Re: rescaling photon beam rate in hdgeant

[Alexander Somov <somov@jlab.org>]

Hi Mark,

E_gamma is a cut off energy for bremsstrahlung photons
In order not to generate too much soft beam photons
(the spectrum goes like ~1/E), one can apply this cut.

For instance, if you generate hadronic events, your energy
threshold  should be ~0.15 GeV (pion production) - makes
no sense to lower it.

For the electromagnetic background studies you can now lower
this threshold in the control.in file directly

-----

By default, the rate is normalized to E_gamma = 0.12 GeV  (it was a
hardcoded  value until recently), i.e.,  it corresponds to a beam
photon rate of  ~11 GHz before the collimator (for the lumi of 10^8).
If you lower the energy threshold, the photon rate, indeed, increases.

The value of the threshold to use depends on what exactly you want to
study.

Cheers,
      Sascha






On Mon, 7 Jul 2008, Mark M. Ito wrote:

> Folks,
>
> Question for y'all on hdgeant. Trying to get myself started.
>
> In the default control.in for hdgeant, there is a comment as follows:
>
> c If Emin is set in the BEAM card (see above), the rate of background
> photons
> c is automatically rescaled as follows:
> c       Rate(E_gamma > Thr)  =  Rate(E_gamma = 0.12 Gev) * K,
> c where K is a scale factor calculated inside uginit.F:
> c       K = 1 for Thr = 0.12 GeV,
> c       K > 1 for Thr < 0.12 GeV.
> c Note, K is calculated assuming the beam energies Emax = 12, Epeak = 9.
>
> which I am finding hard to understand. What is the intent of this
> feature? What is meant by Rate(E_gamma=0.12 GeV)?
>
>   -- Mark
>