**From:** Etam Noah Messomo (*Etam.Noah@cern.ch*)

**Date:** Fri Sep 22 2006 - 14:54:42 CEST

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Hi,

Thanks for clearing up my confusion over the issue of choosing between current and fluence for USRBDX.

I just have one last point to clear up. In my specific case I am interested in monitoring energy crossing a surface (generalised particle 208). To take a simple example, a proton beam is incident on a cylindrical target of a certain length, a fraction of the proton beam's energy is deposited in the target, the remaining fraction escapes. I would like to know the relative contribution of this fraction of escaping energy for each of the surfaces of the target. I am not interested in the energy spectrum so I have one energy bin and one solid angle bin. Am I correct in using current rather than fluence for this specific application?

Cheers,

Etam Noah Messomo

-----Original Message-----

From: owner-fluka-discuss@fisica.unimi.it [mailto:owner-fluka-discuss@fisica.unimi.it] On Behalf Of Alfredo Ferrari

Sent: Thursday, September 21, 2006 10:37 PM

To: fasso@slac.stanford.edu

Cc: Nicole Patricia Lee Pratt-Boyden; Etam Noah Messomo; FLUKA DISCUSSION

Subject: Re: RE: Plotting USRBDX with one angular bin

Dear all

I would like to stress AS MUCH AS possible the importance of what Alberto has just pointed out. This is an extremely common misunderstanding among High Energy Physicists who do not realize that the only meaningful quantity is FLUENCE and NOT CURRENT.

Actually they naively believe that what they want to score is current while they want to score fluence and they don't understand it.

In the past, in my (long) years as ATLAS radiation coordinator I had to put a chapter in a note and explain by voice in many meetings what should be the abc of a (radiation) physicist. I'll try to find the text and send to the list, since it seems there are still misunderstandings around on such basic definition.

BTW, for an isotropic beam, the error you make confusing fluence with current is a factor 2 exact, for a monodirectional beam perpendicular to your surface current is numerically the same as fluence, for a monodirectional beam grazing the surface (parallel), FLUENCE is perfectly defined, finite and equal to the one you would get with whichever other orientation of the test surface, current is zero and your error INFINITE!!

In essence fluence is a property of the (local) radiation field, your test surface being a useful way to "measure" it. Current is as dependent on the surface orientation as it is on the radiation field, and its only purpose is to "count" charged particles (it never has any meaning for neutral ones) in those cases where you don't care about the amount of energy they deposit (which of course is proportional to

FLUENCE) but you operate ie in saturated regime and you get just a pulse for each particle entering your detector, the pulse being independent of the actual energy left by the particle (think about an RPC trigger chamber for example)

Ciao

Alfredo

On Thu, 21 Sep 2006, Alberto Fasso' wrote:

*> In the last mail of Nicole I see reported another mail, from E.N.
*

*> Messomo, which must have been sent to her directly, because I have not
*

*> seen it on the discussion list.
*

*> I would like to point out a very common misunderstandig on that mail.
*

*>
*

*>>> I also started using fluence but found out that with FLUKA, fluence
*

*>>> isn't the time integral of flux, but is rather related to the normal
*

*>>> of the surface/boundary where the detector is defined.
*

*>
*

*> There is nothing strange in the way FLUKA intends fluence: it is just
*

*> what is defined by the International Commission on Radiation Units and
*

*> Measurements (ICRU), in its Report 60 (1998).
*

*> On the other hand, "flux" is commonly (mis-)used in many different ways.
*

*> Even ICRU defines it, but in a way which I have never seen used in practice:
*

*> it is dN/dt, where N=number of particles and t=time, without any
*

*> reference to any surface or boundary crossing.
*

*> So, in any case "fluence" is never the "integral of flux", but the
*

*> integral of "fluence rate". The definition by ICRU is the following
*

*> (capitals are mine):
*

*> ------------------------ Definition
*

*> ---------------------------------------
*

*> "The fluence, Phi, is the quotient of dN/da, where dN is the number of
*

*> particles incident on a sphere of cross-sectional area da, thus
*

*> Phi = dN/da.
*

*> The use of a sphere of cross sectional area da expresses in the
*

*> simplest manner the fact that one considers an area da PERPENDICULAR
*

*> TO THE DIRECTION OF EACH PARTICLE. The quantities fluence and energy
*

*> fluence are applicable in the COMMON SITUATION IN WHICH RADIATION
*

*> INTERACTIONS ARE INDEPENDENT OF THE DIRECTION".
*

*> ----------------------------------------------------------------------
*

*> ----- Relating fluence to the normal of the boundary is a way to
*

*> implement the above definition. If a particle crosses a surface at an
*

*> angle theta, the area "da" to consider is not a small element da' of
*

*> that surface, but an element of a surface perpendicular to the
*

*> particle: da = da'/cos(theta).
*

*> If this was not done, the fluence would not be independent of
*

*> direction, as stated in the definition.
*

*>
*

*>>> If you simply want to score neutrons with no further information on
*

*>>> their directions, which is what I naively assume since you have just
*

*>>> one solid angle bin, you have to use current (FLUKA 'current' has no
*

*>>> relationship with time). You then simply count the number of
*

*>>> neutrons crossing a given surface.
*

*>
*

*> It is correct that "current" amounts to counting particles crossing a
*

*> surface, and also that 'current' has no relationship with time.
*

*> But fluence, too, has no relationship with time. "Information on
*

*> particle direction" is more relevant to current than to fluence,
*

*> contrary to what Messomo seems to think. Indeed, if you turn the
*

*> surface by some angle, current changes but fluence does not (as
*

*> stressed in the official definition). Scoring fluence with several
*

*> angle bins will just give you dPhi/dOmega, i.e. the angular
*

*> distribution of fluence (time integral of particle radiance according to ICRU).
*

*>
*

*>>> (Please ignore all the
*

*>>> above if you know what you're doing and you're sure you have to use
*

*>>> fluence...)
*

*>
*

*> These words don't say it explicitely, but they clearly reveal the
*

*> writer's feeling that current is the "natural" quantity to be used in
*

*> normal work, and fluence some exotic quantity only suited for a few
*

*> very specialized tasks. The truth is just the opposite. Current is
*

*> meaningful only in the rare cases where particles are counted without
*

*> any interest in their interactions. But if one is estimating dose,
*

*> activation, radiation damage, instrument response (all effects
*

*> depending on particle interaction with matter), the quantity to be
*

*> used is fluence and only fluence. See again the ICRU sentences I have written in capitals above.
*

*>
*

*> I would like to conclude this little tutorial on quantities by
*

*> reporting a small note that ICRU adds to the definition of fluence.
*

*> ---------------- Alternative definition
*

*> -------------------------------------
*

*> "In dosimetric calculations, fluence is frequently expressed in terms
*

*> of the lengths of the particle trajectories. It can be shown that the
*

*> fluence, Phi, is given by
*

*> Phi = dl/dV,
*

*> where dl is the sum of the particle trajectory lengths in the volume dV".
*

*> ----------------------------------------------------------------------
*

*> ------- This alternative definition, which I prefer by far because it
*

*> gives a very good insight in the physical meaning of fluence (and it
*

*> is not true that it is used only in dosimetric calculations!) is
*

*> implemented in FLUKA as a track-length estimator (USRTRACK and
*

*> USRBIN).
*

*> The insight is the following: the number of interactions is
*

*> proportional to the total distance travelled by the particles, NOT to
*

*> the number of particles, NOR to the number of particles crossing a
*

*> surface. This becomes more obvious if you measure distances in units of mean free paths.
*

*> Coming back to the USRBDX estimator, if you think the boundary as
*

*> having an infinitesimal thickness, the total path travelled "inside"
*

*> that thickness depends on the cosine of the angle to the normal.
*

*> Current gives just a number with little physical meaning.
*

*>
*

*> Alberto
*

*>
*

*>
*

-- +----------------------------------------------------------------------------+ | Alfredo Ferrari || Tel.: +41.22.767.6119 | | CERN-AB || Fax.: +41.22.767.7555 | | 1211 Geneva 23 || e-mail: Alfredo.Ferrari@cern.ch | | Switzerland || Alfredo.Ferrari@mi.infn.it | +----------------------------------------------------------------------------+

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