Re: [fluka-discuss]: Fissions in very thin target

From: Alberto Fasso <fasso_at_mail.cern.ch>
Date: Tue, 17 Jun 2014 11:24:14 +0200

Sorry!!!

I wrote 5.E-6 x 5.E-6 = 5.E-12

Of course 5.E-6 x 5.E-6 = 2.5E-11 !!!
So, if you run a million neutrons, you have a chance to get about 25
fissions. Still close to "some results"...

Alberto

> Hi James,
>
>> I tried using the lambias card with a reduction factor of 0.001 but still
>> had
>> nothing, so I then used 1.E-6 which gave some results (of the order 1.E-11
>> fissions per beam particle). Is this number realistic?
>
> you can find out the answer by yourself! The probability of fission per cm
> (so-called macroscopic fission cross section) is given by the product
> sigma x Av x rho / A
> where sigma is the fission cross section in cm^2, Av is Avogadro's number,
> rho is the density in g/cm^3 and A is the atomic weight.
> If you look up the fission cross section on ENDF, for example on the
> nndc website http://www.nndc.bnl.gov/, you see that of course it depends on
> energy (I don't know what is the average energy of your neutrons) but is
> of the order of 1.E-4 b = 1.E-28 cm^2
> Av is about 6.E23
> rho is = 19
> A = 238
> 1.E-28 x 6.E23 x 19 / 238 = 5.E-6
>
> The probability of fission in a 50 nm (5.E-6 cm) layer is = 5.E-6 x 5.E-6 =
> 5.E-12
> Enhancing the probability with LAM-BIAS by a factor 1.E6 gives you a
> probability of 5.E-6. Therefore, if you run a million neutrons, you have
> a chance to get about 5 fissions (close to what you call "some results").
> You can play with the above numbers as you like...
>
>> Also, would a BXDRAW routine to look at flux across a region would still be
>> usable in the presence of LAMBIAS?
>
> Yes, provided you take into account the weights of the neutrons crossing
> the boundary, which are likely to be of the order of 1.E-6.
>
> Alberto
>
>
> From: James Ryan <james.ryan-4_at_postgrad.manchester.ac.uk>
> Date: Tue, 3 Jun 2014 13:04:58 +0000
>
> Thanks for the quick response Alberto,
>
> I tried using the lambias card with a reduction factor of 0.001 but still had
> nothing, so I then used 1.E-6 which gave some results (of the order 1.E-11
> fissions per beam particle). Is this number realistic?
>
> Also, would a BXDRAW routine to look at flux across a region would still be
> usable in the presence of LAMBIAS?
>
>
>
> James Ryan
> Nuclear FiRST DTC
> University of Manchester
> james.ryan-4_at_postgrad.manchester.ac.uk
> +447890 486 709
>
>
Received on Tue Jun 17 2014 - 12:33:52 CEST

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