Re: the missing x-rays?

From: Alberto Fasso' <fasso_at_SLAC.Stanford.EDU>
Date: Thu, 26 May 2011 06:10:15 -0700 (PDT)


first, as already pointed out by S. Chatterjee, it is not sufficient to
set the production energy cutoffs (by material), but also the transport cutoffs
(command EMFCUT with SDUM blank, set by region).
In addition, setting the cutoff = 0 (both for production and transport) has
no effect at all. Zero means default: and of course you get the same default
result as before. You must set the cutoffs explicitely to some value > 0, and
not smaller than 1 keV (1.E-6 GeV) for electrons, or 100 eV (1.E-7 GeV) for

The peak "at about 500 keV" is certainly the 511 keV peak from positron
annihilation. Although you have a low energy neutron source, neutron capture
can produce gamma rays some of which can in turn produce e-e+ pairs. The e+ are
transported until they reach the energy cutoff, and then are annihilated
producing two 511 keV gamma rays.


On Tue, 24 May 2011, wrote:

> Dear Fasso,
> I have put in the EMFCUT card to explicitly define that the production cut-off
> energy for electron and photon is 0.But there is still no photon detected,
> with energy lower than about 300 keV.And I have checked the output file,
> only to find the production cut-off energy is still 0.33MeV,shown as follows
> " Ecut =5110E+00 MeV, Pcut =3333E-01 MeV, BIAS = Ray. = S(q,Z) = Pz(q,Z) =,
> which means that the value of cut-off energy is set as default and that my EMFCUT card doesn't work.I feel it strange.
> By the way,I have found there is a photon fluence peak ,which is significantly
> much higher than others,at about 500 keV.However,as the thickness of the GD region
> (see details in my input) decrease,the height of this peak gets down also.
> It doesn't seem to be the X-ray emitted from excited state of Gd.
> So I am wondering why this peak exists.
> The input and output files are attached to this letter.
> Any ideas will be helpful.
> Yours,
> Danny
> ----- Original Message -----
> From: "Alberto Fasso'"<fasso_at_SLAC.Stanford.EDU>
> To:
> Cc:
> Sent: 11:44:29
> Subject: Re: the missing x-rays?
> Yes, it is set by default (which depends on the choice you have made
> with the DEFAULTS command - or without it if you have not put it
> in input).
> Most FLUKA settings have reasonable defaults, but not the electron and photon
> transport and production cutoffs. It is highly recommended that the user
> should ALWAYS set them by means of the command EMFCUT.
> I copy here Note 1) from the manual description of EMFCUT:
> 1) Default values are available for the electron and photon production
> thresholds in electromagnetic interactions, but they are generated
> by a complex logic based on possible other settings (transport
> cutoffs, delta-ray production thresholds, DEFAULTS card, other
> defaults). It is not always guaranteed that the resulting values be
> appropriate to the problem of interest. Therefore, in order to have
> good control of the physics, the user is recommended to provide
> explicit threshold values, or at least to check them on the main
> output.
> Alberto
> > Oh,yeah.According to the output file,the cut-off energy for photon is
> > about 0.333MeV.
> > But I have not assign this cut-off energy, so is it set by default?
> On Fri, 20 May 2011, wrote:
>> Dear sir/madam,
>> Oh,yeah.According to the output file,the cut-off energy for photon is
>> about 0.333MeV.
>> But I have not assign this cut-off energy, so is it set by default?
>> Yours,
>> Danny
>> ----- =E5=8E=9F=E5=A7=8B=E9=82=AE=E4=BB=B6 -----
>> From:
>> To:
>> CC:
>> Subject: Re: the missing x-rays?
>> From the output file you can see that the default Pcut in Gd2O3 and
>> Al2O3 is 0.333 MeV.
>> Hence the photons below this energy are not further transported,
>> but deposits their energy on the spot. So, You
>> have to apply EMFCUT card in the input and set the Pcut
>> value at say 20 keV so that you can detect the
>> photons from 30 keV to 350 keV.
>> Hope this helps.
>> With Regards,
>> S. Chatterjee
>> Scientific Officer,
>> TLD Unit, BARC
>> Variable Energy Cyclotron Centre,
>> 1/AF, Bidhannagar,Kolkata-64
>> Off:+91-33-23184207
>> Fax:+91-33-23346871
>> ----- Original Message -----
>> From:
>> Date: Friday, May 20, 2011 12:15 pm
>> Subject: the missing x-rays?
>>> Dear all,
>>> Now I am using a neutron source (with energy equals about 1ev).
>>> The target is Gd2O3(the mass number of Gd is 156).
>>> Basically,the reaction is as follows:
>>> Gd-156 + n -> Gd-157(*)
>>> Gd-157(*) -> Gd-157 + gamma .
>>> And Gd-157* has a probability of emitting internal conversion
>>> electrons and the
>>> associated x-rays,which leads to
>>> Gd-157(*) -> Gd-157 + conversion e + x-ray .
>>> And it is referred that the energy of these x-rays range
>>> from about 30 kev to 350 kev.I use FLUKA to simulate this process.
>>> I detect the garma fluence transporting from Gd2O3 to Al2O3
>>> (both are regions in my input files), but no gamma with the energy
>>> lower than about 300 kev is detected.
>>> So where are the missing x-rays?
>>> My input files and output files are attached.
>>> Yours,
>>> Danny

Alberto Fasso`
SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025
Phone: (1 650) 926 4762   Fax: (1 650) 926 3569
Received on Thu May 26 2011 - 15:52:14 CEST

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