Re: [GlueX] Radiation lengths in Hall B beam line

[Elton Smith <elton@jlab.org>]

Hi All,

The one item missing is the veto counter (5 mm thick) in front of the
calorimeter. This would be a 0.5/42cm = 0.012 rad lengths. However, we
should read this out into an ADC for use as a software veto.

Cheers, Elton.




Elton Smith
Jefferson Lab
elton@jlab.org
(757) 269-7625

On Tue, 1 Aug 2006, Rafael Hakobyan wrote:

> Dear Daniel,
>
> Thanks a lot for your detailed estimate. As I mentioned this morning,
> the simulation showed such pair
> production that hits the BCAL, and the rate of those cases is very
> small. Of course, in addition to helium bags,
> hydrogen target and two Aluminium foils (71 micrometer), in my
> simulation, I considered the existens of BPM (scintillator of 4mm
> thickness, I took the one that has been used in PRIMEX) and aluminized
> mylar windows as well, but of course the latters (BPM and Al-mylar) will
> not change the result of your estimate, i.e. will not be larger than 2%,
> because your estimate showed that the fraction of photons producing
> pairs is well under 2%.
> Thanks again for your very useful estimate.
> With kind regards,
> Rafael
>
> Daniel Sober wrote:
>
> > Here are some numbers for radiation lengths in the beam line:
> >
> > 2 x 71 micrometer Al foils:  142E-6m/0.089m = 0.0016 RL
> > 18 cm H2 target filled with H2 at STP:  18cm/731000cm = 0.00002 RL
> > (ignore)
> > 25 m He at STP:  (2500cm*0.1786E-3 g/cm^3)/(94.3 g/cm^2) = 0.0047 RL
> >
> > So, if there is vacuum up to the target and helium afterwards, the
> > total is 0.0016+0.0047=0.0063 RL
> > If there is helium both before and after the target, the total is
> > 0.0016+2*0.0047=0.0110 RL
> >
> > Either way, the fraction of photons producing pairs is well under 2%,
> > so it should not be a problem.
> > Other processes will be much less probable; for example the total
> > hadronic photproduction probability on 25 m of He (0.44 g/cm^2) at the
> > gamma+p cross section maximum (~ 0.6 mb at 300 MeV) is about
> > 0.6E-27 cm^2/atom * 0.44 g/cm^2 * 6.022E23 atom/mole / (4 g/mole) * 4
> > nucleons/atom  =   0.00015
> > (more than an order of magnitude lower than pair production).
> >
> > Most of  the pairs will result in both e+ and e- hitting the BCAL, so
> > the energy sum will be the same as for a photon, but with a larger
> > spatial distribution (the characteristic angle, like that of
> > bremsstrahlung, is about m/E, which for 200 MeV gives a separation of
> > about 6 cm at the BCAL if the pair is produced 25 m upstream, i.e. in
> > the target region.)
> >
> > Dan
>
>
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