Re: e beam current

[Alexander Somov <somov@jlab.org>]

Hello Richard,

I double checked the rates and found a
mistake in my normalization (just ignore
my previous mail). The rate difference
between beamgen and Geant is small, about
15%.

Cheers,
        Sascha




On Thu, 19 Feb 2009, Alexander Somov wrote:

>
> Hello Richard,
>
> I have a question regarding the electron beam current:
>
> I am trying to compare the photon rates from the beamgen
> with that predicted by geant (when electrons are passed
> to the diamond directly) in the incoherent region
> 1 < Egamma <3 GeV.
>
>
> The expected number of photons per one electron  in this
> range can be simply estimated
>
> N_gamma /1 el  = (20 10^-4/18.8) [4/3 * ln(3) - 4/3 *(3-1)/12] =
>          1.32 e-4
> Note: 18.8 corresponds to the radiation length of carbon
>      rather than a diamond
>
> The photon rate in the beam energy rand 1 < Egamma < 3 is
> N_gamma =  1.6 10^19 * 2.2 10^-6 * 1.32 10^-4 = 4.65 10^9 gamma/sec,
> where 2.2 10^-6 stands for the electron beam current
>
> When I run Geant I get 4.55 10^9 gamma/sec, i.e., close to the predicted
> value of 4.65 10^9
>
>
> However from the beamgen spectrum we get a smaller rate in this
> energy range (1 < Egamma < 3 GeV), 2.59 10^9 (here I assume that
> the total photon rate for E_gamma > 0.12 GeV is 11 10^9).
>
> The rates ratio R(Geant)/R(beamgen) = 4.55/2.59 ~ 1.76.
>
> For the diamond we would expect smaller radiation length than for
> carbon R_diam ~ 11 - 12 cm (compared with 18.8 cm for carbon), so that
> the rate difference would be even larger
>
> R(geant)/R(beam) ~ 1.76*18.8/12 ~ 2.8
> --------------------------------------
>
> All in all, if we run with a 20 micron diamond the electron
> beam current seems to be about a factor of 2-3 smaller, i.e.
> I_el = 2.2 micro amp /2.8 (?)
>
> Did I miss smth in this estimates (I a cheking them once again) ?
>
> Cheers,
>       Sascha
>
>
> P.S. If the electron beam current is smaller (?) it might
> affect other experiments if they have requirements on the
> lumi
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