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Forum: Biasing and Scoring
Date: 30 Jan, 2014
From: <smharding>

I have a quick question in regards to setting up a bias towards a given spectrum. I posed this question previously on SPENVIS and was given a bit of information before being directed here, so let me recap a bit before I pose my questions:

Given a spectrum, let's say:

/gps/hist/point 0.1 1.16E+06

/gps/hist/point 0.2 4.78E+05

/gps/hist/point 0.3 2.70E+05

/gps/hist/point 0.4 1.65E+05

/gps/hist/point 0.5 1.09E+05

/gps/hist/point 0.7 5.98E+04

/gps/hist/point 1 3.07E+04

/gps/hist/point 2 3.08E+03

/gps/hist/point 3 5.88E+02

So on and so forth

To bias it with regards to energy, we use a /gps/hist/type biase command. Now, here's where it gets a bit fuzzy. I was told on the SPENVIS forums to run something similar to:

/gps/hist/type biase

Followed by a spectrum, and again following it with two columns. The way I was told to calculate the columns, however, could not be explained as well as I would have preferred, but was was told to me was that following the biase command, I would list a spectrum that followed with two columns that looked something like this

/gps/hist/point (calculated probability) (inverse of differential flux)

So in a macro file it would look something like:

/gps/ene/type blah /gps/hist/type blah /min blah /max blah

/primary spectrum

/gps/hist/type biase

/bias spectrum

(normalization factor, etc)

And then whatever other options I wanted to include, assuming I am in fact supposed to use BOTH spectrum.

My questions are:

When calculating the probability given the formula (or technically, approximating the formula provided), a lot of values ended up towards 0.999***. All the way up to one. A few started around 0.2 or some such, but it shot up quickly given that formula. Now, in my mind, these numbers are creating a curve that implies probability, so a point with 0.999 is approximately twice as likely to occur as a point with 0.5 or so. Is this accurate, or am I way off?

My second question is in regards to the second column. I don't understand why it was required that I used the inverse of the differential flux, and an answer to that could not be given. Does anyone on here understand what is necessary to use, or why I might use the inverse differential flux?

And lastly, is there any literature explaining this better that someone could point me to? The Geant4 manual sparsely covers biasing, and what little it does include doesn't really answer my questions...

Thanks so much for any assistance. I realize this is a large multi-part question, and I apologize. But I have so many questions about how to properly use the biasing tool in Geant4 and I cannot seem to find any satisfactory solutions online!

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1 Question: Re: Question regarding event biasing   (smharding - 30 Jan, 2014)
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