Re: Cross section and radiation lenght!

From: Francesco Cerutti <>
Date: Mon, 3 Nov 2008 23:56:05 +0100

Dear Jonathan,

>Looking the aluminum property from the basic output, "Inelastic
>Scattering Length" is 13.68 cm that implies the 9-Be beam flux will be
>reduced crossing the 5 mm Al target to reach 96% of his initial value.

that's right (in real life)

>Using two USRBDX, one for the "incoming" flux and the other for the
>"outgoing" one, we see we have the same flux so it seems there are not
>inelastic reaction.

you are still right: in fact no inelastic reactions are performed for
your beam particles in the simulation. If you look into the .out file, you
will find that the lower threshold for ION nuclear interactions is
set to 100.1 MeV/n, corresponding to the default lower threshold of the
rQMD event generator. You can lower this threshold down to 50.1 MeV/n (not
below, due to applicability limits) through the card PHYSICS

>Then, looking information from USRYIELD it is said that the cross
>section is about 0.45 barn it means 36 cm for the total scattering

For a HEAVYION beam, USRYIELD uses as sigma (if a sigma is requested -
meaningful only for thin target studies, with a beam of well defined
energy - ) the inelastic cross section on the requested material of a
proton with the same velocity, i.e. 0.45 barn in your case, giving an
inelastic scattering length of 36.9 cm. In order to get the actual 9Be
inelastic cross section on Al, you have to add another USRYIELD card with
SDUM=BEAMDEF putting in WHAT(3) the total momentum of your beam ion, i.e.
3.9925 GeV/c.

Hope this helps


Francesco Cerutti
CH-1211 Geneva 23
tel. ++41 22 7678962
fax ++41 22 7668854
Received on Tue Nov 04 2008 - 09:10:57 CET

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