Dear Nikhil,
"you are estimating neutron yield which is n/p. Therefore the error on neutron yield should be Sqrt[n/p] instead of Sqrt[n]/p."
No, this is not correct. Since p (=the number of protons on target) is fixed in each simulation, the only source of error is n, and d(n/p)=dn/p, where I assumed dn=Sqrt[n] (since they are statistical fluctuations). If also p could be a source of error, then
d(n/p)=Sqrt[ (dn/p)^2 + (dp*n/p^2)^2 ]
However since dp=0 we are back to the previous expression.
By the way, if the statistical error was actually Sqrt[n/p], a simulation with 100 protons would have the same statistical error over n/p than one with 10^6 protons, which is clearly impossible.
"Finally, it is perfectly normal for the neutron yield to fluctuate with changing dimensions if one considers the parasitic absorption, given the high neutron absorption cross section of tungsten."
Yes, I agree on this, however in my results the neutron yield was going down increasing the dimension of the target (which is reasonable) then up again (which is a bit weirder, even if I would not say that it's completely impossible: in any case this effect disappeared increasing the statistic).
See you
Emilio
-----Original Messages-----
From: "nikhil shetty" <nikhil.nitk_at_gmail.com>
Sent Time: Monday, October 24, 2016
To: emilio_at_impcas.ac.cn
Cc: fluka-discuss <fluka-discuss_at_fluka.org>
Subject: Re: [fluka-discuss]: Statistical Fluctuations in neutron yield
Hi Emilio,
you are estimating neutron yield which is n/p. Therefore the error on neutron yield should be Sqrt[n/p] instead of Sqrt[n]/p.
This also explains why you observe, from the statistical experiment you did, the difference by a factor of 3 ( ~ Sqrt(10^6/10^5) ).
Finally, it is perfectly normal for the neutron yield to fluctuate with changing dimensions if one considers the parasitic absorption, given the high neutron absorption cross section of tungsten.
Cheers,
Nikhil
On Sun, Oct 23, 2016 at 11:42 AM, <emilio_at_impcas.ac.cn> wrote:
Dear FLUKA experts,
I am simulating the neutron production from a cylindrical W target using a 400 MeV proton beam; using the loop function, I change the radius R and the length L of the cylinder, scoring the total neutron produced.
At first, I tried with 10^5 proton; however I found some weird results (the neutron yield was decreasing and then increasing again changing L); for this reason I increased the primary particles up to 10^6, and the results were more consistent (you can find the imput cards in the attached files).
My problem is that the difference between the two bunch of simulations seems too large to be explained with statistical fluctuation: in the file "Neutron Yield 400 MeV.pdf" you can find the neutron yield n/p with 10^5 (solid curves) and 10^6 (dashed curves) protons; each color corresponds to a different radius. I also included the error bar due to the statistical fluctuation, calculated as Sqrt[n]/p, where n is the number of neutrons ! scored, p is the number of proton. There are several points at 6 and more sigma's, which is not possible with 30 datapoints. In general it looks like the statistical fluctuations are roughly 3 times larger than what expected with Sqrt[n].
Is it not correct to assume that the statistical error would be Sqrt[n]? Am I using some biasing that I am not aware of?
Thank you for your help
Emilio Ciuffoli
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Received on Mon Oct 24 2016 - 18:13:16 CEST