e beam current

[Alexander Somov <somov@jlab.org>]

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