chromium is not present in software center

there is no chromium web browser in software center to download it see the screen shot and this is the out put of terminal :)

bil21al@bil21al-ThinkPad-T60:~$ sudo apt-get install chromium web-browser
[sudo] password for bil21al:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package chromium is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  chromium-bsu

E: Package 'chromium' has no installation candidate
E: Unable to locate package web-browser

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: software-center 5.1.2.1
ProcVersionSignature: Ubuntu 3.2.0-2.5-generic 3.2.0-rc3
Uname: Linux 3.2.0-2-generic i686
ApportVersion: 1.90-0ubuntu1
Architecture: i386
Date: Thu Dec 1 15:38:48 2011
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Alpha i386 (20111129.1)
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: software-center
UpgradeStatus: No upgrade log present (probably fresh install)