdt
is the volume.
Unfortunately, the name "fluence" (and even "flux", as often particle physicists
wrongly call it) conveys the idea of "flowing".
The Merriam-Webster dictionary gives for "flux":
"the rate of transfer of fluid, particles, or energy across a given surface"
Well, in dosimetry that is WRONG!
The Merriam-Webster concept is instead that of CURRENT. Indeed the meaning of
current is only that of counting particles crossing a surface. One cannot define
current in a volume.
> And I could not conceptually understand or visualise what particles/cm^2 in a
> volume means. That's all...
You can understand it if you multiply the fraction above and below by "cm":
particles/cm2 = (cm travelled by particles)/cm3.
The real "thing" is cm/cm3! Remember that "particles" is not a unit, but cm is.
Kind regards,
Alberto
On Wed, 26 Oct 2011, Mina Nozar wrote:
> Thank you Alberto,
>
> I understand this. Speaking as someone from 'outside the field' to me fluence
> only has meaning when one talks about particles incident on a surface or
> particle track lengths inside a volume.
>
> And I could not conceptually understand or visualise what particles/cm^2 in a
> volume means. That's all...
>
>
> Best wishes,
> Mina
>
> On 11-10-24 03:24 PM, Alberto Fasso' wrote:
>> In reality it is not particles/cm^2 in a given volume. It is cm (track
>> length)
>> in a given volume. cm/cm3 gives cm-2. "Particles" is not a unit.
>> The density of particle paths in a given volume is the fluence.
>> (You may find that in a footnote in that ICRU report I sent you time ago).
>>
>> Alberto
Received on Thu Oct 27 2011 - 16:36:30 CEST
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